Ibuprofen and diclofenac sodium adsorption onto functionalized minerals: Equilibrium, kinetic and thermodynamic studies

Abstract

Cationic surfactant - octadecyldimethylbenzyl ammonium chloride (ODMBA) was used for modification of the three natural raw materials – bentonite, kaolin and zeolite. Adsorption of ODMBA by minerals occurred via ion exchange of inorganic cations on minerals with surfactant. Organomodified minerals – organobentonite (OB), organokaolin (OK) and organozeolite (OZ) were studied as adsorbents for removal of two non-steroidal anti-inflammatory drugs - ibuprofen (IBU) and diclofenac sodium (DS) at pH 7. For all systems, the pseudo-second order model showed the best correlation to kinetic experimental data. In all cases, the positive values of ΔH° indicated that the adsorption of both IBU and DS by organomodified minerals was endothermic in nature. Equilibrium data were better described by the Freundlich isotherm model, pointing to adsorbent heterogeneous active sites for adsorption. Under applied experimental conditions, the highest adsorption of both drugs was achieved by OB, while much lower capacities were observed for OK and OZ. Nonlinear isotherms and physicochemical characterization of organomodified minerals after drugs adsorption suggested complex mechanism consisting of hydrophobic interactions between hydrophobic part of the drugs and surfactants alkyl chains, electrostatic interactions of anionic forms of IBU and DS with the cationic “head” of ODMBA, as well as anion exchange of counterion ions from ODMBA micelle and anionic forms of both drugs. Adsorption of both IBU and DS was strongly dependent on the amount of ODMBA ions in the minerals.