Exploring the relationship between Ln leaching and Ln–O binding energy in monazite (Nd, Sm, Eu)

Abstract

There are a large number of αemitters with long-life in nuclear materials inventories and nuclear waste around the world, including Pu and minor actinides (Np, Am, and Cm). 1,2 These long-lived actinides are suggested to be separated than being immobilized in mineral-based ceramics to prevent the migration of radionuclide in the biosphere. 3 Monazite has been proposed as the potential actinides host-matrices owing to its excellent long-term stability. 4-6 An 3+ PO 4 monazite waste forms can be easily obtained from Pu to Es by wet or dry chemistry methods. 7 In contrast, some front actinides (from Th to Np) with the stable tetravalent oxidation state require a coupled substitution of divalent cation for their incorporation in monazite by solid state sintering methods. According to the description by Podor, 8 the formation of M 3+ 1−2x Ca 2+x An 4+x PO 4 (An 4+ = Pu 4+ , U 4+ , Th 4+ ) monazite is restricted by the

Investigation of Nd3+incorporation in Ce‐rhabdophane: Insight from structural flexibility and occupation mechanism

“…The long‐term chemical stability of monazite benefits from the protection of rhabdophane (LnPO 4 ⋅0.667H 2 O) precipitation on the surface of the specimen 22,23 . It is generally believed that the leaching actinides from monazite matrix will be adsorbed immediately by the defective area of monazite surface to the nucleation of Ln x An 1‐ x PO 4 ·0.667H 2 O rhabdophane precipitation 16,24,25 . Recently, Gausse et al.…”

“…22,23 It is generally believed that the leaching actinides from monazite matrix will be adsorbed immediately by the defective area of monazite surface to the nucleation of Ln x An 1-x PO 4 ⋅0.667H 2 O rhabdophane precipitation. 16,24,25 Recently, Gausse et al reported that the standard solubility products (log Ksp) of LnPO 4 ⋅0.667H 2 O rhabdophane in 0.1 mol/L HCl solutions at 298-343 K are approximate 25, implying the feasibility of separation-enrichment of actinides from rhabdophane in acidic solutions. 26 Nano-sized plutonium iron phosphate hydroxide structurally related to the rhabdophane group minerals was found in Hanford site sediments including Pu waste in the 216-Z9 crib sediment in 2014.…”

Response of Nd3+ and Sm3+ precipitating into rhabdophane and the leaching mechanism of associated monazite ceramics

“…The dissolution experiments of La 0.666 Nd 0.167 Sm 0.167 PO 4 monazite ceramics were performed at 363 K in pH = 5, 7, and 9 solutions to assess the chemical stability of ceramics in geological environment and to gain a general understanding of lanthanides leaching activity. Previous studies reveal that the weak leaching resistance of monazite ceramics has been found in different leachates at initial 7 days owing to surface defect caused by mechanical polishing, 32 and that the stable state corresponding to dissolution–diffusion phenomenon has been established after 21 days with an LR value of ∼10 −6 –10 −5 g/m 2 in pH = 5 solution 35 . It should be noted that the updated leachates in previous studies were not digested with strong acid prior to ion concentration measurements.…”

“…Numerous studies reveal understanding the corrosion mechanism of monazite requires a fundamental knowledge of Ln–O bond properties and lanthanides vacancy defect chemistry in monazite surface 21,32 . To deeply investigate the leaching activity of La 0.666 Nd 0.167 Sm 0.167 PO 4 monazite, the formation energies of lanthanides ($${E_f}$$) with and without charge state q are analyzed by the following equations 33 : $$$$\begin{eqnarray} {E_f}\left( {{X^q}} \right) &=& \;{E_{tot}}\left( {{X^q}} \right) - {E_{tot}}\left( B \right)\nonumber\\ && + \mathop \sum \limits_i {n_i}{\mu _i} + q\left( {{\varepsilon _f} + {E_{VBM}}} \right) + {E_{corr}}\end{eqnarray}$$$$where $${E_{tot}}( {{X^q}} )$$ and $${E_{tot}}( B )$$ are the total energy of surface structure with and without vacancy defect ( X ).…”

“…31 Numerous studies reveal understanding the corrosion mechanism of monazite requires a fundamental knowledge of Ln-O bond properties and lanthanides vacancy defect chemistry in monazite surface. 21,32 To deeply investigate the leaching activity of La 0.666 Nd 0.167 Sm 0.167 PO 4 monazite, the formation energies of lanthanides (𝐸 𝑓 ) with and without charge state q are analyzed by the following equations 33 :…”