Study On The Grafting Of Polymer Brushes On Aromatic Polyamide Thin-film Composite Reverse Osmosis Membrane And The Properties Of The Modified Membrane

Aromatic polyamide(APA) reverse osmosis(RO) composite membrane, as the dominant reverse osmosis membrane, has been widely used in the fields of water desalination and purification. Membrane fouling and poor chlorine stability are the two major obstacles that limit the extensive application of aromatic polyamide RO membrane. Surface modification has been proved to be an effective method to improve the anti-fouling and chlorine-resistance of the aromatic polyamide reverse osmosis membrane by adjusting membrane surface properties.This study aimed to enhance the anti-fouling property and chlorine stability of the aromatic polyamide thin-film composite reverse osmosis by surface grafting molecular brush. Methoxypolyethylene glycol(m PEG) was selected as the molecular brush and was grafted onto the surface of the polyamide membrane with glutaraldehyde as linking agent by investigating its influences on membrane surface property and performance. In the experiments, the effects of the modification conditions such as glutaraldehyde concentration, mPEG concentration, treatment temperature and reaction time on the properties of modified membrane were systematically studied. Membrane physico-chemical properties were characterized by attenuated total reflection infrared spectroscopy(ATR-FTIR), thermal field emission scanning electron microscopy(FESEM), atomic force microscopy(AFM), X-ray photoelectron spectroscopy(XPS), static contact angle measurement, Zeta potential measurement. Membrane reverse osmosis performance in terms of salt rejection and water flux was evaluated through cross-flow permeation tests, which were also adopted to estimate the anti-fouling property of the studied membranes with model foulants of humic acid, BSA, tannic acid, DTAB and SDS. Membrane chlorine resistance was evaluated by the static immersion experiments. The following results can be obtained from the experiments:(1) The polymer brush mPEG could be grafted onto the surface of the polyamide RO membrane with GA as linking agent. The new peaks at 1716, 1028 and 926 cm-1 of the ATR-FTIR spectrum of the modified membrane illustrated the presence of mPEG on membrane surface. The new peak at 285.9eV on the XPS spectra of the modified membrane also demonstrated the grafting of mPEG on membrane surface and its change of intensity could be used to calculate the grafting ratio. Membrane surface water contact angle decreased from 59.0° to 42.3° after modification, indicating the significant improvement of surface hydrophilicity. The AFM images showed that the modified membrane exhibited a peak-valley surface structure similar to the virgin membrane with an increased surface roughness. The streaming potential test showed the modification performed in this work resulted in a decreased negative surface charge under neutral pH.(2) The reverse osmosis performance of the modified membrane was largely affected by the modifying conditions. With increasing mPEG concentration, glutaraldehyde concentration and/or reaction time, the permeate flux of modified RO membrane decreased, while the salt rejection increased. On the other hand, the reaction temperature had little effect on membrane performance. The optimum conditions for modification were as follows: mPEG concentration=1.0 w/v%, glutaraldehyde concentration=0.1 w/v%, treatment temperature=65 ℃ and reaction time=1.5 min.(3) Membrane fouling resistance and chlorine resistance could be effectively improved through grafting polymer brush. Fouling experiments illustrated that the modified membrane exhibited improved anti-fouling property, with lower flux decline ratios and higher flux recovery ratios to humic acid, BSA, tannic acid, DTAB and SDS aqueous solutions compared to the virgin membrane. Chlorine exposure experiments also demonstrated that the grafting of mPEG on membrane surface led to enhanced chlorine stability. After chlorination of 24000 ppm·h NaClO, the salt rejection of the virgin membrane decreased sharply from 97.69% to 90.03%, while the salt rejection of the membrane modified with 2.0 w/v% mPEG decline slightlyfrom 97.69% to 96.75%.