Study On Ultrathin High-selectivity NF Membrane Based On Support-free Interfacial Polymerization

Nanofiltration（NF）is an important component of membrane-based separation technology.One of the most typical features of NF is ion separation selectivity,that’s to say,different types of ions have different rejection,which makes NF has obvious advantages in the field of refinement and resource treatment of industrial wastewater.The analysis and enhancement of the selectivity of NF membranes has been a research hotspot in recent years.Starting from the separation mechanism of NF,this thesis studied the influence of the thickness of the separation layer on the separation selectivity of NF membranes based on method innovations and structure control.A series of high-permeability and high-selectivity NF membranes was prepared.In response to the needs of different application fields,this thesis carried out research work on anion selective NF membrane and cation selective NF membrane.In this paper,support-free interfacial polymerization（SFIP）was used to realize the controllable preparation of the ultrathin separation layer.Firstly,the hydrophilic interlayer was constructed on the PSF substrate surface through the complexation of TA-Fe³⁺,which provided a free water-hexane interface for interfacial polymerization（IP）.Subsequently,the ultrathin high-performance polypiperazine amide anion selective NF membrane was prepared by a novel self-designed vaccum filter frame equipment,named Suction-assisted in situ free IP（SAIP）.Thanks to the uniform reaction interface and ultralow amine concentration（0.025%）,the thickness of polyamide nanofilm was reduced to 14.4±0.3 nm.The prepared ultrathin membrane has a high permeability flux(29.58±0.28 Lm^-2h^-1bar^-1)thanks to shortened mass transfer distance,which is about 4.5 times than the control group.The anion selectivity of the ultrathin NF membrane prepared in this paper（112.7±1.5）is significantly higher than that of the traditional NF membrane（43.1±1.4）.Through XPS,AFM and other characterization methods,the reasons for the increase in selectivity were rationally explained based on the size-based exclusion and Donnan effects,and the influence of the thickness of the separation layer on the selectivity of anions was revealed.In view of the flux attenuation problem of the polypiperazine amide NF membrane prepared by SAIP technique,the rigidly SBI was introduced to control the structure of membrane on a molecular scale.The rigid structure of SBI weakened the compaction effect of membrane when operating under pressure.The operating stability of the membrane was significantly enhanced,and the separation performance of membrane was also excellent.Polyethyleneimine（PEI）cationic polyelectrolyte has good hydrophilicity,high charge density and easy protonation,which is widely used to prepare cation seletctive NF membranes.However,the permeability and selectivity of PEI membrane prepared by traditional IP method is very low.This thesis explores the application of SAIP to this type of polymer monomer system.The ultrathin（11.2±0.2 nm）high-performance cation-selective NF membrane was successfully prepared with a low PEI concentration（0.05%）based on SAIP technique.The permeability of prepared membrane was as high as 14.21±0.33 Lm^-2h^-1bar^-1,which is about 10times than the control group.Furthermore,the membrane has excellent cation selectivity.Through the characterization and analysis of membrane structure and the evaluation of membrane performance,the influence of separation layer thickness on membrane performance was discussed.