A novel high-flux, thin-film composite reverse osmosis membrane modified by chitosan for advanced water treatment

Abstract

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. 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 energy can be utilized to increase feed water temperature, and higher temperature feed water gives more a pronounced advantage in trans-membrane flux.

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Introduction

Thin-film composite RO membrane has found numerous applications in desalination and water reuse. Research and development efforts in improving its solute rejection and flow performance of the membrane have made the RO membrane versatile for diverse applications. A thin-film 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 configuration of a thin-film composite reverse osmosis membrane gives the desired properties of high flux of permeate flow, 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 bottom two support layers give the mechanical strength to the thin-film composite RO membrane.^1–3,9 A large number of the thin film composite RO membranes have been prepared from various polymers, such as polyurea, polyamides, polyurea-amides, 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 significant 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 flow performance of TFC RO membrane.^10,11 It has also been studied that the hydrophilic surface improves the flow performance of TFC RO membrane.^12,13

The top layer of thin-film composite RO membrane is the barrier layer responsible for solute rejection. Thus, modification 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 fabricating TFC membranes. The top surface of thin-film composite polyamide membrane can be modified 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 significant attention for use in pervaporation, reverse osmosis and nanofiltration.^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 perm-selective, 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 flux decline that increases the energy consumption of reverse osmosis and ultra-filtration 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 flux 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 flux. The thin-film composite RO membrane has more than 98% selectivity under the standard test condition.²⁴ Commercially available thin-film 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-film composite RO membrane is modified 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 flux has a correlation with the physiochemical properties such as zeta potential, hydrophilicity, chemical composition, and morphology. The increase/decrease in solute rejection and flux must find 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 some trace organic solute and divalent ions from the reverse osmosis membrane.²⁶ Similarly, improved hydrophilicity reflected by decreased contact angle should correspond to increased permeate flux.

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 after chemical treatment on the active polyamide skin layer can alter its performance in terms of flux and NaCl/MgCl₂ solute rejection owing to the supramolecular assembly formed. The temperature sensitivity of such a composite membrane increases compared to thin-film 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 modification in the membrane material.

The controlled treatment of sodium hypochlorite with a polyamide layer of thin-film composite membrane increases the permeate flux. 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 flux.

Materials

Thin-film composite reverse osmosis membrane, sodium hypochlorite, chitosan, sodium m-bisulfite, nitric acid.

Method

A flat-sheet, thin-film 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-film 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-film 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 specified time. Thereafter, a part of the membrane was washed with deionised water and retained in a sodium m-bisulfite 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-bisulfite solution of 5 g l⁻¹ concentration to nullify the presence of active chlorine if any. The TFC RO membrane, sodium hypochlorite treated membrane and chitosan-treated membrane were tested for their flow and solute rejection performance. The treatment conditions, e.g. concentration and time of exposure, are shown in Table 1.

Table 1 Treatment conditions

Sr. no.	Concentration of sodium hypochlorite solution (mg l^−1)	Exposure time of sodium hypochlorite solution (min)	Code	Concentration of chitosan solution (mg l^−1)	Exposure time of chitosan solution (min)	Code
1	1250	30	C1-1250	500	60	C1-1250 CT-500
2	1250	30	C1-1250	1000	60	C1-1250 CT-1000
3	1250	30	C1-1250	5000	60	C1-1250 CT-5000
4	1250	30	C1-1250	10 000	60	C1-1250 CT-10 000
5	1250	60	C2-1250	1000	60	C2-1250 CT-1000
6	1250	60	C2-1250	5000	60	C2-1250 CT-5000
7	1250	60	C2-1250	10 000	60	C2-1250 CT-10 000

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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 filtration was dead-end filtration 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 flux 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 flux. 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 chitosan-treated 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 modification.

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 after 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 after 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.

Attenuated total reflectance 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 modification 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 flux 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 flux 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 flux was observed.

Table 2 Treated membrane performance for NaCl and MgCl₂ rejection and flux

Sr. no.	Membrane samples	NaCl rejection (%)	Flux (gfd)	MgCl2 rejection (%)	Flux (gfd)
1	TFC	92.00	14	86.36	16
2	C1-1250	91.70	18.5	86.32	19
3	C1-1250 CT-500	90.49	23	90.58	21
4	C1-1250 CT-1000	95.00	30	95.06	28
5	C1-1250 CT-5000	91.7	28	89.21	24
6	C1-1250 CT-10 000	90.83	24	85.74	17.5

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It can also be observed from Fig. 1 that NaCl rejection as well as permeate flux increases, compared to virgin TFC RO membrane for the membrane treated with 1000 mg l⁻¹ chitosan for 60 minutes after 1250 mg l⁻¹ sodium hypochlorite for 30 minutes. In this case, flux 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.

Fig. 1 NaCl and MgCl₂ rejection (%) and trans-membrane flux (gfd) of all the membranes.

