Hydration structure of As–III aqueous solutions from ab initio molecular dynamics simulations

Abstract

A comprehensive study of aqueous arsenic acid (H3AsO3) and its deprotonated derivatives, namely H2AsO3− and HAsO32−, is carried out employing Car-Parrinello Molecular Dynamics (CPMD). Calculations are carried out with and without the incorporation of the dispersion correction, and critical comparison of its role on various structural and dynamical properties of the solute-solvent system are discussed. Aqueous H3AsO3 and H2AsO3− species solvated by 60 water molecules (which closely emulates normal pH conditions) are found to be stable, that is, without undergoing proton transfer with the solvent over the 50 ps of MD simulations performed. However, HAsO32− is found to be stable only in the basic media, emulated through the introduction of a hydroxyl ion (OH−) in lieu of one water molecule. The AsO33− species is not stable, and protonates quickly to HAsO32−, even under the basic environment of a hydroxyl ion (OH−) among 59 other water molecules simulated. Detailed analysis of the hydrogen bonding and vibrational spectra provide fresh microscopic insights on the hydration structure and dynamical characteristics of these species in aqueous environment. The spectroscopic insights derived, through comparison with their parent gas-phase species (H3AsO3), would be useful in the detection as well as quantification these species in various aqueous environments, for developing water remediation strategies of these environmentally hazardous species.