Development of composite thin-� lm nano � ltration membranes based on polyethersulfone for water puri � cation

The development of thin-film composite membranes has gained more importance as they exhibit higher permeability, higher solute rejection and improved antifouling behavior. The incorporation of hydrophilic nanomaterials within the composite thin film has reported to effectively increase the hydrophilicity and improve the membrane performance. In this study novel thin-film composite nanofiltration membranes were prepared by interfacial polymerization of trimesoyl chloride, polyvinylpyrolidone (PVP) and montmorillonite (MMT) composite on the porous polyethersulfone membrane. The effect of preparation parameters was studied as; reaction time, PVP concentration and MMT content. The properties of the prepared composite thin-film membranes were analyzed in terms of their chemical structure, surface morphological features, pure water contact angle, zeta potential, pure water permeation flux (PWP) and solutes rejection. The prepared composite thin-film membranes showed smoother surfaces, different surfaces functionality and amphoteric surfaces with points of zero charge at pH 7–7.5. The PWP was found to increase with increasing MMT content up to 0.10 wt%. The rejection of crystal violet dye (CV) was studied using the prepared membranes. The membrane with 0.5% PVP, 0.10% MMT and 6 min reaction time showed PWP of 18.1 L/m2 h bar and CV rejection of 80.01%. The rejection of Na2SO4 and MgCl2 increased with increasing MMT content and reached 96% and 35.4%, respectively. The steady state flux was decreased by 5% with increasing MMT content from 0.10 to 0.12 wt% indicating improved antifouling behavior with MMT.


Introduction
The dying process and dying industries produce a mass of wastewater containing dyes and salts as sodium chloride (NaCl) and sodium sulfate (Na 2 SO 4 ) which are used in the dying process. The wastewater produced from dying industry must be treated and separated using e cient separation methods. The organic dyes do not degrade naturally and the presence of dyes in aqueous systems prevents the entrance of light to water, which affect the ecosystems [1,2]. Crystal violet (CV) is one of the dyes used in paint industry, textile industry and biotechnology, so it comes to the aqueous system through the e uent of these industries. CV has many hazards as it has a known mitotic poisoning nature, mutagenic and teratogenic effects. Many procedures were studied for dye removal from aqueous solutions.
Membrane technology was considered as an attractive separation process for wastewater treatment and decontamination. Membrane technology is developed increasingly and applied in different elds [3].
Membrane separation processes have important characteristics as high e ciency, feasible operation and produce low environmental pollution.
Nano ltration (NF) membranes have been studied for industrial water treatment, water desalination [4][5][6][7][8] and pharmaceutical industries [9,10]. The NF membranes should verify certain permeability, retention, mechanical stability and operational stability. Different procedures have been potentially studied for developing NF membranes through deposition of surface nanocomposite layer or interfacial polymerization of composite layer on porous support [11]. Different methods were applied for membrane development and fabrication of certain unique surface layer structure as; interfacial polymerization, dip coating, and layer by layer assembly [12,13]. Nano ltration membranes have showed e cient performance and wide applications in water puri cation and wastewater treatment [14,15]. The NF technology showed e cient removal of per uorooctanoic acid (PFOA), achieved a signi cant environmental impact, applied in drinking water decontamination and ground water puri cation [16,17]. The process was shown to be affected by size exclusion and electrostatic interactions as well, where PFOA has relatively higher molecular weight (414 g/mol). The rejection of PFOA using NF membranes reached 90% and was found to be affected by membrane surface charge through electrostatic repulsion mechanism [18].
Composite thin lm NF membranes were prepared by interfacial polymerization of sericin polymer and trimesoylchloride (TMC) onto commercial porous support of polysulfone [19]. This study declared that the fabricated composite thin lm supported membranes has smooth surface with isoelectric point of pH 4.1 and showed salt rejection of 22.5%, 40.5%, 40.8% and 95.4% for MgCl 2 , MgSO 4 , NaCl and Na 2 SO 4 , respectively at neutral pH. Composite thin lm of organically bridged silica was deposited on commercial polymeric membrane (NTR-7450) [20] using a low temperature sol-gel spin-coating curing process. The produced composite thin lm NF membranes were studied for the vapor permeation dehydration of water-isopropanol solutions, and the results declared a water ux of 2.3 kg/(m 2 h) and showed highly enhanced separation factor of 2500.
