Mechanistic Divergence of Perrhenate Reduction by Mackinawite through Coprecipitation and Adsorption

Rhenium (Re) is a redox-sensitive trace metal, and understanding its environmental transformation pathways is of considerable interest due to its relevance to paleoenvironmental reconstructions, contaminant remediation, and strategic metal recovery. However, its geochemical behavior in the presence of mackinawite (FeS), a ubiquitous mineral in reducing environments, remains poorly constrained. Here, we compared Re(VII) sequestration via coprecipitation and adsorption with FeS and elucidated the distinct reduction mechanisms involved. In the coprecipitation system, Re(VII) was reduced by S(-II) to form both ReS₂ and ReO₂, while in the adsorption system, Fe(II) served as the primary reductant, yielding ReO₂ as the dominant product. These results demonstrate that Re redox transformation is highly dependent on the FeS formation pathway and the nature of available reductants. The H₂S concentration threshold (∼10^-11 M) above which ReS₂ becomes the dominant phase was further defined, offering a predictive basis for Re sequestration under sulfidic conditions. This work enhances our understanding of Re fate in anoxic environments, informs its application as a paleoredox proxy, and contributes to Re recovery strategies in environmental and resource systems.