Synthesis and Application for Pb²⁺ Removal of a Novel Magnetic Biochar Embedded with Fe_xO_y Nanoparticles

Abstract

Biochar (BC) is a widely studied economic and environment-friendly material. However, its application is limited by its underdeveloped pore structure, small specific surface area, low degree of graphitization, and difficulty in being separated from liquids during the application process. Raw cotton contains almost 100% cellulose and has a high yield for preparing biochar. A novel type of magnetic biochar composite was prepared by the impregnation–calcination method using cotton and iron nitrate nonahydrate(Fe(NO₃)₃·9H₂O). The material was characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TG), and specific surface area testing (BET). The results show that Fe_xO_y nanoparticles with a uniform morphology and an average particle size of less than 20 nm are embedded in the composite; the saturation magnetization strength of the composite material reaches 21.6 emu/g; and compared to the original biochar, the composite material has a larger specific surface area (326 m²/g). As an adsorbent, the composite material has a fast removal rate for Pb²⁺ of 95% in 50 min. The Langmuir model calculation results show that the maximum adsorption capacity of the composite for Pb²⁺ is 252.7 mg/g. Fe_xO_y-BC easily achieves solid–liquid separation and can be recycled for Pb²⁺ wastewater treatment through adsorption–desorption–regeneration.

1. Introduction

Heavy metal pollution in soil and water poses a serious threat to ecosystems and human health. At present, heavy metal treatment technologies for wastewater mainly include chemical precipitation [1], membrane separation [2], electrochemical treatment [3], coagulation [4], biodegradation [5], solvent extraction [6], and adsorption methods [7,8]. Among them, adsorption technology is recognized as one of the most promising environmental remediation methods due to its advantages of low cost, easy operation, high efficiency, and application of renewable materials [9,10,11,12].

In the adsorption material system, biochar (BC) has become a research hotspot due to its easy availability of raw materials, structural stability, and efficient adsorption performance. Biochar is an aromatic solid product formed by the limited oxygen pyrolysis and carbonization of biomass. Its structural characteristics are characterized by a developed multi-stage pore system, high specific surface area, and abundant surface functional groups (carboxyl, carbonyl, phenolic hydroxyl, etc.) [13]. It can capture heavy metal ions effectively through multiple mechanisms, such as ion exchange, chemical precipitation, surface complexation, and physical adsorption [14]. However, the low density and micrometer particle size characteristics of the material severely limit its further application due to its solid–liquid separation efficiency. To this end, the researchers have proposed a magnetic functionalization modification strategy: loading magnetic components (such as Fe₃O₄) onto a biochar matrix to construct composite materials, and using an external magnetic field to achieve rapid solid–liquid separation, which can significantly improve the recyclability of the material [15].

The current preparation technology of magnetic biochar mainly includes chemical methods (such as co-precipitation [16] and hydrothermal synthesis [17]) and physical methods (such as mechanical ball milling [18,19]). Zhang et al. used the co-precipitation method to load nano-Fe₃O₄ onto rice husk biochar under oxygen limited conditions at 400 °C, resulting in a composite material with a specific surface area of 101.9 m²/g [20]. The result shows that the BC-Fe composite material can effectively reduce the bioavailability of cadmium in soil, inhibit the absorption and transport of cadmium by rice, and provide a theoretical basis for the remediation of heavy metal passivation in farmland.

In response to the demand for the collaborative treatment of heavy metal pollution, Liu et al. developed a composite adsorbent based on palm fiber-derived graphene biochar (GB) loaded with nano zerovalent iron (nZVI) [21]. The material was prepared by the liquid-phase reduction method (pyrolysis temperature: 700 °C), achieving the synchronous removal of Cd (II)-As (III): the adsorption capacity of As (III) in an acidic environment (pH = 4) reached 363 mg/g and the removal efficiency of Cd (II) in neutral conditions (pH = 7) was 92.8 mg/g. It is worth noting that this system overcomes the technical bottleneck of traditional zerovalent iron being prone to oxidation. Recently, Peng et al. designed Fe₃O₄-modified corn stover biochar, which achieved a maximum lead adsorption capacity of 113.70 mg/g through the co-precipitation pyrolysis coupling process, demonstrating its excellent cationic heavy metal fixation ability [22].

Although the abovementioned modification strategies significantly improve the adsorption performance of biochar, its application still faces multiple constraints: firstly, the preparation process relies on highly corrosive precursors (such as concentrated hydrochloric acid) and toxic reducing agents (such as NaBH₄), which poses a risk of secondary environmental pollution. Secondly, the multi-step synthesis process involves complex operations, such as high-temperature carbonization, liquid-phase reduction, and surface modification, resulting in high material production costs. In addition, the insufficient bonding strength at heterogeneous interfaces may lead to the leaching of active components. Therefore, the development of a new magnetic biochar material system that combines efficient adsorption performance, environment-friendly characteristics, and simple preparation processes remains a key scientific problem that urgently needs to be addressed in the field of environmental functional materials. It needs to solve fundamental problems, such as green synthesis process development, interface stability enhancement, and full lifecycle cost control.

