**1. Introduction**

Cementation, an electrochemical deposition of noble metal ions by a less noble metal as an electron donor, is usually applied to remove/recover metal ions from dilute aqueous solutions [1–4]. The advantages of cementation are (1) recovery of metals in zero-valent form, (2) simple methods, and (3) low-energy consumption [2,5]. In this method, the overall reaction of cementation is given by Equation (1) [6–8]:

$$m\text{N}^{0} + n\text{M}^{m+} \rightarrow m\text{N}^{n+} + nM^{0} \tag{1}$$

The cementation reaction is divided into anodic (Equation (2)) and cathodic reactions (Equation (3)):

$$\text{Anodic } m\text{N}^0 \to m\text{N}^{n+} + nm\text{e}^- \tag{2}$$

$$\text{Cathodic } nM^{m+} + nme^- \to nM^0 \tag{3}$$

**Citation:** Choi, S.; Jeon, S.; Park, I.; Ito, M.; Hiroyoshi, N. Enhanced Cementation of Co2+ and Ni2+ from Sulfate and Chloride Solutions Using Aluminum as an Electron Donor and Conductive Particles as an Electron Pathway. *Metals* **2021**, *11*, 248. https://doi.org/10.3390/met11020248

Academic Editor: Dariush Azizi Received: 10 January 2021 Accepted: 30 January 2021 Published: 2 February 2021

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The noble metal ions (*Mm+*) are deposited on the surface of a less noble elemental metal (*N*0) spontaneously, and the driving force of this reaction is mainly determined by differences in the standard electrode potentials for *Mn+*/*M*<sup>0</sup> and *Nn+*/*N*<sup>0</sup> redox pairs, and it increases when the electrode potential of *N*<sup>0</sup> is low.

Aluminum (Al) can be considered as a strong reductant (electron donor) used for cementation because of its extremely low standard electrode potential (i.e., E0 Al3+/Al = –1.67 V vs. standard hydrogen electrode (SHE)) [7,9–11]. The practical application of Al for cementation, however, is limited due to the presence of a dense Al oxide layer (Al2O3) on the Al surface, which inhibits electron transfer from Al0 to metal ions [9,12,13]. When the Al oxide layer is removed from the surface, Al can be used as an electron donor for cementation. To remove the Al oxide layer, however, high temperatures, acid/alkaline solutions, or high concentration of chloride ions are needed [2,5,9,14,15], and these extreme conditions make it difficult to use Al as an electron donor in the practical cementation processes.

Recently, the authors investigated the effects of activated carbon (AC) addition on the efficiency of cementation using Al as an electron donor for recovering gold ions from ammonium thiosulfate solution [16,17], and heavy metal ions (Co2+, Ni2+, Zn2+, and Cd2+) from acidic sulfate and chloride solutions. The results showed that cementation efficiencies of the metal ions were significantly enhanced by the addition of activated carbon (AC) even when an insulating Al oxide layer covered on the Al surface [16,17]. This "enhanced cementation using AC/Al-mixture" can be operated under mild conditions; i.e., it does not require extreme operating conditions such as high temperatures, and high concentrations of chemical reagent such as acid, base, and chloride ions. This new method may, therefore, provide a practical way to use Al, one of the strongest reductants (electron donor) for cementation to recover metal ions from dilute solutions.

Although the details of the mechanism of enhanced cementation using the AC/Almixture are not yet fully understood, the results of surface analysis for the cementation products have suggested that AC attached on the Al surface acted as an electron pathway from Al to noble metal ions, even in the presence of a surface Al oxide layer [17]. If this is the case and the essential role of AC is just as an electron pathway, enhanced cementation would occur even when AC is replaced by other (semi)conductors. On the other hand, as AC is a porous material and has a very large specific surface area [18], not only the electroconductivity but also large adsorption capacity of AC for metal ions may play an important role in the enhanced cementation using the AC/Al-mixture. If this is the case, replacing AC to another conductor with a low specific surface area cannot enhance the cementation using Al as an electron donor.

Cobalt (Co) and nickel (Ni) represent important strategic resources in the world market and their use is rapidly growing for renewable energy technologies and rechargeable battery productions, and the importance of the development of technologies for recovering and purifying Co and Ni is continuously increasing [19–24]. Therefore, this study aims to investigate whether the AC could be replaced with other (semi)conductors for recovery of Co and Ni from sulfate and chloride solutions. Titanium dioxide (TiO2) was selected for a semiconductor because of its nontoxic, nonreactive, and high chemical stability, while silicon dioxide (SiO2) was chosen for a nonconductor to clarify the mechanism(s) of the enhanced cementation using the mixture of conductor and Al [25,26].

In the present study, batch-type cementation experiments were conducted using Al as an electron donor to recover Co2+ and Ni2+ from sulfate and chloride solutions and the effects of the addition of AC, TiO2, or SiO2 on the recoveries of these metal ions were investigated. Surface analysis (Auger electron spectroscopy (AES)) for the cementation products were also conducted to elucidate the cementation mechanism.

#### **2. Materials and Methods**

#### *2.1. Materials*

As an electron donor, Al powder (99.99%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used, and AC powder (99.99%, Wako Pure Chemical Industries, Ltd., Osaka, Japan), TiO2 powder (99.0%, rutile form, Wako Pure Chemical Industries, Ltd., Osaka, Japan), and SiO2 powder (99.0%, Wako pure Chemical Industries, Ltd., Osaka, Japan) were used as additives. Particle size distribution of these materials, measured by laser diffraction (Microtrac® MT3300SX, Nikkiso Co. Ltd., Osaka, Japan), is shown in Figure 1. The median diameters (D50) of Al, AC, TiO2, and SiO2 were 21.3, 38.1, 8.5, and 21.2 μm, respectively.

**Figure 1.** Particle size distribution for (**a**) aluminum (Al), (**b**) activated carbon (AC), (**c**) titanium dioxide (TiO2), and (**d**) silicon dioxide (SiO2) used in this study.
