Computational Investigation Into the Adsorption of Pollutants Onto Mineral Surfaces: Arsenate and Dolomite
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0930-JJ05-04
Computational Investigation Into the Adsorption of Pollutants Onto Mineral Surfaces: Arsenate and Dolomite Kat F. Austen1, Kate Wright2, Ben Slater1, and Julian D. Gale2 1 The Royal Institution of Great Britain, DFRL, Albemarle Street, London, London, W1S 4BS, United Kingdom 2 Dept of Applied Chemistry, Nanochemistry Research Institute, Curtin University of Technology, GPO Box U1987, Perth, WA, 6845, Australia
ABSTRACT Computational modeling techniques have been used to investigate the interaction of arsenate with the dolomite (211) surface. The suitability of a variety of techniques has been assessed in the context of their applicability to the problem, in order to determine the least computationally expensive method of modeling the mineral-solution interface. To this end, various methods of solvating arsenate have been investigated, and a reliable solvation energy has been determined for the molecule. The adsorption geometry of the primary arsenate ion at the dolomite surface has been determined under vacuum conditions. Additionally, solvation of the dolomite surface has studied using molecular dynamics, and results show that there is some layering 2Å above the surface, and that dissociation of the water molecules occurs in this layer. INTRODUCTION Arsenic pollution in the environment is a serious anthropological problem[1, 2]. New information on arsenic chemistry and its interaction with species with which it comes into contact is vital in reducing arsenic contamination in water supplies. In most groundwaters it is expected that arsenate will be the species most strongly sorbed onto mineral surfaces[1, 3]. A common naturally occurring carbonate mineral, dolomite (CaMg(CO3)2) is often found in areas with high aqueous arsenic concentrations[1]. Despite this, the interaction between arsenate and carbonate minerals has received little attention, although its interaction with other minerals has been extensively investigated using both computational and experimental techniques[4-6]. The work presented here assesses the suitability of different computational approaches for modeling the mineral-aqueous solution interface and their applicability to the problem of arsenate adsorption onto dolomite surfaces. METHODOLOGY Arsenate Species in Solution The arsenate – water system was studied using two distinct approaches to the inclusion of the aqueous environment: continuum solvation and explicit water with molecular dynamics. Calculations with continuum solvation methods employed the Gaussian03 program [7]. The
arsenate molecule was studied in its various degrees of protonation using the B3LYP[8-10] functional and the 6-311+G(3df,2p) basis set. The molecules were solvated with Conductor-like Polarisable Continuum Model (CPCM) [11] using the United Atom Topological Model[12] default radii and a solvent with dielectric constant of 78.39, the value for water at room temperature and pressure. Calculated raman spectra were obtained for the molecule and the molecular ions under both solvated and vacuum conditions,
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