Equilibrium and kinetic studies of ferricyanide adsorption from aqueous solution by activated red mud

In this study, activated red mud (ARM) was used as a new adsorbent for the removal of ferricyanide anions from aqueous solution. Based on the percentage of ferricyanide removal and ferricyanide adsorption capacity, optimum conditions were evaluated using the response surface method (RSM) and central composite design (CCD). In optimum conditions (pH = 5.6, adsorbent dosage of 2.59 g, ferricyanide concentration of 60 ppm and contact time of 60 min), the percentage of ferricyanide removal and ferricyanide adsorption capacity were obtained as 79.6% and 1.8 mg/g, respectively. The kinetics and equilibrium studies were evaluated by considering the effective parameters including pH and ferricyanide concentration. Kinetic studies were evaluated by kinetic models of pseudo first-order, pseudo-second-order (four different linearized forms), Elovich and intraparticle diffusion. The results of the kinetic study indicated that the mechanism of ferricyanide adsorption onto the ARM adsorbent is a chemisorption interaction by a fast ferricyanide adsorption onto ARM and subsequently the slow diffusion of ferricyanide ions into the ARM inner adsorption sites. The equilibrium studies showed that the adsorption process followed the Langmuir model in which ferricyanide adsorption onto ARM was homogeneous with monolayer adsorption. The results indicated that the activation process of red mud improved adsorbent efficiency and increased the adsorption capacity.