Trace Co²⁺ coupled with phosphate triggers efficient peroxymonosulfate activation for organic degradation

Cobalt-mediated activation of peroxymonosulfate (PMS) has been widely used to remove the refractory organic pollutants from contaminated waters. However, the residual cobalt usually at a trace level inevitably brings about secondary pollution to be disposed of. In this study we found that the presence of phosphate could trigger a more efficient catalytic activation of PMS at trace Co²⁺ dosages (0.17-1.7 μM). Fast degradation of atrazine (ATZ) was observed in the Co²⁺/PMS/phosphate system, with the pseudo first-order kinetic rate constant as high as 5.4 and 15.4 times that in Co²⁺/PMS and phosphate/PMS systems respectively under otherwise similar conditions. The presence of phosphate promoted the production of sulfate radical (SO₄·^-), accompanying the enhanced formation of by-product ¹O₂ simultaneously. Using a competition reaction kinetics approach, the contribution of SO₄·^- to ATZ oxidation was determined as 96.5%, suggesting that SO₄·^- was the main reactive species responsible for ATZ removal. Such favorable effect was partially ascribed to the specific ligand structure of six coordination structure between phosphate and cobalt, which facilitated electron transfer in the Co^III/Co^II reduction. In addition, it was dependent upon the aqueous phosphate levels, and low level (< 0.5 mM) was insufficient to drive the Co^III/Co^II cycle, whereas the higher level (> 15 mM) showed negative effect since the excessive phosphate could quench SO₄·^- and·OH. This study is believed to advance the fundamental understanding of the ligand effect on the cobalt-mediated sulfate radicals-based advanced oxidation process.