Mechanistic insight into the reaction pathway of peroxomonosulfate-initiated decomplexation of EDTA-NiII under alkaline conditions: Formation of high-valent Ni intermediate

The present study aims to gain molecular insights into how peroxomonosulfate (PMS) reacts with ethylenediaminetetraacetic acid (EDTA)-NiII at alkaline pH by conducting experimental tests and density functional theory (DFT) calculations. We first show experimental evidence that the decomplexation of EDTA-NiII and simultaneous formation of Ni hydroxide precipitates can be successfully achieved with the addition of PMS under alkaline conditions. At pH 11.0 or above, the aqueous Ni concentration is substantially reduced from 2.0 to below 0.1 mg L-1 within 60 min. We then propose an alternative reaction pathway in which NiII rather than EDTA is attacked by PMS, leading to the production of high-valent Ni species (in the form of EDTA-NiIV=O). This is a key and highly oxidative species being further self-decomplexed to NiIII hydroxides. Our findings suggest that the involvement of NiIV species might be a viable redox pathway that plays an important role in promoting decomplexation.