Thin Film Composite Reverse Osmosis Membranes With Polyimide Active Layer Prepared By Chemical Imidization

The polyamide(PA)thin film composite(TFC)reverse osmosis(RO)membranes have been playing a leading role in the market of RO membranes based on its excellent separation performance and low economic cost,however,PA material is easily degraded in the aqueous solution containing active chlorine,which leads to the decline of the membrane separation performance and restricts the further development of PA RO membranes.The study of chlorine-resistant RO membranes have attracted more and more attention.Polyimide(PI)membrane has better chlorine resistance compared with PA membrane.Therefore,high performance PA RO membranes were prepared firstly in this paper,then PA material was converted to PI by two chemical imidization methods(steam method and dipping method),and the RO membranes with PI active layer were fabricated.Polyamide RO membranes were prepared by the interfacial polymerization of m-phenylenediamine(MPD)and trimesoyl chloride(TMC)on the polyacrylonitrile support.The result indicated that the rejection of the RO membranes can be improved by PEI modification of PAN support.The rejection of the RO membranes gradually increased and then stabilized with the increase of MPD concentration.The rejection of the RO membranes rised firstly and then declined with the increase of BTC concentration.The rejection of the RO membranes increased gradually and then stabilized with the increase of MPD dipping time.The rejection of the RO membranes increased and then stabilized with the increase of interfacial polymerization.Water flux and rejection of the RO membranes were tradeoff.The additives(such as triethylamine and lauryl sodium sulfate)in the aqueous phase can not improve the separation performance of the RO membranes.The optimum interfacial polymerization conditions were shown as fllows:PAN membranes modified by PEI as the support,the concentration of MPD of 1.5 wt%,the concentration of BTC of 0.03%(w/v),the aqueous phase dipping time of 5 min,the interfacial polymerization time of 80 s,and no other additives in the water phase.The RO membranes prepared under the optimum condition exhibited a typical NaCl rejection of 93.1%and a water flux of 6.8 L/(m~2·h).In the experiment of the preparation of PI active layer RO membranes with the steam chemical imidization method.The results showed that the highest imidization degree of membranes was obtained when the volume ratio of acetic anhydride and pyridine was 1:1.The imidization degree of the membranes increased with the increase of imidization temperature and imidization time.However,when the imidization time was above 8 h,the rejection of membranes decreased obviously.The optimum steam chemical imidization conditions were shown as follows:the volume ratio of acetic anhydride and pyridine of 1:1,the imidization temperature of 60?,the imidization reaction time of 6 h.The RO membranes imidized under the optimum condition exhibited a imidization degree of 37.5%and a typical NaCl rejection of91.3%.Chlorine-resistance of the RO membranes was obviously improved after imidization.ATR-FTIR and XPS confirmed the formation of PI on surface of the RO membranes after imidization.AFM indicated that surface roughness of the membranes decreased and surface morphology of membrane changed after imidization.The work indicated that the imidization of different types of PA membranes could be achieved by the steam chemical imidization.In the experiment of the preparation of PI active layer RO membranes with dipping chemical imidization method,the results showed that the highest imidization degree of membranes was obtained when the volume ratio of acetic anhydride and pyridine was 1.5:1.The addition of acetone to the imidization reagent can protect the active layer of the membranes.SEM indicated that active layer of the membranes was damaged when the imidization temperature was higher than 45?.The imidization degree of the membranes increased with the increase of imidization time.However,when the imidization time was above 24 h,the rejection of the membranes decreased significantly.The optimal dipping imidiation conditions were determined:volume ratio of acetone,acetic and anhydride,pyridine of 5:3:2,imidization temperature of 40?.The RO membranes imidized under the optimum condition exhibited a imidization degree of 34.2%and a typical NaCl rejection of 89.0%.The chlorine-resistance test showed that chloride-resistance of the membranes after imidization was improved obviously.