Waqas Ahmed1,2†
Yunting Wang3†Sehrish Ali2
Avelino Núñez-Delgado4Fengyue Qin1,2Menglu Dong1,2Weidong Li1,2*
Sajid Mehmood1,2,5*- 1Center for Eco-Environment Restoration of Hainan Province, School of Ecology, Hainan University, Haikou, China
- 2Hainan University, Haikou, China
- 3College of the Environment and Ecology, Xiamen University, Xiamen, China
- 4Department of Soil Science and Agricultural Chemistry, Engineering Polytechnic School, Universidade de Santiago de Compostela, Lugo, Spain
- 5School of Tropical Agriculture and Forestry, Hainan University, Haikou, China
The release of lead (Pb) from industrial sources into aquatic environments is a growing concern due to its toxic nature, persistence, and tendency to bioaccumulate, posing significant environmental and health threats. This research involved the synthesis of a manganese-modified bamboo biochar (Mn-BC) using KMnO4 impregnation, and its effectiveness in removing Pb(II) was assessed against that of unmodified biochar (BC). Characterization by SEM, TEM-EDS, XRD, BET, and XPS verified successful Mn integration, leading to surface structural changes, Mn-oxide formation, and increased porosity. Mn-BC exhibited a specific surface area of 121.28 m2 g-1 and a total pore volume of 0.062 cm3 g-1, surpassing the values for BC, thus offering more active sites for Pb(II) adsorption. Batch adsorption tests indicated that Mn-BC attained a maximum Pb(II) adsorption capacity of 153.63 mg g-1, which is nearly five times higher than that of BC (30.22 mg g-1). At an adsorbent application of 1 g L-1, Mn-BC removed over 90% of Pb(II) from the solution, while BC managed only about 30%. The adsorption kinetics were analyzed by using the pseudo first order (PFO), second order (PSO) and intraparticle diffusion kinetic models and were best described by the pseudo-second-order model (49 mg g-1 and R2 = 0.99), suggesting that chemisorption was the primary mechanism, with intraparticle diffusion playing a role. The equilibrium data conformed to the Langmuir isotherm (153.63 mg g-1 and R2 = 0.98), indicating monolayer adsorption on uniform active sites. Thermodynamic analysis showed negative energy values (ΔG°) ranging from −12.4 to −18.7 kJ mol-1, a positive enthalpy and entropy change (ΔH° and ΔS°), suggesting spontaneous adsorption process and endothermic in nature. Regeneration studies revealed that Mn-BC retained 79% of its initial adsorption capacity after five cycles, compared to 64% for BC, highlighting its superior stability and reusability. XPS analysis showed that Pb(II) was immobilized through complexation with oxygen-containing functional groups and Mn species. These results suggest that Mn-BC is a cheaper, stable, and eco-friendly adsorbent with considerable potential for large-scale treatment of Pb(II)-contaminated wastewater.
1 Introduction
The rapid improvement in living standards has significantly contributed to the expansion of metal industries, including mining, metallurgy, and electroplating (Abu-Danso et al., 2018; Ahmed et al., 2021a). This growth has led to the inevitable discharge of wastewater containing heavy metals into aquatic ecosystems, resulting in soil and plant contamination (Tan W. T. et al., 2022). Heavy metals are characterized by their toxicity, persistence, and non-biodegradability, posing substantial environmental and human health risks, and are thus regarded among the utmost critical ecological challenges (Wang H. et al., 2023). Among these metals, lead (Pb) warrants particular attention because of its prevalent application in lead/acid batteries, cables, and the biochemical manufacturing, where its superior conductivity and corrosion resistance render it indispensable (Wang et al., 2023b). Minute Pb content in water can bio-accumulate in creatures, leading to severe health issues such as anemia, and, in extreme cases, death (Zeng et al., 2022). Consequently, Pb is ranked as the major toxic heavy metal, prompting many nations to implement stringent regulations limiting its concentrations in water (Cheng et al., 2022). Therefore, it is of paramount importance to develop effective methods for treating Pb(II) containing wastewater prior to environmental discharge.
