Relationship between Surface Hydroxyl Complexation and Equi-Acidity Point pH of MnO and Its Adsorption for Co and Ni.

MnO 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 (α-MnO, β-MnO, γ-MnO, δ-MnO, δ-MnO, and ε-MnO) were prepared and characterized by XRD, SEM, EDS, XPS, ZETA, and FT-IR. The reasons for the equi-acidity point pH change of MnO 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 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 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, more hydroxyl groups could participate in the complexation reaction by repeatedly controlling the pH, so that MnO could adsorb heavy metals Co and Ni in high-concentration MnSO solution, and the adsorption rates of Co and Ni could reach 96.55 and 79.73%, respectively. The effects of MnO dosage and Mn concentration on the adsorption performance were further investigated, and the products after MnO adsorption were analyzed by EDS and FT-IR. A new process for MnO to adsorb heavy metals Co and Ni in high-concentration MnSO solution was explored, which provided a reference for the deep purification of manganese sulfate solutions.