Heavy metal constituent of medicinal plants: a case study of Moringa oleifera Lam. from selected areas in Accra, Ghana

Background: Anthropogenic activities release metals into the environment that could be absorbed by plants and assimilate into herbal medicines. Objective: This study sought to correlate the outcome of anthropogenic activities that release metals into the environment with levels of metals in medicinal plants using the leaves of Moringa oleifera Lam. (M. oleifera) as a case study. Methods: Leaves of M. oleifera and soil around sampled plants were collected from three locations in the Accra Metropolis: a commercial area, an area with dense vehicular traffic, and a quasi-natural habitat. Samples were analyzed with Energy Dispersive X-ray Fluorescence (ED-XRF) spectroscopy for the presence and levels of arsenic (As), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), vanadium (V) and zinc (Zn). Statistical analyses were performed and the correlation between metal concentrations in leaves and soil was assessed using Spearman’s correlation coefficient. Results: Levels of metals in the leaves and soil samples varied with the site of sample collection. Levels of metals in leaves were significantly lower than permissible limits (p < 0.05) in herbal medicines. Translocation factor (TF) estimates indicated that the M. oleifera leaves exclude the metals under investigation irrespective of the site of collection. However, the TF > 1 in the case of Zn from the quasi-natural habitat signifying bioaccumulation in the leaves. Except for Co, the total concentration of each metal in the soil was not correlated with its concentration in the leaves. Conclusion: The concentration of sampled metals in the leaves and soil varied with the site of sampling and potentially with the anthropogenic activities that release metals into that environment.