The mining of gold and other metals has been an important economic activity in the United States for over 100 hears. Many of the metals of economic interest are found in ore bodies where the metals are present as sulfide minerals. When these ores are exposed to air and water during mining activities, the sulfides are oxidized and the metals are mobilized in acidic drainage water that can have a pH of less than 2. Inactive and abandoned mining sites are known sources of metal pollution to surface and ground waters. These sites are expected to be long term sources of environmental pollution with acid generation persisting for over 100 years. A fundamental understanding of sulfide ore oxidation in natural environments has a number of important gaps in knowledge and engineered solutions to remediating acid drainage are commonly ineffective and expensive. This project examines the acid rock drainage problem from the perspective of microscale biotic and abiotic reactions occurring on the sulfide mineral surface, coupled with macroscopic flow of water and gases. The project will start at the laboratory scale in an analysis of microbial oxidation of pyrite (FeS2) and arsenopyrite (FeAsS). These results will then be compared with the oxidation efficiency in one dimensional column reactors supplied with air and water to quantify the rates of acid and metal release in an unsaturated reactor. A part of these column studies will be an evaluation of various additives from stopping acid production an metal mobilization at the source, including low molecular weight organic acids as biocides, oxidants that can isolate the mineral surface from oxidation reactions, chemicals that promote in situ plugging by precipitate formation, and suspended solids that could be selectively filtered out at sites of acid production. A non-invasive imaging technique will be evaluated for its ability to determine the location of acid generation and to e valuate the effectiveness of remediation activities. The final two years of this effort will shift the emphasis of the research to a field demonstration of remediation technologies.
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