## 2 Materials and Methods  
The synthesis of the standard LTA zeolite was carried out following the methodology described by the International Zeolite Association (IZA), that is, 40 g of H\({}_{2}\)O, 3.585 g of NaOH, 2.5 g of SiO\({}_{2}\), and 3.86 g of sodium aluminate were used. Crystallization occurred statically at 100 \({}^{\circ}\)C for 4 h. This synthesis was defined as the standard LTA.  
The synthesis of LTA-type zeolites using coal fly ash (CF) was carried out using the route proposed by [PERSON] et al. [21]. Two methodologies were employed: synthesis through the conventional hydrothermal treatment and the two-stage alkaline fusion treatment followed by the conventional hydrothermal treatment. This procedure is shown in Figure 1.  
In the hydrothermal process, the mixture of sodium hydroxide, fly ash, and water can be subjected to a much milder temperature (80-200 \({}^{\circ}\)C) [22]. The use of temperature facilitates the dissolution of less stable phases; however, phases such as quartz may remain. The conventional hydrothermal method (open or closed reflux systems) for alkaline fly ash activation has been widely used in previous studies [17; 23; 24]. The other methodology applied in the study of the synthesis of zeolite LTA using fly ash follows in two stages: alkaline fusion followed by hydrothermal treatment. When performing an alkaline fusion, the quartz present in the residue is dissolved totally or partially, depending on the temperature used [18; 25; 26]. After the fusion, the conventional hydrothermal treatment is followed in the synthesis process of the LTA zeolite. Many parameters were studied, some of them for the first time: gel stirring time and temperature; residue ultrasonication; reflux at 80 \({}^{\circ}\)C for 20 or 120 min; addition of Al; Na/T ratio; OH/T ratio; and crystallization time.  
The raw material CF and the synthesized zeolites were characterized by X-ray fluorescence (FRX), X-ray diffraction, and scanning electron microscopy (SEM-FEG). The analyses of the studied samples were performed in a Bruker diffractometer model D2 Phaser(r) using CuK\(\alpha\) radiation (\(\lambda\) = 1.54 A) with a Ni filter, operating at a current of 10 mA and a voltage of 30 kV in an angle range 20 = 3-50\({}^{\circ}\). The analyses were performed with a divergent slit of 0.6 mm and a central slit of 1.0 mm, and the collection of the signals was done with a Lynxeye detector. The scan was carried out per step with an increment of 0.02 and an acquisition time of 0.1 s. The crystallinity of the materials was calculated using the 20 diffraction angles (7.2\({}^{\circ}\); 12.5\({}^{\circ}\); 16.1\({}^{\circ}\); 21.7\({}^{\circ}\); 30\({}^{\circ}\); and 34.2\({}^{\circ}\)) [27] with Equation (1), using the Standard LTA synthesized as a reference.  
\[\%crystallinity=\frac{\textit{sample peak intensity}}{\textit{standard peak intensity}}\times 100 \tag{1}\]  
Scanning electron microscopy (SEM) analysis was performed on a Zeiss Auriga 40 device operating at 3 kV. In the study of the samples, a small portion was taken and placed under a sample holder where scans were acquired at different points within the sample with different magnifications.  
Figure 1: Schematic procedure for material synthesis employing the coal fly ash (CF) residue.