Effect of molecular structure on the adsorption behavior of cationic dyes onto natural vermiculite

An unusual decrease of methylene blue's equilibrium adsorption density (qe) with the increasing initial concentration (C-0) was observed in previous studies. Further studies are required to understand the mechanism related to this phenomenon. For exploring the effect of molecular structure on cationic dye's adsorption, the adsorption behavior of methylene blue (MB) and crystal violet (CV) onto natural vermiculite were investigated. As a comparative dye, CV has the same ionic charge (+1) with MB, but its triangular molecular structure is significantly different with MB's linear molecular structure. The isotherm results indicated that the MB's qe at 25 degrees C decreased from 0.076 to 0.053 mmol g(-1) with the C-0 increased from 0.3 to 0.5 mmol L-1. The Langmuir model was not able to describe its isotherm curves. On the contrary, CV showed a typical Langmuir isotherm with a R-2 of 0.99 at 25 degrees C. Furthermore, the difference between MB and CV's adsorption behavior was also reflected in the column experiment. The breakthrough time of MB exhibited a 71% decrease with the influent concentration increasing from 0.25 to 0.50 mmol L-1, whereas CV only behaved a 45% decrease. Results of this study demonstrate that the molecular structure could be responsible for the unusual decline of MB's adsorption isotherm under high initial concentrations. Due to the linear structure, the molecules of MB could cause the 'adsorbate-adsorbate' steric effect more easily compared to CV, leading a decline of the equilibrium adsorption density. Therefore, the initial concentration of MB should be controlled accordingly to achieve the optimal adsorption performance of natural clays such as vermiculite. (C) 2015 Elsevier B.V. All rights reserved.