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

Abstract 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 CoIII/CoII reduction. In addition, it was dependent upon the aqueous phosphate levels, and low 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.