The removal of heavy metals from an acidic wastewater in treatment wetlands filled with recycled materials

Three lab-scale subsurface flow wetland systems were studied for their efficiencies to provide passive treatment of an acidic wastewater. Built with Perspex and vegetated with Typha domingensis, each system was a hybrid of two treatment units: a vertical flow column followed by a horizontal column. And, each system employed a solid waste material (crushed sea shell, composted municipal green waste, and blast furnace slag) as its main media. A synthetic wastewater was prepared by dissolving sulfuric acid, Pb(CH3COO)2, MnCl2, FeSO4, CuSO4 and ZnSO4 into a distillate water. Experiments were carried out to measure water temperature, pH, conductivity, and concentrations of Cu, Mn, Fe, Pb and Zn in the wastewater, as it passed through each system. The results showed that the pH substantially increased in the wetland columns with the sea shell and slag media, where all five metals were almost completely removed (over 99% removal). Significant and rapid removal of the metals in the vertical flow columns indicated abiotic mechanisms, primarily precipitations as metal (oxy)hydroxides and carbonates triggered by the increase in pH. In contrast, biotic mechanisms (plant uptake and microbe-induced reactions) made little contribution. Among the three wetland media, crushed sea shells were found to be the most effective, as they sustained stable hydraulic conductivity and high metal removal efficiency throughout the experiments. The municipal compost gave the lowest metal removal rates. It was unsuitable to be used as wetland media, unless future studies demonstrate that organic carbon leached from such material can support sulphate-reducing bacteria for ustainable metal removal via biotic routes.