Medicinal plants could potentially be contaminated with heavy metals during growing in the field, processing and/or handling. These heavy metals may be toxic to humans and cause damage to organs such as liver, kidneys and lungs. Currently, there is no data available on the levels of heavy metals in medicinal plants traditionally used for management of infectious diseases in Malawi.
The aim of this study was to determine the levels of heavy metals; lead, chromium, cadmium, zinc, manganese, iron and copper in selected medicinal plants in Malawi.
Four plants namely, Aeschynomene nyassana Taub., Euphorbia whyteana Baker f., Rhus acuminatissima R. Fern. & A. Fern. and Ericae milanjiana Bolus medicinal plants were collected from Southern parts of Malawi and identified at National Herberium and Botanical Gardens of Malawi.
The leaves and roots were shade-dried at room temperature and grounded into fine powder. Two grams of each sample was transferred into silica crucible and mixed with 5 ml HCl. The mixture was subjected to ashing for 3 hours at 450 °C in a furnace. The ash residues were dissolved in HNO3 and made up to 50 ml in a conical flask. The digested material was analysed in triplicates for presence of metals using Atomic Emission Spectrophotometer.
The results showed that heavy metals were present in all the four selected plants. The levels of metals were within the WHO permissible ranges of lead, chromium, cadmium, zinc, manganese, iron and copper. The percentage recovery of heavy metals from each sample was within the acceptable range of 88 to 103 %. Lead was shown to be the most abundant metal while cadmium was the least abundant in all the plants understudy. A statistical analysis of variance at 95 % confidence level showed significant differences in the levels of metals.
This study has shown that selected medicinal plants could be contaminated with heavy metals. While, the metal concentrations are low in the present study, plants growing in heavily polluted environments could accumulate very high levels of metals. This can cause adverse health effects on people consuming these plants. Therefore, this study recommends screening of medicinal plants for heavy metals prior to usage.
Keywords: Levels of heavy metals, Medicinal plants, Microwave Plasma-Atomic Emission Spectrophotometer