**1. Introduction**

Because of considerable migration depth, complex migration mechanism, sizeable spatial variability, strong concealment, and resistance to degradation of heavy metals (HMs), its high-concentration and high-risk pollutants in the soil are extremely difficult to repair and manage and are called "chemical time bombs" [1–3]. In recent years, the remediation of soil HM pollution has been an environmental topic worthy of attention. Studies have shown that HM pollutants may cause changes in soil structure, such as colloidal corrosion between soil particles, decrease in cementation strength, increase in clay content, resulting in increased soil compression, reduced liquid, and plastic limits, reduced shear strength (the reduction can reach about 35%), increased permeability, and the decreased bearing capacity [4–6]. In addition, the non-degradability of HMs leads to their accumulation in plants, animals, and humans along the biological chain [7–11]. Both pose a threat to human safety and health; therefore, it is imperative to control soil

**Citation:** Yang, Z.; Chang, J.; Li, X.; Zhang, K.; Wang, Y. The Effects of the Long-Term Freeze–Thaw Cycles on the Forms of Heavy Metals in Solidified/Stabilized Lead–Zinc–Cadmium Composite Heavy Metals Contaminated Soil. *Appl. Sci.* **2022**, *12*, 2934. https:// doi.org/10.3390/app12062934

Academic Editors: Bing Bai and Dibyendu Sarkar

Received: 6 February 2022 Accepted: 10 March 2022 Published: 13 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

HM pollution [12]. There are two kinds of solutions to manage HM pollution in the soil. The first one is to directly reduce the concentration of HMs in the soil, and the second one is to reduce the possibility of HM migration. That means fixing them in place and reducing their biotoxicity by special treatments [13,14]. In this context, a series of treatment methods for HM-contaminated soils have emerged, such as S/S technology, displacement technology, soil leaching, incineration, solvent extraction, glass curing, and physical separation, emerged. Among them, solidification/stabilization (S/S) technology is the most widely used because of its cost-effective and simple operation [14,15].

The S/S technology of soil usually provides the ability to handle HMs through two aspects. On the one hand, the S/S technology has a direct effect on HMs. Chemical precipitation, physical encapsulation, adsorption, and other means occur between HMs and hydrations products of binders; on the other hand, the S/S technology directly affects soil, and indirectly affects HMs by changing the soil structure. The reduced permeability leads to reduced migration of HMs, then the stability of HMs in the soil can be achieved [15,16]. However, while enjoying the restorative power of S/S technology, one must also consider its environmental impact.

Considering this problem, three inorganic binders, cement, lime, and fly ash, were used in the current study to improve S/S efficiency while reducing carbon emissions and easing the burden of industrial waste disposal [17–19]. However, it should be noted that the use of binders should involve reasonable control of the amount: too little S/S has poor effect, but if the amount is too much it will not only increase the cost, but also change the physical and chemical properties of the soil, and even cause secondary pollution.

Starting from the mechanism of the S/S technology, it can be realized that this technology cannot reduce the concentration of HMs in the soil, but restricts the migration of pollution, or limits the effectiveness of pollutants. Based on this, it is necessary to track the follow-up progress of HMs after the use of S/S technology. In the internal system of S/S HM-contaminated soil, the type of soil [20], the type and content of HMs [21], the type and content of binders [22,23], the curing conditions [23], and the moisture content [24] all have an impact on the long-term stability of HMs. In various complex environments, such as carbonation [25], acid rain leaching [25–27], high salt groundwater infiltration [28,29], wetting and drying cycles [30–32], and F–T cycles [33–36], the stable forms of the contaminants in the S/S-contaminated soil is very likely to change or even fail, which causes the activation of HM pollutants in the soil, causing secondary pollution and affecting its engineering mechanical properties. Therefore, the environmental durability of S/Scontaminated soil is extremely sensitive to environmental changes. The impact of F–T cycles on S/S HM-contaminated soil will continue to be focused on in-depth in this study.

