Soil Freeze–Thaw-Induced Aggregate Structural Transformation Promotes Colloidal Cd Mobilization from Macroaggregates

Freeze–thaw cycles (FTCs) facilitate the mobilization of colloid-bound cadmium (Cd) in mid- to high-latitude soils, thereby increasing the environmental risks of Cd. However, the contribution of different soil aggregate size fractions to colloidal Cd mobilization remains poorly understood, which hinders the development of targeted remediation strategies. Using asymmetric flow field-flow fractionation and 111 Cd isotope tracing, this study demonstrated that 83–89% of colloidal Cd released during FTCs was bound to 100 nm–1 μm organo–clinochlore composite colloids, with 64.1%, 33.2%, and 2.7% of this Cd fraction originating from macroaggregates, microaggregates, and the fine fraction, respectively. Microcomputed tomography showed that macroaggregates contained more abundant and larger pores and pore throats, with pore-throat numbers 6.8 and 41.5 times higher than those in microaggregates and the fine fraction, respectively. Such porous structures enable freezable water inside macroaggregates to freeze readily during FTCs, whereas fine fractions mainly store bound water with depressed freezing points that barely freeze. This stark discrepancy renders macroaggregates susceptible to FTC-induced disruption and further amplifies colloidal Cd release. These findings highlight that limiting colloidal Cd release from macroaggregates is critical for reducing Cd environmental risk and stabilizing Cd in cold-region soils, particularly during the FTC period.