## 2 Materials and Methods  
### Preparation of Brick Materials  
The brick used was made in Bangui (Central African Republic) from kaolinite/illite-rich soils. It was prepared manually by mixing extracted soils and water. The obtained mud was air-dried for 48 h and then heat-treated with dry wood for a period of about 3 days at temperatures ranging from 500 \({}^{\circ}\)C to 800 \({}^{\circ}\)C. The heated material was then cooled progressively to ambient temperature for 3 days. The obtained brick was broken manually into grains using a hammer. Brick particles were sieved with mechanical sieves, and the fraction of particles with sizes varying from 0.7 to 1.0 mm was kept for our experiments. This fraction was washed with Milli-Q water and then decanted; after settling, water was eliminated, and brick grains were dried at 105 \({}^{\circ}\)C.  
In the fabricated brick, quartz (60-65 w%), metakaolinite (20-26 w%) and illite (9-11 w%) were the major minerals, while iron oxide/hydroxide (2-3 w%) and titanium dioxide (1-3 w%) were minor minerals ([PERSON], Thesis in preparation). Alkali-brick synthesis was initially optimized by our research group [53]. Briefly, brick pelletswith size diameters ranging from 0.7 to 1.0 mm were treated in different alkaline solutions (NaOH concentration ranging from 0.10 to 1.50 mol\(\cdot\)L\({}^{-1}\)) at different temperatures (50 \({}^{\circ}\)C, 70 \({}^{\circ}\)C, and 90 \({}^{\circ}\)C) and for a constant reaction time of six days. The recovered materials were then attacked with HNO\({}_{3}\) (2 mol\(\cdot\)L\({}^{-1}\)), and the resulting solutions were analyzed by ICP-OES. Investigations revealed that the quantity of sodium bound to the brick reached a maximum when alkali-brick synthesis was performed with a NaOH concentration of 0.6 mol\(\cdot\)L\({}^{-1}\), knowing that the quantity of Na\({}^{+}\) 'extra-frameworks' ions was considered as a key indicator of the number of reactive sites in the brick for cationic adsorption [53]. This explained why we decided to work here under the following optimized synthesis conditions. Brick grains having 0.7-1.0 mm sizes were treated with sodium hydroxide as follows [57]: 10 g of Bangui brick was reacted in 40 mL of a diluted NaOH solution (0.6 mol\(\cdot\)L\({}^{-1}\)) at room temperature for 24 h under slow shaking at a speed of 120 rpm. The suspension was then treated at 90 \({}^{\circ}\)C for 6 days. The recovered grains were finally rinsed several times with Milli-Q water and dried at 90 \({}^{\circ}\)C for 24 h.  
Alkali-brick samples doped with 'M\({}^{+/++}\)' cations (Cs\({}^{+}\), Rb\({}^{+}\), Co\({}^{2+}\), or Sr\({}^{2+}\)) were prepared as follows. All the experiments were carried out at the natural pH of the initially measured suspension, i.e., 7.74 for Cs\({}^{+}\), 7.42 for Rb\({}^{+}\), 7.37 for Sr\({}^{2+}\), and 7.29 for Co\({}^{2+}\). Experiments were conducted at room temperature. Preliminarily, synthetic solutions which were employed for 'M\({}^{+/++}\)' adsorption onto brick were prepared by diluting 1000 mg\(\cdot\)L\({}^{-1}\) stock solutions of 'M\({}^{+/++}\)' cations with distilled water. The 'M\({}^{+/++}\)' solution of 100 mL (2 mmol\(\cdot\)L\({}^{-1}\)) was put into contact with 0.5 g of alkali-brick grains in a 200 mL glass reactor until equilibrium state was attained (\(\sim\)240 mn). Then, the suspension was vacuum filtered through a 0.45 \(\upmu\)m filter membrane. The recovered grains were rinsed several times with Milli-Q water and dried at 90 \({}^{\circ}\)C for 24 h.  
Brick samples were afterwards heated at the following temperatures: 90 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, and 1150 \({}^{\circ}\)C. For each heated-brick sample, chemical durability/stability was evaluated by conducting leaching experiments in hydrochloride acid solution (1 mol\(\cdot\)L\({}^{-1}\)).