Application of Positively Charged Composite Hollow-Fiber

Article pubs.acs.org/IECR

Application of Positively Charged Composite Hollow-Fiber Nanofiltration Membranes for Dye Purification Xiuzhen Wei,*,†,‡ Songxue Wang,†,‡ Yingying Shi,†,‡ Hai Xiang,§ and Jinyuan Chen*,†,‡ †

College of Biological and Environmental Engineering and ‡Top Key Discipline of Environmental Science and Engineering, Zhejiang University of Technology, HangZhou 310014, P. R. China § College of Bioengineering, Zhejiang Chinese Medical University, Hangzhou 310053, P. R. China ABSTRACT: A positively charged, composite, hollow-fiber nanofiltration (NF) membrane was fabricated by interfacial polymerization using polyethylenimine and trimesoyl chloride as reactive monomers. Rejection rates of different salts and dyes were measured by the NF membranes at various pH values. The effects of dye and salt concentrations on the membrane performance for dye purification were investigated. The dye and salt rejection rates would gradually decrease with an increase in the dye and salt concentrations. Then the influence of the volume concentration factor on dye purification during constant volume batch diafiltration process was studied. The results indicated that the higher of the concentration factor was, the better the concentration effect would be, and the less time would be needed.

1. INTRODUCTION Dyes can be classified as anionic, cationic, and nonionic dyes.1 Synthetic dyes are recalcitrant organic compounds that are extensively used in hair colorants, agricultural research, and different industrial areas, such as paper, textiles, pulp, rubber, leather tanning, and so forth.2 Until now, most dyes have been synthesized using chemical processes and purified using saltingout methods.3 Large amounts of salt remain in dyes after the salting-out processes, which seriously influence the stability and purity of dyes, and thus, reduce the quality of the products.4,5 Additionally, the discharge of huge volumes of high salinity, high concentration dye wastewater into the environment during production and application processes,6 which pollute the environment and endanger human health, is a serious matter.7,8 Thus, new methods to improve the purification processes and dye wastewater treatments are needed. Membrane separation methods are becoming more attractive, due to the modular design of the membrane processes and their purely physical separation processes.9 Nanofiltration (NF) membrane is a pressure-driven membrane with a nominal molecular weight cutoff (MWCO) in the range of 200−1000 Da, which implies an approximate pore size of 0.5−2.0 nm.10 NF membranes possess advantages over RO membranes in many respects, such as lower operation pressures, relatively high water fluxes, and rejection rates of multivalent ions, and low rejection rates of monovalent ions.11,12 Because of these advantages, the NF process has widespread applications in industrial fields, such as wastewater treatment, rejection of micropollutants, desalination, purification of pharmaceuticals, and separation and concentration of solutes, etc.13−17 Additionally, the NF process has proven to be a promising and effective method at separating low molecular weight compounds, such as dyes and divalent salts.18 Compared with conventional processes, the advantages of the NF process include facile operating conditions, low energy requirements, and relatively low maintenance costs.19 The NF methodology for dye purification contains two processes: desalination and © 2014 American Chemical Society

concentration. Desalination refers to the process by which salt molecules pass through the membrane under given conditions, while the dye molecules are retained and cycled back into the feed tank. The main purpose of desalination is to remove the salts from the feed solutions. The latter process refers to water being removed continuously until the dye concentration satisfies the spray drying requirements.20 The NF process is an effective and promising methodology for dye purification. Currently, most composite NF membranes present negative charges. However, the rejection rate of a positively charged NF membrane to divalent cations and cationic dyes is higher than that of a negatively charged NF membrane according to the Donnan exclusion principle.21,22 In recent years, positively charged, composite NF membranes have attracted a lot of attention. Polyethylenimine (PEI) is generally chosen as a monomer to prepare functional polymer membranes. Some researchers also fabricated a positively charged NF membrane using PEI as a reactive monomer. For example, Chiang et al. prepared different NF membranes using PEI and ethylenediamine (EDA) to react with terephthaloyl chloride (TPC) and trimesoyl chloride (TMC), respectively. Their studies indicated that compared with the EDA/TMC membrane, the PEI/TPC membrane showed relatively higher rejection to sodium chloride.23 Sun et al. fabricated positively charged NF membranes using isophthaloyl chloride (IPC) and hyperbranched PEI as reactive monomers on a polyamide-imide (PAI) substrate. The rejection rates of positively and negatively charged dye molecules were both over 99%.24 Qin et al. created positively charged NF membranes by the layer-by-layer assembly technique using poly(sodium 4-styrenesulfonate) (PSS) and PEI on polyacrylonitrile (PAN) ultrafiltration (UF) substrate. The rejection rates of NF membranes were Received: Revised: Accepted: Published: 14036

April 29, 2014 August 16, 2014 August 21, 2014 August 21, 2014 dx.doi.org/10.1021/ie5017688 | Ind. Eng. Chem. Res. 2014, 53, 14036−14045

Industrial & Engineering Chemistry Research

Article

substrate fibers for approximately 10 min. Excess solution was removed and the fibers were dried with pure nitrogen gas. Next, in the same manner, a 0.15 wt % TMC/n-hexane solution was remained in the PES substrate fibers for 40 s. Finally, the fabricated NF membranes were dried and then stored in deionized (DI) water. The possible chemical network structures of PEI/TMC, PES, and TMC are shown in Figure 2.

98.02% for CuSO4, 95.53% for ZnSO4, 95.66% for NiCl2, 94.9% for CdCl2, respectively.25 Moreover, Bai et al. developed a novel NF membrane using PEI/2-hydroxypropyl trimethylammonium chloride chitosan (HACC)/TMC as the reactive monomer. The prepared NF membrane was modified with TiO2 nanoparticles, and the membranes showed relatively higher dye rejection and lower salt rejection.26 It should be noted that most current NF modules are made from composite, flat sheet membranes.27 However, hollow-fiber membranes are more cost-effective for large-scale production and operation and exhibit higher surface to volume ratios, greater packing densities, and better self-support capabilities compared to flat sheet membranes. Thus, increasing amounts of attention will be paid to hollow-fiber membranes. Until now, very few studies have been reported on dye purification using positively charged, composite, hollow-fiber NF membranes. Thus, our study focuses on the dye purification processes using positively charged, composite, hollow-fiber NF membranes. Diafiltration experiments with various dye solutions were performed to investigate the separation performance of NF membranes. Moreover, constant volume batch diafiltration experiments were used to study the effects of different concentration factors on the dye purification processes.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. The poly(ether sulfone) (PES), hollow-fiber UF membranes were purchased from the Development Center of Water Treatment Technology, State Oceanic Administration, Hangzhou, China. PEI with a molecular weight of 70 000 Da, employed as the active monomer in the aqueous phase, was purchased from Aladdin Reagent Co., LLC, Shanghai, China. TMC used as the active monomer in the organic phase, was purchased from Qingdao Benzo Chemical Co., China. n-Hexane used as the organic solvent, was purchased from Shanghai Lingfeng Chemical Reagent Co., China. The dyes, including chromotrope FB, cationic red X-GTL and cationic gold yellow X-GL, were purchased from a local commercial group and were used as feed solutes to study the dye purification processes. The dyes molecular structures are presented in Figure 1. 2.2. Membrane Fabrication. The composite, hollow-fiber NF membrane was fabricated via interfacial polymerization. A 0.25 wt % PEI aqueous solution was guided inside the PES

Figure 2. Chemical structures of monomers and polymer.

