Preparation of cobalt-containing spinel oxides as novel adsorbents for efficient phosphate removal

Abstract

In this paper, a series of cobalt-containing spinel oxides MCo₂O₄ (M = Mg, Mn, Cu) were prepared by a solution combustion method and used as novel adsorbents for phosphate for the first time. MgCo₂O₄ exhibited the highest adsorption capacity of up to 90.02 mg g⁻¹, which was much greater than that of MnCo₂O₄, CuCo₂O₄ and most phosphate adsorbents. The adsorption isotherm and kinetics of MgCo₂O₄ followed the Langmuir monolayer model and the pseudo-second-order model, respectively. The phosphate adsorption of MgCo₂O₄ was found to be endothermic and nonspontaneous. Coexisting ions Cl⁻, NO₃⁻, SO₃²⁻, and HCO₃⁻ did not affect the adsorption of phosphate on MgCo₂O₄, but the presence of CO₃²⁻ would significantly interfere with the adsorption. Little decrease in removal efficiency after three adsorption–desorption cycles suggested its excellent stability and recyclability. Leakages of Co³⁺ and Mg²⁺ from MgCo₂O₄ were negligible when pH was higher than 3.0 during the adsorption process. The mechanism study revealed that the adsorption mechanism involved precipitation, electrostatic interaction and ligand exchange. The large surface area, high surface hydroxyl group content and more positive zeta potential resulted in high adsorption capacity. This work provided an excellent reference to constructing novel phosphate adsorbents and a deep insight into the study of the phosphate adsorption mechanism.