ASSESSMENT OF SELECTED NATIVE PLANTS GROWING ALONG NAIROBI RIVER FOR UPTAKE OF COPPER, ZINC AND CADMIUM

Abstract

Nairobi River has high quantities of heavy metals emanating mainly from industrial and domestic wastes. Phytoremediation is a promising alternative to conventional clean-up methods; however, not enough information is available on plant species suitable for this application, especially in Kenya. Plant screening on contaminated sites is necessary and may lead to the identification of more species. A phytoremediation study was carried out along Nairobi River in six sites; Kikuyu, Kawangware, Chiromo, Gikomba, Njiru, and Fourteen falls. The objective of this study was to ascertain the extent of heavy metal pollution and the potential of Polygonum senegalensis (P. senegalensis), Amaranthus hybridus (A. hybridus) and Eichhornia crassipes (E. crassipes) as phytoremediants. The heavy metals studied were Copper (Cu), Cadmium (Cd) and Zinc (Zn) in the selected native plants (biotic indicators), water and soil (abiotic indicators). The metals were detected using the atomic absorption spectrophotometer (AAS). The observed values of heavy metals in water, soil and plants did not vary significantly (p>0.05) among the sampling sites. The mean concentration of these metals in soil (43.01 ± 0.03 mg/Kg) was higher than the values recorded in water (37.61 ± 0.65 mg/L). The ability of the plants to take up heavy metals (BCF) was evaluated from the ratio of metal concentration in the plants and water. The Bio-concentration factor (BCF) obtained was P. senegalensis: 8.83 ± 0.62, A. hybridus; 8.44 ± 0.59 and E. crassipes: 7.56 ± 0.42. The BCF show that the selected plants accumulate Cu, Zn and Cd from water. The mean concentrations of heavy metals obtained in the plants are: Cu 6.73 ± 0.74 mg/Kg, Zn 16.53 ± 2.59 mg/Kg and Cd 2.57 ± 0.83 mg/Kg. Based on the results observed in the plants, Zn showed the largest accumulation and can be considered as one of the major pollutants of Nairobi River. The results showed differences in accumulation of metals - Zn, Cu and Cd - in different plant organs, roots > stems > leaves. This study showed that P. senegalensis and A. hybridus can accumulate Cu, Zn and Cd even when the concentrations of the metals in the abiotic components (soil and water) of the aquatic environment is low, suggesting that the plants are promising candidates for phytoremediation of aquatic ecosystems polluted with Cu, Zn and Cd.