R6G and MB dyes were purchased from Sigma Aldrich Pte. Ltd. The Na+-bentonite was purchased from Sued-Chemie South East Asia. The cation exchange capacity (CEC) of this bentonite was determined by atomic absorption spectroscopy (AAS) (Perkin-Elmer Model 100). A known amount of bentonite was dispersed in distilled water and placed in a sonication bath for 30 min. The dispersion was then filtered prior measurement of its Na+ content. The value obtained was 1.05 mmol/g, which is consistent with other published research. The organosilane used was phenyltrimethoxysilane (PTMOS, with a purity of 99%, from Fluka). All materials were used without further purification.
Adsorption of MB and R6G by as-received bentonite was performed by a batch method. A dispersion of 10 g/l was prepared and kept for 3 h at room temperature under stirring. The amount of dye added was set to 5, 10, 20, 40, 60, 80 and 100% of the CEC. Later, the clay suspensions were centrifuged and dye concentration in the supernatant was determined by spectrometry