Removal of toluidine blue from water using 1:1 layered clay minerals

Extensive use of color dyes in modern society resulted in their potential release into the environment. Dye removal often involved in sorption or reaction. For sorptive removal, materials with low cost and high efficiency are of great potential. In this study, the removal of toluidine blue (TB), a cationic dye, by 1:1 layered clay minerals kaolinite (KAO) and halloysite (HNC) was systematically studied under different physicochemical conditions. The TB removal was fast and equilibrium could be reached within an hour. The TB removal capacities were 47 and 149 mmol/kg by KAO and HNC slightly larger than their cation exchange capacity (CEC) of 37 and 120 meq/kg. The results indicated that both the CEC and specific surface area (SSA) of the minerals played a critical role in TB removal. Equilibrium solution pH and ionic strength had less effect on TB removal, suggesting preferred affinity of TB over other inorganic cations for mineral surfaces. X-ray diffraction data showed no interlayer expansion, indicating TB sorption sites were on the external surfaces of both minerals. The FTIR results indicated participation of N⁺ on the dimethyl group for TB sorption on negatively charged mineral surface. At the sorption capacities, TB may form a condensed monolayer on KAO surfaces, and a dimer or bilayer coverage on HNC surfaces, as suggested by molecular dynamic simulation. The results from this study suggest that clay minerals of high CEC and large SSA values would be good candidates for the removal of cationic dyes form water.