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Conventional ultrafiltration as effective strategy for dye/salt fractionation in textile wastewater treatment Mei Jiang, Kunfeng Ye, Jiajie Deng, Jiuyang Lin, Wenyuan Ye, Shuaifei Zhao, and Bart Van der Bruggen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02984 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Environmental Science & Technology

Conventional ultrafiltration as effective strategy for dye/salt fractionation in textile wastewater treatment Mei Jiang‡,a, Kunfeng Ye ‡,a, Jiajie Dengb, Jiuyang Lin*,a, Wenyuan Ye*,c, Shuaifei Zhaod, Bart Van der Bruggene ‡These authors contributed equally to this work. a

School of Environment and Resources, Qi Shan Campus, Fuzhou University, No. 2 Xueyuan Road, University Town, 350116 Fuzhou, Fujian, China b

c

Suzhou Nuclear Power Research Institute, Suzhou 215004, China

Fujian Provincial Key Laboratory of Soil Environmental Health and Regulation, College of

Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, China d

Department of Environmental Sciences, Faculty of Science and Engineering, Macquarie University, Sydney, NSW 2109, Australia

e

Department of Chemical Engineering, Process Engineering for Sustainable Systems (ProcESS), KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium

ACS Paragon Plus Environment

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ABSTRACT: Use of tight ultrafiltration (UF) membranes has created a new pathway in fractionation of dye/salt mixtures from textile wastewater for sustainable resource recovery. Unexpectedly, a consistently high rejection for the dyes with smaller sizes related to the pore sizes of tight UF membranes is yielded. The potential mechanism involved in this puzzle remains unclear. In this study, seven tailored UF membranes with molecular weight cut-offs (MWCOs) from 6050 to 17530 Da were applied to separate dye/salt mixtures. These UF membranes allowed a complete transfer for NaCl and Na2SO4, due to large pore sizes. Additionally, these UF membranes had acceptably high rejections for direct and reactive dyes, due to the aggregation of dyes as clusters for enhanced sizes and low diffusivity. Specifically, the membrane with an MWCO of 7310 Da showed a complete rejection for reactive blue 2 and direct dyes. An integrated UF-diafiltration process was subsequently designed for fractionation of reactive blue 2/Na2SO4 mixture, achieving 99.84% desalination efficiency and 97.47% dye recovery. Furthermore, reactive blue 2 can be concentrated from 2.01 to 31.80 g·L-1. These results indicate that UF membranes even with porous structures are promising for effective fractionation of dyes and salts in sustainable textile wastewater treatment.


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1. INTRODUCTION In the textile industry, dye wastewaters with high salinity are generally produced during the synthesis or application of dyes.1 In the synthesis procedure of dyes, a huge amount of inorganic salts (i.e., ~5.0% NaCl or Na2SO4) is generated as byproducts, which substantially compromises the purity of dyes and reduces the brilliance of printing images during their application in textile printing.1,2 On the other hand, the addition of inorganic salt, i.e., NaCl or Na2SO4, (up to 2 kg per kg fabric) to dyeing bath is required to enhance the bonding of dye on the fabrics during the dyeing procedure.3 Specifically, the consumption of fresh water varies from 200 to 400 m3 to produce one ton finished fabric products, resulting in the generation of a large amount of textile wastewater.4,5 This results in an annual discharge of around 280,000 tons of dyes to receiving water bodies, which poses a detrimental impact on public health and ecological systems. The discharge of high amounts of salts along with dyes is a major concern of textile wastewater. Therefore, an appropriate treatment for textile wastewater is of great significance to minimize the discharge of such highly polluted and saline liquor. In view of sustainability, highly loaded wastewaters, e.g., textile wastewater, have been considered more as a resource for water, value-added products, and even energy, rather than a waste.5,6 Therefore, the treatment of high-salinity textile wastewater calls for a conceptual shift from “waste” elimination to sustainable resource recovery. The feasible strategy to address this challenge is to effectively fractionate the dyes and salt from textile wastewater for subsequent recycle of recovered dyes and inorganic salts. Unfortunately, conventional approaches, including coagulation/flocculation,7,8 adsorption,9,10 biological degradation,11 and advanced oxidation processes,12 however, fail to sufficiently extract these resources (i.e., dyes or inorganic salts) through taking the “waste” out of textile wastewater.


