RSC Advances PAPER Cite this: RSC Adv., 2015, 5, 6687 A novel high-flux, thin-film composite reverse osmosis membrane modified by chitosan for advanced water treatment Hiren D. Raval,* Pranav S. Rana and Subarna Maiti Membrane-based desalination is a proven and established technology for mitigating increasing water demand. The high-flux membrane will require lower pressure to produce the given quantity of water and therefore will consume less energy. This work demonstrates a novel method to produce a high-flux membrane by surface modification of thin-film composite reverse osmosis (TFC RO) membrane. TFC RO membrane was exposed to a sodium hypochlorite solution of 1250 mg l 1 for 30 minutes and 60 minutes at pH 11.0, followed by 1000 mg l 1 chitosan for 60 minutes at pH 2.5, and the solute rejection/flux were monitored. It was observed that there is up to 2.5 times increment in flux with ca. 3% increase in solute rejection in the case of chitosan-treated membrane. Although the flux increase is more in membrane with longer exposure to sodium hypochlorite, the decline in solute rejection was also significant. The membrane samples were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) to understand the chemical structural changes in the membrane, atomic force microscopy to understand the morphological changes on membrane surface, zeta potential for surface charge and contact angle analysis to understand the change in hydrophilicity. Received 17th October 2014 Accepted 8th December 2014 The % rise in trans-membrane flux per  C rise in feed water temperature was more in the case of chitosan-modified membrane as compared to virgin TFC RO membrane. The higher temperature sensitivity makes it a good candidate for solar powered reverse osmosis, where low grade thermal DOI: 10.1039/c4ra12610f energy can be utilized to increase feed water temperature, and higher temperature feed water gives www.rsc.org/advances more a pronounced advantage in trans-membrane flux. Introduction Thin-lm composite RO membrane has found numerous applications in desalination and water reuse. Research and development efforts in improving its solute rejection and ow performance of the membrane have made the RO membrane versatile for diverse applications. A thin-lm composite RO membrane consists three layers: the bottom support layer is non-woven polyester fabric, the middle support layer is polysulfone, and the top layer is cross-linked aromatic polyamide layer of less than 200 nm thickness.1–8 Three-layer conguration of a thin-lm composite reverse osmosis membrane gives the desired properties of high ux of permeate ow, high rejection of the undesired materials, such as salts, bacteria, and viruses, and provides good mechanical strength. The polyamide top layer is responsible for the better rejection of unwanted materials and is chosen basically for its high permeability to water and relative impermeability to various dissolved impurities. The Reverse Osmosis Discipline, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientic & Industrial Research (CSIR), Gijubhai Badheka Marg, Bhavnagar-364 002, Gujarat, India. E-mail: hirenraval@ csmcri.org; Fax: +91-0278-2566970 This journal is © The Royal Society of Chemistry 2015 bottom two support layers give the mechanical strength to the thin-lm composite RO membrane.1–3,9 A large number of the thin lm composite RO membranes have been prepared from various polymers, such as polyurea, polyamides, polyureaamides, polyether-amides and many others. Despite extensive research in membrane material, the energy consumption for reverse osmosis has always remained the cause of concern. Water has to counter the osmotic pressure in order to pass through the membrane, and thus high-pressure pumps consume signicant power. The morphological changes