Removal Gaseous Volatile Organic Compounds By A Multiwalled Carbon Nanotubes/Peroxymonosulfate Wet Scrubber:Performance And Mechanism

So far,although the free radical and nonradical mechanisms have been proposed in the carbocatalyst/persulfate systems,the mechanism of persulfate activation is still not declared in detailed due to the complexity of carbocatalyst and complicated influence factors.Moreover,in the study of nonradical pathways involving singlet oxygen and complexes,the specific formation of nonradicals and specific oxidation mechanisms are still blurred.The mechanism of carbocatalyst/persulfate is generally carried out in aqueous phase,and the interface physical chemical properties between gas/liquid phase and the gas/solid/liquid three-phase are essentially different in the carbocatalyst/persulfate systems,because the reaction may occur in the catalyst surface or a gas-liquid interface.Despite the reactive oxygen species and the chemical changes on carbocatalyst surface can be determined,the specific reactive sites are difficult to point out.The combination of experimental and theoretical calculations will subserve the determination of reactive site and promote exploration of mechanism in-depth.Therefore,this article selects multi-walled carbon nanotubes（MWCNTs）as a representative carbocatalys,and removal of gaseous volatile organic pollutants（VOCs）by MWCNTs/peroxymonosulfate（PMS）in a wet scrubber was comprehensively investigated through experiment tests and DFT calculations.The nonradical mechanism based on singlet oxygen and oxidative complexes MWCNTs-PMS*is clarified in detailed,and presents practical application scenarios.At present,the main research results are as follows:（1）The MWCNTs/PMS wet scrubber exhibits excellent removal performance to gaseous VOCs.The removal efficiency of styrene is stable above 98%,and the average mineralization rate is 76%in the first two hours.The removal efficiency of ethyl toluene,benzene,and ethyl acetate was maintained at 90%,91%and 100%,and the average mineralization rate was 88%,75%,and 66%,respectively.The removal efficiency of styrene is maintained at 79%over 32hours through re-adding PMS,but the performance cannot be recovered to initial state due to the loss of carbonyl C═O groups.The removal efficiency of simultaneously removing styrene and toluene is almost consistent with the removal efficiency of removing styrene and toluene alone.The mineralization rate to VOCs could be improved by increasing PMS concentration.（2）The nonradical oxidation mechanism of the MWCNTs/PMS wet scrubber system is clarified.The MWCNTs/PMS wet scrubber was obtained simultaneously to form oxidative complex MWCNTs-PMS*and singlet oxygen（~1O₂）for removing VOCs by quenching experiment,electron paramagnetic resonance（EPR）and Raman spectra.The C═O groups on MWCNTs were proved as the active sites making contributions to the generation of MWCNTs-PMS*complexes and ~1O₂ by X-ray photoelectron spectroscopy（XPS）,Raman and density function theory（DFT）.Based on PTR-TOF-MS results,~1O₂play a key role in breaking benzene ring into CO₂,and a feasible degradation route of styrene in the MWCNTs/PMS wet scrubber was proposed which was further confirmed by DFT calculations.Moreover,the electron characteristics of MWCNTs-PMS*was explained by DFT calculations,and electrons are more susceptible to VOCs on MWCNT-PMS*than MWCNT due to the changes of the electrical properties,accelerating the degradation of VOCs.The degradation process of styrene in the MWCNTs/PMS wet scrubber was determined:MWCNTs promoted the mass transfer of gaseous styrene into dissolved styrene,and the vinyl group of styrene is easily transferred electrons to PMS molecules adsorbed on the surface of the MWCNTs and converts to a benzene ring,and finally are mineralized by ~1O₂ into CO₂ and H₂O.（3）The application scenarios of the MWCNTs/PMS wet scrubber are proposed.The MWCNTs/PMS wet scrubber aromatic VOCs may be more suitable in the ~1O₂and oxidative complexes coupled systems.In practical applications,more pure water should be used and avoids the existence inorganic anions,like H₂PO₄^-,Cl^-,so as not to affect the system’s removal performance on VOCs.