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Nanofiltration (NF) technology primarily offers a possibility for the fractionation of dyes and salts from the textile wastewater, for the sake of resource recovery. NF membranes can effectively retain the dyes but allow for a partial salt transmission, based on size exclusion and the Donnan effect.13,14 Specifically, loose NF membranes as art-of-the-state NF technology have an outstanding fractionation ability for dyes and monovalent salts (i.e., NaCl), due to their loose surface structure for facilitating the salt passage.1,15,16 Ye et al. prepared a polysaccharide NF membrane incorporated by sulfated polyelectrolyte complex with a molecular weight cutoff (MWCO) of 585 Da, with a 99.9% retention to methyl blue coupled with a 15.8% rejection of NaCl.17 Lin et al. systematically investigated the separation performance of Sepro NF 6 (MWCO of 860 Da) with a 99.9% retention for Congo red and a low NaCl rejection of 10%, showing a great potential for dye and salt fractionation.1 The ultra-permeable sulfonated thin-film composite membrane with a MWCO of 950 Da fabricated through interfacial polymerization of 2,2'-benzidinedisulfonic acid and trimesoyl chloride allows for a complete rejection for Congo red and free permeation of NaCl.18 A loose NF membrane with copper nanoparticles via bioinspired poly-dopamine deposition yielded a rejection coefficient of >99.5% to dyes (i.e., Congo red, direct red 23 and reactive blue 2) and elevated NaCl transport (~96.3%).15 However, the negative charges on these loose NF membrane surfaces hamper the free transmission of divalent salts, i.e., Na2SO4, jeopardizing the effective purification of dyes. Alternatively, tight ultrafiltration (UF) membranes with MWCOs of 1000~5000 Da show an acceptably high rejection for dyes with nearly full transmission of inorganic salts, creating a potential pathway for effective fractionation of dyes and divalent salts (i.e., Na2SO4). A Torlonbased tight UF membrane with a MWCO of 1926 Da fabricated by direct phase inversion shows a 99.0% rejection for Alcian blue 8GX and a moderate rejection (7.9%) of Na2SO4.19 Lin et al.


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demonstrated the feasibility of using a tight UF membrane with a MWCO of 4700 Da for fractionation of (direct and reactive) dye/Na2SO4 mixtures.20 However, the moderate rejection of Na2SO4 caused by the Donnan effect would potentially hinder the effective purification of dyes for these tight UF membranes. Generally, the enlargement in membrane pore size would significantly undermine the electrostatic repulsion force of membrane surface, which results in a reduction in the rejection of inorganic salts as well as dye species.13 Unexpectedly, the UF membranes used in previous studies yield a consistently high rejection for dye species with smaller size than the membrane pore radius. This implies that other potential retention mechanisms for dye species by tight UF membranes remain unclear, which requires to be explored. Therefore, UF technology should be systematically explored to illustrate how far it can go for effective fractionation of dye/salt mixtures, in view of sustainable dye recovery and purification of these high-salinity dye liquors. In this work, seven UF membranes with variable pore sizes were applied to fractionate dye/salt (i.e., NaCl and Na2SO4) binary mixtures. Initially, the filtration performance of these UF membranes was investigated in single-component solutions (i.e., salts and dyes) to preliminarily evaluate their feasibility in selective separation of dyes and salts. Subsequently, the mass transfer of these UF membranes in dye/salt mixtures was further studied at different salt concentrations to reveal the filtration mechanisms. Furthermore, an integrated UF-diafiltration process, including pre-concentration, diafiltration and post-concentration steps, for dye/salt mixture was specifically designed, which demonstrates the fractionation performance of regular UF operation for sustainable textile wastewater treatment in industrial application. 2. EXPERIMENTAL SECTION 2.1 Chemicals and membranes


