The solution was prepared using iron chloride hexahydrate (FeCl₃∙6H₂O). Copper sulfate hexahydrate (CuSO₄∙5H₂O), lead nitrate (Pb(NO₃)₂), cadmium chloride (CdCl₂), and anhydrous chromium chloride (CrCl₃) were utilized to prepare metal solutions for the adsorption experiments. The media’s and other solutions’ pH was adjusted using HCl and NaOH.

Camel excrement was collected from a camel farm located near Sharourah, which is in the Najran area of Saudi Arabia. After the plant particles were removed, the manure was dried in the air. The dung was dried in the oven for 48 hours at a temperature of 60˚C. Camel dung was converted into biochar by subjecting it to slow pyrolysis in an electrical muffle furnace at 500˚C for 4 hours in an oxygen-free atmosphere. The charcoal cooled down overnight in the container by free convection. The selected fraction was kept in airtight containers until it was tested after the biochar was sieved to a size of 0.6 to 1.4 mm [28].

The biochar/Fe₃O₄nanocomposite was synthesized by dissolving 2 g of iron chloride hexahydrate (FeCl₃∙6H₂O) in 100 mL of distilled water, adding 3 g of biochar to a 250 mL beaker, and mechanically stirring at 250 rpm for 2 hours forming the biochar/Fe₂O₃ nanocomposite. The mixture had been heating on an 80˚C hot plate for 3 hours. The reaction pH was 8.5. Nanocomposite production was confirmed by liquid color change for dark brown color. Biochar/Fe₂O₃ nanocomposite was isolated from solution by centrifugation at 4000 rpm for 30 minutes after reaction. The biochar/Fe₂O₃ nanocomposite powder has been incubated at 400˚C for two hours. Dry material was powdered and calcined at 600˚C for two hours in an open-air furnace.

Figure 1. The schematic synthesis of biochar/Fe₂O₃ nanocomposite.

The biochar/Fe₃O₄ nanocomposite was studied using several analytical methods, including SEM, FTIR, BET, EDX, TGA, and XRD. FTIR analysis (range 500 - 4000 cm⁻¹) was used to identify the biomolecules that were responsible for the reduction, stabilization, and capping of the biochar/Fe₃O₄ nanocomposite. FTIR spectrum analysis was done using Origin Pro 8.5 software. SEM was used to determine the size and surface appearance of the biochar/Fe₃O₄ nanocomposite. Important information regarding the NPs’ physical characteristics was revealed by this method. The biochar/Fe₃O₄ nanocomposite was analyzed using EDX to determine the presence of Fe and other elements. The EDX data revealed the atomic content of the produced NPs, which reflected their atomic structure. Data were recorded over 2θ range of 20 - 80 with a step increment of 0.02 and interval time of 0.008 s per step. Surface morphology and composition on the surface were visualized and determined using Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDX, Thermos Scientific, US, SU-70). X-ray diffractometer XRD analysis was performed to verify the quality of the biochar/Fe₂O₃ nanocomposite. The TGA analysis used measurement of the weight loss during defined temperature for biochar/Fe₃O₄ nanocomposite. Using the Scherrer equation, the average crystalline size of the produced iron oxide nanoparticle. To find the zeta potential of each biochar and biochar/Fe₃O₄, a 0.002 g sample was dissolved in 25 milliliters of Milli Q water. Using 0.1 M NaOH and 0.1 M HCl, the suspension pH was brought within the range of 2 to 10. The suspensions were ultrasonically distributed for three hours at 25˚C in a bath-type sonicator. The Zetasizer Nano ZS90, Malvern, UK’s JS94G+ micro electrophoresis device was used to test the electrophoresis mobility (Figure 1).

The biochar/Fe₃O₄ nanocomposite was used as the adsorbent in the adsorption studies, and the adsorbate was Pb, and Cd ions. Within a 250 ml conical flask, 0.5 gramme of the adsorbent was dissolved in 100 ml of an aqueous solution that included metal ions at a concentration of 100 mg/l. The present study examined the effects of several variables on the adsorption of Cd, Pb ions, including pH (varying from 2 to 10), starting metal ion concentration (10 - 100 mg/l), and contact period (10 - 120 minutes). An orbital shaker was used to stir the mixture of adsorbents and adsorbate at 150 rpm for a predetermined amount of time at room temperature. Whatman-40 filter paper was used to filter the mixture, and a Flame Atomic Adsorption Spectrometer (FAAS) SHIMADZU AAS AA-7000, Japan, was used to measure the initial and final metal concentrations in the filtrate. The percentage removal efficiency and the capacity for adsorption of each metal ion by the biochar/Fe₂O₃ nanocomposite have been calculated, as indicated in Equation (1) and Equation (2) [29].

The starting and equilibrium concentrations of metal ions after a time, t, are denoted as C_i (mg/L) and C_e (mg/L), respectively. V stands for the volume of the metal ion solution (L), W for the mass of the biochar/Fe₂O₃ nanocomposite used (g), and q_e for the mass of metal ions adsorbed onto the biochar both at equilibrium and after time t.

The pseudo-first- and pseudo-second-order models, which provide mathematical frameworks to comprehend the temporal elements of the adsorption process, were used to investigate adsorption kinetics. The initial adsorption rate and rate constant are estimated via the pseudo-first-order model, which mostly concentrates on early-stage behavior. The pseudo-second order model, however, assumes that the adsorption rate and empty sites are proportionate. A more thorough knowledge of the whole adsorption process, including rate constants and maximum adsorption capacities, is offered by the pseudo-second order model.

The equations of the pseudo-first-order model may be found in Equation (3).

The pseudo-second-order model is expressed in its nonlinear form as:

After the solid adsorbent was washed with water, the desorbed concentrations of the metal ions were analyzed to investigate the leaching of Pb(III) and Cd(II) ions. Following that, Pb(III) and Cd(II) were repeatedly removed using the regenerated adsorbent, and the corresponding removal efficiencies were evaluated at the second run.

Figure 2 shows the results of an FTIR analysis of biochar made from camel manure and biochar/Fe₃O₄ nanocomposite. This analysis compared the functional groups in the biochar and those linked to the biochar/Fe₃O₄ nanocomposite that might aid in the adsorption of heavy metals. Many noticeable peaks were seen in the biochar spectra at wavenumbers 3325, 2931, 1777, 1642, and 1038 cm⁻¹. The presence of the O-H at about 3325 cm⁻¹ supports the presence of alcohols, carboxyl, and phenols in biochar because of the presence of lignin and hemicellulose [30]. It also shows the presence of the broad peak of O-H stretching. The alkyl’s -C-H stretch in biochar is represented by the band at 2931 cm⁻¹. There is probably a C=O (carbonyl) present, which is the cause of the broad peak seen at about 1677 cm⁻¹. The peak at 1690 - 1380 is explained by C=O stretching, which is frequently found in carboxyl and ketone compounds, as well as C=C vibrations in aromatic rings. According to [31], the sulfoxide group (S=O) is represented by the peak at 1034 cm⁻¹. Like this, the biochar/Fe₃O₄ nanocomposite’s FTIR spectra shows clear peaks. A peak was detected at 3321 cm⁻¹, indicating the presence of an O-H functional group. In alkenes, a noticeable peak at 1632 cm⁻¹ that is connected to the C=C stretching vibration was noted. As previously reported [32], the presence of the C-H stretching mode in asymmetric and symmetric aliphatic and aromatic hydrocarbons was also suggested by the identification of a prominent peak at 1450 cm⁻¹. Sulfur oxide and hydroxyl groups were detected in the biochar/Fe₃O₄ nanocomposite by FTIR analysis, as indicated by separate peaks at 1051 and 3321 cm⁻¹, respectively. These results imply the production of a biochar/Fe₃O₄ nanocomposite and the reduction of Fe³⁺ ions; the absorption band at 631 cm⁻¹ offers compelling proof of the successful establishment of Fe-O bonds. Previous research on the FTIR spectroscopy of biosynthesized iron oxide nanoparticles has found absorption bands at wavelengths like 618 [33] and 630 [34], which is in line with our findings. The surface charge of biochar was measured using zeta potential values as a function of pH, as shown in Figure 3. As may be seen, the sites of zero charge for biochar and biochar/Fe₃O₄ are 3.2 and 3.7, respectively. Biochar/Fe₃O₄ has a greater pHPZC value than biochar, which may be explained by the impregnation of Fe₃O₄, which has a pHPZC value of 5 [35]. Higher temperature-produced biochar often has less negative charges because it lacked acidic functional groups [36]. Biochar/Fe₃O₄ showed the highest potential for electrostatic adsorption and the highest negative surface charges in the pH 2 - 10 range [37][38].

Figure 2.FT-IR spectra of camel manure-derived biochar and biochar/Fe₃O₄ nanocomposite, illustrating surface functional groups and Fe-O bond formation.

