Preparation Of Defective Graphitic Carbon Nitride And Its Catalytic Degradation Performance Of Organic Pollutants

The impact of various environmental pollution problems on the sustainable development of society caused by the acceleration of urbanization has become increasingly striking.The advanced photocatalytic oxidation technology that converts renewable solar energy into chemical energy is considered a desirable strategy for solving environmental pollution problems.Recently,in virtue of its unique physical,chemical and optical properties,metal-free graphitic carbon nitride(g-C3N4)is deemed to be an ideal photocatalyst for solving above problems.However,the original g-C3N4 still has many shortcomings,and these factors lead to its low photocatalytic redox efficiency,which limits its long-term development and practical application in the arena of photocatalytic environmental purification.Therefore,it is urgent to modify g-C3N4 to make it a highly efficient and stable photocatalyst.In this paper,the impurity defect engineering and intrinsic defect engineering are applied to modify g-C3N4,which significantly improves its photocatalytic performance of and the activation performance of persulfate,and its performance and oxidation mechanism in removing organic dyes and pesticides in water under visible light were studied.This article is intended for providing inspiration for the establishment of an efficient g-C3N4-based photocatalytic oxidation system.The research content and main conclusions of this article are as follows.(1)To control the band structure and surface defect concentration of g-C3N4,the metal-free sulfur-doped g-C3N4(SCN)was synthesized in situ by one-step thermal polymerization.On this basis,the ball milling method was used to further modify the SCN,and then the influence of the wet and dry environment of the ball mill on the photocatalytic degradation performance of the SCN was deeply discussed.The research results show that the valence band of SCN after wet ball milling is shifted positively due to the quantum size effect,the conduction band is shifted negatively,and the band gap width increases,thus enhancing the redox property of charge carriers.Compared with the dry ball milling method,the particle size of the SCN modified by the wet ball milling method is greatly reduced,thereby increasing the specific surface area and the number of exposed reactive sites of the SCN.In addition,the reduced concentration of amino defects on the surface of the SCN after wet ball milling promotes the effective separation and transfer of photo-generated electron and holes,which is instrumental in diminishing their recombination probability.Therefore,the photocatalytic performance of wet ball milled SCN for methylene blue degradation was 1.5 times and 3.6 times higher than that of SCN and dry ball milled SCN under visible light irradiation,respectively.(2)To improve the degradation ability of the g-C3N4-based photocatalytic system,the bifunctional oxygen-doped g-C3N4(OCN)was prepared by a one-step calcination method.The effect of oxygen doping on the physicochemical properties of g-C3N4 was systematically investigated.On this basis,the photocatalytic activation performance of persulfate(PMS)by OCN to degrade imidacloprid was evaluated under the combined conditions of photocatalysis and activated persulfate oxidation.As a result,O doping in the crystal lattice effectively adjusts the electronic structure,energy band structure and optical properties of g-C3N4.On the one hand,the improved electronic structure is conducive to the formation of dual redox sites of PMS to improve the non-photochemical activation ability.On the other hand,the negative shift of the conduction band edge of OCN is conducive to improving the reduction ability of photogenerated electrons and the PMS activation performance.In addition,due to the thinner layered structure,increased specific surface area,and ameliorated photoelectric properties,the separation and transfer ability of photogenerated electron and hole pairs is enhanced,and the catalytic active sites of PMS activation reactions increase.Radical capture experiments and EPR spectroscopy proved that singlet oxygen is the main active substance for imidacloprid degradation,while superoxide radicals,hydroxyl radicals and holes not only participate in the generation of singlet oxygen,but also participate in the degradation process of imidacloprid.(3)To explore the applicability and practical application potential of the OCN/Vis/PMS composite system,the influence of environmental factors on the degradation of imidacloprid by OCN driven photocatalytic activation of PMS were investigated,including the types of oxidants,PMS dosage,catalyst dosage,pH,humic acid,various inorganic ions and complex actual water matrix.By using OCN-10 as the activator,the best test conditions were determined through degradation experiments:the oxidant was PMS,the dosage of OCN-10 was 0.1 g/L,and the dosage of PMS was 0.3 g/L.The results showed that the higher the pH,the faster the degradation rate of imidacloprid in the OCN-10/Vis/PMS composite system.Besides,sulfate and nitrate ions have no apparent influence on the imidacloprid degradation,while nitrite,dihydrogen phosphate,carbonate,bicarbonate and low concentrations of chloride ions all have a certain inhibitory effect on the degradation of imidacloprid.Instead,alkaline conditions,humic acid and high concentration of chloride ions will greatly promote the imidacloprid degradation.What’s more,OCN-10/Vis/PMS system has good treatment capacity for actual river water,lake water and sewage treatment plant effluent,showing potential application value.Finally,the degradation path of the typical neonicotinoid pesticide imidacloprid is proposed.