Silver/Chitosan nanodispersion (Ag/CS) prepared by gamma ray was used to modify the thin-ﬁlm composite reverse osmosis (RO) membrane. The silver nitrate solutions were added to the chitosan solution with different concentrations of 5 to 20 (wt/wt %) and exposed to gamma radiation at different irradiation doses. The size of the silver/chitosan nanodispersion is investigated by transmission electron microscopy (TEM). The membranes characterized by FT-IR spectroscopy, scanning electron microscope (SEM) and contact angle measurement. Contact angle measurements showed that the surface hydrophilicity of the modified membranes increased by increasing Ag nanoparticles (Ag-NPs) concentration in prepared mixture. This study showed that the Ag-NPs which prepared with gamma technique enhance the permeability of the polyamide membrane. The water ﬂux of the modified membranes, increased progressively at lower concentrations of nanoparticles (5–10 wt/wt%) and then decreased at 20 wt/wt%. Moreover, the modified membrane showed better antibacterial ability compared with the neat membrane. .
Many efforts have been made to tackle the issue of water shortages. The best technique for desalination utilizing RO membrane and the uniform was more affordable whenever contrasted and customary warm desalination forms with high productivity moreover . Membrane technology is considered as an alluring separation approach because of its efﬁciency and vitality sparing procedure and has been connected in water puriﬁcation and softening, wastewater recovery, and food processing, etc. [2-4]. Reverse osmosis is currently a very much acknowledged system for water and wastewater treatment,  thanks to the improvement of high performance membranes with asymmetric structures, either fundamentally symmetric or as a thin-film composite, where a thin skin layer is supported on a microporous substrate . The formation of thin-film composite (TFC) membranes from various polymeric or monomeric amines by interfacial polymerization has been contemplated widely [7-9]. The modification process of membrane surface has improved the efficiency of water treatment plants .
It is known from literatures that the separation performance of the thin-ﬁlm composite membrane is strongly affected by the membrane surface properties (e.g. chemical structure and hydrophilicity) [11–14]. Therefore, a great deal of research efforts has been devoted to modifying the membrane surface properties so as to improve the membrane properties through surface modiﬁcations including physical and chemical treatments [15-19]. Accordingly, various works such as chemical coating, physical blending, plasma treatment, chemical reactions and polymer grafting have been carried out to modify the membranes [20, 21]. Adding inorganic nanoparticles to membrane matrix can enhance the membrane hydrophilicity, strength, permeability and selectivity characteristics [22, 23]
In the experiments, silver/Chitosan nanodispersion (Ag/CS) prepared by gamma ray was modified the TFC membrane. The membrane surface properties in terms of chemical structure, morphological structure and hydrophilicity were characterized by ATR-FTIR analysis, scanning electron microscopy (SEM), and contact angle measurement, respectively, to analyze the changes that resulted from the modiﬁcation. The permeation properties of the neat and modiﬁed TFC polyamide reverse osmosis membranes were evaluated through cross-ﬂow permeation tests by investigating the water ﬂux and NaCl rejection.
Materials and reagents
Chitosan (Deacetylation 93%) was purchased from Oxford Lab Chem. Acetic acid, silver nitrate and isopropanol was obtained from El Nasr pharmaceutical, chemical Co. Egypt. A spiral-wound commercial RO membrane element was purchased to use the commercial thin-ﬁlm composite polyamide (BW60-1892-75) DOW Company.
Preparation of modified membranes
Firstly, the commercial PA RO membrane samples were soaked in de ionized water for a minimum of 10 h, replacing the de ionized water every hour, and then rinsed thoroughly with de-ionized water to remove all preservative materials in the membranes. 400 mg of chitosan was dissolved in 200 mL of acetic acid solution (1 %) to prepared a 0.2 wt.% chitosan solution. Dissolve 40 mg in 100 mL distilled water Stir, slowly add silver nitrate solution into chitosan solution drop by drop and then add 2 ml isopropanol as free radical scavenger for hydroxyl radicals. The mixture is exposed to nitrogen gas for more than 1 h to remove the oxygen; the solution was irradiated under 60Co gamma ray with different irradiation doses. The surface modification of TFC membranes was achieved using the irradiated solutions via the rocker table for 30 min to obtain a uniform thickness layer. After that, the membrane was annealed for 15 min at 70º C. The modiﬁed membrane was thoroughly washed with deionized water to remove the residual solution, and then stored in deionized water for use.
