Poly(vinyl alcohol) / polyamide thin-film composite membranes.

Elharati, M. A. (2009-12)

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2009.

Thesis

ENGLISH ABSTRACT: The aim of this study was to modify the surface of polyethersulfone (PES) ultrafiltration (UF) membranes to produce a more hydrophilic membrane by cross-linking poly(vinyl alcohol) (PVA) with sodium tetraborate (Na2B4O7.10H2O) (SB) on the surface. Key preparation factors were identified as PVA molecular weight, concentrations of the PVA and SB, cross-linking reaction time, number of coatings and the mode of coating. The effect of these factors on the membrane performance (salt retention and permeate flux) is discussed. These PVA-SB membranes typically had 11.46% retention and 413.30 L/m2.h flux for a feed containing 2000 ppm NaCl (0.45 MPa, 20°C, 45 – 50 L/h). The coating was shown to be uniform and stable by Fourier transform infrared spectroscopy (FT-IR) analyses. Coating significantly increased hydrophilicity and a maximum flux increase of 500 L/m2.h was reached. Measurements showed a reduced water contact angle and this confirmed the obvious enhancement of surface hydrophilicity. As a control, the role of the PVA base layer without cross-linking and the effects of its drying and heating on the water permeability of the PES-UF membrane were also studied, in order to ascertain maximum treatment conditions. Retention and permeate flux were determined (feed solution: 2000 ppm NaCl, applied pressure 0.45 MPa, 25°C, 45 – 50 L/h). It was found that the heating had the largest effect on the reduction of water permeability and therefore 50°C was the limit for treatment of this specific PES-UF membrane. Thin-film composite (TFC) membranes were prepared by an interfacial polymerization (IP) reaction between a polyfunctional amine and tri- or di-functional carboxylic chloride and then evaluated for their reverse osmosis (RO) performance. The salt retention of the PVA-SB membranes was improved when covering the cross-linked PVA gel sub-layer with a polyamide (PA) layer. However, the permeate flux decreased to below 30 L/m2.h (2000 ppm NaCl, 1 – 2 MPa, 20°C, 45 – 50 L/h). Two TFC membranes made from trimesoyl chloride (TMC) with m-phenylenediamine (MPD) or 2,6-diaminopyridine (DAP) exhibited retentions of 96.71% to 89.65% and fluxes of 10.93 to 27.91 L/m2.h, depending on the type of diamine used, when tested with a 2000 ppm NaCl solution (2 MPa, 25°C, 45 – 50 L/h). Two TFC membranes made from a n ew 2,5-furanoyl chloride (FC) with MPD or DAP exhibited retentions of 34.22% to 58.54% and fluxes of 49.21 to 25.80 L/m2.h, depending on the type of diamine used, when tested with a 2000 ppm NaCl solution (1 MPa, 25°C, 45 – 50 L/h). Novel PVA-SB-DAP-FC membranes made from the DAP with FC had the highest hydrophilicity value and exhibited >58.54% NaCl retention and 25.80 L/m2.h flux, and 75.08% MgSO4 retention and 34.75 L/m2.h flux, when tested with (2000 ppm feed, 1 MPa, 25°C, 45 – 50 L/h). The effect of the chemical structures of the different amines and carboxylic chlorides used on the RO performances of the TFC membranes prepared by two amines reacting with TMC or FC, on the surfaces of the modified asymmetric PES-UF membranes, was investigated. FT-IR and water contact angle determination were used to characterize the chemical structure, morphology and hydrophilicity of the PA layers of the composite membranes. The response surface methodology (RSM) was used to optimize the preparation conditions that had the largest effects on the RO performance of the PVA-SB-DAP-FC membranes. Good membrane performance could be realized particularly by manipulating three variables: DAP concentration, FC concentration and polymerization time (PT). The regression equation between the preparation variables and the performance of the composite membranes was established. Main effects, quadratic effects and interactions of these variables on the composite membrane performance were investigated. The membranes were characterized in terms of pure water permeation (PWP) rate, molecular weight cut off (MWCO), solute separation and flux. Mean pore size (μp) and standard deviation (σp) of the membranes were determined using solute transport data. The results revealed that PVA-SB membranes have almost the same pure water permeation that PES-UF membranes have. The MWCO of the PES-UF membranes decreased from 19,000 to 13,000 Daltons when the membrane was coated with PVA.

