Activation of peroxydisulfate by magnetically separable rGO/MnFe2O4 toward oxidation of tetracycline: Efficiency, mechanism and degradation pathways

Peroxydisulfate (PDS) activation by a novel magnetically recoverable reduced graphene oxide (rGO)/MnFe2O4 composite was studied for the removal of tetracycline (TC), a widely used antibiotic. The catalytic performance of rGO/MnFe2O4 was greatly higher than that of bare MnFe2O4, and the degradation rate of TC tended to be raised as the content of rGO in composite catalyst increased. After reaction for 80 min, the optimized rGO/MnFe2O4-15 catalyst can degrade 90.1% of TC in water (20 mg/L) with the addition of 1.5 g⋅L−1 PDS. The effect of PDS dose, temperature, initial pH, inorganic anions and humic acid on TC removal was also examined. The quenching experiments and electron paramagnetic resonance (EPR) tests suggested that the radical (SO4•− and ∙OH) and non-radical (1O2) oxidation processes worked together for the degradation of TC in rGO/MnFe2O4-PDS system. PDS was activated to produce SO4•− by MnFe2O4, and rGO can donate electron to facilitate the redox cycle of ≡M(II)/≡M(III) (M refers to Mn or Fe), thus promoting the formation of SO4•−. The carbonyl group in rGO participated in the activation of PDS to generate 1O2. The linear sweep voltammetry indicated that rGO/MnFe2O4 did not act as an electronic medium to promote the electron transfer from TC to PDS. The degradation products were identified by LC–MS and the possible degradation pathways of TC were put forward. Furtherly, the decontamination of natural actual water (tap water, secondary effluent and river water) spiked with TC were evaluated.