*2.1. Elemental Analysis of Fly Ash Extracts*

Waste incineration plants of Ingolstadt, Germany (MVA Ingolstadt), and Zuchwil, Switzerland (KEBAG Zuchwil), kindly provided fly ash extract samples. The samples were filtered using syringe filter holders (0.45 μm, polyether sulfone; Sartorius, Göttingen, Germany). For each sample, 60 elements were analyzed by matrix-adapted inductively coupled plasma optical emission spectrometry (ICP-OES; iCAP 7600 Duo, Thermo Fisher Scientific, Waltham, MA, USA). The corresponding results were validated by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7900, Santa Clara, CA, USA). Chloride concentrations of the fly ash extracts were determined by potentiometric titration with silver nitrate solution (Titrando 904, Metrohm, Herisau, Switzerland).
