Christopher Oze

Genesis of hexavalent chromium from natural sources in soil and groundwater

Naturally occurring Cr(VI) has recently been reported in ground and surface waters. Rock strata rich in Cr(III)-bearing minerals, in particular chromite, are universally found in these areas that occur near convergent plate margins. Here we report experiments demonstrating accelerated dissolution of chromite and subsequent oxidation of Cr(III) to aqueous Cr(VI) in the presence of birnessite, a common manganese mineral, explaining the generation of Cr(VI) by a Cr(III)-bearing mineral considered geochemically inert. Our results demonstrate that Cr(III) within ultramafic- and serpentinite-derived soils/sediments can be oxidized and dissolved through natural processes, leading to hazardous levels of aqueous Cr(VI) in surface and groundwater.

Chromium Geochemistry of Serpentine Soils

Serpentine soils derived from the weathering of ultramafic rocks, mainly ophiolitic serpentinites, are typically characterized by Cr concentrations in excess of 200 mg kg-1, comparatively higher than non-serpentine soils. We review the chemistry of Cr in serpentine soils and their protoliths, focusing on serpentine soils collected from New Caledonia, Oregon, and California. Overall, serpentine soils are slightly acidic (average pH of ∼6), contain a variety Fe(III) oxides (magnetite and hematite), Fe(III) (oxy)hydroxides, phyllosilicates (serpentine and chlorite), and clays (smectites and vermiculites), and contain concentrations of Cr (> 200 mg kg-1), Ni (> 1,000 mg kg-1), and Mn (> 200 mg kg-1) exceeding values of non-serpentine soils. Although Cr concentrations in serpentine soils have been reported as high as 6 wt% in New Caledonia, Cr values in New Caledonia, Oregon, and California serpentine soils evaluated in this study range from 827 to 9,528 mg kg-1. Chromium(III) is the only valence state observed in the serpentine soil solids; however, Cr(VI) has been identified in New Caledonia and California serpentine soil solutions at concentrations below 30 μM. The enrichment and range of Cr concentrations in serpentine soils are directly related to the presence of Cr-spinels, specifically chromite and Cr-magnetite. These phases are resistant to weathering and are preserved in the soil environment; however, oxidation of Cr(III) from Cr-spinels by high-valent Mn oxides, or other strong oxidants, is a potential source of Cr(VI) identified in serpentine soil solutions. Due to the weathering resistant nature of the Cr-spinels, Cr-bearing silicates including clay minerals, Cr-chlorite, Cr-garnet, Cr-mica, and Cr-epidote are more viable sources of Cr identified in vegetation, soil extractions, soil solutions, and related waters.

Carbonate control of H2 and CH4 production in serpentinization systems at elevated P‐Ts

[1] Serpentinization of forsteritic olivine results in the inorganic synthesis of molecular hydrogen (H 2 ) in ultramafic hydrothermal systems (e.g., mid-ocean ridge and forearc environments). Inorganic carbon in those hydrothermal systems may react with H 2 to produce methane (CH 4 ) and other hydrocarbons or react with dissolved metal ions to form carbonate minerals. Here, we report serpentinization experiments at 200°C and 300 bar demonstrating Fe 2+ being incorporated into carbonates more rapidly than Fe 2+ oxidation (and concomitant H 2 formation) leading to diminished yields of H 2 and H 2 -dependent CH 4 . In addition, carbonate formation is temporally fast in carbonate oversaturated fluids. Our results demonstrate that carbonate chemistry ultimately modulates the abiotic synthesis of both H 2 and CH 4 in hydrothermal ultramafic systems and that ultramafic systems present great potential for CO 2 -mineral sequestration.

Biodurability of chrysotile and tremolite asbestos in simulated lung and gastric fluids

Abstract Chrysotile [Mg3Si2O5(OH)4] and tremolite [Ca2Mg5Si8O22(OH)2] asbestos represent two distinct mineralogical categories of regulated asbestos commonly evaluated in epidemiological, toxicological, and pathological studies. Human respiratory and gastric systems are sites of asbestos deposition where chrysotile and tremolite asbestos are undersaturated with respect to biological fluids and dissolution kinetics control the persistence of these minerals in biological environments. Here we examined the biodurabilities (i.e., the resistance to dissolution) of chrysotile and tremolite asbestos in simulated body fluids as a function of mineral surface area over time. Batch experiments in simulated gastric fluid (SGF; HCl and NaCl solution at pH 1.2) and simulated lung fluid (SLF; modified Gamble’s solution at pH 7.4) were performed at 37 °C over 720 h to evaluate the dissolution of chrysotile vs. tremolite asbestos in acidic and near-neutral biological fluids. The rate-limiting step of Si release for both minerals was used to obtain rate constants (k) and reaction orders (n) allowing comparisons of mineral dissolution rates. Both chrysotile and tremolite asbestos are less biodurable in SGF (low pH) compared to SLF (near-neutral pH). Based on equivalent surface area comparisons, the surface chemistry of tremolite is more reactive in lung fluid than chrysotile and vice versa in digestive fluid. However, the relative biodurabilities of these asbestos silicates (from most to least) are tremolite (SLF) > chrysotile (SLF) > tremolite (SGF) > chrysotile (SGF) when accounting for the greater surface area of chrysotile per mass or per fiber compared to tremolite. Overall, this study illustrates the importance of surface area and fiber morphology considerations when evaluating the biodurabilities of asbestiform minerals.

Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces.

Molecular hydrogen (H2) is derived from the hydrothermal alteration of olivine-rich planetary crust. Abiotic and biotic processes consume H2 to produce methane (CH4); however, the extent of either process is unknown. Here, we assess the temporal dependence and limit of abiotic CH4 related to the presence and formation of mineral catalysts during olivine hydrolysis (i.e., serpentinization) at 200 °C and 0.03 gigapascal. Results indicate that the rate of CH4 production increases to a maximum value related to magnetite catalyzation. By identifying the dynamics of CH4 production, we kinetically model how the H2 to CH4 ratio may be used to assess the origin of CH4 in deep subsurface serpentinization systems on Earth and Mars. Based on our model and available field data, low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active.

Anoxic oxidation of chromium

Naturally occurring Cr(VI) has been ascribed to terrestrial Cr(III) oxidation by Mn (di)oxides, generated through reaction of Mn(II) with molecular oxygen (O 2 ). However, hydrogen peroxide (H 2 O 2 ) is a potential oxidant of Cr(III) that may form in serpentinization (high H 2 , low O 2 ) systems where chromite [i.e., the main mineralogical source of Cr(III)] is abundant. Accordingly, here we evaluate H 2 O 2 and chromite interactions in serpentinization systems to determine pathways of Cr(III) oxidation that alters the current paradigm of O 2 -dependent oxidation. Field observations support that metastable H 2 O 2 and Cr(VI) are present in serpentinization-related fluids relatively absent of O 2 . Further, laboratory experiments demonstrate and support that H 2 O 2 is a kinetically facile oxidant of chromite, especially under alkaline conditions, which provides a variety of alternative means by which Cr(VI) may be generated and supplied to the oceans not directly linked to atmospheric O 2 . Thus, Cr(III) oxidation pathways, and their influence on the Cr isotopic record, must account for anoxic Cr(III) oxidation in serpentinization systems as well as a variety of H 2 O 2 -induced Cr(III) oxidation pathways that may occur in both terrestrial and marine systems.