3.1. PMFC to purify water and soil ecosystems from organic compounds and biogenic elements
Wastewater discharges, containing organic and biogenic (nitrogen, phosphorus, carbon) elements in concentrations above MRL (maximum residue limit), significantly affect the ecological balance of aquatic ecosystems. There begin the eutrophication and rapid development of microbiota, it entails a decrease in the dissolved oxygen concentration, causing a decrease in biological diversity [
103]. Additionally, the current active use of oil has a negative impact on ecosystems that have been polluted by its spills during production and transportation. Sludge formed during oil production is discharged into specialized ponds, which "age" under the influence of the environment. So, oxidation of some components, tarring, and evaporation of light fractions occur. These processes lead to an increased stability of oil sludge to the purification; therefore, their disposal is one of the most difficult tasks at present [
104].
Bioremediation, based on the pollutant biodegradation by microorganisms and plants during their vital activity, is one of the most effective methods of wastewater and soil treatment of organic pollutants. It is used for wastewater treatment and processing of biodegradable solid household waste to form biogas [
105,
106]. As it was noted, the PMFC technology is promising in the simultaneous processes of generating electricity [
21] and recycling various pollutants [
107].
Table 2 summarizes the information on the developed PMFC for the disposal of anthropogenic pollutants and their purification.
The authors [
108] used
Pseudomonas,
Azoarcus communis oil destructor bacteria to clean the soil from oil pollution. The addition of bacteria in the PMFC system contributed to better purification of wastewater and soils from hydrocarbons. The maximum specific power (11.56 mW/m
2) was obtained in a system where
Spartina sp was used as a plant. It was almost 5 times higher compared to the control system without plants, the value of which reached about 2 mW/m
2. Generally, power increase (7.5 mW/m
2 and 9.71 mW/m
2, respectively) appeared because of the use of
Typha latifolia (broadleaf cattail) and
Phragmites (common reed) plants due to the formation of rhizodeposites that could be consumed by microorganisms. The internal resistance of the studied systems ranged from 200 Ω to 400 Ω.
Low solubility of petroleum products is one of the problems of effective purification from them. Surface-active substances (surfactants) are additionally introduced into the PMFC systems to increase the bioavailability of the hydrophobic substrate for oil-oxidizing bacteria and, therefore, to increase the degree of biodegradation of oil pollution. It allows to increase the voltage from 184.9 to 377.2 mV [
109]. Besides, the introduction of an additional carbon source available to microorganisms, for example, glucose, contributes to an increase in the system voltage from 184.9 mV to 325 mV [
108]. Biosurfactants should be used instead of using synthetic surfactants (surfactants). Biosurfactants can increase the bioavailability of hydrocarbon substrates, to change the hydrophobic properties and permeability of micro-organisms' membranes [
117]. Moreover, oil-resistant plants should be used to clean the soil from oil in the PMFC systems. They can release a high amount of root exudates. It follows that the further directions in the development of new soil and wastewater treatment systems may consist in the selection of effective bacteria-oil destructors.
PMFC systems can be used for the disposal of organic animal waste. The work [
110] uses the plant stevia honey and urine samples of livestock (goats, cows, and sheep). It is noted that the addition of urine stimulates plant growth and significantly increases the current density of the device. The control sample has fixed the current density about 10 mA/m
2, and this indicator has reached a value of 930 mA/m
2 with the addition of cow urine. Thus, it is noted that the use of cattle urine is a good tool aimed at increasing the PMFC characteristics.
Wastewater treatment uses the technology of microbial associations enrichment with bacteria that are isolated from contaminated soils. For example, the authors [
112] have used an association of microorganisms, designating it as DC5. They are capable of oxidizing textile dyes, which can potentially act as a mediator of electronic transport, thus contributing to an increase in electricity generation during their biodegradation [
119]. The addition of the DC5 association to the PMFC contributes to an increase in the maximum specific power from 177.3 mW/m
2 to 197.94 mW/m
2. This approach makes it possible to improve wastewater treatment from electroactive dyes and at the same time to increase the electricity generation in PMFC systems.
A plant microbial fuel cell is a promising system for wastewater treatment. Thus, the work [
108] uses a reactor for anaerobic purification, where wastewater enters the PMFC. A significant decrease in suspended compounds is observed after two-stage purification. The treated wastewater, taken from the PMFC, has a BOD
5 index lower than the initial one by 71%, the COD index decreases by 74%.
The plants used can influence the efficiency of purification and generation of electricity in the PMFC. The review [
114] has studied the effectiveness of wastewater treatment using the
Canna indica plant (Indian cane) and single-celled algae
Chlorella vulgaris. The system based on the
C. indica plant has a higher voltage (771 mV and 452 mV respectively). The internal resistance for a system with a plant is about 100 Ω, with chlorella algae – 335 Ω. At the same time, the paper notes that
C. vulgaris can decompose the organic compounds, indicating a mix-trophic type of nutrition that provides better purification from organic compounds. Whereas
C. indica has an autotrophic type of power supply, and the degradation of organic compounds occurs at the anode when electroactive microorganisms are introduced into the system, contributing to a higher voltage output and specific power of the system.
