Fabrication of loose inner-selective polyethersulfone (PES) hollow fibers by one-step spinning process for nanofiltration (NF) of textile dyes

doi:10.1016/j.memsci.2017.07.016

Journal of Membrane Science

Volume 541, 1 November 2017, Pages 413-424

https://doi.org/10.1016/j.memsci.2017.07.016 Get rights and content

Highlights

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An inner selective wholly integral asymmetric PES NF hollow fiber is developed.
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Blending SPSf and using a special bore fluid are crucial to produce the fibers.
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The membrane has 70% rejection to PEG 1000 and 96% rejection to indigo carmine.
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The membranes are stable in a continuous running of at least 76 h.
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The membranes can be applied in textile wastewater treatments.

Abstract

The development of a loose inner-selective polyethersulfone (PES) hollow fiber by one-step spinning process for nanofiltration (NF) has been demonstrated in this study. It is found that the addition of a small amount of sulfonated polysulfone (SPSf) and the use of a specially designed bore fluid made of hydrophilic additives such as polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) are of great importance to fabricate this type of NF membranes. Not only are they able to reduce the pore size but also suppress the formation of finger-like macrovoids. After optimizing both dope solution and spinning conditions, the as-fabricated membrane can have a 70% rejection to PEG 1000 and a pure water permeability (PWP) of 12.8 L m⁻² h⁻¹ bar⁻¹. Its rejection of indigo carmine (MW: 466 gmol⁻¹) can reach 96.64%. In addition, it shows good stability in a continuous 76-h operation with good efficiency of dye removal. The newly developed membrane may have great potential in the treatment of textile wastewater.

Graphical abstract

Introduction

Around 80% inhabitants on earth are facing severe water scarcity [1]. Many efforts have been devoted to produce high quality water worldwide, including the treatment of industrial and domestic effluents before discharge or reuse by means of membrane processes. Nanofiltration (NF) is a membrane filtration process placed between reverse osmosis (RO) and ultrafiltration (UF) with a membrane pore size of 0.5–2 nm [2], [3], [4]. Due to its unique rejection mechanisms of size exclusion and charge repulsion [3], NF is able to offer good rejections to a variety of charged solutes with a higher water flux compared to RO membranes [5]. As a result, NF membranes have found extensive usage in the removal of dye solutes, pharmaceuticals and heavy metals from wastewater [6], [7], [8], [9], [10], [11], [12].

Generally, NF membranes have two configurations; namely, flat sheets and hollow fibers. Most commercially available NF membranes are in the form of flat sheets because of easy fabrication. However, hollow fibers have received more attention than flat sheets in terms of R&D because the former offers a higher surface area to volume ratio, higher packing density and more self-support characteristics than the latter [13]. Depending on fabrication methods, NF hollow fiber membranes can be classified as (1) thin-film composite (TFC) membranes and (2) wholly integral asymmetric ones. The TFC membranes are normally formed by the interfacial polymerization of monomeric amines and triacyl halide on a microporous support [14], [15], [16], [17], [18], [19], while the wholly integral asymmetric membranes are fabricated by the Loeb-Sourirajan non-solvent phase inversion method [13].

So far, most commercially available NF membranes are made from interfacial polymerization, but the majority of them are in the configurations of flat sheets and tubular membranes [20], [21]. To our best knowledge, there is no commercially available NF hollow fiber membrane made from interfacial polymerization [22]. This is probably due to the difficulties in conducting the thin-film polycondensation reaction in fine hollow fibers. Although several approaches have been proposed to overcome the difficulties [23], [24], [25], [26], [27], it takes time to develop a cost-effective method to fabricate NF hollow fiber membranes by means of interfacial polymerization.

