2.3.1. Freeze–Thaw Test

The alternately high- and low-temperature test chamber (Chongqing Tester Experimental Instrument Co., Ltd. Chongqing, China) is the instrument for F–T tests. Five kinds of F–T cycles were designed for the specimens, including 3, 7, 14, 30, and 90 F–T cycles (0 F–T cycle was set as the control group). The setting mode of one cycle is shown in Table 2 (24 h for each cycle), in which the freezing temperature of the soil was set at −10 ◦C, and the dissolution temperature was 20 ◦C. To ensure the complete freezing and dissolution of the soil, it was maintained for 11 h after reaching the specified temperature. The F–T cycled specimens were ground and pounded through a 200-mesh sieve to meet the experimental.

**Table 2.** The form of the F–T cycles.


Note: The first time the soil freezes, it starts to drop from room temperature.

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 humicacid-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).


Five hundred and eight grams MgCl2 · 6H2O was weighed and dissolved in distilled water in a 2500 mL plastic bucket. The pH was adjusted by NaOH (ω(NaOH) = 10%).

(h) Sodium acetate (c(CH3COONa·3H2O) = 1.0 mol/L, pH = 5 ± 0.2)

Three hundred and forty grams CH3COONa · 3H2O was weighed and dissolved in distilled water in a 2500 mL plastic bucket. The pH was adjusted by CH3COOH.

(i) Sodium pyrophosphate (c(Na4PO7·10H2O) = 0.1 mol/L, pH = 10.0 ± 0.2)

One hundred and eleven and a half grams Na4PO7· 10H2O was weighed and dissolved in distilled water in a 2500 mL plastic bucket, and the pH was adjusted to 10.0 ± 0.2 with HNO3 where concentrated nitric acid and water are mixed in a volume ratio of 1:1.

(j) Ammonium hydroxide hydrochloride–hydrochloric acid mixture (c(HONH3Cl) = 0.25 mol/L, c(HCl) = 0.25 mol/L)

Forty-three point four grams HONH3Cl 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.

(k) Hydrogen peroxide (ϕ(H2O2) = 30%, pH = 2.0 ± 0.2)

The pH was adjusted by nitric acid where concentrated nitric acid and water are mixed in a volume ratio of 1:1.

(l) Ammonium acetate–nitric acid mixture (c(CH3COONH4) = 3.2 mol/L, c(HNO3) = 4.48 mol/L)

Six hundred and sixteen point six grams CH3COONH4 was weighed and added 500 mL HNO3 (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.


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. 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 ± 5 ◦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 μm, 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.

(3) 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.

(4) 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 ± 5 ◦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.

(5) 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 ± 5 ◦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.

(6) 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 ± 5 ◦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 HNO3 (c) and 1.5 mL HClO4 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 HClO4 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).
