## 2 Materials and Methods
### Testing Methods
#### 2.3.2 The Improved Tessier Method
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 \(25\pm 5\)\({}^{\circ}\)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. Extraction of strong organic bonding fraction  
Three milliliters HNO\({}_{3}\) (d) and 5 mL H\({}_{2}\)O\({}_{2}\) (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 \(83\pm 3\)\({}^{\circ}\)C (stirred once every 10 min during the period). Then, the solution was continued to have 3 mL H\({}_{2}\)O\({}_{2}\) 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 \(25\pm 5\)\({}^{\circ}\)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 HNO\({}_{3}\) (c), and 1 mL HClO\({}_{4}\) (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. Extraction of residue fraction  
The cleaned residue obtained from step (8) was air-dried, ground, weighed, and the correction factor was calculated, and then the sample of 0.2000 g was accurately taken out and placed in the PTFE crucible. The sample was wetted in the crucible by water and 5 mL of hydrochloric acid (f), nitric acid, and perchloric acid mixture in the ratio of 1:1:1 was added. The crucible was heated on an electric heating plate until the white smoke of perchloric acid was fully distributed. Then, 3 mL of hydrochloric acid (b) was added to the crucible. After the crucible wall was washed, the crucible continued to heat until the salt was dissolved, and then cooled to room temperature. Finally, the remaining product was transferred to a colorimetric tube, and the volume was kept at 25 mL and shaken well. The contents of lead, zinc, and cadmium were analyzed by the ICP-OES method. Fourier Transform Infrared Spectrum (FTIR) Analysis Test  
The attenuated total reflection Fourier transform infrared spectroscopy method was adopted in this study. Six kinds of soil were dried at 100 \({}^{\circ}\)C for 24 h and tested by a Fourier infrared spectrometer after passing through a 200-mesh sieve. The mid-infrared region was used for detection in this study. The following procedures were used to analyze the spectrum [51].