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Two types of dyes, including direct dyes, i.e., direct red 80 (DR80) and direct red 23 (DR23, purity: >30%; inorganic salts: 12.6% for SO42-, 5.8% for Cl- and 5.2% for HCO3-) and reactive dyes, i.e., reactive blue 2 (RB2) and reactive orange 16 (RB16), were purchased from SigmaAldrich (Germany) and used as model dyes. Figure S1 of the Supporting Information (SI) shows the chemical structure of these tested dyes. The salts, i.e., NaCl and Na2SO4 with analytical grade, were purchased from Sigma Aldrich (Germany) as inorganic solutes. Dopamine hydrochloride and tris (hydroxymethyl) aminomethane (Tris) purchased from Sigma Aldrich (Germany) were used for membrane modification. All these chemicals were used without further purification. MilliQ water (electrical resistance with 18.2 MΩ·cm) was used throughout all the experiments. The commercial UF membranes, namely GR82PP, UP5 and UP10, were kindly supplied by Alfa-Laval (Denmark) and Microdyn Nadir (Germany), respectively. The UF membrane PAN50 was kindly supplied by Ultura (USA) and used as the substrate for membrane modification by dopamine polymerization to obtain UF membranes with different pore sizes. The properties of these UF membranes are shown in Table 1.

Table 1 Characterization of commercial ultrafiltration membranes Membrane

GR82PP

PAN50

UP10

UP5

Manufacturer

Alfa Laval

Ultura

Microdyn Nadir

Microdyn Nadir

MWCO (Da)

-

~100 K

-

-

PES

PES

1-13

1-13

Material Process pH limitation

Polyethersulfone

Polyacrylonitrile

(PES)

(PAN)

1-13

2-10


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2.2 Membrane modification through dopamine polymerization In order to fabricate UF membranes with different pore sizes, a facile modification by dopamine as a green bio-glue was carried out. Briefly, the PAN50 membrane coupons were washed with MilliQ water and then restored in the MilliQ water for 24 h. Subsequently, the PAN50 membrane coupons were immersed into a 2 g·L-1 dopamine solution, which contained Tris butter (10.0 mM, pH 8.5). Subsequently, poly-dopamine coating on the surface of the PAN-based membranes occurred at ambient temperature and at various time durations of 3, 4, 5, and 10 h. The modified membranes with immersion in the dopamine solution at different time intervals were denoted as DP-3, DP-4, DP-5, and DP-10, respectively. 2.3 Membrane characterization 2.3.1 Membrane morphology Scanning electron microscopy (SEM) measurement through Philips Scanning Electron Microscope XL30 FEG (the Netherlands) was performed to visualize the morphology of these UF membranes. Prior to the measurement, these UF membranes were dried and then sputtercoated with gold nanoparticles. Additionally, these UF membrane samples were fractured in liquid nitrogen for cross-section imaging. All the SEM measurements were performed in high vacuum condition at 10 kV. 2.3.2 Pore size distribution and MWCO The pore size distribution of the tested membranes was determined through the separation of 200 ppm polyethylene glycols (PEGs) with average molecular weights of 2000, 4000, 6000, 10000, and 20000 Da. The relationship between the solute Stokes diameter (ds) and molecular weight (MW) of the PEG can be expressed by the following equation:13,21

ds = 33.46 × 10-12 × MW 0.557

(1)


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The rejection coefficient of these PEGs for these tested UF membranes was measured at a hydraulic pressure of 4 bar. The concentration of PEGs was measured in terms of total organic carbon (TOC) with a Shimazu ASI-5000A TOC analyzer. The rejections coefficients (R) for PEGs were calculated from the measured feed (Cf) and permeate concentration (Cp) using the equation:

 Cp  R ( % ) = 1 −  ×100  Cf 

(2)

In order to obtain the pore size distribution of the tested membranes, a log-normal model between the solute rejection and solute Stokes radius was applied as expressed by the following equation:21 dR ( d p ) dd p

=

1 d p ln σ p

 ( ln d − ln µ )2  p p  exp  − 2   2π 2 ( ln σ p )  

(3)

where µp is the mean effective pore size which is determined at solute rejection R = 50%, and σp is the geometric standard deviation, which is defined as the ratio of dp at R = 84.13% over that at R = 50%. The MWCO of the tested membranes can be back-calculated through the solute Stokes diameter at R = 90% by Equation 1. 2.3.3 Zeta potential measurement The zeta potential of these tested membranes was determined through streaming potential measurements through SurPASS electrokinetic analyzer (Anton Paar GmbH, Austria). Streaming potential measurements were performed using a cylindrical cell with 0.01 mol·L-1 NaCl aqueous solutions at 25°C and pH ranging from 2 to 11, which was adjusted by auto titrations of the NaCl


