3D magnetic nanocomposite modified with molybdenum disulfide: preparation and application in aqueous Cr(vi) remediation

Abstract

Hexavalent chromium (Cr(vi)) poses severe environmental risks due to its high toxicity, persistence, and bioaccumulation. Adsorption technology is recognized as one of the core strategies for Cr(vi) remediation, but the development of low-cost, high-efficiency biochar-based adsorbents remains an urgent demand. In this work, 3D Molybdenum disulfide Magnetic Biochar (3DMMB) was successfully fabricated via a combination of calcination pyrolysis and hydrothermal methods, using traditional Chinese medicine residues (CMR) as the biomass feedstock and iron-based sludge (IS) as an alternative to conventional iron sources. Solution pH was found to exert a significant influence on Cr(vi) removal, with an acidic environment being favorable for Cr(vi) adsorption. Kinetic and isothermal analyses revealed that the adsorption process fits well with the pseudo-second-order kinetic model and Langmuir isotherm model, confirming that the adsorption is an endothermic and spontaneous monolayer process, dominated by chemical adsorption as the rate-limiting step, with a maximum adsorption capacity of 83.33 mg g⁻¹. The potential adsorption mechanisms mainly involve physical adsorption, electrostatic interaction, chemical reduction, and complexation. Notably, 3DMMB exhibits excellent recyclability and regeneration performance, retaining 89.55% of its initial adsorption capacity after six cycles of desorption and regeneration. This study provides a novel and cost-effective approach for the preparation of waste-derived functional adsorbents and realizes selective and efficient purification of Cr(vi)-contaminated wastewater through sulfur-based grafting modification.

Development of waste-derived magnetically recyclable 3D MoS₂–biochar nanocomposites via a calcination–hydrothermal route for synergistic adsorption–reduction remediation of aqueous Cr(vi) with superior adsorption capacity and cyclic regenerability.

1. Introduction

In recent years, heavy metal water pollution has become a serious environmental issue. Hexavalent chromium (Cr(vi)) is highly toxic and carcinogenic, posing a significant threat to the ecosystem and human health.¹ Therefore, it is particularly urgent to carry out efficient and sustainable treatment of Cr(vi)-containing wastewater. Among various treatment technologies, the adsorption method is favored due to its simple operation and high efficiency.² However, most current adsorbents have the problems of high cost and difficulty in recycling, which limits their application.

As a low-cost, eco-friendly adsorbent, biochar has become a research focus. Preparing biochar from solid waste results in waste resource utilization and reduces pollution, conforming to green sustainable development. The traditional Chinese medicine industry generates massive Chinese Medicinal Residues (CMR),³ mostly disposed of by stacking and landfilling. Rich in organic matter, lignocellulose and bioactive components, they are ideal biomass for high-adsorption biochar.

Iron-based sludge (IS) from drinking water treatment plants, containing abundant iron compounds from polyferric flocculation,^4,5 is mainly landfilled after dehydration, wasting metal resources and occupying land. IS can serve as an iron source to magnetize biochar by calcination,⁶ solving adsorbent separation issues and enhancing adsorption performance. However, pristine biomass biochar has a poor adsorption effect, requiring modification to improve heavy metal adsorption capacity and selectivity.^7,8

Molybdenum disulfide (MoS₂) is a layered transition metal sulfide with an S–Mo–S sandwich structure.⁹ The surface of MoS₂ contains massive sulfur-containing groups, which can improve Cr(vi) adsorption capacity through reduction and coordination reactions.¹⁰ The layered structure of MoS₂ enhances adsorbent pore development, specific surface area and active sites significantly.¹¹ Hydrothermal synthesis can be used to directionally prepare MoS₂ with 3D flower-like nanostructures,¹² alleviating 2D layer stacking and improving dispersibility, further enhancing composite adsorption performance. To solve the problem of solid–liquid separation, magnetic particles were introduced into the MoS₂ layers through calcination pyrolysis.¹³

To address these issues, this study uses CMR as a carbon matrix and IS as an iron source to prepare 3D flower-like MoS₂ modified magnetic biochar by calcination pyrolysis and hydrothermal methods. The adsorbent's Cr(vi) adsorption performance and mechanism were systematically studied.

2. Materials and methods

The IS was obtained from a drinking water treatment plant in Huludao, Liaoning, China. The CMR was collected from a pharmacy in Zhengzhou, Henan, China. The CMR and IS were dried to constant weight in a thermostatic drying oven at a fixed temperature of 105 °C for 48 h. The mixture of CMR and IS was calcined in a vacuum tube furnace under a nitrogen (N₂) protective atmosphere to obtain Magnetic Biochar (MB).

