Surface complex structures modelled with quantum chemical calculations: carbonate, phosphate, sulphate, arsenate and arsenite

Hybrid molecular orbital/density functional theory (MO/DFT) calculations on molecular clusters were used to model infrared (IR) vibrational frequencies and interatomic distances obtained via extended X-ray absorption fine structure (EXAFS) spectroscopy. Molecular clusters were found to provide good agreement with experimental observations for the oxyanions carbonate, phosphate, sulphate, arsenate and arsenite. The results show a consistent tendency to form bidentate bridging surface complexes at low pH, but the protonation and hydration states play a significant role in the results obtained from calculation as various protonation states of monodentate surface complexes are also predicted to be stable as pH increases (i.e. the number of H+ ions in the model are decreased). A method for estimating the Gibbs free energy of adsorption (Delta G(ads)) is discussed to complement the comparisons of experimental and theoretical spectroscopic parameters. Calculated Delta G(ads) values were consistent with the interpretations based on modelling observed spectra.