Abstract: Freeze-thawing is an important process in northern cold region of China, and it is an important factor affecting the properties of soil and materials for stabilizing heavy metals. In this study, a test of freeze-thaw cycles was carried out to examine the effects of freeze-thaw on the transformations of cadmium (Cd) form during the remediation of biochar for contaminated soils, with the underlying mechanism being analyzed. The results showed that, in the remediation process of biochar, after 16 cycles of freezing and thawing, the contents of weak acid extracted form Cd, reducible form Cd, and oxidizable form Cd in soil increased by 16.07%, 6.67%, and 15.37%, respectively, while the residue content remained stable. On the whole, the increasing content of available Cd will pose serious ecological risks. Two major reasons were responsible for such alterations. Firstly, without biochar addition, the content of weak acid extracted form Cd and reducible form Cd in soil increased by 11.74% and 13.53%, respectively. Soil capillary porosity increased from 41.72% to 57.63%, soil pH decreased slightly, and the content of soil dissolved organic carbon increased significantly by 15.38% firstly and then decreased by 10.17% over time. The combined effects of the abovementioned factors increased weak acid extracted and reducible form Cd in soil Cd system. Secondly, during the freeze-thaw process, the tubular structure and integrity of biochar were destroyed, the stretching vibrational absorptions of hydroxyl-OH, carbon-hydrogen bond C-H, and aromatic composition were weakened and shifted, and the carbonate composition were decreased, which resulted in the decrease of adsorption sites and adsorption capacity of biochar for Cd.

Key words: biochar, freeze-thaw, cadmium speciation, stabilization remediation.