Adsorption has emerged as a more suitable option due to its high efficiency and cost-effectiveness (Alsuhybani et al., 2020; Ahmed et al., 2021c). Notably, Carbon-based biochars from agricultural residues and industrial wastes are increasingly favored due to their availability, high performance, and dual roles in pollutant removal and waste recycling (Yaashikaa et al., 2021). Nonetheless, pristine/raw biochar generally exhibits lower adsorption potentials, primarily because of limited porosity and insufficient surface functional groups (Feng et al., 2022). A range of techniques has been applied for the remediation of Pb(II), including chemical precipitation (Francisca and Glatstein, 2020), ion exchange processes (Jeon, 2018), and adsorption approaches (Imran et al., 2020), which currently continue to be used. Of these methods, adsorption is generally regarded as the most environmentally friendly and cost-effective approach (Ahmed et al., 2021a). A diverse array of carbon porous resources, including C-nanotubes (Yu et al., 2019), chitosan (Liu et al., 2022), and modified biochar have been investigated as Pb(II) adsorbents. Biochar, in particular, is valued for its high surface area, large porosity, thermostability, abundant functional groups, and strong ion-exchange capacity, leading to its increasing acceptance and widespread use in recent years (Liu N. et al., 2025). Though, the adsorption capability of pristine biochar remains unsatisfactory, necessitating surface modifications to enhance its efficiency. Enhancing the surface characteristics of biochar adsorbents through modification is essential for optimizing their function as sorbent materials. As a result, a variety of engineered biochar-based adsorbents have been developed, such as magnetically modified biochar (Khan et al., 2020), iron-enriched biochar (Tang et al., 2022), algal-derived biochar (Yu et al., 2020) and biochar improved with metal oxides or clay minerals (Sizmur et al., 2017), which have attracted significant interest due to their enhanced capacity for Pb(II) adsorption in aqueous solutions.
Biochar’s porous structure promotes uniform elemental dispersion, and growing expertise is increasingly driving the sustainable reuse of engineering and agronomic residues (Mo et al., 2018). The use of such waste materials not only augments their intrinsic value but also reduces processing costs (Wu et al., 2017). Nevertheless, the typically negative charge of biochar surfaces limits their effectiveness in adsorbing anionic contaminants. Consequently, modified biochar has been the subject of extensive research. Metal-modified biochar, in particular, offers a promising solution by altering the functional groups on biochar surfaces, thereby increasing the number of active sites and improving catalytic or adsorption performance (Fang et al., 2022). Manganese (Mn), a cost-effective transition metal, demonstrates superior performance relative to other metals. Techniques for biochar modification comprise acid treatment, alkali treatment, active modification of metalloid, and various nitrogen fixing (Li et al., 2018). Active metal ion modification is highly efficient, as it increases surface functional group density and enhances metal sequestration via ion exchange and complexation. Oxides (Fe/Mn) are commonly used as modifiers (Xiao et al., 2020). Researchers have consistently reported that manganese oxide possesses a high capacity for metal binding/fixation and offers numerous adsorption spots, thereby enhancing the hydrophilic nature of BC (Li and Cheng, 2023; Yin et al., 2023). Loading manganese oxide onto biochar enriches its surface with hydroxyl groups, like–COOH or phenolic–OH, thereby markedly increasing its ability to adsorb HMs (Wang et al., 2015; Ullah et al., 2023). For instance, BC modified with KMNO4 has been shown to improve cadmium removal efficiency in aqueous solutions, compared with that of unmodified/raw BC (Tan Y. et al., 2022). Similarly, BC modified with manganese has demonstrated a 15 fold higher cephalexin removal rate compared to unmodified BC (Li and Cheng, 2023). Despite their strong Pb adsorption performance, further optimization of manganese modification is needed to improve practical effectiveness and sustainability (Huang et al., 2018). Biochar adsorption efficiency depends on Mn content, modification temperature, and treatment method. Future work should optimize eco-friendly modification to improve stability and long-term performance. Overall, Mn-modified biochar is a low-cost, sustainable option for heavy metal remediation, particularly for reducing Pb contamination. While manganese-modified bamboo biochars have been explored for Pb(II) removal in prior studies (e.g., KMnO4-modified biochars achieving capacities of ∼80–123 mg g-1 (Wang et al., 2012; Mohammadi et al., 2020), this work introduces distinct novelty by focusing on locally abundant tropical bamboo waste from Southern China, where bamboo grows rapidly and generates significant agricultural/industrial residues that are often underutilized or discarded. This regionally tailored approach enables sustainable waste valorization, minimizes transportation and import costs in humid tropical economies with high bamboo availability, and supports circular economy principles in developing contexts facing industrial wastewater challenges (e.g., from battery manufacturing or mining). By employing a simple, low-energy Mn-modification process, we advance cost-effective production suited to resource-limited settings, while offering potential advantages in handling organic-rich, equatorial wastewaters through enhanced surface chemistry specific to tropical bamboo matrices. This area-specific strategy differentiates our contribution and promotes eco-friendly, regionally relevant remediation solutions.
In this context, the present work focused on synthesizing and characterizing manganese-modified bamboo biochar (Mn-BC) and evaluating its performance in removing Pb(II) from aqueous solutions compared to unmodified biochar (BC). The study also explored how critical parameters including initial Pb(II) concentration, solution pH, and temperature affect the adsorption behavior of Mn-BC. The research deals with checking if adsorbents with robust structural stability and strong Pb(II) removal capacity hold promise as cost-effective, sustainable, and efficient materials for treating lead-contaminated wastewater, highlighting their environmental importance.
