Preparation of Magnetic Activated Carbon by Activation and Modification of Char Derived from Co-Pyrolysis of Lignite and Biomass and Its Adsorption of Heavy-Metal-Containing Wastewater

Abstract

Adsorption with activated carbon (AC) is an important method for the treatment of heavy metal wastewater, but there are still certain challenges in the separation and reuse of activated carbon. The preparation of magnetic activated carbon (MAC) by modifying AC is one of the effective means to realize the separation of AC from solution after the adsorption process. In this work, lignite and poplar leaves were used as raw materials for co-pyrolysis, and the co-pyrolysis char was activated and modified to prepare MAC. The structure and properties were characterized by VSM, N₂ adsorption, SEM, XRD, and FT-IR. At the same time, the adsorption performance of MAC on wastewater containing Pb and Cd ions was studied. The results show that the prepared MAC contains Fe₃O₄, and the saturation magnetization (Ms) of the MAC is 13.83 emu/g; the specific surface area of the MAC is 805.86 m²/g, and the micropore volume is 0.23 cm³/g; the MAC exhibited a good porous structure. When the pH value of the solution was 5, the adsorption time was 120 min, the dosage of MAC was 4 g/L, the initial concentration of Pb ion solution was 50 mg/L, and that of Cd ion solution was 25 mg/L, and the adsorption temperature was 30 °C, the adsorption efficiency of Pb, Cd ions were 84.40 and 78.80%, respectively, and the adsorption capacities were 10.55 and 4.93 mg/g, respectively. The adsorption of Pb and Cd ions by MAC conforms to the Langmuir adsorption model, which is a monolayer adsorption. The adsorption process is mainly chemical adsorption, which can be better described by the pseudo-second-order model. The adsorption thermodynamic analysis showed that the adsorption of Pb and Cd ions by MAC was a spontaneous reaction, and the higher the temperature, the stronger the spontaneity.

1. Introduction

With the continuous development of industry and agriculture, pollutants such as waste and wastewater discharged in the production process will cause serious pollution to the environment, especially water resources. How to effectively treat these wastes and wastewater and reduce their harm to the environment is an important research direction. Wastewater containing heavy metals is a major pollutant in industrial production. Heavy metals such as Pb, Cu, and Cd contained in wastewater are non-bio-degradable and can be accumulated in the food chain after being discharged, which will eventually cause serious damage to human health [1,2,3,4,5,6]; therefore, heavy metal wastewater must be treated before discharge.

There are many existing treatment methods for heavy metal wastewater, such as membrane separation, solvent extraction, evaporation concentration, chemical precipitation [7], and the adsorption method. Among them, the adsorption method has a better effect in the treatment of low-concentration heavy metal wastewater and is more economical and effective than other methods. Sobik et al. [8] used poultry manure as raw material to prepare biological AC and carried out Cd ion adsorption experiments. The results show that the AC activated at 725 °C can achieve 100% adsorption efficiency for Cd ion solutions with concentrations of 10, 50, and 100 mg/L after reaching the adsorption equilibrium. Yuan et al. [9] used the AC prepared from waste leather as raw material to adsorb Pb and Cu ions in wastewater. After the AC adsorption reached equilibrium, the adsorption capacity was 231 and 130 mg/g, respectively. Yu et al. [10] prepared AC from peanut shells, cellulose, and hemicellulose to adsorb heavy metal ions in wastewater. The maximum adsorption capacities of the three AC for Pb ions were 196.17, 171.06, and 327.43 mg/g, respectively. Relevant studies have shown that AC exhibits a good adsorption effect on heavy metal ions and can treat wastewater containing heavy metals effectively; however, after the adsorption of AC is completed, it is still a challenge to separate it from the treated solution. The AC is activated and modified to prepare MAC. While retaining the adsorption performance of AC, the effective separation of activated carbon from wastewater can be realized by a magnetic field [11,12,13,14,15,16,17].

Kazak et al. [18] used sucrose and waste red mud as raw materials to prepare MAC by co-pyrolysis at 750 °C, and its saturation magnetization was 15.40 emu/g. When the initial concentration of the solution was 25 mg/L, the saturated adsorption capacity of MAC was 7.12 mg/g, which was used to adsorb Cr ion wastewater. Mohammadi et al. [19] prepared co-containing MAC nanocomposites for adsorption of Cr ions. When the pH value of the solution is 5, the adsorption efficiency can reach 95.3% after 15 min of adsorption. Yang et al. [20] used cation exchange resin as raw material to prepare MAC by ion exchange method, and the saturation magnetization was 2.20 emu/g. MAC adsorbed mercury in flue gas at 120 °C, and the adsorption capacity was 53 μg/g after 3 h. Zhang et al. [21] showed that the adsorption of Pb²⁺ and Cd²⁺ on the MAC prepared by hydrothermal activation and modification of rape straw powder conformed to the pseudo-second-order kinetic model, and the maximum adsorption capacities were 253 and 73 mg/g, respectively. Existing research shows that MAC can be prepared by different raw materials and technologies and has exhibited good results in the field of adsorption of heavy metal wastewater; however, there are few relevant reports on the preparation of MAC from char formed by the co-pyrolysis of biomass and lignite.