Freeze–thaw cycles mainly occur in seasonal frozen regions. The moisture in the soil constantly transforms between liquid and solid with the periodic change of temperature: through changes in the volume of water in different fractions, as well as the free migration of water, the pore structure and density of the soil are changed, which is one of the direct factors affecting the migration effect of HMs. In China, the area of seasonal permafrost is about 5.14 million square kilometers, accounting for 53 percent of the total land area [37]. According to the second national soil erosion remote sensing survey, China's F–T erosion area has reached 13.4% of the country's total area [38]. Because of the vast F–T area, it is of great practical significance to explore the influence of the F–T cycle on the S/S of HM-contaminated soil. After 12 F–T cycles, the leaching concentrations of Pb/Zn-contaminated soil and composite Zn–Pb-contaminated soil increased greatly [34]. The elastic modulus of silty clay and modified clay can be significantly reduced after four F–T cycles [39]. After conducting seven F–T cycles on silty clay, Wang et al. concluded that the number of F–T cycles had a great influence on the mechanical properties of soil [40]. Studies have confirmed that under the long-term effect of simple F–T cycles (180 cycles, 180 days), the unconfined compressive strength loss rate of cemented contaminated soil can reach more than 30%, and the concentration of HM ions in HM leaching solution can increase by more than 20% [18,41,42].

This research has been improved on this basis, and "long-term" is the keyword of this. Studying the condition of HMs in the soil during the long F–T cycle period, observing the S/S effect of the HMs is a good way to ensure the long-term use of the S/S soil. Effective control of HMs in soil under adverse conditions is of great significance to prevent secondary pollution of HMs. A maximum of 90 F–T cycles (90 days) was set up under the same F–T mode to explore the stability of HMs under long-term F–T cycles in this research.

After making up for the problem of short-term F–T, the situation of HMs in the soil has also been improved, and the transformation from single HM pollution to compound HM pollution research has been realized. According to a large number of studies, one source of pollution often produces multiple types of HM pollution [41–43]. As, Cd, Cr, Cu, Mn, Ni, Pb, Zn, and other HMs exist in the farmland soil of the Qixia mining area in Nanjing, China [44]. There are many HMs in the weathered coal gangue, and the surrounding land is also harmed by HMs such as Zn, Cd, and Cr. Research on a single HM-contaminated soil can no longer meet the actual needs. Therefore, in this study, three pollutants, Pb, Zn, and Cd, were added to the soil at the same time.

The adsorption–desorption, precipitation reaction, complexation reaction, and oxidation–reduction reaction between HMs and soil allow HMs to exist in different forms in soil, and different forms of HMs exhibit different toxicity and environmental behavior [45]. When HMs are in a less bioavailable form or a more stable form, their risk in the environment is low. Therefore, the content of each form of HMs can be used as an observation indicator to assess the risk of HMs in soil. The improved Tessier method was used to extract seven fractions of HMs in the soil in this study.

Based on the above analysis, the improved Tessier method was adopted to detect the content of various forms of HMs (Pb, Zn, and Cd) in the soil that had undergone six F–T cycle conditions (0, 3, 7, 14, 30, and 90 cycles) up to 90 days, and the functional groups were analyzed by Fourier transform infrared spectroscopy analysis tests. Meanwhile, the study assumes that the HMs are uniformly distributed in the soil, by controlling the particles of the original soil less than 1 mm, the mixing time of the preparation of contaminated soil longer than 5 min, and the error of X-ray fluorescence analyses (XRF) of the same batch of three concentrations less than 3%.

In this study, the safety of the S/S technology is investigated by using a near-actual high concentration of complex hinged HM-contaminated soil, the morphological changes of HMs as a variable of interest, and the F–T cycle effects that may occur in most of the global land as the influencing factor. The conclusions and laws drawn from this study provide more rigorous theoretical support for predicting the safety of seasonally S/S HM-contaminated soil use and provide favorable conditions for post-remediation reuse of contaminated soil [18].
