Research On Construction And Performance Of Nanofiltration Membranes With Modified Functional Layers Based On Hydrophilic Materials

Nanofiltration technology,recognized as an efficient water treatment technology,plays an important role in the field of wastewater regeneration,desalination and removal of organic matters of drinking water.However,membrane fouling and the “trade-off” effect restrict the further application of nanofiltration.If the “trade-off” effect can’t be broken,the permeability and selectivity performance of nanofiltration membrane won’t get improved at the same time.Meanwhile,membrane fouling results in more operating energy consumption,higher chemical cleaning frequency and shorter lifespan.In order to improve the permeability,selectivity and antifouling performance of membrane,prepare high performance thin film composite nanofiltration membrane by the means of modificating the functional layer of nanofiltration membrane.Cellulose nanocrystals(CNCs),which are excellent hydrophilic modification nanomaterials,have good dispersion in water.Dopamine(DA)is hydrophilic and self-polymerization takes place on the substrate to form dopamine thin layer under weak alkaline conditions in oxygen or air environment.It has a wide range of applications,with little limitation on the substrate material,and has received extensive attention in the field of water treatment.Sulfobetaine methacrylate(SBMA)is superhydrophilic zwitterionic polymer with special salt response characteristics,which has a significant effect on improving the anti-fouling performance of membranes.In this study,high performance composite nanofiltration membrane was prepared by means of supporting layer coating,surface deposition and surface grafting modification with CNCs,DA and SBMA as modified materials.The physical and chemical properties of nanofiltration membrane were studied by means of different characterization methods,and the permeation,selectivity and anti-fouling performance of nanofiltration membrane were tested with appropriate target objects.First,two kinds of membranes with the same material and different mean pore sizes were selected as supporting layer respectively to prepare nanofiltration membranes,and the surface morphology,charge,hydrophilicity and pore size distribution of nanofiltration membranes were compared.The results showed that compared with ultrafiltration as supporting layer,the permeability of nanofiltration membrane prepared with microfiltration membrane as the substrate was improved,but selectivity decreased.Second,improved the performance of nanofiltration membrane with the combination of the modification of the supporting layer and the active layer.Explored the relationship between the coating amount of CNCs,the deposition concentration of DA and the morphology,roughness,hydrophilicity,surface charge and pore size of the prepared membrane,optimized the membrane preparation parameters and broke through the "trade-off" effect of nanofiltration membrane.The results showed that the hydrophilicity and negative charge of membranes were improved with the coating amount of CNCs increasing.When the coating amount of CNCs increased to 4 mg,the nanofiltration membrane roughness decreased,the hydrophilicity increased,the MWCO increased,and the membrane flux significantly increased.When the deposition concentration of dopamine increased to 6.4 g/L,the hydrophilicity of nanofiltration membrane increased,the roughness increased,the MWCO decreased,and the membrane selectivity significantly increased.Compared with the control membrane,the permeation/selectivty performance of the composite nanofiltration membrane prepared by combining with the modification of CNCs interlayer and the surface deposition of DA was effectively improved.The pure water permeability was 23.11 LMH/bar,and the rejection of sodium sulfate was increased to 95.21%.In addition,the rejection of Congo red and Bengal red was both 99.9%,and that of sodium ligninsulfonate was 94.7%.The selectivity performance of TFC-CNC-PDA was also better than that of the control membrane when the permeability was also better.Then,SBMA was grafted on the surface of nanofiltration membrane via an Atom Transfer Radical Polymerization reaction.Explored the influence mechanism of SBMA on the morphology,roughness,hydrophilicity,charge and pore size of the membrane,and tested the effect of the polymer brush on the selectivity and anti-fouling performance of membrane.The results showed that the hydrophilicity of the nanofiltration membrane modified by SBMA was significantly increased and the MWCO was decreased.Compared with the control membrane,SBMA modified nanofiltration membrane significantly improved its ability to retain monovalent salt and to resist pollution of humic acid due to the salt response characteristics of the polymer brush.