Evaluating zinc isotope fractionation under sulfate reducing conditions using a flow-through cell and in situ XAS analysis

Abstract

A flow-through cell experiment was conducted to evaluate Zn isotope fractionation during ZnS precipitation under microbially-mediated sulfate-reducing conditions. Synthetic groundwater containing 0.90 mM Zn was pumped through a cell containing creek sediment that was biostimulated to promote sulfate reducing conditions. Real-time, in situ X-ray absorption spectroscopy (XAS) was applied at the Zn K-edge to collect spectra via a Kapton window in the front of the cell over the course of the experiment. Aqueous effluent samples were collected and analysed to determine concentrations of anions and cations, and Zn isotope ratios. The flow rate was increased step-wise during the experiment to modify the residence time and produce changes in the extent of sulfate reduction, which in turn controlled the extent of ZnS precipitation. Greater enrichment in the heavier isotope in the aqueous phase relative to the input solution was associated with more extensive Zn removal. A Rayleigh curve was fit to the isotope data, where ε = − 0.27 ± 0.06 ‰ (2). Evaluation of Zn isotope fractionation under controlled flow conditions is critical to improve the efficacy of this powerful analytical technique when applied to natural systems or remediation projects in the field.

Zinc Geochemical Cycling in a Phosphorus‐Rich Ocean During the Early Ediacaran

“…However, other lower δ 66 Zn values of our shales precipitated from ferruginous condition could indicate Zn isotope fractionation between Zn sulfides and the water column. In this case, the average δ 66 Zn value of deep water may be close to 0.5‰, based on the average δ 66 Zn bulk value (0.23‰) of our shales and the previous observed Zn isotope fractionation between sulfide and solution with Δ 66 Zn sulfide‐solution = ~−0.3‰ (Archer & Vance, ; Jamieson‐Hanes et al, ; Veeramani et al, ).…”

“…However, other lower δ 66 Zn values of our shales precipitated from ferruginous condition could indicate Zn isotope fractionation between Zn sulfides and the water column. In this case, the average δ 66 Zn value of deep water may be close to 0.5‰, based on the average δ 66 Zn bulk value (0.23‰) of our shales and the previous observed Zn isotope fractionation between sulfide and solution with Δ 66 Zn sulfide-solution =~À0.3‰ (Archer & Vance, 2004;Jamieson-Hanes et al, 2017;Veeramani et al, 2015). Scott et al (2013) proposed that the Zn concentration in seawater was broadly uniform over time and has maintained near-modern levels that were several orders of magnitude above the biolimiting concentration, which has been confirmed by other investigations regarding Zn concentrations and Zn/Fe ratios in banded iron formation (Robbins et al, , 2016.…”

“…For example, Zn sulfides (sphalerite and wurtzite) are typically rich in light Zn isotopes, with Δ 66 Zn sulfide-solution =~À0.3 to À0.6‰ (Fujii et al, 2011). Recent experiments and natural studies have also reported the same fractionation direction, with Δ 66 Zn sulfide-solution =~À0.3‰ (Archer & Vance, 2004;Jamieson-Hanes et al, 2017;Veeramani et al, 2015). This potential mechanism has been widely used to explain lower δ 66 Zn values in sphalerites (Veeramani et al, 2015) and organic-rich shales Vance et al, 2016).…”

Zinc isotopes as tracers of anthropogenic sources and biogeochemical processes in contaminated mangroves

“…Zn sulfides are isotopically lighter compared to other mineral phases (John et al, 2008;Kelley et al, 2009;Veeramani et al, 2015;Jamieson-Hanes et al, 2017) and Aucour and co-workers (2016) found enrichment of light Zn isotopes in roots compared to the soil in a wetland system in France, due to the dissolution of ZnS enriched in light isotopes.…”

“…The changes in sedimentary mineralogy induced by diagenetic processes, tidal cycles and bioturbation affects the Zn mobility in the sediment/water interface and its isotope compositions (Veeramani et al, 2015;Jamieson-Hanes et al, 2017, Deckov et al, 2010. A flow-through cell experiment designed for the study of Zn speciation in situ in natural sediments demonstrated an enrichment of the heavier Zn isotope in the aqueous phase due to the continuous Zn removal via microbially-mediated ZnS precipitation in reducing conditions (Jamieson-Hanes et al, 2017). Leaching experiments of metallurgical slags showed different directions and extent of Zn isotope fractionation depending on the formation or dissolution of secondary phases such as carbonates, hydroxides, or oxides during weathering (Yin et al, 2018).…”