Coal is one of the main energy sources and realizing the clean and effective utilization of low-rank coal such as lignite is an important goal faced by coal utilization. Co-pyrolysis of lignite and biomass, the hydrogen source from biomass can saturate the free radicals generated during the pyrolysis of coal, which not only improves the yield of tar but also obtains pyrolysis gas with high calorific value and low-sulfur char [22]. The char produced by the co-pyrolysis of coal and biomass has a relatively perfect surface pore structure, large specific surface area, and high stability [23]. The char exhibits good adsorption properties and can be used as an activated carbon precursor because of the above characteristics [24,25]. North China is rich in poplar resources, and the yield of poplar leaves is huge. Poplar leaves are a kind of biomass with high H content, low ash content, and are continuously available. Co-pyrolysis of poplar leaves and coal can reduce production costs and effectively utilize natural resources; therefore, the innovation in this work is as follows:

(1)

The co-pyrolysis of lignite and poplar leaves realizes the clean utilization of low-rank coal resources and comprehensively utilizes natural resources such as poplar leaves.

(2)

Activation and modification of co-pyrolysis semi-coke were carried out to prepare MAC with both high specific surface area and high magnetic properties. After the MAC adsorbs heavy metal wastewater, it can be separated from the solution by magnetism, which simplifies the recovery method of activated carbon.

This work uses poplar leaves and lignite as raw materials to obtain char through co-pyrolysis and activates and modifies the pyrolysis char to prepare MAC with higher magnetic properties. The properties and structures of the prepared MAC were characterized by VSM, SEM, N₂ adsorption, XRD, and FT-IR methods. On this basis, the adsorption experiment of the prepared MAC on heavy metal wastewater containing Pb and Cd was carried out, the adsorption performance of MAC on heavy metal ions was analyzed, and the adsorption mechanism was studied.

2. Materials and Methods

2.1. Experimental Materials

The poplar leaves on the campus of the University of Science and Technology Beijing were used as biological raw materials. After indoor natural air drying, the water content was about 5.70%, which is recorded as PL. Huolinhe lignite in Inner Mongolia was used as the coal-based material, and the water content was about 10.90%, which was recorded as LC. Magnetizer FeCl₃·6H₂O (>99.00 wt%) and was purchased from Beijing Coupling Technology Co., Ltd., Beijing, China; Activator ZnCl₂ (>98.00 wt%); cadmium nitrate (>99.99 wt%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China; lead nitrate (>99.00 wt%) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. From Xilong Chemical Co., Ltd., Shantou, China. The water used in the experiment was ultrapure water.

2.2. MAC Preparation

(1)

The LC and PL were mixed uniformly at a mixing ratio of 3:2 and placed in a fixed bed reactor for co-pyrolysis. It was heated from room temperature to 600 °C at a heating rate of 10 °C/min under a nitrogen carrier gas atmosphere and kept at final temperature for 20 min. The char obtained by pyrolysis was treated with HCl-HF two-stage acid for ash removal. First, the char was mixed with 1 mol/L HCl solution at a liquid–solid ratio of 10:1, stirred at room temperature for 4 h, washed with deionized water until neutral, and then dried. Then, we took 10g of treated char, added it to the HF solution with a concentration of 3 mol/L according to a liquid–solid ratio of 10:1, and stirred at room temperature for 4 h; the ash content in the final treated char was reduced from 22.51 to 6.77%.

(2)

Char activation and modification. Approximately 20 g of the acid-washed char was added to 60 mL of 30% ZnCl₂ solution, stirred evenly, and then immersed at room temperature for 16 h. After the immersion was completed, it was dried in an oven at 80 °C for use. Approximately 5 g of the impregnated precursor char was dry blended with 1.96 g of FeCl₃·6H₂O, followed by activation modification in a fixed bed reactor. The activation conditions were as follows: the heating rate was 10 °C/min, the activation temperature was 700 °C, and the activation time was 2 h. The MAC exhibited weak magnetic properties when the activation and modification were carried out at 600 °C; therefore, the activation temperature was finally selected to be 700 °C. After the activation was completed, the sample was cooled to room temperature to obtain magnetic activated carbon with Fe₃O₄ content of 10%, denoted as MAC.

2.3. MAC Characterization

(1)

Magnetic analysis. The MAC was magnetically characterized under a magnetic field of ±2T using a 7404 vibrating magnetometer (VSM) produced by LakeShore in Carson, CA, USA.

(2)

Morphology analysis of activated carbon. Using the TESCAN MIRA LMS scanning electron microscope produced by Tescan Orsay Holding, a.s. in Brno, Czech Republic with a voltage of 30.00 kV, the maximum magnification was 200,000 times, the energy spectrum analysis working distance was 15 mm, and the resolution of 0.9 nm was 15 kV, the surface morphology of the activated carbon was analyzed. The samples were gold sprayed in a vacuum in advance.

(3)

Porosity analysis. The pore size and specific surface area of the MAC were analyzed by using a Mack 2020 automatic specific surface and porosity analyzer produced by Norcross in Georgia, USA. About 0.14 g of sample was vacuum degassed at 100 °C for 8 h. On this basis, N₂ was adsorbed at a liquid nitrogen temperature of −196 °C. The nitrogen adsorption–desorption curve of MAC was obtained, the specific surface area of MAC was calculated by the Brunauer–Emmett–Teller equation, and the pore size distribution of MAC was determined according to the BJH model.

(4)

XRD analysis. The XRD patterns were obtained by Panalytical Empyrean X-ray diffractometer (XRD) from PANalytical B.V., Almelo, Holland with Cu Kα radiation source (40 kV, 30 mA). The data were collected in the range of 10 to 80° with a step size of 0.02°/s. The crystal morphology of iron in the synthesized MACs was characterized by XRD.

