Chromium Stable Isotope Fractionation during Transport: Sorption and Oxidation Experiments

Chromium (Cr) is a common contaminant found in groundwater and surface waters. Sources can be anthropogenic or natural. Cr occurs as Cr(III) and Cr(VI). Cr(VI) is carcinogenic, more toxic and mobile than Cr(III). Reduction of Cr(VI) to Cr(III) is a common remediation strategy but monitoring Cr reduction is difficult because mass balance is complicated by dilution and advection. Cr stable isotopes may be a fast and efficient way to monitor reduction (Johnson et al., 2005). One previous study of Cr(III) oxidation by birnessite (Bain and Bullen, 2005) reported isotopic shifts from -2.50‰ to +0.71‰ in the Cr(VI) product. Also a previous study reported no isotope fractionation during equilibrium sorption (Ellis et al., 2004). However the possibility exists of a kinetic amplification effect during transport. Here we evaluate the possibility of kinetic isotopic effects during: 1) 1-D Cr(VI) transport with sorption using goethite-coated sand in columns, and 2) Cr(III) oxidation by pyrolusite.

The column experiments were designed to determine if any isotope fractionation occurs during transport. The rate of flow was minimized to enhance the effect of sorption and dispersion over advection. Breakthrough (BTC) curves were fitted with CXTFIT to determine basic hydrological parameters. δ⁵³Cr values along the BTC showed no variation within our precision (±0.15‰). The oxidation batch experiments using reagent grade solid MnO₂ were conducted at pH 3. The Cr(VI) formed is enriched in δ⁵³Cr by +1.1‰. This enrichment in the Cr(VI) product cannot be explained by a simple kinetic isotope effect. Some equilibrium isotope effect must be involved, as it would tend to enrich heavy isotopes in the more tightly bonded oxidized species. However, the Cr(VI) and Cr(III) must not be in full isotopic equilibrium, as that should cause a stronger fractionation. We will also present and discuss results from other ongoing experiments.