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Laser / Breakdown detection / Colloid formation / Thorium dioxide / Solubility product / Particle size Summary.A new experimental method is presented for the determination of solubility data, which is based on the laser-induced breakdown detection (LIBD). The method is capable of monitoring the initial colloid generation when the metal ion concentration reaches or just exceeds the solubility at given pH. The application is made to determine the solubility of Th(IV) in acidic solutions at I = 0.5 M (NaCl) and 25 • C. The initial colloid formation is determined as a function the H + concentration in a series of 2.8 × 10 −2 -8.9 × 10 −5 M thorium solutions. The conditional solubility product (log K sp = −49.54 ± 0.22) obtained in this study corresponds to an equilibrium between solution and colloidal thorium dioxide particles. The solubility product at I = 0 (log K • sp = −52.8 ± 0.3) is calculated with the SIT coefficients of the NEA-TDB. It corresponds to the known value for crystalline ThO 2 (cr), in particular if the small particle size of about 20 nm is taken into account. The present results indicate that the high thorium solubilities measured in the previous studies for amorphous Th(IV) hydroxide or hydrous oxide are primarily caused by the inclusion of polynuclear species or Th(IV) colloids of very small size.

The size and shape of colloids released from a natural bentonite into a low-mineralized groundwater are investigated using various colloid characterization methods. For the applied methods such as atomic force microscopy (AFM), laser-induced breakdown detection (UBD), photon correlation spectroscopy (PCS), and flow field-flow fractionation coupled to ICP-mass spectrometric detection (FFFF-ICPMS), the respective raw size data have to be corrected in order to consider chemical composition and shape of the colloids as well as instrumental artifacts. Noncontact mode AFM of the bentonite colloids shows disklike shapes of stacked smectite platelets with a mean height-to-diameter proportion (aspect ratio) of approximately 1/10. A broad particle number size distribution is determined by image processing with a mean particle diameter of 73 nm. In agreement with AFM, a broad size distribution is also found by PCS and FFFF-ICPMS. Likewise, mean particle sizes found by LIBD (67 +/- 13 nm) and FFFF-ICPMS (maximum in the number size distribution, approximately 70 nm) are in fair agreement with the AFM data. Somewhathigher values are obtained by PCS, where mean particle diameters of the intensity-weighted size distributions of larger than 200 nm are found (depending on the algorithm used for data processing). The influence of the disklike particle shape on the results of the individual methods is discussed. As a conclusion, the application of different colloid characterization methods is a prerequisite to get complementary information about colloid size and shape, which is essential for the understanding of natural colloidal systems.

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Laser-induced breakdown spectroscopy (LIBS) was applied to selectively analyze the aqueous suspension of Eu2O3(s) particles in the presence of the Eu3+ aquo ion. A plasma was generated by focusing a pulsed Nd:YAG laser beam (λ = 532 nm) into the sample. The light emission from the plasma was detected by a spectrograph equipped with a gated intensified charge-coupled device (ICCD) in the wavelength range of 275–525 nm. The atomic emission intensity of the Eu2O3(s) suspension was about two orders of magnitude higher than that of the Eu3+ aquo ion. The detection limits for Eu3+(aq) and Eu2O3(s) were found to be 3.3 × 10−5 mol/L and 2.0 × 10−7 mol/L, respectively. Such a difference allows the selective determination of colloidal europium particles. This capability of LIBS was used to study the formation of Eu(OH)3(s) colloids in the aqueous Eu3 solution by varying pH until the solubility limit was exceeded. The appraisal of the threshold pH for the solubility limit led to the determination of the solubility product of colloidal Eu(OH)3(s), which was then calculated to be log K0sp= −25.5 ± 0.4.