2.3. Membrane Characterization. Field-emission scanning electron microscopy (FE-SEM, SIRION-100, FEI, Netherlands) was used to observe the cross sections and inner surfaces of the NF membranes. Surface charge properties of the NF membrane were studied through an electrokinetic analyzer (SurPASS, Anton Paar, Austria). Measurements were performed under 0.4 MPa with 0.001 mol/L aqueous KCl solutions and pH values ranging from 5 to 12. When the streaming potential was obtained, the surface zeta potential could be determined according to the Helmholtz−Smoluchowski equation.28 2.4. Membrane Permeation Performance. NF experiments were performed in a cross-flow filtration apparatus. Each membrane module was composed of eight hollow fibers, and the effective area was about 53 cm2. Before testing, the NF membranes were stabilized at 0.5 MPa for 1 h using DI water. Membrane performance was evaluated by a feed of DI water or solutions containing solutes of different molecular weights, such as magnesium sulfate (MgSO4), sodium chloride (NaCl), sodium sulfate (Na2SO4), magnesium chloride (MgCl2), and polyethylene glycol (PEG). All the concentrations of aqueous solutions were 1 g/L. To maintain a constant concentration, permeation and rejection solutions were recycled back to the feed. All NF permeation performance was carried out at 0.4 MPa with a constant temperature of 25.0 °C. To ensure reproducibility of results, parallel experiments were repeated at least three times. The permeate flux, F (L/m2·h), was calculated according to F=

Figure 1. Chemical structures of three dyes used in the experiments. 14037

V A ·Δt

(1)

dx.doi.org/10.1021/ie5017688 | Ind. Eng. Chem. Res. 2014, 53, 14036−14045


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Figure 3. FE-SEM images of (a) a cross-section of the composite, hollow-fiber NF membrane, (b) an enlarged lumen side cross-section of the composite, hollow-fiber NF membrane, (c) the inner surface of the composite, hollow-fiber NF membrane, and the (d) inner surface of the PES, hollow-fiber UF substrate.

of the rejection rates and fluxes were performed at least three times, and average values are reported in this study. 2.6. Dye Purification Processes. A laboratory-scale apparatus was used to simulate dye purification during constant volume batch diafiltration with hollow-fiber NF membranes.29 The initial volume of aqueous dye/salt solution was 1 L, which contained 1 g cationic red X-GTL dye and 10 g NaCl. Three different constant volume batch diafiltration processes were performed for dye purification. In the three diafiltration processes, when the volume of solution was reduced to 0.6, 0.5, and 0.4 L, respectively, extra pure water was added to the feed tank to recover the original volume of the dye/salt solution. This operation was repeated until a total of 2.4 L of pure water was added to the feed tank for each process. It should be mentioned that in order to keep adding the same volume of pure water in the three different processes, only 0.4 L of pure water was added in the last round of the second process. After the desalination process, the water and salt were removed continuously from the solution until the solution was concentrated to 0.15 L. The volume of the permeate solution was recorded, and the concentrations of dye and salt in the retentate and permeate were measured according to the methods described above. Every operation was carried out under 0.4 MPa pressure and at a constant temperature of 25.0 °C.

where Δt is the operation time (h), A is the effective permeation area (m2), and V is the volume of permeate (L). The rejection rate, R (%), was calculated as ⎛ Cp ⎞ R % = ⎜1 − ⎟100 Cf ⎠ ⎝

(2)

where Cf and Cp (g/L) are the concentrations of feed and permeate solutions, respectively. A total organic carbon analyzer (TOC-V CPN, Shimadzu, Japan) was used to measure the concentrations of different PEG solutions. The salt concentrations were determined by measuring the electrical conductivity, which can be obtained using a conductivity meter DDSJ-308A (Shanghai Precision & Scientific Instruments Co., China). 2.5. Nanofiltration of Dye Solutions. The separation performance of the NF membrane was studied at different conditions, including dye types and concentrations of dye and NaCl in the feed. Dye rejection rates and permeate fluxes were determined as described above. An ultraviolet−visible spectrophotometer (TU-1810, Beijing Purkinje General Instrument Co., China) was used to measured the dye concentrations. To reduce experimental error, a calibration curve for the spectrophotometric absorbance as a function of the dye concentration was established. All experimental measurements 14038



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3. RESULTS AND DISCUSSION 3.1. Membrane Characterization. Representative surface and cross-section SEM images of the membranes are presented

Figure 7. Separation behaviors of the hollow-fiber NF membrane in response to various inorganic salts tested with 1 g/L feed solutions at 0.4 MPa.

