| With the accelerated depletion of freshwater resources and the rapid development of industry,the problem of water pollution has become a growing concern.To solve this problem,technologies such as coagulation-flocculation,gravity separation,ion exchange,adsorption,and membrane filtration have been used to treat wastewater.Among these conventional treatment technologies,membrane filtration technology is widely used due to its high separation efficiency,low energy consumption and low cost effectiveness.Nanofiltration membranes have become the first choice for the treatment of dyes,organics and soluble salts due to their small pore size and low molecular weight cut-off（MWCO）,However,it is still a challenge to prepare nanofiltration membranes with simple process and excellent performance.Since the discovery of dopamine,it has been widely known due to its strong adhesion and ability to act on various substrates.However,since dopamine is expensive,tannic acid,which has a polyphenolic structure and is also inexpensive,was chosen as a modified raw material to construct the nanofiltration layer,and two types of nanofiltration membranes were constructed on PES ultrafiltration membranes by two modification methods.The details are as follows:Based on two hydrophilic substances,tannic acid（TA）and polyethylene polyamine（PEPA）,were used to prepare（TA/PEPA）1.5/PES nanofiltration membrane under the covalent layer-by-layer self-assembly（LBL）method.The reaction mechanism of TA and PEPA was explored by characterizing the change of the chemical structure of the membrane surface,the state of binding between elements.It was found that the coating parameters（the number of bilayers,the monomer concentration,the deposition time）greatly affected the separation performance of the modified membranes.The modified membrane prepared under the optimal conditions showed high separation efficiency and hydrophilicity,the water contact angle of the modified membrane could be reduced to 19°,and it has a high rejection rate for various dyes（CR,BBG,CCB,RB）,all above 97%.At the same time,due to the presence of hydrophilic groups on the membrane surface,the nanofiltration membrane possesses good anti-fouling performance,and the flux recovery rate of the nanofiltration membrane is above 95%in the 3-cycle BSA test.In addition,the nanofiltration membrane has strong stability,in the continuous 12-hour separation experiment of methyl blue,the rejection rate of the dye remained above 97%and remained basically unchanged.A nanofiltration membrane with controllable coating thickness was prepared based on the TA-assisted interfacial polymerization of PEPA with terephthalaldehyde（TPAD）.The elemental composition,binding state,and surface morphology of the membrane surface were analyzed by ATR-FTIR,XPS,and FESEM.The effects of the deposition time,the interfacial reaction time and the curing time on the surface properties investigated.The（T-P-T）/PES membranes prepared under optimal conditions showed strong separation ability,the permeation of the modified membrane can reach 74.76 L·m-2·h-1,and the rejection rate of CR,BBG,CCB and RB are all above 99%.In addition,the（T-P-T）/PES membrane can achieve dye desalination at low salt concentration（1 g/L）and exhibit good stability during long-term dye/salt separation. |
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