Figure 3. Zeta potential of biochar and biochar/Fe₃O₄ as a function of solution pH, indicating the point of zero charge and surface charge behavior.

The phase purity and crystallinity of the biochar/Fe₃O₄ nanocomposite can be discerned through XRD analysis. The research demonstrated consistent XRD patterns of the biochar/Fe₃O₄ nanocomposite, which utilized biochar nanoparticles [28]. The diffraction peaks of the synthesized Fe₃O₄-NPs, as illustrated in Figure 4, were observed at 2θ values of 36.57˚, 43.47˚, 54.52˚, 61.25˚, 63.74˚, 71.91˚, and 84.66˚. These peaks correspond to the crystal planes designated as (111), (200), (311), (400), (422), (511), (440), and (533), respectively. In contrast, several distinct phase peaks were noted within the 2θ range of 30˚ - 90˚ as illustrated in Figure 4, suggesting the extent of crystallinity in the phases observed in the two biochars. The observed weak diffraction peak at 36.57˚ may be ascribed to the presence of amorphous carbon [39]. The diffraction peaks under analysis exhibited a strong correlation with the standard XRD patterns of magnetite (Fe₃O₄), as referenced by JCPDS file no: 00-026-1137, confirming the cubic crystallographic system.

Figure 4.XRD patterns of biochar/Fe₃O₄ nanocomposite.

Figure 5. EDX results of biochar/Fe₃O₄ nanocomposite before (A) and after adsorption of Pb (B) and Cd (C).

SEM-EDX analyses were performed to get a better understanding of the chemical structure of the biochar/Fe₃O₄ nanocomposite after the adsorption process. The goal of these analyses was to uncover the processes of adsorption for Pb(II) and Cd(II). First, as shown in Figure 5, clear peaks of Pb(II) and Cd(II) were observed in the EDX spectra after the adsorption process. At the same time, the peak intensity of iron decreased noticeably, which confirmed that Pb(II) and Cd(II) were successfully adsorbed onto the biochar/Fe₃O₄ nanocomposite. At the same time, the iron components played a significant role in the adsorption process [40]. A technique known as elemental mapping was used to confirm the existence of elements and their distribution throughout the biochar/Fe₃O₄ nanocomposite. Figure 6 demonstrates that the primary elements (C, O, Fe, and Pb/Cd) in the structure of the biochar/Fe₃O₄ nanocomposite were distributed uniformly and continuously over the surface of the biochar/Fe₃O₄ nanocomposite after adsorption. This indicates that the structure of the nanocomposites remained stable throughout the adoption process.

Figure 6. Element mapping analysis of biochar/Fe₃O₄ nanocomposite after adsorption of Pb and Cd.

Figure 7. XPS spectrum of each biochar and biochar/Fe₃O₄ NPs.

The magnetic nanocomposites’ oxidation state and elemental makeup were ascertained using XPS. The total XPS spectra of the magnetic biochar and biochar produced using the method are shown in Figure 7. The biochar’s C1s and O1s bonds, which demonstrate the existence of carbon and oxygen, can be seen. Due to the Fe-O interaction of the Fe₃O₄ nanoparticles, separate Fe (3p_3/2), Fe (2p_3/2), and Fe (2p_1/2) bonds were observed in the spectra of biochar/Fe₃O₄ NPs at about 54, 712, and 725 eV [41]. The inset table (Figure 6) demonstrates that after iron impregnation in biochar/Fe₃O₄ NPs, the atomic percentage of C significantly dropped while the O% significantly rose. Because the organic materials in camel manure-derived biochar are capping onto iron oxide nanoparticles, the surface of biochar/Fe₃O₄ NPs has a greater C content but a lower O and Fe content, which is compatible with the EDX findings.

The C1s and O1s XPS spectra of biochar are shown in Figure 8(a). The C1s spectra of biochar was deconvoluted into five main peaks, which correspond to carbide (282.5 eV), C-H (283.6), C-C/C=C (284.2 eV), C-O-C (286.6 eV), and C=O (288.7 eV), as shown in Figure 8(b) and Table 1 [42]. After being magnetized using a certain method, the components of biochar underwent a transformation. The carbon and C-H bond at peaks about 282 eV in the C1s spectra of biochar/Fe₃O₄ were ascribed to the carbon, whose intensity dramatically increased after the successful impregnation of iron oxide using the co-precipitation approach [43]. The presence of various functional groups on the biochar/Fe₃O₄ surface in the C1s spectrum is confirmed by XPS spectra, which are linked to elemental analysis and FTIR analysis [44]. Additionally, the proportion of oxygen-containing functional groups in biochar/Fe₃O₄ is higher, confirming the surface functionalization of the co-precipitation method.

