Reverse Osmosis and Ultrafiltration
S. Sou ri raja η and Takeshi Matsuura, Editors National Research Council of Canada Reports significant advances in the technological improvement and funda mental understanding of reverse-osmosis-ultrafiltration membranes and pro cesses. Features topics such as new studies in gas separations and pervaporation by RO membranes, fundamental studies in membrane formation and membranes transport, progress in development and industrial use of inorganic membranes, and more. CONTENTS Materials Science of Reverse-Osmosis-Ultrafiltration Membranes · Polyethersulfone Ultrafiltration Membranes · Nature of Dynamically Formed Ultrafiltration Membranes · Polyblend Membranes in Hyperfiltration of Electrolyte Solutions · Structure, Permeability, and Separation Characteristics of Porous Alumina Membranes · Plasma-Polymerized Membranes of 4-Vinylpyridine · Gamma RayInduced Enhancement Effect on Salt Rejection Properties of Irradiated Membranes · Polyether Composite-1000 Spiral-Wound Membrane Element · Transport in Pressure-Drive η Membrane Separation Process · Physicochemical Interpretation of Behav ior of Pressure-Driven Membrane Separation Process · Effect of Hydrolysis on Cellulose Acetate RO Transport Coefficients · Application of Multicomponent Membrane Transport Model to RO Separation Processes · Predictability of Membrane Perform ance in RO Systems Involving Mixed Ionized Solutes in Aqueous Solutions · Solute Separation and Transport Characteristics Through Polyether Com posite-1000 RO Membranes · Hydrodynamic Properties of Skin and Bulk of Asymmetric RO Membranes · Limiting Flux in Ultrafiltration of Macromolecular Solutions · Mineral Ultrafiltration Membranes in Industry · Membrane Bioreactors for High-Performance Fermentation · Single-Stage Seawater Desalting with Thin-Film Composite Membrane Elements · Boiler-Feed Quality Water from Bitumen-Heavy Oil-Oil-in-Water Emulsions · Treatment of Paper-Plant Wastewater by Ultrafiltra tion · Concentration and Recovery of e-Caprolactam from Process Waste Stream · Sanitary Design RO Systems for Pharmaceutical Industry · UInfiltrative Solute Rejection Behavior of Black Liquor · RO and Ultrafiltration Applied to Processing of Fruit Juices · Halogen Interaction with Polyamide RO Membranes · RO Membrane Fouling at Yuma Desalting Test Facility · Fluid Mechanics of Dilute Solutions · RO and Ultrafiltration Membrane Compaction and Fouling Studies Using Ultrafiltration Pretreatment · Gel Volume Deposits on Ultrafiltration Membranes · Pretreatment, Fouling, and Cleaning in Membrane Processing of Industrial Effluents · Gas Permeability of Polypeptide Membranes · SolventExchange Drying of Cellulose Acetate Membranes for Separation ofHydrogen-Methane Gas Mixtures · Pervaporation Membranes · Dehydration of AlcoholWater Mixtures Through Composite Membranes by Pervaporation Based on a symposium sponsored by the Division of Industrial and Engineering Chemistry of the American Chemical Society ACS Symposium Series No. 281 501 pages (1985) Clothbound LC 85-0921-9 ISBN 0-8412-0921-9 U.S. & Canada $89.95 Export $107.95 Order from: American Chemical Society Distribution Office Dept. 36 1155 Sixteenth St., N.W. Washington, DC 20036 or CALL TOLL FREE 800-424-6747 and use your credit card.
Either way, NAA is less sensitive for most metals than furnace AAS. Never theless, workers experienced with the technique generally believe that it is more accurate than the several spec troscopic techniques. Although the judgment is personal, I have not understood why IDMS or NAA should be considered superior to furnace AAS for reference methods. With any of these methods, the most frequent analytical problem results from contamination, which is equally difficult to control in any of the meth ods. Workers who must determine Pb, Cd, Zn, etc., at levels below 1 Mg/L must take considerable precautions to use reagents that have been specially purified and to keep dust from set tling on the samples. If care is taken in the control of contamination, graphite furnace AAS will provide results that are at least as accurate as NAA or IDMS. For most elements, furnace AAS is more sensitive, thus requiring less sample handling. Chromatography. Over the years, gas chromatography (GC) has been used by some workers to determine trace metals in complex organic mix tures. Because GC separates com pounds, the different metallic species are determined. For those compounds for which the technology is appropri ate, very sensitive measurements can be made. However, the technique re quires specialized skills. In the past several years ion chro matography has been developed as an offshoot from modern liquid chroma tography. When ion chromatography is used for metal determination, the metallic compounds are separated and an appropriate detector is used to quantitate the compounds. The tech nique is particularly convenient for anions that are not easily determined by spectroscopic methods (for exam ple, halides, sulfate, and phosphate). Groups of metals can be separated by ion chromatography and detected at levels that begin to be competitive with flame AA or ICP. However, sam ples must be treated so that appropri ate compounds are formed. Ion chro matography is unlikely to be as specif ic as spectroscopic methods, but the cost for equipment is likely to be simi lar to the simplest flame AA instru ments and, of course, the anions can be determined. Choosing a technique Flame AA. Given all of these con siderations, how do you choose an ap propriate technique? If your primary requirements relate to a limited num ber of metals at higher levels, you will almost always be best off with flame AA. If an analyst is new to metal anal ysis or if an experienced spectroscopist is not available to run the lab, I
