Fate of adsorbed arsenic during early stage sulfidization of nano-ferrihydrite

Sulfidization of nano-ferrihydrite (nano-Fhy) by S(-ii) plays an important role in Fe and As biogeochemistry in wetland soils, sediments, and aquifers. However, during early stages, the changes in speciation and redistribution of adsorbed As remain unclear. This study investigated the reduction and transformation of As-bearing nano-Fhy and subsequent As repartitioning among secondary phases induced by an environmentally relevant concentration of sulfide (S(-ii)) (Fe/S ratio = 80, S(-ii) concentration = 0.5 mM) at pH 7. Chemical analysis indicated that the mobility of As associated with iron oxyhydroxide minerals was greatly reduced during the transformation of nano-Fhy. The initially adsorbed As was partially (60-90%) transformed to a phosphate-unextractable phase and approximately 50-90% of As(iii) was oxidized to As(v) during recrystallization of nano-Fhy to lepidocrocite and goethite. The intermediate Fe species formed during electron (e(-)) transfer from S(-ii) to Fe(iii) and subsequent e(-) transfer between Fe(ii) and Fe(iii) may be responsible for oxidization of As(iii). The S K-edge X-ray absorption near edge structure (XANES) results showed that S(-ii) could be oxidized to multiple valence states and its evolution was strongly affected by the concentration and speciation of As. As(iii) can inhibit the oxidation of S(-ii) to sulfate, whereas the presence of As(v) may promote the oxidation of S(-ii) to sulfate compared with the system without As. These findings have important environmental implications for As, Fe, and S redox cycling during early stage sulfidization of nano-Fhy.