in membranes can make water transport faster by reducing the resistance. It has been reported that the thinner polyamide layer improves the ow performance of TFC RO membrane.10,11 It has also been studied that the hydrophilic surface improves the ow performance of TFC RO membrane.12,13 The top layer of thin-lm composite RO membrane is the barrier layer responsible for solute rejection. Thus, modication in the top polyamide layer for improved hydrophilic performance can reduce the power consumption. Further, the top layer of polyamide can react with some hydrophilic chemicals.14 Supramolecular assembly of polyelectrolytes based on electrostatic layer-by-layer deposition is a promising approach for RSC Adv., 2015, 5, 6687–6694 | 6687 RSC Advances fabricating TFC membranes. The top surface of thin-lm composite polyamide membrane can be modied by supramolecular assembly of chitosan on the membrane surface.9 This technique creates a charged skin layer and allows for a better control of the thickness, charge density and hydrophilicity of the active skin layer.9,15,16 Layer-by-layer polyelectrolyte membranes have recently attracted signicant attention for use in pervaporation, reverse osmosis and nanoltration.17–19 However, a technical challenge limits the industrial acceptance of such membranes because of the large number of alternating depositions of oppositely charged polyelectrolytes on a porous substrate for the membrane to become sufficiently permselective, which makes the membrane fabrication process very tedious and time consuming.9,20 The major obstacle of the membrane processes is the fouling and corresponding ux decline that increases the energy consumption of reverse osmosis and ultra-ltration processes.21 Chlorine reacts with the amide bond of the membrane and converts that bond into the N-chloro derivative. However, this effect of chlorine on the polyamide layer depends on which types of amide bonds are present in the structure.11,22,23 The membrane fouling increases energy consumption and results in higher operating cost. Free chlorine reacts with the polyamide membrane. If the exposure of free chlorine is in controlled fashion, e.g. a controlled concentration for a limited time at high pH and room temperature, it can improve the permeate ux with a slight decline in solute rejection.23 In general, two types of membranes presently available in the market are seawater membrane and brackish water membrane. Seawater RO membrane is tuned for higher selectivity, whereas the brackish water RO membrane should be tuned for higher permeate ux. The thin-lm composite RO membrane has more than 98% selectivity under the standard test condition.24 Commercially available thin-lm composite polyamide membrane consisting of a polyamide layer as an active skin layer is available as either fully aromatic or aromatic (cyclo) aliphatic over a polysulfone base membrane.22 First, the thinlm composite RO membrane is modied by the treatment of sodium hypochlorite on the top active skin layer of polyamide. By applying this treatment, the membrane-active polyamide surface becomes more hydrophilic, and its permeability increases than that of a virgin TFC RO membrane.9 The chlorine concentration exposure level on the membrane is measured in the term ppm-hour (x ppm of sodium hypochlorite exposed to y hour-product of x and y). The controlled concentration (500–2000 ppm) and limited exposure time (10–60 min) of sodium hypochlorite onto the TFC RO membrane gives better performance. It is reported that the membrane performance declines because of free chlorine exposure of 1–5 mg l 1 for a period of 1–10 days.25 The performance of the TFC RO membrane-like solute rejection and ux has a correlation with the physiochemical properties such as zeta potential, hydrophilicity, chemical