Adsorption of salicylic acid on bentonite and kaolin and release experiments

The work deals the interactions between two commercial clays, a bentonite and a kaolin, and an iron-salicylate complex. Adsorption experiments were accomplished using a water solution containing Fe(III) 0.0176 M and salicylic acid 0.0253 M. Natural and treated clay samples were characterized by chemical analyses, powder X-ray diffraction, thermal analyses. The time dependence of salicylic acid adsorption by bentonite follows first-order kinetics, with respect to the percentage of salicylic acid adsorbed, in the first 12 h; afterwards the reaction slows down. The reaction is completely exhausted after 2 days and during the next 4 days the concentration of salicylic acid in bentonite does not change from its asymptotic value of 8.0%. The adsorption kinetics of salicylic acid by kaolin highlights a slow adsorption beginning after the fourth day of treatment, and finished after 19 days. The amount of salicylic acid adsorbed was 5.5% of the final complex. The release of salicylic acid adsorbed by bentonite and kaolin was tested in 0.2 N solutions of Na+, K+, Mg2+, and Ca2+. Salicylic acid release rates from Fe(III)-salicylate-containing bentonite were also measured through cellulose acetate membranes by means of Franz-type diffusion cells: an initial slow release of salicylic acid was followed by a fast release phase; after 23 h the concentration of salicylic acid released can be considered constant and the drug desorbed was 1.4% of the amount adsorbed by the bentonite. Even if the desorbed amount of salicylic acid is not very high, the bentonite–salicylate complex could be suitable for an application by gradual release.