Study On Degradation Of Organic Pollutants By Persulfate Activated By Modified δ-MnO₂ Composites

In recent years,advanced oxidation technology based on sulfate radical(SO₄^·-)has gained wide attention due to the advantage of the oxidants which are very stable in the environment,cost-effective,easy to be stored and transported,and have no secondary pollution.Manganese dioxide（MnO₂）is one of the most attractive oxide materials due to its high natural abundance,low toxicity and low cost.Among the different crystal types of MnO₂,δ-MnO₂has a special layered structure,which is conducive to the movement of charged particles in the lattice,resulting in excellent electrochemical performance.However,the catalytic performance ofδ-MnO₂is relatively low while it is directly used to activate perbisulfite（PMS）to remove organic pollutants.Therefore,in this researchδ-MnO₂was modified and doped,the catalytic performance and mechanism of the modifiedδ-MnO₂composites for activating PMS to degrade organic pollutants were studied.The main contents are as follows:（1）The catalytic performance and mechanism of graphite nanosheets（GNs）-supportedδ-MnO₂composites（δ-MnO₂/CNs）in activating PMS to degrade AO7 were investigated.The results showed that the addition of GNs affected the structure and morphology of the catalyst,and the strong metal-support interaction between GNs andδ-MnO₂promoteed the rapid removal of contaminants by inducing abundant surface hydroxyl groups.Compared with singleδ-MnO₂and GNs,δ-MnO₂/GNs catalyst had significantly higher oxidative degradation capacity for PMS.The degradation rate ofδ-MnO₂/GNs+PMS system(0.065 min^-1)is 17 times that ofδ-MnO₂+PMS system(0.0038 min^-1).The characterization of the catalyst before and after the reaction proved that the catalyst had high stability and reusability.The experiments including the free radical quenching,the PMS decomposition,AC impedance（EIS）,attenuated total reflection Flouriert ransformed infrared spectroscopy（ATR-FTIR）,and open circuit potential（OCP）confirmed that AO7 was degraded by direct electron transfer of intermediate complex Mn-O-（HO）OSO₃.（2）The catalytic performance and degradation mechanism of Fe-dopedδ-MnO₂/CNs catalyst for activating PMS to degrade bisphenol A（BPA）were investigated.Through a series of activity comparison of the catalysts with different Fe/Mn ratios,it was confirmed that not only GNs,but also Fe（III）could affect the structure and morphology of the catalyst.When Fe/Mn ratio is 2:1,Fe₂Mn₁/GNs catalysts with smaller particle size,higher surface area,strong interaction with GNs,and abundant surface Fe and Mn species can be obtained.When the dosage of catalyst was 0.2 g/L,the concentration of PMS was 0.65 m M,the initial concentration of BPA was 87.7μM,the p H was 8.2,and the reaction temperature was constant at 25℃,the Fe₂Mn₁/GNs/PMS system could degrade up to 96%BPA in 15 min,and the reaction rate was 0.22 min^-1.The results of the free radical quenching experiment,electron paramagnetic resonance,X-ray photoelectron spectroscopy,PMS decomposition experiment,linear sweep voltammetry（LSV）,temperature-programmed reduction（H₂-TPR）,EIS,etc.,showed that the valence states of Fe and Mn did not change during the degradation process,and there was no formation of reactive oxygen species（ROS）.However,the direct electron transfer mechanism was confirmed in this system.It was proved by galvanic cell experiment and OCP that BPA was mainly adsorbed by manganese species,while PMS was adsorbed by iron species through hydrogen bonding with hydroxyl group.In addition,different from the mechanism of Mn/GNs catalysts that play an important role in electron transfer,in this process only occurs between BPA and PMS with simultaneous surface activation.In this work,the catalytic performance ofδ-MnO₂activating PMS to degrade organic pollutants was improved by introducing carrier and doping metal.The catalytic mechanism was discussed in detail.It provides some theoretical basis for the development of high efficient and rapid oxidation technology catalyzed by persulfate.