Sulfate Radical Induced Catalytic Degradation Of Rhoadmine B With Co Mn Fe Layered Doubled Hydroxide As Heterogeneous Activator Of Peroxymonosulfate

Recently,advanced oxidation processes based on sulfate radical（SR-AOPs）to remove refractory organic pollutants have attracted much attention.Compared with hydroxyl radical（·OH）produced by traditional Fenton reactions,sulfate radicals(SO₄^·-)have higher redox potential,longer survival time,stronger min eralization ability and wider p H range for application.Sulfate radicals can be produced by activation of peroxymonosulfate（PMS）with asymmetric structure and peroxydisulfate（PDS）with symmetric structure through ultraviolet irradiation,heat,alkali ultrasound and transition metals.Compared with PDS,PMS is more easily activated to break O-O bond due to its asymmetrical structure and therefore PMS is chosen as oxidant in this system.The PDS and PMS can be activated by transition metal ion Co²⁺,however,it has biological toxicity and causes secondary pollution to the environment with high prices,which limits its practical application.The heterogeneous catalytic oxidation technology is to put heterogeneous catalysts into the system,which can effectively avoid the environmental problems caused by metal ion precipitation.In this study,we synthesized a novel heterogeneous catalyst Co Mn Fe layered doubled hydroxide（Co Mn Fe-LDH）with Co,Mn and Fe in different proportions.Rhodamine B（Rh B）was selected as target organic contanminnats for exploration the catalytic performance of synthesized materials.The main findings are as follows:（1）Preparation of heterogeneous catalyst Co Mn Fe-LDHCo Mn Fe-LDH were prepared by double drop co-precipitation method.The Co Mn Fe-LDH prepared at the Co/Mn/Fe molar ratio of 16:5:3 showed best degradation effect and decolorization rate in the process of Rhodamine B degradation experiment,hence the Co Mn Fe-LDH at this molar ratio was selected as the follow-up research object.The Co Mn Fe-LDH was characterized by XRD,SEM-EDX,XPS and FTIR.According to the SEM images,the synthesized Co Mn Fe-LDH demonstrated excellent crystallinity and a layered hydrotalcite structure.The XRD pattern of the Co Mn Fe-LDH showed that the obtained catalysts had n o other metal impurities.The FTIR results confirmed the presence of OH and CO₃^2-functional groups of LDHs.And the XPS analysis showed that Co,Mn and Fe metals existed mainly in the form of binary cobalt(Co²⁺),trivalent manganese(Mn³⁺)and trivalent iron(Fe³⁺).Overall,these results indicated that a new type of heterogeneous catalyst Co Mn Fe-LDH with good crystal structure was successfully synthesized.（2）Co Mn Fe-LDH/PMS system for the degradation of Rh BCo Mn Fe-LDH as heterogeneous catalyst showed high catalytic activity for degradation of Rh B.When the dosage of Co Mn Fe-LDH and PMS was 0.1 g/L and 0.2g/L,the Co Mn Fe-LDH/PMS system can completely degrade 100m L Rh B solution（initial concentration:20mg/L）during a certain reaction time（8 min）with no initial p H adjustment at room tempertature（20°C）.The degradation processes conformed to pseudo first-order kinetics,and the reaction rate constant was k=0.4694 min^-1.Co Mn Fe-LDH/PMS system could effectively degrade Rh B in a wide p H range of 3-9 with limited Co²⁺,Mn³⁺and Fe³⁺leaching after reaction.（3）Removal of Rh B by Co Mn Fe-LDH/PMS system under different experimental parametersThe degradation efficiency of Co Mn Fe-LDH/PMS system to Rh B increased with the increase of catalyst Co Mn Fe-LDH and oxidant PMS,and the degradation efficiency also increased with the increase of temperature.Inorganic anions Cl^-and SO₄^2-and natural organic humic acid had a relatively weak impact on the degradation of Rh B,but CO₃^2-had a significant inhibitory effect.With the increase of dye concentration,the degradation efficiency would decrease.The activation energy was 46.08?k J/mol in the Co Mn Fe-LDH/PMS system and TOC removal rate could reach 48.8%after reaction for 30 minutes under the optimal reaction conditions.（4）Degration mechanism of Rh B and stability of Co Mn Fe-LDHThe results of free radical trapping experiments and ESR showed that SO₄^·-,·OH were produced in the Co Mn Fe-LDH/PMS system,and SO₄^·-played a dominant role in the degradation process.According to the results of XPS analysis on the catalysts before and after reaction,the transform of the valence state of metallic elements（Co,Mn and Fe）in Co Mn Fe-LDH lead to the activation of PMS to generate SO₄^·-and·OH.The generated SO₄^·-and·OH attacked the Rh B molecules,leading to the efficient Rh B removal.And the catalyst still showed relatively high catalytic activity after three runs recycle experiment,confirming the good stability of Co Mn Fe-LDH.Based on the conclusions,this study provides a new idea based on LDH activated persulfate system for the treatment of organic contaminants degradation.