3.1.1. Recovery of Co2+ and Ni2+ from Sulfate Solution

Cementation experiments for recovering Co2+ and Ni2+ from sulfate solutions (initial pH = 4) were conducted for 24 h using Al powder as an electron donor, and the effects of the dosage of additives (AC, TiO2, and SiO2) on the efficiency of Co and Ni recoveries were investigated. To access the adsorption of Co2+ and Ni2+ on the additives, experiments without Al were also conducted.

Figures 2a–c and 3a–c show the Co and Ni recovery efficiencies and final pH as a function of SiO2, AC, and TiO2 dosages, respectively. In all experiments, final pH was in the range from 5.1 to 5.6, at which Co2+ and Ni2+ do not precipitate as their hydroxide (Figures S1 and S2).

As shown in Figures 2a and 3a, without Al, the efficiencies of Co and Ni recovery were almost 0% at any dosage of SiO2, suggesting that there was no adsorption of Co2+ and Ni2+ on the SiO2 surface. Even with Al, the Co and Ni recovery efficiencies were also almost 0% regardless of SiO2 dosage, suggesting that cementation of Co2+ and Ni2+ using Al as an electron donor did not occur. This may be due to the presence of an Al oxide layer covering the Al surface, which inhibits the electron transportation from Al to Co2+ and Ni2+ [2,27]. Because the cementation did not occur regardless of SiO2 addition, the results also confirm that physical breakage of the Al oxide layer due to the collision of SiO2 to Al powder in the shaking flask did not cause enhanced cementation.

As shown in Figures 2b and 3b, even without Al, the recovery efficiency of Co2+ and Ni2+ increased with increasing AC dosage, suggesting that these metal ions adsorbed on the AC surface. It has been reported that there are functional groups such as carboxyl and carbonyl groups on the surface of the activated carbon and they act as adsorption sites to metal ions through the reaction described by Equation (5) [18,28,29]. Increase in final pH indicates that not only Co2+ and Ni2+, but also proton (H+) adsorbed on AC [30,31].

$$\text{-C-COOH} + \text{M}^{2+} \rightarrow \text{-C-COOH} + 2\text{H}^{+} \text{ (M}=\text{Co or Ni}\text{)}\tag{5}$$

In the range between 0.05 to 0.2 g AC dosage, recovery efficiency was much higher with Al than without Al; at 0.1 g AC dosage, the efficiency was 56% for Co and 61% for Ni with Al, while it was 31% for Co and 43% for Ni without Al. The difference of metal

recovery efficiency between either with or without Al was 25% for Co and 18% for Ni, which cannot be ignored as an experimental error. This suggests that the addition of AC enhances Co and Ni cementation using Al as an electron donor (Equations (6) and (7)), even though the Al oxide layer remained on the Al surface.

$$\text{\textbullet Co}^{2+} + \text{2Al}^{0} \rightarrow \text{\textbullet Co}^{0} + \text{2Al}^{3+} \tag{6}$$

$$\text{\textbulletNi}^{2+} + \text{2Al}^{0} \rightarrow \text{\textbulletNi}^{0} + \text{2Al}^{3+} \tag{7}$$

Following these equations, it is expected that the stoichiometric amount of Al dissolves when cementation occurs; however, the dissolved Al concentration after cementation was less than 3 ppm (Tables S1 and S2), which means that most of the Al3+ was precipitated as Al-(oxy)hydroxide [7,32].

**Figure 2.** The effects of (**a**) SiO2, (**b**) AC, and (**c**) TiO2 dosages on the recovery efficiency of Co2+ and final pH in sulfate solutions at initial pH 4.0 for 24 h.

**Figure 3.** The effects of (**a**) SiO2, (**b**) AC, and (**c**) TiO2 dosages on the recovery efficiency of Ni2+ and final pH in sulfate solutions at initial pH 4.0 for 24 h.

As shown in Figures 2c and 3c, the recovery efficiency of Co2+ and Ni2+ without Al was almost 0% regardless of TiO2 dosage, indicating that TiO2 has no ability to adsorb Co2+ and Ni2+. When 0.1 g of Al was used together with TiO2, the recovery efficiency continuously increased with increasing TiO2 dosage and reached the maximum value of 52% for Co and 71% for Ni with 0.4 g TiO2. As already discussed, Co2+ and Ni2+ do not precipitate as hydroxides at the pH ranges observed in this series of experiments; the enhanced recovery of Co2+ and Ni2+ with TiO2 and Al suggests that the addition of TiO2 enhanced the cementation of Co2+ and Ni2+ by Al (Equations (6) and (7)). It was also confirmed that the dissolved Ti concentrations were below detection limit, indicating that TiO2 is stable enough to be used as an agent to enhance cementation of Co2+ and Ni2+ with Al in the sulfate solution (Tables S1 and S2).
