Reverse osmosis (RO) membranes based on cellulose acetate (CA), were prepared using a phase inversion technique. To improve the hydrophilicity, salt rejection and water flux of these membranes, a novel grafting of 2-acrylamido-2-methylpropanesulfonic acid (AMPSA) was added on the top surface of the CA-RO membranes. The grafted CA-RO membranes were characterized by Fourier transform infrared spectroscopy (FTIR), contact angle, and scanning electron microscopy techniques. It was found that the contact angles were 58° and 45° for pristine CA and 15 wt% grafted CA-RO membranes, respectively, which suggest an increase in the membrane surface hydrophilicity after grafting. The morphological studies of the surface of the pristine CA-RO membrane revealed a typical ridge-and-valley morphology and displayed a relatively high surface roughness of 337 nm, and a significant decrease at 15 wt% of grafted CA-RO membrane to 7 nm. The effect of the grafting percentages of AMPSA on the water flux and salt rejection was studied using a cross flow RO unit. The salt rejection and water flux of the grafted CA-RO membrane with 15 wt% were 99.03% and 6 L/m2h, respectively.
Water scarcity is one of the most serious global challenges; one-third of the world's population are living in water-stressed countries; by the year 2030, this figure is predicted to rise to nearly two-thirds. The challenge of providing ample, safe drinking water is farther complicated by population growth, contamination of available freshwater resources, and climate change (Elimelech & Phillip 2011). Many decades of successful implementations demonstrate how desalination technology can provide supplementary or main water sources. The desalination process can be roughly categorized into two major types: thermal and membrane separation (Zhou et al. 2014). Reverse osmosis (RO) is currently the most widely used desalination technology due to continuous technological improvements and substantial cost reductions. This technology has the advantages of having modular construction and small carbon footprint, which allows for the combination of additional treatment processes. Presently, two main types of polymeric RO membranes exist in the market: cellulose acetate (CA) membranes and polyamide thin film composite (TFC) membranes. However, TFC membranes, which are the most commonly used type of RO membranes, cannot withstand chlorine exposure, even at low ppm concentrations. On the other hand, CA membranes exhibit some resistance against chlorine at concentrations of up to 5 mg Cl2/L and 0.2 mg Cl2/L for short and long exposure times, respectively. Consequently, CA membranes are used in most of the RO plants located in the Middle East Region while the elevated seawater temperature and water quality promote the risk of membrane biofouling (Khan et al. 2015).
Membrane desalination was improved by the development of an effective process to make defect-free CA membranes that had high fluxes and salt rejections where the resulting CA membrane contained a relatively dense ‘skin’ on the surface with a porous network support (Lonsdale 1982). The disadvantages of the CA membranes are limited pH operating range and lower salt rejections, due to possible biodegradation, hydrolysis by acids and alkalis, and narrow temperature limits. Surface modifications of RO membranes are a cost-effective technique that enhances the membrane performance while conserving its bulk characteristics. There are many surface modification techniques such as free radical, photochemical, radiation, coating and plasma-induced grafting (Savoji et al. 2013). Modification via graft polymerization has many advantages, such as its ease of use and controllable introduction of the graft chains to the surface with the bulk properties unchanged (Lee et al. 2011). Grafting of side chains can be performed in two ways: ‘grafting-from’ and ‘grafting-to’ methods. In the former method, the membrane surface consists of reactive radicals, while in the latter method grafting chains carry the reactive radicals for initiation (Azari et al. 2013). Grafting can be achieved using different chemical techniques, such as chemical oxidation, plasma discharge method, and radiation. Non-toxicity and non-biodegradability, are characteristics possessed by 2-crylamidopropane-2-methyl sulfonic acid (AMPSA), which makes it an ideal choice for use in the water purification industry (Thomas & Asheville 1980). AMPSA has been applied in the medical field as wound dressing material (Dasa et al. 2014). The distinguished properties of AMPSA are a result of high mobility associated with a conformationally flexible structure and water binding ability (Kato et al. 2003). The enhanced surface hydrophilicity of the membranes results in the improvement of the antifouling performance (Kang & Cao 2012). The grafted AMPSA coating aims at producing a surface that will decrease the nonspecific adsorption of various proteins due to its hydrophilicity and provides a highly sulfonated surface (Devi et al. 2014). It was indicated that AMPSA, grafted onto polyurethane membrane, has a negative charge due to the sulfonic acid groups (Coskun et al. 2006).
The main objective of this work is to functionalize and hydrophilize CA-RO membrane surfaces with novel grafting of AMPSA via chemical oxidation. The grafted CA-RO membranes will be characterized by Fourier transform infrared spectroscopy (FTIR), contact angle, atomic force microscopy (AFM), and scanning electron microscopy (SEM) techniques. The performance of the CA-RO membranes with different grafting percentages will be assessed for water desalination using water flux and salt rejection measurements.
MATERIALS AND METHODS
CA (molecular weight of 100,000 g/mol and 39.8 wt% acetyl) was received from Aldrich and 1,4-dioxane was supplied by Panreac Quimica S.A (Barcelona, Spain). Methanol (purity>99.5%) and acetone (purity>99%) were received from Labsolve (Lisbon, Portugal). The acetic acid (purity>99.8%) was supplied by BDH Anala R (UK) and 2-acrylamido-2-methylpropanesulfonic acid was obtained from Fisher, and was used as received. Potassium persulfate, NaCl and NaOH were purchased from Merck, Aldrich and the Egyptian Petrochemical Company, respectively.
