Evidence of chromium dust pollution on the leaves of food and medicinal plants from mining areas of Sekhukhuneland, South Africa

Abstract

Leaves of food and medicinal plant species occurring in the vicinity of chromium (Cr) mines and ferrochrome smelters in Sekhukhuneland, South Africa, were investigated for evidence of Cr dust deposition. Leaf morphology was assessed to identify traits that could enhance dust adhesion. Energy Dispersive X-ray Spectroscopy (EDS) and Scanning Electron Microscopy (SEM) analysis techniques were employed. EDS analysis confirmed Cr on leaf surfaces of 67% of species investigated and recorded Cr in up to 43% of analysed dust particles found on these species. Cr was detected in PM2.5 (0.1‒2.5 µm), PM10 (2.5‒10 µm) and PM > 10 µm. Assessment of leaf macromorphology indicated significantly higher amounts of EDS detected Cr (weight percentage, wt%) on the adaxial surface (p < 0.001) and larger Cr particles on both surfaces (adaxial p < 0.001, abaxial p < 0.05) of larger leaves. Evaluation of SEM images of leaf surface textures revealed that longer trichomes increased total dust deposition on the adaxial leaf surface significantly (p < 0.05). Cr particles were detected on leaf surfaces of 89% of species sampled from localities within 5 km from the nearest mine, indicating that proximity to pollution source enhances Cr particle contamination. The study concludes that plants with larger leaves growing in closer proximity to pollution emitters are more susceptible to Cr dust deposition. Evidence provided here confirms Cr dust deposition on the leaves of useful plants, which therefore necessitates that such leaves should be washed thoroughly with clean water before use, even though this may not remove all particles.