Collaborative Research: Investigation of the Effects of Organic Matter and Sulfur in the Environmental Fate of Mercury

美国淡水中超过80%的鱼类消费建议是由汞引起的。这些汞大部分是由煤的燃烧释放出来的，并从离源远近的大气中沉积下来。一旦沉积，汞对食物网的威胁取决于离子汞转化为(1)甲基汞的程度，甲基汞是一种从浮游生物积累到鱼类并最终进入人类的汞形式，以及(2)单质汞，一种通过挥发从水生系统中去除的汞形式。汞主要通过硫酸盐还原细菌转化为甲基汞，这些细菌在缺氧的水生环境中茁壮成长。这些细菌甲基化汞的能力取决于离子汞的形式——它对硫酸盐还原细菌的可用性是否受到其与有机物的结合或作为硫化汞矿物沉淀的限制。汞通过各种过程的还原转化为单质汞，其中有机物质结合和硫化汞沉淀的影响尚不清楚。这项研究的目的是评估有机物和硫在影响离子汞向甲基汞的转化中的作用，甲基汞是使水生生物获得汞的第一步。这项提议的重点是提高对汞、硫和有机物之间相互作用的理解。这项研究是由两个主要假设驱动的，这两个假设调查了硫循环和氧化还原条件对有机物对汞的反应性的作用。第一个假设考察了硫化物在有机物质中的非生物结合作为有机物质组成的功能，以及金属对新结合的有机硫氧化稳定性的作用。第二个假设涉及有机物质的组成和氧化还原状态对汞从Hg（II）还原为Hg(0)的影响，以及有机物质和无机硫化物中还原硫基团络合对Hg（II）还原的抑制作用。这些假设将通过以下方式进行验证：(1)通过实验室实验和在佛罗里达大沼泽地和阿拉斯加内陆地区进行实地研究，检查硫化物与有机物的结合以及汞（II）到汞(0)的还原，这些地点代表了一系列硫和有机物的条件；(2)通过x射线吸收近边缘结构（XANES）光谱来表征这些材料中的硫和汞。这些努力将通过电化学研究耦合金属-有机物氧化还原反应和高能量分辨率XANES光谱来解析硫官能团而得到加强。研究目标将由一个高中教师培训计划补充，重点是实验室研究和参与正在进行的和新的科学、技术、工程和数学（STEM）教育工作。

Mercury is responsible for more than 80% of the fish consumption advisories in fresh waters of the United States. Most of this mercury is released by the combustion of coal and is deposited from the atmosphere near and far from the sources. Once deposited, the threat of mercury to food webs depends on the extent to which ionic mercury is converted to (1) methylmercury, the form of mercury that is accumulated from plankton to fish and ultimately to humans, and (2) elemental mercury, the form of mercury that is removed from aquatic systems by volatilization. Mercury is converted to methylmercury mainly by sulfate-reducing bacteria that thrive in aquatic environments deprived of oxygen. The ability of these bacteria to methylate mercury depends on the form of the ionic mercury -- whether or not its availability to sulfate-reducing bacteria is limited by its association with organic matter or its precipitation as mercuric sulfide minerals. Mercury is converted to elemental mercury by reduction by a variety of processes for which the influence of organic matter association and precipitation as mercuric sulfide is not well known. The goal of this research is to assess the role of organic matter and sulfur in influencing the conversion of ionic mercury to methylmercury, the first step in making mercury available to aquatic organisms.The focus of this proposal is to improve understanding of the interactions between mercury, sulfur, and organic matter. The research is driven by two main hypotheses that investigate the role of sulfur cycling and redox conditions on the reactivity of organic matter to mercury. The first hypothesis examines the abiotic incorporation of sulfide into organic matter as a function of organic matter composition, and the role of metals on the stability of the newly-incorporated organic sulfur to oxidation. The second hypothesis addresses the effects of the composition and redox state of organic matter on the reduction of mercury from Hg(II) to Hg(0), and inhibition of the reduction of Hg(II) by complexation to reduced sulfur groups in organic matter and inorganic sulfide. The hypotheses will be tested by (1) examining sulfide incorporation into organic matter and reduction of Hg(II) to Hg(0) by organic matter using laboratory experiments and field studies at sites in the Florida Everglades and interior Alaska that represent a range of sulfur and organic matter conditions and (2) characterizing sulfur and mercury in these materials by X-ray absorption near-edge structure (XANES) spectroscopy. These efforts will be enhanced by the use of electrochemistry to study coupled metal-organic matter redox reactions and high energy-resolution XANES spectroscopy to resolve sulfur functionality. The research goals will be complemented by a high-school teacher training program focusing on the laboratory research and participation in ongoing and new science, technology, engineering, and mathematics (STEM) education efforts.