**5. Conclusions**

The improved Tessier method and Fourier transform infrared spectrum (FTIR) analysis test were used to study the effects of long-term F–T on the S/S of Pb–Zn–Cd composite HMcontaminated soil in this study. The soil morphology and FTIR spectra were tested in six environments (without F–T cycles, with 3 cycles, 7 cycles, 14 cycles, 30 cycles, and 90 cycles) in HM-contaminated soil, and the following remarkable conclusions were obtained.

In the S/S Pb–Zn–Cd composite HM-contaminated soil, the addition of three binders, cement, lime, and fly ash, reduces the HM risk by making HMs exist in a more stable form. The characteristic peaks at 463 cm−<sup>1</sup> and 516 cm−<sup>1</sup> of the FTIR spectra indicate the presence of colloids that can adsorb and store HMs, and the characteristic peak of calcium hydroxide that can precipitate HMs appeared at 1651 cm<sup>−</sup>1. However, for different types of HMs, the S/S efficiency is different. The relative content of HMs transferred to stable forms in lead and zinc is 20% greater than that in cadmium.

The content of HMs in a steady state decreased continuously with the increase of frequency of F–T cycles. There were drastic changes before 14 cycles, but with the continuous increase in the frequency of F–T cycles, the increasing trend gradually decreases, and the rate of decrease in the stable form of HMs became minimal after 30 F–T cycles. This is the main reason why F–T cycles would not change the types of functional groups.

Under the action of long-term F–T cycles, the decrease in the stable form content of HMs in the S/S composite HM-contaminated soil is mainly caused by the decrease in the carbonate-bound state content. Its peaks appeared at 874 cm−<sup>1</sup> and 1420 cm<sup>−</sup>1. Ninety F–T cycles caused the carbonate-bound content of Pb, Zn, and Cd to decrease by 18.04%, 8.92%, and 10.74%, respectively.

Through this experiment, it can be seen that after the number of F–T cycles reaches 30, the decrease rate of stable form HM content reaches the minimum; therefore, 30 F–T cycles can be used as the critical test point for indoor tests in actual projects to determine the danger zone. However, for different soils, exactly how the critical number changes is not yet known; in future research, it is recommended that the majority of scholars conduct further research on this part. At the same time, the huge change in the content of the carbonatebound fraction of HMs during the F–T cycles must also recognize the tremendous influence of environmental pH on the S/S effect, and numerous studies have been conducted by domestic and foreign scholars on this content in the hope that a risk control method for the common assessment of various influencing factors can be established in the future.

**Author Contributions:** Conceptualization, Z.Y.; methodology, Z.Y. and Y.W.; investigation, Y.W., X.L., J.C. and K.Z.; writing—original draft preparation, J.C.; writing—review and editing, Z.Y.; supervision, Z.Y., X.L. and K.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number 42177125 and the National Natural Science Foundation of China, grant number 41772306.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors acknowledge the financial support received from the National Natural Science Foundation of China (Grant No. 42177125, and No. 41772306). We also thank Ren Shupei and Wang Yao, graduate students, for their efforts in terms of conducting the laboratory tests.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
