| Polychlorinated naphthalenes（PCNs）,a new kind of persistent organic pollutants,have received widely attention due to its persistence,long-distance transportability,bioaccumulation,biotoxicity.Although the production and use of PCNs are banned in most countries,PCNs are still unintentionally produced and released into the environment through heat-related processes.The transport of PCNs in the atmosphere is one of its main migration pathways,and PCNs in the atmosphere are mainly distributed between the gas-particle phase.Therefore,when studying PCNs in the atmosphere,the surface of atmospheric particles is The polychlorinated naphthalenes are an important research direction.In this work,Octachloronaphthalene（CN-75）,a typical polychlorinated naphthalene,is employed as the target pollutant.And silica gel H is used as simulated soil-derived atmospheric particles to analyze and research the photochemical transformation processes and mechanisms of octachloronaphthalene on the surface of silica gel.In this work,the analytical methodusing GC（gas chromatograph）were firstly optimized,including optimization of the chromatographic column temperature program,split ratio,and ECD detector current.Then the parameters for extraction with ultrasonic were explored,including ultrasonic time,frequency,temperature.and volume of extractant.Silica gel H were characterizedvia scanning electron microscope analysis（SEM）,laser particle size analyzer and Fourier transform infrared spectroscopy（FTIR）.Then effects of influencing factors on the photodegradation of CN-75 were observed,including the volatility of CN-75 and initial concentrations of CN-75,thickness of silica gel,and reaction temperature.Furthermore,the mechanisms of photochemical degradation were explored using radical quenching methods and anaerobic atmosphere reaction,the product analysis was performed with GC-MS（Gas Chromatography-Mass Spectrometry）.The results showed that the ultrasonic extraction conditions were determined as ultrasonic time 5 min,ultrasonic frequency 40 KHz,ultrasonic temperature 20 ℃,and the volume of extractant（n-hexane）was 15 m L,the extraction rate was the best,and the average extraction rate was 97 %.From the characterization analysis,it can be seen that the silica gel H has an irregular block shape,and its surface is loose and porous.The particle size range of silica gel H is 1.5-707.0 μm,most of which are concentrated between 7.8-26.3 μm,and the specific surface area of SSA is 0.479 m2 g-1.The CN-75 on the silica surface can be degraded by 96.07 % under the irradiation of UV-A ultraviolet lamp by 8 h reaction,but under the dark condition without ultraviolet lamp irradiation,CN-75 does not degrade.The initial reaction rate of CN-75 gradually increased with the increase of initial concentration,showing pseudo-first-order reaction kinetics.The thickness of silica gel is in the range of 0.034 mm to 0.172 mm.As the thickness of silica gel increases,the initial photoreaction rate of CN-75 on the silica surface increases first,and then slows down when it reaches 0.103 mm thickness,but the final degradation rate is almost the same.While the reaction temperature was in the range of 5-25 ℃,the degradation rate of CN-75 increased with the increase of temperature.Carbon tetrachloride and p-benzoquinone inhibited the photodegradation of CN-75 by about 5%.In the hypoxic environment,the inhibition rate was about17.04%.Through product analysis,it was found that CN-75 was directly photolyzed,and its photochemical conversion products were mainly its reductive dechlorination products heptachloronaphthalene and hexachloronaphthalene.By calculation,the 2C-14 Cl bond,5C-17 Cl,7C-13 Cl and 8C-18 Cl are more susceptible to radical attack,with an activation free energy barrier of 0.043 Hartree for direct photolysis,0.011 Hartree for hydrogen substitution and 0.004 Hartree for OH substitution.This experiment improves the geochemical process of PCN and can provide support for future research on CN-75. |
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