TFC solvent-resistant nanofiltration membrane prepared via a gyroid-like PE support coated with polydopamine/Tannic acid-Fe(III)

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Abstract

Solvent resistant nanofiltration (SRNF) is now a powerful tool for addressing environmental issues. Hence, we report the fabrication of a thin film composite (TFC) membrane comprised of a novel HDPE support, a polydopamine (PDA)/Tannic acid-Fe(III) interlayer and a polyamide (PA) skin layer. In this respect, high density polyethylene (HDPE)-polystyrene (PS)-styrene-ethylene-butylene-styrene (SEBS) blends were formed with different compositions by mixing via a Brabender machine and making films by using a hot press instrument. Next, a solvent extraction technique was employed for extracting the dispersed phase and making the HDPE membranes. The support membrane with optimum properties was coated with a PDA interlayer. A tannic acid-Fe(III) interlayer was also formed on the as-prepared PDA layer so that the hydrophilicity of the support surface was enhanced to form a defect-free PA layer. The modified support was utilized for the fabrication of the PA top layer by using m-phenylene diamine (MPD) and trimesoyl chloride (TMC) monomers. The prepared TFC membrane provided a significant dye rejection ability (99.9% Direct Yellow, 99.7% Methyl Green, 99.2% Rhodamine B, and 96.1% Methyl Orange), extraordinary solvent resistance ability in harsh solvents, and good methanol (MeOH) Flux (2.25 L/m2.h.bar) in SRNF applications. The skin-substrate adhesion strength of the top layer was also evaluated by a back-ward flush operation. It was demonstrated that the interlayer and the skin layer had an excellent adhesion with the support membrane.

Introduction

Nanofiltration (NF) is one of the efficient pressure-driven membrane methods which have become the center of attentions in recent decades, originating from its separation characteristics which is in the range between reverse osmosis (RO) and ultrafiltration (UF) membranes. Hence, this method is particularly promising for separations on a molecular level [1], [2], [3], [4]. Nanofiltration of non-aqueous solution mixtures, which is known as solvent resistant nanofiltration (SRNF), has nowadays attracted many interests, originating from the appearance of environmental issues and the consequent requirement for finding cleaner and more energy-efficient methods [1], [5]. SRNF technology is able to be utilized in an extensive range of strategic industries, including petrochemical, fine chemical, pharmaceutical, food and semi-conductor, arising from its great potential as an efficient alternative for solvent consuming, energy-intensive, and waste forming distillation, extraction, chromatographic separation, evaporation and crystallization techniques not only for separating desired molecules from solvents but also for recovering solvents and solutes from different waste solutions [6].

Polymers are considered as interesting high potential materials for SRNF applications because of possessing several merits including superb structural diversity, process-ability, low fabrication cost and easy scale-up [7]. Nevertheless, it is well-known that only some of these polymers can be stable in harsh non-aqueous media and many of them swell or dissolve, which leads to a significant loss in membrane selectivity [2]. Hence, it is vital to assure solvent resistance by the polymeric membrane structure, and therefore, the chemical stability of the employed polymer is of key importance [8]. In recent decades, two class of polymeric membranes, i.e. integrally skinned asymmetric (ISA) membranes and thin-film composite (TFC) ones, have been widely employed in the formation of SRNF membranes [9], [10], [11]. Furthermore, if nanoparticles (NPs) are incorporated in the ISA membrane matrix or the thin film, mixed matrix membranes (MMMs) and thin-film nanocomposite (TFN) ones, respectively, can be prepared [12], [13].

Thin film composite (TFC) membranes are made up of two main parts including a cross-linked selective top layer, which is a polyamide (PA) thin film formed by interfacial polymerization (IP) [14], and a porous support [8], [15]. The PA top layer is usually fabricated by using m-phenylenediamine (MPD) and trimesoyl chloride (TMC) as monomers [16], [17]. It is also worth mentioning that sometimes an interlayer is also applied between the above-mentioned parts to bind them with stronger interactions [18], [19]. For this purpose, polydopamine has become the center of attention to surface functionalize membranes and other types of materials. Dopamine, inspired from the adhesive proteins of mussels, is able to self-polymerize in alkaline aqueous media and form hydrophilic polymer films that adhere onto all material surfaces [20]. Tannic acid, a typical kind of natural polyphenols which is extracted from multiple plants, is another material which is especially used with Fe(III) for making a nanoporous TA-Fe layer as a scaffold for further IP reaction to form PA layers. Tannic acid is able to be easily crosslinked with Fe3+ ions through coordination bonds, leading to the formation of a continuous and robust thin film on various substrates. In comparison to most existing surface modification strategies which are composed of multiple steps and toxic chemicals, this route of TA-Fe3+ coordination complexes is simple, rapid and green [21], [22], [23].

