Fabrication Of Positively Charged Nanofiltration Membrane And Its Application In Separation Of Mono-/Divalent Cations

Nanofiltration（NF）membranes have excellent separation performance in single/multi-valent ion separation and salt and organic compound separation due to their unique pore size and charge properties.They are also highly efficient,low-energy,and easy to operate,making them promising for applications in wastewater treatment and resource recovery.However,most commercially available NF membranes are negatively charged,which is not conducive to the separation of single/multi-valent cations.There is a need for developing positively charged NF membranes for cation separation.Polyethyleneimine（PEI）contains a large number of amino groups on its molecular chain,making it an ideal amine monomer material for positive charge modification of NF membranes.This study attempted two approaches to prepare positively charged NF membranes:direct preparation of NF membranes using PEI as a water-phase monomer through interfacial polymerization,and surface modification of commercial NF membranes.In the interfacial polymerization method,a polyvinylidene fluoride（PVDF）ultrafiltration membrane was used as the base membrane for the NF membrane,and PEI and trimesoyl chloride（TMC）were interfacially polymerized on its surface.The effects of substrate formulation,PEI molecular weight,water-phase and organic-phase monomer concentrations on the structure and performance of the NF membrane were studied.The results showed that high molecular weight PEI was beneficial for increasing the surface cross-linking degree and zeta potential of the NF membrane.The optimal formulation was 0.8%PEI（70k Da）and 0.08%TMC,corresponding to a PEI/TMC mass ratio of 15.2:1.The prepared NF membrane PT8 had good performance,with a positively charged surface and an average pore size smaller than that of the commercial NF270 membrane.In long-term filtration tests of mixed salt solutions,PT8 had a stable water permeation flux of6.30-6.85 LMH/bar and a retention rate of over 96%for some divalent cations,with a Mg²⁺/Li⁺separation coefficient of 18.6.In pollution tests with humic acid（HA）,PT8 had a low pollution rate（5.87%）and a high recovery rate（97.57%）,with stronger pollution resistance than NF270 and less affected by pollution on retention performance.In the study of positive modification on commercial membranes,a two-step grafting method was employed to sequentially graft PEI and2,3-epoxypropyltrimethylammonium chloride（EPTAC）onto the membrane surface,increasing the positive charge density of the modified membrane surface.Experimental results showed that compared with EDC/NHS activators,the activation operation with 2-chloro-1-methylpyridinium iodide（CMPI）was simpler and had less water flux decline.As the molecular weight of grafted PEI decreased,the average effective pore size on the membrane surface decreased.The modified membrane using PEI with a molecular weight of 600 Da had an average effective pore size of 0.4052 nm,which was smaller than that of the original membrane（0.4393 nm）,and the surface positive charge increased,resulting in the best separation performance.Through optimization,the optimal grafting conditions for surface modification were determined to be 0.05 wt%CMPI,2 wt%PEI,and 1 wt%EPTAC.The modified membrane had a Mg²⁺retention rate of 94.93%,which was higher than that of the original membrane（57.65%）.Anti-fouling and long-term operation tests showed that the two-step modified nanofiltration membrane with PEI and EPTAC had better separation performance for mono-and divalent cations,anti-fouling performance,and alkali resistance compared to the original membrane.By utilizing the rich amino groups in PEI for interfacial polymerization to prepare nanofiltration membranes,or by synergistically using the quaternary amine groups in EPTAC for surface cationic modification of commercial nanofiltration membranes,the NF membranes’separation performance towards mono-and divalent cation systems can be effectively improved,while showing more excellent pollution resistance.These characteristics of positively charged nanofiltration membranes indicate their wide application in fields such as heavy metal wastewater treatment,water softening,and resource recovery.