Thermodynamic study of sorption of some linear aliphatic monocarboxylic acids from aqueous solution by photografted membranes immobilizing zeolite

Abstract

Sorption studies of propanoic, n-butanoic, n-pentanoic, and n-hexanoic acids from their 1.80 × 10 −4 − 2.75 × 10 −2 M aqueous solutions were carried out at 283–343 K on 1.6-–3.3 μm microporous membranes supporting 9.60 ± 0.06 mg cm −2 of photochemically grafted epoxy-diacrylate composite containing 20.0 wt.% of zeolite 13X onto a cellulose substrate. Two kinds of sorption have been ascertained: 1. i ) adsorption of alkanoic acids in the zeolite channels; 2. ii ) ion exchange paralleled by decrease of hydrogen ions and uptake of sodium ions in solutions. Thermodynamics of these two equilibria have been studied and mean enthalpies and entropies for processes i ) and ii ) evaluated. The influence of pK a of alkanoic acids was evidenced only in equilibrium constants and thermodynamic parameters of reaction ii ), while equilibrium constants for Langmuir adsorption through hydrogen bonds are practically independent of chain length. The model proposed for this complex sorption process supports soundly all experimental evidence. Competition between adsorption and ion exchange may be observed under conditions in which equilibrium constants for these two processes have comparable values. This situation may be reached owing to the endothermic adsorption process and the slightly exothermic hydrogen ion exchange.