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Phase Inversion Directly Induced Tight Ultrafiltration (UF) Hollow Fiber Membranes for Effective Removal of Textile Dyes Gang Han, Yingnan Feng, Tai-Shung Chung, Martin Weber, and Christian Maletzko Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05340 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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Environmental Science & Technology

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Phase Inversion Directly Induced Tight

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Ultrafiltration (UF) Hollow Fiber Membranes for

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Effective Removal of Textile Dyes

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Gang Han,† Yingnan Feng,† Tai-Shung Chung*,†, Martin Weber,‡ Christian Maletzko§

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†Department of Chemical & Biomolecular Engineering, National University of Singapore,

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Singapore 117585

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‡Advanced Materials & Systems Research, BASF SE, RAP/OUB-B001, 67056

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Ludwigshafen, Germany

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§Performance Materials, BASF SE, G-PMF/SU-F206, 67056 Ludwigshafen, Germany

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Correspondence to: T. S. Chung (Email: [email protected])

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Tel: +65-65166645; Fax: +65-67791936

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ABSTRACT: This study has demonstrated the application of tight ultrafiltration (UF)

16

membranes for effective removal of textile dyes from water at a low pressure. Novel UF

17

hollow fiber membranes with well-defined nanopores and surface charges were developed

18

via a single-step spinning process without any post-treatment. The newly developed tight UF

19

hollow fibers not only possess a small mean pore diameter of 1.0-1.3 nm with a molecular

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weight cut-off (MWCO) of 1000-2000 Da but also have a high pure water permeability (PWP)

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of 82.5-117.6 L m-2 h-1 bar-1. Through the synergistic effects of size exclusion and charge

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repulsion, the novel UF hollow fibers can effectively remove various dyes with impressive

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rejections of 93.2-99.9% at 1 bar. At the same time, more than 92% of inorganic salts (i.e., 1 ACS Paragon Plus Environment


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NaCl and Na2SO4) would permeate through the fibers, reducing the detrimental effects of

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concentration polarization and providing an attracted avenue for salts reuse. The tight UF

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hollow fibers also exhibit robust performance in a continuous operation of 170 hours or at a

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high feed recovery of 90%. The fouled fibers can be easily regenerated by backwash of water

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with a flux recovery of larger than 92%. The newly developed tight UF hollow fiber

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membranes display huge potential for treating textile wastewater and other impaired effluents

30

because of their great separation performance and simple fabrication process.

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TOC Art

33 34 35

1. INTRODUCTION

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The growing concerns about water scarcity and the awareness of environmental sustainability

37

have renewed the global interest in effective reuse and treatment of highly polluting

38

wastewater.1 The textile industry is a water intensive industry and the generated wastewater is

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chemically infused effluent which contains of dyestuffs, inorganic salts and other

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chemicals.2,3 The discharge of such highly polluted wastewater into aquatic environments not

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only causes deleterious consequences to the aquatic ecosystems and public health but also

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reduces the amount of useable water.4 As a result, the effective treatment and recovery of the

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severely polluted textile wastewater before returning it to the ecosystem have become an

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important issue. 2 ACS Paragon Plus Environment

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To date, several biological and physicochemical methods have been developed to treat the

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textile wastewater. However, most of the conventional technologies such as flocculation,

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coagulation, oxidation and biological treatments are not able to remove all the chemicals and

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dyes efficiently due to the complexity of the textile wastewater.2,5 Process integration of a

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variety of methods can help improve the separation performance, but it causes a significant

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increase in capital and operating costs.6 Low pressure membrane separation processes, such

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as ultrafiltration (UF) and nanofiltration (NF), have shown huge potentials in water

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purification and wastewater treatment because of their unique advantages of conspicuous

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separation performance, low energy consumption and cost, small footprint, and

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environmental friendly.7-11

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The UF membranes normally possess a large pore size and molecular weight cut-off

56

(MWCO); therefore ultrahigh water permeation flux can be achieved even at low pressures.

