Determination of the cation exchange capacity and the surface area of bentonite, illite and kaolinite by methylene blue adsorption

Dye cations like methylene blue will mainly adsorb on clay minerals by cation exchange. Therefore, the methylene blue adsorption is depending on the exchangeable cations of the clay mineral, on the pH and on the dye cation concentration. The cation exchange capacity of clays by methylene blue adsorption can be determined when the samples are in the sodium exchanged form and the pH is neutral. So one obtains the same values as determined with the ammonium acetate method. The surface area of bentonites can only be calculated with methylene blue adsorption when the montmorillonite surface area per charge corresponds with the area of the methylene blue cation of 130 Å2, i.e. the interlayer charge of the montmorillonites must amount to 0.28–0.33 charges per half unit cell. Our investigations have confirmed the experiences in the application of the methylene blue method that it is necessary to work with optimum dispersion, a fixed clay:water ratio as well as a constant pH and a given initial amount of methylene blue in order to achieve reproducible results. With these assumptions the methylene blue adsorption can be used for the quality control of the same clay.