For the synthesis of 3DMMB, ammonium molybdate and thiourea were dissolved and continuously stirred at a rotation speed of 120 rpm for 30 min at a constant ambient temperature of 25 °C to form a homogeneous solution. A predetermined amount of MB was added to the solution, which was then ultrasonicated for 1 h. The suspension was transferred into a PTFE-lined high-pressure reactor and subjected to a hydrothermal reaction under high temperature and pressure conditions for 6 h. The black solid product was collected by centrifugation and dried in a vacuum freeze dryer, ultimately obtaining the 3D Molybdenum disulfide Magnetic Biochar (3DMMB).

3. Results and discussion

3.1. Sample characterization

3.1.1. BET analysis

According to the IUPAC¹⁴ classification of physisorption isotherms, the N₂ adsorption–desorption isotherm of 3DMMB belonged to a typical type IV isotherm with a distinct H₃ hysteresis loop (Fig. 1a), confirming a layered architecture constructed from slit-shaped mesopores.^15,16 Quantitative BET data show that the specific surface area falls drastically from 73.45 m² g⁻¹ of bare MB to only 8.08 m² g⁻¹ for 3DMMB, accompanied by a remarkable enlargement in pore dimensions. Two structural changes collectively account for the loss of surface area: vertically stacked and agglomerated MoS₂ nanosheets wrap the outer surface of MB and shield most intrinsic micropores that contribute the majority of specific surface area; additionally, Fe₃O₄ magnetic nanoparticles intercalate into the interlayer gaps of MoS₂, further occupying microporous spaces and eliminating original surface adsorption sites. BET pore size calculation manifests that the average pore diameter increases from 16.77 nm (MB) to 59.03 nm (3DMMB). The pore size distribution curve (Fig. 1b) further proves that 3DMMB is dominated by mesopores and macropores ranging from 8.12 nm to 59.47 nm with a central pore size of 32.58 nm.¹⁷

Fig. 1. The N₂ adsorption–desorption curves of 3DMMB (a); the BJH pore size distribution of 3DMMB (b).

There exists an inherent structural trade-off between sacrificed physical surface sites and optimized ion transport channels, which fundamentally explains why 3DMMB retains outstanding Cr(vi) removal efficiency despite its much lower BET surface area. The numerous tiny micropores responsible for the high surface area of pristine MB are structurally unfavorable for Cr(vi) capture. Hydrated Cr(vi) oxyanions possess a large hydration radius, and narrow micropore tunnels produce strong steric hindrance and high intraparticle mass transfer resistance, restricting Cr(vi) ions to merely come into contact with the outermost surface of MB without accessing internal adsorption sites. During composite fabrication, the blockage of these invalid micropores sacrifices total surface area yet generates interconnected wedge-shaped meso/macropore networks. The widened pore channels greatly reduce diffusion barriers, endowing Cr(vi) oxyanions with unobstructed long-distance transport pathways to penetrate deep into the adsorbent matrix.

Crucially, the consumed native surface sites are fully compensated for by newly generated high-efficiency chemical active sites inside the hierarchical pore framework. The interlayer regions of overlapping MoS₂ nanosheets expose massive Mo and S coordination sites with strong reduction and chelation capacity toward Cr(vi), while intercalated Fe₃O₄ nanoparticles supply abundant redox-active Fe sites to accelerate the reduction of toxic Cr(vi) to low-toxicity Cr(iii). In this trade-off system, the quantitative decline of physical surface sites does not weaken adsorption capacity, because the adsorption behavior shifts from surface-dominated physical adsorption to internal-site-governed redox adsorption. The overall removal performance is no longer controlled by the total BET surface area, but synergistically determined by two favorable structural merits: the unimpeded meso/macropore channels that guarantee sufficient ion supply to interior active sites and the high-activity interlayer Mo/S/Fe redox sites that provide strong binding and reduction capacity for Cr(vi). The pore-blocked morphology observed after Cr(vi) adsorption further verifies this trade-off mechanism: Cr(vi) ions successfully migrate through continuous meso/macropore channels and immobilize on interlayer Mo and S active sites, rather than accumulating only on the external material surface.

3.1.2. SEM and TEM analysis

As shown in Fig. 2a, commercial bulk MoS₂ presents an irregular lamellar and blocky structure with severe interlayer stacking. In comparison, the hydrothermally synthesized MoS₂ in this study displays a well-defined layered structure (Fig. 2b). The hydrothermal strategy effectively ameliorates the irregular morphology of traditional MoS₂, alleviates excessive layer stacking, and constructs a highly regular 3D flower-like nanostructure.¹⁸ Such abundant interlayer channels facilitate the diffusion of pollutant ions into internal pores, thereby strengthening the binding interaction between adsorbents and pollutant adsorption sites.

