Bioaccessibility Studies of Potential Toxic Elements (PTEs) In Medicinal Plants from Different Locations in Accra Metropolis, Ghana

Abstract

Medicinal plants represent an important class of various traditional medicines. Over the years, their usage in primary health care intervention has increased in both developed and developing nations. Nowadays herbs are known to be the storehouses of most potential toxic elements (PTEs). In Ghana, most studies on PTEs have focused on only single environmental compartment to characterize medicinal plant contamination. In the present study, an integrated sampling programme that relates PTEs in herbal plants to natural/pristine (Botanical Garden) and anthropogenic (University of Ghana [UG] Campus and Tema Motorway [TM]) environments was carried out in Accra Metropolis. Fifty-six (56) herbal plants (with 20 different species) were collected from the natural and human impacted localities with their corresponding soils. Concentrations of nine (9) PTEs (Ni, As, Cd, Pb, Zn, Fe, Cu, Mn and Cr) were determined after acid digestion of both plant and soil samples. Metal fractionation through a modified three-step European Bureau of References (BCR) extraction procedure and Simple Bioaccessibility Extraction Test (SBET) were employed to evaluate the mobility, bioavailability and bioassessibility of the PTEs. The study revealed a high degree of ethnobotanical novelty of plants at the various sampling locations of which many Ghanaians use as traditional folk medicine. Analysis of the plants yielded 18 genera and 15 families. Euphorbiaceace, caesalpinaceae, phyllanthaceace and solanaceae, each had 2 medicinal plant species (13.33%). The remaining families gave single plant species (6.67%). The physicochemical parameters and PTEs found in the medicinal plants varied greatly among the different environments. The wide variations in PTEs concentrations in the analyzed herbs were attributed to differences in the plant metal uptake and translocation capabilities. Medicinal plants harvested from Botanical Garden had the least metal contamination (57.27%) whereas UG Campus and TM were 70.00% and 86.67% respectively far greater than WHO/FDA international specifications. Fe had the highest concentration among all the 9 PTEs investigated in the medicinal plants followed by Mn and Zn. Soils collected from the various locations on which the herbs were picked were found to contain all the PTEs (Cu, Mn, Cd, Pb, Zn, Cr, Ni, and Fe) studied except As. Comparing the overall mean concentrations with EU soil quality guidelines for urban soils, only Cr exceeded the limit (0.3 ppm). However, the assessment of contamination levels of the soils through pollution and geoaccumulation indices indicated considerable to very high degree of pollution with the maximum value found at site M5 (Tema Motorway). Fractional concentrations of PTEs in the soils through BCR chemical extraction procedure showed different soil-specific patterns for the various availabilities (mobilizable, potentially and effectively bioavailability) with Pb being the least mobile. On the other hand, oral bioaccessibility of the studied PTEs showed variations among the medicinal plants with Fe registering the highest bioaccessible fraction (97.391%) in phyllantus amarus, at UG Campus. Hazard Index (HI) values for all PTEs studied in the medicinal plants indicated no significant risk of non-carcinogenic effects to both adults and children.