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Chromium isotope fractionation analysis is a promising approach for the assessment of microbial Cr(VI) reduction in groundwater. Understanding the mechanisms and other parameters that control Cr isotope fractionation factors (between the product Cr(III) and reactant Cr (VI)) in microbial Cr(VI) reduction is critical to this application. To date, such studies are very limited. Here, the influence of critical factors on observed Cr isotope fractionation during Cr(VI) reduction by Shewanella oneidensis MR-1 under various conditions was investigated. The Cr(VI) concentration and Cr isotope ratio measurements were conducted on unreacted Cr(VI) remaining in solution to determine Cr isotope fractionation factors. The changes in ambient environmental conditions (e.g., pH, temperature) have limited influence on Cr isotope fractionation factors. However, as a result of Cr(VI) consumption as the experiments proceed, the change in bioavailability of Cr(VI) has a significant impact on Cr isotope fractionation factors. For example, in temperature-controlled experiments, Cr isotope fractionation showed two-stage behavior: during Stage I, the values of ε were -2.81 ± 0.19‰ and -2.60 ± 0.14‰ at 18 °C and 34 °C, respectively; during Stage II, as Cr(VI) reduction progressed, Cr isotope fractionation was significantly masked, and the ε values decreased to -0.98 ± 0.49‰ and -1.01 ± 0.11‰ at 18 °C and 34 °C, respectively. Similar two-stage isotope fractionation behaviors were observed in pH-controlled experiments (pH = 6.0 and 7.2) and in experiments with and without the addition of a competing electron acceptor (nitrate). Masking of isotope fractionation in Stage II indicated restrictions on the bioavailability of Cr(VI) and mass-transfer limitations. This study provides an explanation for the variation in Cr isotope fractionation factors during microbial Cr(VI) reduction in the environment, furthering the viability of Cr isotope ratio analysis as an approach in understanding Cr biogeochemical cycling.
This article was published in the following journal.
Name: Water research
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Stable chromium atoms that have the same atomic number as the element chromium, but differ in atomic weight. Cr-50, 53, and 54 are stable chromium isotopes.
Unstable isotopes of chromium that decay or disintegrate emitting radiation. Cr atoms with atomic weights of 46-49, 51, 55, and 56 are radioactive chromium isotopes.
Inorganic compounds that contain chromium as an integral part of the molecule.
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Hydrogen. The first chemical element in the periodic table. It has the atomic symbol H, atomic number 1, and atomic weight 1. It exists, under normal conditions, as a colorless, odorless, tasteless, diatomic gas. Hydrogen ions are PROTONS. Besides the common H1 isotope, hydrogen exists as the stable isotope DEUTERIUM and the unstable, radioactive isotope TRITIUM.