Low-Pressure Nanofiltration (NF) Hollow Fiber. Membranes for Effective Fractionation of Dyes. and Inorganic Salts in Textile Wastewater

Transcription

1 Supporting Information (SI) Low-Pressure Nanofiltration (NF) Hollow Fiber Membranes for Effective Fractionation of Dyes and Inorganic Salts in Textile Wastewater Gang Han, Tai-Shung Chung *,, Martin Weber, Christian Maletzko Department of Chemical & Biomolecular Engineering, National University of Singapore, Singapore Advanced Materials & Systems Research, BASF SE, RAP/OUB-B001, Ludwigshafen, Germany Performance Materials, BASF SE, G-PMF/SU-F206, Ludwigshafen, Germany Correspondence to: T. S. Chung ( Tel: ; Fax: Comprising 15 pages: 9 pages of text, 3 tables and 3 figures. S1

2 1. Materials and Methods 1.1 Materials The Torlon 4000T-MV polyamide-imide (Solvay Advanced Polymers, USA) and the sulfonated polyphenylenesulfone (sppsu) with a molecular weight of 64.2 kda and an ion exchange capacity of meq/g (BASF SE, Germany) polymers were used to fabricate the hollow fiber membranes. Hyperbranched polyethyleneimine (PEI) with a molecular weight of 1800 g/mol (Sigma-Aldrich, Singapore) was utilized as the crosslinker for membrane modification. Figure S1 illustrates their chemical structures. N-methyl-2-pyrrolidinone (NMP) and polyethylene glycol with a molecular weight of 400 Da (PEG 400) were purchased from Merck and applied as the solvent and additive, respectively, for the spinning dope solution. Polyethylene glycols with various molecular weights (PEG, Merck) were employed to characterize the pore size and pore size distribution of the hollow fibers. 2-propanol, NaCl, Na 2 SO 4, MgCl 2 and MgSO 4 (analytical grade) were provided by Merck. Indigo Carmine (INCA, 85 %), Rose Bengal (RB, 95%), and Alcian Blue 8GX (AB-8, 45-65%) ordered from Sigma-Aldrich were applied as the model dyes in this study. Hydrochloric acid (HCl, 37%) and sodium hydroxide (NaOH, >99%) from Merck were used to adjust the ph values of the feed solutions. Deionized (DI) water with a resistivity of 15 MΩ cm was produced by a Milli- Q unit (Millipore). 2.2 Fabrication of the Loose NF Hollow Fiber Membranes Non-solvent induced phase separation (NIPS) process was used to fabricate the Torlon&sPPSU hollow fiber membranes via a dry-jet wet spinning technique. The membrane morphology and microstructure were optimized by controlling the dope composition, spinning parameters and coagulation conditions. Table S1 summarizes the specific spinning parameters and the detailed experimental procedures were similar to those reported in our S2

3 previous work. 1 The addition of a small amount of hydrophilic sppsu into the Torlon dope solution is to (1) optimize the membrane morphology and pore structure by altering the membrane formation during phase inversion, 1-6 (2) improve the membrane hydrophilicity and thus enhance water permeability and (3) introduce negative charge properties on the membrane surface. Since the Torlon&sPPSU hollow fiber is inner selective, hyperbranched polyethyleneimine (PEI) was applied to modify its inner surface. 7 A 2.0 wt% cross-linking solution was prepared by dissolving PEI in a 1:1 (wt%) mixture of 2-propanol and water. Before conducting the surface modification, hollow fiber modules were fabricated and flushed with deionized water to remove the additives containing in the membranes. Then, the PEI cross-linking solution was flowed through the lumen side of the hollow fibers at 75 C for 1 h. After that, the modified membranes were rinsed with deionized water to remove the excess chemicals and then kept in deionized water for further characterizations. The as-spun and PEI modified hollow fibers were termed as Torlon&sPPSU and PEI-Torlon&sPPSU, respectively. 2. Characterizations 2.1 Membrane Morphology The surface and cross-section morphology of the hollow fiber membranes were observed through a Field Emission Scanning Electron Microscope (FESEM, JEOL, JSM-6700F). For cross-section imaging, the freeze-dried membrane samples were fractured in liquid nitrogen. Before FESEM observation, the samples were sputter-coated with platinum nanoparticles using an ion sputtering device (JEOL JFC-1000E). 2.2 Membrane Surface Charge, Hydrophilicity and Chemistry S3

