Novel silica functionalized monosodium glutamate/PVA cross-linked membranes for alkali recovery by diffusion dialysis

In this work, two types of silica functionalized monosodium glutamate (GMSG and VMSG)/ poly(vinyl alcohol) (PVA) were cross-linked by sol-gel process to prepare novel hybrid cation exchange membranes (CEMs). The prepared membranes were systematically characterized by FTIR, ion exchange capacity (IEC), TGA, water uptake, water swelling, mechanical strength and diffusion dialysis (DD) performance for alkali separation using NaOH/Na2WO4 solution. The FTIR peaks around 1260-1350 cm confirmed the secondary C−N linkages. The cross-linking between GMSG/VMSG and PVA was verified by the presence of stretching peaks of Si−O−C, Si−O−Si, C−O−C, and −C(=O)−O−C groups between 1080-1120 cm. TGA results indicated that GMSG membranes showed relatively high thermal stability as compared to VMSG membranes. Water uptake and degree of swelling decreased while IEC values increased with the increase of GMSG/VMSG content in membrane matrix. The mechanical properties of the membranes improved up to 40% GMSG/VMSG content. The NaOH dialysis coefficient (UOH) values improved while values of separation factor (S) declined with the increase of GMSG/VMSG content. Finally, the effect of temperature was studied and it was found that increase in temperature from 25 to 45 °C resulted in increase of diffusion coefficient and decrease of separation factor for both GMSG/VMSG crossed-linked with PVA membranes. Corresponding Author Email: dratifislam@gmail.com Tel: +92 300 6686 506


Introduction
It is a bitter fact that huge quantity of alkaline water is being produced from several industrial wastes like printing, paper, metal ores and leather industries which results in serious environment pollution [1][2][3]. The discharge of alkaline water without treatment process not only wastes useful substances but also disturbs the water pH of rivers which causes severe damage to aquatic life [4]. The traditional methods which have been applied for the treatment of alkaline waste are (a) neutralization of alkali substances with acids (b) lowering the concentration of alkali substances by diluting them and (c) directly burning these substances like in paper and pulp industry. These traditional processes are either energy extensive/require a large amount of commercial acids or a large amount of sludge is produced which poses disposal problems [5,6].
In recent years, membrane-based alkali separation technology has been developed which includes nano-filtration, reverse osmosis, electro-dialysis, ultra-filtration and diffusion dialysis (DD) [3,7]. DD is the most dominant process among all as it is environment friendly, requires less energy and has less installation cost as well. Other advantages include absence of pollution, simple construction of apparatus, recoverable alkali and continuous operation [6,8]. The most important advantage of this process is that it does not require any external driving force as it is spontaneous separation process which relies on simple technique of ion diffusion (movement of ions from high to low concentration) [9]. The alkali separation properties of DD process are remarkably affected by cation exchange membranes (CEM). Recently, a lot of work on CEMs has been done which focuses on various properties of these membranes like NaOH permeability, alkaline resistance, salt rejection, thermo-mechanical stability and selectivity [10]. A lot of polymeric materials have been used, for the development of cation exchange membranes for alkaline recovery, which include poly(vinylidene fluoride), polysulfone, polystyrene, polyetheretherketone, poly(ethylene oxide) and poly(vinyl alcohol [11][12][13][14]. The most critical issues of CEMs having high IEC capacities are their low stability, high degree of swelling and permeability issues in alkaline conditions due to the lack of enough cross-linking [15]. By combining the properties of both inorganic and organic polymer materials, hybrid membranes can be prepared in which inorganic material is dispersed into the organic polymer matrix in order to obtain the required mechanical strength and separation properties [16]. These modifications have made most PVA based hybrid membranes [17,18]. PVA has several advantages like high water solubility, low cost and excellent film forming properties [19]. PVA also possesses −OH groups in abundant which can facilitate extraordinary reaction with -Si-O-CH3 group through sol-gel process. However, due to absence of cation exchange groups in PVA matrix, mixing with other materials having cation exchange group is required to obtain ion exchange capacities [20,21]. Hao et al. studied the effect of carboxyl (−COOH) group content on PVA/SiO2 membranes prepared by sol-gel process for alkali recovery and found that separation factor was increased up to 36.2 [22]. Wu et al. prepared PVA hybrid membrane containing maleic anhydride for NaOH separation performance through DD process and observed noticeable increase in Na + transport, permeability and IEC values with the addition of maleic anhydride containing −COOH groups [23]. Wang et al. studied the influence of mesoporous silica functionalized phosphotungstic acid on PVA membranes for alkali recovery and observed improved thermo-mechanical properties along with improved values of IEC between 0.015 to 0.030 mmol/g [24]. Tong et al. studied separation properties of NaOH/Na2WO4 solution using un-charged PVA-silica hybrid membranes prepared by sol-gel process and found that alkoxysilanes significantly enhanced both mechanical properties and selectivity [2]. Gu [25]. Hence, PVA based hybrid membranes have been developed with different cross-linkers and cation exchange groups to improve the DD performance for alkali recovery [17]. The -Si-OCH3 cross-linked PVA based hybrid cation exchange membranes have been used in many DD applications for alkaline separation [26].
Monosodium glutamate (MSG) is environmental friendly, easily available and cheap food additive having extremely hydrophilic nature due to the presence of −COOH group [27]. The −COOH or −COONa groups (cation exchange component) via ion exchange process can transfer Na + ions, while OH − ions may be transferred through membrane via weak interactions such as hydrogen bonding with the hydroxyl (−OH) groups of PVA and also −Si−OH groups, presents in the membrane [11,28].
In this work, two series of novel silica functionalized monosodium glutamate (GMSG, VMSG) cross-linked PVA hybrid cation exchange membranes have been synthesized which has never been synthesized and studied before for alkaline recovery, according to the best of our literature survey. Moreover, Fourier transform infrared spectroscopy (FTIR), IEC, water uptake, swelling degree, thermo gravimetric analysis (TGA) and mechanical strength of the prepared membranes have been analysed. Finally DD performance properties in term of UOH and selectivity were evaluated via DD test at two different temperatures using NaOH/Na2WO4 solution as a model for alkali waste feed solution.

