Methylparaben Adsorption onto Activated Carbon and Activated Olive Stones: Comparative Analysis of Efficiency, Equilibrium, Kinetics and Effect of Graphene-Based Nanomaterials Addition

This paper describes a comparative study of the adsorption of methylparaben onto commercial activated carbon and olive stones activated by calcination at 300 °C and treatment with 1 M HCl. The influence of the initial concentration of methylparaben, adsorbent dose, stirring speed and pH on the adsorption capacity of methylparaben on both adsorbents was studied. To find out the isotherm model, the kinetic model and the mechanism that best describe the adsorption process on each adsorbent, the experimental equilibrium data were analyzed using six isotherm models (Langmuir, Freundlich, Elovich, Temkin, Jovanovic and Dubinin–Radushkevich), and the experimental kinetic data were analyzed using four kinetic models (pseudo-first order, pseudo-second order, Elovich and Avrami) and two mechanistic models (Weber–Morris and Boyd). For both adsorbents, the Langmuir model best describes the adsorption equilibrium, the kinetics of the process follow a pseudo-first-order model and boundary layer diffusion is the step that mainly controls the adsorption process. The adsorption capacity of methylparaben on activated carbon is about four times higher than that of activated olive stones. The addition of graphene oxide and reduced graphene oxide to both adsorbents increases their methylparaben adsorption capacity, to a greater extent in the case of graphene oxide, being that increase more important in activated carbon than in activated olive stones.