Iron oxide waste form for stabilizing 99 Tc Wooyong Um a,⇑ , Hyunshik Chang a,1 , Jonathan P. Icenhower b , Wayne W. Lukens b , R. Jeffrey Serne a , Nik Qafoku a , Ravi K. Kukkadapu a , Joseph H. Westsik Jr. a a Pacific Northwest National Laboratory, USA b Lawrence Berkeley National Laboratory, USA article info Article history: Received 23 February 2012 Accepted 1 June 2012 Available online 3 May 2012 abstract Crystals of goethite were synthesized with reduced technetium [ 99 Tc(IV)] incorporated within the solid lattice. The presence of 99 Tc(IV) as a substituting cation in the matrix and ‘‘armoring’’ by an additional layer of precipitated goethite isolated the reduced 99 Tc(IV) from oxidizing agents. These products were used to make monolithic pellets to quantify an effective diffusion coefficient for 99 Tc from goethite waste form contacted with a synthetic Hanford IDF (Integrated Disposal Facility) pore water solution (pH = 7.2 and I = 0.05 M) at room temperature for up to 120 days in static reactors. XANES analysis of the goethite solids recovered post-run demonstrated that the 99 Tc in the goethite crystals remains in the reduced 99 Tc(IV) state. The slow release of pertechnetate concentration with time in the static experiments with the monolith followed a square root of time dependence, consistent with diffusion control for 99 Tc release. An apparent diffusion coefficient of 6.15  10 11 cm 2 /s was calculated for the 99 Tc–goethite pellet sample and the corresponding leaching index (LI) was 10.2. The results of this study indicate that technetium can be immobilized in a stable, low-cost Fe oxide matrix that is easy to fabricate and these findings can be use- ful in designing long-term solutions for nuclear waste disposal. Published by Elsevier B.V. 1. Introduction Neutron-induced fission of 235 U-enriched nuclear fuel yields technetium isotopes in relatively large amounts, the most impor- tant of which is technetium-99 ( 99 Tc) [1–3]. Approximately 1 kg of 99 Tc with a long half-life (2.13  10 5 years) is produced for every ton of nuclear fuel ‘‘burned’’ in a typical reactor [4]. The radiological and chemical properties of 99 Tc present some unique problems for nuclear waste disposal. In virtually all near surface conditions, 99 Tc exists in the highly soluble and mobile pertechnetate form ½TcðVIIÞO  4  [5,6]. Environmental concerns have been raised because of the long half-life and high mobility of 99 Tc in oxidizing subsurface environments [7]. The highly soluble pertechnetate oxyanion, 99 TcðVIIÞO  4 , does not sorb onto most terrestrial sediments [8], so 99 TcO  4 migrates at nearly the same velocity as groundwater [9] under common subsurface conditions (i.e., pH close to neutral or slightly alkaline and suboxic conditions). In the natural environ- ment, there may be cases in which pertechnetate may react in locally reducing conditions, caused by microbial activity or with reduced inorganic metal phases, primarily magnetite and sulfides, that will induce reduction and retard the mobility of 99 Tc. Under reducing conditions, pertechnetate can precipitate as 99 Tc(IV)O 2 2H 2 O [10,11], sorb to mineral phases [7], and be retained in different natural environments [12–14]. However, even when reduced to more insoluble forms, the reoxidation of 99 Tc(IV) by changing redox conditions, such as contact with oxygen, can result in release of pertechnetate back into the environment [14], leading to the prediction of high 99 Tc release rates in many performance assessments [15–17]. For example, Lee and Bondietti [10] found that oxidation of an FeS/Tc(IV) precipitate allowed 70% of the 99 Tc to return to solution over a period of 8 months, which could be due to reoxidation back to 99 TcO  4 , or to solubilization of 99 Tc(IV) species at the low pH associated with oxidation of the sulfide [18]. Further, the long half-life of 99 Tc provides ample opportunity for 99 Tc(IV) to reoxidize and frustrates efforts to fashion a long-term stable immobilization matrix. To prevent 99 Tc release into the environment, plans are being drawn to sequester technetium into a stable waste form that will be inert to dissolution and oxidation reactions. Numerous waste forms have been proposed, including cement/grout [16,19], glass [20], hydroceramics [21,22], phosphate-bonded ceramics [21,23], and metal alloys [24]. Evaluating the efficacy of these proposed waste forms is difficult, because data that bear on the long-term immobilization of 99 Tc are sparse. Short-term laboratory tests have, however, underscored a potential pitfall for many of these materials: 99 Tc reduced to the more immobile 99 Tc(IV) form is vul- nerable to the infiltration of oxidizing agents, especially oxygen. 0022-3115/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jnucmat.2012.06.004 ⇑ Corresponding author. Address: Pacific Northwest National Laboratory, PO Box 999, P7-54, 902, Battelle Boulevard, Richland, WA 99354, USA. Tel.: +1 509 372 6227; fax: +1 509 371 6919. E-mail address: wooyong.um@pnnl.gov (W. Um). 1 Present address: Savannah River Ecology Laboratory, P.O. Drawer E, Aiken, SC 29802, USA. Journal of Nuclear Materials 429 (2012) 201–209 Contents lists available at SciVerse ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat