Transformation Of Arsenate Contained Iron Oxides And Mobilization Of Arsenate During Reductive And Oxidative Dissolution

Dissolution of iron oxides played an important role in the mobilization of arsenate in natural water. Under anoxic conditions, the presence of dissolved S(-II) on the mobilization of As(?) during the reductive dissolution of arsenic-contained iron oxides has rarely been investigated. In addition, the mechanism involved in As(?) mobilization during the oxidative dissolution of arsenic-contained iron sulfide is also not clear. In this study, mobilization kinetics of As(?) during reductive dissolution of As(?)-Ferrihydrite and oxidative dissolution of As(?)-Mackinawite were investigated separately. XRD, TEM, FTIR, XPS as well as Raman spectroscopy were used to determine the morphology of the formed secondary minerals. The transformation of iron oxides on the mobilization of arsenate was also investigated.The dissolved S(-II) initiated the reductive dissolution of Ferrihydrite. High S(-II):Fe(III) ratio resulted in a more complete dissolution of Ferrihydrite, and a large fraction of As(?) was released into solution. As(?) loading showed a negative effect on the release of arsenate, with a smaller fraction of arsenate released into solution when more As(?) adsorbed on Ferrihydrite. At pH 8.0, 14.1% and 0.7% of As(?) were released as for 10 and 100 mg L-1 of As(?) loading, respectively. With the increase of S(-II) concentration, more arsenate was released from Ferrihydrite. During the reductive dissolution, secondary minerals such as Goethite and Magnetite were detected, indicating the transformation of Ferrihydrite to Goethite and Magnetite under anoxic conditions in the presence of Fe(II) ion. In addition, excessive S(II)diss in solution would react with Fe(II) to form Mackinawite. The newly formed secondary minerals showed different sorption capacities to the released arsenate. TEM and EDX analysis further confirmed that the released As(?) was primarily repartitioned on the residual Ferrihydrite, the newly-formed Goethite and Magnetite but not FeS.As(?) adsorbed on Mackinawite enhanced the oxidative dissolution of As(?)-Mackinawite. At neutral pH, Mackinawite was oxidized via the solution-phase oxidation and the surface-mediated oxidation mechanisms. Mackinawite would produce Fe(II) and S(-II) during the solution-phase oxidation, and Fe(II) as well as would be further oxidized to Lepidocrocite and Ferrihydrite during the surface-mediated oxidation. While S(-II) was oxidized to elemental S(S0 and S8) and SO42-. The newly-formed Lepidocrocite and Ferrihydrite showed high sorption capacities to the released As(?), and thus reducing the mobilization of As(?). Additionally, formation of arsenic sulfide compounds on Mackinawite as indicated in FTIR, XRD and Raman analysis enhanced the dissolution rate of As(?)- Mackinawite. XPS further confirmed that the content of Fe(III)-S on the surface of As(?)-Mackinawite was 82.07%,which was 9.79% higher than that of Mackinawite. This suggested that formation of As-S compounds reduced the density of electron around Fe, and made the Fe(II) more easily to be oxidized by the dissolved oxygen, which subsequently enhanced the oxidative dissolution of Mackinawite.The significance of this study is to reveal the mobilization mechanism of arsenate during the reductive dissolution of iron oxides and the oxidative dissolution of iron sulfide, which can provide a theoretical basis for the effective control of arsenic contamination under natural conditions.