(5)

FT-IR analysis. The surface functional groups in MACs were measured by Fourier transform infrared spectrometer (FT-IR) of Nicolet iS20 from Thermo Scientific, Waltham, MA, USA. The samples were ground and sieved to below 74 μm, mixed with KBr for tableting treatment prior to testing. Spectra were collected at a resolution of 4 cm⁻¹ for 32 scans in the spectral region 400–4000 cm⁻¹.

2.4. Adsorption Experiments

(1)

The preparation method for wastewater containing heavy metals is as follows: Weigh 1.60 g of Pb (NO₃)₂ and 2.74 g of Cd (NO₃)₂·4H₂O and dissolve them in deionized water. Use a volumetric flask with a capacity of 1L to dilute to the volume and shake well to obtain a concentration of 1 g/L containing Pb and Cd ion solution. The wastewater solution used in subsequent experiments was obtained by dilution of the stock solution.

(2)

Adsorption experiment. A total of 50 mL of wastewater solution was poured into a conical flask and weighed a certain amount of MAC into the solution. The conical flask was shaken at a speed of 200 rev/min in a shaker, and the effects of pH, adsorption time, MAC dosage, solution concentration, and temperature on the adsorption of heavy metals by MAC were investigated. Each set of experiments was repeated three times, and the average value was the final result. After the adsorption was completed, 10 mL of the solution was taken and filtered through a microporous membrane filter with a diameter of 0.45 μm.

(3)

Calculation of adsorption capacity and adsorption efficiency. The concentrations of Pb and Cd ions in the solution before and after adsorption were measured by Plasma 2000 ICP-OES from NCS Testing Technology co., ltd, Beijing, China. (Detection limits: Pb, 0.0285 ppm; Cd, 0.018 ppm. Mean std deviation: Pb, 0.18; Cd, 0.13), and the adsorption capacity and adsorption efficiency of MAC were calculated by Equations (1) and (2) at the time of adsorption equilibrium.

$${\mathrm{Q}}_{\mathrm{e}}=\frac{\mathrm{V}\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{\mathrm{m}}$$

(1)

$$\mathsf{\eta }=\frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_0}\times 100\%$$

(2)

Among them, ${\mathrm{Q}}_{\mathrm{e}}$ represents the adsorption capacity of MAC (mg/g); V represents the volume of wastewater solution (L); ${\mathrm{C}}_0$ represents the initial concentration of the solution (mg/L); ${\mathrm{C}}_{\mathrm{e}}$ represents the concentration of the solution at equilibrium (mg/L); m represents the MAC mass (g); $\mathsf{\eta }$ represents the adsorption efficiency of MAC for ions (%).

2.5. Adsorption Isotherm

(1)

Langmuir model

$${\mathrm{Q}}_{\mathrm{e}}=\frac{{\mathrm{Q}}_{\mathrm{m}}{\mathrm{K}}_{\mathrm{l}}{\mathrm{C}}_{\mathrm{e}}}{{1+\mathrm{K}}_{\mathrm{l}}{\mathrm{C}}_{\mathrm{e}}}$$

(3)

When using the Langmuir model (Equation (3)) for fitting, C_e is used as the X-axis and C_e/Q_e is used as the Y-axis to plot. The data were linearly fitted, with the intercept representing $\frac{1}{{\mathrm{K}}_{\mathrm{l}}{\mathrm{Q}}_{\mathrm{m}}}$ and the slope representing $\frac{1}{{\mathrm{Q}}_{\mathrm{m}}}$. Equation (4) was used to calculate the separation factor to judge the difficulty of the adsorption process. When the separation factor is less than 1, it indicates that the adsorption process is favorable; if the separation factor is greater than 1, it is not conducive to the progress of the adsorption [26].

$${\mathrm{R}}_{\mathrm{l}}=\frac{1}{{1+\mathrm{K}}_{\mathrm{l}}{\mathrm{C}}_0}$$

(4)

Among them, ${\mathrm{Q}}_{\mathrm{e}}$ is the equilibrium adsorption capacity of MAC (mg/g); ${\mathrm{Q}}_{\mathrm{m}}$ is the maximum adsorption capacity that MAC can achieve (mg/g); ${\mathrm{K}}_{\mathrm{l}}$ is the Langmuir constant (L/g); ${\mathrm{C}}_{\mathrm{e}}$ is the concentration of Pb and Cd ions in the liquid phase after adsorption reaches equilibrium (mg/L); ${\mathrm{C}}_0$ is the initial concentration of Pb and Cd ions (mg/L); ${\mathrm{R}}_{\mathrm{l}}$ is the separation factor.

(2)

Freundlich model

$${\mathrm{Q}}_{\mathrm{e}}{=\mathrm{K}}_{\mathrm{f}}{\mathrm{C}}_{\mathrm{e}}^{\frac{1}{\mathrm{n}}}$$

(5)

When using the Freundlich model (Equation (5)) for fitting, plot with ${\mathrm{lnC}}_{\mathrm{e}}$ as the X-axis and ${\mathrm{lnQ}}_{\mathrm{e}}$ as the Y-axis. A linear fit was performed to the adsorption data, with the intercept representing ${\mathrm{lnK}}_{\mathrm{f}}$ and the slope representing 1/n. Among them, ${\mathrm{K}}_{\mathrm{f}}$ is the Freundlich constant (mg/g), which can reflect the adsorption capacity of MAC; 1/n is a dimensionless constant. When 1/n is between 0 and 1, it indicates that the adsorption process is favorable and easily occurs.

