Study On Efficiency And Mechanism Of Microwave Enhanced Sulfate Radical Based Advanced Oxidation Processes

Sulfate radicals-based advanced oxidation processes?SR-AOPs?have been considered as one of the most effective methods to deal with refractory organic wastewater.Peroxydisulfate?PDS?and peroxymonosulfate?PMS?are two conmmon oxidants in these processes.Because of the high molecular symmetry,it is difficult for PDS to be activated by traditional heating activation mode.In addition,there are some problems including high energy consumption and high heat loss about this process.Even through PMS has asymmetric structure and could be easily activated by chemical method,its one sulfonic acid structure limits the number of sulfate radicals to be generated,causing its low utilization rate.Hence,regarding to the activation of PDS and PMS for the generation of persulfate radicals,the present work intended to establish the MW/Fe₃O₄/PDS system,and to prepare Co₃O₄-Bi₂O₃(CBO_MW)as a PMS activator for CBO_MW/PMS system based on the microwave?MW?enhanced chemical reaction theory.Moreover,the work further investigated the catalytic oxidation proformance and dergadation mechanisms of the two systems.The above research will provide the theoretical support for the removal of refractory organic pollutants on the basis of sulfate radical technology.Because H₂O and PDS belong to polar moleculars,the polarization induction between H₂O and PDS could be enhanced through the dipole ratates and dipole conduction under microwave irradiation,which increased the PDS molecular deformability and led to easy PDS activation.Then,according to microwave-enhanced chemical reaction,the MW/Fe₃O₄/PDS system was established with high catalytic activity.Firstly,microwave could activate PDS directly?called as MW/PDS system?.Under the conditions of microwave power of 300 W,PDS initial concentration of 0.216 mmol·L^-1?molar ratio of PDS and PNP of 15:1?,the degradation efficiency of PNP was 96.8% within 14 min.For the MW/PDS system controlled by microwave temperature,the catalytic activity of MW/PDS increased when the microwave temperature increased.When the reaction temperature was 90 ?,the PDS decomposition rate was increased by 58.3% in MW/PDS system compared to the TH/PDS system,and the MW/PDS system showed the best catalytic activity under this condition.In order to reduce the reaction temperature and microwave output power,Fe₃O₄ was introduced into the system because of its magnetic loss under microwave irradiation,leading the establishment of an MW/Fe₃O₄/PDS system.According to the characteristic of Fe₃O₄,it belonged to ball-like structure with wrinkles on its surface with a surface area of 9.41 m2?g^-1,and the high saturation magnetization value was 77.0 emu?g^-1.This demonstrated that Fe₃O₄ could offer many active centre sites and be separated from aqueous solution easily.The degradation efficiency of PNP was 98.2% within 28 min when the reaction temperature was 80 ?.Furthermore,the catalytic activity of MW/natural magnetic/PDS system was carried out and the result showed the similar catalytic activity when the dosage of natural magnetic was 25-fold compared to that of Fe₃O₄.This was caused by the limited surface area of natural magnetic with 0.63 m²?g^-1.As a result,limited active Fe²⁺ was generated during the same period.Thus,the MW/Fe₃O₄/PDS system displayed the superior catalytic activity owing to the facilitat ed PDS decomposition rate by microwave and Fe₃O₄ activation.During the preparation of catalyst,microwave enhanced the hydrolyzation of metal ions and their adsorption effect,which impels further growth of the catalyst to stable complex structure through rapid in-situ nucleation.Hence,Co₃O₄-Bi₂O₃(CBO_MW)was efficiently synthesized by microwave-assissted method for peroxymonosulfate activation on the basis of microwave-enhanced chemical reaction.Compared to traditional heating?TH?method and the same preparation parameters,CBO_MW performed better than CBO_TH on surface area,morphology and catalytic property.The surface area of CBO_MW was 411.7 m m²·g^-1,which was approximately 20-fold to that of CBO_TH.The CBO_MW mainly showed as nanoparticles,which was far smaller than the rod-like CBO_TH with a size of 10 ?m long and 2 ?m wide.This phenomenon might be contributed to the microwave irradiation,which enhanced the interaction between two metal ions for the formation of composite with rapied in-situ nucleation and growth.Since the nucleation was too fast,the crystallinity degree of the composite was reduced and the crystal plane of Bi₂O₃ was exposed.In addition,the rapid nucleation of composite tended to form nanoparticles and the subsequent calcination treatment avoided the agglomeration,which led to aboundant inner pore canal and structure.Then,this treatment enhanced the interface between catalyst and PMS,and facilitated PMS activation.On the basis of CBO_MW prepared by microwave assistant method,the CBO_MW/PMS system was established.For the degradation efficiency of BPA using CBO_MW/PMS system,more than 99.9% of BPA could be removed within 15 min,while only 59.1% of BPA was removed by CBO_TH/PMS.According to the optimization of preparation parameters of CBO_MW,the optimal conditions included microwave temperature of 75?,microwave time of 30 min,calcination temperature of 400? and calcination time of 2 h.In the CBO_MW/PMS system,the optimal conditions for BPA removal included the dosage of CBO_MW of 0.1 g·L^-1 and the dosage of PMS of 0.0263 mmol·L^-1?the molar ratio of PMS and BPA of 3:1?.In this way,more than 99.9% of BPA was removed within 15 min.The superior catalytic activity of CBO_MW/PMS system might contribute to the sufficient interface of catalyst with PMS.As a result,more active radicals were generated and the catalytic capacity of CBO_MW/PMS system was facilitated.As for the established MW/Fe₃O₄/PDS and CBO_MW/PMS systems,the mechanisms of pollutant degradation by sulfate radical systems were investigated.For the study of kinetics,both of the two systems fitted the pseudo-first order reaction kinetics and their corresponding reaction constants were 0.139 min^-1 and 0.380 min^-1,respectively.MW/Fe₃O₄/PDS system and CBO_MW/PMS system exhibited the similar degradation efficiencies for PNP removal.In the respect of environment factor effect for MW/Fe₃O₄/PDS system,CO₃^2-played an inhibiting effect by changing the solution p H and the form of Fe²⁺ during the reaction.The main mechanism of MW/Fe₃O₄/PDS system was due to the generation of sulfate radcials by PDS activation through two ways.Firstly,microwave induced dielectric loss heating and conduction loss heating for H₂O and PDS.Secondly,magnetic loss heating caused on the surface of Fe₃O₄ under microwave irradiation.The main mechanism of CBO_MW/PMS system was due to the generation of sulfate radicals from peroxymonosulfate activation by Co²⁺ which exposed on the surface of Co₃O₄-Bi₂O₃.In addition,the reduction of Co³⁺ by part of Bi³⁺ also provided more Co²⁺ for peroxymonosulfate activation.