Surface Modification Of Polyamide Composite Nanofiltration Membrane And Its Application

As a new kind of pressure-driven separation technology,nanofiltration（NF）membranes have been widely applied in several areas including power,environment,medicine,food,petrochemical industry,sea water soften and others.It is difficult to satisfy the demand of membrane-forming ability,separation performance,stability,economic cost for the common polymer membrane materials.So the method of physical or chemical surface modification are utilized to enhance the performance of NF membranes.This paper is based on the previous work of polyamide composite NF membrane in our group.Interfacial polymerization（IP）and immersion coating method are used to improve the flux and selectivity to Mg²⁺/Li⁺of NF membranes.Two main parts are included in this paper as follows:（1）The polyamide（PA）composite nanofiltration（NF）membrane was prepared using polyetherimide（PEI）ultrafilation membrane as support.Ethylenediamine（EDA）was used as crosslinker and to react with trimesoyl chloride（TMC）.The PA composite NF membrane was modified by piperazine（PIP）and other diamines.The effect of different diamine monomers on the pure water flux of membranes and the effect of PIP and TMC concentration on the performance of membranes were investigated.The optimized PIP modified NF membrane shows the flux of 38.3 L/（m² h）and rejection of 90%to 10 g/L glucose solution.The membrane PA-PIP shows the order of rejection to different salt solutions is MgSO₄（97.4%）>Na₂SO₄（97.3%）>MgCl₂（73.9%）>NaCl（52.4%）,revealing the characteristic of negatively charged NF membrane.The flux recovering ratio to sodium alginate（SA）and bovine serum albumin（BSA）are higher than 85%,which means that the membrane has good anti-fouling property.（2）Branched poly（ethylene imine）（BPEI）was utilized as monomer for surface modification to get a positively charged composite NF membrane.For improving selective separation performance of NF membrane to lithium and magnesium mixture solution as simulative brines by composite NF membrane,then we chose EDTA to modify the as-prepared positively charged NF membrane to combine the Donnan effect and chelating effect.After optimizing conditions of concentration of BPEI and EDTA,the optimal EDTA modified NF membrane PA-B2-E3 showed the separation factor S_Li,Mg of about 9.2 for the LiCl/MgCl₂ mixed solution which has a Mg²⁺/Li⁺mass ratio of 24 and concentration of Mg²⁺of 2.4 g/L under 1.0 MPa.It was confirmed to be relatively stable due to that the flux and separation factor didn’t show decrease during the 12h filtration test.And Mg was found on the surface of membrane after 12h test,which means that EDTA exist on the surface of composite NF membrane as stable form.