composition, and morphology. The increase/decrease in solute rejection and ux must nd answers by the evaluation of the abovementioned physiochemical properties, e.g. the measure of the zeta potential of the membrane correlates to the transport of 6688 | RSC Adv., 2015, 5, 6687–6694 Paper some trace organic solute and divalent ions from the reverse osmosis membrane.26 Similarly, improved hydrophilicity reected by decreased contact angle should correspond to increased permeate ux. The quest for low energy process development requires that changes be made in the membrane chemical structure and morphology in order to make it more hydrophilic and still maintain selectivity. This work provides a new dimension to make the polymer composite by supramolecular assembly that can address this issue. It demonstrates that the morphological changes aer chemical treatment on the active polyamide skin layer can alter its performance in terms of ux and NaCl/MgCl2 solute rejection owing to the supramolecular assembly formed. The temperature sensitivity of such a composite membrane increases compared to thin-lm composite RO membrane, and it can be used synergistically when low-grade thermal energy is available to heat the feed water, e.g. solar thermal. Thus, this paper shows a novel way to develop a low energy intensive process by modication in the membrane material. The controlled treatment of sodium hypochlorite with a polyamide layer of thin-lm composite membrane increases the permeate ux. However, it also causes a limited decline in solute rejection by the formation of n-chloro aromatic compounds. The reaction activates the polyamide layer and generates the free radicals to form the supramolecular assembly. Chitosan is a polymer with many hydroxyl groups present in its structure. If chitosan can be embedded on the polyamide layer, it will certainly form a hydrophilic composite polymer, which may improve the permeate ux. Materials Thin-lm composite reverse osmosis membrane, sodium hypochlorite, chitosan, sodium m-bisulte, nitric acid. Method A at-sheet, thin-lm composite reverse osmosis membrane was prepared by interfacial polymerization of m-phenylenediamine and tri-mesoyl chloride over polysulfone support at the pilot plant, Central Salt and Marine Chemicals Research Institute, Bhavnagar. The same was used for the experiments. A thin-lm composite RO membrane was subjected to a sodium hypochlorite solution of different concentrations at pH 11, followed by chitosan solution at pH 2.5. The concentration of active chlorine in the sodium hypochlorite solution was determined by iodometric titration. The thin-lm composite RO membrane was cut in the size 10 cm width  15 cm length and stuck on a glass plate. The membrane was washed with distilled water and dipped in a solution containing sodium hypochlorite for a specied time. Thereaer, a part of the membrane was washed with deionised water and retained in a sodium m-bisulte solution of 5 g l 1 concentration for testing, and the part was subjected to a chitosan solution of different concentrations at pH 2.5. The chitosan-treated membrane was washed with deionised water and kept in a sodium m-bisulte solution of 5 g l 1 concentration to nullify the presence of active chlorine if any. This journal is © The Royal Society of Chemistry 2015 Paper Table 1 RSC Advances Treatment conditions Sr. no. Concentration of sodium hypochlorite solution (mg l 1) Exposure time of sodium hypochlorite solution (min) 1 2 3 4 5 6 7 1250 1250 1250 1250 1250 1250 1250 30 30 30 30 60 60 60 Code Concentration of chitosan solution (mg l 1) Exposure time of chitosan solution (min) Code C1-1250 C1-1250 C1-1250 C1-1250 C2-1250 C2-1250 C2-1250 500 1000 5000 10 000 1000 5000 10 000 60 60 60 60 60 60 60 C1-1250 CT-500 C1-1250 CT-1000 C1-1250 