Chromium (VI) removal from water by means of adsorption-reduction at the surface of amino-functionalized MCM-41 sorbents

A first study that demonstrates the capability of the amino-functionalized MCM-41 sorbents to reduce Cr(VI) to the less toxic trivalent state is presented. To achieve this aim two mesoporous ordered silica with MCM-41 pores arrangement were synthesized and surface modified by a post-synthetic treatment using 3-aminopropyltriethoxysilane in toluene. The modified silicas were characterized by powder X-ray diffraction (XRD), infrared spectroscopy(FT-IR), scanning electron microscopy (SEM), nitrogen adsorption–desorption experiments, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and thermogravimetric analysis (TGA). Hexavalent chromium adsorption experiments in aqueous media were carried out with the hybrid samples at pH 2 and room temperature until reach equilibrium conditions. The experimental results showed that the aminopropyl-functionalized MCM-41 sorbents adsorb Cr(VI) and partially reduce it to the less toxic Cr(III) state. It is proposed that Cr(VI) adsorption is produced by electrostatic interaction between HCrO4− anions and positive ammonium groups of the sorbent surface and continues with the reduction to Cr(III) while a proton is released from the solid surface to the solution. Afterwards, Cr(III) is partially retained onto the samples surface due to the Lewis basicity of the nitrogen atoms.