The interaction of carbonate and organic anions with the adsorption of inorganic anions on hydrous metal oxides

Date of Completion

January 1999


Agriculture, Soil Science|Geochemistry




Carbonate and organic anions (acetate, formate, oxalate, citrate) are abundant solutes in soils and adsorb on metal (hydr)oxides. The presence of these adsorbed anions affects the adsorption of other anions on these surfaces. This dissertation determines the effect of these anions on the adsorption of sulfate and selenate on Al oxide and goethite. In addition to the expected competitive effects, this research discovered that promotive interactions between anions also occur. These anion-anion promotive interactions can significantly affect the adsorption of important anions such as sulfate and selenate on metal (hydr)oxides. This dissertation quantifies and explains how anion-anion promotive interactions occur. ^ Macroscopic adsorption data show that the presence of carbonate, acetate and formate markedly promotes the adsorption of sulfate and selenate on Al oxide and goethite. Carbonate's promotive effect is concentration and pH dependent, with a maximum at circa 0.2 mM total concentration and pH around 6.9. In contrast oxalate and citrate had a competitive effect. Proton coadsorption was confirmed by pH-stat measurements. Electrophoretic mobility measurements showed stronger surface charge effects by sulfate and selenate than carbonate. ^ Microscopic data (ATR-FTIR and Raman spectra) indicated monodentate inner-sphere surface complexes of carbonate on Al oxide and goethite. Bands corresponding to extra protonated surface groups that accompany adsorbed carbonate were also identified. Sulfate and selenate exist mainly as outer-sphere surface complexes at pH > 6; at pH < 6, inner-sphere surface complexes are also present, particularly on goethite. Based on these data, the promotive effect is the result of the generation of extra adsorption sites by extra protonated surface groups that coexist with adsorbed carbonate, acetate and formate. This generates an increase in reactive adsorption sites. With increasing total anion adsorption densities, competitive interaction also begins to play a role and counteracts the promotive effect. The combined effects of promotive and competitive interactions may render carbonate less competitive than was commonly believed. ^ Carbonate adsorbs in significant amounts on Al oxide and goethite, and alters the adsorptive properties of these surfaces. This has important implications for laboratory protocol for sorption studies (atmospheric exposure) and the transfer of knowledge from simple model systems to more complex field scenarios. ^