Humic substances carboxylate binding site

Humic substances are widely recognised to strongly bind to iron and to influence the redox transformations of iron. These substances have been variously reported to accelerate, retard or have no effect on the rate of Fe(II) oxygenation under a variety of pH and dissolved oxygen conditions [82-87]. However, at typical environmental pH, recent studies have found that DOM in natural waters generally accelerates the oxidation reaction [5,88], which has been attributed in part to the high density of carboxylate binding sites in many humic substances [89]. 

Acid-base titrations of humic substances reflect the nature of the different p/Tfl values, hence the smeared out appearance of these titration curves. While no unique equivalence points are observed, different p regions of carboxylic and phenolic groups can be discerned. Similarly, in metal titrations, metal ions are bound differently by the different ligand groups. The extent of metal-ion binding depends on the ratio of metal ions to humic substances, [M7]/ [L7-]. In titrating humic or fiilvic acids with metal ions (at fixed pH), the metal is bound first to the highest affinity sites. 

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