Comparative Studies On Heterogeneous Fenton-Like Reactions Catalyzed By Halloysite And Kaolinite Supported Fe₃O₄

The heterogeneous Fenton-like oxidation method is an improved technique for the defects of the traditional Fenton oxidation method,which has a promising practical application.In this work,Fe₃O₄nanoparticles were constructed in situ on natural clays with different morphologies,tubular HNTs and lamellar Kaolin,using a one-pot solvothermal method,and heterogeneous Fenton nano-catalysts with different carrier morphologies were obtained.The properties of Fe/H and Fe/K in terms of phase composition,morphological structure and magnetic properties were analyzed by X-ray diffraction（XRD）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,specific surface area analyzer（BET）,Fourier infrared spectroscopy（FT-IR）and vibrating sample magnetometer（VSM）.Using rhodamine B（RhB）as the target dye,the effects of the preparation conditions and degradation experimental conditions of two nano-catalysts,Fe/H and Fe/K,on the heterogeneous Fenton-like reaction efficiency were investigated.Combining with ultraviolet absorption spectroscopy（UV-vis）and based on the experiments of iron ion dissolution,H₂O₂decomposition rate,HO·output and active species capture during the catalytic reaction,their heterogeneous Fenton reaction mechanism was investigated and the differences between them were compared.The characterization results show that the acid washing of HNTs and Kaolin did not destroy their natural crystal structures and surface smoothness,and improved the dispersion of the carriers.While retaining the natural nanotubular structure of HNTs and Kaolin lamellar structure,the Fe₃O₄NPs grow in situ on the external surface of the clay minerals with uniform Fe₃O₄particle size and excellent crystallinity.In addition,both Fe/H and Fe/K have ferromagnetic properties and can be recovered from the catalyst by external magnets.The degradation results showed that Fe/H had the best catalytic performance when HNTs were used as carriers with 50%HNTs content and 20 h preparation time,and when trisodium citrate was added at 7 m M.The degradation rate of RhB could reach 99.6%when the initial p H=3.5,RhB concentration was100 mg/L,H₂O₂concentration was 1.96 m M and the catalyst quantity was 0.5 g/L.The results of RhB degradation in different environments illustrate that the Fe/H composite catalyst can significantly improve the RhB degradation efficiency.When Kaolin was employed as the carrier,the Fe/K composite catalyst had the best catalytic oxidation activity when the preparation conditions were 20%Kaolin content,12 h synthesis time and 5 m M addition of trisodium citrate.When the experimental conditions were p H=3,H₂O₂was 3.92 m M and the catalyst dosage was 0.2 g/L,the RhB at a concentration of 100 mg/L was completely degraded within 30 min.The single-factor experiments showed that for Fe/K,the effects of initial p H,H₂O₂concentration and catalyst dosage on the degradation efficiency had similar trends to those of Fe/H.The reaction mechanism of Fe/H and Fe/K is mainly divided into two parts:first,the leached iron ions in solution catalyze the participation of H₂O₂in the reaction（homogeneous Fenton reaction）,and second,the catalyst surface iron catalyzes the participation of H₂O₂in the reaction（heterogeneous Fenton reaction）.Moreover,the oxidation active species involved in the reaction are the same in both systems.The difference between the two catalysts is that Fe/H degrades RhB adsorbed on the external surface of HNTs,while Fe/K degrades RhB molecules directly and rapidly near the catalyst surface.In addition,the H₂O₂and p H required for the optimal experimental conditions to degrade RhB for both catalysts,as well as the quantity of dissolved iron ions during the reaction,and the contribution of oxidation-active substances to the degradation varied.