PubMed · 13178922
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G HILT. 1954. [Bichromate dermatitis].. https://pubmed.ncbi.nlm.nih.gov/13178922/
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Green rusts, ferrous-ferric iron oxides, occur in many anaerobic soils and sediments and are highly reactive, making them important phases impacting the fate and transport of environmental contaminants. Despite their potential importance in environmental settings, reactions involving green rusts remain rather poorly described. Chromate is a widespread contaminant having deleterious impacts on plant and animal health; its fate may in part be controlled by green rust. Here we examine chromate reduction by a series of green rust phases and resolve the reaction kinetics at pH 7. The overall kinetics of the reactions are well described by the expression d[Cr(VI)]/dt = -k[Cr(VI)][GR], and this model was successfully used to predict rates of reaction at varying chromium concentrations. The rates of reduction are controlled by the concentration of ferrous iron, surface area, and chemical structure of the green rust including layer spacing. On a mass basis, green rust (GR) chloride is the most rapid reductant of Cr(VI) followed by GRCO3 and GRSO4, with pseudo-first-order rate coefficients (k(obs)) (with respect to Cr(VI) concentration) ranging from 1.22 x 10(-3) to 3.7 x 10(-2) s(-1). Chromium(III)-substituted magnetite and lepidocrocite were identified as the major oxidation products. The nature of the oxidation products appears to be independent of the anionic class of green rust, but their respective concentrations display a dependence on the initial GR. The mole fraction of Fe(III) in the Cr(x),Fe(1-x)(OH)3 x nH2O reaction product ranged from 17% to 68%, leading to a highly stabilized (low solubility) phase.
Previous studies have demonstrated that gas-phase H2S can immobilize certain redox-sensitive contaminants (e.g., Cr, U, Tc) in vadose zone environments. A key issue for effective and efficient delivery of H2S in these environments is the reactivity of the gas with indigenous iron oxides. To elucidate the factors that control the transport of H2S in the vadose zone, laboratory column experiments were conducted to identify reaction mechanisms and measure rates of H2S oxidation by iron oxide-coated sands using several carrier gas compositions (N2, air, and O2) and flow rates. Most experiments were conducted using ferrihydrite-coated sand. Additional studies were conducted with goethite- and hematite-coated sand and a natural sediment. Selective extractions were conducted at the end of each column experiment to determine the mass balance of the reaction products. XPS was used to confirm the presence of the reaction products. For column experiments in which ferrihydrite-coated sand was the substrate and N2 was the carrier gas, the major H2S oxidation products were FeS and elemental sulfur (mostly S8(0), represented as S(0) for simplicity) at ratios that were consistent with the stoichiometry of the postulated reactions. When air or O2 were used as the carrier gas, S(0) became the dominant reaction product along with FeS2 and smaller amounts of FeS, sulfate, and thiosulfate. A mathematical model of reactive transport was used to test the hypothesis that S(0) forming on the iron oxide surfaces reduces access of H2S to the reactive surface. Several conceptual models were assessed in the context of the postulated reactions with the final model based on a linear surface poisoning model and fitted reaction rates. These results indicate that carrier gas selection is a critical consideration with significant tradeoffs for remediation objectives.
Abundant extraterrestrial chromite grains from decomposed meteorites occur in middle Ordovician (480 million years ago) marine limestone over an area of approximately 250,000 square kilometers in southern Sweden. The chromite anomaly gives support for an increase of two orders of magnitude in the influx of meteorites to Earth during the mid-Ordovician, as previously indicated by fossil meteorites. Extraterrestrial chromite grains in mid-Ordovician limestone can be used to constrain in detail the temporal variations in flux of extraterrestrial matter after one of the largest asteroid disruption events in the asteroid belt in late solar-system history.