Methylene blue interactions with reduced-charge smectites

The objective of this work was to prepare series of reduced-charge material s from different parent Li-saturated dioctahedral smectites, to investigate the effects of temperature. chemical composition and charge location in sm ectites on the charge reduction and to characterize reduced-charge smectite s (RCSs) using methylene blue (MB) adsorption. The layer charge decrease, i nduced by Li fixation, is correlated with the trends in the spectra of MB-R CS dispersions in the visible region (VIS) spectra. Distribution of the neg ative surface charge of the clay minerals controls the distance between the adsorbed MB cations and thus affects the formation of MB dimers and higher agglomerates. Because each form of MB (monomer, dimer, higher agglomerate, J-aggregates) absorbs light at a different wavelength, the VIS spectra of MB depend sensitively on the charge density at the clay surface. Both catio n exchange capacity (CEC) values and spectra of MB-clay dispersions clearly detect extensive reduction of the layer charge density in reduced-charge m ontmorillonites (RCMs) upon Li-thermal treatment. The extent of charge redu ction depends on the temperature of the thermal treatment. as well as on th e octahedral charge of the montmorillonite. Reduction of the layer charge p roceeds to a much lesser extent for smectites with mainly tetrahedral charg e and high Fe content (Fe-rich beidellite and ferruginous smectite). Both C EC data and MB spectra detect only a slight decrease of the layer charge de nsity, which relates to the low octahedral charge of these minerals. Follow ing heating at higher temperatures (120-160 degreesC), slightly higher Li f ixation is indicated by CEC values: however, no charge reduction is confirm ed by MB spectra. Release of protons accompanying Li+ fixation in Fe-rich s mectites heated at 180 or 200 degreesC was detected in the spectra of MB-cl ay dispersions and confirmed by potentiometric titrations.