3.3.1. Recovery of MnO2 from PLS-1 Solution

#### Purification

Iron precipitation tests were carried out under the same conditions as those used in a study by Blais et al. (2016) with 1.5 times SMR of H2O2 for oxidizing Fe2<sup>+</sup> ions to Fe3<sup>+</sup> at pH = 4, T = 25 ◦C, and stirring rate = 200 rpm [28]. Under these conditions, 98% of Fe in the PLS-1 was precipitated. A concentration of 131 mg Fe/L was recorded in the PLS-1 and a 2.99 mg Fe/L was measured after Fe removal (Table 5). In this step, the concentration of Cu decreased from 42.9 to 26.6 mg/L. After iron removal, cementation was used to remove impurities such as Cd, Co, and Ni. The impurities more electropositive than Zn could be removed by adding metal Zn powder and controlling the pH and temperature of the solution [29]. The most efficient removal of Cd (100%), Co (53%), Cu (100%), and Ni (86%) was obtained after 120 min. The low rate of Co removal in our study corresponds to observations made in other studies [29,30]. Furthermore, these studies also mention the difficulty of removing Co due to the small potential difference between Zn and Co–Zn alloys. In addition, the low concentration of cobalt (13.4 mg/L) also makes it more difficult to remove. Notably, Cd and Cu can be removed at lower temperatures (25–40 ◦C) and at a pH between 2.0 and 2.5. However, a temperature of 80 ◦C and a pH of 4–5 are necessary for the cementation of Ni [30]. The slight increase in Mn (from 28.5 to 31.3 mg/L) and Zn (83.6 to 85.2 mg/L) concentration can be explained by the evaporation of water during the cementation process at 80 ◦C.