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solution with 0.1 M HCl and NaOH. The pH dependent surface charge and the isoelectric point of the UF membranes were determined. 2.4 Filtration performance The filtration performance of these tested UF membranes were evaluated through a lab-scale cross-flow filtration cell, which was specifically designed for flat sheet membranes with an effective area of 22.9 cm2. Figure S2 shows the schematic diagram of this cross-flow filtration cell. Initially, the filtration process was performed with a feed of single-component solution (i.e., NaCl, Na2SO4 or dyes) at variable pressures and solute concentrations. Subsequently, the filtration performance of these tested UF membranes was conducted in the multi-component solutions containing dyes and salts at the presence of variable amounts of salts. In addition, the integrated diafiltration process by UF membrane in a batch mode, including pre-concentration, diafiltration and post-concentration step, was designed to fractionate the dye/salt mixtures. Initially, a 3000 mL RB2/Na2SO4 mixture (i.e., 2 g·L-1 RB2 and 60 g·L-1 Na2SO4) as feed was pre-concentrated without pure water addition by a concentration factor of 3.3. Afterwards, a diafiltration step was performed to desalinate the concentrated solution with 5.4 diavolumes; pure water was continuously added to the feed at the same rate as the permeate to keep the feed volume constant. Finally, post-concentration was further performed at a concentration factor of 5.0 to achieve the dye with high concentration. The overall diafiltration process was performed at 4 bar and 25 ± 1°C. The permeate flux (Jp) was calculated from the time to collect a fixed volume of water at the applied operating pressure bar as: Jp =

V A ⋅ ∆t

(4)


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where V is the volume of permeate collected during the time interval ∆t and A is the effective membrane area. The observed rejection (Rs) of solutes for membranes can be calculated as:

 C − Cp,s  Rs (%) =  f,s  ×100  C f,s  

(5)

where Cf,s and Cp,s are the solute concentration in the feed and permeate, respectively. 2.5 Membrane fouling and cleaning During the diafiltration process, the tested UF membrane would suffer from the severe fouling. The fouled UF membrane was cleaned by pure water or NaOH solution (pH~12.0) until the dye foulant was completely removed. The flux recovery ratio (η) was measured by recording the pure water flux after the cleaning at 4 bar to evaluate the fouling performance and cleaning efficiency:

η ( %) =

J w,c J w,0

×100

(6)

where Jw,0 is the pure water flux of the original UF membrane, and Jw,c is the pure water flux of the fouled UF membrane after cleaning. 2.6 Analytical methods A Shimadzu UV-1601 double beam spectrophotometer (Japan) was used to measure the concentration of dyes. The maximum adsorption wavelength for DR80, DR23, RB2 and RO16 is 528, 507, 628, and 494 nm, respectively. The concentration of salt in the single-component solution (i.e., NaCl or Na2SO4) was evaluated with a conductivity meter (Thermo Scientific Orion Star A212 Benchtop Conductivity Meter, Belgium). The concentration of NaCl or Na2SO4 in the dye/salt mixture solutions was determined by ion chromatography (Dionex ICS-2000 Ion Chromatography System, USA) in terms of Cl- or SO42- ions. In order to eliminate the contamination of the chromatographic column, clean activated carbon was used to completely


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remove the dyes in the dye/salt mixture solutions prior to the measurement of Cl- and SO42ions.16

3. RESULTS AND DISCUSSION 3.1 Membrane characterization The morphology of the tested UF membranes is shown in Figure 1. Obviously, the PAN50 membrane has a porous structure. After the coating of polydopamine, the pores of the PANbased membrane were covered (Figure 1A-E), resulting in a reduction in pore size. When the PAN50 membrane was immersed in the dopamine solution for coating (duration of 3 hours), the mean pore size of the PAN50 membrane significantly decreased to 2.75 nm and its corresponding MWCO dropped from ~100,000 Da to 11,670 Da (Table 2). As the coating time extended, the dopamine layer evolved and gradually became thicker. Finally, the resultant membrane showed a mean pore size of 2.10 nm and a MWCO of 6850 Da after a coating duration of 10 hours. The commercial UP10 membrane from Microdyn Nadir has a porous structure with a MWCO of 17,530 Da. However, the commercial GR82PP membrane from Alfa Laval and UP5 membrane from Microdyn Nadir have a tight structure with no visible pores on the membrane surface, as shown in Figure 1G and H. Furthermore, these two UF membranes have a selective top layer with a thickness of ~1,500 and ~1,200 nm (Figure S3), and corresponding MWCOs of 7310 and 6050 Da, respectively (Figure 2).


