Colloidal properties of humic substances

Humic substances are structurally complex large to macromolecules which occur in soils and natural waters as a consequence of the breakdown of plant and animal residues by microbial activity. A major portion (approx. 50%) of the earthʹs carbon is in the form of humic materials (fulvic and humic acids). The characterisation of humic substances is a major problem due to their heterogeneity both in terms of structure and size and to their tendency to associate in solutions as their concentration increases. Methods of characterisation of humic materials are reviewed and their interactions with substances such as clay particles, herbicides, pesticides and metallic ions which occur in the natural environment considered. The experimental methods of investigating the binding of metallic ions are also reviewed. There have been several approaches to the molecular modelling of metal ion binding based on the representation of humic molecules as colloidal particles with an associated electrical double layer. The various theoretical models have been described and the relative merits of the approaches compared.