A field pilot-scale study on heavy metal-contaminated soil washing by using an environmentally friendly agent-poly-γ-glutamic acid (γ-PGA).

Abstract

In this study, a farmland contaminated by heavy metals (Cu, Zn, Ni, and Cr) was selected to evaluate the performance of poly-γ-glutamic acid (γ-PGA) on the removal of heavy metals in soil washing. The highest heavy metal concentrations at the contaminated site were Cu: 1180mg/kg, Zn: 1450mg/kg, Ni: 287mg/kg, and Cr: 316mg/kg. Batch experiments designed by Taguchi Method were conducted first to assess the effect of different washing conditions on the removal of heavy metals in laboratory. The results of batch experiments show that factors that affected the removal efficiency of heavy metals was of the order γ-PGA concentration > washing time > liquid/soil ratio > rotational speed. The optimal operating parameters for heavy metal removal were γ-PGA 3.5%, liquid/soil ratio 15/1, washing time 60min, and rotational speed 100rpm. Under the optimal conditions, up to 50.7% of the major target metal, Cu, was removed. Heavy metals in the soil were mainly Fe-Mn oxide bound and organically bound. On-site treatment using the optimal operating parameters caused 54.3% of Cu removal. When the soil was washed 3 times by γ-PGA, the removal efficiency of Cu was improved to 74.3%. After the treatment, the change in soil bacterial number was insignificant, indicating that γ-PGA is an environmentally friendly washing reagent.