(3)

Tests for Fitting Models

In order to further verify the applicability of the model obtained by the experiment, the residual standard error (RMSE) was calculated by Equation (6) (the larger the value of R², the smaller the value of RMSE, the better the goodness of fit).

$$\mathrm{RMSE}=\sqrt{\frac{\sum {\left({\mathrm{Y}}_{\mathrm{i}}-\widehat{{\mathrm{Y}}_{\mathrm{l}}}\right)}^2}{\mathrm{N}}}$$

(6)

Among them, ${\mathrm{Y}}_{\mathrm{i}}$ is the true value of Y, $\widehat{{\mathrm{Y}}_{\mathrm{l}}}$ is the predicted value of Y, and N is the number of samples.

2.6. Adsorption Kinetics

(1)

pseudo-first-order model

$$\ln \left({\mathrm{Q}}_{\mathrm{e}}-{\mathrm{Q}}_{\mathrm{t}}\right){=\mathrm{lnQ}}_{\mathrm{e}}-{\mathrm{K}}_1\mathrm{t}$$

(7)

When fitting with a pseudo-first-order model (Equation (7)), time t is used as the X-axis and $\ln \left({\mathrm{Q}}_{\mathrm{e}}-{\mathrm{Q}}_{\mathrm{t}}\right)$ as the Y-axis for plotting. A linear fit was performed on the data, with the intercept representing ${\mathrm{lnQ}}_{\mathrm{e}}$ and the slope representing ${\mathrm{K}}_1$. Among them, ${\mathrm{Q}}_{\mathrm{e}}$ is the equilibrium adsorption capacity of MAC (mg/g); ${\mathrm{Q}}_{\mathrm{t}}$ is the adsorption capacity of MAC at adsorption time t (mg/g); ${\mathrm{K}}_1$ is the pseudo-first-order equilibrium constant (1/min).

(2)

pseudo-second-order models

$$\frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{K}}_2{\mathrm{Q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{e}}}$$

(8)

When fitting with pseudo-second-order models (Equation (8)), time t is used as the X-axis and $\frac{\mathrm{t}}{{\mathrm{Q}}_{\mathrm{t}}}$ as the Y-axis for plotting. A linear fit was performed on the data, with the intercept representing $\frac{1}{{\mathrm{K}}_2{\mathrm{Q}}_{\mathrm{e}}^2}$ and the slope representing $\frac{1}{{\mathrm{Q}}_{\mathrm{e}}}$. ${\mathrm{K}}_2$ is the pseudo-first-order equilibrium constant (1/min).

2.7. Adsorption Thermodynamics

The Gibbs free energy $∆{\mathrm{G}}^0$ is calculated by Equations (9) and (10).

$$∆{\mathrm{G}}^0=-{\mathrm{RTLnK}}_{\mathrm{c}}$$

(9)

$${\mathrm{K}}_{\mathrm{c}}=\frac{{\mathrm{C}}_{\mathrm{B}}}{{\mathrm{C}}_{\mathrm{A}}}$$

(10)

Among them, R is the gas constant (8.314 J/(mol K)); T is the thermodynamic temperature (K); Kc is a constant at the adsorption equilibrium state in the Langmuir model, which can be calculated by Equation (9). ${\mathrm{C}}_{\mathrm{B}}$ and ${\mathrm{C}}_{\mathrm{A}}$ are the concentration of heavy metal ions adsorbed by MAC in equilibrium and the concentration of remaining heavy metal ions in the solution, respectively. The standard enthalpy (ΔH⁰) and standard entropy (ΔS⁰) of the adsorption process can be obtained from Equations (11) and (12).

$${\mathrm{lnK}}_{\mathrm{c}}=\frac{ ∆{\mathrm{S}}^0}{\mathrm{R}}-\frac{∆{\mathrm{H}}^0}{\mathrm{RT}}$$

(11)

$$∆{\mathrm{G}}^0=∆{\mathrm{H}}^0-\mathrm{T}∆{\mathrm{S}}^0$$

(12)

The thermodynamic temperature T is used as the x-axis and $∆{\mathrm{G}}^0$ as the y-axis. The data are linearly fitted, with intercept representing $∆{\mathrm{H}}^0$ and slope representing $-∆{\mathrm{S}}^0$.

2.8. MAC Adsorption–Desorption Cycle

About 0.2 g of MAC was put into a solution containing heavy metal ions in a volume of 50 mL. In the solution at a pH of 5, the Pb ion concentration was 50 mg/L, and that of Cd was 25 mg/L. After adsorption for 2 h, the MAC was separated from the wastewater solution by filtration. The adsorbed MAC was placed into 50 mL of HCl solution with a concentration of 1 mol/L. After shaking for 6 h, the mixture was separated by filtration. The MAC was used for the next cycle after repeated washing with deionized water. The above experiment was repeated four times. The Pb and Cd ion concentrations in the filtrate were measured to determine the adsorption capacity of MAC.

3. Results and Discussion

3.1. Characteristics of Raw Materials

The two raw materials were crushed and sieved to below 74 μm, and the proximate and elemental analyses of the samples are shown in

Table 1. Proximate analysis and elemental analysis of experimental raw materials.

