  In-depth understanding of the sorption mechanisms of organic pollutants, like
the antibiotic sulfanilamide (SAA), in soil requires a combined experimental
and theoretical approach. Therefore, sorption experiments of SAA on
well-characterized samples of soil size-fractions were combined with the
modeling of SAA-soil-interaction via quantum chemical calculations. Freundlich
unit capacities were determined in batch experiments and it was found that they
increase with the soil organic matter (SOM) content according to the order fine
silt > medium silt > clay > whole soil > coarse silt > sand. The calculated
binding energies for mass-spectrometrically quantified sorption sites followed
the order ionic species > peptides > carbohydrates > phenols and lignin
monomers > lignin dimers > heterocyclic compounds > fatty acids > sterols >
aromatic compounds > lipids, alkanes, and alkenes. SAA forms H-bonds through
its different polar centers with polar SOM sorption sites. In contrast
dispersion and {\pi}-{\pi}-interactions predominate the interaction of the
sulfonamide aromatic ring with the non-polar moieties of SOM. Moreover, the
dipole moment, partial atomic charges, and molecular volume of the SOM sorption
sites are the main physical properties controlling the SAA-SOM-interaction. The
correlation between experimental and theoretical results was established by
reasonable estimates of the Freundlich unit capacities from the calculated
binding energies. Consequently, we suggest using this approach in forthcoming
studies to disclose the interactions of a wide range of organic pollutants with
SOM.

Chemical Physics