Impacts of different freeze-thaw treatments on the adsorption and desorption behaviors of Cd in black soil

Seasonal freeze-thaw cycle (FTC) is one of the key processes that affect heavy metal behaviors in soil. However, previous studies are mainly focused on extreme FTC treatments which may exaggerate the real FTC effects in the field. This study aimed to compare the effects of different FTC conditions on the adsorption and desorption behaviors of Cd in the surface black soil. Different minimum freezing temperatures (- 2, - 5, and - 15 °C), FTC rates (1 and 20 °C h^-1), freezing lengths (2 and 24 h), and FTC frequencies (1, 3, and 9) were investigated. The thawing temperature was set at 5 °C. The amplitude for the FTC rate, length, and frequency experiments ranged from 5 to - 2 °C. Our results indicated that the adsorption amounts of Cd presented an order of - 2 °C > - 15 °C > - 5 °C and 24 h > 2 h for different FTC amplitude- and freezing length-treated soils, and the adsorption amounts decreased with increasing FTC rate and frequency. Soil maximum adsorption amount of Cd increased with the increases of FTC frequency, freezing length, and FTC rate, while it decreased with the decreases of freezing temperature. Soil Cd desorption ratio decreased with the increases of FTC frequency, freezing length, and TFC rate, and it increased with the increasing freezing temperature. Our results suggested that FTC conditions can significantly influence the adsorption and desorption behaviors of heavy metal in soil.