Size dispersion and colloid mediated radionuclide transport in a synthetic porous media

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  • Journal of Colloid and Interface Science 324 (2008) 212215

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    00doIntroduction

    Solid particles are omnipresent in the aquifer and their size dis-ibution typically extends over many orders of magnitude fromuatic colloids in the lower nanometer domain to suspended par-cles of many microns in diameter [1]. Detailed knowledge one size distribution is mandatory in many different aspects ofvironmental and aquatic geochemistry [2,3] which might be il-strated by the prominent example of colloid mediated transportcontaminants [410]. Metal ions of very low solubility, like thetravalent actinides, are expected to be removed from solutionprecipitation or surface sorption. However, sorption to colloids,

    hich are mobile in the aquifer, leads to a dramatic enhancementmobility. Plutonium, which was expected to be immobilizedry close to its source by precipitation of Pu(IV)hydroxide, wasund to migrate kilometer distances within years, e.g., close to aclear test sites in Nevada [6] and in the vicinity of the Mayakprocessing plant [11].Hence, colloid mediated transport is considered as one of they uncertainties possibly enhancing the mobility of tri- andtravalent actinides in the far-eld of a deep geological repos-ry for high-level nuclear waste, especially in fractured rockranite) [12]. Although the last decade has brought consider-le progress in the eld of colloid detection methods concern-g colloid size, mass and size distribution determination withine nanometer range [13,14], the colloid transport in natural sys-

    Corresponding author.E-mail address:walther@ine.fzk.de (C. Walther).

    tems still lacks process understanding [1517]. The rock matrixin granite systems and the potential process of matrix diffusionas colloid retention mechanism is discussed frequently [12], butdata in this eld is scarce. To further understand the migrationof colloid-borne actinides, a eld experiment was conducted inthe Grimsel underground laboratory in Switzerland by bringinglaser-induced breakdown detection (LIBD) [1820] for in situ mon-itoring [2123]. For this purpose, different dipole geometries andlengths in a shearzone (MI shearzone) were tested to elucidatethe mobility of colloids and associated metals under the chosenhydraulic conditions [21]. Bentonite colloids (size distribution be-tween ca. 100180 nm) were injected into the fracture. The colloidcontent and mean colloid size of the outow was measured byLIBD, time resolved, and the metal concentration was followed byoff-line ICP-MS analysis. The peak maxima of the colloid and metalbreakthrough curves (Th(IV), Hf(IV), and Tb(III)) appeared about10 min ahead of the nonsorbing (conservative) tracer uranine. Theshift suggested the existence of size and/or charge exclusion effectstypical to colloid migration. However, since only the mean colloiddiameters were measured, the size exclusion effect could not bequantied directly.

    Recent advances of the LIBD technique [14,24], allow us to mea-sure the particle size distribution (PSD) rather than just a meansize of the colloids and, as a consequence, the size dispersion candirectly be observed. In order to avoid uncertainties arising fromill dened conditions in natural matrices, for a rst experiment wechose synthetic porous material of a well-dened pore character-istics. The migration of previously characterized natural polydis-perse colloids, derived from a potential bentonite backll material(FEBEX), was then observed by LIBD.Contents lists availab

    Journal of Colloid an

    www.elsevier.c

    ize dispersion and colloid mediated radioedia

    . Delos a, C. Walther b,, T. Schfer b, S. Bchner b

    mphos XXI Consulting S.L. (formerly Enviros Spain S.L.) Pg. de Rub, 29-31, E-08197 Valldoreixorschungszentrum Karlsruhe, Institut fr Nukleare Entsorgung, 76021 Karlsruhe, Germany

    r t i c l e i n f o a b s t r a c t

    ticle history:ceived 28 February 2008cepted 11 May 2008ailable online 16 May 2008

    ywords:lloide dispersionDrticle size distribution

    Size dispersion effects duringa ceramic column are observ(parts per million) mass conctions (

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    wrato24sevileF2ICplasma is not ignited in pure water but only when a colloidters the focal region and thus single colloids are counted. Thelative number of events per number of laser pulses provides aeasure of colloid number density (e.g., 100 plasmas for 1000ser pulses results in a breakdown probability, BDP, of 10%). Ine present work, the plasma is observed by the detection of theoustic shock wave generated by its rapid expansion [20,29,30].e BDP is measured for increasing pulse energy (i.e., increasing ir-diance) which results in so called s-curves (Fig. 2). The thresholdthese curves is a measure of particle size and the slope of therve scales with particle concentration. Using reference colloidswell-dened diameter, a calibration curve is obtained, which

    lates breakdown thresholds to particle size [18]. A recently de-loped data evaluation scheme for s-curve analysis [24] allows usmeasure the particle size distribution of colloids between 15d 400 nm at concentrations down to about 104 particles/mL.