Lead (Pb), as a typical heavy metal pollutant, poses great harm to human health, and low concentrations can also cause irreversible organic damage to the kidney and liver organs [23,24,25]. In aquatic ecosystems, the high toxicity and difficult degradation characteristics of lead, combined with its bioaccumulation effect, make its efficient removal one of the core challenges in the field of environmental pollution control [26].

In view of the abovementioned issues and challenges, the purpose of this work is as follows:

(i)

Develop new biochar composites. To our knowledge, cotton, as one of the abundant, low-cost, and easily accessible raw materials, has not been reported for the synthesis of magnetic composites. Meanwhile, the cellulose content of cotton is almost 100%, much higher than other wood materials. Under the same quality conditions, the yield of biochar prepared from cotton is higher.

(ii)

Explore simple synthesis methods for magnetic composites. BC-Fe_xO_y was prepared by impregnation calcination using cotton and Fe(NO₃)₃·9H₂O as precursors without the addition of other auxiliary reagents. The process is simple and easy. Characterization shows that Fe_xO_y nanoparticles are embedded in BC, indicating the structural stability of the composite.

(iii)

Investigate the application of composites. Taking the removal of Pd (II) from wastewater as an example, this study explores the adsorption capacity and recycling characteristics of the composite material, proving that BC-Fe_xO_y is an efficient and low-cost new magnetic adsorption composite.

2. Materials and Methods

2.1. Synthesis of Materials

In order to avoid environmental pollution caused by waste, the cotton used in the experiment was sourced from a school’s waste collection station. Although raw cotton is indeed a high-cost commodity, utilizing waste cotton can significantly reduce material costs and environmental burdens. The waste cotton, which was cleaned, shredded, and dried, primarily consisted of cellulose and associated organic matter, comprising approximately 98% of its weight, with an ash content of approximately 2%. The main constituents of ash are sodium, magnesium, and calcium salts. The chemicals used in the experiments, including ethanol (95%), NaOH, CH₃COOH, HCl, Fe(NO₃)₃, and Pb(NO₃)₂, were analytically pure and obtained from Aladdin Reagent (Shanghai, China) Co., Ltd. Each experiment used 18.2 MΩ·cm⁻¹ of deionized water. The main process involved weighing 6.4368 g of cotton, compacting it into a crucible made of quartz (SJM-40, Hanhui Technology Co., Ltd., Shenzhen, China), covering it, and then placing it in a muffle furnace for calcination (JC-MF10-16, Juchuang Environmental Protection Group Co., Ltd., Qingdao, China). The starting temperature was set to be 30 °C. The heating rate was increased by 5 °C per min, and the heating lasted for 110 min. The insulation was maintained at 600 °C for 60 min. The final mass of the biochar was 2.6391 g, which was about 41% of the initial mass, higher than the yield of biochar prepared from rice and corn straw [27].

For the synthesis of magnetic biochar, in a typical experiment, 0.4046 g of Fe(NO₃)₃·9H₂O is weighed and put into a small beaker. Then, 5 mL of deionized water was slowly added with ultrasonic assistance for dissolution. Next, 1.200 g of biochar was placed in another beaker. The Fe(NO₃)₃ solution was then applied dropwise onto the biochar, and subsequently, 2 mL of ultrapure water was added. During this time, a glass rod was used to stir the mixture to guarantee complete impregnation. It was then left to stand at ambient temperature for approximately 2 h. Subsequently, it was dried in an oven at 120 °C. The dried precursor was placed in a crucible and calcined in a muffle furnace for the calcination process. The starting temperature was 30 °C. The heating rate was set to be 5 °C/min, until the temperature reached to 550 °C, while keeping the insulation duration for 60 min. A slightly brownish–black flocculent solid (BC-Fe_xO_y) was obtained. The synthetic process is illustrated in Figure 1.

