Freeze-induced solute segregation generates transient thaw-release pulses during transport

Abstract Freezing can transform an initially homogeneous aqueous solution into a compositionally heterogeneous source whose thawing releases solute nonuniformly. Here, we use sulfolane, a highly water-miscible industrial solvent and groundwater contaminant, as a model solute to examine how freeze-induced segregation can generate transient release pulses during transport. Freeze–thaw experiments showed that liquid thawed at −6.8 °C was strongly enriched in sulfolane relative to the initial solution, indicating preferential concentration of sulfolane in the residual liquid during water crystallization. Differential scanning calorimetry revealed distinct melting contributions from eutectic, non-eutectic freeze-concentrated, and water-rich fractions, consistent with the formation of compositionally distinct thaw-mobile domains. COnductor-like Screening MOdel for Realistic Solvents analysis further indicated that this segregation is better explained by water-selective freezing than by decreasing sulfolane compatibility with liquid water at low temperature. Based on these observations, we developed a reduced thaw–release and transport model in which early mobilization of a freeze-concentrated reservoir, followed by delayed dilution from water-rich melt, generates a rise-and-decay concentration pulse. A first-order mixing/storage term smooths the source signal, and downstream propagation calculations show delayed and attenuated transport in open-water and porous-medium pathways. The model is not intended as a calibrated field-prediction tool. Rather, it provides a phenomenological framework for linking freeze-induced phase segregation to transient source-term generation. Overall, the results support a physical-chemistry mechanism by which freeze–thaw cycling can impose short-duration concentration pulses on transport pathways, with implications for sulfolane mobility in cold-region systems.