Cu(II)-EDTA removal by a two-step Fe(0) electrocoagulation in near natural water: Sequent transformation and oxidation of EDTA complexes

The widely usage of ethylenediaminetetraacetic acid (EDTA) arises environmental concerns on toxic metal mobilization, and challenges the conventional processes in water treatment. In the study Cu(II)-EDTA in near natural water was efficiently removed during a two-step electrocoagulation using Fe(0) anode (Fe-EC), including a transformation to Fe(III)-EDTA induced mainly by structural Fe(II) in anoxic Fe-EC and further degradation in oxic Fe-EC. The degradation of Fe(III)-EDTA was mostly attributed to an oxygen activation mechanism that involving O-2(center dot-) and hydroxyl radical ((OH)-O-center dot) generation, as validated by the quenching experiments and electron spin resonance. Furthermore, O-2(center dot-) generated during Fe(II) oxidation took a dominant role on Fe(III)/Fe(II)-EDTA transformation instead of electrochemical reduction. Six intermediates during the Fe(III)-EDTA degradation were identified by LC-Q-TOF, indicating a pathway of stepwise breakage of N-C bonds. The results revealed in this work is helpful to understand the contribution and fate of EDTA during Fe-EC treatment of metal-EDTA polluted water.