Study on Modification of Nanofiltration Membrane

1. JOURNAL CLUB Study on Modification of Nanofiltration Membrane Hai Yuyan 2012.10.9

2. Designing the Next Generation of Chemical Separation Membranes Douglas L. Gin and Richard D. Noble Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309–0424, USA Science, 2011, 332:674-676

3. Background • Membrane-based chemical separations can have advantages over other methods—they can take less energy than distillation or liquefaction, use less space than absorbent materials, and operate in a continuous mode. • We discuss how membranes work, and some notable new approaches for improving their performance.

4. Dense Membranes orange and green molecules move through the membranes at different rates because they have different permeabilities P. The Robeson plot shows that conventional dense membranes separate mainly via differences in diffusivity, and performance is limited by an “upper bound.” Membranes are either dense or porous, depending on how the molecules move across the barrier. In dense membranes, molecules dissolve into the material and diffuse through it.

5. Porous Membranes Nanoporous membranes separate via molecular size differences. With uniform pore sizes, it is possible to get complete separation (smaller molecules pass through—they have a higher molecular flux; larger ones are completely blocked). With nonuniform pores, the largest pore sizes (i.e., a distribution) dictate the selectivity, and both molecules can pass through.

6. New Approaches • A new approach in the design of dense membranes is to useroom-temperature ionic liquids (RTILs)in various morphologies. RTILs are liquid-phase organic salts (i.e., ionic compounds) with negligible vapor pressure (avoiding evaporation losses), high thermal stability, and intrinsic solubility for certain gases. Unlike conventional polymers, RTILs perform gas separations via solubility differences. • For nanoporous membranes, several methods have recently been developed that afford materials with uniform molecular-size pores. For example, deposition techniques have been successfully used to reduce the pore size of commercial nanoporous polymer and ceramic membranes down to molecular dimensions. • Recent advances in blending organic polymers with inorganic zeolites have afforded viable composite membranes with uniform pore sizes in the 0.3- to 0.7-nm range for light gas separations, such as CO2 , N2 , and CH4.

7. pH-responsive nanofiltration membranes by surface modification Heath H. Himstedt, Kathryn M. Marshall,S. Ranil Wickramasinghe∗ Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, CO 80523-1370, USA Journal of Membrane Science, 366 (2011) 373–381

8. Background • Fouling of nanofiltration membranes remains a major concern that often limits process viability. One method to minimize fouling is to modify the filtration surface and perhaps the pores of the membrane in order to minimize adsorption of dissolved solutes. • A commercially available nanofiltration membrane (NF 270) has been surface modified using UV initiated polymerization to grow polyacrylic acid(聚丙烯酸) nanobrushes from the surface of the membrane.

9. ATR-FTIR The modified membranes contain a new peak at approximately 1720 cm−1 corresponding to vibration of the carboxylic groups(C=O), indicating the attachment of polyacrylic acid nanobrush. The intensity of the peak increases with increasing monomer concentration used during polymerization and UV reaction time.

10. XPS Modified membranes show a clear carboxylic carbon peak at approximately 288 eV. The intensity of the peak increases with increasing monomer concentration used during polymerization and UV reaction time in agreement with the ATR-FTIR data.

11. DI water/HCl/NaOH fluxes

12. Filtrate Flux • Electrostatic interactions are also important. Studies indicate that the zeta potential of NF 270 is around 0 at pH 3.5 but about −20 mV at pH 7.0. The higher concentration of charged solutes present (e.g. Na+ and Cl−) will lead to a higher osmotic pressure compared to DI water resulting in a reduced filtrate flux. • If the pH of the feed is above the pKa=4.25 of acrylic acid, the carboxylic groups will be deprotonated and swell.

13. Rejection of glucose

14. Rejection of glucose • The change in glucose rejection in the presence of other ionic species is due to interactions between the ionic species and the membrane. However in their expanded, charged conformation, the grafted layer affects membrane performance.