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Figure 1. Surface morphologies of the tested UF membranes. (A): PAN50, (B): DP-3, (C): DP4, (D): DP-5, (E): DP-10, (F): UP10, (G): GR82PP, (H): UP5.


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Figure 2. (A) Rejection-PEG molecular weight relationship of the tested UF membranes and (B) pore size distribution.

Table 2. MWCOs and effective pore sizes of the tested UF membranes

UF membranes

MWCO (Da)

µp (nm)

UP10

17530

4.07

DP-3

11670

2.75

DP-4

8770

2.48

DP-5

7880

2.31

GR82PP

7310

2.16

DP-10

6850

2.10

UP5

6050

1.93

3.2 Membrane flux and salt rejection The osmotic pressure of solution can significantly reduce the permeation flux of the membranes in high-salinity wastewater (i.e., dye wastewater), which leads to a low efficiency for water treatment.22 Therefore, the high transmission of inorganic salts, especially Na2SO4, is


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strongly required to facilitate the separation of dye and salt mixtures. Figure 3 shows the filtration performance of the tested UF membranes in NaCl and Na2SO4 solutions with different concentrations.

Figure 3. (A) Permeation flux and (B) salt rejection of the tested UF membranes. After coating polydopamine on the PAN-based membrane surface, the membrane pores were covered, which increased the membrane hydraulic resistance and reduced the membrane flux (Figure 3A). Specifically, the permeability of the PAN-based membrane decreased from 280 to 11.5 L·m-2·h-1·bar-1 after 10-h polydopamine coating. The UP10, GR82PP, and UP5 membranes had permeabilities of 89.7, 37.4, and 25.7 L·m-2·h-1·bar-1, respectively. When filtering the NaCl or Na2SO4 solution with 0.1 and 10.0 g·L-1, the permeability of these tested membranes had no significant decline, compared to that in pure water. This is mainly due to the low salt rejection for these tested membranes with large pore size (Figure 3B), which minimizes the osmotic pressure difference. This gives no significant reduction in the driving force. Furthermore, the rejection of the salts declined with increasing salt concentration, mainly attribute to the reduction in the Debye length of the electrostatic interaction between the charged ions and the negative charges of the membrane, resulting in a reduced Donnan effect and thus elevated salt permeation (Zeta potential in Figure S4).23 For instance, all the tested membranes allow for a complete permeation of a 10.0 g·L-1 NaCl solution. Only slight salt rejections for a 10.0 g·L-1 Na2SO4


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solution were observed. Specifically, the salt rejection of these membranes has a strongly negative dependence on the pore size. This suggests that the Donnan effect can be significantly diminished with increasing the pore size of the membrane.

3.3 Retention of pure dyes 3.3.1 Potential of UF membranes for dye filtration To evaluate the potential of UF technology in dye removal, the dye retention of the tested UF membranes for different dye solutions with 0.1 g·L-1 at an applied pressure of 4 bar was investigated (Figure 4).

Figure 4. (A) Dye retention of the tested UF membranes and (B) relationship between rejection of tested UF membranes and diameter of dyes. As the pore size of the tested UF membranes decreased, the rejection of dyes was enhanced (Figure 4A). For instance, the PAN50 membrane with MWCO of ~100 kDa has a rejection of 0.12% to RO16, while the rejection of 95.91% for RO16 was observed for the UP5 membrane with MWCO of 6050 Da. This can be mainly due to the integration of size exclusion and enhanced Donnan effect, which can be strengthened with reducing pore sizes. Unexpectedly, the PAN50 and UP10 UF membrane with larger MWCO have a remarkably high rejection for DR80