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think flame AA is the technique to choose. And don't let a plasma zealot con vince you that the ICP has no interfer ences and is, thus, easier to use than flame AA. It is free of some of the AA chemical interferences, but it has many more of its own. Certainly, I don't claim the platform Zeeman fur nace is free of interference; otherwise, why have we spent so much time find ing appropriate matrix modifiers? Flame AA is the easiest and most trouble-free metal determination technique. This probably will change gradually with respect to ICP, but it is certainly true today and will be for several years to come. Compared with ICP, flame AA is a much less expensive way to use spec troscopy for metal analysis. Flame AA costs from $10,000 to $40,000, depend ing on the level of automation. The ICP costs from $60,000 to $150,000, depending on speed and automation. A new laboratory not yet using spec troscopic methods for metal analysis can expect to be operational much more rapidly using flame AA. ICP. If you have a well-equipped AA lab, you may want to expand this capability to additional analytes: B, V, P, Zr, W, Nb, Ta, S, and some others. Sequential ICP is the way to do it. It is versatile and flexible. The routine wa ter test lab that determines a large number of elements on each sample and analyzes many samples will be better off with the ICP than with flame AA because the simple matrix of natural waters is easy to handle on the ICP. But the ICP requires more skill than does flame AA. Furnace AA. However, if the most important activity for which new in strumentation is being obtained is to determine metals at ultratrace levels, especially in inorganic or partly inor ganic materials, furnace AAS is called for. The Zeeman system provides ma jor advantages for furnace analysis in almost every case in which real sam ples are being analyzed. Note that the furnace should no longer be considered an accessory to a flame AA instrument. It is a complete ly independent analytical technique requiring optimized instrumentation. In practice, every well-equipped lab doing metal analysis is frequently re quired to reach the low levels in diffi cult matrices that demand the Zeeman platform furnace system. The Zeeman platform furnace A A, technique is ap propriate for every well-equipped metal analysis lab. It has properties that make it unique. The single overriding complaint about the furnace, which curtailed its growth for many years, was that it was greatly error prone because of inter ferences. There are many comments in
published p a p e r s t o t h a t effect, such as t h e two q u o t e d below: Investigators using such common analytical techniques as flame and graphite furnace atomic absorption .. . have made enormous errors in measuring lead concentrations in biological and environmental materials. [Everson J.; Patterson, C. Clin. Chem. 1981, 27, 765.] [Furnace AAS] should at this time still be considered an art rather than a reliable analytical science... . the large number of interferences and the inability of the standard additions technique to compensate for interferences make graphite furnace AAS highly unreliable in many cases. [Boyer, K. W. J. Assoc. Off. Anal. Chem. 1981, 64, 396.] T h e r e is a t e n d e n c y t o believe t h a t Zeeman b a c k g r o u n d correction should be r e c o m m e n d e d only w h e n it is proven t h a t an o r d i n a r y furnace s y s t e m does n o t work. I believe t h i s recomm e n d a t i o n is totally incorrect. I n our experience a quicker, m o r e reliable, a n d less t r o u b l e s o m e p r o c e d u r e can be developed using t h e c o m b i n a t i o n of the stabilized-temperature platform furnace with t h e Z e e m a n b a c k g r o u n d corrector.
Conclusion As I have included negative q u o t a tions from t h e l i t e r a t u r e a b o u t t h e p a s t furnace t e c h n i q u e , I w a n t t o finish with several e x a m p l e s of t h e m a n y more flattering p a p e r s . A r e c e n t p a p e r
THE MARK OF THE EXPERT
c o m p a r e s I C P , t h e D C P , flame AA, furnace AA, a n d electrochemical m e t h o d s for 10 t r a c e m e t a l s in electrol y t e s — t h a t is, in m e t a l chlorides: For the large laboratory the best combination of techniques would be an ICP and electrothermal AAS, whereas for the smaller lab,. . . flame and electrothermal AA would suffice. [Skidmore P. R.; Greetham, S. S. Analyst 1983,108, 171].
nology. Given t h e various characteristics t h a t m a k e u p t h e m o d e r n furnace, including Z e e m a n b a c k g r o u n d correction for difficult samples, t h e g r a p h i t e furnace t e c h n i q u e is now n o more difficult t h a n flame AA a n d p r o b a b l y less difficult t h a n I C P .
A n d finally, George M o r r i s o n , t h e E d i t o r of A N A L Y T I C A L C H E M I S T R Y ,
did a critical review of t h e analytical p r o c e d u r e s available for t r a c e m e t a l d e t e r m i n a t i o n s in biological m a t e r i a l s . His final s u m m a r y is: We see that while many trace element techniques are available for biological analyses, on the basis of the criteria of sample size, pretreatment, sensitivity, accuracy and precision, and cost, the technique of ETA-ÂAS [electrothermal atomization AA spectrometry] offers the greatest opportunity. [Morrison, G. H. Crit. Rev. Anal. Chem. 1979, 8, 287.] T h e r e f o r e , I would conclude t h a t flame AA a n d I C P a r e t h e preferred analytical t e c h n i q u e s a t levels where t h e s e two t e c h n i q u e s a r e useful. T h e choice b e t w e e n t h e two h a s t o d o with t h e p a r t i c u l a r m e t a l a n d t o some ext e n t with t h e m a t r i x . B u t a n y laboratory t h a t m u s t occasionally d o d e t e r m i n a t i o n s a t levels below t h o s e available on flame AA or t h e I C P should be e q u i p p e d to use g r a p h i t e furnace tech-
Walter Slavin is a senior scientist at Perkin-Elmer, working in atomic spectroscopy. He is a 1949 graduate of the University of Maryland in physics and math. He has led Perkin-Elmer's efforts in the field of AAS and has contributed extensively to the development of AA techniques and applications. Slavin has also published widely on instrumentation and applications of fluorescence spectroscopy, far-UVspectroscopy, chromatography, and clinical chemistry.
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