Overlooked oxidative role of Ni(III) in the enhanced mineralization of Ni(II)–EDTA complex by ozonation

The mineralization of organic pollutant by O 3 is prone to be restricted by the accumulative production of oxalate (OA) that is recalcitrant to the attack of hydroxyl radical ( OH). Herein, it was interestingly found that the presence of 0.5 mM Ni(II) enhanced the mineralization efficiency of EDTA by O 3 under alkaline conditions by 22.8%, with the promoting effect of Ni(II) augmented as the increasing loading of Ni(II). Based on the results from scavenging experiment, chemical probe detection and XPS characterization, it was verified that except for the commonly recognized reactive oxidants (i.e., OH and O 3 ) in EDTA mineralization, the overlooked Ni(III) species (i.e., NiOOH) also played an indispensable role in enhancing the mineralization of EDTA. The Ni(III) species was formed via the oxidation of decomplexed Ni(II) by OH and in turn collaborated with OH to decompose EDTA. More importantly, the generated Ni(III) species exerted higher affinity towards OH and O 3 -resistent oxalate, hence elevating EDTA mineralization. This study elucidated the significant role of Ni(III) species in promoting the mineralization efficiency of EDTA, and also proposed a new insight into elevating the treatment performance of ozonation via the collaborative oxidation mechanism-co-mediated by OH and Ni(III) species.