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Sat Sep 28, 2019, 11:43 AM

Properties of Alkali Metal Salts of the Radioactive Pertechnetate Ion.

Last edited Sat Sep 28, 2019, 12:50 PM - Edit history (1)

The paper I will discuss in this brief post is this one: Chemical Trends in Solid Alkali Pertechnetates (McCloy et al, Inorg. Chem. 2017 56 5 2533-2544)

Happily the paper is open sourced under a Creative Commons license and anyone can read it in its entirety in itself.

From time to time, I like to read about the Hanford Reservation Waste tanks, which feature some extremely interesting radiochemistry, because of the complex nature of their contents, which have undergone a rather interesting series of chemical processes on an industrial scale.

Several of the tanks are famously leaking, exciting the bizarre selective attention of anti-nukes. With a few exceptions, anti-nukes are a spectacularly poorly educated set of people, whose poor educations and poor ethical and moral sense lead them to consider the events at the Hanford plant, which was essentially a nuclear weapons plant, as one of the most dire emergencies ever. Thus they love to proudly point out that billions of dollars are being spent to clean up Hanford, even though these billions of dollars will save far fewer lives than spending billions of dollars on improved sanitation for the billions of people who lack access to even primitive forms of it. The reason that cleaning up Hanford will not save very many lives is because very few lives are actually at risk.

It is true that left unattended, some radionuclides, dominated by technetium in the form of the pertechnate ion, will leak into the Columbia River, but at concentrations that are not likely to have profound health consequences. It is well known among medical types that people either eat or are injected with technetium - the 99m isomer which decays in vivo to exactly the same isotope as in the Hanford tanks, Tc-99 - for medical diagnostics and treatment.

To wit, from the introduction to the paper:

Technetium (Tc, element 43) was first isolated in 1937 by the Italian researchers Perrier and Segre from molybdenum foil that had been bombarded with deuterons. Since its discovery, it has been found to have uses in the medical industry (∼85% of all radionuclear scans in the U.S. utilize short-lived 99mTc1), in the steel industry as a corrosion inhibitor,(2) and as a low-temperature superconductor.(3) Additionally, identification of Tc in the spectrum of a variety of star types leads to new research in the solar production of heavy elements.(4, 5) From the standpoint of inorganic chemistry, the research completed on the element since the late 1930s has worked to fill in the gaps of understanding the periodic trends for transitions metals, particularly those related to the formation of chromium and rhenium oxyanions.(6)

Of its 21 isotopes (mass numbers 90111), all of which are radioactive, 99Tc is the most common and abundant, and because of its high mobility under oxidizing conditions in the subsurface environment and long half-life (t1/2 = 2.11 105 years), 99Tc is considered to be a significant environmental hazard.(1, 2)99Tc is common on earth today because it is a byproduct of the fission of uranium and plutonium in nuclear reactors: 6.1% and 5.9% mass yields, respectively. It is the only long-lived Tc isotope produced on a gram scale by this pathway. It is also the daughter product in the decay of 99mTc. For long-term nuclear waste management, 99Tc is the dominant producer of radiation in the period from about 104106 years, in becquerels (Bq) per mass of spent fuel. Because of its radiotoxicity (99Tc is a soft β emitter, 292 keV), high environmental mobility, and long half-life, its immobilization and safe long-term storage is a priority goal to countries that are in the process of disposing of nuclear waste. A current challenge to efficient immobilization of the radioisotope can be linked to a lack of understanding of the behavior of Tc during vitrification, the process of turning nuclear waste into glass, and it localized chemistry in waste glasses.

The bold is mine.

Most of the papers on the interesting chemistry of technetium focus on medical use, but the second most common class of papers focuses on immobilizing technetium in order to treat it as a waste product. I personally object to this second focus, since I regard technetium as an extremely valuable metal, owing to its close similarity, as a consequence of the lanthanide contraction, to its rare cogener rhenium, a valuable element utilized most importantly in the preparation of superalloys, showing high thermal and corrosion resistance.

The migration of radionuclides from the Hanford tanks is of interest to compare to geologically historical migrations of the natural nuclear reactors that operated about two billion years ago, most famously at Oklo in modern day Gabon, but in several other places as well. Analysis of these situations shows that these nuclides traveled hundreds of meters before decaying, although their stable decay products are notably depleted in ruthenium-99, the decay product of technetium-99.

