Abstract:

Objective In order to improve the ion separation efficiency of nanofiltration (NF) membrane and improve hydrophilicity and surface smoothness of chlorinated polyvinyl chloride/polyvinyl butyral (CPVC/PVB) blend membrane, this research investigated the modification of CPVC/PVB blend membrane, and the structure and properties of nanofiltration membrane before and after modification were analyzed to broaden the application of CPVC/PVB blend membrane.

Method CPVC/PVB blend membranes were used as the base membrane, and the surface of the base membranes were modified to improve hydrophilicity using the additives of polyethylene glycol (PEG) and 1,2-propanediol (PG). Using the modified CPVC/PVB blend membranes as the support layer, water phase monomer m-phenylenediamine (MPD) and the oil phase monomer trimesic acid chloride (TMC) were polymerized on the support layer interfacial, by the reaction assistant N-ethylamine piper under the catalysis of azinepropyl sulfonate (AEPPS). The Influences of mass fractions of MPD, TMC and AEEPS on the structure and performance of composite nanofiltration membranes were studied. A series of experiments were carried out using Fourier infrared spectrometer, field emission scanning electron microscope, ultraviolet and visible spectrophotometer, energy spectrum analyzer, Zeta potential tester, and water flux tester. The influences of the infrared spectrum of the blend membrane, surface elements, the cross-section structure of the surface microstructure, the Zeta potential of the membrane surface, and the monomer mass fraction required for the interfacial polymerization on the water flux and desalination performance of the nanofiltration membrane were obtained.

Results After the interface polymerization of CPVC/PVB blend membrane, two characteristic peaks and amino groups were added to the infrared spectrum (Fig. 3). With the increase of the mass fraction of MPD monomer, the proportion of N elements on the membrane surface first increased and then decreased (Tab. 1).The increase in N element is due to the introduction of amino groups on the membrane surface. The decrease in N element content is due to the fact that the surface of the nanofiltration membrane becomes dense with the introduction of amino groups, making it impossible to introduce more amino groups. The surface of the modified nanofiltration membrane became smoother and the pore size became smaller, resulting in the formation of a dense layer, while the section structure had no obvious change (Fig. 4, Fig. 5). According to the Zeta potential test, when pH value was equal to 7, the Zeta potential of the unmodified nanofiltration membrane was negative, and the zeta potential of the modified membrane surface turned positive (Fig. 6). With the increase of the mass fractions of MPD and TMC, the water flux first decreased and then increased. The reason for the decrease in water flux is the formation of a dense functional thin layer on the membrane surface, making it more difficult for water to pass through. The reason for the increase in water flux is that as the content of MPD, TMC and AEEPS increases, the functional thin layer on the surface of the nanofiltration membrane has stabilized and will not become denser. AEEPS contains hydrophilic groups, which can improve the water flux of the membrane.With the increase of the mass fraction of MPD, TMC and AEEPS, the desalination efficiency of nanofiltration membrane was increased first and then decreased (Fig. 7-Fig. 9). After the interfacial polymerization reaction occurs, the positively charged nanofiltration membrane improves the filtration efficiency of divalent cations, thereby improving the desalination efficiency of the nanofiltration membrane. However, with the increase of MPD and AEEPS, excessive amino and hydrophilic groups were introduced, making the surface of the nanofiltration membrane more porous, resulting in an increase in water flux and a decrease in desalination efficiency.

Conclusion After interfacial polymerization, a dense polyamide separation layer was formed on the surface of the CPVC/PVB blended membrane, more amino groups were introduced, the membrane surface was positively charged, and the water flux was improved. When the mass fractions of MPD, TMC and AEPPS were 0.6%, 0.5% and 0.6%, respectively, the three-dimensional morphology of the nanofiltration membrane surface became complete, the surface roughness was reduced, and the structure was more compact, showing the best desalination performance and the best separation performance for divalent cations. Therefore, the nanofiltration membranes have further development and application prospects in the fields of water softening, seawater desalination, and industrial wastewater treatment.

Key words: chlorinated polyvinyl chloride, polyvinyl butyral, positively charged nanofiltration membrane, interface polymerization, microstructure, filter material, water treatment