It is also noteworthy that most of the TFC membranes prepared for SRNF applications have been made by IP reaction on the supports such as cross-linked polyimide [24], hydrolyzed polyacrylonitrile [25], cross-linked silicon [26] and cross-linked polybenzimidazole [27]. Nonetheless, it is an urgent need for the aforementioned polymers to be cross-linked so that they can be chemically stable in a harsh non-aqueous environment [28]. Hence, cross-linking is considered as an extra step which needs great amounts of solvents, time, and post-treatments [29]. Polyethylene (PE) has been applied as an efficient membrane material for the preparation of supports in TFC membranes. PE has numerous outstanding properties such as great chemical and mechanical inertness, ease of processing and low preparation cost [30]. The mechanical and physical properties of PE is considerably dependent on several variables including crystallinity, the extent and type of branching, and the molecular weight. By increasing the crystallinity and molecular weight, and by decreasing the extent of branching, the mechanical strength of PE increases [31], leading to the preparation of a durable and stable SRNF membrane [18]. Additionally, PE is one of the most extensively utilized thermoplastic around the world, and therefore, it is produced in huge amounts, by which commercialization of PE membrane supports for a wide range of SRNF applications can be easily done [30], [31]. PE membranes are capable of affording a uniform pore morphology, which can lead to the fabrication of a defect-free PA layer, as well as great surface porosity, which can result in a great improvement in the mass transport in the interface between the top layer and the support membrane [32]. Park et al. made use of a PE separator as the membrane support for the preparation of an efficient TFC SRNF membrane through IP reaction, based on which it was discerned that the prepared TFC membrane was capable of providing good mechanical and chemical inertness, excellent rejection ability and great organic solvent resistance [33].

Herein, a thin film composite (TFC) membrane made up of a novel HDPE support, a polydopamine (PDA)/Tannic acid-Fe(III) interlayer and a polyamide (PA) skin layer was formed. Accordingly, high density polyethylene (HDPE)-polystyrene (PS)-styrene-ethylene-butylene-styrene (SEBS) blends were first prepared by using a Brabender machine and a hot press instrument. Next, a solvent extraction technique was employed for extracting the dispersed phase and making the HDPE support membranes. The support membrane with the optimum properties was then immersed in a solution containing dopamine monomers so that the support surface was coated with a polydopamine interlayer through a self-polymerization approach. A tannic acid-Fe(III) interlayer was also formed after the formation of the PDA layer so that the hydrophilicity of the support membrane surface could be improved for the purpose of generating a defect-free PA skin layer. It can be expected that the fabricated TA-Fe(III) layer possesses an excellent stability on the support surface owing to the hydrogen bonding interactions present between the PDA layer and TA molecules. In the final step, the modified support membrane was coated with a PA skin top layer by using m-phenylene diamine (MPD) and trimesoyl chloride (TMC).

Polystyrene was obtained from Dow. Polyethylene (HDPE) was purchased from Ilam Petrochemical. SEBS compatibilizer was also obtained from Shell (Table S1). Tetrahydrofuran (THF; 99%), Dimethylformamide (DMF; 99.5%), and methanol (>99.7%) were supplied via Mitsubishi Chemical. Acetone (>99%), tannic acid, Dimethyl sulfoxide (DMSO), sodium dodecyl sulfate (SDS; 99%), methyl orange, isopropyl alcohol (IPA; 99.9%), and m-phenylenediamine (MPD; 99%) were supplied by MERCK. Antioxidants including Irganox 1010 and Irgafos 168 were all bought from BASF. Rhodamine B (>99%), n-hexane (99.5%), direct yellow, dopamine (98%), FeCl3.6H2O (>98%), and trimesoyl chloride (TMC; 98%), were obtained from Sigma-Aldrich. Methyl green and ethanol (99.7%) were obtained from PubChem.

