Risk assessment of metal(loid)-contaminated soils before and after soil washing

A risk assessment of two different metal(loid)-contaminated soils (J1 and J2) collected from a former refinery site was conducted. The soil samples were sequentially washed up to 16 times with 0.1 N ethylenediaminetetraacetic acid (EDTA), 1 N HNO₃, or 1 N HCl and the residual concentrations of As, Pb, Cu, Cd, Zn, Ni, and Cr were determined. The hazardous indices (HI) for non-carcinogenic risk and carcinogenic risk (CR) to humans were evaluated according to the pseudo-total concentrations of metal(loid)s before and after soil washing. It was found that the most effective extractant was 1 N HNO₃ (J1, 82.2 %; J2, 71.8 %), followed by 1 N HCl (J1, 74.1 %; J2, 53.8 %), and 0.1 N EDTA (J1, 35.4 %; J2. 46.9 %). Washing with 1 N HNO₃, 1 N HCl, and 0.1 N EDTA reduced the HI of soil J1 by 46, 41, and 40 %, respectively and the HI of J2 by 42, 70, and 42 %, respectively. For both soils, the HI and CR exceeded the threshold limits (1 and 1 × 10⁻⁶, respectively) proposed by the US Environmental Protection Agency (US EPA). Conclusively, even after 16 soil washes, both soils required further treatment to reduce the HI and CR to within acceptable limits, which is a prerequisite for land usage for residential and non-residential purposes.