Evaluation of poly (ethylene glycol) grafting as a tool for improving membrane performance

A Dissertation entitled Evaluation of Poly (Ethylene Glycol) Grafting as a Tool for Improving Membrane Performance by Tilak Gullinkala Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Doctor of Philosophy Degree in Engineering Dr Isabel Escobar, Committee Chair Dr Maria Coleman, Committee Member Dr Dong-Shik Kim, Committee Member Dr Sasidhar Varanasi, Committee Member Dr Jared Anderson, Committee Member Dr Patricia Komunniecki, Dean College of Graduate Studies The University of Toledo May 2010 Copyright 2010, Tilak Gullinkala This document is copyrighted material Under copyright law, no parts of this document may be reproduced without the expressed permission of the author An Abstract of Evaluation of Poly (Ethylene Glycol) Grafting as a Tool for Improving Membrane Performance by Tilak Gullinkala As partial fulfillment of the requirements for the Doctor of Philosophy Degree in Engineering The University of Toledo May 2010 Although commercially available cellulose acetate membranes are characterized by having high fluxes during filtration as compared to other membrane materials, they are more prone to microbial attack and organic fouling because of their natural cellulose acetate backbone structures Fouling, or the accumulation of foreign substances on the membrane surface, occurs mostly due to hydrophobic interactions between the membrane and the foreign substances, especially natural organic matter (NOM) In order to reduce the hydrophobic interactions and thereby fouling due to NOM, flexible hydrophilic poly(ethylene glycol) (PEG) monomer chains were grafted to the cellulose acetate membrane to increase its hydrophilicity Two methods were used to achieve PEG grafting on the membrane surface In Method I, grafting was achieved by the action of an oxidizing agent for free radical development, followed by monomer for polymerization, and a chain transfer agent (CTA) for termination of the polymerization Two different techniques of introducing the chemicals to the membrane were investigated These were a iii bulk approach, where membranes were immersed in the chemical solutions, and drop approach, where chemicals were added drop wise to the surface of the membrane to avoid polymerization within the pores Both techniques led to improvements in membrane performance, as observed by lower fouling, lower flux declines and lower rates of flux decline, when compared to unmodified membranes While the drop approach displayed slightly higher initial flux values, the bulk method was preferred for its ease of modification and replication Method II was characterized by a greener solvent-free enzymatic polycondensation to graft PEG to the membrane surface NOM feed solutions were used to compare organic fouling between the modified and unmodified membranes Modification led to higher fluxes, lower flux declines, and a more reversible fouling layer easily removed by backwashing during operation Method I and II led to 16 and 17% increase in the pure water flux of the cellulose acetate membrane, respectively Both the methods resulted in improved membrane fouling resistance when using NOM as the feed content iv Acknowledgements It is a pleasure to thank those who made this thesis possible by guiding, motivating and helping me during my doctoral study at University of Toledo I owe my deepest gratitude to my advisor, Prof Isabel C Escobar, for welcoming me into her research group and guiding me throughout my research and study at UT Her enthusiasm in research and ceaseless liveliness has been a motivating factor my doctoral accomplishments Further, she was always accessible and inclined to support and help her students in their research As a result, research life became smooth and rewarding for me It was a delight to interact with Prof Maria Coleman by having her in my dissertation committee I am grateful to other committee members Drs Sasidhar Varanasi, Dong-Shik Kim and Jared Anderson for their advisory role I also want to thank department of chemical and environmental engineering for providing me this wonderful opportunity to pursue doctorate degree in engineering I am grateful to my friend, former colleague and lab-mate, Dr Rama Chennamsetty, as he was the person who introduced me to the world of polymeric membranes and taught me various surface characterization techniques I am indebted to my parents, Sanjeeva Rao and Tulasi Devi, for their endless love and continued support throughout my life I strongly believe that your deep penchant for education made me achieve this