Ozone catalytic oxidation of biologically pretreated semi-coking wastewater (BPSCW) by spinel-type MnFe2O4 magnetic nanoparticles

Abstract

• MnFe 2 O 4 magnetic catalysts had excellent ozone catalytic oxidation performance on the real BPSCW. • The effect of operational parameters in practical applications on catalytic ozonation is investigated. • The removal efficiencies of COD and volatile phenol could reach 85.2% and 94.1%, respectively. • MnFe 2 O 4 magnetic catalysts catalyzed O 3 /H 2 O 2 to produce hydroxyl radicals (∙OH). • MnFe 2 O 4 magnetic catalysts were superparamagnetic (67.8 emu∙g −1 ) and reused without high loss of activity. In this study, spinel-type MnFe 2 O 4 magnetic nanoparticles were successfully synthesized employing the purified sol–gel method and used for the advanced treatment of biologically pretreated semi-coking wastewater (BPSCW) by the O 3 /H 2 O 2 catalytic system. Results showed that the MnFe 2 O 4 magnetic catalysts did not have a regular surface morphology, and their surface contained numerous fine particles that showed a particle accumulation state and formed a good nano-spinel structure. Their surface morphology, ratio, size, and X-ray photoelectron spectroscopy revealed two active metal elements in the catalyst, namely, Fe 3+ and Mn 2+ , which were consistent with the elemental composition and valence of MnFe 2 O 4 . Under optimal reaction conditions, the removal efficiency of O 3 /H 2 O 2 /MnFe 2 O 4 catalytic system for chemical oxygen demand (COD) and volatile phenol (VP) can reach 85.2% and 94.1%, respectively (catalyst dosage = 2.0 g∙L −1 , O 3 dosage = 1.2 mg∙min −1 , H 2 O 2 concentration = 0.15 mol∙L −1 , pH = 7.0, and treatment time = 70 min). The free radical quenching experiment and electron paramagnetic resonance (EPR) experiment verified that hydroxyl radicals (∙OH) played a role in the deep treatment of BPSCW in the O 3 /H 2 O 2 /MnFe 2 O 4 catalytic system. Phosphate experiments proved that lewis acid sites on the surface of the MnFe 2 O 4 magnetic catalysts were the catalytically active sites. The superparamagnetic properties of the MnFe 2 O 4 magnetic catalysts (67.8 emu∙g −1 ) facilitated magnetic separation from the treated wastewater. After 10 cycles, the activity of the MnFe 2 O 4 magnetic catalysts remained high, and the removal efficiencies of COD and VP only decreased by 13.6% and 11.9%, respectively. Considering excellent degradation performance, easy magnetic separation and high stability, this study provided important insights into the practical application of the O 3 /H 2 O 2 catalytic system for the advanced treatment of BPSCW.