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Synthesis and Characterization of Metal Oxide Nanoparticles Anchored Carbon as Hybrid Adsorbents for Effective Heavy Metals-Uptake from Wastewater

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30 November 2023

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01 December 2023

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Abstract
Hybrid materials derived adsorbents have showed a great applicable efficiency in various fields including industrial uses and environmental remediation. Herein, the zinc oxide nanoparticles anchored carbon (ZnO-C) is fabricated and utilized for wastewater treatment by adsorption of Zn(II), Cd(II), Co(II) and Mn(II). The surface and structural characteristics are examined by TEM, SEM, XRD, FTIR, EDS and BET surface area. The Kinetics and equilibrium investigations are applied to optimize the adsorptive removal of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C. Results indicated formation of ZnO-C in crystalline spherical granules with nano-size between 29.3 and 48.8 nm. In addition, the spherical granules are gathered to form clusters. The FTIR indicated that the ZnO-C surface is rich with OH groups and ZnO. The adsorption capacity is reported as 212, 209, 200 and 230 mg/L for Zn(II), Cd(II), Co(II) and Mn(II) respectively. The optimum condition for maximum adsorption were pH between 5 and 6, contact time of 180 min and adsorbent dose of 0.1 g/L. The adsorptive removal data modeling for uptake of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C showed agreement with the assumption of pseudo 2nd order kinetic model and Freundlich isotherm suggesting fast adsorption rate and multilayer mechanism. The achieved adsorption capacity using the prepared ZnO-C is more effective compared to ZnO, carbon, Fe3O4 and Fe3O4-C. Real wastewater samples are applied including valley water, industrial wastewater and rain wastewater and evaluated for applicable uptake of Zn(II), Cd(II), Co(II) and Mn(II) using ZnO-C and Fe3O4-C with high efficiency.
Keywords: 
Subject: 
Chemistry and Materials Science  -   Applied Chemistry

