All chemicals used for experimental studies were of analytical grade. Coal bottom ash sample (CBA) was collected from Jorge Lacerda coal-fired power plant, located in Santa Catarina State, Brazil. Sugarcane waste ash (SWA) was donated by Cosan S.A., Brazil. The residue of aluminum tertiary industry (RA) was provided by the company Latasa Recycling located in São Paulo State, Brazil. A representative RA sample was mixed and homogenized by successive quartering. Sodium hydroxide (97 %) and sodium aluminate (100 %) obtained from Sigma-Aldrich Pty. Ltd. (Australia) were used in the preparation of zeolite synthesis.

Synthesis of zeolite was executed by a two-step process. Firstly, 10 g of ash sample (CBA or SWA) was mixed with NaOH at 1:1.2 (wt%) ratio. The mixture was heated at 500 ºC for 1 h. After cooling at room temperature, commercial sodium aluminate was added to the grinded mixture to control the molar ratio (SiO₂/Al₂O₃ = 1) and mixed with 100 mL deionized water. The mixture was then stirred at room temperature for 16 h and heated at 100 °C for 7 h. After cooled at room temperature, the suspension was filtered and the solid was repeatedly washed with 1 L of deionized water and dried at 105 °C for 16 h [19]. The zeolites synthesized from CBA and SWA were named as ZCBA and ZSWA, respectively.

Fusion treatment was performed by placing 10 g of ash samples either CBA or SWA with RA and NaOH. The ratio (ash + RA):NaOH was fixed at 1:1.2 (wt%). The RA was added to adjust the molar ratio of SiO₂/Al₂O₃ to 1 for zeolite 4A formation. The mixture was heated at 500 ºC for 1 h. The fused sample was allowed to cool to ambient temperature and grounded into a fine powder using a mortar and pestle. Then, deionized water was added, and the mixture was stirred at room temperature for 16 h. The resulting slurry formed was subjected to hydrothermal crystallization at 100 °C for 7 h. After cooled at room temperature, the suspension was filtered and the solid was repeatedly washed with 1 L of deionized water and dried at 105 °C for 16 h. The zeolites synthesized from CBA and SWA and RA were named as ZCBA-RA and ZSWA-RA, respectively. The quantities of reagents used in the synthesis are shown in Table 1.

The mineralogical compositions were determined by X-ray diffraction analyses (XRD) with an automated Rigaku Miniflex 2 diffractometer with Cu anode using Co Kα radiation at 40 kV and 20 mA over the range (2θ) of 5–80° with a scan time of 0.5 °/min. The chemical composition was determined by X-ray fluorescence (XRF) in Rigaku RIX- 3000 equipment. Thermal analysis was performed by thermogravimetry on the Mettler-Toledo equipment model TGA/SDTA 851, with heating of 10 °C/min. Each sample (21.79 mg) was heated from 25 to 1200 ^oC at a heating rate of 5 ^◦C·min⁻¹, under a nitrogen flush (50 mL·min⁻¹). The scanning electron micrographs were performed using a Hitachi TM-3000 microscope. The particle size of the materials was measured using a laser based particle size analyser, namely a Malvern MSS Mastersizer 2000 Ver. 5.54. Cation exchange capacity (CEC) was determined by a previously described procedure [19].

The chemical composition of coal bottom ash and sugarcane waste ash has been reported in previous paper and the data are summarized in Table 2 [20-22]. The particle size distributions of materials are given in Table 3.

Zeolites synthesized was used for adsorption studies. Equilibrium adsorption experiment was performed using various concentrations of cadmium (50 to 1460 mg L^-1) solution. A mixture of 0.25 g of zeolitic material (ZCBA-RA or ZSWA-RA) with 25 mL of metal ion solution of various initial concentrations was shaken at 120 rpm for 24 h at 25 °C. The collected samples were then filtrate and the concentration in the filtrate solution was analyzed using inductively coupled plasma optical emission spectrometry (Spectro ARCOS, Spectro Analytical Instruments, Kleve, Germany). All experiments were performed in duplicate.

The adsorption capacity of the adsorbent was calculated using Eq. (1) [23,24].

\begin{equation} q_{e}=\frac{V\left ( C_{o}-C_{e} \right )}{M} \tag{1} \end{equation}

where \( q_{e} \) is the amount of metal adsorbed per unit mass of adsorbent at equilibrium (mg g^-1), \( C_{o} \) and \( C_{e} \)are the initial and equilibrium concentrations, respectively (mg L^-1); \( V \)the volume of the adsorbate solution added (L) and \( M \) the amount of the adsorbent used (g).

The data of the isotherm adsorption equilibrium were fitted using nonlinear equations of Langmuir and Freundlich models [25,26]. The determination coefficient (Radj²) was used to determine the best-fitting isotherm to the experimental data. In addition, the Chi-square (\( \chi ^{2} \)) test was employed, the lowest values were used to validate the applicability of isotherms tested [27].

The required Al³⁺ amount for the preparation of zeolite was mainly provided by Al-containing waste. The waste from aluminum tertiary industry consists of a black powder (Figure 1) and a detailed characterization was described elsewhere [28]. Briefly, the main chemical composition was Al₂O₃ 27.8 wt.%, Cl 17.4 wt.%, Na₂O 16.7 wt.%, SiO₂ 3.78 wt.% and MgO, KO₂, Fe₂O₃ ≤ 2.37 wt.%. The mainly coarse particle size (D₉₀) was ~450 µm. The total content of aluminum is distributed in different crystalline phases, namely, corundum (Al₂O₃;), sylvite (KCl), spinel (MgAl₂O₄;), halite (NaCl) and fayalite (2FeO.SiO₂). The high Cl and Na content are attributable to the employ of high amounts of salt (mainly NaCl and KCl) in the melting process to get a higher aluminum recovery [3].

The mineralogy result of CBA is displayed in Figure 3a, indicating the presence of quartz, mullite, hematite, magnetite, and halo with amorphous content. On the other hand, the XRD pattern of SWA sample (Figure 3b) presents only peaks relative to crystalline silica, indicating that the burning temperature of the bagasse at the sugar mill was above 800 °C.

The morphologies of CBA and SWA are shown in Figure 6a and Figure 6b, respectively. SEM images of both samples show heterogeneous material with irregular shapes. The spherical shape of coal bottom ash particles (Figure 6a) is typical of coal ashes and can be attributed to their amorphous coating formed because of relatively abrupt cooling after combustion.

Table 4 shows the characterization of the products in terms of cation exchange capacity (CEC) and an estimate of zeolite content in the synthesized materials. The CEC was used to estimate semi-quantitative data of the zeolite content in the synthesized materials by comparing the CEC value obtained with pure commercial zeolite A (4.55 meq g^-1). This estimate considers that CEC values for minor phases are negligible [32].

Eco-friendly adsorbents include the materials obtained from industrial waste recognized as low-cost sources. Among them, zeolites synthesized from wastes have confirmed their ability to remove heavy metals comparatively with natural or commercial zeolites [34,8]. Therefore, the removal of Cd²⁺, which act as endocrine disruptors, from aqueous solutions was performed using zeolitic material synthesized from the two ash samples and aluminum waste (ZCBA-RA and ZSWA-RA). The Langmuir and Freundlich models were adjusted to experimental data through the nonlinear estimation method. Figure 7a and b show the adsorption isotherms for Cd²⁺ on zeolites ZCBA-RA and ZSWA-RA, respectively. The isotherm parameters determined for both adjustments were determined using nonlinear expressions and are shown in Table 5.