Proton and metal ion binding to humic substances

Humic substances are polydisperse mixtures of organic molecules which at least to some extent determine the mobility and bioavailability of heavy metals in soils, sediments and aquatic systems. In order to make a sound risk assessment of the fate of trace metals a good conception and preferably a sound description is essential. In this thesis mechanistic models are presented that explicitely take into account the dominant factors that determine metal ion binding. These factors are the chemical heterogeneity of the humic substances, the variable charge character, and competitive binding of ions.The description of the proton binding behaviour, in absence of metal ions, forms the basis of the metal ion binding model. The proton binding is described with analytical expressions for continuous heterogeneous ligands in combination with a double layer model to account for the electrostatic effects. The parameters for the proton description are obtained from the analysis of proton titration with the so called mastercurve procedure.In order to describe metal ion binding an approximate binding stoichiometry is assumed, in which upon the binding of one metal ion, x protons are released in solution. With respect to site competition two different limiting cases have been considered. In the fully coupled case it is assumed that the different ions bind to the same sites and that the shape of the affinity distribution is the same for all ions. In the uncoupled case each ion has its own binding sites and the affinity distribution may differ for different ions. Both models are capable of describing competitive ion binding. The uncoupled model has the advantage of a lower number of parameters that have to be specified