Fig. 2. The SEM images of commercially purchased MoS₂ (a) and self-fabricated MoS₂ synthesized by the hydrothermal method (b); 3DMMB before (c) and after (d) adsorption.

Fig. 2c clearly shows that spherical Fe₃O₄ nanoparticles are uniformly loaded on the surface, edges, and interlayer gaps of MoS₂ in 3DMMB, demonstrating the successful integration of Fe₃O₄ and MoS₂. The loaded Fe₃O₄ nanoparticles not only endow 3DMMB with excellent magnetic properties,¹⁹ but also introduce abundant additional active sites for Cr(vi) adsorption and removal. The Energy-Dispersive X-ray Spectroscopy (EDS) results exhibit distinct characteristic peaks of Mo and S (Fig. 3), which confirm the existence of MoS₂ in the 3DMMB composite. Elemental mapping analysis further verifies that S, Mo, Fe, and O elements are uniformly distributed in the composite and mainly exist in the forms of MoS₂ and Fe₃O₄, proving the successful fabrication of the 3DMMB material.

Fig. 3. The EDS energy spectra and elemental map of 3DMMB.

After Cr(vi) adsorption, the SEM image of 3DMMB (Fig. 2d) reveals that the original 3D flower-like structure of MoS₂ is partially damaged, and numerous particulate deposits appear on the material surface. Meanwhile, the original pore structure of the adsorbent is partially blocked and filled. These morphological changes indicate that Cr(vi) ions can be effectively captured and immobilized on the 3DMMB surface through multiple synergistic mechanisms, including electrostatic adsorption, chemical reduction, and complexation reaction.

TEM was further utilized to explore the morphological characteristics and surface microstructure of 3DMMB, and the corresponding results are presented in Fig. 4. In Fig. 4a, the translucent gauze-like structure is assigned to the carbon matrix, while the gray striped structures correspond to lamellar MoS₂ nanosheets. Local magnification shows obvious stacking and bending behaviors of MoS₂ nanosheets, which is highly consistent with the 3D flower-like structure constructed from curled two-dimensional nanosheets in SEM results. The black dots distributed in the TEM images are Fe₃O₄ nanoparticles, which are mainly anchored on the surfaces and edges of MoS₂ nanosheets, further verifying the structural characteristics of 3DMMB.

Fig. 4. The TEM of 3DMMB before (a) and after (b) adsorption.

After Cr(vi) adsorption, obvious stacked blocky light and dark domains can be observed in the TEM images of 3DMMB (Fig. 4b). Combined with the above SEM characterization results, the comprehensive analysis confirms that the surface Fe₃O₄ and MoS₂ components of 3DMMB can interact with Cr(vi) via electrostatic attraction, redox reactions and other interaction pathways. Ultimately, chromium species are effectively immobilized on the material surface and form a continuous film-like coating covering the exterior of 3DMMB.

3.1.3. XRD analysis

The crystalline phases of 3DMMB before and after Cr(vi) adsorption were investigated by X-ray diffraction (XRD) analysis, and the results are shown in Fig. 5. A broad diffraction peak was observed at 2θ = 21.77°, which was attributed to the presence of the amorphous SiO₂ phase.²⁰ The peaks at 2θ = 35.7°, 53.4°, and 62.6° corresponded to the (311), (422), and (440) crystal plane reflections of Fe₃O₄ (JCPDS no. 19-0629),²¹ respectively. The characteristic peaks at 2θ = 13.9°, 32.4°, 43.2°, and 57.1° correspond to the (002), (100), (105), and (110) crystal planes of MoS₂ (JCPDS no. 37-1492), consistent with the standard XRD pattern of pure MoS₂.²² These results confirmed the successful synthesis of MoS₂ on the surface of 3DMMB and the strong binding of MoS₂ and Fe₃O₄, thus verifying the successful preparation of the magnetic 3DMMB. After Cr(vi) adsorption, the diffraction peaks of MoS₂ and Fe₃O₄ were still clearly visible, indicating the excellent structural stability of 3DMMB. New diffraction peaks at 2θ = 56.89°, 63.10°, and 71.89° were observed after Cr(vi) adsorption, corresponding to the (511), (400), and (533) crystal planes of FeCr₂O₄ (JCPDS no. 34-0140).²³ Additionally, a new isolated diffraction peak emerges at 44.03°, which matches the (110) plane of standard Cr₂O₃ (JCPDS no. 85-1336),²⁴ suggesting that Cr(vi) was reduced to Cr(iii) and then captured on the material surface by complexation.