    2. Column migration experiments

    For the colloid migration experiments, a Plexiglas column withe dimensions of 80 mm length and 40 mm diameter was lledith ceramic material (Soilmoisture Type B02M2) in the size frac-n 24 mm. The porous ceramic material was characterized byercury intrusion porosimetry and showed a matrix pore size dis-ibution in the range from 30 to 700 nm with a maximum around0 nm. The migration experiments were set-up in an argon glovex. The injection cocktail contained 20 mg/L Febex bentonite col-

    3. Results and discussion

    The colloid size distribution of the injection cocktail determinedby LIBD s-curve analysis is shown in Fig. 1. The number weightedsize distribution of the Febex bentonite colloidal fraction showstwo maxima around 1030 and 100200 nm, respectively. Tak-ing into account the density of clays ( = 2.7 g/cm3) as well asassuming spherical geometry, the surface area and mass per sizeclass can be estimated. Based on these assumptions, the total massis 18.7 g/mL compared to 20 g/mL determined by gravity and Alconcentration analysis (ICP-MS) using the structural formula givenin [31]. The calculated total surface area is in the range of 1011 m2/g.

    A compilation of the s-curves measured for fractions sampled inthe colloid breakthrough of the migration experiment are shown inFig. 2. At small elution volumes (Velu = 12 mL) or at short times,respectively, only a very low number of colloids is detected (Fig. 2,background). Close to the maximum of the breakthrough curve(Velu = 40 mL), the s-curve is shifted noticeably to the left, indi-cating the presence of large colloids. The corresponding particlesize distribution (Fig. 2, bottom left), which is obtained from the s-curve analysis, is centered around 100 nm. With increasing elutionvolume the thresholds of the s-curves shift to the right indicatinga decreasing mean particle size. Close to the end of the colloid-breakthrough peak at Velu = 79 mL more and more small colloidsare eluted (Fig. 2, bottom right).

    HTO and colloids are recovered quantitatively within the analyt-A. Delos et al. / Journal of Colloid and I

    g. 1. Bentonite colloid size distribution of injection cocktail used throughout this stud calculated colloid surface per size class assuming spherical geometry of colloids (ri

    Materials and methods

    1. LIBD

    Only a short summary of the LIBD technique is given here. For atailed description on laser-induced breakdown refer to [25] andr a description on LIBD to a recent review [26]. The methodbased on plasma formation by a focused (d 7 m) pulsed

    ser beam due to dielectric breakdown [19,25,27,28], a processhich works much more ecient in solids than in liquids. Hence,ids suspended in Grimsel ground water [31] with the actinides-241 (4.9 1010 mol/L) and Pu-244 (7.4 1010 mol/L) as

    icofface Science 324 (2008) 212215 213

    ample of injection cocktail was diluted 1:100. Colloid number distribution (left).

    ell as tritiated water (HTO) used as conservative tracer. Bothdionuclides, Am(III) and Pu(IV), are in the injection cocktail ben-nite colloid associated with 84 8% (Pu-244) and 99 8% (Am-1), respectively. The ow rate in the migration experiments wast to 2 mL/min and a total cocktail volume of 1 mL was injecteda an injection device into the column inlet [32]. Fractions col-cted at the column outlet by a fraction collector (Gilson Ltd. Type04) were analyzed off-line by liquid scintillation counting (LSC),P-MS and LIBD s-curve analysis.al errors with 104 5 and 97 5%, respectively. The comparisonthe HTO and colloid mass breakthrough curves (Fig. 3) shows a

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    ditoThtifrlietucrfrg. 2. Raw data s-curves of fractions taken in the colloid breakthrough (top) and two examples of particle size distributions obtained from the s-curves (bottom). Largelloids are eluted rst (left) smaller ones later (right).

    ak arrival time of colloids signicant earlier than that of the con-rvative tracer HTO. This behavior has been observed in the liter-ure [32,33] and is interpreted as a size chromatography effect.e peak arrival times of Pu-244 and Am-241 correspond to thelloid breakthrough showing a colloid mediated transport. How-er, the recoveries for both radionuclides are signicantly lowerith 4653% compared to the colloid recovery. These results showearly the reversibility of Am and Pu binding

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