Influence of sediment quality and microbial community on the functioning capacity of a constructed wetland treating alkaline leachate after 5.5 years in operation

Constructed wetlands (CWs) have been demonstrated as a cost-effective alternative to chemical treatment systems for mine waters, with the microbial communities attributed to promoting carbonation and aiding pH neutralization. However, few data are available for the long-term use of CWs treating alkaline leachates nor the activity of microbes within them. To investigate the feasibility of CW to buffer alkaline pH, a pilot-scale wetland was implemented in 2015 to treat alkaline bauxite residue leachate. After 5.5 years, samples of supernatant water and sediment were taken at 0.5 m increments along the 11 m long wetland. Waters were analysed for pH, EC and metal(loid) content, while sediment was subjected to physico-chemical assessment and element fractionation. Microbial biomass and community were assessed by phospholipid fatty acid analysis (PLFA) and functionality by the Rapid Automated Bacterial Impedance Technique (RABIT). Evidence presented demonstrates that the CW operating for 66 months effectively treats bauxite residue leachate, with reduced influent pH from 11.5 to 7.8. Trace element analysis revealed effective reduction in Al (94.9 %), As (86.7 %) and V (57.6 %) with substrate analysis revealing a frontloading of elevated pH and trace element content in the first 5 m of the wetland. Sediment Al, As and V were present mostly (>94 % of total) in recalcitrant forms. Sediment Na was mostly soluble (48–62 %), but soils were not sodic (ESP < 15 %). Investigations into the microbial community revealed greatest biomass was in the first 5 m of the wetland, where pH, EC and metal contents were greatest. Microbial respiration using endemic Phragmites australis as a substrate demonstrates an ability to cycle recalcitrant carbon sources within a CW system. These novel microbial findings highlight the need for further investigation into the microbial communities in alkaline CWs.