A Recyclable Magnetic Biochar from Corn Cobs and Red Mud for Treating Complex Contaminants Containing Dyes and Heavy Metals

A Recyclable Magnetic Biochar from Corn Cobs and Red Mud for Treating Complex Contaminants Containing Dyes and Heavy Metals | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A Recyclable Magnetic Biochar from Corn Cobs and Red Mud for Treating Complex Contaminants Containing Dyes and Heavy Metals Ruihui Gong, Huidong Li, Yuxin Liu, Jiangzhe Fu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7662320/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 15 Dec, 2025 Read the published version in Environmental Geochemistry and Health → Version 1 posted 12 You are reading this latest preprint version Abstract The treatment of wastewater containing coexisting organic dyes and heavy metals remains challenging due to the competitive adsorption effect, which typically leads to a significant reduction in the removal efficiency of target pollutants, especially heavy metal ions. In this study, a magnetic multifunctional biochar (MMBC-400) was successfully synthesized via the co-pyrolysis of corn cobs and red mud at 400 °C for 2 hours, and it was used for the simultaneous removal of malachite green (MG) and Pb²⁺. The adsorption performance of MMBC-400 for MG and Pb²⁺ in both single and binary systems was systematically investigated through experiments involving different initial conditions, kinetics, isotherms, and cyclic regeneration. MMBC-400 exhibited a high adsorption capacity of 794.72 mg/g for MG, with a removal efficiency of 99.33%. More importantly, in the binary system with a high MG concentration of 500 mg/L, MMBC 400 still maintained a considerable adsorption capacity for Pb²⁺ (129 mg/g) and a removal efficiency of 96.82%, demonstrating strong anti-interference capability. Characterization and model analysis revealed that the adsorption mechanisms of the two pollutants included pore filling, complexation, and ion exchange. Notably, the two pollutants were preferentially adsorbed onto different sites: MG was adsorbed on the carbon matrix of the biochar through π-π interactions, while Pb²⁺ was immobilized on the red mud components via surface complexation. This selective adsorption behavior resulted in limited competitive adsorption between the two pollutants. Furthermore, MMBC-400 showed excellent recyclability. After five consecutive cyclic uses, the removal efficiency of MMBC-400 for both pollutants remained above 85%. Although the single adsorption capacity of MMBC-400 for Pb²⁺ was moderate, it has been proven to be an efficient and practical adsorbent for the treatment of complex polluted wastewater, attributed to its synergistic removal capability, weak competitive effect, and good recyclability. This study provides a valuable strategy for the design of specific adsorbents for complex wastewater systems. Biochar Adsorption Pb2+ Malachite green Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Highlights Synergistic utilization of waste materials to produce MMBC-400 for effective co-removal of composite pollutants MG and Pb²⁺ Due to its unique adsorption mechanism, MMBC-400 maintains a considerable adsorption capacity of 129 mg/g for Pb²⁺ in a binary system with high MG concentration, demonstrating good resistance to interference. Excellent regenerability and reusability, retaining over 85% removal efficiency for both pollutants after five consecutive cycles, showcasing strong potential for practical application. 1. Introduction The discharge of complex wastewater co-contaminated with heavy metals and dyes poses a severe threat to ecosystems and human health[ 1 ]. A significant challenge in treating such wastewater is the competitive adsorption effect between pollutants, which often drastically reduces the removal efficiency of target contaminants. Therefore, developing an adsorbent that can minimize competition, maintain high removal capacity for multiple pollutants, and be easily separated and regenerated is of paramount importance. Among these pollutants,Pb²⁺ and MG are prevalent pollutants in aquatic environments, characterized by high accumulation potential and strong biotoxicity[ 2 , 3 ]. Pb²⁺, a highly toxic heavy metal, can cause serious health issues even at low concentrations, including damage to the nervous system, kidneys, and reproductive organs [ 4 , 5 ]. Conversely, MG, a synthetic triphenylmethane dye, adversely affects aquatic ecosystems by reducing light penetration and gas solubility, thereby inhibiting photosynthesis and compromising the self-purification capacity of water bodies [ 6 , 7 ].Given their distinct physicochemical properties and severe threats, the effective removal of Pb²⁺ and MG from water is crucial[ 8 , 9 ]. To date, various methods have been employed for wastewater treatment, such as photocatalysis[ 10 ], adsorption[ 11 ], coagulation[ 12 ], and membrane separation [ 13 ]. In contrast, adsorption technology has attracted extensive attention due to its simplicity, high efficiency, and low cost[ 14 ]. The search for efficient and low-cost adsorbents has led to the exploration of waste-derived materials. Corn cob is an abundant agricultural waste with high carbon content (45–50%) and significant potential as a biochar precursor[ 15 ], and it has shown a notable affinity for adsorbing dyes like MG[ 16 ]. On the other hand, red mud (RM), a strongly alkaline and iron-rich industrial solid waste from alumina production, possesses excellent adsorption capabilities for heavy metal ions (e.g., Pb²⁺) due to its high dispersibility, porosity, and specific surface area[ 17 , 18 ], Given their complementary functionalities, this study aims to develop a synergistic composite adsorbent using these two waste streams in combination. The designed material is expected to remove dyes through its corn cob-based biochar while immobilizing metals via its red mud component, thereby minimizing competitive effects in treating co-polluted water. However, a critical limitation persists: many reported high-capacity adsorbents excel in removing single pollutants but suffer from severe performance degradation in complex systems due to competitive adsorption. For instance, although a multi-layered chitosan-modified montmorillonite composite achieved an adsorption capacity of 158.79 mg/g for methylene blue (MB)[ 19 ]. and a lignin-bentonite composite showed a maximum capacity of 157.32 mg/g for Cd²⁺[ 20 ], their performance in binary or multi-component systems remains largely unverified and likely compromised. Even when high capacities are reported for both pollutants individually, as in the study by Hajar Abara et al. (189.08 mg/g for Pb²⁺ and 365.31 mg/g for MG)[ 21 ], the critical question of their co-adsorption behavior and competitive effects in a binary system is often not adequately addressed. To address this research gap, this study investigates the adsorption mechanisms of MMBC-400 for MG and Pb²⁺ in both single-component and binary systems. It is hypothesized that MMBC-400 will not only maintain a high adsorption capacity in a high-concentration MG solution (500 mg/L) but also retain a stable affinity for Pb²⁺, exhibiting strong anti-interference capability. This work aims to provide a valuable strategy for developing a practical adsorbent that can resist competitive effects, maintain stable removal capacity for coexisting pollutants, and enable easy separation and regeneration. 2. Materials and methods 2.1. Materials Corn cobs were collected from farmland in Hohhot, Inner Mongolia Autonomous Region, China, and RM was collected from an aluminum plant in Ordos, Inner Mongolia Autonomous Region, China.MG (C 23 H 25 ClN 2 ), hydrochloric acid (HCl), sodium hydroxide (NaOH), and lead nitrate (Pb(NO₃)₂) were supplied by Tianjin Damao Chemical Reagent Factory. All chemical reagents were of analytical grade and prepared with deionized water. 2.2. Synthesis of MMBC-400 Synthesized MMBC-400 was made of raw corn cob and red mud crushed and sieved through 100 mesh, mixed with go 200 ml of ionized water in the ratio of one to one and placed in a magnetic stirrer 800 rmp constant temperature stirring for 2 h. Subsequently, it was oven dried at a constant temperature of 105 ℃, and after drying, it was temperature-controlled burned at 400 ℃ for 2 h in a tube furnace, and then it was milled and sieved through 200 mesh, and it was successfully prepared to be the biochar used for the experiments. Biochar. All the synthesized materials were stored in a desiccator before experimental use. The preparation method was the same as in the previous study[ 22 ]. 2.3. Characterization SEM (German ZEISS Sigma 360) and BET (American Micromeritics ASAP 2460) were used to observe the surface morphology and pore structure of biochar. XPS (American Thermo Scientific K-Alpha) and FTIR (American Thermo Fisher Scientific Nicolet iS20) were employed to analyze surface elemental composition, chemical states, and molecular structures of the materials. XRD (Japanese Rigaku Ultima IV) was utilized to characterize the surface crystal structure. While VSM (American LakeShore 7404) was applied to analyze the magnetic properties of the materials. 2.4. Single adsorption and co-adsorption experiment Adsorption experiments were conducted to evaluate the single and co-adsorption capacities of MMBC-400 for MG and Pb²⁺. In the single-component system, the initial concentration of MG was 500 mg/L, and that of Pb²⁺ was 100 mg/L. Adsorption experiments on dosage, pH value, adsorption time, and initial concentration were carried out, and adsorption kinetics and isotherms were also studied. In the binary co-adsorption system, the initial concentration of MG was 500 mg/L, and that of Pb²⁺ was 100 mg/L. The effects of dosage (10 to 60 mg), pH value, initial concentrations (when the initial concentration of MG was fixed at 500 mg/L, the concentration of Pb²⁺ ranged from 20 mg/L to 250 mg/L; when the initial concentration of Pb²⁺ was fixed at 100 mg/L, the concentration of MG ranged from 100 mg/L to 700 mg/L), and contact time on co-adsorption were investigated. Meanwhile, adsorption kinetics and isotherm models were fitted to analyze the competitive adsorption mechanism. The concentrations of MG were quantified by UV-Vis spectrophotometry at 619 nm, and Pb²⁺ was determined by atomic absorption spectrophotometry (AA-6880F). Each experiment was repeated three times, and the results were recorded as the arithmetic mean of the experimental results. The adsorption capacity and removal rate were calculated using Equ (1) and (2). Where, C₀ (mg/L) and Cₑ (mg/L) represent the initial and equilibrium concentrations of the MG and Pb 2+ solution, respectively; V (L) denotes the volume of the MG and Pb 2+ solution; m (g) indicates the mass of the adsorbent; qₑ (mg/g) is the equilibrium adsorption capacity of the adsorbent for MG and Pb 2+ ; R represents the removal rate. 3. Results and discussion 3.1 Characterization of biochar The SEM results of the biochar are presented in S1. The SEM image of MMBC-400 Fig. S1 (a) reveals a tunnel-like porous structure with clear surface textures, which provides a larger contact area for subsequent adsorption processes. The porous morphology and smooth wrinkled structure of the biochar are favorable for the attachment of metal ions (Fe²⁺/Fe³⁺, Al³⁺) and non-metallic species (K⁺/Ca²⁺, Si⁴⁺) from red mud to the biochar surface[ 23 ]. In Fig. S1 (c), the MMBC-400 composite clearly shows numerous irregular spherical particles heterogeneously embedded in the carbon matrix or distributed on the biochar surface, confirming the successful integration of red mud components into the carbon matrix. The N₂ adsorption-desorption isotherms and pore size distribution of the material are shown in Fig.S2. The adsorption capacity gradually increases with the rise of relative pressure (P/P₀). According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), MMBC-400 exhibits a type IV isotherm with an H3 hysteresis loop, confirming the coexistence of micropores and mesopores. Generally, micropores provide adsorption sites, while mesopores serve as diffusion channels, both of which are applicable for the treatment of organic pollutants[ 24 , 25 ]. The pore structure and specific surface area of the biochar synthesized using red mud and corn cobs are higher than those reported in previous studies on modified corn cobs[ 26 ]. This is because the compact external structure of corn cobs may inhibit the volatile release during the carbonization process[ 27 ]. However, the addition of red mud effectively overcomes this structural limitation, leading to significant pore reorganization, as evidenced by the expanded porosity in MMBC-400. The evolution of the microstructure confirms that red mud-derived iron/aluminum oxides and oxygen vacancies play a key role in porosity optimization. They synergistically promote the pollutant adsorption process by generating abundant active sites[ 28 ]. The hysteresis loop of the synthesized material is shown in Fig.S2(b), indicating that the material exhibits obvious magnetism due to the incorporation of Fe elements from red mud. As the magnetic field strength increases, the magnetization intensity gradually rises until reaching a saturation magnetization value of 10.23 emu/g. This confirms the self-generated magnetism of MMBC-400 during the pyrolysis process. The material possesses natural sedimentation capability, enabling solid-liquid separation without external intervention. Moreover, under the action of an external magnetic field, rapid and thorough solid-liquid separation can be achieved by virtue of the highly sensitive magnetic properties of MMBC-400, which effectively enhances the recyclability of the material and the practicality of operation in wastewater treatment systems[ 29 ]. 