1. Introduction

The heavy metals polluted effluents are originated from industrial activities and cause serious environmental hazard [1]. Heavy metals such as manganese, cadmium, cobalt and zinc are naturally occurring elements. Small amounts of these elements are common in our environment and they are actually necessary for our health. But large amounts of any of them may cause acute or chronic toxicity [2]. Heavy metals in human bodies tend to bioaccumulate, which may result in damaged or reduced mental and central nervous function, and damage to blood composition, lungs, kidneys and liver. For example, cobalt, one of the common toxic metals affecting the environment, is present in the waste water of nuclear power plants and many other industries such as mining, metallurgical, electroplating, paints, pigments and electronic [3]. High levels of cobalt may affect several health troubles such as paralysis, diarrhea, low blood pressure, lung irritation and bone defects [4]. Manganese, is naturally occurring metallic element, which may contaminate groundwater as a result of weathering and the leaching of manganese-bearing rocks into the aquifers. Aquifers in certain regions of Quebec, other portions of Canada, and other countries contain naturally high quantities of manganese (including Sweden, Vietnam, Bangladesh, Morocco, and others) [5,6]. Although manganese is a vital trace element, it may be a strong neurotoxin in excess. The amount of manganese in drinking water is not regulated in the United States or Canada since it is mostly seen as an aesthetic problem. Manganese concentrations more than 100 g/L encourage the discoloration of laundry and hygienic items and provide an unpleasant flavor to drinks [7]. Zinc is extensively present in environmental components including food and water [8,9]. Zinc is a substance having a recommendation value of 5.00 mg/L in drinking water, according to the European Commission Drinking Water Directive, the World Health Organization and its Guidelines for Drinking Water Quality, and the US Environmental Protection Agency (EPA). The over-dose exposure to zinc leads to immediate symptoms such as vomiting and nausea and anemia [10]. Cadmium is one of the most harmful non-essential heavy metals in the environment, and it comes from many sources, including the wastewater from the metal plating industry, nickel-cadmium batteries, phosphate fertilizer, mining, pigments, stabilizers, alloys, petroleum refining, welding, and pulp industries, produce elevated levels of cadmium ions [31,32]. Cadmium toxicity resulted in kidney damage, cancer and lung dysfunction [11]. The cadmium recommended value for drinking water is 0.005 mg/L [12]. Therefore, many research investigations have been conducted to develop tools for wastewater purification. The removal of heavy metals pollutants from water can be achieved by many methods including chemical precipitation, flotation, biosorption, electrolytic recovery, membrane separation, removal by adsorption onto minerals or activated carbon [13,14,15,16]. Usually these methods have been restricted by many factors, such as processing efficiency, operational method, energy requirements, and economic benefit. Adsorption is an effective, straightforward, and affordable technique for removing heavy metals from water [17,18]. An effective sorbent should has a high heavy metal sorption capacity, low cost, renewable, and durable [19]. Although it is costly and ineffective for treating water, carbon materials are reported as an efficient adsorbent for heavy metals uptake [20,21]. The carbon incorporated materials have durability and poses physicochemical stability which enhance the wastewater treatment applications [22].
Recently, nanostructure-based materials have shown higher efficiencies in wastewater treatment applications compared to traditional adsorbents [23,24]. Nanostructure adsorbents such as manganese oxides and titanium oxides, iron oxides and zinc oxides have shown promising results in removal of heavy metals. Zinc oxide is reported to have been reported as proper adsorbent for organic and inorganic pollutants, however, the process suffers from many technical problems related to operating and efficiency [1,25,26,27,28]. The combination of metal oxides with carbon produce a hybrid material with superior properties compared to the single metal oxide oxides or pure carbon. In addition, the nano-adsorbent such as zinc oxide nanoparticles modified with graphene have showed an improved tendency to decompose organic pollutants [29]. The researches in this field is still continuous to improve the efficiency of nano-composite in removal of pollutants, even by adsorption or by photocatalytic degradation. Due to its outstanding optical, electrical, photonic, and microbiological capabilities among others, zinc oxide (ZnO) nanoparticles are widely employed in many new multifunctional materials. The zinc oxide nanoparticles derived materials could be utilized in various manufacturing sectors like rubber, plastics, cosmetics, pharmaceuticals, paints, soap, batteries, electrical equipment, optoelectronics, biomedical science, etc., the demand for zinc oxide is always rising [30,31]. Hybrid materials including carbon and metal oxide nanoparticles have been reported as novel category of materials with enhanced properties for catalysis, adsorption, precontraction and medical applications [32,33,34]. Thus, this work aimed to fabricate zinc oxide anchored carbon (ZnO-C) as hybrid porous adsorbent using solvothermal process to enhance the adsorption capacity for heavy metals uptake. In addition to characterized the produced ZnO-C with SEM, TEM, XRD, surface is and EDS. Furthermore, to compare the achieved adsorption capacity with other adsorbents including zinc oxide, carbon and Fe3O4-C. Moreover, to purify wastewater samples using the developed ZnO-C hybrid adsorbent materials.

2. Experimental

2.1. Materials

The materials including zinc acetate, polyethylene glycol, sodium hydroxide, zinc oxide, hydrochloric acid, zinc nitrate, cobalt nitrate, cadmium nitrate and manganese nitrate are brought with analytical grade from Sigma USA. Carbon and Fe3O4-C are obtained from our laboratory as prepared according our previously published work by Habila et al.

2.2. Fabrication of zinc oxide nanoparticles anchored carbon (ZnO-C)

The solvothermal process was applied to fabricate zinc oxide nanoparticles anchored carbon (ZnO-C). In details, 21 g of zinc acetate, 10 g of polyethylene glycol and 10 g of carbon were well mixed in 1:2 ethanol/water medium, then 100 ml of NaOH (0.1 M) was added to the mixture and stir for an hour. After that, the mixture was kept in oven at 110 °C for 17 hr. The formed precipitate was then isoleted by centrifuge and treaten in muffle furncaes at 750°C for one hour. The obtained ZnO-C was grinded in mortal and washed several times with ethanol and water, dried in oven at 105 °C for 10 hr. the produced ZnO-C was characterized by TEM, SEM, FTIR, XRD, EDS and surface area.