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However, the inefficient rejection and high fouling tendency of the conventional UF

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membranes significantly hinder their separation performance for textile wastewater.12-14 As

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an intermediate process between UF and reverse osmosis (RO) in terms of pore size, NF is

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another low-pressure membrane filtration process that requires a much lower operating

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pressure than RO and offers a better rejection than UF.15,16 Because of the well-defined

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nanopores (i.e., 0.5-2.0 nm in diameter) and specially tailored surface charge characteristics,

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NF membranes are able to effectively reject a variety of inorganic and organic matters via

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both the size sieving and Donnan exclusion mechanisms.17,18 As a result, NF has been

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extensively used to concentrate and purify textile dyes, pharmaceuticals and heavy metals.19-

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24

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predominantly polyamide thin-film composite (TFC) flat-sheet membranes.25 The TFC

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membranes are prepared via a two-step interfacial polymerization reaction which is quite

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laborious and complicated.26 Restrained by the chemistry and surface topology of the

Nevertheless, it is noted that the current commercially available NF membranes are

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polyamide selective layer, the TFC NF membranes usually possess a low water permeability

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and high fouling propensity.27,28 The undesired high rejections against inorganic salts would

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induce significant concentration polarization and thus lower the flux. Furthermore, when

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comparing with the flat-sheet membrane, the hollow fiber configuration provides the

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advantages of a higher surface area to volume ratio, greater packing density, self-supporting

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characteristics and spacer-free module fabrication.26

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In order to simplify the fabrication process of conventional TFC NF membranes and utilize

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the advantages of hollow fiber configuration,29 novel tight UF hollow fiber membranes with

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desirable separation characteristics have been developed in this work via a single-step

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spinning process. By molecularly tailoring the dope formulation and fiber formation, the

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newly developed tight UF hollow fibers not only have a small pore size that falls in the NF

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range and optimized surface charges, but also possess an excellent permeability that is several

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times higher than the conventional TFC NF membranes. As a result, they can provide

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synergistic advantages for resource recovery in textile wastewater treatment through the

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combination of great dye retention and high salt permeation.19 This study would provide

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useful insights for the development of wholly integral asymmetric hollow fiber membranes

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with good rejection and ultrahigh flux for wastewater treatment and water purification.

87 88

2. MATERIALS AND METHODS

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2.1 Fabrication of the Tight UF Hollow Fiber Membranes

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An optimized non-solvent induced phase separation (NIPS) dry-jet wet spinning was adopted

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to fabricate the tight UF hollow fiber membranes.30-33 Figure S1 shows the chemical

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structures of the Torlon and sulfonated polyphenylenesulfone (sPPSU) polymers used for

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fiber preparation, while Table S1 summarizes the spinning conditions. The hollow fibers spun

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from pure Torlon and a mixture of Torlon and sPPSU were named as Torlon and


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Torlon&sPPSU, respectively. The detailed procedures for hollow fiber spinning, post-

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treatment, and module fabrication are disclosed in the Supporting Information (SI).

97 98

2.2 Phase Inversion Studies of Polymer Dope Solutions

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The rheological characteristics of the polymer dope solutions for spinning and their phase

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inversion kinetics and thermodynamics were investigated through modified methods

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described elsewhere.33-35 The detailed specifications of the experimental setup and measuring

102

steps are included in the SI.

103 104

2.3 Membrane Characterizations

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The membrane morphology was observed by a Field Emission Scanning Electron

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Microscope. A SurPASS electrokinetic analyser was utilized to analyze the membrane

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surface charge properties as a function of pH. The surface chemistry of the membranes was

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examined by an X-ray Photoelectron Spectroscopy, while surface hydrophilicity was

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evaluated by water contact angle measurements using a Contact Angle Geniometer at

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23±0.5 °C. The pore size, pore size distribution, and molecular weight cutoff (MWCO) of the

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hollow fibers were determined by solute rejection experiments using polyethylene glycol

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(PEG) as a neutral rejection probe as described in SI and literatures.36,37

113 114

2.4 Pure Water Permeability (PWP), Salt Rejection, and Dye Removal Tests

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A lab-scale cross-flow UF system was employed to measure the pure water permeability

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(PWP), salt rejection, and dye removal performance of the hollow fiber membranes. The salt

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and dye concentrations in both the feed and permeate were measured by conductivity

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measurements and using a UV-vis spectrophotometer, respectively. Table S2 summarizes

