Adsorption Characteristics of Heavy Metals Pb2+ and Zn2+ by Magnetic Biochar Obtained from Modified AMD Sludge

Acid mine drainage (AMD) sludge can be used to prepare adsorbent materials for the removal of heavy metals in water, which is an effective means for its resource utilization. Magnetic modified biochar (MMB), which can be recovered by magnetic separation, was prepared from sludge generated from the carbonate rock neutralization treatment of AMD and rice straw agricultural waste. Unmodified biochar (UMB) was obtained from rice straw and chemically modified and treated by ultraviolet radiation to produce MMB. The Pb2+ and Zn2+ adsorption capacities of UMB and MMB were investigated. Simultaneously, the materials were characterized by SEM, FTIR, BET, and ZETA. The results showed that the specific surface area (130.89 m2·g−1) and pore volume (0.22 m2·g−1) of MMB were significantly increased compared to those of UMB (9.10 m2·g−1 and 0.05 m2·g−1, respectively). FTIR images showed that MMB was successfully loaded with Fe3O4. The adsorption process of Pb2+ and Zn2+ onto MMB was consistent with the Langmuir adsorption isotherm and second-order kinetic models, with maximum adsorption capacities of 329.65 mg·g−1 and 103.67 mg·g−1, respectively. In a binary system of Pb2+ and Zn2+, MMB preferentially binds Pb2+. The adsorption efficiencies of MMB reached >80% for Pb2+ and Zn2+.


Introduction
Acid mine drainage (AMD) was recently listed as the second largest global problem by the United Nations because the toxic ions in AMD can cause damage to the surrounding environment and endanger human health [1]. AMD is a type of acidic wastewater rich in sulfate and many types of metal ions, and is formed by the oxidation of metal sulfide minerals produced in coal mines and sulfur-rich metal mines during or after mining works [2,3]. The current treatment technologies for AMD can mainly be divided into active and passive types [4,5]. Active treatment technology is used to add alkaline chemicals to AMD to increase its pH such that metal ions form insoluble precipitates that can be removed. This technology is expensive to operate and maintain. Passive treatment technology relies on natural media (mainly carbonate rocks) to treat AMD using physical, chemical, and biological mechanisms. Due to the low cost and easy availability of carbonate rocks, they are widely used as a reaction medium in AMD treatment. However, the use of carbonate rocks to neutralize AMD is prone to producing a large amount of reddish-brown iron

Pre-Treatment
Rice straw collected from the paddy fields near Baihua Lake in Guiyang City, Guizhou Province, China, was repeatedly washed with deionized water and dried in an oven at 60 • C to a constant weight, then cut with a pair of scissors and stored in a self-sealing bag. AMD sludge was collected from a wastewater treatment reaction tank at the Shidong Coal Mine in Guiyang City, Guizhou Province, China. AMD sludge was dehydrated in the suction filter, then dried in an oven at 60 • C until it maintained a constant weight. The dried AMD sludge was ground through a 200-mesh sieve and stored in self-sealing bags for later use. Relevant literature confirmed that Fe (26%) was the main constituent element of AMD sludge, and that Si (2.01%) and Ca (0.59%) were at low levels [32].

Preparation Process
The crushed rice straw was placed into a muffle furnace and carbonized at 500 • C for 90 min to obtain unmodified biochar (UMB), which was ground and passed through a 200-mesh sieve. Then, 2 mg·L −1 of HCl solution was mixed with UMB in a liquid:solid ratio of 10:1 (mL:g) in a 250 mL beaker. The mixture was heated at 80 • C for 90 min on a heating plate before being filtered with a separatory funnel after cooling. The filter residue was washed with ultrapure water until the supernatant was neutral and then placed in an oven (60 • C) to dry. The obtained biochar was dried, and the above steps were repeated; however, the HCl solution was replaced with NaOH solution to obtain the modified biochar. Then, 4.5 g of AMD sludge, 2 g of FeSO 4 ·7H 2 O, 15 mL of 2 mg·L −1 of HCl solution, and approximately 3 g of modified biochar were added to 50 mL of aqueous solution. Then, 3 mL of 10% MgCl 2 solution was added, and the pH was adjusted to 8 using NaOH solution. The mixture was stirred ultrasonically for 1 h, then irradiated under an ultraviolet lamp for 2 h. The mixture was then filtered, and the filtrate residue was washed with ultrapure water until the supernatant was neutral, then dried in an oven (60 • C) to produce MMB. The preparation procedures are illustrated in Figure 1.

