Lanthanum functionalized highly ordered mesoporous media: implications of arsenate removal

A highly ordered mesoporous silica, SBA-15, was synthesized and various amounts of lanthanum oxide were incorporated into it. Several characterization techniques, such as X-ray diffraction (XRD), nitrogen gas isotherm, and Fourier transform infrared (FTIR) spectroscopy were used to investigate physicochemical properties of the media and to elucidate their arsenate adsorption behavior. The X-ray diffraction and N-2 isotherm results showed that immoderate substitution of lanthanum in the silica network occurred at 80% lanthanum impregnation, even though the lanthanum was highly dispersed inside the mesopore structures of SBA-15 and lanthanum oxide particles did not form on the outside of the solid. As a result of FTIR, however, there was no structural collapse of the silica framework at 80% lanthanum impregnation since partial substitution of lanthanum precursors with silicon occurred; this may play an important role in structural stabilization, as shown by other studies. Although the arsenate adsorption densities increased with an increase in lanthanum impregnation up to 50% (the most efficient percentage of lanthanum impregnation), it abruptly decreased at 80% due to the substitution of lanthanum with silicon, leading to an overall reduction in arsenate adsorption capacity. At the arsenate concentration of 0.667 mmol(As)/l, the adsorption capacity of 50% lanthanum-impregnated SBA-15 by weight (designated as La(50)SBA-15) was 1.651 mmol(As)/g (123.7 mg(As)/g), which was approximately 10 and 14 times greater than La(III) impregnated alumina and La(III) impregnated silica gel, respectively. (C) 2004 Published by Elsevier Inc.