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their main characteristics of the dyes including structures and molecular weights. The



120

detailed experimental setup, operating conditions, and the determination of water flux and

121

rejections are described in the SI and elsewhere.6

122 123

3. RESULTS AND DISCUSSION

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3.1 Phase Inversion Behaviors of the Dope Solutions

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Table 1 compares the viscosity and solubility parameters of the two spinning dopes, internal

126

and external coagulants as well as the coagulation values of the two dopes. In order to

127

achieve a good permeation flux, a relatively low total polymer concentration of 20 wt% was

128

applied. Although the two dope solutions have relatively low viscosities, an obvious increase

129

in dope viscosity is observed, from 6.7 to 7.7 Pa.s, when adding 2 wt% sPPSU into the

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Torlon dope solution. This is possibly due to the formation of hydrogen bonds among the

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sulfonic acid groups of sPPSU and PEG and NMP molecules through intra- and inter-

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molecular interactions, which enhances chain interaction and entanglement.33,38 The

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increased dope viscosity would affect the phase inversion kinetics and the final membrane

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structure. Thermodynamic stability of the dope solutions is indicated by their coagulation

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values. The introduction of sPPSU into the Torlon dope increases the coagulation value from

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6.7% to 7.2%, suggesting that the Torlon&sPPSU dope can tolerate a larger amount of

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nonsolvent and induce a delayed demixing during phase separation.

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The phase inversion kinetics of the dope solutions were depicted by transmittance tests. As

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illustrated in Figure S2, both dopes display very fast phase separation once they are immersed

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in bore fluid, as indicated by the rapid decrease in transmittance at the initial stage. This

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phenomenon is mainly due to their relatively low dope viscosities and large differences in

142

solubility parameter with the bore fluid. As shown in Table 1, the Torlon and Torlon&sPPSU

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dopes possess almost the same solubility parameters (i.e., 23.5 and 23.4 MPa1/2) which are


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quite different from those of bore fluid (45.3 MPa1/2) and external coagulant (47.8 MPa1/2).

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As a result, both nonsolvents would induce fast precipitation.

146 147

Table 1. The viscosity, solubility parameters and coagulation values of dope solutions, bore

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fluid and external coagulation. Torlon/ PEG400/ NMP (20/ 13/ 67, wt%)

Torlon/ sPPSU/ PEG400/ NMP (18/ 2/ 13/ 67, wt%)

Water/ NMP (90/ 10, wt%)

Viscosity (Pa•s)

6.7

7.7

9.8×10

8.9×10

Solubility 1/2 parameter (MPa )

23.5

23.4

45.3

47.8

Coagulation value (%)

6.7

7.2

-

-

-4

Water

-4

149

150 151

Figure 1. Phase inversion phenomena of flat-sheet films cast from (a) Torlon and (b)

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Torlon&sPPSU dope solutions in order to mimic the hollow fiber membrane formation in

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water.

154 155

The evolution of phase inversion and nonsolvent intrusion were further visually studied and

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shown in Figure 1. Consistent with the dope viscosity and coagulation value characteristics,

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the Torlon dope shows a faster rate of phase inversion and nonsolvent intrusion than 7 ACS Paragon Plus Environment


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Torlon&sPPSU. With the addition of sPPSU, the initiation of macrovoids is delayed at the

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precipitation front and further away from the surface. The delayed nonsolvent intrusion and

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demixing facilitate the formation of a thicker dense layer and the macrovoid-free structure.

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These results are similar to those observations in the literature,32-34,39 where the addition of

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certain amount of hydrophilic materials into dope solutions could not only enhance the dope

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viscosity, hydrophilicity and coagulation value, but also lower the phase separation kinetics

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and suppress the macrovoid formation.

165 166

3.2 Morphology and Hydrophilicity of the Tight UF Hollow Fiber Membranes

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Figure 2 and Figure S3 show the inner surface, outer surface and cross-section morphology of

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the as-prepared hollow fiber membranes. Under the optimal spinning conditions (see Table

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S1), all the fibers have excellent concentricity with an inner diameter of 650-788 µm and

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outer diameter of 960-1177 µm. Due to the fast phase inversion induced by the water-rich

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bore fluid and water external coagulant, both fibers possess a smooth and dense inner

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selective skin and a relatively porous outer surface.

