Efficient decomplexation of heavy metal-EDTA complexes by Co2+/peroxymonosulfate process: The critical role of replacement mechanism

A promising strategy to treat wastewater containing heavy metal complexes (HMCs) is to employ oxidation processes before carrying out the conventional processes. Metal ion-catalyzed oxidation processes (MICOPs) have been confirmed to be suitable for the remediation of wastewater. However, the treatment of HMCs by MICOPs has seldom attracted researchers’ attention and the key mechanisms remain unclear. In this study, CuII-ethylenediamine tetraacetic acid (EDTA) was used to evaluate the decomplexation efficiency of various MICOPs, among which the CoII/peroxymonosulfate (PMS) system provided the best results. In-depth analysis of the decomplexation mechanism of CuII-EDTA indicated that the high affinity of Cu(II) to EDTA inhibits the exchange between CoII and CuII in CuII-EDTA, thus preventing the chelation of the catalytically active Co2+ with EDTA, which leads to the rapid decomplexation of CuII-EDTA. Moreover, through thermodynamic computation, MnII-EDTA, CdII-EDTA, and NiII-EDTA were selected as other HMC candidates to investigate their influence on the oxidation efficiency of the CoII/PMS system. The results confirmed that the oxidation efficiency of CoII/PMS is strongly dependent on the metal-ion replacement. Finally, we propose that the distinct performance of the Fenton(-like) process in decomplexing the NiII-EDTA and CuII-EDTA could be well explained by the metal-ion replacement mechanism.