distinction in my life This thesis would not be possible without you When I look back, you were always there for me whenever the chips were down Our conversations always gave me the strength I needed to perform the task throughout my education You always provided the best possible environment for me to grow up in spite of all the hardships I never had to worry about anything other than my education throughout my schooling I am indebted to you for the same I would like to thank my friends and go-to couple Desi & Anu for being with me throughout the program I always had great time in their presence I also want to thank the team ACES I learnt a new sport and made a few good friends after joining the team v Table of Contents iii Abstract Acknowledgements v Table of Contents vi List of Tables x List of Figures xi xvii List of Abbreviations Introduction 1.1 Membrane Filtration 1.2 Modes of Filtration 1.3 Membrane Fouling 1.3.1 Inorganic Fouling 1.3.2 Organic Fouling 1.3.3 Biofouling 1.4 Fouling Prevention Research Objectives 10 2.1 PEG Grafting 11 2.2 Characterization of the Modification 12 vi 2.3 Evaluation of the Modification Literature Survey 14 3.1 Membrane Materials 12 16 3.1.1 Cellulose Acetate 16 3.2 Fouling: Limitations and Functioning 19 3.3 Hydrophilc Enhancer: Poly (Ethylene Glycol) 23 3.4 Surface Modification 24 3.4.1 25 3.4.2 Plasma Treatment 25 3.4.3 Ultraviolet (UV) Irradiation 27 3.4.4 Chemical Graft Polymerization 28 3.4.5 Ion Beam Irradiation Green Chemistry Method 31 Materials and Methods 34 4.1 Materials 34 4.1.1 Membranes 34 4.1.2 Chemical Reagents 35 4.1.3 Glassware and Labware 36 4.2 Chemical and Morphological Characterizations 37 4.2.1 Atomic Force Microscopy (AFM) 37 4.2.2 X-Ray Photoelectron Spectroscopy (XPS) 38 4.2.3 Fourier Transform Infrared (FTIR) Spectroscopy 38 4.2.4 Scanning Electron Microscopy / Energy Dispersive X-ray 40 Spectroscopy vii 4.3 Filtration Conditions 42 4.3.1 Dead-End Filtration 42 4.3.2 Filtration Protocol 44 4.3.3 Precompaction 44 4.3.4 Feed Solutions 44 4.3.5 UV-Vis Spectroscopy 49 4.3.6 Total Organic Carbon (TOC) Analysis 49 4.4 PEG Grafting 49 4.4.1 Method I 50 4.4.2 Method II 52 Results and Discussion: Method I 54 5.1 Surface Modification 54 5.1.1 Persulfate Functioning 54 5.1.2 Graft Polymerization 58 5.2 Influence of PEG Molecular Weight 62 5.2.1 Addition of PEG to the Surface 62 5.2.2 BSA Filtration 63 5.3 Techniques of Modification 66 5.4 Comparison of Bulk versus Drop Approaches 71 5.4.1 Feed Water: Dextran Solution 5.5 Bulk Approach 5.5.1 71 75 Feed Water: Synthetic Seawater viii 76 5.6 Summary and Discussion of Results Results and Discussion: Method II 84 87 6.1 Surface Modification 87 6.2 Characterization of Modification 89 6.3 Filtration Experiments 94 6.3.1 DI Water Filtration (Precompaction) 94 6.3.2 Dextran Filtration 95 6.3.3 BSA Filtration 99 6.3.4 Natural Organic Matter Filtration 100 6.3.5 NOM Fouling 102 6.4 Summary and Discussion of Results Conclusions and Recommendations 105 107 7.1 Background 107 7.2 Salient Features of Method I 108 7.3 Salient Features of Method II 110 7.4 Comparison of Method I and Method II 112 7.5 Recommendations for Future Work 113 References 116 Appendices A Auxiliary Flux Data 129 B Results and Discussion: Method III 132 C FTIR Spectroscopy Wave numbers 136 ix List of Tables 4.1 Membrane Specifications provided by GEWPT 34 4.2 Bench-top dead-end filtration cell 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properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes, Journal of Membrane Science, 188, 115–128 127 88 De Caro, J., Boudouard, M., Bonicel, J., Guidoni, P., Desnuelle, P., & Rovery, M (1981) Porcine Pancreatic lipase: completion of primary structure, Biochimica et Biophysica Acta, 671, 129-138 89 Kang, G., Liu, M., Lin, B., Cao, Y., & Yuan, Q (2007) A novel method of surface modification on thin-film composite reverse osmosis membrane by grafting poly(ethylene glycol), Polymer, 48, 1165-1170 90 Bagi, K., Simon, L M., & Szajáni, B (1997) Immobilization and characterization of porcine pancreas lipase Enzyme and Microbial Technology, 20, 531-535 128 Appendix A Auxiliary Flux Data Additional data for the synthetic sea water feed filtration through modified and unmodified mwembranes were presented in the following graphs (Figure 8.1 – 8.5) Unmodified Membrane Precompaction 160 Flux, L/(m2-hr) 140 Modified Membrane Precompaction 120 100 80 60 Unmodified Membrane Filtration 40 20 0 50 100 150 Time, 200 250 300 Modified Membrane Filtration Figure A-1: Influence of PEG grafting on membrane flux: run 129 Unmodified Membrane Precompaction 160 