Mercury stable isotope geochemistry as a tool for tracing sources and chemical transformations in the environment
Mercury (Hg) is a redox active global contaminant. Hg has two stable oxidation states in the environment, Hg(0) and Hg(II). Hg(0) is significantly less soluble than Hg(II) and is less reactive. Hg(II) is very soluble and highly reactive; being able to form methylmercury, a potent neurotoxin. Hg is a health concern due to its neurological effects, ability to cross the blood-brain barrier and umbilical cord in pregnant women, and get biomagnified. Anthropogenic inputs of Hg into the environment are dominantly due to coal burning and artisanal gold and silver mining. Hg stable isotope ratios have been developed since the early 2000s in order to understand Hg cycling in the environment. The three studies reported in this dissertation aim at developing Hg isotope ratios as tools for source apportionment, identifying Hg chemical transformations in a contaminated environment, and understanding the interaction between dissolved Hg(0) and Hg(II). In the first study, Hg isotopes were used to apportion different Hg sources into Emory and Clinch Rivers in Tennessee, USA. Following a coal ash spill into the Emory River, elevated Hg concentrations were detected; however, there is a known Hg contamination entering the Clinch from upstream of the spill extent. Hg isotope ratios were used to apportion all the Hg inputs into the Emory and Clinch Rivers. The second study attempted to use Hg isotopes in order to detect and quantify natural chemical transformations of Hg in a contaminated creek. The East Fork Poplar Creek is a contaminated creek that runs through the DOE Y-12 plant in Oak Ridge, Tennessee, USA. The Y-12 plant historically used large amounts of Hg which released into the creek and the surrounding floodplain. Hg isotopes were used to identify the probable chemical transformations occurring naturally in the creek’s waters and quantified them. Finally, laboratory experiments were conducted in order to better understand the interaction between dissolved Hg(0) and Hg(II). Hg(0) and Hg(II) species are expected to be in contact in any environment in which Hg is getting actively chemically transformed; however, no studies have looked into the interaction between these two dissolved species. The results of these exchange experiments show that Hg(0) and Hg(II) have the ability to exchange isotopes very quickly, which will have implications for interpreting field isotope data.
Mercury, Isotopes, Isotopic exchange, Emory River, Clinch River, East Fork Poplar Creek
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign