Freeze-thaw controlled aggregation mechanism of humic acid-coated goethite: Implications for organic carbon preservation

Abstract Organic carbon sequestered in soil aggregate contributes significantly to the carbon preservation on the earth’s surface. However, the turnover of soil aggregate in high latitudes and cold zones is strongly affected by the frequency of freeze–thaw cycles and may be disturbed by climate change. Although the effects of recurrent freeze–thaw cycles on the size, structure, and stability of soil aggregate have been well studied, the aggregation of individual mineral particles adsorbed by organic carbon and its mechanism are not yet conclusive. Here, we report that more than 97 wt% of the humic acid-coated goethite (goethite-HA) particles can aggregate into aggregates of different sizes and morphologies through one freeze–thaw cycle (freezing at −24 °C for 12 h and thawing at 4 °C for 12 h). Combining electron microscopy, infrared absorption spectroscopy and thermogravimetric analysis, we revealed that goethite-HA particles could strongly complex with each other after one freeze–thaw cycle. Compared with previous studies, our results showed that both electrostatic repulsion and steric hindrance (known to prevent aggregation) were overcome by the internal thrust of ice exclusion during freezing, then goethite-HA particles could be interconnected through the carboxyl-iron and hydrogen bonding to form aggregates. We believe that aggregates formed during the freeze–thaw cycles in soils rich in iron (hydr)oxides can serve as shelters for soil organic carbon and may reduce CO2 emissions in these soils.