Effect Of Carbon-based Materials Coating Layers On The Rejection And Antifouling Properties Of Ultrafiltration Membranes

Ultrafiltration?UF?membrane can effectively intercept pollutants in water,such as suspended particles,colloids,macromolecular organics and pathogenic bacteria etc.,has the advantages of low operating pressure,low energy consumption and high degree of equipment integration,and plays an important role as a barrier in water quality security.Nevertheless,membrane fouling still restricts the further application of ultrafiltration technology.Natural organic matter?NOM?is considered as one of the most important membrane pollutants.In order to effectively solve the problem of membrane fouling caused by NOM,the carbon-based materials such as nanocarbon materials,ordered mesoporous carbon?OMC?were utilized to do the surface coating on the ultrafiltration membrane due to their adsorption and catalytic properties.The modified layer was formed and the modified UF membranes coated by the carbon-based materials were fabricated.The effect of carbon-based materials coating layers on the rejection and antifouling properties of ultrafiltration membranes were systematically studied.Carbon nanotubes?CNTs?and Carbon nanofibers?CNFs?were selected as the representative of nanocarbon materials.Taking the typical organic pollutants i.e.,humic acid?HA?,bovine serum albumin?BSA?,and sodium alginate?SA?as the target pollutants,the systematically studies of the effect of nanocarbon materials coating layers on the rejection and antifouling properties of ultrafiltration membranes were conducted.The results showed that the structure of CNTs was coiled-hollow,and CNFs had the solid and rod structure.The modified layer of CNTs and CNFs could effectively raise the interception rate of typical organics to 81.4?91.9%and 26.9?79.2% respecti-vely.The types of organic matter could affect the membrane antifouling properties.Compared with the CNFs layer,CNTs layer can effectively improve the antifouling properties.For the surface water treatment experiments,the results showed that the modified layer significantly raised the removal rate of DOC and UV254 to34.0?54.6% and 34.4?64.1%respectively.The humic acid and fluorescence components were further removed.CNTs modified layer reduced reversible and irreversible pollution by 77.8% and 60.9% respectively,while CNFs modified layer reduced reversible pollution resistance by 38.5%.The model fitting analysis of“membrane pore blocking and cake filtration”showed that CNTs and CNFs modified layer increased the volume of the pollution mechanism transition from 60 mL to 180 mL and 95 mL respectively,which delayed the transition of the pollution mechanism from membrane pore blocking to cake filtration.The modified layer of nanometer carbon material can trap pollutants by preadsorption and pore size interception,so as to avoid the deposition of organic matter on the membrane surface directly,and delay the formation of the cake layer on the membrane surface.The adsorption of modified layer plays a dominant role in the mitigation of humic acid pollution,and the mitigation of sodium alginate pollution is dominated by pore size interception.CMK-3 with hexagonal structure and CMK-8 with cubic structure were selected as the representatives of OMC materials.The effect of OMC material coating layers on the rejection and antifouling properties of ultrafiltration membranes were systematically studied.The results showed that CMK-3 had a typical two-dimensional hexagonal symmetrical structure with long parallel parallel channels inside,while CMK-8 had a three-dimensional porous structure with tightly wound and crowded internal channels.The modified layers of CMK-3 and CMK-8 could effectively improve the retention rate of typical organics to 76.3?87.2%and 49.0?70.7% respectively.The types of organic matter could affect the membrane antifouling properties.Compared with CMK-8 layers,CMK-3 layers can more effectively improve the antifouling abilities.For the surface water treatment experiments,the order for the removal rate of DOC and UV254 is as follows:CMK-3>PAC>CMK-8.Powdered activated carbon?PAC?preferentially removed humics,CMK-8 significantly removed the tryptophan-like proteins,while CMK-3 enhanced the removal of HA and tryptophan-like proteins.The mitigation efficiency of CMK-3 and CMK-8 modified layers to reversible and irreversible membrane fouling was 55.2?63.9%and 65.4?87.8% respectively,while PAC modified layers aggravated reversible membrane fouling.The model fitting analysis of“membrane