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

According to Figure 6, a large number of Si–O–Si stretching vibrations, Si–O vibration absorption peaks, and carbonate stretching vibration can be found in the sample; this proves that substances that can enhance the stability of HMs, such as colloids and carbonate binders, are indeed present in the soil at this time [52,53]. This is why a large number of HMs have previously undergone a transition to the steady fractions.

Meanwhile, the types of functional groups contained in the soil are approximately the same in the six cases. The direct effect of the F–T cycle on the soil is mainly a physical reaction without the addition of new substances; therefore, the number of characteristic peaks did not increase. Meanwhile, the result of chemical speciation analysis shows that when the number of F–T cycles exceeded 14 times, the changing trend of the content of each HM forms was very slow, and gradually stabilized instead of a single decline. Therefore, there was no disappearance of the characteristic peak. Due to the above reasons, the F–T cycle did not change the types of functional groups in the system.

In addition, carbonate stretching vibration was detected in the soil. This is consistent with the fact that there are carbonate binders in the soil, and that when the pH environment in which the HMs are located changes, the HMs react with the carbonates to varying degrees. The F–T cycles, on the other hand, increase the amount of carbon dioxide entering the soil precisely by changing the porosity of the soil, causing changes in the carbonate-bound fraction of the soil.

Throughout the whole process of this study, it is clearly recognized that the change in the content of its stable fractions tends to slow down when the F–T cycles are performed several times. According to the experimental results, this node occurs at 30 cycles. When conducting an HM soil remediation project, the collected soil samples can be S/S in the laboratory and then subjected to a small trial test of 30 F–T cycles to test the morphological components, derive the instability limits, determine whether there is a safety hazard, and determine whether to keep the curing ratios or change the curing ratios.
