Taguchi method for optimization of Cr(VI) removal, isotherm, kinetic and thermodynamic studies.

In this study, Taguchi optimization method was applied to determine the optimum operating conditions for batch adsorption of Cr(VI) from aqueous solution. Initial pH of solution, adsorbent dose, initial hexavalent chromium concentration, contact time and adsorbent type were selected as the variables, and the removal efficiency of Cr(VI) was chosen for the designated response. L₁₈(3⁵) orthogonal array, signal-to-noise (S/N) ratio and analysis of variance statistical procedures were applied to identify the effect of each operating parameter on the removal of Cr(VI) from aqueous solution. The signal-to-noise (S/N) ratio results showed that the optimal combination for Cr(VI) removal was at pH 1.0, adsorbent dose of 3.6 g.L^-1, Cr(VI) concentration of 30 mg.L^-1, contact time of 95 min and olive leaves as adsorbent type. A removal of 95.09% was obtained at these optimum conditions. The analysis of variance of the data revealed that initial pH of solution was the most dominant parameter affecting Cr(VI) removal efficiency, followed by adsorbent type, adsorbent dose, contact time and initial metal concentration. Under optimal conditions, adsorption kinetic of Cr(VI) was studied and modeled using the pseudo first-order, pseudo-second-order and intraparticle diffusion models. It was found that the pseudo-second-order model fitted the adsorption data most with the highest determination coefficient (R² = 0.996). Freundlich isotherm model, with regression coefficient R² of 0.953, fit well with the equilibrium isotherm data. The Langmuir maximum adsorption capacity was found to be 62.5 mg.g^-1. The experimental values of ΔH°, ΔG° and ΔS° revealed that the adsorption process was spontaneous and endothermic.