Modeling Selenate Adsorption Behavior on Oxides, Clay Minerals, and Soils Using the Triple Layer Model.

Selenate adsorption behavior was investigated on amorphous aluminum oxide, amorphous iron oxide, clay minerals:  kaolinites, montmorillonites, illite, and 18 soil samples from Hawaii, and the Southwestern and Midwestern regions of the US as a function of solution pH.  Selenate adsorption decreased with increasing solution pH.  The triple layer model, a chemical surface complexation model, was able to describe Se(VI) adsorption as a function of solution pH by simultaneously optimizing either two outer-sphere Se(VI) surface complexation constants or one inner-sphere and one outer-sphere Se(VI) surface complexation constant.  The fit of the triple layer model to Se(VI) adsorption by oxides, clay minerals, and soils was excellent, as evidenced by very low values of the model variance goodness-of-fit criterion.  The predominantly outer-sphere Se(VI) surface speciation predicted using the triple layer model was in agreement with the weak adsorption behavior previously observed for Se(VI) using electrophoretic mobility measurements and ionic strength dependence of adsorption.  Direct spectroscopic investigations of Se(VI) surface configurations are needed to corroborate the species predicted by the triple layer modeling approach.