Carbonate Adsorption Mechanism on Goethite Studied with ATR–FTIR, DRIFT, and Proton Coadsorption Measurements

Abstract

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.