Ethoxylated Polyethylenimine Gel-Coated on Textile-Grade Acrylic Fiber. A Thermally Regenerable Superfast Sorbent for Water Desalination Manas Chanda, Amitava Sarkar, Jayant M. Modak Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India Received 2 December 2003; accepted 29 January 2004 DOI 10.1002/app.20521 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: A commercial acrylic fiber containing 92 wt % acrylonitrile was hydrolyzed to convert a part of its nitrile (—CN) groups to carboxylic acid (—COOH) groups and then was coated chemically with 80% ethoxylated polyeth- ylenimine (EPEI) resin, followed by crosslinking with glu- taraldehyde. The resulting sorbent, PAN(CO 2 H)(EPEI.XG), containing carboxylic acid groups and weakly basic tertiary amine groups in close proximity on the same fiber is found to simulate the well-known Sirotherm™ resins used for partial desalination of brine solution by adsorbing the salt at ambient temperature and desorbing it at an elevated tem- perature in the same solution. The sorption behavior of the new sorbent was evaluated for solutes NaCl and MgCl 2 , showing saturation capacities of 0.797 and 0.877 meq/g (dry) sorbent fiber, respectively, at 30°C. The equilibrium sorption data show good agreement with both Langmuir and Freundlich isotherms for sorption from single-compo- nent solutions and with Butler–Ockrent and LeVan–Ver- meulen models for bicomponent sorption. Although the equilibrium uptake of NaCl reaches maximum in neutral solutions (pH  6.5), falling at both lower and higher pH, that of MgCl 2 is augmented in alkaline pH due to additional sorption by cation exchange at the ionic sites formed at higher pH. The initial uptake of the salt, which is nearly instantaneous, exceeds the sorption value attainable at equi- librium. The high initial rate of salt uptake fits a shell-core kinetic model for sorption on fiber of cylindrical geometry. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 883– 893, 2004 Key words: desalination; ion-exchangers; resins; adsorption; sorbent fabric; Sirotherm process INTRODUCTION Salinity being one of the most common types of water pollution in the world, the importance of a process for low-cost desalination cannot be overemphasized. The process, designated the Sirotherm process, 1 uses heat rather than costly chemicals and thus has the obvious advantage of lower operating costs, besides adding no chemical load to the effluent. However, unlike the conventional ion-exchange processes involving chem- ical regeneration, the Sirotherm process with thermal regeneration is suitable only for partial removal of salts from waters of moderate salinity. In fact, the operating capacity of the Sirotherm resins restricts the economic upper range of salinities to be treated to between 2000 and 3000 mg/L. This conforms to the treatment needs of effluent from cooling water, the largest volume industrial waste effluent. (Industrial and cooling effluents are usually recycled until a dis- solved salt level of 1000 –3000 mg/L is reached, be- yond which a salt bleed from the system becomes necessary.) The Sirotherm process uses a mixed bed of weakly basic and weakly acidic ion-exchange resins or resins containing both weak acid and weak base functional- ity within the bead for the adsorption of salts from an aqueous solution in a thermally variable manner. Rep- resenting the acidic resin by R A H and the basic resin by R B , the following equilibrium may be written for such a system: R A H + R B + NaCl º R A Na + R B .HCl (1) The Sirotherm process is based on the fact that the equilibrium of eq. (1) is dependent on temperature, the forward reaction being favored at lower tempera- ture to allow salt uptake, and the reverse reaction is favored at a higher temperature so that salt is released when the mixed bed is heated. The difference between the amount of salt retained by the resin hot and cold defines the effective capacity of the resin. Unlike the following equilibrium of a conventional mixed-bed system using strong-electrolyte type resins: Correspondence to: M. Chanda (chanda@chemeng.iisc. ernet.in). Contract grant sponsor: Council of Scientific and Indus- trial Research, New Delhi. Journal of Applied Polymer Science, Vol. 93, 883– 893 (2004) © 2004 Wiley Periodicals, Inc.