Humic acid sample was separated from the bottom sediments of Lake Quarun, in Egypt. It was purified and characterized by elemental
analysis, potentiometric titration, IR, UV-visible and 13C NMR spectroscopies. The product of humic acid was very low (0.009%), gray in color and has low carboxylate capacity (2.4
meq/g). The first derivative of the titration curve indicated one maximum only, which implies one kind of carboxylate groups.
The binding constant of 234Th with Lake Quarun humic acid was determined by solvent extraction. Only one parameter, β1, was required to fit the binding as a function of carboxylate concentration: the Th4+ bound to the carboxylate sites in the gray humic acid forming 1:1 complex only. The binding constant increased with the degree
of ionization and with the pKa of the humic acid.