Interfacial coupling of Mn-Mo sulfides with self-N-doped hydrochar enables efficient Cr(VI) removal: Mechanistic...

In this study, soy sauce residue was utilized as a precursor for preparing N-doped hydrochar through a one-step hydrothermal process, during which MnS and MoS₂ were simultaneously integrated to form a heterojunction on the carbon framework. The obtained composite possessed a flower-like porous structure and enabled fast Cr(VI) removal in acidic solution, achieving nearly complete elimination of 50 mg L^-1 Cr(VI) within 30 min. This behavior could be attributed to the cooperative adsorption and reduction effects arising from MnS, MoS₂, and the surface functional groups. In particular, lattice S^2- species and Mn²⁺ in MnS participated as electron donors during Cr(VI) reduction, whereas MoS₂ facilitated charge transfer across the interface and thereby promoted the redox process. Oxygen- and nitrogen-containing surface groups further promoted electron migration and favored the fixation of the reduced Cr(III) species on the composite. The removal process was consistent with the pseudo-second-order kinetic model and exhibited an endothermic nature. The composite demonstrated pronounced resistance to interference from coexisting ions including Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, and SO₄^2-, whereas elevated carbonate concentrations led to a noticeable suppression of Cr(VI) removal. The composite maintained approximately 76% of Cr(VI) removal after eight cycles, indicating its promising reusability. Overall, this work highlights the potential of coupling MnS/MoS₂ interfacial structures with waste-derived N-doped hydrochar for efficient Cr(VI) remediation in acidic wastewater systems.