Magnetic Fe₃O₄/MnO₂ Composite:Preparation And Degradation Mechanism Of Methylene Blue

Environmental protection has become a hot spot of global concern,and the primary problem that is water pollution closely related to us.Many water treatment technologies are unable to deal with organic pollutants in water because of their stable structure,variety and potential destruction.In recent years,advanced oxidation process has been widely studied and used for its rapid,efficient,simple and low-cost degradation of stubborn organic pollutants in water at room temperature.Flower-like MnO₂and Fe₃O₄complex has become one of the most popular catalysts in advanced oxidation processes,because it combines the advantages of MnO₂high specific surface area to provide more active sites for adsorption and catalysis and Fe₃O₄strong magnetism and easy recovery,and even the two can play a synergistic role in the degradation process.However,most of these complexes require acid to etch Fe₃O₄particles to provide conditions for the growth of MnO₂,and the growth time is relatively long.Therefore,in this paper,high performance Fe₃O₄/MnO₂composite catalyst was prepared to overcome the problems of acid etching and slow growth,and its growth mechanism was studied,and the application of composite catalyst in the degradation of methylene blue was explored in detail.The specific research contents are as follows:（1）In this paper,the Fe₃O₄/α-MnO₂complex was prepared by hydrothermal and ultrasound-assisted methods.Different complex catalysts were obtained by controlling hydrothermal time（0 h,0.5 h,4 h,8 h）and hydrothermal temperature（25℃,80℃,150℃,200℃）,and characterized by XRD,SEM,TEM,HRTEM,FTIR and VSM.The results showed that the growth process of the complex is that Mn Cl₂and KMnO₄react to form a large number of crystal nuclei of MnO₂.With the increase of reaction temperature and reaction time,α-MnO₂·3H₂O gradually forms flower spheres and grows around the well-dispersed Fe₃O₄particles.Increasing the reaction temperature and reaction time,flower globoseα-MnO₂·3H₂O into flakeγ-MnO₂.（2）The degradation of methylene blue（MB）by catalyst activated persulfate（PS）prepared at different hydrothermal time and temperature was studied.The results showed that the Fe₃O₄/α-MnO₂complex catalyst prepared at hydrothermal temperature of 150℃and hydrothermal time of 0.5 h had the best degradation of MB by activated PS.（3）Using Fe₃O₄/α-MnO₂prepared at hydrothermal temperature of 150℃and hydrothermal time of 0.5 h as catalyst,the influence of p H value,PS concentration and catalyst dosage was systematically investigated on the degradation effect of MB,and the degradation mechanism was also explored.The results showed that the best degradation efficiency was when the p H value was 6.5,the concentration of PS was2.25 mmol/L and the amount of catalyst was 0.6 g/L.The degradation rate of MB aqueous solution（30 mg/L）could reach 98%after 180 min.In addition,XPS and FTIR of Fe₃O₄/α-MnO₂catalysts before and after the reaction were compared and analyzed.The results showed that the degradation of MB was mainly caused by the synergistic action of Fe,Mn and surface hydroxyl groups to produce free radicals attacking the chromogenic group of MB.After four times of repeated catalytic experiments,the degradation effect of catalyst on MB can still be maintained above90%,and the structure morphology is maintained well,which lays a foundation for the practical application of composite catalyst.In conclusion,Fe₃O₄/α-MnO₂complex catalyst has good catalytic activity in activating PS to remove MB.This paper provides a simpler and cheaper method for the preparation of flower spherical MnO₂and Fe₃O₄complex.What’s more,it also provides a more detailed mechanism for MB degradation process.