Fig. 5. The XRD patterns of 3DMMB before and after adsorption.

3.2. Adsorption study

3.2.1. Effect of initial solution pH

In the initial pH range of 1 to 10, the influence of the initial water solution pH on the adsorption of Cr(vi) by 3DMMB was analyzed. pH of solution affected the existing forms of Cr(vi) and the functional groups on the 3DMMB surface significantly. Fig. 6b shows that the Cr(vi) adsorption capacity rose from 6.8 mg g⁻¹ to 82.13 mg g⁻¹ as the pH decreased from 10 to 1, indicating that acidic conditions were favorable for Cr(vi) removal.²⁵

Fig. 6. The zeta potential of 3DMMB as a function of pH 1–10 (a); the influence of initial pH on Cr(vi) adsorption capacity (C₀: 250 mg L⁻¹, temperature: 25 °C, reaction time: 6 h) (b).

The point of zero charge (pH_pzc) of 3DMMB was 1.34 (Fig. 6a). When the pH of solution was lower than 1.34, protonation endowed the 3DMMB surface with positive charges, which generated strong electrostatic attraction with negatively charged ions,²⁶ thus achieving efficient Cr(vi) adsorption. As the pH increased, the 3DMMB surface became negatively charged and electrostatic repulsion occurred between Cr(vi) and 3DMMB, which was gradually intensified with the increase in the pH value. In alkaline environments, the OH⁻ ions competed with Cr(vi) ions for available adsorption sites. Meanwhile, the OH⁻ combined with Cr(iii) to form precipitates, covering the active sites of 3DMMB and further suppressing Cr(vi) elimination.²⁷ Therefore, Cr(vi) adsorption by 3DMMB was strongly pH-dependent, and lower pH was more conducive.

3.2.2. Effect of adsorption time and adsorption kinetics

The effect of adsorption time on the adsorption capacity of 3DMMB for Cr(vi) was investigated at initial Cr(vi) concentrations of 100, 200, and 400 mg L⁻¹. Fig. 7a shows that at all three initial Cr(vi) concentrations, the removal rate of Cr(vi) was rapid in the initial stage of adsorption. As the adsorption time increased, the adsorption capacity gradually increased and approached equilibrium. The rapid adsorption in the initial stage was attributed to the abundant active adsorption sites on the surface of 3DMMB, the high concentration of Cr(vi) in the solution, and the fast mass transfer rate.²⁸ With the progress of the adsorption reaction, the active adsorption sites on the surface of 3DMMB were gradually occupied, the concentration of Cr(vi) in the solution decreased, and the adsorption process reached a relatively stable equilibrium state. The adsorption equilibrium time for an initial Cr(vi) concentration of 400 mg L⁻¹ was 24 h.

Fig. 7. The variation of Cr(vi) adsorption by 3DMMB with reaction time (temperature: 25 °C, pH: 2) (a); the linear fitting using the PFO model (b); PSO model (c).

To clarify the adsorption mechanism of Cr(vi) by 3DMMB, the kinetic data were fitted with pseudo-first-order and pseudo-second-order kinetic models.²⁹ The linear fitting plots and corresponding kinetic parameters are presented in Fig. 7b, c and Table 1. At initial Cr(vi) concentrations of 100, 200, and 400 mg L⁻¹, the correlation coefficients (R²) of the pseudo-second-order model were 0.9978, 0.9994, and 0.9995, respectively, which were significantly higher than those of the pseudo-first-order model (0.8587, 0.9219, and 0.9277). Additionally, the equilibrium adsorption capacities calculated from the pseudo-second-order model were 51.76, 72.62, and 85.76 mg g⁻¹, respectively, which were in good agreement with the experimental values. These results indicated that the pseudo-second-order model could better describe the adsorption process of Cr(vi) by 3DMMB, suggesting that the removal of Cr(vi) by 3DMMB was mainly dominated by chemisorption.

Table 1. The kinetic parameters of Cr(vi) adsorption by 3DMMB.

