Assessment of the stabilization of heavy metal contaminants in soils using chemical leaching and an earthworm bioassay

Soil stabilization is a remedial technique that reduces the exposure of the soil environment to soil contaminants. Its efficacy can be assessed by determining whether the environmental availability of a contaminant decreases following treatment. We evaluated several chemical leaching treatments by assessing both contaminant leachability and bioaccumulation in the earthworm Eisenia fetida, and determined the most effective treatment for achieving soil stabilization. Soil samples contaminated with As, Cd, Cu, Pb, and/or Zn were collected from abandoned mine areas and stabilized by adding limestone and steel slag (5% and 2% w/w, respectively). All leaching and earthworm tests were conducted using both contaminated and stabilized soils. In addition to bioaccumulation in earthworms, several toxicity parameters (number of cocoons, growth changes, and survival rates) were also assessed to determine the effects of the treatments on the earthworms. The study showed that treatment of soil with EDTA-NH4OAc resulted in a significant decrease in contaminant leachability following soil stabilization. There was an increase in survival and growth of earthworms exposed to the stabilized soil compared with those exposed to the non-stabilized soil. Bioaccumulation in earthworms was found to be statistically correlated with the leachability of As by EDTA-NH4OAc. We conclude that limestone and steel slag effectively decreased the availability of heavy metals in the soil and reduced the toxicity to earthworms. Leaching with EDTA-NH4OAc has the potential to be predictive in estimating the bioavailability of As in soils, but further studies are needed if it is to be proposed as a standard method.