Density Functional Theory Calculations on the Sorption of Organoarsenicals onto Hydrated Iron (III) Oxyhydroxide Clusters

Abstract

Organic forms of arsenic are introduced into the environment by natural and anthropogenic processes and they pose a threat to human health. The fate of these pollutants depends on their interactions with reactive soils components, such as iron oxyhydroxides. Reaction pathway information and transition states are essential for understanding adsorption mechanisms of pollutants at the liquid–solid interface. Density functional theory (DFT) calculations on the energies, optimal geometries, vibrational frequencies for organic arsenical complexes, p-arsanilic acid (pAsA), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) with iron oxyhydroxide clusters are performed. In addition, changes in Gibbs free energy, enthalpy, and entropy for various types of ligand exchange reactions leading to both inner- and outer-sphere complexes are presented, along with activation barriers and transition states. Results are compared to calculations using the well-studied arsenate system and various experiments that use surface sensitive techniques. DFT calculations show that the formation of inner- and outer-sphere complexes with pAsA, MMA and DMA is thermodynamically favourable, but also that the activation barriers are crucial in understanding the types of complexes that form. It is shown that the mono-substituted arsenicals, pAsA and MMA, are more likely to form inner-sphere complexes, but that the di-substituted DMA, may be hindered to do so because of higher activation barriers along its reaction pathway. The calculations reported herein include explicit and implicit solvation, as well as dispersion corrections, that are shown to be particularly useful for calculations where outer-sphere complexes are involved. Calculations include both a smaller iron oxyhydroxide surface with 2 Fe atoms at its core and an extended iron oxyhydroxide surface with 4 Fe atoms at its core. The 4Fe geometries were shown to be particularly important for outer-sphere and monodentate complexes where the 2Fe complexes suffered from an over-relaxation problem that overestimates the stability of a complex. The Mulliken charge distribution analysis performed on the DMA systems revealed that there is no charge transferred in the formation of the outer-sphere complexes, suggesting outer-sphere complex formation is purely electrostatic while charge is transferred from the ligand to the metal when inner-sphere complexes form. The theoretical infrared spectra for pAsA, DMA and MMA are compared to experimental results and the formation of outer-sphere and inner-sphere complexes are discussed. The theoretical frequencies are also used to calculate a new scaling factor (F=1.0065) to correct the stretching vibrations of the As-O bond of organoarsenicals for anharmonicity.