4 The surface charge property of hollow fibers as a function of ph was characterized through streaming zeta potential measurements performed with a SurPASS electrokinetic analyzer (Anton Paar GmbH, Austria). In order to conduct the measurements, Torlon&sPPSU and PEI-Torlon&sPPSU flat-sheet membranes were prepared by using the same dope for hollow fiber spinning and the same procedures for the PEI modification. During the tests, a 0.01 M NaCl electrolyte solution was constantly pumped through the clamping cell which contained the membrane sample, while the ph of the electrolyte solution was constantly adjusted via auto titrations with 0.1 M NaOH and 0.1 M HCl. Once the zeta potential curve as a function of ph was established, the isoelectric point of the membrane was determined. Water contact angles were also measured on the aforementioned flat-sheet membrane samples by using a Contact Angle Geniometer (Rame Hart, USA) with deionized water as the probe liquid at 23±0.5 C. The membrane samples were freeze dried overnight prior to measurements. A drop of deionized water was introduced on the membrane surface and the contact angle was taken immediately and calculated by the software. At least ten readings were taken at random locations for each sample and the averaged value was reported. The surface chemistry of hollow fibers was examined by an X-ray photoelectron spectroscopy (XPS) performed with a Kratos AXIS UltraDLD spectrometer (Kratos Analytical Ltd., Manchester, UK). The freeze-dried membrane samples were cut along the axis with a blade knife and pasted onto the XPS sample holders with the inner surface faced up for measurements. 2.3 Molecular Weight Cut-Off (MWCO), Mean Pore Size and Pore Size Distribution The MWCO, effective mean pore size and pore size distribution of hollow fibers were determined by solute filtration experiments. 8,9 The rejections of the membranes to 200 ppm PEG solutes with various molecular weights were measured at a hydraulic pressure of 1 bar. S4

5 The feed solution was circulated for 1 h before the concentrations of both the feed and permeate were examined. Between runs of different PEG solutes, the membrane was flushed thoroughly with deionized water. The PEG concentrations of the feed solution (C f ) and the permeate (C p ) were measured by a Total Carbon Analyzer (TOC, Multi N/C 3100, Analytik Jena, Germany). The effective rejection R (%) of each PEG solute was then calculated as: C = p R 1 100% (S1) C f By assuming that the membranes have no steric and hydrodynamic interactions with the PEG solutes, the membrane pore size can be predicted. By plotting PEG rejection R against the solute Stoke diameter on a log-normal probability graph, a linear fit was obtained mathematically. The correlation between the Stokes diameter (d s ) and the molecular weight (M w ) of PEG solutes can be depicted as: w 12 d = M ( M 35,000) (S2) s The membrane MWCO was determined by the molecular weight of the PEG solute when the effective rejection R is 90%. Subsequently, the probability density function curve of the membrane can be constructed based on the following formula: w dr ( d ) dd p p lnσ p (ln d exp 2π 2 lnµ ) 2 p 1 p p = 2 d ( lnσ ) p (S3) where d p is the pore diameter. The effective mean pore size (µ p ) is the pore diameter at which R is 50%, and the geometric standard deviation (σ p ) is the ratio of pore diameters when R values are 84.13% and 50%, respectively. 3. Nanofiltration (NF) Experiments The NF experiments were conducted in a lab-scale cross-flow filtration system. 7 Because the hollow fibers were inner-selective, the feed solution was pumped through the lumen side of S5

