Interactions of Mercury and Other Metals with NOM: Binding by Dissolved OM, Inhibition of Metal Sulfide Precipitation, and Enhancement of Metal Sulfide Dissolution

汞在水生系统中的生态命运在很大程度上取决于溶解有机物（DOM）的浓度、无机配体（特别是硫化物）的浓度，以及将Hg2+转化为甲基汞的硫酸盐还原细菌的存在，甲基汞是一种极易生物积累的剧毒汞形式。最近的研究表明，在与环境相关的条件下，Hg（II）与DOM的结合（KDOM 1023 L kg-1）比之前认为的更强。DOM对汞的强结合是由一小部分含有活性硫醇官能团的DOM控制的，与其他具有地球化学意义的配体相比，它仅次于硫化物。除了与DOM相关的硫醇类基团对Hg（II）的强结合外，DOM-Hg之间的强相互作用在DOM对相对不溶性朱砂（HgS）溶解和沉淀的影响研究中也很明显。有机质通过表面反应促进HgS的溶解，而富含芳香族的DOM有利于表面反应。低浓度（= 3mg C L-1） DOM通过防止纳米胶体硫化汞的聚集，抑制了metachinnabar （HgS）的沉淀。HgS与DOM的相互作用可以通过维持比当前物种形成模型预测的更高的溶解总汞浓度来影响水生环境中汞的地球化学和生物利用度。在这份提案中，Joe Ryan（科罗拉多大学博尔德分校），Kathryn Nagy（伊利诺伊大学芝加哥分校）和George Aiken（美国地质调查局博尔德分校，科罗拉多州）概述了一项计划，以调查汞-有机质相互作用的更具挑战性的方面。将研究与选定金属（从软到硬）的类似相互作用。需要更好地定义这些相互作用，以改进金属物种形成模型，并增加对水生系统中控制金属循环的因素的了解。本研究的具体目标是：(1)量化在环境相关浓度下，溶解和固体有机物对金属结合的有机物、金属性质和硫化物浓度的影响；(2)阐明有机物中硫、氮和氧官能团对金属结合的贡献；(3)研究有机物性质对金属硫化物沉淀的抑制作用，重点是硫化汞。(4)探讨DOM增强金属硫化物溶解的机理。这项研究的主要成果将是确定在与环境有关的浓度下选定的有毒金属的有机物质结合常数，并评估胶体稳定化作为一种有助于溶解金属在水生系统中出现的过程。拟议研究的更广泛影响除了下列特别活动外，还包括在国家会议和同行评议期刊上传播研究成果。联合pi及其研究助理将与广泛使用的地球化学平衡模型（WHAM）的开发者Edward Tipping博士合作，将新的dom -金属结合常数纳入金属形态计算，并与Alain Manceau博士合作，表征生物修复方案中使用的金属与有机物的竞争结合。合作pi将开设一门关于有机物与污染物相互作用的研究生课程。该研究结果将直接应用于水生生态系统的有效管理，并对生态系统恢复计划具有重要意义。艾肯博士将继续参与美国地质调查局的工作，为管理佛罗里达大沼泽地和加州圣华金-萨克拉门托河三角洲的环境监管机构提供科学便利。

The ecological fate of mercury in aquatic systems depends, in large part, on dissolved organic matter (DOM) concentration, the concentrations of inorganic ligands, especially sulfide, and the presence of sulfate-reducing bacteria that convert Hg2+ into methylmercury, a highly toxic form of mercury that is readily bioaccumulated. Recent research shows that Hg(II) binds to DOM more strongly (KDOM 1023 L kg-1) than previously thought under environmentally relevant conditions. Strong binding of mercury by DOM, which is controlled by a small fraction of the DOM containing reactive thiol functional groups, is second only to sulfide when compared to other ligands of geochemical significance. In addition to strong binding of Hg(II) by thiol-like moieties associated with DOM, strong DOM-Hg interactions are apparent from studies of the effects of DOM on the dissolution and precipitation of relatively-insoluble cinnabar (HgS). Organic matter enhances HgS dissolution through surface reactions favored by DOM rich in aromatic moieties. Precipitation of metacinnabar (HgS) is inhibited by low concentrations (=3 mg C L-1) of DOM by prevention of the aggregation of nanocolloidal mercuric sulfide. Interactions of HgS with DOM can influence the geochemistry and bioavailability of Hg in aquatic environments by maintaining higher dissolved total Hg concentrations than predicted by current speciation models.In this proposal, Joe Ryan (University of Colorado at Boulder), Kathryn Nagy (University of Illinois at Chicago), and George Aiken (U.S. Geological Survey, Boulder, Colorado) outline a plan to investigate more challenging aspects of Hg-organic matter interactions. Similar interactions with selected metals (from soft to hard) will be studied. Better definition of these interactions is required for the improvement of metal speciation models and increased understanding of the factors controlling metal cycling in aquatic systems. Specific objectives of this research are to (1) quantify the effects of organic matter, metal nature, and sulfide concentration on metal binding by dissolved and solid organic matter at environmentally relevant concentrations, (2) elucidate the contributions to metal binding by sulfur, nitrogen, and oxygen functional groups in organic matter, (3) examine the effect of organic matter nature on the inhibition of metal sulfide precipitation, with an emphasis on mercuric sulfide, and (4) probe the mechanism of DOM enhancement of metal-sulfide dissolution. Major products of this research will be the determination of organic matter binding constants for selected toxic metals at environmentally relevant concentrations and the assessment of colloidal stabilization as a process contributing to the occurrence of dissolved metals in aquatic systems.Broader impacts of the proposed research include dissemination of the research results at national conferences and in peer-reviewed journals in addition to the following special activities. The co-PIs and their research assistants will collaborate with Dr. Edward Tipping, developer of a widely used geochemical equilibrium model (WHAM), to incorporate new DOM-metal binding constants into metal speciation calculations and with Dr. Alain Manceau to characterize competitive binding of metals with organic matter used in bioremediation schemes. The co-PIs will develop a graduate class on the interactions of organic matter with contaminants. The information resulting from this study will be directly applicable to the effective management of aquatic ecosystems, and has important implications for ecosystem restoration programs. Dr. Aiken will continue to participate in U.S. Geological Survey efforts to make science accessible for environmental regulators managing the Florida Everglades and the San Joaquin-Sacramento River delta in California.