2.3. Optimization of adsorptive removal of Zn(II), Cd(II), Co(II) and Mn(II) for wastewater purification

0.02 g of the metal oxide nanoparticles including ZnO or Fe3O4 or their derived carbon hybrid materials including ZnO-C or Fe3O4-C was mixed with 20 mL of mixed heavy metals solution including Zn(II), Cd(II), Co(II) and Mn(II). Then the pH was optimized to pH using phosphate buffer. The mixture was kept under shaking condition at room temperature for 180 min. Then the aqueous solution was separated by centrifuge. The change in the heavy metal ions concentration was detected by ICP-MS. The adsorptive removal- capacity was evaluated from Equation 1
qe = (C0−Cf). V/M,
where qe is the adsorptive removal-capacity (mg/g) for Zn(II), Cd(II), Co(II) or Mn(II) onto the metal oxide nanoparticles including ZnO or Fe3O4 or their derived carbon hybrid materials including ZnO-C or Fe3O4-C
C0, the initial concentration of Zn(II), Cd(II), Co(II) or Mn(II)
Cf is the final concentration of Zn(II), Cd(II), Co(II) or Mn(II) after adsorption.
V is the volume of the solution of whole adsorption mixture.
M is the mass of the metal oxide nanoparticles including ZnO or Fe3O4 or their derived carbon hybrid materials including ZnO-C or Fe3O4-C
The previously described steps for adsorptive removal were operated several times to assess the effects of pH of adsorption medium, time, dose of metal oxide nanoparticles including ZnO or Fe3O4 or their derived carbon hybrid materials including ZnO-C or Fe3O4-C, as well as the influence of the concentration of Zn(II), Cd(II), Co(II) or Mn(II). The Kinetic models and the equilibrium-isotherms were investigated to study the adsorption rate and predict the adsorption behavior. In addition, wastewater samples were collected from Saudi Arabia, filtered and applied for adsorption evaluation using optimized conditions for uptake of Zn(II), Cd(II), Co(II) or Mn(II) onto including ZnO-C or Fe3O4-C.

3. Results and discussion

3.1. Characteristics of the developed ZnO-C hybrid adsorbent materials

The morphology of the fabricated ZnO-C hybrid adsorbent materials was described by SEM (Figure 1) and TEM (Figure 2) which reveal that spherical structure of the formed granules in nanoscale size of 29.3 - 48.8 nm. In addition, a cluster are notices from the spherical granules aggregate. The BET surface area was detected as 24.84 m²/g while the Langmuir surface area was 37.3470 m²/g. These results are slightly lower than that reported by Al-Rawashdeh et al., reported BET surface areas of the graphene oxide anchored zinc oxide nanoparticles (GO–ZnO) as 36.95 m²/g [35]. In addition, Gu et al., have prepared ZnO nanoparticles for removal of Cr(III) and reported a BET surface area of 26.7 m2/g [36].
The developed ZnO-C is characterized by FTIR (Figure 3) which indicated main peaks between 3300 and 3700 cm–1 due to stretching vibration of the O-H groups. Peaks around 2900 cm–1, which attributed to the aliphatic C-H. While the peak at around 1600 cm-1 is due to carbonyl groups (C═O). The peak between 1400 and 1500 is attributed to O-H binding. The peak between 400 and 470 cm–1 is attributed to the stretching vibration of Zn-O bond. The XRD analysis confirmed the crystalline nature of the formed ZnO-C due to appearance of peaks at 2 thetas of around 31°, 34°, 36°, 47°, 56°, 62° and 69° which are attributed to the planes of (100), (002), (101), (102), (110), (103), and (112), respectively. In addition, the detected high intensity peaks indicated the purity of formed hexagonal structure of ZnO (JPCDS number: 36-1451) [37]. The successful fabrication of pure zinc oxide nanoparticles anchored carbon is expected to improve the formed hybrid materials application for adsorption of heavy metals. Rodríguez et al., developed zinc oxide/graphene nanocomposite and indicated that the removal efficiency of Al and Cu were 19.9 mg/g and 33.5 mg/g, respectively, owing that the decoration of graphene with ZnO nanoparticles enhanced the adsorption capacity [38].