The following are the chemical equations that are involved in the whole preparation process:

$$\mathrm{Waste}\mathrm{cotton}(\mathrm{cellulose})\underset{\mathrm{oxygen}\mathrm{deficit}}{\overset{>500°\mathrm{C}}{\to }}\mathrm{Biochar}$$

$$2\mathrm{F}\mathrm{e}{\left({\mathrm{N}\mathrm{O}}_3\right)}_3·9{\mathrm{H}}_2\mathrm{O}\stackrel{<300°\mathrm{C}}{======}6{\mathrm{H}\mathrm{N}\mathrm{O}}_3+2\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3$$

$$\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3\stackrel{\Delta }{======}{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}$$

$$4\mathrm{F}\mathrm{e}{\left({\mathrm{N}\mathrm{O}}_3\right)}_3\stackrel{>300°\mathrm{C}}{======}2{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3+12{\mathrm{N}\mathrm{O}}_2+3{\mathrm{O}}_2$$

$$6{\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{C}\stackrel{\mathrm{high}\mathrm{temperature}}{=========}4{\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4+{\mathrm{C}\mathrm{O}}_2$$

2.2. Method of Analysis

SEM equipped with an energy spectrometer (TESCAN S8000, Tescan Trading (Shanghai) Co., Ltd., Prague, Czech Republic) and TEM (FEI—TALOS—F200X, Thermo Fisher Scientific (China) Co., Ltd., Shanghai, China) were used to observe the surface morphology. The phase structure was analyzed using XRD (D8 ADVANCE X, Bruker Corporation, Karlsruhe, Germany) with Cu-Kα radiation (λ = 0.154060 nm) at a scanning rate of 0.05 s⁻¹ in the 2θ range from 10° to 80°. The chemical state and mass of the elements were characterized using XPS Escalab 250xi, Thermo Fisher Scientific, Massachusetts, WA, USA). Raman spectroscopy (Raman DXR, Thermo Fisher Scientific, Massachusetts, WA, USA), with a radiation wavelength of 785 nm, was used to analyze the molecular structure and properties.

The mass ratio of the biomass charcoal obtained after pyrolysis to the original biomass raw materials was defined as the yield of biomass charcoal. For BC and BC-Fe_xO_y, a TG analyzer (TGA101, Guangdong Kexiao Instrument Co., Ltd., Guangzhou, China) was used to determine the carbon and Fe_xO_y content. To study the thermal stability of the adsorbents, a thermogravimetric analysis of the two materials was performed in an air atmosphere. The heating rate was 10 °C/min and the temperature range was 50–850 °C. The specific surface area and pore volume were measured using a surface characteristic analyzer (ASAP 2460, McMurdik (Shanghai) Instrument Co., Ltd., Shanghai, China), and a magnetometer (MicroSenseEZ-VSM, MicroSense, LLC, Fremont, CA, USA) was used to test the magnetic field strength.

An atomic absorption spectrometer (TAS-990, Jinpeng Analytical Instrument Co., Ltd., Shanghai, China) was used to detect the concentration of heavy metal ions. FT-IR spectroscopy (Nicolet iS5, Thermo Fisher Scientific, Carlsbad, CA, USA) was employed to test the surface functional groups, and the absorbance of the sample was measured using a KBr tablet.

2.3. Inquiry into Adsorption Capacity

To imitate wastewater containing Pb ions, a lead solution was prepared using lead nitrate. First, an electronic balance was utilized to weigh 500 mg of lead nitrate, which was then diluted in a 1000 mL volumetric flask. Deionized water was added to create a reserve of lead solutions with other concentrations for the adsorption process. In the adsorption experiment, the standard solution of Pb ions should not have an extremely low concentration. If the concentration is too low, it is fully adsorbed by the adsorbent and the calibration curve will become non-straight. This situation may lead to the inaccuracy of the results in general. The adsorption kinetics of Pb²⁺ on BC and BC-Fe_xO_y were studied in 250 mL conical flasks. Some conical flasks contained 100 mL of 200 mg/L Pb²⁺ solution added to them. The pH was adjusted to 5.5 (See Supplementary Materials Figure S1 for the pH effect). A dosage of 0.1 g of BC and BC-Fe_xO_y was added and the conical flasks were sealed. These containers were placed on a 160 rpm constant-temperature oscillator at 25 °C, and 10 mL of suspension was taken at 5, 10, 20, 30, 40, 60, 80, 120, and 240 min and treated through magnetic force. Permanent magnets were placed around the glass tube, and the adsorbed wastewater flowed through the tube at a speed of approximately 10 mL/min. The samples were treated 3–5 times like this. The absorbance of the samples was measured using an atomic absorption spectrometer to determine the concentration of Pb ions after adsorption.

As shown in Equation (1), based on the difference between the initial solution and the equilibrium concentration of Pb²⁺, the adsorption capacity q (mg/g) of Pb²⁺ was calculated in line with a previous study [28].

$$q={\displaystyle \frac{C_0-C_e}{m}}\times V$$

(1)

In the formula, the initial concentration and equilibrium concentration (mg/L) of Pb²⁺ are represented by C₀ and C_e, respectively; the volume of the solution is represented by V (L); and the mass of the added biomass charcoal is represented by m (g).

Equations (2) and (3) are used to process the adsorption data.

