Molecular-level interactions in soils and sediments: the role of aromatic pi-systems

This review intends to deepen our understanding of mechanisms by which molecules with aromatic moieties attach to organic and mineral components of terrestrial environments. We present published evidence for the existence of specific, sorptive interactions of aromatic moieties with environmental sorbents. We find that aromatic pi-systems within organic compounds have the capacity to adsorb to minerals and organic soil and sediment components such as natural organic matter (NOM) and fire-derived black carbon (BC) through specific sorptive forces other than hydrophobic interactions. Polar interactions of aromatic pi-donor and -acceptor compounds show adsorption energies between 4 and 167 kJ mol(-1). Bonding strengths of cation-pi interactions and pi-pi electron donor-acceptor (EDA) interactions appear to be larger than H bonding strengths and comparable to inner- and outer-sphere complex formation. We conclude that, in analogy to polar and ionizable functional groups, components with aromatic pi-donor and -acceptor systems equip organic molecules with a substantial sorptive potential. This observation has important implications for the fate and transport of aromatic contaminants. The resulting sorptive interactions might also play a yet-overlooked functional role in the complex chain of processes which preserve NOM against decomposition.