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

Among the seven different HM fractions extracted by the improved Tessier method, the order of bioavailability for different fractions of HMs in the soil is shown as follows: water-soluble state > ion-exchange state > carbonate-bound fraction > humic-acid-bound fraction > iron and manganese oxidized bond fraction > strong organic bonding fraction > residue fraction. Pb2+, Zn2+, and Cd2+ provided by Pb(NO3)2, Zn(NO3)2·6H2O, and Cd(NO3)2·4H2O are in the ion exchange fraction. HMs whose bioavailability is greater than this form can be considered to have undergone a transition to the stable forms, and are referred to as the stable form, while the ion-exchange fraction and the water-soluble fraction are set as the unstable form.

Figure 3 shows the relative content of HMs in the S/S soil under a non-freeze–thaw condition in the unstable fraction, and the stable fraction. It is obvious that the content of Pb ion, Zn ion, and Cd ion supplied in the soil decreased sharped after the S/S by adding cement, lime, and fly ash. After S/S, the unstable forms of Pb, Zn, and Cd remain as only 1.19%, 1.09%, and 24.89%, respectively.

**Figure 3.** The relative content of heavy metals (HMs) in the solidified/stabilized (S/S) Pb–Zn–Cd composite HM-contaminated soil without F–T cycles.

However, Figure 3 also shows that the S/S efficiency varies for different HMs. The relative contents of the stable forms of Pb and Zn are much larger than that of the stable fractions of Cd. After the S/S process, 98.81% of lead and 98.91% of zinc were converted to a stable fraction, but only 75.10% of cadmium was converted to a stable fraction, with a relative content difference of more than 20%.