Sample	Proximate Analysis/wt. %				Ultimate Analysis/wt. %
	Mad	Aad	Vad	FCad	Cdaf	Hdaf	O *	Ndaf	Sdaf
LC	10.91	12.98	35.47	40.64	53.24	4.15	40.96	1.12	0.53
PL	5.76	9.85	70.27	14.12	41.45	5.4	51.64	1.39	0.12

*: Calculated by difference; ad: Air-dried; daf: Dry ash-free.

. The ultimate analysis shows that the H content of poplar leaves is 5.40%, which is higher than that of lignite (4.15%). In the co-pyrolysis process of poplar leaves and lignite, the hydrogen-containing radicals produced by poplar leaves can be used as hydrogen donors to promote the pyrolysis of lignite.

Figure 1a shows that the inorganic minerals in lignite consist mainly of SiO₂ and some alkaline earth metal phosphates. SiO₂ is an acidic oxide and will not react with acids such as HCl and HNO₃. HF and SiO₂ can interact to form SiF₄; therefore, when the pyrolysis char needs to be acid-eluted to remove ash, HCl-HF two-stage pickling is selected. In Figure 1b, poplar leaves contain a large amount of calcium oxalate (CaC₂O₄(H₂O)), which is an alkaline earth metal compound. Theoretically, the presence of a large number of alkaline earth metals in poplar leaves could promote the pyrolysis of lignite [27,28].

3.2. Characterization of MAC

SEM images and EDS analysis of MAC are shown in Figure 2. The surface of the MAC is rough and exhibits some cavities of various sizes and shapes. This may play an important role in the adsorption of heavy metal ions [29]. In Figure 2b, the elements of C, O, and Fe in the MAC are uniformly distributed, indicating that the magnetic material Fe₃O₄ is uniformly distributed in the MAC. Figure 3a,b show the nitrogen adsorption–desorption curves and pore size distributions of MAC. The nitrogen adsorption–desorption isotherm of MAC is type I, indicating that the pores in the material are mainly micropores [30], and the adsorption behavior can be carried out at a relatively low relative pressure, as shown in Figure 3a.

Table 2. Structural parameters of MAC.

Sample	SBET (m²/g)	Vmicro (cm3/g)	Vmeso (cm3/g)	Vtotal (cm3/g)
MAC	805.86	0.23	0.07	0.3

shows the detailed structural parameters of the MAC. The specific surface area of MAC is 805.86 m²/g; the micropore volume and the mesopore volume are 0.23 and 0.07 cm³/g, respectively. Solutions at different pH values were prepared using HCl and NaOH. The MAC was put into the solution with pH values of 2~10, and the mixture was stirred for 10 h. The pH value in solution with no change after contacting with MAC is the pH_pzc of MAC [31]. The experimental results are shown in Supplementary Materials (Figure S1); the pH_pzc value of MAC was finally determined to be 5.8. When pH_solution > pH_pzc, the surface of the MAC exhibits a negative charge; when pH_solution < pH_pzc, the surface of the MAC exhibits a positive charge.

The characterization results of magnetic activated carbon are shown in Figure 4. Figure 4a shows the hysteresis loops of MAC. The saturation magnetization of the MAC reaches 13.83 emu/g, and the MAC can be separated from the wastewater solution by magnetic separation after adsorption is complete. In

Table 3. Magnetic parameters of MAC.

Sample	Ms (emu/g)	Hc (Oe)	Mr (emu/g)	Mr/Ms (%)
MAC	13.83	24.58	0.14	0.01

, the coercive force Hc and the remanence Mr of the MAC are both small, and the ratio Mr/Ms of the residual magnetization to the saturation magnetization is 0.01, which is much smaller than 0.25; therefore, the product MAC is classified as a superparamagnetic substance [32]. Figure 4b shows the XRD patterns of MAC. The results show that MAC exhibits obvious peaks of Fe₃O₄, which is a crystalline oxide with a cubic structure of an inverse spinel type, and the peaks observed at 2θ of 18.40~74.00°. This indicated that Fe₃O₄ exists in the MACs. In Figure 4c, the peaks located at 3400, 2950, 1770, and 1108 cm⁻¹ in the spectrum are attributable to the stretching vibrations of O-H, C-H, C=O, and C-O-C, respectively. The O-H bonds and C=O bonds can be attributed to the presence of hydroxyl and carboxyl functional groups in MAC. This feature is of great significance for the adsorption of heavy metal ions on the surface of MAC [33]. When adding the magnetizer FeCl₃·6H₂O, the MAC appeared at a new peak at 576 cm⁻¹. This peak is attributable to the stretching vibration of the Fe-O, which also indicates that the Fe₃O₄ exists in the MAC; therefore, the addition of the magnetizer makes the MAC have higher magnetic properties, and the MAC can be separated from the liquid phase by the magnet after the adsorption is completed.

