Freeze-thaw impacts on physicochemical properties and mineral-organic components of various loess soils

Freeze-thaw cycles (FTCs) are a critical factor in the environmental changes of soils in cold regions, significantly impacting their structure and composition through physical stress and chemical migration resulting from water freezing and thawing. Currently, there is a lack of systematic research on the differences in the physicochemical properties and micro-mechanisms of various loess types during FTCs, which limits the theoretical basis for soil management and pollution prevention in cold regions. To address this knowledge gap, this study selected five typical loess types from Northwest China, including sierozem, gray-brown soil, loessial soil, black loess soil, and meadow soil. A novel freeze-thaw simulation was conducted over 10 cycles ranging from -10 °C to 10 °C, allowing for a systematic analysis of the effects of FTCs on soil pH, water content, bulk density, porosity, organic carbon content, cation exchange capacity, inorganic oxides, and heavy metal content. Advanced spectroscopic techniques, including Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), were employed to elucidate the micro-response mechanisms of soil minerals and organic components. The results indicated that FTCs caused a decrease in soil bulk density by 8.3-15.7% and an increase in porosity by 12.4-19.8%, with the most significant structural changes observed in meadow and loessial soils. Organic carbon content declined by 7.1-22.4% across the five soils, with the largest loss of 22.4% in black loess soil. XRD analysis revealed that the full width at half maximum of the 001 crystal plane diffraction peak of illite increased by 17-29%, indicating alterations in the interlayer structure of the mineral. The intensity ratio of the peaks at 1620 and 1030 cm^-1 in the FTIR spectrum decreased by 15-35%, suggesting that FTCs may disrupt the binding interactions of mineral-organic complexes. Concurrently, the leaching fractions of Cd and Pb increased by 10-25% in most soils, coupling heavy metal activation with organic carbon release. This study provides the first systematic elucidation of the synergistic evolution of physicochemical properties and microstructure in different loess types under FTCs, offering a critical scientific basis for soil quality assessment and engineering protection in cold regions.