3.1.2. Recovery of Co2+ and Ni2+ from Chloride Solution

Cementation experiments for recovering Co2+ and Ni2+ from chloride 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 recovery were investigated. To access the adsorption of Co2+ and Ni2+ on the additives, experiments without Al were also conducted. Figures 4a–c and 5a–c show the Co and Ni recovery efficiencies and final pH as a function of AC, TiO2, and SiO2 dosages, respectively.

Similar to the sulfate system (Figures 2 and 3), final pH values of the chloride solutions (Figures 4 and 5) were less than 5.5 for Co and 6.1 for Ni (Tables S3 and S4), which means that removal of Co2+ and Ni2+ from the solutions by the formation of cobalt and nickel hydroxide precipitation does not need to be considered in this series of experiments (Figures S3 and S4).

It has been reported that in the presence of high concentrations of Cl−, the Al oxide layer was dissolved and removed from the Al surface [13,33–35]. If the Al oxide layer is dissolved, a high concentration of dissolved Al would be detected in the solutions, but the observed results (Tables S3 and S4) showed that concentrations of Al were less than 5 ppm under all conditions. This implies that removal of the Al oxide layer did not occur under the experimental condition used here.

As shown in Figures 4a and 5a, when SiO2 was used as an additive, the recovery efficiencies of Co2+ and Ni2+ both with and without Al were almost 0%. This indicates that in chloride solutions, Co2+ and Ni2+ were not adsorbed on SiO2, and the cementation of Co and Ni with Al did not occur.

The results shown in Figures 4b and 5b suggest that adsorption of Co2+ and Ni2+ on AC occurred in chloride solutions, because in the absence of Al, recovery efficiencies of these ions increased with increasing AC dosage. As in sulfate solutions, in the presence of AC, enhancement of metal ion recovery by Al addition was confirmed (Figures 4b and 5b); e.g., at 0.1 g AC dosage, by adding Al, the efficiency increased from 57% to 70% for Co, and it increased from 57% to 70% for Ni. This suggests that enhanced cementation of these metal ions with AC occurred in chloride solutions.

Figures 4c and 5c show that the efficiencies of Co2+ and Ni2+ recovery in the absence of Al were almost 0% at any dosage of TiO2, suggesting that adsorption of these ions on TiO2 can be ignored. In the presence of Al, the efficiencies of Co2+ and Ni2+ recovery increased with increasing TiO2 dosage; without TiO2, the efficiencies were almost 0% for both Co and Ni while they increased to 61% for Co2+ and 99.9% for Ni2+ when 0.4 g TiO2 was added. These results suggest clearly that addition of TiO2 enhanced the cementation of Co and Ni by using Al as an electron donor, and indicated that AC can be replaced with TiO2 even if its surface area is lower than AC [18,36,37].

**Figure 4.** The effects of (**a**) SiO2, (**b**) AC, and (**c**) TiO2 dosages on the recovery efficiency of Co2+ and final pH in chloride solutions at initial pH 4.0 for 24 h.

**Figure 5.** The effects of (**a**) SiO2, (**b**) AC, and (**c**) TiO2 dosages on the recovery efficiency of Ni2+ and final pH in chloride solutions at initial pH 4.0 for 24 h.

#### *3.2. Surface Analysis of Deposited Co and Ni*

To investigate the elemental compositions of the deposited Co and Ni, residues obtained from the Co2+ and Ni2+ recovery experiment from chloride solutions using 0.4 g of TiO2 and 0.1 g of Al were analyzed by AES. Figures 6 and 7 show the AES photomicrographs (Figures 6a and 7a) and scan results of Co (Figure 6b,c) and Ni (Figure 7b,c). In both AES photomicrographs, many small gray particles and light particles are attached together onto the surface of the dark particle. The wide AES spectra of the dark particle (point 1 in Figures 6b and 7b) show strong signals of Al and O, indicating that these particles are assigned to Al powder. The small gray particles correspond to TiO2 because of Ti and O signals observed at point 2 in Figures 6b and 7b. Meanwhile, light particles are observed at point 3 in Figures 6b and 7b are most likely the deposited Co and Ni, respectively.

To identify the elemental composition of the deposited Co and Ni, the narrow AES spectra in the range of 750–785 eV for Co and 830–858 eV for Ni were analyzed (Figures 6c and 7c). These spectra were fitted using reference spectra of Co, CoO, and Co3O4 for Co composition, and Ni and NiO for Ni composition. Fitting results indicate that the deposited Co consisted of metallic Co (93.1%) and CoO (6.9%), while the deposited Ni was composed of metallic Ni (86.2%) and NiO (13.8%). The analysis range of Auger is 0.3–5 nm, which is a near-surface analysis [38], so it is speculated that only the outermost surfaces of deposited Co and Ni were oxidized due to the oxidation of metallic Co and Ni during the dry process.