Pre-Treatment
Rice straw collected from the paddy fields near Baihua Lake in Guiyang City, Guizhou Province, China, was repeatedly washed with deionized water and dried in an oven at 60 °C to a constant weight, then cut with a pair of scissors and stored in a self-sealing bag. AMD sludge was collected from a wastewater treatment reaction tank at the Shidong Coal Mine in Guiyang City, Guizhou Province, China. AMD sludge was dehydrated in the suction filter, then dried in an oven at 60 °C until it maintained a constant weight. The dried AMD sludge was ground through a 200-mesh sieve and stored in self-sealing bags for later use. Relevant literature confirmed that Fe (26%) was the main constituent element of AMD sludge, and that Si (2.01%) and Ca (0.59%) were at low levels [32].

Preparation Process
The crushed rice straw was placed into a muffle furnace and carbonized at 500 °C for 90 min to obtain unmodified biochar (UMB), which was ground and passed through a 200-mesh sieve. Then, 2 mg·L −1 of HCl solution was mixed with UMB in a liquid:solid ratio of 10:1 (mL:g) in a 250 mL beaker. The mixture was heated at 80 °C for 90 min on a heating plate before being filtered with a separatory funnel after cooling. The filter residue was washed with ultrapure water until the supernatant was neutral and then placed in an oven (60 °C) to dry. The obtained biochar was dried, and the above steps were repeated; however, the HCl solution was replaced with NaOH solution to obtain the modified biochar. Then, 4.5 g of AMD sludge, 2 g of FeSO4·7H2O, 15 mL of 2 mg·L −1 of HCl solution, and approximately 3 g of modified biochar were added to 50 mL of aqueous solution. Then, 3 mL of 10% MgCl2 solution was added, and the pH was adjusted to 8 using NaOH solution. The mixture was stirred ultrasonically for 1 h, then irradiated under an ultraviolet lamp for 2 h. The mixture was then filtered, and the filtrate residue was washed with ultrapure water until the supernatant was neutral, then dried in an oven (60 °C) to produce MMB. The preparation procedures are illustrated in Figure 1.    3 and ZnSO 4 ·7H 2 O (analytical grade) in deionized water. The adsorption kinetics of the different materials (UMBs and MMBs) were examined by mixing 0.1 g of material, 25 mL of 500 mg·L −1 Pb 2+ or Zn 2+ experimental solution, and 1 mL of 0.1 mol·L −1 NaNO 3 background electrolyte solution (note: all subsequent experiments were performed under this background electrolyte solution) in a 50 mL centrifuge tube which were then placed in a constant-temperature oscillator and shaken at 25 • C and 200 rpm until sampling. The sampling times were 5, 10, 30, 60, 90, 120, 180, 240, and 300 min. The absorbance of the solution was then measured using an atomic absorption spectrophotometer.

Adsorption Isotherms
Material (0.1 g) was added to a 50 mL centrifuge tube, then 25 mL of different concentrations of Pb 2+ or Zn 2+ were added and stirred at a constant speed at 25 • C for 24 h. The samples were then centrifuged and filtered, and the Pb and Zn concentrations in the solutions were determined to study their isothermal adsorption properties separately.

Effect of pH on Adsorption Properties of Materials
A total of 0.1 g of material was added to a series of 50 mL centrifuge tubes, and then different Pb 2+ or Zn 2+ concentrations solutions were added. The initial pH of the solution was adjusted from 3 to 7 using 0.1 M HCl or 0.1 M NaOH. The subsequent steps were consistent with those followed in the adsorption isotherm experiment.

Effect of Mixed Ions on Material Adsorption
In this experiment, two systems of Pb-Zn and Zn-Pb were set up for investigation. In the Pb-Zn system, the Pb 2+ concentration in the solution was unchanged and the Zn 2+ concentration in the solution gradually increased. In the Zn-Pb system, the Zn 2+ concentration was unchanged and the Pb 2+ concentration in the solution gradually increased. The concentration ratios were 1:0.25, 1:0.5, 1:1, 1:1.5, and 1:2. Each solution was mixed with 0.1 g of MMB and then shaken for 3 h in a constant-temperature water bath vibrator (200 rpm, 25 • C). Finally, the supernatant was retrieved to measure the heavy metal concentrations in the solution using an atomic absorption spectrophotometer.

Separation Experiments
At pH 7 and 25 • C, a series of concentrations of Pb 2+ and Zn 2+ solutions were configured. We added 0.1 g of MMBs into several 50 mL centrifuge tubes, and then added a series of 25 mL Pb 2+ (or Zn 2+ ) solutions at known concentrations at a constant stirring speed and 25 • C for 180 min. The materials were recovered using a magnet after shaking, rinsing, and drying with water repeatedly. Then, the dried materials were added into several 50 mL centrifuge tubes with 25 mL of deionized water. After shaking, the supernatant was taken and the Pb 2+ and Zn 2+ concentrations were determined using an atomic absorption spectrophotometer.

