Local environment of iodine dissolved as iodate in high-pressure aluminoborosilicate glasses: A I K-edge x-ray absorption spectroscopic study.

The use of high-pressure synthesis conditions to produce I-bearing aluminoborosilicate represents a promising issue for the immobilization of 129I radioisotope. Furthermore, iodine appears to be more solubilized in glasses under its iodate (I5+) form rather than its iodide (I-) form. Currently, the local atomic environment for iodine is poorly constrained for I- and virtually unknown for I5+ or I7+. We used I K-edge x-ray absorption spectroscopy conducted at 20 K for determining the local atomic environment of iodine dissolved as I-, I5+, and I7+ in a series of aluminoborosilicate glasses. We determined that I- is surrounded by either Na+ or Ca2+ in agreement with previous studies. The signal collected from EXAFS reveals that I5+ is surrounded invariably by three oxygen atoms forming an IO3 - cluster charge compensated by Na+ and/or Ca2+. The I-O distance in iodate dissolved in glass is comparable to the I-O distance in crystalline compounds at ∼1.8 Å. The distance to the second nearest neighbor (Na+ or Ca2+) is also constant at ∼3.2 Å. This derived distance is identical to the distance between I- and Na+ or Ca2+ in the case of iodide local environment. For one sample containing iodate and periodate, the distinction between the local environment of I5+ and I7+ could not be made, suggesting that both environments have comparable EXAFS signals.

Thermodynamic data of adsorption reveal the entry of CH4 and CO2 in a smectite clay interlayer.

The ability of smectite clays to incorporate gases in their interlayers is shown to be conditioned by interlayer spacing, depending, in turn, on phyllosilicate layer composition and effective size of the charge-balancing cations. As illustrated by earlier in situ X-ray diffraction and spectroscopic characterization of the gas/clay interface, most smectites with small-size charge-balancing cations, such as Na+ or Ca2+, accommodate CO2 and CH4 in their interlayers only in a partially hydrated state resulting in the opening of the basal spacing, above a certain critical value. In the present study CH4 and CO2 adsorption isotherms were measured for Na- and Mg-exchanged montmorillonite up to 9 MPa using a manometric technique. The process of dehydration of these clays was thoroughly characterized by thermogravimetric analysis and powder X-ray diffraction. A dramatic decrease in specific surface area and methane and carbon dioxide adsorption capacities for fully dehydrated samples in comparison to partially dehydrated ones is assigned to the shrinkage of interlayer spacing resulting in its inaccessibility for the entry of CH4 and CO2 molecules. This observation is direct evidence of CH4 and CO2 adsorption capacity variation depending on the opening of smectite clay interlayer spacing.

Alteration of vitrified intermediate level nuclear waste in alkaline media: effects of cementitious materials, pH and temperature.

Alteration experiments involving intermediate level nuclear waste (ILW) glass in contact with hardened cement paste (HCP) were performed to assess its behavior under simulated repository conditions. Batch experiments were conducted at 20 °C and 50 °C in several artificial cement pore water (ACW) samples (pH from 10 to 13), in the presence of HCP (CEM-I, CEM-V and low pH), with a ratio of glass surface to volume of solution of 8000 m-1 and a ratio of mass of HCP to volume of solution of 10 g L-1. Glass alteration rates increase up to ∼4 × 10-2 g m-2 d-1 with pH in contact with HCP, notably with CEM-I. This value decreases by 2 orders of magnitude in low pH cement solution and also for residual alteration rates. The effect of calcium on glass alteration was observed, mainly in Ca(OH)2 saturated solution, with an incubation effect on the release of Si in solution. Experimental data were successfully modeled with the PhreeqC geochemical code. Glass and HCP samples were characterized via SEM/EDX and micro-Raman studies. This work showed that vitrified glass exhibits good performance in terms of low alteration rates (∼10-4 g m-2 d-1), the absence of secondary phases, and the formation of a gel layer at the surface, when in contact with low pH conditions (in the presence or absence of low pH HCP).

Influence of gamma irradiation on uranium determination by Arsenazo III in the presence of Fe(II)/Fe(III).

Arsenazo III is a widely used reagent for the concentration measurement of uranium and other actinides in aqueous samples. This study indicates that, for routine aqueous samples, due to the strong complexing ability with Arsenazo III, Fe(III) can significantly decrease the UV-Vis absorbance of the U(VI)-Arsenazo III complex, whereas the influence of Fe(II) on the absorbance is negligible. However, when Fe(II) is present in a gamma-irradiated U(VI) aqueous sample, it can give rise to the Fenton reaction, which produces oxidizing radicals that decompose the subsequently added Arsenazo III, leading to a sharp decrease in the absorbance of the U(VI)-Arsenazo III complex. The decrease in absorbance depends on the iron content and irradiation dose. Furthermore, the oxidizing radicals from the Fenton reaction induced by gamma irradiation can be continually produced. Even if the irradiated solution has been aged for more than one month in the absence of light at room temperature and without the exclusion of oxygen, the reactivity of the radicals did not decrease toward the subsequently added Arsenazo III. This finding demonstrates that the presence of Fe(II) in gamma-irradiated U(VI) aqueous samples can lead to incorrect U(VI) measurement using the Arsenazo III method, and a new method needs to be developed for the quantitative determination of U(VI) in the presence of gamma radiation and ferrous iron.