Effect of freeze–thawing cycles on aging behavior of phenanthrene, pyrene and their mixture in soil

• Competitive effect on PAHs' extraction was studied.
• The validity of the conceptual freeze–thawing model was confirmed.
• Freeze–thawing and competitive effect play similar role on chemicals' aging.
• More attention should be taken into those newly contaminated soil sites.

This work was initiated to study the competitive sorption effect on phenanthrene and pyrene extraction during the aging process in phaeozem, burozem, aquorizem and krasnozem with or without freeze–thawing cycles. Soils contaminated with 100 μg g− 1 phenanthrene and 100 μg g− 1 pyrene separately and combined were extracted by 10 g L− 1 surfactant SDBS solution at various times over 120 days. The competitive effect on extraction efficiency may either increase or decrease with increasing soil contact time, depending on the properties of the accessible adsorption sites. The increased difference in extraction efficiency change has a positive correlation with soil organic carbon content. The change in extraction efficiency between no freeze–thawing and freeze–thawing in soils contaminated with both hydrocarbons was smaller compared to it with phenanthrene or pyrene alone due to the similar roles freeze–thawing and competitive effect plays, causing contaminant molecules to occupy the high-energy adsorption sites and expanding the glassy domain of soil organic matter. No general conclusions were obtained among the frequency of freeze–thawing cycles, soil moisture and extraction efficiency. This study validates our previous conceptual freeze–thawing model and is expected to help the development of the environmental fate and risk assessment.