Peroxymonosulfate Activated By Magnetic MnFe₂O₄-based Catalysts For The Degradation Of Phenolic Contaminants

Phenolic substances are widely used in medicine,plastics,insecticides and other fields.Due to their high toxicity,difficult degradation and durability,they bring great challenges to environmental governance.Advanced oxidation technology(AOPs)with persulfate(PMS)as an oxidant can produce sulfate radicals(SO4·-)with a high redox potential(2.5～3.1V),strong selectivity and gond adaptability to pH,can convert phenolic pollutants into small molecules with low toxicity and even completely mineralizes phenolic pollutantsUnactivated PMS has weak ability to generate free radicals,while transition metal catalysts can activate PMS to decompose and produce SO4·-and other reactive oxygen species(ROS).Some iron-based catalysts are low toxic,biocompatible,low cost,and stable,which have broad application prospects in industrial wastewater treatment.Among them,the bimetallic or multi-metallic iron-based catalysts with spinel structures have higher catalytic performance due to the synergy effect among various active sites.In addition,the ion-based catalysts with spinel structure have magnetic behavior,and thus the catalysts can be easily seperated from water via external magnetic field.It is convenient for practical application.Manganese(Mn),as a naturally abundant element,has less toxicity and multiple valences(II,III,and IV).When Mn replaces Fe in Fe3O4 to form magnetic MnFe2O4 and the spinel structure can be maintained.In order to obtain a catalyst with high catalytic activity,and easy separation via simple synthetic method,we prepared two MnFe2O4-based catalysts by a simple hydrothermal method and used them to activate PMS for phenolic contaminants degradation in water.The main contents are as follows:1.The ferromagnetic MnFe2O4 with good crystallinity is synthesized via facile hydrothermal method in Chapter 2,which activates PMS for degradation of 2,4-dichlorophenol(2,4-DCP).The effects of MnFe2O4 dosage,PMS concentration and reaction temperature in the catalytic process are investigated.The experimental results showed that the removal rate of 2,4-DCP is only 4.8%in unactivated PMS system;while the removal rate of 2,4-DCP was increased to 17.8%in MnFe2O4/PMS system.That was due to the heterogeneous catalytic process is dominated.The electron spin resonance(ESR)indicate that free radicals(SO4·-and ·OH)and 1O2 are produced for 2,4-DCP degradation during the catalytic process.2.In Chapter 3,β-cyclodextrin(P-CD)was modified on the surface of MnFe2O4 to obtain superparamagnetic O-CD-MnFe2O4 catalyst.The experimental results show that the specific surface area of catalyst increases,and the oxygen vacancies increase after(3-CD modification.Catalytic degradation experiments indicate thatβ-CD-MnFe2O4 has a higher catalytic activity than MnFe2O4,and its normalized apparent rate constant(kobs)is 6 times that of MnFe2O4.Subsequently,a systematic study is conducted on factors such as the PMS concentration,β-CD-MnFe2O4 dosage,solution pH,the adaptability to phenolic pollutants,active oxygen species(ROS),anti-humic acid(HA)interference,and stability of β-CD-MnFe2O4.The experimental results show that β-CD-MnFe2O4 can effectively degrade 2,4-DCP in the pH range of 3-11,and exhibits good cycle stability and good adaptability to various phenolic pollutants.During the catalysis process,ROS are mainly produced on the surface of MnFe2O4;β-CD on the surface of MnFe2O4 can form inclusion compounds with pollutants,and thus shortens the distance between pollutants and ROS,and promotes the degradation rate of pollutants.Due to the limitation of β-CD cavity,the HA with large size cannot compete for ROS and bind catalytic sites.Based on the synergy effect of(3-CD and MnFe2O4,the possible mechanism of β-CD-MnFe2O4 activating PMS is proposed,and the degradation pathway of 2,4-DCP is studied.