2 Materials and methods
2.1 Chemicals and materials
Bamboo-derived biochar (BC), produced through pyrolysis at 500 °C, was sourced from a biochar production company in China. Prior to utilization, the BC underwent thorough washing with pure water (18.2 MΩ cm resistivity) and was subsequently freeze-dried. The biochar was further sieved to a particle size of less than 0.15 mm and subsequently dried in a vacuum drying oven at 60 °C. To synthesize Mn-BC, the biochar was immersed in a 100 mL solution of 0.1 mol L-1 KMnO4 for 24 h and then subjected to pyrolysis at 500 °C for 30 min (Supplementary Figure S1) (Tan et al., 2019). A simulated wastewater solution containing 1,000 mg L-1 of Pb(II) was prepared by dissolving Pb(NO3)2 in deionized water, with the desired concentrations achieved through serial dilution of the stock solution. All chemicals used in this study, including potassium permanganate (KMnO4), lead nitrate [Pb(NO3)2], nitric acid (HNO3), sodium chloride (NaCl), and sodium hydroxide (NaOH), were of analytical purity/grade and procured from Shanghai Sinopharm Chemicals (China).
2.2 Characterization
The microstructure and elemental composition of the prepared biochars were analyzed through the use of scanning electron microscopy (Quanta 400 FEG), transmission electron microscopy (TEM, JEM 2100F) in conjunction with energy-dispersive X-ray spectroscopy (XRD, Rigaku SmartLab 9, Japan). Crystalline phases were identified using X-ray diffraction (XRD, SmartLab9, Rigaku, Japan). The chemical transformation affecting the functional groups of BC and Mn-BC was confirmed by Fourier Transform Infra-Red (FTIR) spectroscopy. Additionally, X-ray photoelectron spectroscopy (XPS) was utilized to examine the exterior chemical states and the valence state of elements using a Thermo Scientific ESCALAB 250Xi instrument. Nitrogen adsorption analysis was performed using an N2 adsorption analyzer. Before conducting the analysis, the adsorbent samples were subjected to degassing at 300 °C for a duration of 24 h. The specific surface area (SBET) was calculated employing the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was determined through Density Functional Theory (DFT). The total pore volume (VT) was derived from nitrogen adsorption at a relative pressure (P/P0) close to 0.99. Zeta potential measurements of the adsorbents were performed in solution using a Malvern Zetasizer Nano-ZS90, adhering to the procedure described by Liu et al. (2022).
2.3 Adsorption kinetics and isotherm experiments
The adsorption efficiency of the synthesized biochars toward Pb(II) was systematically assessed by examining the dosage effects (0.1 g L-1–5 g L-1), contact time (0–48 h), and initial Pb(II) concentration (10–300 mg L-1). A Pb(II) stock solution (1,000 mg L-1) was first prepared and subsequently diluted to the desired concentrations for experimental use. The effect of dosage was examined by the addition of measured quantities of the adsorbent into the centrifuge tubes (15 mL) holding 50 mg L-1 of Pb(II) solution (10 mL), followed by equilibration (24 h). Kinetic trials were directed in the flasks holding 120 mL of Pb(II) solution (50 mg L-1), with aliquots withdrawn at predetermined intervals over a 0–48 h period. For adsorption isotherms, biochar was introduced into Pb(II) solution with varying concentrations (0 mg L-1–300 mg L-1) and equilibrated for 24 h. The collected data were further analyzed using kinetic, isotherm, and thermodynamic models. Additional experimental details are provided in the Supplementary Material.
3 Results and discussion
3.1 Micromorphology, elemental composition, and structure
The microscopic morphology and elemental composition were examined using FESEM-EDS imaging, both prior to and after biochar modification. Figure 1 presents the FE-SEM micrographs of pristine biochar (Figure 1A–C) and Mn-modified biochar (Figure 1D–F) at different magnifications. The unmodified BC exhibits a relatively smooth and layered surface structure with distinct pore cavities and channels (A–C), indicating the inherent porous morphology generated during pyrolysis. In contrast, Mn-BC shows a rougher, fragmented, and more heterogeneous surface (D–F), with abundant mineral particles and aggregates attached to the biochar matrix. The pores in Mn-BC appear partially filled or decorated with fine Mn-containing particles, suggesting successful deposition and surface modification. This structural transformation increases surface roughness and provides additional active sites, which are favorable for enhanced Pb(II) adsorption.
Figure 1. FE-SEM images of BC (A–C) and Mn-BC (D–F).
