Electrostatic discrete functional group models

Electrostatic discrete functional group models. The development of charge on the surface of the humic macromolecule decreases the tendency to dissociate protons from the acid functional groups. To overcome this problem an electrostatic correction factor is introduced into the acid dissociation and complexation constants. This is similar to the approach adopted for the SCMs for inorganic surfaces. 

The model of Tipping et al. (1988) is an example of an electrostatic discrete functional group model. The effects of variable solution ionic strength and pH on the apparent surface acidity constants (polyelectrolyte effects) are accounted for by the incorporation of an electrostatic term exp(—2wzZ) in the equilibrium constants. A brief description of the model is given below. 

The apparent equilibrium constants for two different carboxylic surface sites are written as 

Typical values of pK[nt and pfor a humic acid are 2.67 and 4.46. The introduction of the electrostatic factor into the equilibrium constant is analogous to the coulombic term used in the definition of the intrinsic surface complexation constants. In addition another binding site (WAH) is recognised which is thought to behave as a weak acidic phenolic functional group. Although this site does not contribute to the titratable acidity and, therefore, no pK is needed for proton dissociation, it is involved in metal complexation reactions. The total number of the three monoprotic sites is estimated from titratable acidity and then paired to represent the humic substance as a discrete non-interacting mixture of three dipro-tic acids, which act as the metal complexation sites. The three sites are 

Complexations with metal complexing cations such as aluminium are included by writing the complexations reaction of the metal with a diprotic complexation site, e.g. 

---