The pseudo-first-order equation is:

$$\ln \left(q_e-q_t\right)=\mathrm{l}\mathrm{n}{q}_e-k_1·t$$

(2)

This shows the diffusion-controlled adsorption process.

The chemical adsorption process is illustrated by the pseudo-second-order equation:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2·q_e^2}}+{\displaystyle \frac{1}{q_e}}·t$$

(3)

The quantity of Pb²⁺ adsorbed is represented by q_t (mg/g). The adsorption time is represented by t (min); q_e (mg/g) represents the equilibrium adsorption capacity. The pseudo-first-order kinetic rate constant (min) is represented by k₁. The pseudo-second-order kinetic rate is represented by k₂, in units of g/(mg·min).

2.4. Exploring Isothermal Adsorption Kinetics

Pb²⁺ solutions (100 mL) of different concentrations (100, 150, 200, 250, and 300 mg/L) were poured into several conical flasks. The pH was adjusted to 5.5. Then, 0.1 g of BC and BC-Fe_xO_y were added, and the containers were sealed. They were then placed at 25 °C on a 160 rpm constant-temperature oscillator for 120 min. Subsequently, 10 mL of the suspension was removed and treated with magnetic force. An atomic absorption spectrometer was used to measure the absorbance of the samples and determine the concentration of lead ions after adsorption.

The adsorption process was analyzed by means of Langmuir and Freundlich isotherm models.

The equation for the Langmuir adsorption isotherm model is:

$${\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{C_e}{q_m}}+{\displaystyle \frac{1}{q_m{K}_L}}$$

(4)

The equation for the Freundlich adsorption isotherm model is:

$$\ln q_e=\ln K_F+{\displaystyle \frac{1}{n}}\ln C_e$$

(5)

The equilibrium adsorption capacity is represented by q_e (mg/g); the maximum adsorption capacity is represented by q_m (mg/g); the adsorption equilibrium concentration is represented by C_e (mg/L); and the Langmuir model has a constant K_L (L/mg), which is associated with the affinity of the binding. K_F, which pertains to the adsorption capacity, is the Freundlich model constant (mg¹⁻ⁿ·Lⁿ/g). The Freundlich model has a constant related to the adsorption strength, which is n.

3. Results and Discussion

3.1. Characterization of Biochar and Biochar–Fe_xO_y

SEM analysis of the biochar reveals (as depicted in Figure 2a) that its surface is relatively smooth and flat. EDS analysis indicates that the original biochar primarily consists of carbon, as illustrated in Figure 2b. Following magnetization treatment, numerous regular particles are observed adhering to the surface of the biochar, as shown in Figure 2c. Additionally, Figure 2d demonstrates that the modified biochar contains not only carbon but also iron and oxygen elements.

Selecting a micron-scale region, TEM reveals that the biochar appears sponge-like (Figure 3a). Upon further magnification, as depicted in Figure 3b, the biochar’s surface is devoid of attached particles and features numerous pore structures (blank area). Figure 3c displays the structure of magnetized biochar, markedly distinct from that shown in Figure 3a. Following liquid impregnation and subsequent calcination, nanoparticles were dispersed and embedded onto the biochar’s surface, forming spherical nanoparticles (blank area) with an average size of less than 20 nm (Figure 3d) (refer to Supplementary Materials Figure S2 for a particle size distribution diagram). The morphology of nanoparticles is uniform, with most of them having an elliptical granular structure. Moreover, the formation of nanoparticles did not compromise the pore structure of the biochar.

Further analysis of BC and BC-FexOy was conducted using high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). In Figure 4a, no extensive crystal planes are evident on the biochar, primarily composed of amorphous carbon, with minor regions displaying graphene-like structures. The interplanar spacing of carbon atoms is 0.248 nm (with the lattice constant of graphene being 0.246 nm). Some areas, although graphenized, have obvious defects. Figure 4b displays the HRTEM of BC-FexOy, where the spacing between adjacent lattice fringes indicates the presence of Fe₂O₃ and Fe₃O₄ on the BC’s surface, with lattice fringe spacings of 0.221 nm (320), 0.251 nm (110), and 0.269 nm (104) [29]. Notably, a lattice spacing value of 0.485 nm is attributed to the crystal plane distance of the Fe₃O₄ (111) plane [30]. These results are consistent with the XRD display. Similarly, interspaces are observed in BC-Fe_xO_y. The presence of defects on the adsorbent surface enhances its adsorption capabilities [31]. The SAED patterns clearly demonstrate that the biomass charcoal is not in a monocrystalline form (Figure 4c). The SAED pattern of Fe_xO_y nanoparticles has two clear diffraction rings, corresponding to the (104) and (300) crystal planes. The irregular distribution of other diffraction points indicates that the nanoparticles have a polycrystalline structure (Figure 4d).