3.2 Individual adsorption of MG and Pb 2+ on MMBC-400 3.2.1. The effect of MMBC-400 dosage The dosage of MMBC-400 also affects the adsorption efficiency of MG and Pb²⁺. As shown in Fig. 1 , with the increase of MMBC-400 dosage, the adsorption efficiency of both MG and Pb²⁺ increases, while the adsorption capacity gradually decreases, indicating better adsorption performance. For MG adsorption, the removal rate remains stable when the dosage ranges from 0.04 g to 0.06 g, suggesting that adsorption gradually reaches saturation. The removal rates exceed 99% at dosages of 0.05 g and 0.06 g, with the adsorption capacity reaching 793.512 mg/g. This can be attributed to the increased interaction and collision probability between adsorption sites and MG molecules due to the higher adsorbent content in the solution. Therefore, 50 mg was selected as the optimal dosage for subsequent experiments in the MG system. For Pb²⁺ adsorption, the maximum removal rate of 91.1% and adsorption capacity of 121.4 mg/g were achieved at a dosage of 60 mg, which is explained by the increase in active sites, specific surface area, and the amount of biochar [ 30 ]. The aim of this experiment was to maximize removal using as little biochar as possible, so 60 mg of MMBC-400 was selected as the optimal dosage f or subsequent experiments. 3.2.2. The effect of pH The pH value of the solution is also a crucial factor affecting adsorption, which is directly related to the types of pollutants in the solution and the chemical properties of the adsorbent. With the continuous increase of pH, the MG solution undergoes obvious color reaction and transforms into colorless malachite green alcohol base due to the enhanced alkalinity[ 31 ], which will have a certain impact on adsorption. Therefore, the adsorption of MG by MMBC-400 in this experiment was carried out at a solution pH of 2–10 for 24 hours. Precipitation and coating phenomena of Pb²⁺ occur near neutral and higher pH values, so the adsorption of Pb²⁺ by MMBC-400 was performed at a solution pH of 2–6. As shown in Fig. 2 (a), the adsorption of MG by MMBC-400 exhibits a positive correlation between pH value and adsorption capacity, with the maximum adsorption capacity reaching 794.72 mg/g and the removal rate achieving 99.33%. According to the zeta potential measurement results, the surface charge of MMBC-400 varies at different pH values. When the pH is low, MMBC-400 carries a net positive charge. Therefore, in a highly acidic environment (e.g., pH = 2), the interaction between cationic dyes and the positively charged surface of MMBC-400 is electrostatically repulsive, which reduces the effectiveness of MMBC-400 in removing MG[ 32 ]. When the pH is high, MMBC-400 carries a negative charge, showing attractive interaction with cationic dyes. As shown in Fig. 6 − 2, the adsorption results of Pb²⁺ by MMBC-400 clearly demonstrate that when the solution pH increases from 3 to 5, the adsorption of Pb²⁺ increases rapidly, while it slightly decreases at pH 6. Under the optimal condition of pH 5, the removal efficiency of Pb²⁺ reaches 96.82%, and the adsorption capacity achieves 129 mg/g. This may be because Pb²⁺ forms cationic species in the pH Fig. 2 (b)[ 33 ], Pb²⁺ is adsorbed onto the negatively charged surface of MMBC-400 through strong electrostatic forces. At lower pH values (e.g., pH 2), the surface charge of MMBC-400 is uniform with that of Pb²⁺ in the solution, and electrostatic repulsion exists[ 34 ]. This is unfavorable for the adsorption of Pb²⁺ by MMBC-400. Therefore, pH values of 7 and 5 for the adsorption of MG and Pb²⁺ by MMBC-400 were selected as the optimal conditions for subsequent experiments. In addition, since MMBC-400 contains red mud components, crystalline minerals dissolved at lower pH values may release cations such as K⁺, Ca²⁺, and Mg²⁺. These cations can compete with MG and Pb²⁺ for adsorption sites on the surface of the adsorbent, thereby reducing the removal rate and adsorption capacity[ 35 ]. 3.2.3. The effect of adsorption time Adsorption contact time is also a critical factor in evaluating adsorption performance. The influence of contact time on the adsorption of MG and Pb²⁺ is shown in Fig. 3 . The adsorption processes of MG and Pb²⁺ by MMBC-400 within the temperature range of 293、298、303 K exhibit typical two-stage mass transfer characteristics, Initial stage (0–240 min): A rapid increase in adsorption capacity due to the abundance of surface active sites. This can be attributed to the significant chemical potential difference between the initial solution concentrations (500 mg/L for MG and 100 mg/L for Pb²⁺) and the solid-phase adsorption capacity, which drives molecular diffusion into the mesoporous structure. Additionally, the electrostatic attraction between MMBC-400 and MG cations/Pb²⁺ facilitates rapid binding at the early stage, leading to a steep upward curve. Diffusion-controlled stage: The mass transfer rate slows down significantly due to intraparticle diffusion resistance. The equilibrium adsorption capacities for MG at 293 K, 298 K, and 303 K are 680.2 mg/g, 733.5 mg/g, and 793.4 mg/g, respectively; those for Pb²⁺ are 120.58 mg/g, 130.22 mg/g, and 136.11 mg/g, respectively, indicating a positive temperature effect [ 36 ]。After 240 min, as the contact time increases, the probability of intermolecular collisions rises, and adsorption sites become progressively occupied by MG and Pb²⁺, leading to adsorption equilibrium and a plateau in the curve. 3.2.4. The effect of initial concentration As shown in Fig. 4 , the adsorption behavior of MG and Pb²⁺ by MMBC-400 exhibits significant concentration gradient effects and temperature dependency. Figure 4 (a), experimental data show that at three different temperatures, when the initial concentration of MG increases from 100 mg/L to 700 mg/L, the equilibrium adsorption capacity at 298 K linearly increases from 152.3 mg/g to 793.6 mg/g. This may be because when the MG concentration is low, the concentration gradient between malachite green in the solution and the biochar surface is small, resulting in a low diffusion rate and limited adsorption capacity. With the increase of concentration in the solution, the diffusion rate accelerates, enabling more effective collisions, and the adsorption sites are gradually filled [ 37 , 38 ]. As shown in Fig. 4 (b), the effect of initial solution concentration on the adsorption of Pb²⁺ by MMBC-400 demonstrates that the adsorption capacity gradually increases with the increase of Pb²⁺ concentration. However, when the concentration reaches 300 mg/L, the curve tends to flatten, with adsorption capacities of 322.12 mg/g, 289.7 mg/g, and 210.02 mg/g, respectively. In the Pb²⁺ system, at lower initial concentrations, Pb²⁺ is first adsorbed onto the abundant reactive regions of MMBC-400. The adsorption capacity of Pb²⁺ is low at lower concentrations and gradually increases rapidly with the increase of ion concentration. This is because after a single ion layer with lower concentration is formed on the adsorbent surface, the further formation of this layer is severely hindered at higher concentrations due to the interaction between metal ions on the surface and those in the solution. Additionally, at low metal ion concentrations, the ratio of Pb²⁺ to the available surface area of the adsorbent is large; subsequently, the adsorption capacity becomes independent of the initial concentration of Pb²⁺. As the Pb²⁺ concentration gradually increases, Pb²⁺ is continuously adsorbed onto MMBC-400, making the available adsorption sites of the adsorbent fewer. Therefore, the adsorption capacity of Pb²⁺ at higher concentrations gradually increases and then tends to flatten[ 39 , 40 ]. Adsorption temperature is also a key parameter affecting the performance of adsorbents. As can be seen from the figure, an increase in temperature is more conducive to the adsorption of pollutants. Increasing the adsorption temperature can enhance the mobility of molecules, thereby improving the possibility of interaction between the adsorbent and MG as well as Pb²⁺. The above experiments confirm that MMBC-400 is an excellent biochar material for removing the dye MG and Pb²⁺ in single-component adsorption systems. However, the coexistence of inorganic and organic pollutants is common in the environment[ 41 ]. The adsorption capacity of MMBC-400 may change when MG and Pb²⁺ coexist due to different adsorption mechanisms. This study further investigates the removal capability of MMBC-400 in the co-contaminated system of MG and Pb²⁺. 3.3. Simultaneous adsorption of Pb 2+ and MG on MMBC-400 3.3.1 Adsorbent dosage As shown in the dosage diagram of the MG-Pb²⁺ binary system Fig. 5 (a, b) with the adsorbent dosage increasing from 10 mg to 60 mg, the overall removal rates of both MG and Pb²⁺ first rise rapidly and then tend to plateau. This phenomenon can be attributed to the following reasons: at lower dosages (10–30 mg), the number of adsorption sites is insufficient, and the pollutant concentration far exceeds the adsorption capacity, leading to a rapid increase in removal rates. At higher dosages (40–60 mg), the active sites tend to reach saturation, and the diffusion resistance increases, resulting in a slowed growth of removal rates due to the adsorbent excess effect. However, the decrease in unit adsorption capacity may be caused by the intensified agglomeration of adsorbents for MG and Pb²⁺, which reduces the utilization efficiency of specific surface area, leaving some adsorption sites underutilized or competitively occupied. In the binary adsorption diagram, at lower dosages (10, 20, and 30 mg), Pb²⁺ is preferentially adsorbed, with its removal rate significantly higher than that of MG, while MG surpasses Pb²⁺ in removal rate at a later stage. When the dosage exceeds 40 mg, the removal rate of MG overtakes that of Pb²⁺. This can be explained by the strong coordination of Fe/Al oxides in red mud with heavy metals, such as surface hydroxyl complexation (as shown in the Equ.3) and ion exchange—Ca²⁺/Na⁺ from red mud in MMBC-400 raw materials can exchange with Pb²⁺[ 42 ]. The later surpassing of MG can be attributed to the enhanced π-π stacking effect of the porous structure of corn cob-derived biochar on organic dyes at higher dosages. Additionally, the molecular size of MG is smaller than that of hydrated Pb²⁺ ions, enabling it to more easily enter micropores. Therefore, 40 mg was selected as the optimal dosage for subsequent experiments in the binary system. ≡Fe-OH₂⁺ + Pb²⁺ → ≡Fe-O-Pb⁺ + 2H⁺ (3) ≡Fe-O-Pb⁺ + Dye-N⁺(CH₃)₂ → ≡Fe-O···⁺N(CH₃)₂-Dye + Pb²⁺ (4) 3.3.2 pH of the solution The pH value of the solution is a critical factor that significantly affects the removal efficiency of pollutants by biochar. As shown in the Fig. 5 (c, d)., the adsorption of MG by MMBC-400 exhibits a gradual increase in the pH range of 2–3. This can be attributed to the protonation of the material surface, which carries a positive charge and thus generates electrostatic repulsion with MG. Meanwhile, the high concentration of H⁺ competes for adsorption sites, and the aromatic structure of corn cob-derived biochar provides a small amount of π-π stacking interactions. In the pH range of 4–6, the adsorption capacity gradually reaches a peak. This is likely because the material surface undergoes deprotonation and carries a negative charge, resulting in electrostatic attraction with MG. Additionally, the -N(CH₃)₂ groups in MG form stronger hydrogen bonds with the -OH/O⁻ groups on the surface of MMBC-400, and the π-π conjugation is maximized[ 43 ]. In the same system, the adsorption trend of Pb²⁺ by MMBC-400 shows a gradual increase in the pH range of 2–4, reaching a peak. This is because the hydroxyl groups on the surface of Fe₂O₃ and Al₂O₃ (components of red mud in the prepared MMBC-400) provide abundant cation exchange sites for Pb²⁺, as shown in Equ.4. Meanwhile, the ion exchange between Ca²⁺/Na⁺ in red mud plays a dominant role[ 44 ]. In the pH range of 5–6, OH⁻ in the solution reacts with Pb²⁺ to form soluble Pb(OH)⁺ or Pb₂(OH)³⁺, thereby reducing the concentration of free Pb²⁺. When pH > 5.5, Pb(OH)₂ precipitates are generated, resulting in an artificially high apparent removal rate that does not reflect the actual adsorption. As shown in the Fig. 5 (c, d). for the competitive adsorption of MG and Pb²⁺ by MMBC-400 in the binary system, Pb²⁺ dominates the adsorption within the pH range of 2–3. This is likely due to the occupation of metal-binding sites in red mud components of MMBC-400 by Pb²⁺, resulting in the exclusion of MG. At pH 4–5, Pb²⁺ maintains a high adsorption capacity, while the adsorption of MG is enhanced through π-π interactions and hydrogen bonding, leading to a peak of synergistic removal at pH 5. Specifically, the removal efficiency of MG reaches 98.4% with an adsorption capacity of 787.56 mg/g, and the removal efficiency of Pb²⁺ reaches 93.8% with an adsorption capacity of 128.24 mg/g. At pH 6, the electrostatic adsorption of MG is maximized, whereas the actual adsorption of Pb²⁺ decreases due to the formation of hydroxyl complexes induced by precipitation, resulting in the removal efficiency of MG surpassing. 