Chromium dust deposition on plant leaves in Sekhukhuneland, South Africa, and the importance of leaf traits

Abstract

Chromium (Cr) is a hazardous heavy metal and its highest oxidized state, hexavalent Cr (Cr(VI)), can be carcinogenic. The ultramafic outcrops of Sekhukhuneland, South Africa, which belong to the Rustenburg Layered Suite, are mined heavily for Cr, yet Cr contamination of the local vegetation is understudied. The present study aims to understand if Cr pollution in a mining environment is adsorbed or accumulated by plant leaves with a primary focus on commonly used food and medicinal plant species in Sekhukhuneland. The following techniques were applied to attain the main research objectives: (i) to comprehend the global trend on environmental Cr pollution and Cr accumulation by useful plants, a systematic review was conducted, (ii) to investigate the presence of Cr dust on leaf surfaces, Energy dispersive x-ray spectroscopy (EDS) based elemental analysis of leaf surfaces was performed, (iii) plant morphology, leaf macromorphology and Scanning electron microscope (SEM) imaged leaf micromorphology were studied to assess their influence on foliar Cr dust deposition, (iv) combination of bi- and multivariate statistical techniques were applied to identify Cr sources on leaf surfaces and assess the interaction between plant morphology and Cr pollution, (v) to evaluate the contribution of dust to foliar Cr content and human health risk, total Cr and Cr(VI) was quantified in selected useful plant leaves, and finally (vi) to evaluate the phytoremediation potential of dominant local species, foliar metal bioaccumulation factors were determined. The systematic review suggested a global occurrence of Cr accumulation above the international permissible limits by useful plants, especially in polluted areas. Findings from the present study confirmed human activity sourced Cr dust deposition on the useful plant leaf surfaces. It was found that larger leaves and taller plants could be susceptible to foliar Cr dust deposition. Plant morphological traits, i.e. leaf area, epicuticular wax, stomata size and plant height, and Cr pollution in terms of proximity and number of polluters, however, collectively determine foliar Cr dust deposition. Cr content in leaves of food and medicinal plants exceeded international safe limits by several folds suggesting Cr contamination. Deposited dust contributed significantly towards the total Cr content of leaves (p<0.001). Lifelong carcinogenic health risk was estimated for total Cr and Cr(VI) content of leafy vegetables. Accumulation of several metals in leaves by the local metallophytes especially the indigenous species indicated the prospect of phytoremediation. Tailing soils are major Cr sources including dust borne Cr to local vegetation. Results confirmed Cr enrichment of plant leaves due to Cr dust deposition and Cr accumulation in leaf tissue. Cr pollution and associated human health risk should be of priority concern in Sekhukhuneland. At the same time, extended research on local metallophytes may suggest sustainable techniques to limit further dispersion of hazardous elements from polluted sites.