Hydrous ferric arsenate transformation coupled with As, Fe, and S environmental cycling in sulfidic systems under anoxic and circumneutral conditions.

Despite many studies on the environmental cycling of As, Fe, and S, sulfide (S(-II))-induced hydrous ferric arsenate (HFA) transformation remains to be elucidated. Herein, we investigated the anaerobic reaction of HFA with S(-II) at three environmental concentrations (1, 10, and 50 mM) at pH 48. Changes in solid-phase As, Fe, and S speciation were investigated by XRD, FTIR, Raman, XPS, synchrotron XANES, SEM, and TEM. The results demonstrated that 1 mM S(-II) induced significant As mobilization from HFA, and secondary goethite formed only at pH 8. Greater S(-II) addition caused rapid reductions in As(V) and Fe(III) and generated substantial amounts of elemental sulfur, SO, SO, and SO, subsequently decreasing As mobilization through secondary As-bearing phase formation. Solid characterization revealed that 10 mM S(-II) produced a scorodite‑sulfur mixture at pH 4, while schwertmannite, goethite, hematite, and tooeleite formed at pH 8. At pH 4 and 6, 50 mM S(-II) produced secondary semicrystalline symplesite, lepidocrocite, and tooeleite; in contrast, a substantial fraction of FeS plus minor amounts of hematite and goethite were generated at pH 8. These results deepen our understanding of the geochemical cycling of As, Fe, and S and will help establish an effective As fixation method for effluent treatment.