CT-5000 C1-1250 CT-10 000 C2-1250 CT-1000 C2-1250 CT-5000 C2-1250 CT-10 000 The TFC RO membrane, sodium hypochlorite treated membrane and chitosan-treated membrane were tested for their ow and solute rejection performance. The treatment conditions, e.g. concentration and time of exposure, are shown in Table 1. The membrane samples were cut into 4.9 cm diameter circular shapes and placed in the testing kit. The testing was done in a standard testing kit, and the mode of ltration was dead-end ltration at 30  C temperature. The saline water solution was made with concentrations of 2000 mg l 1 NaCl and 2000 mg l 1 MgCl2. The pressure was maintained at 250 psi for 20 minutes to bring the membrane to its normal functioning state and stabilized state. The permeate ux was collected for 20 minutes. Conductivities of feed as well as permeate were measured. Four such samples were tested in a kit and their average was considered for solute rejection and ux. To understand the temperature sensitivity of the treated membrane, the testing experiments were also done at 45  C for 2000 mg l 1 NaCl feed solution. Membrane characterization The surface charge of the TFC RO membrane, 1250 mg l 1 sodium hypochlorite treated membrane and 1250 mg l 1 sodium hypochlorite treated followed by 1000 mg l 1 chitosantreated membrane samples were measured by the instrument ZetaCAD to understand the change in the surface charge of the membrane as a result of the treatment. Atomic force microscopy (AFM) images of abovementioned membrane were taken to understand the surface morphological changes as a result of membrane modication. The hydrophilicity of the membrane is determined by evaluating its contact angle. The contact angle of TFC RO membrane, exposed to 1250 mg l 1 sodium hypochlorite treated membrane for 30 minutes and 1000 mg l 1, 5000 mg l 1 and 10 000 mg l 1 chitosan-treated membrane for 60 minutes aer 30 minutes of sodium hypochlorite treatment, and exposure to 1250 mg l 1 sodium hypochlorite treated membrane for 60 minutes and 1000 mg l 1, 5000 mg l 1 and 10 000 mg l 1 chitosan-treated membrane for 60 minutes aer 60 min of sodium hypochlorite treatment were analyzed by a drop-shape analyzer, KRUSS/DSA-100, at different locations on the sample, and the average contact angle was reported. This journal is © The Royal Society of Chemistry 2015 Attenuated total reectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra of the TFC RO membrane, 1250 mg l 1 sodium hypochlorite treated membrane and 1250 mg l 1 sodium hypochlorite treated followed by a 1000 mg l 1 chitosan-treated membrane were taken to understand the chemical structural modication of the membrane as a result of the treatment. Results and discussion Table 2 demonstrates the performance of virgin TFC RO membrane, 30 minutes sodium hypochlorite treated TFC RO membrane and subsequent 1000, 5000 and 10 000 mg l 1 chitosan-treated membranes. This table shows that the permeate ux increases from 14 gfd to 18.5 gfd with 1250 mg l 1 sodium hypochlorite treatment for 30 minutes in conformity with our previous work.23 The permeate ux further increases to 23 gfd with 500 mg l 1 chitosan treatment, whereas it increases to 30 gfd with 1000 mg l 1 chitosan treatment. However, on increasing the chitosan concentrations further to 5000 mg l 1 and 10 000 mg l 1, the decline in permeate ux was observed. It can also be observed from Fig. 1 that NaCl rejection as well as permeate ux increases, compared to virgin TFC RO membrane for the membrane treated with 1000 mg l 1 chitosan for 60 minutes aer 1250 mg l 1 sodium hypochlorite for 30 minutes. In this case, ux becomes greater than two times, and the rejection also increases from 92% to 95%. In all other cases, the NaCl rejection was lower as compared to virgin TFC RO