## 2 Materials and Methods  
Bottom ash samples from two different waste incineration plants in Germany (plant A and plant B, both using grate technology with a wet extraction system) were taken right after leaving the wet extraction unit of the incinerator and the subsequent removal of ferrous and non-ferrous metals through magnet and eddy current separation (sample A: 15 June 2021, sample B: 22 March 2023). Sample size was in the range of 400-600 kg each, large enough to generate homogenous test samples for the investigations performed in the lab. Batch tests were performed according to DIN 19529 [39] (L/S = 2 L/kg) and DIN EN 12457 [40] (L/S 10 L/kg). A representative sample mass of approximately 2.5 kg for each test was split into several subsamples, which were shaken in several glass bottles for 24 h. After 15 min of settling, the supernatants were recombined. Pressure filtration was subsequently conducted through a 0.45-\(\upmu\)m cellulose nitrate membrane filter for liquid-solid separation. All leaching tests were performed at least twice. Column tests were performed according to DIN 19528 [41] up to an L/S ratio of 10 L/kg. According to the standard, an L/S ratio of only 4 L/kg is required for basic characterization (or even L/S = 2 L/kg for the compliance test option in Germany). In column tests, the eluate is generated continuously over the complete test duration. Concentrations can therefore be determined for individual L/S ratios. DIN 19528 prescribes the L/S ratios as 0.3, 1.0, 2.0, and 4.0 L/kg: in this study, we also examined L/S = 7 L/kg and 10 L/kg. For a comparison of results from batch and column tests at different L/S ratios, the respective concentrations in mg/L were converted to leached amount E in mg/kg by multiplying by the respective liquid-to-solid ratio (L/S in L/kg). For the batch tests, this ratio is commonly 2 or 10 L/kg. To determine the total concentration of most elements as cations (Al, As, B, Ba, Ca, Co, Cd, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sn, Sr, Ti, Tl, V, Zn), a digestion using aqua regia was performed according to DIN ISO 11466 [42]. For this purpose, 0.5 g of the ground sample was placed in a PTFE container, and 12 mL of aqua regia (HNO3 65% 3:1 HCl 37%) was added. Subsequently, the container was subjected to microwave digestion using an Albert Romera ATC-400 microwave for 20 min. The cations were analyzed using a Thermo Scientific iCAP 7000 ICP-OES equipped with an ASX-200 autosampler. Prior to sample measurements, calibration using external standards was necessary. For lower cation concentrations, additional ICP-MS measurements were carried out using a Thermo Scientific iCAP Qc instrument with the same aqua regia digestions. For anion determination (F\({}^{-}\), Cl\({}^{-}\), NO\({}_{2}\)\({}^{-}\), Br\({}^{-}\), NO\({}_{3}\)\({}^{-}\), CO\({}_{4}\)\({}^{2-}\), PO\({}_{4}\)\({}^{3-}\)), extractions with Na\({}_{2}\)CO\({}_{3}\) were performed. In this procedure, 1 g of the sample and 10 g of Na\({}_{2}\)CO\({}_{3}\) were brought to a boil with 100 mL of water under agitation and maintained at this temperature for 10 min. Subsequently, hot filtration was conducted, and the final volume was adjusted to 250 mL. Ion chromatography was performed using a Thermo Scientific Dionex Integration system equipped with a Dionex Suppressor ADRS 600, an AS-AP autosampler, and a KOH gradient to determine the concentration of anions. IBA was leached in the acidic pH range with maleic acid at pH 4. Maleic acid was chosen as the acidifying agent as suggested in the practice note of the German associations ITAD and IGAM for the assessment of HP 14 properties [43] of IBA [44; 45]. In brief, 1 g of sample (milled to <0.25 mm grain size) was used in a triple approach. An amount of 40 mL of an extraction solution, containing 0.1 mol/L maleic acid and 0.1 mol/L sodium acetate, was added to the samples in centrifuge tubes. To adjust the pH to 4.0, 1.5 mol/L nitric acid or 1.0 mol/L sodium hydroxide solution was slowly added.