Nitric Acid Dehydration Using Mixed Perfluorosulfonate and -carboxylate Ionomer Membranes Richard L. Ames,* ,†,‡ J. Douglas Way, ‡ Elizabeth A. Bluhm, † Daniel M. Knauss, § Rajinder P. Singh, ‡ and Jesse E. Hensley ‡ Los Alamos National Laboratory, NMT-2, P.O. Box 1663, Los Alamos, New Mexico 87575, and Chemical Engineering and Chemistry and Geochemistry, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401 Investigations confirmed the feasibility and potential advantage of using mixed perfluorosul- fonate/carboxylate membranes for nitric acid dehydration. Experimentation consisted of in situ generation of pure sulfonate and mixed sulfonate/carboxylate (as high as 71 mol % carboxylate) polymer films from a perfluorosulfonyl fluoride precursor membrane, the characterization of the material, and a study of the transport characteristics of these membranes. The carboxylate concentration was determined using direct Fourier transform infrared and X-ray fluorescence spectroscopy. Nitric acid dehydration transport tests confirmed that bulk fluxes decreased and the water separation factor dramatically increased as the carboxylate side-chain content increased to 53 mol %. Introduction Perfluorocarboxylate ionomer membranes are being considered at Los Alamos National Laboratory (LANL) for uses in nitric acid dehydration and low-level waste solution processing. LANL performs actinide processing in an aqueous system with nitric acid and hydrochloric acid based unit operations. As a result of the ever- increasing cost of waste disposal and limited waste disposal resources, LANL facilities use reprocessed or recycled nitric acid in many systems not requiring acid of high purity. A nitric acid recycle system (NARS) has been designed around a distillation column where bottoms from the column are recycled to plant facilities and overheads are discarded as dilute aqueous waste. Increased reprocessing requirements have made it necessary to add an additional purification unit to the NARS that will eliminate reprocessing of dilute acid overheads. One proposed option uses a perfluoro iono- mer membrane for acid dehydration because the per- fluorosulfonate and composite perfluorosulfonate/car- boxylate ionomer films have been investigated for the dehydration of a number of mixtures including acetic acid, methanol/water, and nitric acid. 1-7 Nitric acid dehydration via pressure filtration and pervaporation has been investigated as a unit operation replacement for distillation in the generation of highly concentrated acid (beyond the nitric acid/water azeo- trope). 1,8 Sportsman and co-workers claimed that per- fluorosulfonate ionomer membrane films would not only dehydrate concentrated acid feeds (distillation bottoms) but also dehydrate dilute waste streams (distillation overheads), while excluding ionic contaminants, and operate in either a pressurized or pervaporation envi- ronment. 1,2 They conducted preliminary research on the use of perfluorosulfonate/carboxylate composite mem- branes (Nafion 90209) for the dehydration of concen- trated nitric acid (>10 wt %) and demonstrated that the polymer ionomer material offered improved water sepa- ration capabilities when compared to pure sulfonate ionomer films. Following the acid dehydration by Sports- man, an objective described herein was to produce thin mixed sulfonate/carboxylate membranes from commer- cially available films and demonstrate the films’ ability to dehydrate nitric acid. Perfluorocarboxylate and mixed carboxylate/sulfonate films can be synthesized from the same perfluoro precursor material as the sulfonate form. 9-15 Tetrafluo- roethylene (TFE) copolymerization forms the perfluoro precursor and the addition of a functionalized form of TFE allows the introduction of the side chain. Both of the sulfonate and carboxylate (Nafion) products are made from the perfluorosulfonyl fluoride precursor polymer film shown in Figure 1, where m is 0, 1, or 2, p is from 1 to 10, q is from 3 to 15, and M is usually a halogen (F in this instance) or hydrogen. 9,10 For ex- ample, the precursor material shown in Figure 1 with a sulfonyl fluoride side chain can be oxidized to a carboxylate derivative: This reaction can be carried out at an elevated temper- ature (50-60 °C) or in the presence of a metal catalyst (salts of Fe, V, or Cu) at room temperature. Hydrolysis of the sulfonyl fluoride precursor generates a sulfinic acid or sulfonate form of the membrane: Materials with a mixture of both carboxylate and sulfonate pendant chains (eqs 2 and 3) can be generated by controlled exposure of the precursor to oxidizing and reducing reagents. * To whom correspondence should be addressed. Tel.: 1-505- 606-0165. Fax: 1-505-665-1780. E-mail: rames@lanl.gov. † Los Alamos National Laboratory. ‡ Chemical Engineering, Colorado School of Mines. § Chemistry and Geochemistry, Colorado School of Mines. -O-(CF 2 ) 2 -SO 2 F (1) -O-CF 2 -COOH (2) -O-(CF 2 ) 2 -SO 3 H (3) 3672 Ind. Eng. Chem. Res. 2005, 44, 3672-3680 10.1021/ie0488391 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/16/2005