Vincent Noël

Physico-Chemical Heterogeneity of Organic-Rich Sediments in the Rifle Aquifer, CO: Impact on Uranium Biogeochemistry

The Rifle alluvial aquifer along the Colorado River in west central Colorado contains fine-grained, diffusion-limited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfate-reducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeter-scale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uranium-contaminated aquifers, may be regionally important to uranium persistence.

XAS evidence for Ni sequestration by siderite in a lateritic Ni-deposit from New Caledonia

Abstract Mineralogical and spectroscopic analyses were conducted on a lateritic Ni-deposit from Southern New Caledonia. Results show that Ni is incorporated in siderite (FeCO3) found between 37 and 40 m depth in the laterite and saprolite units of the regolith. SEM-EDXS analyses of siderite-rich samples indicate that a significant amount of nickel can be hosted by this crystalline phase (~0.8 wt% NiO). Ni and Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopic analyses of the siderite-rich samples from the regolith as well as comparison with synthetic Ni-bearing and Ni-free siderites demonstrate isomorphous substitution of Ni2+ for Fe2+ in the siderite structure. Linear combination fitting (LCF) of the Ni K-edge EXAFS data reveals that this Ni-bearing siderite species accounts for more than 90% of the total Ni pool (1 wt% NiO) in the siderite-rich horizons of the regolith. In addition, LCF analysis of the EXAFS spectra indicates that goethite and serpentine are the major Ni hosts in the upper horizons (laterite) and lower horizons (saprolite) of the regolith, respectively. Formation of siderite, an uncommon mineral species in such oxidized environments, is attributed to the development of swampy conditions in organic-rich lateritic materials that accumulated at the bottom of dolines. These results thus show the importance of siderite as a host for nickel in lateritic Ni deposits that have been affected by late hydromorphic and reducing conditions.

Understanding controls on redox processes in floodplain sediments of the Upper Colorado River Basin

Floodplains, heavily used for water supplies, housing, agriculture, mining, and industry, are important repositories of organic carbon, nutrients, and metal contaminants. The accumulation and release of these species is often mediated by redox processes. Understanding the physicochemical, hydrological, and biogeochemical controls on the distribution and variability of sediment redox conditions is therefore critical to developing conceptual and numerical models of contaminant transport within floodplains. The Upper Colorado River Basin (UCRB) is impacted by former uranium and vanadium ore processing, resulting in contamination by V, Cr, Mn, As, Se, Mo and U. Previous authors have suggested that sediment redox activity occurring within organic carbon-enriched bodies located below the groundwater level may be regionally important to the maintenance and release of contaminant inventories, particularly uranium. To help assess this hypothesis, vertical distributions of Fe and S redox states and sulfide mineralogy were assessed in sediment cores from three floodplain sites spanning a 250km transect of the central UCRB. The results of this study support the hypothesis that organic-enriched reduced sediments are important zones of biogeochemical activity within UCRB floodplains. We found that the presence of organic carbon, together with pore saturation, are the key requirements for maintaining reducing conditions, which were dominated by sulfate-reduction products. Sediment texture was found to be of secondary importance and to moderate the response of the system to external forcing, such as oxidant diffusion. Consequently, fine-grain sediments are relatively resistant to oxidation in comparison to coarser-grained sediments. Exposure to oxidants consumes precipitated sulfides, with a disproportionate loss of mackinawite (FeS) as compared to the more stable pyrite. The accompanying loss of redox buffering capacity creates the potential for release of sequestered radionuclides and metals. Because of their redox reactivity and stores of metals, C, and N, organic-enriched sediments are likely to be important to nutrient and contaminant mobility within UCRB floodplain aquifers.

Redox Controls over the Stability of U(IV) in Floodplains of the Upper Colorado River Basin

Aquifers in the Upper Colorado River Basin (UCRB) exhibit persistent uranium (U) groundwater contamination plumes originating from former ore processing operations. Previous observations at Rifle, Colorado, have shown that fine grained, sulfidic, organic-enriched sediments accumulate U in its reduced form, U(IV), which is less mobile than oxidized U(VI). These reduced sediment bodies can subsequently act as secondary sources, releasing U back to the aquifer. There is a need to understand if U(IV) accumulation in reduced sediments is a common process at contaminated sites basin-wide, to constrain accumulated U(IV) speciation, and to define the biogeochemical factors controlling its reactivity. We have investigated U(IV) accumulation in organic-enriched reduced sediments at three UCRB floodplains. Noncrystalline U(IV) is the dominant form of accumulated U, but crystalline U(IV) comprises up to ca. 30% of total U at some locations. Differing susceptibilities of these species to oxidative remobilization can explain this variability. Particle size, organic carbon content, and pore saturation, control the exposure of U(IV) to oxidants, moderating its oxidative release. Further, our data suggest that U(IV) can be mobilized under deeply reducing conditions, which may contribute to maintenance and seasonal variability of U in groundwater plumes in the UCRB.

Sulfidation mechanisms of Fe(III)-(oxyhydr)oxide nanoparticles: a spectroscopic study

We used synchrotron-based X-ray absorption spectroscopy, transmission electron microscopy, and wet chemical analyses to study the sulfidation mechanism(s) and sulfur oxidation products from the reaction of ferrihydrite, goethite, and hematite nanoparticles with dissolved sulfide at different S/Fe molar ratios under anaerobic condition. Our results suggest that surface area alone does not explain the differences in reactivity of Fe(III)-(oxyhydr)oxide nanoparticles with dissolved sulfides; differences in atomic-level surface structure are also likely to play an important role. The higher reactivity of ferrihydrite leads to a faster sulfidation rate compared to that of goethite and hematite. We found that polysulfides as well as elemental sulfur are the major reaction products in the sulfidation of all three Fe(III)-(oxyhydr)oxide nanoparticles studied. We also found that thiosulfate and sulfate formed during the sulfidation of goethite and hematite but did not form in the case of ferrihydrite, suggesting that the slower reaction kinetics of goethite and hematite favors the formation of solid-phase thiosulfates and elemental sulfur in our experiments. In addition, our results revealed that the S/Fe ratio is a critical variable in the sulfidation reaction. Iron dissolution rates for ferrihydrite, goethite, and hematite nanoparticles were found to increase up to a S/Fe ratio of ≤0.5 and decline above this ratio, suggesting formation of FeS species. Similarly, Fe dissolution rates increased with increasing S/Fe ratios and remained an order of magnitude higher for ferrihydrite than for goethite and three times higher for ferrihydrite than for hematite. Sulfur-K-edge X-ray absorption near edge structure (XANES) spectroscopy revealed for the first time the mass distribution of these solid-phase sulfur oxidation products. In addition, we used Fe-K-edge XANES and extended X-ray absorption fine structure (EXAFS) spectroscopic analysis to follow the kinetics of FeS formation for the three types of Fe(III)-(oxyhydr)oxide nanoparticles, with varying S/Fe ratios. Ferrihydrite transformed completely to FeS in our experiments, but only 58% of the goethite and only 18% of the hematite transformed to FeS. These results have important environmental implications for Fe- and S-redox cycling and contaminant mobility and provide experimental evidence for the impact of S/Fe ratio on contaminant mobility in the systems studied, either by releasing surface-sorbed contaminants due to Fe(III)-reductive dissolution at lower S/Fe ratios or by trapping or co-precipitation of contaminants with FeS precipitation at higher S/Fe ratios.