From the leaking tanks at Hanford, a similar effect is being observed.

It is very likely that, as at Oklo, many of the shorter lived nuclides will decay before they travel the distance reach the Columbia River, although some will not. Consider the Cs-137 of a tank filled in the 1950's. About 77% of the cesium dumped into the tanks in 1955 have decayed to stable barium. Moreover, much of the radiocesium in some of the tanks has been removed for use as radiation sources, meaning that the concentration is low. After years and years of effective ion exchange in components of the soil, slowing the migration, invariably coupled with dilution, it is unlikely that concentrations of cesium comparable to the natural radioactivity associated with essential potassium will effectively harm anyone. If it did or does, the number of persons so injured will be dwarfed by the number of people who die today from air pollution, which will be about 19,000 people. They may ultimately harm someone, but compared to other risks experienced by humanity, notably climate change, these risks are exceedingly small.

This is why the people who love to complain about events are Hanford are so morally stupid. They think that their poor educations, and lack of even a shred of education about radiochemistry, implies that Hanford - a nuclear weapons plant largely unconnected with nuclear power - trumps concern about the 7 million people who will die this year who will die from air pollution. And "trumps" is the right word. Their selective attention is Trumpian in its stupidity.

Anyway, the paper is open sourced, and anyone can read it.

I was drawn to it because I was thinking about the technetium at Hanford - there are about 1200 kg of these valuable metal there, and I found myself wondering about the alkali metal salts of the pertechenetate ion.

The part that caught my eye was the experimental section, which I reproduce here:

Tc was obtained from the U.S. DOE Isotopes Program, Oak Ridge National Laboratory (ORNL), Oak Ridge, TN. As received, Tc was in the form of solid NH4TcO4, black and badly decomposed from its own β radiation. The as-received material (approximately 10 g) was transferred to a 125 mL glass Erlenmeyer flask. All chemicals used in this work, except 99Tc, were analytical reagent grade. Water was taken from a Millipore deionizer, 18 MΩ cm or higher resistivity. Approximately 50 mL of concentrated NH4OH and several milliliters of 30% H2O2 were added. The mixture was stirred with a magnetic stir bar and gently warmed, with a small glass funnel serving as a spray trap on the top of the flask. NH4TcO4 dissolved in a few minutes to give a colorless solution of NH4TcO4. The solution was warmed at near boiling for 1 h to decompose H2O2, and then the spray trap was removed to allow the water and NH4OH to evaporate. After nearly all of the water had evaporated, fine crystalline NH4TcO4 was centrifuged out of the solution, washed with dry ethanol to remove water, and then dried to a constant weight at ∼90 C.
Ammonium pertechnetate was converted into aqueous HTcO4 by cation exchange. Crystalline ammonium pertechnetate was dissolved in water and then passed through a column of hydrogen-form cation resin (Dowex 50W-X8, 50100 mesh, 15 mL resin volume, in water). This volume of resin has a cation capacity of about 1.8 g of NH4TcO4, loaded to 50% of theoretical capacity. By loading to only 50% of the cation capacity of the resin, no cations break through the column to contaminate the pertechnic acid. The cation column effluent was periodically checked with pH strips to confirm that the effluent was always acidic. The column effluent was transferred to a 125 mL round-bottom flask. The solution was stirred and heated, while dry nitrogen gas was gently blown over the surface of the solution to evaporate the water. The nitrogen gas quickly evaporated the water and kept the temperature low enough to avoid bubbles breaking at the surface, even with the heat set high, so that no spray from bursting bubbles could carry Tc out of the flask. This apparatus permits dilute pertechnic acid to be safely evaporated and concentrated without causing airborne β contamination in the fume hood.

Sodium pertechnetate was prepared by neutralizing aqueous HTcO4 with aqueous NaOH to pH 7. The product NaTcO4 solution was then evaporated under flowing nitrogen in a 125 mL round-bottom flask, with heat and stirring, until only wet-crystalline NaTcO4 remained in the flask. The solid NaTcO4 was dried to a constant weight at 120 C to make the anhydrous salt. Potassium pertechnetate was prepared in the same way, using KOH. Cesium and rubidium pertechnetates were prepared by combining stoichiometric amounts of Rb2CO3 and Cs2CO3, weighed as dry solids, with accurately weighed amounts of NH4TcO4 dissolved in water. The product solutions of RbTcO4 and CsTcO4 were evaporated to expel water and ammonium carbonate. When the volume was 23 mL, the crystalline products were centrifuged out of solution and then dried to a constant weight at 120 C.

