## 2 Materials and Methods  
IBA (IBA 1-3) was sampled at three waste incineration plants and at one substitute fuel plant (IBA 4) in Germany, all using grate firing for incineration. Sample size was in the range of 600-800 kg. SSI ash (SSA) was sampled at a German municipal sewage sludge incineration plant with 3 lines using stationary fluidized bed technology. The MSWI fly ash (FA) was provided by the operator of a MSWI plant in southern Germany also using grate technology. The fly ash was separated from the flue gas in a multicyclone after the boiler and before the spray dryer and the bag filter. Therefore, no flue gas cleaning products were contained in this fly ash. The crushed sand (CS) was purchased from a company in the construction materials recycling sector in southern Germany. The material is economically non-recyclable and accumulates as a by-product during the crushing process during the recycling of construction and demolition waste (C&D waste). The IBA samples required careful and systematical processing due to strong inhomogeneity of the material. In the present study only the grain size fraction <16 mm was examined. FA, SSA, and CS required no crushing (ball milling only) because the grain size of the original material was already <2 mm. In the first step, the entire sample quantity from four steel drums was formed into a cone on a heap. The cone was divided into four piles of approximately the same size using stainless steel blades and dividing plates. Two opposite piles were then combined and stored for further dividing steps, while the remaining two piles were returned to the barrels. This procedure was repeated until a manageable quantity of approx. 20 kg was produced. All residues were shoveled back into the drums for further analysis. The partial samples were dried on a dividing plate. After drying, scrap iron particles were removed from the samples using a magnet (scrap removal). The samples were then crushed to a particle size of <2 mm using a jaw crusher (Retsch BB300, Haan, Germany). Non-ferrous metal particles and unburned materials (e.g., paper or non-woven polymer materials) were removed from the crushed material by visual inspection. Compared to the initial large sample quantity, a manageable amount of material was now available, which could be further divided after sorting out impurities using a rotary sample divider. The crushed material was then finely ground in a ball mill (vibrating cup mill Retsch MM2000, Haan, Germany). It was important to separate the magnetic particles again with a magnet after grinding. Visible non-magnetic metal particles also had to be removed. In the final step, the resulting fine grain was sieved again to <0.25 mm and prepared for analysis. A 0.5 g portion of the sample was weighed into a polypropylene plastic centrifuge tube and the four extraction steps were carried out:  
Fraction 1 (F1, weak acid soluble): (1) 40 mL acetic acid (0.11 M) per 1 g ash, i.e., L/S = 40 L/kg; (2) shaking for 16 h at 25 \({}^{\circ}\)C on an orbital shaker (Heidolph Instruments, Schwabach, Germany), liquid-solid separation by centrifugation at 1500 rpm, removal of the supernatant, washing of the residue with 5 mL of distilled water, and centrifuging separately. It is important that the final pH is 4 to ensure that carbonates are digested [26; 27]. The original recipe was devised for the application of sequential extraction with soil and sediments. However, the alkalinity of IBA is much higher than that of soil due to a high content of CaO, Ca(OH)\({}_{2}\), and CaCO\({}_{3}\) in the ashes. Necessary adjustments of the pH were performed with concentrated acetic acid. Fraction 2 (F2, reducible fraction, substances bound to iron and manganese oxides): (1) 40 mL NH\({}_{2}\)OH\(\times\)HCl (0.5 M); (2) shaking for 16 h at 25 \({}^{\circ}\)C. Residues of the previous extraction were added to the hydroxyapatite hydrochloride (CAS No. 5470-11-1). The samples were shaken in an orbital shaker for 16 h (overnight) at room temperature at 30 rpm.