Simultaneous redox transformations of arsenite (As(III)) and chromate (Cr(VI)) were investigated on semiconducting nano-materials, ferrihydrite and titanium dioxide, resulting in transformations to arsenate (As(V)) and chromite (Cr(III)). Using nanophase aluminum oxyhydroxide (an insulator) as a comparison, X-ray absorbance near edge spectroscopy (XANES), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and geochemical batch reactions were preformed to ascertain the fate of the contaminants on the surface of the nano materials.  XANES showed that near complete (ferrihydrite 92%) As(III) and Cr(VI) simultaneous redox transformations could be preformed on the semiconducting surfaces, while the insulating material was associated with less than 50% redox conversion. We conclude that the semiconducting surfaces are able to go to further completion due to surface mediated facile electron transfer. ATR-FTIR was used to study the ferrihydrite result more closely, showing competitive adsorption of As(III) and Cr(VI) and after 60 mins complete loss of the characteristic 924 Cr(VI) mode and growth of 810 mode associated with As(V). Furthermore, the exposure of the surface to simulated solar light led to further As(III) oxidation. Additional geochemical batch reaction experiments with all three nano materials showed that the redox reaction reached completion on the surface of the semiconducting materials, but not in the solution phase. When applying simulated solar light to the batch reactions we experimentally observed an increase in the solution phase redox products, increasing in concentration from aluminum oxyhydroxide, to ferrihydrite and finally titanium dioxide showing the most As(V) in solution.

Simultaneous redox transformations on semiconducting nanomaterials

246th National Meeting (2013)

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