Discrepancies in thorium oxide solubility values: a new experimental approach to improve understanding of oxide surface at solid/solution interface

Vandenborre, Johan; Abdelouas, Abdesselam; Grambow, Bernd

doi:10.1524/ract.2008.1531

Cited by 9 publications

(5 citation statements)

References 19 publications

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“…This may explain the observed differences in solubilities. The other solids named ThO 2 (cr) are commercially available , or are sintered sphere fuel for a high-temperature reactor (HTR). , …”

Section: Introductionmentioning

confidence: 99%

Discrepancies in Thorium Oxide Solubility Values: Study of Attachment/Detachment Processes at the Solid/Solution Interface

2010

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The solubility of thorium under oxide and/or hydroxide forms has been extensively studied for many years. Nevertheless, a large discrepancy in the solubility values is noticed in the literature. We study Th atom exchange between thorium oxide surfaces and various aqueous solutions (0.01 mol·L(-1) NaCl for 0.0 < pH < 5.2) to address this issue. By solid-state characterization [X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and atomic force microscopy], we determined that 80% of the XPS accessible near the surface region of sintered thorium oxide is represented by the less reactive ThO(2)(cr) grains. The remaining 20% corresponds to ThO(x)(OH)(y)(H(2)O)(z), which is largely associated with grain boundaries. Only the latter fraction is involved in solid/solution exchange mechanisms. Local conditions (thorium concentrations, pH values, etc.) in grain boundaries lead to an adjustment of the "local solubility constraints" and explain the thorium concentration measured in our experiments. For pH <5.2, the thorium concentration and pH gradient between the bulk solution and grain-boundary regions imply that the solubility values mainly depend on the availability and accessibility of ThO(x)(OH)(y)(H(2)O)(z). We have performed two solubility experiments with a (232)ThO(2)(cr) solid in a 0.01 mol·L(-1) NaCl solution for 300 days. In a first experiment, we measured (232)Th concentrations in dissolution experiments in order to determine the detachment rates of Th atoms from the solid surface. In a subsequent step, we added (229)Th to the solution in order to measure the surface attachment rate for dissolved Th atoms. This allowed an assessment of the net balance of Th atom exchange at the solid/solution interface. The empirical solubility data do not correspond to the thermodynamic bulk phase/solution equilibrium because measured solution concentrations are controlled by site-specific exchange mechanisms at the solid/solution interface. Therefore, for sparingly soluble solids, one needs to quantify site-specific surface attachment and detachment rates if one wants to assess solubility constraints.

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Section: Introductionmentioning

confidence: 99%

Discrepancies in Thorium Oxide Solubility Values: Study of Attachment/Detachment Processes at the Solid/Solution Interface

2010

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Section: Resultsmentioning

confidence: 99%

“…Over a time period of up to one year the pentamers are stable 'steady state' species, though we cannot be certain that the solutions reached thermodynamic equilibrium since there is evidence that kinetics might be very slow in the case of Th solids. 27 In slightly oversaturated solutions Ostwald ripening leads to formation of nm-sized colloids at the expense of polymers. 14 The second important finding is that the relative abundance of polymers, i.e., the percentage of Th 4+ ions contained in a size fraction, is not so much a function of absolute Th concentration but rather a function of the degree of saturation.…”

Section: Resultsmentioning

confidence: 99%

Thorium nanochemistry: the solution structure of the Th(iv)–hydroxo pentamer

et al. 2012

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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.

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“…Thorium (IV) forms strong complexes with humic and fulvic acids, and its adsorption to organic matter, clays and oxides increase with pH, limiting its mobility and bioavailability. The waters AV10 (April 2009), AV1 (July 2009), AV7 (April 2009), AV5 (April 2009) are saturated in crystalline ThO 2 , which means if the pH values decrease, more Th can be released into solution (Vandenborre et al 2008).…”

Section: Speciation Of As Pb Th and U In Surface Waters And Groundwmentioning

confidence: 99%

Potential toxic elements in stream sediments, soils and waters in an abandoned radium mine (central Portugal)

Antunes

Neiva

Albuquerque

et al. 2017

Environ Geochem Health

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The Alto da Várzea radium mine (AV) exploited ore and U-bearing minerals, such as autunite and torbernite. The mine was exploited underground from 1911 to 1922, closed in 1946 without restoration, and actually a commercial area is deployed. Stream sediments, soils and water samples were collected between 2008 and 2009. Stream sediments are mainly contaminated in As, Th, U and W, which is related to the AV radium mine. The PTEs, As, Co, Cr, Sr, Th, U, W, Zn, and electrical conductivity reached the highest values in soils collected inside the mine influence. Soils are contaminated with As and U and must not be used for any purpose. Most waters have pH values ranging from 4.3 to 6.8 and are poorly mineralized (EC = 41-186 lS/cm; TDS = 33-172 mg/L). Groundwater contains the highest Cu, Cr and Pb contents. Arsenic occurs predominantly as H 2 (AsO 4)and H(AsO 4) 2-. Waters are saturated in goethite, haematite and some of them also in lepidocrocite and ferrihydrite, which adsorbs As (V). Lead is divalent in waters collected during the warm season, being mobile in these waters. Thorium occurs mainly as Th(OH) 3 (-CO 3)-, Th(OH) 2 (CO 3) and Th(OH) 2 (CO 3) 2 2-, which increase water Th contents. Uranium occurs predominantly as UO 2 CO 3 , but CaUO 2 (CO 3) 3 2and CaUO 2 (-CO 3) 3 also occur, decreasing its mobility in water. The waters are contaminated in NO 2-, Mn, Cu, As, Pb and U and must not be used for human consumption and in agricultural activities. The water contamination is mainly associated with the old radium mine and human activities. A restoration of the mining area with PTE monitoring is necessary to avoid a public hazard.

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Discrepancies in thorium oxide solubility values: a new experimental approach to improve understanding of oxide surface at solid/solution interface

Cited by 9 publications

References 19 publications

Discrepancies in Thorium Oxide Solubility Values: Study of Attachment/Detachment Processes at the Solid/Solution Interface

Discrepancies in Thorium Oxide Solubility Values: Study of Attachment/Detachment Processes at the Solid/Solution Interface

Thorium nanochemistry: the solution structure of the Th(iv)–hydroxo pentamer

Potential toxic elements in stream sediments, soils and waters in an abandoned radium mine (central Portugal)

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