This modification enhanced the interaction between the adsorption sites of biochar and metal ions, thereby promoting the formation of new functional groups (Tan et al., 2018). Numerous fine particles and powder-like deposits were observed on the surface of Mn-BC, which are likely MnOx species formed from the decomposition of manganese salts at elevated temperatures (Huang et al., 2019). These subtle nanoscale modifications such as increased surface roughness and dispersed nano-scale manganese oxide aggregates are generally not visible in standard SEM images taken at lower magnifications, yet they substantially boost the accessible surface area and active site density while preserving the overall macroscopic structure of the biochar.
Figure 2 presents TEM-EDS elemental mapping of pristine biochar (Figure 2A) and Mn-loaded biochar (Figure 2B), along with their corresponding EDS spectra (C and D). In the pristine BC, carbon (C), nitrogen (N), and oxygen (O) signals are distributed relatively uniformly, reflecting the native carbonaceous matrix with intrinsic O- and N-containing functional groups. Trace or negligible manganese signals were observed in BC. In contrast, the Mn-BC mapping reveals substantial clustering of Mn (yellow), co-localized with O (blue), while C and N remain widespread. This indicates that manganese species have been successfully introduced and are associated with oxygen-rich domains, likely forming Mn-oxide or oxyhydroxide deposits attached to the biochar surface. The EDS spectrum for Mn-BC (D) shows distinct Mn peaks (Mn-K), in addition to the baseline C and O, confirming the presence of Mn in the modified material. Together, these results confirm successful Mn incorporation into the biochar, forming discrete Mn-containing features that could enhance adsorption performance by providing additional reactive sites. The observed increase in the oxygen content of Mn-BC can be ascribed to the incorporation of manganese-based complexes (Mn-Ox or/and Mn-Cx) generated through modification process. This interpretation is validated by the associated rise in elemental oxygen and carbon levels post-modification (Wang and Liu, 2018; Gautam et al., 2023).
Figure 2. TEM-EDS spectrum patterns of BC (A) and Mn-BC (B).
The nitrogen adsorption–desorption isothermal analysis and porous distribution curves together with the BET surface analysis data (Table 1) clearly demonstrate the effect of Mn modification on the textural properties of BC. Both BC (Figures 3a,b) and Mn-BC (Figures 3c,d) exhibit type IV isotherms with an H3-type hysteresis loop, indicating mesoporous structures. Compared with BC, Mn-BC shows a higher adsorption capacity across the relative pressure range, reflecting the increase in surface area and pore accessibility after Mn loading. This observation is supported by BET analysis, where Mn-BC exhibits a larger specific surface area (121.28 m2/g) and higher total pore volume (0.062 cm3/g) compared to BC (76.17 m2/g and 0.042 cm3/g, respectively) (Table 1). These findings are in agreement with the observations reported by (Kim et al., 2019). The average pore diameter of both materials remains within the mesoporous range, although a slight variation is observed following Mn incorporation. The pore size distribution curves further confirm that both samples are dominated by mesopores with a sharp peak below 5 nm and a minor distribution of larger pores extending up to 50 nm (Luo et al., 2019). Overall, the combination of increased surface (outer) area, enhanced pore volume, and favorable pore structure in Mn-BC indicates that Mn impregnation substantially improves the adsorption potential of the material, making it more effective than pristine BC for environmental remediation applications. The XRD patterns (Figure 4a) revealed that pristine biochar (BC) exhibited mainly broad and weak peaks, indicating its predominantly amorphous carbon structure, while distinct crystalline peaks appeared in Mn-modified biochar (Mn-BC), which were attributed to manganese oxide phases such as MnO2 and Mn3O4. These crystalline reflections confirm the successful loading of Mn species onto the biochar surface, thereby introducing additional active spots/sites for adsorption and redox reactions (Liu C. et al., 2025). The pyrolysis temperature of was chosen to optimize KMnO4 decomposition on bamboo biochar, yielding a stable mixed MnO2/Mn3O4 phase under N2 atmosphere. This temperature balances the high surface reactivity and complexation capacity of MnO2 with the structural stability and redox properties of Mn3O4, contributing to the excellent Pb(II) adsorption capacity and regeneration performance. Lower temperatures produce more amorphous Mn oxides with higher surface area but reduced stability, while higher temperatures favor less reactive phases (e.g., Mn2O3), potentially decreasing overall efficiency. This tailored temperature selection enhances the material’s practical performance for tropical bamboo-derived adsorbents in heavy metal removal. The zeta potential analysis (Figure 4b) further demonstrated that BC surfaces were generally negatively charged across the tested pH range, which limits their ability to capture anionic contaminants. In contrast, Mn-BC showed a shift in surface charge, with more positive zeta potential values under acidic to neutral conditions because of the presence of Mn–O and Mn–OH functional groups. This modification not only increased the electrostatic attraction toward anions such as phosphate but also broadened the functional versatility of the adsorbent, explaining the enhanced performance of Mn-BC (also for Pb(II)) compared to unmodified biochar (Zeng et al., 2025). FTIR spectra (Supplementary Figure S2) of BC and Mn-BC reveal surface functional group changes, confirming successful Mn modification (Hassan et al., 2014). Both biochar materials exhibit rich–OH stretching at 3,500–3,900 cm-1 (Jung et al., 2015) and C–H stretching (2,700 and 3,100 cm-1), with intensified peaks in Mn-BC. Characteristic C=O stretching appears at 1,555 and 1,410 cm-1 (Akin et al., 2023), strongest in Mn-BC, alongside–CH in-plane bending (1,000–680 cm-1) and enhanced Mn-BC peak at 650 cm-1 (substituted benzene ring). Shifts in Mn-BC bands indicate altered surface properties (Fan et al., 2018), while new peaks at 1,650 cm-1 and 1,225 cm-1 signify C=O/C=C in polycyclic aromatics and aromatic C–C stretching, denoting high aromatization (Chen et al., 2015; Tao et al., 2019). Moreover, a new peak at 520 cm-1 confirms MnOx incorporation (Tan et al., 2019).
