Surface Complexes Modeled With AB Initio Calculations.
James D. Kubicki, The Pennsylvania State University, 308 Deike Building, University Park, PA 16802
Adsorption of oxyanions is a common and important process in soils. Whether affecting the availability of nutrients or the transport of contaminants, the interactions of species such as carbonate, arsenate, phosphate, and sulfate with metal oxide and clay 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 give examples of how these principles can be applied. Carbonate is a common constituent of soil and groundwaters; hence, its adsorption to mineral surfaces can affect the behavior of other oxyanions and metals. Combined ATR FTIR and molecular modeling studies demonstrate the existence of a bridging bidentate surface complex in addition to a H-bonded outer-sphere complex on goethite. The role of solvation in modeling the IR spectra suggests that a strongly physisorbed layer of water exists on the Fe-oxide surface when carbonate is adsorbed in significant quantities.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. Adsorption of arsenic to Al- and Fe-oxides surfaces can be important in the transport of As in the environment and in water treatment. Recent studies have observed strong adsorption of As(V) onto Al- and Fe-oxides, but As(III) has been observed to adsorb less to Al-oxides than Fe-oxides. 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. Vibrational frequencies of the model surface complexes will be compared to the observed spectra of As adsorbed onto these oxides. 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.Sulfate is an important nutrient in soils that can become highly concentrated due to the weathering of sulfides or to the application of alum as a phosphate sequestering agent. 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.