###### Abstract  
Packed bed bioreactors were used to remove soluble manganese from a synthetic mine water as the final stage of an integrated bioremediation process. The synthetic mine water had undergone initial processing using a sulfidogenic bioreactor (pH 4.0-5.5) which removed all transition metals present in elevated concentrations (Cu, Ni, Zn and Co) apart from manganese. The aerobic bioreactors were packed with pebbles collected from a freshwater stream that were coated with black-colored, Mn(IV)-containing biofilms, and their capacity to remove soluble Mn (II) from the synthetic mine water was tested at varying hydraulic retention times (11-45 h) and influent liquor pH values (5.0 or 6.5). Over 99% of manganese was removed from the partly processed mine water when operated at pH 6.5 and a HRT of 45 h. Molecular techniques (clone libraries and T-RFLP analysis) were used to characterize the biofilms and identified two heterotrophic Mn-oxidizing microorganisms: the bacterium _Leptoltrix discophora_ and what appears to be a novel fungal species. The latter was isolated and characterized in vitro. manganese; bioremediation; acid mine drainage; Mn-oxidizing microorganisms +
Footnote †: journal: Applied Sciences  
1  
Manganese-Removing Bioreactors and Integration into a Composite Model  
Medular System for Remediating and Recovering Metals from Acidic Mine Waters  
Waterst. Appl. Sci.2021142  
are widely distributed in natural environments and belong to several phyla including _Firmicutes_, _Actinobacteria_, _Bacteroidetes_ and _Proteobacteria_[5]. A number of Mn (II)-oxidizing fungal species belonging to the phyla Ascomycota and Basidiomycota have also been described [6]. Manganese (II) is soluble within a wide pH range and its occurrence in surface water and groundwater can be problematic due to some of its undesirable effects on human health if ingested in high concentrations and on domestic water supplies when it is oxidized to Mn (IV) (e.g., by obstructing water distribution networks). As with many other transition metals, manganese can occur in elevated concentrations in coal and metal mine wastewaters and its removal from these sources is also desirable since, in addition to iron and aluminum, manganese hydrolysis contributes to the total mineral acidity of mine waters [7]. Manganese has some chemical features that make its removal from wastewaters sometimes costly and impractical [1,7,8], including: (i) abiotic oxidation of Mn (II) to insoluble Mn (IV) requires elevated pH (>8), and often large amounts of alkaline material (such as lime; CaO) are required to achieve this; (ii) biological Mn (II) oxidation does not proceed rapidly in presence of Fe (II), and therefore it is not removed efficiently using aerobic wetlands where Fe (II) concentration can exceed 1 mg L\({}^{-1}\) and (iii) the relatively large solubility of its sulfide phase (log Ksp of \(-13.3\) at 25 \({}^{\circ}\)C; [8]) means that, unlike transition metals such as copper and zinc, it is not effectively removed in compost wetlands and sulfidogenic bioreactors. However, aerobic packed bed bioreactor systems colonized by Mn (II)-oxidizing microorganisms have demonstrated the biological removal of soluble Mn (II) from synthetic media [7] and mine waters [9]. [PERSON] and [PERSON] [10] developed a bioremediation process to both recover base metals and mitigate a synthetic, moderately acidic and metal-rich mine water based on the chemistry of that in a working copper mine in northern Brazil. The mine water contained 7.5 mM copper and lower concentrations (<0.25 mM) of other transition metals (Zn, Ni, Co and Mn), and had a pH of 5.0. The process used a single low pH continuous flow packed bed anaerobic sulfidogenic reactor. Excess H\({}_{2}\)S produced by the bioreactor was delivered to an off-line vessel containing the synthetic mine water, removing >99% copper (as CuS) and causing the pH to fall to 2.1, precluding the formation of other metal sulfides that have larger solubility products.