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DIVISION S-2—SOIL CHEMISTRY

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

^a Connecticut Agric. Exp. Stn., P.O. Box 1106, New Haven, CT 06504
^b Dep. of Plant Science, U-4067, Univ. of Connecticut, Storrs, CT 06269

* Corresponding author (c.schulthess{at}uconn.edu)

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 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.

Abbreviations: AcO, acetate • ATR-FTIR, attenuated total reflectance Fourier transformed infrared • CV, coefficient of variation • DRIFT, diffuse reflectance infrared Fourier transformed • EM, electrophoretic mobility • IC, ion chromatography • TIC, total inorganic C • TOC, total organic C • _i, amount of i adsorbed

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