Fourier transform infrared spectroscopy (FTIR) was employed for analysis the surface composition of the neat and modified TFC polyamide RO membranes, respectively. The membrane tested samples for ATR-FTIR analysis were dried at 25℃ under vacuum before characterization. ATR-FTIR spectra were recorded on a Nicolet Aratar 370 FTIR spectrometer with a ZnSe crystal as the internal refection element with an angle of incidence of 45°.
Transmission electron microscopy (TEM) estimations were performed with a JEOL, JEM 100CX, Japan, demonstrate the size of silver inside the prepare solusion. Finely grounded sample was dispersed in 10 ml distilled water followed by sonication to get a solution of metal nanoparticles. Approximately 20–25 µl of this solution was dropped on a 3 mm copper grid, drying at room temperature. The copper grid was thereafter inserted into transmission electron microscope
Surface morphologies of the composite membranes were observed by scanning electron microscopy of the membrane surface was examined with JEOL JSM-5400 scanning electron microscopy (Japan), available at NCRRT.
The hydrophilicity of the membranes was determined by the contact angle measured utilizing a DSA100 contact angle analyzer (KRUSS, Germany). All modified membrane samples and neat membrane were dried to remove the water in the TFC membranes and tested in at least three measurements.
Evaluation of membrane performance
The RO performance of polyamide/chitosan/silver (PA/CS/Ag) membrane was evaluated through estimating both reverse osmosis parameters; permeate flux (L/m2.h) and salt rejection (%). Permeate flux and salt rejection were estimated utilizing cross-flow filtration using different aqueous feed solutions containing different concentrations (ppm) of NaCl with a pH range 7± 0.4 at 25◦C. The permeate flux and salt rejection were evaluated from the cross-flow experiment M20. The permeation flux (J) through a membrane area (A) was calculated as the volume (Q) collected during a time period ∆t  from the Eq. (1): J = Q/ A. ∆t (1)
Also, the salt rejection (Rs %) was calculated by measuring the electric conductivity of both feed and permeate solutions using JENWAY Laboratory bench conductivity meter (4510) and calculated as follows using Eq. (2): Rs % = (Cf – Cp /Cf) x 100 (2)
Where Cf and Cp are the concentrations of the feed and permeate water, respectively.
Results and discussion
Characterization of membranes
The neat TFC membrane elucidate peak bands appeared indicating the PA barrier layer onto the PSF support layer. Two peak bands appear around 1597 and 1248 cm−1 for the C=O stretching vibration and C–N group of the amide II on TFC membrane, respectively . The stretching peak at 3341–3473 cm−1 can be assigned to N–H and O–H which overlapped together [26, 27]. After modification the intensify percent of hydroxyl group decreases. The CONH2 absorption band is also observed at near 1650 cm−1 in the FTIR spectrum. The shift was observed at 1705 cm−1 in the modified membrane with Ag nanodispersion. This shift may indicate the binding of Ag nanoparticles to N–H bond .
SEM is a powerful technique for studying the morphology of the different membranes and also it gives a reasonable image of the change in the surface morphology as a result of modifications. The surface of the neat polyamide membrane appears to slight changes were observed in SEM images between the original and modified membranes at 10 KGy. However, this structure became more porous and denser after modification with the modified solution prepared which exposed to gamma ray 20, 30 and 40 KGy. Those changes indicated that the membrane surface morphology was influenced by modification.
