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

酸性矿山排水工程(AMD)是通过化学和细菌催化氧化黄铁矿和其他硫化物源岩，从废岩中浸出的酸性、铁、硫酸盐和重金属对水的污染。据估计，仅在美国，废弃矿场就有450亿吨废石，导致数千公里的溪流和水生栖息地受到污染，生物多样性严重丧失(General Account Office，1996)。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，挖掘和拆除、添加石灰)。此外，这些地点中的许多都很小，而且分散在山区；因此，在安装反应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值下，黄铁矿氧化菌--典型的嗜酸、自养和好氧菌株，如氧化亚铁硫杆菌--催化产酸。有人建议研究通过促进本地异养菌种群的生长来抑制岩石-水界面上的黄铁矿氧化，异养菌将消耗氧气并释放有机成分，这些有机成分能够络合金属，捕获沉淀物，最终引起产生碱度的反应，如厌氧铁(III)和硫酸盐呼吸作用，阻断活性部位，并使长期的部位恢复。这一探索性研究项目旨在了解岩石-水界面的微生物群落结构和基本生物地球化学与废石层中的流动和输送过程的耦合。将进行单一碎石和废石柱规模的实验，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，进而影响氧气消耗、铁和硫循环以及金属淋溶。我们还将研究水文变量(如地层饱和条件、滞水时间)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生成的影响。除了了解岩石介质、排水水流、微生物群落和地球化学反应之间的相互关系外，我们的实验还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传输模型，以量化单一岩石尺度上的基本微生物-地球化学-水文过程，并为扩大到岩柱和野外尺度提供一个框架。这个模型将被用来设计碳添加和修复战略，用于试点领域的应用。据估计，美国西部有超过10万个废弃矿场产生酸性矿山废水。这项研究成果的工程应用将是一种新的成本效益高的新工艺，通过局部添加良性的可溶有机基质来长期修复产酸废石。受AMD影响的流域将给栖息地和当地社区带来相关好处，使其能够进行娱乐、旅游和可靠的水资源。由于设想的工艺设计是基于基本的生物地球化学和流动工艺，这样的修复策略可以进行修改，以应用于各种产酸现场。在科罗拉多州和其他拥有历史采矿活动的山区州，AMD是一个重要的公众关注的话题。在有大量AMD排放的流域中，有两个当地利益攸关方团体已经与科罗拉多大学的研究人员建立了联系，并表示对可持续的场地恢复方法感兴趣，这些方法可以最大限度地减少对社区的破坏，并且可以低成本实施。然而，目前对所提出的恢复方法尝试田间应用的基本机制了解太少。这项探索性研究的结果将使碳添加的实地试验成为可能，并使大学研究人员、矿业公司、公共土地机构和受AMD影响的流域的居民能够合作。最后，学院调查人员致力于通过参与科罗拉多大学博尔德分校针对未被充分代表的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.