Chlorination And Surface Modification Of Aromatic Composite Polyamide Reverse Osmosis Membrane

The reverse osmosis technology is more and more widely applied into water treatment following the growing demand for freshwater resources from the human society and the increasing intensification of water pollution. However, the application of reverse osmosis membranes is largely limited by RO fouling, especially bio-fouling. To prevent bio-fouling, researchers of reverse osmosis usually adopt preliminary sterilization by adding active chlorine into the feed liquids. The polyamide composite RO membranes, which occupy the largest market of RO membranes, easily stimulate chlorinated degradation with free chlorine, thereby inducing membrane failure and service life reduction. Thus, to prolong the service life of RO membranes, we studied the chlorine degradation of RO membranes, and applied surface modification of commercial RO membranes to improve their chlorine resistance, anti-fouling and antibacterial ability.We studied the chlorinated degradation of commercial RO membrane. We found when chlorination intensity is constant, the high-concentration short-time chlorination compared with low-concentration long-time reaction affected the performances of RO membranes more severely. To understand the reasons, we studied the surface physicochemical properties of RO membranes after chlorination. We found the high-concentration short-time chlorination led to larger reduction of membrane hydrophilicity and more production of N-Cl on membrane surfaces. Experiments on reversible reproduction of chlorinated RO membranes showed that alkali soaking could effectively promote the chlorination-produced N-Cl to be reproduced into N-H, thus raising the membrane surface hydrophilicity and modestly recovering membrane osmosis. Moreover, the high-concentration short-time chlorination could lead more seriously irreversible chlorination.Through the amino terminal reaction between epoxy resin E-40 and RO membranes, the E-40 was coated onto the membrane surfaces. The modification did not destroy the separation structures of RO membranes, and enhanced its chlorine resistance. X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) confirmed the success of modification. The optimal modification conditions were temperature at 60 ?, modifier concentration=1%(wt%), and time=20 min. Osmosis performance experiments showed the modification reduced the membrane flux, but improved membrane rejection. Chlorination degradation experiments showed that the modified membranes outperform the original membranes with higher chlorine resistance, especially at low concentrations.Through the self-polymerization of dopamine on membrane surfaces, we grafted the polyethylene imine onto membrane surfaces via the Michael additive reaction or/and Schiff reaction. The modification enhanced chlorine resistance, anti-fouling and anti-biofouling abilities of membrane at the same time. XPS and ATR-FTIR. confirmed the success of modification. Analysis of membrane surface physicochemical properties shows that the modification improved the membrane surface hydrophilicity, but did not destroy the surface peak-valley structures, and only slightly aggravated membrane surface roughness. The modification drove the membrane surface electric property into positive charge. The modification also reduced the membrane flux, but improved membrane rejection. The chlorination experiment showed that the water flux of PDA-PEI modified membrane were almost stable during the chlorination, while the water flux of unmodified membrane reduced. The modification endowed membrane better and regenerative chlorine resistance ability. Contamination simulations show that the modification efficiently improves the anti-fouling resistance of RO membranes. And endowed the membrane some anti-biofouling ability.Through the electrostatic attraction between charged ions in modified solution and surface of the reverse osmosis membrane, the AgC1 nanoparticles was coated onto the membrane surfaces via alternate soaking technique. The modification endow the membrane with excellent antibacterial properties, and enhanced its water flux and salt rejection. EDS, ATR-FTIR and SEM confirmed the success of modification. Analysis of membrane surface elements shows that the surface AgC1 loading can be controlled through the number of modifications. SEM visually shows that AgCl nanocrystals uniformly deposit on membrane surfaces, and with the increasing number of modifications, the crystals loaded on membrane surfaces are improved in terms of both quantity and (slightly) particle size. Experiments of contact angle and Zeta show that the increasing number of modifications led to the enhancement in membrane surface hydrophilicity and the transition into negative charge. Stability experiments show that AgC1 crystals stably exist on membrane surfaces. Contamination simulations show that the modification efficiently improves the anti-fouling resistance of RO membranes. Antibacterial performance experiments show that the modified membranes are highly antibacterial. Even at the initial stage of contact, the modified membranes can efficiently sterilize and inhibit the bacterial growth on membrane surfaces and in the surroundings. Even a small load of AgCl could endow the membranes with high antibacterial effect, which is slightly improved with the increase of coating.