3.2. XRD, XPS, and Raman Analysis

XRD is displayed in Figure 5. The broad peak of BC between 15° and 30° indicates that biochar is mainly composed of amorphous carbon, consistent with the results in Figure 4a. After magnetization, the peak intensity of BC at the corresponding position significantly decreases, which is attributed to the surface of biomass charcoal being heavily loaded with iron oxide [32]. At 2θ, values of 24.1°, 33.2°, 35.6°, 40.8°, 49.5°, 54.2°, 62.5°, and 64.1° diffraction peaks corresponding to the (012), (104), (110), (113), (024), (116), (214), and (300) crystal planes of Fe₂O₃ (JCPDS 89-0597) were displayed. Fe₃O₄ (JCPDS 19-0629) shows characteristic peaks of iron oxide at 2θ of 35.6° and 62.5°. The peak appearing at 26.1° originates from the graphene (002) crystal plane. The crystal planes corresponding to typical characteristic peaks are consistent with those shown in Figure 4a,b.

The surface composition and chemical states of major elements of BC and BC-Fe_xO_y were detected by XPS spectroscopy, as shown in Figure 6a (the blue line represents BC-Fe_xO_y, and the red line represents BC). In BC, the atomic content of carbon and oxygen is 83.28% and 15.35%, while in BC-Fe_xO_y, the content of carbon, oxygen, and iron is 73.02%, 21.54%, and 4.08%, respectively. High-resolution C 1s spectra of BC-Fe_xO_y show that C-C, O-C-O, and O-C=O appear at 284.2 eV, 285.5 eV, and 288.2 eV, respectively (Figure 6b). The high-resolution O 1s spectrum of BC-Fe_xO_y shows peaks at 532.8 eV, 531.5 eV, and 529.2 eV, which are attributed to C=O, C-O, and Fe-O (Figure 6c). The corresponding high-resolution O 1s spectrum of BC only shows two peaks: C=O and C-O (inset of Figure 6c). The peak of the high-resolution spectrum of Fe 2p at 709.8 eV binding energy is attributed to the peak at Fe 2p_3/2, 723.1 eV corresponding to the peak at Fe 2p_1/2, as shown by the green line in Figure 6d. After peak fitting using Gaussian and Tougaard type baselines, the red line in Figure 6d represents the characteristic peak of divalent iron, and the blue line represents the characteristic peak of trivalent iron. The peak values of 711.9 eV and 724.7 eV are derived from the Fe 2p_3/2 and Fe 2p_1/2 peaks of iron oxide. Values of 718.1 eV and 728.7 eV are typical Fe 2p_3/2 satellite peaks and Fe 2p_1/2 peaks of divalent iron, while 719.1 eV and 732.5 eV peaks belong to Fe 2p_3/2 and Fe 2p_1/2 satellite peaks of trivalent iron [33]. The XPS results indicate that the nanoparticle elements embedded in BC are mainly C, O, and Fe, with iron exhibiting two valence states: divalent and trivalent.

As illustrated in Figure 7, the two prominent peaks at approximately 1342 cm⁻¹ and 1602 cm⁻¹ are identified as the D and G bands. The D band reflects the characteristics of disordered sp³ carbon with structural defects, suggesting the presence of defects near the material’s edges. The G band arises from hybrid sp² carbon within the biochar structure, significantly influencing the extent of defects and disorder within the carbon layer [34]. The I_D/I_G ratio is commonly employed to characterize the degree of graphitization in carbon materials [35]. The measured I_D/I_G value for BC-Fe_xO_y is 0.94, higher than that of the BC (0.75). The elevated I_D/I_G value suggests that the introduction of Fe_xO_y nanoparticles has enhanced the graphitization of biochar and increased the defects and disorder, thereby activating more active sites within the BC and facilitating the adsorption of Pb²⁺ [36]. Furthermore, the peaks at 150 cm⁻¹, 400 cm⁻¹, and 634 cm⁻¹ are assignable to Fe₂O₃ and Fe₃O₄ [37]. The Raman characterization findings align with the XRD results.

3.3. Physicochemical Properties of BC and BC-Fe_xO_y

Upon heating from 30 °C to 850 °C at a rate of 10 °C/min, the thermal stability and composition of BC and BC-Fe_xO_y were studied by thermogravimetric analysis. As shown in Figure 8a, at low temperatures (usually below 100 °C), the changes in the curve arise from the weight loss of a small amount of water in the sample and the volatilization of adsorbed gases, which are shown as endothermic processes on the DSC chart. As the temperature increased, the biochar began to react with oxygen. Above 370 °C, the reaction intensified, and this exothermic process can be observed in the DSC curve. Due to the decomposition of hydroxyl groups, there were changes in the two curves of biochar and Fe_xO_y at 410 °C and 442 °C. BC and BC-Fe_xO_y were heated above 553 °C and 579 °C, respectively, and the thermogravimetric and DSC curves did not show a decreasing trend, indicating that the biochar in BC and BC-Fe_xO_y was completely removed. The residual substances of BC and BC-Fe_xO_y are 12.5 wt% and 23.8 wt%, respectively, with Fe_xO_y accounting for 11.3 wt% of the total weight of BC-Fe_xO_y.