3.3.3 Solution concentration and contact time As shown in the Fig. 6 (a) for the binary system at 298 K, Pb²⁺ achieves rapid adsorption within 0–10 min. This is likely attributed to the instantaneous coordination between ≡ M-OH groups on the surface of Fe/Al oxides and Pb²⁺, as well as the rapid displacement of soluble Ca²⁺/Na⁺ in red mud, with red mud contributing predominantly. However, due to the limited adsorption sites for Pb²⁺, adsorption equilibrium is gradually reached at 120 min. Figure 6 (b), the adsorption of MG by MMBC-400 proceeds in two stages: the rapid stage (0–120 min) involves macropore/mesopore diffusion, where MG molecules enter the pores of the biochar material—mainly contributed by corn cobs—and the aromatic structure of the biochar adsorbs the benzene rings of MG through surface π-π stacking. The slow stage (120–480 min) is dominated by micropore filling, where the -N(CH₃)₂ groups in MG form multiple hydrogen bonds with -COOH/-OH groups on the surface of MMBC-400. It can be seen that, due to its constituent raw materials, MMBC-400 exhibits excellent adsorption performance for both MG and Pb²⁺ in the binary system. However, a competitive effect exists between MG and Pb²⁺, Fig. 6 (c). In the first 30 min, Pb²⁺ preferentially occupies the metal-binding sites of red mud, inhibiting the adsorption of MG on red mud components. After 60 min, MG avoids competition through the pores of biochar, and independent adsorption channels are activated. When MG reaches equilibrium after 240 min, Pb²⁺ begins to be displaced by the hydrophobic groups of MG, resulting in a slight decrease in its adsorption efficiency. As shown in the Fig. 6 (d) when the MG concentration is fixed at 500 mg/L and the Pb²⁺ concentration increases from 20 mg/L to 250 mg/L, the removal efficiency of MG reaches the highest at 20 mg/L of Pb²⁺, gradually decreases with the increase of Pb²⁺ concentration, hits the lowest at 200 mg/L of Pb²⁺, and partially recovers at 250 mg/L. This phenomenon may be explained by the pore shielding effect induced by high-concentration Pb²⁺: Pb²⁺ ions and [Pb(OH)]⁺ species block mesopores, preventing MG from entering the pores. When the ion concentration reaches saturation at 250 mg/L, part of MG is adsorbed on the carbon surface through hydrophobic interactions. However, the removal efficiency of Pb²⁺ decreases continuously. This is because the total cation exchange capacity of MMBC-400 is overloaded, accompanied by electrostatic repulsion. Moreover, the high concentration of Pb²⁺ causes the surface of the material to be positively charged, inhibiting the subsequent adsorption of Pb²⁺. When the concentration of Pb²⁺ is kept constant at 100 mg/L and the concentration of MG increases from 100 mg/L to 700 mg/L, Fig. 6 (e), the removal efficiencies of both MG and Pb²⁺ decrease continuously. At an MG concentration of 100 mg/L, there is no significant adsorption competition. As the MG concentration gradually increases, the removal efficiencies decrease due to mild competition for functional groups and competition for pore channels between the two. When the MG concentration exceeds 500 mg/L, the dye layer hinders the diffusion of Pb²⁺. When the MG concentration reaches 700 mg/L, most of the MMBC-400 surface is covered, resulting in a relatively low removal efficiency. 3.4. Adsorption modeling 3.4.1 Equilibrium isotherm Adsorption isotherms are used to study the equilibrium relationship between adsorbents and adsorbates under a certain temperature condition. In this study, the Langmuir and Freundlich models were employed. From the fitting results, Table S1 it can be seen that in the binary system, the fitting degree of the Langmuir model for Pb²⁺ (R² = 0.999) is higher than that for MG (R² = 0.977), indicating that the adsorption of Pb²⁺ on MMBC-400 is a monolayer adsorption on a uniform surface, which occurs at a fixed number of localized sites on the adsorbent surface with no interaction between the adsorption sites[ 45 ]. In the Freundlich model, the fitting parameter for MG is higher than that for Pb²⁺, suggesting that the adsorption of MG by MMBC-400 is suitable for multi-layer adsorption on a heterogeneous surface. This adsorption occurs on the adsorbent surface, which is inherently heterogeneous, resulting in the formation of multi-layer coverage on the surface[ 46 ]. These results indicate that the adsorption of MG and Pb²⁺ by MMBC-400 in the binary system conforms to different adsorption mechanisms, and the competitive adsorption between them is weak. 3.4.2 Adsorption kinetics Adsorption kinetics is an indicator parameter for understanding adsorption mechanisms, adsorbent performance, and adsorption rates[ 47 ], Fig. 7 (a,b,c,d,) Adsorption kinetics is an indicator parameter for understanding adsorption mechanisms, adsorbent performance, and adsorption rates [ 40 ]. In this study, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were used to evaluate the co-adsorption kinetic data of single-component and binary systems. The Table.S2 shows the mutual influence of adsorption kinetics of MMBC-400 on MG and Pb²⁺ in the binary system. At 298 K and under the same adsorption time, the pseudo-first-order parameter (r² = 0.948) for MG is significantly higher than that for Pb²⁺ (r² = 0.693) under the same conditions. The low fitting degree of Pb²⁺ suggests that the chemical complexation on the surface of red mud is an instantaneous reaction, which does not conform to the physical adsorption assumption of the pseudo-first-order model. The moderate fitting for MG indicates that the initial stage is controlled by film diffusion, while the micropore diffusion deviates from linearity in the later stage. In the pseudo-second-order kinetic parameters table, both MG and Pb²⁺ show good fitting results, with Pb²⁺ exhibiting a nearly perfect fit, confirming that the adsorption of Pb²⁺ by MMBC-400 is dominated by chemical adsorption, and surface complexation is the rate-controlling step. The high fitting degree for MG also indicates that its adsorption is mainly chemical adsorption, which may be attributed to the π-π stacking in the early stage and the formation of hydrogen bonds between -N(CH₃)₂ groups and -OH groups in the later stage. The intra-particle diffusion model is the optimal model for identifying adsorption diffusion mechanisms. In the intra-particle diffusion model, if the line passes through the origin, the adsorption process is controlled by intra-particle diffusion; if the data do not pass through the origin, external mass transfer and intra-particle diffusion may occur simultaneously (Fig. 7 (e,f)). Therefore, the entire adsorption process is controlled by both external mass transfer and intra-particle diffusion[ 48 ]. It can be observed that after 240 min, as MG reaches equilibrium, the adsorption efficiency of Pb²⁺ decreases slightly due to displacement by the hydrophobic groups of MG. The data for Pb²⁺ form a single straight line, indicating that the surface reaction is free of diffusion limitations. In contrast, the intra-particle diffusion of MG is divided into three stages: the first stage involves rapid filling of mesopores; the second stage is characterized by inhibited micropore diffusion due to Pb²⁺; and the third stage gradually reaches equilibrium. 3.4.3 Adsorption thermodynamics To investigate the thermodynamic behavior of MG and Pb²⁺ adsorption onto MMBC-400, thermodynamic studies were conducted in a binary system. The thermodynamic parameters are presented in the Table S3. According to the data, the adsorption of MG onto MMBC-400 is an endothermic (ΔH = 50.41 kJ/mol) and entropy-increasing (ΔS° = 241.5 J/mol·K) process. These parameters align with the synergistic mechanism of ion exchange and π-π stacking revealed by structural characterization. The adsorption of Pb²⁺ onto MMBC-400 is also endothermic, possibly due to the dehydration of Pb²⁺ during ion exchange (an endothermic step). The release of water molecules upon adsorption onto the solid surface increases the system entropy, driving the adsorption process. Additionally, the adsorbed Pb²⁺ may gain greater vibrational entropy on the solid surface, further contributing to the entropy increase. Notably, all ΔG values are negative, indicating spontaneous adsorption. The increasingly negative values with rising temperature suggest enhanced spontaneity, consistent with previous studies[ 17 ]. 3.5. Adsorption mechanisms of individual and simultaneous mixture of MG and Pb(II) on MMBC-400 The FTIR spectra of MMBC-400 before and after adsorption of MG and Pb²⁺ are shown in Fig. 8(a), displaying significant spectral changes. After adsorption, the peak width and intensity of -OH (3414 cm⁻¹) increased in both MG and Pb²⁺ systems, accompanied by enhanced absorption intensity, indicating the formation of hydrogen bonds during the adsorption process. In both the MG system and the MG-Pb²⁺ binary system, aliphatic C-H bending vibrations were observed at 2850–2900 cm⁻¹ with increased intensity[ 49 ]. This may be attributed to the monomeric adsorption of MG in the single-component system, while in the binary system, the presence of Pb²⁺ might lead to the formation of certain compounds between MG and Pb²⁺, resulting in enhanced peak intensity. However, no such peak change was observed in the Pb²⁺ system. The Al-O oscillation at 600 cm⁻¹ shifted to 818.6 cm⁻¹, indicating the coordination complexation between Al³⁺ and MG molecules[ 50 ]. The characteristic absorption bands at 1710 cm⁻¹ (C = O stretching vibration) and 1230 cm⁻¹ (C-O stretching mode) confirmed the presence of carboxyl functional groups (-COOH)[ 51 ], he peak intensity increased in the Pb²⁺ system but decreased in the MG system; in the binary system, the peak disappeared possibly due to the formation of (-COO)₂Pb. In the MG system, bands consistent with the aromatic structure of MG appeared at 1611 cm⁻¹ (aromatic ring stretching) and 1517 cm⁻¹ (C = C skeletal vibration), confirming the successful immobilization of pollutants with enhanced intensity[ 52 ], However, in the binary system, the reduced peak intensity indicated inhibited π-π stacking. A new peak at 480 cm⁻¹ (Pb-O) appeared in both the Pb²⁺ system and the binary mixed system, providing direct evidence for the successful adsorption of Pb²⁺ onto MMBC-400. The XPS spectra of MMBC-400 before and after adsorption of MG and Pb²⁺ are shown in Fig.S3.and Fig.S4 The adsorption mechanism can be inferred from the analysis of O1s, C1s, Fe2p, and Pb4f spectra. The C1s peak at 284.8 eV, corresponding to C-C/C = C, showed no significant change after the individual adsorption of MG and Pb²⁺, indicating that the aromatic ring skeleton of the biochar is stable and not easily deformed. The C-O bond at 286 eV[ 53 ], exhibited significant changes after the adsorption of MG and Pb²⁺, suggesting that oxygen-containing functional groups are involved in the binding of MG. Additionally, the reduced intensity of the C-O peak is attributed to the formation of complexes between carboxyl groups (-COOH) and Pb²⁺ (e.g., Pb-OOC-R). The peak area also decreased in the binary system. In the Pb²⁺ system, the C = O peak shifted from 288 eV to a higher binding energy of 289.18 eV, indicating the transfer of lone-pair electrons from carboxylic oxygen to Pb²⁺ through coordination. The peak at 532 eV, corresponding to hydroxyl groups (-OH), showed a significant decrease in intensity in the Pb²⁺ system, indicating the consumption of -OH due to coordination with Pb²⁺ Fig.S4(d) [ 54 ], Meanwhile, a new peak at 533.5 eV appeared after Pb²⁺ adsorption, corresponding to Pb-O bonds (e.g., Pb₃(CO₃)₂(OH)₂). The peak at 534 eV, corresponding to carboxylic oxygen (O-C = O), shifted after MG adsorption, suggesting hydrogen bonding between -N(CH₃)₂ in MG and carboxyl groups. There was no significant change in the lattice oxygen peak (529 eV), indicating the stable framework structure of red mud. Due to the presence of nitrogen in MG, a distinct N peak appeared after adsorption in the MG-containing system, as shown in the N1s spectrum, Fig.S3(g) and Fig.S5(c). The peak at 399.5 eV corresponds to -NH₂, which originates from the primary amine groups in MG molecules. The peak at 401 eV, assigned to NH₃⁺, may result from the protonation of partial -NH₂ groups. Additionally, the metal-N coordination peak at 398.5 eV indicates that -NH₂ forms coordination bonds with Fe³⁺ or Al³⁺[ 55 ]. In the binary system, the presence of Pb²⁺ may affect the adsorption of MG through competitive occupation of adsorption sites: Pb²⁺ is likely to occupy some negatively charged sites, reducing the electrostatic adsorption of MG and thereby decreasing the amount of adsorbed MG molecules, which leads to a weakened N signal. However, the increased peak area of -NH- suggests the possibility of synergistic adsorption, where Pb²⁺ may form "cationic bridges" on the material surface to promote the adsorption of MG[ 56 – 58 ]. In the Fe2p spectrum, the Fe2p peaks before adsorption show the presence of Fe³⁺ 2p₃/₂ at 710 eV (hematite Fe₂O₃), Fe2p₁/₂ at 725.2 eV, and satellite peaks at 718.3/733.1 eV, confirming that hematite (Fe₂O₃) is the dominant species. After adsorption of MG in the single-component system, the area of the Fe²⁺ peak increases significantly, indicating that a redox reaction occurs between Fe³⁺ and MG molecules, with partial reduction of Fe³⁺. The weakening of Fe³⁺ satellite peaks and a slight decrease in the area of the main peak suggest that Fe-O bonds may form due to the coordination of -NH₂ groups with MG molecules[ 56 – 58 ]. After adsorption of Pb²⁺ in the single-component system, the increased proportion of Fe²⁺ indicates partial reduction of Fe³⁺, which may participate in the reductive adsorption of Pb²⁺. The broadened peak width at 710.5 eV corresponds to the formation of Fe-O-Pb bonds, implying possible interfacial interactions between Fe and Pb²⁺. As shown in the Fig.S4.and Fig.S5, after adsorption in both single-component and binary systems, characteristic peaks of heavy metal Pb²⁺ appear as Pb4f doublets: Pb 4f₇/₂ at 139.28 eV (corresponding to Pb²⁺ species such as ≡ Fe-O-Pb, (-COO)₂Pb, or Pb(OH)⁺) and Pb4f₅/₂ at 144.18 eV. The spin-orbit splitting energy is consistent with previous studies[ 59 ], confirming the successful adsorption of Pb²⁺ by MMBC-400. MMBC-400 synergistically immobilizes Pb²⁺ through carboxyl complexation, ion exchange, and precipitation reactions. Overall, Pb²⁺ is effectively and successfully adsorbed in both single-component and binary mixed systems. The disappearance or reduction of K⁺/Ca²⁺ peaks in the full spectra of both MG and Pb²⁺ systems indicates that ion exchange occurs between MG/Pb²⁺ and K⁺/Ca²⁺. In the XRD analysis Fig.S6(b), it can be observed that the forms of Fe, namely Fe₂O₃ and Fe₃O₄, remain stable in both single and binary systems, indicating the chemical stability of the material. Pb₃(CO₃)₂(OH)₂ was detected in both MMBC-400-Pb²⁺ and MMBC-400-MG-Pb²⁺ systems (Fig. 7 (b)), suggesting that Pb²⁺ is adsorbed on MMBC-400 through surface precipitation. Specifically, Pb²⁺ reacts with -OH groups on MMBC-400 to form Pb(OH)₂ precipitates, which further react with additional Pb²⁺ and CO₃²⁻ to generate Pb₃(CO₃)₂(OH)₂, as shown in Equ. (5,6)[ 60 ], In the binary system, the presence of MG competes with Pb²⁺ for adsorption sites, leading to a decrease in the intensity of Pb₃(CO₃)₂(OH)₂ precipitation. $$\:{\text{Pb}}^{\text{2+}}\text{+}{\text{2OH}}^{\text{-}}\text{→}{\text{Pb(OH)}}_{\text{2}}$$ 5 $$\:{\text{Pb(OH)}}_{\text{2}}\text{+2}{\text{Pb}}^{\text{2+}}\text{+2}{\text{CO}}_{\text{3}}^{\text{2-}}\text{→}{\text{Pb}}_{\text{3}}{\text{(}{\text{CO}}_{\text{3}}\text{)}}_{\text{2}}{\text{(OH)}}_{\text{2}}$$ 6 In summary, based on the analysis results of XRD, FTIR, and XPS, the adsorption mechanism of MG onto MMBC-400 involves multiple interactions, including π-π stacking, covalent bonding, coordination complexation, and redox reactions, which collectively contribute to the immobilization of MG, consistent with previous studies[ 22 ]. The strong adsorption capacity of MMBC-400 on Pb²⁺ involves not only ion exchange and electrostatic effects, but also the combined effects of precipitation, complexation and pore filling. In the coexistence system of MG and Pb²⁺, competitive adsorption occurs. Initially, Pb²⁺ preferentially occupies ≡ Fe-OH/-COO sites, inhibiting the hydrogen bonding adsorption of MG. Due to the different molecular diameters of Pb²⁺ and MG, hydrated Pb²⁺ ions may block mesopores, delaying the diffusion of MG. The coverage of MG micelles leads to surface positive charging, weakening electrostatic attraction. Additionally, the precipitation of Pb₃(CO₃)₂(OH)₂ may affect the adsorption of MG, and ion exchange between Ca²⁺/K⁺ and MG/Pb²⁺ also contributes to competition. 