Table 1. BET surface area, pore volume and average pore size of BC and Mn-BC.
Figure 3. BET isotherm and pore width distribution of BC (a,c) and Mn-BC (c,d).
Figure 4. XRD (a) and zeta potentials (b) of BC and Mn-BC.
3.2 Performance of Pb(II) removal
3.2.1 Impact of adsorption dosage
Figure 5 illustrates the impact of biochar (BC) and manganese-modified biochar (Mn-BC) dosages (0.1–5 g L-1) on Pb(II) adsorption, quantified by the percentage removal efficiency and adsorption capability (mg g-1). The findings demonstrate that Mn-BC outperforms unmodified or raw BC. As the dosage increases, the removal efficiency of Mn-BC rapidly rises, surpassing 90% at 1 g L-1 and approaching complete removal at higher dosages, whereas BC achieves only approximately 30% removal at the maximum dosage (Figure 5). In contrast, adsorption capacity is maximized at low dosages and diminishes with increasing dosage. Mn-BC achieves a peak adsorption/removal capacity of 110 mg g-1 at the dosage rate of 0.1 g L-1, which decreases to 20 mg g-1 at higher dosages, while BC exhibits a similar but significantly lower trend. This inverse relationship is attributed to the higher Pb(II) to adsorbent ratio at low dosages, optimizing per-gram uptake, whereas excess adsorbent at higher dosages results in underutilized binding sites. The observed decrease in adsorption capacity (qe) with increasing adsorbent dosage, while removal efficiency rises, stems from the reduced Pb(II)-to-adsorbent ratio at higher dosages, leading to underutilization of binding sites as excess adsorbent provides more sites than needed for the fixed Pb(II) concentration. Furthermore, higher dosages may induce particle aggregation or overlapping of active sites, diminishing the effective surface area for adsorption (Fan et al., 2020; Liu and Zhang, 2022). This inverse relationship is a common feature in batch heavy metal adsorption studies using biochars and related materials. Overall, Mn-BC markedly enhances Pb(II) removal efficacy and adsorption capability compared to BC, corroborating the beneficial effect of manganese modification on adsorption performance. An increase in adsorbent dosage enhances the availability of active sites, thereby promoting more effective removal of contaminants from the acidic aqueous solution (Ahmed et al., 2021b; 2023; Hu et al., 2023).
Figure 5. Effect of biochar dosage (0.one to five g L-1) on Pb(II) adsorption using BC and Mn-BC.
3.2.2 Adsorption studies
The adsorption trial results were analyzed using the PFO, PSO, and intraparticle diffusion kinetic models. Figure 6 depicts the influence of contact time (0–48 h) on Pb(II) adsorption by BC and Mn-BC, alongside kinetic model fitting. Pb(II) adsorption increased rapidly (Figure 6a) during the initial hours and gradually reached equilibrium, with Mn-BC demonstrating a significantly higher adsorption capacity (49 mg g-1) compared to BC (14 mg g-1). The adsorption data align more closely with the pseudo-second-order (PSO) model than with the pseudo-first-order (PFO) model, suggesting that chemisorption, involving electron sharing or exchange, governs the Pb(II) uptake (Chen et al., 2017; Ahmed et al., 2021d). Figure 6b presents the intraparticle diffusion (IPD) model, where adsorption occurs in three distinct stages: an initial sharp rise (Step 1) representing surface adsorption, a slower increase (Step 2) attributed to intraparticle diffusion, and a final plateau (Step 3) corresponding to equilibrium. The multilinear plots confirm that intraparticle diffusion plays a vital role in the adsorption process but it is not exclusively responsible for the overall rate limitation (Table 2). Overall, the results underscore that Mn-BC exhibits superior adsorption efficiency, with Pb(II) removal primarily driven by chemisorption and complemented by intraparticle diffusion (Feng et al., 2022).
