Using chromium stable isotopes to monitor chromium reactive transport: Oxidation experiments and field studies
Chromium (Cr) is a common contaminant found at a number of sites globally. Cr sources can be either anthropogenic or natural. Cr occurs in nature in two valences, with Cr(VI) being carcinogenic, more toxic and mobile than Cr(III). Reduction of Cr(VI) to Cr(III) is a common remediation strategy, but monitoring Cr reduction using only concentration and speciation analyses is a difficult endeavor because concentrations can be affected by dilution and advection in addition to reduction. Cr isotopes are fractionated during reduction processes, so monitoring isotopic changes in Cr may be a faster and more effective way to monitor reduction. This research involved both measuring Cr isotopic fractionation caused by Cr(III) oxidation and a field study in an area of well documented Cr contamination problem. ^ Back reaction of Cr(III) to Cr(VI) is possible under some conditions and we need to be able to characterize that reaction when monitoring Cr(VI) contamination. Previous work measured isotopic fractionation factors for Cr(VI) reduction and through this study we now have fractionation factors for Cr(III) oxidation. Cr(III) oxidation experiments using pyrolusite (β-MnO 2) at pH 3 and 4 were performed. Cr(VI) formed in the first stages of the experiments is enriched in δ53Cr by 1.1‰, and is independent of pH or matrix or electrolyte used. This enrichment in the Cr(VI) product relative to the reactant cannot be explained by a simple kinetic isotopic effect, because during kinetic interactions one would expect the product to become enriched in the lighter Cr isotopes relative to the reactant. Moreover, the results do not fit a Rayleigh distillation curve behavior. We also suspect that this is not a reflection of isotopic equilibrium, as that should cause stronger fractionation. Seemingly a combination of equilibrium and kinetic isotopic effects govern the multistep oxidation reaction. ^ The second part of the study was conducted in León valley, Guanajuato, México. In 1975, high Cr concentrations were detected in groundwater. Previous work proposed an anthropogenic origin for a high concentration plume near the Química Central (QC) factory located in Buenavista (BV), while the larger but less contaminated Cr plume located near San Juan de Otates (SJO) is assumed to be caused by weathering of ultramafic rocks. In this study surface and groundwater samples collected in 2007 and 2008 in both areas showed enriched isotopic values ranging from +0.124‰ to +1.40‰ near QC and +0.749‰ to +2.853‰ in San Juan de Otates. The samples suggested a decrease in concentrations of Cr(VI). In BV concentrations ranged from 0.005 mg/l and 121 mg/l for surface and groundwater in 2007 to 0.002 mg/l and 95.1 mg/l for surface and groundwater samples in 2008. In 2007 Cr(VI) concentrations in San Juan de Otates ranged from 0.01 mg/l to 0.012 mg/l in surface and from 0.01 mg/l to 0.016 mg/l in groundwater, while in 2008 Cr(VI) concentrations were <0.001 mg/l in surface water. ^ Fractionation in Cr isotopes measured in leaching experiments from the waste material piles and δ53Cr values from groundwater, suggests that: (1) Cr in the waste piles was fractionated, during "weathering", (2) the decrease in δ53Cr values is caused by an unknown process, and/or (3) indicates the presence of new Cr sources with unique Cr isotopic compositions. Cr (VI) concentrations and Cr isotopes were also measured in a nearby landfill. Low Cr(VI) concentrations and enriched δ 53Cr values suggest that reduction is occurring in the landfill. Additional sampling suggests that weathering of ultramafic rocks in San Juan de Otates outcrop resulted in Cr(VI) concentrations in surface and groundwater below USEPA and Mexican Maximum Permissible Contaminant Levels in 2007 and 2008.^
Villalobos Aragon, Alejandro, "Using chromium stable isotopes to monitor chromium reactive transport: Oxidation experiments and field studies" (2009). ETD Collection for University of Texas, El Paso. AAI3371752.