Prerpints.org logo
Preprint
Review

Biochar-Derived Persistent Free Radicals: A Plethora of Environmental Applications in a Lights and Shadows Scenario

Altmetrics

Downloads

89

Views

34

Comments

0

A peer-reviewed article of this preprint also exists.

This version is not peer-reviewed

Submitted:

14 March 2024

Posted:

15 March 2024

You are already at the latest version

Alerts
Abstract
Biochar (BC) is a carbonaceous material obtained by pyrolysis at 200−1000 °C in the limited presence of O2 of different vegetable and animal biomass feedstocks. BC has demonstrated great potential, mainly in environmental applications, due to its high sorption ability and persistent free radicals (PFRs) content. These characteristics enable BCs to carry out the direct and PFRs-mediated removal/degradation of environmental organic and inorganic contaminants. The types of PFRs possibly present in BC depend mainly on the pyrolysis temperature and the kind of pristine biomass. Since they also can cause ecological and human damage, a systematic evaluation of the environmental behavior, risks, or management techniques of BC-derived PFRs is urgent. PFRs generally could consist of a mixture of carbon- and oxygen-centered radicals, and of oxygenated carbon-centered radicals, depending on the pyrolytic conditions. Here, to promote a more productive and beneficial use of BC and related PFRs and stimulate further studies to make them environmentally safer and less hazardous to humans, we have first reviewed the most common methods used to produce BC, its main environmental applications, the primary mechanisms by which BC remove xenobiotics, as well as the reported mechanisms for PFRs formation in BCs. Secondly, we have discussed the PFRs environmental migration and transformation; we have reported the main PFRs-mediated applications of BCs to degrade inorganic and organic pollutants, the correlated environmental potential risks, and the possible strategies to limit them.
Keywords: 
Subject: Environmental and Earth Sciences  -   Waste Management and Disposal

1. Introduction

Biochar (BC) is a stable carbon-rich black solid produced from vegetable or animal biomass feedstocks when pyrolyzed. Pyrolysis is a procedure that involves the heating of substrates at 200-1000 °C under oxygen-limited conditions [1]. The term “biochar” derives from the combination of “bio-,” which stands for “biomass,” and “char,” meaning “charcoal.” In recent years, BC has received wide attention due to its potential application in carbon sequestration, soil amendment/remediation, wastewater treatment, and catalysis [2]. Batch and column sorption experiments have shown that certain types of BC have good adsorption performance for heavy metals, dyes, or phosphate from aqueous solutions and are being investigated as cost-effective, promising, and eco-friendly alternative adsorbent materials [3]. As a soil improver, BC can reduce soil acidity and help maintaining soil moisture and nutrient levels. Through its carbon sequestration action, BC performs climate restoration. Moreover, due to its strong adsorption capacity, BC can remove environmental xenobiotics, thus preventing their uptake in plants, animals, and humans [4,5,6,7]. Additionally, BC derived from thermal treatment of organic material generally contains persistent free radicals (PFRs) bound to the external or internal surfaces of its solid particles’ [8,9]. Such BC-bound PFRs, which are reactive due to unpaired electrons, can persist for minutes and up to several months, in contrast to traditional transient radicals [10], thus conferring BC the capacity to degrade organic pollutants through the generation of other reactive oxygen species (ROS) and sulfate radicals [11,12,13]. In this context, BC-bound PFRs have been investigated to activate persulfate (S2O82−) to obtain sulfate radicals, which efficiently degraded phenolic compounds and polychlorinated biphenyls [14], acid orange 7 [15], and sulfamethoxazole [16,17]. In the presence of PFRs, hydrogen peroxide (H2O2) or oxygen (O2) are activated to produce hydroxyl radicals (OH•) and superoxide radical (O2•-), which succeeded in efficiently degrading chloro-biphenyl [18], diethyl phthalate [19,20], and ciprofloxacin [21]. By the PFRs-mediated activation of peroxyl mono sulfate (PMS), radical species such as SO4•-, •OH, and O2•-, as well as non-radical species such as 1O2 formed that were the main contributors to the degradation of antibiotics [22]. On the other hand, by stimulating the production of ROS, PFRs can inhibit seed germination and retard the growth of roots and shoots [23]. Additionally, the BC production itself may cause the release of xenobiotics such as polycyclic aromatic hydrocarbons (PAHs), toxic inorganic elements, and dioxins, thus posing potential risks to human health and the environment [1]. The scientific community should evaluate BC and BC-bound PFRs’ positive and negative impacts before their extensive ecological applications. Although the environmental behavior and risks of BC and BC-associated PRFs are increasingly attracting the researchers’ attention, in the last ten years, the study about their toxicity remains still very limited (96 publications), if compared to that concerning PFRs in general (542 publications) and their degradation capability (402 publications) (Figure 1).
However, to safeguard the environment from BC-related PFRs' potential adverse effects, it is necessary to comprehensively and systematically consider their environmental risks, formation mechanisms, and controlling factors [13], as well as the corresponding possible mitigation actions. Studies have shown that the type of biomass feedstock used to produce BC is pivotal in determining the physicochemical properties of the resulting BC, which also strongly affect the formation and characteristics of PFRs. To date, the types of biomasses used to prepare BC and involved in the investigation of BC-bound PFRs mainly include lignocellulosic biomasses (hemicellulose, cellulose, and lignin), such as pine needles, wheat straw, lignin, cow manure, rice husk, and maize straw [24,25,26]. Bamboo is an emerging starting material perfect for synthesizing BC and activated carbon (AC) due to its inexpensive cost, high biomass yield, and accelerated growth rate [27]. Anyway, only a few researchers and scientists have used bamboo as a unique source for developing BC so far, as established by the number of publications on bamboo-derived BC developed in the last ten years (331) vs. those on BC derived by different other sources (13630) (Figure 2).
Among the 331 publications on bamboo-derived BC, most parts were about their adsorption activity (119), followed by those on their degradation capacity (87). At the same time, few studies were conducted on their possible toxic action (Figure 3).
In this scenario, to promote a more productive and beneficial use of BC and related PFRs and stimulate further studies to make them environmentally safer and less hazardous to humans, we have first reviewed the most common methods used to produce BC, its main environmental applications, as well as the reported mechanisms for PFRs formation in BC. The main factors influencing the physicochemical properties of BC have also been reported. Secondly, we have discussed the PFRs environmental migration and transformation, the main PFRs-mediated applications of BC to remediate inorganic and organic pollutants described in the last five years, the correlated potential risks to the environment and humans, and the possible strategies to limit them.

2. Biochar (BC)

The constant growth in world population translates into a continued increase in the global energy requirement by all sectors and a dramatic decrease in fossil fuels, the primary energy source [28,29,30]. Furthermore, the effect of the CO2 emissions on the environment determines additional global energy issues, which make the replacement of fossil fuels necessary and urgent [31]. In this regard, biochar (BC) mainly obtained from organic waste, and possessing the capability of sequestering carbon, represents a rich carbon source and an alternative to fossil fuels [32,33]. Table 1 report the biomasses commonly used for BC production [34,35,36,37,38,39,40,41,42,43,44,45,46,47].
BC obtained by the combustion of biomasses reported in Table 1 have demonstrated to possesses nonpareil physicochemical properties such as large surface area, high porosity, presence of several functional groups, high cation exchange capacity (CEC), and long-term stability, etc. (Figure 4). Such properties make BC suitable for various applications, including but not only carbon sequestration, soil amendment, energy storage, catalysis [48,49,50,51,52,53,54,55,56] (Table 1). Additionally, BC is cost-effective, has an eco-friendly nature, and is endowed with reusability (Figure 4) [57,58]. Mainly, BC is increasingly gaining attention by many researchers as material to efficiently remove various environmental contaminants, including antibiotics, thus reducing the emergence of microbial resistance [53,55].
Among the biomass waste materials appropriate for BC production, crop residues from agriculture, forestry, municipal solid waste, food, and animal manures have a high potential [59,60,61,62,63,64].

2.1. Main Methods to Produce Biochar

As reported in the following Table 2, BC can be quickly and fast prepared using thermochemical conversion techniques such as pyrolysis, hydrothermal carbonization, gasification, flash carbonization, and torrefaction [65,66], being pyrolysis the most adopted (Section 2.1.1).

2.1.1. Pyrolysis

Pyrolysis is a thermochemical process where the organic compounds present in the biomass are decomposed at a specific temperature [72]. Mainly, during pyrolysis, the thermal decomposition of organic materials occurs in an oxygen-free or oxygen-limited environment under the temperature range of 250-1000 °C [73]. In these conditions, the lignocellulosic components of biomass, such as cellulose, hemicellulose, and lignin, go through chemical reactions like depolymerization, fragmentation and cross-linking depending on the adopted temperatures. Three are the principal possible products, including solid, liquid, and gas physical state materials. The solid products comprise BC and ash, the liquid ones encompass bio-oils and tar, whereas the gaseous products (syngas) comprise carbon dioxide, carbon monoxide, hydrogen, and C1-C2 hydrocarbons [67]. As shown in Figure 5, during pyrolysis, the process parameters, including temperature, types and nature of biomass, residence time, heating rate, pressure, etc., could strongly affect BC's yield and its physicochemical characteristics [74,75]. Moreover, although BC derived from different biomasses are all entirely made of carbon content and ash, their elemental composition, as well as their physical characteristics and properties, could differ enormously based on the type of biomass, reaction conditions, and type of reactors used during the carbonization process [76] (Figure 5). Consequently, every experimental condition and the starting raw material should be considered for the proof-of-concept of the future industrial application of the BC.
The most used reactors for the chemical transformation of different biomasses include paddle kilns, bubbling fluidized beds, wagon reactors, tubular ovens, and agitated sand rotating kilns. However, temperature remains the primary operating process condition that governs the yield in BC vs those of the oily and gaseous products. Usually, BC yield decreases, and syngas production increases when the pyrolysis temperature is improved [77]. Based on the heating rate, temperature, residence time, and pressure, pyrolysis can be categorized as fast or slow, as summarized in Table 3. Generally, fast pyrolysis is employed to maximize the liquid product yield, while slow pyrolysis is employed to maximize the solid product yield [78].

2.2. Biochar Characterization and Main Properties

The characterization of BC aiming at determining its elemental composition, its surface functional groups, stability and structure is usually carried out employing various modern techniques such as Scanning Electron Microscopy (SEM), Transmission electron microscopy analysis (TEM), Elemental Analysis (EA), Energy-Dispersive X-ray Spectroscopy (EDXS), X-ray Fluorescence spectroscopy (XRF), Fourier Transform Infrared Spectrometer (FTIR), Thermo Gravimetric Analysis (TGA), X-Ray Diffraction (XRD), Brunauer-Emmett-Teller (BET), Transmission Electron Microscopy (TEM), Nuclear Magnetic Resonance (NMR), Raman Spectroscopy, X-ray photoelectron spectroscopy (XPS), Electron Energy Loss Spectroscopy (EELS), Cation Exchange Capacity (CEC), and Electron Paramagnetic Resonance (EPR) or Electron Spin Resonance (ESR) spectroscopy [38]. Table 4 summarizes the analytical techniques typically used to characterize BC regarding their physicochemical, surface, and structural characterization.
As mentioned, the source of feedstock and the heat treatment temperatures during preparation are two significant factors that determine the physiochemical properties of BC.
The properties of pristine biomass that mainly influence the related BC include moisture content, ash content, calorific value, percentage of lignin, cellulose, hemicellulose, fractions of fixed carbon, and volatile components [78]. High-yield BC with high porosity is achievable using biomasses owing more lignin and less cellulose. Additionally, the volatile component, water content, particle size, and shape of the original biomass can also affect the property of BC [78]. Table 5 reports the general chemical and physical features of BC, while Table 6 reports some characteristics of BCs produced from specific feedstocks at various production temperatures.

2.2.1. The Question of Temperature

As already reported, the pyrolysis temperature and feedstock greatly influence the physicochemical properties of BC, including pH, specific surface area, pore size, CEC, volatile matter, ash, and carbon content. CEC and volatile matter decrease with increasing pyrolysis temperature, whereas pH, specific surface area, ash, carbon content, and pore volume increase with the increase in pyrolysis temperature [81]. Increasing temperature also causes the decrease of the number of acidic functional groups, especially carboxylic functional groups, and causes the appearance of carbonylic functional groups and alkalinity [82]. Particularly, unpaired negative charges forming during pyrolysis at higher temperatures enable BC to accept protons [82]. Although BC’s alkalinity increases with higher pyrolysis temperatures, thus improving its capacity to neutralize acids in soils, lower temperatures are necessary to preserve functional groups and obtain BC with higher CEC [83]. Low water content in BC, which reduces the possible microbial activity promoting self-healing and degradation, is achievable at a higher temperature. However, the highly porous structure of BC obtained in such conditions causes the ready adsorption of moisture from the surroundings, thus increasing water content, re-enabling microbial activity, and contributing to self-heating and degradation [81].
During biomass decomposition to BC, the total surface area changes like the porosity due to the escaping of volatile gases and increases with increasing temperature [84]. In this regard, a large surface area affects CEC and water holding capacity (WHC). Curiously, during pyrolysis, the hydro-properties of initial biomass undergo several modifications depending on the pyrolysis temperature, which can translate into contradicting findings. Notably, with increasing temperature, due to a decrease in functional oxygenated groups and an increase in aromatic structure, the material’s affinity to water is altered, the hydrophobicity of BC becomes higher than that of pristine biomass, and its capacity to retain water should be lower. On the contrary, thanks to increased porosity that changes the amount of water that can be adsorbed, BCs produced at high temperatures can hold more water in their porous structure than BC prepared at lower ones [85].
The mechanical stability of biomass usually decreases during pyrolysis and correlates inversely with the porosity and directly to the density of the BC and temperature. The electric conductivity increases with higher thermal treatment, improving the graphitic carbons' crystallinity and the carbons-packed domains' density [86]. BC with high mechanical stability can be produced from feedstocks with high density and lignin content, making lignin, the constituent, more resilient to decomposition and loss of structural complexity. On the contrary, BC with higher grindability can be obtained by the torrefaction of biomass with a larger amount of hemicellulose (e.g., agricultural residues) compared to woody biomass. The decomposition of biomass to BC causes a reduction in its bulk density and an increase in its porosity and, therefore, a decrease in its thermal conductivity depending on the pyrolysis temperature. Concerning the electric properties of BC, the reduction in oxygenated functional groups and the appearance of conjugated double bonds cause an increase in conductivity and electromagnetic shielding efficiency, which make BC suitable as an additive in various composite materials (e.g., building materials such as cement). Further, the effectiveness of shielding against electromagnetic interference is enhanced concerning the pristine biomass.

2.3. Possible Biochar Applications

The various properties of BC reported above, including the high carbon content, larger surface area, well-developed porous structures, and the surface sufficiently enriched with functional groups, render it potentially pertinent for various applications. In Table 7, we have reported the current possible environmental BC applications.
BC production could be an alternative to mitigate climate change by carbon sequestration in soil, thus retaining half of the carbon fixed in biomass during photosynthesis and reducing CO2, NO2, and CH4 emissions [54]. Mainly, BC shows long-term stability in soil. The mean carbon residence time in BC has been estimated to be around 90-1600 years, depending upon the labile and intermediate stable carbon components [54]. Due to these characteristics, BC can sequester carbon in soil, thus decreasing carbon dioxide emissions into the atmosphere and those of nitrous oxide and methane by biotic and abiotic mechanisms [54]. Experiments have demonstrated that emissions of greenhouse gases (including CH4 and N2O) can be avoided by pyrolyzing waste biomasses [88]. Concurrently, the pyrolysis process balances fossil fuel consumption by producing bioenergy.
Interesting, BC has been estimated to be capable of tackling 12% of current anthropogenic carbon emissions. Furthermore, thanks to its high carbon content, BC can work as a soil conditioner, mainly by improving soil's physicochemical and biological properties. BC increases soil water retention capacity by ~18%, reduces nutrient leaching [49], and neutralizes acidic soils, thus enhancing plant productivity, seed germination, plant growth, and crop yields. Additionally, wet BC prevents the soil desiccation [49]. While it has been reported that soils treated with BC demonstrated improved microbial population and activity [89], null or positive effects were observed in the earthworm population in soils amended with wood-based BC [90].
The production of BC itself is an economical and mutually beneficial strategy to manage and eliminate waste from animals and plants and reduce the pollution associated with it [89]. Furthermore, when waste biomass deriving mainly from animal manure and sewage sludge is pyrolyzed, the hazardous microbial population possibly present is killed, thus reducing its possible negative impact on the environment and humans. Unfortunately, toxic heavy metals from sewage sludge and municipal solid waste could persist in BC, which must be carefully checked and handled correctly before long-term soil application [91].
A remarkable potential use of BC, still too little investigated and controversial, is the production of bioenergy, as an alternative to fossil fuel, that could lower carbon emissions. In this regard, while slow pyrolysis allows a lower yield of liquid fuel and more BC, fast pyrolysis provides more liquid fuel (bio-oil) and less BC [92]. Evidence has demonstrated that BC can be successfully applied in environmental remediation because it is capable of adsorbing both organic and inorganic contaminants, such as pesticides, herbicides, PAH, dyes, and antibiotics, as well as non-biodegradable metals ions, that are highly toxic to all living organisms [53,55]. BC can enhance the composting process by improving its physicochemical properties and microbial activities and promoting the decomposition of organic matter. Also, more investigations are needed to evaluate BC compost's agricultural/environmental performance [50]. Table 8 summarizes some advantages and disadvantages associated with the production and use of BC.
As evidenced in Table 8, we are reimbursed with additional advantages by producing BC from biomass, including waste biomass. The cost necessary to produce BC is six-fold lower than that of commercially available activated carbon (AC), which, unlike BC, is deprived of some properties of BC, such as its ion exchange capacity [93]. Generally, BC does not require further processing to be activated, and thanks to its non-carbonized fraction and maintained oxygen-containing groups, such as carboxyl, hydroxyl, and phenolic surface functional groups, BC is capable of adsorbing both organic as well as inorganic contaminants, and to interacts with soil contaminants [53,55]. BCs produced from sewage sludge and manure have a high nutrient content for soils, thus enriching their quality [49]. Anyway, apart from the advantages of using BC, there is a series of possible fallouts, as reported, that need consideration. Among these, long-term removal of crop residues, like stems, leaves, and seed pods, for producing BC could reduce the overall soil health by diminishing the number of soil microorganisms and disrupting internal nutrient cycling with a possible negative impact on soil biota, including short-term adverse effects on earthworm population density. In this scenario, there is a dire need for further extensive research so that any possible issues associated with its usage can be aptly resolved.

2.3.1. Xenobiotics Removal by Biochar (BC)

As reported in the previous section, BC is a porous material, and its porosity, depending on the production temperature, allows it to interact with water nutrients and other materials, including inorganic metal cations and organic pollutants. Due to its enhanced porous structure, surface area, functional groups and mineral components, BC is an optimal absorbent material for solutions. Although BC produced through pyrolysis, has a relatively moderate adsorption capacity (3.6 g/g – 6.3 g/g for BC prepared at a temperature range of 300–700 °C) [94], it can be enhanced by modifying its physicochemical properties through acid, alkali, or oxidizing treatments, while the surface area can be altered mainly using acid treatments [95,96,97]. As an adsorbent, BC can absorb organic and inorganic contaminants such as PAH, and phthalate acid esters, and its help in improving the treatment of sewage wastewater containing organic xenobiotics has been widely reported [98]. In this context, several are the main mechanisms used by BC for capturing inorganic or organic pollutants, which have been included and discussed in Table 9.
Interesting, BCs produced at higher temperatures exhibited higher sorption efficiency for the remediation of organic and metallic contaminants in soil and water. Additionally, it is worth mentioning that the sorption of organic xenobiotics by BC is more favored than that of inorganic ones. Concerning complexation with metal cations, the smaller is the ionic radius of metals, the greater will be the adsorption capacity by BC.

