Effect of Carbonate on the Adsorption of Selenate and Sulfate on Goethite

Abstract

Although it is generally recognized that goethite surfaces present in natural environments have carbonate species adsorbed, knowledge of their effect on the reactivity of goethite toward other anions remains limited. Accordingly, the effect of CO3 on the adsorption of SeO4 and SO4 on goethite is determined. Carbonate promotes the adsorption of SeO4 and SO4 between pH 6 and 8. The maximum effect occurs at relatively low CO3 concentrations (0.2 mM), and the promotive effect decreases and eventually becomes competitive with further increase in CO3 adsorption. Acetate (AcO) and formate also appear to promote adsorption of SeO4 and SO4. Based on existing Fourier transformed dsorption of SeO4 and SO4 Based on existing Fourier transformed infrared (FTIR) spectroscopic data and new electrophoretic mobility data, CO3 may promote the adsorption of other oxyanions by forming extra reactive protonated sites that coexist with the adsorbed CO3. Acetate and formate presumably interact through the same mechanism. Goethite surfaces present in natural environments may have substantial amounts of CO3 adsorbed. Depending on the CO3 levels, this may enhance, decrease, or have a neutral effect on the adsorption of moderate affinity anions such as SeO4 and SO4. Accordingly, the combined effects of promotive and competitive interactions may render CO3 less competitive overall than was commonly presumed. Similarly, the presence of AcO and formate may be less competitive with other anions due also to their promotive interaction effect.