membrane. Thus, optimum performance was achieved in sr. no. 4 in Table 2. Table 3 demonstrates the performance of a virgin TFC RO membrane, 60 minutes sodium hypochlorite treated TFC RO membrane and subsequent 1000, 5000 and 10 000 mg l 1 chitosan-treated membrane. It can be inferred from Table 3 and Fig. 2 that the transmembrane ux in the case of the NaCl and MgCl2 rejection experiments increases about 2.5 times with 60 minutes sodium hypochlorite treatment. It further increases to 38% by exposing the sodium hypochlorite treated membrane to 1000 mg l 1 chitosan solution for 60 minutes; however, it decreases slightly on exposure of higher concentrations of chitosan solution, e.g. 5000 mg l 1 and 10 000 mg l 1 for 60 minutes each. NaCl and MgCl2 rejection decreases to ca. 7% with 60 minutes exposure of sodium hypochlorite, but increases ca. RSC Adv., 2015, 5, 6687–6694 | 6689 RSC Advances Table 2 Paper Treated membrane performance for NaCl and MgCl2 rejection and flux Sr. no. Membrane samples NaCl rejection (%) Flux (gfd) MgCl2 rejection (%) Flux (gfd) 1 2 3 4 5 6 TFC C1-1250 C1-1250 CT-500 C1-1250 CT-1000 C1-1250 CT-5000 C1-1250 CT-10 000 92.00 91.70 90.49 95.00 91.7 90.83 14 18.5 23 30 28 24 86.36 86.32 90.58 95.06 89.21 85.74 16 19 21 28 24 17.5 Fig. 1 NaCl and MgCl2 rejection (%) and trans-membrane flux (gfd) of all the membranes. 1% with chitosan exposure and remains nearly unaffected with a change in concentration of chitosan solution. Thus, optimum results were obtained in sr. no. 3, where the TFC RO membrane was subjected to 1250 mg l 1 of sodium hypochlorite solution for 1 hour and 1000 mg l 1 of chitosan solution for 1 hour. It can be seen from Fig. 1 and 2 that 1000 mg l 1 chitosan is the optimum concentration level for achieving a high-ux, good-rejection membrane. Moreover, MgCl2 rejection improved ca. 9% in the case of 30 minutes, 1250 mg l 1 sodium hypochlorite and 60 minutes, 1000 mg l 1 chitosan exposure. The zeta potential of the treated membrane were evaluated for the TFC RO membrane, 1250 mg l 1, 30 minutes sodium hypochlorite treated membrane and 1000 mg l 1, 60 minutes chitosan-treated membrane to understand the change in surface charge responsible for divalent rejection as indicated in Table 4. Table 3 Table 4 shows that surface charge remains nearly unaffected for the sodium hypochlorite treated membrane, whereas it decreases from 33.8 mV to 37.42 mV for the chitosan-treated membrane. This explains the improvement in the magnesium chloride rejection performance of the membrane. Atomic force microscopy images were taken of the treated membrane to understand the changes in surface morphology. Thin lm composite RO membrane Roughness analysis. It is evident from Fig. 3A and B, 4A and B, 5A and B and Table 5 that membrane roughness decreases from 225 nm to 154 nm with the treatment of sodium hypochlorite and further decreases to 117 nm with the treatment of chitosan. Surface skewness decreases with sodium hypochlorite treatment; however, it increases with chitosan treatment. Contact angles were measured to evaluate the change in hydrophilicity of the membrane with the treatment as shown in Table 6. Treated membrane performance for NaCl and MgCl2 rejection and flux Sr. no. Membrane samples NaCl rejection (%) Flux (gfd) MgCl2 rejection (%) Flux (gfd) 1 2 3 4 5 TFC C2-1250 C2-1250 CT-1000 C2-1250 CT-5000 C2-1250 CT-10 000 92.00 85.70 86.51 86.23 85.90 14 35.5 49 32.5 32 86.36 84.17 85.46 86.14 84.50 16 32 43.5 30 30 6690 | RSC Adv., 2015, 5, 6687–6694 This journal is © The Royal Society of Chemistry 2015 Paper Fig. 2 RSC Advances NaCl and MgCl2 rejection (%) and trans-membrane flux (gfd) of all the membranes. Table 4 Surface charge of treated membrane Membrane TFC C1-1250 C1-1250 CT-1000 Zeta potential (mV) 33.8 32.81 37.42 Conductivity (mS