Construction Of Photocatalytic Oxidation System Based On Ag₃PO₄/GO And Its Removal Mechanism Of Refreatory Organics

A visible light catalytic oxidation system（Ag3PO4/GO）based on the composites of graphene oxide（GO）and silver phosphate（Ag 3PO4）is widely used for the treatment of refractory organics due t o its strong visible light response and high quantum efficiency.However,the weak photocatalytic activity and poor oxidation capacity of Ag3PO4/GO give rise to low degradation and mineralization rate of refractory organic matter.According to the previous studies,the photocatalytic activity of the system can be improved by modifying the catalysts with carbon materials and noble metals,while the oxidation capacity of the catalytic system can be enhanced by constructing heterojunctions and adding auxiliary oxidants.In this paper,a novel photocatalytic oxidation system（AG-Ag3PO4@g-C3N4/PDS/Vis）was built up,which is based on Ag3PO4/GO photocatalyst via the co-decoration of reduced graphene oxide（r GO）and silver nanoparticles（Ag NPs）,the construction of heterojunction with graphitic carbon nitride（g-C3N4）,and the addition of peroxydisulfate（PDS）.The underneath mechanism of enhanced degradation and mineralization performance of AG-Ag3PO4@g-C3N4/PDS/Vis system were studied using carbamazepine（CBZ）as an indicator.In this thesis,various studies have been done on optimal preparation conditions,morphology,and composition characteristics of Ag 3PO4/GO photocatalyst,as well as the activity and optimal reaction conditions for degrading CBZ.Optimal preparation conditions of Ag3PO4/GO are as follows:the concentration of Ag NO3 and Na2HPO4·12H2O are both 100g/L,and the concentration and content of GO are 1.0 g/L and 1.0%,respectively.Moreover,under the optimal reaction conditions,the removal rate of CBZ（5 mg/L）from water could reach 100%in 30 min by this system.The rate constant for CBZ degradation is 0.12 min^-1,which is much higher than traditional catalysts such as Ti O2 or Bi VO4.The composites of GO and Ag3PO4 can improve the dispersibility of Ag3PO4 particles,which enhances visible light absorption,boost photogenerated charge separation,and improves the photocatalytic activity of the Ag3PO4/GO system.To further improve the photocatalytic activity,the prepared Ag 3PO4/GO photocatalyst was simultaneously modified with r GO and Ag NPs via a one-step hydrothermal method in this thesis.The AG-Ag3PO4 photocatalyst was constructed.The results showed that the best performance of AG-Ag3PO4 was obtained when the molar ratio of Ag NO3/Na2HPO4 is 3:1.5,hydrothermal time is9 h,and r GO content is 1.0%.With the catalyst dosage of 0.5 g/L,the removal rate of 5 mg/L CBZ in water reached 100%within 15 min using AG-Ag3PO4.The rate constant reaches 0.51 min^-1,which is 4.25 times that of the Ag3PO4/GO system.However,the TOC removal rate of the AG-Ag3PO4 system is still as low as 7.85%.The simultaneous modification of r GO and Ag NPs effectively enhanced the visible light absorption of the system and improved its photogenerated charge separation ability.Specifically,the modification with an appropriate amount of Ag NPs（1.9%of Ag⁰）is crucial for enhancing photocatalytic activity.Considering the low TOC removal rate of the AG-Ag3PO4 system,the catalytic oxidation performance of the system still needs f urther improvement.Therefore,the heterojunction of AG-Ag3PO4@g-C3N4 was constructed and the PDS was added in this thesis,which built up the novel photocatalytic oxidation system,AG-Ag3PO4@g-C3N4/PDS/Vis.The results showed that the new catalytic oxidation system significantly improved the degradation rate of CBZ and the removal rate of TOC.The rate constant of removing 5 mg/L CBZ in water was as high as 0.45 min^-1,and the TOC removal rate reached 87.15%.The study indicates a synergistic effect between the AG-Ag3PO4 and g-C3N4components of AG-Ag3PO4@g-C3N4,while PDS can capture the photogenerated electrons from AG-Ag3PO4@g-C3N4,which significantly improved the mineralization rate of CBZ in the AG-Ag3PO4@g-C3N4/PDS/Vis system.The mechanism of CBZ degradation and the improved mineralization rate of CBZ in the novel photocatalytic oxidation system was discussed in this thesis.The results showed that the main ROS for the degradation of CBZ by Ag3PO4/GO and AG-Ag3PO4 are both photogenerated holes（h⁺）and superoxide radicals（·O2^-）,while the main ROS of AG-Ag3PO4@g-C3N4 is h⁺,·O2^-,and singlet oxygen（¹O2）.Specifically,AG-Ag3PO4@g-C3N4 is a Z-type heterojunction,and the photogenerated electrons in the valence band can reduce the PDS to generate more O2^-and ¹O2.There are two main paths for attacking ROS to decompose the C=C unsaturated bond on the seven-membered ring of CBZ.First,after CBZ is attacked by O2^-and h⁺to form an epoxy bond,it can be converted into a fused-ring macromolecular substance through ring condensation and deamidation.It can also be converted into small molecules via electrophilic attack.Second,the electrophilic cycloaddition reaction and ring-opening cleavage reaction between CBZ and electrophilic reagent ¹O2 convert CBZ into small molecules.The presence of ¹O2 in AG-Ag3PO4@g-C3N4/PDS/Vis can enhance its electrophilic attack ability,further crack CBZ into small molecules,and achieve high mineralization of CBZ.According to the above studies in this thesis,an Ag3PO4/GO-based photocatalytic system that can efficiently and rapidly remove r efractory organics in water was developed,providing theoretical guidance for treating refractory organics in water.