Adsorption of an anionic dye from aqueous medium by organoclays: equilibrium modeling, kinetic and thermodynamic exploration

Abstract

Adsorption of an anionic dye, remazole brilliant blue R (RBBR) from aqueous solution, on organically modified attapulgite (ATP) and bentonite (BNT) clays was investigated. Beneficiated attapulgite and bentonite clay samples were modified by the cationic surfactant cetyltrimethylammoniumbromide (CTAB) by the ion exchange method. The efficiency of these adsorbents for the uptake of RBBR was examined by a series of adsorption isotherms using a batch technique at three different temperatures. A modified Freundlich equation has been proposed to describe the isotherms and compared with Langmuir, Freundlich and Redlich–Peterson (RP) models. Predicated values of RBBR adsorbed on modified clay using Freundlich, modified Freundlich, and RP models were identical along with the normalized standard deviation for all three models. These values were lower than those of Langmuir model. Values of r² for RP and modified Freundlich were found to be identical and better than that of Langmuir for all the isotherms. Adsorption of RBBR followed a pseudo second order kinetic model. Negative values of free energy change, ΔG indicated spontaneity of the adsorption of RBBR on both of organoclays. An increase in adsorption of RBBR with an increase in temperature may be attributed to activated adsorption and hence an endothermic process. This was further supported by positive values of heat of adsorption, ΔH. The adsorption capacity of modified attapulgite is lower than that of modified bentonite. However, considering the requirement of CTAB for modification of the clay, attapulgite is the more economical adsorbent than modified bentonite.