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Predicting long-term actinide mobility

Nanoscopic approaches to aqautic plutonium chemistry

Thomas Fanghänel of the Institut fur Nukleare Entsorgung, Karlsrühe Research Center, Germany, makes a point during his discussion of nanoscopic approaches to aquatic plutonium chemistry.

The Institut fur Nukleare Entsorgung (INE) in Karlsrühe Research Center, Germany, focuses on the basic research relevant to the assessment of prospective repositories for radioactive waste, including both technical and scientific chemical aspects. The predictive modeling of long-term actinide mobility in the geosphere is contingent on basic knowledge of the aquatic chemistry of actinides. In this context, plutonium is of special interest.

Plutonium appears in spent fuel in small amounts (about 1 percent), but after the decay of short-lived fission products, plutonium-239 represents the dominant radioactive inventory for thousands of years.

On the one hand, the solubility constraints of plutonium have led to the perception that this element will be immobilized easily in the repository environment. On the other hand, this particular property of low solubility induces the formation of colloids (tiny, nano-sized particles-either real colloids or pseudo colloids), which may strongly enhance plutonium's potential for mobility in the aquifer. These characteristic properties are of cardinal importance for our work.

Challenges

Different oxidation states (typically IIIÐVI) of plutonium can coexist in aqueous solution under the appropriate conditions, with the relative abundance of each oxidation state depending on the chemical conditions such as pH (acidity or alkalinity), Eh, and ionic strength. Plutonium exhibits a complicated redox behavior that permits transformation of one oxidation state into another. For example, the collision of two plutonium(IV) ions generates one plutonium(III) and one plutonium(V) ion, which subsequently can be oxidized to a plutonium(VI) ion by an additional collision with a plutonium(IV) ion.

Since different oxidation states exhibit different chemical properties, a reliable speciation (with regard to the oxidation states) is required. Our work focuses on the tetravalent plutonium ion, which, in aquatic solution, is known to be unstable even at low pH in dilute concentrations, not only because of disproportionation

This instability of dilute plutonium(IV) has made the appraisal of its thermodynamic solubility in aquatic systems extremely difficult, resulting in a large number of controversial results. Thus, assessing the chemical reactions of plutonium(IV) in dilute concentrations in the low pH region requires sensitive speciation and colloid characterization methods.

Our present work combines two different laser spectroscopic speciation approaches, providing the possibility of assessing the chemical reactions of plutonium(IV) at concentrations of only a few micromoles per liter or even lower.

Researchers from the Karlsrühe Research Center Institut fur Nukleare Entsorgung are using a "homebuilt" laser setup to study the colloidal transport of plutonium in aqueous solutions. In the front row, from left to right, are: Claudia Bitea and Alice Seibert. In the back row, from left to right, are: Christian Marquardt, Clemens Walther, and Jong Il Yun.

Experimental Approaches

We use two methods in our research:laser-induced breakdown detection (LIBD) for colloid quantification and laser-induced photoacoustic spectroscopy (LPAS) for chemical speciation.

LIBD is based on nonlinear interaction of colloids with a tightly focused laser beam (see sidebar). This leads to the formation of a hot, dense plasma, detected either optically (via light emission) or acoustically (via its expansion-generation shockwave). Each plasma event corresponds to a single colloid and, when compared with the number of laser shots, this provides a measure of colloid concentration.

Compared with dynamic light scattering (the most frequently applied colloid detection technique), the sensitivity, particularly for colloids smaller than 50 nanometers, is enhanced by more than six orders of magnitude. Compared with the so-called single-particle counter, a static light-scattering device, the accessible size range is considerably extended toward smaller colloids.

For particles of 20-nanometer diameter, for instance, the sensitivity corresponds to detecting a pinhead one-millimeter in diameter in a good-sized hotel swimming pool.

