Photochemical Mercury Transformations: Influence of Aquatic Conditions and Nanosized Minerals

NON-TECHNICAL SUMMARY: Bioaccumulation of mercury in fish species is the reason for 80% of the fish consumption advisories in the U.S. Globally, anthropogenic sources of mercury emissions exceed natural sources by about a factor of 3. This excess mercury undergoes complex transformations and one fate is bioaccumulation, affecting both wildlife and humans. The southeastern U.S. is one of the areas particularly affected. Study of the environmental fate of mercury has received significant attention over the past couple of decades, and consequently, our understanding has steadily progressed. However, the chemical and biological behavior of mercury is complex and the environments it is found in are diverse. A thorough and quantitative description of mercury transformations in many aquatic systems remains elusive. The general approach of the proposed project is to perform laboratory studies of mercury transformations in aqueous systems with particular attention to quantifying all forms of mercury-containing products that result. The conditions investigated include the effects of light in driving transformations and the role of particles, such as metal oxide nanoparticles, in influencing reaction pathways and products. These effects are generally not focused on as potentially important influences on mercury chemistry. Laboratory findings will also be tested in water and sediment samples obtained around the state. This study should result in an improved understanding of mercury chemistry that will help address existing knowledge gaps and improve our ability to make sound decisions regarding mercury contamination issues as well as better assess environmental effects of natural and anthropogenic nanoparticles in aquatic ecosystems.

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APPROACH: Photochemical experiments will be performed using a light source which approximates natural sunlight and with a monochromator for selected wavelengths. The equipment includes a 450W xenon arc lamp (OSRAM), cooling system, optical filters, and turntable reactor (Ace Glass). Sample containers for the turntable reactor are quartz tubes. Larger batch reactors will be used as needed. Instrumentation currently available for mercury measurements is a Perkin-Elmer FIMS-200 which uses cold vapor generation and atomic absorbance (CVAAS). This instrument and practices proposed for this work are consistent with EPA Method 1631, except that atomic absorption rather than atomic emission is the method of detection. EPA Method 1631 reports minimum detection limits of 0.2 ng/L or 0.2 ppb (in the absence of interferences) and minimum quantification limits of 0.5 ng/L or 0.5 ppb. In our lab, we have demonstrated detection limits of about 2.5 ppb as Hg using the less sensitive CVAAS instrument without the sample concentration steps that may be employed in Method 1631. It is worth noting that most EPA methods are "performance based," meaning laboratories are "permitted to modify the Method to overcome interferences or lower the cost of measurements if all performance criteria are met," and requirements for establishing equivalency are described. The EPA method for methylmercury or monomethyl mercury is Method 1630, which employs distillation and ethylation prior to CVAFS detection. Since approximately 1996, methods similar to EPA Method 1630 have been shown to be subject to artifactual formation of methylmercury during sample treatment steps (see Leermakers et al., 2005 for a summary). Samples containing NOM are among the most susceptible to such artifacts. Therefore, total and monomethyl mercury measurements will be performed based on methods described by Siciliano et al. (2005), and this technique has been used in our lab (Xu 2009). The modifications described by Siciliano use of sulfhydryl-derivatized cotton to preconcentrate methylmercury. While artifactual methylmercury generation has been reported for this process (Celo et al., 2004), Siciliano et al. found this to be 0.0023% or less (small compared to methylmercury levels observed). In the proposed work, artifactual methylmercury formation will be monitored and alternative approaches, such as thiourea extraction of ionic mercury species from several matrices (Vermillion and Hudson, 2007), will be adopted and adapted to this study if appropriate. Elemental mercury will be measured by trapping on gold, followed by CVAAS, using an approach similar to that described by Lalonde et al. (2004). Other variables and analytes (pH, ionic strength, DO, chloride) will be measured using standard techniques such as commercial meters and ion chromatography. Quality control will be practiced through the measurement of replicates, blanks, standard solutions, and standard reference materials. ICP-MS instrumentation has become available recently (approximately 6 months); ultimately, ICP-MS detection of mercury may be adopted.