Table 3 Treated membrane performance for NaCl and MgCl₂ rejection and flux

Sr. no.	Membrane samples	NaCl rejection (%)	Flux (gfd)	MgCl2 rejection (%)	Flux (gfd)
1	TFC	92.00	14	86.36	16
2	C2-1250	85.70	35.5	84.17	32
3	C2-1250 CT-1000	86.51	49	85.46	43.5
4	C2-1250 CT-5000	86.23	32.5	86.14	30
5	C2-1250 CT-10 000	85.90	32	84.50	30

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It can be inferred from Table 3 and Fig. 2 that the trans-membrane flux 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. 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.

Fig. 2 NaCl and MgCl₂ rejection (%) and trans-membrane flux (gfd) of all the membranes.

It can be seen from Fig. 1 and 2 that 1000 mg l⁻¹ chitosan is the optimum concentration level for achieving a high-flux, 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 4 Surface charge of treated membrane

Membrane	Zeta potential (mV)	Conductivity (mS cm^−1)	Temperature (°C)
TFC	−33.8	0.15	12.08
C1-1250	−32.81	0.176	12.07
C1-1250 CT-1000	−37.42	0.155	11.86

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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 film 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.

Fig. 3 (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.

Fig. 5 (A and B) AFM images of 1000 mg l⁻¹ chitosan-treated membrane after 1250 mg l⁻¹ sodium hypochlorite treatment.

Table 5 Roughness analysis

Membrane	Roughness average (nm)	Root mean square roughness (nm)	Surface skewness
TFC	225	276	0.403
C1-1250	154	188	0.265
C1-1250 CT-1000	117	144	0.473

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Contact angles were measured to evaluate the change in hydrophilicity of the membrane with the treatment as shown in Table 6.

Table 6 Contact angle data of treated membrane

Sr. no.	Membrane	Contact angle (left)	Contact angle (right)
1	TFC	48	50
2	C1-1250	45	44
3	C1-1250 CT-1000	32	33
4	C1-1250 CT-5000	34.73	36.7
5	C1-1250 CT-10 000	41	39.1
6	C2-1250	30.5	31
7	C2-1250 CT-1000	29	29
8	C2-1250 CT-5000	31	31.5
9	C2-1250 CT-10 000	32	31

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It is evident from Table 6 that the contact angle of the high-flux membrane is lower as compared to the low-flux membrane. The average contact angle of 60 minutes, 1000 mg l⁻¹ chitosan treated membrane after 30 minutes and 60 minutes 1250 mg l⁻¹ 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 modification in chemical structure, ATR-FTIR 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 ATR-FTIR spectra of TFC, sodium hypochlorite treated TFC and chitosan-treated membrane.

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 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 modification attributed to of unsaturation. These chemical structural changes make the membrane more hydrophilic in nature as endorsed by contact angle analysis. The chemical structural modification suggests hydrophilic supramolecular assembly of chitosan over polyamide, which is responsible for improved membrane performance.

Table 7 The peak intensities modification as a result of chemical treatment

Sr no.	% Transmittance	Wavenumber	Functional group	Type of vibration	Intensity
1	86.22	1010.50	Ether (–C–O–C)	Stretch	Strong
2	93.72	1133.92	Ester (–C–O–O–R)	Stretch	Two bands or more
3	83.39	1226.49	Aliphatic amines (–C–N–)	Stretch	Medium-weak
4	70.89	1504.19	Aromatic (–C=O)	Stretch	Medium-weak
5	103.37	3640.90	Hydroxyl (–O–H)	—	—

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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 flux per °C rise in temperature as compared to TFC RO membrane with a 3% rise in flux 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 flexible polymer structure at higher temperatures could have helped to increase the flux 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.

Table 8 Temperature sensitivity of treated membrane

Membrane	NaCl rejection	Flux (gfd)	Temperature (°C)	% Rise in flux per °C rise in temperature
TFC	92.00	14.00	30
TFC	91.50	22.40	45	4.00
C1-1250	91.70	18.50	30
C1-1250	91.05	30.50	45	4.32
C1-1250 CT-1000	95.00	30.00	30
C1-1250 CT-1000	95.63	55.00	45	5.56
C2-1250	85.70	35.50	30
C2-1250	85.25	54.13	45	3.50
C2-1250 CT-1000	86.51	49.00	30
C2-1250 CT-1000	86.34	75.00	45	3.54

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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 reflected 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 confirmed the modification 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-film composite RO membrane can be modified to form a very high-flux 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 flux increases from 14 gfd to 30 gfd with an increase in NaCl solute rejection from 92% to 95%, whereas flux 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 flux increases from 14 gfd to 49 gfd with a decline in NaCl solute rejection from 92% to 86.51%, whereas flux 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.

3. Zeta potential of the thin-film 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 modification 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 after 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 flux per °C rise in feed water temperature. Such a modified 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 Scientific 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.

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