Adsorption equilibrium, kinetics, and thermodynamics assessment of the removal of the reactive red 141 dye using sesame waste

Abstract

Adsorption of the reactive red 141 dye from aqueous solutions onto sesame hull waste, as a kind of useless agricultural waste, in a batch process was studied. The biosorbent was characterized by scanning electron microscope and Fourier transform infrared spectroscopy. There were hydroxyl groups, carboxyl groups, etc. on the surface of the sorbent, as shown by Fourier transform infrared spectroscopy. Adsorption was investigated as a function of the particle size, the amount of adsorbent, the pH, the initial dye concentration, and the agitation speed with time. The maximum removal of dye molecules was found to be 95% at pH 1.1, the initial dye concentration of 30 mg L−1, the adsorbent dose of 4 g L−1, and the temperature of 20°C. The experimental equilibrium data were analyzed, showing that the adsorption behavior of the reactive red dye could be described well reasonably by Langmuir isotherm model with the maximum sorption capacity of 27.55 mg g−1for the biosorbent. Kinetics studies also revealed that the adsorption of the dye followed pseudo-second-order kinetics, with the regression, R2, of 0.994. The adsorption process was exothermic with a mean change in the enthalpy of −15.48 kJ mol−1, and spontaneous with a mean free energy change of −15.38 kJ mol−1. Due to the exothermic nature of the adsorption process, the removal percentage of the dye was improved remarkably from 50% at the temperature of 323.15 K to over 70% at the temperature of 298.15 K. The acidic aqueous solution with the initial pH <2 favored the adsorption of the anionic reactive red dye by the sesame waste biosorbent (with pHpzc of 5.1). Through the reusability study of the sorbent, the removal percentages of dye were found to be 94, 91, 87, and 80% for 1st, 2nd, 3rd, and 4th cycles of adsorption, respectively.