2.2.1. Preparation of Co2+ and Ni2+ Solutions

The sulfate solutions containing 1 mM metal ions were prepared by dissolving CoSO4·7H2O (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) or NiSO4·6H2O (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) in deionized (DI) water (18 MΩ·cm, Mill-Q® Integral Water Purification System, Merck Millipore, Burlington, Vermont, USA). For the preparation of 1 mM metal chloride solutions, CoCl2·6H2O (99.0%, Kishida Chemical Co., Ltd., Osaka, Japan), or NiCl2·6H2O (98.0%, Kishida Chemical Co., Ltd., Osaka, Japan) was dissolved in DI water. The initial pH of sulfate and chloride solutions was adjusted to 4.0 using 1 M H2SO4 and HCl (Wako Pure Chemical Industries, Ltd., Osaka, Japan), respectively. The total concentration of SO4 <sup>2</sup><sup>−</sup> and Cl<sup>−</sup> were fixed to 0.1 M using Na2SO4 (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) and NaCl (99.5%, Wako Pure Chemical Industries, Ltd., Osaka, Japan) to normalize their effects on experiments.

#### 2.2.2. Cementation Tests

The cementation tests were carried out in a 50 mL Erlenmeyer flask using a thermostat water bath shaker (Cool bath shaker, ML-10F, Taitec Corporation, Saitama, Japan) with 40 mm of shaking amplitude and 120 min−<sup>1</sup> of shaking frequency at 25 ◦C for 24 h. (Note that these parameters were selected based on our preliminary experiments). Ten milliliters of the prepared solution were added to the flask, then ultrapure nitrogen gas (99.9%) was introduced for 15 min before experiments to maintain an oxygen-free environment. One-tenth gram of Al powder and/or a predetermined amount (0.01, 0.05, 0.1, 0.2, 0.4 g)

of additive (i.e., AC, TiO2, and SiO2) were added to the solution. Ultrapure nitrogen gas (99.9%) was further introduced to the flask for 5 min, then the flask was tightly capped with a rubber cap and sealed with parafilm, and an experiment was conducted. After 24 h, the suspension was filtered using a syringe-driven membrane filter (pore size: 0.2 μm, LMS Co., Ltd., Tokyo, Japan); final pH of the filtrate was measured. The filtrate was diluted with 0.1 M HNO3, and the concentrations of metal ions were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICPE-9820, Shimadzu Corporation, Kyoto, Japan). The recovery efficiency of Co2+ and Ni2+ was calculated based on Equation (4):

$$\text{Recovery efficiency}, \ R = \frac{\mathcal{C}\_i - \mathcal{C}\_f}{\mathcal{C}\_i} \tag{4}$$

where *Ci* and *Cf* are the initial and final concentrations of metal ions, respectively.

#### 2.2.3. Surface Analysis

The solid products obtained by filtration were washed 5 times with DI water, dried in a vacuum oven at 40 ◦C for 24 h, and then analyzed by Auger electron spectroscopy (AES) using JAMP-9500F (JEOL Co., Ltd., Tokyo, Japan). The dried residue was mounted on an AES holder using conductive carbon tape. The analysis was conducted under the following conditions: ultrahigh vacuum condition, ~1 × <sup>10</sup>−<sup>7</sup> Pa; probe energy, 10 kV; and probe current, 19.7 nA. The spectra were analyzed by using Spectra Investigator AES software.

#### **3. Results and Discussion**