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In addition, both Torlon and Torlon&sPPSU fibers exhibit a sandwich cross-section

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morphology that comprises a thin spongy-like layer in the middle of the fiber wall and two

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layers of finger-like macrovoids located at the inner and outer edges. This structure is

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attributed to the relatively low dope viscosity and intensive intrusion from both inner and

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external coagulants during the phase inversion. With the addition of sPPSU into the Torlon

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dope, the length and number of macrovoids throughout the cross-section of the

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Torlon&sPPSU fiber gradually decrease. This cross-section morphology observed from

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FESEM is consistent with the phase inversion behavior of the corresponding dope illustrated

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in Figure 1. Due to the delayed demixing characteristics, the Torlon&sPPSU fiber shows a

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thicker and denser sublayer with a fully sponge-like structure underneath the inner surface


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(see Figure 2). This sponge-like layer is helpful to provide small micropores for good

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selectivity while the finger-like macrovoids across the fiber wall are critical to minimize the

185

transport resistance for water permeation. Thus, the resultant fibers may have a high rejection

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and great water permeability.

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188 189

Figure 2. FESEM images of the inner surface and cross-section of Torlon and

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Torlon&sPPSU hollow fiber membranes.

191 192

The XPS results displayed in Figure S4 and Table S3 show the unique S element peaks on

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the inner surface of the Torlon&sPPSU fiber, confirming the successful incorporation of

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sPPSU into the membrane. Since more hydrophilic sPPSU is introduced into the Torlon fiber,

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the membrane hydrophilicity is further improved. As a result, the water contact angle drops

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from 81.7° for the Torlon fiber to 75.6° for the Torlon&sPPSU fiber, as shown in Table 2.

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The enhanced surface hydrophilicity may lower the water transport resistance and reduce

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membrane fouling for the latter.40

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Table 2. Summary of mean pore diameter (µp), geometric standard deviation (σp), pure water

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permeability (PWP), molecular weight cut-off (MWCO), water contact angle, and isoelectric

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point of the hollow fiber membranes. Membrane ID

PWP (LMH/bar) at 1 bar

Water contact angle (°)

pH for isoelectric point

Mean pore diameter, µp (nm)

Geometric standard deviation (σp)

MWCO (Da)

Torlon

117.6±10.6

81.7±3.5

3.7

1.31

1.53

1926.2

Torlon&sPPSU

82.5±10.1

75.6±3.0

3.6

1.02

1.30

1001.5

203 204

3.3 Pore Size, Pure Water Permeability (PWP), Molecular Weight Cut-Off (MWCO),

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and Surface Charge Properties of the Hollow Fiber Membranes

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Table 2 summarizes the mean effective pore diameter µp, PWP, and MWCO of the two

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hollow fiber membranes, while Figure 3 shows their pore size distributions. The Torlon fiber

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has a relatively broad pore size distribution with a mean pore diameter of 1.3 nm.

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Consequently, it has a high PWP of 117.6 LMH/bar and a large MWCO of 1926.2 Da. When

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sPPSU is blended into the Torlon dope, the mean pore diameter of the resultant

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Torlon&sPPSU fiber drops to 1.0 nm and its pore size distribution becomes narrower. Thus,

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its MWCO decreases dramatically to 1001.5 Da. This is possibly because of the increased

213

dope viscosity and hydrophilicity which result in a relatively denser and thicker selective skin.

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Nevertheless, a high PWP of 82.5 LMH/bar is still achieved by the Torlon&sPPSU fiber

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mainly due to the enhanced hydrophilicity and the formation of macrovoids across the fiber

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wall. Clearly, sPPSU acts as an effective mediator in terms of selective-layer thickness and

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pore structure during the phase inversion for membrane formation. The mechanical stability

218

of the developed fibers was further assessed by measuring their PWP at various hydraulic

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pressures. As shown in Figure S5, both fibers can withstand a high pressure of larger than 10

220

bar. Compared to the PWP measured at 1 bar, the PWP values of Torlon and Torlon&sPPSU 10 ACS Paragon Plus Environment

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fibers decrease with an increase in operating pressures. This can be attributed to membrane

222

compaction under high pressures.16,32

223

224 225

Figure 3. The probability density function curves of the Torlon and Torlon&sPPSU hollow

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fiber membranes.

