## 3 Results
### Sequential Extraction
However, the SSI ash, at only 435 mg/kg, has a significantly lower solubility, especially in relation to the total Zn concentration. This suggests that other Zn compounds that are not soluble in weak organic acids are predominantly present in SSA. At 4005 mg/kg, a larger proportion of the total Zn content is dissolved in the fly ash, although this accounts for less than half of it. In the second fraction, F2, all ashes are similar; the solubility of Zn is relatively low, similar to that of Cu in F2. In contrast to copper, however, the solubility of Zn under oxidative conditions in fraction F3 is also low and, therefore, plays a subordinate role in the overall analysis. It was striking that the majority of Zn species were only soluble in sample FA in fraction F4 at 6012 mg/kg, which means that the composition of the Zn species in the fly ash must differ significantly from that of the IBA, where only a small proportion was still present in the residue. Zn could also hardly be detected in the crushed sand (CS) at up to 53 mg/kg, similar to Cu. In the case of Pb, hardly any trend differences could be detected between IBA and FA, see Figure 1. The picture here was similar to that for Cu, with 40-180 mg/kg dissolved in fraction F1 in these five samples and fraction F2 being negligibly small. At 100-176 mg/kg, more Pb species were again dissolved in fraction F3. The majority of Pb was only dissolved in fraction 4 with up to 944 mg/kg in FA. In contrast to Cu and Zn, the total concentration of Pb in SSA is relatively low at only 60 mg/kg. CS also only contains a total of 36 mg/kg Pb. As the selective extraction and the aqua regia digestion have already shown, Ni is present to an even lesser extent in HMVA, but also in FA, see Figure 1. The ratio between the individual fractions showed a low solubility of 12-49 mg/kg in fraction F1, which, however, is usually much higher than in fractions F2 and F3. The majority of the Ni species (82-349 mg/kg) only dissolves in the residual fraction F4. In comparison, the Ni concentrations in SSA and CS are again lower than in IBA and FA, at a total of 52 and 32 mg/kg, respectively. To classify the toxicity (see Section 3.2) of the ashes and the crushed sand, the solubilities of Cd and Cr are also shown in Figure 2. It should be noted that ICP-MS was used as the quantification method due to the low concentrations, which nevertheless have a major influence on the classification of the samples with regard to hazards and toxicity. With the exception of fly ash with 42 mg/kg, Cd is only soluble in fraction F1 in very low concentrations of 0-2 mg/kg. Fraction F2 is again negligibly low; in fraction F3, the solubility is below 1 mg/kg; an exception here is again FA, with approx. 4 mg/kg. In fraction F4, too, only low solubilities of 0-6 mg/kg could be detected. Cr is present more frequently overall in the ashes, whereby almost all species for this element are only dissolved in fraction F4. Fraction F1 also has a relevant solubility of up to 53 mg/kg; fraction F2 is negligibly small; and fraction F3, with up to 27 mg/kg, is also relevant for the overall classification of the samples with regard to toxicity. With a solubility of up to 794 mg/kg, the residue of fraction F4 contains the largest proportion of Cr species, as already mentioned. The results clearly showed that Zn and Cd are elements that are mainly transferred to the fly ash in the process of municipal solid waste incineration [32; 33]. Crushed sand from C&D waste recycling showed only negligible heavy metal contamination.