3.2. Utilization of developed hybrid materials for adsoprion application

The metal oxide nanoparticles including ZnO and Fe3O4 or their derived carbon hybrid materials including ZnO-C and Fe3O4-C were evaluated for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) from aqueous solution as presented in Figure 5. The adsorption capacity of ZnO was 101, 116, 85, and 111 mg/g for Zn(II), Cd(II), Co(II) and Mn(II), respectively while for Fe3O4 was 125, 101.3, 116 and 83 mg/g for Zn(II), Cd(II), Co(II) and Mn(II), respectively. The adsorption capacity for carbon was 100, 91, 106 and 90 for Zn(II), Cd(II), Co(II) and Mn(II), respectively. On the other hand, the adsorption capacity of metal oxide anchored carbon hybrid adsorbent materials exhibited higher performance for removal of heavy metals due to synergic effect. In addition, the developed ZnO-C exhibited the higher adsorption capacity as 212, 209, 200 and 230 mg/L for Zn(II), Cd(II), Co(II) and Mn(II) respectively, compared to Fe3O4-C which possess adsorption capacity of 177, 173, 165 and 167 for Zn(II), Cd(II), Co(II) and Mn(II), respectively. These results agree with finding indicated by Hadadian et al., who reported that combination of zinc oxide nanoparticles with graphene improve the adsorption capacity for nickel removal [39].

3.3. Optimizing the most influencing factors

The common effective factors for adsorption efficiency during wastewater treatment are pH, adsorbent dose and time of contact. Therefore, various investigations have been applied to optimize the adsorptive removal of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C. The pH is studied in the range from 2 to 7 (Figure 6). The maximum adsorption capacity was achieved in the acidic medium between 5 and 6. These results agreed with previous adsorption methods using ZnO based adsorbent for removal of Cu(II) which achieved at pH between 4 and 4.8 [40]. In addition, pH is reported between 3-7 by Gu et al., for removal of Cr(III) onto ZnO nanoparticles [36]. In addition, the influence of ZnO-C dose is investigated in the range 0.1-0.6 g/L and the related adsorption capacity is presented in Figure 7. The maximum adsorption capacity is reported at dose of 0.1 g/L, reporting values of 215, 213, 206 and 231 mg/g for Zn(II), Cd(II), Co(II) and Mn(II), respectively. Then by increasing the ZnO-C dose, the adsorption capacity decreased. This could be attributed to unoccupied sites is increased by increasing ZnO-C at constant heavy metals ions concentration. The similar trend for the influence of dose on the adsorption capacity is previously reported by Habila et.al., for removal of arsenic and mercury onto CNT/SDS-Alumina nanoparticles [41]. Moreover, the influence of time of contacting of Zn(II), Cd(II), Co(II) and Mn(II) with the ZnO-C were investigated in the range between 5 and 1200 min. By increasing time from 5 to 180, a gradually improvement in the adsorption capacity is noticed (Figure 8). By further increasing the time of contact from 180 to 1200 min, the adsorption capacity remain constant due to the steady stage equilibrium in which rate of adsorption is equal to the rate of desorption processes. Herein, the reported equilibrium time of 180 min is considered as fast adsorption rate which indicate the effectiveness of the prepared ZnO-C for removal of Zn(II), Cd(II), Co(II) and Mn(II) with capacity of 183, 180, 163 and 196 mg/L.

