Geochemical Factors Controlling the Fate of Fe, Al, and Zn in Coal-Mine Drainage in the Anthracite Coal Region, Pennsylvania, USA

Abstract

Metal sulfides are exposed to aqueous and oxidizing conditions in coal mines, producing sulfuric acid and dissolved metals which drain into waterways as coal-mine drainage (CMD). This metal-rich effluent impairs the ecologic and economic value of surface waters; however, current CMD remediation options can be invasive and costly. This study characterized how CMDs could change over time based on hydrologic setting (above/below-drainage) and assessed controls on metal concentrations and sludge stability to better guide treatment decisions.The first study evaluated long-term (37 year, 1975-2012) changes in 10 above- and 14 below-drainage discharges in the anthracite region of Pennsylvania. Median Fe and SO42- concentrations decreased by 56% and 33%, respectively, with a median pH increase of 0.5 log units for below-drainage discharges, indicating improvement in water quality. Above-drainage discharges exhibited a median pH increase of 1.6 log units, but no significant change in Fe or SO42- concentrations, suggesting above-drainage discharges have less potential for improvement compared to below-drainage discharges. The second study determined the efficiency of treatment options (aeration, H2O2, control) on metals removal from a net alkaline, anoxic CMD. H2O2 treatment removed all Fe and Al, but minimal Zn, while aeration removed all Fe, and ~80% of Zn and Al. A kinetic-adsorption model built in PHREEQC indicated Zn and Al were adsorbed to hydrous FeIII oxides (HFO). The increase in pH and decrease in aqueous Zn-carbonate complex formation during CO2 outgassing through aeration enhanced Zn adsorption to HFO, while the pH increase favored Al desorption from HFO. In the third study, the control of Fe precipitation rate and water-quality parameters on trace-metal removal and sludge stability was assessed for aeration, H2O2, and control treatments on CMD. Changes in metal concentrations and water-quality parameters suggested pH and pCO2 were more important factors in trace-metal removal than Fe precipitation rate, crystallinity, or sludge density. Extractions indicated lower crystallinity of Fe solids formed during H2O2 treatment compared to aeration and control treatments. Lack of complete dissolution of Zn in easily reducible fractions indicated potential for Zn mineral formation or coprecipitation with crystalline FeIII solids in addition to Zn adsorption to HFO