NF membranes with surface composite thin lm have been prepared by interfacial polymerization of polyethyleneimine and trimesoyl chloride onto microporous supporting of polyethersulfone [21]. The prepared membranes showed improved performance as; high salt rejection (95.1% for MgCl 2 , 94.4% for MgSO 4 , 85.1% for NaCl and 80.5% for Na 2 SO 4 ) and higher pure water permeation ux of 24.5 L/(m 2 h).
Poly(vinyl alcohol)/poly(vinylidene uoride) composite membranes modi ed with TiO 2 were prepared through dip-coating and applied for dye removal and wastewater treatment [22]. These membranes showed sharp performance enhancement in dye removal, salt rejection and antifouling properties.
Montmorillonite (MMT) is a layered silicate and can be easily assembled into stacked layers with de ned interlayer distance providing good separation possibilities in membrane techniques. MMT has been studied as reinforcement material in polymeric composites and different matrices for various applications. The MMT polymeric nanocomposites showed important properties enhancements as liquid barrier and mechanical stability [23][24][25][26]. MMT has good swelling properties, high cation exchange capacity and has been studied for water treatment. The embedding of nanoparticles within NF membranes has been provided higher hydrophilicity, higher membrane performance and improved antifouling properties as well [27,28]. It has been reported that the incorporation of MMT within PVC membrane showed improved performance with respect to water ux, salt rejection and antifouling properties [29]. Different studies have been performed for membrane modi cation using TiO 2 nanoparticles [30]. The nanoparticles modi ed membranes showed highly improved antifouling characteristics and high absorption properties. PVA/PSE membrane modi ed by depositing surface layer of TiO 2 nanoparticles were prepared and showed higher ux and higher salt rejection than the unmodi ed PVA/PSE [31].
In this study commercial polyethersulfone (PES) membrane was modi ed by interfacial surface polymerization of composite thin lm of trimesoylchloride (TMC)/polyvinylpyrrolidone (PVP)-Monmorillonite (MMT). The composite thin lm membranes properties were optimized in terms of reaction time, PVP concentration, MMT content and consequently the membrane performance. The structural features and surface properties of the produced membranes were analyzed by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), atomic force microscope (AFM) and surface zeta potential. The membranes performance was studied in terms of pure water permeation, salt rejection and crystal violet (CV) dye rejection. The antifouling properties were studied through antifouling experiment.

Experimental
Materials: Polyethersulfone (PES) ultra ltration membranes with diameter of 47 mm and pore size of 0.45 µm was purchased from GVS, USA, m-phenylenediamne (m-MPD) was obtained from Sigma Aldrich, trimethoyl chloride (TMC) was obtained from Sigma Aldrich, and hexane, dimethylformamide (DMF) as solvents were obtained from Across organics. Montmorillonite (MMT) was purchased from Sigma-Aldrich.
Composite thin lm membranes preparation: The PES membranes were soaked in distilled water for 5 min followed by immersion in m-PDA solution (2 wt. % in deionized) for 2 min. PVP solution of (1 wt. % in DMF) was mixed with varied amount of MMT, Membrane characterization: The ltration performance of the fabricated membranes was measured by a laboratory designed dead end ltration device. The effective testing membrane area was 12.56 cm 2 . Firstly, the membrane was prestabilized for 20 min with the feed sample. Appropriate concentration of CV dye 100 mg/L or the concerned salts (MgCl 2 and Na 2 SO 4 500 mg/L) were applied as feed solutions. These solutions were passed through the membrane at 40 psi, where the permeation ux (J, L/m 2 ·h) and rejection (R, %) were calculated by the following equations: Where V is the permeated volume, A is the effective membrane area and ∆t is the permeation time period.
where C f is the solute concentration in the feed solution and C p is solute concentration in the permeate solution.
The salt solution concentrations after and before permeation were measured by an electrical conductivity meter, where, CV dye concentrations were measured using UV-visible spectrophotometer.

Antifouling experiment
The antifouling properties of the composite thin-lm membranes were studied through permeation experiments using aqueous solution of sodium alginate (SA) with 100 mg/L as fouling agent solution.
The fouling results of the membranes were presented in terms of the normalized permeate uxes with time J t /J o , where, J o and J t are the water uxes at initial and after time t of the fouling test, respectively.
The values of the normalization ux should declare the antifouling behavior of the studied membrane and re ect the fouling agent deposition onto the membrane surface.

