Lara Duro

The oxidative dissolution mechanism of uranium dioxide. I. The effect of temperature in hydrogen carbonate medium

Abstract The oxidative dissolution of uranium (IV) dioxide has been experimentally investigated as a function of hydrogen carbonate concentration at 4 different temperatures (10, 25, 45, and 60°C) by using a continuous thin-layer flow-through reactor. The experimental results have been interpreted as evidence for a bicarbonate-promoted oxidative dissolution mechanism which can be differentiated in to 3 steps: 1) initial oxidation of the uranium dioxide solid surface; 2) binding of HCO3− at the U(VI) sites of the oxidized layer; and 3) detachment of the U(VI)-carbonato surface complex. From this mechanism a general rate equation has been derived: r= k 1 k 2 { >UO 2 } tot [ O 2 ][HCO 3 − ] k −1 +k 2 [ HCO 3 − ]+k 1 [ O 2 ] Which allows to rationalization of some of the discrepancies found in the literature concerning the dependence of the dissolution rate of uranium dioxide on the hydrogen carbonate concentration. The application of this bicarbonate-promoted oxidative dissolution mechanism allows us to predict unirradiated UO2 and spent nuclear fuel dissolution rates which are in satisfactory agreement with rates determined experimentally. Some differences observed in spent fuel dissolution rates have been attributed to radiolysis effects.

The dissolution of biotite and chlorite at 25°C in the near-neutral pH region

We studied the dissolution of biotite and chlorite in laboratory systems with flow-through and batch reactors. The initial dissolution of biotite in the near-neutral pH region, under N 2(g) atmosphere is highly non-stoichiometric. A slow linear release of iron during a period of weeks indicates a surface-chemical-reaction-controlled mechanism of release for iron. The release of potassium is much faster than that of iron. A parabolic dependence of accumulated release with time suggests a diffusion-controlled mechanism of potassium release. The rates of magnesium, aluminium and silicon release are between those for potassium and iron and approach that of iron with time. There is no significant influence of (bi)carbonate or pH on biotite dissolution rate or stoichiometry in the pH region 7 < pH < 8.5. The release rates of elements from chlorite are close to stoichiometric and similar to the iron release rate from biotite. In closed batch reactors at near-basic pH the composition of test solutions in contact with biotite is apparently controlled by gibbsite (Al), kaolinite (Si) and Fe(III)-(hydr)oxide. We estimated a turn-over time (10 1 -10 2 yr) for molecular oxygen and a time scale (10 months) to develop characteristic Fe(II) concentrations for a granitic groundwater.

Denitrification in presence of acetate and glucose for bioremediation of nitrate‐contaminated groundwater

With the current increasing interest in aquifer denitrification, recent attention has been given to cost‐effective in‐situ treatments such as Enhanced In‐Situ Biological Denitrification (EISBD), which intends to stimulate the indigenous bacterial activity by injecting an external organic substrate and/or nutrients to the aquifer matrix. Within this context, laboratory batch assays have been conducted to develop a strategy for in‐situ denitrification of a nitrate‐contaminated aquifer in Argentona, Catalonia (Spain). The assays were run under aerobic and anaerobic conditions at a temperature of 17°C to better simulate the conditions of the aquifer. Acetate and glucose were added to assess their potential to promote heterotrophic denitrifying bacteria activity. Overall, the results revealed that indigenous micro‐organisms had the potential of reducing nitrate under appropriate conditions. Nitrate removal was complete and faster under anaerobic conditions, though high nitrate removals were also attained under initial aerobic conditions when a readily organic compound was amended at a sufficient dosage. The results also revealed that a significant amount of the available organic carbon was consumed by processes other than denitrification, namely aerobic oxidation and other microbial oxidation processes. To sum up, the results of this study demonstrated that addition of organic compounds into the groundwater is a promising method for in‐situ bioremediation of nitrate in the Argentona aquifer. This approach could potentially be applied to a number of situations in which nitrate concentration is elevated and where indigenous micro‐organisms with potential to reduce nitrate are present within the aquifer material.

Review of the complexation of tetravalent actinides by ISA and gluconate under alkaline to hyperalkaline conditions.

