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The adsorption reaction of bicarbonate at the goethite–water interface was investigated by determining the speciation and coordination of adsorbed carbonate species using in situ attenuated total reflectance (ATR)–Fourier transformed infrared (FTIR) and diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopies, and the proton coadsorption by pH‐stat measurements. The spectra of the adsorbed carbonate species indicated monodentate inner‐sphere surface complexes. Only the carbonate anion species was detected as the adsorbed species in the pH range of 4.8 to 7.0. The DRIFT spectra indicated the existence of additional protonated surface groups associated with adsorbed carbonate. The proton‐to‐bicarbonate coadsorption stoichiometry was 0.54:1 in 0.011 M NaCl and 0.86:1 at very low ionic strength. These proton stoichiometry values appear to be higher than stoichiometry that have been reported for other bivalent oxyanions. The adsorption reaction of carbonate and the concurrent proton adsorption reactions on goethite are proposed.

The presence of anionic cosolutes often influences anion adsorption on soil mineral surfaces. The possible effect of common solutes, such as CO3 species, on the adsorption of other anions on metal (hydr)oxides is poorly understood. Accordingly, we determined the effect of CO3, formate, acetate, oxalate, and citrate on the adsorption of SO4 and SeO4 on Al oxide. Carbonate markedly promotes the adsorption of SO4 and SeO4 between pH 6 and 8. The maximum effect occurs at relatively low CO3 concentrations (0.3 mM) and the promotive effect decreases with further increase in CO3 adsorption. Formate and acetate also promote adsorption of SO4 and SeO4, while oxalate and citrate have a competitive effect, especially at lower pH values. 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 exist with the adsorbed CO3 Acetate and formate presumably interact through the same mechanism. This promotive mechanism was ineffective in the case of adsorption of a high affinity anion such as PO4 Finally, the adsorptive properties of Al oxide, as we know it from simple model systems, can significantly be altered in soils, where CO3 and organic anions are ubiquitous solutes. For example, the adsorption maximum of SO4 > SeO4 became in the presence of CO3