Removal of Rhodamine B from aqueous solutions and wastewater by walnut shells: kinetics, equilibrium and thermodynamics studies

An adsorption study of Rhodamine B (RB) dye from aqueous solutions was carried out using walnut shells pretreated by different methods. In addition to the effects of the pretreatment, the effects of various parameters like pH, adsorbent dose, contact time, initial dye concentration and temperature on the adsorption of RB was studied. The adsorption process was highly pH dependent and a maximum adsorption was achieved at pH 3.0. The best fit for the rates of dye adsorption was a pseudo-second-order kinetic model with good correlation coefficients (R2>0.99). Langmuir isotherms were used to determine that the maximum loading capacity of the different walnut shells and the RB capacities ranged from 1.451–2.292 mg·g−1. The dye adsorption was also evaluated thermodynamically. Positive standard enthalpy (ΔH°) values were obtained indicating that the RB adsorption process is endothermic as well as ΔG° and ΔS° values showed that adsorption process is spontaneous with an increased randomness at the solid-liquid interface. Desorption studies were carried out to explore the feasibility of regenerating the used walnut shells and it was found that 97.71%–99.17% of the retained RB was recovered with 0.1 mol·L−1 NaOH solution. The walnut shells were also successfully used to remove RB from industrial effluents.