Electrochemical Remediation of Copper-Contaminated Soils Enhanced by Ethylenediaminetetraacetic Acid: an in-situ X-ray Absorption Spectroscopic Study

Speciation of copper with ethylenediaminetetraacetic acid (EDTA) during electrotreatments of a contaminated soil nearby printed circuit board recycling plants has been studied by in-situ extended X-ray absorption fine structural (EXAFS) and X-ray absorption near edge structural (XANES) spectroscopies. The least-square fits of the copper XANES spectra indicated that the main copper species in the contaminated soil were CuCO3 (41%), CuO (40%) and adsorbed copper (Cu/SiO2) (19%). By EXAFS studies, it was found that the bond distance of Cu-(O)-Si was 3.25 Å with a coordination number (CN) of 1.0 in the second shells, suggesting a chemical interaction between copper and the soil surfaces. In the presence of EDTA (0.05 M), a Cu-EDTA complex having the equatorial and axial Cu-O bond distances of 1.96 Å and 2.21 Å, respectively was observed. Interestingly, after 180 min of the electrotreatments, the axial Cu-O bond distance was increased by 0.1 Å. The perturbation might be attributed to the possibility that the weak-field carboxylic acid groups of EDTA in the equatorial plane of Cu(II) were replaced by the strong-field water molecules. In addition, about 36% of total copper in the soil was dissolved into the aqueous phase and 67% (or 24% of total copper) of which were migrated to the cathode under the electric field (5 V/cm). This work demonstrates the utilization of in-situ X-ray absorption spectroscopy for tracking and revealing speciation and possible reaction pathways of copper with EDTA in the contaminated soil under an electric field.