Figure 8b shows the magnetization curve of BC-Fe_xO_y. The saturation magnetization is 21.6 emu/g. The superparamagnetic behavior and high magnetic saturation strength exhibited by the magnetic biochar at room temperature indicate its susceptibility to external magnetic fields, which can be used to achieve solid–liquid separation and adsorbent recovery.

N₂ adsorption/desorption analysis was carried out to show the specific surface area and pore structure. Typically, a greater specific surface area and pore volume usually lead to better adsorption efficiency. Figure 9a presents the N₂ adsorption/desorption isotherms of BC and BC-Fe_xO_y. BC has a specific surface area of 219 m²/g; when doped with magnetic particles, BC-Fe_xO_y has a specific surface area of 326 m²/g. Nanoparticles of iron oxide possess a large specific surface area (around 66 m²/g), which might result in an increase in the specific surface area of biochar [38]. As can be seen from Figure 9b, the pore volumes of BC and BC-Fe_xO_y are 0.126 cm³/g and 0.232 cm³/g as per order. This may be because, for the same mass, the smaller the nanoparticles are, the larger their specific surface area will be, and the uniformly dispersed magnetic particles hardly block the pore structure on the BC surface [39]. In addition, the figure shows that when P/P₀ > 0.5, the isotherm shows an IV curve with a hysteresis loop, which suggests the existence of mesoporous channels that are advantageous for the adsorption of heavy metal ions.

Table 1. Physical and chemical properties of BC and BC-Fe_xO_y.

Materials	Magnetization Strength (emu/g)	Specific Surface (m2/g)	Pore Volume (m3/g)
BC	-	219	0.126
BC-FexOy	21.6	326	0.232

shows the physicochemical properties of the two materials.

3.4. Exploration of Adsorption Kinetics

The change in the adsorption capacity of the two materials for Pb²⁺ over time is presented in Figure 10, and it can be seen that BC-Fe_xO_y has a much higher adsorption capacity than BC. Pb²⁺ adsorption has two distinct phases: fast and less rapid. In the initial stage (0–50 min), the biochar’s adsorption ability for Pb²⁺ rises steeply, attaining approximately 95% of the saturated adsorption capacity. Subsequently, the adsorption enters a slower stage and attains equilibrium at approximately 120 min. The main reason for the occurrence of the two stages is that, in the initial stage of adsorption, biochar indicates the presence of a large number of holes and oxygen-containing functional groups that rapidly bind to lead ions, leading to a rapid decrease in the lead ion concentration. As the time increased, the number of holes and functional groups that could adsorb lead ions decreased, resulting in a decrease in the adsorption rate [40]. When the adsorption of Pb ions by biochar is saturated, the system reaches an equilibrium state.

The experimental data were linearly fitted with ln(q_e − q_t) against t. Figure 11a shows the fitting results of the pseudo-first-order kinetic models for lead ion adsorption by BC and BC-Fe_xO_y. The curves obtained from the fitting show that the linear regression coefficients R² for the adsorption of the two substances are 0.3939 and 0.4972. Figure 11b shows the t and t/q linear fit curves for the pseudo-second-order kinetic models regarding the adsorption of Pb²⁺ in wastewater by BC and BC-Fe_xO_y, and the linear correlation coefficients R² of this model for the data from the adsorption experiments are 0.967 and 0.997, respectively, indicating that this model can represent the adsorption processes of BC and BC-Fe_xO_y more precisely during the whole adsorption process. It is indicated that chemisorption rather than physisorption controls the adsorption of Pb²⁺ by these two biochar types [41]. The chemisorption process mainly consists of the sharing or swapping of surface functional groups and electrons, which leads to the creation of valence forces for adsorbing metal ions [42]. The relevant parameters of pseudo-first-order and pseudo-second-order kinetics after linear fitting are presented in

Table 2. The relevant parameters of pseudo-first-order kinetics after linear fitting.

Pseudo-First-Order	Intercept		Slope		Statistics
	Value	Standard Error	Value	Standard Error	Adj. R2
BC-FexOy	4.39251	0.27467	−0.00919	0.00308	0.4972
BC	4.88914	0.13414	−0.00384	0.00154	0.39394

and

Table 3. The relevant parameters of pseudo-second-order kinetics after linear fitting.