3.6. Regeneration of MMBC-400 after adsorption of MG and Pb 2+ Evaluating the reusability of MMBC-400 is crucial for its feasibility and economic efficiency in practical applications. As shown in Fig.S7., using an EDTA-ethanol mixed solution as the desorption agent for MG and Pb²⁺, the adsorption performance remained relatively stable in the first two cycles. By the third cycle, the removal rates slightly decreased from 98.4% and 93.8% to 93.43% and 90.58%, respectively. This gradual reduction in adsorption efficiency may be attributed to pore collapse or loss of functional groups in MMBC-400. Additionally, partial oxidation of the adsorption sites on MMBC-400 could hinder further adsorption. However, the adsorption capacity and removal efficiency of MMBC-400 remained relatively stable over five cycles. Even after five cycles, the removal rates still maintained above 85%, indicating that MMBC-400 possesses strong regenerability and holds great application potential in practical wastewater treatment. 4. Conclusion This study successfully developed a magnetic multifunctional biochar MMBC 400 through the synergistic utilization of corn cob and red mud, two abundant industrial and agricultural wastes. The primary innovation of this work lies in demonstrating that MMBC-400 effectively minimizes competitive adsorption in a binary system of MG and Pb²⁺, a common yet challenging scenario in real wastewater. Despite a moderate adsorption capacity for Pb²⁺ (129 mg/g) in single systems, MMBC-400 exhibited exceptional performance in the co-existing system, retaining high removal efficiencies of 98.4% for MG and 93.8% for Pb²⁺. This indicates that the adsorbent possesses strong anti-interference capability, which is attributed to its site-specific adsorption mechanisms: the corn cob-derived carbon matrix primarily facilitates MG removal via π-π interactions and pore filling, while the red mud components selectively immobilize Pb²⁺ through surface complexation and precipitation. Furthermore, MMBC-400 showcases excellent regenerability and reusability, maintaining over 80% removal for both pollutants after five consecutive cycles, underscoring its potential for practical application. However, this study has certain limitations. The adsorption performance and mechanisms were specifically investigated for the MG and Pb²⁺ binary system. The effectiveness of MMBC-400 against other prevalent dyes and heavy metal ions, as well as in more complex multi-component wastewater matrices, requires further validation. Additionally, the long-term stability of the material and a comprehensive economic assessment for large-scale application remain to be explored. In summary, the findings provide a valuable theoretical foundation for designing efficient, waste-derived adsorbents targeted at complex water pollution. MMBC-400 stands out not for a singularly high adsorption capacity, but for its balanced, synergistic, and sustainable approach to co-contaminant removal, offering a promising strategy for practical wastewater treatment. Declarations Declaration of Competing Interest The authors hereby declare that the disclosed information is correct and that no other situation of real, potential, or apparent conflict of interest is known to them. The authors undertake to inform any change in these circumstances. Author Contribution CRediT authorship contribution statementRuihui Gong: Writing–original draft, Methodology, Formal analysis, Data curation, Conceptualization. Huidong Li: Writing–review & editing, Funding acquisition, Formal analysis, Conceptualization. Yuxin Liu: Supervision, Software, Investigation, Data curation. Jiangzhe Fu: Resources, Investigation. Acknowledgements This work was supported by Natural Science Foundation of Inner Mongolia Autonomous Region of China (2024LHM05041), National Natural Science Foundation of China (42067031) and Inner Mongolia Science and Technology Plan Project (2025KYPT0025) References S.Z. Mohammadi, M.A. Karimi, D. Afzali, F. 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4","display":"","copyAsset":false,"role":"figure","size":531636,"visible":true,"origin":"","legend":"\u003cp\u003eThe effect of initial concentration on individual adsorption of MG(a) and Pb\u003csup\u003e2+\u003c/sup\u003e(b) using MMBC-400\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7662320/v1/9b61e04896987efd93f6f24b.png"},{"id":92845811,"identity":"5c47a2e6-4425-4ff5-89ce-0a6d1358e060","added_by":"auto","created_at":"2025-10-06 09:34:42","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1195782,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorption efficiency (a), removal rate (b), of MG, adsorption efficiency (c), removal rate (d)of Pb\u003csup\u003e2+\u003c/sup\u003e by MMBC-400 in a binary system.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7662320/v1/f1d2b76a212517ddc02423cc.png"},{"id":92845796,"identity":"8b7443ca-16f9-4a05-af4f-51ce66962f43","added_by":"auto","created_at":"2025-10-06 09:34:41","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":858404,"visible":true,"origin":"","legend":"\u003cp\u003eIn binary systems, the contact time effect on the adsorption of MG (a), Pb²⁺ (b), and MG and Pb²⁺ (c) using MMBC-400, and the solution concentration effect on the adsorption of MG (d) and Pb²⁺ (e) using MMBC-400 are shown.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7662320/v1/a334a7ad4aa66d243689695e.png"},{"id":92845801,"identity":"e0c38844-4a6e-4e44-90d8-e39a02c178c4","added_by":"auto","created_at":"2025-10-06 09:34:41","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1083693,"visible":true,"origin":"","legend":"\u003cp\u003eIn the binary system, the quasi-first-order kinetics (a), quasi-second-order kinetics (b), and intra-particle diffusion (e) of MG, the quasi-first-order kinetics (c), quasi-second-order kinetics (d), and intra-particle diffusion (f) of Pb²⁺\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7662320/v1/5276dae5f8012fdb18585e69.png"},{"id":98814008,"identity":"9e897159-a70e-4013-9de0-e3f53350aa4d","added_by":"auto","created_at":"2025-12-22 16:09:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6433003,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7662320/v1/b75054ff-44eb-4a50-8288-cbc4f2801b46.pdf"},{"id":92846974,"identity":"a09e1d5d-ba89-4424-add1-0a21eeadd896","added_by":"auto","created_at":"2025-10-06 09:42:42","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":5612573,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-7662320/v1/f32275a0b3afef96e89fe7ab.docx"},{"id":92845794,"identity":"aa77b28b-41c9-41c1-8317-a3ccd26b0179","added_by":"auto","created_at":"2025-10-06 09:34:41","extension":"jpg","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":157164,"visible":true,"origin":"","legend":"","description":"","filename":"Graphicalabstract.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7662320/v1/ccaf1ca44849a0f4c49be61d.jpg"}],"financialInterests":"No competing interests reported.","formattedTitle":"A Recyclable Magnetic Biochar from Corn Cobs and Red Mud for Treating Complex Contaminants Containing Dyes and Heavy Metals","fulltext":[{"header":"Highlights","content":"\u003cul\u003e\n \u003cli\u003eSynergistic utilization of waste materials to produce MMBC-400 for effective co-removal of composite pollutants MG and Pb\u0026sup2;⁺\u003c/li\u003e\n \u003cli\u003eDue to its unique adsorption mechanism, MMBC-400 maintains a considerable adsorption capacity of 129 mg/g for Pb\u0026sup2;⁺ in a binary system with high MG concentration, demonstrating good resistance to interference.\u003c/li\u003e\n \u003cli\u003eExcellent regenerability and reusability, retaining over 85% removal efficiency for both pollutants after five consecutive cycles, showcasing strong potential for practical application.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eThe discharge of complex wastewater co-contaminated with heavy metals and dyes poses a severe threat to ecosystems and human health[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. A significant challenge in treating such wastewater is the competitive adsorption effect between pollutants, which often drastically reduces the removal efficiency of target contaminants. Therefore, developing an adsorbent that can minimize competition, maintain high removal capacity for multiple pollutants, and be easily separated and regenerated is of paramount importance. Among these pollutants,Pb\u0026sup2;⁺ and MG are prevalent pollutants in aquatic environments, characterized by high accumulation potential and strong biotoxicity[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Pb\u0026sup2;⁺, a highly toxic heavy metal, can cause serious health issues even at low concentrations, including damage to the nervous system, kidneys, and reproductive organs [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Conversely, MG, a synthetic triphenylmethane dye, adversely affects aquatic ecosystems by reducing light penetration and gas solubility, thereby inhibiting photosynthesis and compromising the self-purification capacity of water bodies [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].Given their distinct physicochemical properties and severe threats, the effective removal of Pb\u0026sup2;⁺ and MG from water is crucial[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. To date, various methods have been employed for wastewater treatment, such as photocatalysis[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], adsorption[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], coagulation[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], and membrane separation [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. In contrast, adsorption technology has attracted extensive attention due to its simplicity, high efficiency, and low cost[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The search for efficient and low-cost adsorbents has led to the exploration of waste-derived materials. Corn cob is an abundant agricultural waste with high carbon content (45\u0026ndash;50%) and significant potential as a biochar precursor[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], and it has shown a notable affinity for adsorbing dyes like MG[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. On the other hand, red mud (RM), a strongly alkaline and iron-rich industrial solid waste from alumina production, possesses excellent adsorption capabilities for heavy metal ions (e.g., Pb\u0026sup2;⁺) due to its high dispersibility, porosity, and specific surface area[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], Given their complementary functionalities, this study aims to develop a synergistic composite adsorbent using these two waste streams in combination. The designed material is expected to remove dyes through its corn cob-based biochar while immobilizing metals via its red mud component, thereby minimizing competitive effects in treating co-polluted water.\u003c/p\u003e\u003cp\u003eHowever, a critical limitation persists: many reported high-capacity adsorbents excel in removing single pollutants but suffer from severe performance degradation in complex systems due to competitive adsorption. For instance, although a multi-layered chitosan-modified montmorillonite composite achieved an adsorption capacity of 158.79 mg/g for methylene blue (MB)[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. and a lignin-bentonite composite showed a maximum capacity of 157.32 mg/g for Cd\u0026sup2;⁺[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e], their performance in binary or multi-component systems remains largely unverified and likely compromised. Even when high capacities are reported for both pollutants individually, as in the study by Hajar Abara et al. (189.08 mg/g for Pb\u0026sup2;⁺ and 365.31 mg/g for MG)[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], the critical question of their co-adsorption behavior and competitive effects in a binary system is often not adequately addressed.\u003c/p\u003e\u003cp\u003eTo address this research gap, this study investigates the adsorption mechanisms of MMBC-400 for MG and Pb\u0026sup2;⁺ in both single-component and binary systems. It is hypothesized that MMBC-400 will not only maintain a high adsorption capacity in a high-concentration MG solution (500 mg/L) but also retain a stable affinity for Pb\u0026sup2;⁺, exhibiting strong anti-interference capability. This work aims to provide a valuable strategy for developing a practical adsorbent that can resist competitive effects, maintain stable removal capacity for coexisting pollutants, and enable easy separation and regeneration.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials\u003c/h2\u003e\u003cp\u003eCorn cobs were collected from farmland in Hohhot, Inner Mongolia Autonomous Region, China, and RM was collected from an aluminum plant in Ordos, Inner Mongolia Autonomous Region, China.MG (C\u003csub\u003e23\u003c/sub\u003eH\u003csub\u003e25\u003c/sub\u003eClN\u003csub\u003e2\u003c/sub\u003e), hydrochloric acid (HCl), sodium hydroxide (NaOH), and lead nitrate (Pb(NO₃)₂) were supplied by Tianjin Damao Chemical Reagent Factory. All chemical reagents were of analytical grade and prepared with deionized water.