Relationship between Surface Hydroxyl Complexation and Equi-Acidity Point pH of MnO(2) and Its Adsorption for Co(2+) and Ni(2+)

[Image: see text] MnO(2) 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 MnO(2) (α-MnO(2), β-MnO(2), γ-MnO(2), δ(1)-MnO(2), δ(2)-MnO(2), and ε-MnO(2)) were prepared and characterized by XRD, SEM, EDS, XPS, ZETA, and FT-IR. The reasons for the equi-acidity point pH change of MnO(2) and the complex mechanism of surface hydroxylation on metal ions were discussed. The results showed that the equi-acidity point pHs of different crystalline MnO(2) 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 MnO(2) 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 MnO(2), more hydroxyl groups could participate in the complexation reaction by repeatedly controlling the pH, so that MnO(2) could adsorb heavy metals Co(2+) and Ni(2+) in high-concentration MnSO(4) solution, and the adsorption rates of Co(2+) and Ni(2+) could reach 96.55 and 79.73%, respectively. The effects of MnO(2) dosage and Mn(2+) concentration on the adsorption performance were further investigated, and the products after MnO(2) adsorption were analyzed by EDS and FT-IR. A new process for MnO(2) to adsorb heavy metals Co(2+) and Ni(2+) in high-concentration MnSO(4) solution was explored, which provided a reference for the deep purification of manganese sulfate solutions.