Results And Discussion
Preparation of composite thin PES-TMC/PVP membranes: Composite thin lm membranes of PES-TMC/PVP-MMT were prepared on the front side of porous support PES membrane through interfacial polymerization. The lm thickness was controlled by limiting the reaction time between PES and TMC/PVP-MMT and the `membrane performance was studied as a function of lm thickness. The thin lm has an important role in the structural properties and morphology of the membrane interface and consequently the membrane performance [32].
The reaction time, PVP concentration and MMT content, were optimized with respect to the membrane performance. The effect of reaction time and PVP concentration on the membrane performance are given in Table 1.
( ) h.bar was obtained. The C.V. rejection was found to be clearly improved from 68.9% at reaction time of 2 min to be 79.5% at reaction time 6 min, which refer to the increased thickness of the interface, polymerized composite thin lm. The effect of MMT content in the composite thin lm on the membrane performance was studied at xed reaction time of 6 min and xed PVP concentration of 0.5%, and the results are given in Table 2. The results given in Table 2  increase with MMT and consequently enhance the water permeation through the thin-lm [33][34][35]. The results given in Table 2 clearly illustrate the signi cant effect of the interfacial thin lm content on the membrane performance (water ux and salt rejection). It could be deduced that the improved structural and morphological properties affect the hydraulic resistance and so affect the water permeation.
The effect of MMT content on the permeation ux was presented in Fig. 1, re ecting important role of MMT on the pure water permeability.
The membrane surface hyrophilcity was measured through determining the pure water contact angel against the MMT content in the composite thin lm and the results are given in Fig. 2 The surface morphological features of the produced composite thin lm membranes were studied using SEM and AFM. The SEM images of PES and PES-TMC/PVP-MMT are presented in Fig. 4(a-f). The images show that the interfacial polymerization of TMC/PVP-MMT onto the PES surface produces highly dense and smooth surface and declared the formation of surface active layer of TMC/PVP-MMT onto the porous surface of PES. The SEM images of PES-TMC/PVP-MMT demonstrate the formation of thin lm with considerable thickness, and no particles agglomeration. The presence of MMT showed no clear folds or wrinkles, where PVP could coat the surface composite thin lm. The SEM images refer to the fast growth of the surface thin lm, which indicate that the polymerization may occurred not only on the PES surface but also within the MMT layer which cause more layer thickness and decrease the MMT interlayer distance. However, the improved surface properties due to the presence of MMT content appeared up to 0.1 wt% and the effect diminishes at higher MMT content of 0.12 wt%. The presence of MMT with low content in the thin lm composite may increase the thermodynamic incombatibilty between polymer and solvent due its hydrophilic nature, which could affect the surafce morphological properties [36]. The hydrophilic properties of MMT particles may also affect the entrance of water into the membrane body and facilitate the water-solvent exchange and so decrease the formation of sponge like structure, while instead larger nger like macro-voids could be formed [37].
Furthermore, the variation of pure water permeability relative to the variation was of pure water contact angel depending on MMT content refer to the potential surface structural and morphological changes due to the interfacial polymerization of the composite thin lm [38][39][40].
The AFM images of the prepared membranes are given in Fig. 4(g-j) depecting further information about the membrane surface morphology. The images clearly depict that the surface of the composite surafce layer appeared smoother after the formation of thin lm composite. The surafce roughness was determined as the root mean square roughness (RMS) to indicate the surface roughness of the prepared composite membranes and was found to be 51.16 nm for PES and 48.28 nm for surface modi ed membarne PES-TMC/PVP-MMT.
These results refer to a smooth surface layer produced through interfacial polymerization, which may due to the limited depth of the reaction and the formation of barrier layer. The smooth surface represent an advantage for the membrane fouling resistance [36].
The surface charge is an important parameter depending on the membrane content which could affect the membrane performance. The prepared composite membrane surface charges was studed by measuring the potential at different pH (3.5-10) and the results were given in Fig. 5. Obiviously, the prepared membranes with interfacially formed surface composite thin lm showed amphoteric surafce with point of zero charge at pH 7-7.5. The results showed that the membranes are positivly charged at pH below 7 and are negatively charged at pH higher than 7.5 and the isoelecrtic points were found shifted towards higher pH with increasing the MMT content.
