**Jessica Lizeth Reyes-Ledezma, Eliseo Cristiani-Urbina and Liliana Morales-Barrera \***

Departamento de Ingeniería Bioquímica, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Av. Wilfrido Massieu s/n, Unidad Profesional Adolfo López Mateos, Ciudad de Mexico 07738, Mexico; jessicalrl22@hotmail.com (J.L.R.-L.); ecristianiu@yahoo.com.mx (E.C.-U.) **\*** Correspondence: lmoralesb@ipn.mx; Tel.: +52-55-5729-6000 (ext. 57827)

Received: 19 October 2020; Accepted: 23 November 2020; Published: 25 November 2020

**Abstract:** The wastewater of the many industries that use divalent cobalt (Co2+)-containing compounds has elevated levels of this metal. Thus, novel technology is needed to efficiently remove Co2<sup>+</sup> ions from aqueous solutions. Biosorption is a low-cost technique capable of removing heavy metals from contaminated water. This study aims to evaluate the performance of KH2PO4-pretreated *Lemna gibba* (*PLEM*) as a biosorbent of Co2<sup>+</sup> in aqueous solutions tested under different conditions of pH, particle size, and initial Co2<sup>+</sup> concentration. Kinetic, equilibrium, and thermodynamic studies were conducted. The capacity of biosorption increased with a greater initial Co2<sup>+</sup> concentration and was optimal at pH 7.0 and with small-sized biosorbent particles (0.3–0.8 mm). The pseudo-second-order sorption model best describes the experimental data on Co2<sup>+</sup> biosorption kinetics. The Sips and Redlich-Peterson isotherm models best predict the biosorption capacity at equilibrium. According to the thermodynamic study, biosorption of Co2<sup>+</sup> was endothermic and spontaneous. The effect of pH on the biosorption/desorption of Co2<sup>+</sup> suggests that electrostatic attraction is the main biosorption mechanism. SEM-EDX verified the presence of Co2<sup>+</sup> on the surface of the pretreated-saturated biosorbent and the absence of the metal after desorption.

**Keywords:** divalent cobalt; *Lemna gibba*; biosorption; desorption; SEM-EDX
