Iron and manganese removal in wetland treatment systems: rates, processes and implications for management

Mine water drainage is a significant environmental problem throughout the world and constructed wetlands are being increasingly used to treat such contaminating discharges. Iron and manganese removal within a wetland treatment system at Whittle Colliery, UK which receives alkaline waters, was monitored to determine rates and processes within the different components of the system. In addition hourly samples were taken for a period of 24 h to determine the effect of photosynthetic organisms on metal removal. Significant iron removal occurred, with removal rates of 98% at Whittle. Oxidation of iron to form iron hydroxide precipitates was the dominant removal process in the proximal sections of the treatment system (i.e. oxidation ponds and initial reaches of the wetland), whereas biotic removal processes appeared to become more important in distal parts of the systems, where iron concentrations were much lower. Significant manganese removal was also observed, although this did not become substantial until iron concentrations had fallen below 5 mg/L. The process by which manganese was removed is not clear, but bacterially-mediated oxidation may be involved. Further elucidation of the relative importance of abiotic and biotic removal processes within treatment systems is important in the design and long-term management of constructed systems.