2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 153-4
Presentation Time: 2:20 PM


BURROWS, Jill E.1, CRAVOTTA III, Charles A.2 and PETERS, Stephen C.1, (1)Earth and Environmental Sciences, Lehigh University, 1 W Packer Ave, Bethlehem, PA 18015, (2)Pennsylvania Water Science Center, U.S. Geological Survey, 215 Limekiln Rd, New Cumberland, PA 17070, jillerinburrows@gmail.com

Net-alkaline, anoxic coal-mine drainage (CMD), containing approximately 20 mg/L of FeII, 4 mg/L of Mn, and 0.05 mg/L of Al, Zn, and Ni, was subjected to a control without mechanical aeration, three aeration rates (Aer 1 12.6 ml/s; Aer 2 16.8 ml/s; Aer 3 25.0 ml/s), and another control dosed with hydrogen-peroxide (H2O2). The field experiments were conducted to test the hypothesis that aeration increases CO2 outgassing with consequent increases in pH, FeII oxidation, hydrous FeIII oxide (HFO) formation, and trace-metals removal through adsorption and coprecipitation with HFO. During the 5.5-hr experiments, the pH increased from 6.4 to 6.7, 7.1, 7.6, and 8.1 for the control, Aer 1, Aer 2, and Aer 3, respectively, whereas the pH decreased to 6.3 for the H2O2 treatment. Aggressive aeration accelerated removal of dissolved CO2, Fe, Zn, and Al; H2O2 promoted rapid removal of all dissolved Fe and Al, and 13% of dissolved Zn, but had no effect on dissolved CO2. Concentrations of Mn, Ni, and other trace elements were not affected by the treatments.

Kinetic modeling with PHREEQC simulated the effects of aeration on pH, CO2, Fe, Zn, and Al concentrations, and the importance of adsorption and coprecipitation with HFO. Zn adsorption was enhanced by aeration because of increased pH and HFO formation, and decreased Zn-carbonate complexation. Changes in Al concentration were inconsistent with solubility control by kaolinite, Al hydroxides, Al hydroxysulfates, or Al-containing HFO; therefore, Al adsorption to HFO was hypothesized. During the Aer 3 experiment, dissolved Al was completely removed within 1 hr, but increased to approximately 20% of the initial concentration after 2.5 hr when pH exceeded 7.5. The dissolved Al concentrations in the Aer 3 experiment can be explained by adsorption on HFO at pH <7.5 and desorption at higher pH where Al(OH)4- is the predominant dissolved species. The lack of pH increase in the H2O2 treatment and nearly instantaneous oxidation of dissolved FeII promoted rapid adsorption of Al; however, the H2O2 treatment promoted only limited Zn removal. Thus, chemical oxidation without pH adjustment may be most effective for treating high Fe and moderate Al concentrations in CMD; whereas, aeration or chemical oxidation with pH adjustment to ~7.5 could be effective for treating high Fe and moderate Zn concentrations in CMD.