Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide.

Here, Cr(VI) effects on Tc-immobilization by Fe(OH)2(s) are investigated while assessing Fe(OH)2(s) as a potential treatment method for Hanford low-activity waste destined for vitrification. Batch studies using simulated low-activity waste indicate that Tc(VII) and Cr(VI) removal is contingent on reduction to Tc(IV) and Cr(III). Furthermore, complete removal of both Cr and Tc depends on the amount of Fe(OH)2(s) present, where complete Cr and Tc removal requires more Fe(OH)2(s) (∼200 g/L of simulant), than removing Cr alone (∼50 g/L of simulant). XRD analysis suggests that Fe(OH)2(s) reaction and transformation in the simulant produces mostly goethite (α-FeOOH), where Fe(OH)2(s) transformation to goethite rather than magnetite is likely due to the simulant chemistry, which includes high levels of nitrite and other constituents. Once reduced, a fraction of Cr(III) and Tc(IV) substitute for octahedral Fe(III) within the goethite crystal lattice as supported by XPS, XANES, and/or EXAFS results. The remaining Cr(III) forms oxide and/or hydroxide phases, whereas Tc(IV) not fully incorporated into goethite persists as either adsorbed or partially incorporated Tc(IV)-oxide species. As such, to fully incorporate Tc(IV) into the goethite crystal structure, additional Fe(OH)2(s) (>200 g/L of simulant) may be required.

Reduction and Simultaneous Removal of 99Tc and Cr by Fe(OH)2(s) Mineral Transformation.

Technetium (Tc) remains a priority remediation concern due to persistent challenges, including mobilization due to rapid reoxidation of immobilized Tc, and competing comingled contaminants, e.g., Cr(VI), that inhibit Tc(VII) reduction and incorporation into stable mineral phases. Here Fe(OH)2(s) is investigated as a comprehensive solution for overcoming these challenges, by serving as both the reductant, (Fe(II)), and the immobilization agent to form Tc-incorporated magnetite (Fe3O4). Trace metal analysis suggests removal of Tc(VII) and Cr(VI) from solution occurs simultaneously; however, complete removal and reduction of Cr(VI) is achieved earlier than the removal/reduction of comingled Tc(VII). Bulk oxidation state analysis of the final magnetite solid phase by XANES shows that the majority of Tc is Tc(IV), which is corroborated by XPS measurements. Furthermore, EXAFS results show successful, albeit partial, Tc(IV) incorporation into magnetite octahedral sites. Cr XPS analysis indicates reduction to Cr(III) and the formation of a Cr-incorporated spinel, Cr2O3, and Cr(OH)3 phases. Spinel (modeled as Fe3O4), goethite (α-FeOOH), and feroxyhyte (δ-FeOOH) are detected in all reacted final solid phase samples analyzed by XRD. Incorporation of Tc(IV) has little effect on the spinel lattice structure. Reaction of Fe(OH)2(s) in the presence of Cr(III) results in the formation of a spinel phase that is a solid solution between magnetite (Fe3O4) and chromite (FeCr2O4).

Removal of TcO4(-) ions from solution: materials and future outlook.

Technetium mainly forms during artificial nuclear fission; it exists primarily as TcO4(-) in nuclear waste, and it is among the most hazardous radiation-derived contaminants because of its long half-life (t1/2 = 2.13 × 10(5) years) and environmental mobility. The high water solubility of TcO4(-) (11.3 mol L(-1) at 20 °C) and its ability to readily migrate within the upper layer of the Earth's crust make it particularly hazardous. Several types of materials, namely resins, molecular complexes, layered double hydroxides, and pure inorganic and metal-organic materials, have been shown to be capable of capturing TcO4(-) (or other oxoanions) from solution. In this review, we give a brief description about the types of materials that have been used to capture TcO4(-) and closely related oxyanions so far and discuss the possibility of using metal-organic frameworks (MOFs) as next-generation ion-exchange materials for the stated application. In particular, with the advent of ultra-stable MOF materials, in conjunction with their chemical tunability, MOFs can be applied to capture these oxyanions under real-life conditions.

