## 3 Results and Discussion
### Chemical Composition of BA  
The chemical compositions (major elements) of the five different fractions of BA, obtained by XRF, are provided in Table 1. The compositions are described as the weight percentages of each element.  
Figure 3: Water content in untreated MSWI bottom ash.  
The chemical analysis of the five different fractions of BA showed that the main elements (>10%) were silicon (Si) and calcium (Ca). The bottom ash also contained aluminum, magnesium, sodium, iron, and sulfur (1-8%). The above-mentioned elements of bottom ash were found in the oxide form (oxides, silicates, and sulfates).  
A comparison between the chemical compositions of the different fractions of BA and the Earth's crust is depicted in Figure 4.  
Figure 4 shows that the amounts of the major elements found within all five different fractions of BA were close to the composition of the Earth's crust (with basaltic and other geological materials) [27; 28]. We also found a wide range of metals that are hazardous to human health and the environment, such as titanium, chromium, manganese, nickel, copper, zinc, lead, and others. A comparison of the amounts of five heavy metals (Mn, Cu, Zn, Sr, and Pb) in the different untreated BA fractions is presented in Figure 5. In the <40 mm fraction, we performed tests in natural weathering (N) and laboratory aging (L) conditions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multicolumn{5}{c}{**Fraction of MSWI BA (mm)**} \\ \cline{2-6}  & **<5.6** & **5.6–11.2** & **11.2–24.2** & **22.4–40.0** & **<40.0** \\ \hline Na & 2.890 \(\pm\) 0.627 & 2.179 \(\pm\) 0.113 & 2.194 \(\pm\) 0.544 & 2.027 \(\pm\) 0.339 & 2.603 \(\pm\) 0.154 \\ \hline Mg & 1.844 \(\pm\) 0.400 & 1.390 \(\pm\) 0.072 & 1.400 \(\pm\) 0.348 & 1.293 \(\pm\) 0.216 & 1.661 \(\pm\) 0.098 \\ \hline Al & 2.708 \(\pm\) 0.075 & 2.597 \(\pm\) 0.072 & 2.963 \(\pm\) 0.070 & 4.028 \(\pm\) 0.185 & 3.047 \(\pm\) 0.225 \\ \hline Si & 12.982 \(\pm\) 0.088 & 19.947 \(\pm\) 0.120 & 22.098 \(\pm\) 0.127 & 21.925 \(\pm\) 0.275 & 17.520 \(\pm\) 0.295 \\ \hline P & 0.876 \(\pm\) 0.016 & 0.537 \(\pm\) 0.014 & 0.391 \(\pm\) 0.013 & 0.454 \(\pm\) 0.032 & 0.632 \(\pm\) 0.043 \\ \hline S & 2.727 \(\pm\) 0.019 & 1.422 \(\pm\) 0.012 & 0.843 \(\pm\) 0.009 & 0.779 \(\pm\) 0.020 & 1.946 \(\pm\) 0.041 \\ \hline K & 0.792 \(+\) 0.007 & 0.735 \(\pm\) 0.007 & 0.899 \(\pm\) 0.008 & 1.260 \(\pm\) 0.022 & 0.809 \(\pm\) 0.021 \\ \hline Ca & 19.050 \(\pm\) 0.115 & 14.453 \(\pm\) 0.085 & 12.880 \(\pm\) 0.073 & 12.965 \(\pm\) 0.155 & 16.268 \(\pm\) 0.245 \\ \hline Ti & 0.826 \(\pm\) 0.022 & 0.595 \(\pm\) 0.019 & 0.478 \(\pm\) 0.018 & 0.442 \(\pm\) 0.044 & 0.610 \(\pm\) 0.065 \\ \hline Fe & 4.690 \(\pm\) 0.032 & 4.272 \(\pm\) 0.029 & 3.917 \(\pm\) 0.026 & 4.510 \(\pm\) 0.068 & 4.398 \(\pm\) 0.085 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Major elements of five different fractions of untreated MSWI bottom ash (wt %).  
Figure 4: Major chemical elements of MSWI BA in comparison to the element abundances in the Earth’s crust, based on [27].  
The chemical composition analysis showed that zinc (Zn) was present in untreated BA in high concentrations (2596-5504 mg kg\({}^{-1}\)). The maximum concentrations of three metals (Cu, Pb, and Sr) were found in the 5.6-11.2 mm fraction, with values of 3223 \(\pm\) 129, 805 \(\pm\) 40, and 1000 \(\pm\) 25 mg kg\({}^{-1}\), respectively. The highest concentration of Zn (5504 \(\pm\) 152 mg kg\({}^{-1}\)) was found in the smallest BA fraction (<5.6 mm).  
Figure 5 shows that bottom ash particle size and heavy metal distribution did not have a direct relationship. These results showed that the bottom ash fractions did not have a substantial effect on the amounts of heavy metals.