Ethoxylated Polyethylenimine Gel-Coated on Textile-Grade Acrylic Fiber. A Thermally Regenerable Superfast Sorbent for Water Desalination

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 polyethylenimine (EPEI) resin, followed by crosslinking with glutaraldehyde. The resulting sorbent, $PAN(CO_2H)(EPEI.XG)$, 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 temperature 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-component solutions and with Butler–Ockrent and LeVan–Vermeulen models for bicomponent sorption. Although the equilibrium uptake of NaCl reaches maximum in neutral solutions (pH \sim 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 equilibrium. The high initial rate of salt uptake fits a shell-core kinetic model for sorption on fiber of cylindrical geometry