Redox-Active SnMoS_x Chalcogels for Multi-Electron Reductive Separation of Chromate and Pertechnetate

High-valent oxyanions, such as chromate (Cr^VIO₄ ^2-) and pertechnetate (⁹⁹Tc^VIIO₄ ^-), are highly soluble, weakly coordinating, and exceptionally stable, making their removal from water and nuclear waste extremely challenging. Here, we report a redox-coupled metathesis strategy to synthesize a sulfide-rich SnMoS_x nanoparticle-aggregated chalcogel and demonstrate its redox-mediated, high-capacity removal of both Cr(VI) and ⁹⁹Tc(VII) from aqueous solutions, including Hanford low-activity nuclear waste condensate streams. Synchrotron X-ray pair distribution function and X-ray absorption spectroscopy reveal Mo─S motifs resembling Mo₂(S₂)₆ and Mo₃S(S₂)₆ clusters, while Sn─S coordination approximates Sn₂S₇-like units, generating a framework enriched in reactive polysulfides. This unique coordination and redox landscape enables a concurrent near-quantitative removal of Cr(VI) (>99%) and efficient capture of Tc(VII) (>73%) from representative nuclear waste simulants. Moreover, SnMoS_x exhibits a Cr⁶⁺ removal capacity of 672 mg g^{- 1} from aqueous solutions. Mechanistic analysis shows that Cr(VI) undergoes multi-electron reduction to insoluble Cr(III) via electron transfer from Mo⁵⁺/Mo⁴⁺ and S_x ^2- centers, with a similar pathway proposed for Tc(VII). These results establish SnMoS_x chalcogels as a distinct class of porous, redox-active materials capable of transforming and immobilizing recalcitrant high-valent oxyanions, offering a general strategy for the separation of highly stable inorganic oxoanions from chemically complex solutions.