Synergism And Mechanism Of Ag₃PO₄/Fe₃O₄/GO Bifunctional Catalyst In Visible Light-persulfate Coupling System

With the rapid development of medicine,chemical industry,printing and dyeing and other industries,the types and concentrations of refractory organic pollutants in water are increasing day by day.Some single water treatment technologies which have many limitations and defects,have been unable to meet the national emission standards.It has become an effective way to solve the defects of single technology by using the existing and relatively mature single water treatment technology as the coupling water treatment technology.Among them,the combination process with photocatalytic water treatment technology as the main body has gradually become the development direction of coupling water treatment technology with great application prospect.In such combination process,photocatalytic/Fenton coupling technology is suitable for practical engineering applications because of low energy consumption and normal temperature and pressure.However,there are still some drawbacks in the application of the technology,such as the low utilization of visible-light,the difficulty of catalyst recovery,the dependence of acidic environment on the reaction,the lack of theoretical basis for influencing factors and micro mechanism.In view of the above disadvantages and deficiencies,it is necessary to improve the photocatalytic/Fenton coupling technology to meet the increasingly stringent water quality discharge standards and the urgent demand for safe water quality environment.In this work,the design and preparation of the composite catalytic materials and the construction of the combined process were carried out to improve the coupling technology.The specific implementation mode is as follows:The visible light responsive catalyst Ag₃PO₄ was used to replace the UV responsive catalyst TiO₂,and the combination of Ag₃PO₄/Fe₃O₄/GO was used to improve the stability and recycling of the catalyst.The persulfate which can produce SO₄^-·was used to replace H₂O₂ which can produce·OH in the traditional Fenton system,so as to prolong the life of the active species,widen the application range of pH and improve the oxidative degradation ability of the system.The prepared Ag₃PO₄/Fe₃O₄/GO has bifunctional catalytic activities of visible-light and persulfate,that is,under visible-light irradiation,it can be used as photocatalyst;in the presence of PDS,it can be used as PDS catalyst;under the coexistence of visible-light and PDS,it can be used as a bifunctional catalyst.Then,Ag₃PO₄ photocatalysis and PDS catalysis were combined in the form of heterogeneous catalysis through the bifunctional catalyst.The heterogeneous catalysis was dominated by photocatalysis and coexisted by persulfate catalytic oxidation（Vis-PDS）.The coupling technology can effectively utilize visible-light,the catalyst was easy to be separated and recovered,and the pH range was wide.Furthermore,it could play the role of efficient degradation of pollutants through different mechanisms under different conditions.The main innovative achievements of the research are as follows:1.The preparation of Ag₃PO₄/Fe₃O₄/GO composite took Ag₃PO₄ as the main material,making full use of the PDS catalytic activity,magnetic recovery,conductivity and large specific surface area of Fe₃O₄/GO.Moreover,the structure information,composition,morphology,nanostructure,element and distribution,oxidation state and magnetic properties of surface elements were identified and analyzed by various characterization methods.After that,taking rhodamine B（RhB）and p-chlorophenol（p-CP）as the target pollutants,the photocatalytic performance,PDS catalytic performance and synergistic effect of Ag₃PO₄/Fe₃O₄/GO were investigated.Based on the mechanism of photocatalysis and PDS catalysis,the mechanism of synergistic degradation of pollutants by Vis-PDS coupling system was proposed.2.The introduction of Fe₃O₄/GO can effectively enhance the absorbance of Ag₃PO₄ in visible-light region and the reaction activity.The photocatalytic property of Ag₃PO₄/Fe₃O₄/GO was related to the mass fraction of Fe₃O₄/GO.The heat resistance,photocatalytic activity and structural stability of Ag₃PO₄/Fe₃O₄/GO are relatively good.There was a certain degree of photo etching in the process of photocatalytic reaction.In the photocatalytic system,h⁺and O₂^-·played the major role,while·OH played the secondary role.3.The mechanism of photocatalytic degradation of RhB by Ag₃PO₄/Fe₃O₄/GO was revealed.That was,GO retained the active center of Ag₃PO₄ nanoparticles and promoted the efficient decomposition of organic pollutants.The p/n heterojunction constructed by GO and Ag₃PO₄ promoted charge separation and prolongs the lifetime of photocarriers.Meanwhile,the photogenerated e^-on Fe₃O₄/GO reduced the O₂adsorbed on the catalyst surface to O₂^-·,which is an oxidant and can degrade pollutants.The h⁺on the valence band of Ag₃PO₄ oxidized and decomposed the pollutants directly.Meanwhile,it can oxidize the OH^-and H₂O on the catalyst surface to generate·OH to degrade the pollutants.4.It had been proved that Ag₃PO₄/Fe₃O₄/GO possessed better PDS catalytic activity than Fe₃O₄/GO,and the catalytic performance,characteristic groups and molecular structure of the catalyst were stable.Ag⁺and Fe²⁺in the catalyst material played the role of PDS catalyst together,and the catalytic capacity of the former on PDS was more significant.In addition,SO₄^-·and·OH were the main active groups of PDS catalytic system.5.It was clarified that the coupling system can effectively enhance the reactivity through synergism,and the synergy coefficient Syn was 59.8%.The mass ratio of Fe₃O₄/GO to Ag₃PO₄ had an effect on the catalytic performance of the coupling system,and the synergistic catalytic performance of Ag₃PO₄/Fe₃O₄/GO-0.05 was the best.Moreover,the activity,surface element composition and oxidation state of the catalyst in the coupling system were stable before and after reaction.Only a small amount of Ag and Fe elements in the catalyst were lost during the whole reaction process.6.The effects of kinetic parameters such as catalyst dosage,initial pH,concentration of pollutant and PDS on the three catalytic systems were determined.Compared with the single system,the pH range of the coupling system is wider,PDS consumption is less,and oxidation degradation capacity is stronger.The operation mode of the coupling system was the highest when it operated simultaneously.In addition,the common inorganic ions had different degrees of inhibition on the catalytic performance of the coupling system,the anion inhibition was significant which was in the following order:CO₃^2->HCO₃^->NO₃^->SO₄^2-.However,the inhibition of Mn²⁺and Mg²⁺was not obvious.7.It was explained that h⁺、SO₄^-·、·OH、¹O₂ and O₂^-·played a role in the degradation of p-CP by coupling system and the contribution rates are ranked according to h⁺>SO₄^-·>·OH>¹O₂>O₂^-·order.On this basis,the synergistic mechanism of the coupling system was expounded in detail according to the semiconductor type and the degree of separation of photo generated e^--h⁺pairs.From the perspective of chemical kinetics,electrode potential was used as criterion the to determine the forward reaction of Ag²⁺+Fe²⁺→Ag⁺+Fe³⁺in the coupling system.It was explained that the circulation reaction of Ag⁺and Ag²⁺,Fe²⁺and Fe³⁺could promote the regeneration of Ag⁺and Fe²⁺effectively and maintain the catalytic activity and stability of the catalyst.Finally,the reaction limits and mineralization rates of p-CP in coupling system were discussed.The main degradation intermediates of p-CP in the coupling system were analyzed as 2-butenoic acid,p-Benzoquinone and decanoic acid,and the degradation pathways were theoretically speculated.