Synthesis Of Fenton Oxidation Catalysts And Their Application In Dye Wastewater Treatment

The textile industry is a traditional pillar industry of China's national economy and an important civilian production industry.It is also an industry with obvious international competitive advantages,whichplays an important role in prospering markets,expanding exports,absorbing surplus labor,increasing farmers'income,and promoting economic development.However,the large amount of printing and dyeing wastewater with high colority,complex composition and difficult biodegradation in the production process of the textile industry has caused serious damage to the natural environment,and become a recognized water pollution control problem at home and abroad.Fenton oxidation method is one of the efficient and inexpensive advanced treatment processes for organic wastewater.Under acidic conditions,hydrogen peroxide(H₂O₂)reacts with ferrous ions(Fe²⁺)and produces highly oxidative hydroxyl radicals(·OH),which can quickly oxidize and degrade most organic pollutants.However,the traditional homogeneous Fenton oxidation method has many disadvantages such as the high acidity of the reaction system,the non-reusability of the catalysts,and the generation of iron sludge which may cause secondary pollution.These defects make it difficult for large-scale application in industrial production of Fenton oxidation method.Therefore,the research and application of heterogeneous Fenton oxidation catalysts have received extensive attention.In this paper,firstly the activity of the traditional homogeneous Fenton oxidation method for the degradation of small heterocyclic compounds was investigated using tetrahydrofuran as a model compound and the reaction conditions were optimized.Subsequently,the degradation rate of 7 common nitrogen-containing compounds such as 2-imidazolidinone,pyrrole,caprolactam,imidazole,1,3-dimethyl-2-imidazolidinone,piperazine and piperidine with the Fenton oxidation method were tested under the best conditions:the concentrations of Fe²⁺and H₂O₂were 161 and384 ppm,separately.It was found that the conversion rate of these substances reached 97.6,92.5,85.4,73.5,50.5,24.6 and 25.2%within 60 minutes,relatively.According to the structure of the above substrates,we speculate thatthe alkalinity and strong coordination ability of the nitrogen atoms in the heterocyclic compounds have a huge inhibitory effect on the activity of Fenton oxidation reaction.A variety of composite carbon-supported iron-based catalysts were prepared by impregnation method and pyrolysis method respectively.Taking 2-imidazolidinone and rhodamine B as model compounds,the performances and stabilities of these catalysts in heterogeneous Fenton oxidation reaction were tested.It was found that these catalysts can completely degrade organic compounds under suitable conditions,but the stabilities are generally poor and the catalysts cannot be reused.Finally,a composite catalyst Fe-N_x@g-C₃N₄with high Fenton oxidation activity and stability was successfully synthesized through a controlled pyrolysis process using a mixture of Fe-BTC and melamine as a raw material,and the preparation process of the catalyst was optimized.It was found that the Fe-N_x@g-C₃N₄-4-400 catalyst obtained by pyrolysis at 400 ? with the mass ratio of Fe-BTC and melamine of 1:4 in the raw material has the best heterogeneous Fenton oxidation activity under neutral conditions,with which 100 ppm of rhodamine B could be completely degraded within 120 minutes.Besides,the catalyst has good stability and can maintain high activity after 7 consecutive repeated uses.The results of characterization showed that one part of the iron species in this composite catalyst still maintains a structure similar to Fe-BTC and is evenly wrapped in the g-C₃N₄layer produced by the pyrolysis of melamine,while large part of Fe-BTC and melamine are co-pyrolyzed to form the stable Fe(?)-N_xspecies.The Fe(?)-N_xstructure not only keeps Fe stable in the catalyst,but also increases the electron density of Fe and accelerates the electronic delivery and the redox cycle of Fe(?)and Fe(?).Thus Fe-N_x@g-C₃N₄-4-400has ultra-high Fenton oxidation activity and stability.In summary,the process conditions and activity rules of the homogeneous Fenton oxidation reaction in the degradation of small heterocyclic compounds were preliminarily explored,and the activities and stabilities of some heterogeneous Fenton oxidation catalysts in degrading2-imidazolidinone and rhodamine B were investigated.Based on these results,a composite catalyst Fe-N_x@g-C₃N₄-4-400 with a special structure containing Fe(?)(existing in Fe-BTC)and Fe(?)(existing in Fe(?)-N_x)was synthesized,and the activity and stability of this catalyst were investigated in the degradation reaction of rhodamine B and other various pollutants.These studies can provide references for the preparation of heterogeneous Fenton oxidation catalysts and their application in the deep oxidation of wastewater.