Radiotracer exchange studies for gaining direct insight into the equilibrium characteristics of elementary processes determining humic-bound metal transport

The mobility of toxic or radioactive contaminant metals in the subsurface hydrosphere can be essentially governed by their interaction with humic colloids. Predictions on migration processes need thorough consideration of kinetic aspects in the ternary system metal / humic substance / mineral surface. In reactive transport models, reversibility is commonly presumed. However, for adsorption of humic matter, strong hysteresis is observed (hardly any desorption upon dilution), and recoveries in column experiments are far from complete. Complexation of higher-valent metals with humic substances is accompanied by slow processes leading to an increase in complex inertness, i.e., a growing resistance to dissociation.
In view of these uncertainties, the aim of our studies was to elucidate the reversible / irreversible character of interactions controlling humic-bound transport. For this purpose, the principle of tracer exchange was employed to gain insight into the dynamics of equilibria within the ternary system. A radioactive probe, introduced as a reactant into pre-equilibrated systems, will represent the overall equilibrium if there is a dynamic exchange. In case of a static equilibrium, the tracer will not get involved. The chosen model system for these experiments consisted of terbium(III) (as an analogue of trivalent actinides), humic acid and kaolinite. ¹⁶⁰Tb as a radioisotope was produced by neutron activation of ¹⁵⁹Tb. Humic acid was radiolabelled by azo coupling with [¹⁴C]aniline.
After introducing trace amounts of [¹⁴C]humic acid into pre-equilibrated adsorption systems of kaolinite and non-labelled humic acid in the state of surface saturation, quantitative exchange was found to take place. Evidently, adsorption equilibria of humic colloids are not static, notwithstanding their size and multiple bonding, albeit an exchange time of more than 4 weeks was required. Isotope exchange of ¹⁵⁹Tb / ¹⁶⁰Tb on saturated humic acid was completed within a very short time frame, independently of the time of pre-equilibration ¹⁵⁹Tb / humic acid. However, if the tracer ¹⁶⁰Tb was introduced prior to saturation with ¹⁵⁹Tb, the expected partial desorption of ¹⁶⁰Tb occurred at much lower rates, decreasing with time of pre-equilibration. Inertisation phenomena are thus confined to the strongest sites of humic molecules. Analysing the time-dependent course of isotope exchange according to first-order kinetics indicated that up to 3 years are needed to attain equilibrium.