Article pubs.acs.org/est
Soluble Microbial Products Decrease Pyrite Oxidation by Ferric Iron at pH < 2 Tesfayohanes Yacob,* Sachin Pandey, JoAnn Silverstein, and Harihar Rajaram Departments of Civil, Environmental, and Architectural Engineering, University of Colorado, 428 UCB, ECOT 441, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: Research on microbial activity in acid mine drainage (AMD) has focused on transformations of iron and sulfur. However, carbon cycling, including formation of soluble microbial products (SMP) from cell growth and decay, is an important biogeochemical component of the AMD environment. Experiments were conducted to study the interaction of SMP with soluble ferric iron in acidic conditions, particularly the formation of complexes that inhibit its effectiveness as the primary oxidant of pyrite during AMD generation. The rate of pyrite oxidation by ferric iron in sterile suspensions at pH 1.8 was reduced by 87% in the presence of SMP produced from autoclaved cells at a ratio of 0.3 mg DOC per mg total soluble ferric iron. Inhibition of pyrite oxidation by SMP was shown to be comparable to, but weaker than, the effect of a chelating synthetic siderophore, DFAM. Two computational models incorporating SMP complexation were fitted to experimental results. Results suggest that bacterially produced organic matter can play a role in slowing pyrite oxidation.
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INTRODUCTION Weathering of sulfidic mine waste rock can reduce pH to levels where acid mine drainage (AMD) is formed by chemical oxidation of pyrite by ferric iron, producing ferrous iron and sulfuric acid, eq 1. Regeneration of ferric iron by iron oxidizing bacteria maintains the AMD cycle as long as reactive minerals, oxygen, and water are present, eq 2.
Slowing of pyrite oxidation has been shown to result from addition of organic carbon to stimulate the growth of acidophilic and acid-tolerant heterotrophs.7−9 A secondary effect of increased heterotrophic growth is increased internal carbon cycling in the rock ecosystem and resulting interaction of soluble microbial products (SMP) with iron. Addition of cell free extracts from cultures of A. ferrooxidans and Acidiphilium acidophilum reduced pyrite oxidation by ferric iron at pH values of 2.5, possibly by complexation of Fe(III).8 The interaction of organic ligands and metals in various environments is well documented. Low molecular weight organic acids produced by Arthrobacter growing on hornblende promoted iron dissolution from the mineral surface.10 Soluble microbial products from a wastewater anaerobic digester have been shown to complex copper and nickel ions, with low molecular weight fractions, 1−10 kDa, having the highest capacity.11,12 Natural organic matter has also been shown to chelate and complex ferric iron and other metals.13−20 Lower molecular weight fractions (0.1−5 kDa) containing aromatic, carboxylic acid, and amino acid groups have been shown to have the greatest affinity for dissolved iron.14,15 However,
FeS2 + 14Fe3 + + 8H 2O → 15Fe2 + + 2SO4 2 − + 16H+ (1)
4Fe2 + + O2 + 4H+ → 4Fe3 + + 2H 2O
(2)
Mining for metals and coal produces waste rock and soil at an estimated rate of over 15 000 Mt/year. Globally, several hundred billion tons of solid mine waste cover a surface area of approximately 100 million hectares, equivalent to or greater than all the solids displaced by natural processes such as landslides and earthquakes.1 Waste rock at many mine sites is deposited above the tree line where the microbial populations are dominated by chemolithotrophic bacteria, primarily iron and sulfur oxidizing organisms using electrons from iron sulfide minerals and fixing both CO2 and N2.2,3 These primary producers sustain a complex population of heterotrophic organisms whose growth is probably limited by the internal cycling of organic carbon and nitrogen products of the chemolithotrophs.4−6 © 2013 American Chemical Society
Received: Revised: Accepted: Published: 8658
February 22, 2013 May 21, 2013 June 18, 2013 June 18, 2013 dx.doi.org/10.1021/es400828g | Environ. Sci. Technol. 2013, 47, 8658−8665
Environmental Science & Technology
Article
retentate volume ratio was used to ensure proper filtration. The
