Photocatalytic Degradation Of Sulfonamide Antibiotics In Water By Modified Graphitic Carbon Nitride Nanosheets Under Visible Light Irradiation

As a group of common broad-spectrum antibiotics,sulfonamide antibiotics(SAs)are widely used in clinical medicine and aquaculture industry.However,it is difficult to degrade in the ecological environment and SAs have obvious cumulative effect,it has become one of the environmental problems faced by China and even the world.Photocatalysis is one of the advanced oxidation technologies,which has attracted extensive attention due to its mild reaction conditions,environmentally friendly reaction process and simple operation.Photocatalytic technology based on semiconductor materials can use clean and pollution-free solar energy to control the environmental pollution.It is the core of photocatalytic technology to develop efficient solar responsive semiconductor photocatalytic materials.At present,the most studied photocatalytic semiconductor material is titanium dioxide,which can only show catalytic activity under ultraviolet light irradiation,but the ultraviolet energy in the sunlight is less than 5%of the total energy emitted by the sun.Therefore,the development of novel photocatalysts with visible light response and high catalytic activity is of far-reaching significance under the background of the current energy shortage.Graphitic carbon nitride(CN),with advantages of reliability,simplicity,low cost and visible light response,is a promising photocatalyst with great potential in environmental pollution control.However,small specific surface area,low utilization of visible light and easy recombination of photo-generated carriers greatly inhibits the photocatalytic activity of CN.Compared with CN,CN nanosheets have larger specific surface area,more active sites,smaller thickness and shorter photo-generated charge diffusion distance,which are conducive to the migration for the photo-generated charge carriers.In this paper,the structure-activity relationship of the photocatalytic degradation of SAs based on the modified CN nanosheets was studied from the aspects of morphology modification,doping of heteroatom,engineering of active sites on the surface and 2D/2D heterostructure construction.The influencing factors and reaction mechanism of visible photocatalytic degradation SAs by these modified CN nanosheets were also revealed.The main conclusions are as follows:(1)Amino-rich CN nanosheets(CNNS)was prepared by collecting the gaseous products from thermal polymerization of urea.Cyanic acid can spontaneously and rapid polymerize to generate cyanuric acid at approximately 175°C in the gaseous phase.The cyanuric acid then reacted with ammonia to produce ammelide,and melamine was obtained by the further reaction of ammelide and ammonia.As a small and polar molecule,the large amount of ammonia intercalated into the interplanar space of the melamine and broke the weak van der Waals bonds during the copolymerization process of melamine in the gaseous phase.The gaseous products fell down into the crucibles as the temperature decreased.Finally,ultrathin CNNS were obtained in the larger porcelain crucible.The characterization results shown that the prepared nanosheets retained the basic crystal characteristics and elemental composition of CN,and had regular and uniform sheet morphology,large specific surface area and more active sites.The amino groups on the surface of CNNS could be used as the transmission channel of photo-generated electrons,which was conducive to the separation of photo-generated carriers.Under visible light irradiation,it has excellent photocatalytic degradation performance and photochemical stability for sulfathiazole in water.(2)Porous oxygen-doped CN nanosheets(CNO)were synthesized by calcination the high-pressure thermal polymerization of urea.The polymerization degree of urea could be controlled by the time of the high-pressure thermal polymerization,thus controlling the oxygen content in CN.The characterization results shown that a part of the N atoms in CN structure was replaced by O atoms and the O atoms were successfully doped into the framework of CN.Compared with CN,the doping of O atoms not only broadened the optical absorption and inhibited the recombination of photo-generated carriers,but also changed the energy band structure of CN,which made it easier to produce active radicals and had enhanced visible-light photocatalytic degradation of sulfamerazine in water.Moreover,there is no significant decrease in activity of CNO after four cycles of experiments.(3)The nickel oxide(Ni O_x)modified-CN(CN-m Ni O_x)was synthesized by one-step thermal polymerization using urea and nickel(II)acetylacetonate as precursors.The results of morphology shown that CN-m Ni O_x had folded layered structure and Ni O_xwas highly dispersed on CN.CN-m Ni O_x exhibited the higher photocatalytic activity than the photocatalyst prepared by the thermal polymerization of nickel nitrate and urea in the degradation of sulfathiazole under visible light irradiation.The enhanced photocatalytic activity resulted mainly from the introduction of acetylacetone,which was not only conducive to the dispersion of Ni O_x,but also helpful to form well contact interface between Ni O_x and CN,and optimize the migration pathways for the photo-generated charge carriers,thus improving the separation efficiency of photo-generated charge carriers.The results of species trapping experiments indicated that·O₂^-played a key role in photocatalytic degradation of sulfathiazole over CN-m Ni O_x.The excellent photodegradation activity of sulfathiazole could also be achieved with the effect of quality parameters(initial concentration,p H,inorganic ions and organic matter)and water quality(Yangtze River water in Yanziji,Qixia District,Nanjing and tap water in Xuanwu District,Nanjing).(4)The metal-free 2D/2D heterojunction photocatalysts were successfully obtained by the electrostatic self-assembly method using CNNS and carboxyl-rich covalent triazine-based frameworks nanosheets(CTFNS).The results of structure and morphology shown that CTFNS distributed uniformly on CNNS and CTFNS was closely combined with CNNS.The resulting CTFNS/CNNS heterojunction shown significantly enhanced photocatalytic activity for degradation of sulfamethazine under the irradiation of simulated solar light.The excellent photocatalytic performance of CTFNS/CNNS heterojunction was related to the intensive interaction at the interface of CTFNS and CNNS.The two components in CTFNS/CNNS heterojunction was combined by the electrostatic attractive interaction between carboxyl groups from the surface of CTFNS and amino groups from the surface of CNNS.The intimate contact could be used as the transmission channel of photo-generated electrons,which could greatly promote the separation efficiency of photo-generated carriers and improve the photocatalytic activity.In addition,CTFNS/CNNS heterojunction exhibits an excellent photocatalytic and structural stability during four recycling processes.