Manganese (Mn), the second most abundant transition metal in the lithosphere, is required (as a micronutrient) by all living organisms but can also be an environmental pollutant. Soluble Mn(II) tends to impart a metallic taste to water and, when oxidized and precipitated as Mn(IV), can stain laundry and block water distribution networks. Manganese has also been shown to have a chronic biological toxic effect at relatively low concentrations and an upper concentration of 30 µg Mn(II) L–¹ for surface and ground waters had been imposed as the Environmental Quality Standard in the U.K .. Remediation of Mn(II)-contaminated waters involves the oxidation of soluble Mn(II) to insoluble Mn(IV). Standard chemical (active) methods for Mn removal can be expensive and may produce undesirable by-products. However, some microorganisms can accelerate Mn-oxidation by up to 5 orders of magnitude above that of abiotic oxidation, and may prove useful for passive remediation, such as those that utilize bioreactors. This work aimed to prepare systems based on these organisms to remove Mn(II) from a variety of contaminated waters.
A series of Mn-abundant packed bed bioreactors were developed and tested for their abilities to remove soluble Mn(II) from contaminated waters. Manganese(II)-oxidising microorganisms were sourced from an oligotrophic stream in north Wales. One litre capacity packed-bed aerated reactors were set up that contained Mn-rich, biofilmcoated pebbles taken from the stream. The bioreactor had the capacity to remove 160 µg Mn(II) L-¹ min-¹ from Mn(II)-amended stream water. Two species of Mn(II)- oxidising fungi and one Mn(II)-oxidising bacterial species were isolated from this bioreactor. Biomolecular analysis of community DNA revealed that the fungal isolates comprised ~ 80 % of the initial total eukaryotic community within the bioreactor. In contrast, the bacterial community was much more diverse, and the bacterial isolate accounted for 30 % of the total bacterial community, as assessed by semi-quantitative PCR-based analysis (terminal restriction fragment length polymorphism). The fungi were identified as belonging to the order Pleosporales (Ascomycetes), and one was related (99 % 18S rRNA gene sequence identity) to a known Mn(II)-oxidising fungus. The bacterial isolate was closely related to the α-proteobacterium, Bosea thiooxidans and is the first reported Mn(II)-oxidising member of the Bosea genus. Both fungal and bacterial isolates contained multicopper oxidase genes, but the contribution of these genes to the oxidation of Mn(II) by these isolates was not resolved.
The bioreactor using the stream pebbles was shown to be capable of removing Mn(II) from contaminated waters. Although the manganese-oxidizing microorganisms identified were all heterotrophic, no organic amendment of test (stream) water was necessary to affect Mn(II)-oxidation, suggesting that small concentrations of dissolved organic carbon are sufficient to sustain the activities of the fungi and bacteria. The packed bed bioreactor was used successfully to remove Mn(II) from waters from which other soluble transition metals, aluminium and arsenic had previously been removed using a combined chemical/ biological approach, and also mine water that had been partially remediated by a commercial 'active' (lime-dosed) remediation system. The most suitable support material for the immobilisation of fungal isolates was dolomitic limestone, while the bacterial isolate was successfully immobilised on polyester-mesh bags. Both the immobilised fungus and bacterium were used to set up bioreactors for Mn(II)-remediation, and the fungal reactor proved to be capable of removing Mn(II) (~ 160 µg L-¹ min–¹ of Mn(II)) from water of varying pH (4 to 7). This work provided baseline data that could be used to develop pilot- and full-scale bioreactors that could be cost-effective for remediating mine waters and other surface and ground waters that contain elevated concentrations of soluble manganese.