*4.1. Effect of Solidification/Stabilization on the Speciation of Different Heavy Metals*

The results in Section 3.1 show the transfer of HMs to stable forms after the addition of binders. After the hydration reaction of the binders, their hydration products will interact with HMs, increasing the inertness of HMs and reducing the migration ability of HMs.

The first mechanism for immobilization of HMs is a chemical reaction, in which lead, zinc, and cadmium react with Ca(OH)2, calcium silicate hydrates, and so on, and are fixed

to the binder components. Furthermore, the S/S technology also has a certain physical effect on the treatment process of HMs through the improvement of soil properties and the progress of the chemical reaction of the binders [55]. The decrease in soil permeability will hinder the migration of HMs, and the colloids produced by the curing agent will physically encapsulate the HMs [56]. Soil S/S technology, as a risk management technology, has a strong effect on reducing the risk of HMs. However, it can be clearly found that for different HMs in the soil, the conversion efficiency is different, with cadmium having the lowest conversion efficiency, with a difference of 20% compared to the other two HMs. The factors causing this phenomenon may have the following three aspects.

First, the properties of each HM are different [55]. Compared with cadmium ions, lead ions are more electronegative and have poor reaction activity [57]. Second, the S/S of HMs is closely related to the pH value of the soil, and different kinds of HM have different optimal pH values for S/S [58–60]. Most lead ions precipitate at a pH of about 9.8 and remain stable within a pH of 12 [61]. Zinc hydroxide begins to precipitate when pH is above 5 [62], and crystalline Cd(OH)2 is precipitated in soil when pH is increased to above 6.2 [63]. Finally, the initial content of the three elements added to the undisturbed soil is different, the added cadmium is the least, and the progress of S/S will be less targeted.