Figure 8. Biochar and Biochar/Fe₃O₄ NPs’ C1s and O1s deconvolution.

Metal oxides in biochar are responsible for the binding energy in the O1s region between 529.1 eV and 530.71 eV [45]. K₂O, CaO, MgO, and other metal oxides may be present in biochar [46] (Table 2). Because so much iron is added, the linkages in biochar/Fe₃O₄ are mostly inorganic oxygen bound to iron (FeO, Fe₂O₃, Fe₃O₄) [43]. According to the FTIR data, the newly observed signal at 532 eV in biochar/Fe₃O₄ was ascribed to -OH or molecular water, which mostly originated from moisture.

Table 1. Information of structure and binding energy values of the prepared biochar and biochar/Fe₃O₄ NPs.

Figure 9 displays the Fe 2p spectra, and Table 1 provides a summary of the data for each deconvoluted peak. Seven peaks were found in the deconvoluted Fe 2p spectra of biochar/Fe₃O₄, and they are shown in Figure 9. 2p_3/2and 2p_1/2 of Fe³⁺cwere represented by the three spectral bands at 712.7 eV, and 727.99eV, respectively. Fe²⁺ species 2p_3/2 and 2p_1/2 may be responsible for the other two peaks, which had binding energy levels of 710.34 eV and 724.15 eV, respectively. combined with the last two peaks, which were matching peaks of Fe³⁺ and Fe²⁺ at 719.87eV and 732.9 eV, respectively, showing that the Fe₃O₄ phase in biochar/Fe₃O₄ was successfully formed [47]. Given that FeO·Fe₂O₃ is another way to define stoichiometric Fe₃O₄, the Fe²⁺/Fe³⁺ ratio ought to be 1/2 [48]. The deconvoluted satellite peaks of 0.572 in this study’s Fe²⁺/Fe³⁺ findings show that the co-precipitation process generated pure Fe₃O₄.

Table 2. Metal contents in biochar and biochar/Fe₃O₄ (mg/g).

Figure 9. Deconvolution of Fe 2p of the biochar/Fe₃O₄.

One of the most important factors affecting the surface charge and dissociation of functional groups on the adsorbent during the adsorption process is the pH of the solution. Moreover, the degree of ionization and speciation of the metal ions is also influenced by the pH of the solution [49]. The adsorptive removal of both Pb(II) and Cd(II) from the aqueous solution was evidently pH-dependent, as shown in Figure 10(a), Figure 10(b). This suggests that the adsorption was strongly connected to the electrostatic forces between Pb(II)/Cd(II) and biochar/Fe₃O₄ [50][51].

For Pb(II) and Cd(II) recovery, the biochar/Fe₃O₄ outperformed the biochar in almost the whole pH range investigated. In the case of Pb(II), for example, both biochar and biochar/Fe₃O₄ removal percentages were low (pH 2), but for biochar/Fe₃O₄, the percentage removal increased dramatically from pH 2 to pH 6 and reached equilibrium, which was 96.2% at pH 6. However, the amount of Pb(II) removed from biochar only slightly increased (from 1.6% to almost 16.2%). At pH 2, biochar and biochar/Fe₃O₄ showed the least amount of Cd(II) elimination. But with biochar/Fe₃O₄, removal increased from as low as 2.1% to almost 68.4% as pH increased from 2 to 4, reaching a maximum removal efficiency of 92.82% at equilibrium pH 6.

Figure 10. Effect of initial solution pH on Pb(II) and Cd(II) removal efficiency and corresponding iron leaching behavior of biochar/Fe₃O₄.

The removal of Cd(II) on biochar exhibited a modest increase of 14.18% as the pH shifted from 2 to 8, which stands in contrast to the findings related to Pb(II). At pH 10, a notable increase in Cd(II) removal was observed for both types of biochar, likely due to the precipitation of Cd(OH)₂. The limited adsorptive removal observed at low pH can be attributed to the elevated concentration of H+ ions in the solution. These ions compete with Pb(II) and Cd(II) for bonding with the active sites of biochar [52]. This competition leads to the saturation of the bonded active sites, making them unavailable for interaction with other cations. The results of the zeta potential analysis indicated that biochar exhibited a pH of 3.18. These ions compete with Pb(II) and Cd(II) for bonding with the active sites of biochar, leading to the saturation of these sites and making them unavailable for interaction with other cations. The rise in pH led to a decrease in H+ concentration, and the deprotonation of functional groups on the biochar surface resulted in a shift from a positive to a negative surface charge. This change enhanced the adsorption of Pb(II) and Cd(II) onto the biochar (Figure 3). In addition to the acidic functional groups such as phenolic, hydroxyl, and carboxylic acid that are anchored on the surface of biochar, iron oxides are capable of forming iron hydroxide complexes in aqueous solution [53].