Table 1. Radius and Diffusion Coefficient of Different Ions

Figure 4. Zeta potential of the hollow-fiber NF membrane.

ion

ionic radius (nm)

hydrated radius (nm)

diffusion coefficient (10−9 m2/s)

Mg2+ Na+ Cl− SO42−

0.065 0.095 0.181 0.290

0.428 0.358 0.332 0.379

0.706 1.334 2.032 1.065

Figure 5. Variations of pure water flux under different operation pressures for the hollow-fiber NF membrane.

Figure 8. Effects of pH on different salt rejection rates of the membrane at 0.4 MPa.

Table 2. Rejection Rate and Permeate Flux to Different Dyes of the NF Hollow-Fiber Membrane at 0.4 MPa and 25.0 °C dye chromotrope FB cationic red XGTL cationic gold yellow X-GL

Figure 6. PEG rejection rates of the hollow-fiber NF membrane tested with aqueous solutions of 25 mg/L PEG at 0.4 MPa.

molecular weight (g/mol)

maximum absorption wavelength (nm)

dye retention rate (%)

permeate flux (L/ m2·h)

502.4

516

98.84

28.3

502.0

530

99.81

29.9

433.5

441

96.37

29.5

distributed small pores, while the inner surface of the NF membrane (Figure 3c) was smooth, uniform, and dense, with no pore structures in the active layer. The surface zeta potential of the NF membrane was determined and shown in Figure 4. The isoelectric point (IEP) of the NF membrane was observed at pH 8.1. At a pH value below the IEP, the thin film active layer exhibits positive

in Figure 3. As shown in Figure 3a, the main part of the crosssection exhibited finger-like structures. Figure 3b shows that a dense layer was composited onto the inner surface of the membrane. In Figure 3d, we can see that the inner surface of the UF substrate showed the presence of many uniformly 14039



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Figure 12. Impact of NaCl concentration on the rejection rate and permeate flux of the hollow-fiber NF membrane tested with aqueous solutions of 100 mg/L cationic red X-GTL at 0.4 MPa.

Figure 9. Effects of pH on different dye rejection rates of the membrane at 0.4 MPa.

3.2. Measurement of Membrane Separation Properties. Pure water flux, MWCO values, permeate flux, and different salt rejection rates were measured, and the impact of pH on salt rejection rates was investigated. Pure water flux of the NF membrane was measured at various operation pressures. As presented in Figure 5, the pure water flux increased linearly as operating pressure increased. At 0.4 MPa, the pure water flux was approximately 38 L/m2·h. The MWCO of the NF membrane was measured through permeation experiments, using PEG with various molecular weights (4000, 2000, 1000, 600, 400, and 200 Da) as model solutes. As shown in Figure 6, the MWCO of the NF membrane was about 511 Da. Separation properties of the NF membranes for different inorganic salts (NaCl, MgCl2, MgSO4, and Na2SO4) were examined in a series of permeation experiments. The salt rejection rates and permeation fluxes are shown in Figure 7. The sequence of different salt rejection rates was MgCl2 > MgSO4 > Na2SO4 > NaCl. The rejection rate was higher for MgCl2 (97.5%) than Na2SO4 (75.9%), which suggests that the hollow-fiber NF membrane was positively charged.31 In terms of the Donnan exclusion principle, the rejection rate of divalent cations (Mg2+) with a higher co-ion charge was greater than that of monovalent cations (Na+), and the rejection rate of divalent anions (SO42−) with a higher counterion charge was lower than that of monovalent anions (Cl−). Additionally, a rejection rate of Na2SO4 greater than that of NaCl was observed. The steric hindrance effect must be considered.32 As shown in Table 1,33,34 divalent anions (SO42−) have larger hydrated radii and lower diffusion coefficients than monovalent anions (Cl−). Thus, in terms of steric hindrance, the rejection of SO42− was greater than Cl−. In summary, for the sequence of rejection rates MgCl2 > MgSO4 > Na2SO4, the Donnan exclusion played an important role in the separation tests, while steric hindrance influenced the order of the rejection rates for Na2SO4 and NaCl in the separation process. Accordingly, the hollow-fiber NF membranes exhibited positive charges and the separation properties of the hollow-fiber NF membrane for inorganic salts was affected by both steric hindrance and Donnan exclusion. The effects of pH on salt rejection rates were further studied and the results are shown in Figure 8. We knew from Figure 4 that the IEP of the NF membrane was at pH 8.1. Therefore, at a pH value below the IEP, the NF membranes exhibit positive

