Performance And Mechanism Of MnFe₂O₄@HTC Activated Persulfate System For Algae Removal

Eutrophication of water bodies has caused algae outbreaks to threaten the health of water ecology and the safety of drinking water.Traditional algae removal processes are mainly physical fishing and chemical oxidation methods,which have problems such as poor effect,high cost and complex technology.In recent years,activated persulfate（PS）advanced oxidation technology has attracted widespread attention due to its high redox potential,wide p H range,low energy consumption,and easy storage and transportation.Mn Fe₂O₄ has stable catalytic performance,cheap raw material and simple preparation methods.It can activate PS to produce oxidizing active substances,and has strong degradation ability for most organic compounds,including toxic and non-biodegradable compounds.However,ferrite has problems such as easy agglomeration in water and low catalytic efficiency.To solve such problems,hydrothermal carbon（HTC）is a common good carrier as a stable renewable energy material.In this thesis,the dominant algae species Microcystis aeruginosa during cyanobacteria outbreaks was used as the research object.The magnetic Mn Fe₂O₄@HTC composite catalyst was prepared with Mn Fe₂O₄supported on the surface of HTC,and the efficiency and reaction mechanism of Mn Fe₂O₄@HTC/PS system for removing algae were explored.In this paper,a kind of magnetic Mn Fe₂O₄@HTC composite catalyst was prepared by loading Mn Fe₂O₄ on the surface of the hydrothermal carbon（HTC）with hydrothermal-coprecipitation method.XRD,SEM,EDS,FTIR and BET methods were used to characterize the material.The results demonstrated that the composite had rough surface,and it was in the shape of a connected sphere.The particle size concentrated between 10 nm and 100 nm.The specific surface area of the material was 448 m²g^-1,and the average pore size was 7.7 nm.Compared with HTC monomer,the specific surface area and pore volume of the material increased significantly.The pore size distribution was more conducive to the adsorption of microalgae system.There were elements C,Mn,Fe and O on the surface of the composite material,and the content ratio of Mn and Fe was about 1:2.The XRD results accord with the characteristic peak of Mn Fe₂O₄.In algae removal efficiency of Mn Fe₂O₄@HTC/PS system,the effects of four parameters,including Mn Fe₂O₄/HTC load ratio,PS dosage,initial p H and initial algal concentration,on the algae cells removal were investigated and the optimal working condition of the system was determined.The results showed that when the initial concentration of algae was 1.4×10⁹～1.5×10⁹ L^-1,the dosage of catalyst was 0.2 g L^-1,the loading ratio was Mn-Fe3@HTC(Fe=213.2 mg g^-1,Mn=105.79 mg g^-1)and the dosage of PS was 0.4 g L^-1,the algae removal effect reached 99%in 30 min.The change of p H had little effect on the system,and the removal rate of algae could reach more than 90%in the range of 3～10.The removal process of algae in the system conformed to the first-order kinetic characteristics,and the reaction rate constant k=0.138,R²=0.9097.After three cycle experiments,the removal rate of algae was 58%,and the dissolution of metal ions in Mn Fe₂O₄ was found in the catalyst.The dissolution amount of Mn and Fe accounted for about 6%of the total amount of Fe and Mn loaded on the catalyst.During the reaction,the OD₆₂₀ of the solution decreased persistently,and the UV₂₅₄continued to rise and stabilize,indicating that the Mn Fe₂O₄@HTC/PS system could rupture the algae cells and release the cellular contents such as microcystins.The Mn Fe₂O₄@HTC/PS system could effectively degrade MC-LR and released cell contents.Under optimal conditions,the initial MC-LR concentration was 32.06μg L^-1,and the total MC-LR removal rate could reach 91.6%within 60 min.The process of Mn Fe₂O₄@HTC/PS system to degrade MC-LR was the simultaneous release of algal toxins by the rupture of algal cells and the oxidative degradation of algal toxins.There were the production of new substances and the degradation of old substances.Based on the analysis of different systems,free radical screens and XPS,the algae removal mechanism of Mn Fe₂O₄@HTC/PS system was explored.The results showed that the main reaction mechanism of the system was the non-radical pathway of activating PS,with holes and ¹O₂ predominant,accompanied by comprehensive effects such as adsorption,flocculation and homogeneous catalysis.In this system,Mn Fe₂O₄@HTC material could adsorb algal cells on its surface,catalyze PS to produce holes,¹O₂,·O₂^-,SO₄^-·and·OH by the valence cycle of Mn and Fe as well as the synergistic reaction effect of HTC,and then made algal cells to burst and die.