Freeze-thaw cycles lead to enhanced colloid-facilitated Pb transport in a Chernozem soil

Freeze-thaw cycles in soils lead to break up of soil aggregates and the formation of dispersible soil colloids. Leaching events following freeze-thaw cycles can therefore mobilize and transport colloids through the soil profile. Here, we investigated the effect of freeze-thaw cycles on the subsequent mobilization of colloids in a Pb contaminated soil, and we quantified the amount of colloid-facilitated Pb transport. Soil contaminated with Pb (250 mg/kg or 1000 mg/kg) was packed into 15 cm tall columns, and the soil water content adjusted to field capacity (0.306 kg/kg). Columns were subjected to freeze-thaw cycles of 12 h freezing at-20 degrees C followed by 12 h of thawing at 25 degrees C. Then, the soil columns were leached with distilled water, and the effluent was analyzed for colloids, soluble Pb, and colloidal-bound Pb. Freeze-thaw cycles were found to generate dispersible soil colloids and lead to colloid-facilitated Pb transport. Colloid and Pb mobilization increased with increasing number of freeze-thaw cycles. The majority (83-97%) of the Pb that leached out of the columns was colloid-bound. Our findings suggest that freeze-thaw cycles in high latitude areas can mobilize heavy metals, which are otherwise immobile, through colloid-facilitated transport. More frequent freeze-thaw cycles in high-latitude regions, as predicted by climate change models, thus increases the risk of metal leaching from contaminated soils and can lead to subsequent ground water pollution.

Automated summary

Freeze-thaw cycles lead to the mobilization of colloids and heavy metals in soil, increasing the risk of metal leaching and subsequent groundwater pollution. The amount of colloids and lead transported increases with the number of freeze-thaw cycles.