An experimental study of the dissolution rates of Nd-britholite, an apatite-structured actinide-bearing waste storage host analogue

Chaïrat, Claire; Oelkers, Eric H.; Schott, Jacques; Lartigue, Jean-Éric

doi:10.1016/j.jnucmat.2006.01.022

Cited by 30 publications

(18 citation statements)

References 39 publications

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“…For example, in the dissolution experiments for the Nd-britholite performed by Chairat et al (2006), it was found that Nd leaching rates were significantly slower than compared to the other elements in the britholite mineral structure and Nd concentrations that were less than 10 −12 M at near neutral conditions. They, furthermore, indicated that Nd concentrations in their experimental solution appear to be limited by precipitation of secondary phosphate phases and concluded that a potential actinide release from analogous waste repository would be similarly limited by the precipitation of sparingly soluble phases such as monazite and rhabdophane.…”

Section: Resultsmentioning

confidence: 99%

The influence of pH and temperature on the aqueous geochemistry of neodymium in near surface conditions

Çetiner

2008

Environ Monit Assess

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Geochemical calculations were employed for the solubility and speciation of neodymium in a model soil solution as a function of pH. The calculations were based on the recently determined stability constants for Nd and solubility product for the Nd end-member of mineral monazite (NdPO(4)). Simulations were carried out at near neutral pH (pH 6.0 to pH 7.5) and 25 degrees C at the atmospheric CO(2) partial pressure. Additional calculations were also performed to assess Nd mobility at the extreme temperature conditions (300 degrees C) at neutral pH. Our results suggest that relatively dilute (Ionic Strength, I = 0.1), low-temperature waters may transport very small quantities of rare earth elements and actinides to the surficial environment at near neutral pH conditions. Evidently, higher temperature or extreme fluid composition may have a greater potential for mobilization of these elements. The results are pertinent to researchers interested in engineering applications for the precipitation of Nd and surrogate actinides from aqueous nuclear wastes.

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Section: Resultsmentioning

confidence: 99%

The influence of pH and temperature on the aqueous geochemistry of neodymium in near surface conditions

Çetiner

2008

Environ Monit Assess

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“…This fluoride depletion is confirmed by the initial preferential departure of fluoride from FAP sur- face during dissolution experiments. Surface titrations of the FAP suggest that fluoride removal is coupled to the incorporation of hydroxide consistent with an ion exchange mechanism (Chaïrat et al, 2006(Chaïrat et al, , 2007. This exchange is supported by the similar location of hydroxyl and fluoride ions in apatite structure, and by existence of a solid-solution between FAP and hydroxyapatite (HAP) (Schaeken et al, 1975).…”

Section: Neutral and Basic Solutionsmentioning

confidence: 94%

Kinetics and mechanism of natural fluorapatite dissolution at 25°C and pH from 3 to 12

Chaïrat

Schott

Oelkers

et al. 2007

Geochimica et Cosmochimica Acta

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146

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The dissolution rates of natural fluorapatite (FAP), Ca 10 (PO 4 ) 6 F 2 , were measured at 25°C in mixed-flow reactors as a function of pH from 3.0 to 11.7, and aqueous calcium, phosphorus, and fluoride concentration. After an initial preferential Ca and/or F release, stoichiometric Ca, P, and F release was observed. Measured FAP dissolution rates decrease with increasing pH at 3 6 pH 6 7, FAP dissolution rates are pH independent at 7 6 pH 6 10, and FAP dissolution rates again decrease with increasing pH at pH P 10. Measured FAP dissolution rates are independent of aqueous Ca, P, and F concentration at pH % 3 and pH % 10.Apatite dissolution appears to be initiated by the relatively rapid removal from the near surface of F and the Ca located in the M1 sites, via proton for Ca exchange reactions. Dissolution rates are controlled by the destruction of this F and Ca depleted surface layer. The destruction of this layer is facilitated by the adsorption/penetration of protons into the surface at acidic conditions, and by surface hydration at neutral and basic conditions. Taking into account these two parallel mechanisms, measured fluorapatite forward dissolution rates can be accurately described using2þ a 1:4 F À a 0:6 OH À a 6 H þ Kex þ 3:69 Â 10 À8 ½BCaOH þ 2 0:6AE0:1where a i refers to the activity of the ith aqueous species, ½BCaOH þ 2 denotes the concentration of hydrated calcium sites at the surface of the leached layer (mol m À2 ), and K ex and K ads stand for the apparent stability constants of the Ca 2+ /H + exchange and adsorption/penetration reactions, respectively.

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“…Moreover, apatite minerals are currently being considered as both possible additives to backfill material (Perrone et al, 2002) or as potential actinide and iodine storage hosts for the confinement of nuclear wastes (e.g. Boyer et al, 1998;Guy et al, 2002;Chaïrat et al, 2004;Chaïrat et al, 2006).…”

Section: Introductionmentioning

confidence: 99%

Fluorapatite surface composition in aqueous solution deduced from potentiometric, electrokinetic, and solubility measurements, and spectroscopic observations

Chaïrat

Oelkers

Schott

et al. 2007

Geochimica et Cosmochimica Acta

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International audienceThe surface chemistry of fluorapatite in aqueous solution was investigated using electrokinetic techniques, potentiometric titrations, solubility measurements, and attenuated total reflection infrared spectroscopy. All methods indicate the formation of Ca/F depleted, P enriched altered layer via exchange reactions between H+ and Ca2+, and OH? and F? at the fluorapatite (FAP) surface. Observations suggest that this leached layer has a di-calcium phosphate (CaHPO4) composition and that it controls the apparent solubility of FAP. Electrokinetic measurements yield an iso-electric point value of 1 ± 0.5 consistent with a negatively charged FAP surface at pH > 1. In contrast, surface titrations give an apparent pH of point of zero charge of ?7.7, consistent with a positively charged surface at pH < 7.7. These differences are shown to stem from proton consumption by both proton exchange and dissolution reactions at the FAP surface. After taking account for these effects, FAP surface charge is shown to be negative to at least pH 4 by surface titration analysis

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An experimental study of the dissolution rates of Nd-britholite, an apatite-structured actinide-bearing waste storage host analogue

Cited by 30 publications

References 39 publications

The influence of pH and temperature on the aqueous geochemistry of neodymium in near surface conditions

The influence of pH and temperature on the aqueous geochemistry of neodymium in near surface conditions

Kinetics and mechanism of natural fluorapatite dissolution at 25°C and pH from 3 to 12

Fluorapatite surface composition in aqueous solution deduced from potentiometric, electrokinetic, and solubility measurements, and spectroscopic observations

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