Figure 10. Impact of dye concentration on the rejection rate and permeate flux of the hollow-fiber NF membrane tested with aqueous solutions of cationic red X-GTL at 0.4 MPa.

Figure 11. FE-SEM image of the inner surface of the NF membrane after the dye concentration experiment.

charges resulting from some protons coordinated with amine groups. However, these composite membranes possess negative charges if the pH is above the IEP, as deprotonated carboxyl groups are present.30 The results indicated that the NF membranes exhibited positive charges in neutral solutions. 14040



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Figure 13. Variations in permeate flux of the hollow-fiber NF membrane at 0.4 MPa during the three batch desalination and concentration processes.

charges and the sequence of the salt rejection rates was MgCl2 > MgSO4 > Na2SO4 > NaCl. It is different from the sequence for typical positively charged NF membranes (MgCl2 > MgSO4 > NaCl > Na2SO4). If the pH value was higher than the IEP, the NF membrane was negatively charged. At pH 10, the salt rejection sequence was Na2SO4 > MgSO4 ≈ MgCl2 > NaCl, which is still not consist with the rejection sequence of a typical negatively charged NF membrane. The NaCl rejection rate was relatively low all along and changed slightly with an increase in pH. However, the NF membranes showed relatively higher rejection rates of MgSO4 and MgCl2 in the entire pH range. This phenomenon was attributed to Mg2+ with a relative larger hydrated radius and lower diffusion coefficient. Therefore, we concluded that the separation properties of the NF membranes prepared with PEI and TMC are determined by both the Donnan exclusion effect and steric hindrance effect. However, the steric hindrance effect may play a relatively more important role than the Donnan exclusion effect. 3.3. Membrane Separation Properties for Dyes. 3.3.1. Permeate Fluxes and Rejections of Different Dyes. The steady permeate fluxes and dye rejections of the NF membrane for different dye solutions are shown in Table 2. The rejection rates of the different dyes followed this sequence: cationic red X-GTL > chromotrope FB > cationic gold yellow X-GL. This result can be explained by the effects of both Donnan exclusion and steric hindrance. At neutral pH, both the cationic dye molecules and the inner surface of the NF membranes exhibit positive charges, and Donnan exclusion promoted the rejection of dye molecules. Thus, the positively charged membrane rejected the cationic dye (cationic red XGTL) molecules more efficiently than it did the anionic dye (chromotrope FB) molecules. However, the rejection rate of

chromotrope FB exceeded that of cationic gold yellow X-GL, which contradicts the Donnan exclusion principle. This occurs because cationic gold yellow X-GL has a lower molecular weight than chromotrope FB. Thus, due to steric hindrance effects, the rejection rate was lower for cationic gold yellow XGL. Additionally, the permeate flux of NF membranes was relatively higher for aqueous solutions of cationic red X-GTL and cationic gold yellow X-GL than chromotrope FB. This behavior is due to electrostatic repulsions between cationic dye molecules and a positively charged membrane surface, which weaken the adsorption of the dye. Because of these results, in the experiments that followed, cationic red X-GTL was used as the objective dye molecules. The effects of pH on different dye rejection rates were also studied, and the results were presented in Figure 9. The rejection of chromotrope FB increased with the pH increasing. Chromotrope FB is anionic dye, and the rejection rate increased with increasing pH owing to the Donnan exclusion effect. Both cationic red X-GTL and cationic gold yellow X-GL present positive charges. The rejection rates of cationic red XGTL and cationic gold yellow X-GL decreased when the operation pH was higher than the IEP. When the pH value increased from 5 to 10, the membrane charges changed from positive to negative which resulted in the decline of rejections for the cationic dyes according to the Donnan exclusion. However, the NF membranes showed a relative high rejection for chromotrope FB and cationic red X-GTL in the entire pH range due to chromotrope FB and cationic red X-GTL having relative higher molecular weights than cationic gold yellow XGL. From the results we also concluded that the membrane separation properties are determined by both Donnan exclusion and steric hindrance. 14041



