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.

Octahedral manganese trioxide was prepared from manganese sulfate solution with air as oxidant and compound additive as pH regulator. The results showed that the product had high crystallinity, stable baseline and significant diffraction peak. The micro morphology was formed by stacking regular octahedral structure, the particle size was uniform, and it was about 200nm, the pores were rich, the specific surface area was 6.5 m2/g, and the tap density was 2.11g/cm3. All performance indexes of the product met the excellent standard of soft ferrite manganese trioxide (GBT21836-2008).

Manganese oxides showed many special physicochemical properties in many fields such as electrochemistry, adsorption and catalysis. They were widely used in cathode materials for lithium batteries, molecular sieves, catalytic materials and adsorbents. In this paper, in situ oxidation of manganese sulfate solution was conducted with H2O2 as oxidant, and the characterization means of XRD, SEM and BET were used. The purpose was to study the effects of different regulation mechanisms on the physical and chemical properties of manganese oxides such as morphology, phase composition, surface properties and specific surface area. The adsorption properties of γ-MnO2 for Co and Ni in manganese ore leaching solution were tested. The results showed as follows. Under alkaline conditions, the main product of manganese sulfate solution oxidized by H2O2 was Mn3O4 spherical particles with a radius of about 50 nm, these particles had micropores or mesopores, the oxidation reaction rate was rapid, and the specific surface area and N2 adsorption capacity changed with the change of reaction conditions.The temperature had a great influence on the micro morphology of the product.The micro morphology was slender nanorod when the temperature was 20 °C. With the increase of temperature, the length of nanorod became shorter. When the temperature rises to 50 °C, the rod became spherical. When the pH value decreased from 9 to 7, the diffraction peak of each crystal plane in the product Mn3O4 decreased gradually. The diffraction peak of γ-MnO2 appeared when the pH value decreased to 5. All the products were γ-MnO2 when the pH value decreased to 3. With the increase of Mn2+ concentration, the grain size decreased and agglomeration was easy to occur. The optimum conditions were obtained as follows: the temperature was 30 °C, pH was 3, reaction time was 90 min, the mole ration of H2O2 to Mn2+ was 1:1, and Mn2+ concentration was 10 g l−1. Under the optimum conditions, γ-MnO2 with high specific surface area (172.41 m2 g−1) was prepared. This γ-MnO2 has a good adsorption effect on Co and Ni in manganese ore leaching solution, and the adsorption efficiency could be reached 94.75% and 95.67%. This study would provide a reference for the preparation of manganese oxides with different physical and chemical properties.