Synthesis Of Different Morphologies Of MnO₂ And Fe₃O₄@MnO₂ Core-shell Nanocomposites And Their Performance Of Catalytic Degradation Of Methylene Blue

As the world’s largest producer and consumer of dyestuffs,China handles a large amount of printing and dyeing wastewater every year.According to the latest annual report data of environmental statistics,textile industry is the industry with the highest chemical oxygen demand emission among the 42 industrial industries counted.Since printing and dyeing materials have become more and more complex in composition and more stable in physical and chemical properties with the gradual improvement of social development,traditional biological,physical and chemical methods have been difficult to treat new types of dye wastewater.In recent years,the new advanced oxidation technology based on SO₄^·-has attracted the attention of researchers at home and abroad,which has the following advantages:（1）SO₄^·-has a long lifetime and can be stable in a wide p H range;（2）higher redox potential;（3）the presence of the oxidant in the form of solid particles at room temperature,which is stable and easy to store and transport.Transition metal activation is the most convenient and efficient way to activate such oxidants.MnO₂has good catalytic performance and is environmentally friendly and resource-rich,and is considered to be a new type of environmentally friendly catalyst.Therefore,this paper takes peroxymonosulfate（PMS）and MnO₂as the starting point and designs two hydrothermal systems,one is to prepare MnO₂with different morphologies by changing the concentration of potassium permanganate in the hydrothermal system,and the other is to prepare core-shell structure Fe₃O₄@MnO₂by simple hydrothermal method,and uses the experimentally prepared samples to activate PMS to degrade the azo dye methylene blue（MB）,as follows:（1）MnO₂with different morphologies containingγ-,α-andδ-crystalline structures was prepared using potassium permanganate and hydrochloric acid as raw materials and by changing the amount of potassium permanganate added to the hydrothermal system at 120°C.The effect of the content of potassium permanganate in the hydrothermal system on the crystalline shape of MnO₂was investigated:a lower content of potassium permanganate producedγ-MnO₂+α-MnO₂,and a proper increase of the amount of potassium permanganate,the crystalline shape was completely transformed toα-MnO₂.After the concentration of potassium permanganate in the hydrothermal system exceeded 3.3 g/L,the crystalline shape was transformed fromα-MnO₂toδ-MnO₂,and a complete transformation occurred at a concentration of 13.3 g/L.In addition,the performance of different MnO₂activated PMS for MB degradation was investigated,and the removal rate of MB was up to more than 90%for all systems after adding PMS for 30 min,indicating that MnO₂can efficiently activate PMS for MB degradation.Furthermore,the catalytic degradation process of all samples followed a pseudo first-order kinetic model,and the sea urchin-like material withα-MnO₂+δ-MnO₂crystalline form was the optimal catalyst.（2）Fe₃O₄@MnO₂magnetic core-shell structure nanocomposites were prepared by a simple hydrothermal method（ultrasound assisted）,and the materials were characterized by XRD,VSM,SEM,TEM,HRTEM,mapping,BET,and XPS.The synthesized Fe₃O₄@MnO₂core-shell composites were used to activate PMS for MB degradation,and the effects of catalyst addition,PMS addition,and the initial p H of the solution in the degradation system on MB degradation were systematically investigated.The results showed that the removal efficiency of MB was greater than96%at 30 min using a small amount of catalyst（0.03 g）and PMS（0.018 g）in the p H range of 3.14-11.3.The catalytic mechanism was proposed by XPS spectroscopy and quenching experiments.In the system of Fe₃O₄@MnO₂/PMS,SO₄^·-,^·OH,and ¹O₂were all reactive radicals that together promoted the rapid degradation of MB,among which ¹O₂played a dominant role.The catalysts were characterized by XRD and SEM before and after the reaction,and the experiments proved that the composites have high stability,and the degradation rate of MB by the catalysts remained basically unchanged after four repeated catalytic experiments,which have strong reusability.（3）Fe₃O₄@MnO₂core-shell composites with different morphologies were prepared by varying the controllable factors in the Fe₃O₄@MnO₂synthesis process,i.e.,potassium manganate concentration and hydrothermal reaction time.The Fe₃O₄@MnO₂with different morphologies was characterized by XRD,EDS,and SEM,which proved that the preparation of the Fe₃O₄@MnO₂core-shell structure by this hydrothermal system is highly feasible...