(8) Extraction of strong organic bonding fraction

Three milliliters HNO3 (d) and 5 mL H2O2 (k) were accurately added to the residue obtained from step (7). The solution was shaken well and kept for 1.5 h in a constant temperature water bath at 83 ± 3 ◦C (stirred once every 10 min during the period). Then, the solution was continued to have 3 mL H2O2 added (k) and kept warm in the water bath for 1 h and 10 min (stirring once every 10 min during this period). After cooling to room temperature, 2.5 mL ammonium acetate–nitric acid solution (a) was added, and the samples were diluted to about 25 mL and stirred for 1 min, then placed at 25 ± 5 ◦C overnight, and centrifuged at 4000 r/min for 20 min. The clear liquid was poured into a 50 mL colorimetric tube, diluted with distilled water to scale, and shaken well. Then, a 50 mL beaker was used. First, 25 mL separated clear liquid, 10 mL HNO3 (c), and 1 mL HClO4 (e) was poured in, then, with a surface dish, it was heated on the electric heating plate at low temperature until it was nearly dry, and at high temperature until the concentrated white smoke was cleared. After this, 5 mL HCl (b) was added while hot, and the dish was washed. The beaker was heated at a low temperature until the salts dissolved. After the solution cooled, 25 mL of water was placed into a colorimetric tube and shaken well. A 5 mL solution was separated into 10 mL colorimetric tubes for ICP-OES analysis for Pb, Zn, and Cd. About 20 mL of distilled water was added to wash the sediment, and centrifuged for 10 min at a speed of 4000 r/min. The aqueous phase was discarded. The cleaning process was carried out twice, leaving the residue at the end.
