SOLUTION ION EFFECTS ON THE SURFACE EXCHANGE OF SELENITE ON CALCITE

The adsorption of SeO3 on CaCO3(s) was evaluated in equilibrium CaCO3(aq) suspensions over a range in pH and SeO3 concentration in the absence and presence of the cosolutes, Mg, SO4, and PO4. Selenite was adsorbed by CaCO3(s) in a manner consistent with its exchange on anionic specific surface sites saturated with CO32-/HCO3- that were quantified by H14CO3 isotopic exchange. This exchange process was described very well using the computer program FITEQL and half-reactions for the surface exchange of HCO-3, CO3-, HSeO3-, and SeO32-. The cosolutes SO4 and PO4 decreased SeO3 sorption by direct competition or site blockage, and these effects were also predicted using the surface exchange model. By contrast, the sorption of SeO3 was not influenced by Mg because the two ions sorbed over different pH regions and exchanged with different surface sites that appeared to be distinct and noninteractive. Evaluation of SeO3 sorption over a wide concentration range suggested that the surface of CaCO3(s) was heterogeneous and that SeO3 had access to less than 5% of the total anion specific surface sites. © 1990.