Humic material/substances metal chelation

The mesh structure formed by humic substances is capable of trapping smaller chemical species. For example, minor amounts of acyclic alkanes are found in most samples of humic and fulvic acids, and some of the fatty acids associated with humics may be similarly trapped components rather than bonded to the macromolecular backbone. Humic substances also usually contain a variety of metals, which are incorporated into the macromolecular structure. Metal ions can be surrounded by and bonded to suitable chelating groups, chiefly carboxylic acids, on humic molecules that stabilize the ions and allow them to be transported with the organic material. This important property of humic substances is examined again later (Section 7.6.5) in relation to the environmental fate of heavy metals. 

The most important humic substances in water are the lower-molecular-mass fulvic acids. These species tend to chelate Fe " ion, producing a yellow material called gelbstoffe (German for yellow stuff), which is responsible for much of the undesirable color found in some water. Metal ions bound with fulvic acid are hard to remove from water and, since iron is a very undesirable water impurity, drastic measures such as destruction of the fulvic acid with chlorine may be required to remove the chelated iron. Figure 3.14 illustrates a typical fulvic acid molecule. 

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