Synthesis of silica functionalized monosodium glutamate (GMSG, VMSG)
Two types of silica functionalized monosodium glutamate (GMSG, VMSG) were separately prepared by the reaction of monosodium glutamate with 3-glycidoxypropyltrimethoxysilane and vinyltrimethoxysilane, respectively. First, 1 g (5.9 mmol) of monosodium glutamate was added to 10 mL of deionized water with continuous stirring at 75 °C for 30 min in a 50 mL round bottom flask and then calculated amount of 3-glycidoxypropyltrimethoxysilane (1.39 g/ 5.9 mmol) was added to this mixture and stirring was continued on a hot plate for 5 h at 75 °C in order to complete the reaction and obtain viscous solution of GMSG. Similarly, 1 g (5.9 mmol) of monosodium glutamate and 0.87 g (5.9 mmol) of vinyltrimethoxysilane were processed to obtain viscous solution of VMSG.  The resulting gel mixtures were casted on petri dishes and dried at room temperature for about four days. The obtained membranes were carefully peeled off from the petri dishes with the help of knife blade. The membranes were then processed for thermal treatment in a vacuum oven for 5 h at 120 °C to ensure the complete reaction conversion into the hybrid membranes [29]. The obtained membranes were labelled as GMSG20, GMSG40, GMSG60 and VMSG20, VMSG40, VMSG60 series with different wt. % content of GMSG and VMSG inside the membrane matrix, which were 20, 40 and 60 wt. % of PVA as shown in Table 1. The thickness of the membranes was maintained to around 120 to 150 μm.

Fourier transform infrared (FTIR) spectroscopy
FTIR spectroscopic analysis of the PVA hybrid membrane was performed using IR Shimadzu, Prestige-21 (Japan) spectrometer equipped with Horizontal Attenuated Total Reflectance (HATR) accessory. The experiment was performed in the wave number range of 4000-400 cm −1 and 100 scans were collected per spectrum at the resolution of 4 cm −1 with air as its background.

Ion exchange capacity (IEC)
Membrane IEC value was calculated to investigate the content of interchangeable ionic (−COOH or −COONa) group in membrane using acid base titration method. The dried sample was weighed (Wdry) and soaked in 1.0 M HCl solution for two days at room temperature to convert the ion exchange groups into H + form. Afterwards, the sample was washed with plenty of deionized water to ensure the complete removal of acid residues and immersed in 0.05 M solution of NaOH for 48 h. This NaOH solution was titrated against 0.05 M HCl solution with phenolphthalein as an indicator to determine the decreased amount of NaOH. The IEC (mmol/g) was measured by the amount of HCl per dry weight of membrane by equation 1 [30,31].
Where, IEC is the Ion exchange capacity (mmol/g), CHCl, VHCl and Wdry represent the molar concentration of HCl solution, volume of HCl consumed and dry membrane weight (g), respectively.

