Phytoaccumulation of Heavy Metals from Municipal Solid Waste Leachate Using Constructed Wetland

Abstract

solid waste management is a growing issue in both developed and developing countries. One of the chronic environmental hazards of solid waste is the leachate generation. Safe disposal of leachate, generated either through engineered or un-engineered landfill sites, has become a major environmental problem. Leachate can be highly toxic in nature due to the presence of heavy metals and other toxic pollutants, therefore it needs efficient treatment before disposal. The domains of municipal solid waste leachate treatment have been unattended in developing countries like Pakistan, creating increasingly concerning situation. To that end, the aim of the present research was to study phytoaccumulation of heavy metal of leachate using potential of different hyperaccumulator species the laboratory and in constructed wetland. To enhance the efficiency of metal removal different adsorbents were used also substrates in the constructed wetland. Moreover, wetland was operated in batch and continuous flow modes at various retention times and loading rates. The experiments of this study were carried out in two phases. The first phase consisted of four lab-scale experiments whereas the second phase constituted the pilot scale investigations. The first laboratory scale experiment was carried out to investigate growth potential of plants (P. australis, T. latifolia and V. zizanioides) in different dilutions of leachate. In the following experiment, the effect of two substrates: crushed brick and steel slag on removal of Cu and Zn was investigated in a lab-scale wetland planted with P. australis and T. latifolia. The percentage removal of Zn and Cu by T. latifolia in crushed brick was 71 and 95%, respectively whereas in the case of steel slag it was 72 and 94%, respectively. P. australis in presence of crushed brick removed 78 and 99% of Zn and Cu, respectively whereas in presence of crushed brick and 73 and 80% in presence of steel slag, respectively. Comparison of percentage removal by plants P. australis and T. latifolia in substrates namely crushed brick and steel slag and chelators namely EDTA and citric acid was also investigated during third lab-scale experiment. At 15 mg L-1 dose of Cu, approximately 90% removal of Cu was observed for both chelators and both plants while at dose of 5 mg L-1, the observed removal was 99% for both the plants spiked with both chelators. Kinetics study for COD and metal removal by plants P. australis and T. latifolia, and the substrates crushed brick, steel slag and limestone as well as by their combination was conducted in fourth lab-scale experiment. The results showed that T. latifolia in presence of brick (r2 = 0.85) and slag (r2 = 0.90) removed Cu significantly. Whereas, Cu removal by P. australis was also efficient (r2 = 0.98) in presence of crushed brick. The pilot scale constructed wetland during the second phase of this study was operated in batch and continuous mode at different COD and metal (Cu, Zn and Pb) loading rates and varying HRTs (21, 14, 35 and 5 days). Wetland comprised of five chambers with an area of 2.15 m2 for each chamber. In batch mode, each chamber of multi-chambered wetland acted as discrete chamber and removal of metal and COD exceeded 90% of the applied concentration in each chamber. Besides, in the case of continuous operation mode, all chambers were inter-connected and removal of more than 90% of applied concentration was achieved in first chamber alone while only 10% of the applied concentration or lesser than that got removed in the following four chambers. The result suggests that in continuous mode, all chambers after first chamber were lightly loaded as more than 90% of applied loads were removed in first chamber.