227 228

Figure S6 shows the zeta potential versus pH curves of the two hollow fiber membranes

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and their isoelectric points derived from the plots are listed in Table 2. The isoelectric point

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of the Torlon fiber is pH=3.7, similar to our previous reported results.41 As a consequence,

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the Torlon fiber is slightly positively charged below pH 3.7 because of the protonation of the

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amine groups, while it becomes negative charged above pH 3.7 due to the deprotonation of

233

the carboxyl group. Since additional sulfonic acid groups are introduced onto the membrane

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surface when sPPSU is blended, the zeta-potential curve of the Torlon&sPPSU fiber shifts to

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the left, and its isoelectric point slightly drops to pH=3.6. This implies that the

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Torlon&sPPSU fiber becomes more negatively charged than Torlon particularly at higher pH

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values. As the charge characteristics play an import role in membrane rejection in NF

238

processes, it is believed that both hollow fibers are able to reject negative charged molecules

239

effectively via the Donnan exclusion mechanism.



240 241

3.4 Rejections to Electrolytes and Dye Solutes

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Figure 4 portrays the rejections of the hollow fibers against various electrolytes using 1000

243

ppm NaCl, MgCl2, Na2SO4 and MgSO4 solutions as the feeds at 1 bar. In general, the two

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fibers have relatively low rejections of less than 10% to four inorganic salts. This is mainly

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because of their relatively loose structure and large pore sizes. In the textile industry, the

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inorganic salts such as NaCl and Na2SO4 are usually used to enhance the dye uptake by the

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fabric and to maximize the exhaustion of dye molecules.10,19 Attributed to the well-defined

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pore sizes and unique charged surface properties, more than 90% of the inorganic salts

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permeate through the fibers, offering great potential to reuse these salts in the next dying

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processes. For the treatment of high-salinity wastewater, this ultralow salt rejection would

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also help increase the permeation flux since the effects of osmotic pressure and concentration

252

polarization effects are minimized.42

253

254 255

Figure 4. The rejections of Torlon and Torlon&sPPSU hollow fiber membranes against

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various electrolytes. (Feed: 1000 ppm electrolyte; trans-membrane pressure: 1 bar).

257 258

Table 3. Rejections and water permeability of Torlon and Torlon&sPPSU hollow fiber

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membranes to different dyes. 12 ACS Paragon Plus Environment

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Membrane ID

Dye

Molecular weight (g/mol)

PWP (LMH/bar)

Rejection (%)

Solution pH

Torlon

RB

1018

88

93.2

4.5

AB-8

1299

76

99.0

5

PY-H

1632

90

99.9

6

RB

1018

65

99.1

4.5

AB-8

1299

71

99.9

5

PY-H

1632

80

99.9

6

Torlon&sPPSU

260 261 262

The feed solution was prepared by dissolving the dyes in deionized water with a concentration of 200 ppm; testing pressure was 1 bar.

263

The ability of the two hollow fiber membranes for dye removal was firstly evaluated by

264

using single-component dye solutions. Table 3 summarizes the characteristics of the dye

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solutions prepared by directly dissolving the solid dyes in deionized water and the water flux

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and rejections of the fibers to Rose Bengal (RB), Alcian Blue 8GX (AB-8), and Procion

267

Yellow (PY-H). Generally, both fibers show high water fluxes and outstanding rejections to

268

all the dyes. The order of rejections is in agreement with the order of dye molecular weights.