Repeated Adsorption Experiments
After adsorption, the MMB was collected via centrifugation. It was then washed with ultrapure water and dried, and directly applied in the next adsorption experiment. The adsorption-regeneration experiments were repeated four times. The experiment conditions were 100 mg·L −1 Pb 2+ , 500 mg·L −1 Zn 2+ , solid-to-liquid ratio of 4 g·L −1 , and oscillation for 3 h at 25 • C.

Material Characterization and Sample Testing
A Spectrum 100 infrared spectrometer (PerkinElmer, Waltham, MA, USA) was used to record the infrared spectrum of the materials and analyze their functional group composition. The specific surface area (BET-N 2 method) and pore size distribution of the materials before and after modification were compared using an ASAP-2020 surface area analyzer (Mike Instruments, Pocatello, ID, USA). The surface structure characteristics of samples under different magnifications before and after modification were observed using an S-570 scanning electron microscope (Hitachi Company, Tokyo, Japan). An atomic absorption spectrophotometer (TAS-990, Beijing Purkinje General Instrument Co., Ltd., Beijing, China) was used to determine the Pb 2+ and Zn 2+ concentrations in the solution. DelasaNanoC (McMurritk, NSW, Australia) was used to determine the electro-kinetic potential of the samples.

Adsorption Kinetics
The Pb 2+ and Zn 2+ adsorption kinetics of the two materials are shown in Figure 2. The Pb 2+ or Zn 2+ adsorption rates of the two materials were very fast in the first 30 min. As the adsorption sites on the surface of the two materials approached saturation, the adsorption rate decreased and finally reached an equilibrium after 1 h. MMB has a greater Pb 2+ and Zn 2+ adsorption capacity, which can be attributed to abundant active sites, that is, the large surface area and pore structure. Factors limiting the adsorption rates include the diffusion of the adsorbent molecules at the interface, electrostatic attraction by the adsorbent surface/repulsion, adsorption potential binding ability of the adsorbent surface, and control of surface chemical reactions [33,34].
Toxics 2023, 11, x FOR PEER REVIEW A Spectrum 100 infrared spectrometer (PerkinElmer, Waltham, MA, USA) w to record the infrared spectrum of the materials and analyze their functional grou position. The specific surface area (BET-N2 method) and pore size distribution of terials before and after modification were compared using an ASAP-2020 surface alyzer (Mike Instruments, Pocatello, ID, USA). The surface structure character samples under different magnifications before and after modification were obser ing an S-570 scanning electron microscope (Hitachi Company, Tokyo, Japan). An absorption spectrophotometer (TAS-990, Beijing Purkinje General Instrument C Beijing, China) was used to determine the Pb 2+ and Zn 2+ concentrations in the s DelasaNanoC (McMurritk, NSW, Australia) was used to determine the electro-kin tential of the samples.

Adsorption Kinetics
The Pb 2+ and Zn 2+ adsorption kinetics of the two materials are shown in Figur Pb 2+ or Zn 2+ adsorption rates of the two materials were very fast in the first 30 min adsorption sites on the surface of the two materials approached saturation, the ads rate decreased and finally reached an equilibrium after 1 h. MMB has a greater P Zn 2+ adsorption capacity, which can be attributed to abundant active sites, that is, t surface area and pore structure. Factors limiting the adsorption rates include the d of the adsorbent molecules at the interface, electrostatic attraction by the adsorb face/repulsion, adsorption potential binding ability of the adsorbent surface, and of surface chemical reactions [33,34]. The parameters obtained from the adsorption kinetics models (pseudo-firs pseudo-second-order, Elovich, and double constant) are shown in Table 1. The cor coefficients of the four models fitted for Pb 2+ adsorption by UMB were 0.84, 0.96, 0 0.88, and those of MMB were 0.75, 0.92, 0.75, and 0.72. The pseudo-second-orde had the highest fit for MMB adsorption of Pb 2+ or Zn 2+ , which was consistent w linear form of the already reported pseudo-second-order model [35]. This indica the pseudo-second-order model could better describe the kinetic properties of P sorption by UMB and MMB. For Zn 2+ adsorption by UMB and MMB, the correlati ficients (R 2 ) of the pseudo-second-order model were 0.97 and 0.98, respectively indicated that the pseudo-second-order model could well describe the adsorpti cesses of Zn 2+ by UMB and MMB. The fitting results showed that MMB mainly re chemisorption to achieve Pb 2+ and Zn 2+ adsorption, and its adsorption rate was controlled by the chemical reaction on the material surface [36,37]. The parameters obtained from the adsorption kinetics models (pseudo-first-order, pseudo-second-order, Elovich, and double constant) are shown in Table 1. The correlation coefficients of the four models fitted for Pb 2+ adsorption by UMB were 0.84, 0.96, 0.93, and 0.88, and those of MMB were 0.75, 0.92, 0.75, and 0.72. The pseudo-second-order model had the highest fit for MMB adsorption of Pb 2+ or Zn 2+ , which was consistent with the linear form of the already reported pseudo-second-order model [35]. This indicated that the pseudo-second-order model could better describe the kinetic properties of Pb 2+ adsorption by UMB and MMB. For Zn 2+ adsorption by UMB and MMB, the correlation coefficients (R 2 ) of the pseudo-second-order model were 0.97 and 0.98, respectively, which indicated that the pseudo-second-order model could well describe the adsorption processes of Zn 2+ by UMB and MMB. The fitting results showed that MMB mainly relied on chemisorption to achieve Pb 2+ and Zn 2+ adsorption, and its adsorption rate was mainly controlled by the chemical reaction on the material surface [36,37].