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Synthesis of MMBC-400\u003c/h2\u003e\u003cp\u003eSynthesized MMBC-400 was made of raw corn cob and red mud crushed and sieved through 100 mesh, mixed with go 200 ml of ionized water in the ratio of one to one and placed in a magnetic stirrer 800 rmp constant temperature stirring for 2 h. Subsequently, it was oven dried at a constant temperature of 105 ℃, and after drying, it was temperature-controlled burned at 400 ℃ for 2 h in a tube furnace, and then it was milled and sieved through 200 mesh, and it was successfully prepared to be the biochar used for the experiments. Biochar. All the synthesized materials were stored in a desiccator before experimental use. The preparation method was the same as in the previous study[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Characterization\u003c/h2\u003e\u003cp\u003eSEM (German ZEISS Sigma 360) and BET (American Micromeritics ASAP 2460) were used to observe the surface morphology and pore structure of biochar. XPS (American Thermo Scientific K-Alpha) and FTIR (American Thermo Fisher Scientific Nicolet iS20) were employed to analyze surface elemental composition, chemical states, and molecular structures of the materials. XRD (Japanese Rigaku Ultima IV) was utilized to characterize the surface crystal structure. While VSM (American LakeShore 7404) was applied to analyze the magnetic properties of the materials.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4. Single adsorption and co-adsorption experiment\u003c/h2\u003e\u003cp\u003eAdsorption experiments were conducted to evaluate the single and co-adsorption capacities of MMBC-400 for MG and Pb\u0026sup2;⁺. In the single-component system, the initial concentration of MG was 500 mg/L, and that of Pb\u0026sup2;⁺ was 100 mg/L. Adsorption experiments on dosage, pH value, adsorption time, and initial concentration were carried out, and adsorption kinetics and isotherms were also studied. In the binary co-adsorption system, the initial concentration of MG was 500 mg/L, and that of Pb\u0026sup2;⁺ was 100 mg/L. The effects of dosage (10 to 60 mg), pH value, initial concentrations (when the initial concentration of MG was fixed at 500 mg/L, the concentration of Pb\u0026sup2;⁺ ranged from 20 mg/L to 250 mg/L; when the initial concentration of Pb\u0026sup2;⁺ was fixed at 100 mg/L, the concentration of MG ranged from 100 mg/L to 700 mg/L), and contact time on co-adsorption were investigated. Meanwhile, adsorption kinetics and isotherm models were fitted to analyze the competitive adsorption mechanism. The concentrations of MG were quantified by UV-Vis spectrophotometry at 619 nm, and Pb\u0026sup2;⁺ was determined by atomic absorption spectrophotometry (AA-6880F). Each experiment was repeated three times, and the results were recorded as the arithmetic mean of the experimental results. The adsorption capacity and removal rate were calculated using Equ (1) and (2).\n\u003cp\u003e\u003cimg 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67rY2NhQqzoYhhH6us1mU+TzeQFAmKYpVldX3X1q27bIZrPuulmWpT69q7j7slfVatVdZwCR3wO5n0ulkhDt52uaFrrPe1neMAz3/4F8nnf/qlZXV4VpmmoxEdHYsCwr8Dglc4FhGMI0TaFpmnsclsdLr2azKQzD6Lnt6GawrUkP5AdVD/C2bbt1QY1EP+I0oqVSSei6rhb7NJtNXwgJIxuxQe4wL8uyhKZpQtO00PDSbDaFruuRn0klt0O3UGrbtjAMQy0WwhOqoj5/JpMR6CGsSXH2ZT9kKOoWiOQ6qN8B+R97dXXVV97L8qurqx3bTtO0wAOJaO+LqP1FRLSVNjY2Ao/3si3x5oFms+n70aw+R3heL6z969VwWpMY5IdUA5HwNJKDbuyiGlHbtoWmaYFp1EuedVEbNlWz2RzozvKSDWa3MCTl83mRyWTU4q6azabQNE3k83m1ylUqlQK3ldyOaoMeZHV1VWiaphZHitqXUcEgqt6yLIGIQCRDsbr9ZahXQ2gvy8uQ5P3/IZ8fxDTNwP9LRETjwjCMwLaoVCp1HBcl+cM67FhsmmboD/NejXQMUVzPPfecWjQSFy5cQKvVwssvv6xWuQqFAtbX16HrOt5++2212iedTkPTNOzdu1et6kuj0cAbb7wBADh//nzk6+/atQsvvfSSWtxVOp3G7Owszp07h0ajoVYDAK5evRq4rTKZDFqtFjKZTOT4qZ07d2J2dlYt7tvRo0dD+5cdx8Hc3BxqtZpa5frRj36kFvk0Gg2sra0BQMf2n5ychK7rsG3bHYvV6/KffPKJb5luisUi9u3bh5mZGbWKiGgslMtlrK+v49ChQ2oVFhYWOo6L0ptvvqkW+bz22mtYX18PPd73YiwD0UcffQQA0DRNrRoaOTha13VMTk6q1UC7UTt37hwA4NKlS0in0+oiHfbv368W9U0GN8MwsLCwoFYH2rNnj1oUSYaou3fvqlVuSFK3lfzSA8Df/vY3X12YYQTga9eu4eLFix3/SWQYOnnyZF8B4sGDBwAAwzDUKgDA1NQUAOAf//gHsInl9+3bpywRrF6v46OPPsLZs2fVKiKisfHBBx8AfbSJzz77rFoEAPjZz34GALh165Za1bOxC0S1Ws1tUP/4xz921KlXpUU9up0F8PrnP/+JVqvlNkxBLly4AADQdR2vvPKKWh3o9u3balFfGo0Grly5AgA4efKkWh3o1KlTm2r8ZYi6evWqWhV6ddmZM2cAANlsNlZgnJmZ6bjSYBDS6TRu377tC0XeMBR15irKN998A7Tfp5tHjx4Bm1j+Jz/5CQDgv//9r1v34MEDX6ByHAfHjx8P3D9EROPCcRz3DLn6IzrK119/DU3T8Pzzz6tVgOeM+8rKSt9Xp49NIJJzEhw4cABoN6hqQzkzM+O75UacR9wg8OWXXwIRv8xlw5rJZNSqkfGerQnqrhqkHTt2AIAbUL2WlpY63r9er8O2bQDAq6++6qvbCt5QJC/7H0QY6sUXX3yhFnUll5+fn4dhGLh48aI7bUKr1cKf//xnd9m33noLJ0+edA8IlUoFU1NTSKVSmJ+f7/vgQEQ0CF999RXQ5Qx5GNlzc/78+a4/JmVvknyfzdryQHTgwAGkUin89Kc/xenTp5HNZlGtVnH58mV10aGSYzZ27dqlVgHtxr7VagEAXnjhBbV6ZO7duwe0v1jdviCD4O3T9Z5pq9fr0DStI+nL7h4AoWl+1NLpNFZWVvC73/0O09PTIw1DADAxMaEWdeVd/vbt29B1HT/84Q+xtLQEy7LcM5MynMvPU6vV8Itf/AJ/+MMf0Gw2Yds25ubm3NciItoq9+/fB2KcIVe9//772L9/f+TQENkNJ99ns7Y8EFWrVQghYJomAOCzzz7DM888oy4GDLnLLMp3333n/r1z505f3Sh9++23wCa+WIN0/fr1wO4y2d2DLV4/L8dxkMlkUCqV8Nlnn3WMKRq23bt3q0VdeZdPp9Mol8sQQuDhw4du+KnX67h48aJvjNZf//pXoD04MZ1O4+TJkwMbaEhENGr1eh3vvvsurl27plYNzZYHIunatWvQNA3r6+v405/+pFYDQ+4yGwdhgW/cfumvrKx0dJcNmuz6SaVSoVe5RfGOGVpYWOgYU9SPsDOJKrlcr8t3I8cNeUPnysqK+6MCnqvk/vWvf7llRETbgRwfee/evZH+uB6bQJROp93uoCtXrsS6ZcIoPfXUU+7fjx8/9tUNyo4dO2CaZsfDO65JdlNt1fiQer0OwzA6ussA4Omnn3b/7mf9UqkU3nzzTQghUK1WN3VZftAA6qCB1pslr2yQV4+pZLlcrtflwxQKBfzqV78KvURV9fnnn6tFRERjy3EcHD16FFevXo19nBsYdWKiUZGT6qmTyXlveRE1qV+vuk3mJyfC6zbZYpzZqaVqtdoxU/EgeD9D0Mydqm4TK0aRE0t6t1c+nw/dL3Iyx7j7rlQqdXyGbDbrm6BQTlqoLtdtX4r2vgpbBznlu/rd85KvHzYZmPBM0KlOKCbXWZ0srNflVdVqNXSmarRvjyLJ9e9n/xMRDYI8HqmT1arksVk9RkaRx9Zux/Q4gluTEZCNWdAHkDNVx5mFuRfdGlE583NYgyOU54c1tqI9nfgwGyIZzOR9rsLk8/nQ7eedEj3sM8vP622oo24PIYNl1L6zLKtjG8pg4C3fbCDqto4iRn2cmarDQlPQrTjEJpb3krdgUbeDpM5iLddf3cZERKMW9ONaJcNQ2DHLsqzQY6R87ajjepTwtRsi2QAg5BesPPjLZRYXF7s2rnFFNaLyvmDdWO37h6F9psi7XhsbGyKbzQZ+Ji/v51cbxziannupyRvLyi9Cs9kUVvvmeWHbzPu+cpsEUUPBxsZG4LTrKnkfME3TxOLioq8RX11dFZlMJvBLL7eLl1wHNQhE7ct+yVAeFTqz2azQNM0N9nJ9w84i9rq8ZLZvjhtGbo9SqeQeWKLCKxHRqMhjalC7JI9Z8n6N6kPX9dC2Wf5oDvth34vhtCYhvI2Y+lCDwcbGhhs8wpbpVVQjKs+aqI2vyrZtkc/n3dN0aAeTrHK3+SBql5Cu65HPCWNZlshkMr7tZJqm2ygGMU3Ttx0XFxdDv0jq9ujWXaaqVqvuZ5XrZhiGyOfzodtX1/WOdZHroIral5vlDaveR7fvnrwRrvyM3YKL2OTyUSFbtMOmfF3TNEO3MxHRqHU7a+1tS8MeYcfAbq/bq8G2JmMuqhG1Y97cdbPk+3sDkBpQhkl+cSRE9OlqETd3HTQEBI+gkCRi7EsiIhovRsjNXfuRyWQix1/GNTZXmY2DyclJnD9/HktLS31dJRXmnXfegWmaWzYVwKeffgq0r+JKpVKwbRsPHz5UFwPaE/9NTEzgxIkTatVQ/fznP3f/rtVqsG0bv/3tb33LEBHR9nP16lWsrKy4N7HuV6PRwMrKysBuX8RApFhYWMDs7Czee+89taov8m7nauMu7+8yCo1GA6ZpQs7RNDk5iUaj0XEJuuM4uHjxIm7evDnSOSBUMkCOenZpIiIavL1798KyrMCJfTcjn8/DsqyBXZ7PQBRA3jZEDQr9uHHjBqDc6VfOou2dv2eYDh48iLW1NXeiw0ajAV3XfYFjK+eAME3TvZlpsVjE2trayG/hQkREwzM/P4+TJ0+iUCioVT3J5XI4dOjQQH8wp8T3YzcSoVaruTePjfOxK5UKHj9+PJANnsvlcOfOHV8XVbFYxOnTp2HbduBEh8MwNTXl3oBV13Xf+jQaDdy4cQPHjh3bsjNDqVTK/bvbPup1XxIR0fio1+tYW1vruIl7HMViEaZpDvxHOwPRiORyOcBz9gntcDI1NYXbt297lqQ4tnJfEhHRk4ddZlukXC7Dtm12CREREY2BxAYi781TR+HFF1/0jd05cuQIFhcXR9ZV9iQoFovuPpNnh4iIiAYhUV1mWy2Xy+HKlSsAAMuyBjI2iYiIiPrHQERERESJl9guMyIiIiKJgYiIiIgSj4GIiIiIEo+BiIiIiBKPgYiIiIgSj4GIiIiIEo+BiIiIiBKPgYiIiIgSj4GIiIiIEo+BiIiIiBKPgYiIiIgSj4GIiIiIEo+BiIiIiBKPgYiIiIgSj4GIiIiIEo+BiIiIiBKPgYiIiIgSj4GIiIiIEo+BiIiIiBLv/wE5IyB/4mJjCQAAAABJRU5ErkJggg==\" style=\"width: 431px; height: 107.007px;\" width=\"431\" height=\"107.007\"\u003e\u003c/p\u003e\u003c/p\u003e\u003cp\u003eWhere, \u003cem\u003eC₀\u003c/em\u003e (mg/L) and \u003cem\u003eCₑ\u003c/em\u003e (mg/L) represent the initial and equilibrium concentrations of the MG and Pb\u003csup\u003e2+\u003c/sup\u003e solution, respectively; \u003cem\u003eV\u003c/em\u003e (L) denotes the volume of the MG and Pb\u003csup\u003e2+\u003c/sup\u003e solution; \u003cem\u003em\u003c/em\u003e (g) indicates the mass of the adsorbent; \u003cem\u003eqₑ\u003c/em\u003e (mg/g) is the equilibrium adsorption capacity of the adsorbent for MG and Pb\u003csup\u003e2+\u003c/sup\u003e; \u003cem\u003eR\u003c/em\u003e represents the removal rate.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Characterization of biochar\u003c/h2\u003e\u003cp\u003eThe SEM results of the biochar are presented in S1. The SEM image of MMBC-400 Fig.\u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e(a) reveals a tunnel-like porous structure with clear surface textures, which provides a larger contact area for subsequent adsorption processes. The porous morphology and smooth wrinkled structure of the biochar are favorable for the attachment of metal ions (Fe\u0026sup2;⁺/Fe\u0026sup3;⁺, Al\u0026sup3;⁺) and non-metallic species (K⁺/Ca\u0026sup2;⁺, Si⁴⁺) from red mud to the biochar surface[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. In Fig.\u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e(c), the MMBC-400 composite clearly shows numerous irregular spherical particles heterogeneously embedded in the carbon matrix or distributed on the biochar surface, confirming the successful integration of red mud components into the carbon matrix.\u003c/p\u003e\u003cp\u003eThe N₂ adsorption-desorption isotherms and pore size distribution of the material are shown in Fig.S2. The adsorption capacity gradually increases with the rise of relative pressure (P/P₀). According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), MMBC-400 exhibits a type IV isotherm with an H3 hysteresis loop, confirming the coexistence of micropores and mesopores. Generally, micropores provide adsorption sites, while mesopores serve as diffusion channels, both of which are applicable for the treatment of organic pollutants[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The pore structure and specific surface area of the biochar synthesized using red mud and corn cobs are higher than those reported in previous studies on modified corn cobs[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. This is because the compact external structure of corn cobs may inhibit the volatile release during the carbonization process[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. However, the addition of red mud effectively overcomes this structural limitation, leading to significant pore reorganization, as evidenced by the expanded porosity in MMBC-400. The evolution of the microstructure confirms that red mud-derived iron/aluminum oxides and oxygen vacancies play a key role in porosity optimization. They synergistically promote the pollutant adsorption process by generating abundant active sites[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe hysteresis loop of the synthesized material is shown in Fig.S2(b), indicating that the material exhibits obvious magnetism due to the incorporation of Fe elements from red mud. As the magnetic field strength increases, the magnetization intensity gradually rises until reaching a saturation magnetization value of 10.23 emu/g. This confirms the self-generated magnetism of MMBC-400 during the pyrolysis process. The material possesses natural sedimentation capability, enabling solid-liquid separation without external intervention. Moreover, under the action of an external magnetic field, rapid and thorough solid-liquid separation can be achieved by virtue of the highly sensitive magnetic properties of MMBC-400, which effectively enhances the recyclability of the material and the practicality of operation in wastewater treatment systems[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Individual adsorption of MG and Pb\u003csup\u003e2+\u003c/sup\u003e on MMBC-400\u003c/h2\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e3.2.1. The effect of MMBC-400 dosage\u003c/h2\u003e\u003cp\u003eThe dosage of MMBC-400 also affects the adsorption efficiency of MG and Pb\u0026sup2;⁺. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, with the increase of MMBC-400 dosage, the adsorption efficiency of both MG and Pb\u0026sup2;⁺ increases, while the adsorption capacity gradually decreases, indicating better adsorption performance. For MG adsorption, the removal rate remains stable when the dosage ranges from 0.04 g to 0.06 g, suggesting that adsorption gradually reaches saturation. The removal rates exceed 99% at dosages of 0.05 g and 0.06 g, with the adsorption capacity reaching 793.512 mg/g. This can be attributed to the increased interaction and collision probability between adsorption sites and MG molecules due to the higher adsorbent content in the solution. Therefore, 50 mg was selected as the optimal dosage for subsequent experiments in the MG system. For Pb\u0026sup2;⁺ adsorption, the maximum removal rate of 91.1% and adsorption capacity of 121.4 mg/g were achieved at a dosage of 60 mg, which is explained by the increase in active sites, specific surface area, and the amount of biochar [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. The aim of this experiment was to maximize removal using as little biochar as possible, so 60 mg of MMBC-400 was selected as the optimal dosage f or subsequent experiments.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e3.2.2. The effect of pH\u003c/h2\u003e\u003cp\u003eThe pH value of the solution is also a crucial factor affecting adsorption, which is directly related to the types of pollutants in the solution and the chemical properties of the adsorbent. With the continuous increase of pH, the MG solution undergoes obvious color reaction and transforms into colorless malachite green alcohol base due to the enhanced alkalinity[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e], which will have a certain impact on adsorption. Therefore, the adsorption of MG by MMBC-400 in this experiment was carried out at a solution pH of 2\u0026ndash;10 for 24 hours. Precipitation and coating phenomena of Pb\u0026sup2;⁺ occur near neutral and higher pH values, so the adsorption of Pb\u0026sup2;⁺ by MMBC-400 was performed at a solution pH of 2\u0026ndash;6.\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a), the adsorption of MG by MMBC-400 exhibits a positive correlation between pH value and adsorption capacity, with the maximum adsorption capacity reaching 794.72 mg/g and the removal rate achieving 99.33%. According to the zeta potential measurement results, the surface charge of MMBC-400 varies at different pH values. When the pH is low, MMBC-400 carries a net positive charge. Therefore, in a highly acidic environment (e.g., pH\u0026thinsp;=\u0026thinsp;2), the interaction between cationic dyes and the positively charged surface of MMBC-400 is electrostatically repulsive, which reduces the effectiveness of MMBC-400 in removing MG[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. When the pH is high, MMBC-400 carries a negative charge, showing attractive interaction with cationic dyes. