Assembly Of Layered Carbon Materials Modified Ceramic Membrane And Water Purification By Coupled Ozone-catalytic Membrane Process

Ultrafiltration membrane water purification technology has been widely adopted in wastewater reclamation technology.Whereas,it showed poor removal performance of micro-organic contaminants and the serious membrane fouling limiting its development.These problems result in the ecological risk in the effluent from ultrafiltration membrane technology,complex membrane operation condition and high cost.Therein,ceramic ultrafiltration membranes with good chemical stability can be coupled with ozonation technology,which has a high oxidation capacity to degrade micro-organic contaminants to a certain extent and simultaneously mitigate membrane fouling.Catalytic ozonation membrane was fabricated by loading metal oxides with catalytic ozonation activity onto the surface of ceramic membranes to form a catalytic functional layer.It’s effective in addressing the problems of weak and selective molecular ozone.However,the metal ions were released leading to a weakening of the catalytic function and the formation of secondary pollution.Layered carbon materials,like graphene and carbon nitride,are potential materials to fabricate catalytic functional layers in coupled ozone-catalytic membrane processes.They not only showed the potential activity to catalyze ozone decomposition,but also do not create the secondary risk of metal ion leaching.In addition,the principle of catalytic ozonation for water purification by layered carbon materials will be discussed in details.In summary,this paper evaluates the catalytic ozonation performance of four typical layered carbon materials（graphene oxide（GO）,reduced graphene oxide（rGO）,carbon nitride prepared from melamine and urea（M-g-C₃N₄ and U-g-C₃N₄））and explores their catalytic mechanisms.The structural of rGO obtained from the screening were doped with metal-free heteroatoms（N,P,B and S）.It reveals the structure-activity relationship between the surface-active sites and the free radical’s evolution.The best performance N-rGO was employed as the active component to optimize the assembly of catalytic membranes.In-situ catalytic ozonation degradation of micro-pollutants,COD degradation and organic membrane fouling self-cleaning performance were evaluated while the interfacial catalytic mechanism and membrane fouling mitigation mechanism were revealed.In order to optimize the catalytic materials for the catalytic membrane,the performance of the above layered carbon materials was evaluated by selecting p-chlorobenzoic acid（p-CBA）and benzotriazole（BZA）as the target organics pollutants.GO(k_obs,p-CBA=0.229 min^-1,k_obs,BZA=0.226 min^-1)and rGO(k_obs,p-CBA=0.183 min^-1,k_obs,BZA=0.298 min^-1)performed significantly better than M-g-C₃N₄(k_obs,p-CBA=0.099 min^-1,k_obs,BZA=0.087 min^-1)and U-g-C₃N₄(k_obs,p-CBA=0.116 min^-1,k_obs,BZA=0.156 min^-1).However,the structure of GO is extremely unstable in catalytic ozonation reactions.Ozone and the formation of hydroxyl radicals（·OH）destroy the layer structure of GO.RGO exhibits the best catalytic ozonation performance with good layer structure stability.The oxygen-containing functional groups,free electrons in the defective sites andπ-electrons in the sp² hybrid carbon structure of layered carbon materials are able to catalyze the decomposition of ozone to form·OH and superoxide radicals(O₂^·–).It achieves rapid degradation of the target pollutant.Therefore,the core of the catalytic ozonation of organic pollutants by layered carbon materials lies in the distribution and transfer of electrons in the structure of the layered carbon material.Based on the above catalytic mechanism,the electronic structures of the screened rGO were moderated by virtue of N,P,B and S atoms with different electronegativities.Their catalytic ozonation performance was evaluated as well.The results indicated that except sulfur doping,nitrogen(k_obs,p-CBA=0.694 min^-1,k_obs,BZA=0.392 min^-1),phosphorus(k_obs,p-CBA=2.425 min^-1,k_obs,BZA=0.703 min^-1)and boron(k_obs,p-CBA=0.654 min^-1,k_obs,BZA=0.347 min^-1)doping significantly improved the catalytic ozonation activity of rGO.While P-rGO and B-rGO are able to maintain good layer structure stability during the reaction with severe loss of heteroatoms.Only N-rGO exhibits the best structural stability.The free electrons in the defective sites,doped sites,and the oxygen-containing functional groups contribute to the production of·OH while theπ-electrons in the sp² hybrid carbon structure and the surface-OH groups facilitate the production of O₂^·–.A set of N-rGO catalytic membranes（N-rGO-CM）with water purification,self-cleaning and reduction of toxic by-products was prepared by physically loading N-rGO,which has superior catalytic ozonation properties onto the surface of ceramic membranes.Density functional theory（DFT）calculations results disclosed that ozone molecule was adsorbed by pyridinic N and pyrrolic N atoms on the N-rGO-CM surface.It then underwent direct catalytic decomposition to surface-bound atomic oxygen(*O_ad)and peroxide(*O_2,ad),ultimately producing·OH.The graphitic N（in-frame or edge）extracted ozone onto the N-rGO-CM surface to enhance ozone-derived direct oxidation.It triggers that the ozone utilization of N-rGO-CM increased to 79.5%compared to the 37.75%of ceramic membranes.Based on the analysis of the influence of membrane interface parameters（catalytic layer thickness,roughness and hydrophobicity）on the performance of catalytic ceramic membranes,this study established the relationship between membrane interface parameters and catalytic ozonation performance and self-cleaning of membrane fouling and thus providing new methods and ideas for the assembly and performance regulation of catalytic membranes.The self-cleaning performance of N-rGO-CM coupled with ozone was evaluated against organic membrane fouling using the secondary effluent as the treatment target.The rapid degradation of EfOM and the transformation of molecular characteristics are important factors for mitigate membrane fouling by N-rGO-CM coupled with ozone revealed by means of multiple spectroscopies.The molecular transformation pathways of EfOM were revealed by Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR MS）.It consisted of three main types of reactions,namely decarbonization,deheteroatomisation and oxygenation dehydrogenation,with a total of 18 reaction pathways.Two-dimensional correlation spectroscopy（2D-COS）was employed to reveal the self-cleaning mechanism of membrane fouling.The N-rGO-CM coupled with ozone can preferentially enhance the removal of membrane foulants that tend to form membrane pore blockages or adhere to the membrane surface,such as humic acid-like substances with medium or low molecular weight.It enables in-situ self-cleaning of membrane fouling.Based on the membrane fouling mitigation behavior and pathways described above,N-rGO-CM with ozone is able to maintain a J/J₀ of 70.17±1.77%after a long-term use,with a flux recovery performance 1.96 times higher than that of commercial ceramic membrane coupled ozone filtration.In conclusion,a set of catalytic membranes with synergistic decontamination and efficient self-cleaning in water purification is designed based on the catalytic ozonation mechanism by layered carbon materials while the method of interracial regulation of the catalytic membranes is revealed in this study.The combination of the catalytic membrane interface ROS generation pathway,ROS mitigation behavior towards membrane foulants and the transformation mechanism of EfOM represents a new mechanism for the mitigation of membrane fouling by N-rGO-CM coupled with ozone.This provides some theoretical guidance and data support for the development and design of new catalytic membranes and the practical application of ultrafiltration membrane coupled catalytic ozonation processes.