Limestone drains to increase pH and remove dissolved metals from acidic mine drainage |
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| Institution: | 1. Hydrologist and ECO Associate, U.S. Geological Survey, 840 Market Street, Lemoyne, PA 17043, USA;2. Wallis Engineering, 119 E. 8th Street, Vancouver, WA 98660, Canada;1. U.S. Geological Survey, 215 Limekiln Rd., New Cumberland, PA 17070, United States;2. Pennsylvania Department of Environmental Protection, Harrisburg, PA 17101, United States;1. Research Institute on Mines and Environment (RIME), University of Quebec in Abitibi-Temiscamingue (UQAT), Rouyn-Noranda, QC, Canada;2. RIME-Polytechnique Montreal, Department of Civil, Geological, and Mining Engineering, Polytechnique Montreal, Montreal, QC, Canada;1. Division of Sustainable Resources Engineering, Graduate School of Engineering, Hokkaido University, Japan;2. Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Japan |
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| Abstract: | Despite encrustation by Fe and Al hydroxides, limestone can be effective for remediation of acidic mine drainage (AMD). Samples of water and limestone (CaCO3) were collected periodically for 1 a at 3 identical limestone-filled drains in Pennsylvania to evaluate the attenuation of dissolved metals and the effects of pH and Fe- and Al-hydrolysis products on the rate of CaCO3 dissolution. The influent was acidic and relatively dilute (pH<4; acidity <90 mg) but contained 1–4 mg·L?1 of O2, Fe3+, Al3+ and Mn2+. The total retention time in the oxic limestone drains (OLDs) ranged from 1.0 to 3.1 hr. Effluent remained oxic (O2>1 mg·L?1) but was near neutral (pH=6.2–7.0); Fe and Al decreased to less than 5% of influent concentrations. As pH increased near the inflow, hydrous Fe and Al oxides precipitated in the OLDs. The hydrous oxides, nominally Fe(OH)3 and Al(OH)3, were visible as loosely bound, orange-yellow coatings on limestone near the inflow. As time elapsed, Fe(OH)3 and Al(OH)3 particles were transported downflow. The accumulation of hydrous oxides and elevated pH (>5) in the downflow part of the OLDs promoted sorption and coprecipitation of dissolved Mn, Cu, Co, Ni and Zn as indicated by decreased UK concentrations of the metals in effluent and their enrichment relative to Fe in hydrous-oxide particles and coatings on limestone. Despite thick (～1 mm) hydrous-oxide coatings on limestone near the inflow, CaCO3 dissolution was more rapid near the inflow than at downflow points within and the OLD where the limestone was not coated. The high rates of CaCO3 dissolution and Fe(OH3) precipitation were associated with the relatively low pH and high Fe3+ concentration near the inflow. The rate of CaCO3 dissolution decreased with increased pH and concentrations of Ca2+ and HCO3? and decreased Pco2. Because overall efficiency is increased by combining neutralization and hydrolysis reactions, an OLD followed by a settling pond requires less land area than needed for a two-stage treatment system consisting of an anoxic limestone drain an oxidation-settling pond or wetland. To facilitate removal of hydrous-oxide sludge, a perforated-pipe subdrain can be installed within an OLD. |
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