*3.2. Effect of F–T Cycles on the Speciation of HMs*

Figure 4 reveals the trend of the relative content of the two types of fractions of HMs in the S/S Pb–Zn–Cd composite HM-contaminated soil with the action of F–T cycles. The changing trend of the curve in it indicates that the F–T cycle causes HMs to transform from stable to unstable forms, but the reduction of stable HM becomes slow after reaching 14 cycles. After 90 F–T cycles, the relative contents of stable forms of Pb, Zn, and Cd in the soil decreased by 1.27%, 3.11%, and 8.16%, respectively, while the decrease reached more than 50% of the total decrease after 14 F–T cycles.

**Figure 4.** The variation of the relative content of HM forms with F–T cycles in the S/S Pb-Zn-Cd composite HM-contaminated soil. (**a**) unstable HM forms; (**b**) stable HM forms.

The variation of the relative content of stable fractions of three HMs in S/S soil with the number of F–T cycles is shown in Figure 5. It can be seen that the relative content of the carbonate-bound fraction of HMs decreases significantly with F–T cycles. The effect is significant within the 14 F–T cycles, the slope of the curve is large, and the rate of decline is fast. However, with the increase of the F–T cycles times, the rate of decline of carbonate-bound forms becomes relatively slow. After 14 F–T cycles, the relative contents of carbonate-bound fraction of Pb, Zn, and Cd decreased by 13.46%, 7.35%, and 8.68%, respectively, while the relative contents of corresponding types only decreased by 2.06% in the process from 14 F–T cycles to 90 F–T cycles, 4.58%, and 1.56%.

**Figure 5.** The variation of relative content of each speciation of HMs with F–T cycles in the S/S Pb–Zn–Cd composite HM-contaminated soil. (**a**) Zn; (**b**) Pb; (**c**) Cd.

## *3.3. Effect of F–T Cycles on the Functional Groups in S/S Soil*

Figure 6 shows the FTIR spectra of S/S Pb–Zn–Cd composite HM-contaminated soil under six F–T cycle conditions (0, 3, 7, 14, 30, and 90 cycles). According to their own spectra, a large amount of literature data, and standard Fourier infrared spectrogram, the types of functional groups in each condition were distinguished.

**Figure 6.** FTIR spectra of S/S Pb–Zn–Cd composite HMs-contaminated soil under 6 F–T cycle conditions (0, 3, 7, 14, 30, and 90 cycles).

The locations of the characteristic peaks of the FTIR spectra under the six F–T conditions are roughly the same, indicating that within 90 F–T cycles, the types of target substances (soil components, binder components, hydration products, etc.) detected in the S/S Pb–Zn–Cd composite HMs-contaminated soil did not change.

Seven hundred and ninety-seven per centimeter, and Seven hundred and eighty-seven per centimeter are caused by Si–O–Si stretching vibrations, which are typical double peaks of quartz minerals, which is an important part of the soil itself, and the added binders. The peaks at 516 cm−<sup>1</sup> and 463 cm−<sup>1</sup> are the Si–O vibration absorption peaks of the gel [52,53]. There are two main reasons for the existence of the characteristic peak of colloid. One is that the effective component of cement, tricalcium silicate, and dicalcium silicate, generates a large amount of hydrated calcium silicate colloid after hydration; the other is that the hydration product of cement, calcium hydroxide, continues to react with lime and fly ash to form colloids [54]; among them, there was residual calcium hydroxide, and the characteristic peak appears in 1651 cm−1. The existence of colloids has made a great contribution to the S/S of HMs, such as the large amounts of adsorption of HM ions, and the filling of soil pores, which hinders the flow of HMs. The formation of calcium hydroxide also underwent a substitution reaction with HM ions, causing HM ions to precipitate.

Eight hundred and seventy-four per centimeter and one thousand four hundred and twenty per centimeter are caused by carbonate stretching vibration. These two obvious characteristic peaks appeared in the FTIR spectra under every F–T condition (0, 3, 7,14, 30, and 90 F–T cycles). This is mainly produced by the entry of carbon dioxide into the atmosphere and is affected by the pH of the soil. There is a huge relationship with the formation of HM carbonate-bound fraction.

## **4. Discussion**