Adsorption Isotherms
The Langmuir model assumes that the molecules adsorbed on the surface have monolayer distributions, the surface has a fixed number of adsorption sites, and that adsorbed molecules do not interact. Figures 3 and 4 and Tables 2 and 3 demonstrate that the Langmuir model could better describe the process of Pb 2+ and Zn 2+ adsorption by MMB. According to the calculated isotherm parameters, the model-calculated values of the equilibrium adsorption of Pb 2+ by the material before and after modification were 34.11 mg·g −1 and 329.65 mg·g −1 , respectively, which were similar to the measured values of 25.58 mg·g −1 and 292.20 mg·g −1 , respectively. The modeled values of equilibrium adsorption of Zn 2+ before and after the modification were 18.70 mg·g −1 and 103.67 mg·g −1 , respectively, which were also similar to the measured values of 15.62 mg·g −1 and 92.17 mg·g −1 , respectively. The correlation coefficient (R 2 ) for the linear form of Pb 2+ /Zn 2+ adsorption for both materials was close to 1. This indicated that the MMB surface had many energetically undifferentiated adsorption sites, and that the adsorption process of Pb 2+ or Zn 2+ in solution by MMB was more consistent with the Langmuir model [38]. The Langmuir model assumes that the molecules adsorbed on the surface hav olayer distributions, the surface has a fixed number of adsorption sites, and that ad molecules do not interact. Figures 3 and 4 and Tables 2 and 3 demonstrate that th muir model could better describe the process of Pb 2+ and Zn 2+ adsorption by MM cording to the calculated isotherm parameters, the model-calculated values of the rium adsorption of Pb 2+ by the material before and after modification were 34.11 and 329.65 mg·g −1 , respectively, which were similar to the measured values of 25.58 and 292.20 mg·g −1 , respectively. The modeled values of equilibrium adsorption of fore and after the modification were 18.70 mg·g −1 and 103.67 mg·g −1 , respectively were also similar to the measured values of 15.62 mg·g −1 and 92.17 mg·g −1 , respe The correlation coefficient (R 2 ) for the linear form of Pb 2+ /Zn 2+ adsorption for both als was close to 1. This indicated that the MMB surface had many energetically u entiated adsorption sites, and that the adsorption process of Pb 2+ or Zn 2+ in solu MMB was more consistent with the Langmuir model [38].       To further evaluate the adsorption effect of the prepared materials on Pb 2+ and Zn 2+ , the absorption ability and conditions of MMB were compared to those of previously reported adsorbents ( Table 4). The results showed that the prepared MMB had strong adsorption effects on Pb 2+ and Zn 2+ .

Effects of pH and Zeta Potential on Adsorption
Changes in the pH of the solution affect the active groups on the surface of materials, leading to changes in their protonation and thus affecting the adsorption process. As Pb 2+ and Zn 2+ produce white precipitation under alkaline conditions, the pH range set for this experiment was 3.0-7.0 to avoid interference from precipitation. As seen in Figure 5, the overall increase in solution pH from 3.0 to 7.0 resulted in an increasing trend in Pb 2+ and Zn 2+ adsorption on the surface of MMB/UMB. Furthermore, the adsorption capacity of MMB for Pb 2+ and Zn 2+ was significantly higher than that of UMB under the same pH conditions. The zeta potential test showed that the electronegativity of MMB was at the maximum (−18.11 mV) at pH 7.0. At this point, the deprotonation of MMB made the surface of MMB negatively charged, which promoted attraction to the positively charged Pb 2+ and Zn 2+ through electrostatic interaction, and the maximum adsorption amount was reached.  At a pH below 5.0, the MMB surface became positively charged and electrostat pulsion occurred. Additionally, the oxygen-containing groups, such as carbonyl an droxyl groups, on the surface of MMB easily bound H + in aqueous solution, which occupied the adsorption sites; the large amount of H + in solution competed with Pb 2+ Zn 2+ ) for adsorption, which hindered the migration of Pb 2+ (Zn 2+ ) to the surface of M and then decreased the adsorption efficiency. As the pH of the solution increased 7.0), the amount of H + decreased, and its competing adsorption ability weakened increase in dissociation of oxygen-containing groups on the surface of MMB increase ratio of hydroxyl and carbonyl groups in the form of -COO-and -O-and increase negative charge of MMB. This enhanced the attraction of MMB to positively cha Pb 2+ and Zn 2+ ions.

