Morgan, Luke Robert (2017-12). MINERAL TRANSFORMATION OF ACID MINE DRAINAGE IN A LIMESTONE ENVIRONMENT, ZIMAPAN, MEXICO. Doctoral Dissertation.
After mining operations extract the desired profitable minerals, the large amount of waste remaining can pose a potential environmental hazard due to high concentrations of heavy metals and acid generated by the oxidation of sulfide minerals. The corresponding unearthed mine waste is commonly stored in large open heaps, exposed to weather, leading to cascading geochemical changes that require additional land reclamation. Such a problem occurred in Zimapan, Mexico, known for its mining of copper, zinc, and lead. Operations for approximately 60 years left over 1 million tonnes of mine tailings leading to acid mine drainage (AMD), consequently now in drastic need of remediation. Zimapan lies in an area of limestone-rich bedrock and sediment, which is commonly ground and used to neutralize acidity. Unfortunately, the region still has acidic leachates present and AMD persists as a problem. The purpose of this research was to better understand mineral transformation, acid generation, and heavy metal mobilization/immobilization in mine tailings in Zimapan, Mexico. This specific region has a unique arid climate and encompasses a limestone geological environment, making it suitable to be a geochemical model applicable for other mine sites around the world with similar environments. The objectives of this study were: 1) to characterize mineral phases and nanoparticles, 2) assemble a comprehensive mineralogical report of the tailing sites located in Zimapan, Mexico, and 3) to identify the arsenic and iron speciation and geochemical processes affected by the carbonate-rich environment. Mineralogical analysis determined the presence of jarosite, plumbojarosite, goethite, ferrihydrite, hematite, galena, lepidocrocite, scorodite, arsenopyrite, pyrite, garnet, albite, sphalerite, calcite, dolomite, gypsum, mica, chlorite, montmorillonite, and vermiculite in mining waste and the surrounding soil. Where the tailings were stored, finding soil and water pH<1 was not rare, but the average was pH 2.5. These highly acidic conditions occurred after the oxidation and dissolution of iron sulfide minerals with sufficient precipitation or moisture, allowing for metal desorption and increasing their mobility. Likewise, the arid environment helped precipitate soluble salts and increase the formation of stable iron oxides that are able to absorb metals. Heavy metal contamination was prevalent, notably: copper, zinc, lead, and arsenic. With a dangerous amount of arsenic (maximum concentration found was 110,000 mg/kg) it was primarily seen in forms of As^1- in arsenopyrite and As^5+ in scorodite and absorbed iron oxides and to other minerals such as jarosite. Dissolution and re-precipitation of solid phases directly affect heavy metal transformations and translocation. Poorly crystalline nanoparticles of iron oxides and silica are easily susceptible to geochemical changes due to their high surface area, structural defects, high surface charge, and variation of elements in their structure. Fortunately, metal contaminants in the tailing waste were incorporated into mineral phases before and after oxidation, or sorbed to oxides after oxidation. Lower concentrations of contamination were found in soluble sulfates, suggesting they are not available for release after their original dissolution and yet still exist in the new precipitate. In the soluble sulfates, samples on average had about 25% Fe, up to 6854 mg/kg Cu, up to 465,104 mg/kg Zn, up to 4544 of As, and up to 670 mg/kg of Pb present. Lead showed low bioavailability, consistently found associated with jarosite phases. Similarly, arsenic was incorporated in arsenopyrite, scorodite, or sorbed to iron oxides. Primary ferrous sulfide minerals, such as pyrite and pyrrhotite, were abundant. There were also large amounts of ferric iron in iron oxides and sulfates such as copiapite, goethite, and jarosite. ?