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u0026thinsp;\u0026minus;\u0026thinsp;2, the adsorption results of Pb\u0026sup2;⁺ by MMBC-400 clearly demonstrate that when the solution pH increases from 3 to 5, the adsorption of Pb\u0026sup2;⁺ increases rapidly, while it slightly decreases at pH 6. Under the optimal condition of pH 5, the removal efficiency of Pb\u0026sup2;⁺ reaches 96.82%, and the adsorption capacity achieves 129 mg/g. This may be because Pb\u0026sup2;⁺ forms cationic species in the pH Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b)[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], Pb\u0026sup2;⁺ is adsorbed onto the negatively charged surface of MMBC-400 through strong electrostatic forces. At lower pH values (e.g., pH 2), the surface charge of MMBC-400 is uniform with that of Pb\u0026sup2;⁺ in the solution, and electrostatic repulsion exists[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. This is unfavorable for the adsorption of Pb\u0026sup2;⁺ by MMBC-400. Therefore, pH values of 7 and 5 for the adsorption of MG and Pb\u0026sup2;⁺ by MMBC-400 were selected as the optimal conditions for subsequent experiments.\u003c/p\u003e\u003cp\u003eIn addition, since MMBC-400 contains red mud components, crystalline minerals dissolved at lower pH values may release cations such as K⁺, Ca\u0026sup2;⁺, and Mg\u0026sup2;⁺. These cations can compete with MG and Pb\u0026sup2;⁺ for adsorption sites on the surface of the adsorbent, thereby reducing the removal rate and adsorption capacity[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e3.2.3. The effect of adsorption time\u003c/h2\u003e\u003cp\u003eAdsorption contact time is also a critical factor in evaluating adsorption performance. The influence of contact time on the adsorption of MG and Pb\u0026sup2;⁺ is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The adsorption processes of MG and Pb\u0026sup2;⁺ by MMBC-400 within the temperature range of 293、298、303 K exhibit typical two-stage mass transfer characteristics, Initial stage (0\u0026ndash;240 min): A rapid increase in adsorption capacity due to the abundance of surface active sites. This can be attributed to the significant chemical potential difference between the initial solution concentrations (500 mg/L for MG and 100 mg/L for Pb\u0026sup2;⁺) and the solid-phase adsorption capacity, which drives molecular diffusion into the mesoporous structure. Additionally, the electrostatic attraction between MMBC-400 and MG cations/Pb\u0026sup2;⁺ facilitates rapid binding at the early stage, leading to a steep upward curve. Diffusion-controlled stage: The mass transfer rate slows down significantly due to intraparticle diffusion resistance. The equilibrium adsorption capacities for MG at 293 K, 298 K, and 303 K are 680.2 mg/g, 733.5 mg/g, and 793.4 mg/g, respectively; those for Pb\u0026sup2;⁺ are 120.58 mg/g, 130.22 mg/g, and 136.11 mg/g, respectively, indicating a positive temperature effect [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]。After 240 min, as the contact time increases, the probability of intermolecular collisions rises, and adsorption sites become progressively occupied by MG and Pb\u0026sup2;⁺, leading to adsorption equilibrium and a plateau in the curve.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\u003ch2\u003e3.2.4. The effect of initial concentration\u003c/h2\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the adsorption behavior of MG and Pb\u0026sup2;⁺ by MMBC-400 exhibits significant concentration gradient effects and temperature dependency. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a), experimental data show that at three different temperatures, when the initial concentration of MG increases from 100 mg/L to 700 mg/L, the equilibrium adsorption capacity at 298 K linearly increases from 152.3 mg/g to 793.6 mg/g. This may be because when the MG concentration is low, the concentration gradient between malachite green in the solution and the biochar surface is small, resulting in a low diffusion rate and limited adsorption capacity. With the increase of concentration in the solution, the diffusion rate accelerates, enabling more effective collisions, and the adsorption sites are gradually filled [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b), the effect of initial solution concentration on the adsorption of Pb\u0026sup2;⁺ by MMBC-400 demonstrates that the adsorption capacity gradually increases with the increase of Pb\u0026sup2;⁺ concentration. However, when the concentration reaches 300 mg/L, the curve tends to flatten, with adsorption capacities of 322.12 mg/g, 289.7 mg/g, and 210.02 mg/g, respectively.\u003c/p\u003e\u003cp\u003eIn the Pb\u0026sup2;⁺ system, at lower initial concentrations, Pb\u0026sup2;⁺ is first adsorbed onto the abundant reactive regions of MMBC-400. The adsorption capacity of Pb\u0026sup2;⁺ is low at lower concentrations and gradually increases rapidly with the increase of ion concentration. This is because after a single ion layer with lower concentration is formed on the adsorbent surface, the further formation of this layer is severely hindered at higher concentrations due to the interaction between metal ions on the surface and those in the solution. Additionally, at low metal ion concentrations, the ratio of Pb\u0026sup2;⁺ to the available surface area of the adsorbent is large; subsequently, the adsorption capacity becomes independent of the initial concentration of Pb\u0026sup2;⁺. As the Pb\u0026sup2;⁺ concentration gradually increases, Pb\u0026sup2;⁺ is continuously adsorbed onto MMBC-400, making the available adsorption sites of the adsorbent fewer. Therefore, the adsorption capacity of Pb\u0026sup2;⁺ at higher concentrations gradually increases and then tends to flatten[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Adsorption temperature is also a key parameter affecting the performance of adsorbents. As can be seen from the figure, an increase in temperature is more conducive to the adsorption of pollutants. Increasing the adsorption temperature can enhance the mobility of molecules, thereby improving the possibility of interaction between the adsorbent and MG as well as Pb\u0026sup2;⁺.\u003c/p\u003e\u003cp\u003eThe above experiments confirm that MMBC-400 is an excellent biochar material for removing the dye MG and Pb\u0026sup2;⁺ in single-component adsorption systems. However, the coexistence of inorganic and organic pollutants is common in the environment[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. The adsorption capacity of MMBC-400 may change when MG and Pb\u0026sup2;⁺ coexist due to different adsorption mechanisms. This study further investigates the removal capability of MMBC-400 in the co-contaminated system of MG and Pb\u0026sup2;⁺.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Simultaneous adsorption of Pb\u003csup\u003e2+\u003c/sup\u003e and MG on MMBC-400\u003c/h2\u003e\u003cdiv id=\"Sec15\" class=\"Section3\"\u003e\u003ch2\u003e3.3.1 Adsorbent dosage\u003c/h2\u003e\u003cp\u003eAs shown in the dosage diagram of the MG-Pb\u0026sup2;⁺ binary system Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a, b) with the adsorbent dosage increasing from 10 mg to 60 mg, the overall removal rates of both MG and Pb\u0026sup2;⁺ first rise rapidly and then tend to plateau. This phenomenon can be attributed to the following reasons: at lower dosages (10\u0026ndash;30 mg), the number of adsorption sites is insufficient, and the pollutant concentration far exceeds the adsorption capacity, leading to a rapid increase in removal rates. At higher dosages (40\u0026ndash;60 mg), the active sites tend to reach saturation, and the diffusion resistance increases, resulting in a slowed growth of removal rates due to the adsorbent excess effect. However, the decrease in unit adsorption capacity may be caused by the intensified agglomeration of adsorbents for MG and Pb\u0026sup2;⁺, which reduces the utilization efficiency of specific surface area, leaving some adsorption sites underutilized or competitively occupied. In the binary adsorption diagram, at lower dosages (10, 20, and 30 mg), Pb\u0026sup2;⁺ is preferentially adsorbed, with its removal rate significantly higher than that of MG, while MG surpasses Pb\u0026sup2;⁺ in removal rate at a later stage. When the dosage exceeds 40 mg, the removal rate of MG overtakes that of Pb\u0026sup2;⁺. This can be explained by the strong coordination of Fe/Al oxides in red mud with heavy metals, such as surface hydroxyl complexation (as shown in the Equ.3) and ion exchange\u0026mdash;Ca\u0026sup2;⁺/Na⁺ from red mud in MMBC-400 raw materials can exchange with Pb\u0026sup2;⁺[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. The later surpassing of MG can be attributed to the enhanced π-π stacking effect of the porous structure of corn cob-derived biochar on organic dyes at higher dosages. Additionally, the molecular size of MG is smaller than that of hydrated Pb\u0026sup2;⁺ ions, enabling it to more easily enter micropores. Therefore, 40 mg was selected as the optimal dosage for subsequent experiments in the binary system.\u003c/p\u003e\u003cp\u003e\u003cem\u003e\u0026equiv;Fe-OH₂⁺ + Pb\u0026sup2;⁺ \u0026rarr; \u0026equiv;Fe-O-Pb⁺ + 2H⁺\u003c/em\u003e (3)\u003c/p\u003e\u003cp\u003e\u003cem\u003e\u0026equiv;Fe-O-Pb⁺ + Dye-N⁺(CH₃)₂ \u0026rarr; \u0026equiv;Fe-O\u0026middot;\u0026middot;\u0026middot;⁺N(CH₃)₂-Dye\u0026thinsp;+\u0026thinsp;Pb\u0026sup2;⁺\u003c/em\u003e (4)\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section3\"\u003e\u003ch2\u003e3.3.2 pH of the solution\u003c/h2\u003e\u003cp\u003eThe pH value of the solution is a critical factor that significantly affects the removal efficiency of pollutants by biochar. As shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c, d)., the adsorption of MG by MMBC-400 exhibits a gradual increase in the pH range of 2\u0026ndash;3. This can be attributed to the protonation of the material surface, which carries a positive charge and thus generates electrostatic repulsion with MG. Meanwhile, the high concentration of H⁺ competes for adsorption sites, and the aromatic structure of corn cob-derived biochar provides a small amount of π-π stacking interactions. In the pH range of 4\u0026ndash;6, the adsorption capacity gradually reaches a peak. This is likely because the material surface undergoes deprotonation and carries a negative charge, resulting in electrostatic attraction with MG. Additionally, the -N(CH₃)₂ groups in MG form stronger hydrogen bonds with the -OH/O⁻ groups on the surface of MMBC-400, and the π-π conjugation is maximized[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. In the same system, the adsorption trend of Pb\u0026sup2;⁺ by MMBC-400 shows a gradual increase in the pH range of 2\u0026ndash;4, reaching a peak. This is because the hydroxyl groups on the surface of Fe₂O₃ and Al₂O₃ (components of red mud in the prepared MMBC-400) provide abundant cation exchange sites for Pb\u0026sup2;⁺, as shown in Equ.4. Meanwhile, the ion exchange between Ca\u0026sup2;⁺/Na⁺ in red mud plays a dominant role[\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. In the pH range of 5\u0026ndash;6, OH⁻ in the solution reacts with Pb\u0026sup2;⁺ to form soluble Pb(OH)⁺ or Pb₂(OH)\u0026sup3;⁺, thereby reducing the concentration of free Pb\u0026sup2;⁺. When pH\u0026thinsp;\u0026gt;\u0026thinsp;5.5, Pb(OH)₂ precipitates are generated, resulting in an artificially high apparent removal rate that does not reflect the actual adsorption.\u003c/p\u003e\u003cp\u003eAs shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c, d). for the competitive adsorption of MG and Pb\u0026sup2;⁺ by MMBC-400 in the binary system, Pb\u0026sup2;⁺ dominates the adsorption within the pH range of 2\u0026ndash;3. This is likely due to the occupation of metal-binding sites in red mud components of MMBC-400 by Pb\u0026sup2;⁺, resulting in the exclusion of MG. At pH 4\u0026ndash;5, Pb\u0026sup2;⁺ maintains a high adsorption capacity, while the adsorption of MG is enhanced through π-π interactions and hydrogen bonding, leading to a peak of synergistic removal at pH 5. Specifically, the removal efficiency of MG reaches 98.4% with an adsorption capacity of 787.56 mg/g, and the removal efficiency of Pb\u0026sup2;⁺ reaches 93.8% with an adsorption capacity of 128.24 mg/g. At pH 6, the electrostatic adsorption of MG is maximized, whereas the actual adsorption of Pb\u0026sup2;⁺ decreases due to the formation of hydroxyl complexes induced by precipitation, resulting in the removal efficiency of MG surpassing.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\u003ch2\u003e3.3.3 Solution concentration and contact time\u003c/h2\u003e\u003cp\u003eAs shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) for the binary system at 298 K, Pb\u0026sup2;⁺ achieves rapid adsorption within 0\u0026ndash;10 min. This is likely attributed to the instantaneous coordination between \u0026equiv;\u0026thinsp;M-OH groups on the surface of Fe/Al oxides and Pb\u0026sup2;⁺, as well as the rapid displacement of soluble Ca\u0026sup2;⁺/Na⁺ in red mud, with red mud contributing predominantly. However, due to the limited adsorption sites for Pb\u0026sup2;⁺, adsorption equilibrium is gradually reached at 120 min. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b), the adsorption of MG by MMBC-400 proceeds in two stages: the rapid stage (0\u0026ndash;120 min) involves macropore/mesopore diffusion, where MG molecules enter the pores of the biochar material\u0026mdash;mainly contributed by corn cobs\u0026mdash;and the aromatic structure of the biochar adsorbs the benzene rings of MG through surface π-π stacking. The slow stage (120\u0026ndash;480 min) is dominated by micropore filling, where the -N(CH₃)₂ groups in MG form multiple hydrogen bonds with -COOH/-OH groups on the surface of MMBC-400. It can be seen that, due to its constituent raw materials, MMBC-400 exhibits excellent adsorption performance for both MG and Pb\u0026sup2;⁺ in the binary system.