  The fate of organic pollutants in the environment is influenced by several
factors including the type and strength of their interactions with soil
components especially SOM. However, a molecular level answer to the question
How organic pollutants interact with SOM? is lacking. In order to explore
mechanisms of this interaction, we have developed a new SOM model followed by
carrying out molecular dynamics (MD) simulations in parallel with sorption
experiments. The new SOM model comprises free SOM functional groups (carboxylic
acid and naphthalene) as well as SOM cavities (with two different sizes),
representing the soil voids, containing the same SOM functional groups. To
examine the effect of the hydrophobicity on the interaction, the organic
pollutants hexachlorobenzene (HCB, non-polar) and sulfanilamide (SAA, polar)
were considered. The experimental and the theoretical outcomes explored four
major points regarding sorption of SAA and HCB on soil. 1. The interaction
depends on the SOM chemical composition more than the SOM content. 2. The
interaction causes a site-specific adsorption on the soil surfaces. 3. Sorption
hysteresis occurs, which can be explained by inclusion of these pollutants
inside soil voids. 4. It was observed that the hydrophobic HCB is adsorbed on
soil stronger than the hydrophilic SAA. Moreover, the theoretical results
showed that HCB forms stable complexes with all SOM models in the aqueous
solution while most of SAA complexes are accompanied by dissociation. The
SOM-cavity modeling showed a significant effect on binding of organic
pollutants to SOM.
