Molecular and microscale-level insights into the potential mobilization of toxic elements in Pb/Zn smelting contaminated soils under freeze-thaw cycles.

Freeze-thaw (F-T) cycles have significant influences on the mobilization behavior of potentially toxic elements (PTEs) in soils. However, the interactive effects of soil minerals and dissolved organic matter (DOM) on PTEs mobilization under F-T cycles remain underexplored. Therefore, this study aimed to investigated the mobilization behavior of PTEs and the composition characteristics of DOM in soils through simulated F-T cycles experiments, using fluorescence and mass spectroscopy characterization. The results indicated that the mobilization of As, Cd, Cu, Pb and Zn could be driven by F-T cycles in soils, due to the transformation of their speciation. Among molecular formulas, DOM were dominated by CHO and CHOS, which decreased by 1.38 -3.00 % and 4.03 -7.81 % after F-T cycles, respectively. Among DOM compounds, lignin was the most abundant in soils, which increased by 5.13 -5.39 % after F-T cycles. These results suggested that F-T cycles have significant impacts on the molecular transformation of DOM in soils. In addition, DOM could interact with reactive Fe, Al and Ca bearing minerals to form Al, Ca, Fe-OM associations. Further evidence indicated that DOM composition and its interaction with reactive minerals were the key factors affecting PTEs mobilization driven by F-T cycles in soils. Overall, these findings deepened the mechanistic understanding of PTEs mobilization in soils under F-T cycles, which had significant implications for the risk management and remediation of contaminated soils.