## 2 Materials and Methods
### Testing Methods
#### 2.3.2 The Improved Tessier Method
Ultrasound was performed at the working mode of 5 min with 5 min intervals at a frequency of 40 kHz, and the total working time was 30 min, during which the water temperature in the ultrasonic cleaner was \(25\pm\ 5\ ^{\circ}\)C. Then, the material in the beaker was transferred into a centrifuge tube for centrifugation (20 min at a speed of 4000 r/min), the precipitated supernatant was filtered with a pore diameter of 0.45 \(\upmu\)m, and then the liquid was poured into a 25 mL colorimetric tube. An Optima 8000 inductively coupled plasma emission spectrometer was used to detect the contents of lead, zinc, and cadmium in the extracted liquid. Residue cleaning  
The remaining residue was added to about 100 mL distilled water to wash the precipitation, and centrifuged at 4000 r/min for 10 min. The aqueous phase was discarded, and the residue was left. Extraction of ion-exchange fraction  
Twenty-five milliliters of magnesium chloride (g) solution were accurately added to the residue left in step (3), shaken well, and placed in an ultrasonic cleaner that had been placed into water. Ultrasound was performed at a frequency of 40 kHz for 30 min (ultrasonic was performed for 5 min every 5 min during this period, and the water temperature in the ultrasonic cleaner was controlled at \(25\pm 5\ ^{\circ}\)C). After the ultrasound, they were taken out, and centrifuged at 4000 r/min for 20 min. A total of 5 mL of liquid was taken into a 10 mL colorimetric tube and added to 0.5 mL of hydrochloric acid (a). After that, the distilled water was used for constant volume to scale. After shaking the colorimetric tube well, the ICP-OES theory was used to determine the content of HMs in liquid. The step of residue cleaning was repeated at last. Extraction of carbonate-bound fraction  
Sodium acetate (h) was selected as the extraction agent of carbonate-bound fraction. A total of 25 mL of it was added to the residue which came from step (4), shaken well, and placed in an ultrasonic cleaner with a frequency of 40 kHz for 1 h (ultrasonic was performed for 5 min every 5 min during the period, and the water temperature in the ultrasonic cleaner was controlled at \(25\pm\ 5\ ^{\circ}\)C). Then they were taken out and centrifuged at 4000 r/min for 20 min. Separated 5 mL liquid was poured into a 10 mL colorimetric tube. After adding 0.5 mL HCl (a) to the colorimetric tube, the distilled water was brought to scale, and shaken well. An Optima 8000 inductively coupled plasma emission spectrometer was used to analyze the contents of lead, zinc, and cadmium, and step (3) was repeated finally. Extraction of humic-acid-bound fraction  
Fifty milliliters sodium pyrophosphate solution (i) was accurately added to the residue obtained in step (5). After shaking well, it was taken to the ultrasonic cleaner with a frequency of 40 kHz for 40 min (ultrasonic 5 min every 5 min during this period, and the water temperature in an ultrasonic cleaner is controlled at \(25\pm 5\ ^{\circ}\)C) and placed for 2 h. Then, the solution was centrifuged at 4000 r/min for 20 min. A 50 mL beaker was filled with 10 mL clear liquid and 5 mL HNO\({}_{3}\) (c) and 1.5 mL HClO\({}_{4}\) were added (e). The surface dish was used to cover the beaker, and the beaker was heated and steamed on the electric heating plate until the occurrence of HClO\({}_{4}\) white smoke exhaust. Then, 1 mL HCl (b) was added, the surface dish was washed, and the dissolved salt was heated, removed, and cooled. Finally, a 10 mL colorimetric tube was scaled by distilled water. After shaking well, an Optima 8000 inductively coupled plasma emission spectrometer was used to analyze the contents of lead, zinc, and cadmium. The remaining residue was treated as described in step (3). Extraction of iron and manganese oxidized bond fraction  
Fifty milliliters hydroxylamine hydrochloride solution (j) was added to residue obtained from step (6) accurately.