Two advanced oxidation pathways of modified iron-shavings participation in ozonation

•Two advanced oxidation pathways of iron-shavings participation in ozonation were thoroughly studied.•γ-FeOOH was formed by precipitation on the iron-shavings surface to catalyze ozonation.•Inner-Fenton reaction pathway was verified through quantification measurement of hydrogen peroxide.•The mechanism of catalytic ozonation was verified by qualitative detection of hydroxyl radicals and C-centra radicals. In this study, the performance of ozonation, catalytic ozonation and activation of molecular oxygen (AMO) process was examined with oxalate as the target refractory organics. Modified iron-shavings was used as catalyst for catalytic ozonation in degradation of oxalate at ambient temperature (25 ± 2 °C). Under catalytic ozonation, the removal of oxalate was as high as 90%, which was much better than ozone alone. Initial pH was a key factor affecting the mechanisms of oxalate degradation. In AMO process, with the existence of Fe (II) there occurred an inner-Fenton reaction, hydrogen peroxide (H2O2) was produced at initial pH2.27 and its concentration ranged from 120 to 200 μmol/L. Heterogeneous catalytic ozonation occurred in alkaline solution with hydroxyl radicals (·OH) production, which was indirectly oxidation instead of inner-Fenton reaction. Carbonate would be produced in alkaline solution during catalytic ozonation of oxalate which would in turn inhibit the process. Lepidocrocite (γ-FeOOH) was formed through the dissolved iron ions precipitation and considered as the main component of catalyst, which was confirmed via field emission scanning electron microscope with energy-dispersive spectrometer (SEM-EDS), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Hydroxyl radicals produced via different paths were detected out by electron paramagnetic resonance (EPR) and were considered as strong oxidant in catalytic ozonation and AMO process.