C 0, mg L−1	Pseudo-first-order			Pseudo-second-order
	q e, mg g−1	k 1, h−1	R 2	q e, mg g−1	k 2, g mg−1 h−1	R 2
100	40.19	0.2029	0.8587	51.76	0.0193	0.9978
200	45.26	0.1867	0.9219	72.62	0.0137	0.9994
400	44.11	0.1623	0.9277	85.76	0.0116	0.9995

3.2.3. Adsorption isotherms

The effect of initial Cr(vi) concentration on the adsorption capacity of 3DMMB was investigated at reaction temperatures of 10, 25, and 35 °C. As shown in Fig. 8a, the Cr(vi) uptake increased with rising temperature, demonstrating that elevated temperatures facilitated Cr(vi) adsorption. Meanwhile, Fig. 8a shows that the removal capacity of Cr(vi) was enhanced with the increase in initial Cr(vi) concentration.

Fig. 8. The relationship between equilibrium concentration (C_e) and Cr(vi) adsorption uptake (q_e) (reaction time: 24 h, pH: 2) (a); linear fitting using the Langmuir model (b) and Freundlich model (c).

To further understand the adsorption behavior of Cr(vi) on 3DMMB, the adsorption isotherm data were fitted with Langmuir and Freundlich models.³⁰ The linear fitting curves and corresponding fitting parameters are displayed in Fig. 8b, c and Table 2. At reaction temperatures of 10, 25, and 35 °C, the correlation coefficients (R²) of the Langmuir model were 0.9989, 0.9986, and 0.9991, respectively, which were significantly higher than those of the Freundlich model. This indicated that the adsorption of Cr(vi) on 3DMMB was better described by the Langmuir model, suggesting that the adsorption process was monolayer adsorption with no mutual interaction between adjacent adsorption sites. According to the Langmuir model, the saturation adsorption capacities of 3DMMB for Cr(vi) at 10, 25, and 35 °C were calculated to be 67.11 mg g⁻¹, 76.92 mg g⁻¹, and 83.33 mg g⁻¹, respectively.

Table 2. The fitting parameters of the isotherm models.

T/°C	Langmuir			Freundlich
	q m, mg g−1	K L, L mg−1	R 2	K F	1/n	R 2
10	67.11	0.543	0.9989	47.951	0.071	0.9809
25	76.92	1.062	0.9986	60.725	0.035	0.7577
35	83.33	1.694	0.9991	71.300	0.016	0.6819

The thermodynamic parameters³¹ were calculated and the corresponding data are shown in Table 3. The ΔG⁰ was negative and the value of ΔG⁰ increased with the rising temperature, indicating that the Cr(vi) adsorption by 3DMMB was a spontaneous process. The positive ΔH⁰ indicated that the adsorption process was endothermic, and the positive ΔS⁰ suggested that the adsorption process was entropically favorable.

Table 3. The fitting parameters of the thermodynamic exploration.

T/K	ΔG0 (kJ mol−1)	ΔH0 (kJ mol−1)	ΔS0 (kJ mol−1 K−1)
283 K	−1.96	13.43	0.05
298 K	−2.77
308 K	−3.32

3.2.4. Regeneration and reusability

Magnetic modification of the adsorbent enabled efficient separation of the spent adsorbent from the solution, which could effectively avoid secondary pollution and facilitate the recycling of the adsorbent. 3DMMB exhibited excellent magnetic properties with a saturation magnetization of 10.14 emu g⁻¹ (Fig. 9a), which allowed for rapid solid–liquid separation within 30 s under an external magnetic field. The regeneration performance of 3DMMB was studied using 0.5 mol per L NaOH and 0.1 mol per L HCl as eluents for desorption and regeneration after Cr(vi) adsorption. The adsorption capacity of 3DMMB after each regeneration cycle is shown in Fig. 9b. After the first regeneration cycle, the adsorption capacity of 3DMMB decreased by only 1.74%, essentially maintaining its original adsorption performance. With the increase in regeneration cycles, the adsorption capacity gradually decreased; however, 3DMMB still retained 89.55% of its initial adsorption capacity after six cycles of desorption and regeneration. Hence, 3DMMB was suitable for regeneration and exhibited excellent recycling performance.

Fig. 9. Magnetization of 3DMMB (a); the reusability study of 3DMMB (b) (C₀ = 200 mg L⁻¹, pH = 2); surface SEM morphology of 3DMMB after six regeneration cycles (c).

