Sorption of arsenate and arsenite on RuO2•xH2O:: A spectroscopic and macroscopic study

The sorption reactions of arsenate (As(V)) and arsenite (As(Ill)) on RuO2.xH(2)O were examined using macroscopic and spectroscopic techniques. Constant solid:solution isotherms were constructed from batch sorption experiments and sorption kinetics assessed at pH 7. X-ray absorption near edge spectroscopy (XANES) was employed to elucidate the solid-state speciation of sorbed As. At all pH values studied (pH 4-8), RuO2.xH(2)O showed a high affinity for As regardless of the initial As species present. Sorption was higher at all pH values when the initial As species was As(III). Oxidation of As(III) (250 mg/L solution) to As(V) was virtually complete (98-100%) within 5 s. XANES results showed the presence of only As(V) on the RuO2.xH(2)O regardless of the initial As oxidation state. There was no change in the As oxidation state on the solid phase for 4 weeks in both oxic and anoxic environments. It is speculated that changes in the RuO2.xH(2)O structure, due to oxidation reactions, caused the higher total As sorption capacity when As(III) was the initial species. The As sorption capacity of RuO2.xH(2)O is greater than that of other metal oxides reviewed in this study. The ability of RuO2.xH(2)O to rapidly oxidize As(III) is much greater than other oxides, such as MnO2.