Removal of Pertechnetate-Related Oxyanions from Solution Using Functionalized Hierarchical Porous Frameworks.

Efficient and cost-effective removal of radioactive pertechnetate anions from nuclear waste is a key challenge to mitigate long-term nuclear waste storage issues. Traditional materials such as resins and layered double hydroxides (LDHs) were evaluated for their pertechnetate or perrhenate (the non-radioactive surrogate) removal capacity, but there is room for improvement in terms of capacity, selectivity and kinetics. A series of functionalized hierarchical porous frameworks were evaluated for their perrhenate removal capacity in the presence of other competing anions.

Zirconium-Based Metal-Organic Framework for Removal of Perrhenate from Water.

The efficient removal of pertechnetate (TcO4(-)) anions from liquid waste or melter off-gas solution for an alternative treatment is one of the promising options to manage (99)Tc in legacy nuclear waste. Safe immobilization of (99)Tc is of major importance because of its long half-life (t1/2 = 2.13 × 10(5) yrs) and environmental mobility. Different types of inorganic and solid-state ion-exchange materials have been shown to absorb TcO4(-) anions from water. However, both high capacity and selectivity have yet to be achieved in a single material. Herein, we show that a protonated version of an ultrastable zirconium-based metal-organic framework can adsorb perrhenate (ReO4(-)) anions, a nonradioactive surrogate for TcO4(-), from water even in the presence of other common anions. Synchrotron-based powder X-ray diffraction and molecular simulations were used to identify the position of the adsorbed ReO4(-) (surrogate for TcO4(-)) molecule within the framework.

Computational Investigation of Technetium(IV) Incorporation into Inverse Spinels: Magnetite (Fe3O4) and Trevorite (NiFe2O4).

Iron oxides and oxyhydroxides play an important role in minimizing the mobility of redox-sensitive elements in engineered and natural environments. For the radionuclide technetium-99 (Tc), these phases hold promise as primary hosts for increasing Tc loading into glass waste form matrices, or as secondary sinks during the long-term storage of nuclear materials. Recent experiments show that the inverse spinel, magnetite [Fe(II)Fe(III)2O4], can incorporate Tc(IV) into its octahedral sublattice. In that same class of materials, trevorite [Ni(II)Fe(III)2O4] is also being investigated for its ability to host Tc(IV). However, questions remain regarding the most energetically favorable charge-compensation mechanism for Tc(IV) incorporation in each structure, which will affect Tc behavior under changing waste processing or storage conditions. Here, quantum-mechanical methods were used to evaluate incorporation energies and optimized lattice bonding environments for three different, charge-balanced Tc(IV) incorporation mechanisms in magnetite and trevorite (∼5 wt % Tc). For both phases, the removal of two octahedral Fe(II) or Ni(II) ions upon the addition of Tc(IV) in an octahedral site is the most stable mechanism, relative to the creation of octahedral Fe(III) defects or increasing octahedral Fe(II) content. Following hydration-energy corrections, Tc(IV) incorporation into magnetite is energetically favorable while an energy barrier exists for trevorite.

Rhenium solubility in borosilicate nuclear waste glass: implications for the processing and immobilization of technetium-99.

The immobilization of technetium-99 ((99)Tc) in a suitable host matrix has proven to be a challenging task for researchers in the nuclear waste community around the world. In this context, the present work reports on the solubility and retention of rhenium, a nonradioactive surrogate for (99)Tc, in a sodium borosilicate glass. Glasses containing target Re concentrations from 0 to 10,000 ppm [by mass, added as KReO(4) (Re(7+))] were synthesized in vacuum-sealed quartz ampules to minimize the loss of Re from volatilization during melting at 1000 °C. The rhenium was found as Re(7+) in all of the glasses as observed by X-ray absorption near-edge structure. The solubility of Re in borosilicate glasses was determined to be ~3000 ppm (by mass) using inductively coupled plasma optical emission spectroscopy. At higher rhenium concentrations, additional rhenium was retained in the glasses as crystalline inclusions of alkali perrhenates detected with X-ray diffraction. Since (99)Tc concentrations in a glass waste form are predicted to be <10 ppm (by mass), these Re results implied that the solubility should not be a limiting factor in processing radioactive wastes, assuming Tc as Tc(7+) and similarities between Re(7+) and Tc(7+) behavior in this glass system.

