Binding of Small Organic Molecules by Soluble Humic Substances

Abstract

Sorption on stationary surfaces and complexation with soluble or suspended species may have an opposite effect on the fate of small organic molecules. Transport, bioavailability and often, susceptibility to biotic degradation are retarded by sorption to stationary surfaces. Interaction with soluble macromolecules, on the other hand, increases the apparent solubility (i.e., the total concentration in the liquid phase) of small molecules and with it their mobility. Yet, the nature of the interaction of small molecules with macrospecies is likely to be independent of the size of the binding partner. This interaction is strongly affected by the properties of the binding site (which should be about as large as the interacting small molecule) and much less so by the whole macrospecies. The mechanisms and energetics of the interaction of a wide range of macrospecies with small organic molecules can thus be defined with the aid of the same few rules and described or quantified in a unified way. In contrast, steric factors, such as the extent of coiling of organic strands, will influence the accessibility and rate of approach of small molecules to binding sites. Hence, the dynamics of binding is related to the shape, size and sequence of the building blocks of the macrospecies. Soluble humic substances (HS) may have a larger impact on the fate of small organic molecules than any other group of the soluble macrospecies found in the environment. This is due to the high affinity between humic compounds indigenous to the environment and most xenobiotic organic molecules and to the pervasiveness of HS in aqueous systems. Below, the complexation of small organic molecules with soluble HS is examined. Both the basic similarity of the binding process in the various small molecule-macrospecies systems (regardless of the identity or size of the interacting partners) and the way the properties of a particular soluble HS influence its complexation with small organic molecules, are central themes of this review. The strong effect of environmental parameters such as pH and ionic strength and composition on the complexation is demonstrated.