Characterizing Chromium Isotope Fractionation During Reduction of Cr(VI): Batch and Column Experiments

Chromium (VI) is a pervasive groundwater contaminant that poses a considerable threat to human health. Remediation techniques have focused on the reduction of the highly mobile Cr(VI) to the sparingly soluble, and less toxic, Cr(III) species. Traditionally, remediation performance has been evaluated through the measurement of Cr(VI) concentrations; however, this method is both costly and time-consuming, and provides little information regarding the mechanism of Cr(VI) removal. More recently, Cr isotope analysis has been proposed as a tool for tracking Cr(VI) migration in groundwater. Redox processes have been shown to produce significant Cr isotope fractionation, where enrichment in the ⁵³Cr/⁵²Cr ratio in the remaining Cr(VI) pool is indicative of a mass-transfer process. This thesis describes laboratory batch and column experiments that evaluate the Cr isotope fractionation associated with the reduction of Cr(VI) by various materials and under various conditions. Laboratory batch experiments were conducted to characterize the isotope fractionation during Cr(VI) reduction by granular zero-valent iron (ZVI) and organic carbon (OC). A decrease in Cr(VI) concentrations was accompanied by an increase in δ⁵³Cr values for the ZVI experiments. Data were fitted to a Rayleigh-type curve, which produced a fractionation factor α = 0.9994, suggesting a sorption-dominated removal mechanism. Scanning electron microscopy (SEM), X-ray absorption near-edge structure (XANES) spectroscopy, and X-ray photoelectron spectroscopy (XPS) indicated the presence of Cr(III) on the solid material, suggesting that reduction of Cr(VI) occurred. A series of batch experiments determined that reaction rate, experimental design, and pre-treatment of the ZVI had little to no effect on the Cr isotope fractionation. The interpretation of isotope results for the organic carbon experiments was complicated by the presence of both Cr(VI) and Cr(III) co-existing in solution, suggesting that further testing is required. A laboratory column experiment was conducted to evaluate isotopic fractionation of Cr during Cr(VI) reduction by OC under saturated flow conditions. Although decreasing dissolved Cr(VI) concentrations also were accompanied by an increase in δ⁵³Cr values, the isotope ratio values did not fit a Rayleigh-type fractionation curve. Instead, the data followed a linear regression equation yielding α = 0.9979. Solid-phase analysis indicated the presence of Cr(III) on the surface of the OC. Both the results of the solid-phase Cr and isotope analyses suggest a combination of Cr(VI) reduction mechanisms, including reduction in solution, and sorption prior to reduction. The linear characteristic of the δ⁵³Cr data may reflect the contribution of transport on Cr isotope fractionation

Sequestration of Zn into mixed pyrite-zinc sulfide framboids: A key to Zn cycling in the ocean?

Zinc (Zn) is an important micronutrient in the ocean, and fixation of Zn into organic, trace element-rich sediments is an important contributor to Zn cycling in the ocean. Framboidal sulfides are considered to be the major host for Zn in such settings. The sequestration of Zn into framboids via biotic or abiotic processes is not fully understood, which presents difficulties for interpretation of Zn isotope values in sediments. In this work, we describe a novel type of framboid with mixed pyrite and zinc sulfide (sphalerite or wurtzite) microcrystals from meta-pelites of the Otago Schist, New Zealand. A combination of optical microscopy, scanning electron microscopy (SEM) and nanoscale secondary ion mass spectrometry (NanoSIMS) were utilized to assess the association between Zn, pyrite and organic matter in framboids. The distribution of Zn in framboids is variable. Most pyrite microcrystals include minor amounts of Zn. Trace Zn is also observed to co-locate with organic matter, which occurs on the boundaries of pyrite microcrystals. Finally, Zn is found as single zinc sulfide microcrystals or zinc sulfide rims around pyrite microcrystals within individual framboids. These textures have not been recorded before, to our knowledge. The sequence of events that sequesters Zn into framboids may affect Zn isotope fractionation from seawater to continental margin sediments

Evaluating zinc isotope fractionation under sulfate reducing conditions using a flow-through cell and in situ XAS analysis

A flow-through cell experiment was conducted to evaluate Zn isotope fractionation during ZnS precipitation under microbially-mediated sulfate-reducing conditions. Synthetic groundwater containing 0.90 mM Zn was pumped through a cell containing creek sediment that was biostimulated to promote sulfate reducing conditions. Real-time, in situ X-ray absorption spectroscopy (XAS) was applied at the Zn K-edge to collect spectra via a Kapton (R) window in the front of the cell over the course of the experiment. Aqueous effluent samples were collected and analysed to determine concentrations of anions and cations, and Zn isotope ratios. The flow rate was increased step-wise during the experiment to modify the residence time and produce changes in the extent of sulfate reduction, which in turn controlled the extent of ZnS precipitation. Greater enrichment in the heavier isotope in the aqueous phase relative to the input solution was associated with more extensive Zn removal. A Rayleigh curve was fit to the isotope data, where epsilon= -0.27 +/- 0.06% ( 2 sigma). Evaluation of Zn isotope fractionation under controlled flow conditions is critical to improve the efficacy of this powerful analytical technique when applied to natural systems or remediation projects in the field

Dual Mechanism Conceptual Model for Cr Isotope Fractionation during Reduction by Zerovalent Iron under Saturated Flow Conditions

Chromium isotope analysis is rapidly becoming a valuable complementary tool for tracking Cr­(VI) treatment in groundwater. Evaluation of various treatment materials has demonstrated that the degree of isotope fractionation is a function of the reaction mechanism, where reduction of Cr­(VI) to Cr­(III) induces the largest fractionation. However, it has also been observed that uniform flow conditions can contribute complexity to isotope measurements. Here, laboratory batch and column experiments were conducted to assess Cr isotope fractionation during Cr­(VI) reduction by zerovalent iron under both static and saturated flow conditions. Isotope measurements were accompanied by traditional aqueous geochemical measurements (pH, Eh, concentrations) and solid-phase analysis by scanning electron microscopy and X-ray absorption spectroscopy. Increasing δ⁵³Cr values were associated with decreasing Cr­(VI) concentrations, which indicates reduction; solid-phase analysis showed an accumulation of Cr­(III) on the iron. Reactive transport modeling implemented a dual mechanism approach to simulate the fractionation observed in the experiments. The faster heterogeneous reaction pathway was associated with minimal fractionation (ε = −0.2‰), while the slower homogeneous pathway exhibited a greater degree of fractionation (ε = −0.9‰ for the batch experiment, and ε = −1.5‰ for the column experiment)

Zinc Isotope Fractionation as an Indicator of Geochemical Attenuation Processes

Isotope ratio measurements have been used to trace environmental processes, especially in subsurface environments. In this study, we evaluated the potential to use zinc (Zn) stable isotope ratios as indicators of attenuation processes, including sorption and precipitation. Zn isotope fractionation was observed during distinctly different precipitation processes. Isotope measurements confirmed an increasing trend in aqueous δ⁶⁶Zn in solution during sphalerite (ZnS) formation, but a decreasing trend in δ⁶⁶Zn during the precipitation of hydrozincite [Zn₅(CO₃)₂(OH)₆] and hopeite [Zn₃(PO₄)₂·4H₂O]. In contrast, time-dependent sorption of Zn onto ferrihydrite at a fixed pH did not cause isotopic fractionation in the solution over the duration of the experiments. These findings suggest potential applications of stable isotope measurements in aqueous environments for determining reaction pathways (e.g., precipitation with common groundwater constituents) leading to Zn attenuation