Zinc leachability in contaminated soil stabilized/solidified by cement-soda residue under freeze-thaw cycles

The effectiveness of stabilization/solidification (S/S) method for the remediation of heavy metal contaminated soil is challenged by suffering freeze-thaw (F-T) cycles under long-term conditions. On this basis, the current paper performed semi-dynamic leaching test to assess the long-term stability of zinc contaminated soil treated with cement-soda residue under F-T cycles. Sequential F-T cycles increased the cumulative fraction of leached (CFL) Zn2+ by 4.4% and 9.4% for sample Zn0.05 and Zn1.0, respectively, but had minimal impact on the pH value of leached solution. After experiencing 10 F-T cycles, the effective diffusion coefficient (De) of Zn2+ increased from 1.6 × 10−11 to 1.37 × 10−9 cm2/s for sample Zn0.05, and 1.5 × 10−9 to 1.1 × 10−8 cm2/s for sample Zn1.0. The environmental risk of the treated soils after subjecting to F-T cycles would also be enhanced, which was predicted by the variations of the leachability index (LX). Zn2+ leachability in the specimen with high initial Zn2+ concentration was initially controlled by diffusion, and that with low concentration was dissolution. Over the long period of leaching, surface wash-off became dominant. Microstructural analysis based on X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) tests highlighted the absence of chemical changes in the mineral compositions, as well as the redistribution of the soil particles and pore sizes in the specimen. •Freezing-thawing cycles had detrimental impacts on the stability of Zn2+ in solidified/stabilized soil.•Zn2+ leachability was closely associated with the variations of pH value in the leached solution.•The leaching mechanism of Zn2+ changed over leaching time and initial Zn2+ concentrations during freezing-thawing cycles.•Deteriorations induced by freezing-thawing cycles is predominantly involved in physical processes.