Dynamics of metal-humate complexation equilibria as revealed by isotope exchange studies – a matter of concentration and time

Complexation with dissolved humic matter can be crucial in controlling the mobility of contaminant metals. For speciation and transport modeling, a dynamic equilibrium process is commonly assumed, where association and dissociation run permanently. This is, however, questionable in view of reported observations of a growing resistance to dissociation over time. In this study, the isotope exchange principle was employed to gain direct insight into the dynamics of the complexation equilibrium, including kinetic stabilization phenomena. Terbium(III) was used as an analogue of trivalent actinides. Isotherms of binding to humic acid, determined by means of 160Tb as a radiotracer, were found to be identical regardless of whether the radioisotope was introduced together with the bulk of stable 159Tb or subsequently after pre-equilibration for up to 3 months. The existence of a dynamic equilibrium was thus evidenced since all available binding sites are occupied in the plateau region of the isotherm. If the small amount of 160Tb was introduced prior to saturation with 159Tb, the expected partial desorption of 160Tb occurred at much lower rates than those observed for the equilibration process in the reverse procedure. In addition, the rates showed a distinct dependence on the time of pre-equilibration. Obviously, stabilization phenomena are confined to the most reactive sites of humic molecules. Analyzing the time-dependent course of isotope exchange according to first-order kinetics indicated that up to 2 years are needed to attain equilibrium.