Effect of Mixed Ions on MMB Adsorption Ability
In this experiment, two systems (Pb-Zn and Zn-Pb) were established. In the P system, the Pb 2+ concentration in the solution was kept constant and the Zn 2+ concentr in the solution was gradually increased to study the removal of Pb 2+ by MMB. In the Pb system, the Zn 2+ concentration was kept constant and the Pb 2+ concentration in sol was gradually increased to study the removal of Zn 2+ by MMB. Figure 6 shows tha adsorption ability of MMB decreases with the increase in the ion concentration due t addition of competing ions in both systems. As the initial concentration of the interf ion Zn 2+ increased from 25 to 150 mg·g −1 , the decrease in the adsorption ability of M for Pb 2+ was marginal and slow. Furthermore, when the concentration of the interf ion increased to 200 mg·g −1 , the Pb 2+ adsorption by MMB decreased significantly. Th dicated that the low Zn 2+ concentration in the Pb-Zn system had little influence o ability of MMB to adsorb Pb 2+ and that high concentrations significantly inhibited adsorption by MMB. In the Zn-Pb system, Zn 2+ adsorption by MMB significantly creased with the increase in the concentration of interfering ions (Pb 2+ ). The results sho that the presence of Pb 2+ in these systems significantly inhibited the Zn 2+ adsorptio MMB. At a pH below 5.0, the MMB surface became positively charged and electrostatic repulsion occurred. Additionally, the oxygen-containing groups, such as carbonyl and hydroxyl groups, on the surface of MMB easily bound H + in aqueous solution, which then occupied the adsorption sites; the large amount of H + in solution competed with Pb 2+ (and Zn 2+ ) for adsorption, which hindered the migration of Pb 2+ (Zn 2+ ) to the surface of MMB and then decreased the adsorption efficiency. As the pH of the solution increased (5.0-7.0), the amount of H + decreased, and its competing adsorption ability weakened. The increase in dissociation of oxygen-containing groups on the surface of MMB increased the ratio of hydroxyl and carbonyl groups in the form of -COO-and -O-and increased the negative charge of MMB. This enhanced the attraction of MMB to positively charged Pb 2+ and Zn 2+ ions.

Effect of Mixed Ions on MMB Adsorption Ability
In this experiment, two systems (Pb-Zn and Zn-Pb) were established. In the Pb-Zn system, the Pb 2+ concentration in the solution was kept constant and the Zn 2+ concentration in the solution was gradually increased to study the removal of Pb 2+ by MMB. In the Zn-Pb system, the Zn 2+ concentration was kept constant and the Pb 2+ concentration in solution was gradually increased to study the removal of Zn 2+ by MMB. Figure 6 shows that the adsorption ability of MMB decreases with the increase in the ion concentration due to the addition of competing ions in both systems. As the initial concentration of the interfering ion Zn 2+ increased from 25 to 150 mg·g −1 , the decrease in the adsorption ability of MMB for Pb 2+ was marginal and slow. Furthermore, when the concentration of the interfering ion increased to 200 mg·g −1 , the Pb 2+ adsorption by MMB decreased significantly. This indicated that the low Zn 2+ concentration in the Pb-Zn system had little influence on the ability of MMB to adsorb Pb 2+ and that high concentrations significantly inhibited Pb 2+ adsorption by MMB. In the Zn-Pb system, Zn 2+ adsorption by MMB significantly decreased with the increase in the concentration of interfering ions (Pb 2+ ). The results showed that the presence of Pb 2+ in these systems significantly inhibited the Zn 2+ adsorption by MMB. The competitive selective adsorption between ions is related to the hydration radius and electronegativity of heavy metals [46]. The hydration radius of Pb 2+ (4.01 Å) is smaller than that of Zn 2+ (4.30 Å), whereas the electronegativity of Pb 2+ (2.33) is higher than that of 2+ Figure 6. Effect of the binary (Pb and Zn) mixed system on MMB adsorption. The competitive selective adsorption between ions is related to the hydration radius and electronegativity of heavy metals [46]. The hydration radius of Pb 2+ (4.01 Å) is smaller than that of Zn 2+ (4.30 Å), whereas the electronegativity of Pb 2+ (2.33) is higher than that of Zn 2+ (1.65) [47]. The radius of hydrated metal ions affects adsorption selectivity, whereby the smaller the radius of hydrated metal ions, the more easily they are adsorbed by the adsorbent [48,49]. Thus, Pb 2+ was more dominant in competitive adsorption. This also showed that, in addition to the influence of the adsorption mechanism, the properties of individual heavy metal ions also plays an important role in adsorption behavior in competitive systems.

