## 2 Materials and Methods
### Testing Methods
#### 2.3.2 The Improved Tessier Method  
According to the Geological Survey Standards of China Geological Bureau (DD2005-03), the improved Tessier method was adopted to determine the distribution of seven fractions of three HMs of S/S Pb-Zn-Cd HM-contaminated soil under six conditions. This method is to further divide the organically bound fractions in the Tessier sequential extraction method proposed by Tessier into strongly organically bound fractions and humic-acid-bound fractions, and additionally add the water-soluble fraction [49; 50]. The seven forms in the method are water-soluble fraction, ion exchange fraction, carbonate-bound fraction, iron and manganese oxidation fraction, weak organic fraction, strong organic fraction, and residue fraction. The main reagents used are as follows, according to Technical Standards for Geological Survey of China Geological Survey (DD2005-03). Hydrochloric acid (\(\rho(\mathrm{HCl})=1.18\) g/mL);  
Hydrochloric acid in which concentrated hydrochloric acid and water are mixed in a volume ratio of 1:1;  
Nitric acid (\(\rho(\mathrm{HNO_{3}})=1.41\) g/mL);  
Nitric acid (c(\(\mathrm{HNO_{3}})=0.02\) mol/L);  
Perchlorate (\(\rho(\mathrm{HCl_{4}})=1.66\) g/mL);  
Hydrofluoric acid (\(\rho(\mathrm{HF})=1.15\) g/mL);  
Magnesium chloride (c(MgCl\({}_{2}\cdot\)6H\({}_{2}\)O) = 1.0 mol/L, pH = 7.0 \(\pm\) 0.2)  
Five hundred and eight grams MgCl\({}_{2}\cdot\) 6H\({}_{2}\)O was weighed and dissolved in distilled water in a 2500 mL plastic bucket. The pH was adjusted by NaOH (\(\omega(\mathrm{NaOH})=10\%\)). Sodium acetate (c(CH\({}_{3}\)COONa\(\cdot\)3H\({}_{2}\)O) = 1.0 mol/L, pH = 5 \(\pm\) 0.2)  
Three hundred and forty grams CH\({}_{3}\)COONa \(\cdot\)3H\({}_{2}\)O was weighed and dissolved in distilled water in a 2500 mL plastic bucket. The pH was adjusted by CH\({}_{3}\)COOH. Sodium pyrophosphate (c(Na\({}_{4}\)PO\({}_{7}\cdot\)10H\({}_{2}\)O) = 0.1 mol/L, pH = 10.0 \(\pm\) 0.2)  
One hundred and eleven and a half grams Na4 PO\({}_{7}\cdot\) 10H\({}_{2}\)O was weighed and dissolved in distilled water in a 2500 mL plastic bucket, and the pH was adjusted to 10.0 \(\pm\) 0.2 with HNO\({}_{3}\) where concentrated nitric acid and water are mixed in a volume ratio of 1:1. Ammonium hydroxide hydrochloride-hydrochloric acid mixture (c(HONH\({}_{3}\)Cl) = 0.25 mol/L, c(HCl) = 0.25 mol/L)  
Forty-three point four grams HONH\({}_{3}\)Cl was weighed and 104 mL hydrochloric acid was added, where concentrated hydrochloric acid and water are mixed in a volume ratio of 1:1 and dissolved in distilled water in a 2500 mL plastic bucket. Hydrogen peroxide (\(\varphi(\mathrm{H_{2}O_{2}})=30\%\), pH = 2.0 \(\pm\) 0.2)  
The pH was adjusted by nitric acid where concentrated nitric acid and water are mixed in a volume ratio of 1:1. Ammonium acetate-nitric acid mixture (c(CH\({}_{3}\)COONH\({}_{4}\)) = 3.2 mol/L, c(HNO\({}_{3}\)) = 4.48 mol/L)  
Six hundred and sixteen point six grams CH\({}_{3}\)COONH\({}_{4}\) was weighed and added 500 mL HNO\({}_{3}\) (c). Both were dissolved in a 2500 mL plastic bucket with distilled water. Using the above reagent, the concentration of HMs in soil under seven forms could be accurately determined according to the following nine steps. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Temperature Form** & From 20\({}^{\circ}\) to \(-\) 10\({}^{\circ}\) & Remaining \(-\)10\({}^{\circ}\) & From \(-\) 10\({}^{\circ}\) to 20\({}^{\circ}\) & Remaining 20\({}^{\circ}\) \\ \hline
**Time/h** & 0–1 & 2–12 & 12–13 & 14–24 \\ \hline \hline \end{tabular} Note: The first time the soil freezes, it starts to drop from room temperature.  
\end{table} 
Table 2: The form of the F–T cycles. A sample of 2.5000 g was accurately weighed and placed in a centrifuge tube. Extraction of water-soluble fraction. Distilled water was used as the extraction agent for water-soluble HMs. First, 25 mL of distilled water was added to the beaker and shaken well, then the beaker was placed into the ultrasonic cleaner.