Relationship between Surface Hydroxyl Complexation and Equi-Acidity Point pH of MnO2 and Its Adsorption for Co2+ and Ni2+

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 (alpha-MnO2, beta-MnO2, gamma-MnO2, delta(1)-MnO2, delta(2)-MnO2, and epsilon-MnO2) were prepared and characterized by XRD, SEM, EDS, XPS, ZETA, and FT-IR. The reasons for the equiacidity 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 equiacidity 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 equiacidity 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.