Study On The Effect Different Crystal Types Of Manganese Dioxide In Activation Of Peroxymonosulfate For Degradation Of Rhodamine B

With the development of advanced oxidation technologies,the advanced oxidation technology that based upon the sulfate free radical?SO4·-?has made great progress.Nowadays the research on the heterogeneous catalysis of transition metal oxides has been gradually developed.The advantages of this method are safe and efficient.In this project,three crystal forms manganese dioxide?MnO₂?were prepared by hydrothermal method.Peroxymonosulphate?PMS?was treated as oxidant and Rhodamine B was treated as the target pollutant.The effects were examined in heterogeneous MnO₂/PMS system for degradation of Rhodamine B in water.Then it was be investigated the influences of changes of preparation conditions and reaction conditions for crystal form of MnO₂ and catalytic degradation of Rhodamine B.Finally it was inferred the reasons of differences of the catalytic performance and mechanism of catalytic reaction by combining a variety of characterization and analysis to find out the best crystal form.Three crystal forms manganese dioxide?MnO₂?were prepared by hydrothermal method and characterized by X-ray diffraction?XRD?and scanning electron microscope?SEM?.The optimum preparation condition of the ?-MnO₂ was dissolving 8 mmol Mn SO4·H₂O and 8 mmol KMn O4 then put in 160 °C of hydrothermal temperature and 6 h of hydrothermal time.The morphology of ?-MnO₂ was fibre like needle.The optimum preparation condition of the ?-MnO₂ was dissolving 16 mmol Mn SO4·H₂O and 16 mmol?NH4?2S2O8 then put in 120 °C of hydrothermal temperature and 48 h of hydrothermal time.The morphology of ?-MnO₂ was fibre like ?-MnO₂.Its length was shorter than ?-MnO₂ and the width was much thicker than ?-MnO2.The optimum preparation condition of the ?-MnO₂ was dissolving 2 mmol Mn SO4·H₂O and 12 mmol KMn O4 then put in 160 °C of hydrothermal temperature and 24 h of hydrothermal time.The morphology of ?-MnO₂ was layered bouquet.Then the effect of three crystal forms of MnO₂ in activation of PMS for degradation of Rhodamine B was investigated.The reaction conditions were p H=7 at room temperature,the PMS concentration of 1 mM,the MnO₂ concentration of 0.5 g/L,and Rhodamine B concentration of 50 mg/L.Rhodamine B could be degraded completely at 60 min when the catalyst was ?-MnO₂.The catalyst was reused for 4 times and the catalytic efficiency decreased by 6.8%.The performance was very stable.97.4% of Rhodamine B could be degraded completely at 60 min when the catalyst was ?-MnO₂.The catalyst was reused for 4 times and the catalytic efficiency decreased by 9.3%.The performance was stable.98.9% of Rhodamine B could be degraded completely at 60 min when the catalyst was ?-MnO₂.The catalyst was reused for 4 times and the catalytic efficiency decreased by 8.3%.The performance was relatively stable.The presence of SO4·-and ·OH in the reaction system was confirmed by the addition of the quenching agent in the reaction process.It was found that the ?-MnO₂ had the largest specific surface area and the total pore volume of them by comparison.So the catalytic effect of ?-MnO₂ was best.According to the X-ray photoelectron spectroscopy?XPS?characterization results,that the catalytic mechanism was: the catalysts in the catalytic reaction was converted between Mn???and Mn???and produced SO4·-,and the content of Mn???of hydrothermal method ?-MnO₂ was higher than the other two forms of MnO₂.So ?-MnO2 could produced more SO4·-than the other two forms of MnO₂ and had best effect of catalytic degradation.