Arsenic release and speciation during the oxidative dissolution of arsenopyrite by O2 in the absence and presence of EDTA

The oxidative decomposition of arsenopyrite is an important source of As in surface environment. This study investigated the oxidative dissolution of arsenopyrite by O2 and aqueous arsenic transformation at different pHs, dissolved oxygen (DO) contents, and temperatures in the absence and presence of EDTA. The oxidative dissolution was greatly inhibited at neutral and alkaline pH in the absence of EDTA. However, in the presence of EDTA, the oxidative dissolution rate increased linearly from pH 4 to 7. The highest dissolution rate was 3-4 times higher than that at pH 4 and 1-2 orders of magnitude higher than that at pH 7 in the absence of EDTA. This is possibly due to the lack of Fe oxyhydroxides on the surface of arsenopyrite. In the pH range of 7-10, the oxidative dissolution rate decreased linearly, possibly due to the formation of goethite and/or hematite coating. The oxidation of released arsenite (AsIII) to arsenate (AsV) took place simultaneously during the oxidative dissolution of arsenopyrite in the presence of dissolved Fe without EDTA, while no obvious aqueous AsIII oxidation was observed in the presence of EDTA, indicating that aqueous Fe species play an important role in AsIII oxidation.