A Systematic Study of Thin Film Composite RO membranes Verl Murugaverl Department of Chemistry and Biochemistry University of Denver

An increasing scarcity and the demand for fresh water sources stimulated the drive for alternative resources like ocean and waste water. Drinking water standards have also become more stringent over the years and therefore the need for efficient water purification methods. With the development of high performance synthetic polymeric materials, filtration techniques using membranes have emerged as viable means for water purification. The steady advancement in membrane technology research has resulted in more efficient and robust membranes for water treatment through advanced membrane materials and configurations. The newer generation reverse osmosis (RO) membranes such as thin- film composite (TFC) membranes have highly improved flux and salt rejection characteristics.

Thin Layer Composite Membrane Overview A composite membrane is created by stacking various thin layers of materials as shown on the right, TFC RO membranes are made of 3 layers; an active membrane layer (50 um), a microporous support (250 um) and a non woven polyester mesh spacer (2 mm). Active membrane is formed directly on the microporous support by interfacial polymerization.

Polyamide (PA) thin-film composite membranes, having a polyamide discriminating layer on a porous support are widely used in RO process for water treatment Typical commercial PA TFC membranes are prepared by interfacial polymerization of a difunctional aromatic amine, such as meta- phenylene diamine (mPD), and a trifunctional aromatic acyl halide, such as trimesoyl chloride (TMC).

The Problem Reverse Osmosis (RO) plants have been operating with polyamide (PA) spiral-wound thin-film composite membranes since 1977 The major flaws in current RO technology are membrane fouling and membrane chemical stability. Often controlling the fouling problem leads to chemical stability issues. The feed water is commonly treated with chlorine prior to RO filtration step for sanitation and to control fouling. Unfortunately, commercially available RO membranes are vulnerable to chlorine attack and degrade causing catastrophic membrane failure. Improving the RO membrane’s tolerance to chlorine will have tremendous impact on the cost and efficiency of water treatment process.

Approach Understand the structure property relationship of Polyamide membranes. Understand the mechanism of chlorine attack. Difficult to do molecular level characterization of synthetic polymers especially a cross linked polymer.  Synthesize novel model amides (polymeric mimics)  Study the properties of these model amides  Synthesize polymerizable monomers  Study the membrane properties in a RO setting

Amides Synthesized and Tested

Monomers Synthesized and Tested

Comparison of Water Flux and Salt Rejection Properties of Commercial versus New Cl-resistant Membranes (Unit 1, August 12, 2005 to February 24,2006) Operating Conditions  Feedwater chlorine concentration – 1 mg/L  pH – 6.3  Pressure – 2400 kPa  Temperature – 23 o C  Conductivity – µS/cm

Comparison of Water Flux and Salt Rejection Properties of Commercial versus the New Cl-resistant Membranes (Unit 1, July 21, 2006 – January 7, 2007) Operating Conditions  Feedwater chlorine concentration – 1 mg/L  pH – 6.3  Pressure – 2400 kPa  Temperature – 23 o C  Conductivity – µS/cm

Comparison of Water Flux and Salt Rejection Properties of Commercial versus the New Cl-resistant Membrane (Unit 2, July 21, 2006 – January 7, 2007) Operating Conditions  Feedwater chlorine concentration – 1 mg/L  pH – 6.3  Pressure – 2400 kPa  Temperature – 23 o C  Conductivity – µS/cm

Overview Microporous Polymeric membranes are used as the support layer in Thin Film Composite (TFC) membranes in Ultrafiltration (UF) and Reverse Osmosis (RO) for water purification. TFC-RO membranes slowly deteriorate due to chlorine mediated damage as they are exposed to chlorine disinfectant. Much of the study has been focused on chlorine attack on the polyamide (PA) active membrane layer (the top layer) of the TFC. Whether chlorine attack on the support layer as a possible contributing factor for the overall failure of the TFC membrane has not been studied. Goals  Develop analytical techniques to study the effect of chlorine on microporous polymeric supports. Use MALDI-TOF-MS and NMR Spectroscopy as novel methods in studying such polymers. Determine the location of chlorine attack on the polymer.  Ultimately develop chlorine resistant microporous support polymers.

Microporous Polymers These polymers need to display certain properties to qualify as a support layer:  must be porous, must provide mechanical strength, and must strongly adhere to the active PA membrane. Commonly used microporous polymeric supports:  Polysulfone (PS); average MW= 20kDa, repeating unit of 442 Da.

