Copper isotopic fractionation during adsorption on manganese oxide: Effects of pH and desorption

We report new results for Cu2+ adsorption on delta-MnO2 and the resulting changes in Cu isotopic composition due to isotopic fractionation. The adsorption experiments were designed as a pH series (3.22-6.94) and an adsorption-desorption series. Cu-63 adsorbs preferentially on delta-MnO2, and the degree of isotopic fractionation is 0.45 +/- 0.18 parts per thousand (2 sigma, n = 12). Preferential desorption of Cu-65 from delta-MnO2 was also observed. The isotopic data are consistent with a closed isotopic equilibrium model, and the degree of fractionation indicates no systematic variation with pH or reaction time. Previously reported extended X-ray absorption fine structure study predicted the preferential adsorption of Cu-65 on delta-MnO2, because of the formation of Cu2+ surface complexes (with 3-4 ligands) from dissolved Cu2+ (with 5 ligands); however, the opposite was observed in this study. The equilibrium in isotopic fractionation is affected by the bonding environment; hence, we suggest that dissolved Cu2+ species have a stronger bond than adsorbed ones.