Electrochemical treatment of spent solution after EDTA-based soil washing

The use of EDTA in soil washing technologies to remediate soils contaminated with toxic metals is prohibitive because of the large volumes of waste washing solution generated, which must be treated before disposal. Degradation of EDTA in the waste solution and the removal of Pb, Zn and Cd were investigated using electrochemical advanced oxidation processes (EAOP) with a boron-doped diamond anode (BDDA), graphite and iron anodes and a stainless-steel cathode. In addition to EAOP, the efficiency of electro-Fenton reactions, induced by the addition of H₂O₂ and the regulation of electrochemical systems to pH 3, was also investigated. Soil extraction with 15 mmol kg⁻¹ of soil EDTA yielded waste washing solution with 566 ± 1, 152 ± 1 and 5.5 ± 0.1 mg L⁻¹ of Pb, Zn and Cd, respectively. Treatments of the waste solution in pH unregulated electrochemical systems with a BDDA and graphite anode (current density 67 mA cm⁻²) were the most efficient and removed up to 98 ± 1, 96 ± 1, 99 ± 1% of Pb, Zn and Cd, respectively, by electrodeposition on the cathode and oxidatively degraded up to 99 ± 1% of chelant. In the electrochemical system with an Fe anode operated at pH 3, the chelant remained preserved in the treated solution, while metals were removed by electrodeposition. This separation opens up the possibility of a new EDTA recycling method from waste soil washing solution.