Subcritical water treatment of explosive and heavy metals co-contaminated soil: Removal of the explosive, and immobilization and risk assessment of heavy metals

Co-contamination of explosives and heavy metals (HMs) in soil, particularly army shooting range soil, has received increasing environmental concern due to toxicity and risks to ecological systems. In this study, a subcritical water (SCW) extraction process was used to remediate the explosives-plus-HMs-co-contaminated soil. A quantitative evaluation of explosives in the treated soil, compared with untreated soil, was applied to assess explosive removal. The immobilization of HMs was assessed by toxicity characteristic leaching procedure tests, and by investigating the migration of HMs fractions. The environmental risk of HMs in the soil residue was assessed according to the risk assessment code (RAC) and ecological risk indices (Er and RI). The results indicated that SCW treatment could eliminate the explosives, >99%, during the remediation, while the HM was effectively immobilized. The effect of water temperature on reducing the explosives and the risk of HMs in soil was observed. A marked increase in the non-bioavailable concentration of each HM was observed, and the leaching rate of HMs was decreased by 70-97% after SCW treatment at 250 °C, showing the effective immobilization of HMs. According to the RAC or RI, each tested HM showed no or low risk to the environment after treatment.