Rate-dependent Isotopic Fractionation of Zn During Electrodeposition

Abstract:

Isotopic fractionation has been found to vary with reaction rate for a variety of elements in settings including crystal precipitation and redox reactions. Understanding the mechanisms of fractionation in these systems will help expand the use of non-traditional stable isotopes as proxies of environmental processes. Rate-dependent fractionation is particularly attractive as a “speedometer” that could reveal rates of reaction in natural systems if fractionation is sufficiently well constrained. Previous laboratory-based results suggest that isotopic fractionation during redox reactions increases as the reaction rate slows.

We conducted electrochemical reduction experiments with Zn and a Rotating Disk Electrode (RDE) to better constrain the isotopic fractionation during slow electrochemical reactions and at low temperatures. The RDE system allows us to adjust the reaction rate by setting an electrochemical potential without needing to adjust chemical concentrations and to change chemical flux to the electrode by altering the rotation rate. Using a solution of ZnSO₄ in methanol, we conducted Zn plating experiments at potentials ranging from 25 to 400 mV below the Zn²⁺-Zn reduction potential and temperatures from 20° to -45°C. We measured the isotopic composition of the Zn plated from a large (>200-fold) excess of ZnSO₄ solution to determine the fractionation associated with electrochemical reduction.

These experiments represent significantly lower reaction rates than previous aqueous Zn plating experiments but do not show greater isotopic fractionation, suggesting more complex behavior with changing chemistry. Within our dataset, lower reaction rates do correspond to greater fractionation, but the maximum fractionation is consistent with equilibrium values predicted from vibrational frequency calculations. Therefore, unlike previous results, ours do not require a mechanism to explain kinetic fractionation in excess of equilibrium values. Finally, we note that we do not see the expected increase in fractionation with decreasing temperature; at -45°C, Δ⁶⁶Zn between the solution and deposited material was around -1 ‰ while experiments at 20° and 0°C yielded fractionations of -1.5 to -2.6 ‰.