Thermal gravimetric analysis (TGA)
TGA was performed using Mettler Toledo analyser at a heating rate of 10 °C/min and over a temperature range from room temperature to 800 °C. Nitrogen gas at a flow rate of 15 mL/min was supplied to obtain the inert atmosphere.

Mechanical properties
Mechanical properties like tensile strength and elongation at break of the hybrid membranes were determined using U-CAN Dynatex, tensile tester (UT-2080, Taiwan) having crosshead speed of 100 mm/min. The experiment was performed at 25 ± 2% relative humidity and 25 °C using dumbbell shaped sample having 25 mm length and 5 mm width.

Water uptake and swelling degree
Water uptake was measured to investigate the hydrophilicity of membrane. Small pieces of membranes were dried in a vacuum oven at 75 °C for 24 h. The dried pieces of membranes were weighed and soaked in deionized water at room temperature for 72 h. Afterwards, the pieces were withdrawn, wiped out with bloating paper to remove extra water and weighed again. The Water uptake (%) was determined [32] using equation 2.
water uptake (%) = Where, W1 and W2 represent dried membranes weight and wet membrane weight respectively.
Swelling degree is used to investigate the stability of membrane in hot water. Degree of swelling of the membrane was determined using water immersion method, by membrane swelling behaviour in water. The weight gain of the dry membrane was measured for a certain time after immersed in water at 65 °C [33]. Pre-weighed dry membrane pieces (W1) was dipped in water at 65 °C and then taken out after 72 h. As a final point, the extra surface water from the membrane was wiped dry quickly with blotting paper and then wet piece of membrane was weighed (W3).
Swelling degree (%) was measured using the following equation

Diffusion dialysis
To investigate the potential application of the membrane for alkali recovery, DD experiment was performed using NaOH/Na2WO4 aqueous solution [34]  The dialysis coefficient (U) was calculated using equation 4 [31,35].
Where, M, represents the amount of substance transported in moles, A is the effective area of membrane (m 2 ), t stands for time in h and ΔC is the logarithm average concentration difference (mol/m 3 ) between the two compartments which was calculated using equation 5 [13,36].
Where C 0 f and C t f are the concentrations of feed solutions at time 0 and t, respectively and concentration of dialysate at t time is C t d.
The separation factor (S) determined by the ratio of (two dialysis coefficients) UOH and Uwo4 2− in the solution and was calculated using equation 6 [37].

FTIR spectra analysis
The FTIR spectra of GMSG and VMSG cross-linked PVA hybrid membranes are demonstrated in Fig. 1. Absorption band between 1260-1350 cm −1 represents the stretching vibration of secondary C−N group [25,38] which confirms the formation of GMSG/VMSG (Scheme 1 a).
The broad band around 3300 cm −1 is assigned to stretching vibration of −OH group of PVA and cross-linker [39]. The peaks appearing at 2930 cm −1 and 1427 cm −1 represent -CH stretching and bending vibrations, respectively [40][41][42][43][44][45] . The characteristics sharp peak at 1723 cm −1 is attributed to stretching vibration of carbonyl (−C=O) group from GMSG/VMSG [46,47], the intensity of which increases with the increase of GMSG/VMSG contents in the membrane matrix. The small peak at 1645 cm −1 is attributed to the stretching vibration of C=C [48], which

Ion exchange capacity (IEC)
The IEC value was measured using the titration method to confirm the charged nature of the CEMs and results are shown in Fig. 2. The IEC was mainly contributed from the −COOH/−COONa groups (Scheme 1 a, b) present inside membrane matrix. By increasing GMSG and VMSG amount, the −COOH/−COONa content increases which results in gradual increase of both theoretical and experimental IEC values as shown in Table 1. Hence, GMSG, VMSG can be applied as new types of cross-linking agents for the synthesis of hybrid cation exchange membranes. The experimental IEC values increases from 0.47 mmol/g to 1.05 mmol/g, which is slightly higher than the reported hybrid PVA/poly(AA-co-γ-MPS) membrane having IEC value 0.76 mmol/g to 0.90 mmol/g [11]. The higher experimental IEC values of membranes are due to having −COOH and −COONa groups present in GMSG/VMSG cross-linked membranes. It is also interesting to note that -COONa/−COOH groups remain stable during the sol-gel reaction and only -Si-O-CH3 groups react with −OH groups of PVA [52]. The experimental IEC values depend upon the amount of fixed charge groups in membranes so it can also be claimed that increasing the amount of GMSG, VMSG in the membranes result in increase of IEC values.

