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

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 (IO 4 − , PI) activation process mediated by hydrated Mn II ions and highlighted the critical involvement of the in-situ formed colloidal MnO 2 (Mn IV ). Mn II ions were directly oxidized to colloidal Mn IV in the Mn II /PI system, and the nascent Mn IV could in turn activate PI to form Mn IV -PI* complex as the reactive oxidant. Besides, several kinds of ligands were introduced into the Mn II /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 18 O isotope-tracer experiments, we proposed that the high-valent manganese-oxo (Mn V -oxo) species was the dominant reactive oxidant generated in the Mn II /NTA/PI system, and the metastable Mn III complex was the precursor of the Mn V -oxo species. Notably, maintaining the Mn II high-spin d 5 center and stabilizing the in-situ formed Mn III 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 Mn II and might also inspire the designing of heterogeneous catalyst modification for the water treatment technology. • Nascent colloidal Mn IV contributed to PI activation in Mn II /PI system. • NTA could significantly enhance the performance of PI activation via soluble Mn II . • Mn III -NTA complex was the primary catalytic intermediates in the Mn II /NTA/PI system. • High-valent Mn V -oxo species was identified as the reactive oxidant in Mn II /NTA/PI system.