Equilibrium and kinetics studies for adsorption of direct blue 71 from aqueous solution by wheat shells

This paper deals with the application of wheat shells (WS), an agricultural by-product, for the removal of direct blue 71 (DR) from aqueous solution. The characteristics of WS surface, such as surface area, Bohem titration and scanning electron microscopy (SEM) were obtained. The removal of direct blue 71 onto WS from aqueous solution was investigated by using parameters, such as pH, temperature, adsorbent dose, contact time and initial concentration. The adsorption process attains equilibrium within 36 h. The extent of dye removal decreased with increasing adsorbent dosage and also increased with increasing contact time, temperature, in solution concentration. Optimum pH value for dye adsorption was determined between 6 and 8. The experimental data were analysed by the Langmuir and Freundlich models of adsorption. It was found that the Langmuir equation fit better than the Freundlich equation. Maximum adsorption capacity (Qm) was calculated as at different temperatures (293, 303 and 313 K) 40.82, 45.66 and 46.30 mg g−1, respectively. In addition, the adsorption data obtained at different temperatures of DR by WS were applied to pseudo first-order, pseudo second-order and Weber-Morris equations, and the rate constants of first-order adsorption (k1), the rate constants of second-order adsorption (k2) and intraparticle diffusion rate constants (k3) at these temperatures were calculated, respectively. The rates of adsorption were found to conform to pseudo second-order kinetics with good correlation (R2 ≥ 0.9904). Also, free energy of adsorption (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes were determined to predict the nature of adsorption. Furthermore, the results indicate that WS could be employed as a low-cost alternative to other adsorbents in the removal of direct blue 71 from aqueous solution.