Uranium fate during crystallization of magnetite from ferrihydrite in conditions relevant to the disposal of radioactive waste

Abstract

Uranium incorporation into magnetite and its behaviour during subsequent oxidation has been investigated at high pH to determine the uranium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to U(VI) aq containing cement leachates ( pH 10.5-13.1) and crystallization of magnetite was induced via addition of Fe(II) aq . A combination of XRD, chemical extraction and XAS techniques provided direct evidence that U(VI) was reduced and incorporated into the magnetite structure, possibly as U(V), with a significant fraction recalcitrant to oxidative remobilization. Immobilization of U(VI) by reduction and incorporation into magnetite at high pH, and with significant stability upon reoxidation, has clear and important implications for limiting uranium migration in geological disposal of radioactive wastes.

Environmental implications and applications of engineered nanoscale magnetite and its hybrid nanocomposites: A review of recent literature

“…A combination of XRD, chemical extraction and XAS techniques provide direct evidence that U(VI) is reduced and incorporated into the magnetite structure, possibly as U(V), with a significant fraction recalcitrant to oxidative remobilization. Immobilization of U(VI) by reduction and incorporation into magnetite at high pH, and with significant stability upon reoxidation, has clear and important implications for limiting uranium migration in geological disposal of radioactive wastes [358]. No information about the size of magnetite was given in this study, but it may be relevant to MNPs as well.…”

Neptunium(V) and Uranium(VI) Reactions at the Magnetite (111) Surface

“…The XANES spectra from the magnetite crystals equilibrated with both the U MOPS and NaHCO 3 buffer solutions were similar to the U(VI) standard rather than the U(IV) standard. This suggests that the uranium speciation on the magnetite crystals was dominated by U(VI) even though magnetite has previously been shown to be able to reduce U(VI) to U(IV) [12,33]. Furthermore, the best fits to the EXAFS analyses (Figure 2b,c, and Table 1) include 2 oxygen backscatterers at 1.77-1.79Å (reducing the number of oxygen backscatters resulted in an increase in the R-factor) diagnostic for uranyl speciation.…”

“…This suggests that the uranium speciation on the magnetite crystals was dominated by U(VI) even though magnetite has previously been shown to be able to reduce U(VI) to U(IV). [12,33] Furthermore, the best fits to the EXAFS analyses (Figure 2b,c, and Table 1) include 2 oxygen backscatterers at 1.77 -1.79Å (reducing the number of oxygen backscatters resulted in an increase in the R-factor) diagnostic for uranyl speciation. Additionally, the fit to the EXAFS from the magnetite crystal equilibrated with the U NaHCO3 buffer solutions included 5 oxygen backscatterers at 2.31 Å and the fit to the EXAFS from the magnetite crystal equilibrated with the U MOPS buffer solution included 6 oxygen backscatterers at 2.21 and 2.41 Å.…”

“…In natural and engineered environments iron oxide minerals are ubiquitous [9] and have the potential to affect the mobility of radionuclides significantly through both incorporation and surface complexation processes [10][11][12][13][14][15][16][17][18][19][20][21]. In aerobic environments the redox states of the actinides (U, Np and Pu) are dominated by relatively soluble hexa-and pentavalent species, while under anoxic and progressively more reducing conditions, poorly soluble tetra-and trivalent species dominate [22].…”

“…Moreover, under reduced environments, magnetite (Fe (II) Fe (III) 2 O 4 ) will be a significant iron oxide phase present in many geological disposal situations [9] and form as a corrosion product from zero valent iron in construction materials and stored wastes. Magnetite has been shown to affect the mobility of uranium [12,17,[23][24][25][26][27], neptunium [16,28] and plutonium [13,17] through Fe(II) oxidation coupled to reduction of these actinides to less soluble (tri-and) tetravalent species including incorporation (U) and adsorption (U, Np, Pu) processes. It is postulated that the reduction of U(VI) occurs concurrently to oxidation of magnetite to maghemite [23,29,30].…”