Promoted hexavalent chromium removal using biochar-supported cobalt-doped molybdenum disulfide: Interfacial transformation, aging, and mechanistic insights

Hexavalent chromium, Cr(VI), is a highly toxic and mobile water pollutant whose removal is critical for environmental and human health. In this study, a rice-husk-derived biochar-supported cobalt-doped molybdenum disulfide composite (BC@Co-MoS2) was developed for Cr(VI) removal, with emphasis on its interfacial structure, storage-induced evolution, and removal mechanism. The biochar support improved Co-MoS2 dispersion and suppressed severe restacking, providing a heterogeneous reactive interface. One month of natural aging increased the 15-min Cr(VI) removal efficiency from approximately 31.9% to 93.8%. Controlled storage experiments showed that this performance change was associated with oxidative exposure rather than storage time alone. Naturally aged BC@Co-MoS2 produced 240 mg L-1 sulfate after 120 min, 4.8 times that of the inert-stored material, accompanied by proton-releasing behavior and oxidation-related evolution of the Mo-S domains. When self-acidification was suppressed by maintaining the solution at pH 6.0, the difference in Cr(VI) removal between fresh and aged materials decreased to approximately 2 percentage points, indicating that the apparent enhancement was governed predominantly by a pH-mediated effect rather than increased intrinsic removal capacity. Time-dependent chromium speciation revealed the transient formation of dissolved Cr(III), while Cr 2p XPS confirmed Cr(III) on the spent composite, supporting a coupled Cr(VI) reduction-retention pathway. These findings demonstrate that storage-induced interfacial evolution can substantially alter the apparent reactivity of MoS2-based composites and should be considered when evaluating their performance and stability.