pore blocking and cake filtration”showed that the OMC modified layer makes the membrane pore blocking dominate in the filtration process of HA and BSA,while the volume of the pollution mechanism transition increases slightly in the SA filtration.The adsorption and pore size interception of the modified layer both help to alleviate membrane pollution,and CMK-3 has a stronger adsorption capacity.Pollutants can enter the mesoporous through axial and radial directions,while the relatively large internal resistance of CMK-8 is not conducive to the adsorption of pollutants.In order to strengthen the rejection and antifouling properties,a system of carbon-based materials modified layers absorption-catalytic peroxymonosulfate?PMS?oxidation on the surface of UF membrane was set up.Effects of four carbon-based materials?CNTs,CNFs,CMK-3 and CMK-8?modified layers adsorption-catalytic oxidation systems on UF membranes properties were compared.The results showed that the carbon-based materials can promote the decomposition of the PMS and the order of the catalytic efficiency is as follows:CMK-3>CNTs>CMK-8>CNFs.The results of electron paramagnetic resonance and quenching experiments of oxidizing active substances showed that both radical and nonradical oxidation pathways exist in the reaction systems.SO₄^·-,?·OH,O₂^·-and ¹O₂ are the main reactive oxygen species.The removal rate of HA,BSA and the organic mixture increased to 76.1?92.3%?51.8?89.4%and 72.6?86.4%respectively.but the removal rate of SA reduced to 41.4?57.6%.Adsorption and catalytic oxidation further alleviated the reversible membrane fouling caused by HA,and the pollution resistance was reduced by 32.6?54.0%,but the further mitigation effect on BSA contamination was limited.The performance of ultrafiltration membrane against SA pollution was greatly improved,the specific flux at the filtration end increased from 0.031 to0.105?0.286,and the reversible pollution resistance decreased by 72.0?91.2%.For the surface water treatment experiments,Surface water treatment tests showed that the removal rates of pollutant DOC and UV254 increased to 36.2?74.1%and 43.2?84.0%respectively,and the fluorescence components in the effluent were also effectively removed.Under the action of several oxidizing active species,the molecular weight distribution of organic pollutants is changed,and the macromolecular biopolymers are decomposed into small molecular substances,so as to be better absorbed and trapped by the modified layer.The adsorption and catalytic oxidation of the modified layer reduced the reversible pollution resistance by 58.2?83.2%and the irreversible pollution resistance by 36.2?76.3%.The pollution mitigation effect of CNTs and CMK-3 was better than that of CNFs and CMK-8.The types of organic matter could affect the membrane fouling control effect.Compared with the single carbon-based materials modification,the catalytic oxidation of the modified layer can further alleviate the reversible membrane fouling caused by HA.However,the control effect of BSA was not improved.In the case of pollution caused by SA and organic mixture,the membrane fouling control efficiency was greatly improved,which solved the problem that the carbon-based materials modified layer had a poor effect on the fouling control.The surface water treatment experiments showed that the catalytic oxidation could further improve the removal efficiency of DOC and UV254,and the fluorescence components in the effluent were also significantly removed.The molecular weight distribution of organics was changed,with the macromolecular biopolymers degraded into small molecules.Under the synergistic action with adsorption and catalytic oxidation of the modified layer,the membrane fouling resistance was reduced to different degrees,with the reversible fouling resistance reduced by 58.2-83.2%and the irreversible fouling resistance reduced by36.2-76.3%.For the four carbon-based materials modified layers,CNTs and CMK-3 had significantly better membrane pollution mitigation effects than CNFs and CMK-8.In this paper,carbon-based materials modified layers were prepared on the surfaces of UF membranes.The mechanisms of retention and membrane fouling mitigation by modified layers were proposed,and the mechanism of adsorption-catalytic oxidation by modified layers was explained.The preparation and process operation of the modified layers were optimized.Overall,the membrane retention efficiency was improved and the service life was prolonged,which provides theoretical and technical supports for engineering applications.