269

Although the Torlon fiber has a MWCO of 1926 Da and an effective mean pore diameter of

270

1.31 nm, it not only shows excellent rejections of 99.0% and 99.9% to AB-8 and PY-H with

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larger molecular weights of 1299 and 1632 g/mole, respectively, but also has a relatively high

272

rejection of 93.2% to RB with a molecular weight of 1018 g/mole. This impressive separation

273

performance can be attributed to the following two factors: 1) the membrane has a negative

274

charged surface which can effectively reject the negatively charged dyes via electrostatic

275

repulsion, and 2) the dye molecules form larger aggregates via hydrogen bonding so that the

276

fiber can effectively reject them via size exclusion.43 Since the Torlon&sPPSU fiber

277

possesses a smaller pore diameter and a more hydrophilic and negatively charged surface

278

than the Torlon fiber, the former shows higher rejections to all dyes (99.1-99.9% vs. 93.2-

279

99.9%) but slightly lower fluxes than the latter (65-80 LMH vs. 76-90 LMH at 1 bar). 13 ACS Paragon Plus Environment


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281 282

Figure 5. Permeation flux and rejection of Torlon and Torlon&sPPSU hollow fiber

283

membranes as a function of pH for (a) RB and (b) PY-H dyes, respectively. (Feed: 200 ppm

284

RB and PY-H solutions; trans-membrane pressure: 1bar).

285 286

Figure 5 depicts the water permeation flux and rejection of the Torlon and Torlon&sPPSU

287

fibers at different pH values using 200 ppm RB and PY-H solutions as feeds. It is interesting

288

to observe that pH plays a significant role on membrane separation performance. Both fibers

289

exhibit higher water fluxes but lower dye rejections in an alkaline feed solution (i.e., pH=10)

290

than an acidic solution (i.e., pH=2.0). As shown in Figure 5 (a), the two fibers display high

291

water fluxes of 100 and 82 LMH with a high rejection of 99.9% to the RB solution at pH=2.0.

292

These might be due to the synergistic combination between size exclusion and charge

293

repulsion at low pH.43 From pH 2.0 to 4.5, the charge of the fibers is changed from a positive

294

to a slightly negative state (see Figure S6). Under such a condition, the electrostatic repulsion

295

between the membrane and RB molecules becomes weaker, thus the flux and rejection are

296

slightly decreased. When the pH is further increased to a high value of 10, the water fluxes

297

rise rapidly to 115 and 70 LMH, while their rejections drop sharply to 78% and 90% for the

298

Torlon and Torlon&sPPSU fibers, respectively. The increased fluxes and the decreased

299

rejections may result from the effects of membrane swelling at an extreme alkaline condition

300

because this basic condition would induce greater chain mobility and enlarge the pore size.21 14 ACS Paragon Plus Environment

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For the PY-H dye solution, it is surprising to find that the two fibers have low water fluxes

302

of 10-30 LMH at a low pH of 2.0. This may arise from the fact that the PY-H molecules are

303

in an anionic state under this acidic condition due to the low pKa value of the sulfonic acid

304

group (pKa: ~1.0).44 Therefore, the anionic dye molecules can easily adsorb and accumulate

305

on the positively charged fiber surface and result in severe membrane fouling and flux

306

reduction.42,45 As the pH value is increased, the membrane surface gradually becomes

307

negatively charged. As a result, the dye deposition on membrane surface is reduced and the

308

water fluxes are increased to 105 LMH and 90 LMH for Torlon and Torlon&sPPSU fibers at

309

pH=10, respectively. Because the PY-H dye has a large molecular size, both fibers show

310

great rejections of large than 99% to it at all pH values even though membrane swelling

311

happens at such high pH.

312 313

3.5 Long-Term Performance of the Tight UF Hollow Fibers for Dye Removal

314

In order to study the effects of dye concentration on separation performance of the newly

315

developed hollow fiber membranes for dye removal, the filtration was firstly conducted under

316

the concentrate mode by continuously recycling the feed while collecting the permeate until

317

reaching the predetermined feed recovery rate using the Torlon&sPPSU fiber. Figure 6 (a)

318

and (b) show the variations of permeation flux and rejection as a function of recovery rate.

319

For the PY-H feed solution, the permeation flux reduces slightly with an increase in recovery

320

because of the increase of dye concentration (i.e., osmotic pressure) in the feed. Since the

321

PY-H has a large molecular size and there is charge repulsion between it and the membrane

322

surface, the dye adsorption, pore blocking and fouling on the surface are minimized. As a

323

result, the rejection to PY-H is almost independent on recovery rate. It remains at a high

324

value of greater than 99% even when the recovery reaches 90%. This signifies the stable

325

rejection of the Torlon&sPPSU fiber to PY-H at a wide range of feed concentration.