The speciation methods are based on linear light absorption. Plutonium's oxidation states III, IV, V, and VI exhibit characteristic absorption bands. The absorption strength is directly proportional to the amount of species present. From the (visible) absorption spectra, a quantitative speciation is obtained, typically by UV-visible (UV-VIS) spectroscopy, which measures the extinction (i.e., absorption plus scattering losses) of white light wavelength resolved after passing through the sample.

A time evolution of laser-induced plasma plumes incited on single colloids as observed by an ultrafast (200 picoseconds) charge-coupled device camera.

However, this method is limited to measuring plutonium(IV) concentrations of approximately 10 micromolar. LPAS lowers this limit by a factor of up to 100 by detecting the effects of the light absorbed by the plutonium ions (photothermal method) rather than the transmitted light. A 10-nanosecond light pulse of a tunable dye-laser is guided through the sample, and the energy of the absorbed photons gives rise to a rapid temperature increase with subsequent expansion and generation of an acoustic shock wave.

Analog to the LIBD method, this acoustic wave is detected by a piezo detector, but because its energy content is rather low (less than one nanojoule), the signal is electronically amplified prior to data recording. The acoustic (and electronic) signal is linearly proportional to the absorption strength at the specific wavelength of the laser beam. Absorption spectra are obtained by scanning the laser over the desired spectral range. In addition to the increased sensitivity down to the micromole-per-liter range, LPAS (in contrast to UV-VIS) is not influenced by the presence of colloids, which give rise to light scattering in the blue end of the spectrum.A plutonium-242 solution of pure oxidation state (IV) was prepared by electrochemical reduction in a 0.5-molar hydrochloric acid to plutonium(III), followed by careful re-oxidation back to plutonium(IV). The oxidation state was monitored by UV spectroscopy and the plutonium concentration was determined by liquid scintillation spectroscopy. The pH was increased with an associated decrease in plutonium concentration by very slow, stepwise dilution with 0.5-molar sodium chloride solution. The pH adjustment ultimately led to the formation of plutonium(IV) colloid kernels, as observed by a well-defined, sharp increase of the LIBD signal. Such colloid formation is the most sensitive indication that the solubility limit has been exceeded.

Macroscopically, the solubility limit is defined as the concentration at which the total amount of a substance is no longer present in ionic form but instead forms a precipitate. Here, our precipitate takes the form of colloids, which are so small that they remain suspended because of Brownian motion. Analogue experiments were conducted at different concentrations of the plutonium-stem solution. With decreasing plutonium(IV) concentration, the pH of colloid kernel formation increased.

Evaluation

Depending on pH, the tetravalent plutonium ion is more or less hydrolyzed, meaning that it is surrounded by up to four hydroxide (OHÐ) molecules. Only in very acidic conditions (a pH less than zero), does a non-hydrolyzed Pu4+ aquo-ion exist in solution. The hydrolysis does not change the spectral properties of Pu4+, and so it cannot be detected directly by LPAS. But from the crossing-points of colloid formation (log of plutonium(IV) concentration versus pH), we obtain the solubility curve with slope = Ð2 indicating that the plutonium(IV) dihydroxo complex Pu(OH)22+ represents the dominant species.

Laser-induced photoacoustic spectroscopy (LPAS) detects oxidation state of plutonium ions in solution by means of light absorption. This figure shows plutonium(IV) undergoing disproportionation with time. The signal on the left at 470 nanometer (nm) is due to the electronic absorption of plutonium(IV). This signal decreases with time, while the plutonium(III) signal at 605nm increases by the same amount.

This mononuclear plutonium species undergoes further colloid formation and is thus in equilibrium with colloids. Knowing the hydrolysis constant of Pu(OH)22+ from the literature, the solubility product (log Ksp = -59 at zero ionic strength) of plutonium colloids (presumably oxy-hydrate) can be derived from our data.

Since the amount of plutonium(IV) colloids is relatively small at a given pH of colloid kernel formation, there remains a considerable quantity of (hydrolyzed) plutonium(IV) ionic species undergoing disproportionation with time. This reaction is directly observed by LPAS for plutonium(IV) and (III), at a plutonium concentration within the region of colloid formation above the solubility curve.