Preparation of CA-RO membranes
The CA-RO membrane was prepared using a mixture of dioxane (27.62 g), acetone (10.57 g), and acetic acid (5.07 g) as solvents while methanol (8.45 g) was used as a nonsolvent in addition to CA (8.45 g). This mixture of solution was left under stirring for 24 h at room temperature, until the CA completely dissolved. The solution was put into an ultrasonic bath for 30 min to remove the air bubbles entrapped in the polymer solution. The RO membranes were obtained through the spreading of the solution onto a glass plate using the knife of an automatic applicator, at room temperature and 42% relative humidity. The thickness of the film was previously selected (250 μm), and spread at a constant speed (10 mm/s) with an automatic film applicator (Zehntner 2300-Swiss). After casting the solution with evaporation time of the solvent of 60 s, the CA membrane (cast onto the glass plate) was immersed for 15 min in a deionized water ice bath. The formed CA membrane was then placed in a water bath at about 4 °C for 2 h to eliminate the effect of capillary pressure and then washed with distilled water to completely remove the residual solvents (Han & Bhattacharyya 1995; Werner et al. 2011). The prepared CA-RO were then post-treated for 10 min at about 80–85 °C. Then these membranes were soaked in deionized water for 24 hours and air-dried for 24 h before characterization.
Grafting of AMPSA onto the CA-RO membrane surface
Grafting mechanism of AMPSA onto CA-RO surface membrane
RESULTS AND DISCUSSION
Structure investigation of grafted CA-RO membranes
Conversely, the FTIR spectrum of AMPSA showed the transmission band, also called the amide I mode, which resonated at 1,672 cm−1, while the band at 1,641 cm−1 was assigned to the alkene C = C stretch, and the band at 1,765 cm−1 for the C = O, and the N–H stretch, amide II mode, absorbed around 3,470 cm−1, and the C–O stretch absorbed at 1,222 cm−1. The 1,036 cm−1 band of the sulfonic group of AMPSA was also observed. The bands at 1,560 cm−1 and at 634 cm−1 were assigned to C-N- and S-O groups, respectively (Durmaz & Okay 2000; Coskun et al. 2006).
The FTIR spectra of the grafted CA with different weight percentages of AMPSA are shown in Figure 3(b). The broad absorption band around 3,496 cm−1 was ascribed to the overlapping peaks of NH and OH groups. The bands at 2,960 and 2,884 cm−1 were attributed to CH3 and CH2 stretching. The band observed at 1,430 cm−1 is the C-N bend. The disappearance of the band at 1,640 cm−1 indicated the grafting formation at the C = C sites (Yetimoglu et al. 2007).
Morphological properties of grafted CA-RO membranes
The thickness of the dense layer at the bottom of the CA-RO membrane that is grafted with 10 wt% AMPSA decreased; meanwhile the finger-like structure did not change (Figure 4(b). Conversely, the top layer of this membrane became smoother than the pristine CA membrane. From the cross section of the CA-RO membrane grafted with 15 wt%, it was clear that the thickness of the top dense layer increased, while the bottom layer decreased. The finger-like structure became distorted with tear-shaped macrovoids that extended from the compact skin layer towards the permeate side (Richards et al. 2012). In addition, the top layer of this membrane revealed an irregular rhombohedral structure (as shown in inset of Figure 4(c)) (Abdul et al. 2014). This led to the decrease of water flux and increase of the operation pressure. The cross sections of the CA-RO membranes grafted with 20 and 25 wt% indicated that the thickness of the top and the bottom dense skin layer increased (Figure 4(d) and 4(e)). The surfaces of these highly grafted membranes became smoother and the pores in the bottom layer were blocked by the grafting of AMPSA inside these pores. The monomer had penetrated through the active layer of the membrane, particularly for higher degrees of grafting (Dai et al. 2004).
Hydrophilic properties of CA-RO grafted membranes
Salt rejection and water flux of grafted CA-RO membranes
For the CA-RO membrane grafted with 10 wt%, the salt rejection increased to 97.2% at 14 bar and then decreased to 85.6% at 20 bar. Meanwhile, for the CA-RO membranes grafted with 15 wt%, the salt rejection increased to 99.03% at 11 bar and decreased to 96.9% at 15 bar. This was due to the higher thickness of the active top dense layer while the bottom layer decreased in thickness. At high grafting weight percentages of 20 and 25, the operating pressure increased to 16 and 24 bar, respectively, to produce a salt rejection decrease from 66.6% to 36.3%. These results are in agreement with those obtained from the SEM images, where the thickness of the top active dense skin layer was deformed and the bottom layer increased. The grafting process may destroy or add flaws to the membrane surface, resulting in an increased water flux (Bentvelzen et al. 1973).
The data for the CA-RC membrane grafted with 15% AMPSA showed great promise for large scale water desalination. Future work will focus on a pilot scale study for commercial applications.
The surface of the CA-RO membranes was successfully modified by grafting a hydrophilic AMPSA monomer. The cross section of the CA-RO membrane grafted with 15 wt% of AMPSA showed that the thickness of the top dense layer had increased while the bottom layer had decreased. The dominating finger-like structure was distorted to form tear-shaped macrovoids. The surfaces of theses membranes became smoother and the pores in the bottom layer were blocked. Moreover, the CA-RO membrane surface hydrophilicity was enhanced to have a contact angle of 45° when grafted with 15 wt% of AMPSA as three hydrophilic groups (sulfonic, carboxylic and amide) were present. From the atomic force images, it was noted that the pristine CA-RO membranes displayed a relatively high surface roughness of 337 nm and showed a significance decrease at 15 wt% of grafted CA-RO membrane to 7 nm. The optimum salt rejection and water flux of 99.03% and 6 l/m2h were obtained for the grafted CA-RO membrane with 15 wt% of AMPSA. These data showed great promise for designing an effective RO system for large scale applications.
This work is supported by the Science and Technology Development Fund in Egypt (project ID 3988).