Study On Selective Adsorption And Catalytic Degradation Of Formaldehyde Wastewater By Heterogeneous Fenton Catalyst

Refractory organic wastewater has complex components,high toxicity,poor biochemical properties,and the traditional biochemical technology is difficult to meet the standards.As an improvement of the traditional homogeneous Fenton technology,heterogeneous Fenton technology has excellent treatment effect on this kind of wastewater.However,due to the strong oxidation of hydroxyl radicals generated in the heterogeneous Fenton oxidation reaction process,it is inevitable that biodegradable substances in the wastewater will also be oxidized and degraded,which indirectly leads to the increase of cost and waste of carbon source.So achieving Fenton’s selective catalytic degradation of target pollutants can significantly reduce the amount of chemicals used,thus reducing costs and preserving carbon sources.In this paper,a heterogeneous Fenton reaction catalytic system was established.The selective adsorption technology was combined with heterogeneous Fenton technology.The target pollutants were adsorbed to the catalytic active point through selective adsorption,and then degraded,and finally the selective heterogeneous Fenton technology was formed.In this study,the basic performance of ZSM-5 molecular sieve and the selective adsorption of formaldehyde simulated wastewater were investigated.The iron-loaded ZSM-5 molecular sieve catalyst was prepared and characterized by specific surface area analysis,scanning electron microscope and energy spectrum analysis,Fourier transform infrared spectroscopy analysis and X-ray crystal diffraction analysis.For preparation of heterogeneous fenton catalyst performance study,examines the different catalytic condition influences the performance of catalyst to formaldehyde simulated wastewater degradation,optimum reaction conditions were determined,and under the optimum reaction conditions for the repeatability and stability of the catalyst,the in situ regeneration of catalyst performance,as well as the catalyst in the form of particles and powder effect on the degradation of performance.The main active substances in the reaction system were investigated and the mechanism of formaldehyde degradation was speculated.Finally,the degradation performance of the catalyst to high concentration of formaldehyde simulated wastewater was investigated under the optimal reaction conditions.The study reached the following conclusions:（1）Based on the investigation of the basic properties of ZSM-5 molecular sieve,the pseudo-second-order adsorption kinetics model and Langmuir adsorption isotherm model can better describe the adsorption process of ZSM-5 molecular sieve.Through the investigation of formaldehyde and glucose concentration ratio,solution pH value,temperature and ZSM-5molecular sieve dosage on the removal effect of simulated formaldehyde wastewater,the adsorption of the best reaction conditions:formaldehyde concentration/glucose concentration is1/1,acidic conditions,reaction temperature is 25℃,the dosage of molecular sieve is 10g/L.（2）The formaldehyde removal effect and selectivity of ZSM-5 molecular sieve catalyst modified by NaOH reaming solution are both worse,so it is not suitable for further reaming modification of catalyst.ZSM-5 zeolite catalyst modified by nickel nitrate,cobalt nitrate and copper acetate solution has stronger formaldehyde removal effect,and the modification effect of cobalt ion is the best.By investigating the influence of impregnation concentration,impregnation temperature,impregnation liquid-solid ratio and impregnation time on the iron loading capacity of ZSM-5 zeolite catalyst,the optimal reaction conditions of iron loading were obtained as follows:iron ion impregnation concentration is 0.8mol/L,impregnation liquid-solid ratio is 15:1,impregnation temperature in water bath is 50℃,impregnation time is 4h,iron loading capacity reaches 75.95mg/g.（3）According to the specific surface area analysis,the ZSM-5 molecular sieve is mainly microporous structure;Scanning electron microscopy and energy spectrum analysis showed that the structure of ZSM-5 catalyst did not change significantly after iron loading,adsorption and Fenton reaction,and the original morphology was well retained.As can be seen from the infrared spectrum,the ZSM-5 molecular sieve loaded with iron did not appear new groups,iron ions did not change the internal group structure of the ZSM-5 molecular sieve,only loaded on the surface;By X-ray crystal diffraction analysis,it can be seen that the phase composition of ZSM-5 zeolite catalyst does not change after iron loading,adsorption and Fenton reaction.（4）By investigating the effects of H₂O₂concentration,solution pH value,reaction temperature and catalyst dosage on the removal effect of simulated formaldehyde wastewater,the optimal reaction conditions of heterogeneous Fenton catalytic degradation were obtained as follows:H₂O₂concentration 385mmol/L,pH=3,reaction temperature 55℃,catalyst dosage10g/L,and formaldehyde removal rate of more than 90%.Under the optimum reaction conditions,after 360 min,the retention rate of glucose and the removal rate of formaldehyde reached 90.23%,and the selectivity of the catalyst to glucose and formaldehyde was 1.85%and 98.15%respectively.In the adsorption system,the retention rate of glucose was 81.50%,the removal rate of formaldehyde was 22.54%,the selectivity of the catalyst to glucose was 13.58%,and the selectivity to formaldehyde was 86.42%.The selectivity coefficient of the catalyst was calculated,Dformaldehyde=2.3984,Dglucose=3.9962.（5）Using the prepared iron-loaded ZSM-5 zeolite catalyst for five reproducibility experiments under the optimal reaction conditions,the formaldehyde removal rate is stable to93%,the retention rate of glucose is 91.77%,the reproducibility and selectivity are good.The iron dissolution of the catalyst was determined.The average iron dissolution of the catalyst was0.048mg/g for five times,and the average dissolution rate was 0.062%.It still had good stability in the process of repeated use.The comparison of the powdered and granular forms of the catalyst shows that the two forms have no obvious effect on the removal rate of formaldehyde.The catalyst prepared in this experiment is not limited by the form and can play a good degradation effect.（6）The active substances in the reaction system of this experiment were determined.From the experimental results,it can be concluded that hydroxyl radical plays a leading role in the reaction system,and the reaction mainly occurs on the surface of the catalyst.And the degradation mechanism of formaldehyde on ZSM-5 zeolite catalyst is speculated,which is mainly heterogeneous surface catalysis mechanism.The cycle of adsorption--degradation--desorption--re-adsorption--re-degradation--re-desorption is carried out on the surface of the catalyst.（7）Best catalytic reaction was investigated under the condition of the performance of the catalyst simulated wastewater of high concentration of formaldehyde,formaldehyde concentration of formaldehyde in simulated wastewater will increase to 1000 mg/L,by the experimental results it can be seen that this experiment of the preparation of iron ZSM-5 zeolite catalyst wastewater of high concentration of formaldehyde still has good biodegradation performance,to formaldehyde removal rate steady at 95%.