Details
| Original language | English |
|---|---|
| Article number | 37 |
| Journal | Contributions to Mineralogy and Petrology |
| Volume | 174 |
| Issue number | 4 |
| Publication status | Published - 29 Apr 2019 |
Abstract
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 Δ 65Cu Cu (0) - Cu (II) aq= - (0.27 ± 0.04) × 10 6T- 2+ (0.16 ± 0.34) , 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.
Keywords
- Cu isotopes, Electrochemical reduction, Kinetics, Metallic Cu, Thermodynamic equilibrium
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geophysics
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Contributions to Mineralogy and Petrology, Vol. 174, No. 4, 37, 29.04.2019.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Cu isotope fractionation during reduction processes in aqueous systems
T2 - Evidences from electrochemical deposition
AU - Qi, Dong Mei
AU - Behrens, Harald
AU - Lazarov, Marina
AU - Weyer, Stefan
N1 - Funding information: This work was supported by the German Academic Exchange Service (DAAD-57076462) and Graduate School GeoFluxes. We thank Ulrich Kroll for his invaluable technical supports. We are grateful to two anonymous reviewers for their thoughtful and constructive reviews on this manuscript.
PY - 2019/4/29
Y1 - 2019/4/29
N2 - 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 Δ 65Cu Cu (0) - Cu (II) aq= - (0.27 ± 0.04) × 10 6T- 2+ (0.16 ± 0.34) , 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.
AB - 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 Δ 65Cu Cu (0) - Cu (II) aq= - (0.27 ± 0.04) × 10 6T- 2+ (0.16 ± 0.34) , 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.
KW - Cu isotopes
KW - Electrochemical reduction
KW - Kinetics
KW - Metallic Cu
KW - Thermodynamic equilibrium
UR - http://www.scopus.com/inward/record.url?scp=85065030500&partnerID=8YFLogxK
U2 - 10.1007/s00410-019-1568-4
DO - 10.1007/s00410-019-1568-4
M3 - Article
AN - SCOPUS:85065030500
VL - 174
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
SN - 0010-7999
IS - 4
M1 - 37
ER -