Adsorption kinetics and thermodynamics of an anionic dye onto sepiolite

The adsorption kinetics of CI reactive blue 221 (RB221), an anionic dye, onto sepiolite was investigated in aqueous solution in a batch system with respect to stirring speed, contact time, initial dye concentration, pH, and temperature. Experimental results have shown that the acidic pH, increasing initial dye concentration and temperature favours the adsorption. Experimental data related to the adsorption of RB221 on sepiolite under different conditions were applied to the pseudo-first-order equation, the pseudo-second-order equation and the intra-particle diffusion equation, and the rate constants of first-order adsorption (k1), the rate constants of second-order adsorption (k2) and intra-particle diffusion rate constants (kint) were calculated, respectively. The experimental data fitted very well the pseudo-second-order kinetic model and also followed the intra-particle diffusion model up to 20 min, whereas diffusion is not only the rate controlling step. The activation energy of system (Ea) was calculated as 7.73 kJ mol−1. The thermodynamics parameters of activation such as Gibbs free energy, enthalpy, entropy were also evaluated and found that ΔG∗, ΔH∗, and ΔS∗ are 47.9 (49.4, 50.0, 51.3) kJ mol−1, 5.30 (5.21, 5.13, 5.05) kJ mol−1, and −145.3 (−148.8, −142.7, −143.1) J K−1 mol−1 at 20 (30, 40, 50) °C, respectively. The results indicate that sepiolite could be employed as a low-cost material for the removal of textile dyes from effluents.