Figure 6. Effect contact time (0–48 h) and adsorption kinetics fitted models towards Pb(II) removal by BC and Mn-BC, (a) fitting of pseudo-first-order and pseudo-second-order models; (b) fitting of intraparticle diffusion model.
Table 2. Experimentally determined kinetic parameters for the Pb(II) adsorption of BC and Mn-BC.
The influence of varying initial Pb(II) concentrations (10–300 mg L-1) on adsorption performance was investigated (Figure 7a). To gain further insight into the adsorption behavior of lead by BC and Mn-BC, the experimental data were fitted to the Langmuir, Freundlich, and Temkin isotherm models (Figure 7b; Table 3). Figure 7a illustrates the effect of initial Pb(II) concentration and the adsorption isotherm fitting for BC and Mn-BC. Pb(II) uptake by both adsorbents increases with rising initial concentrations (10–300 mg L-1), but Mn-BC exhibits a significantly higher adsorption capacity (up to 153.63 mg g-1) compared to BC (30.22 mg g-1). This difference highlights the superior affinity of Mn-BC for Pb(II), likely due to enhanced surface functional groups and improved porosity after Mn modification. Figure 7b shows the isotherm fitting, where the Langmuir model provides the best fit to the experimental data (R2 = 0.98), suggesting monolayer adsorption (Wang et al., 2023c) on a homogeneous surface (Table 3). In contrast, the Freundlich and Temkin models show weaker correlations, particularly at higher concentrations. These results confirm that Mn-BC is more effective for Pb(II) removal, and its adsorption behavior is best described by the Langmuir isotherm, indicating strong and uniform binding sites for Pb(II). This study used synthetic Pb(II) solutions in controlled batch experiments to optimize conditions and elucidate the adsorption mechanisms of Mn-modified biochar (Mn-BC), which demonstrated superior performance compared to unmodified biochar. We acknowledge that model solutions do not capture the complexities of real wastewater, including competing ions, organic matter, humic substances, suspended solids, and variable pH, which may reduce efficiency through site competition, pore blockage, or altered kinetics. Scaling to practical applications also involves challenges such as continuous-flow operation, breakthrough behavior, regeneration, and economic feasibility. Nevertheless, the enhanced surface chemistry of Mn-BC suggests strong potential for real-world use. Future work will focus on testing Mn-BC in actual Pb(II)-containing industrial wastewater, column studies for dynamic performance, regeneration in complex matrices, and modifications to improve selectivity and interference resistance.
Figure 7. Effect of initial Pb(II) concentration (a) and adsorption isotherms (b) of BC and Mn-BC.
Table 3. Experimentally determined isotherm parameters for the Pb(II) adsorption of BC and Mn-BC.
3.2.3 Thermodynamic study
Thermodynamic analysis was conducted to evaluate the spontaneity of Pb(II) adsorption, with ΔH°, ΔG°, and ΔS° calculated at 298, 308, and 318 K (Table 4). The consistently negative ΔG° values (ranging from −12.4 to −18.7 kJ mol-1) confirmed that Pb(II) adsorption onto both biochars proceeded spontaneously, while the positive ΔH° values (23.6–31.4 kJ mol-1) demonstrated the endothermic process (Song et al., 2020). Moreover, ΔG° values became less negative with increasing temperature, indicating that adsorption was more thermodynamically favorable at elevated temperatures due to enhanced ion exchange and stronger surface complexation interactions. The corresponding ΔS° values (95.2–121.6 J mol-1 K−1) further suggested increased randomness (solid/solution interface). Comparable thermodynamic behaviors have been observed in previous studies on Mn-modified and functionalized biochar, confirming that such endothermic and spontaneous mechanisms are characteristic of heavy metal adsorption systems.
Table 4. Thermodynamic indexes for the Pb(II) adsorption.
