Cu isotope fractionation during reduction processes in aqueous systems: evidences from electrochemical deposition

Redox processes are ubiquitous in Earth science, and redox transitions often lead to large fractionations of the stable isotopes of many transition metals such as copper. To get insights into the mechanisms of isotope fractionations induced by electrochemical processes, we examine the behavior of copper isotopes during the reduction reaction Cu2+ + 2e− = Cu0. All experiments have been conducted by applying a controlled current between the working electrode and the auxiliary electrode, i.e., the galvanostatic electrodeposition technique, in aqueous CuSO4 solutions. Controlling parameters were tested by varying electrolyte concentration (0.01–1 mol kg−1), stirring speed (0–500 rpm), current (0.1–0.5 A), time (35–600 s), and temperature (5–80 °C). In all cases, the plated Cu metal is enriched in the light isotope (63Cu) with respect to the solution. At room temperature, the Cu isotopic fractionation between the electroplated Cu and electrolyte is found to increase with electrolyte concentration and stirring speed, and to decrease with current and run duration. These trends can be interpreted by three competing processes: copper transport in the solution, kinetics of electrochemical reduction of copper ions and surface diffusion at the electrode, i.e., transport becomes important at low copper concentration, low stirring speed, high currents and large amount of copper precipitation. Copper isotope fractionation has a maximum near 35 °C, decreasing both towards higher and lower temperatures. In the temperature range of 35–80 °C, the dependence of temperature on isotope fractionation can be described by $$ \Delta^{65} {\text{Cu}}_{{{\text{Cu}}\left( 0 \right) - {\text{Cu}}({\text{II}}){\text{aq}}}} = - \left( {0.27 \pm 0.04} \right) \times 10^{6} T^{ - 2} + \left( {0.16 \pm 0.34} \right), $$ where ∆65CuCu(0)–Cu(II)aq (‰) represents the copper isotopic composition differences between the product (electroplated copper) and the reactant (electrolyte solution, CuSO4(aq)), and T is the temperature in K. At low temperature (down to 5 °C), a noticeable deviation from this trend suggests a change in the controlling mechanism, i.e., transport in the solution becomes important. Our findings are best explained by a two-step reduction process including reduction from Cu(II) to Cu(I) and a subsequent reduction of Cu(I) to Cu(0). The good agreement of our high-temperature data with the results from Ehrlich et al. (2004), who used a different experimental approach to precipitate Cu(I) mineral from CuSO4 solution, implies that transformation of Cu(II) to Cu(I) dominates the isotope fractionation observed during electrochemical reduction of Cu(II) to Cu(0). These findings support that copper isotopes can be used as effective tracers of redox processes. They may have implications to processes in hydrothermal systems and the formation of ore deposits, e.g., volcanic-hosted massive sulfides, as well as to processes in near surface aquatic environment and related supergene processes.