Equilibrium and thermodynamic investigation of methylene blue adsorption on thermal- and acid-activated clay minerals

The use of a natural Moroccan clay as an adsorbent for removal of the cationic dye methylene blue from aqueous solutions has been investigated. The starting material was activated by thermal treatment and by acid leaching with nitric acid in order to obtain samples with modified adsorption properties. The effect of the experimental conditions on the adsorption behaviour was studied by varying the treatment temperature from 373 to 1,073K in the case of the thermally treated solids, the nitric acid concentration for the acid-activated solids and the adsorption temperature from room temperature to 368K for both these solids. Data obtained from further batch studies were analysed using the Freundlich, Langmuir and Toth isotherm equation models. The thermodynamic parameters of the adsorption process were calculated, and the resulting Gibbs free energy indicated its spontaneity. The adsorption capacity of the clay materials makes them suitable as a low cost alternative for the removal of cationic dyes during wastewater treatment.