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

Abstract

Heavy metals (HMs) exist in nature in different forms, and the more unstable the form of an HM, the higher its toxicity and bioavailability. The content of HMs in stable fractions can increase significantly through the stabilization/solidification (S/S) technology. Still, external environments such as freeze–thaw (F–T) cycles will affect the stability of HMs directly. Therefore, a long-term F–T study of S/S Pb–Zn–Cd composite HM-contaminated soil was conducted under six conditions (0, 3, 7, 14, 30, and 90 cycles) with each F–T cycle process up to 24 h. The improved Tessier method was employed, and the results show that the S/S technology makes HMs transform to a more stable fraction. Still, the transformation efficiency is different for each HM. More than 98% of lead and zinc were converted to stable forms, while for cadmium, there are only 75.1%. Meanwhile, the S/S HMs were rapidly transformed into unstable forms at 0–14 cycles, but after 14 cycles, the transformation speed was significantly reduced. Among stable forms, it is mainly that the carbonate-bound fraction of HMs changes to unstable forms, and the characteristic peaks of carbonate stretching vibration were found at 874 cm−1, and 1420 cm−1 by Fourier infrared spectroscopy proves the presence of carbonate-bound substances. As a result of this study, the change trend of contaminated soil with S/S HMs under the effect of long-term F–T cycle was revealed, and the crisis point of pollution prevention and control was found, which provides some theoretical basis for the safety of soil remediation project.