Contamination of useful plant leaves with chromium and other potentially toxic elements and associated health risks in a polluted mining-smelting region of South Africa

Abstract

• Foliar Al, Co, Cr and Hg concentrations exceeded international permissible levels. • Cr(VI) was detected in unwashed leaves and soils from outcrops and polluted sites. • Among all PTEs, only Cr had a Hazard Quotient value > 1 for unwashed leaves. • Estimated weekly intake of Hg exceeded recommended PTWI. • Carcinogenic risks were high for Cr(VI), total Cr and Ni. Foliar exposure to dust-borne potentially toxic elements (PTEs) is often overlooked in chromium (Cr) mining and industrial regions. A comparative analysis of unwashed (UL) and washed leaves (WL) was conducted to investigate the contribution of dust to Al, As, Cd, Co, Cr (total and hexavalent Cr (Cr(VI)), Cu, Hg, Mn, Mo, Ni, Pb, Sb, V and Zn contents of leafy vegetables and medicinal plants from chromitite outcrops and Cr mining-smelting localities in Sekhukhuneland, South Africa. For leafy vegetables, health risks were estimated for all PTEs except Al and Hg, for which weekly intakes were calculated. Mean total Cr levels in UL and WL exceeded the international permissible levels by up to 22- and 9-fold, respectively, linking foliar Cr contamination to dust deposition and a lesser extent, accumulation in tissue. The most hazardous Cr species, Cr(VI) was detected in soil-plant (UL) systems of outcrops and polluted sites. Unwashed leaves had higher mean concentrations of 71% of the assessed PTEs (except Cd, Co, Hg and Sb) than WL, indicating dust as a source of PTEs. Only Cr had a Hazard Quotient (HQ) > 1 (UL). The HQ of Cr and Co contributed the most to the Hazard Index of UL (1.70) and WL (1.39). Carcinogenic risk was > 1 × 10 ‒4 for Cr(VI) (UL) and > 1 × 10 ‒3 for total Cr (UL > WL) and Ni (WL > UL). Estimated weekly intake of Hg (WL > UL) exceeded Provisional Tolerable Weekly Intake. Lifelong consumption of unwashed and even washed leafy vegetables might be hazardous in the study locality. Regular monitoring of PTE levels in the dust-soil-plant system is necessary for Cr mining-smelting regions.