MCM-41-based composite with enhanced stability for Cr(VI) removal from aqueous media

Abstract

Two mesoporous ordered silica with MCM-41 pores arrangement were synthesized and surface modified by a post-synthetic treatment. The first composite has aminopropyl functionalities while the other features aminopropyl and methyl surface groups. Both composites were used as sorbents for Cr(VI) removal from aqueous media at pH ꞊ 2. In order to test the stability and durability of the sorbents, the samples were subjected to alkaline extreme conditions (pH ꞊ 10) at different times and after that tested again for Cr(VI) removal. The bi-functionalized MCM-41 retains its Cr(VI) elimination capacity unchanged after 16 h of alkaline treatment, while the aminopropyl-MCM-41 sample suffer a drop of about 30 %. The fresh and alkaline treated samples were characterized by X ray diffraction at low angles, nitrogen sorption at 77 K, Fourier transform infrared, Thermal analysis. The presence of the methyl moieties increases the hydrophobicity of the composite`s surface, which inhibit the approaching of the water molecules avoiding the hydrolysis of Si-O-Si bonds of the MCM-41 pores walls. Besides, this protective effect is reflected in the textural properties changes before and after alkaline treatments: while the specific surface area of aminopropyl-MCM-41 goes from an initial value of 793 to 495 m2.g-1 (-37.6 %), the aminopropyl-methyl-MCM-41 shows a change from 702 to 650 m2.g-1 (-7.2%). From these results it can be concluded that a second functionalizing agent that expose methyl-residues increases the stability of an aminopropyl-MCM-41 based-material, increasing the potential of this type of nanostructured composites for aqueous media applications.