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Vanadium contamination is a growing environmental hazard worldwide. Aqueous vanadate (H x V V O 4 (3−x)− (aq) ) concentrations are often controlled by surface complexation with metal (oxyhydr)oxides in oxic environments. However, the geochemical behavior of this toxic redox-sensitive oxyanion in anoxic environments is poorly constrained. Here, we describe results of batch experiments to determine kinetics and mechanisms of aqueous H 2 V V O 4 − (100 μM) removal under anoxic conditions in suspensions (2.0 g L −1 ) of magnetite, siderite, pyrite, and mackinawite. We present results of parallel experiments using ferrihydrite (2.0 g L −1 ) and Fe 2+ (aq) (200 μM) for comparison. Siderite and mackinawite reached near complete removal (46 μmol g −1 ) of aqueous vanadate after 3 h and rates were generally consistent with ferrihydrite, whereas magnetite removed 18 μmol g −1 of aqueous vanadate after 48 h and uptake by pyrite was limited. Removal during reaction with Fe 2+ (aq) was observed after 8 h, concomitant with precipitation of secondary Fe phases. X-ray absorption spectroscopy revealed V(V) reduction to V(IV) and formation of bidentate corner-sharing surface complexes on magnetite and siderite, and with Fe 2+ (aq) reaction products. These data also suggest that V(IV) is incorporated into the mackinawite structure. Overall, we demonstrate that Fe(II)-bearing phases can promote aqueous vanadate attenuation and, therefore, limit dissolved V concentrations in anoxic environments.

Chromium isotopes are potentially useful indicators of Cr(VI) reduction reactions in groundwater flow systems; however, the influence of transport on Cr isotope fractionation has not been fully examined. Laboratory batch and column experiments were conducted to evaluate isotopic fractionation of Cr during Cr(VI) reduction under both static and controlled flow conditions. Organic carbon was used to reduce Cr(VI) in simulated groundwater containing 20 mg L(-1) Cr(VI) in both batch and column experiments. Isotope measurements were performed on dissolved Cr on samples from the batch experiments, and on effluent and profile samples from the column experiment. Analysis of the residual solid-phase materials by scanning electron microscopy (SEM) and by X-ray absorption near edge structure (XANES) spectroscopy confirmed association of Cr(III) with organic carbon in the column solids. Decreases in dissolved Cr(VI) concentrations were coupled with increases in δ(53)Cr, indicating that Cr isotope enrichment occurred during reduction of Cr(VI). The δ(53)Cr data from the column experiment was fit by linear regression yielding a fractionation factor (α) of 0.9979, whereas the batch experiments exhibited Rayleigh-type isotope fractionation (α = 0.9965). The linear characteristic of the column δ(53)Cr data may reflect the contribution of transport on Cr isotope fractionation.

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Brines are commonly found at depth in sedimentary basins. Many of these brines are known to be connate waters that have persisted since the early Paleozoic Era. Yet questions remain about their distribution and mechanisms for retention at depth in the Earth's crust. Here we demonstrate that there is insufficient topography to drive these dense fluids from the bottom of deep sedimentary basins. Our assessment based on driving force ratio indicates that sedimentary basins with driving force ratio > 1 contain connate waters and frequently host large evaporite deposits. These stagnant conditions appear to be relatively stable over geological time and insensitive to factors such as glaciations, erosion, compaction, and hydrocarbon generation.

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Dissolved Se(VI) removal by three commercially available zero-valent irons (ZVIs) was examined in oxic batch experiments under circumneutral pH conditions in the presence and absence of NO 3 – and SO 4 2– . Environmentally relevant Se(VI) (1 mg L –1 ), NO 3 – ([NO 3 —N] = 15 mg L –1 ), and SO 4 2– (1800 mg L –1 ) were employed to simulate mining-impacted waters. Ninety percent of Se(VI) removal was achieved within 4–8 h in the absence of SO 4 2– and NO 3 – . A similar Se(VI) removal rate was observed after 10–32 h in the presence of NO 3 – . Dissolved Se(VI) removal rates exhibited the highest decrease in the presence of SO 4 2– ; 90% of Se(VI) removal was measured after 50–191 h for SO 4 2– and after 150–194 h for SO 4 2– plus NO 3 – depending on the ZVI tested. Despite differences in removal rates among batches and ZVI materials, Se(VI) removal consistently followed first-order reaction kinetics. Scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analyses of reacted solids showed that Fe(0) present in ZVI undergoes oxidation to magnetite [Fe 3 O 4 ], wüstite [FeO], lepidocrocite [γ-FeOOH], and goethite [α-FeOOH] over time. X-ray absorption near-edge structure spectroscopy indicated that Se(VI) was reduced to Se(IV) and Se(0) during removal. These results demonstrate that ZVI can be effectively used to control Se(VI) concentrations in mining-impacted waters.