3.2.4 Reusability and repeatability evaluation
Figure 8 shows the practicality and recyclability of BC and Mn-BC over five consecutive adsorptions–desorption cycles. Regeneration of both biochars was tested in order to check their reusability potentials in actual situations and environments. To do that, biochars were repeatedly washed using 1.0 M HCl and then vacuum dried (GJ101, Guanjue Electric heating equipment Co., Ltd. Suzhou, China). Both adsorbents exhibit a gradual decline in adsorption efficiency with repeated use, reflecting the partial loss of active binding sites and possible structural or surface modifications during regeneration (Liu et al., 2025a). Mn-BC consistently outperforms BC in all cycles, retaining nearly 79% adsorption capability after the fifth cycle, compared to about 64% for BC. This superior reusability indicates that Mn modification enhances the structural stability and regeneration potential of biochar, making Mn-BC more reliable for practical and sustainable Pb(II) removal applications. Overall, the results highlight that Mn-BC not only achieves higher initial adsorption efficiency but also maintains better long-term performance through multiple regeneration cycles (Table 5). Further investigation is required to enhance the cost-effectiveness and efficiency of biochar modification, thereby improving its practical application in wastewater treatment. Developing targeted strategies for the large-scale implementation of these innovative technologies could offer substantial societal benefits. Although regeneration results suggest good reusability, future studies should quantify dissolved Mn concentrations in both adsorption effluents and desorption solutions (especially under acidic conditions) using ICP-MS or AAS to fully evaluate Mn leaching risk and confirm environmental safety. Such measurements will be particularly important at pH ≤ 5.0 and during acid regeneration to ensure no secondary pollution occurs.
Figure 8. Regeneration capacities of BC and Mn-BC with five consecutive regeneration cycles.
Table 5. Comparison of Mn-BC’s adsorption capacity with other adsorbents.
3.2.5 Proposed mechanisms of Pb(II) adsorption
To better elucidate the mechanisms underlying Pb(II) adsorption, XPS was employed to analyze the chemical states and elemental composition of Mn-BC both before and after interaction with lead. Figure 9 presents the XPS survey spectrum (a) and the high-resolution Pb 4f spectrum (b) of BC and Mn-BC both before and after Pb(II) adsorption. In Figure 9a, the survey scan of Mn-BC prior to adsorption displays characteristic peaks of C 1s, O 1s, and Mn 2p, confirming the presence of carbon, oxygen, and manganese functional groups. After Pb(II) adsorption, distinct peaks corresponding to Pb 4f and Pb 4d emerge, providing clear evidence of Pb incorporation onto the adsorbent surface. This confirms that Pb(II) ions were effectively immobilized on Mn-BC through surface interactions. Figure 9b shows the high-resolution Pb 4f spectrum, where two strong peaks at binding energies of approximately 138.5 eV (Pb 4f7/2) and 143.4 eV (Pb 4f5/2) are observed. These peaks verify the presence of Pb on the adsorbent surface and suggest that Pb exists in chemically bound states, most likely through complexation with oxygenated functional groups and interactions with Mn species. Overall, the XPS analysis confirms the successful adsorption of Pb(II) by Mn-BC and supports the mechanism involving both surface functional groups and Mn-mediated binding. Figure 10 presents the high-resolution XPS spectra of Mn-BC before and after Pb(II) adsorption, illustrating the alterations in the O 1s, C 1s, and Mn 2p peaks. Figure 10a,d depict the O 1s spectra: prior to adsorption, peaks corresponding to O–H, C–O, and C–O–C/C–O–H groups are discernible, whereas after-adsorption, significant shifts in binding energy and variations in peak intensity are observed. These changes suggest robust interactions between Pb(II) and oxygen-containing functional groups, likely through surface complexation. Figure 10b,e display the C 1s spectra, where the peaks for C–C and C–H and C–O–C/C–O–H persist after adsorption but exhibit slight shifts, indicating potential interactions of Pb(II) with carbon-bound oxygen groups (Reguyal and Sarmah, 2018; Ahmed et al., 2021b). Figure 10c,f illustrate the Mn 2p spectra, showing Mn 2p3/2 and Mn 2p1/2 peaks. Post Pb(II) adsorption, shifts in these peaks are noted (Shen et al., 2020; Tang et al., 2022), implying the involvement of Mn species in binding or redox-related interactions with Pb(II). Overall, the observed spectral shifts confirm that Pb(II) adsorption onto Mn-BC occurs through multiple mechanisms, primarily involving oxygen functional groups (–OH, C–O, C–O–C) and Mn active sites, which collectively contribute to the enhanced binding and stabilization of Pb(II) on Mn-BC (Wang et al., 2025). Figure 11 schematically illustrates all relevant mechanisms reported in the literature, the experimental evidence emphasizes the dominance of chemically driven processes (complexation, ion exchange, and precipitation) over electrostatic attraction in this Mn-BC system. To provide a more conclusive understanding of the adsorption mechanisms beyond qualitative peak shifts, quantitative XPS data further support the proposed interactions. Comparative analysis of surface elemental compositions revealed a noticeable decrease in the relative atomic percentage of oxygen (particularly in deconvoluted O 1s components assigned to free–OH and Mn–O groups) after Pb(II) adsorption, accompanied by the emergence of Pb signals and corresponding reductions in unbound Mn and oxygenated carbon contributions. These changes quantitatively indicate consumption of active oxygen- and manganese-containing sites during Pb(II) uptake, consistent with inner-sphere complexation, ion exchange, and possible partial redox involvement of Mn species. Such site-specific depletion aligns well with the enhanced Pb(II) affinity observed for Mn-BC and corroborates the multi-mechanism process involving surface complexation with oxygenated functional groups (–OH, C–O, C–O–C), Mn-mediated binding, and supplementary precipitation/ion exchange pathways (Wan et al., 2020; Chang, 2025). Although XPS provides strong evidence for Pb–O bonding indicative of inner-sphere complexation and/or surface precipitation, the lack of post-adsorption XRD analysis limits direct identification of crystalline Pb phases (e.g., Pb(OH)2 or Pb–Mn precipitates). Future studies should incorporate XRD on saturated samples to confirm precipitation contributions and characterize newly formed phases under varying pH and loading conditions.
