A NOVEL THIN FILM COMPOSITE REVERSE OSMOSIS MEMBRANE MODIFIED BY IONIC LIQUID

* Corresponding author: Email: hirenraval@csmcri.res.in , raval.hiren@rediffmail.com , Fax: +91-2782566970 Abstract Thin Film Composite Reverse Osmosis (TFC RO) membranes have undergone significant changes since inception; particularly the top polyamide layer has been tuned for optimal performance. The present paper demonstrates the novel approach to alter the polyamide membrane performance by subjecting it to ionic liquids. Ionic liquids 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl], 1-Methyl-3-Octylimidazolium Chloride [C8MIM][Cl] and 1-Butyl-3-Methylimidazolium Bromide [BMIM][Br] were used to alter the membrane performance. About a 6.5% increase in MgSO4 rejection and about an 87% increase in water-flux were noted when the membrane was subjected to 3000 mg/L [BMIM][Cl] after 2000 mg/L sodium hypochlorite each for 2 hours. Also, the decline in contact angle from 52.86o to 43.12o by this treatment demonstrated higher hydrophilicity. Atomic force microscope images showed a decline in surface roughness with the treatment. Scanning electron micrographs were taken to understand the changes in morphology of thin film composite reverse osmosis membranes with ionic liquid treatment. Attenuated total reflectance, infrared spectroscopy and nuclear magnetic resonance analysis were done to evaluate the changes in chemical structure and it was found that the treatment resulted in chemical structural modification of thin film composite reverse osmosis membranes with ionic liquid treatment.


INTRODUCTION
The process of reverse osmosis, which was first commercially developed in the mid-1960s, has undergone significant development since then (Glater, 1998).Reverse osmosis, as it is commonly known, is now used extensively in various separation processes, including brackish and sea water desalination.Concentration polarization is one of the major hindrances, which causes a reduction in salt rejection and flux in reverse osmosis thin film composite membranes (Tang et al., 2010).Concentration polarization refers to the gradual increase in concentration of non-permeating components near the surface of the membrane, effectively reducing its capacity for salt rejection and flux (Morao et al., 2008).The top layer of a reverse osmosis membrane is dense.The thickness of this layer is an important consideration in concentration polarization.Thinning the top layer of the membrane can weaken the effects of concentration polarization and thereby theoretically increase the water flux (Wang et al., 2009).Thin film composite reverse osmosis membranes are either cellulose acetate or polyamide membrane.However, the latter is widely used because of its higher operating pH range, wider temperature range, higher stability to biological attack and ease of surface modification (Petersen et al., 1993;El-Saied et al., 2003;Younos et al., 2005;Wethern et al., 1995;Asano, 1998;Kulkarni et al., 1996).
Various experiments have been performed for surface modification of reverse osmosis membranes using different modifying agents.These include the use of sodium hypochlorite and polysaccharides like chitosan (Raval et al., 2016;Raval et al., 2017;Raval et al., 2015).Some toxic, corrosive, flammable or environmentally harmful agents such as concentrated sulfuric acid and hydrofluoric acid (Gilron et al., 2001;Belfer et al., 1998).Room Temperature Ionic Liquids [RTIL] are salts in the liquid state in which ions are poorly coordinated and they exist in the liquid state below 100 0 C. Ionic liquids have recently gained popularity in use in various chemical applications owing to their unique properties.Ionic liquids are powerful solvents and find applications in decomposing polymers and as additives in nano-filtration and other membranes (Swatloski et al., 2002;Lua et al., 2009;Yung et al., 2010;Mohammad et al., 2015).However, the application of ionic liquid for surface modification of thin film composite reverse osmosis membranes is largely unknown (Schmidt et al., 2008).
This paper aims at using ionic liquids as surface modifiers for commercial thin film composite reverse osmosis membranes and explores their effect on membrane performance.From various different ionic liquids 1-Butyl-3-Methylimidazolium Chloride (Molecular Weight = 230.78g/mol) were purchased from Sigma -Aldrich Co. LLC.Sodium hypochlorite (Molecular Weight = 74.44 g/mol) was purchased as laboratory reagent with available chlorine of 4% -6% w/v from RFCL limited, New Delhi (India).Magnesium sulphate (divalent salt) was purchased from Tokyo Chemical Industry Co., LTD, Tokyo, Japan.The chemical structure of the polyamide layer of thin film composite reverse osmosis membranes is shown in Figure 1 (Rangarajan et al., 2011).

