Degradation Of Azo Dyes In Wastewater By SiO₂@nZVI@Fe₃O₄ Fenton-like Technology

The textile industry in China is well-developed,and the treatment of dyeing and printing wastewater has attracted widespread attention.Azo dyes are organic compounds with azo double bonds（-N=N-）connecting aromatic groups at both ends,and they are widely used in textile dyeing and printing as the most extensively applied synthetic dyes.Azo dyes are difficult to remove once they enter the water environment and can decompose to produce various carcinogenic aromatic amines,causing severe harm to both biological and human health.The nano zero-valent iron（n ZVI）-based Fenton-like technology has good degradation effects on organic matter and is commonly used for treating organic pollutants.However,due to the aggregation and oxidation of n ZVI,modification of n ZVI is an important research direction.Fe₃O₄support can improve the magnetic recovery of n ZVI,while Si O₂coating can improve the dispersion and oxidation resistance of n ZVI.At present,there are few reports on the simultaneous use of Fe₃O₄and Si O₂to modify n ZVI to synthesize iron based composite material Si O₂@n ZVI@Fe₃O₄,and coupling with H₂O₂to construct a Si O₂@n ZVI@Fe₃O₄heterogeneous Fenton system for the removal of azo dyes in wastewater.Therefore,this article first synthesized the composite material Si O₂@n ZVI@Fe₃O₄through liquid phase reduction and St（?）ber hydrolysis methods.Using this material as a catalyst,a Fenton like system was constructed to degrade azo dye wastewater.The effects of various environmental factors on the degradation rate were investigated,and the possible mechanism of Si O₂@n ZVI@Fe₃O₄Fenton like system on the degradation of three azo dyes was explored.The main conclusions are as follows:（1）Characterization results show that Si O₂@n ZVI@Fe₃O₄has a chain-like structure with an average particle size of about 100 nm and some aggregation.The material mainly contains elements such as Fe,O,Si,and C,and is primarily composed of Fe₃O₄with a small amount of n ZVI.Its surface exhibits partial oxidation,and the Fe mainly exists in the form of Fe²⁺and Fe³⁺.The pore size distribution of Si O₂@n ZVI@Fe₃O₄is mainly between 1～40 nm,and the surface Si O₂coating preserves most of the pores for pollutant entry and contact with the iron-based catalyst for Fenton reactions.The magnetism of Si O₂@n ZVI@Fe₃O₄has a saturation magnetization strength of 22.04 emu/g,which is lower than that of n ZVI and Fe₃O₄but still has good magnetism for magnetic recovery and utilization.The material surface primarily has functional groups such as hydroxyl（-OH）and chemical bonds such as C-H bonds,Si-O bonds,and Si-O-Si bonds,proving that the residual tetraethyl orthosilicate（TEOS）exists in the composite material,and that the undetermined Si O₂is also successfully coated on the surface of n ZVI@Fe₃O₄catalyst.（2）Si O₂@n ZVI@Fe₃O₄itself has a certain adsorption removal effect on azo dyes,and when combined with H₂O₂to form a Fenton system,both the removal rate and reaction rate of azo dyes are significantly improved.The removal rate of azo dyes in the Si O₂@n ZVI@Fe₃O₄Fenton-like system increases first and then decreases with the increase of H₂O₂concentration.The maximum removal rates of Congo Red and Orange II are 99.68%and 98.14%at an H₂O₂concentration of 15 mmol/L,and the maximum removal rate of Acid Black 210 is 99.73%at an H₂O₂concentration of 20 mmol/L.The highest removal rates of Congo Red,Orange II,and Acid Black 210 are 96.89%,99.01%,and 99.26%,respectively,when the dosage of Si O₂@n ZVI@Fe₃O₄is 0.8 g/L.At p H=3,the removal rates of Congo Red,Orange II,and Acid Black 210 are 99.73%,98.23%,and 98.47%,respectively,which are significantly higher than those at p H=5,7,and 9.As the initial concentration of azo dyes increases,the removal rate decreases slightly,and the reaction rate decreases significantly.The effect of temperature on the reaction is not significant,and the removal rate increases slightly as the temperature rises.Adding excessive TEOS（tetraethyl orthosilicate）during material preparation will increase the residual TEOS and reduce the removal rate.When the added amount increases from 18.8 g/L to 37.6 g/L,the removal rate of Congo Red decreases from 97.89%to 93.17%.The reaction follows pseudo-first-order kinetics,with correlation coefficients R²all greater than 0.94,indicating that the removal of the three azo dyes by the Si O₂@n ZVI@Fe₃O₄Fenton-like system conforms to the pseudo-first-order kinetic process.（3）Repeated use experiment results show that although the reaction rate decreases gradually with the increase of reuse times,the removal rates of Orange II and Acid Black 210decreased from 95.12%and 98.70%to 98.65%and 97.77%,the decline is not significant.This indicates that Si O₂@n ZVI@Fe₃O₄has good magnetic recovery performance and relatively stable catalytic performance.（4）Free radical quenching experiments show that the oxidation of HO·and the reduction of n ZVI play an important role in the removal of azo dyes in Si O₂@n ZVI@Fe₃O₄Fenton systems,¹O₂playing an auxiliary role.Combining free radical quenching experiments and UV-visible absorption spectroscopy,it is found that the removal mechanism of the Si O₂@n ZVI@Fe₃O₄Fenton-like system involves the breaking of azo bonds in the dye structure under the influence of n ZVI and HO·,causing the solution to decolorize.As the reaction time prolongs,the benzene and naphthalene rings break,and the dye is eventually completely mineralized.