Produce water treatment and discharge is a critical public health issue in most oil producing countries, where efficient and economical treatment options are being sought after. The main focus is centred on the removal of heavy metals and hydrocarbon residues. 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 three serially connected horizontal sub-surface flow (HSF) constructed wetlands. Each wetland had only one of the three plant species but all were supplied with wastewater from a common source. From October 2010 to March 2011, the growth performance of the candidate plants was measured, and their accumulation and translocation rates of heavy metals (Fe, Cu, Zn, Pb, Hg) determined using atomic absorption spectrophotometer. 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 was most efficient (p<0.001) at removing Fe, Cu mostly removed by T. latifolia (p< 0.001) whilst T. geniculate appeared to be the best remover of lead (p< 0.021). Both T. geniculata and T. latifolia appeared to remove zinc better than L. flava (p<0.021), but there was no statistical difference in the removal rates of mercury 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 heavy metal contaminated water.

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