Study On Catalytic And Mass Transfer Synergistic Effect And Mechanism Of VOCs Removal By UV/Fenton Oxidation

In recent years,a large number of exhaust pollutants discharged by industrial production and human activities have received widespread attention.Among them,volatile organic compounds(VOCs)derived from a wealth of sources,including petrochemical industry,automobile exhaust gas,waste disposal,building materials release,etc.Most of the VOCs are toxic or odorous,which can cause environmental pollution and pose a threat to human health.Therefore,the effective treatment of VOCs has become a research hotspot.Fenton oxidation method has been widely used to treat refractory organics in various types of wastewater due to the advantage of strong oxidizing ability,environment-friendly and simple operation.Recent studies have found that Fenton oxidation is equally effective in treating refractory organics in exhaust gas.However,,when using the Fenton oxidation method to remove the mixed waste gas,its removal efficiency for hydrophobic organic gas is rather low due to the high mass transfer resistance of liquid film and poor gas-liquid mass transfer efficiency.Therefore,improving the gas-liquid mass transfer efficiency of hydrophobic organic gas is the key to effectively remove complex mixed organic waste gas by Fenton oxidation process.In this paper,the enhancement effect of surfactants and dispersed particles on the gas-liquid mass transfer efficiency of insoluble organic gas,as well as the effect on Fenton oxidation removal efficiency,were studied.A diffusion-mass transfer-reaction model was established to reveal the synergistic mechanism of particle enhanced mass transfer and UV/Fenton catalytic oxidation.Specific research contents are as follows:1.Comparing the removal effects of UV/Fenton oxidation on three typical VOCs with different solubility(ethyl acetate>toluene>n-octane).The removal efficiency of ethyl acetate and toluene remained above 80%within 150 min,while that of n-octane decreased to below 10%within 20 min,indicating that the gas-liquid mass transfer efficiency is the key factor in the degradation process of insoluble VOCs.The apparent Henry coefficients of n-octane in pure water and in solution with anionic surfactant sodium dodecyl benzene sulfonate(SDBS),cationic surfactant bromohexadyl pyridine(CPB)and the non-ionic surfactant Octaphenyl Polyoxyethyiene(OP-10)were determined.The results showed that when OP-10 with low critical micelle concentration is added,the apparent Henry coefficient of n-octane decreased significantly,and the UV/Fenton oxidation removal efficiency can still remain 50%within 50 min,demonstrating that the non-ionic surfactant can effectively enhance the gas-liquid mass transfer efficiency of insoluble VOCs.However,it showed that the the surfactant itself was continuously decomposed by UV/Fenton oxidation,which indicated that the enhanced effect of surfactant on removal of insoluble VOCs by UV/Fenton oxidation is limited.2.The influence of activated carbon particles on the bubble behavior and gas-liquid mass transfer efficiency of the insoluble gas n-octane were investigate by adding activated carbon particles with different solid content into pure water.When the solid content of activated carbon increased from 0%to 0.10%,the gas-liquid mass transfer coefficient of n-octane increased from 0.0057 s-1 to 0.14 s-1.Moreover,the introduction of activated carbon particles into pure water can reduce the rise rate of bubbles.The activated carbon particles were further introduced into the removal of the n-octane gas reaction system of UV/Fenton oxidation,the removal efficiency was increased from less than 10%to more than 50%.The synergistic effect of enhanced mass transfer by activated carbon on UV/Fenton oxidation was investigated in terms of hydroxyl radical concentration,intermediates,carbon mass conservation,Hatta number,enhancement factor,synergistic factor and oxidation reaction process.The synergistic factor can increase from 0.5 to 11.7 in 40 min.Intermediate analysis showed that it is more inclined to fully oxidized fatty acids in the liquid phase after adding activated carbon particles,while the other intermediate species and their concentration were both significantly reduced.It was confirmed that the reaction process of n-octane removal by UV-Fenton oxidation also changed due to the enhancement of gas-liquid mass transfer efficiency,and the reaction complete degree was higher,which verified again that the synergistic effect of UV/Fenton oxidation on n-octane removal was significantly improved.3.A one-dimensional diffusion-mass transfer reaction model was established for the synergistic process of particle enhanced gas-liquid mass transfer and UV/Fenton catalytic oxidation.In addition,the accuracy of the predicted results was verified by comparing with the measured data.The key parameters that determine the VOCs removal efficiency of UV/Fenton oxidation are reaction rate constant,gas-liquid mass transfer rate constant and gas-liquid distribution coefficient.In which the latter two parameters are mainly affected by the introduction of activated carbon particles.In considering only enhancement mass transfer efficiency with activated carbon particles,separately considering the enhancement mass transfer and UV/Fenton oxidation,the synergistic effect for enhancement mass transfer and UV/Fenton oxidation,the consistency of the model fitting results and the experimental results were compared.It was found that the fitting accuracy improves greatly after introducing the synergistic factor in the model,which explains the experimental result reasonably that UV/Fenton can be stable at more than 60%for a long period of time(about 50 min).4.Further,the synergistic effect between the enhanced mass transfer of CNT and Fe3O4 catalytic oxidation was investigated in the heterogeneous UV/Fenton system.The effect of Fe3O4,CNTs+Fe3O4 and Fe3O4/CNTs on the dynamic removal efficiency of n-octane on heterogeneous Fenton system were studied.The results showed that the removal efficiencies of n-octane in the three systems were 17.4%,64.3%and 69.5%at 250 min,respectively,which confirmed that CNTs can enhance the gas-liquid mass transfer effect in heterogeneous UV/Fenton system.The synergistic effect of catalytic oxidation and mass transfer were more significant by comparing Fe3O4 alone,Fe3O4+CNTs and Fe3O4/CNTs.Moreover,the removal efficiency for insoluble n-octane was higher.Finally,the degradation mechanism of n-octane by heterogeneous Fenton was investigated based on the intermediates and free radicals.