2.3.2. Not Only Adsorption

It is reported that the principal mechanism by which BC removes toxic heavy metals and other contaminants, including organic pollutants, is adsorption. Its adsorptive efficiency mainly depends on the type and number of functional groups, surface area, CEC, etc. Anyway, previous research studies and reviews on BC have evidenced the presence on the surface or inside its particles, of free radicals known as persistent free radicals (PFRs), whose nature depends strongly on the pyrolysis conditions and whose formation and characteristics mainly differ based on the feedstock types. In this regard, several recent studies have mainly focused on the role of BC-related PFRs in the degradation of organic xenobiotics, in addition to their adsorptive capacity. Odinga et al., in their recent work, reviewed the application of BCs-derived PFRs in environmental pollution remediation [13], while Fang et al. investigated the reactivity of PFRs in BC and their catalytic ability to activate persulfate to degrade pollutants [14]. On the other hand, Odinga et al. also considered and commented on the possible environmental risks of PFRs from BCs, which represent the shadows associated with these chemicals and need further study, knowledge, and regulation before their extensive application [13].

3. Biochar-Derived Free Radicals

As previously mentioned, BC has a broad prospective use in the treatment of environmental xenobiotics, in soil amendment, in photocatalytic and photothermal systems, for photothermal conversion, as electrical and thermal devices, as 3D solar vapor-generation devices for water desalination [99,100,101,102]. All these potentials are due to its high surface area and rich pore structure, which determines great physical absorptivity. They also depend on the chemical characteristics of BC, including the presence of PFRs [103,104]. In this regard, it is of paramount importance to clarify the formation mechanism of the free radicals in BC for the optimal management of their properties and their more efficient and safer utilization [105].

3.1. Persistent Free Radicals (PFRs)

An atom or molecule with at least a lone pair of electrons is a chemical species characterized by significant instability and high chemical activity and is referred to as a free radical species [88]. Usually, free radicals are highly unstable and rapidly react with each other, thus destroying as soon as they form, with a consequent very short half-life. Whereas, it has been found that in BC, some free radicals named persistent free radicals (PFRs), like the radicals that naturally occur in the environment known as environmental persistent free radicals (EPFRs), can remain stable for months and play a crucial role in subsequent reactions of oxidative degradation carried out by BC containing them [11,88,106,107] (Figure 6).
Unlike other free radicals, PFRs are resonance-stabilized since they are bound to the external or internal surface of solid particles of BC. They can be analyzed by electron paramagnetic resonance spectroscopy (EPR) [11]. Figure 7a provides an example of EPR analysis of PFRs present on a solid N-doped hydro char prepared in a tube furnace at a temperature of up to 600 °C for 1 h under N2 atmosphere [108].
Their lifetime under vacuum appears infinite, while they react with molecular oxygen in the air, resulting in decay with time and simultaneous production of reactive oxygen species (ROS). In this regard, PFRs act as transition metals like Fe2+, stimulating ROS production in aqueous systems. Unlike PFRs, ROS are detectable by EPR only when captured by a proper radical scavenger as 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Figure 7b provides an example of the EPR spectrum of the unstable free radical superoxide (O2•-) when trapped by DMPO to form a long-lived nitroxide radical (DMPO−OOH). As in BC, by the EPR technique, PFRs were previously detected in combustion-generated particulate matter (PMs), sediments, and soils. PFRs are categorized into three classes, i.e., oxygen-centered PFRs (OCPFRs), carbon-centered PFRs (CCPFRs), and oxygenated carbon-centered radicals (CCPFRs-O). The EPR analyses provide three parameters: the PFRs concentration, the g-value, and the line width [88] (Figure 8).
PFRs concentration is calculated from the double integral of the EPR spectrum and can reflect the content of PFRs in BC [109]. The g-value of PFRs is a constant specific to a particular compound, reflects its hybrid nature, and provides information about the type of radical [110]. The PFRs line width in the EPR spectrum measures the peak-to-peak width. It is affected by spin–spin interactions (including electron–proton interaction and electron–electron interaction), the heteroatom effect, and the anisotropy of the spectrum [88].
The line width reflects the relaxation time of spinning electrons [111]. It has been reported that the oxidation processes that can occur using BCs mainly depend on PFRs and these parameters [106,112,113]. These parameters are, in turn, affected significantly by pyrolysis conditions, biomass types, the elemental composition of pristine biomass, and the presence of external transition metals (Table 10).
Qin et al. [114] found that the PFR concentrations in the same BC obtained at different temperatures and in different kinds of BC obtained at the same temperature were significantly different. Tao et al. [117], as well as Xiang [118] and Huang et al. [10], found that in BCs by different feedstocks, the PFRs concentration first increased with increasing temperature, reaching a maximum around 500-600°C and then decreased with further increase of temperature. The relations between the feedstocks’ properties or the BCs’ composition with the PFRs concentration were also demonstrated [115], and non-lignocellulosic-biomass produced lower PFRs concentrations than lignocellulosic-biomass under the same pyrolytic conditions, perhaps due to their lower H/C and O/C atomic ratios [116]. The types of PFRs that can be produced during pyrolysis change during the pyrolysis process and with a temperature rise, as reported in Table 10. The study speculated that the reduction in oxygen content during biomass pyrolysis may account for the progressive conversion of oxygen-centered radicals to carbon-centered radicals [116].

3.1.1. Mechanism Proposed for PFRs Formation during Biomass Pyrolysis

The several environmental sources of PFRs include atmospheric particulate matter (PM), contaminated soil, materials from thermal treatments of plastic and hazardous waste, tar balls, products of pyrolysis of biodiesel and biomass waste feedstocks at high temperatures [11]. Concerning BC-derived PFRs, it was observed that they mainly form in the post-flame and cool-zone regions of combustion systems and other thermal conversion processes. Although the actual mechanism by which PFRs form during pyrolysis remains not fully clarified, transition metals capable of electron transferring and substituted aromatics molecules present in lignin have been recognized as critical factors of PFRs formation. Anyway, high concentration of PFRs have been detected also in the product of combustion of non-aromatic cellulose in absence of transition metals [117]. Based on the temperature of pyrolysis processes, during the production of BC highly heterogeneous composite structures, comprising both labile and recalcitrant organic molecules, such as PAH, furans, and dioxins, as well as inorganic fractions, including oxides, cations, anions, and free radicals occur [119]. These fractions, products of incomplete combustion od biomass, may gradually form PFRs by different pathways including or not transition metals. As formed PFRs could be either only surface-stabilized or surface-stabilized in metal-radical complexes [13]. Generally, the breaking of covalent bonds by heat, light, electricity, and chemical energy possibly sustained by transition metals is essential to form free radicals, and during the pyrolysis process of biomass, its main constituents that are cellulose, hemicellulose, and lignin, undergo different reaction pathways at various destructive pyrolysis temperatures of 300 °C, 300–400 °C, and 350–450 °C. Figure 9 attempts to describe the possible series of events occurring during the biomass pyrolysis which could lead to the PFRs’ formation and that are chemically described in Scheme 1 (concerning lignin) and Scheme 2 (concerning cellulose and emicellulose).
First, the C-O and C-C covalent bonds of constituents of lignin vie electron transfer by transition metals or not are broken under heat to form free radical fragments, phenols, chinones, and other products of incomplete combustion. Simultaneously, the cleavage of glucoside bonds of cellulose and hemicellulose present in biomass feedstocks occurs causing depolymerization and formation of other radicals. These first radicals can couple to form bio-oil, pyrolysis syngas (CO2, CO, CH3CH3 and CH4), and BC simultaneously or abstract hydrogen from other molecules forming further radicals [115,120]. Several chemical reactions can occur including dehydration, decarboxylation with further emission of CO2, CO and H2O, aromatization, intra-molecular condensation leading to the formation of the crystalline graphene structure and graphitic radicals. During pyrolysis, the elemental composition of biomass undergoes changes that cause mutations in the types of radicals that, upon entrapment onto the BCs’ surface, and/or formation of metals-radical complexes form stable PFRs [10].
According to what reported in literature, the possible types of PFRs comprise (i) transition metal mediated PFRs or (ii) PFRs inside organic matrices forming during biomass pyrolysis to give BC [121]. The transition metal mediated PFRs formation starts with the initial physisorption of an aromatic substituted molecule or of its degradation intermediate radicals generated at 150-400 °C or under UV irradiation onto the transition metal oxides such as ZnO, NiO, CuO, Fe2O3, and TiO2 or transition metal ions [122]. Then, a chemisorption occurs by forming a chemical bond eliminating water or hydrogen chloride. Finally, a single electron is transferred from substituted aromatics to the center of transition metals, leading to the simultaneous reduction of metal and forming PFRs [122], whose stability is attributed to the synergy of metals and aromatic compounds [122]. Transition metal accepts an electron, and its valence changes from high to low during this process.
Unlike PFRs discussed previously, PFRs formed inside the matrix of organic moieties are not related to the presence of transition metals [121]. Still, they are highly dependent on the organic matter, while their concentration is significantly and positively correlated with the elemental carbon content [121]. In this case, PFRs are compared in thermally treated particles, and the breaking of chemical bonds in the precursor molecules during pyrolysis is the primary reason. At the initial pyrolysis stage, the homolytic cleavage of weak linkage bonds like the α- and β-alkyl aryl ether bonds, C-C, and C-O linkage resulted in forming free radicals in BC. The outer-surface free radicals would rapidly react and dissipate, resulting in a decrease in EPR signals. The free radical concentrations then increased with the extended pyrolysis and during the cooling stage, thus accumulating many free radicals on the BC surface [121] and dramatically growing the EPR signals. The free radicals formed in the matrix of the produced BC are probably protected from reacting with each other or other chemicals and thus stabilized.
As above mentioned, the type of biomass and its elemental composition, the presence of oxygenated functional groups, the pyrolysis conditions (temperature, heating time, and heating rate), and the presence of external transition metal as well as phenolic compounds strongly affect both the concentration, structure, and type of PFRs present in BC. Notably, no radical is produced during the first stage of pyrolysis, providing the transition char (< 300 °C). Subsequently, in the second stage of pyrolysis (300-500 °C), amorphous char is produced, and oxygen-centered radicals and oxygenated carbon-centered radicals appear. In the third stage of pyrolysis at 500-700 °C, composite char is created where the concentration of PFRs, including carbon-centered and oxygenated carbon-centered radicals, drastically decrease. Finally, when turbostratic char is produced (> 700 °C) little or no PFRs are further produced [13]. In the EPR, the g-factor values, even if they could change due to the presence of metal ions and temperature changes, are specific for a type of radical. Table 11 reports the main types of radicals recognizable in BC and their specific g-values.

3.2. PFRs: Lights and Shadows

3.2.1. PFRs Lights

It has been demonstrated that PFRs originating in BC by combustion in the presence or absence of external transition metals could play a vital role in several beneficial reactions, such as PFRs-mediated remediation and degradation of organic and inorganic pollutants by different actions and mechanisms, including oxidative and reductive processes (Table 12).
For instance, PFRs on BC can activate hydrogen peroxide (H2O2) or oxygen (O2), as well as persulfate (S2O82−) to produce different free oxygenated radicals (ROS) capable of efficiently degrade organic contaminants such as chloro-biphenyl [18], phenolic compounds and polychlorinated biphenyls [7], diethyl phthalate [19], thiacloprid [104], and bisphenol A [11]. Moreover, organic chemicals can also be directly degraded on the BC surface by macromolecular free radicals without adding any radical activators [55]. Semiquinone-type radicals present in BC can oxidize As (III) [124]. At the same time, BCs can also exhibit highly effective removal of Cr (VI) by reduction to Cr (III) using PFRs for industrial wastewater remediation [125,126,127,128,129].
Unfortunately, PFRs, by generating surface-bound hydroxyl radicals and free hydroxyl radicals in aqueous solution also in the absence of H2O2, can induce various types of cardiovascular and pulmonary disease through ROS-induced oxidative stress (OS) [11]. PFRs and OH radicals detected in the biological fluids generated ROS that induced an oxidant injury and modulated toxic responses in biological tissues [132]. Moreover, quinoid redox cycling is another possible path causing the formation of ROS from material containing semiquinone-type radicals, which could exert toxicity like that exercised by the combustion products present in cigarette smoke [133]. Although BC has beneficial effects on agricultural soil, PFRs in BCs could inhibit plant germination and growth when used in soil remediation. BC addition as a soil amendment has been reported to positively affect plant germination, growth, and yield [134]. In contrast, a negative impact has also been documented when BC-bounded PFRs induce ROS, which can inhibit seed germination and retard growth of root and shoot [18,21]. As shown in Figure 10, the formation and presence of PFRs in the BC produced by several biomasses have been widely documented and studied since 2014.
In this regard, in Table 13, we have reported a random selection of the main experimental works regarding the PFRs found in BCs obtained by different biomasses conveyed in the last five years (2019-2023). Table 13 also summarizes their reported applications including mainly the oxidative degradation of organic environmental pollutants (51 papers), the removal of hazardous inorganic compounds from wastewater such as As (III) and Cr (VI) (12 papers), the degradation of biological samples including bacteria (3 papers), hormones (4 works), genes of bacterial resistance (1 paper), and their use as electrical devices due to their electron and electron donor capacity (EAC and EDC).
As for the mechanisms, many publications regarded the activation, sometimes photocatalytic, of PS, PMS, and PDS by BC. The employed BC was derived from different feedstock biomasses (bagasse powder, poplar, and pine sawdust, cellulose, lignin, Blue algae, waste straw, and other sources as reported in Table13), not doped or doped with nitrogen atoms or different metals including Fe, Mn, Co, Ni, Zn. In these processes, the electron transfer promoted by metals and/or PFRs of diverse nature, based on the pyrolysis conditions, generated ROS such as SO4•-, •OH, •O2-, •O2H and non-radical species (1O2), which carried out the oxidative degradation of different organic xenobiotics, including drugs, dyes, antibiotics, hormones, as well as phenols or aromatic derivatives. Many other publications reported the use of BC to activate or photochemically activate O2 or H2O2 (Fenton-like systems) via metal and/or PFRs-mediated single electron transfer. The generated ROS (•OH, •O2-, •O2H) and oxygen non-radical species (1O2) successfully oxidized several organic pollutants, degraded hormones and eDNA and, in some cases, showed antibacterial effects against E. coli and S. aureus. Moreover, the capacity of BC to transfer electrons via transitional metals or PFRs was used to oxidize As (III) to As (V) or reduce Cr (VI) to Cr (III), thus resulting in helping to remove hazardous inorganic contaminants from industrial wastewater.

3.2.2. BC-Associated PFRs Shadows: Cytotoxicity and Biotoxicity

Despite the plethora of possible beneficial applications of BC, PFRs, as well as other free radicals and toxic substances that compose it, such as heavy metals, PAHs, dioxins, and perfluorochemicals (PFCs), are released into the environment during the pyrolysis process, thus representing a potential risk to the environment and humans [194]. Additionally, as well as other contaminants, the possible carbon allotropes forming during pyrolysis are severe contaminants in air, water, and soil [195]. Black carbon, carbon black (CB), carbon nanotubes, graphene, quantum dots, and fullerenes can possess distinct toxicity depending on many factors, including type of allotrope, particle size, form, structural defects, coating molecules, grade of functionalization [195]. Understanding the toxicity of such carbon nanomaterials and nanoproducts possibly present in BC is essential for human and environmental health, safety, and public acceptance. In this regard, recent studies have focused their attention on the adverse effects of BC due to its particle size and the various interactions with the environment that could occur [196,197]. Upon its application, BC may produce harmful environmental effects due to aging by oxidative or biological processes leading to changes in its properties [198,199]. Additionally, higher toxicity has been reported for BC with micro or nano dimensioned particles. It has been reported that the presence of micro-BC (MBC) or nano-BC (NBC) can promote the release of heavy metal ions into the medium when applied to soil [194]. Kim et al. (2018) observed that BC particles with a particle size of less than 0.45 μm could increase the release and mobility of As in soil [200]. Regarding the biotoxicity of MBC/NBC, it has been previously reported that particle-induced oxidative stress is a crucial mechanism of MBC/NBC cytotoxicity, which increases as the particle size decreases. Also, the PFRs concentration on the surface of particles with an aerodynamic diameter of less than 1 μm is the highest [121,201]. While, several reviews and studies exist on the production and modification of BC, the reaction mechanisms, and the beneficial active role of BC in environmental remediation, the adverse effects and potential risks of BC have only recently been evidenced. The comprehensive phenomena and mechanisms involved in BC toxicity still require elucidation, especially in environmental media different from soil, including water and the atmosphere. It is imperative to systematically study and discuss the possible adverse environmental effects of BC application concerning various media, including water and atmosphere, by determining the corresponding occurrence, detection, assessment, and avoidance measures. Worryingly, the current knowledge concerning the possible adverse effects on the environment and biota deriving from the extensive application of PFRs originating in BC is even more limited [202]. Although they are emerging as contaminants of increasing concern, their formation, fate, toxicity, and health risks are poorly known [202]. Thermal treatment, a common remediation technique to clean industrial soils, induces the formation of PFRs, which could paradoxically increase soil toxicity, contrary to the original remediation objective. For example, there is still little knowledge on the formation and toxicity of PFRs in soils contaminated by polycyclic aromatic hydrocarbons (PAHs) [203]. BC-derived PFRs, as well as those present in the environment and deriving from combustion and soil restoration, the burning of coal, wood, straw, cigarettes, oil, and other fuels, and from the restoration of organic contaminated soil, can enter the human body mainly through three pathways including respiratory tract, skin exposure and ingestion [132]. PFRs are not toxic to living beings and the environment, but they can stimulate the formation of other harmful substances and free radicals, including various types of ROS, when in the environment or in vivo [203]. As well documented, ROS can interfere with the normal redox and metabolic processes, thus causing oxidative stress in biota [204]. Additionally, it has been reported that exposure to PFRs may induce cell degeneration or apoptosis and may affect the normal functions of the heart or lungs of humans [203]. So far, cytotoxicity and biotoxicity are the two categories of toxicity reported as attributable to PFRs (Table 14).
Usually, toxicity tests are carried out in research laboratories using both environmental samples and lab-prepared PFRs, such as those generated by MCP230 (a mixture of CuO and chlorophenol at 230 ◦C), DCB230 (a mixture of CuO, 0.2 μm amorphous silica and 1,2-dichlorobenzene at 230 ◦C), CGUFP (combustion generated ultrafine particle) or other mixtures of transitional metals and substituted aromatic compounds. For cytotoxicity experiments, cultured cells extracted from the bronchial epithelium and rats are used, while biotoxicity essays are carried out on plants, fishes, rabbits, and worms. Generally, it was observed that exposure to PFRs causes oxidative stress. More specifically, cytotoxicity tests evidenced cell variation, decreased number and activity, the disparity in protein expression, and DNA damage. Biotoxicity experiments revealed abnormalities in development and behaviour, disease, and organ and tissue damage. Although BC can serve as an environmentally sustainable soil amendment due to its ability to enhance several chemical properties of soil, such as pH, electrical conductivity, CEC, and organic carbon content, thus contributing to the overall improvement of nutrient retention in the soil, BC with high concentrations of PFRs negatively affect crop growth.
Additionally, it has been found that PFRs in the aquicultural solution inhibited the germination rate of different crops by ROS induction [132]. The oxidative stress brought by the production of ROS can also damage the plasma membrane of the root system and hinder plant root growth. Moreover, PFRs induced neurotoxicity of Cryptobacterium hidradenoma, transforming it into a neurotoxin for soil organisms and posing a threat to their survival.

