Analysis of heavy metal concentrations in selected species of genus tephrosia in South Eastern and Coastal regions in Kenya

Abstract

About 70-80% of the world population relies on herbal products, especially in the developing countries. Herbal products are available in various dosage forms for different types of ailment. The genus Tephrosia is highly used worldwide as a remedy for numerous ailments, including malaria, cancer, healing bleeding piles and wounds. Contamination of herbal medicines by heavy metal is of great concern worldwide. Heavy metals are known to affect the normal physiological function of plants or animals at elevated concentrations. In this study, the level of six heavy metals (Cd, Cr, Cu, Fe, Pb, Zn) were determined in soils and plant parts (i.e. aerial part and root) of nine selected species of the genus Tephrosia: namely T. purpurea, T. villosa, T. hildebrandtii, T. holstii, T. polyphylla, T. pumila, T. subtriflora, T. elata, T. aequilata growing in South Eastern (Kilungu and Emali) and Coastal regions (Manyani, Kilifi and Ukunda) of Kenya. Soil properties such as pH, organic matter and soil texture were also determined for soils from the study sites. The accumulation of the metals from soil to plants was also calculated in terms of Enrichment Factor (EF), Translocation Factor (TF) and Bioaccumulation Factor (BF). Plant samples were collected from the selected sites, washed, air- dried and milled. The samples were then digested by wet digestion method and analyzed by Atomic Absorption Spectrophotometric technique using Spectra AA-10. Results showed that the medicinal herbs contained heavy metals namely: iron, chromium, copper, zinc and lead. The concentration (mg/kg) ranges of heavy metals in the aerial part were found to be as follows: chromium (0.06 to 1.49), iron (308.03 to 2077.57), copper (5.06 to 9.11), zinc (7.58 to 20.77), cadmium (≤ 0.006), and lead (0.21 to 7.18). The concentration (mg/kg) ranges of heavy metals in the roots were found to be as follows: chromium (<0.01 to 1.56), iron (83.23 to 2104.9), copper (3.32 to 14.07), zinc (3.1 to 29.22), cadmium (≤ 0.006), and lead (≤ 0.02 to 10.85). The concentration (mg/kg) of heavy metals in the soil sample was found to be as follows: chromium (0.07 to 119.14), iron (2438.9 to 12, 485. 57), copper (1.33 to 23.71), zinc (2.28 to 28.40), cadmium (≤ 0.006), and lead (0.33 to 4.53). The bioaccumulation factor ranged from 0.00 to 7.42 with chromium recording the lowest BF and lead recording the highest BF at Manyani in T. pumila. Enrichment factor ranged from 0.00 to 5.33 with chromium recording the lowest EF and zinc recording the highest EF at Kilifi in T. purpurea. Translocation factor ranged from 0.00 to 16.67. BF, TF, and EF ranged from metal to metal. Among the soil properties, pH ranged from 4.70 to 7.71, whereas the percentage Total Organic Carbon ranged from 0.68 to 1.49. There was a negative correlation between soil pH and the mean heavy metal concentrations; a positive correlation between the TOC and the mean heavy metal concentrations. From the comparison of the results with the defined permissible limits, it was concluded that the levels of heavy metals in the herbal plants were within the permissible range for consumed medicinal herbs as defined internationally by the three different agencies namely the USEPA, Codex Alimentarius Commission and WHO/FAO. Further studies could be conducted to determine the metal speciation in the plant preparations since toxicity of metals varies with oxidation states. To reduce bioaccumulation of heavy metals in human, a regular monitoring programme is recommended to assess heavy metals in herbal medicines. Areas with high levels of heavy metals such as Emali should not be used as sources for herbal medicine. These data are of immediate interest to herbalists and medicinal practitioners in Kenya. Further, awareness creation activities should be promoted to sensitize the physicians, and the general public about the risk associated with consumption of herbal drugs contaminated with heavy metals.