Regulating Active Layer To Fabricate Reverse Osmosis Membranes With High Flux And High Rejection

Aromatic polyamide(PA)reverse osmosis(RO)membranes are widely applied in water treatment,but the membranes still face the challenge of high energy consumption and low water quality due to unsatisfactory permselectivity.Optimizing the PA active layer structure is an effective method to improve RO membrane permselectivity.In this work,focusing on interfacial polymerization(IP)and post treatment,the following strategies for optimizing the active layer structure to prepare RO membranes with high flux and high salt rejection were developed.The mechanical force was used to regulate solvent activation and optimize the active layer structure.The mechanical force can strengthen the interaction between PA oligomers on the membrane surface and PA,reducing the dissolution of PA oligomers and alleviating the swelling of PA during activation process.The characterization results showed that the membrane prepared by regulating solvent activation had more PA oligomers and a PA layer with alleviated swelling compared to the membrane prepared by solvent activation directly.The membrane prepared by regulating solvent activation had a higher rejection and comparative water flux compared with the membrane prepared by solvent activation directly.For the feed of 2000 mg/L NaCl at the pressure of 1.55 MPa,the water flux and rejection of the modified membrane can reach 48.4 L m-2 h-1 and 99.39%(the rejection improved by 0.43%),respectively.For the feed of 32000 mg/L NaCl at the pressure of 5.5 MPa,the water flux and rejection can reach 65.0 L m-2 h-1 and 99.43%(the rejection improved by 0.22%),respectively.The additive was introduced during IP process to regulate the active layer structure and enhance the membrane performance.1-methylimidazole(1-MI)was used as a novel additive in aqueous phase solution to prepare PA-RO membranes.After the introduction of 1-MI,the RO membrane had a thinner and denser active layer,and the membrane surface had more carboxylic acid groups.The variation of PA active layer structure was attributed to that 1-MI can react with the monomer trimesoyl chloride(TMC)in organic phase solution,which increased the back surface pore number of the active layer,accelerated the reaction rate of IP,and enabled the self-inhibition of IP to reach earlier.The reaction product of 1-MI and TMC hydrolyzed easily,thus more carboxylic acid groups were remained on the membrane surface.Under different feed concentration,the RO membrane prepared by introducing 1-MI had a high water flux and an excellent rejection.When 0.3%w/v 1-MI was introduced in aqueous phase,for the feed of 2000 mg/L NaCl at the pressure of 1.55 MPa,the flux and rejection of the developed membrane were 72.9 L m-2 h-1 and 99.06%,respectively,increased by 65%and 0.35%compared with unmodified membrane,respectively.The RO membrane was prepared by combining the optimization of IP with additive and the regulation of solvent activation with mechanical force.The results showed that regulating solvent activation had no significant effect on the structure and performance of the membrane prepared by introducing 1-MI as the additive during IP.This may be due to the higher denseness of the PA active layer prepared with 1-MI as additive.Then,the membrane was prepared by combining the optimization of IP with co-solvent as the additive and the regulation of solvent activation with mechanical force.Dimethyl carbonate(DMC)and hexamethylphosphoramide(HMPA)were used as the organic/aqueous phase co-solvent to regulate IP,which increased the water flux of the RO membrane.The regulation of solvent activation by mechanical force was applied to the membrane prepared with DMC and HMPA as organic/aqueous phase co-solvent,maintaining high flux of the membrane,and improving the membrane rejection effectively.When 1.5%w/v DMC was introduced in the organic phase solution,for the feed of 2000 mg/L NaCl at the pressure of 1.55 MPa,the flux of the modified membrane increased from 50.0 L m-2 h-1 to 76.9 L m-2 h-1,and the rejection dropped from 98.94%to 98.07%compared with the membrane prepared without DMC;further,the flux of the DMC-modified membrane prepared with regulated solvent activation by mechanical force was still at a high level of 74.4 L m-2 h-1,and the rejection increased to 98.70%.Based on the experimental and theoretical analysis,it was concluded that the regulated solvent activation by mechanical force was suitable for the membrane prepared by the optimizated IP process with co-solvent.Lastly,the influence mechanism of regulated solvent activation by mechanical force on the structure and performance of the membranes prepared with different IP process was analyzed systematically.