ABSTRACT

Natural organic matter (NOM) is a highly complex, polydisperse mixture of naturally occurring polyelectrolytic organic compounds whose average residence time in the hydrosphere is substantially longer than that of ordinary biomolecules such as carbohydrates and proteins [1]. NOM includes, but is not limited to, aquatic humic and fulvic acids, and much that is known about NOM is based on studies of those humic substances. The chemical and biological properties (rates and mechanisms of transformation, bioavailabilities, toxicities) of other natural and xenobiotic constituents of natural waters are often substantially affected by their interactions with NOM. Those interactions, in turn, are influenced by the physicochemical properties of NOM. Most distinctly, NOM is acidic. Its acidic functional groups contribute to the acid-base balance of natural waters, form complexes with trace metal cations of environmental and biological importance, and dictate its ligand-based adsorption to metal (hydr)oxides. In natural waters, the net average charge of NOM is determined by dissociation of acidic functional groups and complexation of other cations by those functional groups, and all of these reactions are controlled by pH and ionic strength. The role of ionic strength, in particular, has been difficult to parameterize properly, mainly because the actual distribution of charge (the real proportions of singly-, doubly-, and multiply-charged ions) in NOM is not known.