Functional Evolution of Elemental Sulfur in Cr(VI) Bioreduction during Sulfidogenesis

Elemental sulfur (S⁰)-driven sulfidogenic process is a promising approach for heavy metal remediation. Its remediation capacity is generally attributed to biogenic sulfide, whose formation is kinetically hindered by the limited bioavailability of S⁰. Whether S⁰ itself contributes to metal reduction during the pre-sulfidogenic stage remains unknown. Here, we established a sulfidogenic system consisting of Shewanella oneidensis MR-1, S⁰, and lactate for the Cr(VI) removal. Addition of 1 g/L S⁰ increased the overall Cr(VI) removal efficiency from 17.5% to 37.6%. More notably, S⁰ enhanced the Cr(VI) removal rate 2.1-fold during stage I (0-6 h), whereas parallel sulfur bioreduction experiments showed that this period was insufficient for detectable sulfide production. S K-edge XANES further revealed structural changes in S⁰ without the formation of detectable reduced sulfur species. Raman spectroscopy, electrochemical analyses, density functional theory calculations, thermodynamic analysis, and comparative experiments with two structural forms of S⁰ collectively supported the possible formation of a transient electron-accepting S⁰ intermediate capable of facilitating electron transfer from MR-1 to Cr(VI). In the subsequent stage, sulfide production correlated with enhanced Cr(VI) reduction, consistent with the conventional sulfide-mediated pathway. These findings suggest a time-dependent mechanism in which S⁰ may facilitate Cr(VI) reduction through an electron-mediated pathway prior to sulfide formation, followed by conventional sulfide-driven reduction. This study expands the mechanistic understanding of S⁰-driven sulfidogenic remediation and provides a theoretical basis for optimizing sulfidogenic remediation technologies.