These results suggest that Co and Ni were deposited on TiO2 particles attached to the Al surface and TiO2 can act as an electron pathway from Al to Co2+ and Ni2+, even if the Al oxide layer remains on the Al surface. These results showed that physical separation (i.e., ultrasonification) could be applied as the postcementation process for Co/Ni–TiO2 particle and Al separation. Afterward, it is expected that only Co and Ni would be dissolved in aqueous solutions, while TiO2 would not be dissolved because TiO2 is more stable than Co and Ni.

**Figure 6.** Auger electron spectroscopy (AES) results of the residue obtained after cementation of Co2+ from chloride solution using TiO2/Al: (**a**) photomicrograph, (**b**) wide scan energy spectrum of each point, and (**c**) the narrow scan energy spectrum of the Co peak with fitting spectra of Co, CoO, and Co3O4.

**Figure 7.** Auger electron spectroscopy (AES) results of the residue obtained after cementation of Ni2+ from chloride solution using TiO2/Al: (**a**) photomicrograph, (**b**) wide scan energy spectrum of each points, and (**c**) the narrow scan energy spectrum of the Ni peak with fitting spectra of Ni and NiO.

#### **4. Conclusions**

This study investigated whether activated carbon (AC) could be replaced with other additives such as TiO2 and SiO2 for the enhanced cementation of Co2+ and Ni2+ using aluminum (Al) in sulfate and chloride solutions. In summary, the Co2+ and Ni2+ recovery efficiencies using Al in sulfate and chloride solutions were almost 0% because of the presence of an Al oxide layer on an Al surface. The adsorption of Co2+ and Ni2+ occurred when using only AC, while it did not occur when using only TiO2 and SiO2. When using an AC/Al-mixture or TiO2/Al-mixture, the Co2+ and Ni2+ recovery efficiencies from sulfate and chloride solutions were enhanced compared to using Al, AC, TiO2, and SiO2/Almixture. From the results of AES analysis, Co and Ni were mostly deposited as zero-valent forms on TiO2 attached to Al surface. This work establishes that using a conductor (AC) or a semiconductor (TiO2) could enhance the recovery of Co2+ and Ni2+ by Al-based cementation even under mild conditions (e.g., low temperature, 25 ◦C; mild pH conditions, pH 4–5; no Cl− or a low concentration). Moreover, it is expected that other conductive materials could also be used for the removal and/or recovery of metal ions using Al.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2075-4 701/11/2/248/s1, Table S1: The concentration of Al, Ti, and Si ions after cementation experiment of Co2+ in sulfate solution at initial pH 4.0 at 25 ◦C for 24 h, Table S2: The concentration of Al, Ti, and Si ions after cementation experiment of Ni2+ in sulfate solution at initial pH 4.0 at 25 ◦C for 24 h, Table S3: The concentration of Al, Ti, and Si ions after cementation experiment of Co2+ in chloride solution at initial pH 4.0 at 25 ◦C for 24 h, Table S4: The concentration of Al, Ti, and Si ions after cementation experiment of Ni2+ in chloride solution at initial pH 4.0 at 25 ◦C for 24 h, Figure S1: The activity–pH diagram for 1 mM Co2+ species with 0.1 M SO4 <sup>2</sup><sup>−</sup> at 25 ◦C (created

using the GWB Professional Ver. 12.0.3 software), Figure S2: The activity–pH diagram for 1 mM Ni2+ species with 0.1 M SO4 2– at 25 ◦C (created using the GWB Professional Ver. 12.0.3 software), Figure S3: The activity–pH diagram for 1 mM Co2+ species with 0.1 M Cl– at 25 ◦C (created using the GWB Professional Ver. 12.0.3 software), Figure S4: The activity–pH diagram for 1 mM Ni2+ species with 0.1 M Cl– at 25 ◦C (created using the GWB Professional Ver. 12.0.3 software).

**Author Contributions:** Conceptualization, S.C. and S.J.; methodology, S.C., S.J. and N.H.; formal analysis, S.C., S.J., I.P., M.I. and N.H.; investigation, S.C.; writing—original draft preparation, S.C.; writing—review and editing, S.C., S.J., I.P., M.I. and N.H.; supervision, N.H.; project administration, N.H.; funding acquisition, S.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was financially supported by the Japan Society for the Promotion of Science (JSPS) grant-in-aid for Research Activity start-up (grant numbers: 19K24378).

**Data Availability Statement:** Data available on request due to restrictions, as the research is ongoing.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

