Relationship between Surface Hydroxyl Complexation and Equi-Acidity Point pH of MnO2and Its Adsorption for Co2+and Ni2+

Abstract

MnO2 has shown great potential in the field of adsorption and has a good adsorption effect on heavy metal ions in aqueous solution, but there have been problems in the adsorption of heavy metal ions in high-concentration metal salt solutions. In this paper, different crystal forms of MnO2 (α-MnO2, β-MnO2, γ-MnO2, δ1-MnO2, δ2-MnO2, and ϵ-MnO2) were prepared and characterized by XRD, SEM, EDS, XPS, ZETA, and FT-IR. The reasons for the equi-acidity point pH change of MnO2 and the complex mechanism of surface hydroxylation on metal ions were discussed. The results showed that the equi-acidity point pHs of different crystalline MnO2 were different. The equi-acidity point pH decreased with the increase of reaction temperature and electrolyte concentration, but the reaction time had no effect on it. The equi-acidity point pHs of MnO2 were essentially equal to the equilibrium pH values of adsorption and desorption between surface hydroxyl and metal ions on them. The change of equi-acidity points was mainly due to the complexation of surface hydroxyl, and the equi-acidity point pHs depended on the content of surface hydroxyl and the size of the complexation ability. According to the equi-acidity point pH characteristics of MnO2, more hydroxyl groups could participate in the complexation reaction by repeatedly controlling the pH, so that MnO2 could adsorb heavy metals Co2+ and Ni2+ in high-concentration MnSO4 solution, and the adsorption rates of Co2+ and Ni2+ could reach 96.55 and 79.73%, respectively. The effects of MnO2 dosage and Mn2+ concentration on the adsorption performance were further investigated, and the products after MnO2 adsorption were analyzed by EDS and FT-IR. A new process for MnO2 to adsorb heavy metals Co2+ and Ni2+ in high-concentration MnSO4 solution was explored, which provided a reference for the deep purification of manganese sulfate solutions.