Activation Of Peroxymonosulfate By Polymer Semiconductor Photocatalysts Under Visible Light Irradiation For The Degradation Of Organic Dyes

Sulfate radicals based advanced oxidation technologies has recently received much attention for its capability of destructing many organic pollutants. Researchers have carried out a lot of research on the heterogeneous activation of metal catalysts, however, found that the metal ions are easy to dissolve to the water body from metal catalysts and cause the second pollution. Therefore, the non-metallic catalysts have attracted more attention in the degradation of organic pollutants. Polymer semiconductor graphitic carbon nitride (g-C3N4) and based on crystal polyimide (PI) both have broad application prospect due to their abundant sources, low-cost fabrication, easy processibility and non-metallic introduction. Activation of peroxymonosulfate (PMS) to degrade contaminants by metal-free catalysts has attracted the attention of more people. It is of profound significance to develop green and efficient systems for PMS activation by polymer semiconductor photocatalysts to degrade organic pollutants.In this paper, the polymer photocatalsysts g-C3N4 and PI were synthesized. X-ray diffraction(XRD), scanning electron microscopy(SEM),N2 adsorption/desorption isotherms, fourier transform infrared spectroscopy(FT-IR), X-ray photoelectron spectroscopy (XPS)and UV-vis diffusereflectance spectrum were used to characterize the properties of the material. The degradation for acid orange (AO7) by the two kinds of heterogeneous catalysts were investigated for the activation of PMS under visible light irradiation.The effect of several parameters on AO7 degradation was also explored. The stability of the catalysts was studied. The intermediate products after AO7 degradation were analyzed by UV-Vis spectra, Liquid chromatography-mass spectrometer (ESI-MS) and Gas chromatography-mass spectrometer(GC/MS), and the degradation pathways of acid orange were put forward. The mechanism of activation of PMS by catalysts was illustrated. The main research contents of this paper are showed as follows:(1) Semiconductor g-C3N4 were prepared by thermal decomposition method.The characterization showed that the catalyst was in high degree of polymerization, and exhibited excellent optical properties.The removal rate of AO7 was 86% within 30min. The reclycing experiment suggested that g-C3N4 presented a long-term stability and could be reused for 6 times maintaining high catalytic activity. It was found from inhibitor experiments that a large number of sulfate radical SO4-· was generated in the system, which plays a dominate role for the degradation of AO7. Based on intermediate detections, it was found that AO7 was firstly decomposed to naphthaquinone, and afterward further degraded into smaller compounds. The effects of several parameters including the concentration of catalyst, PMS and organic dye, and initial solution pH on its catalytic activity were also investigated.(2) Polymer based materials, PI were synthesized by directly thermal condensation method. The structural characterization of PI showed that PI possessed high degree of polymerization, but small specific surface area. Optical performance indicated that PI presented wide absorption of ultraviolet and visible light. AO7 degradation results revealed that the PI/PMS/Vis system was highly active for AO7 decomposition.The degradation rate of AO7 was 93% 30min later. When PI was used for 5 times, the removal rate of AO7 was 95%, indicating that PI was efficient, stable and reusable. The results of Kinetic experiments suggested that the degradation rate of AO7 enhanced with the increase of PI and PMS concentration, but decrease of dye concentration and initial solution pH. Based on the analyses of intermediate products, the degradation pathway of AO7 was proposed. The mechanism of activation of PMS by PI was analyzed according to inhibiting experiments of quenching agents.(3) In order to increase the specific surface area of PI, and to enhance the photocatalytic activity for organic dyes, attapulgite-supported polyimide (PI/ATP) was prepared. The effects of calcination temperature and PI loading on AO7 degradation in the presence of PMS under visible irradiation were explored. It was found that PI/ATP with 15%PI loading and calcination temperature of 300℃ could degrade AO7 with a concentration of 20mg/L within 20min completely. PI/ATP also exhibited highly active for other organic dyes with different chemical structures. PI/ATP could be used for five successive cycles with no obvious structural changes indicating its good stability. Analysis of intermediate products suggested that the PI-ATP/PMS/Vis system fristly destroy the azo structure of AO7, and ultimately decomposed into small-molecular compounds such as coumarin. The higher loading of PI-ATP, PMS and lower concentration of AO7 as well as acid medium improved the efficiency of AO7 degradation.