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

Abstract

A series of replicate flow-through cell experiments was conducted to characterize Cr isotope fractionation during Cr(VI) treatment by granular zero-valent iron (ZVI). Synthetic groundwater containing 50 mg L-1 Cr(VI) was pumped upward through a custom-made cell packed with ZVI under anaerobic conditions. The geochemical evolution of the system was monitored using pH and redox measurements, while aqueous effluent samples were retained for analysis of cations and Cr isotopes. Real-time, in situ X-ray absorption near edge structure (XANES) spectroscopy collected via a Kapton® window in the cell provided additional information on the speciation of the reaction products. Increases in  53 Cr values corresponding to decreases in Cr(VI) concentration suggested the occurrence of redox processes. Spectroscopic results correlated well with the isotope data, indicating reduction of Cr(VI) to Cr(III). The isotope data did not appear to follow a single trend. A two-stage system was proposed to explain the complex isotope trend, where the rapid Cr removal was associated with very little fractionation ( =-0.2 ‰), whereas slower removal was associated with a greater degree of fractionation ( =-1.2 to-1.5 ‰). Reactive transport modeling was used to quantify distinct isotope fractionation values (), differentiated by a significant change in the Cr removal rate.

Global variability of chromium isotopes in seawater demonstrated by Pacific, Atlantic, and Arctic Ocean samples

“…The underlying reason for this difference requires additional study, but one possibility is that the rate of reduction and removal exerts a control over the fractionation factor, as suggested by Jamieson-Hanes et al (2014).…”

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

“…A flow-through cell (FTC) was fabricated using high-density polyethylene (HDPE) following a similar design to that used in Jamieson-Hanes et al (2014). The dimensions of this cell were greater than that used in a previous study, with the open-faced indentation measuring 3.8 × 7.6 × 1.3 cm (37.5 mL internal volume, 28.9 cm 2 face area; Figure 1).…”

Chromium geochemistry of the ca. 1.85 Ga Flin Flon paleosol

“…Only the reduction in aqueous Cr(VI) to Cr(III) is well‐studied empirically and experimentally to date, with apparent fractionation factors ranging from −0.4 to −5.0 ‰ (Δ Cr(III)solid‐Cr(VI)aqueous ) depending on the reaction kinetics and electron acceptor involved in Cr(VI) reduction (e.g. Ellis, Johnson, & Bullen, ; Izbicki, Ball, Bullen, & Sutley, ; Schoenberg et al., ; Zink et al., ; Døssing, Dideriksen, Stipp, & Frei, ; Jamieson‐Hanes, Lentz, Amos, Ptacek, & Blowes, ; Scheiderich, Amini, Holmden, & Francois, ). By contrast, the limited number of available reports addressing the oxidation of Cr(III) to Cr(VI) by Mn oxides has shown isotopic fractionation in both directions, with a range of δ 53/52 Cr from −2.5 to +1.1 ‰ in generated Cr(VI) (Bain & Bullen, ; Zink et al., ; Joshi, Wang, Ellis, Johnson, & Bullen, ).…”