Bicarbonate-enabled high-valent cobalt-oxo and singlet oxygen generation for pathway switching from radical to non-radical oxidation in Co(II)/calcium sulfite system

HCO 3 − coordination-enabled high-valent cobalt-oxo pathway absent without HCO 3 − • Quantitative Co(IV) contribution elevation from 2.5% to 28% with bicarbonate • Oxidation pathway switch from radical to non-radical regime by bicarbonate • CoCO 3 SO 3 2− complex identified as kinetically competent intermediate • Efficient performance across real waters with Co dosage below discharge limit Bicarbonate, a ubiquitous constituent of natural waters, is typically regarded as a radical scavenger in sulfate-radical-based advanced oxidation processes (SR-AOPs); however, its coordinative interaction with the catalytic metal center and the consequent modulation of oxidative pathways remain poorly understood. In this work, the role of HCO 3 − in the Co(II)-catalyzed activation of calcium sulfite (CaSO 3 ) was systematically investigated. HCO 3 − species is identified to coordinate with Co(II) to assemble a ternary Co(II) − HCO 3 − −S(IV) complex that facilitates the rate-limiting Co(II)-to-Co(III) oxidation, while the subsequent Co(III) − HCO 3 − complexation prevents Co(III) precipitation and sustains the catalytic cycle. Critically, this coordination is a prerequisite for Co(IV) generation: by depressing the Co(IV)/Co(II) redox potential, it unlocks the otherwise forbidden oxygen-atom transfer pathway, yielding a non-radical oxidant entirely absent without HCO 3 − . Together with 1 O 2 , this HCO 3 − -exclusive Co(IV) pathway operates in parallel with the SO 4 •− radical pathway, redirecting the dominant oxidation mechanism from a radical-driven regime toward a non-radical-driven regime, accompanied by a pronounced reduction in apparent activation energy (37.46 kJ mol −1 ). This system functions effectively across a wide pH, exhibits high tolerance to common anionic interferents, and achieves efficient performance across multiple authentic water matrices. In short, this work establishes HCO 3 − as a coordination-directed switch that sustains the catalytic cycle while uniquely gating access to the Co(IV) oxidation pathway, offering a ligand-engineering strategy for tuning reactive species speciation in S(IV)-based AOPs. The HCO 3 − enhanced- and altered- CaSO3 activation by Co(II). Download: Download high-res image (301KB) Download: Download full-size image