3.4. Kinetic and Equilibrium modeling

Applying adsorption theories is important to study the characteristics of the adsorbent’s hybrid materials to enhances the performance and develop a well-controlled process for wastewater treatments [41,42]. To deeply assess the interaction between the developed ZnO-C and Zn(II), Cd(II), Co(II) and Mn(II), various models are applied such as pseudo 1st order, pseudo 2nd order, Langmuir isotherm and Freundlich isotherm as stated in the related equations provided in Table 1.
The pseudo 1st order kinetic model (Figure 9) (Table 2) reveal a calculated adsorption capacity of 227.1, 233.1, 191.7 and 258.4 mg/g for Zn(II), Cd(II), Co(II) and Mn(II), respectively, which significantly different from the experimental capacity (183, 180, 163 and 196 mg/L for Zn(II), Cd(II), Co(II) and Mn(II), respectively) mg/L for Zn(II), Cd(II), Co(II) and Mn(II), respectively. The pseudo 2nd order kinetic model (Figure 9) (Table 2) reveal a calculated adsorption capacity of 196.1, 188.7, 181.8 and 200.0 mg/g for Zn(II), Cd(II), Co(II) and Mn(II), respectively, which are near from the experimental capacity (183, 180, 163 and 196 mg/L for Zn(II), Cd(II), Co(II) and Mn(II), respectively). Therefore, the pseudo 2nd order model is the suitable model to describe the adsorption process. These results indicated that the adsorption process onto the developed ZnO-C occur over three steps (migration of Zn(II), Cd(II), Co(II) and Mn(II) in the adsorption solution, arrangement at the ZnO-C surfaces, and migration through the pores)[43]. By applying the Langmuir isotherms plot (Figure 11) for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C, the calculated constant (Table 3) and the related correlation coeffcienct indicated that the isnot applicable for describing the adsorption process. While plotting the Freundlich isotherm (Figure 12), the calculated constants (Table 3) indicated strong correlation which confirm the multilayer adsorption and heterogenous surfaces according to Freundlich assumptions.

3.5. Purification of wastewater using the deviled hybrid adsorbent materials

As a result of environmental pollution, various water resources have been contaminated with toxic metals. In order to reduce their negative impacts, the treatment of effluents with high contaminate ratio is applied prior to reaching natural water system [44,45]. Various wastewater samples are brought from Riyadh city, Saudi Arabia. The developed adsorption process was utilized using ZnO-C and Fe3O4-C for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) from real wastewater matrix (Table 4). The removal efficiency was not less than 91% for tested samples. These achieved adsorptive removal performance confirm that the developed hybrid materials are effective for wastewater treatment in real field with high added value. By comparing the obtained results with literature [46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68] (Table 5), most of applied materials exhibited the maximum adsorption capacity in the in the pH range of low acidic medium and neutral medium. In addition, the performance efficiency of the evaluated hybrid martials in this work including ZnO-C and Fe3O4 are superior compared to most of tabulated adsorbents from the literature. However, few adsorbents materials exhibited higher adsorption capacity such as f the ZnO nanoparticles [54], hydroxyapatite/pectin hybrid material [64] and Zinc oxide/ graphene oxide composite (ZnO/GO) [68].

4. Conclusion

The zinc oxide nanoparticles anchored carbon (ZnO-C) hybrid materials have been prepared with crystalline structure and spherical granules in nanoscale size of 29.3 - 48.8 nm. The prepared porous structure of ZnO-C poses surface area of 24.84 m²/g. The optimized conditions for Zn(II), Cd(II), Co(II) and Mn(II) uptake were at pH between 5 and 6, contact time 180 min and the ZnO-C dose of 0.1 g/L. For comparison, various adsorbents materials including ZnO, Fe3O4, carbon, ZnO-C and Fe3O4-C were utilized for wastewater treatment by adsorption of Zn(II), Cd(II), Co(II) and Mn(II). However, the ZnO-C exhibited the superior adsorption capacity. Real wastewater sample including valley-water, industrial wastewater and rain-water are successfully treated by adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C and Fe3O4-C indicating high removal efficiency (more than 91%) for evaluated samples. The results achieved in this work lead to further future investigations to develop novel transition metal oxide nanoparticles decorated carbon as hybrid porous materials for enhanced wastewater treatment applications. In addition, the investigated materials in this work could be investigated for additional applications such photocatalytic degradation of organic pollutants and/or removal of various pollutant’s categories such as radioactive waste or greenhouse gases.

