The adsorption of humic acid on bentonite from Milos Island (Greece) acid-treated with dilute H2SO4 Solutions over a concentration range between 0.25 and 13 M has been Studied. Bentonite activated with 3 M Sulfuric acid (AAS) showed a higher efficiency in removing humic acid from aqueous solutions and was selected for further investigation. The specific surface area of acid-activated bentonite was estimated using the methylene blue adsorption method. The Morphology of untreated, activated, and HA-sorbed bentonite was studied Under scanning electron microscope (SEM). The effects of contact time, adsorbate concentration, adsorbent dose, and temperature on the adsorption Of humic acid onto bentonite activated with 3 M H2SO4 were Studied using a batch adsorption technique. Acidic pH and high ionic strength proved to be favorable for the adsorption efficiency. Pseudo-first-order, pseudo-second-order, and intraparticle diffusion models were used to describe the kinetic data and the rate constants were evaluated. The experimental isotherm data were analyzed using Langmuir, Freundlich, and Temkin equations and the isotherm constants were determined. Thermodynamic parameters (Delta H degrees, Delta S degrees, and Delta G degrees) of adsorption of humic acid onto acid-activated bentonite with 3 M Sulfuric acid were also evaluated. (C) 2009 Elsevier Inc. All rights reserved.