Biosorption of co²⁺ ions from aqueous solution by k₂hpo₄-pretreated duckweed lemna gibba

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⁺ 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⁺ in aqueous solutions tested under different conditions of pH, particle size, and initial Co²⁺ concentration. Kinetic, equilibrium, and thermodynamic studies were conducted. The capacity of biosorption increased with a greater initial Co²⁺ 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⁺ biosorption kinetics. The Sips and Redlich-Peterson isotherm models best predict the biosorption capacity at equilibrium. According to the thermodynamic study, biosorption of Co²⁺ was endothermic and spontaneous. The effect of pH on the biosorption/desorption of Co²⁺ suggests that electrostatic attraction is the main biosorption mechanism. SEM-EDX verified the presence of Co²⁺ on the surface of the pretreated-saturated biosorbent and the absence of the metal after desorption.