\u003c/p\u003e\u003cp\u003eHowever, a competitive effect exists between MG and Pb\u0026sup2;⁺, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c). In the first 30 min, Pb\u0026sup2;⁺ preferentially occupies the metal-binding sites of red mud, inhibiting the adsorption of MG on red mud components. After 60 min, MG avoids competition through the pores of biochar, and independent adsorption channels are activated. When MG reaches equilibrium after 240 min, Pb\u0026sup2;⁺ begins to be displaced by the hydrophobic groups of MG, resulting in a slight decrease in its adsorption efficiency.\u003c/p\u003e\u003cp\u003eAs shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(d) when the MG concentration is fixed at 500 mg/L and the Pb\u0026sup2;⁺ concentration increases from 20 mg/L to 250 mg/L, the removal efficiency of MG reaches the highest at 20 mg/L of Pb\u0026sup2;⁺, gradually decreases with the increase of Pb\u0026sup2;⁺ concentration, hits the lowest at 200 mg/L of Pb\u0026sup2;⁺, and partially recovers at 250 mg/L. This phenomenon may be explained by the pore shielding effect induced by high-concentration Pb\u0026sup2;⁺: Pb\u0026sup2;⁺ ions and [Pb(OH)]⁺ species block mesopores, preventing MG from entering the pores. When the ion concentration reaches saturation at 250 mg/L, part of MG is adsorbed on the carbon surface through hydrophobic interactions. However, the removal efficiency of Pb\u0026sup2;⁺ decreases continuously. This is because the total cation exchange capacity of MMBC-400 is overloaded, accompanied by electrostatic repulsion. Moreover, the high concentration of Pb\u0026sup2;⁺ causes the surface of the material to be positively charged, inhibiting the subsequent adsorption of Pb\u0026sup2;⁺.\u003c/p\u003e\u003cp\u003eWhen the concentration of Pb\u0026sup2;⁺ is kept constant at 100 mg/L and the concentration of MG increases from 100 mg/L to 700 mg/L, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(e), the removal efficiencies of both MG and Pb\u0026sup2;⁺ decrease continuously. At an MG concentration of 100 mg/L, there is no significant adsorption competition. As the MG concentration gradually increases, the removal efficiencies decrease due to mild competition for functional groups and competition for pore channels between the two. When the MG concentration exceeds 500 mg/L, the dye layer hinders the diffusion of Pb\u0026sup2;⁺. When the MG concentration reaches 700 mg/L, most of the MMBC-400 surface is covered, resulting in a relatively low removal efficiency.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003e3.4. Adsorption modeling\u003c/h2\u003e\u003cdiv id=\"Sec19\" class=\"Section3\"\u003e\u003ch2\u003e3.4.1 Equilibrium isotherm\u003c/h2\u003e\u003cp\u003eAdsorption isotherms are used to study the equilibrium relationship between adsorbents and adsorbates under a certain temperature condition. In this study, the Langmuir and Freundlich models were employed. From the fitting results, Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e it can be seen that in the binary system, the fitting degree of the Langmuir model for Pb\u0026sup2;⁺ (R\u0026sup2; = 0.999) is higher than that for MG (R\u0026sup2; = 0.977), indicating that the adsorption of Pb\u0026sup2;⁺ on MMBC-400 is a monolayer adsorption on a uniform surface, which occurs at a fixed number of localized sites on the adsorbent surface with no interaction between the adsorption sites[\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. In the Freundlich model, the fitting parameter for MG is higher than that for Pb\u0026sup2;⁺, suggesting that the adsorption of MG by MMBC-400 is suitable for multi-layer adsorption on a heterogeneous surface. This adsorption occurs on the adsorbent surface, which is inherently heterogeneous, resulting in the formation of multi-layer coverage on the surface[\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. These results indicate that the adsorption of MG and Pb\u0026sup2;⁺ by MMBC-400 in the binary system conforms to different adsorption mechanisms, and the competitive adsorption between them is weak.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section3\"\u003e\u003ch2\u003e3.4.2 Adsorption kinetics\u003c/h2\u003e\u003cp\u003eAdsorption kinetics is an indicator parameter for understanding adsorption mechanisms, adsorbent performance, and adsorption rates[\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e], Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a,b,c,d,) Adsorption kinetics is an indicator parameter for understanding adsorption mechanisms, adsorbent performance, and adsorption rates [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. In this study, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were used to evaluate the co-adsorption kinetic data of single-component and binary systems. The Table.S2 shows the mutual influence of adsorption kinetics of MMBC-400 on MG and Pb\u0026sup2;⁺ in the binary system. At 298 K and under the same adsorption time, the pseudo-first-order parameter (r\u0026sup2; = 0.948) for MG is significantly higher than that for Pb\u0026sup2;⁺ (r\u0026sup2; = 0.693) under the same conditions. The low fitting degree of Pb\u0026sup2;⁺ suggests that the chemical complexation on the surface of red mud is an instantaneous reaction, which does not conform to the physical adsorption assumption of the pseudo-first-order model. The moderate fitting for MG indicates that the initial stage is controlled by film diffusion, while the micropore diffusion deviates from linearity in the later stage.\u003c/p\u003e\u003cp\u003eIn the pseudo-second-order kinetic parameters table, both MG and Pb\u0026sup2;⁺ show good fitting results, with Pb\u0026sup2;⁺ exhibiting a nearly perfect fit, confirming that the adsorption of Pb\u0026sup2;⁺ by MMBC-400 is dominated by chemical adsorption, and surface complexation is the rate-controlling step. The high fitting degree for MG also indicates that its adsorption is mainly chemical adsorption, which may be attributed to the π-π stacking in the early stage and the formation of hydrogen bonds between -N(CH₃)₂ groups and -OH groups in the later stage.\u003c/p\u003e\u003cp\u003eThe intra-particle diffusion model is the optimal model for identifying adsorption diffusion mechanisms. In the intra-particle diffusion model, if the line passes through the origin, the adsorption process is controlled by intra-particle diffusion; if the data do not pass through the origin, external mass transfer and intra-particle diffusion may occur simultaneously (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(e,f)). Therefore, the entire adsorption process is controlled by both external mass transfer and intra-particle diffusion[\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. It can be observed that after 240 min, as MG reaches equilibrium, the adsorption efficiency of Pb\u0026sup2;⁺ decreases slightly due to displacement by the hydrophobic groups of MG. The data for Pb\u0026sup2;⁺ form a single straight line, indicating that the surface reaction is free of diffusion limitations. In contrast, the intra-particle diffusion of MG is divided into three stages: the first stage involves rapid filling of mesopores; the second stage is characterized by inhibited micropore diffusion due to Pb\u0026sup2;⁺; and the third stage gradually reaches equilibrium.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section3\"\u003e\u003ch2\u003e3.4.3 Adsorption thermodynamics\u003c/h2\u003e\u003cp\u003eTo investigate the thermodynamic behavior of MG and Pb\u0026sup2;⁺ adsorption onto MMBC-400, thermodynamic studies were conducted in a binary system. The thermodynamic parameters are presented in the Table S3. According to the data, the adsorption of MG onto MMBC-400 is an endothermic (ΔH\u0026thinsp;=\u0026thinsp;50.41 kJ/mol) and entropy-increasing (ΔS\u0026deg; = 241.5 J/mol\u0026middot;K) process. These parameters align with the synergistic mechanism of ion exchange and π-π stacking revealed by structural characterization.\u003c/p\u003e\u003cp\u003eThe adsorption of Pb\u0026sup2;⁺ onto MMBC-400 is also endothermic, possibly due to the dehydration of Pb\u0026sup2;⁺ during ion exchange (an endothermic step). The release of water molecules upon adsorption onto the solid surface increases the system entropy, driving the adsorption process. Additionally, the adsorbed Pb\u0026sup2;⁺ may gain greater vibrational entropy on the solid surface, further contributing to the entropy increase. Notably, all ΔG values are negative, indicating spontaneous adsorption. The increasingly negative values with rising temperature suggest enhanced spontaneity, consistent with previous studies[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec22\" class=\"Section2\"\u003e\u003ch2\u003e3.5. Adsorption mechanisms of individual and simultaneous mixture of MG and Pb(II) on MMBC-400\u003c/h2\u003e\u003cp\u003eThe FTIR spectra of MMBC-400 before and after adsorption of MG and Pb\u0026sup2;⁺ are shown in Fig.\u0026nbsp;8(a), displaying significant spectral changes. After adsorption, the peak width and intensity of -OH (3414 cm⁻\u0026sup1;) increased in both MG and Pb\u0026sup2;⁺ systems, accompanied by enhanced absorption intensity, indicating the formation of hydrogen bonds during the adsorption process. In both the MG system and the MG-Pb\u0026sup2;⁺ binary system, aliphatic C-H bending vibrations were observed at 2850\u0026ndash;2900 cm⁻\u0026sup1; with increased intensity[\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. This may be attributed to the monomeric adsorption of MG in the single-component system, while in the binary system, the presence of Pb\u0026sup2;⁺ might lead to the formation of certain compounds between MG and Pb\u0026sup2;⁺, resulting in enhanced peak intensity. However, no such peak change was observed in the Pb\u0026sup2;⁺ system. The Al-O oscillation at 600 cm⁻\u0026sup1; shifted to 818.6 cm⁻\u0026sup1;, indicating the coordination complexation between Al\u0026sup3;⁺ and MG molecules[\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. The characteristic absorption bands at 1710 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;O stretching vibration) and 1230 cm⁻\u0026sup1; (C-O stretching mode) confirmed the presence of carboxyl functional groups (-COOH)[\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e], he peak intensity increased in the Pb\u0026sup2;⁺ system but decreased in the MG system; in the binary system, the peak disappeared possibly due to the formation of (-COO)₂Pb. In the MG system, bands consistent with the aromatic structure of MG appeared at 1611 cm⁻\u0026sup1; (aromatic ring stretching) and 1517 cm⁻\u0026sup1; (C\u0026thinsp;=\u0026thinsp;C skeletal vibration), confirming the successful immobilization of pollutants with enhanced intensity[\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], However, in the binary system, the reduced peak intensity indicated inhibited π-π stacking. A new peak at 480 cm⁻\u0026sup1; (Pb-O) appeared in both the Pb\u0026sup2;⁺ system and the binary mixed system, providing direct evidence for the successful adsorption of Pb\u0026sup2;⁺ onto MMBC-400.\u003c/p\u003e\u003cp\u003eThe XPS spectra of MMBC-400 before and after adsorption of MG and Pb\u0026sup2;⁺ are shown in Fig.S3.and Fig.S4 The adsorption mechanism can be inferred from the analysis of O1s, C1s, Fe2p, and Pb4f spectra. The C1s peak at 284.8 eV, corresponding to C-C/C\u0026thinsp;=\u0026thinsp;C, showed no significant change after the individual adsorption of MG and Pb\u0026sup2;⁺, indicating that the aromatic ring skeleton of the biochar is stable and not easily deformed. The C-O bond at 286 eV[\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e], exhibited significant changes after the adsorption of MG and Pb\u0026sup2;⁺, suggesting that oxygen-containing functional groups are involved in the binding of MG. Additionally, the reduced intensity of the C-O peak is attributed to the formation of complexes between carboxyl groups (-COOH) and Pb\u0026sup2;⁺ (e.g., Pb-OOC-R). The peak area also decreased in the binary system. In the Pb\u0026sup2;⁺ system, the C\u0026thinsp;=\u0026thinsp;O peak shifted from 288 eV to a higher binding energy of 289.18 eV, indicating the transfer of lone-pair electrons from carboxylic oxygen to Pb\u0026sup2;⁺ through coordination. The peak at 532 eV, corresponding to hydroxyl groups (-OH), showed a significant decrease in intensity in the Pb\u0026sup2;⁺ system, indicating the consumption of -OH due to coordination with Pb\u0026sup2;⁺ Fig.S4(d) [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e], Meanwhile, a new peak at 533.5 eV appeared after Pb\u0026sup2;⁺ adsorption, corresponding to Pb-O bonds (e.g., Pb₃(CO₃)₂(OH)₂). The peak at 534 eV, corresponding to carboxylic oxygen (O-C\u0026thinsp;=\u0026thinsp;O), shifted after MG adsorption, suggesting hydrogen bonding between -N(CH₃)₂ in MG and carboxyl groups. There was no significant change in the lattice oxygen peak (529 eV), indicating the stable framework structure of red mud. Due to the presence of nitrogen in MG, a distinct N peak appeared after adsorption in the MG-containing system, as shown in the N1s spectrum, Fig.S3(g) and Fig.S5(c). The peak at 399.5 eV corresponds to -NH₂, which originates from the primary amine groups in MG molecules. The peak at 401 eV, assigned to NH₃⁺, may result from the protonation of partial -NH₂ groups. Additionally, the metal-N coordination peak at 398.5 eV indicates that -NH₂ forms coordination bonds with Fe\u0026sup3;⁺ or Al\u0026sup3;⁺[\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. In the binary system, the presence of Pb\u0026sup2;⁺ may affect the adsorption of MG through competitive occupation of adsorption sites: Pb\u0026sup2;⁺ is likely to occupy some negatively charged sites, reducing the electrostatic adsorption of MG and thereby decreasing the amount of adsorbed MG molecules, which leads to a weakened N signal. However, the increased peak area of -NH- suggests the possibility of synergistic adsorption, where Pb\u0026sup2;⁺ may form \"cationic bridges\" on the material surface to promote the adsorption of MG[\u003cspan additionalcitationids=\"CR57\" citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn the Fe2p spectrum, the Fe2p peaks before adsorption show the presence of Fe\u0026sup3;⁺ 2p₃/₂ at 710 eV (hematite Fe₂O₃), Fe2p₁/₂ at 725.2 eV, and satellite peaks at 718.3/733.1 eV, confirming that hematite (Fe₂O₃) is the dominant species. After adsorption of MG in the single-component system, the area of the Fe\u0026sup2;⁺ peak increases significantly, indicating that a redox reaction occurs between Fe\u0026sup3;⁺ and MG molecules, with partial reduction of Fe\u0026sup3;⁺. The weakening of Fe\u0026sup3;⁺ satellite peaks and a slight decrease in the area of the main peak suggest that Fe-O bonds may form due to the coordination of -NH₂ groups with MG molecules[\u003cspan additionalcitationids=\"CR57\" citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e]. After adsorption of Pb\u0026sup2;⁺ in the single-component system, the increased proportion of Fe\u0026sup2;⁺ indicates partial reduction of Fe\u0026sup3;⁺, which may participate in the reductive adsorption of Pb\u0026sup2;⁺. The broadened peak width at 710.5 eV corresponds to the formation of Fe-O-Pb bonds, implying possible interfacial interactions between Fe and Pb\u0026sup2;⁺. As shown in the Fig.S4.and Fig.S5, after adsorption in both single-component and binary systems, characteristic peaks of heavy metal Pb\u0026sup2;⁺ appear as Pb4f doublets: Pb 4f₇/₂ at 139.28 eV (corresponding to Pb\u0026sup2;⁺ species such as \u0026equiv;\u0026thinsp;Fe-O-Pb, (-COO)₂Pb, or Pb(OH)⁺) and Pb4f₅/₂ at 144.18 eV. The spin-orbit splitting energy is consistent with previous studies[\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e], confirming the successful adsorption of Pb\u0026sup2;⁺ by MMBC-400. MMBC-400 synergistically immobilizes Pb\u0026sup2;⁺ through carboxyl complexation, ion exchange, and precipitation reactions. Overall, Pb\u0026sup2;⁺ is effectively and successfully adsorbed in both single-component and binary mixed systems. The disappearance or reduction of K⁺/Ca\u0026sup2;⁺ peaks in the full spectra of both MG and Pb\u0026sup2;⁺ systems indicates that ion exchange occurs between MG/Pb\u0026sup2;⁺ and K⁺/Ca\u0026sup2;⁺.