Complexation of carbonate species at the goethite surface: Implications for adsorption of metal ions in natural waters

Headspace PCo2 was measured with an infrared gas analyzer over an equilibrated goethite suspension to determine adsorption of carbonate species in the pH range 3 to 8. For a 2 g/L goethite suspension in 0.1 N NaClO4 (~3 10−4 M surface sites), the fraction of carbonate species adsorbed increased from 0.15 at pH 3 to a maximum of 0.56 at pH 6. In 0.01 N NaClO4, the fraction of carbonate species adsorbed at pH 6 increased to 0.67. The total concentration of CO2 in the suspension increased from about 0.4 to 0.6 10−4 M in the pH range of these experiments. The development of surface charge at the goethite surface was determined in the pH range 4 to 11 by potentiometric titration under controlled low CO2 conditions. No hysteresis was observed between the acid and base legs of titrations in 0.10, 0.03, and 0.01 N NaClO4 resulting in a pHpzc of 8.9. The carbonate species adsorption data were modelled using the least squares optimization program FITEQL for the diffuse double-layer model and the triplelayer model using stoichiometries of the type Fe-OCOOH and Fe-OCOO− for surface bound carbonate species. The model results are consistent with separate experiments showing a significant reduction in chromate adsorption on goethite as the partial pressure of CO2 was increased from <5 to 450 and 40,000 μatm. Our data suggest that mineral oxide surface sites which control solid/solute partitioning of metal ions in natural systems may be largely bound to adsorbed carbonate species.