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

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.