\u003c/p\u003e\u003cp\u003eIn the XRD analysis Fig.S6(b), it can be observed that the forms of Fe, namely Fe₂O₃ and Fe₃O₄, remain stable in both single and binary systems, indicating the chemical stability of the material. Pb₃(CO₃)₂(OH)₂ was detected in both MMBC-400-Pb\u0026sup2;⁺ and MMBC-400-MG-Pb\u0026sup2;⁺ systems (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(b)), suggesting that Pb\u0026sup2;⁺ is adsorbed on MMBC-400 through surface precipitation. Specifically, Pb\u0026sup2;⁺ reacts with -OH groups on MMBC-400 to form Pb(OH)₂ precipitates, which further react with additional Pb\u0026sup2;⁺ and CO₃\u0026sup2;⁻ to generate Pb₃(CO₃)₂(OH)₂, as shown in Equ. (5,6)[\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e], In the binary system, the presence of MG competes with Pb\u0026sup2;⁺ for adsorption sites, leading to a decrease in the intensity of Pb₃(CO₃)₂(OH)₂ precipitation.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{\\text{Pb}}^{\\text{2+}}\\text{+}{\\text{2OH}}^{\\text{-}}\\text{\u0026rarr;}{\\text{Pb(OH)}}_{\\text{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$\\:{\\text{Pb(OH)}}_{\\text{2}}\\text{+2}{\\text{Pb}}^{\\text{2+}}\\text{+2}{\\text{CO}}_{\\text{3}}^{\\text{2-}}\\text{\u0026rarr;}{\\text{Pb}}_{\\text{3}}{\\text{(}{\\text{CO}}_{\\text{3}}\\text{)}}_{\\text{2}}{\\text{(OH)}}_{\\text{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eIn summary, based on the analysis results of XRD, FTIR, and XPS, the adsorption mechanism of MG onto MMBC-400 involves multiple interactions, including π-π stacking, covalent bonding, coordination complexation, and redox reactions, which collectively contribute to the immobilization of MG, consistent with previous studies[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The strong adsorption capacity of MMBC-400 on Pb\u0026sup2;⁺ involves not only ion exchange and electrostatic effects, but also the combined effects of precipitation, complexation and pore filling. In the coexistence system of MG and Pb\u0026sup2;⁺, competitive adsorption occurs. Initially, Pb\u0026sup2;⁺ preferentially occupies\u0026thinsp;\u0026equiv;\u0026thinsp;Fe-OH/-COO sites, inhibiting the hydrogen bonding adsorption of MG. Due to the different molecular diameters of Pb\u0026sup2;⁺ and MG, hydrated Pb\u0026sup2;⁺ ions may block mesopores, delaying the diffusion of MG. The coverage of MG micelles leads to surface positive charging, weakening electrostatic attraction. Additionally, the precipitation of Pb₃(CO₃)₂(OH)₂ may affect the adsorption of MG, and ion exchange between Ca\u0026sup2;⁺/K⁺ and MG/Pb\u0026sup2;⁺ also contributes to competition.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec23\" class=\"Section2\"\u003e\u003ch2\u003e3.6. Regeneration of MMBC-400 after adsorption of MG and Pb\u003csup\u003e2+\u003c/sup\u003e\u003c/h2\u003e\u003cp\u003eEvaluating the reusability of MMBC-400 is crucial for its feasibility and economic efficiency in practical applications. As shown in Fig.S7., using an EDTA-ethanol mixed solution as the desorption agent for MG and Pb\u0026sup2;⁺, the adsorption performance remained relatively stable in the first two cycles. By the third cycle, the removal rates slightly decreased from 98.4% and 93.8% to 93.43% and 90.58%, respectively. This gradual reduction in adsorption efficiency may be attributed to pore collapse or loss of functional groups in MMBC-400. Additionally, partial oxidation of the adsorption sites on MMBC-400 could hinder further adsorption. However, the adsorption capacity and removal efficiency of MMBC-400 remained relatively stable over five cycles. Even after five cycles, the removal rates still maintained above 85%, indicating that MMBC-400 possesses strong regenerability and holds great application potential in practical wastewater treatment.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study successfully developed a magnetic multifunctional biochar MMBC 400 through the synergistic utilization of corn cob and red mud, two abundant industrial and agricultural wastes. The primary innovation of this work lies in demonstrating that MMBC-400 effectively minimizes competitive adsorption in a binary system of MG and Pb\u0026sup2;⁺, a common yet challenging scenario in real wastewater. Despite a moderate adsorption capacity for Pb\u0026sup2;⁺ (129 mg/g) in single systems, MMBC-400 exhibited exceptional performance in the co-existing system, retaining high removal efficiencies of 98.4% for MG and 93.8% for Pb\u0026sup2;⁺. This indicates that the adsorbent possesses strong anti-interference capability, which is attributed to its site-specific adsorption mechanisms: the corn cob-derived carbon matrix primarily facilitates MG removal via π-π interactions and pore filling, while the red mud components selectively immobilize Pb\u0026sup2;⁺ through surface complexation and precipitation. Furthermore, MMBC-400 showcases excellent regenerability and reusability, maintaining over 80% removal for both pollutants after five consecutive cycles, underscoring its potential for practical application. However, this study has certain limitations. The adsorption performance and mechanisms were specifically investigated for the MG and Pb\u0026sup2;⁺ binary system. The effectiveness of MMBC-400 against other prevalent dyes and heavy metal ions, as well as in more complex multi-component wastewater matrices, requires further validation. Additionally, the long-term stability of the material and a comprehensive economic assessment for large-scale application remain to be explored. In summary, the findings provide a valuable theoretical foundation for designing efficient, waste-derived adsorbents targeted at complex water pollution. MMBC-400 stands out not for a singularly high adsorption capacity, but for its balanced, synergistic, and sustainable approach to co-contaminant removal, offering a promising strategy for practical wastewater treatment.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003ch2\u003eDeclaration of Competing Interest\u003c/h2\u003e\u003cp\u003eThe authors hereby declare that the disclosed information is correct and that no other situation of real, potential, or apparent conflict of interest is known to them. The authors undertake to inform any change in these circumstances.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eCRediT authorship contribution statementRuihui Gong: Writing\u0026ndash;original draft, Methodology, Formal analysis, Data curation, Conceptualization. Huidong Li: Writing\u0026ndash;review \u0026amp; editing, Funding acquisition, Formal analysis, Conceptualization. Yuxin Liu: Supervision, Software, Investigation, Data curation. Jiangzhe Fu: Resources, Investigation.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThis work was supported by Natural Science Foundation of Inner Mongolia Autonomous Region of China (2024LHM05041), National Natural Science Foundation of China (42067031) and Inner Mongolia Science and Technology Plan Project (2025KYPT0025)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eS.Z. Mohammadi, M.A. Karimi, D. Afzali, F. 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Kwon, Fabrication of Fe-doped biochar for Pb adsorption through pyrolysis of agricultural waste with red mud, Chemosphere 370 (2025) 143930. https://doi.org/10.1016/j.chemosphere.2024.143930.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Biochar, Adsorption, Pb2+, Malachite green","lastPublishedDoi":"10.21203/rs.3.rs-7662320/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7662320/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"The treatment of wastewater containing coexisting organic dyes and heavy metals remains challenging due to the competitive adsorption effect, which typically leads to a significant reduction in the removal efficiency of target pollutants, especially heavy metal ions. In this study, a magnetic multifunctional biochar (MMBC-400) was successfully synthesized via the co-pyrolysis of corn cobs and red mud at 400 °C for 2 hours, and it was used for the simultaneous removal of malachite green (MG) and Pb²⁺. The adsorption performance of MMBC-400 for MG and Pb²⁺ in both single and binary systems was systematically investigated through experiments involving different initial conditions, kinetics, isotherms, and cyclic regeneration. MMBC-400 exhibited a high adsorption capacity of 794.72 mg/g for MG, with a removal efficiency of 99.33%. More importantly, in the binary system with a high MG concentration of 500 mg/L, MMBC 400 still maintained a considerable adsorption capacity for Pb²⁺ (129 mg/g) and a removal efficiency of 96.82%, demonstrating strong anti-interference capability. Characterization and model analysis revealed that the adsorption mechanisms of the two pollutants included pore filling, complexation, and ion exchange. Notably, the two pollutants were preferentially adsorbed onto different sites: MG was adsorbed on the carbon matrix of the biochar through π-π interactions, while Pb²⁺ was immobilized on the red mud components via surface complexation. This selective adsorption behavior resulted in limited competitive adsorption between the two pollutants. Furthermore, MMBC-400 showed excellent recyclability. After five consecutive cyclic uses, the removal efficiency of MMBC-400 for both pollutants remained above 85%. Although the single adsorption capacity of MMBC-400 for Pb²⁺ was moderate, it has been proven to be an efficient and practical adsorbent for the treatment of complex polluted wastewater, attributed to its synergistic removal capability, weak competitive effect, and good recyclability. This study provides a valuable strategy for the design of specific adsorbents for complex wastewater systems.\n","manuscriptTitle":"A Recyclable Magnetic Biochar from Corn Cobs and Red Mud for Treating Complex Contaminants Containing Dyes and Heavy Metals","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-06 09:34:36","doi":"10.21203/rs.3.rs-7662320/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-10-14T02:25:05+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-06T09:45:14+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-26T07:49:56+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"241437879865799557813767358302942564452","date":"2025-09-25T12:13:44+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-25T01:06:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"159532710877524408299338889879141265449","date":"2025-09-23T00:26:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"229416398553806700006583958180359826882","date":"2025-09-23T00:13:35+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"337110623207499680119428441607832993149","date":"2025-09-23T00:10:19+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-23T00:07:02+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-22T15:21:00+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-09-20T18:03:46+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Geochemistry and Health","date":"2025-09-20T04:39:13+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-geochemistry-and-health","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"egah","sideBox":"Learn more about [Environmental Geochemistry and Health](https://www.springer.com/journal/10653)","snPcode":"10653","submissionUrl":"https://submission.nature.com/new-submission/10653/3","title":"Environmental Geochemistry and Health","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"1be84abe-3203-4bf9-a794-060a3e159ded","owner":[],"postedDate":"October 6th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-12-22T16:03:19+00:00","versionOfRecord":{"articleIdentity":"rs-7662320","link":"https://doi.org/10.1007/s10653-025-02933-8","journal":{"identity":"environmental-geochemistry-and-health","isVorOnly":false,"title":"Environmental Geochemistry and Health"},"publishedOn":"2025-12-15 15:58:31","publishedOnDateReadable":"December 15th, 2025"},"versionCreatedAt":"2025-10-06 09:34:36","video":"","vorDoi":"10.1007/s10653-025-02933-8","vorDoiUrl":"https://doi.org/10.1007/s10653-025-02933-8","workflowStages":[]},"version":"v1","identity":"rs-7662320","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7662320","identity":"rs-7662320","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}