Study On Persulfate Activation By CoFe₂O₄@Nitrogen-doped Porous Carbon With Core-shell Structure For Degradation Of Rhodamine B Dye Wastewater

This research aims to prepare a kind of highly efficient,stable,and easy to separate and recycle magnetic catalytic material to activate persulfate to overcome the technical challenges in the treatment of triphenylmethane dye wastewater and the restriction of traditional activation methods on the practical application of persulfate advanced oxidation technology.Considering the unique advantages of zeolite-like imidazole ester framework material（ZIF）as a typical Metal Organic Framework Material（MOF）,Core-Shell magnetic catalytic materials were prepared by one-step pyrolysis with cobalt iron bimetallic ZIF-9 as precursor and graphite oxide（GO）as structure directing agent through solvothermal method.In this study,Rhodamine B（Rh-B）and persulfate（PMS）were selected as the target degradation compound and oxidant,respectively.The effects of cobalt-iron ratio,GO dosage,pyrolysis temperature and pyrolysis time on the decolorization efficiency of Rh-B were investigated.The preparation conditions were optimized,the CoFe₂O₄@nitrogen-doped porous carbon（CoFe₂O₄@NPC）catalyst with core-shell structure was successfully prepared.Various analytical technologies were used to characterize the CoFe₂O₄@NPC prepared under the optimal conditions,and the catalytic performance of the catalyst was evaluated in detail.Based on the above results,the degradation efficiency of each factor on Rh-b in the catalytic oxidation system was investigated comprehensively.Finally,the activation mechanism of CoFe₂O₄@NPC for PMS and the degradation mechanism of Rh-B in the catalytic oxidation system were discussed.The specific experimental results are as follows:（1）The optical conditions were cobalt-iron 1:2,GO dosage 2 mg/m L,pyrolysis temperature 600 ^oC and pyrolysis time 3 h,and the CoFe₂O₄@NPC catalyst had the highest catalytic activity.The catalyst had a specific surface area of 114.18 m²/g and had a rich pore structure.The dispersion and stability of CoFe₂O₄nanoparticle was significantly improved,which could be attributed to the anchoring role of all kinds of structured nitrogen such as pyrrole nitrogen,pyridine nitrogen and graphitic nitrogen,and a small amount of oxygen-containing functional groups of nitrogendoped porous carbon through C-N-M and C-O-M bonds.The decolorization process of Rh-B dye wastewater by the CoFe₂O₄@NPC/PMS catalytic oxidation system followed pseudo-second-order kinetics.The rate constant K₂ was 0.05364 L/（mg·min）,and the activation energy（Ea）was 4.230 k J/mol,much lower than other systems.The decolorization efficiency of Rh-B was all above 95%during five successive reutilization experiments.Moreover,the dissolution amounts of cobalt ion and iron ion were 0.89 mg/L and 0.83 mg/L,respectively,and redox peak wasn’t observed in the cyclic voltammetry curve.These results indicated that the catalyst had excellent catalytic activity and stability.（2）The optimal parameters were the dosage of CoFe₂O₄@NPC 60 mg/L,the dosage of PMS 300 mg/L,pH=6,and the initial concentration of Rh-B 100 mg/L,the catalytic oxidation system has the best degradation efficiency on Rh-B dye wastewater.The dominating free radical in the catalytic oxidation system is SO₄^-·,and the highly effective activation of PMS by CoFe₂O₄@NPC was owing to both Co²⁺and Fe³⁺in CoFe₂O₄ highly dispersed and stable in the nuclear layer on the one hand,and structural nitrogen delocalizedπelectrons in the nitrogen-doped porous carbon shell on the other hand.In addition,the nitrogen-doped porous carbon was rich in sp²hybrid aromatic structure,which facilitated the adsorption of Rh-B on its surface throughπ-πstacking interaction,and further improved the catalytic degradation efficiency of the catalytic oxidation system on Rh-B.Based on the results of full wavelength scanning tests of UV-visible spectrophotometer,it could be inferred that the degradation process of Rh-B attacked by SO₄^-·included four processes:N-deethylation,chromophore cracking,ring opening and mineralization.