Density functional theory calculations on the adsorption of monomethylarsonic acid onto hydrated iron (oxyhydr)oxide clusters

Abstract

Monomethylarsonic acid (MMA) is an important arsenic pollutant that was extensively used as a herbicide/pesticide. Once introduced to the environment, MMA can enter the food chain or be recycled to the more mobile forms of arsenic that can contaminate nearby water sources. Density functional theory with dispersion corrections (DFT-D3) calculations provide a molecular-level understanding of the fate of pollutants such as MMA, particularly their interactions with reactive soil components such as iron (oxyhydr)oxides. Energies, optimal geometries, and vibrational frequencies for hydrated MMA complexes with iron (oxyhydr)oxide clusters were computed and ligand exchange reactions were constructed to investigate the thermodynamics of inner- and outer-sphere complex formation. The Gibbs free energies and enthalpies of adsorption show that both inner- and outer-sphere complex formation is thermodynamically favourable, with monodentate complexes being most favourable. The calculated AsFe inter-atomic distances for MMA complexes are shown to be 3.23–3.55Å for inner-sphere complexes and 5.17–5.75Å for OS complexes. The calculated infrared frequencies were correlated to experimental values in the 740–900cm−1 range containing stretching frequencies of AsO bonds. A comparison between MMA and other organoarsenicals reveals that adding methyl groups to arsenate affects the adsorption process and hence the mobility of organoarsenicals in the environment.