N-vinyl Formamide Surface Modified Polypropylene Membrane For Removal Of Organic Micropollutants From Water

With the rapid development of industrial,agricultural and pharmaceutical markets,organic micropollutants(OMPs),such as organic raw materials,plasticizers,pesticides,and pharmaceuticals,have been found in ground waters,surface waters,and even wastewater treatment plants(influents and effluents)worldwide.Although the presence of such contaminants in water sources may be at trace levels,these contaminants could be enriched in animals and humans and induce chronic toxicity,increasing the risk of many serious diseases like cancer,leukemia,teratogenesis and endocrine disorders.A range of methods including oxidation,photolysis,UV-degradation,nanofiltration,reverse osmosis,and adsorption has been used for their remediation from aqueous systems.However,adsorption technology is considered to be the most effective and commonly used method for removing OMPs from water quickly because of its low cost,facile operation,environmental friendliness,and easy integration with wastewater treatment plants.Commonly used activated carbon adsorbents have many shortcomings in removing OMPs from water,such as,poor adsorption capacity,weak adsorption affinity,slow adsorption rate,and difficult regeneration process.Newly developed adsorbents,such as,porous polymers,nanomaterials,MOFs,COFs,Mxene,have good OMPs adsorption performance,but they are limited by high cost,poor batch stability,and difficulty in industrial scale-up.Besides,in order to maintain a large specific surface area and suitable pore morphology,these adsorbents are usually in the form of powder with a small diameter,which would increase the pressure drop and energy consumption of the packed column,and reduce the water flux and treatment efficiency.Adsorptive membranes,combine the advantages of adsorption materials and separation membranes,may overcome these shortcomings of powder adsorbents because of its large flux,low pressure,high efficiency,low cost,and easy industrial scale-up.However,although adsorption membranes have attracted much attention in recent years,there are still many problems,such as,weak adsorption affinity,low adsorption capacity,unclear adsorption mechanism,and poor removal of trace OMPs.In this work,commercial polypropylene fiber membrane(PPM)is employed as the substrate,and the N-vinyl formamide(NVF)which has a stable structure,biomedical safety,and multiple hydrogen bonding sites,is selected as the functional monomer.As a result,the poly(Nvinylformamide)(PNVF)modified PPM(NPPM)with good hydrophilicity,high flux and excellent adsorption performance was prepared.The influence of different driving forces on the adsorption performance was investigated,and the adsorption process and mechanism of OMPs,especially trace OMPs,was revealed.Therefore,this study may open a pathway to develop strong adsorption affinity and broad-spectrum adsorptive membranes with low-cost,high-efficiency,and easy industrial scale-up,for targeted removal of OMPs from water.Main contents are as follows:1.NVF was grafted onto the surface of PPM to prepare PNVF modified polypropylene fiber membrane(NPPM)via one-step UVinitiated surface polymerization.The NPPM has excellent water permeability and ultra-high water flux.After grafting PNVF,the water contact angle of the membrane is decreased from 136° to 24°,and the water flux of the single-layer membrane reach 40,000 L/(m2h),which is 4 times that of PPM.The NPPM effectively removes a variety of OMPs from water,and its adsorption performance is 5-50 times that of PPM and commonly used PES,PA,and PTFE.Moreover,the NPPM exhibits excellent stability,regeneration and reusability.The adsorption process is more in line with the pseudo-second-order kinetic model and the Langmuir isotherm model,and the grafted PNVF could form a strong hydrogen bond with BPA,which increases the adsorption enthalpy change and enhances the adsorption affinity.2.The effects of temperature,pH,hydrophobicity of the compound,and membrane substrate on the adsorption performance were investigated,which shows that the adsorption of OMPs by the NPPM is a synergistic effect of hydrophobic interaction and hydrogen bonding.Furthermore,the strong hydrophobic interaction provided by PPM plays a fundamental role in the adsorption process of polypropylene-based