*Article* **Equilibrium, Kinetics and Thermodynamics of Chromium (VI) Adsorption on Inert Biomasses of** *Dioscorea rotundata* **and** *Elaeis guineensis*

**Angel Villabona-Ortíz 1 , Ángel González-Delgado <sup>2</sup> and Candelaria Tejada-Tovar 1,\***


**Abstract:** Adsorption equilibrium and kinetics on lignocellulosic base adsorbents from oil palm bagasse (OPB) and yam peels (YP) were studied for the removal of hexavalent chromium present in aqueous solution, in a batch system, evaluating the effect of temperature, adsorbent dose and particle size on the process. Isotherms were fitted to Langmuir, Freundlich and Dubinin–Radushkevich isothermal models. Kinetic data were adjusted to the pseudo-first-order, pseudo-second-order and Elovich models. Thermodynamic parameters were estimated by the van't Hoff method. From characterization of adsorbents, the presence of a porous surface typical of lignocellulosic materials was found, with hydroxyl, amine and carboxyl functional groups. It was also found that the highest adsorption capacity was obtained at 0.03 g of adsorbent, 55 ◦C and 0.5 mm, reporting an adsorption capacity of 325.88 and 159 mg/g using OPB and YP, respectively. The equilibrium of adsorption on OPB is described by Langmuir and Freundlich isotherms, while that of YP is described by Dubinin– Radushkevich's model, indicating that the adsorption is given by the ion exchange between the active centers and the metallic ions. A maximum adsorption capacity was obtained of 63.83 mg/g with OPB and 59.16 mg/g using YP, according to the Langmuir model. A kinetic study demonstrated that equilibrium time was 200 min for both materials; kinetic data were described by pseudo-secondorder and Elovich models, thus the mechanism of Cr (VI) adsorption onto the evaluated materials is dominated by a chemical reaction. The thermodynamic study determined that the elimination of YP is endothermic, irreversible and not spontaneous, while for OPB it is exothermic, spontaneous at low temperatures and irreversible.

**Keywords:** bio-adsorption; Langmuir; Freundlich; Dubinin–Radushkevich
