Efficiency of Different Chemical Extractants in Evaluating Heavy Metal Bioavailability and Ecological Risk in Waste-Impacted Soils: A Comparative Study

Abstract

This study evaluated extraction efficiencies and investigated how the choice of extractant influenced contamination assessments. Soil samples were taken from areas impacted by auto mechanic, paint processing, and abattoir wastes and analyzed for Pb, Cd, Cr, Ni, and V using three extractants: aqua regia (AR), nitric–perchloric acids (NP), and ethylenediamine tetraacetic acid (EDTA). Overall mean concentrations were computed from all samples collected at all waste disposal sites and control sites across depth intervals of 0–20, 20–60, and 60–80 cm. Mean Pb concentrations extracted with aqua regia (AR), nitric–perchloric acids (NP), and EDTA were 1534.0, 1419.0, and 1192.0 mg/kg, respectively. Similar trends were observed for Cd, Cr, Ni, and V, where aqua regia consistently yielded higher concentrations than nitric–perchloric acids and EDTA, reflecting the stronger digestion capability of aqua regia in releasing metals bound to resistant mineral phases. Results were compared with regulatory limits set by the Food and Agriculture Organization (FAO). Pb and Cd exceeded FAO limits in all methods (indicating severe soil degradation), Ni slightly exceeded in aqua regia, but was below the limits in NP and EDTA, while V and Cr remained below permissible limits in all extractions. Heavy metal concentration decreased with soil depth, indicating surface enrichment that heightens exposure risk for plants, humans, and soil organisms. The results indicate that reliance on strong-acid digestion extractants may overestimate health and ecological risks, while dependence on milder extractants may underestimate contamination levels and associated hazards. Using multiple extractants provides a more practical assessment of soil contamination.