Enhanced removal of coumarin by a novel O₃/SPC system: Kinetic and mechanism

A high-efficient, eco-friendly and applicable oxidant of percarbonate (SPC, Na₂CO₃·1.5H₂O₂) is applied as oxidant in ozonation (O₃) process. In this work, coumarin (COU) decomposition by O₃ and O₃/SPC was proposed in terms of diverse parameters, including ozone concentration, SPC dosage and pH, with regard to the pseudo-first-order kinetic model. About 96.5% of 25 mg L^-1 COU was removed by 209.3 mg L^-1 SPC and 2.08 mg L^-1 O₃ within 30 min, whereas 49.8% of COU was exhausted in O₃ system alone. High ozone concentration and pH could enhance COU removal, and overdose SPC has an inhibition effect as CO₃^2- could react with ·OH. Furtherly, the generation of hydroxyl radicals (OH) via a chain reaction in O₃ was ascertained based on indirect alcohols quenching tests and direct electron spin resonance (ESR) spin-trapping tests, though high initial SPC concentration led to no trapping of OH by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in O₃/SPC. According to the concentration of dissolved ozone and prior study, a fast reaction kinetic model was estimated firstly for O₃/SPC system and well fitted with different condition. The hydroxylated intermediates of 7-hydroxylcoumarin and 6,7-hydroxylcoumarin were recognized and the degradation pathways were proposed. The findings of the study can broaden the research direction of SPC and provide a new application information for SPC in environmental pollution control.