Cation-mediated cross-linking in natural organic matter: a review

Interactions between cations and natural organic matter (NOM) are central for the stability of organic matter, formation of supramolecular NOM structure, formation of organo-mineral associations, soil aggregation and binding of organic contaminants. The effect of multivalent cations on environmental functionalities of NOM strongly depends on the relative importance between intramolecular complexation and intermolecular cross-linking, the degree of which will be determined by the spatial arrangement of the hydrophilic functional groups in NOM. This literature review seeks to evaluate the current state of the art regarding the relevance of intermolecular cross-links via bridges of multivalent cations. Cross-linking has been suggested to explain among others aggregate stability, retarded dissolved organic matter release, reduced organic matter (OM) solubility as well as increase in degree and nonlinearity of sorption or organic chemicals to NOM. Although the cross-linking mechanism has been suggested in numerous studies, it has not yet been verified directly. The dynamics of the intermolecular cross-links, their persistence as well as their interplay with OM and their influence on stability and bioavailability of organic chemicals is up to now unknown. The major challenge in this context is the development of a suitable combination of experimental and instrumental techniques and relating the results to molecular and physicochemical models on the basis of targeted combination of spectroscopic, molecular modelling and thermoanalytical methods.