Treatment of membrane with sodium hypochlorite and ionic liquid
Polyamide thin film composite reverse osmosis membranes were cut into a strip of 10 cm x10 cm, cleaned with deionized water.We stuck the membrane strip on a glass plate to ensure that only the top layer was subjected to treatment and the other components remained air tight and free of any chemical contact.We dipped the membrane into a 2000 mg/L solution of sodium hypochlorite for a period of 2 hours.Membrane was then taken out and once again thoroughly washed with deionized water and then cut into two strips each of 5 cm x 10 cm.

Membrane performance
Experiments were carried out in a standard reverse osmosis experimentation kit.The modified membranes were subjected to magnesium sulfate solution of 4000 µS in deionized water at 250 psig pressure.The membranes were stabilized for 2 hours and then permeate was collected for 20 minute at 250 psig pressure to measure water-flux and conductivity as per the standard procedure.

Membrane characterization
We characterized the membranes by Scanning electron micrographs (SEM) to study the surface morphology, nuclear magnetic resonance (NMR) and attenuated total reflectance infrared spectroscopy (ATR-FTIR) and to understand the chemical structural modification, atomic force micrographs (AFM) to study the top surface roughness features and contact angle analysis for measuring surface hydrophilicity.

Divalent salt rejection
Results as shown in Figure 3 and Figure 4 indicate that of the various different ionic liquids 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl] gives the optimum results in terms of flux as well as divalent salt rejection.Also it can be inferred from Figure 4 and Table 1 that 3000 mg/L concentration of 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl] gives 6.5% increase in MgSO 4 rejection and about 87% increase in water-flux as compared to virgin thin film composite reverse osmosis membranes and other concentrations of It can be inferred from Table 1 that divalent salt rejection increases until 3000 mg/L 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl] concentration and then decreases.Water flux also follows the similar trend.It can be deduced from the data in Table 1 that

Contact angle of membranes
Contact angles of membranes were measured using the drop shape analyser (DSA 100) provided by Krüss Optronic -Germany.The contact angle is a measure to study the hydrophilicity of the membrane, i.e., the contact angle of a water drop will decline with increased hydrophilicity.Contact angles for various membranes are mentioned in Table 2.It can be seen that the contact angle declines from 52.86° to 43.12° for 3000 mg/L 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl] as compared to virgin thin film composite reverse osmosis membrane.However, it increases further with higher concentration treatment, i.e., 4000 mg/L and 5000 mg/L of 1-Butyl-3-Methylimidazolium Chloride
Presence of ionic liquid can be observed in Figure 5 (b) & (c).White spots can be clearly identified in Figure 5 (b) that shows the structural modification with ionic liquid treatment.The cross-section in Figure 5 (c) and (d) shows the thinning of the polyamide layer with the ionic liquid treatment.Top polyamide layer thickness was reduced to 60 nm as compared to average polyamide layer thickness, i.e., about 200 nm as reported in case of interfacial polymerization with hexane solvent (Ghosh et al., 2008).The decline in thickness of the membrane is because of interaction between polyamide and ionic liquid.Decline in thickness of the polyamide layer may be attributed to π-π interactions of ionic liquid with polyamide.

Atomic Force Microscope Images (AFM)
The topography and surface roughness properties of the membrane modified by 1-Butyl-

3-Methylimidazolium
Chloride [BMIM][Cl] were measured using an atomic force microscope (NTEGRA Aura, NT-MDT Instruments, Russia).The images of virgin thin film composite membrane, and 500 mg/L, 3000 mg/L and 4000 mg/L 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl] ionic liquid treated membranes are shown in Figure 6.The membrane with the highest water-flux and magnesium sulphate rejection, i.e., membrane treated with 3000 mg/L 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl], showed the surface area ratio of 21.34% and roughness of 42.7 nm as shown in Table 3.It shows the change in roughness with ionic liquid treatment.

