Changes in layer charge of clay minerals upon acid treatment as obtained from their interactions with methylene blue

Adsorption of methylene blue (MB) on acid-treated clay minerals was used to follow the changes in layer charge (LC). Decreased values of cation exchange capacity (CEC) upon HCl attack confirmed reduction of LC in all studied systems. Second derivative absorption spectra (SDS) were used for better resolution of the individual bands in the spectra and for easier distinguishing between absorption of different dye species, such as the monomers absorbing near 675 nm, dimers (600 nm), H-aggregates (570 nm), and J-aggregates (775 nm). The changes in the SDS of smectites showed increasing rate of their dissolution in HCl with rising substitution for octahedral Al. The MB spectra of SWy-2 revealed heterogeneity of layer charge distribution in this montmorillonite. Formation of MB monomers confirmed that smectite/illite was more stable in HCl than illite despite of its higher content of swelling layers. The MB spectra were found to better reflect the changes in the LC of acid-treated clay minerals with non-swelling layers than the CEC values. The spectra of well-ordered kaolinite did not change at all within acid-treatment, while a slight increase of monomers band was observed for disordered kaolinite after 36 h. The obtained results confirmed that MB adsorption is a very sensitive tool for characterization of layer charge of different clay minerals modified upon acid treatment.

Dye cations are adsorbed at the surface of smectites by an ion-exchange reaction, at the sites of negative charge. Dye adsorption is accompanied by their aggregation, which depends on the layer charge density. The charge distribution affects the distances between the neighboring dye cations.Figure optionsDownload as PowerPoint slideHighlights
► Layer charge (LC) changes in acid-treated smectites, illite/smectites and kaolinites.
► Second derivative spectra of methylene blue adsorbed on HCl-treated clay minerals.
► Intensities of H-aggregates and monomers bands reflect LC reduction upon dissolution.
► Reasonable estimation of LC modification for acid-treated illite/smectites.