## 3 Results and Discussion
### Leaching Tests
#### 3.2.2 pH dependence Leaching Behavior
Figure 4: Concentrations (mg/kg) of Cr, Pb, Zn, Cd, Ba, Cu, and Ni in the leachate after the pH dependence test of the geopolymers with EAFD at 75 \({}^{\circ}\)C and 225 \({}^{\circ}\)C.  
Elements that are close to oxygen in electronegativity tend to form oxyanions i.e., the transition metal Mo which from MoO\({}^{2-}\)\({}_{4}\)[58]. Looking at Figure 6, the maximum solubility was found at pH 10. The alkaline V-shape for Mo leaching might indicate binding of Mo to sulphoaluminates. Considering the geochemical behavior, Powellite (CaMoO\({}_{4}\)) precipitated in all cases. Based on the existing findings, Mo was subjected to geochemical speciation modeling, assuming its presence as oxyanion. Leaching behavior of Cd and Zn as a function of pH follows a cationic pattern. For elements controlled by solubility or sorption, cationic pattern involves increasing dissolution and/or desorption of metal-bearing mineral phases as the pH decreases and diminishing leaching due to precipitation and/or increasing sorption as the pH increases. As pH decreases the leaching amount of the cationic elements increase due to increasing attacking on the metal-bearing mineral phases as acid strength increases [59]. Sulphate and carbonate minerals are the two main minerals that may precipitate with leached Cd element. In this case, the carbonate can control the leaching of Cd in the aqueous solution. In this case, Otavite (CdCO\({}_{3}\)) precipitates at all pH ranges [33]. Leaching of Pb is controlled by Pb (OH)\({}_{2}\) in accordance with previous studies appearing on the literature [57; 60]. Such a mechanism corresponds to cationic element mobilizing in the form of Pb\({}^{+2}\). It is expected that the leaching of Zn be controlled by the dissolution/precipitation reaction in the soil matrix at any pH condition. In this case, zincite (ZnO) may control the leaching of the geopolymeric matrix in the pH range of 2-9 (Figure 5). Oxyanion pattern was remarkable for As which is in accordance with the literature [59]. The same occurs with the Se within the studied pH range. The leaching behavior of both elements as a function of pH can be explained by the species in the leachate and pH -dependent surface charge of fly ash. Negatively charged oxyanions, As and Se in the form of SeO\({}_{3}\)\({}^{2-}\), \(\text{HSeO}_{3}\)\({}^{1-}\), and \(\text{AsO}_{4}\)\({}^{3-}\) precludes adsorption onto the negatively charged surface of the geopolymer contributing to higher As and Se concentrations at alkaline pH [59]. Apart from the geochemical models plotted in Figure 6, Visual Minteq also provides information about the speciation distributions in terms of percentage of each specie on a molar basis. The trace pollutants Cd, Mo, Sb, As, Se, Zn, and Pb speciation was visualized in Figure 6 for the geopolymers with 20% of steel powder at 75 \({}^{\circ}\)C in all the studied pH range. It seems how the cationic elements such as Cd, Zn, and Pb are mainly released as their respective cations (+2) oxidation state. Nevertheless, oxyanions As, Mo, and Se, arsenates, molybdates, and selenates are responsible for the As, Mo, and Se leaching at all pH values.