Willows and Reeds for Biomediation of Landfill Leachate: Redox Potential in the Root Zone

Constructed wetlands can be used for bioremediation of landfill leachate (LL) making it safe to discharge into the environment. Wetland plants (reed and willow), contribute to pollutant removal, particularly of organic and nitrogen loads. Root exudates stimulate microbial activity and elevate oxygen levels in the rhizosphere which promotes nitrification. This study investigated the effects of reed and willow on bioremediation of LL in comparison with an unplanted control by measuring redox potential levels in the rhizosphere of microcosm systems in a greenhouse. Redox potential in the reed rhizosphere was consistently the highest, with the willow rhizosphere consistently the lowest. Redox potential fluctuated in the willow rhizosphere during daylight hours, with large decreases in the morning. Levels of NH4+ decreased dramatically in the first day of the experiment and remained at similar low levels in all tanks. Removal of ammonia took place in the control tank with peaks in NO2- and NO3-, but levels of NO3- remained high. Removal of ammonia was also observed in the reed tank with a peak in NO2-, but there was no peak in NO3-, as well as in the willow tank, but there were no peaks in NO2- or NO3-. Final levels of totalnitrogen, nitrate and chemical oxygen demand where considerably lower in the reed and willow tank than the unplanted control.