Polycyclic aromatic hydrocarbons sorption on soils: some anomalous isotherms

Area of Science:

• Environmental Chemistry
• Soil Science
• Organic Geochemistry

Background:

• Polycyclic Aromatic Hydrocarbons (PAHs) are persistent organic pollutants found in soils.
• Understanding PAH sorption mechanisms is crucial for environmental risk assessment.
• Soil organic matter (SOM) significantly influences the fate and transport of PAHs.

Purpose of the Study:

• To investigate the sorption kinetics and equilibrium of four PAHs (acenaphthene, phenanthrene, anthracene, fluoranthene) on soil.
• To evaluate the influence of temperature and soil organic matter on PAH sorption.
• To compare the sorption behavior of individual PAHs versus a mixture.

Main Methods:

• Sorption experiments were conducted for individual PAHs and a mixture.
• Kinetic models (Elvoich, Lagergren, second order, double exponential) were used to analyze sorption rates.
• Sorption isotherms were determined across a temperature range (298–333 K).
• The role of soil organic matter (SOM) in PAH sorption was examined.

Main Results:

• Sorption kinetics exhibited a fast initial stage followed by a slower process.
• Anomalous sorption isotherms were observed for phenanthrene, anthracene, and fluoranthene at specific temperatures, attributed to SOM release and trapping.
• The anomalous behavior was mitigated when PAHs were studied as a mixture, with acenaphthene potentially binding released SOM.

Conclusions:

• PAH sorption on soil is a complex process influenced by kinetics, temperature, and interactions with soil organic matter.
• The 'trapping' mechanism by released SOM can lead to non-ideal sorption behavior for individual PAHs.
• Studying PAHs as mixtures provides a different perspective, highlighting the competitive sorption dynamics and the role of compound solubility.