Macroscopic Experimental and Modeling Evaluation of Selenite and Selenate Adsorption Mechanisms on Gibbsite.

Selenite Se(IV) and selenate Se(VI) adsorption behavior was investigated on gibbsite as a function of solution pH and solution ionic strength.  Adsorption of both Se redox states decreased with increasing solution pH.  Electrophoretic mobility measurements showed downward shifts in point of zero charge indicative of the formation of inner-sphere Se(IV) and Se(VI) surface complexes.  However, the downward shift was less than for strongly adsorbing anions such as phosphate and arsenate, suggesting that outer-sphere surface complexes are also present.  The triple layer model, a chemical surface complexation model, was well able to describe Se(IV) and Se(VI) adsorption as a function of solution pH and solution ionic strength by simultaneously optimizing both inner-sphere and outer-sphere surface complexation constants.  Direct spectroscopic investigations of selenite surface configuration are needed to corroborate the species suggested by the macroscopic experiments and obtained from the triple layer model optimizations.