## 4 Applicability and Effect of Citric Acid-Based Treatment
### The Behavior of Notable Components: Calcium, Iron, and Aluminum  
Following the observations of [PERSON] et al. [28], calcium is anticipated to be the easiest cation to form a dissolved complex in citric acid for various silicate groups. A similar tendency is confirmed in all results in Figures 5-10, where a notable reduction in the calcium content is anticipated. The decreasing calcium concentration is even measurable in Figures 5 and 6 with no external agitation, indicating how likely it is to be dissolved. However, the reduction in calcium content might not be solely determined by the contribution of calcium-bearing silicate phases. In this case, two calcium-containing substances are also present in MF010-Min measured by XRD in Figure 3: anhydrite (CaSO4) and calcite (CaCO3). Considering the anhydrite polymorphism, which is unfortunately not specified in the XRD results, three dehydrated structures are commonly well known [33], which can also potentially be formed during MSW incineration [34]: anhydrite III (\(\gamma\)-CaSO4), anhydrite II (\(\beta\)-CaSO4), and anhydrite I (\(\alpha\)-CaSO4). However, considering the sequence after the incineration, including water quenching, weathering, classification, and drying, \(\alpha\)-CaSO4 is not expected because of its stability at temperatures above 1180 \({}^{\circ}\)C [35]. On the other hand, considering the \(\gamma\)-CaSO4 and \(\beta\)-CaSO4, it is also necessary to include two hydrate phases of CaSO4: dihydrate (CaSO4\(\cdot\)2H2O) and hemihydrate (CaSO4\(\cdot\)3H2O) [36]. Those hydrates are essential in the context of IBA based on observation from [PERSON] et al. [37], who suggested that three primary phases containing calcium sulfate in weathered IBA are gypsum (dihydrate), anhydrite, and ettrinigite. Given that anhydrite is still measured as a primary calcium sulfate substance, it is reasonable to presume that the remaining anhydrite was the insoluble phase of \(\beta\)-CaSO4. It is supported by the argument that if only soluble anhydrite (\(\gamma\)-CaSO4) is present, rehydration should take place upon water quenching due to its metastable behavior [38,39] into at least hemihydrate and transform into ettringite during weathering [40]. Moreover, despite the reverse establishment of \(\gamma\)-CaSO4 being thermodynamically possible through the decomposition of ettringite [41] and gypsum dehydration [42], the formation of gypsum and hemihydrate is proven to be kinetically favorable compared with that of \(\gamma\)-CaSO4. Specifically, as recently elucidated by [PERSON] et al. [43], when approximately 100 \({}^{\circ}\)C was used as the temperature during the preparation of MF010-Min (drying and milling), a mixture of \(\beta\)-CaSO\({}_{4}\) and hemihydrate was detected instead of \(\gamma\)-CaSO\({}_{4}\) upon dehydration of the gypsum. In the case of ettringite decomposition, despite its formation originating from hemihydrate, its reverse reaction during the preparation of MF010-Min is not necessarily comparable since the pH value determines its decomposition products. In this case, assuming that the formation of ettringite starts from fresh IBA until a particular initial stage of weathering, the ettringite is relatively stable since the pH value of raw IBA is approximately 12-13 [44], which is in the range of ettringite stability [45; 46]; otherwise, gypsum tends to form at lower pH values. Considering these dynamics in the effect of pH, since the pH value of raw IBA decreases with prolonged weathering [47] and is aggravated by the drying and selective milling process, the decomposition of ettringite yields a mixture of gypsum and hemihydrate. However, it is also worth mentioning that both \(\gamma\)-CaSO\({}_{4}\) and hemihydrate are metastable phases, possess similar crystal structures, and differ only in the presence of hydrate water [48]. Based on the above theoretical exploration, considering the results of this study and only the stable phase of calcium sulfate, dry-processed IBA should contain gypsum and anhydrite II (\(\beta\)-CaSO\({}_{4}\)).