Tin-113 and Selenium-75 radiotracer adsorption and desorption kinetics in contrasting estuarine salinity and turbidity conditions.

Batch experiments were performed to study adsorption and desorption of 75Se and 113Sn radiotracers at environmentally representative concentrations of ~0.3 ng L-1 and ~3 ng L-1, respectively. The radiotracers were incubated with wet bulk sediments from the Gironde Estuary and the Rhône River, combining freshwater and coastal seawater salinity (S = 0, S = 32) and three different Suspended Particulate Matter (SPM) concentrations (10 mg L-1, 100 mg L-1, 1000 mg L-1) to simulate six hydrologically contrasting situations for each particle type. Results showed no measurable adsorption for 75Se under the experimental conditions, whereas >90% of 113Sn rapidly adsorbed onto the particles during the first hours of exposure. Adsorption efficiency increased with increasing SPM concentration and seemed to be slightly greater for the Rhône River sediments, potentially related to the intrinsic mineral composition. Desorption of spiked sediments exposed to filtered, unspiked freshwater and seawater only occurred for 113Sn (<15% of the previously adsorbed 113Sn) in the Garonne River sediments. This study provides insights to the potential environmental behaviour of hypothetical radionuclide releases of Se and Sn into highly dynamic and contrasting aquatic systems. Multiple accidental scenarios for the case of the Gironde Estuary and the Rhône River are discussed. These scenarios suggest that the environmental fate of soluble radionuclides like Se will be associated to water hydrodynamics and potentially more bioavailable whereas highly particle-active radionuclides like Sn will follow natural river/estuarine sedimentary regimes. Information on reactivity of radionuclides is important for improving the precision of current approaches aiming at modelling environmental radionuclide dispersion in continent-ocean transition systems.

Protective Buffering Capacity of Restorative Dental Materials In Vitro.

PURPOSE: To investigate the buffering capacity of restorative materials during a simulated carious and intrinsic erosive attack. MATERIALS AND METHODS: Cavities with a volume of 130 µl were milled (Cerec MC XL) out of blocks of Ceram X Mono (CM), Quixfil (QX), Filtek Supreme (FS), Apa Fill 3 (AF), an experimental dual-curing composite containing a bioactive glass (EX), Dyract eXtra (DY), Beautifil (BE), Equia Fil (EQ), Telio CAD (TL) (negative control), TheraCal (TC; positive control), and extracted teeth (ED). 80 µl of lactic acid (pH 4.5) and hydrochloric acid (pH 2.6) were each pipetted into the cavities of two samples of each material. Change of pH in the solutions was measured continuously for 12 min using a calibrated pH electrode. RESULTS: CM, AF, and FS (final pH 3.0-3.2) neutralized hydrochloric acid to a significantly lesser extent than did BE, EQ, DY and QX (final pH 5.0-5.6) (p < 0.05, ANOVA Scheffé). The lactic-acid buffering capacity of CM, BE, and AF was equivalent (final pH 6.3-7.4) to that of ED (7.5), but was surpassed by FS (pH 8.0). pH values for EX and TC (final pH 9.2-11.3) increased significantly (p < 0.05) in response to both acids. CONCLUSIONS: Conventional restorative materials do not buffer better than human teeth. However, the experimental composite demonstrates that buffering against carious and intrinsic erosive acid attacks is technically feasible.

Thorium nanochemistry: the solution structure of the Th(IV)-hydroxo pentamer.

Tetravalent thorium exhibits a strong tendency towards hydrolysis and subsequent polymerization. Polymeric species play a crucial role in understanding thorium solution chemistry, since their presence causes apparent solubility several orders of magnitude higher than predicted by thermodynamic data bases. Although electrospray mass spectrometry (ESI MS) identifies Th(IV) dimers and pentamers unequivocally as dominant species close to the solubility limit, the molecular structure of Th(5)(OH)(y) polymers was hitherto unknown. In the present study, X-ray absorption fine structure (XAFS) spectroscopy, high energy X-ray scattering (HEXS) measurements, and quantum chemical calculations are combined to solve the pentamer structure. The most favourable structure is represented by two Th(IV) dimers linked by a central Th(IV) cation through hydroxide bridges.

Investigation of polynuclear Zr(IV) hydroxide complexes by nanoelectrospray mass-spectrometry combined with XAFS.

Polynuclear species of zirconium in acidic aqueous solution are investigated by combining X-ray absorption spectroscopy (XAFS) and nanoelectrospray mass spectrometry (ESI-MS). Species distributions are measured between pHC 0 and pHC 3 for [Zr]=1.5-10 mM. While the monomer remains a minor species, with increasing pH the degree of polymerization increases and the formation of tetramers, pentamers, octamers, and larger polymers is observed. The high resolution of the mass spectrometer permits the unambiguous determination of polynuclear zirconium hydroxide complexes by means of their isotopic patterns. The relative abundances of mononuclear and polynuclear species present simultaneously in solution are measured, even if one of the species contributes only 0.1% of the Zr concentration. For the first time it has been directly observed that the hydrolysis of polynuclear Zr species is a continuous process which leads to charge compensation through the sequential substitution of water molecules by hydroxide ligands until doubly charged polymers dominate at conditions (H+ and Zr concentrations) close to the solubility of Zr(OH)4(am). The invasiveness of the electrospray process was minimized by using very mild declustering conditions, leaving the polynuclear species within a solvent shell of approximately 20 water molecules.