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 containing similar to 20 mg/L Fe-II and similar to 0.05 mg/L Al and Zn was subjected to parallel batch experiments: control, aeration (Aer 1 12.6 mL/s;.Aer 2 16.8 mL/s; Aer 3 25.0 mils), and hydrogen peroxide (H2O2) to test the hypothesis that aeration increases pH, Fell oxidation, hydrous Fe-II oxide (HFO) formation, and trace-metal removal through adsorption and coprecipitation with HFO. During 5.5-hr field experiments, 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, but decreased to 6.3 for the H2O2 treatment. Aeration accelerated removal of dissolved CO2, Fe, Al, and Zn. In Aer 3, dissolved Al was completely removed within 1 h, but increased to similar to 20% of the initial concentration after 2.5 h when pH exceeded 7.5. H2O2 promoted rapid removal of all dissolved Fe and AI, and 13% of dissolved Zn. Kinetic modeling with PHREEQC simulated effects of aeration on pH, CO2, Fe, Zn, and AL Aeration enhanced Zn adsorption by increasing pH and HFO formation while decreasing aqueous CO2 available to form ZnCOR and Zn(CO3)(2)(2-) at high pH. Al concentrations were inconsistent with solubility control by Al minerals or Al-containing HFO, but could be simulated by adsorption on HFO at pH < 7.5 and desorption at higher pH where Al(OH)4 was predominant. Thus, aeration or chemical oxidation with pH adjustment to similar to 7.5 could be effective for treating high -Fe and moderate-Zn concentrations, whereas chemical oxidation without pH adjustment may be effective for treating high-Fe and moderate-Al concentrations. Published by Elsevier Ltd.