Removal mechanisms and efficiencies of selected heavy metals in constructed wetlands

The removal efficiencies and mechanisms of Pb, Cr, Cd and Zn removal were investigated in batch type constructed wetlands comprising water hyacinth (Eichhornia crassipes (Mart.) Solms) using a mass balance analysis. This study was conducted for 15 weeks in four 590 l capacity tanks containing 3.1, 5, 0.2 and 1.1 mg/l of Pb, Cr, Cd and Zn respectively. Removal efficiencies from the wastewaters were in the range of 92.1-99.6%, 31.3-100%, 46.3-94.4% and 25.1-94.8% for Pb, Cr, Cd and Zn, respectively. The results showed that water hyacinth roots play a crucial role in the removal of Pb and Cr by means of precipitation, though phytoremediation was insignificant. However, phytoremediation through rhizofiltration seemed to govern the removal of Cd and Zn. We conclude that constructed wetlands comprising water hyacinth is a promising low cost option for removing the above heavy metals within the ranges investigated.