Unusual extraction behaviour of crown ether when intercalated in bentonite

Abstract

Dicyclohexano-18-crown-6 (DCH18C6) has been intercalated in bentonite and the product, Ben-Crown, characterized by microelemental analysis, TG-DTA analysis, X-ray diffraction and IR spectroscopy. The permanent negative charge present in the bentonite layer has been exploited for more efficient extraction of cesium and strontium by Ben-Crown at nitric acid concentrations (<1.0 M) that gave only a meagre extraction when either DCH18C6 or bentonite alone were employed. The extraction of cesium and strontium has been investigated as a function of time, temperature, and the concentrations of nitric acid, metal ion and sodium nitrate. An unusual extraction phenomenon was observed at low concentrations of nitric acid (<1.0 M) in the absence of any organophilic agents. Rapid extraction of the metal ion was followed by the establishment of an equilibrium, which occurred after 150 min. Distribution coefficients (K_d) of 599 and 1007 ml g⁻¹ were obtained, respectively, for the extraction of cesium and strontium from 0.1 M nitric acid by Ben-Crown; K_d decreased with an increase in the temperature or in the concentrations of nitric acid and sodium nitrate. The extraction data were fitted by the Langmuir adsorption model and the apparent experimental exchange capacity obtained by linear regression analysis was in good agreement with the amount of crown ether (0.22 mmol g⁻¹) intercalated in bentonite. The enthalpy change (ΔH°) for the extraction of cesium and strontium was found to be exothermic.