ABSTRACT

We report new results for Cu²⁺ adsorption on δ-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. ⁶³Cu adsorbs preferentially on δ-MnO₂, and the degree of isotopic fractionation is 0.45 ± 0.18‰ (2σ, n = 12). Preferential desorption of ⁶⁵Cu from δ-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 ⁶⁵Cu on δ-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.