Author Contributions

Conceptualization, Mohamed Habila and Zeid ALOthman; Formal analysis, Mohamed Habila, Zeid ALOthman and Ahmed Badjah-Hadj-Ahmed; Investigation, Abdullah ALanazi, Mohamed Habila and Ahmed Badjah-Hadj-Ahmed; Methodology, Abdullah ALanazi, Mohamed Habila and Ahmed Badjah-Hadj-Ahmed; Resources, Zeid ALOthman; Supervision, Zeid ALOthman and Ahmed Badjah-Hadj-Ahmed; Validation, Mohamed Habila, Zeid ALOthman and Ahmed Badjah-Hadj-Ahmed; Writing – original draft, Abdullah ALanazi; Writing – review & editing, Mohamed Habila and Ahmed Badjah-Hadj-Ahmed.
Funding: This project was funded by the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdul Aziz City for Science and Technology, Kingdom of Saudi Arabia, award No. 13-ENV1186-02.

Acknowledgments

The author acknowledges the National Plan for Science, Technology, and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia for its grant with award number 13-ENV1186-02.

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Preprints 91970 g001
Figure 1. SEM characterization of ZnO-C at magnification of (A) 250 and (B) 100000.
Figure 1. SEM characterization of ZnO-C at magnification of (A) 250 and (B) 100000.
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Figure 2. TEM characterization of ZnO-C at scale of (A) 200 nm and (B) 100 nm.
Figure 2. TEM characterization of ZnO-C at scale of (A) 200 nm and (B) 100 nm.
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Figure 3. FTIR of ZnO-C.
Figure 3. FTIR of ZnO-C.
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Figure 4. XRD of ZnO-C.
Figure 4. XRD of ZnO-C.
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Figure 5. investigating the adsorption capacity of varios adsorbent s for removal of Zn(II), Cd(II), Co(II) and Mn(II), pH 6, time 180 min and adsorbent dose 0.15 g/L.
Figure 5. investigating the adsorption capacity of varios adsorbent s for removal of Zn(II), Cd(II), Co(II) and Mn(II), pH 6, time 180 min and adsorbent dose 0.15 g/L.
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Figure 6. influence of pH on the removal of Zn(II), Cd(II), Co(II) and Mn(II) using ZnO-C, (time 180 min and adsorbent dose 0.15 g/L).
Figure 6. influence of pH on the removal of Zn(II), Cd(II), Co(II) and Mn(II) using ZnO-C, (time 180 min and adsorbent dose 0.15 g/L).
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Figure 7. influence of ZnO-C dose on the removal of Zn(II), Cd(II), Co(II) and Mn(II) using ZnO-C (pH 6, time 180 min).
Figure 7. influence of ZnO-C dose on the removal of Zn(II), Cd(II), Co(II) and Mn(II) using ZnO-C (pH 6, time 180 min).
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Figure 8. influence of contact-time on the removal of Zn(II), Cd(II), Co(II) and Mn(II) using ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
Figure 8. influence of contact-time on the removal of Zn(II), Cd(II), Co(II) and Mn(II) using ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
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Figure 9. the pseudo 1st order plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
Figure 9. the pseudo 1st order plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
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Figure 10. the pseudo 2nd order plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
Figure 10. the pseudo 2nd order plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
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Figure 11. the Langmuir plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6, 25 C˚ and co time 180 min).
Figure 11. the Langmuir plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6, 25 C˚ and co time 180 min).
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Figure 12. the Freundlich plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6, 25 C˚ and co time 180 min).
Figure 12. the Freundlich plot for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6, 25 C˚ and co time 180 min).
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Table 1. kinetics and equilibrium equation for the applied as pseudo 1st order, pseudo 2nd order, Langmuir isotherm and Freundlich isotherm.
Table 1. kinetics and equilibrium equation for the applied as pseudo 1st order, pseudo 2nd order, Langmuir isotherm and Freundlich isotherm.
Model Equation Constants
Pseudo 1st order model l o g q e − q t = log ⁡ q e − K 1 2.303 .   t k1: the rate constant of the pseudo 1st order (min−1).
