A mechanistic study of metal sorption at the mineral-water interface using Cu(II)-EPR spectroscopy

Cu(II) sorption on kaolinite (KGa-1B) was studied by using EPR spectroscopy over various pH, ionic strength, and Cu(II) concentration conditions in order to understand the mechanism of sorption. The influence of pH, Na(I) and Cu(II) concentrations on the EPR signal were observed. The Cu(II) sorbed on fully hydrated surface showed an isotropic absorption line of giso=2.19 at room temperature measurement. It was at its maximum at neutral pH and decreased either at acidic or basic pH conditions. It was proportional to the Cu(II) concentration, but was inversely proportional to Na(I) concentration in the experimental condition. The observation of the isotropic signal indicated that Cu(II) formed a weakly bound outer-sphere type surface complex on the kaolinite surface whose amount changed with experimental conditions. The results suggested that solution variables such pH, concentrations of radionuclides and competing cations are important factors in determining the amount and long-term fate of outer-sphere surface complex formed on the kaolinite surface