Surface complexation model for multisite adsorption of copper(II) onto kaolinite

We measured the adsorption of Cu(II) onto kaolinite from pH 3–7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO4Hn)n−6 and binuclear (Cu2O6Hn)n−8 inner-sphere complexes on variable-charge ≡AlOH sites and as Cu2+ on ion exchangeable ≡X--H+ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)6 polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)2Cu(OH)20, tridentate (≡Al3O(OH)2)Cu2(OH)30 and ≡X--Cu2+ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions 2≡AlOH+Cu2++2H2O=(≡AlOH)2Cu(OH)20+2H+3(≡AlOH)+2Cu2++3H2O=(≡Al3O(OH)2)Cu2(OH)30+4H+ and equation(A1)≡X−--H++Cu2+=≡X−--Cu2++X+.≡X−--H++Cu2+=≡X−--Cu2++X+.