Research On The Mechanism Of Photocatalytic Degradation Of Glyphosate By ZnO-based Nanomaterials

Glyphosate is currently the world’s most used broad-spectrum herbicide,with an annual global use of about 7 million tons,and in 2017,the World Health Organization’s International Agency for Research on Cancer listed glyphosate as a Class 2A carcinogen.In the natural environment,the main intermediate of biodegradation is aminomethylphosphonic acid（AMPA）has stronger toxicity and stability,and glyphosate and AMPA exceeding safe standard concentrations have also been detected in natural water.Therefore,how to remove glyphosate efficiently and cleanly and reduce the formation of AMPA has always been the focus of research at home and abroad.Photocatalytic technology is a green and efficient advanced oxidation technology,which rapidly degrades pollutants in water by absorbing sunlight to catalyze the production of oxidizing active species.As one of the most studied photocatalysts,ZnO has non-toxicity,high quantum yield,low synthesis cost and excellent biological and chemical properties,but ZnO is prone to photocorrosion in the photocatalytic process,resulting in a significant reduction in its photocatalytic performance,and the rapid recombination of ZnO photogenerated electrons will also reduce its photocatalytic efficiency.In this paper,ZnO is modified by different methods to clarify the mechanism of ZnO composite against photocorrosion.At the same time,the built-in electric field is constructed through ZnO’s own piezoelectric effect to improve the separation efficiency of photogenerated electrons,further promote the photocatalytic degradation of glyphosate by ZnO,and finally explore the interfacial adsorption effect of hydrogen peroxide-assisted ZnO photocatalytic and efficient degradation of glyphosate and the bond breaking pathway of glyphosate,which provides a new idea for the safe and efficient degradation of glyphosate in water by ZnO.The main research contents and conclusions are as follows:1.ZIF-8@ZnO composites were synthesized by changing the amount of organic ligand 2-methylimidazole by simple hydrothermal method.Through the photocatalytic cycle degradation experiment and the comparison of degradation rate constants,the significant improvement of the photocorrosion resistance of ZIF-8@ZnO composites was verified.By measuring the impedance,photocurrent,fluorescence spectroscopy（PL）and other electrochemical characterizations,the separation efficiency of photogenerated electron holes of ZIF-8@ZnO composites is significantly higher than that of ZnO,and the band gap width and the position of the conduction band valence band of ZIF-8@ZnO composites are calculated by UV-VIS diffuse reflection and Motschottky.It is clarified that the improvement of photocorrosion resistance of ZIF-8@ZnO composites is due to the transfer of photogenerated holes to the surface of ZIF-8 by the heterojunction structure formed by the composites,which prevents the decomposition and damage of ZnO by holes and improves the photocorrosion resistance of ZIF-8@ZnO composites.At the same time,the photocatalytic degradation of glyphosate by composite materials was analyzed,the main active species was determined as·O2^-by capture experiments,and ion chromatography（IC）and high performance liquid chromatography（HPLC）were used to predict that the degradation pathway of glyphosate was mainly through the breaking of C-N bonds to generate AMPA.2.The fine rod and coarse rod ZnO were synthesized by hydrothermal method and solvothermal method,and the degradation efficiency of the two ZnO on glyphosate under the conditions of 265 nm ultraviolet,40 k Hz,80 w ultrasonic field,and photoultrasound were determined.Through Zeta potential analysis,it was concluded that the positively charged and negatively charged glyphosate of fine rod ZnO had stronger electrostatic adsorption under the condition of pH=4.5,and the specific surface area determination showed that the fine rod ZnO had a larger specific surface area,which provided more active sites for the degradation of glyphosate.By determining the number of free radicals generated by different ZnO,it was shown that the superoxide radicals generated by ZnO in thin rods were much higher than those generated by coarse rods.Capture experiments also proved that the main oxidation-active species in piezoelectric photocatalysis are superoxide radicals.At the same time,X-ray diffraction（XRD）and scanning electron microscopy（SEM）were used to characterize and analyze,and the excellent crystal plane synergy effect of fine rod ZnO generated more superoxide radicals under piezoelectric photocatalytic conditions,which was the main reason for the improvement of the degradation efficiency of glyphosate.The degradation products were analyzed by HPLC,IC and mass spectrometry（MS）,and the results showed that during the piezoelectric photocatalytic degradation of glyphosate,the formation of the intermediate product AMPA was reduced due to the significant electrostatic adsorption of glyphosate by the（100）crystals exposed by the fine rod ZnO,and 25%of the non-toxic intermediate product glycine was generated.This provides a theoretical basis for ZnO to degrade glyphosate efficiently and safely.3.The photocatalytic degradation of glyphosate by adding hydrogen peroxide-assisted ZnO to efficiently degrade glyphosate was determined to maintain the efficiency of ZnO photocatalytic degradation of glyphosate at different pH and in different ionic environments,which proved the good applicability of hydrogen peroxide-assisted ZnO photocatalytic degradation of glyphosate in a wide range of pH and different environments.At the same time,the capture experiment and the free radical quantification experiment showed that the addition of hydrogen peroxide would promote the separation of electron holes in the photocatalytic process of ZnO and increase the formation rate of superoxide radicals,thereby greatly improving the degradation efficiency of ZnO to glyphosate.Raman and FT-IR infrared spectroscopy confirmed the coordination adsorption effect of ZnO on glyphosate,and the phosphate group of glyphosate had a strong coordination effect with the Zn site of ZnO,revealing the law that ZnO and Glyphosate molecule C-PO₃H₂ end coordination weakened C-P bond to reduce the formation of AMPA intermediate products.At the same time,a large number of free radicals will continue to degrade AMPA into small molecules,and the mineralization rate is as high as 70%in 2 hours.This provides a new idea for the practical application of ZnO photocatalytic complete mineralization degradation of glyphosate.