3.3. Adsorption Performance

3.3.1. Effect of Solution pH Value

Figure 5 shows the effect of solution pH values on the adsorption capacity of MAC under the conditions of adsorption time of 120 min, MAC dosage of 4 g/L, Pb ion concentration of 50 mg/L, Cd ion concentration of 25 mg/L, and temperature of 30 °C. In Figure 5, when the pH value is 2, the removal effects of Pb and Cd ions exhibit a lower value; the adsorption efficiency is 28.00 and 20.00%, respectively. With the gradual increase in pH value, the adsorption efficiency and adsorption capacity of MAC on the two ions increased significantly. When the pH value was 4, the adsorption effect of MAC on Pb ions experienced an optimal performance, the adsorption capacity was 10.52 mg/g, and adsorption efficiency was 84.20%. When the pH value was 5, the adsorption of Cd ions by MAC reached a high level, the adsorption capacity was 4.93 mg/g, and the adsorption efficiency was 78.80%. This is mainly because when the pH value is at a low level, the solution contains a higher concentration of H⁺, which adsorbs on the surface of the MAC, occupies the adsorption site, and competes with Pb and Cd ions for adsorption. At the same time, the adsorption of H⁺ makes the functional groups on the surface of the MAC positively charged and mutually repels the positively charged Pb and Cd ions, which is not conducive to the adsorption [34]. With the increase in pH value, the concentration of H⁺ in the solution decreased greatly, and the competitive adsorption of Pb and Cd ions by H⁺ was weakened. At the same time, the positive charge on the surface of the MAC gradually decreased. The MAC surface exhibited negative charges after the pH value was higher than the pH_pzc of MAC. The repulsion between MAC and Pb (Cd) ions was reduced and gradually turned into electrostatic attraction, which is conducive to the adsorption of Pb and Cd ions on the surface of MAC.

The pH value in the solution further increased, the adsorption efficiency of the two ions exhibited similar profiles, and the MAC had reached the adsorption equilibrium at this time. When the pH value reached 7, the adsorption efficiency of Pb and Cd ions increased again. This is mainly due to the reaction of the two ions with OH^- in the solution, resulting in the precipitation of Pb(OH)₂ and Cd(OH)₂, resulting in a decrease in the measured ion concentration in the solution; therefore, the Pb²⁺ and Cd²⁺ in the wastewater were adsorbed by MAC, and the optimum pH range was 5~6. At this time, the adsorption efficiency of both ions experienced a higher value.

3.3.2. Effect of Adsorption Time

Figure 6 shows the effect of adsorption time on the adsorption capacity of MAC under the conditions of solution pH value of 5, MAC dosage of 4 g/L, Pb ion concentration of 50 mg/L, Cd ion concentration of 25 mg/L, and temperature of 30 °C. It can be seen from Figure 6a that for Pb ions, within 60 min of adsorption time, the adsorption efficiency and adsorption capacity increased significantly; the adsorption efficiency reached 74.00%, and the adsorption capacity increased to 9.25 mg/g. During the adsorption process from 60 to 120 min, the adsorption efficiency and adsorption capacity continued to increase, but the growth rate was relatively slow, the adsorption efficiency increased to 84.40%, and the adsorption capacity increased to 10.55 mg/g. In Figure 6b, the adsorption of Cd ions showed a similar trend to that of Pb ions. Within 80 min, the adsorption efficiency increased from 16.00 to 77.20%, and the adsorption capacity increased from 1.00 to 4.83 mg/g. This is mainly due to the fact that in the early stage of adsorption, the concentration of heavy metal ions in the solution is relatively high, which shows a large difference with the ion concentration on the surface of the MAC. The potential difference is large, which can offset the mass transfer resistance at the solid–liquid interface [35]. At the same time, the number of active sites on the MAC surface experienced a higher value, and the adsorption process was faster, which was favorable for the adsorption. With the gradual progress of adsorption, the adsorption efficiency of heavy metals by MAC gradually slowed down and finally reached adsorption saturation. Pb and Cd ions achieved higher adsorption efficiency and adsorption capacity at 120 and 80 min, respectively; therefore, 120 min for the adsorption reaction is the optimal time.

3.3.3. The Effect of MAC Dosage

Figure 7 shows the effect of increasing the dosage of MAC from 1 to 7 g/L on the adsorption effect. Other conditions in the experiment were that the pH value of the solution was 5, the adsorption time was 120 min, the Pb ion concentration was 50 mg/L, the Cd ion concentration was 25 mg/L, and the temperature was 30 °C. In Figure 7, the adsorption efficiency of MAC for Pb and Cd in wastewater increased with the increase in MAC dosage, and it maintains a relatively stable state after reaching a certain level. When the dosage of MAC was 7 g/L, the adsorption efficiency of Pb and Cd showed the maximum values, which were 97.03 and 88.80%, respectively; however, with the increase in dosage, the adsorption capacity of MAC on the two ions gradually decreased, the adsorption capacity of Pb ion decreased from 11.40 to 6.93 mg/g, and that of Cd ion decreased from 6.20 to 3.17 mg/g. This is because, with the increase in the dosage of MAC during the adsorption process, the adsorption sites also increase correspondingly, which can combine with more Pb and Cd ions, resulting in a higher adsorption efficiency; however, the number of heavy metal ions in the solution is limited, not all adsorption sites can be combined with ions, and cannot all reach saturation [36]. The increase in the dosage of MAC will increase the cost of wastewater treatment, and it will not be able to realize its rational and effective utilization, reducing the economic feasibility of practical application of wastewater treatment; therefore, the determination of the dosage is crucial for the adsorption process. When the dosage of MAC was 3.5 and 4 g/L, respectively, the adsorption efficiency for Pb and Cd ions both experienced a higher level while maintaining a high adsorption capacity, which could not only meet the needs of wastewater treatment but also not cause a waste of resources.

