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Radiotracer exchange studies on the reversibility of interaction processes related to humic-bound metal transport
Lippold, H.; Lippmann-Pipke, J.;
The mobility of actinides or other contaminants in the subsurface hydrosphere is considerably influenced by their interaction with natural colloids. Besides inorganic particles, aquatic humic substances are ubiquitous in natural waters, and their complexing ability can dominate the speciation of toxic or radioactive metals [1, 2]. Since humic carriers are subject to a solid-liquid distribution depending on geochemical parameters, an adequate assessment of migration processes needs thorough consideration of all interactions within the ternary system metal – humic substance – solid surface, including adsorption / retardation of humic colloids. Reactive transport models have been developed, taking all these processes into account [3-5]. As a prerequisite, reversibility is commonly assumed.

There is, however, a lack of clarity as to whether full reversibility is actually given for the whole ternary system, especially concerning interactions of humic matter with mineral surfaces and metals. For adsorption of humic substances, strong hysteresis has been observed (hardly any desorption upon dilution) [6-8], and recoveries in column experiments have been found to be far from complete [9, 10]. Regarding metal-humic interaction, it has been reported that complexation of higher-valent metals is accompanied by slow processes leading to an increase in complex inertness, i.e., a growing resistance towards dissociation in the presence of competing ligands or metals [11-13].

In view of these uncertainties, the aim of the present study was to elucidate the reversible / irreversible character of processes 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. In case of reversibility, a dynamic equilibrium exists, i.e., a permanent run of adsorption and desorption (or complex formation and dissociation) at equal rates. Such an exchange can be detected by introducing a radiotracer into pre-equilibrated systems where all binding sites are occupied.

The chosen model system for these experiments consisted of terbium(III) (as an analogue of trivalent actinides), humic acid (Aldrich) or fulvic acid (isolated from bog water), and kaolinite (KGa-1b standard material). 160Tb as a radioisotope was produced by neutron activation of 159Tb at the TRIGA Mark II reactor of the University of Mainz. Humic and fulvic acids were radiolabelled by an azo coupling reaction with [14C]aniline [14].

To investigate the dynamics of adsorption equilibria, kaolinite suspensions were first contacted with non-radioactive Tb(III) or humic / fulvic acid at a range of concentrations, covering an adsorption isotherm including the plateau region. Subsequent to a pre-equilibration phase, a small amount of the radiotracer (160Tb(III) or 14C-labelled humic / fulvic acid, respectively) was added. After admitting different time periods for equilibration, tracer exchange was evaluated from the concentration decrease in the supernatant. In additional batch experiments, desorption of humic and fulvic acid upon dilution was examined within a comparable time frame.

Reversibility of Tb(III)-humate complexation was investigated in a similar way. Since humic acid precipitates completely on loading with Tb(III), adsorption systems were generated. Here, times for tracer exchange were kept constant, and pre-equilibration times were varied instead.

As expected, adsorption of Tb(III) onto kaolinite was found to be a fast dynamic equilibrium process. Identical adsorption isotherms were obtained regardless of whether the radiotracer was introduced instantaneously together with the non-radioactive metal or subsequently after 2 days of pre-equilibration. For humic and fulvic acid, such dynamic exchange was proven to exist as well, but at considerably lower rates. In case of subsequent tracer addition, the plateau sections of the isotherms were significantly lowered (notably, not to zero). When equilibration times were increased (from 6 hours to 4 weeks), the plateaus approached the respective isotherm for instantaneous tracer addition. Finally, both isotherms coincided, i.e., the dynamic equilibrium was quantitatively represented by the tracer.

In desorption experiments with humic or fulvic acid, initiated by diluting the supernatant after an adsorption phase, no release was observed in the course of 4 weeks, which seems to be contradictory to the above results. One may conclude that the absence of desorption upon dilution is not necessarily indicative of a static equilibrium without any exchange. Thus, models for humic-bound transport are certainly applicable under appropriate conditions. Nonetheless, when comparing the kinetics of exchange to the kinetics of adsorption for humic / fulvic acid, rate constants differ by one order of magnitude [15]. This discrepancy must be taken into account when conditions of a steady local equilibrium are assigned to a maximum flow velocity.

For complexation of Tb(III) with humic acid, we did not find any indications of stabilisation processes affecting the reversibility. Increasing complex inertness has been observed for a variety of metals such as Al(III), Eu(III), Am(III), Th(IV) or U(VI), on time scales ranging from 2 days up to several months (see [13] for a review). In our tracer exchange experiments, Tb(III)-humate complexes were pre-equilibrated for 1 to 90 days before 160Tb(III) was added and a subsequent equilibration period of 1 day was admitted. In all cases, the binding isotherms were indistinguishable from the binding isotherm obtained for instantaneous tracer addition, i.e., an unresisted dynamic exchange was indicated by the tracer, even after long contact times prior to its introduction. Obviously, increasing complex inertness is not a general phenomenon occurring across all higher-valent metals.

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[14] A. Mansel and H. Kupsch, Appl. Radiat. Isot. 65, 793 (2007).
[15] H. Lippold et al., in preparation (2013).
  • Poster
    14th International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere (MIGRATION 2013), 08.-13.09.2013, Brighton, United Kingdom

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