Research On The Mechanism Of The Degradation Of Organic Pollutants Via Persulfates Activated By MnO₂ With Different Crystalline Structures

With the rapid development of urbanization and industrialization,organic contaminants such as p-Nitrophenol and its derivatives discharged from industrial refineries become an urgent environmental issue.Advanced oxidation processes（AOPs）have considered as effective technologies to degrade organic compounds in aqueous media,because they can generate highly reactive radicals,such as ·OH and SO₄^·-radicals,that quickly oxidize a broad range of organic pollutants.Recently,sulfate radical has received an increasing interest in both research and application for the remediation of organic pollutants in groundwater and wastewaters because SO₄^·-is more powerful,selective,effective and possess longer half-life oxidant for the degradation of persistent organic pollutants than ·OH.Furthermore,as the origin of SO₄^·-,persulfates,including peroxymonosulfate（PMS）and peroxydisulfate（PDS）,are relatively stable,facile for the storage and transportation,environmental friendly,and cost-effective.Many methods was used to generate SO₄^·-by activating persulfates,but till now,metal oxides or supported metal oxides were considered as a highly efficient,very stable and without second contaminative heterogeneous catalysts.manganese oxide,what is rich in the soil and possessed low toxicity,including 20 kinds of polymorphs,like MnO,MnO2,Mn2O3,Mn3O4,other multivalent nature and non stoichiometric chemical composition of the manganese oxide was much more interesting and complex than a single oxide.Based on the previous reports,there were plenty of mechanism about the PMS activation by manganese oxides,but no certain mechanism were raised,especially mechanism about the PMS activation by MnO2 with different crystalline forms.Based on this,our research group obtained MnO2 with different crystalline forms through different methods,deeply discussing PMS activation mechanism and organic contaminant degradation mechanism.Concrete content include:（i）why different crystalline forms of MnO2 show different catalytic performance for the PMS activation;（ii）how to activate PMS to produce SO₄^·-or ·OH radicals by MnO2 with different crystalline forms;（iii）how to degrade organic pollutants by the SO₄^·-or ·OH radicals.Firstly,our research group synthesized nearly amorphous δ-MnO2（OL-1）nanoparticles by liquid phase synthesis,then adding graphene oxide（GO）as the regulating agent of morphology and phase transitions,successfully obtaining linear,uniform,pyknotic and novle α-MnO2（OMS-2）nanofibers by using hydrothermal method lastly.Comparing catalytic properties of OMS-2 and OL-1,which states that when significant 4-NP degradation of OMS-2 achieved 96.96% with PMS in 80 min,while OL-1 could only reach 31.11% in the range of 80 min.Two kinds of catalysts were conforming to pseudo first order kinetic model during 4-NP degradation process.The degradation rate of KOMS-2（0.086 min-1）was about 21.5 times of KOL-1（0.004 min-1）.The activation energy Ea of OMS-2 is 28.53 kJ/mol.And we systematically and deeply investigated the PMS activation mechanism by the as-prepared OMS-2 and OL-1 using more advanced measurements,such as EPR,XPS,LC,Raman,H2-TPR,cyclic voltammetry,LC-MS and so on.Experiment results stated that the main reasons for the different catalytic activities of OMS-2 and OL-1 in the PMS activation are as following.1)Seeing from the aspect of catalyst structure mechanism,OMS-2 possessed three valences of Mn（Ⅱ）,Mn（Ⅲ）and Mn（Ⅳ）,while OL-1 only owned two valences of Mn（Ⅲ）and Mn（Ⅳ）.OMS-2 had more transformation channel than OL-1 when activated PMS;2)Looking from the oxidation and reduction ability of catalyst surface,OMS-2 had wider redox potential,namely 0.191 V,while OL-1 owned only 0.150 V;3)Test results of H2-TPR showed that OMS-2 had a larger number of active sites than OL-1;4)Thinking from PMS activation mechanism,whether OMS-2 or OL-1 was produced SO₄^·-and ·OH as the main radicals during PMS activation.With NB and BA as probes of SO4-· and ·OH,the reaction rate was k（OMS-2 SO4-·）=0.00224 min-1,k（OL-1 SO4-·）=0.00191 min-1,k（OMS-2 ·OH）=0.01615 min-1 and k（OL-1 ·OH）=0.00899 min-1,respectively.OMS-2 possessed faster radical producing rate contrasting OL-1.What’s more,our group compared efficiency of heterogeneous catalysis of OMS-2 and conventional homogeneous catalysis.Results proved heterogeneous catalytic efficiency of OMS-2 coordination PMS is much higher than the homogeneous catalytic efficiency by Mn2+ coordination PMS.Finally,our group researched 4-NP degradation pathway deeply.Testing result ascertained that 4-NP degradation pathway in whether OMS-2 or OL-1 catalyst process is nearly same,but 4-NP degradation rate by OMS-2 was higher than OL-1.