The stability of magnetic biochar was assessed by investigating the leaching of Fe ions from biochar/Fe₃O₄ during adsorption experiments with Pb(II) and Cd(II) across various pH levels. Figure 10(c) and Figure 10(d) illustrate that the leaching of iron exhibited a consistent pattern for both Pb(II) and Cd(II), characterized by a rapid decrease from pH 2 to pH 4, while the loss remained minimal beyond pH 4. At a pH of 2, the quantity of iron extracted from biochar/Fe3O4 was measured at 6.62 mg/g in Pb(II) solutions and 7.62 mg/g in Cd(II) solutions. Considering the findings and the results regarding the effect of pH on sorption, pH 5 was selected for both Pb(II) and Cd(II) for the subsequent experiments discussed below.

Evaluation of the effect of adsorbent dose on Pb(II) and Cd(II) adsorption revealed that removal efficiency for both Pb(II) and Cd(II) rose as adsorbent dosage increased (Figure 11). With practically all the Pb(II) and Cd(II) eliminated, the removal efficiency remained essentially the same with adsorption dosages of 4 g/L for Pb(II) and 6 g/L for Cd(II). The concentration of residual metal ions in the solution is large when the absorbent surface is saturated with metal ions at low adsorbent dosages. As the dose of the adsorbent was raised, the removal of metal ions increased as well, until the concentrations of surface and solution metal ions balanced out. But when the dose of the adsorbent increased, the quantity of metal ions that biochar was able to absorb per unit mass dropped significantly. The dose was set at 2 g/L and 4 g/L for Pb(II) and Cd(II) adsorption tests in the ensuing experiments to achieve high removal efficiency and to fully use the biochar.

Figure 11. Effect of dose on (a) Pb(II) and (b) Cd(II) adsorption on biochar/Fe₃O₄ (pH = 5, starting concentration = 100 mg/L, contact period = 24 hours).

The pseudo first-order and pseudo second-order kinetic models were used to examine experimental data to further assess the adsorption efficiency (Figure 12). To study the effect of contact duration on Pb(II) and Cd(II), we used a starting concentration of 100 mg/L at pH 5, as shown in Figure 12. There are two noticeable steps to the adsorption kinetics of Pb(II) and Cd(II): the first is a fast uptake, and the second is a gradual uptake. One possible explanation is that the biochar/Fe₃O₄ nanocomposite gradually becomes saturated with the available active sites over time, making it difficult for Pb(II)/Cd(II) to be adsorbed onto the biochar. Biochar exhibited a delayed adsorption of Pb(II)/Cd(II) because, whereas 91% removal efficiency for Pb(II) was achieved within 30 minutes, 91% sorption of Cd(II) happened about 4 hours, leading to an adsorption of around 42% and a tendency to become more saturated with time. Adsorption of roughly 43 mg/g for Pb(II) and 27 mg/g for Cd(II), respectively, were achieved after 30 minutes and 4 hours, respectively, according to the corresponding removal efficiencies. The adsorption capacities of Pb(II) (46.33 ± 0.25 mg/g) and Cd(II) (28.51 ± 0.41 mg/g) did not show any notable improvement after 4 hours and 24 hours, respectively. Thus, it can be inferred that, given the experimental circumstances utilized, biochar/Fe₃O₄ has a higher speed and capacity adsorption affinity for Pb(II) compared to Cd(II).

The experimental results for both metal ion solutions more closely fit the pseudo-second-order model than the pseudo-first-order model, according to the comparison of R² values, which is shown in Table 3. The strong agreement between the measuredq_e and the anticipatedq_e values for both Pb(II) and Cd(II) ions adsorbed over biochar/Fe₃O₄ further supports the validity of the pseudo-second-order model. These results are consistent with previous research that has shown that pseudo-second-order adsorption kinetics may be applied to Pb(II) and Cd(II) adsorption onto biochar made from plant and/or animal biomass [54]-[56].

