Construction And Performance Study Of Thin-layer Composite Polypiperazine Amide Nanofiltration Membrane

Thin-layer composite（TFC）polyamide（PA）membranes,consisting of a dense,ultrathin active layer and a macroporous support layer,are widely used in membrane separation processes such as nanofiltration（NF）.At present,most TFC PA membranes are produced by interfacial polymerization to generate an active layer on top of a microporous support,which is responsible for selective transport.Traditional interfacial polymerization suffers from the ultrafast reaction and uncontrollable monomer diffusion rates.As a result,the performance of the preprepared TFC PA membranes has been limited by the intrinsic trade-off between permeability and selectivity as well as the insufficient separation of monovalent/divalent salts.Suitable regulation of the interfacial polymerization process can effectively realize the desired microscopic structures for high-performance nanofiltration membranes,Herein,this thesis proposes a simple in-situ regulation of the interfacial polymerization,which relies on a self-assembled monolayer of photo-polymerizable amphiphiles at the water/oil interface.To this end,a polymerizable anionic amphiphilic molecule（MUSS）and a polymerizable zwitterionic amphiphilic molecule（MAC）were designed and synthesized.The properties of the amphiphile-laden interface can be finely tuned by precisely controlling the interfacial structures of the polymerizable amphiphiles.As a result,the thickness and the topological morphologies of the PA active layer can be regulated,leading to the formation for a high-performance.The key research contents and results of this thesis are as follows:（1）Preparation of high-performance nanofiltration membranes by MUSS amphiphiles regulated interfacial propertiesThe procedure for the preparation of high-performance nanofiltration membranes involves the modification of the water/oil interface by a monolayer of the polymerizable amphiphile MUSS,which can be subsequently photo-polymerized to optimize the structure of the interface before interfacial polymerization.We study the influence of several parameters such as the concentration of MUSS and the polymerization time on the resultant membrane performance.We systematically relate these parameters to the flux,salt rejection,and mixed salt separation capacity of the membrane as evaluated using the cross-flow nanofiltration.The results show that the MUSS-modified interfacial polymerization results in the distinctly different surface morphology and roughness of the film as compared with those derived from traditional procedures.The surface of the film evolves from nodular to peak-valley structure,and the surface roughness of the film is increased by 2.5times.Compared with conventional TFC PA membranes,the PA layers produced by MUSS regulation display reduced thickness and enhanced charge.Filtration performance of the membranes show a decrease in molecular weight cut-off,an increase of pure water flux by 124%,and a rejection rate of dianions remains above 99.2%.The long-term stability of mixed-salt separation show that the modified TFC NF membrane can maintain the separation accuracy of monovalent/divalent salts for 24 h under the mixed-salt feeding condition.The separation factor of the composite membrane for chloride ion/sulfate ion is as high as 108.5.The high selectivity mains without the significant sacrifice of the water permeation flux,as evidenced by a stable flux of 24 Lm^-2h^-1bar^-1and a decay rate of 4.19%.The above results demonstrate that the regulation of interfacial polymerization by a procesure of MUSS photopolymerization can simultaneously enhance the TFC PA membrane flux and selectivity.（2）Preparation of high-performance nanofiltration membranes by MAC amphiphiles regulated interfacial propertiesTo explore the effect of the amphiphile charge on the regulation of the interfacial polymerization process,a polymerizable zwitterionic amphiphilic molecule,i.e.,MAC,was designed and synthesized to control the preparation of TFC PA membranes.The effects of MAC addition concentration and photoinitiation time on the surface morphology and surface properties of the membrane were systematically studied,and the flux,salt rejection and mixed salt separation capacity of the membrane were evaluated in the process of cross-flow nanofiltration.The research results show that the pure water flux of the TFC PA membrane prepared by MAC regulation is as high as 20.6 Lm-2h-1bar-1,and the rejection of magnesium sulfate is as high as 99.2%.Compared with conventional TFC PA membranes,the simultaneous improvement of permeability and selectivity is achieved.However,the ability of zwitterionic MAC to enhance the membrane performance is slightly weaker than that of anionic MUSS.Such a difference may be due to the fact that the charge of the hydrophilic head group of the amphiphilic molecule can significantly affect the packing density at the water/oil interface.（3）Preparation of high-performance nanofiltration membranes based on other types of amphiphilic moleculesTo further study the influence of the other structural factors of amphiphiles on the regulation of the interfacial polymerization process,we selected cationic（MTAB）,special zwitterionic（MAO）,imidazole zwitterionic（MIS）and Gemini polymerizable amphiphiles（PAC,G-6）,to control the preparation of TFC PA membranes.The flux and salt rejection of the composite membranes were evaluated by cross-flow nanofiltration.It is found that the ion selectivity improvement of cationic polymerizable amphiphiles on thin-layer composite nanofiltration membranes is limited,far less than that of anionic and zwitterionic systems.As for the systems of the non-ionic and cationic Gemini amphiphiles,both the flux and rejection rate of the as-prepared TFC NF membranes are found to decrease.The deleterious effects of cationic amphiphiles stem from the preclusion of the pre-concentration concentration of piperazine monomer by charge repulsion from the presence of cations from the amphiphile at the interface,which increases the instability of the interfacial polymerization reaction and subsequently the proportion of defects in the polyamide layer.As for non-ionic amphiphilic molecules,their presence increases the thickness of the PA layer.This effect is equivalent to introducing impurities during the interfacial polymerization process,which greatly reduces the membrane performance.The regulation capability of the imidazole-type zwitterionic amphiphile is generally weaker than that of MAC.Due to the structural hindrance of the imidazole ring,the packing density of the amphiphilic molecules at the interface is reduced,and the regulation effect is reduced.In summary,this thesis aims to circumvent the permeability selectivity trade-off of traditional TFC PA membranes produce by interfacial polymerization.We introduce a feasible approach to regulate this process by taking advantage of self-assembled polymerizable amphiphiles that can be photopolymerized to modulate the structures at the interface.TFC NF membranes with enhanced flux and selectivity have been successfully prepared,which significantly benefits the highly desired membranes-based water purification.