The Role of Electron Donor-Acceptor Complexes in Organo-Mineral Interactions

A long tradition divides the organic materials that occur in terrestrial environments into hydrophilic and hydrophobic compounds. Often, inference about natural organic matter (NOM) retention and/or contaminant behavior is largely based on these qualifications, assigning weak bonding/retention mechanisms to hydrophobic moieties and assuming energetically stronger interactions between hydrophilic substances and other, charged or hydroxylated surfaces.

This contribution reviews the significance of electron-donor acceptor (EDA) complexes between natural or xenobiotic organic compounds and phyllosilicates as retention mechanisms in soils and subsurface environments. A large fraction of organic compounds present in these environments carry electron-deficient aromatic structures that may serve as π-acceptors. Prominent examples are quinone structures in NOM as well as notorious and emerging contaminants such as nitroaromatic compounds (NACs) and fullerenes (e.g. C₆₀), respectively. The high affinity of NACs for layer silicates has long been attributed to n-π EDA complexation of the electron-deficient π-system with siloxane oxygens. More recently, however, the importance of EDA interactions as a relevant mechanism for NAC retention in soils and sediments has become a matter of debate. New experimental approaches and computational studies indicate that, particularly for phyllosilicates with a low layer charge (montmorillonite), other mechanisms such as specific NAC-cation interactions and non-specific dispersive forces play the predominant role in the retention of NACs.

We combine these new results with existing knowledge of the dominant adsorption processes and provide a mechanistic definition and evaluation of the proposed EDA complex formation between organic compounds and phyllosilicate surfaces. Explicit conclusions on both the existence and the potential contribution of specific EDA complexes for NAC stabilization cannot be drawn based on current evidence. Further research is needed to investigate the nature of NAC adsorption as well as the capability of organic electron-acceptors other than NAC to undergo EDA interactions with mineral surfaces.