Preventing Pyrite Oxidation: A Geomicrobial Strategy for Source Control of Acid Mine Drainage

酸性矿井水（AMD）是指黄铁矿矿物和其他硫化物源岩在化学和细菌催化氧化作用下，从废岩中浸出的酸性、铁、硫酸盐和重金属对水体的污染。仅在美国，废弃矿场估计就有450亿吨废石，导致数千公里的溪流和水生生境受到污染，并严重丧失生物多样性（总审计署，1996年）。目前修复AMD场地和二次污染区域的技术成本高昂，并造成进一步的环境破坏（例如，挖掘和移除，添加石灰）。此外，许多这些地点很小，分散在山区；因此，在安装反应性屏障或湿地等大型处理过程之前，许多溪流都会受到影响。在低pH下，黄铁矿氧化细菌——通常是嗜酸、自养和需氧菌株，如氧化亚铁硫杆菌——催化酸的产生。研究人员提出了一种假设，即通过促进异养细菌的本地种群的生长，可以在岩石-水界面抑制黄铁矿的氧化，这些异养细菌会消耗氧气并释放出能够复合金属的有机成分，捕获沉淀物，并最终引起产生碱性的反应，如厌氧铁（III）和硫酸盐呼吸，阻断反应位点并实现持久的位点恢复。知识价值。本探索性研究项目旨在了解岩石-水界面微生物群落结构和基本生物地球化学与废岩层流动和运输过程的耦合关系。将在单个岩石碎片和废岩柱的尺度上进行实验，以测试添加可生物降解的有机基质是否会导致优势细菌种群从自养代谢转变为异养代谢，从而影响氧气消耗、铁和硫循环以及金属浸出。我们还将研究水文变量（如地层饱和度条件、水滞留时间）和岩石性质（如孔隙度、黄铁矿含量）对异养细菌生长和抑制AMD生成的有效性的影响。除了了解岩石介质、排水流、微生物群落和地球化学反应之间的相互关系外，我们的实验将能够量化抑制细菌黄铁矿氧化所需的碳，并评估碳添加引起的生物地球化学变化的长期可持续性。将开发一个机械反应输运模型，以量化单个岩石尺度上的基本微生物-地球化学-水文过程，并为升级到岩柱和野外尺度提供框架。该模型将用于设计碳添加和补救策略的试点领域规模的应用。更广泛的影响。据估计，在美国西部有超过10万个废弃矿场产生酸性矿井废水。本研究结果的工程应用将是一种新的经济有效的方法，通过局部添加良性可溶性有机底物来长期修复产酸废石。这将给受amd影响的流域的栖息地和当地社区带来相关的好处，从而实现娱乐、旅游和可靠的水资源。由于所设想的工艺设计是基于基本的生物地球化学和流动过程，因此这种修复策略可以修改以适用于各种产酸现场。AMD是科罗拉多州和其他具有历史采矿活动的山地州公众关注的重要话题。在有大量AMD排放的流域，有两个当地的利益相关者团体已经与科罗拉多大学的研究人员建立了联系，并对可持续的场地恢复方法表示了兴趣，这种方法可以最大限度地减少对社区的破坏，并且可以以低成本实施。然而，目前对所提出的修复方法的基本机制了解太少，无法尝试现场规模的应用。这项探索性研究的结果将允许进行加碳的现场试验，并使大学研究人员、矿业公司、公共土地机构和受AMD影响的流域居民能够合作。最后，教师调查人员致力于通过参与科罗拉多大学博尔德分校少数族裔学生的研究项目，如科罗拉多科学、数学和工程多样性倡议，让不同的学生群体参与研究活动。

PROJECT SUMMARYAcid mine drainage (AMD) is the contamination of water by acidity, iron, sulfate and heavy metals leached from waste rock by chemical and bacterially catalyzed oxidation of pyrite minerals and other sulfidic source rock. There are an estimated 45 billion tons of waste rock in abandoned mine sites in the United States alone, resulting in pollution of thousands of kilometers of streams and aquatic habitat and a significant loss of biodiversity (General Accounting Office, 1996). Current technologies for remediation of AMD sites and secondary contamination areas are costly and cause further environmental disruption (e.g., excavation and removal, addition of lime). Moreover, many of these sites are small and dispersed over mountainous terrain; so that many streams are impacted before a large treatment process such as a reactive barrier or wetland can be installed. At low pH, pyrite-oxidizing bacteria - typically acidophilic, autotrophic, and aerobic strains such as Acidithiobacillus ferrooxidans -- catalyze acid generation. Research is proposed to investigate the hypothesis that pyrite oxidation may be inhibited at the rock-water interface by enhancing the growth of native populations of heterotrophic bacteria, which will consume oxygen and release organic constituents that are able to complex metals, trap precipitates and, eventually, give rise to alkalinity-generating reactions such as anaerobic Fe(III) and sulfate respiration, blocking reactive sites and enabling long-lasting site restoration.Intellectual Merit. This exploratory research project aims to understand the coupling ofmicrobial community structure and fundamental biogeochemistry at the rock-water interface with flow and transport processes through waste rock formations. Experiments at scales of a single rock fragment and waste-rock columns will be conducted to test whether addition of biodegradable organic substrate will result in a shift in the dominant bacterial populations from autotrophic to heterotrophic metabolism that, in turn, will impact oxygen consumption, iron and sulfur cycling and metal leaching. We will also investigate the influence of hydrologic variables (e.g. formation saturation conditions, water detention time) and rock properties (e.g. porosity, pyrite fraction) on the effectiveness of heterotrophic bacterial growth and inhibition of AMD generation. In addition to understanding the interrelations between rock media, drainage flow, the microbial community, and geochemical reactions, our experiments will enable quantification of the carbon required to inhibit bacterial pyrite oxidation and evaluate the long-term sustainability of the biogeochemical changes caused by carbon addition. A mechanistic reactive transport model will be developed to quantify fundamental microbial-geochemical-hydrologic processes at the single rock scale and provide a framework for upscaling to the rock column and field scales. This model will be used to design carbon addition and remediation strategies for pilot field-scale applications.Broader Impacts. It has been estimated that there are over 100,000 abandoned mine sites in theWestern US producing acid mine drainage. The engineering application of the results of this research will be a novel cost-effective process for long-term remediation of acid-generating waste rock by localized addition of a benign soluble organic substrate. There will be associated benefits to the habitat and to local communities in AMD-impacted watersheds enabling recreation, tourism, and reliable water resources. Because the process design envisioned is based on fundamental biogeochemical and flow processes, such a remediation strategy could be modified for application at a variety of acid-generating field sites. AMD is a topic of significant public concern in Colorado and other mountain states with historic mining activity. There are two local stakeholder groups in watersheds with significant AMD discharges that already have ties to researchers at the University of Colorado and have expressed interest in sustainable site restoration methods that minimize disruption of their communities and can be implemented at low cost. However, at this time, too little is understood about both the fundamental mechanisms of the proposed restoration method to attempt field-scale applications. The results from this exploratory research would allow a field trial of carbon addition and enable collaboration of university researchers, mining companies, public land agencies, and residents in AMD impacted watersheds. Finally, the faculty investigators are committed to involving a diverse student population in research activities by participating in research programs for underrepresented minority students at the University of Colorado, Boulder such as the Colorado Diversity Initiative in Science, Math and Engineering.