Dechelation of Cd-EDTA complex and recovery of EDTA from simulated soil-washing solution with sodium sulfide.

Washing water containing poorly degradable heavy metal-EDTA complexes is produced by ethylenediaminetetraacetic acid (EDTA) washing of heavy metal-contaminated soil. A series of batch experiments were performed to explore the dechelation of heavy metal-EDTA complexes and the recovery of EDTA from simulated soil-washing solution using sulfide precipitation with Na2S. The results showed that the effect of Na2S dosage on the dechelation of Cd-EDTA solution was greater than that of other factors (reaction temperature, time, and pH) and excess EDTA suppressed cadmium removal. Additionally, the Cd removal efficiency reached 99.99 ± 0.001%, and the residual amount of Cd-EDTA was below the detection limit of ion chromatography (IC) measurements under the following optimal conditions: an initial Cd-EDTA solution pH of 6, a sodium sulfide-to-Cd-EDTA ratio of 2:1, a reaction temperature of 25 °C, and a contact time of 20 min. Furthermore, the results of analysis using inductively coupled plasma optical emission spectroscopy (ICP-OES), Fourier transform infrared (FT-IR) spectroscopy, and ion chromatography (IC) confirmed that the dechelation process could be completed in less than 20 min and that EDTA was completely recovered as trisodium EDTA (HNa3EDTA). The mechanism for the dechelation of Cd-EDTA and recovery of EDTA using sulfide precipitation with Na2S was also proposed. The recovered EDTA with acidification had the same ability to extract heavy metals from soil with fresh EDTA. This study may facilitate the recycling of soil-washing wastewater and reduce the cost of extracting heavy metals from soil using EDTA.