FRACTIONATION OF CU AND ZN ISOTOPES DURING ADSORPTION ONTO AMORPHOUS FE(III) OXIDES: EXPERIMENTAL MIXING OF ACID-ROCK DRAINAGE AND AMBIENT RIVER WATER

Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxides is examined in experimental mixtures of metal-rich acid-rock drainage and relatively pure river water, and during batch adsorption experiments using synthetic ferrihydrite in complex synthetic acid-rock drainage and simple NaNO₃ solutions. Metal adsorption as a function of pH is well described using a diffuse double layer model. Isotopic measurements of dissolved Cu (⁶⁵Cu/⁶³Cu) and Zn (⁶⁶Zn/⁶⁴Zn) show systematic changes that indicate significant fractionation of the stable metal isotopes during sorption. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (α_{soln – solid}) are 0.99927 + 0.00008 for Cu and 0.99948 + 0.00004 for Zn or, alternately, the separation factors (Δ_soln-solid) are -0.73 + 0.08 ‰ for Cu and -0.52 + 0.04 ‰ for Zn. These results indicate that the heavier isotope preferentially adsorbs onto the oxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous Fe(III) oxide from this and other studies suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define the strength of aqueous metal interactions with oxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by amorphous Fe oxides with Cu > Zn > Fe(II). This process-based understanding of stable metal isotopic fractionation can be used to successfully design field programs, interpret field results, and improve our understanding of the transport and fate of elements in the environment.