cm 1) Temperature ( C) 0.15 0.176 0.155 12.08 12.07 11.86 It is evident from Table 6 that the contact angle of the highux membrane is lower as compared to the low-ux membrane. The average contact angle of 60 minutes, 1000 mg l 1 chitosan treated membrane aer 30 minutes and 60 minutes 1250 mg l 1 Fig. 3 sodium hypochlorite treatment are 32.5 (sr. no. 3) and 29 (sr. no. 7), respectively, that shows the increased hydrophilicity as compared to TFC RO membrane whose average contact angle is 49 (sr. no. 1). Thus, the membrane hydrophilicity increases as a result of sodium hypochlorite and chitosan treatments. To understand the modication in chemical structure, ATRFTIR spectra of the TFC RO membrane, 30 minutes, 1250 mg l 1 sodium hypochlorite treated membrane and 60 minutes, 1000 mg l 1 chitosan-treated membrane were taken, as shown in Fig. 6. Fig. 6 and Table 7 demonstrate the chemical structural changes in the TFC RO membrane as a result of chitosan treatment. The presence of –OH group in the 1000 mg l 1 (A and B) AFM images of TFC RO membrane. Fig. 4 A and B: AFM images of 1250 mg l 1 sodium hypochlorite treated TFC RO membrane. This journal is © The Royal Society of Chemistry 2015 RSC Adv., 2015, 5, 6687–6694 | 6691 RSC Advances Fig. 5 Table 5 Paper (A and B) AFM images of 1000 mg l 1 chitosan-treated membrane after 1250 mg l 1 sodium hypochlorite treatment. Roughness analysis Membrane Roughness average (nm) Root mean square roughness (nm) Surface skewness TFC C1-1250 C1-1250 CT-1000 225 154 117 276 188 144 0.403 0.265 0.473 Table 6 Contact angle data of treated membrane Sr. no. Membrane Contact angle (le) 1 2 3 4 5 6 7 8 9 TFC C1-1250 C1-1250 CT-1000 C1-1250 CT-5000 C1-1250 CT-10 000 C2-1250 C2-1250 CT-1000 C2-1250 CT-5000 C2-1250 CT-10 000 48 45 32 34.73 41 30.5 29 31 32 Fig. 6 Contact angle (right) 50 44 33 36.7 39.1 31 29 31.5 31 chitosan-treated membrane shows that the chitosan has chemically reacted with the polyamide structure and formed –OH bonds. The sharp peak at wave number 1504 cm 1 shows the chemical changes in –C]O bond in an aromatic polyamide structure as –C]O bond is vulnerable to structural modication attributed to of unsaturation. These chemical structural changes make the membrane more hydrophilic in nature as endorsed by contact angle analysis. The chemical structural modication suggests hydrophilic supramolecular assembly of chitosan over polyamide, which is responsible for improved membrane performance. ATR-FTIR spectra of TFC, sodium hypochlorite treated TFC and chitosan-treated membrane. 6692 | RSC Adv., 2015, 5, 6687–6694 This journal is © The Royal Society of Chemistry 2015 Paper RSC Advances Table 7 The peak intensities modification as a result of chemical treatment Sr no. % Transmittance Wavenumber Functional group Type of vibration Intensity 1 2 3 4 5 86.22 93.72 83.39 70.89 103.37 1010.50 1133.92 1226.49 1504.19 3640.90 Ether (–C–O–C) Ester (–C–O–O–R) Aliphatic amines (–C–N–) Aromatic (–C]O) Hydroxyl (–O–H) Stretch Stretch Stretch Stretch — Strong Two bands or more Medium-weak Medium-weak — Table 8 Temperature sensitivity of treated membrane Membrane NaCl rejection Flux (gfd) Temperature ( C) TFC TFC C1-1250 C1-1250 C1-1250 CT-1000 C1-1250 CT-1000 C2-1250 C2-1250 C2-1250 CT-1000 C2-1250 CT-1000 92.00 91.50 91.70 91.05 95.00 95.63 85.70 85.25 86.51 86.34 14.00 22.40 18.50 30.50 30.00 55.00 35.50 54.13 49.00 75.00 30 45 30 45 30 45 30 45 30 45 Temperature sensitivity Table 8 demonstrates that the temperature sensitivity of the C1-1250 CT-1000 membrane, i.e. the TFC RO membrane treated with 1250 mg l 1 sodium hypochlorite for 30 minutes and 1000 mg l 1 of chitosan for 60 minutes is the highest at 5.56% rise in ux per  C rise in temperature as compared to TFC RO membrane with a 3% rise in ux per  C rise in temperature. However, on