Production of Hydroxyl radicals from oxygenation of simulated AMD due to CaCO-induced pH increase.

Many karst areas are impacted by acid mine drainage (AMD) which is characterized by low pH, high concentrations of dissolved Fe(II) and toxic contaminants. During the flow of AMD in karst areas, the increase in pH facilitates the oxygenation of Fe(II). Whereas, the oxidizing capacity for Fe(II) oxygenation in AMD is poorly understood. In light of the recent finding that hydroxyl radicals (OH) can be produced from Fe(II) oxygenation, this study experimentally measured the cumulative concentrations of OH produced from oxygenation of simulated AMD (8.93 mM Fe, pH 3) in the presence of limestone (CaCO). With the increase in CaCO dosages from 0.67 to 2.78 g/L, Fe(II) oxidation rate increased accordingly, but the maximum concentration of cumulative OH appeared at the CaCO dosage of 1.39 g/L, being 59.3 μM within 24 h. The production of OH was mainly attributed to the rise of AMD pH due to dissolution of limestone, rendering the appearance of adsorbed Fe(II) on the newly formed lepidocrocite and ferrihydrite and complexed Fe by carbonate (i.e., siderite). Oxygenation of these Fe(II) species was accountable for the production of OH. An appropriate pH (i.e., 5-6) was required for the moderate rate of Fe(II) oxidation, corresponding to the maximum production of OH. The OH produced from AMD oxygenation can concurrently oxidize the contaminants of arsenic and p-aminobenzenesulfonamide. Findings from this study suggest that the oxidizing impact of OH on contaminants transformation as well as organic carbon mineralization should be concerned for the oxygenation of AMD in karst areas.