Microscopic Morphological Changes in UMB and MMB
To investigate the changes in the morphological structure of the materials before and after the modification, SEM was performed to study UMB and MMB, and the results are shown in Figure 7. Figure 7a,b show that the surface of UMB was smoother and flatter than that of MMB. After modification, the surface structure of MMB was considerably changed. The surface of MMB became significantly rougher, with more effective adsorption sites and many tiny particles of iron elements attached to the surface (Figure 7d). Figure 7c,d also show that a layer of flocs and many new white flocs [50] (maybe a mixture of Pb 2+ or Zn 2+ and iron) were attached to the surface of MMB after the adsorption of Pb 2+ and Zn 2+ .
The competitive selective adsorption between ions is related to the hydratio and electronegativity of heavy metals [46]. The hydration radius of Pb 2+ (4.01 Å) is than that of Zn 2+ (4.30 Å), whereas the electronegativity of Pb 2+ (2.33) is higher tha Zn 2+ (1.65) [47]. The radius of hydrated metal ions affects adsorption selectivity, w the smaller the radius of hydrated metal ions, the more easily they are adsorbe adsorbent [48,49]. Thus, Pb 2+ was more dominant in competitive adsorption. T showed that, in addition to the influence of the adsorption mechanism, the prop individual heavy metal ions also plays an important role in adsorption behavior petitive systems.

Microscopic Morphological Changes in UMB and MMB
To investigate the changes in the morphological structure of the materials be after the modification, SEM was performed to study UMB and MMB, and the re shown in Figure 7. Figure 7a,b show that the surface of UMB was smoother an than that of MMB. After modification, the surface structure of MMB was cons changed. The surface of MMB became significantly rougher, with more effective tion sites and many tiny particles of iron elements attached to the surface ( Fig  Figure 7c,d also show that a layer of flocs and many new white flocs [50] (maybe a of Pb 2+ or Zn 2+ and iron) were attached to the surface of MMB after the adsorptio and Zn 2+ .

Specific Surface Area and Pore Size Distribution of UMB and MMB
As shown in Figure 8 and Table 5, more fine pores on the surface of MMB were formed following chemical modification, and the specific surface area and pore volume of MMB were enhanced. The measured specific surface area of MMB was 130.89 m 2 ·g −1 , the average pore size was approximately 6.58 nm, and the pore volume was 0.22 cm 3 ·g −1 , which represented a 14.38-fold increase in specific surface area and a 4.4-fold increase in pore volume compared to that of UMB.