The salt rejection behavior of the PES-TMC/PVP-MMT with varied MMT content ratio was studied through permeation experiments for salt solutions containing MgCl 2 and Na 2 SO 4 . The results presented in Fig. 6 showed that the rejection for Na 2 SO 4 was found to be greatly higher than that for MgCl 2 . The rejection of electrolyt solution is related to membrane pore size and the electrostatic interaction between the membrane and the diffusion coe cient of the salts [20,41]. The observed high rejection for Na 2 SO 4 than MgCl 2 could be explained due to membrane surface charge and the effect of diffusion coe cient of studied salts as well. Where, the diffusion coe cient in the membrane can be considered to be approximately as in aqueous solutions. The diffusion of Na 2 SO 4 is slightly lower than that for MgCl 2 which contribute in the higher rejection of Na 2 SO 4 .
Crystal violet dye removal characteristics of PES-TMC/PVP-MMT membrane: The rejection behavior of CV dye from aqueous solution was studied using the prepared composite thin lm membranes PES-TMC/PVP-MMT containing different MMT content ratio (PES-NF3, PES-NF4, PES-NF5). The permeation of CV dye solution (100 mg/l) was studied at pH 7 and 0.276 MPa. The dye solution permeate ux was studied at different permeation time and the results are given in Fig. 7. The results in Fig. 7 showed that the PES-TMC/PVP-MMT membranes effectively remove the CV dye especially NF5 and the dye solution ux decreased clearly with increasing the MMT content in the interfacial polymerized thin lm and reached steady state. It was reported that both the electrostatic interaction and steric hindrance highly affect both the rejection and permeation of CV dye via PES-TMC/PVP-MMT membrane [42,43]. The results showed slow rate of ux decrease, which re ect the potential antifouling behavior of the composite thin lm based membrane towards the CV aqueous solution and the possible high rejection of higher molecular weight organic dyes. This nding could be explained to be due to the surface smoothness of the surface thin lm and the electrostatic interaction under these experimental conditions and the dye predominate species under the experiment pH. The results in Fig. 8 showed that the membranes uxes for SA solutions decreased sharply followed by slow decrease with time. These reults could be due the deposition of the fouling agent molecules on the surface of membranes till reaching the steady state between 15-25 h. The steady state ux for PES-NF5 membrane decreased with ratio of 5% for SA aqueous solutions. These ndings re ect the role of MMT content in the composite thin lm in improving the antifouling behavior of the membrane. The membrane with higher MMT content (PES-NF5) showed better performance in fouling resistance. It was previously reported that the fouling agents could be adsorbed onto the membrane surface via hydrophobic actions, hydrogen bond, elecrostatic interactions and van der Waals forces. Consequently, the fouling effect could be vanshed through decreasing the adsorptive driving forces and enhancing the repulsive interactions of membrane surface with the fouling molecules [44][45]. Notably, the membrane surface tends to be negatively charged under the speci ed testing conditions so, certain electrostatic repulsive forces could occurre between membrane surface and the fouling agent, resulting in lower fouling effect.
The presence of the MMT within the interfacial composite thin lm showed improvement in antifouling behavior, resulting in higher ux recovery ratio (FRR). These results could be due to the increased hydrophilic spots within the composite thin lm, which inhibit the interaction of fouling molecules with the membrane active surface groups. It has been reported that the surface hydrophilicicty is a limiting parameter for surface adsorption behavior [46-47]. The increased hydrophilicity by the presence of MMT was con rmed also by the decreased water contact angel with increasing of MMT concentration.
The fouling reversibility and membrane durability are mainly dependent on the weak bonding between the fouling molecules and the membrane surface. Consequently, the membranes could be easily washed out and re-applied in repeated fouling experiments to analyze the membrane stability and durability. The SA solution was applied as feed solution till steady state followed by washing all the permeation equipment and the used membrane by distilled water. The washed membrane was applied in pure water permeation to evaluate the pure water ux and determine the ux recovery ratio (FRR %) as below: where J w2 is the pure water ux after fouling process and J w1 is the pure water ux before the fouling process.
The fouling process was repeated to analyze the membrane durability and stability and the results were given in Table 4. The results in Table 4 clearly show that the NF4 and NF5 membranes are relatively stable until the fourth fouling-cleaning cycle. The obtained results showed enhanced membrane stability, durability and enhanced FRR performance, which re ects the potential application of the PES-TMC/PVP-MMT membrane for water treatment.

Conclusion
In this study the preparation of thin lm composite nano ltration membranes was studied by interfacial polymerization of TMC/PVP-MMT on the porous support PES. Considerable lm thickness with improved properties was prepared at 6 min reaction time, 0.