15. UV-Photo Graft Functionalization of Polyethersulfone Membrane with Strong Polyelectrolyte Hydrogel and Its Application for Nanofiltration Roy Bernstein,Enrique Anton,and Mathias Ulbricht Lehrstuhl fu ̈ r Technische Chemie II, Universita ̈ t Duisburg-Essen, 45117 Essen, Germany Department of Chemical and Environmental Engineering, University of Oviedo, 33006 Oviedo, Spain ACS Applied Materials & Interfaces, 2012, 4: 3438 – 3446

16. Background • The feasibility of charged nanofiltration (NF) membranes fabrication using polyelectrolytes as the active layer is being explored in the past few years. This is primarily done through two methods. The first one is synthesis of a polyelectrolyte, either inside the pores of an ultrafiltration (UF) base membrane, thus obtaining a pore-filling composite membrane, or on the outer surface of an UF membrane, resulting in a thin-film composite membrane. The second method is through the deposition of polyelectrolyte, the “layer by layer” (LBL) technique, on or within a porous polymeric support, or inorganic support. Yet, these membranes still have some drawbacks compared with the commercially available NF membranes that withhold their further expansion. • The photoirradiation-induced radical graft copolymerization technique was recently successfully applied for surface modification of hydrogels on UF membranes. This technique has several advantages: it generates a rapid reaction and is performed under mild conditions with various monomers using simple equipment at a relatively low cost.

17. A strong polyelectrolyte hydrogel was graft copolymerized on a polyethersulfone (PES 聚醚砜) ultrafiltration (UF) membrane using vinyl sulfonic acid (VSA 乙烯磺酸)as the functional monomer, and N,N'-methylenbisacrylamide (MBAA) as the cross-linker monomer. This was carried out in one simple step using the UV photoirradiation method. VSA MBAA

18. Degree of grafting The degrees of modification measured by the two techniques have a similar trend: a linear increase in the DG with cross-linker concentration. The DG without cross-linker was very low. This is probably a consequence of wetting or diffusion limitation due to incompatibility between the charged monomer and the hydrophobic surface.

19. Degree of grafting The increase of the DGs with modification time is monotonic. However, the DGg rises fast in the early stages and then the increase moderates. Therefore, it can be assumed that in the early stages it is mainly the cross-linker monomer that is grafted to the surface, and then, either because of the cross-linker ’ s two double bonds or a change in the surface properties, the functional monomer (VSA) grafting is enhanced.

20. It was found that when the modification was carried out using low UV intensity the modification degree and the membrane performance were better than for modification at high UV intensity. Polymerization at too high intensities can be monomer diffusion limited immediately in the early stages, due to the high initiator radicals concentration, whereas for polymerization at low UV intensities, the diffusion limitation occurs at later stage. Degree of grafting

21. salt rejection The salt rejection was in the order Na2SO4 > MgSO4 ≈ NaCl > CaCl2 , as expected for rejection based on Donnan exclusion for negatively charged NF membranes. Because rejection of uncharged solutes with charged NF membranes derives mainly from steric exclusion.

22. Conclusion • It is well known that the hydrophilic surface has lower tendency to fouling[1]. • 对自制的聚酰亚胺纳滤膜(BTDA-ODA 型，其自身含有光敏性基团)，进行紫外辐照接枝改性。随着光照时间的增长，改性膜的接枝率逐渐增加，意味着膜表面微孔孔径随光照时间的增长而减小[2]. • 研究了不同操作条件对自制的氟酐型聚酰亚胺纳滤膜分离螺旋霉素-乙酸丁酯萃取液性能的影响[3]。 • 在紫外光照射条件下把丙烯酸或甲基丙烯酸甲酯接枝到自制的氟酐型聚酰亚胺纳滤膜上[4]. [1] Ahmad Rahimpour Korean J. Chem. Eng., 2011, 28: 261-266. [2] 陈洪杰（孔瑛）. 聚酰亚胺纳滤膜改性的研究[D]. 东营: 中国石油大学(华东), 2010. [3] 宋力航, 孔瑛, 杨金荣, 史德青, 李阳初. 化学工业与工程, 2009, 26(1): 50-53. [4] 李林英, 丛晓英. 内蒙古石油化工, 2009, 1: 8-9. • Using the UV photoirradiation method for membrane surface modification.

23. Thank you !