Isosaccharinic (ISA) and gluconic acids (GLU) are polyhydroxy carboxylic compounds showing a high affinity to metal complexation. Both organic ligands are expected in the cementitious environments usually considered for the disposal of low- and intermediate-level radioactive wastes. The hyperalkaline conditions imposed by cementitious materials contribute to the formation of ISA through cellulose degradation, whereas GLU is commonly used as a concrete additive. Despite the high stability attributed to ISA/GLU complexes of tetravalent actinides, the number and reliability of available experimental studies is still limited. This work aims at providing a general and comprehensive overview of the state of the art regarding Th, U(IV), Np(IV), and Pu(IV) complexes with ISA and GLU. In the presence of ISA/GLU concentrations in the range 10(-5)-10(-2) M and absence of calcium, An(IV)(OH)x(L)y complexes (An(IV)=Th, U(IV), Np(IV), Pu(IV); L=ISA, GLU) are expected to dominate the aqueous speciation of tetravalent actinides in the alkaline pH range. There is a moderate agreement among their stability, although the stoichiometry of certain An(IV)-GLU complexes is still ill-defined. Under hyperalkaline conditions and presence of calcium, the species CaTh(OH)4(L)2(aq) has been described for both ISA and GLU, and similar complexes may be expected to form with other tetravalent actinides. In the present work, the available thermodynamic data for An(IV)-ISA/GLU complexes have been reviewed and re-calculated to ensure the internal consistency of the stability constants assessed. Further modelling exercises, estimations based on Linear Free-Energy Relationships (LFER) among tetravalent actinides, as well as direct analogies between ISA and GLU complexes have also been performed. This approach has led to the definition of a speciation scheme for the complexes of Th, U(IV), Np(IV) and Pu(IV) with ISA and GLU forming in alkaline to hyperalkaline pH conditions, both in the absence and presence of calcium.

The applicability and limitations of thermodynamic geochemical models to simulate trace element behaviour in natural waters. Lessons learned from natural analogue studies

Abstract Natural analogue investigations aim to understand key phenomena and processes in natural systems related to those expected to occur in radioactive waste repositories. One of the key applications of natural analogue studies has been the possibility to test the geochemical models to be used to describe the migration of radionuclides in a future radioactive waste repository system. To this end, several geochemical modelling testing exercises (commonly denoted as blind predictive modelling, BPM) have formed an integral part of these studies over the last decade. We have reviewed, discussed and compared the results obtained from geochemical modelling BPM exercises carried out within six natural analogue studies: Pocos de Caldas, Cigar Lake, Maqarin, El Berrocal, Oklo and Palmottu. To make this comparison meaningful, we present the main geochemical characteristics of each site in order to highlight the most relevant mineralogical and hydrochemical differences. The elements selected for discussion are: Sr, Ba, Sn, Pb, Se, Ni, Zn, REEs, Th and U. We have based our discussion on the results obtained from the calculated aqueous speciation as well as by comparing solubility calculations with the actually observed concentrations. Results can be differentiated into two categories of elemental behaviour: 1. those elements like Th and U under reducing conditions that can be fairly well described by assuming solubility control exerted by pure solid phases as their oxyhydroxides; 2. elements such as Sr, Zn, REEs and U under oxidising conditions for which the association to major geochemical components of the system must be considered in order to explain their concentrations in groundwaters. Additionally, we discuss the main improvements made to the thermodynamic databases and the geochemical calculation methodologies due to the BPM exercises. Furthermore, the most important characterisation geochemical data needed to complete predictive solubility and speciation calculations are identified.

