Metal Isotope Fractionation Associated with Cu and Zn Attenuation by Zero-Valent Iron in Anaerobic Flow-Through Cell Experiments

Abstract

Metal stable isotope fractionation is an emerging tool in the environmental sciences for studying biogeochemical pathways for cycling of metals in the environment. Copper and zinc are essential elements for many biological functions, but can accumulate in high concentrations toxic to living organisms through anthropogenic activities such as mining. Permeable reactive barrier (PRB) technologies have been implemented at many field sites to remove inorganic contaminants, such as Cu and Zn, from groundwater. A common reactive material used to construct PRBs is zero-valent iron (ZVI), which is useful for its propensity to create strongly reducing conditions that favour the reduction and co-precipitation of metals. Flow-through cells (FTCs) were used to examine the isotope composition of Cu and Zn while these metals were removed from solution by ZVI. Attenuation of dissolved Cu resulted in the enrichment of the heavy isotope (65Cu) in solution, where δ65Cu initially peaked at 1.30 ‰ and decreased towards the mean influent isotope value (δ65Cuinput = 0.27 ‰). Copper isotope ratios in the FTC effluent displayed Rayleigh-type behavior (ɛ = -0.31 ‰). X-ray absorption near edge structure (XANES) spectroscopy was used to identify the solid phases present in the FTCs. In linear combination fitting (LCF), reduced Cu and ZnO were found to primarily contribute to the solid species in respective Cu and Zn FTCs. Reduced Cu0 consistently contributed to the fit for Cu XANES sample spectra present at the central position of the Cu FTC. Similar portions of Cu0 and Cu2O were identified at bottom and top positions of the Cu FTC, indicating that Cu isotope fractionation could be attributed to reduction of Cu during immobilization by ZVI. As Zn breakthrough in the FTC effluent occurred, an increase in δ66Zn values was observed, increasing initially from -0.59 ‰ towards the mean δ66Zninput value of -0.19 ‰ throughout the experiment. The fractionation values (ɛ) for the Zn FTC II was 0.32 ‰. Linear combination fitting indicated contributions from ZnO and Zn(OH)2 in the fitting of four Zn sample spectra. A standard for Zn adsorbed to ferrihydrite also contributed to the fit for the Zn XANES spectra from the bottom location of the FTC. These XAS measurements suggest a combination of adsorption and precipitation mechanisms contributed to the majority of the depletion of the heavy Zn isotope associated with Zn attenuation by ZVI. Characterizing the isotope fractionation linked to Cu and Zn removal by ZVI in PRB systems may contribute to an enhanced knowledge of how isotopes can be correlated to geochemical processes. This insight may be influential for understanding the controls on metal mobility at contaminated sites, and how to devise more efficient and economical groundwater monitoring programs.