Study On Photocatalytic Degradation Of Triazophos In Oolong Tea Beverage

Tea has a long-history and rich-variety in China,and it has been popular for thousands of years.However,the problem of food safety is becoming more and more serious recently,and the problem of pesticide residues in tea beverage has been paid more and more attention.In this study,triazophos,which is a kind of common pesticide that easy to be solved in water,was selected to be an object to explore the result of nano TiO₂ photocatalytic degradation in Oolong tea beverage,so as to provide some reference for the practical application in the future.In this study,the self-made reactor was adopted to explore the photoelectronic semiconductor material nanometer titanium dioxide to degrade the triazophos residues in Oolong tea beverage under the catalysis condition of ultraviolet light.There are three main aspects in this study:（1）the method of low concentration of triazophos detection by GC was built for exploring the degradation rules of triazophos with ultraviolet light catalysis under different conditions and getting all the kinetic equations,and the final products of triazophos was analyzed by GC-MS,the degradation processes and photocatalytic reaction pathways of triazophos were deduced;（2）explored the effect of Oolong tea’s quality with ultraviolet light catalysis under different conditions,which measured by the changes of theanine and tea polyphenol content,and analyzed the changes of Oolong tea beverage before and after degradation by GC-MS at the best condition to determine the applicability of the method;（3）The nano TiO₂ loaded by quartz glass tubes were characterized by the scanning electron microscope 、 X ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）for analyzing the microscopic mechanism of photocatalytic degradation.The results were as follows:（1）The tea beverage was extracted by acetonitrile,purified by PTP solid phase extraction column and acetonitrile: toluene3:1（V/V）,then used GC with NPD as the detector with the air flow rate at 150 m L/min、hydrogen flow rate at 22 m L/min、nitrogen flow rate at 27 m L/min,initial temperature at 100℃,Keep 2min,then at the heating rate of 15℃/min made the temperature at 180℃,Keep 2min,then at the heating rate of 10℃/min made the temperature at 220℃,Keep 2min,then at the heating rate of 5℃/min made the temperature at 240℃,Keep 1min,which could detect triazophos well.When the additive amount of triazophos were 0.5ppm-2ppm,the recovery rate of triazophos were 99.38%-100.60%,the relative standard deviation of triazophos were 5.98%-6.66%,the detection limit of triazophos is 0.01mg/L;（2）Chose parameters as the different concentration of Oolong tea beverage、different rate of circulation of the reaction system、different ultraviolet light intensity and the different load capacity of TiO₂、the different temperature and time of calcination、the different load number of piles of TiO₂、the different ratio of the content of TiO₂ and PVPK30 to explore the effect of p hotocatalytic degradation of the triazophos residues in the Oolong tea beverage,and got the best conditions as 1g/100 m L of the Oolong tea beverage concentration、800m L/min of the rate of circulation of the reaction system、26mol/m2 s of the ultraviolet light intensity and 0.15% of mass fraction of TiO₂、500℃ and 2h of calcination、one layer of loaded TiO₂、1:1 the content ratio of TiO₂ and PVPK30,which could degrade more than 98% of triazophos residues in 10 min,which means the reaction rate got rose when nano TiO₂ was added,and got increased by 144% comparing with the pure ultraviolet degradation.It can be deduced from the degradation product of triazophos analyzed by GC-MS,the P-O ester bond was broken by photocatalysis degradation.（3）The content of tea polyphenol in Oolong tea beverage was relatively stable before and after degradation,whether it was pure UV degradation or nano TiO₂ photocatalyst,the retention rate of tea polyphenols were all more than 90% under all degradation conditions,which were totally in line with national standards;the theanine of Oolong tea beverage showed some linear reduction rules when the concentration of Oolong tea was different,and the retention rates after degradation were in direct proportion to the concentration of Oolong tea beverage,but there were no orderliness under other conditions such as different rate of circulation of the reaction system、different ultraviolet light intensity and the different load capacity of TiO₂.While the theanine in Oolong tea beverage showed varying degrees of reduction,the factor of UV intensity effected most on the content of theanine,the retention rate of which was only 65.57%.Relatively speaking,changes of the contents of tea polyphenols and theanine after degradation were smaller after adding nano TiO₂ catalyst.（4）It could be found that the aromatic components of the Oolong tea beverage mainly were aldehydes,ketones,esters,carboxylic acids,hydrocarbons and other compounds.Those substances all have their own characteristic flavor,and all the aroma ingredients mixed together,formed the specific aroma of Oolong tea.Under the optimum conditions of the photocatalytic degradation,there was no significant change in the composition of aroma components of Oolong Tea beverage before and after degradation,but some contents such as hydrocarbons,carboxylic acids,phenols and so on were reduced,therefore,the photocatalytic degradation had a certain effect on the aroma of tea beverage,which led the flavor of tea beverage to a certain extent weaker,but the tea beverage fragrance remains as a whole.（5）The XRD analysis showed that calcination can affect the particle size of nano TiO₂ crystal,the higher the temperature is,the larger the grain size of anatase is,the smaller mass fraction of anatase is,the smaller half peak width is,the optimum conditions of nano titanium dioxide particle size is 25.76 nm,the mass fraction of anatase is 80.05%,indicating that it is the most suitable particle size and mass fraction of anatase of the degradation conditions of triazophos residues in Oolong Tea.And according to the analysis of SEM,it is found that the nano titanium dioxide is uniformly and firmly loaded on the qua rtz glass tube with a slight agglomeration,but the dispersion of titanium dioxide nanoparticles is also good.（6）The XPS characterization was carried out to analyze the nano TiO₂ under300℃、2h and 500℃、2h of calcination conditions,the results showed that the content of hydroxyl oxygen of 500℃、2h of calcination was 15.56% while the content of hydroxyl oxygen of 300℃、2h of calcination was only 13.58%,and the Ti3+ of nano titanium dioxide under 500℃、2h of calcination is significantly higher than 300℃、2h of calcination conditions,and the ratio of the content of Ti3+ and Ti4+ at 500℃ reached 34.25:1,which meant that the content of Ti3+ in 500℃ is 6.9 times of the content in 300℃,which further demonstrated that the enhancement effect of 500℃、2h calcination on nanometer TiO₂ photocatalytic activity.