Amino organosilane grafted ordered mesoporous alumina with enhanced adsorption performance towards Cr(VI)

Ordered mesoporous alumina (MA) was successfully modified with three amino organosilanes including 3aminopropyltriethoxysilane (1N), N-(beta-aminoethyl)-gamma-aminopropylmethylbimethoxysilane (2N) and N-3-trimethoxysilylpropyldiethylenetriamine (3N) via a facile grafting method, and the as-prepared amino organosilane grafted MA-1N, MA-2N and MA-3N show enhanced adsorption performance towards Cr(VI) removal from wastewater. Their physicochemical properties and the MA before modification were comparatively characterized by FT-IR, TEM, XRD, N-2 adsorption-desorption, CHNS elemental analysis, zeta potential measurements and XPS. Their adsorption performance was also comparatively studied along with the effect of contact time, adsorption isotherms, multi-metal ion adsorption, interference of co-existing anions and regeneration ability in batch experiments. It was found that their adsorption kinetics data were better fitted by the pseudo-second order model; adsorption isotherms were better described by the Langmuir isotherm for MA-1N, the Freundlich isotherm for MA-2N and the Temkin isotherm for MA-3N. Among them, MA-2N shows the maximum adsorption capacity of 137.9 mg g(-1) which is more than twice the 59.4 mg g(-1) of MA. The residual concentration of Cr(VI) with a concentration of 50 mg L-1 when treated with MA-2N meets the emission standard of the World Health Organization (WHO). Moreover, MA-2N shows better selectivity toward Cr(VI), and can reduce relatively more Cr(VI) to low toxicity Cr(III). More results were found that Cr(VI) is adsorbed on the surface through a monodentate ligand or bidentate ligand, and then Cr(VI) is reduced to Cr(III) by an adjacent electron donor, after which Cr(III) is co-precipitated with the adsorbents. All the amine-grafted samples show good reusability for 5 cycles. These results indicated that the amino organosilane grafted ordered MA with high adsorption capacity, good selectivity and favourable reusability is a promising candidate for Cr(VI) removal, in combination with its low cost and energy saving preparation process.