## 3 Results  
### Sequential Extraction  
The sequential extraction of mineral waste is an advanced method that allows a more in-depth analysis of the mobility and environmental availability of heavy metals such as copper, zinc, lead, and nickel. This method makes it possible to distinguish the bonding forms of these metals more precisely. While standard leaching with water identifies the readily soluble fractions, sequential extraction splits the metals into several fractions based on their binding to different matrix components of the mineral waste. This differentiated approach is of great importance as it provides insights into the conditions under which metals can be mobilized, such as changes in pH or redox conditions. This is particularly relevant for the elements copper, zinc, lead, and nickel as potential toxic elements. Sequential extraction thus provides valuable information on the potential bioavailability and mobility of these metals, which is crucial for the assessment of environmental risks and the development of environmentally sound waste treatment and disposal strategies. It is important to emphasize that the fractionation was performed sequentially, with the residues of each step being used for the subsequent extraction. Overall, sequential extraction enables a more comprehensive assessment of the chemical speciation of heavy metals in incineration ashes compared to selective extraction. Through this method, targeted measures can be developed to minimize the environmental impact of MSWI bottom ashes by considering the exact fractions and mobility patterns of the investigated metals. In the present study, an SSI ash, an MSWI fly ash and crushed sand from C&D waste have additionally been included in the investigation. Figure 1 shows the extracted amounts of the elements Cu, Zn, Pb, and Ni in the individual fractions F1 to F4. All concentrations have been measured with ICP-OES and calculated to leached content in mg/kg. The elements Cd and Cr exhibited much lower concentrations, and have been measured with ICP-MS (see Figure 2). The results for the elements As, Sb, V, Mo, Sn, and Co are displayed in the Supporting Information (SI, Figure S1). Figure 1: _Cont._As can be seen from Figure 1, for Cu, the four incineration bottom ashes IBA1-4 show different solubilities between 267 and 1084 mg/kg in fraction F1. Sample SSA is comparable to IBA2 in all fractions F1-F4; the fly ash shows a low solubility of Cu in fraction F1, although the total content is the highest. Thus, there are fewer Cu species soluble in weak organic acids in the fly ash (FA). Cu species that are soluble in the reducible fraction F2 are hardly present in any of the ashes analyzed; their solubility is limited to a maximum of 185 mg/kg. However, it must always be taken into account that the compounds soluble in fraction F1 are no longer present at this point. Under oxidative conditions in F3, at 173-759 mg/kg, significantly more Cu compounds dissolve than previously in fraction F2. The largest proportion, at 798-3248 mg/kg, is the residue that was quantified in fraction F4  
Figure 1: Extracted amounts of the elements Cu, Zn, Pb, and Ni in fractions F1–F4 after sequential extraction. The unit of the y-axis in all cases is mg/kg (note different scales). Figure 2: Extracted amounts of the elements Cr and Cd in fractions F1–F4 after sequential extraction, measured using ICP-MS. The unit of the y-axis in all cases is mg/kg (note different scales). using aqua regia digestion. In comparison to the ashes, the examined crushed sand showed a very low solubility of Cu species in all four fractions of up to 13 mg/kg only. As the selective extraction already showed, Zn species in waste incineration ashes are already highly soluble in weak organic acids such as the 0.1 M acetic acid used here (see Figure 1). In each of the four IBA samples, the majority of the total Zn content dissolved in this first fraction F1 at 990-2877 mg/kg.