Flux, L/(m2-hr) 140 Modified Membrane Precompaction 120 100 Unmodified Membrane Filtration 80 60 40 50 100 150 200 250 300 Modified 350 Membrane Filtration Time, Figure A-2: Influence of PEG grafting on membrane flux: 30 run Unmodified Membrane Precompaction 160 L/(m2-hr) 120 Flux, 140 80 Modified Membrane Precompaction 100 Unmodified Membrane Filtration 60 40 100 200 Time, 300 400 Modified Membrane Filtration Figure A-3: Influence of PEG grafting on membrane flux: hour run 130 Flux, L/(m2-hr) Unmodified Membrane Precompaction 180 160 140 120 100 80 60 40 20 Modified Membrane Precompaction Unmodified Membrane Filtration 100 200 Time, 300 Modified 400Membrane Filtration Figure A-4: Influence of PEG grafting on membrane flux: hour run Unmodified Membrane Precompaction 160 Flux, L/(m2-hr) 140 120 Modified Membrane Precompaction 100 80 60 Unmodified Membrane Filtration 40 20 -50 50 150 250 350 Time, 450 Modified 550 Membrane Filtration Figure A-5: Influence of PEG grafting on membrane flux: hour run 131 Appendix B Results and Discussion: Method III 8.1 Surface Modification Free radical graft polymerization of poly ethylene glycol (PEG) on the membrane surface was carried out through the application of benzoyal peroxide (BPO) as the free radical initiator Reaction was carried out in a round bottom flask at 55-650C PEG 200 was used as the monomer side chain in the reaction Methyl alcohol was used as the solvent medium in the proposed graft polymerization BPO dissociation is shown in Figure 8.1 O O O O O O O O Figure B-1: BPO dissociation at high temperatures (60- 900C) Once the BPO dissociates, PEG chain propagation takes place on the membrane surface as well as in the liquid phase due to the extreme reactivity of benzoyal radicals Reaction was carried out for to hours for the polymerization to take place 132 8.2 Characterization of Modification SEM analyses of the modified membranes clearly indicated the formation of a thorough PEG layer on the membrane surface as shown in Figure 8.2 A and B Surface features gained on the membrane due to PEG layer formation through modification were clearly perceivable in the SEM images On the other hand, unmodified membrane SEM images were characterized by lack of any noticeable surface features as shown in 6.3 A and B In some modification experiments, it was observed that some membranes underwent loss of selective layer due to the high temperatures and toxic solvents involved in the reaction Figure B-2 A and B: SEM images of the modified membrane showing a thorough PEG layer on the membrane surface 133 Figure B-3 A and B: SEM images of unmodified cellulose acetate membrane showing lack of surface features on the surface FTIR analysis of the modification indicated the PEG grafting on the membrane surface PEG grafting was confirmed by strong OH stretching and CO stretching as shown in Figure 8.4 Unmodified Membrane Modified Membrane CO Stretching 500 1000 1500 2000 OH Stretching 2500 3000 3500 Wave Number, ν Figure B-4: FTIR analysis of Method II modification 134 4000 4500 8.3 BSA Filtration Flux of the BSA feed solution did not increase, but slightly decreased due to the modification This could be due to the formation of a thorough PEG layer on the surface, which could contribute to the pressure drop severely The BSA flux data is shown Figure 8.5 Though total rejection of BSA was observed, flux loss due to extensive in situ PEG grafting led to the flux drop via modification 60 Unmodified Membrane Precompaction Flux (L/sqm/hr) 50 40 Unmodified Membrane Filtration 30 Modified Membrane Filtration 20 10 0 200 400 Time (min) 600 Modified Membrane 800 Filtration Figure B-5: BSA flux comparison between unmodified and modified membranes 135 Appendix C FTIR Spectroscopy: Wave Numbers Please use the following table when referring to FTIR spectra presented in the document Table C-1: Wave numbers information of FTIR spectroscopy Wavenumber, ν Functional group Membrane Spectrum Range 420-530 1034 420-530 1060-1025 1030-950 1065-1015 C-O-C in ethers CH2 OH in primary alcahols Carbon ring in cyclic compounds CH OH in cyclic alcahols 1100 1126 1120-1080 1150-1070 1200-1015 C OH in alcahols C O C in aliphatic ethers C OH in alcahols 1220 1280 1225-1200 1240-1070 1280-1150 C O C in vinyl ethers C O C in ethers C O C in esters, lactones 1300 1335-1295 SO2 antisymmetric stretch 1366 1388 1380-1370 1400-1310 CH3 in aliphatic compounds COO- group; symmetric stretch 136 1600 1735 1610-1560 1750-1740 COO- group; antisym stretch C==O in esters 2840 2900 2990-2850 CH symmetric and antisymmetric stretch 3480 3420-3250 OH stretch in solids 137