Chromium Isotope Fractionation During Reduction of Cr(VI) Under Saturated Flow Conditions

Abstract

Chromium isotopes are potentially useful indicators of Cr(VI) reduction reactions in groundwater flow systems; however, the influence of transport on Cr isotope fractionation has not been fully examined. Laboratory batch and column experiments were conducted to evaluate isotopic fractionation of Cr during Cr(VI) reduction under both static and controlled flow conditions. Organic carbon was used to reduce Cr(VI) in simulated groundwater containing 20 mg L(-1) Cr(VI) in both batch and column experiments. Isotope measurements were performed on dissolved Cr on samples from the batch experiments, and on effluent and profile samples from the column experiment. Analysis of the residual solid-phase materials by scanning electron microscopy (SEM) and by X-ray absorption near edge structure (XANES) spectroscopy confirmed association of Cr(III) with organic carbon in the column solids. Decreases in dissolved Cr(VI) concentrations were coupled with increases in δ(53)Cr, indicating that Cr isotope enrichment occurred during reduction of Cr(VI). The δ(53)Cr data from the column experiment was fit by linear regression yielding a fractionation factor (α) of 0.9979, whereas the batch experiments exhibited Rayleigh-type isotope fractionation (α = 0.9965). The linear characteristic of the column δ(53)Cr data may reflect the contribution of transport on Cr isotope fractionation.

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

“…However, it is also important to evaluate more complex systems, such as the experiment conducted in this study, to extend the understanding of Zn isotope behaviour to a system where multiple geochemical processes may be involved. In addition, it is essential to consider the effect of saturated flow conditions, as flow dynamics can influence the degree of isotope fractionation (Jamieson-Hanes et al, 2012;Jamieson-Hanes et al, 2015). Results from this study demonstrate that the application of Zn isotope ratio measurements, combined with XAS and aqueous geochemical measurements, can provide valuable insight into the complex geochemical systems encountered in field studies.…”

Examination of Cr(VI) treatment by zero-valent iron using in situ, real-time X-ray absorption spectroscopy and Cr isotope measurements

“…Laboratory experiments have demonstrated that the degree of fractionation is dependent on the mechanism of removal and the electron donor, as summarized in Table 1. Materials used to induce reduction in these experiments include ferrous iron, both as Fe(II) aq and in the form of Fe(II)-bearing minerals (Ellis et al, 2002;Berna et al, 2010;Døssing et al, 2011;Kitchen et al, 2012;Basu and Johnson, 2012), organic carbon (Kitchen et al, 2012;Jamieson-Hanes, Gibson et al, 2012), Cr-reducing bacteria (Sikora et al, 2008;Han et al, 2012), and hydrogen peroxide (Zink et al, 2010). A large range of values has been reported for Cr isotope fractionation during reduction of Cr(VI) ( Table 1).…”

“…Enriched  53 Cr values have also been observed in field samples collected at both anthropogenically and naturally contaminated sites, but system heterogeneities and complex flow regimes have posed considerable challenges for data interpretation (Ellis et al, 2002;Izbicki et al, 2008;Berna et al, 2010;Raddatz et al, 2011;Villalobos-Aragón et al, 2012;Wanner, Eggenberger, Kurz, et al, 2012;Izbicki et al, 2012). Jamieson-Hanes, Gibson, et al (2012) explored the potential influence of transport by comparing Cr(VI) reduction by organic carbon in a batch and a column experiment. A significant difference was observed in the isotope behaviour under static and saturated flow conditions, and was attributed to the presence of multiple Cr removal mechanisms.…”

“…The objective of this study was to determine whether the complex Cr isotope behaviour observed in Jamieson-Hanes, Gibson, et al (2012) using organic carbon is observed during reduction of Cr(VI) using other reactive materials. In this study, granular ZVI was used to treat Cr(VI) under saturated flow conditions.…”

Groundwater Contamination Studies by Environmental Isotopes: A review

“…Equilibrium fractionations between Cr (VI) and Cr (III) were estimated by Schauble [171], who predicted Cr isotope fractionations of >1‰ between Cr species with different oxidation states. Ellis et al [172], Sikora et al [173], Berna et al [174], Zink et al [175], Dossing et al [176], Basu and Johnson [177], Han et al [178], Jamieson-Hanes et al [179], and Kitchen et al [180] studied the fractionation of Cr isotopes in a series of Cr(VI) reduction experiments. The results showed a general trend of accumulating heavier Cr isotopes in the unreacted Cr(VI) species and an accumulation of the lighter ones in the produced Cr(III), whereas Cr(III) did not undergo rapid isotopic exchange during Cr(III) oxidation runs [175].…”