Aspects of cationic dye molecule adsorption to palygorskite

This paper presents a study on the adsorption of cationic dyes, methylene blue and crystal violet, from aqueous solutions (distilled water and actual groundwater) onto palygorskite clay. In particular, we discuss the effect of dissolved ions, pH, particle size, adsorbent mass, and dye initial concentration on adsorption capacity. It is clearly shown that both dyes adsorption by palygorskite in groundwater is dramatically more effective than in distilled water, i.e., adsorption of cationic dyes by palygorskite is not affect by the postulated competition of the dissolved ions, but in contrary the presence of these ions results in significant improvement of adsorption. Langmuir and Freundlich isotherms failed to represent palygorskite adsorption of basic dyes in groundwater. Therefore, multilayer adsorption isotherms such as BET are proposed. Two simplified kinetic models, pseudo-first order and pseudo-second order, were tested to investigate adsorption mechanisms. It was found that the kinetics of adsorption of the two basic dyes onto the surface of the palygorskite are best described by the pseudo-second order model supporting chemisorption (chemical reaction) as rate controlling mechanism which indicates that the adsorption process is irreversible. Below pH=9, pH has no apparent effect on adsorption; however, for pH greater than 9, the adsorption capacity increases with the pH. Effect of particle size analysis revealed that decreasing the clay particle size has increased the adsorption capacity of the basic dyes.