Fig. 6b explicitly illustrates that 3DMMB achieves a prominent Cr(vi) adsorption capacity of 82.13 mg g⁻¹ at pH = 1, powerfully demonstrating its outstanding Cr(vi) capture efficiency even in extreme strong acid environments. To further assess its structural endurance in acidic media, all adsorption kinetic and thermodynamic experiments were implemented at pH = 2. No decomposition, fracture or collapse of the carbon skeleton was detected over the whole adsorption reaction. Post-adsorption SEM and TEM characterization studies further corroborate the robust structural integrity of 3DMMB: although partial pore channels are blocked by and filled with granular Cr(vi)-bearing precipitates, the integral carbon framework and partial distinctive 3D flower-like architecture are well maintained. We supplemented SEM characterization of 3DMMB following six regeneration cycles (Fig. 9c). The characterization only revealed mild pore clogging on the material surface after repeated cycling, whereas its well-defined 3D hierarchical architecture remains fully preserved. These findings demonstrate the exceptional structural robustness and recyclable adsorption performance of 3DMMB in acidic wastewater treatment environments.

3.3. Adsorption mechanism exploration

3.3.1. FTIR analysis

The surface functional groups of MB and 3DMMB were investigated by Fourier transform infrared spectroscopy (FTIR), and the results are shown in Fig. 10. Peaks at 1032 cm⁻¹ and 798 cm⁻¹ were observed, which corresponded to the stretching vibration of Si–O–Si,³² indicating that the carbon matrix structure of 3DMMB was stable. The characteristic peak at 468 cm⁻¹ corresponded to the Mo–S stretching vibration,³³ confirming the successful loading of MoS₂ onto the surface of 3DMMB. The peaks at 702 cm⁻¹ and 913 cm⁻¹ were assigned to the stretching vibrations of Mo O and Mo–O,³⁴ indicating partial oxidation of Mo ions. The peaks of Fe–O appeared at 565 cm⁻¹ and 556 cm⁻¹ before and after MoS₂ loading, suggesting that MoS₂ and Fe₃O₄ formed a stable composite structure through chemical bonding under high-temperature conditions, which was consistent with the XRD results.

Fig. 10. The FTIR spectra of MB and 3DMMB before and after adsorption.

To further analyze the changes in surface functional groups and chemical bonds during Cr(vi) removal, the shifts in the FTIR absorption peaks of 3DMMB before and after Cr(vi) adsorption were investigated. After Cr(vi) adsorption, the characteristic peaks at 1393 cm⁻¹, 1623 cm⁻¹, 2930 cm⁻¹, and 3436 cm⁻¹ shifted to 1462 cm⁻¹, 1625 cm⁻¹, 2928 cm⁻¹, and 3430 cm⁻¹, respectively, indicating that functional groups such as C–H, –COOH, and –OH participated in the adsorption of Cr(vi) by 3DMMB through electrostatic interaction and complexation. Additionally, after Cr(vi) adsorption, the characteristic peaks of Mo–S and Fe–O shifted to 459 cm⁻¹ and 547 cm⁻¹, respectively, and new peaks at 810 cm⁻¹ and 908 cm⁻¹ appeared, which corresponded to the Cr–O stretching vibration and validated the formation of Cr₂O₃ nanoparticles.³⁵ These results suggested that MoS₂ and Fe₃O₄ played important roles in the removal of Cr(vi), acting as electron donors to reduce Cr(vi) to Cr(iii), which was then adsorbed onto the surface of 3DMMB through complexation.

3.3.2. XPS analysis

X-ray Photoelectron Spectroscopy (XPS) was adopted to characterize the surface elemental composition and chemical states of 3DMMB before and after Cr(vi) adsorption. XPS results revealed that the main elements on the 3DMMB surface were C, N, O, Fe, Mo and S, which verified the successful immobilization of Fe, Mo and S on the material surface. Combined with SEM and XRD analysis, it was deduced that Fe, Mo and S primarily existed in the forms of Fe₃O₄ and MoS₂, further demonstrating that 3DMMB was successfully synthesized via combined calcination–pyrolysis and hydrothermal strategies. After Cr(vi) adsorption, the characteristic peaks of Fe, Mo and S declined remarkably, and a new characteristic peak assigned to Cr 2p emerged. This phenomenon proved that chromium ions were effectively captured by 3DMMB, and diverse surface functional groups as well as chemical bonds of 3DMMB participated in the Cr(vi) removal process.

The Cr 2p XPS spectra are shown in Fig. 11e; the peaks at 577.66 eV and 587.57 eV were assigned to the Cr 2p orbital, confirming that the chromium species had successfully adsorbed on the 3DMMB surface. Further peak analysis of Cr 2p was conducted, the peaks at 577.83 eV and 587.54 eV belonged to Cr(vi), while the peaks at 576.89 eV and 586.57 eV were attributed to Cr(iii).³⁶ Based on the peak area statistics, Cr(iii) accounted for 53.26% among the total chromium peak area after adsorption, whereas Cr(vi) accounted for 46.74%. The above results indicated that Cr(vi) was removed by 3DMMB through physical adsorption and electrostatic interaction. Meanwhile, some of Cr(vi) was reduced to Cr(iii), and the resulting Cr(iii) was immobilized on the surface of 3DMMB through complexation.

