A Donnan model in combination with a site-binding model is applied to
interpret proton binding to humic substances as a function of pH at di
fferent salt levels. The Donnan model accounts for the salt effect on
the charging behavior; it can only be successfully applied if it is as
sumed that the specific volume of humic substances is strongly depende
nt on the ionic strength. Empirical relationships have been derived th
at relate the volume of the Donnan phase with the ionic strength. For
fulvic acid,the Donnan approach is physically not very realistic. The
approach presumably mimics the behavior of the diffuse double layer ar
ound the relatively small fulvic acid molecules. For humic acids, howe
ver, the approach is physically realistic. The Donnan approach is a re
latively simple and numerically efficient technique to account for the
effects of ionic strength on the charging behavior. The ionic composi
tion of the gel phase follows directly from the technique. The insight
obtained on the swelling behavior of the humic acids may be of use fo
r interpreting binding of organic pollutants in natural systems.