Biogeochemical Cycling of Copper in Acid Mine Drainage

Open Access
Author:
Kimball, Bryn Elizabeth
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 04, 2009
Committee Members:
  • Susan Louise Brantley, Dissertation Advisor
  • Susan Louise Brantley, Committee Chair
  • Jennifer Macalady, Committee Chair
  • Matthew Scott Fantle, Committee Member
  • Brian Dempsey, Committee Member
Keywords:
  • microscopy
  • isotopes
  • iron-oxidizing microorganisms
  • acid mine drainage
  • copper
  • spectroscopy
Abstract:
Metal contamination in surface water and soil environments is largely due to acid rock drainage (ARD) or acid mine drainage (AMD), which results from dissolution of metal sulfide minerals. This dissertation focuses on characterizing the biogeochemical processes that solubilize and sequester copper (Cu) in environments impacted by AMD. Copper isotope measurements are a novel approach to characterizing Cu mobility. A review of published Cu isotope data (Chapter 2) show that, on average, Cu(I)-sulfides and secondary Cu(I)/(II) minerals are depleted in 65Cu (based on 65Cu/63Cu) relative to AMD-impacted streamwater, seawater, and large rivers. Based on published experiments, including those described in this dissertation, aqueous Cu(II) is enriched in 65Cu relative to aqueous and solid Cu(I) phases (Chapter 2). Enrichment of aqueous Cu in 65Cu relative to Cu-bearing minerals is best explained by a redox isotope effect, whereby isotopically heavy Cu(I) in sulfide minerals preferentially oxidizes to Cu(II), which is then released to solution. Experiments also show that Cu isotope fractionation (up to 4‰) due to a redox isotope effect can be overprinted by additional isotope effects that are related to mineralogy, kinetics, ligand complexation, and biological processes, which, in combination, may account for the large variation of Cu isotope composition (Δ65Cu = 12‰) in environmental samples. Chalcopyrite is the most abundant Cu-bearing mineral on earth, and most commonly occurs in hydrothermal sulfide deposits. Under oxic conditions, dissolution of chalcopyrite releases dissolved Cu and Fe and acidifies surrounding solutions. Understanding how this and other sulfide minerals dissolve is important for abating anthropogenic release of metals in association with mining activies. In Chapter 3, chalcopyrite dissolution kinetics are synthesized and analyzed in order to derive empirical chalcopyrite dissolution rate laws. Using multiple linear regression analysis, we determine a rate law for nonoxidative chalcopyrite dissolution and for oxidation of chalcopyrite by Fe(III). Chapter 4 is a survey of Cu isotope compositions in primary chalcopyrite and enargite and leached Cu in an AMD-impacted area in southwestern Colorado, USA. To aid our interpretation of Cu isotope fractionation measured among field samples, we also conducted chalcopyrite and enargite leach experiments in the absence and presence of the Proteobacterium Acidithiobacillus ferrooxidans. When bacteria are present, leached Cu is isotopically lighter than chalcopyrite (∆aq- mino = -0.57 ± 0.14‰, where mino refers to the starting mineral), and isotopically identical to enargite (∆aq- mino = 0.14 ± 0.14‰). The biotic fractionation differences between chalcopyrite and enargite are likely due to differing mineralogy. Even though microorganisms are present at the field site, Cu isotope fractionation in the field resembles that in abiotic chalcopyrite leach experiments. In abiotic experiments, leached Cu was isotopically enriched in 65Cu relative to chalcopyrite (∆aq- mino = 1.37 ± 0.14‰) and enargite (∆aq- mino = 0.98 ± 0.14‰). Mass balance calculations support the likelihood that partitioning of isotopically heavy dissolved Cu into microbial-associated phases is less significant in the open field system compared to the closed laboratory experiments. Copper isotope measurements in abiotic experiments allow us to infer that heavy Cu is preferentially oxidized at the interface between the isotopically homogeneous mineral and a surface oxidized layer. The isotopically heavy oxidized Cu is then solubilized. The result is leached Cu that is isotopically heavier than the starting material during abiotic leaching. During biotic leaching, additional processes likely affect the Cu isotope composition of leached Cu. Based on the hypothesis that interaction of isotopically heavy dissolved Cu with A. ferrooxidans cells caused part of the biotic fractionation measured in sulfide leach experiments (Chapter 4), we characterized Cu association with cells and related precipitates using transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), X-ray fluorescence microscopy (μ-XRF), and micro-x-ray diffraction (μ-XRD). The results, described in Chapter 5, show that within cells, polyphosphate granules have the highest Cu concentration. The μ-XRF observations show that whole cells of A. ferrooxidans grown in 1 mM Cu may contain 0.11 ± 0.01 to 6.19 ± 0.28 fg Cu/cell. Outside of cells, Cu may co-precipitate with jarosite minerals (identified with μ-XRD) that form as a result of cell growth. Incorporation of isotopically heavy Cu into jarosite is consistent with equilibrium isotope fractionation theory and the observed bioleach fractionation measured (Chapter 4). Further work is warranted to determine whether isotopic, chemical, and structural characteristics of jarosite function as a biosignature of microbial Fe-oxidation.