Fig. 11. The XPS spectra of 3DMMB (a); the detail survey of C 1s (b); O 1s (c); N 1s (d); Cr 2p (e); Fe 2p (f); Mo 3d (g); S 2p (h) before and after Cr(vi) adsorption.

As displayed in Fig. 11b, the fitted peaks at 284.47 eV, 285.20 eV and 286.16 eV corresponded to C C/C–C, C–N⁺ and C–O functional groups, respectively.³⁷ This finding confirmed that abundant C-, N- and O-containing functional groups were distributed on the 3DMMB surface, which provided sufficient active sites for Cr(vi) adsorption. After Cr(vi) adsorption, these characteristic peaks shifted to 284.68 eV, 285.08 eV and 286.18 eV accordingly, and the relative contents of carbon-based functional groups changed obviously from 47.73%, 40.47% and 11.80% to 45.43%, 29.66% and 24.90%. Such variations implied that large amounts of C–N⁺ and C–O groups were consumed through electrostatic interaction and redox reactions with HCrO₄⁻ and Cr₂O₇²⁻, which fully evidenced that carbon-containing functional groups played a vital role in the Cr(vi) adsorption process of 3DMMB.

The O 1s XPS spectra of 3DMMB were fitted into three distinct peaks (Fig. 11c). The peak at 530.65 eV was assigned to lattice oxygen, which originated from S–O, Mo–O and Fe–O bonds.³⁸ The peaks at 531.59 eV and 533.14 eV corresponded to –OH and physically/chemically adsorbed water molecules, respectively. After Cr(vi) adsorption, all O 1s characteristic peaks underwent obvious shifts, indicating that oxygen-containing functional groups participated in Cr(vi) elimination.

The N 1s XPS spectrum of 3DMMB is presented in Fig. 11d. The peaks at 396.91 eV and 399.1 eV were attributed to N– and –NH groups, respectively. Additionally, the N 1s peak at 394.99 eV overlapped with the characteristic signal of Mo 2p,³⁹ indicating that thiourea decomposed under hydrothermal conditions at 210 °C, and the N and Mo atoms formed Mo–N chemical bonds successfully. The Mo 2p peak shifted to 394.54 eV after Cr(vi) adsorption, while the peaks of N– and –NH shifted to 399.29 eV and 398.19 eV respectively, and the intensity of the –NH peak was enhanced. The peak analysis indicated that the nitrogen-containing functional groups provide abundant active sites for the adsorption process of Cr(vi) on 3DMMB, thereby facilitating the removal process.⁴⁰

Fig. 11f shows the Fe 2p XPS spectrum; the peaks of 712.28 eV (Fe 2p_3/2) and 725.77 eV (Fe 2p_1/2) corresponded to Fe(iii) and the two characteristic peaks of Fe(ii) are at 710.80 eV (Fe 2p_3/2) and 724.40 eV (Fe 2p_1/2),⁴¹ while the peak at 720.31 eV was assigned to the satellite peak of Fe(ii). The peak area was calculated, and the initial relative contents of Fe(iii) and Fe(ii) were 25.94% and 74.06%, respectively. The peak position of Fe changed after Cr(vi) adsorption, and the relative proportions of Fe(iii) and Fe(ii) changed to 68.83% and 31.17%. The results indicated that the iron-containing groups were involved in the removal of Cr(vi) by 3DMMB, and the Fe(ii) reduced Cr(vi) to Cr(iii) as an electron donor, while it was oxidized to Fe(iii).⁴²

In the Mo 3d XPS spectrum (Fig. 11g), the peaks at 229.05 eV and 232.21 eV were assigned to the Mo 3d_5/2 and Mo 3d_3/2 orbitals of Mo(iv) ⁴³ and a distinct peak at 226.05 eV is attributed to the S 2s orbital, confirming the successful synthesis of MoS₂ on 3DMMB. The peaks at 233.10 eV and 236.15 eV corresponded to the Mo 3d_5/2 and Mo 3d_3/2 of Mo(vi), indicating that slight oxidation and incomplete sulfidation occurred during the preparation of 3DMMB. The intensity of the S 2s peak was weakened after adsorption, indicating that the sulfur-containing groups participated in the Cr(vi) adsorption process by 3DMMB. The peaks of Mo(vi) and Mo(iv) were shifted slightly after Cr(vi) adsorption, and the content of Mo(iv) decreased from 83.36% to 55.89%, while that of Mo(vi) increased from 16.63% to 44.11%. This result indicated that the Mo(vi) played an important role in the Cr(vi) removal by 3DMMB, reducing Cr(vi) to Cr(iii), while the Mo(vi) was oxidized to Mo(iv).