Hydrophilicity of the membranes
Contact angle examination is one among the essential parameters for deciding membrane surface charges and hydrophilic/hydrophobic characters that have excellent influence on membrane both water permeability and condition for fouling. While as contact angle becomes lower this suggests that the membrane surface turns to be more deliquescent because the water particles an excellent tendency to wet the surface layer. This, in turn, raises the membrane water flux .
The contact angle (θ) estimations of PA membrane and modified PA reverse osmosis membranes. The PA membrane demonstrates contact angle equals to 53° this indicates week membrane surface wettability with water and moderately hydrophilic properties. After surface modification, the hydrophilicity of modified membrane was enhanced, and it diminished to 36.3° this demonstrated the PA layer which be more polar and hydrophilic because the presence of chitosan and silver nanoparticles
The main target of modification of the commercial RO membranes is to increase the salt rejection and water flux, which are the most important parameters that the authors work on them and also main reasons behind the intense development in this area.
Effect of AgNO3 concentration
For polyamide RO membrane, the salt rejection was about 84% at salinity 10000 ppm. The percentage increased with the increase of the silver nitrate concentrations, this due to formation of nano particles on membrane surface, which promotes the salt rejection. The salt rejection reached the largest percent at AgNO3 concentration of 10 (wt/wt).The water flux of PA membrane is 42 (l/m2.h). After modified with silver nanoparticles the flux increase reached to 48.5 (l/m2.h).
Effect of applied pressure
Performance of the PA and modified membrane, i.e. the permeation ﬂux of the solution through the membrane and salt rejection, was carried out using a 10000 ppm aqueous NaCl solution as function of the operation pressure. The results of flux increase bit by bit as associate degree operative pressure will increase, and by the additional increase of the operation pressure, water flux will increase dramatically scrutiny with increase in salt rejection. It will process by the means that, permeate is directly reminiscent of the networking pressure and the matter diffusion across the membrane [25, 26].
Effect of irradiation doses
The performance of the modified membrane, i.e. the permeation ﬂux of the solution through the membrane and salt rejection, was carried out using 0.2% chitosan concentration and AgNO3 10 (wt/wt) at 20 bar at different salinity as a function of the radiation doses. Generally, both the ﬂux and the salt rejection increased with increasing dose up to 30 KGy. It was also found that modified membrane exhibited lower ﬂuxes and rejection values at high irradiation doses. The lower ﬂux may be related to the structural compactness of the PA membranes, resulting from the speciﬁc interaction between silver and PA matrix.
Fouling properties of membranes
Microorganisms, for example, Escherichia coli and Staphylococcus aureus effectively stick to film surfaces and develop as they assimilate supplements, for example, proteins and polysaccharides that accumulate on the membrane surface. The attached microorganisms excrete an extracellular polymeric substance (EPS) and, thus, create a biofilm on the membrane surface (Hyoungwoo, et al., 2012). The membrane prompts natural obstruction on the membrane that causes flux decay.
The results show that the PA/CS/Ag modified membrane significantly reduced the number of viable E. coli and S. aureus cells by 90% and 83% respectively, which is higher than that of the commercial polyamide membrane which reduced it by 33% and 25%. Silver nanoparticles showed a high inactivation of bio-fouling on membrane surface . These results can be attributed to a synergetic effect of the NPs materials, which have different approaches to bacterial inactivation.
Modified membranes were prepared by silver nano dispersion. The modified solution was prepared with gamma ray technique at different irradiation doses. The membrane showed a high hydrophilic surface, as the contact angle with water reached (~36.3°). The morphology of modified nanocomposite membrane also examined using FT-IR and SEM. Furthermore, water flux and a salt rejection for the resulted membranes were assessed, where performance of the newly modified membrane gave about 48.2 L m2h as a permeate water flux and a salt rejection of ≥95.5% was obtained for a saline water (10000 ppm of NaCl) at an applied pressure about 20 bars with a 14% increase in salt rejection comparing to the neat TFC membrane.
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