(7) Extraction of iron and manganese oxidized bond fraction

Fifty milliliters hydroxylamine hydrochloride solution (j) was added to residue obtained from step (6) accurately. After they were shaken well, the ultrasound was carried out in ultrasonic cleaner with a frequency of 40 kHz for 1 h (ultrasonic for 5 min every 5 min during this period, water temperature in an ultrasonic cleaner is controlled at 25 ± 5 ◦C). Then, the solution was removed and centrifuged at 4000 r/min for 20 min. Finally, 10 mL clear liquid was separated into a colorimetric tube. The contents of lead, zinc, and cadmium were analyzed by the ICP-OES method. The remaining precipitation was transferred to a 50 mL centrifuge tube with 50 mL distilled water and centrifuged for 10 min at a speed of 4000 r/min. The aqueous phase was discarded, and the residue was left after cleaning twice.
