Preparation Of Tunable G-C₃N₄-based Heterojunction And Its Visible Light Photocatalytic Degradation For Contaminants Of Emerging Concern（CEC）in Environmental Water

Contaminants of emerging concern（CEC）is frequently detected in natural water,drinking water and effluent from wastewater treatment Plants（MWWTP）.The long-standing CEC pose a potential threat to aquatic organisms,water resources,and human health.Therefore,it is urgent to remove trace pollutants in water.Photocatalytic technology is considered to be one of the greenest and most effective strategies to solve environmental problems.Graphite carbon nitride（g-C₃N₄）is known as a new generation of visible light catalyst because of its low cost,easy preparation,proper electronic band structure,good stability and visible light activity.However,the g-C₃N₄ suffers from several shortcomings,including low specific surface area,insufficient visible light utilization,and,particularly,rapid recombination of photogenerated charge carriers,which limits its application.To address these problems,this thesis is based on the modification of g-C₃N₄,including morphology control,co-catalyst control and preparation of multiple heterojunction composite materials.Starting from changing the specific surface area,increasing the active sites,broadening the visible light response range,and suppressing the electron-hole recombination rate,a tunable g-C₃N₄-based heterojunction photocatalyst was constructed for the photocatalytic degradation of organic pollutants in environmental water.The specific content of the paper is as follows:（1）Porous and edge curled g-C₃N₄ nanosheets with special morphology were prepared by gradient thermal polymerization in N₂ atmosphere using urea as precursor.The special morphology g-C₃N₄ has a large specific surface area and abundant active sites,which effectively promotes the separation of electron-hole pairs.Under simulated visible light irradiation,photocatalytic degradation of tetracyclines and quinolones in environmental water,the removal rate was greater than 85%,The active free radicals in the photocatalytic degradation reaction were predicted,and the photocatalytic degradation mechanism of g-C₃N₄ nanosheets with special morphology was proposed.By changing the preparation method,the morphology-controlled g-C₃N₄ nanosheet is obtained,which provides a better idea for improving the research of morphologically controllable g-C₃N₄ photocatalyst.（2）Using ultra-thin MoS₂ nanosheets as a cocatalyst,a MoS₂/g-C₃N₄ heterostructure aqueous dispersion was constructed through a simple ultrasonic adsorption method.Under simulated visible light irradiation,the photocatalytic degradation rates of chlorimuron-methyl and tetracycline antibiotics in environmental water by ultrathin MoS₂/g-C₃N₄ heterojunction photocatalyst were greater than 90%,8.8 and 13.4 times higher than those of g-C₃N₄,respectively.Combined with the high performance liquid chromatography-mass spectrometry（HPLC-MS）,the degradation products and degradation pathways of chlorimuron-methyl were inferred.Through free radical capture experiments,the active free radicals in the photocatalytic degradation reaction were predicted,and the mechanism of ultra-thin MoS₂/g-C₃N₄ heterojunction photocatalytic degradation of organic pollutants was proposed.（3）The hydrothermal treatment of melamine is used as the precursor to prepare the sheet g-C₃N₄ by thermal condensation polymerization.Tetrabutyl titanate was introduced into g-C₃N₄ suspension and g-C₃N₄/TiO₂ heterojunction photocatalyst was constructed by solvothermal method.Due to the close contact between the heterojunction interface,the synergistic effect of the two catalysts is enhanced.The photocatalytic degradation rate of carbamazepine and paracetamol in environmental water by 1.5wt%g-C₃N₄/TiO₂ heterostructure was high as 93%and 98%under simulated sunlight irradiation,which is 2.63 and 7.67 times that of g-C₃N₄,respectively.Research shows that the reactive·OH,h+,· O2-play the major role in the photocatalytic degradation process.（4）A series of ternary g-C₃N₄/g-C₃N₄/Pr6O11 composite photocatalytic materials were prepared by thermal polymerization with urea,melamine and praseodymium nitrate as the precursor.The introduction of an appropriate amount of praseodymium into the g-C₃N₄/g-C₃N₄ homotype heterojunction produces lattice defects,which not only reduced the recombination rate of photo-generated carriers,but also improves its photocatalytic activity.The 10wt%g-C₃N₄/g-C₃N₄/Pr6O11 heterojunction catalyst has high photocatalytic activity for the degradation of sulfonamide antibiotics under visible light irradiation,and its degradation removal rate can reach more than 85%.Combined with high performance liquid chromatography-mass spectrometry（HPLC-MS）detection,the intermediate products and possible degradation pathways of sulfachloropyridazine and sulfamethoxine were analyzed.In addition,the quenching effects of different scavengers displayed that the reactive h+play the major role in photocatalytic degradation of sulfonamides.