Highly efficient co-removal of zinc and manganese during passive treatment of mine drainage: Mechanisms, microbiology and application

Previous investigations have reported the coeval removal of zinc (Zn) and manganese (Mn) under aerobic circumneutral pH conditions during the passive treatment of mine waters, but the removal mechanisms are less well established. Here 7 months of data are presented on Mn and Zn removal from a field based passive mine water treatment trial on mine water of pH 5.42, containing Mn 1.5 mg/l, and Zn 0.8 mg/l. The final treatment train comprised two aerobic vertical flow reactors (VFRs) operated in series, the first with granite/pyrolusite media and the second with limestone media (retention times ∼19 h and ∼ 3 h respectively). Only limited Mn and Zn removal was observed in the granite/pyrolusite VFR reactor. When limestone vertical flow reactors were added to the treatment train after month 4, Mn removal was quickly established with coeval removal of Zn. The Mn concentrations decreased from a mean of 1169 μg/l and 1154 μg/l in the influents to the two limestone reactors, to 245 μg/l and 234 μg/l in the effluents, respectively. Zn concentrations decreased from a mean of 791 μg/l and 935 μg/l (influent) to 228 μg/l and 236 μg/l in the effluents, respectively, with effluent concentrations on occasion reaching as low as 15 μg/l. Detailed analyses of the precipitates bulk and surficial chemistry indicate Zn removal in association with manganese oxides (MnOx) accreted on the surface of limestone as the predominant removal mechanism. Sequential extraction data indicates a significant proportion of Zn also being associated in the “adsorbed/carbonate-associated” phase. Interestingly, based on the difference in influent and effluent concentrations, the overall ratio of Mn:Zn removal was close to 2:1 suggestive of the precipitation of a MnZn oxide mineral directly from the mine water. Microbial community analyses reveal distinct structure in the accumulated ochreous sludge in the granite/pyrolusite and on the grains of limestone in the limestone reactors. The granite/pyrolusite VFR sludge was populated with iron oxidising microbes including Gallionella and Pseudomonas, of which Pseudomonas can also oxidize Mn. The limestone reactors were populated with several genera of Mn oxidising bacteria including Pseudomonas and Leptothrix, suggesting biotic mechanisms are important. These data reveal that Zn may be removed to concentrations was low as 15 μg/l when the Zn is co-removed with Mn within limestone reactors. This opens opportunities for this mechanism to be exploited within nature-based passive treatment systems for removal of Zn.