Nanofiltration Membranes Fabricated Via Contra-diffusion Method And Their Pore Size Tuning

Nanofiltration membranes(NFMs)have been widely used in water treatment,chemical separation and food industry.However,all existing fabrication methods of NFMs have their shortcomings,for example,limited diversity of chemical structures of the selective layers,mediocre performance of NFMs prepared by some methods,as well as low time-and cost-efficiency of many methods.These shortcomings significantly limit the applications of NFMs.Herein,we develop a novel contra-diffusion strategy for NFM fabrication.Two precursor solutions of the selective layer are separately placed on either side of thesupport membrane.Precursor A will diffuse through the support membrane into precursor B solution and in situ form the preformed selective layer with precursor B via chemical reactions or physical interactions.The preformed selective layer will hinder the diffusion of the precursors,so further diffusion will mainly take place at the defects of the preformed selective layer,which will in situ form insoluble materials to seal these defects.This contra-diffusion process will finally lead to a thin,uniform and defect-free selective layer.The contra-difftusion strategy has several advantages in NFM fabrication,including diverse selective layer structures and properties oriented from the wide range of applicable precursors,satisfying performance of the as-prepared NFMs oriented from the self-limiting mechanism,relatively high precursor utilization rate,as well as facile operation with relatively high time-and cost-efficiency.In order to show the wide range of applicable precursors,we checked dopamine/polyethyleneimine(Mw = 600 Da)as small molecule precursors,sodium alginate as a polymer precursor and graphene oxide as a nanomaterial precursor for NFM fabrication via the contra-diffusion strategy.We studied the contra-diffusion process and the formation of the selective layers.The properties and performances of the as-prepared NFMs were also compared with other NFMs in literatures in detail.In order to obtain NFMs with narrow pore size distribution for precise molecular separation,we then provide a general strategy to narrow down the pore size distribution of any existing NFMs by pore wall modification.A "regulating" molecule solution is filtrated by an NFM,during which the "regulating" molecules react with the NFM selective layer and tether on it.If the "regulating" molecule has a proper size,they can enter the large pores in the NFM selective layer and modify the pore walls to reduce the pore size during the filtration process.At the same time,few "regulating"molecules can enter those small pores in the NFM selective layer,leaving them unaffected.Such a process will lead to a narrowed pore size distribution of the NFM,which we call here the "self-regulated" mechanism.We chose mercaptopropanesulphonate ion solution filtrated by polydopamine/polyethyleneimne codeposited NFM and as a model to study the pore wall modification process and its influence on the pore size and its distribution as well as the influence on the nanofiltration performance.