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and DR23 with small molecular sizes. Normally, the Donnan effect plays a negligible role in enhancing the rejection of charged solutes when a porous structure is present on the membrane surface (Figure 3). This implies that alternative filtration mechanisms can be responsible for the greatly enhanced dye rejection for UF membranes. The most plausible mechanism is the aggregation of dye molecules.24 Navarro and Sanz investigated the aggregation behavior of direct dyes by electrochemical measurements, demonstrating that an aggregation of direct dyes can occur at a low dye concentration. The tested direct dye (i.e., direct red 1) with ca. 3.10×10-6 mol/L yields an average aggregation number of 8 at 25 °C, which significantly increases the size of dye clusters, due to the hydrophobic interactions between the aromatic rings of adjacent dye molecules.24 A similar aggregation phenomenon of other direct dyes (i.e., direct yellow 162) was also observed by Ferus-Comelo and Greaves.25 This can well explain the greatly enhanced rejection for DR80 and DR23 obtained by PAN50 and UP10 membranes. Furthermore, the other two reactive dyes, i.e., RB2 and RO16, also show a much higher measured rejection coefficient by UF membranes than that theoretically calculated from the log-normal model (Figure 4B). This demonstrates that the hydrophobic interaction for the formation of dye clusters also takes place for reactive dyes. However, the aggregation degree of reactive dyes is less than that of direct dyes, which results in a lower rejection. Therefore, conventional UF technology can be an effective strategy for dye retention, due to the aggregation of dyes. Furthermore, combining the high salt transmission and dye retention, the UF membranes offer a new avenue to fractionate the dye/salt mixture for dye purification in textile industry, as an alternative to NF membrane. However, more investigations about the properties of the dyes and their interaction with water molecules should be performed to determine the transport mechanism. 3.3.2 Effect of operation pressure and dye concentration for dye filtration


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As demonstrated in Figure 4, UF is an effective strategy to remove dyes. Figure 5 shows the effect of operating pressure and dye concentration on the filtration with the GR82PP membrane with MWCO of 7310 Da; this is a membrane with high selectivity and acceptably high flux.


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Figure 5. Dye rejection and flux for GR82PP membrane as a function of dye concentration and operation pressure. (A): DR80; (B): DR23; (C): RB2; (D): RO16. Figure 5 indicates that the GR82PP membrane with a MWCO of 7310 Da has a very high rejection (>98.9%) for DR80, DR23 and RB2, showing a high potential for dye retention. Generally, the rejection of dyes increases with increasing the dye concentrations. This is mainly due to the equilibrium of the dye aggregation ( nD

n dimer 2

n tr im er ⋅⋅⋅ 3

Dn ) in the dye

solutions. As the concentration of dye increases, the aggregation of dye clusters is substantially intensified. This is confirmed by UV-vis adsorption spectra for different dyes (Figure S5). As shown in Figure S5, the extra maximum adsorption peaks can be observed with increasing dye concentration, indicating the presence of diverse dye clusters. Unexpectedly, the rejection of DR23 decreased with the increasing dye concentration at low operating pressure (i.e., 2 bar), plausibly ascribed to the increasing amount of impurities (i.e., SO42-, Cl- and HCO3-), which can intensify the electrostatic shielding effect for reduced rejection of DR23.1,20 However, the aggregation degree differs from the dye species. The smaller molecular size and lower aggregation degree of RO16 result in lower rejections.


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Additionally, a decreased dye rejection of the GR82PP UF membrane was observed with increasing operating pressure. Generally, an elevated rejection for solute of the membrane can be expected at higher operating pressures due to the enhanced dilution effect, which yields high water flux to reduce the effective solute concentration in the permeate.16 On the other hand, with the increasing operating pressure, the concentration polarization was intensified which can significantly deteriorate the dye rejection in this case. However, increasing operating pressure has no pronounced effect on the rejection of DR80, DR23 and RB2, resulting in a slight reduction (99%), which allows for a high dye yield. In the subsequent diafiltration stage, the GR82PP membrane allows for an effective purification of dye/salt mixtures. The concentration of Na2SO4 decreased from 60.16 to 0.83 g·L-1 at 5.4 diavolumes. The concentration of RB2 almost remains constant, slightly decreasing from 6.55 to 6.34 g·L-1. With decreasing Na2SO4 concentration, the electrostatic shielding effect of the GR82PP membrane was reduced for enhanced rejection of RB2 and Na2SO4. Specifically, the rejection of Na2SO4 in the diafiltration step ranges from 1.3% to 42.1%. Following the diafiltration stage, a post-concentration step was carried out. The concentration of Na2SO4 increased from 0.83 to 1.58 g·L-1, which is due to the moderate rejection of Na2SO4 (i.e., ca. 43%), while the concentration of RB2 increased linearly from 6.34 to 31.80 g·L-1 at a concentration factor of 5.0. In this stage, the GR82PP membrane yielded an extremely high rejection (>99.8%) for RB2. Thus, the diafiltration process can effectively fractionate the RB2/Na2SO4 mixture with a desalination efficiency of 99.84%, which substantially enhances the purity of RB2. Due to the extremely high rejection of RB2, the overall diafiltration process yielded a dye loss of 2.53% (Figure 7B). The flux of the GR82PP membrane in the pre-concentration stage dropped from 56.5 to 41.7 L·m-2·h-1, which was due to the integrated effects of the formation of a dye cake layer and the increasing osmotic pressure (Figure 7C). In the diafiltration stage, the decrease of Na2SO4 concentration in the feed diminishes the aggregation of RB2 and alleviates the formation of a dye cake layer, which can significantly promote the permeation flux. Specifically, the flux of


