Study On The Efficacy And Mechanism Of Cobalt-based Nanoparticles Activated PMS For Catalytic Degradation Of Methylene Blue

As a category of organic pollutants in the aquatic environment,dyes not only threaten aquatic plants and animals,but also influence human health.Dyes are difficult to be degraded by traditional wastewater treatment processes,so advanced oxidation process（AOPs）are often used to treat dye wastewater.Peroxymonosulfate（PMS）is often used in AOPs,as a common oxidant that can be activated to produce reactive oxygen species（ROSs）with high redox potential,and the key for PMS activation is the development of efficient and stable catalysts.In particular,cobalt-based catalysts were used to activate PMS becaues of their high catalytic degradation efficiency.In this study,a cobalt-based catalyst with a nitrogen-doped carbon shell layer structure was prepared by a simple method using methylene blue（MB）,a typical representative of dyes,as the characteristic pollutant,and the catalytic degradation process of MB was analyzed by combining density flooding theory.The main studies were as follows:（1）A nitrogen-doped carbon-encapsulated cobalt-based nanoparticle catalyst（Co@NC-750）with a core-shell structure was prepared by pyrolysis,and the degradation efficiency of MB was used as an evaluation index for the optimization of the catalyst by changing the preparation parameters.The results show that the material achieves optimal degradation of MB with a molar mass ratio of cobalt and melamine as 0.25:1 and a pyrolysis temperature up to750°C.In addition,scanning electron microscopy and transmission electron microscopy were combined to test the morphology of the materials;specific surface area analysis,X-ray diffraction,fourier infrared spectroscopy and Raman spectroscopy were used to investigate the structure and physical and chemical properties of the catalysts;and X-ray photoelectron spectroscopy was used to analyze the surface elemental composition of the catalysts.（2）The degradation efficiency of MB under different conditions was investigated.In the catalytic degradation system containing 20 mg/L Co@NC-750,100 mg/L PMS at room temperature,the removal rate of 10 mg/L MB could reach 99.9%in 10 min with a primary rate constant of 0.605 min^-1.The concentration change of PMS was basically consistent with the degradation trend of MB,and the operating conditions of the system and the parameters of the water matrix would have different effects on the degradation efficiency of MB.The rapid degradation of MB was achieved in a wide p H range of 3.0-9.0.Cl^-,HCO₃^-and HA would have different inhibitory effects on the catalytic system,the degradation of MB was not affected by SO₄^2-and NO₃^-.（3）The Co@NC-750/PMS system could still remove more than 95.0%of MB within 30min after five cycles,indicating the good recyclability of the system.The degradation efficiency of the Co@NC-750/PMS system for both dyestuff and antibiotic organic pollutants could reach more than 99.9%within 10 min,and their rate constants all exceeded 0.605 min^-1,indicating the high efficiency of the Co@NC-750/PMS system for the removal of organic pollutants,and the wide range of organic pollutants removed.Even in river water and UV effluent of wastewater plants,the degradation rate of MB can reach 87.9%and 84.6%,indicating the potential application of the Co@NC-750/PMS system in real water bodies.（4）The main reaction mechanism of Co@NC-750 activated PMS for MB degradation was investigated.By density flooding theory,it was calculated that when the four O atoms of PMS are adsorbed on the surface of Co@NC-750,it has the smallest adsorption energy and its adsorption structure is the most stable.The analysis combined with the experimental results of radical quenching experiments and electron paramagnetic resonance experiments indicated that¹O₂,SO₄^·-,·OH and O₂^·-promoted the removal of MB together,and the non-radical ¹O₂ played a major role in the Co@NC-750/PMS/MB system,with the radicals SO₄^·-,·OH and O₂^·-as minor ROSs.By liquid mass spectrometry,a dozen intermediates were identified in the degradation of MB,and two possible degradation pathways of MB were obtained by analysis.Under the attack of ROSs,MB was gradually cleaved to produce various intermediates,which are transformed into small molecule organic compounds through demethylation,deamidation,ring opening and oxidation,and finally converted to CO₂ and H₂O by mineralization.