Pseudo-Second-Order	Intercept		Slope		Statistics
	Value	Standard Error	Value	Standard Error	Adj. R2
BC-FexOy	0.01116	0.00242	0.00148	2.8696 × 10−5	0.99699
BC	0.03695	0.01103	0.00216	1.408 × 10−4	0.96704

. The small standard estimation error (SE) of the utilized model indicates a high fitting accuracy.

3.5. Exploration of Adsorption Isotherm

The adsorption capabilities and affinities of adsorbents for metal ions can be evaluated using the equilibrium adsorption isotherm [39]. According to the experimental findings, at 25 °C, as the concentration of Pb²⁺ increased, the adsorption capacity of both normal and magnetized biomass charcoal increased, whereas the adsorption efficiency decreased (refer to Supplementary Materials Figure S3). The adsorption isotherms show that both materials have strong adsorption capabilities for Pb²⁺.

Linear fits of the Langmuir (Figure 12a) and Freundlich (Figure 12b) adsorption isotherms for Pb²⁺ adsorption in wastewater by biochar and magnetized biochar are shown in Figure 12. Using Equations (4) and (5), the slopes and intercepts were calculated to obtain the crucial parameters for the Langmuir and Freundlich adsorption isotherms, which are presented in detail in

Table 4. Related parameters of the Langmuir adsorption isotherm model and Freundlich adsorption isotherm model.

Materials	Langmuir Model			Freundlich Model
	KL	qm (mg/g)	R2	n	KF	R2
Biochar	0.57	149.8	0.9935	2.72	52.29	0.6266
Biochar–FexOy	0.69	252.7	0.9963	2.89	112.3	0.9063

.

The R² values of BC and BC-Fe_xO_y in the Langmuir models were 0.9935 and 0.9963, respectively, exceeding the R² values of 0.6266 and 0.9063 in their Freundlich models, which implies that the Langmuir model can represent the adsorption process of heavy metal lead ions in wastewater by BC and BC-Fe_xO_y more precisely, with the characteristic of single-layer adsorption. The BC-Fe_xO_y has a maximum adsorption capacity of 252.7 mg/g, which is 1.69 times that of BC (149.8 mg/g). The term 1/n in Equation (5) denotes the adsorbent surface’s heterogeneity and adsorption intensity, 1/n > 1 indicates difficult adsorption, while 1/n < 0.5 signifies ease of adsorption [43]. The 1/n values for BC and BC-FexOy were 0.367 and 0.346, respectively, suggesting both materials effectively adsorbed Pb²⁺.

Additionally, as shown in

Table 5. Comparison of Pb²⁺ adsorption by different modified biochar types.

Biochar Materials	Adsorption Capacity (mg/g)	Refs.
Microbial modification of iron-carrying biochar	113.70	[22]
Alginate-modified rice husk biochar	112.30	[29]
Acid ammonium persulfate oxidize biochar	135.40	[44]
PSB-modified sludge biochar	12.17	[45]
H3PO4-modified chicken feather biochar	24.41	[46]
Chitosan-modified pine biochar	134.00	[47]
α-Fe2O3-modified biochar (Fe2O3/BC)	390.60	[48]
Biochar embedded FexOy nanoparticle composite	252.7	This work

, when comparing the adsorption efficacy of various modified biochar types for Pb²⁺, BC-Fe_xO_y exhibits an advantageous adsorption performance for Pb²⁺. Peng et al. prepared the Fe₃O₄-loaded biochar adsorbent using agricultural waste corn straw [22]. Perhaps due to its larger surface area and more functional groups, BC-Fe_xO_y has a better adsorption performance than Peng’s results. This underscores the significant influence of raw materials, synthesis processes, and methods on the performance of the adsorbents.

3.6. Characterization of FT-IR Spectroscopy

FT-IR spectroscopy can be used to characterize adsorbents and reveal functional groups in the adsorbent structure. This is important for understanding how adsorbents remove metals [49,50]. Figure 13 displays the FT-IR spectra of BC and BC-Fe_xO_y before and after Pb²⁺ adsorption. The two samples possessed comparable functional groups. The -OH groups showed stretching vibrations in the range of 3424–3461 cm⁻¹. In comparison to BC, the functional group location of magnetized biochar has a red shift and its absorption peak is strengthened, which implies an increase in hydroxyl groups during the magnetization process [51]. At 2920 cm⁻¹~2922 cm⁻¹, both BC and BC-Fe_xO_y showed -CH₂ vibrational bands. The stretching vibration peaks of C=O and C=C in the carboxyl, hydroxyl, and lactone groups are observed between 1570 and 1587 cm⁻¹ [52]. The peak of the C-O stretching vibration can be found in the range from 1300 to 1000 cm⁻¹. The C-H bending and C-C bond vibrations are represented in the 900–600 cm⁻¹ range. A new characteristic peak at 569 cm⁻¹ was observed for BC-Fe_xO_y, which was due to the stretching vibration of Fe-O groups. This indicates that, on the surface of the modified biochar, iron oxide forms chelation or bidentate bridge coordination bonds [53].

