Effects of Fe(II)-induced transformation of scorodite on arsenic solubility

Scorodite (FeAsO4 center dot 2H(2)O) is a pivotal secondary ferric arsenate that immobilizes most of arsenic (As) in acidic As contaminated environments, but secondary As pollution may occur during dissolution of scorodite in environments involving redox changes. Reductive dissolution of scorodite by coexisting dissolved Fe2+ (Fe(II)(aq)) under anaerobic conditions and its effects on the behavior of As have yet to be examined. Here, this study monitored the changes in mineralogy, solubility and speciation of As during scorodite transformation induced by Fe(II) under anaerobic conditions at pH 7.0 and discussed the underlying mechanisms. Mossbauer and X-ray diffraction (XRD) analysis showed the formation of parasymplesite and ferrihydrite-like species during scorodite transformation, which was highly controlled by Fe(II)aq concentrations. 1 mM Fe(II)(aq) enhanced As mobilization into the solution, whereas As was repartitioned to the PO43--extractable and HCl-extractable phases with 5 and 10 mM Fe(II). The neo-formed parasymplesite and ferrihydrite-like species immobilized dissolved As(V) through adsorption and incorporation. Additionally, As(V) reduction occurred during Fe(II)-induced scorodite transformation. Our results provide new insights into the stability and risk of scorodite in anaerobic environments as well as the geochemical behavior of As in response to Fe cycling.

Automated summary

Scorodite can immobilize most of the arsenic in acidic arsenic-contaminated environments, but reductive dissolution of scorodite may occur during redox changes. This study investigated the transformation of scorodite induced by dissolved Fe(II) under anaerobic conditions and its effects on arsenic behavior. The results showed that the formation of parasymplesite and ferrihydrite-like species during scorodite transformation was controlled by Fe(II)aq concentrations. The newly formed species immobilized dissolved As(V) through adsorption and incorporation. Additionally, As(V) reduction occurred during Fe(II)-induced scorodite transformation.