Adsorption of Methylene Blue from aqueous solutions by amorphous aluminosilicate gels and zeolite X

Abstract

Methylene Blue adsorption isotherms from aqueous solution at 25 °C, and diffuse reflectance spectra of the adsorbed dye have been used to examine aluminosilicate gels and zeolite X formed during hydrothermal synthesis. These techniques require little sample pretreatment, and during the measurements the dye cannot penetrate significantly into the zeolite X pore system and so are particularly relevant for examining the external surfaces of fragile solid materials. Despite this, characteristic changes were observed in these properties after the same ageing times as changes in the bulk phase sensitive properties of XRD, FTIR and water adsorption capacity occurred. Dye molecule aggregation at gel surfaces was indicated by the isotherm shape, although the spectra were like those for monomers in solution, whereas spectra, but not isotherm shape, suggested aggregation on zeolite X. Spectral variation with surface concentration could not be described for either system on the basis of surface aggregation, and unique spectra for monomeric and associated species. Nucleation gel and seed-crystal preparations were also studied, and had properties distinct from both amorphous gel and well crystallised zeolite.