Three-dimensional Porous CoFe₂O₄ With Tunable Composition For PMS Activation:Preparations,Properties,and Mechanisms

In recent years,the discharge of organic pollutants from modern industries has caused water pollution problems to become more and more serious.Sulfate radical（·SO₄^-）has broad application prospects in the treatment of organic pollutants in water bodies due to its strong oxidizing property and wide application range.The bimetallic oxide CoFe₂O₄ can activate permonosulfate（PMS）to produce·SO₄^-,which then degrades organic pollutants in water.However,the small size and high surface energy of CoFe₂O₄nano-materials make it easy to agglomerate,which leads to the decrease of its specific surface area and the reduction of active sites.Therefore,the improvement of catalytic activity is limited.In addition,to increase the surface activation reaction rate,the in-depth study of the mechanism of CoFe₂O₄ for activating PMS is also a relatively important research topic.To this end,this paper constructs a three-dimensional porous structure of CoFe₂O₄-based catalytic material through polymer-assisted vacuum freeze drying technology.The structure has both the characteristics of micro-nano structure and three-dimensional macro-porous structure,which can increase the specific surface area of the material and increase the surface active sites,while reducing the powder agglomeration and improving the catalytic activity.Its three-dimensional porous structure is also conducive to improving the mass transfer capacity of the reaction process.In order to further study the mechanism of CoFe₂O₄ activation of PMS,the paper used non-redox metal substitution and doping strategies to regulate the composition of three-dimensional porous materials,and explored the activation mechanism of Co and Fe bimetals.The specific research is as follows:（1）Preparation of three-dimensional porous CoFe₂O₄ material.Using a mixed solu tion of cobalt nitrate（Co（NO₃）₃·6H₂O）,ferric nitrate（Fe（NO₃）₃·9H₂O）and polymer pol yvinylpyrrolidone（PVP）as the precursor,a three-dimensional porous CoFe₂O₄ material was prepared by vacuum freeze-drying technology combined with high-temperature calc ination.It was found that the pores of the sample became narrower and the specific s urface area increased while the content of PVP was increased in the precursor solution;the pores of the sample were shrunk and the specific surface area was decreased as the calcination temperature increases.When the calcining temperature is 600℃,the sa mple has a larger specific surface area(10.0 m²g^-1),which is 2.5 times that of the sa mple calcined at 800℃.（2）The performance of three-dimensional porous CoFe₂O₄ in activating PMS to degrade OrangeⅡ.The preparation conditions,such as the content of PVP in the precursor solution and the calcination temperature,affect the performance of CoFe₂O₄ samples in activating PMS,the key factor of which is the preparation-dependent specific surface area.The CoFe₂O₄ material prepared with 5 g PVP in the precursor solution and a calcination temperature of 600℃has the highest specific surface area,and the best performance in activating PMS.The degradation rate of OrangeⅡ（20 mg/L）reached 97%within 40 minutes.The main reason is that the activation process of PMS is a surface reaction process,and a larger specific surface area can provide more active sites for the reaction and increase the activation reaction rate with PMS.（3）The performance of photo-CoFe₂O₄synergistic activation of PMS to degrade OrangeⅡ.The effects of different light wavelengths on the activation of PMS were studied.The degradation rate of OrangeⅡreaches 95%within 20 minutes when the PMS is activated with CoFe₂O₄ and visible light（λ≥420 nm）.However,it takes 35 minutes to achieve the same removal rate when using CoFe₂O₄ alone to activate PMS.At the same time,CoFe₂O₄ has a wide spectrum utilization rate,and the use of light with a wavelength greater than or equal to 800 nm can also significantly promote the entire degradation process.For this reason,increasing light irradiation is one of the important ways to improve the performance of CoFe₂O₄activated PMS.（4）Research on the mechanism of CoFe₂O₄activation PMS based on element substitution/doping strategy.It is found that the catalytic activity is:Co Ga₂O₄>Co Al₂O₄>CoFe₂O₄>Zn Fe₂O₄>Mg Fe₂O₄ by comparing the performance of CoFe₂O₄ samples with Mg Fe₂O₄,Zn Fe₂O₄,Co Al₂O₄ and Co Ga₂O₄ samples to activate PMS to degrade OrangeⅡ.It shows that under the same crystal structure,the performance of the cobalt-based catalyst is significantly better than that of the iron-based catalyst.The mechanism of CoFe₂O₄ activation of PMS is proposed:Co²⁺on the surface of CoFe₂O₄ is the activation center of PMS,and Fe³⁺may have a certain inhibitory effect on Co²⁺during the activation process.The results based on element doping strategies also further support the above conclusions.