Zwitterionic dye rhodamine B (RhB) uptake on different types of clay minerals

Increased use of color dyes in textile industries resulted in great needs of treating dye-containing wastewater. As such, researches and developments for effective removal of dyes from wastewater have been conducted extensively and good progresses have already been made. However, there are still needs to understand the mechanisms of interactions between dyes and solids and to evaluate factors controlling such interactions. In this study, the interactions between different types of clay minerals and rhodamine B (RhB), a zwitterionic dye, were examined under different physic-chemical conditions. The solution pH, ionic strengths, and temperature had minimal influence of RhB uptake on these clay minerals. Uptake of RhB was limited to the external surfaces for the non-swelling clay minerals kaolinite and palygorskite. In contrast, both external and interlayer spaces are responsible for the uptake of RhB on swelling clay mineral montmorillonite. The RhB molecules intercalated into the interlayer of montmorillonite may adopt a monolayer or a bilayer configuration under low and high uptake levels. Under neutral pH condition, the RhB molecules exist as zwitterionic species in solution. The extensive uptake of RhB by the clay minerals, up to their cation exchange capacity (CEC) values, was attributed to electrostatic interactions between cationic end of the amine and the negatively charged clay minerals surfaces. The significant RhB uptake suggested that clay minerals are also good sorbents for the removal of zwitterionic dyes.