The work [
116] has investigated the possibility of removing two antibiotics, tetracecline and sulfamexosol, from wastewater in the PMFC system. According to the obtained data, the greatest degree of removal is achieved during the first day of the PMFC operation. The removal efficiency has reached 99%. It has been shown that both antibiotics can accumulate in insignificant amounts in electrode compartments, which is due to the electrosorption [
120]. It occurs because of the formation of a double electric layer (DEL) on the surface of cathodes and anodes, while tetracecline and sulfamexosol do not accumulate in plants. In systems with
C. indicia, the specific power is on average 55% higher than in systems without plants. The resulting power density is 124.89 mW/m
2, and the internal resistance ranges from 600 Ω to 800 Ω for all systems. Generally, the conducted research testifies to the prospects of PMFC systems for the removal of pharmaceutical preparations in wastewater.
Canna indica plant-based PMFC system was described in [
117], which was used to remove sodium dodecyl beznesulfonate (SDBS) from a model wastewater mixture. According to the results, the removal efficiency of SDBS was 56.8%, and power values of 4.01 mW/m
2 and voltage of 230 mV with a resistance of about 200 Ω were achieved at SDBS concentration of 5 mg/L. It should be noted that increasing the SDBS concentration had a negative effect on the PMFC systems, reducing the power and increasing the internal resistance.
Thus, PMFC can be used for wastewater treatment, which may contain not only biogenic elements, but also antibiotics and petroleum hydrocarbons, including PAHs. At the same time, note should be maid towards the selection of the optimal composition of the microorganism association to reduce the time of their adaptation to pollutants and higher electricity generation. It bears emphasis that there are fluctuations in the internal resistance of various systems (from 100 Ω to 800 Ω), which is associated with different designs, the distance between the anode and cathode, the electrical conductivity of the electrolytes used. High internal resistance negatively affects the power output of the BES [
121].
3.2. PMFC application for removal of heavy metals from soil and aquatic ecosystems
Soil pollution with heavy metals (HM) poses a threat to the environment and agriculture [
122]. Heavy metals negatively affect agricultural crops, reducing their yield. Phytoremediation methods are used to clean soils from heavy metals, the principle of which is based on biosorption and HM accumulation by various plant components. The removal rate of heavy metals from soils is about 35% of the initial HM concentration during the soil phytoremediation [
123]. Microorganisms are also able to reduce concentrations of HM ions by forming chelated complex compounds with them, which is due to the production of siderophores, organic acids and extracellular polymeric substances [
124]. However, as it has been shown earlier, bio-remediation has not become widespread in poor countries due to its relatively high cost [
10]. Therefore, the use of PMFC technology can become a compromise solution in poor countries not only because of the purification of contaminated soils from HM, but also due to the generation of environmentally friendly electricity [
125].
Table 3 presents the parameters of some well-known PMFC systems used for soil purification from heavy metals.
Raygrass has been used to remove Cr
2O
72- in the PMFC system [
126]. According to the results, the removal efficiency can reach 99% under various conditions. At the same time, most of the reduced to Cr (III) precipitates in the form of Cr(OH)
3. Meanwhile, an increase in the concentration of Cr
2O
72- from 9 mg/dm
3 to 19 mg/dm
3 increases the current density by about two times (up to 55 mA/m
2).
The study [
127] has tried to reduce the absorption of soap by rice culture
Ozyra sativa L. (seeded rice), since the rice consumption is one of the main routes of arsenic entry into the human body. The results show that the use of PMFC technology reduces the arsenic accumulation in rice by up to 67.9% due to the obstruction of As (III) migration to the plant roots. The output power equals 22.2 mW/m
2.
The article [
128] illustrates the PMFC development based on
Ozyra rufipogon (wild rice) and
Typha orientalis (oriental cattail) to remove cadmium from the soil. It is shown that cadmium absorption is carried out mainly by plant roots. The addition of biochar has contributed to the better removal of cadmium from the soil. The use of PMFC reduces the cadmium mobility by binding to carbonates, iron oxides and organic compounds. When the PMFC operates in the closed-circuit mode, it is noted that the percentage ratio of the Cd
2+ exchange fraction is significantly lower than when the PMFC operates in the open circuit mode. It is caused by bioelectrochemical processes. Additionally, higher values of the generated voltage are observed (350 mV vs. 137 mV) when using rice. At the same time, the voltage has decreased from 400 mV to 150 mV in the control system (without Cd (II)) with cattail by 112, and the voltage has increased to 400 mV in the system with rice by the end of the experiment. Such a feature may be due to the different composition of the rhizodeposits.
The study [
130] has used seeded rice exploiting such HM as Cd, Cr-, Cu and Ni. It shows that a decrease in the concentration of HM in PMFC (compared with the control rate) is possible to be achieved during the experiment. The authors note that the removal of such heavy metals as Cu is probably due to the transition under the action of an electric field of Cu to the cathode region, where they react with oxygen and precipitate in the oxide form. Besides, bacterial biofilms can absorb Cu through the cell membrane. Similar mechanisms of reduced mobility are observed for Cr and Ni. The maximum current is 1.20 mA, while the fluctuations of this value have been caused by a change in the oxygen volume into the anode compartment from the rice roots. The generated capacity of 22.2 MW/m
2 allows to conclude that the PMFC system can be applied for soil purification from some heavy metals in their joint presence.
The above data prove that the soil purification from heavy metals in PMFC systems is practically not inferior to phytoremediation. In some cases, a decrease in the mobility of some HM [
131] can be detected because of their conversion into poorly soluble compounds due to the course of bioelectrochemical processes in PMFC systems.
Thus, the use of PMFC technology to combat soil pollution still needs further study, since the processes and mechanisms that occur should be considered separately for various heavy metals and plants. The use of PMFC, according to the research, is promising due to the economic effect that is caused by electricity generation and low cost phytor-mediation technologies [
132].