Carbonate induced dissolution of uranium containing precipitates under cement leachate conditions

Retardation of uranium and thorium by a cementitious backfill developed for radioactive waste disposal

The solubility of uranium and thorium has been measured under the conditions anticipated in a cementitious, geological disposal facility for low and intermediate level radioactive waste. Similar solubilities were obtained for thorium in all media, comprising NaOH, Ca(OH)₂ and water equilibrated with a cement designed as repository backfill (NRVB, Nirex Reference Vault Backfill). In contrast, the solubility of U(VI) was one order of magnitude higher in NaOH than in the remaining solutions. The presence of cellulose degradation products (CDP) results in a comparable solubility increase for both elements. Extended X-ray Absorption Fine Structure (EXAFS) data suggest that the solubility-limiting phase for uranium corresponds to a becquerelite-type solid whereas thermodynamic modelling predicts a poorly crystalline, hydrated calcium uranate phase. The solubility-limiting phase for thorium was ThO₂ of intermediate crystallinity. No breakthrough of either uranium or thorium was observed in diffusion experiments involving NRVB after three years. Nevertheless, backscattering electron microscopy and microfocus X-ray fluorescence confirmed that uranium had penetrated about 40 μm into the cement, implying active diffusion governed by slow dissolution-precipitation kinetics. Precise identification of the uranium solid proved difficult, displaying characteristics of both calcium uranate and becquerelite.

Cross-sectional biomonitoring of metals in adult populations in post-war eastern Croatia: differences between areas of moderate and heavy combat

AIM

To determine differences in metal and metalloid exposure between residents of areas in eastern Croatia exposed to heavy fighting during the war in Croatia and residents of areas exposed to moderate fighting.

METHODS

Concentrations of aluminum (Al), arsenic (As), barium (Ba), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), nickel (Ni), lead (Pb), uranium (U), vanadium (V), and zinc (Zn), reported to be associated with military operations, were determined in hair, serum, and urine samples using inductively-coupled plasma mass spectroscopy. A total of 127 and 46 participants from areas of heavy and moderate fighting, respectively, were included.

RESULTS

Compared with participants from areas exposed to moderate fighting, participants from areas exposed to heavy fighting had significantly higher serum concentrations of Al (87.61 vs 42.75 μg/L, P=0.007), As (5.05 ∓ 1.79 vs 4.16 ∓ 1.55 μg/L, P=0.003), Ba (7.12 vs 6.01 μg/L, P=0.044), and V (17.98 vs 16.84 μg/L, P=0.008); significantly higher urine concentrations of As (43.90 vs 11.51 μg/L, P<0.001) and Cd (0.67 vs 0.50 μg/L, P=0.031); and significantly higher hair concentrations of Al (12.61 vs 7.33 μg/L, P<0.001), As (0.32 vs 0.05 μg/L, P<0.001), Cd (0.03 vs 0.02 μg/L, P=0.002), Fe (22.58 vs 12.68 μg/L, P=0.001), Pb (1.04 vs 0.69 μg/L, P=0.006), and V (0.07 vs 0.03 μg/L, P<0.001).

CONCLUSION

Differences between populations from eastern Croatian areas exposed to heavy and populations exposed to moderate fighting point to the need for extensive monitoring of metal and metalloid exposure, emphasizing the role of biomonitoring through ecologic and preventive activities.

Solubility kinetics of a synthetic uranium(VI) oxide at different pH and controlled O2 partial pressure and evaluation of substantial interface properties

The solubility behaviour of a synthetic uranium(VI) oxide was studied under inert conditions in aqueous NaClO4 solution at 25 ± 2 °C. The dissolution kinetics were investigated in respect of pH in solution and O2 partial pressure in the atmosphere. The actual uranium concentration was determined by ICP-MS measurements. Under acidic conditions we observed a moderate solubility which increases by lowering of the pH of solution yielding to a solubility constant of log10 K sp = −24.38 ± 0.07. The influence of the ionic strength, I, on the dissolution kinetics could be observed which was evaluated using a theoretical model considering the electrical double layer at the surface of the oxide as well as all possible reaction pathways. As a result we could determine the acid-base equilibrium constants of the surface groups and the overall reaction rate, k +, of dissolution process as a function of pH and I. On this basis we are able to determine the pzc of the uranium oxide, to be 7.28. It is in good accordance with a minimum in the tendency of the solid to dissolve which was observed in the pH range 6 to 8.

Uranium(VI) solubility and speciation in simulated elemental human biological fluids

The complete understanding of the human body response to uranium contamination exposure is vital to the development of exposure analysis and subsequent treatments for overexposure. Thermodynamic modeling has traditionally been used to study environmental metal contaminant migration (especially uranium and other radionuclides), allowing examination of chemical processes difficult to study experimentally. However, such techniques are rarely used in the study of metal toxicology. Chemical thermodynamics has a unique and valuable role in developing models to explain metal metabolism and toxicology. Previous computational models of beryllium in simulated biological fluids have been shown to be useful in predicting metal behavior in the human body. However, previous studies utilizing chemical thermodynamics in understanding uranium chemistry in body fluids are limited. Here, a chemical thermodynamic speciation code has been used to model and understand the chemistry of uranium in simulated human biological fluids such as intracellular, interstitial, and plasma fluids, saliva, sweat, urine, bile, gastric juice, pancreatic fluid, and a number of airway surface fluids from patients with acute lung conditions. The results show predicted uranium solubility, and speciation varies markedly between each biological fluid due to differences in fluid composition, ionic strength, and pH. The formation of uranium hydroxide, phosphate (sodium/potassium autunite), and calcium uranate was observed in most of the fluids. The results of this work, supported by experimental validation, can aid in understanding the metabolism and toxic effects of uranium with potential applications to biological monitoring as well as chelation treatment of uranium body burden.