Influence Mechanisms Of Surface Microstructure On Fe₂O₃ Activation Of Peroxymonosulfate

In recent years,the activation of peroxymonosulfate（PMS）to degrade organic pollutants in water has attracted much attention.One of the most common PMS activators are metal oxides,in which Fe₂O₃ has the characteristics of good stability,non-toxicity and wide range of sources.However,the activation efficiency of Fe₂O₃ is low due to the slow redox cycle of Fe²⁺/Fe³⁺in the process of activating PMS.Therefore,the surface microstructure（oxygen vacancies,surface hydroxyl groups）ofα-Fe₂O₃ was adjusted to improve the activation efficiency.The influence mechanisms of surface microstructure onα-Fe₂O₃activation of PMS is discussed.The results are expected to expand a new approach for catalyst design to activate PMS.First,α-Fe₂O₃ containing different oxygen vacancies was synthesized by the precipitation-calcination method.The catalysts were elongated needles,and theα-Fe₂O₃ synthesized by calciningα-Fe OOH at 300°C（Fe₂O₃-300°C）had more oxygen vacancies.The results of activating PMS to degrade sulfamethoxazole（SMX）showed that Fe₂O₃-300°C activated PMS most effectively.The major reactive specie was ¹O₂,oxygen vacancies were the active sites in the catalyst.Oxygen vacancies could enhance the mobility of surrounding O^2-to generate reactive oxygen species（O*）,which in turn reacted with PMS to generate ¹O₂.The efficiency of PMS activation increased with the rise of PMS concentration,catalyst dosage,temperature,and the decrease of SMX concentration.The optimum reaction p H was 8.0 and the activation energy was 35.0 k J mol^-1.In addition,Fe₂O₃-300℃/PMS system also showed good removal performance for bisphenol A and ciprofloxacin.SMX degradation process produced 7degradation intermediates.The decrease of removal of SMX after five rounds was only 14.5%,indicating that Fe₂O₃-300℃had good stability.Second,theα-Fe₂O₃ obtained by calciningα-Fe OOH at 500℃（Fe₂O₃-500℃）was modified with Na OH solution.The microscopic morphology and crystal structure of catalyst remained unchanged after modified.But when the Na OH concentration used became higher,the specific surface area of the catalyst was larger and the pore size became smaller.Meanwhile,the oxygen vacancy content decreased,while the surface hydroxyl content and surface zero charge point increased.HSO₅^-binded more easily to the catalyst surface due to electrostatic interactions,so the effect of the modified catalyst in activating PMS became better.The catalyst modified with 0.1 M Na OH（Fe₂O₃-500℃-0.1）had the most surface hydroxyl groups and the highest PMS activation efficiency.The major active specie in the reaction was ¹O₂,the oxygen vacancies of the catalyst were the main reaction sites and surface hydroxyl groups were involved in the activation processand.The SMX degradation rate tended to be stable after p H>7.0,and the activation energy was 34.6 k J mol^-1.Moreover,the degradation rate of SMX was greatly affected by HCO₃^-、HPO₄^2-and SO₄^2-,but not by Cl^-,NO₃^-,ionic strength and dissolved oxygen.8 kinds of SMX degradation products were detected.The results of five repeated use experiments showed that the catalyst had good stability and the removal rate of SMX only decreased by 14.5%.Finally,the efficiency of PMS activation,p H applicability,reaction mechanism,SMX degradation pathway and stability of Fe₂O₃ with different surface microstructure were compared,finding that the activation effect of Fe₂O₃-300℃was better than that of Fe₂O₃-500℃-0.1,but the activation energy of Fe₂O₃-500℃-0.1 was lower than that of Fe₂O₃-300℃.The optimal p H range of both systems was 7～9,and the degradation effect of the two systems was stable.The three possible degradation paths shared by SMX in the non-radical system were deduced,which provided a new idea for the follow-up study of the non-radical system in the PMS activation technology.