4. Risk Prevention Strategies and Conclusions

This review has evidenced that BC and mainly PFRs, generated during the pyrolysis processes performed to produce it could be double-edged weapons. BC is reported to be an eco-friendly and low-cost black gold with many beneficial properties, including the capability to remove organic and inorganic pollutants from water by adsorption processes and/or through its PFRs. Anyway, several studies have reported that PFRs can be very dangerous to the environment and humans by a ROS-dependent mechanism. In addition to being produced by various common xenobiotics, PFRs can be easily converted into secondary pollutants, causing further biotoxicity. The still too little studied transport and transformation of PFRs in the environment can also affect the behavior of other substances, leading to potential environmental hazards that are not yet fully understood. Therefore, the further exploration of the ecological impact of PFRs and the development of prevention and control measures are necessary. In this regard, although some progress has been made in the environmental risk and biotoxicity studies concerning PFRs, research is still in the initial stage, and there is an urgent need for systematic and in-depth studies on the production and transport of PFRs. A more in-depth understanding of the influence of the environmental conditions on their occurrence is needed to control the external factors for reducing the output of PFRs and promoting the degradation of xenobiotics. A more rational knowledge about their toxicity mechanisms is necessary to have a more precise toxicological equivalence regulation. A better strategy to prevent the risks associated with PFRs is avoiding exposure to them by reducing contact with combustion sources, such as vehicle exhaust and cigarette smoke. Proper air filtration systems removing PFRs from indoor air and tearing protective masks or respirators can lower the possibility of contact with PFRs in outdoor environments. Additionally, treatments for limiting the adverse health effects associated with PFRs exposure, such as using antioxidants, which can neutralize ROS, could be another strategy to protect humans from the adverse impacts of PFR exposure.