3.3.4. The Effect of Initial Concentration and Temperature

Figure 8 shows the effects of concentrations of Pb and Cd ion and adsorption temperature on the adsorption capacity of MAC when the pH of the solution is 5, the adsorption time is 120 min, and the dosage of MAC is 4 g/L. In Figure 8, with the increase in ion concentration in the solution, the adsorption capacity of MAC for both ions increased at the same temperature. When the adsorption temperature was 30 °C, the initial concentration in the solution was 150 mg/L of Pb ion and 75 mg/L of Cd ion, the adsorption capacity of Pb was 11.61 mg/g, and that of Cd was 5.95 mg/g. At this time, the adsorption efficiency of the two ions in the wastewater experienced an opposite trend. The adsorption efficiency of Pb was 30.93% and that of Cd was 31.73%. This is mainly because with the increase in the initial concentration of Pb and Cd ions, the opportunity for the two ions to contact the active sites on the MAC surface increases, and the adsorption capacity increases [37]; however, when the dosage of MAC is determined, the number of active sites is also limited. When the adsorption sites were gradually occupied by Pb and Cd ions and reached a saturated state, the MAC could not combine with more ions; therefore, the higher the ion concentration in the liquid phase after the adsorption is completed, the adsorption efficiency gradually decreases.

3.4. Analysis of Adsorption Mechanism

3.4.1. Adsorption Isotherm

In order to understand the adsorption mechanism and distribution of Pb and Cd ions on MAC, the Langmuir model and the Freundlich model were used to simulate the adsorption isotherm of the experimental data at 20, 30, and 40 °C, respectively. The simulation results of the Langmuir model and the Freundlich model for the adsorption process are shown in Figure 9 and Figure 10, and the fitting parameters are listed in

Table 4. Pb ion adsorption isotherm model fitting parameters.

Temp./°C	Langmuir Model				Freundlich Model
	Qm (mg/g)	Kl	R2	RMSE	Kf	1/n	R2	RMSE
20	10.65	0.44	0.9992	0.0139	7.17	0.09	0.848	0.0296
30	11.85	0.45	0.9995	0.0145	8.66	0.07	0.8357	0.0296
40	12.50	0.51	0.9995	0.0136	9.01	0.07	0.7964	0.0353

and

Table 5. Fitting parameters of adsorption isotherm model for Cd ion.

Temp./°C	Langmuir Model				Freundlich Model
	Qm (mg/g)	Kl	R2	RMSE	Kf	1/n	R2	RMSE
20	5.76	0.4	0.9989	0.0261	3.5	0.12	0.8416	0.0403
30	6.25	0.43	0.9987	0.0263	3.79	0.13	0.8532	0.0433
40	7.06	0.45	0.9991	0.0177	3.95	0.15	0.9054	0.0433

. The Langmuir model exhibits a larger R² value and a smaller RMSE value, indicating that the Langmuir model is more consistent with the adsorption process of Pb and Cd ions by MAC, which can be attributed to the monolayer adsorption. Among the fitting parameters based on the Langmuir model, the Q_m values of Pb ions are 10.65, 11.85, and 12.50 mg/g, respectively, and that of Cd ions are 5.76, 6.25, and 7.06 mg/g at 20, 30 and 40 °C, respectively, which are close to the experimental data. The maximum adsorption capacity increased with the increase in adsorption temperature, indicating that the adsorption of Pb and Cd ions by MAC was an endothermic reaction, and the maximum adsorption capacity of Pb was higher than that of Cd. According to Equation (5), the separation factor ${\mathrm{R}}_{\mathrm{l}}$ value of Pb ion is 0.02~0.18, and the separation factor ${\mathrm{R}}_{\mathrm{l}}$ value of Cd ion is 0.03~0.32 under the condition of 30 °C, which are all less than 1, and under the other temperature conditions, the obtained separation factor ${\mathrm{R}}_{\mathrm{l}}$ value is also less than 1. This indicates that the adsorption process of Pb and Cd ions by MAC is favorable and easy to occur [38].

In order to further evaluate the adsorption effect of the prepared MAC on Pb and Cd ions, the adsorption effect was compared with the reported adsorbents in

Table 6. Comparison of Cd and Pb ions removal capacity for MAC to other adsorbent materials cited in the literature.

Adsorbent	Pb (mg/g)	Cd (mg/g)	Refs
AC prepared by Salix matsudana carbon	58.82	40.98	[39]
Mangosteen shell	-	3.15	[40]
AC modified with tartrazine	25.50	13.20	[41]
AC prepared by hazelnut husks	13.05	6.65	[42]
Commercial AC	16.84	17.23	[43]
AC prepared by chicken feather	24.41	7.84	[44]
AC prepared by bamboo	0.67	0.19	[45]
AC prepared by pinus barks	-	10.83	[46]
MAC prepared by lignite with poplar leaves	12.50	7.06	This study

. The results show that the adsorption effect of the prepared MAC on Pb and Cd ions is comparable to other adsorbents.

3.4.2. Adsorption Kinetics

Under the conditions that the pH value of the solution is 5, the dosage of MAC is 4 g/L, the concentration of Pb ion is 50 mg/L, the concentration of Cd ion is 25 mg/L, and the adsorption temperature is 30 °C, the experimental adsorption data of Pb and Cd ions by MAC under different adsorption time were analyzed by using pseudo-first-order and pseudo-second-order models. Additionally, the relationship between adsorption time and adsorption capacity was also studied. The simulation results of the pseudo-first-order and pseudo-second-order models for the adsorption process are shown in Figure 11 and Figure 12 and

Table 7. Fitting parameters of adsorption kinetic model.