The disproportionation reaction can be written as 3Pu(IV) + 2H2O D 2Pu(III) + Pu(VI) + 4H+, which requires that, for three consumed plutonium(IV) ions, two plutonium(III) ions and one plutonium(VI) ion must be formed. The ratio of plutonium(IV) decrease and plutonium(III) / plutonium(VI) increase should scale as 3:2:1.

Our LPAS measurement however, corresponds to a ratio of 1, indicating a decrease of plutonium(IV) that is too low by at least 50 percent.

To explain this discrepancy, we must keep in mind that plutonium(IV) colloids are in equilibrium with the plutonium(IV) ionic species, and that the plutonium(IV) tied up in colloids is spectroscopically invisible in the LPAS experiment.

Over the course of 70 hours, as plutonium(IV) ionic species are consumed by the disproportionation reaction, the fraction of (spectroscopically invisible) plutonium(IV) present as colloids redissolved, and this addition to the spectroscopically observable plutonium(IV) fraction was sufficient to account for the discrepancy. Once this was taken into account, the true decrease of plutonium(IV) could be calculated, and satisfied the required 3:2 ratio of the reaction. This was confirmed by LIBD measurements, where the colloid dissolution with time was observed directly.

At lower concentrations (below 10 micromolar total plutonium concentration), LPAS is no longer capable of detecting small amounts of plutonium(IV) colloids present in the solution. However, such minute concentration of colloids (less than one micromolar) can still be detected by LIBD, and the dissolution with time has been demonstrated for solutions of one micromolar total plutonium.

A combination of LIBD and LPAS facilitates a sound assessment of chemical reactions of plutonium(IV) involved at a rim of its solubility-constrained pH for the total plutonium(IV) concentration down to micromoles per liter and for its colloids, about 100 times less (down to a particle concentration of 10 nanomoles per liter).

The present experiment shows how complicated the thermodynamic assessment of plutonium(IV) solubility is. For this reason, there is a wide scattering of the plutonium(IV) solubility product published in the literature-either for its oxide or for its hydroxide, with the differences being a few orders of magnitude. Our research has demonstrated how the use of novel spectroscopic approaches can reduce the uncertainties that have, up to now, hindered the assessment of plutonium(IV) solubility.

The basics of LIBD

Laser-induced breakdown detection (LIBD) is based on plasma formation due to dielectric breakdown in the high-field region of a focused pulsed laser beam when a colloid is present. We commonly experience low-energy plasmas in everyday life, ranging from "flames" over neon lights to the blinding flash of lightning.

The plasma used for LIBD begins with a single (so-called "lucky") electron, which is created by the high-electric field of the laser. In the language of quantum physics, this is equivalent to the nonresonant absorption of four to six photons (multiphoton ionization or MPI) within only some picoseconds. This process is very unlikely, and it takes on the order of 1015 photons to free one electron.

By interaction with the electric field, this electron is accelerated (by inverse bremsstrahlung) to energies sufficient to ionize neighboring atoms by means of collisions, resulting in a second electron (second "generation"), which undergoes the same process. Within typically only 30 such generations, a hot, dense plasma is created, which is observed by either its light emission or by detection of the acoustic shock wave generated by its rapid expansion.

Because each plasma event corresponds to one single colloid, the relative number of events per number of laser shots provides a measure of colloid concentration. The photon flux required for breakdown decreases with the increasing number of molecules inside the colloid. By making use of that dependence, the colloid size can be ascertained by varying the laser pulse energy.

This article was contributed by Clemens Walther, Claudia Bitea, Jong Il Yun, Jae Il Kim, Thomas Fanghänel, Christian M. Marquardt, Volker Neck and Alice Seibert of the Institut fur Nukleare Entsorgung, KarlsrŸhe Research Center


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