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GR82PP membrane increases from 41.7 to 59.5 L·m-2·h-1. During the post-concentration stage, the evolution of a dye cake layer due to the increasing concentration of RB2 mainly limits the flux of the GR82PP membrane, but a moderate flux can be still obtained for effective purification. In order to effective recover the membrane flux, the cleaning for the fouled GR82PP membrane was performed (Figure S7). As shown in Figure S7, the flux recovery ratio of 95.14% after water flushing can be obtained, demonstrating the low fouling propensity. This is consistent with the case for filtrating the humic acid solution (Figure S8). Additionally, with the cleaning by NaOH solution, a complete flux recovery (i.e., 99.63%) was yielded by the GR82PP membrane. Therefore, these results further demonstrate that the conventional UF technology can be a promising alternative to NF technology for effective fractionation of dye/salt mixtures, in view of sustainable textile wastewater treatment.

ASSOCIATED CONTENT The following files are available free of charge. Molecular structures of the model dyes tested in this study (Figure S1); Schematic of UF setup for fractionation of dye/salt mixtures (Figure S2); Cross-section morphology of the tested UF membranes (Figure S3); Zeta potential of the tested UF membranes (Figure S4); UV-vis adsorption for four dyes at various concentrations (Figure S5); Digital pictures for dye/salt mixtures with different NaCl and Na2SO4 concentrations (Figure S6); Flux recovery of the fouled GR82PP ultrafiltration membrane after pure water rinsing or NaOH cleaning (Figure S7); Relative flux of the GR82PP ultrafiltration membrane in the filtration of pure water and a 0.1 g·L-1 humic acid solution at 4 bar and 25 °C (Figure S8) (PDF).


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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (J. Lin), [email protected] (W. Ye)

ACKNOWLEDGMENT J. Lin and W. Ye would like to thank the funding support from the National Natural Science Foundation of China (Grant Nos: 21706035 and 21707018), the Natural Science Foundation of Fujian Province (Grant No.: 2017J01413), the Fujian Agriculture and Forestry University Program for Distinguished Young Scholar (Grant No.: xjq201704) and Fuzhou University (Grant Nos: XRC-1622 and 2017T017) for this work.

REFERENCES (1) Lin, J.; Ye, W.; Zeng, H.; Yang, H.; Shen, J.; Darvishmanesh, S.; Luis, P.; Sotto, A.; Van der Bruggen, B., Fractionation of direct dyes and salts in aqueous solution using loose nanofiltration membranes. Journal of Membrane Science 2015, 477, 183-193. (2) Xue, C.; Chen, Q.; Liu, Y.; Yang, Y.; Xu, D.; Xue, L.; Zhang, W., Acid blue 9 desalting using electrodialysis. Journal of Membrane Science 2015, 493, 28-36. (3) Erkanlı, M.; Yilmaz, L.; Çulfaz-Emecen, P. Z.; Yetis, U., Brackish water recovery from reactive dyeing wastewater via ultrafiltration. Journal of Cleaner Production 2017, 165, 12041214.


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Table of Content

The conventional UF membranes with MWCOs ranging from 6,050 to 17,530 Da yield a high dye rejection and free salt passage, showing a high potential in fractionation of dye and salt mixtures.


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Salt

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