The desorption characteristics of polycyclic aromatic hydrocarbons in Lanzhou loess soil under freeze-thaw action cycles

In order to study the migration characteristics of new pollutants Sixteen priority control polycyclic aromatic hydrocarbons (PAHs) during soil freeze-thaw cycles, a series of experiments were designed and operated using Lanzhou loess：the experiment results showed that freeze-thaw cycles significantly enhanced the desorption capacity of loess for both PAH mixtures and monomers, increasing their activity in soil. At desorption equilibrium, the desorption amount in the freezethaw group was 2.48 times greater than that in the non-freeze-thaw control group, with this effect being particularly significant before reaching equilibrium. During the isothermal desorption process, the desorption rate was positively correlated with PAH concentration, and the freeze-thaw group exhibited a more pronounced linear relationship between solid-phase and liquid-phase concentrations. The desorption hysteresis coefficient was significantly lower in the freezethaw group. For monomers with molecular weight≤FL, both the desorption rate and capacity gradually increased with the initial PAH concentration, while the desorption hysteresis correspondingly decreased. In contrast, monomers with higher molecular weight exhibited different trends. The desorption isotherms for the 16 PAH mixture were well-fitted by both the Freundlich and linear models for both groups. The Kd value for the freeze-thaw group was 65.48% lower than that of the non-freeze-thaw group, while the KF value was 91.05% lower, and the 1/n value increased by 25.64%, indicating reduced nonlinearity in desorption under freeze-thaw conditions. The Freundlich model demonstrated a better fit for higher molecular weight PAH monomers.