Studies in sorption of benzene by Kenya diatomite

Abstract

The BET surface area of Kenya diatomite has been determined by using a gravimetric method. The surface area of the diatomite was

found to be 30.1 m2 g-1 and acid treatment with warm 3N hydrochloric acid increased the surf'ac e area by 20%. Samples of untreated and acid treated diatomite were heated in air at 300, 400, 700 and 1000oc. The surface areas of both untreated and acid-treated samples decreased with increasing temperature of heating to the same value of 2.2 m2 g-1 when heated to 1000 C. Benzene adsorption isotherms at 25.00c have been determined for all samples. Hysteresis occurred in all cases except for samples heated at 10000c. The untreated diatomite samples exhibit Low-pressure hysteresis, while the isotherms on acid-treated samples had reversible portions in the pressure range 0 - 0.37po , where Po is the saturation vapour pressure of benzene at 250 C. The benzene isotherms have also been plotted according to the BET theory. Good straight lines were obtained. From the BET plots, surface areas have been estimated using 40 A02 as the area of cross-section of an adsorbed benzene molecule. Agreement with nitrogen surface areas was obtained for some samples. The radii of capillaries have been calculated with the aid of the Kelvin equation, from both adsorption and desorption boundary curves of the hysteresis loops. In all cases, it was found that ra < rd < 2ra while ( pa /po)2 < pd/po; where ra and pa are the radius and pressure from the adsorption branch and rd and Pd are the radius and pressure from the desorption branch of the hysteresis loops. Distributions of capillary radii have also been calculated from desorption scanning curves of the hysteresis loops. Some regularity in the formation of the capillaries is revealed in the linear relationship between the radius of the predominant small capillary and that of the corresponding starting large capillary. No correction for thickness of the adsorbed film was made in Calculation of capillary radii, owing to the difficulty of obtaining a meaningful ‘t' plot for benzene.