Molecular Modeling of Surface Complexes

Whether affecting the availability of nutrients or the transport of contaminants, the interaction of species such as arsenate, phosphate, and sulfate with mineral surfaces need to be understood in order to predict the kinetics of transport. Oxyanion surface complexes have been studied using a variety of techniques, such as FTIR and EXAFS. This presentation will examine the principles behind modeling the oxyanion surface complexes and examples of how these principles can be applied. This work focuses on molecular orbital calculations of previously proposed surface complexes for As(III) and As(V) onto Al- and Fe-oxides. Comparison of the calculated and observed vibrational frequencies for the aqueous As species suggests that the molecular orbital approach can describe As bonding. Models of the bridging bidentate surface complexes of As(III) and As(V) with Al- and Fe-oxides are consistent with the hypothesis of stronger As(III) bonding to the Fe-oxide. ATR-FTIR complemented by molecular orbital calculations was used to investigate the structure of sulfate on gibbsite. Correlations of the observed and calculated vibrational frequencies are excellent. Details of the binding mechanism can be derived by finding the model surface complex that provides the best correlation to observed frequencies. In this manner, issues such as the number of Al-O-S linkages and protonation state of the sulfate can be determined as a function of pH. Quantum chemical calculations were applied to resolve controversies about phosphate surface complexes on iron hydroxides. Six possible surface complexes were modeled: deprotonated, monoprotonated, and diprotonated versions of bidentate and monodentate complexes. The calculated frequencies were compared to experimental IR frequency data. In addition, reaction energies were calculated for adsorption from aqueous solution. Four possible species are a diprotonated bidentate complex, either a deprotonated bidentate or a monoprotonated monodentate complex, and a deprotonated monodentate complex.