Author Contributions

The authors (S.A. and O.G.P.) contributed equally to this work.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Zhang, R.; Zhang, R.; Zimmerman, A.R.; Wang, H.; Gao, B. Applications, Impacts, and Management of Biochar Persistent Free Radicals: A Review. Environmental Pollution 2023, 327, 121543. [CrossRef]
  2. Wang, J.; Wang, S. Preparation, Modification, and Environmental Application of Biochar: A Review. Journal of Cleaner Production 2019, 227, 1002–1022. [CrossRef]
  3. Premarathna, K.S.D.; Rajapaksha, A.U.; Sarkar, B.; Kwon, E.E.; Bhatnagar, A.; Ok, Y.S.; Vithanage, M. Biochar-Based Engineered Composites for Sorptive Decontamination of Water: A Review. Chemical Engineering Journal 2019, 372, 536–550. [CrossRef]
  4. Beesley, L.; Moreno-Jiménez, E.; Gomez-Eyles, J.L.; Harris, E.; Robinson, B.; Sizmur, T. A Review of Biochars’ Potential Role in the Remediation, Revegetation and Restoration of Contaminated Soils. Environmental Pollution 2011, 159, 3269–3282. [CrossRef]
  5. Jeffery, S.; Verheijen, F.G.A.; van der Velde, M.; Bastos, A.C. A Quantitative Review of the Effects of Biochar Application to Soils on Crop Productivity Using Meta-Analysis. Agriculture, Ecosystems & Environment 2011, 144, 175–187. [CrossRef]
  6. Kinney, T.J.; Masiello, C.A.; Dugan, B.; Hockaday, W.C.; Dean, M.R.; Zygourakis, K.; Barnes, R.T. Hydrologic Properties of Biochars Produced at Different Temperatures. Biomass and Bioenergy 2012, 41, 34–43. [CrossRef]
  7. Qin, Y.; Li, G.; Gao, Y.; Zhang, L.; Ok, Y.S.; An, T. Persistent Free Radicals in Carbon-Based Materials on Transformation of Refractory Organic Contaminants (ROCs) in Water: A Critical Review. Water Research 2018, 137, 130–143. [CrossRef]
  8. Vejerano, E.P.; Rao, G.; Khachatryan, L.; Cormier, S.A.; Lomnicki, S. Environmentally Persistent Free Radicals: Insights on a New Class of Pollutants. Environ. Sci. Technol. 2018, 52, 2468–2481. [CrossRef]
  9. Volpe, R.; Bermudez Menendez, J.M.; Ramirez Reina, T.; Volpe, M.; Messineo, A.; Millan, M.; Titirici, M.-M. Free Radicals Formation on Thermally Decomposed Biomass. Fuel 2019, 255, 115802. [CrossRef]
  10. Huang, Y.; Guo, X.; Ding, Z.; Chen, Y.; Hu, X. Environmentally Persistent Free Radicals in Biochar Derived from Laminaria Japonica Grown in Different Habitats. Journal of Analytical and Applied Pyrolysis 2020, 151, 104941. [CrossRef]
  11. Ruan, X.; Sun, Y.; Du, W.; Tang, Y.; Liu, Q.; Zhang, Z.; Doherty, W.; Frost, R.L.; Qian, G.; Tsang, D.C.W. Formation, Characteristics, and Applications of Environmentally Persistent Free Radicals in Biochars: A Review. Bioresource Technology 2019, 281, 457–468. [CrossRef]
  12. Wang, R.-Z.; Huang, D.-L.; Liu, Y.-G.; Zhang, C.; Lai, C.; Wang, X.; Zeng, G.-M.; Gong, X.-M.; Duan, A.; Zhang, Q.; et al. Recent Advances in Biochar-Based Catalysts: Properties, Applications and Mechanisms for Pollution Remediation. Chemical Engineering Journal 2019, 371, 380–403. [CrossRef]
  13. Odinga, E.S.; Waigi, M.G.; Gudda, F.O.; Wang, J.; Yang, B.; Hu, X.; Li, S.; Gao, Y. Occurrence, Formation, Environmental Fate and Risks of Environmentally Persistent Free Radicals in Biochars. Environment International 2020, 134, 105172. [CrossRef]
  14. Fang, G.; Liu, C.; Gao, J.; Dionysiou, D.D.; Zhou, D. Manipulation of Persistent Free Radicals in Biochar To Activate Persulfate for Contaminant Degradation. Environ. Sci. Technol. 2015, 49, 5645–5653. [CrossRef]
  15. He, J.; Xiao, Y.; Tang, J.; Chen, H.; Sun, H. Persulfate Activation with Sawdust Biochar in Aqueous Solution by Enhanced Electron Donor-Transfer Effect. Science of The Total Environment 2019, 690, 768–777. [CrossRef]
  16. Wang, H.Z.; Guo, W.Q.; Yin, R.L.; Du, J.S.; Wu, Q.L.; Luo, H.C.; Liu, B.H.; Sseguya, F.; Ren, N.Q. Biochar-induced Fe(III) reduction for persulfate activation in sulfamethoxazole degradation: insight into the electron transfer, radical oxidation and degradation pathways. Chem. Eng. J. 2019, 362, 561-569.
  17. Liang, J.; Xu, X.; Qamar Zaman, W.; Hu, X.; Zhao, L.; Qiu, H.; Cao, X. Different Mechanisms between Biochar and Activated Carbon for the Persulfate Catalytic Degradation of Sulfamethoxazole: Roles of Radicals in Solution or Solid Phase. Chemical Engineering Journal 2019, 375, 121908. [CrossRef]
  18. Fang, G.; Gao, J.; Liu, C.; Dionysiou, D.D.; Wang, Y.; Zhou, D. Key Role of Persistent Free Radicals in Hydrogen Peroxide Activation by Biochar: Implications to Organic Contaminant Degradation. Environ. Sci. Technol. 2014, 48, 1902–1910. [CrossRef]
  19. Fang, G.; Zhu, C.; Dionysiou, D.D.; Gao, J.; Zhou, D. Mechanism of Hydroxyl Radical Generation from Biochar Suspensions: Implications to Diethyl Phthalate Degradation. Bioresource Technology 2015, 176, 210–217. [CrossRef]
  20. Fang, G.; Liu, C.; Wang, Y.; Dionysiou, D.D.; Zhou, D. Photogeneration of Reactive Oxygen Species from Biochar Suspension for Diethyl Phthalate Degradation. Applied Catalysis B: Environmental 2017, 214, 34–45. [CrossRef]
  21. Luo, K.; Yang, Q.; Pang, Y.; Wang, D.; Li, X.; Lei, M.; Huang, Q. Unveiling the Mechanism of Biochar-Activated Hydrogen Peroxide on the Degradation of Ciprofloxacin. Chemical Engineering Journal 2019, 374, 520–530. [CrossRef]
  22. Zhang, Y.; Xu, M.; He, R.; Zhao, J.; Kang, W.; Lv, J. Effect of Pyrolysis Temperature on the Activated Permonosulfate Degradation of Antibiotics in Nitrogen and Sulfur-Doping Biochar: Key Role of Environmentally Persistent Free Radicals. Chemosphere 2022, 294, 133737. [CrossRef]
  23. Liao, S.; Pan, B.; Li, H.; Zhang, D.; Xing, B. Detecting Free Radicals in Biochars and Determining Their Ability to Inhibit the Germination and Growth of Corn, Wheat and Rice Seedlings. Environ. Sci. Technol. 2014, 48, 8581–8587. [CrossRef]
  24. Ahmad, M.; Rajapaksha, A.U.; Lim, J.E.; Zhang, M.; Bolan, N.; Mohan, D.; Vithanage, M.; Lee, S.S.; Ok, Y.S. Biochar as a Sorbent for Contaminant Management in Soil and Water: A Review. Chemosphere 2014, 99, 19–33. [CrossRef]
  25. Qin, J.; Chen, Q.; Sun, M.; Sun, P.; Shen, G. Pyrolysis Temperature-Induced Changes in the Catalytic Characteristics of Rice Husk-Derived Biochar during 1,3-Dichloropropene Degradation. Chemical Engineering Journal 2017, 330, 804–812. [CrossRef]
  26. Yang, J.; Pignatello, J.J.; Pan, B.; Xing, B. Degradation of P-Nitrophenol by Lignin and Cellulose Chars: H2O2-Mediated Reaction and Direct Reaction with the Char. Environ. Sci. Technol. 2017, 51, 8972–8980. [CrossRef]
  27. Ginoble Pandoli, O.; Santos de Sá, D.; Nogueira Barbosa Junior, M.; Paciornik, S. Bamboo-Based Lignocellulose Biomass as Catalytic Support for Organic Synthesis and Water Treatments. In: Palombini, F.L.; Nogueira, F.M. (eds) Bamboo Science and Technology. Environmental Footprints and Eco-design of Products and Processes. 2023, Springer, Singapore, pp. 297-327. [CrossRef]
  28. Jahirul, M.I.; Rasul, M.G.; Chowdhury, A.A.; Ashwath, N. Biofuels Production through Biomass Pyrolysis —A Technological Review. Energies 2012, 5, 4952–5001. [CrossRef]
  29. Yaashikaa, P.R.; Senthil Kumar, P.; Varjani, S.J.; Saravanan, A. A Review on Photochemical, Biochemical and Electrochemical Transformation of CO2 into Value-Added Products. Journal of CO2 Utilization 2019, 33, 131–147. [CrossRef]
  30. Saidur, R.; Abdelaziz, E.A.; Demirbas, A.; Hossain, M.S.; Mekhilef, S. A Review on Biomass as a Fuel for Boilers. Renewable and Sustainable Energy Reviews 2011, 15, 2262–2289. [CrossRef]
  31. Yaashikaa, P.R.; Kumar, P.S.; Varjani, S.; Saravanan, A. A Critical Review on the Biochar Production Techniques, Characterization, Stability and Applications for Circular Bioeconomy. Biotechnology Reports 2020, 28, e00570. [CrossRef]
  32. Lehmann, J. Biochar for environmental management: an introduction. Biochar for Environmental Management. Sci. Technol. 2009, 25, 15801-15811.
  33. Mohanty, S.K.; Valenca, R.; Berger, A.W.; Yu, I.K.M.; Xiong, X.; Saunders, T.M.; Tsang, D.C.W. Plenty of Room for Carbon on the Ground: Potential Applications of Biochar for Stormwater Treatment. Science of The Total Environment 2018, 625, 1644–1658. [CrossRef]
  34. Ayla Norris Smidth. A New Boost for Biochar as a Natural Climate Solution. 2023. Available online at https://blog.nature.org/science-brief/a-new-boost-for-biochar-as-a-natural-climate-solution/#:~:text=Biochar%20is%20a%20type%20of%20charcoal%20made%20from,other%20biomass%20would%2C%20leading%20to%20more%20carbon%20sequestration (accessed on December 27, 2023).
  35. Yu, D.; Yu, Y.; Tang, J.; Li, X.; Ke, C.; Yao, Z. Application Fields of Kitchen Waste Biochar and Its Prospects as Catalytic Material: A Review. Science of The Total Environment 2022, 810, 152171. [CrossRef]
  36. Yu, S.; Zhang, W.; Dong, X.; Wang, F.; Yang, W.; Liu, C.; Chen, D. A Review on Recent Advances of Biochar from Agricultural and Forestry Wastes: Preparation, Modification and Applications in Wastewater Treatment. Journal of Environmental Chemical Engineering 2024, 12, 111638. [CrossRef]
  37. Wijitkosum, S. Biochar Derived from Agricultural Wastes and Wood Residues for Sustainable Agricultural and Environmental Applications. International Soil and Water Conservation Research 2022, 10, 335–341. [CrossRef]
  38. Yao, Y.; Gao, B.; Inyang, M.; Zimmerman, A.R.; Cao, X.; Pullammanappallil, P.; Yang, L. Biochar Derived from Anaerobically Digested Sugar Beet Tailings: Characterization and Phosphate Removal Potential. Bioresource Technology 2011, 102, 6273–6278. [CrossRef]
  39. Puettmann, M.; Sahoo, K.; Wilson, K.; Oneil, E. Life Cycle Assessment of Biochar Produced from Forest Residues Using Portable Systems. Journal of Cleaner Production 2020, 250, 119564. [CrossRef]
  40. Papageorgiou, A.; Azzi, E.S.; Enell, A.; Sundberg, C. Biochar Produced from Wood Waste for Soil Remediation in Sweden: Carbon Sequestration and Other Environmental Impacts. Science of The Total Environment 2021, 776, 145953. [CrossRef]
  41. Laird, D. Biochar Amendments Make the Harvesting of Crop Residue for Bioenergy Production Sustainable. Nutrient Cycling in Agroecosystems 2023. [CrossRef]
  42. Li, N.; He, M.; Lu, X.; Yan, B.; Duan, X.; Chen, G.; Wang, S.; Hou, L. Municipal Solid Waste Derived Biochars for Wastewater Treatment: Production, Properties and Applications. Resources, Conservation and Recycling 2022, 177, 106003. [CrossRef]
  43. Shi, W.; Wang, H.; Yan, J.; Shan, L.; Quan, G.; Pan, X.; Cui, L. Wheat Straw Derived Biochar with Hierarchically Porous Structure for Bisphenol A Removal: Preparation, Characterization, and Adsorption Properties. Separation and Purification Technology 2022, 289, 120796. [CrossRef]
  44. Foong, S.Y.; Chan, Y.H.; Chin, B.L.F.; Lock, S.S.M.; Yee, C.Y.; Yiin, C.L.; Peng, W.; Lam, S.S. Production of Biochar from Rice Straw and Its Application for Wastewater Remediation − An Overview. Bioresource Technology 2022, 360, 127588. [CrossRef]
  45. Gunamantha, M.; Widana, G.A.B. IOP Conf. Ser.: Earth Environ. Sci. 2018, 131 012055.
  46. Rathnayake, D.; Schmidt, H.-P.; Leifeld, J.; Mayer, J.; Epper, C.A.; Bucheli, T.D.; Hagemann, N. Biochar from Animal Manure: A Critical Assessment on Technical Feasibility, Economic Viability, and Ecological Impact. GCB Bioenergy 2023, 15, 1078–1104. [CrossRef]
  47. Drané, M.; Zbair, M.; Hajjar-Garreau, S.; Josien, L.; Michelin, L.; Bennici, S.; Limousy, L. Unveiling the Potential of Corn Cob Biochar: Analysis of Microstructure and Composition with Emphasis on Interaction with NO2. Materials 2024, 17. [CrossRef]
  48. Luo, L.; Wang, J.; Lv, J.; Liu, Z.; Sun, T.; Yang, Y.; Zhu, Y.-G. Carbon Sequestration Strategies in Soil Using Biochar: Advances, Challenges, and Opportunities. Environ. Sci. Technol. 2023, 57, 11357–11372. [CrossRef]
  49. Brassard, P.; Godbout, S.; Lévesque, V.; Palacios, J.H.; Raghavan, V.; Ahmed, A.; Hogue, R.; Jeanne, T.; Verma, M. 4 - Biochar for Soil Amendment. In Char and Carbon Materials Derived from Biomass; Jeguirim, M., Limousy, L., Eds.; Elsevier, 2019; pp. 109–146 ISBN 978-0-12-814893-8.
  50. Godlewska, P.; Schmidt, H.P.; Ok, Y.S.; Oleszczuk, P. Biochar for Composting Improvement and Contaminants Reduction. A Review. Bioresource Technology 2017, 246, 193–202. [CrossRef]
  51. Xiang, W.; Zhang, X.; Chen, J.; Zou, W.; He, F.; Hu, X.; Tsang, D.C.W.; Ok, Y.S.; Gao, B. Biochar Technology in Wastewater Treatment: A Critical Review. Chemosphere 2020, 252, 126539. [CrossRef]
  52. Rawat, S.; Wang, C.-T.; Lay, C.-H.; Hotha, S.; Bhaskar, T. Sustainable Biochar for Advanced Electrochemical/Energy Storage Applications. Journal of Energy Storage 2023, 63, 107115. [CrossRef]
  53. Qiu, B.; Shao, Q.; Shi, J.; Yang, C.; Chu, H. Application of Biochar for the Adsorption of Organic Pollutants from Wastewater: Modification Strategies, Mechanisms and Challenges. Separation and Purification Technology 2022, 300, 121925. [CrossRef]
  54. Kalu, S.; Kulmala, L.; Zrim, J.; Peltokangas, K.; Tammeorg, P.; Rasa, K.; Kitzler, B.; Pihlatie, M.; Karhu, K. Potential of Biochar to Reduce Greenhouse Gas Emissions and Increase Nitrogen Use Efficiency in Boreal Arable Soils in the Long-Term. Frontiers in Environmental Science 2022, 10. [CrossRef]
  55. Jiang, T.; Wang, B.; Gao, B.; Cheng, N.; Feng, Q.; Chen, M.; Wang, S. Degradation of Organic Pollutants from Water by Biochar-Assisted Advanced Oxidation Processes: Mechanisms and Applications. Journal of Hazardous Materials 2023, 442, 130075. [CrossRef]
  56. Lyu, H.; Zhang, Q.; Shen, B. Application of Biochar and Its Composites in Catalysis. Chemosphere 2020, 240, 124842. [CrossRef]
  57. Gayathri, R.; Gopinath, K.P.; Kumar, P.S. Adsorptive Separation of Toxic Metals from Aquatic Environment Using Agro Waste Biochar: Application in Electroplating Industrial Wastewater. Chemosphere 2021, 262, 128031. [CrossRef]
  58. Hemavathy, R.V.; Kumar, P.S.; Kanmani, K.; Jahnavi, N. Adsorptive Separation of Cu(II) Ions from Aqueous Medium Using Thermally/Chemically Treated Cassia Fistula Based Biochar. Journal of Cleaner Production 2020, 249, 119390. [CrossRef]
  59. El-Naggar, A.; El-Naggar, A.H.; Shaheen, S.M.; Sarkar, B.; Chang, S.X.; Tsang, D.C.W.; Rinklebe, J.; Ok, Y.S. Biochar Composition-Dependent Impacts on Soil Nutrient Release, Carbon Mineralization, and Potential Environmental Risk: A Review. Journal of Environmental Management 2019, 241, 458–467. [CrossRef]
  60. El-Naggar, A.; Lee, S.S.; Rinklebe, J.; Farooq, M.; Song, H.; Sarmah, A.K.; Zimmerman, A.R.; Ahmad, M.; Shaheen, S.M.; Ok, Y.S. Biochar Application to Low Fertility Soils: A Review of Current Status, and Future Prospects. Geoderma 2019, 337, 536–554. [CrossRef]
  61. Hu, X.; Xu, J.; Wu, M.; Xing, J.; Bi, W.; Wang, K.; Ma, J.; Liu, X. Effects of Biomass Pre-Pyrolysis and Pyrolysis Temperature on Magnetic Biochar Properties. Journal of Analytical and Applied Pyrolysis 2017, 127, 196–202. [CrossRef]
  62. Senthil Kumar, P.; Abhinaya, R.V.; Gayathri Lashmi, K.; Arthi, V.; Pavithra, R.; Sathyaselvabala, V.; Dinesh Kirupha, S.; Sivanesan, S. Adsorption of Methylene Blue Dye from Aqueous Solution by Agricultural Waste: Equilibrium, Thermodynamics, Kinetics, Mechanism and Process Design. Colloid Journal 2011, 73, 651–661. [CrossRef]
  63. Kumar, P.S.; Senthamarai, C.; Deepthi, A.S.L.S.; Bharani, R. Adsorption Isotherms, Kinetics and Mechanism of Pb(II) Ions Removal from Aqueous Solution Using Chemically Modified Agricultural Waste. Canadian Journal of Chemical Engineering 2013, 91, 1950–1956.
  64. Varjani, S.; Kumar, G.; Rene, E.R. Developments in Biochar Application for Pesticide Remediation: Current Knowledge and Future Research Directions. Journal of Environmental Management 2019, 232, 505–513. [CrossRef]
  65. Bridgwater, A.V. Review of Fast Pyrolysis of Biomass and Product Upgrading. Biomass and Bioenergy 2012, 38, 68–94. [CrossRef]
  66. Ng, W.C.; You, S.; Ling, R.; Gin, K.Y.-H.; Dai, Y.; Wang, C.-H. Co-Gasification of Woody Biomass and Chicken Manure: Syngas Production, Biochar Reutilization, and Cost-Benefit Analysis. Energy 2017, 139, 732–742. [CrossRef]
  67. Cantrell, K.B.; Hunt, P.G.; Uchimiya, M.; Novak, J.M.; Ro, K.S. Impact of Pyrolysis Temperature and Manure Source on Physicochemical Characteristics of Biochar. Bioresource Technology 2012, 107, 419–428. [CrossRef]
  68. Funke, A.; Ziegler, F. Hydrothermal Carbonization of Biomass: A Summary and Discussion of Chemical Mechanisms for Process Engineering. Biofuels, Bioproducts and Biorefining 2010, 4, 160–177. [CrossRef]
  69. Klinghoffer, N.B.; Castaldi, M.J.; Nzihou, A. Influence of Char Composition and Inorganics on Catalytic Activity of Char from Biomass Gasification. Fuel 2015, 157, 37–47. [CrossRef]
  70. P.C.A. Bergman, A.R. Boersma, R.W.R. Zwart, J.H.A. Kiel, Torrefaction for Biomass Co-Firing in Existing Coal-Fired Power Stations; Report No. ECNC05013, Energy Research Centre of The Netherlands (ECN), Petten, The Netherlands, 2005, pp. 71.
  71. Nunoura, T.; Wade, S.R.; Bourke, J.P.; Antal, M.J. Studies of the Flash Carbonization Process. 1. Propagation of the Flaming Pyrolysis Reaction and Performance of a Catalytic Afterburner. Ind. Eng. Chem. Res. 2006, 45, 585–599. [CrossRef]
  72. Basu, P. Chapter 5 - Pyrolysis. In Biomass Gasification, Pyrolysis and Torrefaction (Third Edition); Basu, P., Ed.; Academic Press, 2018; pp. 155–187 ISBN 978-0-12-812992-0.
  73. Devi, M.; Rawat, S.; Sharma, S. A Comprehensive Review of the Pyrolysis Process: From Carbon Nanomaterial Synthesis to Waste Treatment. Oxford Open Materials Science 2021, 1, itab014. [CrossRef]
  74. Cha, J.S.; Park, S.H.; Jung, S.-C.; Ryu, C.; Jeon, J.-K.; Shin, M.-C.; Park, Y.-K. Production and Utilization of Biochar: A Review. Journal of Industrial and Engineering Chemistry 2016, 40, 1–15. [CrossRef]
  75. Yaashikaa, P.R.; Senthil Kumar, P.; Varjani, S.J.; Saravanan, A. Advances in Production and Application of Biochar from Lignocellulosic Feedstocks for Remediation of Environmental Pollutants. Bioresource Technology 2019, 292, 122030. [CrossRef]
  76. Gontijo, L.O.L.; Junior, M.N.B.; Santos de Sá, D.; Letichevsky, S.; Pedrozo-Peñafiel, M.J.; Aucélio, R.Q.; Bott, I.S.; Diniz Lopes Alves, H.; Fragneaud, B.; Oliveira Maciel, I.; et al. 3D Conductive Monolithic Carbons from Pyrolyzed Bamboo for Microfluidic Self-Heating System. Carbon 2023, 213, 118214. [CrossRef]