Pseudo-second-order kinetics show that chemisorption controls adsorption or that metal ions and biochar/Fe₃O₄ share electrons [56]. Its shows biochar/Fe₃O₄ may effectively adsorb these metal ions, making it an intriguing candidate for aqueous solution contamination removal.

Figure 12. Pb(II) and Cd(II) adsorption kinetics and isotherm on biochar/Fe₃O₄ (pH = 5, starting concentration = 100 mg/L, dosage = 2 g/L for Pb, 4 g/L for Cd).

Table 3. Adsorption Kinetics Parameters for Pb(II) and Cd(II) onto biochar/Fe₃O₄.

To get a fundamental grasp of the removal capabilities of the adsorbent created in this study, the removal efficacy of different adsorbents was investigated utilizing Pb(II) and Cd(II) ions as the adsorbates. Figure 13 compares the removal efficacy of biochar derived from camel dung with biochar/Fe₃O₄ nanoparticles. Pb(II) ion removal on biochar was 12.25%, while Cd(II) ion removal was 10.67%. Biochar/Fe₃O₄ demonstrated a higher ability to remove both metal ions than biochar derived from camel dung, and co-precipitation magnetization enhanced Pb(II) and Cd(II) uptake overall. The biochar/Fe₃O₄ demonstrated an exceptionally high removal efficiency for both Pb(II) (97.46%) and Cd(II) (99.17%) ions under the experimental circumstances, which were almost completely adsorbed by the material. This suggests that biochar/Fe₃O₄ has a significant deal of promise for heavy metal removal from aqueous solution.

Figure 13. Comparison of biochar and biochar/Fe₃O₄ nanoparticles for Pb(II) and Cd(II) (Initial concentration: 100 mg/L, Dosage: 4 g/L, pH = 6.0) adsorption.

The stability of magnetic biochar was assessed by investigating the leaching of iron from biochar/Fe₃O₄ nanoparticles within a pH range of 2 to 6. Figure 14 illustrates that the leaching of iron exhibited a consistent pattern, with a significant decrease occurring from pH 2 to pH 3, followed by negligible loss beyond pH 3 for biochar/Fe₃O₄ nanoparticles. At a pH of 2.0, the quantity of iron released from biochar/Fe₃O₄ was measured at 5.92 mg/g. The increased leaching amount in biochar/Fe₃O₄ can be attributed to the reduced iron content in biochar/Fe₃O₄. Approximately 0.3 mg/g of iron was still released from biochar/Fe₃O₄ at pH 3. Consequently, the biochar/Fe₃O₄ demonstrates greater stability under acidic conditions.

Figure 14. Leaching of iron of biochar/Fe₃O₄.

The results indicated that the surface functional groups of biochar/Fe₃O₄ nanoparticles experienced qualitative alterations before and during adsorption. Moreover, it was apparent that the hydroxyl and carboxyl groups, along with triple bonds or accumulated double bonds, and the incorporation of Fe₃O₄ particles were predominantly consumed during the adsorption of Pb(II) and Cd(II) to create inner complexes (-COO-M, -O-M, C:M²⁺, and Fe-O-M, where M represents Pb or Cd), resulting in the attenuation or displacement of the characteristic bands of COOH, O-H, and Fe-O. Additionally, co-precipitation happened to generate the basic PbCO₃ component during the adsorption process, which aligned well with the results from the XRD study.

The SEM-EDX, XRD, and FTIR results allow us to suggest some possible ways that Pb(II) and Cd(II) might stick to the surface of biochar/Fe₃O₄ nanoparticles. Some potential adsorption processes of Pb(II) and Cd(II) on the surface of biochar/Fe₃O₄ nanoparticles may be suggested based on the results of SEM-EDX, XRD, and FTIR investigations.

First, the strong electrostatic attraction between the negatively charged functional groups, such -COO- and -OH-, on the surface of biochar/Fe₃O₄ nanoparticles and the positively charged Pb(II) and Cd(II) seen in Figure 15 illustrates the importance of the bonding process. Second, since iron oxides have a strong affinity for lead and cadmium, they are crucial to the adsorption of Pb(II) and Cd(II) by biochar/Fe₃O₄ nanoparticles. Finally, the adsorption of Pb(II) onto biochar/Fe₃O₄ nanoparticles may be improved by the co-precipitation of Pb(II) on the surface of the nanoparticles.

Figure 15. Potential adsorption mechanisms of biochar/Fe₃O₄ nanoparticles.