Effect of phosphate on ferrihydrite transformation and the associated arsenic behavior mediated by sulfate-reducing bacterium.

Although PO43- is commonly found in association with iron (oxyhydr)oxide, the effect of PO43- on ferrihydrite reduction, mineralogical transformation, and associated As behavior in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this study, batch experiments, together with geochemical, mineralogical, and biological analyses, were conducted to elucidate these processes. The results showed that SRB can reduce ferrihydrite via direct and indirect processes, and PO43- promoted ferrihydrite reduction by supporting SRB growth at low and medium PO43- loadings. However, at high loadings, PO43- stabilized the ferrihydrite. PO43- shifted the transformation of ferrihydrite from magnetite and mackinawite to vivianite, which scavenges As effectively by incorporating As into its particle. In systems with 0.5 mM SO42-, PO43- exerted a weak effect on As mobilization. However, in systems with 10 mM SO42-, substantial amounts of As were released into the solution, and PO43- impacted As behavior strongly. Low PO43- loadings increased the mobilization of As because of the competitive adsorption of PO43- on mackinawite. Medium and high PO43- loadings were beneficial for As immobilization because of the substitution of mackinawite by vivianite. These findings have important implications for understanding the biogeochemistry of iron (oxyhydr)oxide and As behavior in SRB-containing sediments.