Composition, structure, and luminescence of montmorillonites saturated with different aggregates of methylene blue.

The distribution of various aggregates (dimers, trimers, and tetramers) of methylene blue (MB) formed in aqueous solution at various concentrations of dye has been calculated using the equilibrium aggregation constants betaq. Two montmorillonite samples with different cation exchange capacities, surface areas, and interlayer distances d001, Na-SWy, and Ca-Cheto, were saturated with methylene blue (MB) solutions with various ratios between monomers and higher aggregates of dye. The total amount of MB in the intercalated montmorillonite samples (MB-SWy and MB-Cheto) increases with increasing concentration of dye in water solutions, i.e., with increasing aggregates/monomers ratio of MB in water solution. In all intercalated montmorillonite samples with methylene blue except guest qth aggregate cations [MBqq+] low contents of Na+ (in MB-SWy) and Ca2+ (in MB-Cheto) cations were also determined. A very good positive correlation between the basal spacing d001 and the MB/montmorillonite molar ratio was revealed for saturated MB-montmorillonite samples. Structural analysis using a combination of diffraction data with molecular modeling revealed the differences in the interlayer arrangement of MB guests in MB-SWy and MB-Cheto intercalates. Also, fluorescence measurements showed the strong effect of the silicate layer charge on the spectroscopic behavior of MB guests intercalated in montmorillonite. Methylene blue exhibits a certain luminescence in MB-SWy samples with cation exchange capacity 0.80 meq g-1 and almost no luminescence in MB-Cheto samples with higher cation exchange capacity 1.50 meq g-1.

The rhodamine B intercalation of montmorillonite.

Using photometric methods the dissociation constants and weight fractions of rhodamine B dimer in water solutions at different concentrations were determined. The montmorillonite (SWy) samples were fully intercalated with rhodamine B (RhB) solutions at various monomer/dimer ratios. The amount of rhodamine B in fully intercalated montmorillonite (RhB-SWy) increases with increasing concentration of dye in water solutions, i.e., with increasing dimer/monomer ratio. The sum of exchangeable guest cations in RhB-SWy is approximately constant (0.900 meq g(-1)) for all samples, because RhB-SWy samples with prevailing dye monomer also contain higher amounts of non-exchanged alkali elements. The experimental data are supported by calculated structure models that illustrate the changes in RhB-SWy structure depending on monomeric and/or dimeric arrangement of guests. The analysis of the calculated structure models confirmed the existence of two phases with different basal spacings, d approximately 1.8 and approximately 2.3 nm, revealed by X-ray diffraction.

Fluorescence of reduced charge montmorillonite complexes with methylene blue: Experiments and molecular modeling.

The intensity of fluorescence of montmorillonites fully saturated by methylene blue (MB) is very poor due to energy dissipation in MB aggregates. A series of reduced charge montmorillonites (RCM) were prepared from Na-homoionic SWy and Ca homoionic SAz with the aim to decrease the MB aggregation. Fine tuning MB adsorption degree by charge reduction and MB concentration enabled controlled production of different dye species from aggregates via dimers to monomers. It was shown that the intensity of the fluorescence of low-loaded MB-RCM complexes is enhanced by several orders of magnitude with respect to dye-saturated original montmorillonites. XRD analyses, molecular modeling, and diffuse reflectance spectroscopy revealed that low MB-loaded RCMs are very probably adsorbed mainly on the external montmorillonite surface as isolated dye molecules. Such a state cannot be achieved in the solid state without very careful tailoring of the host-guest interaction.

Cesium uptake from aqueous solutions by bentonite: a comparison of multicomponent sorption with ion-exchange models.

The removal of cesium from concentrated aqueous solutions into Ca/Mg-bentonite for a wide range of bentonite-to-water (m/V) ratios was studied experimentally and theoretically. Using the batch technique, the equilibrium of Cs uptake was measured. The nonlinear character of cesium sorption substantially influenced by the m/V ratio was observed. The experimental data were evaluated using the multicomponent Langmuir isotherm and an ion-exchange model based on the ion-exchange reaction between Cs+ and M2+ (Ca2+/Mg2+) initially sorbed on bentonite. Constants k1,Cs = 0.521 mmol.g-1, k2,Cs = 968 L.mol-1, and k2,M = 592 L.mol-1 were obtained for Cs uptake described by multicomponent Langmuir isotherm. For the ion-exchange model, the thermodynamic equilibrium constant K = 75.5 mL.g-1 with a standard deviation of sK = 17.4 mL.g-1 was determined. Using the t test, the calculated data of multicomponent Langmuir and ion-exchange isotherms were fit to experimental data, and the best agreement was obtained for the ion-exchange model. The results show that Cs uptake by bentonite could be substantially decreased in systems with a high bentonite-to-water (m/V) ratio as a consequence of the presence of desorbed divalent cations in the liquid phase.