A Study On The Degradation Performance And Mechanism Of Organic Pollutants By Persulfate Activation Using Iron-based Magnesium-aluminum Hydrotalcite Catalysts

In order to cope with the growing problem of water pollution,the Fenton system and the resulting Fe-based advanced oxidation system have received extensive attention due to their high efficiency,environment friendly nature and cost effectiveness.However,the undesirable Fe^III/Fe^II redox cycles restrict it from sustainable activity during practical applications.In this paper,two homogeneous and heterogeneous Fe-based catalysis systems are constructed based on MgAl-MoS₄ LDH obtained by ion exchange method from MgAl-CO₃ LDH,expecting to control the redox cycles of Fe^III/Fe^II and Mo^VI/Mo^IV after enriching thiol groups(S^2-)in the interlayers of FeMgAl-MoS₄ or MgAl-MoS₄.In this study,MgAl-MoS₄ and FeMgAl-MoS₄catalysts with typical LDH structure were prepared by hydrothermal method and ion exchange method.Then Fe³⁺+MgAl-MoS₄ LDH/PMS and FeMgAl-MoS₄ LDH/PMS systems were used to degrade 4-chlorophenol?4-CP?and p-hydroxybenzoic acid?PHB?respectively.And the catalytic oxidation performance and reaction mechanism of the two systems were toatolly studied.The main contents and results are as follows:?1?MgAl-MoS₄ LDH was successfully prepared by hydrothermal method and ion exchange method.The removal rate of 4-CP in 60 minutes by using Fe³⁺+MgAl-MoS₄ LDH as catalyst to activate PMS was more than 98%.After comparison,the optimal conditions for the system were as follows:MgAl-MoS₄ LDH dosage was 0.1 g/L,Fe³⁺dosage was 5 ppm,PMS dosage was 3 mM.And 10 ppm 4-CP can be substantially completely degraded in 60minutes under the optimal reaction conditions.?2?The Fe³⁺+MgAl-MoS₄ LDH/PMS system is an out-surface reaction,which relies on the electrostatic interaction between PMS,4-CP and MgAl-MoS₄ LDH particles.The main active species in the process of degrading 4-CP is SO₄·^-and·OH,where SO₄·^-made a major contribution.The process of activating PMS by Fe³⁺+MgAl-MoS₄ was proposed.PMS first combines with some active sites on the surface of MgAl-MoS₄ LDH to form a complex during the activation process,while Fe^III in the solution is adsorbed to the surface of MgAl-MoS4 and reduced to Fe^II.Then PMS is activated by electron transfer to produce SO₄·^–to degrade 4-CP.?3?FeMgAl-MoS₄ LDH was successfully prepared by hydrothermal method and ion exchange method.The removal rate of PHB was 100%in 60 minutes by using FeMgAl-MoS₄LDH as catalyst to activate PMS.After comparison,the optimal conditions for the catalytic system were as follows:the dosage of FeMgAl-MoS₄ LDH was 0.25 g/L,the dosage of PHB was 10 ppm,and the dosage of PMS was 5 mM.Under the optimal reaction conditions,10ppm PHB can be completely degraded within 60 minutes.Compared with other advanced oxidation systems,the Fe-MoS₄/PMS system maintains a substantially uniform oxidation performance over a wide range of pH?3.0-10.0?.The effect of common anions or humic acids on the system is also very slight.?4?The degradation of PHB by FeMgAl-MoS₄ LDH/PMS system is an out-surface reaction,which depends on the electrostatic interaction between PMS,PHB and FeMgAl-MoS4 LDH particles.The main active species in the reaction process are SO₄·^-and·OH,where SO₄·^-made a major contribution rate of 57%and·OH made a contribution rate of 35%.A reasonable mechanism for the activation of PMS in Fe-MoS₄/PMS system is proposed.Firstly,after PMS undergoes complexation and electron transfer on the surface of Fe-MoS₄ catalyst,the surface bound?Fe^II/Mo^IV is oxidized to?Fe^III/Mo^VI and SO₄·^–species was produced to degrade organic substrates.Subsequently,the S^2-species is oxidized to S⁰ or SO₃^2-/SO₄^2-in a series of reactions.The electrons generated in this process will be used to reduce?Fe^III/Mo^VII to surface bound?Fe^II/Mo^IV species,thereby starting a new PMS activation process.This cycle will continue until all of the S^2-in the Fe-MoS₄ catalyst is oxidized.