Applicability of alkaline precipitation for the recovery of EDDS spent solution

This paper presents an innovative procedure for the recovery of SS-ethylenediamine-N,N'-disuccinic acid (EDDS) solution used for soil washing processes. The procedure is derived from that applied for the recovery of ethylenediamine-tetraacetic acid (EDTA), modifying and optimizing the choice of the chemical agents used for the protonation of the chelant, the exchange of the metals, and the final precipitation of the unwanted compounds. To select the reagents and test the proposed approach, an experimental study was conducted on real EDDS spent solutions, obtained washing a Cu and Zn real contaminated soil. According to the results obtained, the precipitation ranges from 30% to more than 90% for both Cu and Zn, depending on the adopted reagent sequence, and on the molar ratio between the salt and the chelant contained in the spent solution. Data were in agreement with chemical equilibrium predicted in ideal conditions. The recovered solutions had a reduced ability to remove the contaminants when applied in a new soil washing cycle (15% less for Cu and 30% less for Zn) because of the high concentration of alkaline metal ions required for the precipitation. At the same time, they were more biodegradable compared to non-treated solutions, confirming that EDDS-metal chelates may represent a threat for biological wastewater processes.