Phytoremediation of Irrigation Water Using Limnocharis Flava, Typha Latifolia and Thalia Geniculata in a Constructed Wetland

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Irrigation water quality is a critical public health issue in most developing countries, where farmers continue to rely on wastewater for vegetable production due to limited access to sources of clean water. Phytoremediation, the technique that utilizes a plant’s inherent ability to accumulate metals, is fast emerging as a relatively cheap and environmentally friendly alternative to conventional wastewater treatment methods. A corollary of this is an urgent need to identify plant species with the appropriate suite of characteristics for phytoremediation. The purpose of this study was to evaluate and compare the phytoremediation potentials of Limnocharis flava, Thalia geniculata and Typha latifolia using a horizontal sub-surface flow (SSF) constructed wetlands. The system comprised a storage tank, sedimentation tank, three parallel treatment columns and an effluent tank. Each column in turn had two rectangular serially arranged cells or ponds connected by inlet and outlet pipes, and were both planted with only one of the three plant species. All cells were supplied with irrigation water from a common source. Duplicate plant and water samples were collected from October 2010 to March 2011, and analyzed for Fe, Cu, Zn, Pb and Hg using the atomic absorption spectrophotometer. Bioaccumulation and translocation factors varied greatly among species for different metals. The results showed substantial accumulation of the trace metals by the plants, with Fe (~1600mg kg-1) and Pb (5.71mg kg-1) as the most and least accumulative metals respectively. L. flava and T. geniculata hyper-accumulated Hg. Mean removal efficiencies ranged from 40-80%, 48-54%, 44-54%, 18-32% and 8-38% respectively for Fe, Hg, Zn, Pb and Cu. The removal efficiencies of the species differed depending on the metal. L. flava, T. latifolia and T.geniculata were most efficient (p < 0.001) at removing Fe, Cu and Pb respectively. Both T. geniculata and T. latifolia appeared to remove zinc better than L. flava (p < 0.021), but there was no statistically significant difference in the removal rates of Hg by the plants. Similar trends were observed for the bioaccumulation factor, which increased substantially with time. The plants accumulated most of the metals in their roots. The findings demonstrate the capabilities of the three phytoremediants for improving the quality of irrigation water used for vegetable production.
A Thesis submitted to the Department of Theoretical and Applied Biology, Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfillment of the requirements for the award of the degree of Master of Philosophy, June-2011