Sorption-desorption behavior of phenanthrene elucidated by pyrolysis- gas chromatography-mass spectrometry studies of soil organic matter

Commonly used partitioning models of hydrophobic organic contaminant sorption in soil, which treat all soil organic matter (SOM) as having identical structure, are unable to explain differences in organic carbon- normalized sorption coefficients (K(OC)) among sorbents, isotherm nonlinearity, and sorption-desorption hysteresis. This study relates one index of SOM composition, structural fragments quantified by pyrolysis-gas chromatography-mass spectrometry, to aqueous and supercritical carbon dioxide (SC CO₂) sorption-desorption parameters. Results show positive correlations between aqueous K(OC)s and hydrocarbon fragment peak areas and negative correlation to N- and O-containing peaks, which is consistent with hypotheses attributing sorption of phenanthrene to hydrophobic sorbent domains. Positive correlation between Freundlich n values in SC CO₂ and hydrocarbon fragments with negative correlation to N- and O-containing fragments suggests that energetic heterogeneity of polar environments controls nonlinearity in this solvent of limited polarity. Aqueous sorption-desorption hysteresis appears to be surpressed by N- and O-containing moieties and correlates with decreased thermal desorption of phenanthrene at 800°C. The SC CO₂ extraction efficiency and, to a lesser degree, the desorption response when methanol is added as a cosolvent indicate that polar functional groups play a role in retarding phenanthrene desorption during SC CO₂ extraction. Organic matter pyrolysis under varying time and temperature conditions indicates that pyrolysis fragments that do not significantly correlate with functional trends likely evolve by a different pyrolytic mechanism and are generally poorly correlated with sorption-desorption properties. The level of structural detail utilized in structure-function correlations in this work exceeds previous efforts to relate sorption behavior to sorbent structure. However, the work reveals that certain sorption parameters, notably Freundlich n values in aqueous systems, require even more detailed characterization of sorbent structures.