ATR -Fourier Transform Infrared spectroscopic analysis
Attenuated total reflectance Fourier transform infrared spectroscopy (Spectrum GX FTIR Spectrometer, PerkinElmer, USA) was used to analyse the chemical structure of the virgin thin film composite [Cl] ionic liquid.The ATR-FTIR spectra of both the membranes are shown in Figure 7.The identifiable peaks are mentioned in Table 4.
It can be seen from Figure 7 and Table 4 that the amide bond of polyamide layer has been modified by ionic liquid treatment.The peaks at 1635 cm -1 and 1589   cm -1 correspond to amide I and amide II, respectively.Amide I is comprised of CO stretching and amide II represents NH in-plane bending.It can be inferred that CC aromatic ring stretching has been affected by the treatment of ionic liquid on the TFC RO membrane.
Ether and C=C linkages have been modified by ionic liquid treatment of the polyamide layer.In this way, the chemical structural changes in the polyamide layer   peaks at 21.6 ppm and 26.6 ppm are related to the butyl chain of ionic liquid, where the associated peak at 118.4 ppm can be assigned to aromatic imidazolium carbons.Shifting in the ppm value is due to the change of the chemical environment of carbons from a polymeric aromatic environment.
Figure 9 shows 1 H NMR overlay spectra of the same.The origin of the new associated peak at 6.1 ppm is related to aromatic imidazolium protons of the ionic liquid.
This clearly demonstrates that the polyamide layer has been modified by the ionic liquid through weak π -π or cation-π interactions.
Thus, the characterization of the membrane reveals the chemical, surface morphological and roughness changes and hydrophilic surface formation upon addition of ionic liquid into the polysulfone membrane matrix.

CONCLUSION
Different ionic liquids were used to modify the top polyamide layer of thin film composite reverse osmosis membranes.It was observed that the top polyamide layer of the thin film composite reverse osmosis membrane became thinner.Moreover, the contact angle of such modified membrane declined from 52.86 o to 43.12 o , which demonstrated increased hydrophilicity.These factors contributed to an increase in water-flux.Divalent salt rejection of the modified membrane (3000 mg/L 1-Butyl-3-Methylimidazolium Chloride [BMIM][Cl] exposed for 2 hours after treatment with 2000 mg/L sodium hypochlorite for 2 hours) increased from 89.3% to 95.10% with an increase in water-flux from 2.08 LMH/Bar to 3.89 LMH/Bar.The performance increment was the highest with 1-Butyl-3-Methylimidazolium Chloride [BMIM] [Cl] as compared the other ionic liquids such as 1-Butyl-3-Methylimidazolium Bromide [BMIM][Br] and 1-Methyl-3-Octylimidazolium Chloride [C 8 MIM] [Cl].The modified membrane had decreased surface roughness, with an increased surface area ratio.ATR-FTIR spectra demonstrated the modification in the amide bond and prove the chemical modification of structure with ionic liquid treatment. 13C-and 1 H-NMR spectra demonstrated new peaks confirming the chemical structural modification with ionic liquid treatment.Scanning electron micrographs showed the modification in the morphology of the membrane with the treatment.Thus, the present paper demonstrates novel thin film composite reverse osmosis membrane with improved water flux, higher magnesium sulfate rejection and lower thickness of top polyamide layer upon ionic liquid treatment.It opens the possibility of future work in the area.

Figure 2 .
Figure 2. Schematic of membrane surface modification by ionic liquids.

Figure 4 .
Figure 4. Water-flux comparison of different ionic liquids with varying concentrations.

Figure 3 .
Figure 3. Divalent ion rejection comparison of various ionic liquids with varying concentrations.

Table 1 .
Membrane salt rejection and divalent rejection with different dosage of [BMIM][Cl] ionic liquid.