The shifting of the peaks within the ranges of 1570–1587 cm⁻¹ and 1300–1000 cm⁻¹ after Pb²⁺ adsorption indicates a complexation reaction between Pb²⁺ and oxygen-containing groups on the biochar. For BC and BC-Fe_xO_y, upon Pb²⁺ adsorption, the O-H stretching vibration peak intensity at 3424~3461 cm⁻¹ increased markedly, which shows that ion-exchange and complex adsorption reactions took place during the adsorption process [54,55]. Furthermore, it was discovered that the alterations in the vibration peaks on the surface of the magnetic biochar suggest that Fe_xO_y might play a role in Pb²⁺ adsorption.

3.7. Absorption Mechanism

Figure 14 shows the adsorption mechanism of composite materials for Pb²⁺ according to the above research. The primary adsorption procedure could be like this:

(1)

During physical adsorption, lead ions diffused on the surface of the composite material. As they move, these lead ions are gradually adsorbed into the micropores of the composite material and effectively fixed. This process utilizes the adsorption capacity of the composite material micropores to achieve the effective removal of lead ions.

(2)

The oxygen-containing functional groups that are abundant in biochar play a crucial role. These functional groups have unique chemical properties and can undergo a series of physical and chemical reactions, such as complexation and ion exchange, with heavy metal ions. Specifically, they form stable complexes with heavy metal ions through chelation, effectively binding these ions. Through ion exchange, the ions in the functional groups are replaced with heavy metal ions, further promoting the removal of heavy metal ions. Through the abovementioned mechanism, Pb²⁺ was effectively removed from wastewater.

(3)

The embedding of Fe_xO_y nanoparticles into biochar not only increased the specific surface area and porosity of the composite, improving the adsorption efficiency, but also removed Pb²⁺ through precipitation/co-precipitation by interacting with the hydroxyl functional groups (Fe-OH) on the surface of Fe₃O₄ [56].

3.8. Reusability Needs to Be Evaluated

It is crucial to evaluate the reusability of ideal adsorption materials. BC-FexOy was regenerated with eluent 0.5 mol·L⁻¹ hydrochloric acid and ethanol in a volume ratio of 1:1. After the ionization of hydrochloric acid, a large amount of H⁺ is generated, which can undergo ion exchange with Pb²⁺, causing Pb²⁺ to desorb from BC-Fe_xO_y. The detached material can be recycled [57]. Ethanol can remove organic impurities adhered to the adsorbent surface, enhancing elution effectiveness. Under optimal experimentation (0.1 g BC-Fe_xO_y, 100 mL of 200 mg/L Pb²⁺ simulant wastewater, 25 °C, PH 5.5), adsorption and desorption were used to evaluate the reusability of BC-Fe_xO_y. It was found that after 5 cycles, only 12.16% of Pb²⁺ could be adsorbed (Figure 15). This might be due to the fact that a small number of adsorption sites were destroyed or blocked [53].

4. Conclusions

The research results show that biochar prepared from cotton as a precursor exhibits significant preparation advantages and adsorption characteristics. During the pyrolysis preparation process, cotton biochar showed a high yield of 41%, which is significantly advantageous over traditional wood materials. Through various characterization methods, it was confirmed that Fe_xO_y nanoparticles (<20 nm) were successfully loaded onto the obtained magnetic composite biochar (BC-Fe_xO_y), with a magnetization strength of 21.6 emu/g and an excellent magnetic recovery performance. The specific surface area (326 m²/g) and pore volume (0.232 cm³/g) of the material were increased by approximately 1.49 times and 1.84 times, respectively, compared to unmodified BC, indicating that the introduction of nanoparticles effectively optimized the pore structure.

The lead adsorption experiment showed that the adsorption process of BC-Fe_xO_y on Pb²⁺ followed the Langmuir isotherm model (R² > 0.99), with a maximum adsorption capacity of 252.7 mg/g, which was 68.8% higher than the original BC (149.8 mg/g). FT-IR characterization reveals that the abundant oxygen-containing functional groups, such as hydroxyl and carboxyl groups on the surface of the material, can serve as active sites, achieving efficient adsorption through coordination and ion exchange with Pb²⁺. The pore structure generated by magnetic composite provides more diffusion channels for heavy metal ions.

Dynamic simulation experiments have confirmed that BC FexOy maintains high adsorption efficiency after five consecutive adsorption desorption cycles. Combining the renewable characteristics of the raw materials indicates that the material has the potential for large-scale application in the field of wastewater treatment.