Thermodynamic studies of the sorption of ammonia on natural and modified chrysotile asbestos

Abstract

The sorption of ammonia was studied at 273 and 298 K on Californian and Quebec chrysotile heat-treated at 150, 300, 500 and 700 degrees C. Another ammonia sorption study involving the two chrysotiles treated at 500 degrees C at isotherm temperatures of 266, 273, 288, 298 and 308 K was carried out. The variation of the calorimetric heats of adsorption with surface coverage and adsorption/desorption isotherms were determined. From these data experimental differential molar entropies of adsorption were calculated. The results indicated that the high heats of adsorption observed initially may have been due to a weak chemisorption reaction perhaps involving ammonium complex formation with impurities on the chrysotile surface. Hydrogen bonding and van der Waals forces could possibly have accounted for the lower heats of adsorption observed at higher surface coverages The present studies appeared to belong to an unusual class of adsorption phenomena where the heat of adsorption was less than the heat oT llquifactlon of the ammonia. This was attributed to the two hypotheses of significant entropy Involvement in the adsorption and capillary adsorption. Minor studies of the adsorption/desorption of ammonia on Californian chrysotile treated with sodium nitrate and boiling water were also performed. Heats of adsorption at all degrees of surface coverage were approximately three times those in the other experiments. This was attributed to the highly hydroxylated surface which may have facilitated chemisorption. A group of experiments were carried out to determine the desorption efficiencies of the two chrysotiles. The Quebec samples generally proved to desorb the ammonia more efficiently. All adsorbents were characterised by low-temperature (77 K) nitrogen adsorption/desorption Isotherms from which surface area and pore size distribution data were obtained. Also, direct examination of the topographical and structural features of the adsorbents was made by transmission electron microscopy and selected area electron diffraction.