Estimation of the concentrations of trace metals in natural systems: The application of codissolution and coprecipitation approaches to El Berrocal (Spain) and Poços de Caldas (Brazil)

Abstract Trace element concentrations in natural systems indicate that, in most cases, solubility controls are not exerted by pure solid phases. The cycling of trace metals in the environment is coupled to the chemistry of the major components. To model this behaviour we have developed a methodology based on codissolution and coprecipitation approaches. We have applied this methodology to the El Berrocal (Spain) and Pocos de Caldas (Brazil) sites. Trace metals under investigation were: Ba, Cu, Mn, Ni, Sr, and U. The models took into account: (1) the observed association of copper and uranium with iron oxyhydroxides (both in El Berrocal and in Pocos de Caldas), (2) the interaction of barium and manganese with calcite in El Berrocal, and (3) the association between strontium and fluorite in Pocos de Caldas. In most cases, the results were in good agreement with field data, showing that application of these approaches can reproduce more accurately the measured concentration of trace metals in groundwater than the assumption of a pure solid phase controlling the concentration of trace metals in solution. However, some problems still remain, such as the description of nickel concentrations in groundwater.

Experimental study and modeling of uranium (VI) transport through ferrous olivine rock columns

The Lovasjärvi intrusion (SE-Finland) contents a high percentage of ferrous olivine (> 65%). This material has been suggested as a redox-active backfill-additive in deep nuclear waste repositories, due to the large Fe(II) proportion in its mineral composition. In order to understand the processes involved in the redox buffering capacity of this material the transport of uranium (VI) through olivine columns was studied. The results showed considerable retardation factor for the U(VI), particularly in carbonate-free media. The experimental data were simulated by means of reactive transport modeling. The best agreement between the experimental and calculated data was obtained considering that the interaction of U(VI) with the olivine surface occurred at two different types of sorption sites. One type accounts for the sorption capacity of the olivine mineral, and a second type accounts for the sorption on amorphous Fe(OH)3(s) formed at expenses of the oxidative dissolution of olivine.

Conceptual and numerical modeling of radionuclide transport and retention in near-surface systems.

Scenarios of barrier failure and radionuclide release to the near-surface environment are important to consider within performance and safety assessments of repositories for nuclear waste. A geological repository for spent nuclear fuel is planned at Forsmark, Sweden. Conceptual and numerical reactive transport models were developed in order to assess the retention capacity of the Quaternary till and clay deposits for selected radionuclides, in the event of an activity release from the repository. The elements considered were carbon (C), chlorine (Cl), cesium (Cs), iodine (I), molybdenum (Mo), niobium (Nb), nickel (Ni), radium (Ra), selenium (Se), strontium (Sr), technetium (Tc), thorium (Th), and uranium (U). According to the numerical predictions, the repository-derived nuclides that would be most significantly retained are Th, Ni, and Cs, mainly through sorption onto clays, followed by U, C, Sr, and Ra, trapped by sorption and/or incorporation into mineral phases.

Dissolution studies of soddyite as a long-term analogue of the oxidative alteration of the spent nuclear fuel matrix

The thermodynamic and kinetic dissolution properties of a synthetically obtained soddyite have been determined at different bicarbonate concentrations. This uranium-silicate is expected to be a secondary solid phase of the oxidative alteration pathway of uranium dioxide in waters with low phosphate content and, consequently, it is likely to constitute one of the long-term uranium solubility limiting solid phases. The experimental data obtained at the end of the experiments correspond fairly well to the theoretical model calculated with a log K{sub S0}{sup 0} of 3.9 {+-} 7. On the other hand, the general trend of the total uranium in solution measured in the experiments as a function of time has been fitted by using a kinetic equation obtained from the principle of detailed balancing of the dissolution reaction. In addition, the EQ3/6 code has also been used to model the uranium concentrations as a function of time. In both modeling exercises comparable results were obtained. The dissolution rate, normalized to the total surface area used in the experiments as measured with the BET method, gave an average value of 6.8 ({+-}4.4) 10{sup {minus}14} mol/cm{sup 2} s.

Fluorimetric determination of traces of uranium(VI) in brines and iron(III) oxides using separation on an activated silica gel column

Abstract A method for the determination of trace uranium levels in brines and waters with a high content of iron(III) based on the separation of uranium from a mixture of masking agents by sorption on an activated silica gel column was developed in connection with the Scintrex UA-3 uranium analyser. The detection limit of uranyl ions was 1.4 ng. This value can be achieved even with solutions containing 56 mg of iron, 480 mg of magnesium and/or 1000 mg of chloride. The relative standard deviation of the method is 5%. An analysis requires less than 20 min.