Heterogeneous Catalytic Ozonation Of Wastewater:Catalysts And New Conceptual Ozonation Reactor

Catalytic ozonation processes are able to overcome the basic disadvantages of ozone that selectively oxidizes unsaturated organic compounds and partially mineralizes pollutants.Heterogeneous ozonation catalysts are generally composed of metal oxides and carbon-based materials,based on which the control of surface active sites receives increasing research efforts.It would provide theoretical basis for designing effective ozonation catalysts.In addition,conventional ozonation reactors commonly operate in a way transferring gaseous ozone from reactor bottom.Even though the employment of in-situ ozone reaction focusing on chemical reactivity or microbubble ozonation concentrating on mass transfer could accelerate the effectiveness of these reactors,the theoretical basis for enhancing ozone transfer and pollutants removal by means of fluid mechanics still needs development.The establishment of structure-activity relationship between fluid mechanics and mass transfer/chemical reaction will be beneficial to the industrial application of high performance ozonation reactor.Therefore,it is of importance to identify the main factors influencing catalyst activity and verify the structure-activity relationship of ozonation reactor.In this work,the catalytic activities of spinel ferrite,coking sludge based biochar,carbon nanotube?CNT?in ozone reaction were investigated,and the operation performance of microbubble ozone fluidized bed reactor enhanced with countercurrent contact was analyzed.A series spinel ferrites including ZnFe₂O₄,CoFe₂O₄,CuFe₂O₄ and Ni Fe₂O₄ were synthesized aiming at enhancing ozone reaction with phenol,bio-treated coking wastewater?BTCW?or oxalic acid,and fundamentally elucidating the mechanism of catalytic ozone decomposition.The mechanisms of biochar catalytic ozone decomposition and phenol degradation were revealed.The effect of CNT surface active sites on ozone decomposition and generation/scavenging mechanism of hydroxyl radical?·OH?were investigated.Then,CNT suspension was transformed into a CNT membrane filter system,by which the mechanistic influence of hydrophilic/hydrophobic properties of para-chlorobenzoic acid?p-CBA?and atrazine on membrane surface-mediated reaction were discussed.Finally,countercurrent reactor was manufactured,for the purpose of explicating the fluid mechanics that regulate ozone mass transfer and phenol degradation.ZnFe₂O₄?ZnFO?was prepared via hydrothermal and sol-gel methods,focusing on the efficiency and mechanism of phenol and BTCW degradation by catalytic ozonation.The crystal structure,elemental composition,surface morphology of ZnFO were characterized by XRD,XPS,SEM.ZnFO catalytic activities in ozonation were evaluated by the change of aqueous phenol,dissolved ozone,·OH concentrations and solution pH.It was found that the higher catalytic activity of hydrothermally synthesized ZnFO could be ascribed to higher surface hydroxyl group density.An empirical kinetic model was established via discussing the influence of reaction conditions on phenol removal.ZnFO catalyzed ozonation of BTCW was carried out by measuring total organic carbon?TOC?,UV-vis and three-dimensional fluorescence.Results indicated that organic compounds substituted with electron-donating groups such as-OH and-NH₂ were more efficiently oxidized by ozone or·OH.Given that Zn Fe₂O₄ was capable of catalyzing ozone decomposition into·OH,CoFe₂O₄,CuFe₂O₄,NiFe₂O₄ and ZnFe₂O₄ synthesized by sol-gel method were employed to inspect the catalytic activity and mechanism in ozonation of oxalic acid.The crystal structure and surface morphology of ferrites were analyzed by XRD,FTIR and SEM.The catalytic activity and stability of ferrites were investigated by oxalic acid mineralization and metal ion leaching,respectively.The reduction properties and electron conductivities of ferrites were examined by H₂-programmed temperature reduction and cyclic voltammetry,respectively.It was demonstrated that CoFe₂O₄ has the most prominent electron conductivity,suggesting higher catalytic activity in ozone decomposition and radical generation.According to XPS and surface acid-base titration analysis,A?II?-A?III?-A?II?redox cycle and protonated surface hydroxyl group were active sites.Thus,it could be suggested that,as for metal oxide catalysts,metal ions with variable valences play vital roles in regulating catalytic activity.The activity and mechanism of phenol ozonation catalyzed by biochar produced from pyrolysis of coking sludge that operated under different temperatures were studied.Biochar's thermal stability,elemental composition,and functional groups were characterized by TG-DSC,elemental analysis and FTIR.The efficiency and stability of biochar catalyzed ozone reaction with phenol were examined.Empirical kinetic model for phenol removal was developed based on the effect of reaction conditions.The mechanism of phenol ozonation catalyzed by biochar was proposed based on analyzing dissolved ozone concentration,free radical inhibition and influence of inorganic anions on phenol removal.Results indicated that dissolved ozone was predominantly adsorbed on biochar surface,followed by surface carbonyl mediated decomposition leading to superoxide anion radical generation.It could be emphasized that,as for sludge-based biochar,the ratio of surface carbonyl group determined by sludge pyrolysis temperature plays significant role in regulating catalytic activity.CNT was functionalized by nitric acid,and surface oxygen-containing group mediated·OH generation and inhibition were investigated.As a proof-of-concept,CNT deposited on filter membrane that could chemically oxidize pollutants was validated.In CNT suspension,CNT surface controlled·OH generating and scavenging mechanisms were explored according to R_CT and[?p-CBA]/[?O₃].It was demonstrated that the equilibrium of·OH production and consumption by CNT surface could be established while CNT concentration was 5 mg L?¹.The contribution of ozone self-decomposition to overall p-CBA degradation was evaluated by completely consuming the delivered ozone.As for CNT membrane filter which functionalized in a flow through mode,the mole concentration of p-CBA degradation via oxidation increased linearly with influent p-CBA concentrations,due to non-adsorptive property of p-CBA with CNT.By contrast,the mole concentration of atrazine degradation via oxidation rised logarithmically with influent atrazine concentrations,resulted from prominent adsorption of atrazine by CNT.Apart from heterogeneous catalysts that strengthen ozonation efficiency via surface mediated reaction,fluid mechanics in ozonation reactor could also regulate ozone reaction.Using microbubble fluidized bed reactor enhanced with countercurrent contact,the promoting effect of fluid mechanics on ozone dissolution and phenol oxidation were examined.While operating in ultrapure water,the influence of bubbling modes,ozone concentration and ozone flow rate both transferred from ozone generator on dissolved ozone concentration were analyzed.Results suggested that ozone transfer coefficient from gaseous to aqueous phase could be improved via increasing the driving force responsible for liquid film diffusion and contact time.Taking radical inhibition into account,the degradation efficiency of phenol was evaluated under different operation modes.The structure-activity relationship of reactor was evaluated according to the effect of fluid mechanics,including gas holdup,liquid circulation velocity and bubble rising velocity,on ozone transfer and phenol removal.Most importantly,results verified the essential role of microbubble in achieving countercurrent contact.In light of above work,the controlling factors responsible for catalytic activities of metal oxides and biochar in ozonation were concluded according to catalysts evaluation.The membrane filter concept was proposed for the purpose of achieving effective nanoparticle recycling.Also,the promoting effect of countercurrent contact principle on mass transfer and oxidation were verified.These obtained results could potentially provide theoretical support for the industrial application of ozone reaction.