Figure 9. Survey scan (a) and Pb 4f spectrum of BC and Mn-BC before and after Pb(II) sorption.
Figure 10. XPS spectrum for Mn-BC before and after Pb(II) sorption.
Figure 11. Schematic overview of the multi-mechanism Pb(II) adsorption processes on Mn-Bc.
4 Conclusion
This research highlights the superior adsorption capabilities of manganese-modified bamboo biochar (Mn-BC) for Pb(II) ions compared to unmodified biochar. The Mn-BC variant features an expanded specific surface area of 121.28 m2 g-1 and an increased total pore volume of 0.062 cm3 g-1, surpassing the 76.17 m2 g-1 and 0.042 cm3 g-1 of the raw biochar, respectively, thus offering more active sites for adsorption. Experimental data reveal that Mn-BC achieves a maximum adsorption capacity of approximately 153.63 mg g-1, which is nearly quintuple that of the pristine biochar, which stands at about 30.22 mg g-1. The adsorption process adheres to the second-order kinetic model, indicating that chemisorption is the primary mechanism. Thermodynamic evaluations further affirm the process’s spontaneity (ΔG° < 0) and endothermic nature (ΔH°). Reusability assessments show that Mn-BC maintains around 79% of its initial adsorption capacity even after five consecutive cycles, in contrast to the 64% retention observed with pristine biochar. XPS analysis identifies functional groups (–OH, –COOH) and manganese species as key contributors to Pb(II) binding. In conclusion, Mn-BC presents itself as a cost-effective, stable, and environmentally sustainable adsorbent with significant potential aimed at the wide-scale remediation of Pb(II)-contaminated environments.
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Author contributions
WA: Investigation, Software, Formal Analysis, Funding acquisition, Visualization, Writing – original draft, Writing – review and editing, Data curation, Methodology, Validation, Conceptualization, Project administration. YW: Methodology, Formal Analysis, Writing – review and editing, Data curation, Software. SA: Writing – review and editing, Software, Visualization, Data curation, Formal Analysis, Methodology. AN-D: Writing – review and editing. FQ: Visualization, Methodology, Writing – review and editing. MD: Writing – review and editing, Formal Analysis, Software. WL: Visualization, Project administration, Funding acquisition, Investigation, Supervision, Writing – review and editing, Resources. SM: Visualization, Methodology, Formal Analysis, Writing – review and editing, Software.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This research was economically supported by the Hainan Province Science and Technology Research Fund (RZ2300001281), Launch Fund of Hainan University High level Talent (RZ2100003226), Hainan Province Science and Technology Special Fund (ZDYF2021SHFZ071 and ZDYF2021XDNY185).
Acknowledgements
The authors would like to thank the School of ecology, Hainan University for conducting this study.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2026.1770860/full#supplementary-material
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Keywords: adsorption, biochar, Pb(II), reusability and stability, surface functional groups, wastewater treatment
Citation: Ahmed W, Wang Y, Ali S, Núñez-Delgado A, Qin F, Dong M, Li W and Mehmood S (2026) Adsorption characteristics and mechanism insights of manganese modified biochar for Pb (II) adsorption in wastewater. Front. Environ. Sci. 14:1770860. doi: 10.3389/fenvs.2026.1770860
Received: 18 December 2025; Accepted: 21 January 2026;
Published: 11 February 2026.
Reviewed by:
Weixiong Lin, Zhaoqing University, ChinaMuhammad Younas, Wuhan Textile University, China
Copyright © 2026 Ahmed, Wang, Ali, Núñez-Delgado, Qin, Dong, Li and Mehmood. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Weidong Li, d2VpZG9uZ2xpQGhhaW5hbnUuZWR1LmNu; Sajid Mehmood, ZHJzYWppZEBoYWluYW51LmVkdS5jbg==
†These authors have contributed equally to this work