AFRIKAANSE OPSOMMING: Die doel van hierdie studie is die modifikasie van die oppervlakte van poliëtersulfoon ultrafiltrasie (PES-UF) membrane om meer hidrofiliese membrane te berei deur die kruisbinding van polivinielalkohol (PVA) met natriumtetraboraat ((Na2B4O7.10H2O) (NaB) op die membraanoppervlakte. Sleutelfaktore in die bereidingsproses is geïdentifiseer, naamlik: PVA molekulêre massa, PVA en NaB konsentrasies, kruisbindingsreaksietyd, die aantal bestrykingslae, en die manier waarop die bestrykingslae aangewend is. Die invloed van hierdie faktore op die membraanontsouting en vloed is ondersoek, en word hier bespreek. Hierdie PVA-NaB membrane het die volgende tipiese resultate getoon: 11.46% ontsouting en 413.30 L/m2.h vloed (Kondisies: 2000 dpm NaCl oplossing, 0.45 MPa toegepaste druk, 20 °C, vloeitempo 45–50 L/h). Die deklaag was uniform en stabiel, soos bepaal d.m.v. FTIR. Die aanwesigheid van die deklaag het die hidrofilisiteit verhoog en 'n maksimum vloed van 500 L/m2.h is behaal. Die waterkontakhoek is ook gemeet; 'n laer waarde het 'n verbetering in die hidrofilisiteit van die oppervlakte bevestig. Die rol van die PVA basislaag, sonder kruisbinding (kontrole), en die effek van uitdroging en verhitting hiervan, is ook bestudeer, om sodoende optimale behandelingskondisies te bepaal. Membraanontsouting en vloed is bepaal (Kondisies: 2000 dpm NaCl oplossing, 0.45 MPa toegepaste druk, 25 °C, vloeitempo 45–50 L/h). Verhitting het die grootste effek gehad op die afname in vloed. Daar is bevind dat 'n maksimum temperatuur van 50°C geskik is vir die behandeling van hierdie spesifieke PES-UF membraan. Dunfilmsaamgestelde (DFS) membrane is berei d.m.v. 'n tussenvlakpolimerisasiereaksie tussen 'n polifunksionele amien en 'n di- of tri-funksionele karbonielchloried, en daarna is die tru-osmose (TO) gedrag bepaal. Die ontsouting van die PVA-NaB membrane was hoër nadat die kruisgebinde PVA jel sub-laag met 'n poliamied (PA) laag bedek is. Die vloed het egter afgeneem, tot onder 30 L/m2.h (Kondisies: 2000 dpm NaCl oplossing, 1–2 MPa toegepaste druk, 20 °C, vloeitempo 45–50 L/h). Twee DFS membrane is berei met trimesoïelchloried (TMC), naamlik met m-fenieldiamien (MFD) of 2,6-diaminopiridien (DAP). Afhangend van die diamien wat gebruik is, is die volgende ontsoutingsresultate en vloede verkry: 96.71% tot 89.65% en 10.93 to 27.91 L/m2.h (Kondisies: 2 000 dpm NaCl oplossing, 2 MPa toegepaste druk, 25 °C, v loeitempo 45–50 L/h). Twee DFS membrane is ook berei met 'n nuwe verbinding, 2,5-furanoïelchloride (FC), en MFD of DAP. Afhangend van die diamien wat gebruik is is die volgende ontsoutingsresultate en vloede behaal: 34.22% tot 58.54% en 49.21 tot 25.80 L/m2.h (Kondisies: 2000 dpm NaCl oplossing, 1 MPa toegepaste druk, 25 °C, vloeitempo 45–50 L/h). Die PVA-NaB-DAP-FC membrane het die hoogste hidrofilisiteit getoon: 58.54% NaCl ontsouting en 25.80 L/m2.h vloed, en 75.08% MgSO4 ontsouting en 34.75 L/m2.h vloed (2000 ppm NaCl oplossing, 1 MPa toegepaste druk, 25 °C, vloeitempo 5–50 L/h). Die invloed van die chemiese struktuur van die verskillende diamiene en karboksielsuurchloriedes wat gebruik is in die bereiding van die DFC membrane op die oppervlakte van die gewysigde PES-UF membrane is in terme van TO ondersoek. FTIR en kontakhoekbepalings is gebruik om die chemiese struktuur, morfologie en hidrofilisiteit van die PA lae van die saamgestelde membrane te bepaal. Die eksperimentele oppervlakte ontwerp metode is gebruik om die bereidingskondisies vir die TO aanwending van die PVA-NaB-DAP-FC membrane te optimiseer. Goeie resultate is verkry deur die volgende veranderlikes te manipuleer: DAP en FC konsentrasies en die tydsduur van die polimerisasie. 'n Regressie-vergelyking tussen die bereidingsverandelikes en die funksionering van die saamgestelde membrane is bepaal. Die volgende is ook ondersoek vir hul effek op die funksionering van die saamgestelde membrane: hoof-effekte, vierkantseffekte, en interaksie tussen veranderlikes. Die eienskappe van die membrane wat bepaal is, is: deurlatingstempo van suiwer water (DSW), molekulêre massa-afsnypunt (MMAP), skeiding van opgeloste sout en vloed. Deurlating van opgeloste sout data is gebruik om gemiddelde poriegrootte (μp) en standaard afwyking (σp) van die membrane te bepaal. Resultate het getoon dat die PVA-NaB membrane amper dieselfde DSW gehad het as die PES-UF membrane. Die MMAP van die PES-UF membrane het afgeneem van 19,000 tot 13,000 Daltons na behandeling met PVA.

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