Specific Surface Area and Pore Size Distribution of UMB and MMB
As shown in Figure 8 and Table 5, more fine pores on the surface of MM formed following chemical modification, and the specific surface area and pore vol MMB were enhanced. The measured specific surface area of MMB was 130.89 m 2 · average pore size was approximately 6.58 nm, and the pore volume was 0.22 which represented a 14.38-fold increase in specific surface area and a 4.4-fold incr pore volume compared to that of UMB.   Figure 9 shows the infrared spectra of MMB before and after Pb 2+ and Zn 2+ a tion. As shown in Figure 9, the peaks at 505 cm −1 and 492 cm −1 correspond to the vi of Fe-O bonds [51], which further confirmed the successful loading of Fe3O4 onto The increased binding of MMB to -OH in the presence of magnetic iron particles r in a broad and extended O-H stretching vibrational peak. A carbonyl (-COOH) vibr peak occurred at 3426 cm −1 [52]. After Pb 2+ and Zn 2+ adsorption by MMB, their res O-H peaks were reduced. It may be that Pb 2+ (or Zn 2+ ) was attracted to the surro environment by electrostatic adsorption. Then, some Pb 2+ (or Zn 2+ ) diffused to the of MMB, which underwent reactions with H + and the carbonyl group. A previou   Figure 9 shows the infrared spectra of MMB before and after Pb 2+ and Zn 2+ adsorption. As shown in Figure 9, the peaks at 505 cm −1 and 492 cm −1 correspond to the vibration of Fe-O bonds [51], which further confirmed the successful loading of Fe 3 O 4 onto MMB. The increased binding of MMB to -OH in the presence of magnetic iron particles resulted in a broad and extended O-H stretching vibrational peak. A carbonyl (-COOH) vibrational peak occurred at 3426 cm −1 [52]. After Pb 2+ and Zn 2+ adsorption by MMB, their respective O-H peaks were reduced. It may be that Pb 2+ (or Zn 2+ ) was attracted to the surrounding environment by electrostatic adsorption. Then, some Pb 2+ (or Zn 2+ ) diffused to the surface of MMB, which underwent reactions with H + and the carbonyl group. A previous study [53] confirmed that Pb 2+ adsorption by magnetic biomass carbon was accompanied by the ionic exchange of Pb 2+ with H + in O-H and complexation with oxygen-containing functional groups, such as -OH and -COOH, on the surface of the magnetic biomass carbon. C=O was shown at 1620 cm −1 [54,55], and there was no significant change in the peak before and after adsorption, indicating that the carbon group was not involved in the reaction. The peak at 1097.3 cm −1 corresponded to M-OH (M is Fe) in the MMB, which exhibited an -OH bending vibration, which was likely because M-OH dissociated H + and reacted with Pb 2+ and Zn 2+ to form surface complexes [56]. The strength of electrostatic interactions is closely related to the pH of the solution [57]. Figure 5 shows the zeta potential of the material adsorbing Pb 2+ and Zn 2+ under different pH conditions. It was found that, in a neutral pH environment, a large number of grafted carbonyl groups on the surface of MMB underwent a deprotonation reaction (-COOH group lost H + to form -COO-, -OH lost H + to form -O-), increasing the degree of dissociation of functional groups and the negative charge. This enhanced the coordination bond force and electrostatic adsorption capacity between MMB, Pb 2+ , and Zn 2+ . fore and after adsorption, indicating that the carbon group was not involved in th tion. The peak at 1097.3 cm −1 corresponded to M-OH (M is Fe) in the MMB, which ited an -OH bending vibration, which was likely because M-OH dissociated H + a acted with Pb 2+ and Zn 2+ to form surface complexes [56]. The strength of electrost teractions is closely related to the pH of the solution [57]. Figure 5 shows the zeta po of the material adsorbing Pb 2+ and Zn 2+ under different pH conditions. It was foun in a neutral pH environment, a large number of grafted carbonyl groups on the sur MMB underwent a deprotonation reaction (-COOH group lost H + to form -COO lost H + to form -O-), increasing the degree of dissociation of functional groups a negative charge. This enhanced the coordination bond force and electrostatic adso capacity between MMB, Pb 2+ , and Zn 2+ . In summary, the Pb 2+ and Zn 2+ adsorption mechanism of MMB mainly involved ical adsorption, ion exchange, electrostatic attraction, and complexation reactions. binary system, where Pb 2+ and Zn 2+ coexisted, Pb 2+ and Zn 2+ competed for the adso sites on the surface of MMB, and MMB preferentially bound to Pb 2+ . The principle di of the adsorption mechanism of MMB of Pb 2+ and Zn 2+ is shown in Figure 10. In summary, the Pb 2+ and Zn 2+ adsorption mechanism of MMB mainly involved physical adsorption, ion exchange, electrostatic attraction, and complexation reactions. In the binary system, where Pb 2+ and Zn 2+ coexisted, Pb 2+ and Zn 2+ competed for the adsorption sites on the surface of MMB, and MMB preferentially bound to Pb 2+ . The principle diagram of the adsorption mechanism of MMB of Pb 2+ and Zn 2+ is shown in Figure 10.
C=O was shown at 1620 cm −1 [54,55], and there was no significant change in the peak before and after adsorption, indicating that the carbon group was not involved in the reaction. The peak at 1097.3 cm −1 corresponded to M-OH (M is Fe) in the MMB, which exhibited an -OH bending vibration, which was likely because M-OH dissociated H + and reacted with Pb 2+ and Zn 2+ to form surface complexes [56]. The strength of electrostatic interactions is closely related to the pH of the solution [57]. Figure 5 shows the zeta potential of the material adsorbing Pb 2+ and Zn 2+ under different pH conditions. It was found that, in a neutral pH environment, a large number of grafted carbonyl groups on the surface of MMB underwent a deprotonation reaction (-COOH group lost H + to form -COO-, -OH lost H + to form -O-), increasing the degree of dissociation of functional groups and the negative charge. This enhanced the coordination bond force and electrostatic adsorption capacity between MMB, Pb 2+ , and Zn 2+ . In summary, the Pb 2+ and Zn 2+ adsorption mechanism of MMB mainly involved physical adsorption, ion exchange, electrostatic attraction, and complexation reactions. In the binary system, where Pb 2+ and Zn 2+ coexisted, Pb 2+ and Zn 2+ competed for the adsorption sites on the surface of MMB, and MMB preferentially bound to Pb 2+ . The principle diagram of the adsorption mechanism of MMB of Pb 2+ and Zn 2+ is shown in Figure 10.

Experimental Analysis of Desorption and Regenerative Adsorption by MMB
We analyzed the reuse characteristics of MMB and the recovery of heavy metal resources by studying the cyclic adsorption of MMB. The experiments involving the adsorptiondesorption cycling of Pb 2+ and Zn 2+ ions by MMB are shown in Figures 11 and 12, and Table 6.