  77. Swagathnath, G.; Rangabhashiyam, S.; Parthsarathi, K.; Murugan, S.; Balasubramanian, P. Modeling Biochar Yield and Syngas Production During the Pyrolysis of Agro-Residues. In Proceedings of the Green Buildings and Sustainable Engineering; Drück, H., Pillai, R.G., Tharian, M.G., Majeed, A.Z., Eds.; Springer Singapore: Singapore, 2019; pp. 325–336.
  78. Ibitoye, S.E.; Mahamood, R.M.; Jen, T.-C.; Loha, C.; Akinlabi, E.T. An Overview of Biomass Solid Fuels: Biomass Sources, Processing Methods, and Morphological and Microstructural Properties. Journal of Bioresources and Bioproducts 2023, 8, 333–360. [CrossRef]
  79. Tan, H.; et al. A Review On The Comparison Between Slow Pyrolysis And Fast Pyrolysis On The Quality Of Lignocellulosic And Lignin-Based Biochar IOP Conf. Ser.: Mater. Sci. Eng. 2021, 1051 012075. [CrossRef]
  80. Kumar, A.; Bhattacharya, T. Biochar and Its Application 2018.
  81. Tomczyk, A.; Sokołowska, Z.; Boguta, P. Biochar Physicochemical Properties: Pyrolysis Temperature and Feedstock Kind Effects. Reviews in Environmental Science and Bio/Technology 2020, 19, 191–215. [CrossRef]
  82. Ding, W.; Dong, X.; Ime, I.M.; Gao, B.; Ma, L.Q. Pyrolytic Temperatures Impact Lead Sorption Mechanisms by Bagasse Biochars. Chemosphere 2014, 105, 68–74. [CrossRef]
  83. Banik, C.; Lawrinenko, M.; Bakshi, S.; Laird, D.A. Impact of Pyrolysis Temperature and Feedstock on Surface Charge and Functional Group Chemistry of Biochars. Journal of Environmental Quality 2018, 47, 452–461. [CrossRef]
  84. Zhao, S.-X.; Ta, N.; Wang, X.-D. Effect of Temperature on the Structural and Physicochemical Properties of Biochar with Apple Tree Branches as Feedstock Material. Energies 2017, 10. [CrossRef]
  85. Suliman, W.; Harsh, J.B.; Abu-Lail, N.I.; Fortuna, A.-M.; Dallmeyer, I.; Garcia-Pérez, M. The Role of Biochar Porosity and Surface Functionality in Augmenting Hydrologic Properties of a Sandy Soil. Science of The Total Environment 2017, 574, 139–147. [CrossRef]
  86. Gontijo, L.O.; Junior, M.N.B.; de Sá, D.S.; Letichevsky, S.; Pedrozo-Peñafiel, M.J.; Aucélio, R.Q.; ... Pandoli, O.G. 3D conductive monolithic carbons from pyrolyzed bamboo for microfluidic self-heating system. Carbon, 2023, 118214.
  87. Sudarsan, J.S.; Prasanna, K.; Shiam Babu, R.; Sai Krishna, V.M.V. Chapter 11 - Biochar: A Sustainable Solution for Organic Waste Management a Way Forward towards Circular Economy. In Recent Trends in Solid Waste Management; Ravindran, B., Gupta, S.K., Bhat, S.A., Chauhan, P.S., Tyagi, N., Eds.; Elsevier, 2023; pp. 215–230 ISBN 978-0-443-15206-1.
  88. Zhou, B.; Liu, Q.; Shi, L.; Liu, Z. Electron Spin Resonance Studies of Coals and Coal Conversion Processes: A Review. Fuel Processing Technology 2019, 188, 212–227. [CrossRef]
  89. Zhang, S.; Cui, J.; Wu, H.; Zheng, Q.; Song, D.; Wang, X.; Zhang, S. Organic Carbon, Total Nitrogen, and Microbial Community Distributions within Aggregates of Calcareous Soil Treated with Biochar. Agriculture, Ecosystems & Environment 2021, 314, 107408. [CrossRef]
  90. Ma, L.; Song, D.; Liu, M.; Li, Y.; Li, Y. Effects of Earthworm Activities on Soil Nutrients and Microbial Diversity under Different Tillage Measures. Soil and Tillage Research 2022, 222, 105441. [CrossRef]
  91. Hu, W.; Gao, W.; Tang, Y.; Zhang, Q.; Tu, C.; Cheng, J. Remediation via Biochar and Potential Health Risk of Heavy Metal Contaminated Soils. Environmental Earth Sciences 2022, 81, 482.
  92. Kant Bhatia, S.; Palai, A.K.; Kumar, A.; Kant Bhatia, R.; Kumar Patel, A.; Kumar Thakur, V.; Yang, Y.-H. Trends in Renewable Energy Production Employing Biomass-Based Biochar. Bioresource Technology 2021, 340, 125644. [CrossRef]
  93. Sakhiya, A.K.; Anand, A.; Kaushal, P. Production, Activation, and Applications of Biochar in Recent Times. Biochar 2020, 1–33.
  94. Nguyen, H.N.; Pignatello, J.J. Laboratory Tests of Biochars as Absorbents for Use in Recovery or Containment of Marine Crude Oil Spills. Environmental Engineering Science 2013, 30, 374–380. [CrossRef]
  95. Jothirani, R.; Kumar, P.S.; Saravanan, A.; Narayan, A.S.; Dutta, A. Ultrasonic Modified Corn Pith for the Sequestration of Dye from Aqueous Solution. Journal of Industrial and Engineering Chemistry 2016, 39, 162–175. [CrossRef]
  96. Suganya, S.; Senthil Kumar, P.; Saravanan, A.; Sundar Rajan, P.; Ravikumar, C. Computation of Adsorption Parameters for the Removal of Dye from Wastewater by Microwave Assisted Sawdust: Theoretical and Experimental Analysis. Environmental Toxicology and Pharmacology 2017, 50, 45–57. [CrossRef]
  97. Saravanan, A.; Kumar, P.S.; Renita, A.A. Hybrid Synthesis of Novel Material through Acid Modification Followed Ultrasonication to Improve Adsorption Capacity for Zinc Removal. Journal of Cleaner Production 2018, 172, 92–105. [CrossRef]
  98. Luo, Z.; Yao, B.; Yang, X.; Wang, L.; Xu, Z.; Yan, X.; Tian, L.; Zhou, H.; Zhou, Y. Novel Insights into the Adsorption of Organic Contaminants by Biochar: A Review. Chemosphere 2022, 287, 132113. [CrossRef]
  99. Lou, L.; Wu, B.; Wang, L.; Luo, L.; Xu, X.; Hou, J.; Xun, B.; Hu, B.; Chen, Y. Sorption and Ecotoxicity of Pentachlorophenol Polluted Sediment Amended with Rice-Straw Derived Biochar. Bioresource Technology 2011, 102, 4036–4041. [CrossRef]
  100. Tan, X.; Liu, Y.; Zeng, G.; Wang, X.; Hu, X.; Gu, Y.; Yang, Z. Application of Biochar for the Removal of Pollutants from Aqueous Solutions. Chemosphere 2015, 125, 70–85. [CrossRef]
  101. Yu, X.-Y.; Ying, G.-G.; Kookana, R.S. Reduced Plant Uptake of Pesticides with Biochar Additions to Soil. Chemosphere 2009, 76, 665–671. [CrossRef]
  102. Liang, C.; Niu, H.-Y.; Guo, H.; Niu, C.-G.; Yang, Y.-Y.; Liu, H.-Y.; Tang, W.-W.; Feng, H.-P. Efficient Photocatalytic Nitrogen Fixation to Ammonia over Bismuth Monoxide Quantum Dots-Modified Defective Ultrathin Graphitic Carbon Nitride. Chemical Engineering Journal 2021, 406, 126868. [CrossRef]
  103. Kumbhar, D.; Palliyarayil, A.; Reghu, D.; Shrungar, D.; Umapathy, S.; Sil, S. Rapid Discrimination of Porous Bio-Carbon Derived from Nitrogen Rich Biomass Using Raman Spectroscopy and Artificial Intelligence Methods. Carbon 2021, 178, 792–802. [CrossRef]
  104. Zhang, K.; Sun, P.; Faye, M.C.A.S.; Zhang, Y. Characterization of Biochar Derived from Rice Husks and Its Potential in Chlorobenzene Degradation. Carbon 2018, 130, 730–740. [CrossRef]
  105. Chen, D.; Xu, J.; Ling, P.; Fang, Z.; Ren, Q.; Xu, K.; Jiang, L.; Wang, Y.; Su, S.; Hu, S.; et al. Formation and Evolution Mechanism of Persistent Free Radicals in Biochar during Biomass Pyrolysis: Insights from Biochar’s Element Composition and Chemical Structure. Fuel 2024, 357, 129910. [CrossRef]
  106. Wu, C.; Fu, L.; Li, H.; Liu, X.; Wan, C. Using Biochar to Strengthen the Removal of Antibiotic Resistance Genes: Performance and Mechanism. Science of The Total Environment 2022, 816, 151554. [CrossRef]
  107. Huang, C.; Qin, F.; Zhang, C.; Huang, D.; Tang, L.; Yan, M.; Wang, W.; Song, B.; Qin, D.; Zhou, Y.; et al. Effects of Heterogeneous Metals on the Generation of Persistent Free Radicals as Critical Redox Sites in Iron-Containing Biochar for Persulfate Activation. ACS EST Water 2023, 3, 298–310. [CrossRef]
  108. Yu, J.; Zhu, Z.; Zhang, H.; Shen, X.; Qiu, Y.; Yin, D.; Wang, S. Persistent Free Radicals on N-Doped Hydrochar for Degradation of Endocrine Disrupting Compounds. Chemical Engineering Journal 2020, 398, 125538. [CrossRef]
  109. Ma, L.; Syed-Hassan, S.S.A.; Tong, Y.; Xiong, Z.; Chen, Y.; Xu, J.; Jiang, L.; Su, S.; Hu, S.; Wang, Y.; et al. Interactions of Cellulose- and Lignin-Derived Radicals during Pyrolysis: An in-Situ Electron Paramagnetic Resonance (EPR) Study. Fuel Processing Technology 2023, 239, 107536. [CrossRef]
  110. Retcofsky, H.L.; Hough, M.R.; Clarkson, R.B. Nature of the Free Radicals in Coals, Pyrolyzed Coals, and Liquefaction Products, 1979. Journal Volume: 24:1; Conference: 177. national meeting of the American Chemical Society, Honolulu, HI, USA, 1 Apr 1979.
  111. Liu, J.; Jiang, X.; Shen, J.; Zhang, H. Chemical Properties of Superfine Pulverized Coal Particles. Part 1. Electron Paramagnetic Resonance Analysis of Free Radical Characteristics. Advanced Powder Technology 2014, 25, 916–925. [CrossRef]
  112. Luo, K.; Pang, Y.; Wang, D.; Li, X.; Wang, L.; Lei, M.; Huang, Q.; Yang, Q. A Critical Review on the Application of Biochar in Environmental Pollution Remediation: Role of Persistent Free Radicals (PFRs). Journal of Environmental Sciences 2021, 108, 201–216. [CrossRef]
  113. Zhang, Y.; Sun, X.; Bian, W.; Peng, J.; Wan, H.; Zhao, J. The Key Role of Persistent Free Radicals on the Surface of Hydrochar and Pyrocarbon in the Removal of Heavy Metal-Organic Combined Pollutants. Bioresource Technology 2020, 318, 124046. [CrossRef]
  114. Qin, J.; Cheng, Y.; Sun, M.; Yan, L.; Shen, G. Catalytic Degradation of the Soil Fumigant 1,3-Dichloropropene in Aqueous Biochar Slurry. Science of The Total Environment 2016, 569–570, 1–8. [CrossRef]
  115. Huang, D.; Luo, H.; Zhang, C.; Zeng, G.; Lai, C.; Cheng, M.; Wang, R.; Deng, R.; Xue, W.; Gong, X.; et al. Nonnegligible Role of Biomass Types and Its Compositions on the Formation of Persistent Free Radicals in Biochar: Insight into the Influences on Fenton-like Process. Chemical Engineering Journal 2019, 361, 353–363. [CrossRef]
  116. Wang, Y.; Gu, X.; Huang, Y.; Ding, Z.; Chen, Y.; Hu, X. Insight into Biomass Feedstock on Formation of Biochar-Bound Environmentally Persistent Free Radicals under Different Pyrolysis Temperatures. RSC Adv. 2022, 12, 19318–19326. [CrossRef]
  117. Tao, W.; Duan, W.; Liu, C.; Zhu, D.; Si, X.; Zhu, R.; Oleszczuk, P.; Pan, B. Formation of Persistent Free Radicals in Biochar Derived from Rice Straw Based on a Detailed Analysis of Pyrolysis Kinetics. Science of The Total Environment 2020, 715, 136575. [CrossRef]
  118. Xiang, C.; Liu, Q.; Shi, L.; Liu, Z. A Study on the New Type of Radicals in Corncob Derived Biochars. Fuel 2020, 277, 118163. [CrossRef]
  119. Chintala, R.; Subramanian, S.; Fortuna, A.-M.; Schumacher, T.E. Examining Biochar Impacts on Soil Abiotic and Biotic Processes and Exploring the Potential for Pyrosequencing Analysis. Chapter 6, In Biochar Application Essential Soil Microbial Ecology, Komang Ralebitso-Senior, T.; Orr, C.H. Eds; Elsevier, 2016, pp. 133-162, ISBN 9780128034330. [CrossRef]
  120. Takeshita, A.; Uemura, Y.; Onoe, K. Quantification of Persistent Free Radicals (PFRs) Formed in Thermally Decomposed Cellulose and Its Correlation with Residual Carbon Amount. Journal of Analytical and Applied Pyrolysis 2020, 150, 104883. [CrossRef]
  121. Pan, B.; Li, H.; Lang, D.; Xing, B. Environmentally Persistent Free Radicals: Occurrence, Formation Mechanisms and Implications. Environmental Pollution 2019, 248, 320–331. [CrossRef]
  122. Lomnicki, S.M.; Truong, H.; Vejerano, E.P.; Dellinger, B. Copper Oxide-Based Model of Persistent Free Radical Formation on Combustion-Derived Particulate Matter. Environmental science & technology 2008, 42 13, 4982–4988.
  123. Gasim, M.F.; Choong, Z.-Y.; Koo, P.-L.; Low, S.-C.; Abdurahman, M.-H.; Ho, Y.-C.; Mohamad, M.; Suryawan, I.W.; Lim, J.-W.; Oh, W.-D. Application of Biochar as Functional Material for Remediation of Organic Pollutants in Water: An Overview. Catalysts 2022, 12. [CrossRef]
  124. Zhong, D.; Jiang, Y.; Zhao, Z.; Wang, L.; Chen, J.; Ren, S.; Liu, Z.; Zhang, Y.; Tsang, D.C.W.; Crittenden, J.C. pH Dependence of Arsenic Oxidation by Rice-Husk-Derived Biochar: Roles of Redox-Active Moieties. Environ. Sci. Technol. 2019, 53, 9034–9044. [CrossRef]
  125. Zhang, K.; Sun, P.; Zhang, Y. Decontamination of Cr(VI) Facilitated Formation of Persistent Free Radicals on Rice Husk Derived Biochar. Frontiers of Environmental Science & Engineering 2019, 13, 22. [CrossRef]
  126. Zhu, S.; Huang, X.; Yang, X.; Peng, P.; Li, Z.; Jin, C. Enhanced Transformation of Cr(VI) by Heterocyclic-N within Nitrogen-Doped Biochar: Impact of Surface Modulatory Persistent Free Radicals (PFRs). Environ. Sci. Technol. 2020, 54, 8123–8132. [CrossRef]
  127. Wang, X.; Xu, J.; Liu, J.; Liu, J.; Xia, F.; Wang, C.; Dahlgren, R.A.; Liu, W. Mechanism of Cr(VI) Removal by Magnetic Greigite/Biochar Composites. Science of The Total Environment 2020, 700, 134414. [CrossRef]
  128. Tang, Z.; Kong, Y.; Zhao, S.; Jia, H.; Vione, D.; Kang, Y.; Gao, P. Enhancement of Cr(VI) Decontamination by Irradiated Sludge Biochar in Neutral Conditions: Evidence of a Possible Role of Persistent Free Radicals. Separation and Purification Technology 2021, 277, 119414. [CrossRef]
  129. Zhu, Y.; Wei, J.; Li, J. Decontamination of Cr(VI) from Water Using Sewage Sludge-Derived Biochar: Role of Environmentally Persistent Free Radicals. Chinese Journal of Chemical Engineering 2023, 56, 97–103. [CrossRef]
  130. Baltrėnaitė-Gedienė, E.; Lomnicki, S.; Guo, C. Impact of Biochar, Fertilizers and Cultivation Type on Environmentally Persistent Free Radicals in Agricultural Soil. Environmental Technology & Innovation 2022, 28, 102755. [CrossRef]
  131. Tai, Y.; Sun, J.; Tian, H.; Liu, F.; Han, B.; Fu, W.; Liu, Z.; Yang, X.; Liu, Q. Efficient Degradation of Organic Pollutants by S-NaTaO3/Biochar under Visible Light and the Photocatalytic Performance of a Permonosulfate-Based Dual-Effect Catalytic System. Journal of Environmental Sciences 2023, 125, 388–400. [CrossRef]
  132. Kelley, M.A.; Hebert, V.Y.; Thibeaux, T.M.; Orchard, M.A.; Hasan, F.; Cormier, S.A.; Thevenot, P.T.; Lomnicki, S.M.; Varner, K.J.; Dellinger, B.; et al. Model Combustion-Generated Particulate Matter Containing Persistent Free Radicals Redox Cycle to Produce Reactive Oxygen Species. Chem. Res. Toxicol. 2013, 26, 1862–1871. [CrossRef]
  133. Squadrito, G.L.; Cueto, R.; Dellinger, B.; Pryor, W.A. Quinoid Redox Cycling as a Mechanism for Sustained Free Radical Generation by Inhaled Airborne Particulate Matter. Free Radical Biology and Medicine 2001, 31, 1132–1138. [CrossRef]
  134. Alburquerque, J.A.; Salazar, P.; Barrón, V.; Torrent, J.; del Campillo, M. del C.; Gallardo, A.; Villar, R. Enhanced Wheat Yield by Biochar Addition under Different Mineral Fertilization Levels. Agronomy for Sustainable Development 2013, 33, 475–484. [CrossRef]
  135. Jiang, S.-F.; Ling, L.-L.; Chen, W.-J.; Liu, W.-J.; Li, D.-C.; Jiang, H. High Efficient Removal of Bisphenol A in a Peroxymonosulfate/Iron Functionalized Biochar System: Mechanistic Elucidation and Quantification of the Contributors. Chemical Engineering Journal 2019, 359, 572–583. [CrossRef]
  136. Luo, H.; Lin, Q.; Zhang, X.; Huang, Z.; Liu, S.; Jiang, J.; Xiao, R.; Liao, X. New Insights into the Formation and Transformation of Active Species in nZVI/BC Activated Persulfate in Alkaline Solutions. Chemical Engineering Journal 2019, 359, 1215–1223. [CrossRef]
  137. Pi, Z.; Li, X.; Wang, D.; Xu, Q.; Tao, Z.; Huang, X.; Yao, F.; Wu, Y.; He, L.; Yang, Q. Persulfate Activation by Oxidation Biochar Supported Magnetite Particles for Tetracycline Removal: Performance and Degradation Pathway. Journal of Cleaner Production 2019, 235, 1103–1115. [CrossRef]
  138. He, J.; Xiao, Y.; Tang, J.; Chen, H.; Sun, H. Persulfate Activation with Sawdust Biochar in Aqueous Solution by Enhanced Electron Donor-Transfer Effect. Science of The Total Environment 2019, 690, 768–777. [CrossRef]
  139. Danping, W.U.; Fangfang, L.I.; Jing, Z.; Peng, W.; Min, W.U. Photocatalysis degradation of rhodamine B by dissolved organic matter of biochars[J]. Chinese Journal of Environmental Engineering 2019, 13(11), 2562-2569. [CrossRef]
  140. Wang, H.; Guo, W.; Yin, R.; Du, J.; Wu, Q.; Luo, H.; Liu, B.; Sseguya, F.; Ren, N. Biochar-Induced Fe(III) Reduction for Persulfate Activation in Sulfamethoxazole Degradation: Insight into the Electron Transfer, Radical Oxidation and Degradation Pathways. Chemical Engineering Journal 2019, 362, 561–569. [CrossRef]
  141. Zhu, K.; Wang, X.; Geng, M.; Chen, D.; Lin, H.; Zhang, H. Catalytic Oxidation of Clofibric Acid by Peroxydisulfate Activated with Wood-Based Biochar: Effect of Biochar Pyrolysis Temperature, Performance and Mechanism. Chemical Engineering Journal 2019, 374, 1253–1263. [CrossRef]
  142. Li, L.; Lai, C.; Huang, F.; Cheng, M.; Zeng, G.; Huang, D.; Li, B.; Liu, S.; Zhang, M.; Qin, L.; et al. Degradation of Naphthalene with Magnetic Bio-Char Activate Hydrogen Peroxide: Synergism of Bio-Char and Fe–Mn Binary Oxides. Water Research 2019, 160, 238–248. [CrossRef]
  143. Luo, K.; Pang, Y.; Yang, Q.; Wang, D.; Li, X.; Wang, L.; Lei, M.; Liu, J. Enhanced Ciprofloxacin Removal by Sludge-Derived Biochar: Effect of Humic Acid. Chemosphere 2019, 231, 495–501. [CrossRef]
  144. Deng, R.; Luo, H.; Huang, D.; Zhang, C. Biochar-Mediated Fenton-like Reaction for the Degradation of Sulfamethazine: Role of Environmentally Persistent Free Radicals. Chemosphere 2020, 255, 126975. [CrossRef]
  145. Liu, G.; Zhang, Y.; Yu, H.; Jin, R.; Zhou, J. Acceleration of Goethite-Catalyzed Fenton-like Oxidation of Ofloxacin by Biochar. Journal of Hazardous Materials 2020, 397, 122783. [CrossRef]
  146. Liu, B.; Guo, W.; Wang, H.; Si, Q.; Zhao, Q.; Luo, H.; Ren, N. Activation of Peroxymonosulfate by Cobalt-Impregnated Biochar for Atrazine Degradation: The Pivotal Roles of Persistent Free Radicals and Ecotoxicity Assessment. Journal of Hazardous Materials 2020, 398, 122768. [CrossRef]
  147. Grilla, E.; Vakros, J.; Konstantinou, I.; Manariotis, I.D.; Mantzavinos, D. Activation of Persulfate by Biochar from Spent Malt Rootlets for the Degradation of Trimethoprim in the Presence of Inorganic Ions. Journal of Chemical Technology & Biotechnology 2020, 95, 2348–2358. [CrossRef]
  148. Feng, Z.; Zhou, B.; Yuan, R.; Li, H.; He, P.; Wang, F.; Chen, Z.; Chen, H. Biochar Derived from Different Crop Straws as Persulfate Activator for the Degradation of Sulfadiazine: Influence of Biomass Types and Systemic Cause Analysis. Chemical Engineering Journal 2022, 440. [CrossRef]
  149. Kim, D.-G.; Ko, S.-O. Effects of Thermal Modification of a Biochar on Persulfate Activation and Mechanisms of Catalytic Degradation of a Pharmaceutical. Chemical Engineering Journal 2020, 399, 125377. [CrossRef]
  150. He, W.; Zhu, Y.; Zeng, G.; Zhang, Y.; Wang, Y.; Zhang, M.; Long, H.; Tang, W. Efficient Removal of Perfluorooctanoic Acid by Persulfate Advanced Oxidative Degradation: Inherent Roles of Iron-Porphyrin and Persistent Free Radicals. Chemical Engineering Journal 2020, 392, 123640. [CrossRef]
