Schulten and colleagues developed a polymeric SOM model, based on bio-and geochemical, NMR-spectroscopic and mass spectrometric analyses . It consisted of a network of long-chain alkyl structures interspersed with aromatic rings . Subsequent refinements incorporated carbohydrate and protein units . It had the advantage of providing a multitude of interaction sites and functionalities, to represent an average HS molecular structure. This enabled the study of the adsorption of xenobiotic compounds onto HS . However, the idea that SOM is composed of high-weight molecular species is now considered obsolete, even though the models do not imply the existence of these specific structures .