Selenium Adsorption on an Amorphous Iron Oxide in the Presence and Absence of Dissolved Organic Carbon.

In the Western United States, seleniferous soils are a problematic occurrence. The unique conditions present in this region result in Se that is bioavailable to vegetation in high concentrations. Previous work confirmed competitive interactions occurred between other oxyanion species and dissolved organic carbon (DOC) present in soils. In order to examine competitive adsorption reactions between Se and DOC, a batch reactor was employed for both selenite and selenate on a synthetic, amorphous iron oxide surface, 2-line ferrihydrite. This was conducted in the presence and absence of DOC acids for pH 5 to 9 in order to determine their effects on Se solubility. Dissolved citric and salicylic acids were chosen to represent common aliphatic and aromatic acids present in soils, respectively. Following an equilibration period of 2 h, zeta-potentials were determined in order to identify changes in the point of zero charge (PZC) of ferrihydrite that are associated with inner-sphere complexation. Citric acid exhibited more competitive behavior against both selenite and selenate, shown through increases in soluble Se concentrations. Competition was more pronounced for selenite at pH 5 through 8 and selenate at pH 5 and 6. Little competitive behavior was observed for salicylic acid with selenite and selenate. Changes in the zeta-potential suggested strong, inner-sphere complexation occurred for selenite and citric acid on ferrihydrite.