Table 1. Description of the BCR Sequential Extraction Procedure Used for HTEs Speciation.

fraction	BCR sequential extraction procedure
F1	0.11 M CH3COOH (40 mL)	16 h at laboratory temperature	exchangeable hazardous elements (e.g., carbonates and highly water-soluble and/or under acidic conditions)
F2	HONH2HCl (40 mL)	16 h at laboratory temperature	hazardous elements (e.g., bound to either Fe and/or Mn oxyhydroxides)
F3	8.8 M H2O2 (10 mL) + 1 M NH4C2H3O2 (50 mL)	1 h at laboratory temperature and 1 h at 85 °C, +1 h at 85 °C, +16 h at laboratory temperature	hazardous elements (e.g., associated with both organic matter and sulfides)
F4	HNO3/HF/HClO4	two-step acid digestion method devised by Querol et al.	hazardous elements (e.g., associated with crystalline and amorphous phases)