## 4 Applicability and Effect of Citric Acid-Based Treatment
### The Behavior of Traced Components: Copper, Zinc, and Manganese  
The respective XRF and XRD results in Table 1 and Figure 3 indicate that significant phases containing predominantly Cu, Zn, and Mn are not explicitly detected because of their lower contents in MF010-Min. Consequently, the particles or phases that comprise at least one of those three elements are most likely distributed randomly in the studied fraction. In other words, it is convincing to presume that those elements are either chemically or physically bound with the major components, some of which are explored in the previous sections. Furthermore, since the reaction characteristics during the washing process are dominated by surface reactions, interfacial interactions between Cu-, Zn-, and Mn-bearing particles should become a prerequisite for their dissolution. In this instance, the physical isolation of the enriched phases containing Cu, Zn, or Mn could become a limitation during the washing process. In the case of Cu, four different commonly possible water-insoluble compounds containing copper could be expected [34]: metallic (Cu) [79], oxides (CuO and Cu\({}_{2}\)O) [80], sulfides (CuS and Cu\({}_{2}\)S) [40], and a carbonate hydroxide phase (Cu\({}_{2}\)(OH)\({}_{2}\)CO\({}_{3}\)) [81], which exhibit different behaviors upon contact in a citric acid solution. Generally, metallic Cu does not readily react with citric acid until specific oxidation agents are included [82]. In this instance, the reaction between citric acid and oxides of copper to form copper citrate is highly anticipated [83; 84]. A similar effect can also be expected from Cu\({}_{2}\)(OH)\({}_{2}\)CO\({}_{3}\) (malachite), which can be dissolved in citric acid [85]. In contrast, CuS is negligibly affected by weak acids and requires a strong acid before being soluble [86]. Accordingly, the change in copper concentration during the washing process in Figures 5-10 was most likely caused by the dissolved copper oxides, both from copper oxide particles or the surface of metallic copper. Moreover, the contribution of copper carbonate might be relatively minor, as reported by [PERSON] et al. [87]. It is also amplified by the fact that no notable change in copper can be measured in correlation with highly soluble calcium carbonate during washing without agitation in Figures 5, 6 and 9. Compared with copper-bearing particles, zinc-rich substances are expected to be associated with some silicate phases [34], which are typically insoluble in water but behave differently in acidic solutions, including citric acid. In addition to those silicate phases, four water-insoluble zinc-rich components can also be found in IBA [87] and potentially contribute to the notable change in zinc concentration from Figures 5-10: metallic (Zn) [88], oxide (ZnO) [37], phosphate (Zn\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)) [89], and carbonate (ZnCO\({}_{3}\)) [44]. Despite being generally insoluble in water, Zn can be readily dissolved in citric acid [90]. However, considering its volatility at high temperatures, the existence of Zn in IBA should tendentially be well isolated during the process; otherwise, it will be concentrated in fly ash instead, as reported by [PERSON] et al. [91]. Hence, in the case of bottom ash, more stable zinc compounds are anticipated. It includes ZnO, Zn\({}_{3}\)(PO\({}_{4}\))\({}_{2}\), and ZnCO\({}_{3}\), which are also soluble in citric acid [92; 93; 94] and should contribute to the change in zinc concentration. However, despite their ability to react with citric acid, external agitation is necessary to enhance the reaction, as supported by the results of Figures 5, 6 and 9, in which no notable change in zinc was measured without stirring. It could also be related to its volatility, where zinc-rich phases are more likely generated during incineration. As a result, their availability would be bound near the surface of the carrier particles. Thus, external agitation is needed to enhance the interaction during the washing process.