Remediation of an electroplating contaminated soil by EDTA flushing : chromium release and soil dissolution

Purpose: Remediation of soils contaminated with Cr (as Cr(III) complexes/precipitates and/or Cr(VI) oxyanion) and cationic metals (Cu, Ni, Zn, and Pb) by ethylenediaminetetraacetate (EDTA) flushing has been challenging and rarely investigated. This study aimed to evaluate the efficiency of EDTA flushing for metal extraction of soil from an electroplating site, with a specific focus on chromium release and soil dissolution. Materials and methods: Column flushing tests were performed on a sandy soil contaminated by electroplating activities in the field. Three EDTA concentrations (5, 10, and 20 mM) and flow interruptions were employed to investigate the operation of EDTA flushing. Results and discussion: Results demonstrated that Cr, Cu, and Ni were continuously released along with dissolution of Fe, Al, Mg, and Mn throughout the entire flushing process (up to 600 pore volumes), whereas Zn and Pb removal primarily occurred in the first 50-200 pore volumes. By comparing the Cr and Fe release patterns, the observed Cr release by EDTA flushing possibly resulted from a combination of dissolution of Fe oxides, dissolution of metal-chromate precipitates, and ligand competition for the surface sites (substitution reaction). The latter two mechanisms appeared to be more influential at the early stage. It was also revealed that soil dissolution was predominant, and metal extraction became inefficient at the later stage of flushing, especially with the concentrated EDTA solution. On the other hand, when the flushing process was temporarily paused (i. e., flow interruptions), Cr, Cu, Ni, and Zn concentrations elevated, whereas Pb levels in the effluent decreased, indicating the significance of rate-limited metal exchange of newly formed metal-EDTA complexes. Conclusions: In consideration of EDTA utilization efficiency and potential ecological risks, diluted EDTA solution is recommended for field applications.Department of Civil and Environmental Engineerin