longer exposure of sodium hypochlorite as in the case of C2-1250 CT-1000, the temperature sensitivity falls to 3.54% per  C rise in temperature. This suggests that the supramolecular assembly formed by chitosan over a polyamide layer is temperature sensitive where the segmental mobility of the exible polymer structure at higher temperatures could have helped to increase the ux through the membrane. The elevated temperature up to 45  C does not harm the TFC RO membrane. It is also widely accepted, and the commercial membrane suppliers specify the temperature operating range up to 45  C. However, it opens the opportunity to further investigate the performance of TFC RO membranes at higher temperatures on a continuous basis for longer duration. Such membranes can be very useful in solar powered reverse osmosis where the low grade thermal energy can be captured from solar photovoltaic cells by feed water to reverse osmosis, thus increasing its temperature as demonstrated in the previous report.27 Thus, improved hydrophilicity is reected by decreased contact angle, surface charge becomes more negative with the treatment showing the improvement in divalent ion separation, and decreased roughness and supramolecular assembly formed by chitosan as understood from ATR-FTIR has also conrmed This journal is © The Royal Society of Chemistry 2015 % Rise in ux per  C rise in temperature 4.00 4.32 5.56 3.50 3.54 the modication in the morphology and chemical structure of the membrane. Such a composite membrane opens the possibility of development of ultra-low-energy membrane process. Conclusion Energy consumption for reverse osmosis can be decreased by making a very high permeability reverse osmosis membrane and putting it to use. The thin-lm composite RO membrane can be modied to form a very high-ux membrane by successive chemical treatment of sodium hypochlorite and chitosan to make the hydrophilic supramolecular assembly. Such a composite membrane demonstrated lower contact angle and higher hydrophilicity. The zeta potential of such a membrane also decreased, which shows improvement in divalent ion separation. The following conclusions were drawn: 1. The trans-membrane ux increases from 14 gfd to 30 gfd with an increase in NaCl solute rejection from 92% to 95%, whereas ux increased from 16 gfd to 28 gfd with an increase in MgCl2 rejection from 86.36% to 95.06% when the membrane was exposed to 1250 mg l 1 sodium hypochlorite at pH 11.0 for 30 minutes and 1000 mg l 1 chitosan solution at pH 2.5 for 60 minutes. 2. The trans-membrane ux increases from 14 gfd to 49 gfd with a decline in NaCl solute rejection from 92% to 86.51%, whereas ux increased from 16 gfd to 43.5 gfd with a slight decline in MgCl2 rejection from 86.36% to 85.46% when the membrane was exposed to 1250 mg l 1 sodium hypochlorite at pH 11.0 for 60 minutes and 1000 mg l 1 chitosan solution at pH 2.5 for 60 minutes. This shows that longer sodium hypochlorite exposure compromises membrane selectivity. RSC Adv., 2015, 5, 6687–6694 | 6693 RSC Advances 3. Zeta potential of the thin-lm composite RO membrane decreased from 33.8 mV to 37.42 mV and average roughness decreased from 225 nm to 117 nm when the membrane was exposed to 1250 mg l 1 sodium hypochlorite at pH 11.0 for 30 minutes and 1000 mg l 1 chitosan solution at pH 2.5. This shows improvement in divalent ions as the charge becomes more negative. 4. The average contact angle decreased from 49 to 32.5 when the membrane was exposed to 1250 mg l 1 sodium hypochlorite at pH 11.0 for 30 min and 1000 mg l 1 chitosan for 60 minutes at pH 2.5. This demonstrates that the hydrophilicity of the membrane increased. 5. The presence of the –OH group and modication in the –CO– group in the polyamide structure shows the supramolecular assembly of chitosan over polyamide. 