326

327 328

Figure 6. (a) Normalized water flux and (b) rejection of the Torlon&sPPSU hollow fiber

329

membrane as a function of feed recovery; (c) long-term performance tests and (d) cleaning of

330

the Torlon&sPPSU hollow fiber membrane. (Feed: 200 ppm RB (pH=4.5) and PY-H

331

(pH=6.0) solutions; trans-membrane pressure: 1bar).

332 333

The membrane performance changes when a RB solution is employed as the feed because

334

RB has a much smaller molecular size than PY-H. The flux firstly drops slightly to 95% of

335

the initial value at a low recovery of around 10%, then rapidly decreases to 80% when the

336

recovery reaches 45% and finally becomes stable till to a high recovery of 90%. In addition,

337

with the increases in recovery and feed concentration, the rejection against RB slightly drops

338

to 89% at a recovery of 90%. The declines in both flux and rejection are possible caused by

339

the combinative effects from (1) enhanced adsorption, pore blocking and osmotic pressure of


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feed solution, and (2) the reduced Donnan exclusion effect at higher dye concentrations,

341

which result in lowered flux and rejection.

342

A total recirculation mode was also operated to further examine the long-term membrane

343

performance, in which the feed concentration was kept constant. Figure 6 (c) and (d) shows

344

the evolution of water flux and dye rejection as a function of time in a continuous test of 170

345

h at 1 bar using 200 ppm RB (pH=4.5) and PY-H (pH=6.0) as the feed solutions, separately.

346

Both permeation fluxes decrease rapidly during the first 70 h and then become less steep. For

347

the RB solution, the water flux drops 65 LMH to 30 LMH after the 170-h operation, while it

348

decreases from 80 to 35 LMH for the PY-H solution. These represent flux declines of around

349

53-56%. Since the feed concentration and volume are kept constant, the flux reduction is

350

mainly caused by dye adsorption, pore blocking and cake layer formation.46,47 Since a high

351

flux of >37 LMH is achieved at the relatively stable stage (see Figure 6 (d)), this implies that

352

the developed tight UF fiber has low fouling propensity. On the other hand, the membrane

353

exhibits an excellent dye rejection of higher than 99% over the entire testing period. The

354

consistent and effective removal of dyes indicates the outstanding stability of the

355

Torlon&sPPSU fiber.

356

In addition, as shown in Figure 6 (d), the fouled fiber can be easily regenerated and the

357

water flux can be almost fully recovered by simple water backwashing, suggesting the high

358

fouling reversibility. Therefore, this work provides useful insights for the design and

359

fabrication of next-generation loose NF hollow fiber membranes via a single-step spinning

360

process. A comprehensive study of filtration performance and fouling behaviors on these two

361

new membranes will be carried out using real textile wastewater in the future.

362 363

Supporting Information



364

Materials (Figure S1); phase inversion studies of polymer dope solutions (Figure S2); hollow

365

fiber spinning (Table S1); characteristics of the dye molecules used in this study (Table S2);

366

SEM images (Figure S3), XPS data (Figure S4 and Table S3) and surface charge properties

367

(Figure S6) of the hollow fibers; the mechanical stability studies of the hollow fibers (Figure

368

S5); and UF process and operating conditions and performance evaluation.

369 370

ACKNOWLEDGMENTS

371

This work is granted by the Singapore National Research Foundation under its Environmental

372

&Water Research Programme and administered by PUB, Singapore’s national water agency.

373

It is funded under the projects entitled "Membrane Development for Osmotic Power

374

Generation, Part 1. Materials Development and Membrane Fabrication" (1102-IRIS-11-01)

375

and NUS Grant no. R-279-000-381-279; "Membrane Development for Osmotic Power

376

Generation, Part 2. Module Fabrication and System Integration" (1102-IRIS-11-02) and NUS

377

Grant no. R-279-000-382-279. The authors would like to thank BASF SE, Germany for

378

funding this work with a Grant no. of R-279-000-411-597. The authors would also like to

379

thank Mr. C.Z. Liang, Mr. J. Chang, Mr. S. Jin and Mr. B.W. Feng for all their kind help on

380

experiments work.

381 382

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