Ligands-triggered evolution of catalytic intermediates during periodate activation via soluble Mn(II) for organic contaminants’ abatement

Abstract

Homogenous catalysis technology could be flexibly combined with existing water treatment facilities, which presents broad applicational prospects and deserves systematic study. This work reinvestigated the periodate (IO4−, PI) activation process mediated by hydrated MnII ions and highlighted the critical involvement of the in-situ formed colloidal MnO2 (MnIV). MnII ions were directly oxidized to colloidal MnIV in the MnII/PI system, and the nascent MnIV could in turn activate PI to form MnIV-PI* complex as the reactive oxidant. Besides, several kinds of ligands were introduced into the MnII/PI oxidation system to explore their influences. Nitrilotriacetic acid (NTA) outperformed other selected ligands and could significantly enhance PI activation to degrade organic contaminants, which could achieve a removal efficiency of ∼99% within seconds. Interestingly, the presence of NTA completely altered the catalytic pathways rather than just accelerating the generation of reactive oxidants. Combining the results of sulfoxide-probe transformation tests and the 18O isotope-tracer experiments, we proposed that the high-valent manganese-oxo (MnV-oxo) species was the dominant reactive oxidant generated in the MnII/NTA/PI system, and the metastable MnIII complex was the precursor of the MnV-oxo species. Notably, maintaining the MnII high-spin d5 center and stabilizing the in-situ formed MnIII intermediates were the crucial premises for the evolution of the catalytic oxidation process triggered by functional ligands. This work clarified the critical involvement of functional ligands during PI activation via soluble MnII and might also inspire the designing of heterogeneous catalyst modification for the water treatment technology.