Adsorptive Removal of Brilliant Cresyl Blue by Tamarix aphylla Leaves: Equilibrium, Kinetics and Thermodynamics

Abstract

Tamarix aphylla leaves were used as low cost adsorbent material for removal of brilliant cresyl blue dye from aqueous media using batch adsorption technique. Three kinetic models include first-order, pseudo-second order and intra-particle diffusion were applied to analyze brilliant cresyl blue adsorption process. The kinetic rates were best fitted to the pseudo-second order model. The correlation coefficients (R²) of pseudo-second order reaction were 0.9996 and 0.9991 at initial concentration 28.1 mg/l and 32.3 mg/l of brilliant cresyl blue, respectively. The intra-particle diffusion studies proved that the intra-particle diffusion is not only the mean determining step. Adsorption isotherms of brilliant cresyl blue onto Tamarix aphylla leaves at different temperatures were investigated. Langmuir and Freundlich models were applied in the present study to describe the adsorption isotherm. The isotherms of adsorption were fitted to the Langmuir equation better than Freundlich equation. The maximum adsorption capacities (q_e) for brilliant cresyl blue on Tamarix aphylla leaves calculated from Langmuir isotherm were 526, 556 and 455 mg/g at contact temperatures 20, 30 and 40 ^oC, respectively. The thermodynamic parameters such as Gibbs free energy ΔG⁰, standard enthalpy ΔH⁰ and entropy change ΔS⁰ were investigated. The negative values of ΔG⁰ (-7.11-10-48 Kj/ mol. K) proved that the adsorption process of brilliant cresyl blue onto Tamarix aphylla leaves is spontaneous in nature and the degree of spontaneity increase as the temperature increase. The positive value of ΔH⁰ (45.90 Kj/ mol. K) indicate the endothermic behavior of the adsorption system and the positive value of ΔS⁰ (0.18 Kj/ mol. K) suggest the increasing randomness of the adsorption system.