Study on Optimal Conditions for Recovery of EDTA from Soil Washing Effluents

The recovery of ethylenediaminetetraacetic acid (EDTA) from washing effluents is essential to reduce the cost of EDTAenhanced soil washing and the production of wastewater. This study evaluated a recovery method, in which Pb or Zn was first dissociated from Pb- or Zn-EDTA complex through the replacement reaction by adding FeCl3 and then removed as phosphate precipitates through adding Na2HPO4. Finally, Fe(III) was removed as Fe(OH)3 precipitates through adding Ca(OH)2. As a result, EDTA was recovered as Ca-EDTA for further use in soil washing process. The optimal conditions for EDTA recovery, including the molar ratios of FeCl3 and Na2HPO4 to EDTA as well as the pH after adding Na2HPO4 and adding Ca(OH)2, were well investigated. Under the optimal conditions, 96% of Pb or 83% of Zn was removed from the Pbor Zn-EDTA, respectively. The four-cycle recovery and reuse of EDTA experiments indicated the recovered EDTA from soil washing effluents did not lose much chelating capacity for Pb removal. However, there is a loss of 15% of its chelating capacity in the first cycle reuse for Zn-contaminated soil washing due to substantial Zn residual in the recovered EDTA solution.