## 3 Results
Leaching at a fixed pH of 4 with maleic acid was included in the present study to compare leached amounts under \"natural\" conditions, i.e., from neutral to alkaline pH values to the acidic range, where the heavy metal release is significantly higher. However, the presence of maleic acid, i.e., possible complexation reactions with the organic ligand, was not included in the geochemical modelling with Visual MINTEQ. The solid lines in Figure 3 are almost horizontal in the pH range from 3 to 5 because the solid phases entered into the program were lower than the real concentrations in the samples. The observed leaching results of Sb could not be modelled as function of pH with the applied simple approach. There is, therefore, no solid line in Figure 3d. The pH- dependent leaching of Sb is influenced by sorption on iron oxides and amorphous Al minerals, which are always present in IBA [53]. However, sorption was also not included in the model here. Nonetheless, precipitation was modelled by the addition of solid phase Ca-antimonate to the database, using a value for log K\({}_{\rm s}\) of \(-\)12.55 [54]. Antimony (Sb) exists as the less toxic Sb(V) in IBA [55], i.e., specifically as oxyanion Sb(OH)\({}_{6}\)\({}^{-}\)[27,56]. It shows a completely different dependence on pH from Cu, Zn, and Pb ( see Figure 3d). In contrast, a correlation between the concentration of Ca and Sb was already found in landfill leachates in the 1990s [48]: high Sb concentration at low Ca concentration and vice versa, suggesting the precipitation of Sb(V) by Ca ions. The authors gave a stability constant log K = \(-\)15.5 for Ca(Sb(OH)\({}_{6}\))\({}_{2}\), changed in a later work to log K = \(-\)12.55 [54]. This finding was confirmed by various research groups [27,53,56] and was also reproduced in the present investigation, at least with the batch tests performed at an L/S ratio of 10 L/kg(see Figure 4). Here, the Ca concentrations were mostly in the narrow range between 100 and 250 mg/L (displayed as leached amount in mg/kg). In the batch tests with L/S = 2 L/kg, the data show more scattering. However, a reasonable agreement between the experimental data and the modelling results (Visual MINTEQ, using log K of Ca-antimonate = \(-\)12.55) was found, i.e., high Sb concentrations at low Ca concentration and vice versa. Figure 3: Leached amount (E) in mg/kg as a function of pH for the elements Cu (**a**), Zn (**b**), Pb (**c**), and Sb (**d**). Filled symbols are results at an L/S ratio of 10 L/kg, unfilled symbols at an L/S ratio of 2 L/kg. Green: sample A; blue: sample B. The leached amount of soluble components such as Na and Cl showed little variance with storage time and thus with the pH of the leachate. Large differences were observed, of course, between the samples with outdoor and indoor storage. Na and Cl dropped by more than 90% in sample A, which was stored outdoors (see Figure S2 in SI). The effect was much lower for sample B, due to less rainfall (factor 4: sample A 348 L/m\({}^{2}\), sample B 87.0 L/m\({}^{2}\)), see the precipitation curve in Figure S2 in the SI. The measured sulfate concentrations changed drastically during the course of the aging experiment, indicating that, at least in the beginning with fresh IBA, gypsum (CaSO\({}_{4}\)\(\times\) 2 H\({}_{2}\)O) is not the dominant phase present. For sample A, the sulfate concentrations were below 100 mg/L in the first 4 weeks in both standardized batch tests (i.e., leached amount \(<\)1000 mg/kg at L/S = 10 L/kg and \(<\)200 mg/kg at L/S = 2 L/kg), although the solubility of CaSO\({}_{4}\) is 2000 mg/L [26] (see Figure 5). This effect was also observed with sample B, but less pronouncedly. Maximum sulfate concentrations between 1300 and 1400 mg/L were observed after 10-12 weeks of storage time outdoors; the sulfate concentrations of the samples indoors did not exceed 1000 mg/L.