  151. Cao, W.; Zeng, C.; Guo, X.; Liu, Q.; Zhang, X.; Mameda, N. Enhanced Electrochemical Degradation of 2,4-Dichlorophenol with the Assist of Hydrochar. Chemosphere 2020, 260, 127643. [CrossRef]
  152. Xu, H.; Zhang, Y.; Li, J.; Hao, Q.; Li, X.; Liu, F. Heterogeneous Activation of Peroxymonosulfate by a Biochar-Supported Co3O4 Composite for Efficient Degradation of Chloramphenicols. Environmental Pollution 2020, 257, 113610. [CrossRef]
  153. Li, X.; Jia, Y.; Zhou, M.; Su, X.; Sun, J. High-Efficiency Degradation of Organic Pollutants with Fe, N Co-Doped Biochar Catalysts via Persulfate Activation. Journal of Hazardous Materials 2020, 397, 122764. [CrossRef]
  154. Xie, Y.; Hu, W.; Wang, X.; Tong, W.; Li, P.; Zhou, H.; Wang, Y.; Zhang, Y. Molten Salt Induced Nitrogen-Doped Biochar Nanosheets as Highly Efficient Peroxymonosulfate Catalyst for Organic Pollutant Degradation. Environmental Pollution 2020, 260, 114053. [CrossRef]
  155. Yan, J.; Yang, L.; Qian, L.; Han, L.; Chen, M. Nano-Magnetite Supported by Biochar Pyrolyzed at Different Temperatures as Hydrogen Peroxide Activator: Synthesis Mechanism and the Effects on Ethylbenzene Removal. Environmental Pollution 2020, 261, 114020. [CrossRef]
  156. Mian, M.M.; Liu, G.; Zhou, H. Preparation of N-Doped Biochar from Sewage Sludge and Melamine for Peroxymonosulfate Activation: N-Functionality and Catalytic Mechanisms. Science of The Total Environment 2020, 744, 140862. [CrossRef]
  157. Veiga, P.A. da S.; Schultz, J.; Matos, T.T. da S.; Fornari, M.R.; Costa, T.G.; Meurer, L.; Mangrich, A.S. Production of High-Performance Biochar Using a Simple and Low-Cost Method: Optimization of Pyrolysis Parameters and Evaluation for Water Treatment. Journal of Analytical and Applied Pyrolysis 2020, 148, 104823. [CrossRef]
  158. Sun, P.; Zhang, K.-K.; Zhang, Y.; Zhang, Y. Sunflower-Straw-Derived Biochar-Enhanced Fe（Ⅲ/S2O82- System for Degradation of Benzoic Acid. Environmental Science 2020, (05), 2301-2309.
  159. Zeng, L.; Chen, Q.; Tan, Y.; Lan, P.; Zhou, D.; Wu, M.; Liang, N.; Pan, B.; Xing, B. Dual Roles of Biochar Redox Property in Mediating 2,4-Dichlorophenol Degradation in the Presence of Fe3+ and Persulfate. Chemosphere 2021, 279, 130456. [CrossRef]
  160. Yang, F.; Zhu, Q.; Gao, Y.; Jian, H.; Wang, C.; Sun, H. Effects of Biochar-Dissolved Organic Matter on the Photodegradation of Sulfamethoxazole and Chloramphenicol in Biochar Solutions as Revealed by Oxygen Reduction Performances and Free Radicals. Science of The Total Environment 2021, 781, 146807. [CrossRef]
  161. Min, L.; Zhang, P.; Fan, M.; Xu, X.; Wang, C.; Tang, J.; Sun, H. Efficient Degradation of P-Nitrophenol by Fe@pomelo Peel-Derived Biochar Composites and Its Mechanism of Simultaneous Reduction and Oxidation Process. Chemosphere 2021, 267, 129213. [CrossRef]
  162. Mer, K.; Sajjadi, B.; Egiebor, Nosa.O.; Chen, W.-Y.; Mattern, Daniell.L.; Tao, W. Enhanced Degradation of Organic Contaminants Using Catalytic Activity of Carbonaceous Structures: A Strategy for the Reuse of Exhausted Sorbents. Journal of Environmental Sciences 2021, 99, 267–273. [CrossRef]
  163. Cao, Y.; Jiang, S.; Kang, X.; Zhang, H.; Zhang, Q.; Wang, L. Enhancing Degradation of Atrazine by Fe-Phenol Modified Biochar/Ferrate(VI) under Alkaline Conditions: Analysis of the Mechanism and Intermediate Products. Chemosphere 2021, 285, 131399. [CrossRef]
  164. Zhang, R.; Zheng, X.; Zhang, D.; Niu, X.; Ma, J.; Lin, Z.; Fu, M.; Zhou, S. Insight into the Roles of Endogenous Minerals in the Activation of Persulfate by Graphitized Biochar for Tetracycline Removal. Science of The Total Environment 2021, 768, 144281. [CrossRef]
  165. Yi, Y.; Luo, J.; Fang, Z. Magnetic Biochar Derived from Eichhornia Crassipes for Highly Efficient Fenton-like Degradation of Antibiotics: Mechanism and Contributions. Journal of Environmental Chemical Engineering 2021, 9, 106258. [CrossRef]
  166. Zhang, Y.; Xu, M.; Liang, S.; Feng, Z.; Zhao, J. Mechanism of Persulfate Activation by Biochar for the Catalytic Degradation of Antibiotics: Synergistic Effects of Environmentally Persistent Free Radicals and the Defective Structure of Biochar. Science of The Total Environment 2021, 794, 148707. [CrossRef]
  167. Wang, X.; Zhang, P.; Wang, C.; Jia, H.; Shang, X.; Tang, J.; Sun, H. Metal-Rich Hyperaccumulator-Derived Biochar as an Efficient Persulfate Activator: Role of Intrinsic Metals (Fe, Mn and Zn) in Regulating Characteristics, Performance and Reaction Mechanisms. Journal of Hazardous Materials 2022, 424, 127225. [CrossRef]
  168. Luo Kun; Yang Chen; Li Xue; Pang Ya; Yang Qi Mn-Doped Biochar Derived from Sewage Sludge for Ciprofloxacin Degradation. Journal of Environmental Engineering 2022, 148, 04022048. [CrossRef]
  169. Li, H.; Liu, Y.; Jiang, F.; Bai, X.; Li, H.; Lang, D.; Wang, L.; Pan, B. Persulfate Adsorption and Activation by Carbon Structure Defects Provided New Insights into Ofloxacin Degradation by Biochar. Science of The Total Environment 2022, 806, 150968. [CrossRef]
  170. Dai, Z.; Zhao, L.; Peng, S.; Yue, Z.; Zhan, X.; Wang, J. Removal of Oxytetracycline Promoted by Manganese-Doped Biochar Based on Density Functional Theory Calculations: Comprehensive Evaluation of the Effect of Transition Metal Doping. Science of The Total Environment 2022, 806, 150268. [CrossRef]
  171. Rangarajan, G.; Farnood, R. Role of Persistent Free Radicals and Lewis Acid Sites in Visible-Light-Driven Wet Peroxide Activation by Solid Acid Biochar Catalysts – A Mechanistic Study. Journal of Hazardous Materials 2022, 438, 129514. [CrossRef]
  172. An, X.; Chen, Y.; Ao, M.; Jin, Y.; Zhan, L.; Yu, B.; Wu, Z.; Jiang, P. Sequential Photocatalytic Degradation of Organophosphorus Pesticides and Recovery of Orthophosphate by Biochar/α-Fe2O3/MgO Composite: A New Enhanced Strategy for Reducing the Impacts of Organophosphorus from Wastewater. Chemical Engineering Journal 2022, 435, 135087. [CrossRef]
  173. Jiang, Xinyi and Xiao, Ye and Xiao, Jiana and Zhang, Weihua and Rongliang, Qiu, The Effect of Persistent Free Radicals in Sludge Derived Biochar on the Removal of P-Chlorophenol. Available at SSRN: https://ssrn.com/abstract=3992617 (accessed on 04 January 2024).
  174. Pan, Y.; Peng, Z.; Liu, Z.; Shao, B.; Liang, Q.; He, Q.; Wu, T.; Zhang, X.; Zhao, C.; Liu, Y.; et al. Activation of Peroxydisulfate by Bimetal Modified Peanut Hull-Derived Porous Biochar for the Degradation of Tetracycline in Aqueous Solution. Journal of Environmental Chemical Engineering 2022, 10, 107366. [CrossRef]
  175. Yin, Q.; Yan, H.; Liang, Y.; Jiang, Z.; Wang, H.; Nian, Y. Activation of Persulfate by Blue Algae Biochar Supported FeOX Particles for Tetracycline Degradation: Performance and Mechanism. Separation and Purification Technology 2023, 319, 124005. [CrossRef]
  176. Badiger, S.M.; Nidheesh, P.V. Applications of Biochar in Sulfate Radical-Based Advanced Oxidation Processes for the Removal of Pharmaceuticals and Personal Care Products. Water Science and Technology 2023, 87, 1329–1348. [CrossRef]
  177. Yu, Y.; Zhong, Z.; Guo, H.; Yu, Y.; Zheng, T.; Li, H.; Chang, Z. Biochar–Goethite Composites Inhibited/Enhanced Degradation of Triphenyl Phosphate by Activating Persulfate: Insights on the Mechanism. Science of The Total Environment 2023, 858, 159940. [CrossRef]
  178. Pei, S.; Zhao, Y.; Li, W.; Qu, C.; Ren, Y.; Yang, Y.; Liu, J.; Wu, C. Critical Impact of Pyrolysis Temperatures on Biochars for Peroxymonosulfate Activation: Structural Characteristics, Degradation Performance and Mechanism. Chemical Engineering Journal 2023, 477, 147274. [CrossRef]
  179. Zhang, Y.; He, R.; Zhao, J.; Zhang, X.; Bildyukevich, A.V. Effect of Aged Biochar after Microbial Fermentation on Antibiotics Removal: Key Roles of Microplastics and Environmentally Persistent Free Radicals. Bioresource Technology 2023, 374, 128779. [CrossRef]
  180. Yang, Z.; An, Q.; Deng, S.; Xu, B.; Li, Z.; Deng, S.; Zhao, B.; Ye, Z. Efficient Activation of Peroxydisulfate by Modified Red Mud Biochar Derived from Waste Corn Straw for Levofloxacin Degradation: Efficiencies and Mechanisms. Journal of Environmental Chemical Engineering 2023, 11, 111609. [CrossRef]
  181. Zhang, Y.; He, R.; Zhao, J. Removal Mechanism of Tetracycline-Cr(Ⅵ) Combined Pollutants by Different S-Doped Sludge Biochars: Role of Environmentally Persistent Free Radicals. Chemosphere 2023, 317. [CrossRef]
  182. Luo, K.; Lin, N.; Li, X.; Pang, Y.; Wang, L.; Lei, M.; Yang, Q. Efficient Hexavalent Chromium Removal by Nano-Cerium Oxide Functionalized Biochar: Insight into the Role of Reduction. Journal of Environmental Chemical Engineering 2023, 11, 110004. [CrossRef]
  183. Zhong, D.; Zhao, Z.; Jiang, Y.; Yang, X.; Wang, L.; Chen, J.; Guan, C.-Y.; Zhang, Y.; Tsang, D.C.W.; Crittenden, J.C. Contrasting Abiotic As(III) Immobilization by Undissolved and Dissolved Fractions of Biochar in Ca2+-Rich Groundwater under Anoxic Conditions. Water Research 2020, 183, 116106. [CrossRef]
  184. Huang, Y.; Gao, M.; Deng, Y.; Khan, Z.H.; Liu, X.; Song, Z.; Qiu, W. Efficient Oxidation and Adsorption of As(III) and As(V) in Water Using a Fenton-like Reagent, (Ferrihydrite)-Loaded Biochar. Science of The Total Environment 2020, 715, 136957. [CrossRef]
  185. Zhu, Y.; Wei, J.; Li, J. Biochar-Activated Persulfate Oxidation of Arsenic(III): Nonnegligible Roles of Environmentally Persistent Free Radicals. Journal of Environmental Chemical Engineering 2023, 11, 111033. [CrossRef]
  186. Yu, J.; Zhu, Z.; Zhang, H.; Chen, T.; Qiu, Y.; Xu, Z.; Yin, D. Efficient Removal of Several Estrogens in Water by Fe-Hydrochar Composite and Related Interactive Effect Mechanism of H2O2 and Iron with Persistent Free Radicals from Hydrochar of Pinewood. Science of The Total Environment 2019, 658, 1013–1022. [CrossRef]
  187. Zhu, N.; Li, C.; Bu, L.; Tang, C.; Wang, S.; Duan, P.; Yao, L.; Tang, J.; Dionysiou, D.D.; Wu, Y. Bismuth Impregnated Biochar for Efficient Estrone Degradation: The Synergistic Effect between Biochar and Bi/Bi2O3 for a High Photocatalytic Performance. Journal of Hazardous Materials 2020, 384, 121258. [CrossRef]
  188. Chen, Y.; Duan, X.; Zhang, C.; Wang, S.; Ren, N.; Ho, S.-H. Graphitic Biochar Catalysts from Anaerobic Digestion Sludge for Nonradical Degradation of Micropollutants and Disinfection. Chemical Engineering Journal 2020, 384, 123244. [CrossRef]
  189. Xu, H.; Han, Y.; Wang, G.; Deng, P.; Feng, L. Walnut Shell Biochar Based Sorptive Remediation of Estrogens Polluted Simulated Wastewater: Characterization, Adsorption Mechanism and Degradation by Persistent Free Radicals. Environmental Technology & Innovation 2022, 28, 102870. [CrossRef]
  190. Wang, T.; Zheng, J.; Cai, J.; Liu, Q.; Zhang, X. Visible-Light-Driven Photocatalytic Degradation of Dye and Antibiotics by Activated Biochar Composited with K+ Doped g-C3N4: Effects, Mechanisms, Actual Wastewater Treatment and Disinfection. Science of the Total Environment 2022, 839. [CrossRef]
  191. Shi, J.; Wang, J.; Liang, L.; Xu, Z.; Chen, Y.; Chen, S.; Xu, M.; Wang, X.; Wang, S. Carbothermal Synthesis of Biochar-Supported Metallic Silver for Enhanced Photocatalytic Removal of Methylene Blue and Antimicrobial Efficacy. Journal of Hazardous Materials 2021, 401, 123382. [CrossRef]
  192. Lian, F.; Yu, W.; Zhou, Q.; Gu, S.; Wang, Z.; Xing, B. Size Matters: Nano-Biochar Triggers Decomposition and Transformation Inhibition of Antibiotic Resistance Genes in Aqueous Environments. Environ. Sci. Technol. 2020, 54, 8821–8829. [CrossRef]
  193. Liang, Z.; Min, W.U.; Guo-juan, W.U. Electron exchange capacity of rice biochar at different preparation temperatures[J]. CHINA ENVIRONMENTAL SCIENCECE, 2019, 39(10), 4329-4336.
  194. Xiang, L.; Liu, S.; Ye, S.; Yang, H.; Song, B.; Qin, F.; Shen, M.; Tan, C.; Zeng, G.; Tan, X. Potential Hazards of Biochar: The Negative Environmental Impacts of Biochar Applications. Journal of Hazardous Materials 2021, 420, 126611. [CrossRef]
  195. Kharisov, B.I.; Kharissova, O.V. Carbon Allotropes in the Environment and Their Toxicity. In Carbon Allotropes: Metal-Complex Chemistry, Properties and Applications. Springer, Cham, 2019. [CrossRef]
  196. El-Naggar, A.; Lee, M.-H.; Hur, J.; Lee, Y.H.; Igalavithana, A.D.; Shaheen, S.M.; Ryu, C.; Rinklebe, J.; Tsang, D.C.W.; Ok, Y.S. Biochar-Induced Metal Immobilization and Soil Biogeochemical Process: An Integrated Mechanistic Approach. Science of The Total Environment 2020, 698, 134112. [CrossRef]
  197. Cui, H.; Li, D.; Liu, X.; Fan, Y.; Zhang, X.; Zhang, S.; Zhou, J.; Fang, G.; Zhou, J. Dry-Wet and Freeze-Thaw Aging Activate Endogenous Copper and Cadmium in Biochar. Journal of Cleaner Production 2021, 288, 125605. [CrossRef]
  198. Rombolà, A.G.; Fabbri, D.; Baronti, S.; Vaccari, F.P.; Genesio, L.; Miglietta, F. Changes in the Pattern of Polycyclic Aromatic Hydrocarbons in Soil Treated with Biochar from a Multiyear Field Experiment. Chemosphere 2019, 219, 662–670. [CrossRef]
  199. Joseph, S.; Camps-Arbestain, M.; Lin, Y.; Munroe, P.; Chia, C.H.; Hook, J.M.; Zwieten, L.V.; Kimber, S.; Cowie, A.L.; Singh, B.P.; et al. An Investigation into the Reactions of Biochar in Soil. Soil Research 2010, 48, 501–515.
  200. Kim, H.-B.; Kim, S.-H.; Jeon, E.-K.; Kim, D.-H.; Tsang, D.C.W.; Alessi, D.S.; Kwon, E.E.; Baek, K. Effect of Dissolved Organic Carbon from Sludge, Rice Straw and Spent Coffee Ground Biochar on the Mobility of Arsenic in Soil. Science of The Total Environment 2018, 636, 1241–1248. [CrossRef]
  201. Jia, C.; Luo, J.; Fan, J.; Clark, J.H.; Zhang, S.; Zhu, X. Urgently Reveal Longly Hidden Toxicant in a Familiar Fabrication Process of Biomass-Derived Environment Carbon Material. Journal of Environmental Sciences 2021, 100, 250–256. [CrossRef]
  202. Liu, J.; Gao, N.; Wen, X.; Jia, H.; Lichtfouse, E. Plant and Algal Toxicity of Persistent Free Radicals and Reactive Oxygen Species Generated by Heating Anthracene-Contaminated Soils from 100 to 600 °C. Environmental Chemistry Letters 2021, 19, 2695–2703. [CrossRef]
  203. Xu, Y.; Lu, X.; Su, G.; Chen, X.; Meng, J.; Li, Q.; Wang, C.; Shi, B. Scientific and Regulatory Challenges of Environmentally Persistent Free Radicals: From Formation Theory to Risk Prevention Strategies. Journal of Hazardous Materials 2023, 456, 131674. [CrossRef]
  204. Yi, J.-F.; Lin, Z.-Z.; Li, X.; Zhou, Y.-Q.; Guo, Y. A Short Review on Environmental Distribution and Toxicity of the Environmentally Persistent Free Radicals. Chemosphere 2023, 340, 139922. [CrossRef]
  205. Jaligama, S.; Patel, V.S.; Wang, P.; Sallam, A.; Harding, J.; Kelley, M.; Mancuso, S.R.; Dugas, T.R.; Cormier, S.A. Radical Containing Combustion Derived Particulate Matter Enhance Pulmonary Th17 Inflammation via the Aryl Hydrocarbon Receptor. Particle and Fibre Toxicology 2018, 15, 20. [CrossRef]
  206. Harmon, A.C.; Hebert, V.Y.; Cormier, S.A.; Subramanian, B.; Reed, J.R.; Backes, W.L.; Dugas, T.R. Particulate Matter Containing Environmentally Persistent Free Radicals Induces AhR-Dependent Cytokine and Reactive Oxygen Species Production in Human Bronchial Epithelial Cells. PLOS ONE 2018, 13, e0205412. [CrossRef]
  207. Reed, J.R.; dela Cruz, A.L.N.; Lomnicki, S.M.; Backes, W.L. Environmentally Persistent Free Radical-Containing Particulate Matter Competitively Inhibits Metabolism by Cytochrome P450 1A2. Toxicology and Applied Pharmacology 2015, 289, 223–230. [CrossRef]
  208. Chuang, G.C.; Xia, H.; Mahne, S.E.; Varner, K.J. Environmentally Persistent Free Radicals Cause Apoptosis in HL-1 Cardiomyocytes. Cardiovascular Toxicology 2017, 17, 140–149. [CrossRef]
  209. Zhao, S.; Miao, D.; Zhu, K.; Kelin, T.; Wang, C.; Sharma, V.; Jia, H. Interaction of Benzo[a]Pyrene with Cu(II)-Montmorillonite: Generation and Toxicity of Environmentally Persistent Free Radicals and Reactive Oxygen Species. Environment International 2019, 129, 154–163. [CrossRef]
  210. Thevenot, P.; Saravia, J.; Jin, N.; Giaimo, J.; Chustz, R.; Mahne, S.; Kelley, M.; Hebert, V.; Dellinger, B.; Dugas, T.; et al. Radical-Containing PM0.2 Initiates Epithelial-to-Mesenchymal Transitions in Airway Epithelial Cells. American journal of respiratory cell and molecular biology 2012, 48. [CrossRef]
  211. Zhang, X.; Gu, W.; Ma, Z.; Liu, Y.; Ru, H.; Zhou, J.; Zang, Y.; Xu, Z.; Qian, G. Short-Term Exposure to ZnO/MCB Persistent Free Radical Particles Causes Mouse Lung Lesions via Inflammatory Reactions and Apoptosis Pathways. Environmental Pollution 2020, 261, 114039. [CrossRef]
  212. Balakrishna, S.; Saravia, J.; Thevenot, P.; Ahlert, T.; Lominiki, S.; Dellinger, B.; Cormier, S.A. Environmentally Persistent Free Radicals Induce Airway Hyperresponsiveness in Neonatal Rat Lungs. Particle and Fibre Toxicology 2011, 8, 11. [CrossRef]
  213. Huang, H.-S.; Ma, M.-C.; Chen, J.; Chen, C.-F. Changes in Renal Hemodynamics and Urodynamics in Rats with Chronic Hyperoxaluria and after Acute Oxalate Infusion: Role of Free Radicals. Neurourology and Urodynamics 2003, 22, 176–182. [CrossRef]
  214. Reinke, L.A.; Moore, D.R.; Nanji, A.A. Pronounced Hepatic Free Radical Formation Precedes Pathological Liver Injury in Ethanol-Fed Rats. Alcoholism: Clinical and Experimental Research 2000, 24, 332–335. [CrossRef]
  215. Burn, B.R.; Varner, K.J. Environmentally Persistent Free Radicals Compromise Left Ventricular Function during Ischemia/Reperfusion Injury. American Journal of Physiology-Heart and Circulatory Physiology 2015, 308, H998–H1006. [CrossRef]
  216. Wang, P.; You, D.; Saravia, J.; Shen, H.; Cormier, S.A. Maternal Exposure to Combustion Generated PM Inhibits Pulmonary Th1 Maturation and Concomitantly Enhances Postnatal Asthma Development in Offspring. Particle and Fibre Toxicology 2013, 10, 29. [CrossRef]
  217. Guan, X.; Truong, L.; Lomnicki, S.; Tanguay, R.; Cormier, S. Developmental Hazard of Environmentally Persistent Free Radicals and Protective Effect of TEMPOL in Zebrafish Model. Toxics 2021, 9, p12.
  218. Zhang, Y.; Guo, X.; Si, X.; Yang, R.; Zhou, J.; Quan, X. Environmentally Persistent Free Radical Generation on Contaminated Soil and Their Potential Biotoxicity to Luminous Bacteria. Science of The Total Environment 2019, 687, 348–354. [CrossRef]
  219. Zhang, X.; Chen, Y.; Hu, D.; Zhao, L.; Wang, L.; Wu, M. Neurotoxic effect of environmental persistent free radicals in rice biochar to Caenorhabditis elegans. China Environ. Sci. 2019, 39, 2644-2651.
