Integrating Methods for Characterizing the Passive Treatment of Mercury and Selenium in Groundwater and Sediment
Gibson, Blair Donald
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Standard geochemical analysis methods, such as aqueous geochemistry analysis and mineralogical analysis, frequently are utilized to evaluate the effectiveness of passive treatment systems, though they do not necessarily provide information regarding the mechanism of removal. Two emerging analytical techniques have shown promise by providing additional information to improve characterization of treatment systems: X-ray absorption spectroscopy (XAS) and stable isotope analysis. In this thesis, these novel analytical techniques were integrated with standard geochemical measurements to better characterize contaminated sites as well as potential treatment technologies used to mitigate aqueous contaminant mobility. Laboratory experiments were used to evaluate the removal of Se(VI) form simulated groundwater using granular Fe0 (GI) and organic carbon (OC). Greater than 90 % removal of Se(VI) was observed for systems containing GI after 5 days of reaction time and only 15 % removal was observed in systems containing OC. Synchrotron radiation-based XAS analysis of the treatment materials indicated the presence of both Se(IV) and Se(0) on the edges of GI grains after 6 hours reaction time, with no evidence of oxidized Se after 5 days of reaction. Several analytical techniques were integrated to characterize sediment contaminated with Hg and other contaminants through previous industrial practices. Analysis of the sediment by XAS indicated the possible presence of mercury selenide and copper sulfide. Resuspension tests were performed in oxic and anoxic conditions to simulate the effects of changing geochemical conditions of Hg release from sediments during dredging operations. The results indicated a higher release of Hg under oxic conditions in some sediment locations, suggesting that oxidative degradation of organic carbon or oxidative dissolution of Hg sulfides contributed to Hg release. The treatment of aqueous Hg(II) was evaluated with a variety of treatment media, including clay and GI. Treatment with GI was rapid, with 90 % removal observed after 2 hours reaction time. Extended X-ray absorption fine structure (EXAFS) analysis indicated the presence of Hg-O bonding on GI, suggesting that Hg was bound to Fe oxides formed on the surface of corroded GI. A new conceptual model for tracking the stable isotope fractionation of sulfur was coupled to the reactive transport model MIN3P to determine the effects of secondary transformations on sulfur cycling in passive treatment systems. Minor differences were noted when comparing the transport model-derived fractionation factor to calculations using a simplified Rayleigh distillation model, possibly indicating the effect of SO4 precipitation. The incorporation of stable isotope modeling provides a framework for the modeling of other isotope systems in treatment technologies.