Technetium Removal Using Tc-Goethite Coprecipitation

Abstract

Executive SummaryThis report describes the results from laboratory tests performed at Pacific Northwest National Laboratory for the U.S. Department of Energy (DOE) EM-31 Support Program (EMSP) subtask, "Low temperature waste forms coupled with technetium removal using an alternative immobilization process such as Fe(II) treated-goethite precipitation" to increase our understanding of 99 Tc long-term stability in goethite mineral form and the process that controls the 99 Tc(VII) reduction and removal by the final Fe (oxy)hydroxide mineral forms.The overall objectives of this task were to 1) evaluate the transformation process of Fe (oxy)hydroxide minerals to more crystalline goethite (α-FeOOH) Tc removal was not considered to be from surface adsorption, but incorporation into transformed mineral product. Although a further analysis is essential to confirm 99 Tc speciation in the final transformed product from Fe(OH) 2 (s), mineral transformation from Fe(OH) 2 (s) can be used to effectively remove 99 Tc(VII) in alkaline pH conditions germane to off-gas scrubber secondary waste and low-activity waste streams. Slightly more transformation products were also found from Fe(OH) 2 (s) with a solution-to-solid ratio of 100 than with a ratio of 1000. The transformation product from ferrihydrite was similar, and solely goethite mineral was produced in most of the conditions. It is clear that comparison results from both Fe(OH) 2 (s) and ferrihydrite substrates show that high slurry pH and high temperature conditions favor more goethite formation from the transformation reaction. Unfortunately, we could not conduct 99 Tc removal using ferrihydrite as an initial substrate in this task, but based on previous results (Um et al. 2010), ferrihydrite can also be used to remove 99 Tc with additional aqueous Fe(II). Transformation product from magnetite was very limited and there were only small amounts of maghemite and goethite formed from magnetite reacted as initial substrate under high pH and temperature conditions. In addition, negligible 99 Tc removal (<5%) from solution was also found in magnetite slurry without aqueous Fe(II) addition.Even after long-term leaching in the Integrated Disposal Facility (IDF) pore water solution for two years, very limited amounts of 99 Tc and Fe(total) were detected in the IDF leachates. The limited 99 Tc release was attributed to more stabilized 99 Tc(IV) present in 99 Tc-goethite lattice even after long-term exposure to oxygen, while zero detected Fe(total) concentration in the IDF leachate indicated that the iv structure of the final goethite mineral was stable in a circumneutral pH condition similar to the IDF pore water. Reduced 99 Tc(IV) incorporated in the goethite was unlikely to be reoxidized to 99 Tc(VII), even when the final 99 Tc-goethite product (sample 2-5) was exposed to oxidizing conditions for three years (two years in IDF solution plus one year in air). In addition, the measured reductive capacity of both 99 Tc-goethite 2-2 sample (prepared without additional armoring pro...

Weathering of ancient and medieval glasses—potential proxy for nuclear fuel waste glasses. A perennial challenge revisited

“…Hence, it is suggested that alteration of the glasses under thoroughly reducing conditions sustained over very long time will keep the Tc ions confined to an insoluble secondary phase akin to pyrolusite, thus preventing their hazardous accumulation in the biosphere. Recent research suggests that Tc(IV) ions can be sorbed onto FeS 2 surfaces or incorporated in iron oxides such as hematite (α‐Fe 2 O 3 ) or magnetite (Fe 3 O 4 ) as well as iron oxihydroxides such as goethite (α‐FeOOH) . The mechanism relies on direct substitution of Tc(IV) onto Fe(III) lattice sites that may be retained stably under environmental conditions to be expected in a nuclear waste disposal vault.…”

“…Recent research suggests that Tc(IV) ions can be sorbed onto FeS 2 surfaces 102 or incorporated in iron oxides such as hematite (a-Fe 2 O 3 ) 103 or magnetite (Fe 3 O 4 ) 104 as well as iron oxihydroxides such as goethite (a-FeOOH). 105,106 The mechanism relies on direct substitution of Tc(IV) onto Fe(III) lattice sites that may be retained stably under environmental conditions to be expected in a nuclear waste disposal vault.…”

Technetium Immobilization Forms Literature Survey

“…If such Tc incorporation into these solid phases takes place, then the mobility of Tc may depend on the stability of the host phase and not necessarily upon the reoxidation kinetics. a The coordination number refers to the number of associated ligands and LS and HS stand for "low-spin" and "high spin" configurations Um et al (2010Um et al ( , 2011aUm et al ( , 2012Um et al ( , and 2013Westsik et al 2011) have been conducting experimental studies on sequestering Tc in goethite and other iron oxide solid phases. Their first studies showed that goethite could sequester significant amounts (>90% of the initial Tc mass) of Tc present in simple and caustic brine simulants as long as additional aqueous Fe(II) was added and the pH of the slurry was adjusted to neutral to mildly caustic conditions.…”

“…This post-Tc-reduction/incorporation step forms a physical armoring layer that provides a second barrier to leaching and reoxidation of Tc(IV) (Um et al 2011a). Um et al (2013) showed that Fe(OH) 2 (s) readily reduced and then sequestered Tc and transformed to a mixture of magnetite, maghemite, 1 and goethite even at room temperature and circumneutral pH conditions. Removal of 99 Tc from solution by Fe(OH) 2 (s) was fast, and more than 95% of the initial 99 Tc (10 -5 M) was removed from Tc(VII)-spiked distilled water even without aqueous Fe(II) addition.…”

“…It was noticed that final Tc sequestering results from both Fe(OH) 2 (s) and ferrihydrite substrates show that the high slurry pH and high-temperature conditions favor more goethite formation and slightly higher Tc removal. For the ferrihydrite tests performed in Um et al (2013), only the initial 10 -5 M Tc(VII) condition was tested and aqueous Fe(II) addition was used during the Tc(VII) sequestration process. The removal rate for Tc(VII) when aqueous Fe(II) was added to the ferrihydrite slurry was ~90%.…”