  220. Stephenson, E.J.; Ragauskas, A.; Jaligama, S.; Redd, J.R.; Parvathareddy, J.; Peloquin, M.J.; Saravia, J.; Han, J.C.; Cormier, S.A.; Bridges, D. Exposure to Environmentally Persistent Free Radicals during Gestation Lowers Energy Expenditure and Impairs Skeletal Muscle Mitochondrial Function in Adult Mice. American Journal of Physiology-Endocrinology and Metabolism 2016, 310, E1003–E1015. [CrossRef]
  221. Lee, G.I.; Saravia, J.; You, D.; Shrestha, B.; Jaligama, S.; Hebert, V.Y.; Dugas, T.R.; Cormier, S.A. Exposure to Combustion Generated Environmentally Persistent Free Radicals Enhances Severity of Influenza Virus Infection. Part Fibre Toxicol 2014, 11, 57. [CrossRef]
  222. Briedé, J.J.; Godschalk, R.W.L.; Emans, M.T.G.; de Kok, T.M.C.M.; van Agen, E.; van Maanen, J.M.S.; van Schooten, F.-J.; Kleinjans, J.C.S. In Vitro and In Vivo Studies on Oxygen Free Radical and DNA Adduct Formation in Rat Lung and Liver during Benzo[a]Pyrene Metabolism. Free Radical Research 2004, 38, 995–1002. [CrossRef]
  223. Balakrishna, S.; Lomnicki, S.; McAvey, K.M.; Cole, R.B.; Dellinger, B.; Cormier, S.A. Environmentally Persistent Free Radicals Amplify Ultrafine Particle Mediated Cellular Oxidative Stress and Cytotoxicity. Particle and Fibre Toxicology 2009, 6, 11. [CrossRef]
Preprints 101353 g001
Figure 1. Number of publications on PFRs and their degradation, absorption, and possible toxic actions of the last ten years according to PubMed dataset. The survey was carried out using the following keywords: permanent free radicals (rose bars); permanent free radicals AND degradation (purple bars); permanent free radicals AND absorption (yellow bars); permanent free radicals AND toxicity (light blue bars).
Figure 1. Number of publications on PFRs and their degradation, absorption, and possible toxic actions of the last ten years according to PubMed dataset. The survey was carried out using the following keywords: permanent free radicals (rose bars); permanent free radicals AND degradation (purple bars); permanent free radicals AND absorption (yellow bars); permanent free radicals AND toxicity (light blue bars).
Preprints 101353 g001
Preprints 101353 g002
Figure 2. Number of publications on BCs and on bamboo-derived BCs in the last ten years according to PubMed dataset. The survey used the following keywords: biochar (rose bars) and bamboo biochar (purple bars).
Figure 2. Number of publications on BCs and on bamboo-derived BCs in the last ten years according to PubMed dataset. The survey used the following keywords: biochar (rose bars) and bamboo biochar (purple bars).
Preprints 101353 g002
Preprints 101353 g003
Figure 3. Number of publications on bamboo-derived BCs, as well as their absorption, degradation, and toxic properties of the last ten years according to PubMed dataset. The survey was carried out using the following keywords: bamboo biochar (rose bars); bamboo biochar AND adsorption (green bars); bamboo biochar AND degradation (yellow bars); bamboo biochar AND toxicity (light blue bars).
Figure 3. Number of publications on bamboo-derived BCs, as well as their absorption, degradation, and toxic properties of the last ten years according to PubMed dataset. The survey was carried out using the following keywords: bamboo biochar (rose bars); bamboo biochar AND adsorption (green bars); bamboo biochar AND degradation (yellow bars); bamboo biochar AND toxicity (light blue bars).
Preprints 101353 g003
Preprints 101353 g004
Figure 4. Some of the advantages and physicochemical properties of BCs.
Figure 4. Some of the advantages and physicochemical properties of BCs.
Preprints 101353 g004
Preprints 101353 g005
Figure 5. Parameters that mainly influence the pyrolysis process outcomes and the prepared BC elemental composition, physicochemical properties, and possible applications.
Figure 5. Parameters that mainly influence the pyrolysis process outcomes and the prepared BC elemental composition, physicochemical properties, and possible applications.
Preprints 101353 g005
Preprints 101353 g006
Figure 6. The lifetime of different free radicals.
Figure 6. The lifetime of different free radicals.
Preprints 101353 g006
Preprints 101353 g007
Figure 7. EPR analysis of PFRs generated on a N-doped hydro char prepared in a tube furnace to 600 °C for 1 h under N2 atmosphere (a) [108]; EPR spectrum of the unstable free radical superoxide (O2•-) when trapped by DMPO to form a long-lived nitroxide (DMPO−OOH) (b).
Figure 7. EPR analysis of PFRs generated on a N-doped hydro char prepared in a tube furnace to 600 °C for 1 h under N2 atmosphere (a) [108]; EPR spectrum of the unstable free radical superoxide (O2•-) when trapped by DMPO to form a long-lived nitroxide (DMPO−OOH) (b).
Preprints 101353 g007
Preprints 101353 g008
Figure 8. Information deriving from EPR analyses.
Figure 8. Information deriving from EPR analyses.
Preprints 101353 g008
Preprints 101353 sch001
Scheme 1. Possible mechanisms leading to the formation of BC-bounded PFRs from lignin. The orange sphere represents biomass, while the black sphere represents BC whose hypothetic structures depending on pyrolysis condition have been shown at the bottom of the Scheme.
Scheme 1. Possible mechanisms leading to the formation of BC-bounded PFRs from lignin. The orange sphere represents biomass, while the black sphere represents BC whose hypothetic structures depending on pyrolysis condition have been shown at the bottom of the Scheme.
Preprints 101353 sch001
Preprints 101353 sch002
Scheme 2. Possible mechanisms leading to the formation of BC-bounded graphitic PFRs from cellulose (left side) and emicellulose (right side). The orange sphere represents biomass, while the black sphere represents BC whose hypothetic structures depending on pyrolysis condition have been shown at the bottom of the Scheme.
Scheme 2. Possible mechanisms leading to the formation of BC-bounded graphitic PFRs from cellulose (left side) and emicellulose (right side). The orange sphere represents biomass, while the black sphere represents BC whose hypothetic structures depending on pyrolysis condition have been shown at the bottom of the Scheme.
Preprints 101353 sch002
Preprints 101353 g009
Figure 9. PFRs formation process.
Figure 9. PFRs formation process.
Preprints 101353 g009
Preprints 101353 g010
Figure 10. Number of publications on BCs-derived PFRs from 2014 according to Scopus dataset (reviews and chapters in books included). The survey used the following keywords: persistent AND free AND radicals AND biochar.
Figure 10. Number of publications on BCs-derived PFRs from 2014 according to Scopus dataset (reviews and chapters in books included). The survey used the following keywords: persistent AND free AND radicals AND biochar.
Preprints 101353 g010
Preprints 101353 g011
Figure 11. Number of publications for each type of EPRs application found in literature considering a randomly selected population of 72 papers published in 2019-2023.
Figure 11. Number of publications for each type of EPRs application found in literature considering a randomly selected population of 72 papers published in 2019-2023.
Preprints 101353 g011
Table 1. Main sources and general application of BCs.
Table 1. Main sources and general application of BCs.
Source biomass Ref. Applications Refs.
Crop residue [34] Carbon sequestration [48]
Kitchen waste [35]
Forestry [36] Soil amendment [49]
Agricultural waste [37]
Sugar beet tailings [38] Composting [50]
Forest residues [39]
Waste wood [40] Wastewater treatment [51]
Bioenergy crops [41]
Municipal solid waste [42] Energy production/storage [52]
Wheat straw [43] Adsorbing xenobiotics [53]
Rice straw [44] Reducing greenhouse emission gas [54]
Food manure [45] Xenobiotics degradation [55]
Animal manure [46] Catalysis [56]
Corn cob [47]
Table 2. Main BC production methods, temperature conditions, and yields.
Table 2. Main BC production methods, temperature conditions, and yields.
Pyrolysis Temperature (°C) Residence Time Biochar (%) Bio-oil (%) Syngas (%) Refs
200−700 0.5-2 sec 35 30 35 [67]
500−1000 Hours/day 12 75 13
HC 180−300 1−16 h 50−80 5−20 2−5 [68]
Gasification 750−900 10−20 s 10 5 85 [69]
Torrefaction 290 10−60 min 80 0 20 [70]
Flash carbonization 300−600 < 30 min 37 -- -- [71]
HC = Hydrothermal carbonization.
Table 3. Fast and slow pyrolysis details.
Table 3. Fast and slow pyrolysis details.
Fast Pyrolysis Slow Pyrolysis Ref.
Target Product Bio-oils Biochar [79]
Reactors Bubbling fluidized bed
Ablative reactor
Rotary cone		 Fixed bed pyrolysis reactor
Auger pyrolysis reactor
Warming rate 10-10000°C/min 0.1-10°C/min
RT at⇧ temperature 0.5-2 seconds > 1 hour
Aeration Oxygen-free Oxygen-free or limited
Advantages ⇧ Yield of bio-oil ⇧ Yield of BC
Accepted a wide range of particle size
Disadvantages ⇓ BC yield
Required fine particles of biomass feed
(1-2 mm)
Prefer biomass with low moisture
content (<10%)		 Further treatment of gases is
needed due to high CO
concentrations
Applications ⇧ Potential for energy applications Improvement of soil quality
RT = Residence time; ⇧ = high, higher; ⇓ = low, lower.
Table 4. Techniques typically used to characterize BCs in terms of their physicochemical, surface, and structural characterization.
Table 4. Techniques typically used to characterize BCs in terms of their physicochemical, surface, and structural characterization.
Biochar characterization techniques
Physicochemical Characterization Surface Characterization Structural/Molecular Characterization
Chemical Physical Investigation Technique Investigation Technique
pH Surface area (BET) Morphology SEM Thermal behavior TGA
Cation
exchange
capacity (CEC)		 Size (nm) (TEM) Functional groups FTIR/Raman Structural arrangements XRD
Electrical
conductivity		 Bulk density Surface elements SEM, EDX, XFR Aromaticity NMR/Raman
Pore size (BET) Surface oxygen Boehm titration Free radicals EPR/ESR
Table 5. Main chemical and physical features of BCs.
Table 5. Main chemical and physical features of BCs.
Properties Discussion
Chemical properties Atomic ratio ⇓ O/C and H/C ratio for untreated biomass
Elemental composition ⇑ Carbon content (>95%) *⇓ Hydrogen content (<5%) *⇓ Oxygen content (<2%) *
Energy content ⇑ Energy content with temperature (From 15-20 MJ/kg a to 30–35 MJ/kg b at 700 °C
Fixed carbon (FC) **Volatile matter (VM) ⇑ in FC (from 10% a to 90%b at 700 °C)⇓ in VM (from 90% a to 10% b at 700°C)
Structural composition Partially decomposed cellulose cNear totally decomposed hemicellulose cPartially decomposed lignin c
Release of O2 and H2⇓ Oxygenated functional groups in BC (OH and C=O groups) *⇑ Highly stable aromatic structures in BC *(the maximum aromaticity at 500-800 °C)⇑ Alkalinity and ability to neutralize acids in soils *⇑ Unpaired negative charges that enable BC to accept protons
pH value ⇑ pH-value (from 5-7.5 a to 10-12 b at > 500 °C)⇑ Ash
Cation exchange capacity (CEC) ⇑ CEC for BCs produced at relatively ⇓ low temperatures
Ash content (SiO2, CaO, K2O, P2O5, Al2O3, MgO) ⇑ With temperature
Self-heating degradation during storage ⇓ Highly volatile content in BC⇓ Risk of self-heating⇑ Thermal stability⇓ Risk of spontaneous combustion⇓ Water content and microbial
Physical properties Density and porosity ⇑ Weight-based energy density * at ⇑ temperature⇓ Bulk density * (the volume-specific weight of a bulk material in a heap or pile)⇑ Porosities at ⇑ temperature
Surface area ⇑ Total surface area * (< 800°C)⇓ Total surface area * (> 800-1000°C)
Pore volume distributionPore size distribution ⇑ Total pore volume * with ⇑ temperatureMacropores (1000–0.05 μm)Mesopores (0.05–0.002 μm)Micropores (0.05–0.0001 μm (more than 80% of the total pore volume)
HydrophobicityWater holding capacity (WHC) ⇑ Hydrophobicity⇓ Affinity to water⇑ Porosity and amount of water that can be absorbed
Mechanical stability ⇓ Mechanical stability during carbonatization⇓ Structural complexity during carbonization
Grindability ⇑ Grindability compared to the parent material
Thermal conductivityHeat capacity ⇓ Thermal conductivity in BC(from 1300 J/(kgK) a to 1000 J/(kgK) b at 500°C)
Electromagnetic properties ⇑ Conductivity⇑ Electromagnetic shielding efficiency
* Depending on pyrolysis temperature: a higher degree of carbonatation at higher temperatures; ** after removing the volatile components, the carbon content that remains in the solid structure is called fixed carbon; arow biomass; b BC; c depending on biomass and pyrolysis temperatures involved (< 650 °C decomposes almost all of the holocellulose (cellulose and hemicellulose); the temperatures required for decomposing lignin are several hundred degrees higher than that for holocellulose; CEC: is defined as the amount of exchangeable cations (e.g., Ca2+, Mg2+, K+, Na+, NH4+) that a material can capture, which directly depends on the surface structure, and presence of functional groups providing surface negative charges; ⇧ = high, higher or increase; ⇓ = low, lower or decrease.
Table 6. Properties of BCs produced from various feedstocks at various production temperatures (adapted from [80]).
Table 6. Properties of BCs produced from various feedstocks at various production temperatures (adapted from [80]).
Feedstock Pyrolysis temp. Yield Ash pH C H O N Surface area
(∘C) (%) (%) (%) (%) (%) (%) (m2 g 1)
Canola straw 400 27.4 – – 45.7 – – 0.19 –
Corn cobs 500 18.9 13.3 7.8 77.6 3.05 5.11 0.85 0.0
Corn stover 450 15.0 58.0 – 33.2 1.40 8.60 0.81 12.0
Corn stover 500 17.0 32.8 7.2 57.3 2.86 5.45 1.47 3.1
Cottonseed hull 200 83.4 3.1 – 51.9 6.00 40.5 0.60 –
Cottonseed hull 800 24.2 9.2 – 90.0 0.60 7.00 1.90 322.0
Fescue straw 100 99.9 6.9 – 48.6 7.25 44.1 0.64 1.8
Fescue straw 700 28.8 19.3 – 94.2 1.53 3.60 0.70 139.0
Oak bark 450 – 11.1 – 71.2 2.63 12.9 0.46 1.9
Oakwood 400–450 – 2.9 – 82.8 2.70 8.05 0.31 2.7
Orange peel 150 82.4 0.5 – 50.6 6.20 41.0 1.75 22.8
Orange peel 700 22.2 2.8 – 71.6 1.76 22.2 1.72 201.0
Peanut shell 300 36.9 1.2 7.8 68.27 3.85 25.89 1.91 3.1
Peanut shell 700 21.9 8.9 10.6 83.76 1.75 13.34 1.14 448.2
Peanut straw 400 28.2 – – 42.90 – – 1.50 –
Pine needles 100 91.2 1.1 – 50.87 6.15 42.27 0.71 0.7
Pine needles 700 14.0 2.2 – 86.51 1.28 11.08 1.13 490.8
Pine shaving 100 99.8 1.2 – 50.60 6.68 42.70 0.05 1.6
Pine shaving 700 22.0 1.7 – 92.30 1.62 6.00 0.08 347.0
Pinewood 700 – 38.8 6.6 95.30 0.82 3.76 0.12 29.0
Poplar wood 400 32.0 3.5 9.0 67.30 4.42 – 0.78 3.0
Rice husk 500 – 42.2 – 42.10 2.20 12.10 0.50 34.4
Saw dust 450 – 1.1 5.9 72.00 3.50 24.41 0.08 –
Saw dust 550 – 2.8 12.1 85.00 1.00 13.68 0.30 –
Soybean stover 300 37.0 10.4 7.3 68.81 4.29 24.99 1.88 5.6
Soybean stover 700 21.6 17.2 11.3 81.98 1.27 15.45 1.30 420.3
Soybean straw 400 24.7 – – 44.10 – – 2.38 –
Spruce wood 400 36.0 1.9 6.9 63.50 5.48 – 1.02 1.8
Spruce wood 525 – 4.7 8.6 78.30 3.04 – 1.17 40.4
Wheat straw 400 34.0 9.7 9.1 65.70 4.05 – 1.05 4.8
Wheat straw 525 – 12.7 9.2 74.40 2.83 – 1.04 14.2
Chicken litter 620 43-49 53.2 - 41.50 1.20 0.70 2.77 -
Poultry litter 350 54.3 30.7 8.7 51.07 3.79 15.63 4.45 3.9
Poultry litter 700 36.7 46.2 10.3 45.91 1.98 10.53 2.07 50.9
Tire rubber 200 93.5 15.0 - 74.70 6.38 3.92 - -
Tire rubber 800 43.0 10.5 - 86.0 0.87 2.16 0.47 50.0
Table 7. Main possible applications of BC.
Table 7. Main possible applications of BC.
Application Mechanisms Refs.
Climate change mitigation Sequestering carbon in soil
⇓ CO2 emissions into the atmosphere
⇓ NO2 emissions
⇓ CH4 emissions
Tackling 12% of current anthropogenic carbon emissions		 [54]
Soil improvement ⇑ Physicochemical and biological properties of soils
⇑ Water retention capacity of soil
⇓ Nutrient leaching
⇓ Acids in soils
⇑ Microbial population and microbial activity in soils
Positive impacts on the earthworm population
Preventing desiccation		 [49]
Waste management By pyrolyzing waste biomass * [87]
Energy production By conversion of waste biomass to BC by fast pyrolysis, thus
providing liquid fuel (bio-oil)		 [52]
Capturing contaminants By adsorption of both organic pollutants and/or
metal ions from soil and water		 [53,55]
Composting ⇑ Physicochemical properties of composting
⇑ Composting microbial activities
⇑ Organic matter decomposition 		[50]
* Including crop residues, forestry waste, animal manure, food processing waste, paper mill waste, municipal solid waste, and sewage sludge; ⇧ = high, higher, improved, enhanced; ⇓ = low, lower, reduced, decreased.
Table 8. Advantages and disadvantages associated with the production and use of BC.
Table 8. Advantages and disadvantages associated with the production and use of BC.
Advantages Disadvantages
Obviate to significant modification on Earth Gaseous aerosol emissions during improper pyrolysis
Enhanced soil productivity Environmental pollution from dust, erosion and leaching of BC particles
Higher food security Ash could be at risk for respiratory diseases.
Solution of xenobiotics danger BC can sequester water and nutrients no further available for crops
Addressing waste management Not desired sorption of residual herbicides and pesticides
Reduced utilization of fossil fuels Long-term removal of crop residues for producing BCs can reduce overall soil health by diminishing the number of soil microorganisms and
disrupting internal nutrient cycling
Less expensive than activated carbon (AC) Possible negative impact on soil biota
Improvement of living microbiology in soil Short-term adverse effects on earthworm population density
Greater WHC than AC No universal reduction in nitrous oxide emissions
Enhanced food web in soil
Improved aeration in the soil
Reduced loss of nutrients through leaching
AC = Activated carbon; WHC = water holding capacity.
Table 9. Main mechanisms by which BC can capture inorganic or organic contaminants.
Table 9. Main mechanisms by which BC can capture inorganic or organic contaminants.
Capturing Mechanism Influencing factors #, Details °, Examples § Ref.
Sorption * ⇑ Surface area #
Microporosity of BC #
pH # 		[98]
Hydrogen bond formation ** For polar compounds °,**
Electrostatic attraction/repulsion For cationic compounds °
Interaction between positively charged cationic organic contaminants and negatively charged BC surfaces °,**
Electrostatic outer sphere
complexation 		Due to metallic exchange with K+ and Na+ available in BC °,**