Wholly integral asymmetric hollow fiber membranes can provide such a solution to the above issue. It not only abridges the complicated process in interfacial polymerization of hollow fibers, but also simplifies the spinning process by using just a dope solution. Several materials such as polyimide, polyethersulfone (PES), cellulose/cellulose acetate (CA) and polybenzimidazole (PBI) have been employed to fabricate NF hollow fiber membranes [2], [3], [5], [7], [9], [10], [28]. However, compared to polyimide and PBI, it is difficult to fabricate PES NF membranes through a one-step spinning process. The major obstacle is to reduce the membrane pore size to an NF range. To our best knowledge, only two studies on the fabrication of PES NF hollow fiber membranes could be found by means of one-step spinning process in the literature [29], [30]. Both of them produced outer-selective NF hollow fiber membranes. Since inner-selective NF hollow fiber membranes have advantages of a better distribution of the feed and less channelling phenomenon than outer-selective ones, the aims of this study are to (1) efficiently reduce the membrane pore size and fabricate loose inner-selective PES NF hollow fiber membranes by a one-step spinning process; and (2) investigate the effects of dope composition, bore fluid and spinning conditions on pore size and water permeability of the formed PES membranes.

PES was chosen as the membrane material in this study because it not only possesses good chemical, thermal and hydrolysis stability [31] but also has a reasonable cost compared to PBI and polyimides. To control the pore size, two methods would be explored. Firstly, a sulfonated polysulfone (SPSf) would be utilized to blend with PES as the spinning material. Not only can they blend well because of similar basic chemistry [32], but also potentially reduce the pore size, improve water permeability and rejection of the membrane [31], [33]. Secondly, a special bore fluid consisting of hydrophilic additives, polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG), would be employed in this study. According to previous studies [24], [34], [35], [36], hydrophilic bore fluids may affect the phase inversion near the inner skin and manipulate the pore size. Therefore, the effects of bore fluid chemistry on membrane morphology would be studied and the separation performance of the resultant hollow fibers for dye separation would be evaluated. This study may provide useful insights to design next-generation NF hollow fiber membranes for the recycle of textile wastewater and the removal of total organic carbon (TOC) from wastewater.

Section snippets

Materials

To prepare the polymer dope for spinning, PES (E6020P, M_W 53 K gmol⁻¹, BASF, Germary) and SPSf (M_W 90–95 K gmol⁻¹, 20% sulfonation degree, FHM China) were added to a homogeneous mixture of PEG (M_W 400 gmol⁻¹, Merck) and N-methylpyrrolidone (NMP, 99.5%, Merck, Germay). PVP K90 from Fluka (M_W 360 K gmol⁻¹) was used to prepare the bore fluid. After spinning, a mixture of glycerol and deionized (DI) water was employed to treat the as-spun membrane in order to minimize pore collapse. To test the pore size

Overall cross-section morphology of PES and PES/ SPSf membranes

To investigate the influences of SPSf and different bore fluids on hollow fiber formation, the first batch of hollow fibers were spun using different bore fluids at the same take-up speed of 0.9 m/min and air gap of 10 cm. The SPSf amount in the PES/ SPSf membranes was fixed at 10 wt% of the total polymer content. Fig. 2 shows the cross-section morphology as a function of bore fluid chemistry, i.e., water, NMP/ water, NMP/ PEG 400/ water and mixtures of NMP, PEG 400, PVP K90 and water. The

Conclusions

An inner selective NF hollow fiber membrane made of PES/ SPSf has been fabricated in a single-step spinning process for dye removal from wastewater. To effectively reduce the membrane pore size and form a sponge-like macrovoid-free structure, the effects of SPSf and a specially designed bore fluid, Bore X, were investigated. The dope composition was also optimized in order to reduce the membrane pore size. The following conclusions can be made:

(1)
By blending 10 wt% SPSf into PES dopes, a dope with

Acknowledgement

This research is supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Program, administered by PUB, Singapore's National Water Agency under the project “Advanced Development of Nanofiltration-Hollow Fiber membranes and their applications” (1301-IRIS-60) with NUS grant No. R-279-000-451-279. The authors would also like to thank Mr. C.Z. Liang, Ms. L. Luo, Dr. D. Hua, Dr. J.C. Su, Dr. X. Li, Dr. S. Japip and Dr. Y.K. Ong for all

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Fabrication of loose inner-selective polyethersulfone (PES) hollow fibers by one-step spinning process for nanofiltration (NF) of textile dyes

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials

Overall cross-section morphology of PES and PES/ SPSf membranes

Conclusions

Acknowledgement

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