In the high-resolution S 2p XPS spectrum (Fig. 11h), the characteristic peaks at 161.85 eV and 163.11 eV are assigned to the S 2p_3/2 and S 2p_1/2 spin–orbit states of S²⁻, while the peaks at 164.92 eV and 169.06 eV correspond to the S 2p_3/2 and S 2p_1/2 spin–orbit states of SO₄²⁻.⁴⁴ The relative abundances of S²⁻ and SO₄²⁻ are 91.17% and 8.83%, respectively, indicating that S mainly exists in the form of S²⁻, which effectively verifies the successful synthesis of MoS₂ on 3DMMB. Meanwhile, the presence of SO₄²⁻ suggests partial oxidation of S²⁻ on the surface of 3DMMB. After Cr(vi) adsorption, the S 2p spin–orbit peaks shift: the S²⁻ S 2p_3/2 and S 2p_1/2 peaks shift to 161.46 eV and 162.76 eV and the SO₄²⁻ S 2p_3/2 and S 2p_1/2 peaks shift to 168.67 eV and 169.77 eV. The relative proportion of S²⁻ decreases from 91.17% to 86.01%, while that of SO₄²⁻ increases from 8.83% to 13.99%. This indicates that a redox reaction occurs between S²⁻ and Cr(vi) during the adsorption process: Cr(vi) is reduced to Cr(iii), while S²⁻ is oxidized to SO₄²⁻.

In summary, the main adsorption mechanisms for Cr(vi) removal by 3DMMB include physical adsorption, electrostatic interaction, chemical reduction, and complexation. Based on the above comprehensive analysis, the main reaction equations for the Cr(vi) adsorption process by 3DMMB are shown in eqn (1)–(5).

MoS₂ + 6HCrO₄⁻ + 18H⁺ → MoO₄²⁻ + 2SO₄²⁻ + 6Cr³⁺ + 12H₂O 1

MoS₂ + 3Cr₂O₇²⁻ + 18H⁺ → MoO₄²⁻ + 2SO₄²⁻ + 6Cr³⁺ + 9H₂O 2

3Fe²⁺ + HCrO₄⁻ + 7H⁺ → 3Fe³⁺ + Cr³⁺ + 4H₂O 3

3Fe²⁺ + CrO₄²⁻ + 8H⁺ → 3Fe³⁺ + Cr³⁺ + 4H₂O 4

Fe³⁺ + Cr³⁺ + H₂O → Cr_xFe_1−x(OH)₃ + 3H⁺ 5

4. Conclusions

In this study, an eco-friendly biochar 3DMMB was successfully fabricated using CMR as the carbon matrix, IS as the magnetizing raw material, and MoS₂ as the sulfur-based grafting agent. A 3D flower-like structure was directionally constructed via the hydrothermal method, which alleviated the traditional stacking problem of flake structure in MoS₂. The abundant wedge-shaped mesopores and macropores of 3DMMB promoted pollutant diffusion and adsorbate–adsorption site interactions for efficient Cr(vi) removal. The Cr(vi) adsorption by 3DMMB fitted the pseudo-second-order kinetic model and Langmuir models, indicating chemical-dominated monolayer adsorption. XPS analysis confirmed that Fe(ii), Mo(vi), and S²⁻ reduced Cr(vi) to Cr(iii) and were simultaneously oxidized to Fe(iii), Mo(iv), and SO₄²⁻, respectively. Overall, 3DMMB exhibited excellent magnetic separation performance, regeneration capacity, and Cr(vi) adsorption–reduction ability, providing a promising strategy for the resource recovery of solid waste and the high-efficiency remediation technologies for Cr(vi)-contaminated wastewater.

Author contributions

Xiaohan Duan: conceptualization, formal analysis, writing – original draft preparation; Jian Zhang: methodology, supervision; writing – review and editing; Yao Wang: data curation, investigation; Baoxi Miao: data curation, investigation; Yanxu Yu: methodology, formal analysis; Shuoze Guo: project administration, writing – review and editing; Yating Liu: investigation, visualization; Wei Song: formal analysis, investigation; Junkai Zheng: visualization, writing – original draft preparation; Jingxi Tie: conceptualization, supervision, writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the manuscript.