Modification Of Graphitic Carbon Nitride Photocatalyst And Its Enhancement Mechanism Of Removal Of Typical Organic Pollutants From Water

Harmful organic pollutants in water including dyes and antibiotics,seriously endanger the ecological environment and human health.Therefore,the removal of organic dyes and antibiotics in water is of necessity for the protection of ecological environment and human health.Graphite phase carbon nitride（g-C₃N₄）,as a new type of non-metallic semiconductor photocatalytic material,has the advantages of simple production,stable structure and visible-light response,which has been widely used in photocatalytic removal of organic pollutants in water and gas pollutants.However,g-C₃N₄ has some disadvantages,such as small surface area,high recombination rate of photogenerated electrons and holes,and consequent the poor photocatalytic activity toward environmental pollutants.In this work,the g-C₃N₄-based catalyst with enhanced performance was prepared by multiple calcinations,acetic acid post-treatment and heterogeneous structure construction.The effects of material microstructure and optical properties of the g-C₃N₄-based catalysts on the removal performance of organic pollutants in water were studied,and the possible mechanism of the enhanced removal performance of typical organic pollutants in water was preliminarily explored.The main research contents are as follows:A simple and effective thermal stripping method was used to obtain g-C₃N₄ with a high specific surface area by calcining the melamine precursor multiple times at the same temperature.The influence of calcination times on the performance of g-C₃N₄ for the removal of organic pollutants from water was mainly investigated.The results showed that after calcination four times at 580℃,the specific surface area of g-C₃N₄increased from 20.98 m²/g to 55.47 m²/g.The block-shaped g-C₃N₄ nanosheets turned into thinner aggregated nanosheets,and the volume increased significantly.The performance of the samples for the removal of organic pollutants from water was significantly enhanced after multiple calcinations.The photocatalytic rate constant of g-C₃N₄ calcined three times（CN-3）for the photocatalytic degradation of Rh B was about 0.085 min^-1,which was 6.5 times that of g-C₃N₄ calcined once（CN-1）;the photocatalytic rate constant of CN-3 for the degradation of methyl orange was about0.0073 min^-1,which was 2.1 times that of CN-1.Combined with optical property investigation,the valence band of CN-3 increased the due to the quantum size effect and the oxidation ability of h⁺is enhanced.According to the active species trapping experiments,the main active species in the photocatalytic degradation of Rh B are h⁺and·O₂^-..（2）Carbonyl（C=O）is introduced on the terminal amino group of g-C₃N₄（G-CN）via a condensation reaction in acetic acid solution under 80°C of oil-bath stirring（HAC-CN）.After the post-treatment of acetic acid,the photocatalytic degradation efficiency of antibiotics over HAC-CN is significantly enhanced with the quasi-first-order rate constants of 0.046 min^-1,0.0120 min^-1,0.0086 min^-1,and 0.0058 min^-1 for Rhodamine B（Rh B）,tetracycline hydrochloride（TC）,sulfamethoxazole（SMZ）,and ciprofloxacin（CPFX）,respectively,which are 1.42,1.50,1.50,and 1.99 times those over G-CN.The characterization and analysis indicate that C=O might offer more collection sites for photogenerated electrons to facilitate the separation of photogenerated electron-hole pairs.Combined with the calculation of the conduction band and valence band of HAC-CN,and the active species detection tests,h⁺,·OH,·O₂^-,and ¹O₂ contribute to the TC degradation,especially·O₂^-and ¹O₂ is the dominant oxidized radical.（3）g-C₃N₄ was first prepared by calcination and stripping method,and then g-C₃N₄/Bi OI heterojunction materials with different ratios were obtained via electrostatic self-assembly strategy at 80℃oil bath.The effects of the preparation temperature and precursors ratios on the performance of g-C₃N₄/Bi OI heterojunction for removal of Rh B in water and formaldehyde gas were mainly studied.The results showed that the composite sample CB-15-80-2 obtained under the optimal conditions could achieve 100%of Rh B photocatalytic degradation in 40 minutes,which was 1.99 times faster than that of individual Bi OI,and 2.3 times faster than that of individual g-C₃N₄.The enhanced photocatalytic performance of g-C₃N₄/Bi OI catalyst was mainly attributed to the heterojunction that could increase the separation of photo-generated electrons and holes and enhance the catalyst’s redox ability.