In this research work, we made use of the same method employed in our previous studies [34]. Blending immiscible polymers including PE and PS, antioxidants of Irganox 1010 and Irgafos 168 along with the compatibilizer of SEBS was done by using a Brabender machine, fixed at 200 °C and 60 rpm, for 14 min. Next, after complete mixing, the blend was taken out of the Brabender chamber, and for freezing-in the obtained morphology, it was rapidly quenched with cold water.

In the next step, the prepared blends were converted to sheets (thickness = 200 ± 10 μm) through a hot press machine, fixed at 200 °C and 180 bar for 7 min. Similar to the blending route, for freezing-in the attained morphology, the prepared sheets were rapidly quenched with cold water. Blends with different PE, PS and SEBS compositions were prepared for the purpose of discovering the optimum composition as the support for making a TFC membrane with outstanding abilities (Table 1).

The fabrication of PE membrane supports was done similar to the method employed in our previous studies [25], [50]. In short, the blends prepared in the previous section were dipped in THF under stirring for 80 h, by which the dispersed phase including PS and SEBS was extracted from the blend, leading to the formation of gyroid-like co-continuous membranes (Scheme 1) whose pore sizes were equal to the phase sizes of the dispersed phase. As we utilized different compositions for the preparation of the blends, different membranes with a wide range of pore sizes, porosities and water fluxes were prepared in the range of nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF), as listed in Table 1.

Based on Table 1, the more the PS content increased, the more pore sizes and porosities also increased, leading to higher fluxes by the membranes. Accordingly, the 75-25-30, 85-15-30 and 95-5-30 supports, which were made by the lowest content of PS among all the membranes, exhibited the lowest pore sizes and porosities, and so, possessed no water fluxes by applying the pressure of 20 bar. Additionally, it is noteworthy that water flux of supports decreased through increasing the SEBS content in the blends composition, as a result of improving the compatibility of PE and PS, and hence, reducing the pore sizes. It should be also noted that the use of no compatibilizers in the prepared blends (70-30-0 support) resulted in the preparation of membranes with much different water fluxes and porosities, probably for the non-compatibility and wide range of pore sizes. Here, the 70-30-30 composition showed better properties as the TFC support when compared with the other compositions, and therefore, the TFC membranes were prepared by the 70-30-30 support.

The surface of the PE support, prepared by our new method, was modified through immersing in a MeOH solution (60 mL) for 50 min, followed via contacting with 1.9 mg/mL dopamine solution for approximately 14 h, where a solvent of 10 mM Tris-HCl buffer solution was used with the pH of 8.5. In the next step, the PDA-coated PE membranes were taken out of the dopamine solution and washed with distilled water in order to remove all the residual MeOH molecules, followed via drying at 50 °C for 10 h (Fig. S1).

For the purpose of fabricating a TA-Fe(III) layer on the surface of the as-synthesized PE@PDA membrane, preparation of tannic acid and Fe(III) aqueous solutions with certain concentrations was done, in which a fixed ratio of TA/Fe (1:3) was employed. It was discerned that the best results were provided via the concentrations of 2.5 g/l and 7.5 g/L for tannic acid and FeCl3, respectively, and therefore, they were considered as the optimum ones. Accordingly, the surface of the as-synthesized PDA-coated PE membrane was contacted with a tannic acid aqueous solution for 240 s, and subsequently, the residual tannic acid solution was removed and washed with distilled water. In the next step, the membrane surface was contacted with FeCl3 solution for 150 s, and subsequently, the residual FeCl3 solution was eliminated and the membrane surface was washed with distilled water. In the final step, the prepared PE@PDA-TA membrane dried at RT for 35 min.

For fabricating the TFC membrane skin layer, the as-fabricated PE@PDA-TA support was first immersed in a 0.07 wt.% SDS aqueous solution for 1 day. Next, the membrane was placed between two round Teflon templates in which the top layer faced upwards. The aqueous diamine solution was fabricated through dissolving 2.5 wt.% of MPD and 0.07 wt.% SDS in distilled water, followed via pouring onto the surface of the membrane and contacting the surface for 9 min. It is noteworthy that the reason for introducing the SDS surfactant into the MPD aqueous solution is the reduction of the tension in the interface of the support and water molecules [5]. In the next step, the diamine solution on the membrane surface was gently drained off, followed by eliminating the excess droplets by using a roller. Subsequently, 0.14 wt.% of TMC was entirely dissolved in n-hexane for 20 min, and slowly poured onto the surface of the amine-loaded support and left to contact for 180 s. Finally, after assuring the completion of the IP reaction, the TMC solution was poured off and the prepared PE@PDA-TA-TFC membrane dried in an oven at 40 °C for 23 min. the total route employed for preparing the PE@PDA-TA-TFC membrane is depicted in Scheme 2.