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Figure 14. Variations in the dye concentrations of the retentate and rejection rates of the hollow-fiber NF membrane at 0.4 MPa during the three batch desalination and concentration processes.

weakened.36 However, there were large amounts of Cl− after NaCl was added into the dye solution. What’s more, the Cl− would couple with the positive charge on the NF membranes surface, which shields the positive charges and reduces the electrostatic repulsion between the NF membrane and cationic red X-GTL. This would weaken the Donnan exclusion effect between the NF membrane and cationic red X-GTL and lead to relative lower dye rejection. Additionally, the permeate flux decreased from approximately 30.8 to 26.5 L/m2·h with increased salt concentration. The decreased permeate flux can be attributed to enhanced concentration polarization, which leads to higher osmotic pressure between both sides of the membrane.37 3.4. Dye Purification Processes. The simulated constant volume batch diafiltration process for dye purification was studied using hollow-fiber NF membranes with 1 L of an aqueous solution containing 10 g of NaCl and 1 g of cationic red X-GTL. Figure 13 shows the variation in permeate flux during the operation time of these three processes using the NF membranes. During each round of the desalination process, the permeate flux declined with operation time. However, the permeate flux largely recovered after the addition of pure water to the solution. These results can be attributed to the increased dye adsorption and solution viscosity, which resulted from the increased solution concentration that occurred during each batch desalination process. However, the concentration decreased after adding pure water to the solution, which significantly increased the permeate flux. Additionally, during the entire desalination and concentration process, the initial

3.3.2. Impact of Dye Concentration. The impact of dye concentration on dye purification was studied using different concentrations of cationic red X-GTL solutions which contained 10 g/L NaCl. The equilibrium permeates flux and dye rejection rate are presented in Figure 10. The permeate flux of the NF membrane decreased from 29.1 to 25.5 L/m2·h with increasing dye concentration. This decrease occurred because more dye molecules deposited onto the surface of the NF membranes as the dye concentration increased. The inner surface morphology of the NF membranes used for dye purification was characterized by FE-SEM, which was presented in Figure 11. From the image we can see obviously that some dye molecules deposited onto the surface of NF membranes. This deposition, in turn, increased the osmotic pressure, concentration polarization, and membrane fouling.35 However, these factors have slight effects on the rejection rates of different dye solutions, which were all more than 99.9%. 3.3.3. Impact of NaCl Concentration. The impact of NaCl concentration on dye purification was studied using aqueous solutions of cationic red X-GTL dye with different NaCl concentrations. As shown in Figure 12, when the NaCl concentration increased from 1 to 2 g/L, the salt rejection rate decreased from 47.8% to 19.3% and the dye rejection rate decreased from 99.4% to 96.8%. Cationic red X-GTL would ionize a SO42− and present a positive charge after it was dissolved in water. Of course, Na+ and Cl− in aqueous solution would couple with cationic red X-GTL; at higher feed salt concentration, the dyes would disperse more uniformly. Therefore, the fouling tendency of the membrane would be 14042



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Figure 15. Variations in NaCl concentrations of the retentate and rejection rates of the hollow-fiber NF membrane at 0.4 MPa during the three batch desalination and concentration processes.

