Redox transformation and partitioning of arsenic during the hydrothermal aging of FeS-As coprecipitates under anoxic condition.

In sulfidic anoxic environments, iron sulfides are widespread solid phases that play an important role in the arsenic (As) biogeochemical cycle. This work investigated the transformation process of FeS-As coprecipitates, the concurrent behavior, and the speciation of associated As under anoxic conditions. The results showed that FeS-As coprecipitates could convert to greigite and pyrite. The transformation degree of the produced solid phases was dependent upon the pH conditions and initial As species. These results showed that the As mobilization was closely associated with the solid phase transformation. The solid phase transformation went from disordered mackinawite to crystalline mackinawite, then greigite and finally pyrite. The As in the coprecipitates underwent a process of release, fixation, and release again. Both reduction of As(V) and oxidation of As(III) were observed in the aqueous and solid phases during reactions. Our study may have important implications for further understanding of As behavior and Fe/S cycling that may occur under an anoxic environment more comprehensively.