adsorptive membranes.Besides,polyacrylamide,polyacrylicacid poly(N-vinylpyrrolidone),poly(N-vinylformamide),polystyrenesulfonate,and polybutylacrylate modified polypropylene fiber membranes were prepared and the adsorption mechanism was revealed,which demonstrates that the construction of dynamic hydrophobic domains on the hydrophobic interface could significantly improve the adsorption performance.Therefore,it may balance the contradiction that hydrophilic modification would reduce adsorption sites and hydrophobic modification would reduce water flux,which opens up new ideas for the preparation of adsorptive membranes with high water flux and adsorption capacity.3.The adsorption performance of different membranes on a broad range of trace OMPs(10 μg/L)was investigated.The NPPM is able to reduce OMP molecules in water samples by up to 1-3 orders of magnitude,which is superior to that of the PPM,PTFE,PES and PA.Even at an ultrahigh water flux(12700 L/(m2h)),the removal efficiency for trace BPA remains 95%,indicating that NPPM has ultra-strong affinity and ultra-high adsorption rate.The removal effect of different functional polymer modified membranes on trace OMPs demonstrates that the dynamic hydrophobic domains formed at the hydrophobic interface are the key factor for the adsorption of trace OMPs.Furthermore,the adsorption process of trace OMPs by dynamic hydrophobic microdomains was revealed,and the adsorption mechanism called hydrogen bonding-induced hydrophobic assembly was proposed and revealed using pyrene fluorescence,and two-dimensional nuclear Overhauser effect spectroscopy(2D-NOESY),and simulation.Thus,the strategy may open a pathway to design adsorptive membranes with ultra-strong adsorption affinity for the capture of trace OMPs from water.4.To introduce more hydrophobic sites,the epoxy ring-opening reaction was used to covalently introduce graphene oxide(GO)on the partially hydrolyzed NPPM to prepare a GO-modified NPPM composite membrane(NPPM-GO).The NPPM-GO has excellent water permeability,ultra-high water flux(38000 L/(m2h),single layer),good chemical and physical stability and regeneration performance.The adsorption performance of the NPPM-GO for various OMPs is 3-60 times that of the NPPM,and 25-110 times that of the PPM.The adsorption process of NPPM-GO is in line with the pseudo-second-order kinetic model and Langmuir isotherm model,and the measured adsorption capacity of BPA is 41.7 mg/g,which is higher than that of the NPPM.In addition,the NPPM-GO has a strong adsorption affinity and could effectively remove a variety of trace OMPs from water with a removal rate of larger than 80%.5.To introduce more hydrogen bonding sites,the epoxy ring-opening reaction was used to introduce double-end epoxidized polyethylene glycol(DEPEG)on the partially hydrolyzed NPPM to prepare a PEG-modified NPPM composite membrane(NPPM-PEG).The adsorption process of the NPPM-PEG is more in line with the pseudo-second-order kinetic model,and the adsorption rate is 2.4 and 3.2 times that of the NPPM-GO and the NPPM,respectively.The adsorption process of the NPPM-PEG is in line with the Langmuir isotherm model,and the measured adsorption capacity of BPA is 51.6 mg/g,which is higher than the NPPM,NPPM-GO and other similar adsorptive membranes.Moreover,the NPPM-PEG has strong adsorption affinity,which could effectively capture trace OMPs from water.6.To construct more interfacial hydrophilic/hydrophobic microdomains,NVF-St copolymers with different NVF contents were prepared by free radical copolymerization of NVF and styrene.AFM,TEM and SEM images show that the copolymer with high NVF content could form a nanoscale hydrophilic/hydrophobic microphase separation structure.Such structure allows the copolymers to be swelled in water and self-pored by freeze-drying,yielding a large specific surface area and excellent adsorption performance.The adsorption capacity of the copolymer for BPA reaches 226 mg/g,which is higher than that of the NPPM,NPPM-GO,NPPM-PEG,and most of the activated carbon and polymer adsorbents reported in the literature.The copolymer could be used to prepare a new type of adsorptive membrane through coating,which makes up for the deficiency of ultraviolet grafting,broadens the selection range of substrates,and provides a new idea for preparing high-performance adsorptive membranes.