Heavy Metal Ion	Exp. Value	Pseudo-First-Order Model				Pseudo-Second-Order Model
	Qe,exp	K1	Qe,cal	R2	RMSE	K2	Qe,cal	R2	RMSE
	(mg/g)	(min−1)	(mg/g)			g/(mg·min)	(mg/g)
Pb2+	10.7	0.03	7.63	0.9577	0.3321	0.01	11.57	0.9989	0.2268
Cd2+	5.1	0.02	3.57	0.9556	0.3018	0.01	5.61	0.998	0.2352

. When MAC was used to adsorb Pb and Cd ions, the R² from the pseudo-first-order model were 0.9577 and 0.9556, respectively, and that of the pseudo-second-order model were 0.9989 and 0.9980, respectively.

At the same time, the RMSE value of the pseudo-second-order model (0.2268,0.2352) is smaller than that of the pseudo-first-order model (0.3321,0.3018), which indicates that the pseudo-second-order model can better fit the experimental data. Using the pseudo-second-order model, the theoretical adsorption capacities of Pb and Cd ions by MAC were calculated to be 11.57 and 5.61 mg/g, respectively, which were close to the experimental values. The results show that the pseudo-second-order model can better describe the adsorption process of MAC for Pb and Cd ions, and the adsorption process is mainly chemical adsorption [47].

3.4.3. Adsorption Thermodynamics

In order to further explain the adsorption process of MAC for heavy metal ions, under the conditions of pH value of 5, MAC dosage of 4 g/L, Pb ion concentration of 50 mg/L, Cd ion concentration of 25 mg/L, and adsorption time of 120 min. The adsorption process of Pb and Cd ions was thermodynamically analyzed by MAC at different temperatures. The fitting curve is shown in Figure 13, and

Table 8. Thermodynamic parameters of MAC adsorption of heavy metal ions.

Heavy Metal Ion	Temp.	∆G0	∆H0	∆S0	R2
	(K)	(KJ/mol)	(KJ/mol)	(J/(mol·K))
Pb2+	293	−3.38	23.12	90.4	0.9997
	303	−4.25
	313	−5.18
Cd2+	293	−2.65	16.33	64.8	0.9999
	303	−3.31
	313	−3.95

shows the thermodynamic parameters. For the adsorption process of Pb ions by MAC, $∆{\mathrm{G}}^0$ at three temperatures is −3.38, −4.25, −5.18 KJ/mol, respectively, and $∆{\mathrm{G}}^0$ in the adsorption process of Cd ions is −2.65, −3.31, −3.95 KJ/mol. The $∆{\mathrm{G}}^0$ values at different temperatures were all less than 0, indicating that the adsorption of Pb and Cd ions by MAC was a spontaneous process. As the temperature increased from 20 to 40 °C, the $∆{\mathrm{G}}^0$ value gradually decreased, and the spontaneity increased. The standard enthalpy values of Pb and Cd ions in the adsorption process are 23.12 and 16.33 KJ/mol, respectively, which are all greater than 0, indicating that the adsorption process of MAC was endothermic [48,49]. Likewise, related studies have brought similar conclusions that the adsorption of Pb and Cd ions by AC is endothermic with accompanying chemical reactions [39]. The specific adsorption mechanism is shown in Figure 14.

3.5. MAC Adsorption–Desorption Cycle

Studying the cyclic adsorption of MAC is helpful in analyzing the reuse characteristics of MAC and the recovery of heavy metal resources. The adsorption–desorption cycling experiments of Pb and Cd ions by MAC are shown in

Table 9. Four adsorption–desorption cycles of MAC.

Capacity	Cycles	Pb (mg/g)	Cd (mg/g)
Adsorption/Desorption	1	10.00/9.65	4.93/4.77
	2	9.53/9.21	4.68/4.51
	3	9.05/8.67	4.31/4.10
	4	8.49/7.94	4.04/3.77

. The adsorption capacity of MAC for Pb ions decreased from 10.00 to 8.49 mg/g, and that of Cd ions decreased from 4.93 to 4.04 mg/g. After four cycles, the adsorption capacities for Pb and Cd ions still retained 84.90 and 81.95%, respectively; therefore, MAC exhibits good reproducibility and can be used to repeatedly adsorb Pb and Cd ions. The reuse of MAC and the recovery of heavy metals are achievable, which can effectively reduce the investment cost of the adsorbent.

4. Conclusions

The MAC was prepared from co-pyrolysis of lignite and poplar leaves. Its nitrogen adsorption–desorption isotherm is determined via the Langmuir model, indicating that the material is dominated by micropores with a specific surface area of 805.86 m²/g. The MAC has abundant porous structures, which is an important condition for MAC to have a better adsorption effect. The saturation magnetization (Ms) of MAC was 13.83 emu/g, and the MAC could be separated from the liquid phase by a magnet after the adsorption was completed. The pH value of the solution, the adsorption time, the addition amount of MAC, the initial concentration of the ionic solution, and the adsorption temperature all had a certain effect on the adsorption efficiency of Pb and Cd ions by MAC. The adsorption of Pb and Cd ions by MAC conforms to the Langmuir model, which is a monolayer adsorption, and the maximum monolayer adsorption capacity is comparable to the adsorption effect of other adsorbents in the literature. Additionally, the adsorption process of Pb and Cd ions by MAC is mainly chemisorption, which can be better described by a pseudo-second-order model. The adsorption of Pb and Cd ions by MAC is a spontaneous reaction, and the higher the temperature, the stronger the spontaneity. MAC showed good reproducibility, and the adsorption capacities of Pb and Cd ions remained at 84.90 and 81.95% after four cycles, respectively.