Aspects of methylene blue (MB) adsorption by various clay minerals and zeolite samples were examined and found that the aspects of adsorption were very different between the type of minerals and even between same clays having different exchangeable cations.
(1) Kaolinite and montmorillonite adsorb, regardless of temperature, fairly constant amounts of MB from the aqueous solutions containing relatively small excess of the dye. Sodium-saturated clays adsorb slightly more MB, a monovalent cation, than the cation exchange capacities (CEC), while H (Al)-saturated clays adsorb less MB than CEC. Adsorption of MB by H (Al)-montmorillonite amounts only to sixty per cent of the amount adsorbed by Na-montmorillonite. More MB were adsorbed when excess MB were added to the sample clays in one time than when small amounts were added successively until slight excess.
(2) Basal spacing of Na-montmorillonite increased initially by about 3.5Å and later about 8.5Å in the process of the adsorption. This seems to show that MB molecules penetrate into the interlayers of Na-montmorillonite forming a flat monolayer and two layers in the two stages of adsorption, respectively. Alternatively, the expanding process of Na-montmorillionite accompanied with the MB adsorption could be explained as change in oriention of MB molecules adsorbed within the interlayers. Basal spacing of H (Al)-montmorillonite increases by about 2Å by MB adsorption in comparison with the minimum presumably showing in this case MB forms a flat monolayer.
(3) MB adsorption in allophane samples proceeded satisfying the Langmuir type adsorption isotherms. The adsorption attained to the maximum only under considerably high MB concentration of the supernatant. An experiment showed that more MB were adsorbed at high (32°C) than at low (2°C) temperature. In every case the amount of adsorption was less than the CEC, and the relative order of amounts of adsorption did not agree with that of CEC values.
(4) MB molecules were adsorbed on to the external surface of zeolite samples (mordenite) but did not penetrate into the pores of zeolites. A little larger amount of adsorption in one sample was attributed to the presence of montmorillonite and amorphous silica as admixtures.
(5) It is concluded that MB adsorption on to clay surface is primarily cation exchange reaction and simultaneously adsorption by probably Van der Waals' force occurs.
(6) MB molecules adsorbed on clay surface occupied relatively large areas and considered to be adsorbed in monolayer. Appreciable multilayer adsorption or micell (aggregate) adsorption were not observed.