Metallic technetium sequestration in nickel core/shell microstructure during Fe(OH)2 transformation with Ni doping.

This study investigates the impacts of Ni doping on technetium-99 (Tc) sequestration in aqueous solutions through transformation of Fe(OH)2(s) to iron spinel (magnetite) under alkaline conditions. Extensive solid characterization was performed for the mineral phases produced, as well as the Tc/Ni speciation and distribution within these phases. X-ray diffraction results show that iron spinel was the dominant mineral product without detectable Ni incorporation. The doped Ni ions mainly precipitated as fine Fe/Ni oxide/hydroxide particles, including strongly reduced nanometer-sized spheroidal Ni-rich and metallic Ni phases. High-resolution analytical scanning transmission electron microscopy using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy on the produced solid samples (focused ion beam-prepared specimens) revealed three Tc distribution domains dominated by nanocrystals and, especially, a Tc-rich metallic phase. Instances of metallic Tc were specifically found in spheroidal, Ni-rich and metallic nanoparticles exhibiting a core/shell microstructure that suggests strong reduction and sequential precipitation of Ni-Tc-Ni. Mass balance analysis showed nearly 100% Tc removal from the 4.8 × 10-4 M Tc solutions. The finding of the metallic Tc encapsulation indicates that Tc sequestration through Ni-doped Fe(OH)2(s)-to-iron spinel transformation process likely provides an alternative treatment pathway for Tc removal and could be combined into further waste treatment approaches.

Impact of Cr and Co on 99Tc retention in magnetite: A combined study of ab initio molecular dynamics and experiments.

The effect of co-mingled dopants, Co(II) and Cr(III), on Tc(IV) incorporation and retention in magnetite under varying temperatures (75-700 °C) was explored using ab initio molecular dynamics simulations, batch experiments, and solid phase characterization. Tc(IV) stabilization was achieved with a magnetite surface oversaturated with or containing an equal number of Tc and Cr. Under oversaturation conditions, the forced formation of a Cr2O3 phase on the magnetite surface may help prevent Tc release. Upon Co addition, and depending on the relative concentration of Tc, Cr, and Co at the magnetite surface, Co was found to preferentially stabilize Cr rather than Tc and suppress the formation of the protective Cr2O3 surface phase. Only systems with similar Cr/Co concentrations or relatively high Cr concentrations stabilized Tc within magnetite. As such, the relative concentration of Tc, Cr, and Co was identified as a critical parameter for maximizing dopant efficacy towards Tc stabilization in magnetite.

99Tc immobilization from off-gas waste streams using nickel-doped iron spinel.

Technetium-99 (99Tc) incorporation within stable spinel phases is a novel method for 99Tc removal and immobilization from waste streams. In this study, transformation of Ni-doped Fe(OH)2(s) to spinel minerals, e.g. trevorite (NiFe2O4), is explored as a method for removing 99Tc from Hanford Waste Treatment and Immobilization Plant (WTP) primary off-gas waste stream simulant. The Fe(OH)2(s) transformation process was found to reduce 99Tc(VII) to 99Tc(IV) and incorporate reduced Tc(VI) into the produced spinel simultaneously. Nickel doping was applied in the mineral transformation to inhibit potential reoxidation of 99Tc(IV). Solid phase characterization by XRD and XANES confirmed the formation of nickel substituted ferric-spinel, and suggest incorporation of 99Tc(IV) in the final spinel. Furthermore, in the primary off-gas stream, which contains both redox-sensitive contaminants Cr(VI) and 99Tc(VII), results from solution analysis and solid digestion indicate that nearly 100% Cr and over 80% 99Tc can be simultaneously removed by adding Fe(OH)2(s) to solution with a solid to solution ratio of 5 g/L under near neutral and alkaline conditions. The 99Tc removal approach developed herein provides an alternative treatment method to eliminate the proposed recycle process of the off-gas waste stream, which ultimately can reduce WTP mission cost and operation time.