Pseudo 2nd order model t q e = 1 K   q e 2 + 1 q e .   t   K the rate constant of the pseudo 2nd (min−1).
Langmuir isotherms C e q e = ( 1 Q m a x 0 )   C e + 1 Q m a x 0   K L Qomax (mg/g): maximum adsorption capacity
KL (L/mg): a constant associated with the affinity ZnO-C and adsorbed heavy metals ions
Freundlich isotherms l o g   q e = log ⁡ K + 1 n   C e KF (mg/g)/(mg/L)n: Freundlich constant;.
n (dimensionless) is the Freundlich intensity parameter.
Table 2. the pseudo 1st order and pseudo 2nd order constants for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
Table 2. the pseudo 1st order and pseudo 2nd order constants for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C (pH 6 and adsorbent dose 0.15 g/L).
Pseudo-First-Order Pseudo-Second-Order
qe,exp (mg/g) K1(min−1) qe,cal(mg/g) R2 k2(g/mg.min) qe,cal(mg/g) R2
Cd(II) 183.0 0.000192 196.1 0.957 0.00634 227.1 0.996
Ni(II) 180.0 0.000131 188.7 0.936 0.003821 233.1 0.994
Mn(II) 163.0 0.000119 181.8 0.944 0.000391 191.7 0.993
Pb(II) 196.0 0.00022 200.0 0.979 0.006079 258.4 0.997
Table 3. Equillibrium isothers constants for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C.
Table 3. Equillibrium isothers constants for adsorption of Zn(II), Cd(II), Co(II) and Mn(II) onto ZnO-C.
Adsorbate (heavy metals ions) Langmuir constants Freundlich constants
KL Q max. R2 KF n R2
Zn(II) 0.029 212.77 0.88 3.21 0.54 0.95
Cd(II) 0.040 161.29 0.83 2.55 1.97 0.93
Co(II) 0.004 1111.11 0.27 2.55 1.42 0.90
Mn(II) - - - 2.07 1.03 0.94
Table 4. utilization of ZnO-C and Fe3O4-C hybrid materials for real wastewater purification.
Table 4. utilization of ZnO-C and Fe3O4-C hybrid materials for real wastewater purification.
Adsorbent Wastewater Initial concentration before treatment (mg/L) Detected concentration After treatment (mg/L) Removal Efficiency %
Mn Co Cd Zn Mn Co Cd Zn Mn Co Cd Zn
Carbon- Fe3O4 Vally Water 3.56 5.85 4.16 8.81 0.14 0.11 0.23 0.21 96 98 95 98
Industrial Wastewater 15.64 21.04 17.04 11.81 0.43 1.40 1.14 0.79 97 93 93 93
Rain Wastewater 1.50 2.51 1.91 3.43 0.06 0.00 0.08 0.30 96 100 96 91
Cabon-ZnO Vally Water 3.56 5.85 4.16 8.81 0.05 0.18 0.10 0.14 99 97 98 98
Industrial Wastewater 15.64 21.04 17.04 11.81 1.05 0.08 0.09 1.07 93 100 99 91
Rain Wastewater 1.50 2.51 1.91 3.43 0.06 0.05 0.01 0.11 96 98 99 97
Table 5. comparing adsoprion performance of ZnO-C and Fe3O4-C hybrid materials with materials from literature.
Table 5. comparing adsoprion performance of ZnO-C and Fe3O4-C hybrid materials with materials from literature.
Adsorbent Adsorbate Optimum Qe Rf
Acid modified carbon-based adsorbents Cd(II) ion pH = 7
contact time=120min		 M-CNTs= 2.02 mg/g
M-AC= 1.98 mg/g
M-CNFs= 1.58 mg/g
M-FA= 1.22 mg/g		 [46]
Alumina-decorated multi-walled carbon nanotubes (MWCNTs) Cd (II) ion
trichloroethylene (TCE)		 pH = 7
contact time=240min		 Cd (II) ion= 27.21 mg/g
TCE = 19.84 mg/g
 [47]
Natural kaolinite clay Pb(II), Cd (II), Ni(II) and Cu(II) pH = 5.5-7
contact time= 30 min		 Pb=2.35 mg/g
Cd= 0.88 mg/g
Ni= 0.90 mg/g
Cu= 1.22 mg/g
 [48]
Functionalized carbon nanotubes and magnetic biochar Zn(II) pH = 10
contact time= 120 min		 functionalized CNT= 1.05 mg/g
magnetic biochar= 1.18 mg/g		 [49]
Poly(acrylic acid) multi-walled carbon nanotubes (MWCNT-g-PAA) Co(II) pH = 6
contact time= 300 min		 3.55×10−4molg−1 [50]
Natural and modified clay
 Mn(II)
Cd(II)		 pH =1-6
contact time= 60 min		 NT-25/Cd(II)= 11.2 mg/g
NT-25/Mn(II)= 6.0 mg/g		 [51]
Natural phosphate(NP) Cd(II) pH = 5
 26mg/g [52]
straw biochar (WSB)and acid treated wheat straw biochar (AWSB) Cd(II) pH = 6
contact time (5–180 min)		 WSB= 31.65 mg/g
AWSB= 74.63 mg/g		 [53]
ZnO nanoparticles Zn(II)
Cd(II)
Hg(II)		 pH = 5.5
 357 mg/g for Zn(II)
387 mg/g for Cd(II)
714 mg/g for Hg(II)		 [54]
TiO2 nanoparticles Pb, Cd, Cu, Ni, Zn pH = 8
 - [55]
Sugarcane leaves (SCL) Ni2+
Cr3+
Co2+ 		pH = 8 for Cr3+
 51.3 mg/g for Ni2+
62.5 mg/g for Cr3+
66.7 mg/g for Co2+ 		[56]
graphene oxide-bovine serum
Albumin(GO-BSA)		 Co(II) pH = 6
 184 mg/g [57]
activated Saudi
clays		 Co(II) - 12.9 mg/g for
treated Tabbuk clay
12.55 mg/g for treated Bahhah clay
 [58]
intact and modified Ficus carica leaves (FCLs)
 Co(II) pH = 6
 33.9 mg/g [59]
Polyaniline/sawdust composite Mn(II)
 pH = 10
contact time= 30 min		 58.824 mg/g [60]
Poly (sodium acrylate)- graphene oxide (PSA-GO) double network hydrogel Mn(II)

