Complexation of Carbonate Species at the Goethite Surface - Implications for Adsorption of Metal-Ions in Natural-Waters

Headspace P(CO2) 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 (approximately 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 pH(pzc) 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 triple-layer 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 muatm. 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.