Natural and acid-activated sepiolite samples were functionalized with (3-Mercaptopropyl)trimethoxysilane. By X-ray powder diffraction, Fourier transform infrared spectroscopy, and differential thermal analysis it was shown that functionalization of sepiolites by grafting silane reagents occurs mainly on the surface, whereby their crystalline structure remained unchanged. The efficiency of the adsorbents in Cr(VI) removal from aqueous solutions follows the order: functionalized acid-activated sepiolite > functionalized natural sepiolite > acid-activated sepiolite > natural sepiolite. As the number of silanol groups on the sepiolite surface increased during the acid activation, the silane functionalization increased and this led to an increase in Cr(VI) adsorption. The adsorption capacity strongly depended on the pH of the solution from which the adsorption occurred. Maximum Cr(VI) removal was ca. 8.0 mg Cr(VI) per g of functionalized acid-activated sepiolite at initial pH 3, when the pH of solution reached 4.7, and ca. 2.7 mg Cr(VI) per g of functionalized natural sepiolite at initial pH 2, when the pH of the solution reached 2.5. Cr(VI) removal by the functionalized sepiolites was mainly attributed to electrostatic attraction between the protonated mercapto groups and the negatively charged Cr(VI) species. The sorption isotherms showed that Cr(VI) removal by both functionalized sepiolites for all initial investigated solution pH values is well described using the Dubinin-Radushkevich model. The thermodynamic data suggest spontaneity of the major physical adsorption process at 298 K. (C) 2010 Elsevier B.V. All rights reserved.