Experimental Analysis of Desorption and Regenerative Adsorption by MMB
We analyzed the reuse characteristics of MMB and the recovery of heavy metal resources by studying the cyclic adsorption of MMB. The experiments involving the adsorption-desorption cycling of Pb 2+ and Zn 2+ ions by MMB are shown in Figures 11 and 12, and Table 6.   From Figure 11 and Table 6, the Pb 2+ and Zn 2+ adsorption process of MMB was reversible, and the regenerative adsorption performance was high. After four cycles of adsorption-regeneration-resorption in 100 mg·L −1 Pb 2+ and Zn 2+ solutions, the Pb 2+ removal rate decreased from 95.23% to 72.23% and that of Zn 2+ decreased from 95.27% to 68.29%; however, the removal rate remained >80% in all three experiments. For the 500 mg·L −1 Pb 2+ and Zn 2+ solutions, the Pb 2+ removal rate from MMB decreased from 94.42% to 64.34%, Figure 11. Reusability experiments of MMB adsorption of (a) Pb 2+ and (b) Zn 2+ .

Experimental Analysis of Desorption and Regenerative Adsorption by MMB
We analyzed the reuse characteristics of MMB and the recovery of heavy metal resources by studying the cyclic adsorption of MMB. The experiments involving the adsorption-desorption cycling of Pb 2+ and Zn 2+ ions by MMB are shown in Figures 11 and 12, and Table 6.   From Figure 11 and Table 6, the Pb 2+ and Zn 2+ adsorption process of MMB was reversible, and the regenerative adsorption performance was high. After four cycles of adsorption-regeneration-resorption in 100 mg·L −1 Pb 2+ and Zn 2+ solutions, the Pb 2+ removal rate decreased from 95.23% to 72.23% and that of Zn 2+ decreased from 95.27% to 68.29%; however, the removal rate remained >80% in all three experiments. For the 500 mg·L −1 Pb 2+ and Zn 2+ solutions, the Pb 2+ removal rate from MMB decreased from 94.42% to 64.34%,  From Figure 11 and Table 6, the Pb 2+ and Zn 2+ adsorption process of MMB was reversible, and the regenerative adsorption performance was high. After four cycles of adsorption-regeneration-resorption in 100 mg·L −1 Pb 2+ and Zn 2+ solutions, the Pb 2+ removal rate decreased from 95.23% to 72.23% and that of Zn 2+ decreased from 95.27% to 68.29%; however, the removal rate remained >80% in all three experiments. For the 500 mg·L −1 Pb 2+ and Zn 2+ solutions, the Pb 2+ removal rate from MMB decreased from 94.42% to 64.34%, and that of Zn 2+ decreased from 96.78% to 71.41% after four cycles. The decrease in removal efficiency may have been attributed to the blocking of the pore structure and a decrease in the number of binding sites [58].
The leaching experiment of MMB after Pb 2+ and Zn 2+ adsorption was conducted using deionized water as the leaching solution ( Figure 12). The results showed that the leaching rate of MMB increased with an increase in the initial heavy metal concentration in solution, and that the leaching rate of MMB was maintained at <5% when the concentration was <100 mg·L −1 . Thus, MMB had good adsorption stability for low Pb 2+ and Zn 2+ concentrations in solutions.
Under the applied magnetic field (the circular surface of the magnet with a 1.5 cm diameter), MMB was rapidly separated from air and water molecules and adsorbed stably on the magnet, which is important for future resource recycling.

Conclusions
MMB was prepared from modified AMD sludge and rice straw, and the specific surface area was greatly improved compared to UMB and reached approximately 130.89 mg·g −1 , which was 14.38-times higher than that of UMB. Langmuir and the secondary kinetic model well described the adsorption process of the materials for the two heavy metals, and showed the adsorption of MMB mainly relied on chemisorption and that the process followed monolayer adsorption. The maximum Pb 2+ and Zn 2+ adsorption capacities at 25 • C and pH 7 were 329.65 and 103.67 mg·g −1 , respectively.
In the Pb 2+ and Zn 2+ binary system, the maximum adsorption capacity of MMB was lower than that in the single system. MMB had a stronger bond to Pb 2+ than to Zn 2+ in this system. Mechanisms underlying the MMB adsorption of heavy metals included physical adsorption, ion exchange, electrostatic attraction, and complexation.
MMB showed good reproducibility, and after three cycles of adsorption-regeneration, the adsorption efficiency of the material for Pb 2+ and Zn 2+ reached >80%. Additionally, it did not undergo extensive desorption, and re-released the adsorbed heavy metals into the environment, suggesting that MMB can be a potential, environmentally friendly adsorbent for treating heavy metal contamination.
Although this study contributes to the resource utilization of AMD sludge and provides novel ideas for the removal of heavy metals (Pb 2+ and Zn 2+ ), actual heavy metalcontaminated wastewater has a complex composition. Therefore, the practical application of the MMB material requires further study.

Data Availability Statement:
We have full control of all primary data, and we agree to allow the journal to review our data if requested.