Adsorption kinetics and thermodynamics of azo-dye Orange II onto highly porous titania aerogel

We report herein a kinetic and thermodynamic study of the adsorption of azo-dye Orange II from aqueous solutions onto titania aerogels. Aerogels structure was confirmed by FTIR and N2 adsorption revealed their specific surface area (500 m2/g), pore volume (2.86 cm3/g) and pore size (mean 13.9 nm). Adsorption tests were conducted in batch reactors under various conditions where the effect of pH, temperature, contact time, dye concentration, and adsorbent dose were studied. Experiments performed at pH 2 show the optimal adsorption due to the best surface charge interactions. The temperature shows a weak influence with a decrease in the adsorption uptake as the temperature increases. Adsorption kinetics is shown to be very fast and follows a pseudo second-order indicating the coexistence of chemisorption and physisorption with the intra-particle diffusion being the rate controlling step. The experimental data fit perfectly with Sips isotherms and reveal the ability of titania aerogel to adsorb 420 mg of Orange II per gram of adsorbent at the optimal conditions. The thermodynamic study reveals the activation energy (42.1 kJ mol−1) and the changes in Gibbs free energy (1.2 kJ mol−1), enthalpy (−16.4 kJ mol−1), and entropy (−58 J mol−1 K−1). The entire regeneration of the titania aerogel adsorption sites at pH 11 and 30 °C shows a total recovery of the dye and the efficient reusability and the economic interest of these adsorbing materials for environmental purposes.