Cd(II)
 pH = 6
 Mn(II) :
165.5 mg/g
Cd(II): 238.3mg/g		 [61]
Surfactant Modified Alumina (SMA) Mn(II)
 pH = 4.04-8.05
Contact time= 30 min
 2.04 mg/g [62]
Activated carbon from bean pods waste Mn(II)
As (III)		 pH = 5-6
Contact time: 30 min		 Mn(II)
=23.4 mg/g
As (III)=1.01 mg/g		 [63]
hydroxyapatite/pectin hybrid material Zn(II)
 pH = 5
 330.4 mg/g [64]
Polyaniline Nanocomposite Coated on Rice Husk(PAn/RH) Zn(II)
 pH = 3
Contact time: 20 min		 24.3 mg/g [65]
Dendrimer-conjugated magnetic nanoparticles Zn(II)
 pH= 7 24.3 mg/g [66]
Zinc oxide nanoparticles
(ZnO-NPs)		 Cr3+ pH = 3-7
Contact time:20 min		 88.547 mg/g [67]
Zinc oxide/ graphene oxide composite (ZnO/GO) Pb(II) pH = 5
Contact time:160 min		 909.09 mg/g [68]
ZnO-C Zn(II), Cd(II), Co(II), Mn(II) pH=2
Contact time: 180 min		 This work
Fe3O4-C Zn(II), Cd(II), Co(II), Mn(II) pH=2
Contact time: 180 min		 This work
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