permeate flux of the last round in processes 1, 2, and 3 was approximately 25 L/m2·h, 26.5 L/m2·h, and 28 L/m2·h, respectively. The total operating time for each process was approximately 12, 11.6, and 11.2 h, respectively. Thus, process 3 had the highest permeate flux and the shortest operating time. The variations in dye rejection rate and concentrations of retentate during the operation of the dye purification process are shown in Figure 14. The dye rejection rate and concentration of retentate steadily changed during the entire desalination process. The dye retentate concentration increased significantly with operating time during each round of the desalination process, which allowed more dye molecules to deposit and form a gel layer on the membrane surface. Thus, the dye rejection rate increased due to the additional resistance provided by the gel layer. However, after adding pure water to the solution, the rejection rate and concentration of dye decreased sharply. After finishing the desalination process, the solution continued to concentrate to the same volume under the same conditions. Finally, the dye rejection rates were stable above 99%, and the dye concentrations of the retentates increased to 3.9 g/L, 4.2 g/L, and 4.6 g/L for processes 1, 2, and 3, respectively. Thus, process 3 provided the best results for dye purification, and the average dye rejection rate reached 99.5% in the entire process. The variations in the NaCl rejection rate and concentrations of retentate during the operation of the three processes are presented in Figure 15. During each round of desalination process, the NaCl concentration in the retentate increased gradually. However, this concentration tended to decrease in the whole desalination process. The variation of the NaCl rejection rate was exactly opposite to that of the NaCl

concentration of retentate in the desalination process. The repulsive electrostatic force decreased as the NaCl and dye concentrations increased during each round of the desalination process, according to the Donnan exclusion, which led to the decline in NaCl rejection rate. Additionally, the decline of the NaCl rejection rate can result from the enhanced concentration polarization effect. The NaCl and dye concentrations decreased after adding pure water, which increased the NaCl rejection rate according to the same principle. For all three whole desalination processes, the NaCl rejection rates were less than 39%, and the NaCl removal rates reached approximately 78.7%, 81.1%, and 86.6% for processes 1, 2, and 3, respectively. After finishing the concentration process, the removal rates of NaCl from the dye/salt mixtures in the three processes were 93.1%, 94.7%, and 96.4% for processes 1, 2, and 3, respectively. Thus, process 3 provided the best desalting effect when the whole process of dye purification was completed.

4. CONCLUSIONS In this research, dye purification processes were studied using the positively charged, composite, hollow-fiber NF membranes prepared via interfacial polymerization. The hollow-fiber NF membranes possessed an MWCO of around 511 Da, an IEP at pH 8.1, and a pure water flux of 38 L/m2·h under operating pressure of 0.4 MPa. The rejection sequence of the positively charged, composite, hollow-fiber NF membranes was MgCl2 > MgSO4 > Na2SO4 > NaCl at neutral pH. The rejection rates of cationic red X-GTL, chromotrope FB, and cationic gold yellow X-GL were 99.8%, 98.8%, and 96.4%, respectively, at neutral pH. With the increase of NaCl and dye concentrations, permeate flux and rejection rates of dye and NaCl decreased, 14043



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while the rejection rate of cationic red X-GTL was higher than 99%. The simulation of a constant volume batch diafiltration process was used to purify cationic red X-GTL. After the entire desalination and concentration process, process 3 provided the best purification and highest concentration factor. The concentration of dye in the final solution reached 4.6 g/L, which was 4.6 times more concentrated than the original feed. Additionally, over 96.4% of the NaCl was taken away from the solution. These results suggest the impressive potential of positively charged, composite, hollow-fiber NF membranes for applications in dye purification.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Top Key Discipline of Environmental Science and Engineering (Grant No. 20130305) and the Zhejiang Provincial Department of Environmental Protection (Grant No. 2012B013, 2013A029). The authors sincerely thank the Natural Science Foundation of Zhejiang Province (Grant No. LY13H280009, LY14B070007).

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Application of Positively Charged Composite Hollow-Fiber

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