Density Functional Theory Calculations on Hydrated Dimethylarsinic Acid and Iron Oxide Clusters.

摘要

Dimethylarsinic Acid (DMA) or (CH3)2AsO 2H is an important organoarsenical compound detected in arsenic speciation studies of environmental samples and synthesized during pyrolysis of oil shale. DMA was used historically as a herbicide on large agricultural fields and can be detected in the leachates of landfills rich in waste containing arsenic such as glass, alloys, and semiconductors, as well as biologically pre-treated municipal solid waste. Under certain soil conditions DMA can become bio-available and has the potential to be recycled to more toxic inorganic forms of arsenic. Bioavailability of DMA is largely controlled by the extent of its interactions with reactive components in soil. Little is known about these interactions, particularly with iron oxides that have high affinity to arsenic compounds and are ubiquitous components of soil.;In this thesis, density functional theory (DFT) calculations are used to obtain energies, optimal geometries and vibrational frequencies for hydrated DMA-iron oxide clusters. Calculations were performed using Gaussian 09, running on Sharcnet, with the B3LYP and BMK functionals and the 6-31G(d) and 6-311+G(d,p) basis sets. Solvation is simulated by adding explicit water molecules, as well as using the integral equation formalism polarizable continuum model (IEFPCM) and the universal solvation model (SMD).;Various ligand exchange reactions are constructed to investigate the thermodynamics of inner- and outer-sphere complex formation. The Gibbs free energies of adsorption (Gads) for these reactions are calculated and results indicate that both inner- and outer-sphere complex formation is thermodynamically favourable with bidentate complexes being most favourable. Similarly, the Gibbs free energies of desorption (Gdes) are calculated for various desorption reactions of DMA due to interactions with phosphate ions and it is determined that desorption favourability of DMA increases in the order of bidentate < monodentate < outer-sphere.;These theoretical studies are used to explain experimental infrared spectral peaks and provide geometrical parameters useful for modeling x-ray absorption data using extended X-ray absorption fine structure (EXAFS) showing DMA-Fe inter-atomic distances to be within 3.3 - 3.4 Å for the bidentate complexes, within 3.4 - 4.9 Å for monodentate complexes and 4.8 - 6.8 Å for outer-sphere complexes.