Commercial Polysulfones

 Polyetherimide (PEI); average MW of 15kDa, repeating unit of Da.  Polybutylene Terephthalate (PBT), Repeating unit 220 Da

Development of Methods Due to their inherent physical and chemical properties, synthetic polymers are among the most difficult materials to characterize:  Poor solubility, large molecular weights, polydispersion, random incorporation of monomeric units etc. Characterization of a structure as complex as synthetic polymer molecule is a formidable problem. Our approach to outwit this problem was to characterize the polymers through the soluble, smaller members of the polymers.  The smaller units should have the identical repeat units and end groups as the larger counterparts therefore they are chemically identical.  Analytical data from the model smaller units can be extrapolated to the bulk of the polymer. Two analytical techniques were considered:  MALDI-TOF-MS  NMR  These two techniques complement each other while uniquely suited for providing structural information.

Materials and Methods All materials including polymers(treated and untreated ), MALDI matrices and NMR solvents were provided by the Water Treatment Research Group of the Bureau of Reclamation.  Accelerated Chlorine Exposure Polymer beads were pulverized (to increase surface area) and stirred in a pH 6.5, 5x10 3 mg/l solution of NaOCl for 24 hrs to simulate an exposure equivalent of 5 years of chlorine disinfectant. Solid particles were filtered and washed with DI water and then dried.  MALDI-TOF-MS A Bruker, Reflex IV instrument equiped with a 337nm N 2 laser was used both in the reflectron and linear geometry in the positive ion mode. Sample preparation : About 8 different solvents systems and 8 different matrices were tried in various combinations to analyze 5 different polymers (analytes). Dried droplet method was used to spot the sample to the MALDI sample plate. Final analyte concentration in the sample was about 1-10 pmole.

An Overview of MALDI-TOF-MS The mechanism of MALDI process is not very well understood, extremely sensitive to sample preparation. Capable of analyzing macro molecules ( up to 300kDa), femto mole sensitivity, and sub-isotopic resolution in the lower mass ranges. Ideally suited for macromolecular characterization:  MW determination, identification of repeating units and end groups, MW Dispersion, and analyzing modifications to the polymeric back bone.

Results and Discussion Various solvent mixtures were tried in combination with different matrices and different analyte concentrations.  Solvent systems DMF, THF, CDCl3, 1,1,1,3,3,3-hexafluoro-2-propanol, DMSO, 1,1,2-trichlorotrifluoroethane, acetonitrile, acetone, isopropanol  Matrices α-cyano-4-hydroxycinnamic acid, sinapinic acid, 2,5-dihydroxybenzoic acid, norharmane, dithranol, 2’-(4-Hydroxyphenylazo)benzoic acid  Analyte Analyte size ranging from ~1-100 pmol were tried. Polyetherimide (PEI)  Ideal solvent for analyte was DMF.  Best results were obtained using α-cyano-4-hydroxycinnamic acid as matrix in ACN:H 2 0:TFA (50:50:0.1) as solvent. Polysulfone  The ideal matrix was α-cyano-4-hydroxycinnamic acid matrix in ACN:CDCl 3 :MeOH (50:25:25) as solvent.  Analyte was dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol.

MALDI-TOF-MS Analysis  MS data of polymers showed molecular weights no higher than 6000 Da.  The mass of repeating units calculated from the MS spectra were consistent with the chemical structure of the polymer as claimed by the manufactures.  No significant changes in MW distribution was observed between the treated and untreated polymer samples.  Any incorporation of deletion of atoms or molecules would have resulted in mass shifts giving different set of MW distributions.  The end groups identified from the spectra were consistent with the polymerization process.

Polyetherimide ( α-cyano matrix)

Polysulfone Untreated, α-cyano, analyte- hexaflouro. Matrix-ACN:CDCl3:MeOH Cl- treated

Polysulfone

NMR C13 and proton NMR run on standard and chlorinated polymers Untreated PolysulfoneCl treated polysulfone

NMR Untreated Polyetherimide Cl treated polyetherimide

Conclusions Developed a viable method to characterize polymeric support materials based on MALDI-TOF-MS and NMR. MALDI-TOF-MS analysis of synthetic polymers are not very straight forward, success of analysis depends on many factors:  choice of solvents.  Choice of matrix.  sample preparation technique. NMR is a useful technique to study synthetic organic polymers as long as compatible solvent system can be found. Our data indicate that there is no evidence of chlorine attack on polysulfone or polyimide polymers, under the chlorination conditions used. However, this may not be conclusive since the chlorination conditions adopted in this study to simulate the exposure may not represent the true exposure conditions of RO.