Hydrophobic interactions *** For non-polar compounds °
Diffusion Non-ionic compounds can diffuse into the
non-carbonized and carbonized fractions of BC °
Formation of surface complexes ** pH #
Ionic radius #
Between metal cations and -OH, -COOH on BCs °
Precipitation Lead precipitates as lead-phosphate-silicate in BC §
Co-precipitates and inner-sphere complexes can form between
metals and organic matter/mineral oxides of BC §
* From water/soil onto biochar; ** for BCs produced at relatively lower temperatures; *** for BCs produced at higher temperatures; ⇧ = high, higher, improved, enhanced.
Table 10. Factors influencing PFRs formation in BC.
Table 10. Factors influencing PFRs formation in BC.
Parameter Influencing Factors Specifications Observations Ref.
PFRs
concentration		 Biomass type Cow manure, rice husk, others (< 500°C) ≠ Concentrations [114,115]
Non-lignocellulosic biomass with ⇓ H/C and O/C ⇓ Concentration [116]
Lignocellulosic biomass ⇑ Concentration
Temperature 300°C, 700°C ≠ Concentrations [114]
Maximum concentration at 600°C [117]
Maximum of concentration at 500-600°C [10,118]
Transition metals Adsorb onto biomass and transfers electrons from polymer to metal center during pyrolysis ⇑ Concentration [18]
Type of PFRs Temperature 200-300°C Oxygen centered radicals [10]
400°C A mixture of oxygen and
carbon-centered radicals
500-700°C Exclusively carbon
centered radicals
Table 11. Features and g-values of the main PFRs forming in BC.
Table 11. Features and g-values of the main PFRs forming in BC.
Radicals g-value Features
Carbon-centered radicals < 2.003 Susceptible to oxidation in air
Carbon-centered radicals adjacent to an
oxygen atom
(oxygenated carbon-centered radicals)		 2.003–2.004 Susceptible to oxidation in air
Oxygen-centered radicals > 2.004 More stable in an atmospheric environment
Semiquinone radicals (oxygen-centered) > 2.0045 More resistant to react with molecular oxygen in ambient environment
Phenoxy radicals
(oxygenated carbon-centered radicals)		 2.0030–2.0040 Susceptible to oxidation in air
Cyclopentadienyls
(carbon-centered radicals)		 < 2.003 Susceptible to react with molecular oxygen in ambient environment
Table 12. Main actions and mechanisms by which PFRs remediate/degrade environmental xenobiotics.
Table 12. Main actions and mechanisms by which PFRs remediate/degrade environmental xenobiotics.
EPFRs actions Degraded substances * Mechanism Refs.
Activation of H2O2 by
single electron transferring		 SMX, CIP, SMT, TC, OG, MNZ, ERF
benzene 		Oxidation by the production of ROS
(OH•#, HO2•, O2•-)		 [18,123]
Activation of O2 by single electron transferring Degradation of organic compounds
Chloro-biphenyl
Phenolic compounds
Polychlorinated biphenyls
Diethyl phthalate
Thiacloprid
Bisphenol A		 Oxidation by the production of
radical superoxide (O2•-)		 [7,11]
[19,104]
Activation of persulfate (S2O82−) X-3B, SMT, CTC, SMX, TC, MB, SDZ, OG Oxidation by the production of
sulfate radicals (SO4 −•)		 [123]
Direct activity of
macromolecular radicals on the BC surface		 Direct degradation of organic chemicals Oxidation [55]
Direct activity of
semiquinone-type radicals		 As (III) removal Oxidation [124]
Direct activity of PFRs Removal of Cr (VI) Reduction to Cr (III) [125,126,127,128,129]
Catalytic effects Detoxification of environmental
xenobiotics		 Generation of activated species
Stimulation of the microbial
biotransformation		 [55]
Ions’ exchange Enhancement of agricultural soil
performance		 Maintenance of CEC in soils [130]
Electron-hole pair
formation		 Photo-catalytically degradation of contaminants under Vis irradiation Electrons in free radicals can be transformed from the valence band to the conduction band under irradiation [131]
* Degraded or removed; SMX = sulfamethoxazole; CIP = ciprofloxacin; SMT = sulfamethazine; TC = tetracycline; OG = orange; MNZ = metronidazole; # free or surface-bond; KET = ketoprofen; CTC = chloro-tetracycline; SDZ = sulfadiazine; MB = methylene blue; ERF = enrofloxacin (photocatalytic degradation); X-3B = reactive brilliant red X-3B.
Table 13. BC-derived PFRs and their applications as described previously reported in the years 2019-2023.
Table 13. BC-derived PFRs and their applications as described previously reported in the years 2019-2023.
Biomass Pyrolysis °C/Time BC-name Active radicals Radical Mechanisms Application 1
Degraded compound 2 		Refs.
Sawdust 700°C/1h Fe0-BC-700 SO4•- PFRs OH• Activation of PMS by Fe0
Activation of PMS by PFRs		 BPA 2 [135]
Waste wood 500°C 700°C Fe0-BC SO4•- PFRs OH• Production of PFRs by Fe0
Activation of PS by Fe0
Activation of PS by PFRs		 TDWW 2 [136]
Camellia seed husks 400°C/2h OBC-Fe3O4 SO4•- PFRs OH• Activation of PS TC 2 [137]
Sawdust 300°C 700°C SBC SO4•- PFRs OH• Activation of PS AO-7 2 [138]
N.R. 200°C 500°C N.R. PFRs • O2- UV-induced interaction PFRs/DOM and • O2- production RhB 2 [139]
Sewage sludge 500°C/4h HNO3-BC PFRs • O2- •OH •O2H Activation of H2O2 CIP 2 [21]
Wheat straw 500°C/2h BC/Fe (III) SO4•- PFRs OH• Activation of PS by PFRs SMX 2 [140]
Sawdust 700°C BC700 SO4•- PFRs OH• Activation of PDS by PFRs CA 2 [141]
Pine needle 500°C/2h Fe/Mn/BC •OH Activation of H2O2 by Fe (II), Mn (II) and PFRs
(FeMn/BC/H2O2 photo-Fenton system)		 Naphthalene 2 [142]
Sewage sludge 500°C/4h SS-BC PFRs • O2- •OH •O2H Activation of O2 and H2O2 by PFRs
Degradation of PNP by PFRs		 CIP 2 [143]
Swine manure 600°C SBC OCPFRs CCPFRs-O
• OH •O2H		 Activation of oxygenated species by OCPFRs and CCPFRs-O
(heterogeneous Fenton-like systems SBC/ H2O2)		 SMT 2 [144]
Wheat straw 300°C 600°C BC300 BC600 •OH •O2H Goethite (Gt)-mediated activation of H2O2
(Fenton-like system)		 OFX2 [145]
Wheat straw 500°C/2h 800°C/2h CoBCX SO4•- PFRs OH• Cobalt and PFRs mediated activation of PMS via O2 ATZ2 [146]
Various crop straws 450, 550
650°C		 BC450,550
BC650		 SO4•- • O2- OH• BC mediated activation of PS by electron transferring SDZ2 [147]
Tobacco steam 300℃ 500℃
700℃		 T-BC ROS OCPFRs mediated activation of O2 in the water PNP2 [148]
Pruning wastes of apple trees 400°C550°C
700°C		 BC400, BC550 BC700 SO4•− PFRs BC and PFRs mediated activation of PS ACT2 [149]
Camphor leaves 400°C/6h Fe (TPFPP)/BC SO4•- PFRs OH• PFRs-mediated electrons transferring to iron porphyrin-loaded BC 3 PFOA2 [150]
Corn stalks 240°C/4h hydrochar •OH Electrode and PFRs mediated generation of ROS 2,4-DCP2 [151]
Wheat straw 450°C/4h Co3O4-BC SO4•- PFRs OH• Co3O4-BCmediated activation of PMS CAP2 FF2 TAP2 [152]
Wheat straw
Urea
Iron salts		 800°C/1h Fe-N-BC SO4•- PFRs •OH • O2- Fe, N co-doped BC and PFRs mediated activation of O2 and PS AO72 [153]
Candida utilis 700°C/2h NCS-x SO4•- PFRs OH• Activation of PMS by nitrogen-doped biochar nanosheets (NCS-x) using molten salt (NaCl and KCl) in the pyrolysis process BPA2 BPF2 BPS2 BPAF2 [154]
Pine needles 500°C nFe3O4/BC PFRs •O2H •OH • O2- Activation of H2O2 by nano-magnetite supported biochar via Fe (III)/Fe (II) cycling and electron transfer with the PFRs Ethylbenzene2 [155]
Sewage sludge 800°C/3h SM-(0.5:1) SO4•- PFRs OH• Activation of PMS by nitrogen-doped sludge biochar with different ratios of melamine in acidic Cationic/anionic dyes2 [156]
Elefant grass 350°C 600°C
900°C
30-120 min		 EG OCPFRs OCPFRs mediated oxidation CV2 [157]
Sunflower-straw N.R. SSBC SO4•- PFRs OH• Enhanced Fe (II) activation of PS via BC and PFRs Benzoic acid2 [158]
Pine chips 500°C OP5
RP5		 SO4•- PFRs •OH • O2-
•O2H		 EDC-involved structures, Fe3+ and BC (PFRs) mediated activation of PS in a Fenton-like reaction system using H2O2 and NaBH4 2,4-DCP2 [159]
Rice straw 350°C 500°C
700°C		 BCs
MBCs
BDOMs		 PFRs •OH Direct photocatalytic degradation in BCs and MBCs
solutions by Xenon-lamp
Oxygen reduction by FPRs of BCs and MBCs
BDOMs mediated generation of ROS		 SMX2 CAP2 [160]
Pomelo peels 600°C Fe@PP-Hy-Py PFRs •OH • O2- Amorphous Fe (0) mediated formation of PFRs
Fe (0) mediated reduction of PNP
EPFRs mediated oxidation of PNP via ROS
(O2 and H2O2) activation		 PNP2 [161]
Softwood pine 823-873 K US-BC
BC-P
BC-P-DEA
US-BC-P-DEA
US-BC-P-DEA		 PFRs •OH • O2- •O2H Reinforcement of PFRs concentration doping BCs with Ni and Pb
Activation of H2O2 by PFRs		 Phenol2 [162]
Camphor leaves 500 °C/1h Fe (VI)/BC-2 Fe(Ⅴ)/Fe(Ⅳ) PFRs •OH Fe (VI)-BC (PFRs) mediated electron transferring and generation of ROS AZT2 [163]
Bagasse powder 800 °C DBC800 PBC800-A SO4•- PFRs •OH• O2-
•O2H		 Enhanced BCs mediated activation of PS
Improved PFRs generation by natural endogenous
minerals		 TC2 [164]
Eichhornia
crassipes
Iron salts		 400 °C/2h MBC PFRs •OH• O2-•O2H Fe (II)-BC mediated activation of H2O2
(Fenton-like system)		 MNZ2 [165]
Poplar and pine sawdust 300-500°C PO xxx
PI xxx		 SO4•- PFRs •OH• O2- Activation of PMS by CCEPFRs-O and CCEPFRs in BC TC2 CTC2 DOX2 [166]
S. alfredii
 Air-dried Metal@P •O2H PFRs generation by the thermochemical behaviour of Mn and Zn
Electron transfer
Activation of PDS by PFRs in Fe/Zn@PB9/PDS system
AOPs		 Imidacloprid2 [167]
Sludge
 N.R. N.R. SO4•- PFRs •OH• O2- Production of ROS via PFRs Mn-mediated electron transfer through Mn-doped sludge-based biochar (BC) mediated the CIP2 [168]
Cellulose
Lignin		 200-1000°C C200, C500 C1000
L200, L500 L1000		 SO4•-PFRs • O2- Activation of PS adsorbed onto BCs via PFRs, oxygen-containing functional groups, and defective structures of BCs OFX2 [169]
Chestnut shell
KMnO4 		700 °C/1h
400 °C/1h		 Mn-BC PFRs Mn-improved electron-transfer OTC2 [170]
Spent coffee
TiO2 		300 °C
500 °C
600 °C		 SBC500 PFRs •OH• O2-•O2H Activation of H2O2 by Ti-doped H2SO4-modified biochar (SBCs)
(Photo-Fenton-like system)		 MO2 [171]
RS 550 °C/2h
 BC-α-Fe2O3/MgO PFRs •OH• O2-•O2H UV light activation of PFRs
Production of O2 upon NPA degradation
O2 activation by PFRs		 NPA2 [172]
Sewage sludge 400 °C/2h SDBC PFRs •OH• O2-•O2H O2 activation by PFRs promoted by HNO3 or NaOH
environmental		 p-Chlorophenol2 [173]
Peanut hull 700 °C/2h BC-Fe-1-Zn SO4•- PFRs •OH Activation of PS by bimetal-modified peanut hull-derived biochar via Fe and Zn oxides and oxygen-containing functional groups active sites TC2 [174]
Blue algae 700 °C Z-700
FeOX@BC		 SO4•- PFRs •OH •O2-•O2H • O2- production by FeOX
(zero-valent iron and iron oxide)
C=C, C=O, O-C=O, Fe-O functional groups and PFRs promoted the activation of PDS		 TC2 [175]
Biomasses 300-1000 °C N.R. SO4•- PFRs •OH • O2- Activation of PS and PMS by physical and chemical modified BCs using acid/alkali treatment and metal doping via PFRs PPCPs2 [176]
Chicken feathers 350°C/4h
800°C/4h		 MBC35@FH MBC80@FH SO4•- PFRs •OH • O2- Activation of PDS by the transformation of Fe species, oxygen-containing functional groups, pyrrolic nitrogen, and PFRs to produce ROS TPhP2 [177]
Pine needles 300-900 °C BC300-900 SO4•- PFRs •OH • O2- Activation of PMS by BC via ROS production or electron transfer capability OFX2 ENR2 FLE2 [178]
PolyS 220°C/2h $
500°C /2h #
900°C /2h # 		BC500 + PS BC900 + PS
BC500
BC900		 SO4•- PFRs •OH • O2- Activation of PMS using CCEPFRs on BC-aged by microbial fermentation for ROS production SDZ2 OFX2 DOX2 [179]
Red mud
Wheat crop		 700°C /2h MRBC SO4•-PFRs • OH PDS activation by the active sites of MRBC, such as Fe (II) and PFRs LFX2 [180]
Various sludges 300-900°C
2h		 S-HPBC
S-PBC
S-HBC		 SO4•- PFRs •OH • O2- Activation of PS by PFRs mediated electrons transferring activity
Electrons transferring to Cr (VI) by PFRs		 TC2
Cr (VI) ⇒ Cr (III) 1 		[181]
Peanut shells 500 °C/4 h BC-Ce OFGs, CCPFRs Electrons transferring to Cr (VI) by OFGs, CCPFRs, oxygen vacancies and graphitic structure in BC-Ce promoted by CeO2 Cr (VI) ⇒ Cr (III) 1 [182]
Rice husk 400°C/1h BC400 OH• H2O2 (pH acid) Activation of O2 by phenol OH and
semiquinone types PFRs		 As (III) ⇒ As (V) 1 [124]
Semiquinone-type PFRs
Quinoid C═O
H2O2 		(pH alkaline) Activation of O2 by phenol OH and
semiquinone types PFRs
Rice husk 550°C RH-BC PRFs Promotion of OCPFRs by BC-inducted Cr (VI) degradation Cr (VI) ⇒ Cr (III) 1 [125]
Stalk 450°C/90 min NBC PFRs N-doped BC mediated evolution of PFRs for the transformation of Cr (VI) Cr (VI) ⇒ Cr (III) 1 [126]
Rice husk 500°C/2h MGBs PFRs •OH • O2- Efficient surface Fe (III)/Fe (II) cycling via electron transfer with the PFRs of magnetic greigite/BC composites (MGBs) Cr (VI) ⇒ Cr (III) 1 [127]
Sludge 220 °C/2h BC OCPFRs UV-Vis photo-irradiation enhanced the production of PFRs
Action of OCPFRs as electron donors to transform Cr (VI) into Cr (III)		 Cr (VI) ⇒ Cr (III) 1 [128]
Sludge 120 °C SBC120 OCPFRs OCPFRs mediated electrons transferring to Cr (VI) in neutral solutions Cr (VI) ⇒ Cr (III) 1 [129]
270 °C SBC270 CCPFRs CCPFRs mediated electrons transferring to Cr (VI) in neutral solutions
Rice husk 400°C/1h rUBC, rDBC Quinoid C=O PFRs Quinoid C=O and PFRs mediated oxidation of As (III) As (III) ⇒ As (V) 1 [183]
Maize straw
powder		 500°C/2h FhBC PFRs • O2- •OH Fe and PFRs mediated activation of O2 and H2O2 As (III) ⇒ As (V)1 [184]
Sewage sludge 270 °C/2h SBC SO4•- PFRs •OH • O2- Activation of PS by SBC via PFRs mediated electrons transferring As (III) ⇒ As (V) 1 [185]
Pinewood 600°C/1h Fe/HC • O2- •OH Activation of O2 and H2O2 by CCPFRs Estrogens 2 [186]
Rice straw 500°C/1h BiPB •OH PFRs Generation of •OH by Bi/Bi2O3 and PFRs Estrone 2, * [187]
Anaerobic digestion sludge 400°C 600°C
800°C 1000°C		 ADSBC 400
ADSBC 600
ADSBC 800
ADSBC 1000		 SO4•- PFRs OH• BC-mediated activation of PDS Dyes2 Estrogens2
Sulfonamides2 E. coli2
Others2 		[188]
Walnut shell 700 °C/1h BC700 PFRs Oxidation by PFRs-mediated electron transfer E12 E22 E32 [189]
Caragana
korshinskii 		650 °C/3h ACB-K-gC3N PFRs h+•OH• O2- Electron photogeneration and PFRs mediated
H2O and O2 activation		 S. aureus2E. coli2
RhB2 TA2 NOR2 CAP2 		[190]
Pinewood 600°C Ag0-PBC PFRs •OH • O2- UV-light promoted excitation of the electron-hole pairs and
Subsequently, the production of ROS
Enhanced ROS generation by PFRs		 MB2 E. coli2 [191]
Rice straw 400°C 700°C
120 min		 Nano-BC PFRs •OH • O2- Oxidation and damage by ROS eDNA2 [192]
Rice straw 500℃ RS-BC Quinones Phenols PFRs By electron acceptor capacity (EAC)
By electron donor capacity (EDC)		 ⇆ Redox property 1
Electronic storage 1 		[193]
BCs = biochar; MBCs = modified-biochar; BDOMs = biochar-derived dissolved organic matters; PMS = peroxy-mono-sulfate; BPA = bisphenol A; PS = persulfate; TDWW = textile dyeing wastewater; TC = tetracycline; (TDWW); SBC = sawdust biochar; AO-7 = acid orange 7; RhB = rhodamine B; DOM = dissolved organic matter in BC; CIP = ciprofloxacin; SMX = sulfamethoxazole; PDS = peroxydisulfate; CA = clofibric acid; WW = wastewater; ⇆ = reversible; PNP = p-nitrophenol (water pollutant); SMT = sulfamethazine; SDZ = sulfadiazine; OCPFRs = oxygen-centered environmental persistent free radicals; CCPFRs-O = carbon-centered environmental persistent free radicals with oxygen atoms; * photocatalytic; CCPFRs = carbon-centered environmental persistent free radicals; BiPB = bismuth-containing BC; PFX = pefloxacin; OTC = oxytetracycline; CTC = chlorotetracycline; OFX = ofloxacin; AZT = atrazine; TMP = trimethoprim; AOPs = advanced oxidation processes; ACT = acetaminophen; PFOA = perfluorooctanoic acid; 3 degradation efficiency in presence of ascorbic acid (AA); 2,4-DCP = 2,4-dichlorophenol; CAP = chloramphenicol; FF = florfenicol; TAP = thiamphenicol; CV = crystal violet dye; MB = methylene blue; MNZ = metronidazole; DOX = doxorubicin; xxx = refers to the temperature of pyrolysis process; NPA = N-phosphono methyl iminodiacetic acid (organophosphorus pesticide (OP); NOR = norfloxacin; E1 = estrone; E2 = 17-estradiol; E3 = estriol; PPCPs = pharmaceuticals and personal care products; TPhP = triphenyl phosphate; ENR = enrofloxacin; FLE = fleroxacin (FLE); $ HTC = hydro-thermal carbonization; # HTP = high temperature pyrolysis; PolyS = polystyrene; OFGs = oxygen-containing functional group; S-HPBC = S-doped hydrothermal + pyrocarbon BC; S-HBC = S-doped hydrochar, S-PBC = S-doped pyrocarbon. Figure 11 shows the relative abundances of the types of PFRs applications concerning the 72 case studies considered here.
Table 14. Potential toxic hazards caused by PFR exposure.
Table 14. Potential toxic hazards caused by PFR exposure.
Target Danger Material source Refs.
Cells ⇑ Lungs’ T (Th1, Th2, Th17) cells PM, DCB230, MCP230 [205,206]
⇓ P450 activity PM, MCP230 [207]
Cardiomyocytes’ apoptosis DCB230 [208]
⇓ Survival of gastric epithelial cells BaP–Na montmorillonite [209]
Loss of normal morphology of pulmonary epithelial cells DCB230 [210]
Mitochondrial depolarization DCB230 [206]
Changes in VEGF ZnO/MCB [211]
Enzymes
Proteins
Genes		 Altered expression activity of Cyp1a, Cyp2b, Cyp2e1, Cyp2d2, Cyp3a and other genes DCB230, MCP230 [205]
⇑ Expression levels of peroxiredoxin-6
Cofilin 1, annexin A8		 MCP230, CGUFP, ZnO/MCB [206]
⇓ of GSH, GPx, SOD ZnO/MCB [212]
Organs and tissues Altered normal renal hemodynamics and urodynamics N.R. [213]
Liver damage N.R. [214]
Impair left ventricular function DCB230 [215]
Airway hyperresponsiveness
Lung inflammation		 MCP230 [216]
Individuals Abnormalities in zebrafish DCB230 [217]
⇓ Growth and reproduction of luminescent bacteria PM [218]
Altered behavior of Caenorhabditis elegans Biochar [219]
⇓ Energy consumption MCP230 [220]
Disease ⇑ Severity of the flu DCB230 [221]
Asthma MCP230 [206]
Cardiovascular disease and dysfunction DCB230 [208]
Other damage Oxidative stress DCB230, ZnO/MCB [212]
DNA damage BaP [222]
Lipid peroxidation MCP230 [223]
⇑ Improved, increased; ⇓ decreased, inhibited; PM = particulate matter; DCP230 = 1,2-dichlorobenzene at 230 ◦C; MCP230 = mono-chlorophenol at 230 ◦C; MCB = mono-chlorobenzene; CGUFP = combustion generated ultrafine particle; N.R. = not reported; GSH = glutathione; GPx = glutathione. Peroxidase; SOD = superoxide dismutase; VEGF = vascular endothelial growth factor.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.
Copyright: This open access article is published under a Creative Commons CC BY 4.0 license, which permit the free download, distribution, and reuse, provided that the author and preprint are cited in any reuse.
Prerpints.org logo

Preprints.org is a free preprint server supported by MDPI in Basel, Switzerland.

facebook logotwitter logolinkedin logo
MDPI Initiatives
Important Links
Subscribe

Disclaimer

Terms of Use

Privacy Policy

© 2024 MDPI (Basel, Switzerland) unless otherwise stated