Thermodynamics of Dimethylarsinic Acid and Arsenate Interactions with Hydrated lron-(Oxyhydr)oxide Clusters: DFT Calculations

摘要

DimethyUrsink Add (DMA) belongs to an important class of organoarsenical compounds commonly detected in arsenic speciation studies of environmental samples and pyrotysis products of fossil fuels. Transformation of DMA under certain conditions leads to the formation of other forms of arsenic, which could be more tone than DMA to biota, and more efficient in deactivating catalysts used in petrochemical refining. Published surface sensitive X-ray and infrared spectroscopic work suggested that DMA simultaneously forms inner- and outer-sphere complexes with iron-(oxyhydr)oxides. Computational work on the compilation of arsenicals with various surfaces of environmental and industrial interest provides useful information that aids in the interpretation of experimental spectroscopic data as well as predictions of thermodynamic favorability of surface interactions. We report herein Gibbs free energies of adsorption, ΔG_(adss), for various ligand exchange reactions between hydrated complexes of DMA and Fe-(oxyhydr)oxide dusters calculated using density functional theory (DFT) at the B3LYP/6-311 +G(d,p) level. Calculations using arsenate were also performed for comparison. Calculated Δs-(O,Fe) distances and stretching frequencies of Δs-O bonds are also reported for comparison with experimental spectroscopic data. Gibbs free energies of desorption, ΔG_(des) due to reactions with phosphorus species at pH 7 are reported as welL Our results indicate that the formation of both inner- and outer-sphere DMA complexes is thermodynamkally favorable, with the former having a more negative ΔG_(ads). Values of ΔG_(des). indicate that desorption favorability of DMA complexes increases in this order: bidentate < mondentate < outersphere. The significance of our results for the overall surface complexation mechanism of DMA is discussed.