## 3 Results  
Table 1 shows the total content of substances in the two bottom ash samples used for the aging experiment compared with data from the literature. The experiments were performed with the grain size fraction <22 mm; material >22 mm was excluded from the investigation. The mean values of triplicate analyses are presented for samples A and B. However, it is known that IBA is an extremely inhomogeneous material. For comparison, data from the literature (mean values, together with minimum and maximum values) are also shown in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline
**Element** & **Sample A** & **Sample B** & **Literature 1** [46]** \\ \hline Ca & 67,793 & 76,417 & 130,833 (50,825–198,289) \\ Si 2 & 224,100 & 161,800 & 82,713 (6106–96,078) \\ Fe & 38,530 & 52,932 & 58,714 (34,216–11,822) \\ Al & 15,098 & 16,490 & 47,232 (30,527–75,089) \\ Na & 10,916 & 9599 & 21,379 (12,308–34,971) \\ Mg & 4781 & 4655 & 12,429 (637–34,372) \\ K & 3663 & 2504 & 7748 (4854–12,722) \\ P & 3247 & 4378 & 5633 (2531–12,556) \\ Ti & 2687 & 2661 & 4244 (2873–7479) \\ S & 2853 & 3785 & 3862 (131–16,808) \\ Cu & 1216 & 2883 & 3275 (738–17,620) \\ Zn & 1671 & 2504 & 3241 (1142–9370) \\ Pb & 475 & 418 & 1309 (197–6441) \\ Mn & 546 & 929 & 1173 (644–2248) \\ Ba & 774 & 1483 & 1102 (760–297) \\ Cr & 28 & 180 & 353 (115–852) \\ Sr & 132 & 213 & 271 (267–369) \\ B & 89.5 & 26.1 & 198 (30–532) \\ Ni & 85.0 & 100.8 & 185 (38–850) \\ Sn 2 & 134.7 & 213.0 & 181 (52–737) \\ Sb & 54.1 & 88.4 & 73 (18–250) \\ V & 16.5 & 15.3 & 41.2 (19–248) \\ Co & 35.7 & 32.9 & 31.8 (11–103) \\ Mo & 4.4 & 8.2 & 30.1 (5-84) \\ \hline \end{tabular}
\end{table} 
Table 1: Total content of substances in the test material in mg/kg (extraction with aq. regia, exception labelled). Ferrous and non-ferrous metals were removed from the IBA samples on site, at a technical scale. Figure 1: Flow chart of the methodology applied in the present study. Standardized batch tests were performed at L/S ratios of 2 and 10 L/kg. The sample size for the batch tests were 500 and 100 g per liter, respectively. Effects of inhomogeneity are here less relevant than in the analysis of the total content using aqua regia with a sample size of 0.5 g. The presence of, e.g., a small metallic Cu or brass particle could lead to high Cu or Zn values after digestion even after careful lab sample preparation, whereas the impact in the leaching test would be small. The observed pH values of the leaching tests shortly after sampling (\"fresh\" IBA) were between 12.3 and 12.5, without any significant differences between the two L/S ratios. In the course of the aging experiment (\"aged\" IBA), the pH values dropped to values below 10 for the material stored outdoors and below 11 for the material stored indoors. Further, the decline in pH in the leaching tests was faster for the outdoor samples than for the samples stored indoor. The alkaline pH values are due to the presence of CaO and Ca(OH)\({}_{2}\) in IBA, which reacts with CO\({}_{2}\) from the air or in rain forming CaCO\({}_{3}\)[47]. As a result, pH values decrease to lower values. Figure 2 shows the pH curves (separated for indoors and outdoors) as a function of storage time. Obviously, the aging process is more efficient under outdoor conditions involving rainwater. The reason for that could be a more effective penetration of CO\({}_{2}\) containing rainwater through the material, compared to the heterogeneous gas-solid reaction only. The complete numerical data set can be found in the Supporting Information (SI), SI Table S1. The leaching of most heavy metals is a function of pH. In the present investigation, the release of heavy metals was observed at the pH values at the end of the standardized batch test, ranging between 12.5 and 8.8 (sample A after 281 days; see Figure 2), and at a pH of 4 (leaching with maleic acid). In order to compare the experimental results obtained at different L/S ratios and during leaching with maleic acid, the concentrations were converted to leached amount (E) in mg/kg.