Highly efficient degradation of phenolic compounds by Fe(II)-activated dual oxidant (persulfate/calcium peroxide)...

This study demonstrates the feasibility, reaction mechanisms, and potential of practical applications of a dual oxidant (DuOx) system comprising calcium peroxide (CP) and persulfate (PS) catalyzed using Fe(II) [PS/CP/Fe(II)]. The DuOx system was superior in phenol degradation to single oxidant systems, i.e., PS/Fe(II) or CP/Fe(II), with 95.5% phenol removal under an optimum condition of a phenol/PS/CP/Fe(II) molar ratio of 1/1/5/6 ([Phenol]₀=0.5 mM). Based on scavenger studies and electron spin resonance (ESR) spectroscopy, the phenol removal in the DuOx system was barrierless, with negative activation energy assisted by robust reactive species. The phenol degradation results in the presence of methanol, t-butanol, l-histidine, and NaN₃. The ESR spectroscopy indicates that phenol degradation is attributed dominantly to ¹O₂ generated by recombining O₂^•- and radicals, such as hydroxyl (HO^•) and sulfate (SO₄^•-). The performance of the DuOx system was highly efficient in pH 3-11, up to 10 mM Cl^-, SO₄^2-, or NO₃^-, and up to 50 mg/L humic acids but was strongly suppressed by more than 10 mM HCO₃^- and H₂PO₄^-. In addition, the DuOx system was efficient in phenol removal in natural groundwater as well as removing and mineralizing other phenolic compounds (PCs) such as bisphenol A, chlorophenol, dichlorophenol, trichlorophenol, and nitrophenol. These results provide insights into the reactions induced by the DuOx system and confirm its applicability of in situ chemical oxidation in refractory organic pollutants.