ENHANCED AL AND ZN REMOVAL FROM COAL-MINE DRAINAGE DURING RAPID OXIDATION AND PRECIPITATION OF FE OXIDES AT NEAR-NEUTRAL PH

Net-alkaline, anoxic coal-mine drainage (CMD), containing approximately 20 mg/L of Fe^II, 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, Fe^II oxidation, hydrous Fe^III 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 Fe^II 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.