Arsenate and Arsenite Reaction Kinetics with Ferric Hydroxides Using Quantum Chemical Calculations

Abstract

The knowledge of the mechanism involved in the process of adsorption and desorption of arsenate and arsenite with ferric hydroxides is important to address the water toxicity problems and to tackle the adverse effect of these substances in nature. An essential outcome of previous studies on the kinetics of the arsenate adsorption on aluminum and iron oxide was that the adsorption is a two-phase (bi-phase) process. Quantum mechanical calculations using density functional theory were used to determine the thermodynamic variables governing the adsorption process to get an insight into the stability of the complexes formed. The previous investigation showed that the positively charged ferric hydroxide cluster had better stability at neutral pH. The chemisorbed charged monodentate complexes had Gibbs free energy of reaction -55.97 kcal/mol where the bidentate complex formation had Gibbs free energy of reaction -62.55 kcal/mol. The bidentate complex having a negative charge had more Gibbs free energy of reaction compared to uncharged one. The results of the study indicate that Gibbs free energy for the reaction has a significant role in controlling the kinetics of the adsorption and sorption process of arsenate on ferric hydroxide clusters.