6. 1000 mg l 1 chitosan-treated membrane for 60 minutes at pH 2.5 aer 1250 mg l 1 sodium hypochlorite-treated membrane at pH 11.0 was more temperature-sensitive, compared with the TFC RO membrane and demonstrated a 5.56% rise in ux per  C rise in feed water temperature. Such a modied membrane may be used in solar photovoltaic powered desalination, in which the feed water can capture thermal energy from a solar photovoltaic panel to control its temperature and thus gets heated. The low-grade captured thermal energy can be utilized in synergistic fashion to increase the permeability of the membrane. The study presented here opens the opportunity for further work in the area. Acknowledgements CSIR-CSMCRI PRIS no. CSIR-CSMCRI-174/2014. The authors thankfully acknowledge the funding support of the Council of Scientic and Industrial Research, India. The authors acknowledge the suggestions of Dr AVR Reddy to improve the manuscript quality. The authors acknowledge Dr Babulal Rebary for AFM analysis, Mr Viral Vakani for ATR-FTIR, Mr Veerababu for help in zeta potential analysis, and Mr Ravi for help in contact angle analysis. References 1 S. H. Maruf, D. U. Ahn, J. Pellegrino, J. P. Killgore, A. R. Greenberg and Y. Ding, J. Membr. Sci., 2012, 405–406, 167–175. 2 C. Y. Tang, Y.-N. Kwon and J. O. Leckie, Desalination, 2009, 242, 149–167. 3 C. Y. Tang, Y.-N. Kwon and J. O. Leckie, J. Membr. Sci., 2007, 287, 146–156. 4 J. Benavente and M. I. Vázquez, J. Colloid Interface Sci., 2004, 273, 547–555. 6694 | RSC Adv., 2015, 5, 6687–6694 Paper 5 P. S. Singh, S. V. Joshi, J. J. Trivedi, C. V. Devmurari, A. P. Rao and P. K. Ghosh, J. Membr. Sci., 2006, 278, 19–25. 6 A. Tiraferri, N. Y. Yip, W. A. Phillip, J. D. Schiffman and M. Elimelech, J. Membr. Sci., 2011, 367, 340–352. 7 B.-H. Jeong, E. M. V. Hoek, Y. Yan, A. Subramani, X. Huang, G. Hurwitz, A. K. Ghosh and A. Jawor, J. Membr. Sci., 2007, 294, 1–7. 8 A. Simon, L. D. Nghiem, P. Le-Clech, S. J. Khan and J. E. Drewes, J. Membr. Sci., 2009, 340, 16–25. 9 J. Xu, X. Feng and C. Gao, J. Membr. Sci., 2011, 370, 116–123. 10 T. Kouketsu, S. Duan, T. Kai, S. Kazama and K. Yamada, J. Membr. Sci., 2007, 287, 51–59. 11 V. Freger, J. Gilron and S. Belfer, J. Membr. Sci., 2002, 209, 283–292. 12 D. M. Sullivan and M. L. Bruening, J. Membr. Sci., 2005, 248, 161–170. 13 J. C. van Dijk, T. N. Olsthoorn, L. Zou, J. Q. J. C. Verberk and S. G. J. Heihman, Surface hydrophilic modication of RO membrane by plasma polymerization for low organic fouling, 2010. 14 M. Ulbricht, J. Polym. Sci., 2006, 47, 2217–2262. 15 S. U. Hong, R. Malaisamy and M. L. Bruening, J. Membr. Sci., 2006, 283, 366–372. 16 L. Krasemann, A. Toutianoush and B. Tieke, J. Membr. Sci., 2001, 181, 221–228. 17 Y.-L. Liu, C.-H. Yu and J.-Y. Lai, J. Membr. Sci., 2008, 315, 106–115. 18 W. J. Lau, A. F. Ismail, N. Misdan and M. A. Kassim, Desalination, 2012, 287, 190–199. 19 J.-F. Blanco, J. Sublet, Q. T. Nguyen and P. Schaetzel, J. Membr. Sci., 2006, 283, 27–37. 20 G. Zhang, W. Gu, S. Ji, Z. Liu, Y. Peng and Z. Wang, J. Membr. Sci., 2006, 280, 727–733. 21 G. Kang, M. Liu, B. Lin, Y. Cao and Q. Yuan, J. Polym. Sci., 2007, 48, 1165–1170. 22 G.-D. Kang, C.-J. Gao, W.-D. Chen, X.-M. Jie, Yi-M. Cao and Q. Yuan, J. Membr. Sci., 2007, 300, 165–171. 23 H. D. Raval, J. J. Trivedi, S. V. Joshi and C. V. Devmurari, Desalination, 2010, 250, 945–949. 24 H. D. Raval, V. R. Chauhan, A. H. Raval and S. Mishra, Desalin. Water Treat., 2012, 48, 349–359. 25 N. P. Soice, A. R. Greenberg, W. B. Krantz and A. D. Norman, J. Membr. Sci., 2004, 243, 345–355. 26 N. Misdan, W. J. Lau and A. F. Ismail, Desalination, 2012, 287, 228–237. 27 H. D. Raval, S. Maiti and A. Mittal, J. Renewable Sustainable Energy, 2014, 6, 033138. This journal is © The Royal Society of Chemistry 2015