MACROMOLECULAR PROPERTIES OF SOIL HUMIC SUBSTANCES: FACT, FICTION, AND OPINION

Abstract

A critical review of 'random coil' and 'self-associating aggregate' models and of earlier concepts of the macromolecular structure of humic substances (HS) is undertaken as a result of the development of more recent information and hypotheses. Initially, the modes of formation of HS are considered and the influence that these might have on molecular properties is discussed. This is followed by an analysis of the strengths and weaknesses of size-exclusion chromatography (SEC) techniques that is central to the studies under consideration. Emphasis is placed on the importance of overcoming interferences, attributable largely to charge and sorption effects, in order to achieve meaningful applications of SEC. An overview is given of the results obtained by SEC and ultracentrifugation studies of a carefully fractionated collection of humic acid samples. The development of the proposal of the flexible, random-coil model for the macromolecular conformation of HS in solution based on this work is described. This is followed by a review of the evidence put forward in support of the 'self-associating aggregate' hypothesis of molecular structure, and emphasis is placed on areas of concern that question the validity of this model. Aspects of the work that require investigation and verification are identified, and proposals are made for the further studies required.