Study On Photocatalytic Degradation Of Several Benzoylurea Pesticides By G-C₃N₄ Under Simulated Solar Irradiation

The large-scale production and application of pesticides provides an important guarantee for the national food security,whereas it also poses a serious threat to the ecological environment and human health.As a substitute for organophosphorus and organochlorine pesticides with high toxicity and residue,benzoylurea pesticides have been widely welcomed for their selective inhibition of chitin,low toxicity and friendliness to mammals in the market.However some kinds of pesticides among them with poor solubility,good stability,and persistent residues presented the significant adverse effects on the non-target organisms such as fruits/vegetables,livestocks and aquatic organisms.They were often included the shrinkage of community,declination of reproductive rates,malformation of organs,disorder of endocrines and even genetic mutation.Thus it is necessary to thoroughly evaluate the degradability of different BUPs and study their accelerating degrading processes in natural light,water or soil that is meaningful to R&D and administration of environ-governance.Using a xenon lamp to simulate the visible light sources,for example sunlight,we initiatively developed a new glass-based nano-C₃N₄ film as a mild visible light-driven degradation process:（1）firstly the preparation,characterization and properties of photocatalytic nano-films were carried out.Then the phthalate ester was picked as a typical environmental pollutant and its degradation rate could reach 85-100% under 3⁵ hrs sunlight irradiation.The non-metal nanofilm can be easily separated after reaction and can be recycled for many times with no obvious decrease;（2）the degradation kinetics and mechanism of seven BUPs（teflubenzuron,diflubenzuron,lufenuron,hexaflumuron,novaluron,chlorfluazuron and flufenoxuron）were further investigated under simulated visible light irradiation.The results showed that the pesticides could be degraded to some extents in 2³ hrs under visible light and the photo-acceleration effect was increased by about 30%-60% compared with the control;（3）the seven photodegradation constants of BUPs were calculated and agreed with the first-order reaction kinetics model.The conformation and energy levels of pesticides were therotical simulated with Gaussian 09 and their photodegradation rates were determined as follows: chlorofluoro substituents（teflubenzuron > diflubenzuron）,chlorofluoroalkoxy substituents（lufenuron > hexaflumuron > novaluron）,chlorofluoro benoxy/pyridoxyl-substituents（chlorfluazuron > flufenoxuron）;（4）the mechanism of photodegradation was reasonably inferred by UHPLC-MS analysis of the residual components of BUPs’ photodegradation.It was found that seven pesticides showed the similar degradation routines and there is a main degradation product（m/z=301.14）identified as 2-fluorobenzamide.Urea-bridge group is the most vulnerable to fracture,oxidation and dehalogenation;（5）the toxicity sequence of the above three-substituted BUPs was:diflubenzuron>teflubenzuron,lufenuron>novaluron>hexaflumuron,and chlorfluazuron >flufenoxuron.Finally the growth inhibition of Scenedesmus obliquus was observed in the wastewater after photodegradation.The toxicity of the photolysis products driven by the nano-film reduces to the environment-friendly range.In addition,nitrobenzene is an important intermediate and raw material of pesticides and explosives.Its production leads to the discharge of a large amount of strong acid or alkaline wastewater,which has obvious three-way toxic effects.Now we prepared g-C₃N₄/Ti O2 composite catalysts using cheap industrial titanium dioxide and melamine as raw materials and showed more sensitive to visible light.Their structure is a complex of graphite phase carbon nitride and anatase crystals.The response of Ti O2 can be effectively extended to the range from ultraviolet to visible-light.At least 80% of nitrobenzenes can be degraded without additives after 1.5hrs illumination.It demonstrates good and stable catalytic ability even in repeated use in acidic,neutral or alkaline environment.It was suggested that the materials derived from non-metallic C₃N₄ can be applied on a safe and environmentally friendly production and usage of pesticides or corresponding raw material and intermediates in the future.