6 the fiber module, while the permeate exited from the shell side of the module. A relatively low volumetric feed flow rate of 0.15 L/min (1.7 m/s) was applied to eliminate the pressure drop. Before measurements, the hollow fibers were conditioned at the testing pressure for 2 h. In terms of permeation flux and solute rejection, the filtration performance of the hollow fibers was characterized by NF experiments using (1) deionized water to measure the pure water permeability (PWP); (2) single-component solutions containing an inorganic salt (i.e., NaCl, Na 2 SO 4, MgCl 2 or MgSO 4 ) or a dye at various solute concentrations and (3) 200 ppm dye solutions at various ph values. The main characteristics of these acid and basic textile dyes are tabulated in Table S2. The ph of the dye solutions was adjusted by NaOH (1.0 M) and HCl (1.0 M) solutions and was determined via a ph meter (Horiba ph meter D-54, Japan). After that, the dye/salt fractionation performance was evaluated by using multicomponent feed solutions consisting of 200 ppm INCA or AB-8 dye in the presence of different amounts of Na 2 SO 4. For the long-term performance and fouling tests, around 1.0 L of an AB-8/Na 2 SO 4 solution consisting of 200 ppm AB-8 and 2000 ppm Na 2 SO 4 was used as the feed. The mixture was continuously run through the fibers until the predetermined testing duration or feed recovery rate was reached. During the total recirculation operation mode, permeate of the fibers was bypassed into the feed tank to maintain the feed composition. The permeate sample was collected at certain time intervals to monitor the membrane performance. The feed water recovery rate (R e ) under the concentrate operation mode was calculated by the following equation: V R e = 100% (S4) V f, i where V is the permeate volume and V f,i is the initial volume of the feed solution. Membrane cleaning was conducted by physical backwash of freshwater at 1 bar for 1 h. All filtration tests were carried out at 23±1 C. In order to ensure the repeatability of the S6

7 experiments, at least three fibers were tested for each testing condition and the averaged results were reported. It is worthy to note that the error bars were not presented in the figures in order to make them easy to read. 4. Membrane flux and solute retention The membrane pure water permeability, PWP (L m -2 h -1 bar -1, abbreviated as LMH/bar), was measured by using deionized water as the feed. PWP was calculated using the equation: Q PWP= (S5) A P where Q is the water permeation volumetric flux (L/h), A is the effective membrane surface area (m 2 ), and P is the transmembrane hydraulic pressure (bar). The concentrations of the dye solute in the feed and permeate solutions were determined by the UV vis integral method. 10,11 This method was based on the UV vis integral (integrated range: nm) that was obtained through scanning the sample by a UV vis spectrophotometer (Pharo 300, Merck). Since the UV vis integral has a linear relationship with dye concentration only when the concentration is below 100 ppm, the concentrated dye solutions were diluted to less than 100 ppm in order to achieve a reasonable accuracy. The salt concentration of single electrolyte solution was measured by an electric conductivity meter (Schott Instruments, Lab 960, Germany). In order to measure the salt concentration of the dye/salt mixture, the solution was filtrated via a membrane filter (Millipore, Singapore Pte. Ltd., MWCO 500 or 1000 Da) that can remove the dye molecules but completely permeate the salt. The permeation flux, J w (L m -2 h -1, abbreviated as LMH), was calculated as: J w V = (S6) A t S7