Brabender (Misagh Afzar, Iran) was utilized for blending the polymers and the compatibilizer. Hot press (CUEC) technique was also employed for making films with a certain thickness via the as-prepared blends. The mechanical properties of all the prepared membrane supports were evaluated by using a tensile instrument (Santam, model STM-50, length = 20 mm, width = 10 mm, 20 mm/min test speed).

The study of other properties of the membranes was done via the following analyses: field‐emission scanning electron microscopy (FESEM, top surface and cross-section), SEM Mapping and energy dispersive X-ray detector (EDS) analyses (SIGMA VP, ZEISS, Germany); attenuated total reflectance Fourier transform-infrared spectroscopy (ATR FT-IR, Nicolet-6700, Thermo Fisher); X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI 6000C ECSA); contact angle (CA, CA-500A); atomic force microscope graphing (AFM, Nano Wizard II; Germany).

The investigation of membrane supports porosity was performed via an n-butanol uptake route based on Eq. (1) [35]:ε=(mwet-mdry)/ρs×d×100%(1)

In this Eq., ε corresponds to the membrane supports porosity, mdry and mwet are related to the mass of the dry and wet PE supports, respectively, s corresponds to the PE supports area, ρ is ascribed to the alcohol density, and d represents the PE supports thickness.

Moreover, it should be mentioned that the pore size of the PE supports was evaluated via image-j software (the size of 60 pores was measured to evaluate the average pore size).

Here, dye rejection and solvent flux measurements were carried out by using a dead-end cell, as depicted in Fig. S2. The dye rejection experiments were done via filling the cell by dye solutions, as shown in Table S2, with a fixed concentration of 30 ppm in a way that the solution contacts with the top surface of the membrane applied in the cell. Then, the solution was pushed by the pressure of 11 bar in order to permeate through the membrane. Finally, both dye rejection and solvent flux were determined by using Eq. (2) and (3), respectively.R=CF-CPCF×100%(2)F=VA×t(3)

In the above-mentioned Equations, the parameters are defined as follows: R represents the rejection (%), Cf and Cp represent the feed and permeate solution concentrations, respectively, which were calculated through UV–Vis. analysis (UV-1900, Shimadzu, Japan), F corresponds to the flux (L/m2.h), V defines the collected volume (L), A represents the effective surface area (m2) and t represents the collecting time (h).

The prepared SRNF membranes were dipped in Dimethylformamide in order to carry out a solvent activation for washing away low-molecular-weight PA fragments so that the solvent flux can be improved [36]. We set the Dimethylformamide temperature at 70 °C to speed up the activation procedure. The activation process was done for 35 min, followed by taking out of the solvent and dipping in MeOH in order to wholly eliminate Dimethylformamide molecules. The long-term process was also done in a similar way, except that it was performed at RT for 70 days in order to examine the stability of SRNF membranes.

Section snippets

HDPE support characterization

Figs. S3 and S4 shows the surface SEM images of all the prepared membrane supports. According to the results, the higher fluxes provided via the 55-45-30 and 65-35-30 supports can be due to their bigger pore sizes, which are obvious in their SEM images. Moreover, it is worth pointing out that the presence of big pores on the supports surfaces prevents the formation of defect-free, uniform PA thin film composites, as depicted in Fig. S5 [23]. Hence, the 55-45-30 and 65-35-30 supports were not

Conclusions

A thin film composite (TFC) membrane was made up of a novel HDPE support, a polydopamine (PDA)/Tannic acid-Fe(III) interlayer and a polyamide (PA) skin layer. The HDPE support membrane was fabricated by first preparing HDPE-PS-SEBS blends by using a Brabender machine and a hot press instrument, and then, extracting the dispersed phase. The support membrane with the optimum properties was then coated with a PDA and a TA-Fe(III) interlayer, respectively, and finally, it was coated with a PA skin

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support received from Iran National Science Foundation (INSF; grant number 99032465).

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