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 sorpti on in soil, which treat all soil organic matter (SOM) as having identical-s tructure, are unable to explain differences in organic carbon-normalized so rption coefficients (K-OC) among sorbents, isotherm nonlinearity, and sorpt ion-desorption hysteresis. This study relates one index of SOM composition, structural fragments quantified by pyrolysis-gas chromatography-mass spect rometry, to aqueous and supercritical carbon dioxide (SC CO2) sorption-deso rption 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 betwee n Freundlich n values in SC CO2 and hydrocarbon fragments with negative cor relation to N- and O-containing fragments suggests that energetic heterogen eity 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 desor ption of phenanthrene at 800 degrees C. The SC-CO2 extraction efficiency an d, to a lesser degree, the desorption response when methanol is added as a cosolvent indicate that polar functional groups play a role in retarding ph enanthrene desorption during SC CO2 extraction. Organic matter pyrolysis un der varying time and temperature conditions indicates that pyrolysis fragme nts that do not:significantly correlate with functional trends likely evolv e by a different pyrolytic mechanism and are generally poorly correlated wi th 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 t hat certain sorption parameters, notably Freundlich n values in aqueous sys tems, require even more detailed characterization of sorbent structures.