8 where V (L) is the volume of permeate collected during the time interval t (h). The rejection of salt (R s ) or a dye (R d ) was calculated as: C (%) = 1 p R 100 (S7) C f where C f and C p are the solute concentrations in the feed and permeate, respectively. REFERENCES (1) Han, G.; Feng, Y.; Chung, T. S.; Weber, M.; Maletzko, C. Phase inversion directly induced tight ultrafiltration (UF) hollow fiber membranes for effective removal of textile dyes. Environ. Sci. Technol. 2017, DOI: /acs.est.7b (2) Li, S.; Cui, Z.; Zhang, L.; He, B.; Li, J. The effect of sulfonated polysulfone on the compatibility and structure of polyethersulfone-based blend membranes. J. Membr. Sci. 2016, 513, (3) Gao, J.; Thong, Z., Wang, K. Y.; Chung, T. S. Fabrication of loose inner-selective polyethersulfone (PES) hollow fibers by one-step spinning process for nanofiltration (NF) of textile dyes. J. Membr. Sci. 2017, 541, (4) Feng, Y.; Han, G.; Zhang, L.; Chen, S. B.; Chung, T. S.; Weber, M.; Staudt, C.; Maletzko, C. Rheology and phase inversion behavior of polyphenylenesulfone (PPSU) and sulfonated PPSU for membrane formation. Polymer 2016, 99, (5) Wang, D. M.; Lai, J. Y. Recent advances in preparation and morphology control of polymeric membranes formed by nonsolvent induced phase separation. Curr. Opin. Chem. Eng. 2013, 2, (6) Xia, Q. C.; Wang, J.; Wang, X.; Chen, B. Z.; Guo, J. L.; Jia, T. Z.; Sun, S. P. A hydrophilicity gradient control mechanism for fabricating delamination-free dual-layer membranes. J. Memb. Sci. 2017, 539, S8

9 (7) Sun, S. P.; Hatton, T. A.; Chung, T. S. Hyperbranched polyethyleneimine induced cross-linking of polyamideimide nanofiltration hollow fiber membranes for effective removal of ciprofloxacin. Environ. Sci. Technol. 2011, 45, (8) Van der Bruggen, B.; Vandecasteele, C. Modelling of the retention of uncharged molecules with nanofiltration. Water Res. 2002, 36, (9) Aimar, P.; Meireles, M.; Sanchez, V. A contribution to the translation of retention curves into pore size distributions for sieving membranes. J. Memb. Sci. 1990, 54, (10) Han, G.; Liang, C. Z.; Chung, T. S.; Weber, M.; Staudt, C.; Maletzko, C. Combination of forward osmosis (FO) process with coagulation/flocculation (CF) for potential treatment of textile wastewater. Water Res. 2016, 91, (11) Liang, C. Z.; Sun, S. P.; Zhao, B. W.; Chung, T. S. Integration of nanofiltration hollow fiber membranes with coagulation flocculation to treat colored wastewater from a dyestuff manufacturer: a pilot-scale study. Ind. Eng. Chem. Res. 2015, 54, S9

10 Figure S1. Chemical structures of (a) Torlon 4000T-MV polyamide-imide (Torlon), (b) sulfonated polyphenylenesulfone (sppsu), and (c) hyperbranched polyethyleneimine (PEI). S10

11 Figure S2. (a) Pure water permeability (PWP) and (b) rejections to NaCl of Torlon&sPPSU and PEI-Torlon&sPPSU hollow fiber membranes as a function of operating pressure. S11

12 Figure S3. Cleaning efficiency of freshwater backwash for Torlon&sPPSU and PEI- Torlon&sPPSU hollow fiber membranes after 168-h tests. The cleaning was performed by back flushing freshwater for 1 h at 1 bar. S12

13 Table S1. Spinning parameters of the Torlon&sPPSU hollow fiber membrane Spinning parameters Polymer dope composition (wt %) Torlon&sPPSU 18%Torlon+2%sPPSU2.5+13%PEG+67%NMP Bore fluid (wt %) Water/NMP (90/10) Dope flow rate (ml/min) 2 Bore fluid flow rate (ml/min) 1.5 Air gap length (cm) 6 Take up speed (m/min) 3.5 Fiber ID/OD (µm) 788/1177 External coagulant Water Spinneret dimension (mm) i.d./ o.d. ( ) S13

14 Table S2. Characteristics of the dyes used in this study S14

15 Table S3. XPS analysis of Torlon&sPPSU and PEI-Torlon&sPPSU hollow fiber membranes Fiber code Atomic concentration (%) N/O C N O S Torlon&sPPSU PEI-Torlon&sPPSU S15