TGA analysis
The thermal stability of the prepared membranes was determined using TGA and the results are reported in the form of thermograms as shown in Fig. 3. Each thermogram shows following degradation steps. The first step, in the temperature range of 30-190 °C is attributed to dehydration and removal of bounded and unbounded water molecules from membrane [41]. The second step from 190 to 370 °C is due to the degradation of the −COOH and −COONa groups [53]. The next step from 370 to 490 °C is attributed to the elimination of −OH groups from the PVA [54]. Final degradation stage occurring from 490 to 800 °C is due to the carbonization of degraded product into ash residue by the degradation of main polymer backbones [55] . It has been observed that by increasing GMSG/VMSG content in the membrane, the weight loss keeps on increasing due to increase of cross-linked and compact structure owing as a result of abundance of −COOH/−COONa groups concentration inside the membrane matrix.  Table 2. This is due to the increasing amount of -COOH/−COONa function groups which starts to degrade at relatively lower temperature. It was also noted that temperature at 50% weight loss (T50%) was 382, 437, 383 and 417°C for GMSG20, GMSG60, VMSG40, and VMSG60 membranes, respectively. The enhancement in T50% by increasing the amount of GMSG and VMSG loading is due to increase of silica linkages which offer resistance to degradation.

Mechanical strength
Mechanical properties of membranes such as tensile strength (TS) and elongation at break (Eb) were determined and obtained results are shown in Table 3

Water uptake and swelling degree
The water uptake and swelling degree values of the prepared membranes are shown in Table 3 and shown in Fig  which can significantly improve the membrane stability [11,22]. Hence, increased linkages provide strength to resist the swelling degree. GMSG (20-60) membranes exhibit lesser degree of swelling as compared to that of the VMSG (20-60) membranes, which is due to the low content of (−COOH/−COONa) of ion exchange groups and greater cross-linking ability of GMSG. The favourable swelling resistance in the GMSG membrane is attributed to the advantages of GMSG over VMSG.

DD performance at 25 °C
The prepared membranes were employed to separate NaOH/Na2WO4 aqueous solution (1M NaOH/0.1M Na2WO4) as model aqueous waste feed for analyzing the DD performance and their potential use for application in alkali recovery. The dialysis coefficient of NaOH (UOH) and separation factor (S) results at 25 °C for membranes having different wt.% of GMSG or VMSG in PVA matrix are shown in Fig. 6.
It has been observed that UOH values increase gradually by increasing GMSG/VMSG amount, which is attributed to −COOH/−COONa groups in membrane matrix which are necessary for the DD process as they are able to migrate Na + ions through membrane to dialysate, therefore the through the membrane matrix [62]. The transport mechanism of OH − ions through the PVA membrane can be better explained by adsorption-desorption mechanism. This mechanism involves three steps: adsorption of OH − ions into the membrane by making the hydrogen bond with the −OH groups presents in the membrane, breaking the hydrogen bond and desorption of OH − ions from the membrane [25]. The complementary effect of -OH group of PVA and -COOH group from GMSG, VMSG form a network of hydrogen bonding with the OH − ion, enhance the permeability of OH − ions. The VMSG membrane series showed higher UOH values (due to its higher swelling) as compared to corresponding GMSG cross-linked membrane series as indicated in Fig. 6. These results were in agreement with low cross-linking and high IEC values which are beneficial to transport OH − ions. Furthermore, ion exchange capacity of membranes highly influences on UOH values.  It is also interesting to that with the increase in temperature to 45 °C; S values obtained are lower than the values obtained at 25 °C. The decrease in S values is due to the different diffusion mechanism of OH − and WO4 2− ions at high temperature. The diffusivity of WO4 2− ion is very low at normal temperature for its large size and higher valence. However, increase in temperature can accelerate the diffusion of both OH − and WO4 2− ions. The increase in diffusivity of WO4 2− ions with temperature is more significant than OH − ions [11,34] resulting in decrease of membrane selectivity at high temperature.
Finally, the comparison of our work was done with other similar membranes reported in the literature for alkali separation. From the comparison, it can be easily seen that our membranes perform better than many other membranes and can be potential candidates for alkali separation applications in the industry.  properties, indicating their great potential for industrial applications particularly for recovery of alkali from waste water by diffusion dialysis.

Funding
There is no special funding for this project.