What I found interesting is the state of the ammonium pertechnate as received, which was black. This indicates radiochemical self reduction of the metal to TcO2, which is largely insoluble, suggesting that in high radiation fields, the migration is retarded by self-reduction and reoxidation.

Secondly I was intrigued by the description of the preparation of the cesium salt, which involved crystallization from water. While the solubility of cesium pertechnetate was not given, this means that the solubility is limited, an interesting fact of potential utility in considering the chemistry of used nuclear fuels.

I hope you're having a pleasant Saturday. I'm feeling a little under the weather myself, but reading this paper made me feel a little better.

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Reply Properties of Alkali Metal Salts of the Radioactive Pertechnetate Ion. (Original post)
NNadir Sep 28 OP
Chin music Sep 28 #1
eppur_se_muova Sep 28 #2
NNadir Sep 28 #3
NNadir Sep 28 #4

Response to NNadir (Original post)

Response to NNadir (Original post)

Sat Sep 28, 2019, 02:01 PM

2. Well, I had to look up the solubilities of some analogs ...

KMnO4 is not all *that* soluble -- ~0.4M at sat'n, RT. CsMnO4 has only ~5% of that. Hard to find data on other analogs, but I assume the perrhenates are known.

Too bad Tc(VI) doesn't appear to be isolable -- I would expect BaTcO4 to be *really* insoluble, as are BaMnO4 and BaMoO4.

((For those who don't yet know, "104106 years" is supposed to indicate 10,000 to 1,000,000 years, but DU doesn't do superscripts anymore. Blame the election night hacker.))

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Response to eppur_se_muova (Reply #2)

Sat Sep 28, 2019, 02:47 PM

3. This 1972 patent reports the solubility of potassium perrhenate as 12 g/l, as opposed to the Na...

...salt as 250 g/liter.

US 3704211

It's a patent, so it's not necessarily filled with truth.

Still, it's indicative of the general trend. I would not be surprised to learn that some of the sludges in the Hanford tanks actually contain cesium pertechnetate, although if I came across this in my readings on the Hanford tanks, I would think I'd remember it and I don't.

Almost all papers modelling technetium chemistry that do not actually involve technetium utilize rhenium. Many papers with actual technetium chemistry also consider rhenium. Because of the lanthanide contraction their atomic radii are almost identical. This implies that their chemical separations would be challenging, as is the case with zirconium and hafnium, and especially tantalum and niobium.

This has little current industrial interest, but I could imagine situations where it would be, were one to make machinable tungsten alloys by substituting rhenium with technetium. In a neutron flux, some of the tungsten would be transmuted into rhenium, particularly in "breed and burn" type reactors with irradiation periods of decades, or - should they ever go commercial - fusion reactors.

Recycling the alloys might thus require this separation.

Thanks for pointing out the sad matter of the lack of subscripts and superscripts at DU. It's unfortunate for those of us who use the science forum. I really don't understand why that code represented a security risk, but I'm not a programming type.

The half-life of technetium (the 99 low energy isomer) is, according to the BNL Nuclear Data site, 211,100 years. This means it is definitely usable in alloys, particularly those utilized in high temperature nuclear applications.

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Response to eppur_se_muova (Reply #2)

Sat Sep 28, 2019, 03:34 PM

4. You got me thinking, and I found this old paper...

...from 1966 which reports the use of CsCl as a "precipitating solution" for perrhenate.

Rate of oxygen exchange between the perrhenate ion and water (Murmann, J. Phys. Chem.1967 71 4974-978)

From the text:

Preparation of Materials. NaRe04 enriched inO18 was prepared from a concentrated (1 g of Re/ml) solution of HRe04 (S. W. Shattuck Co.) by mixing with an equal volume of 8 X 018-enriched water, redistilled from KMn04. After equilibration for 2 hr, which was known from previous work to cause complete exchange, reagent grade Na2C03(s) was slowly added in slight excess. Then a small excess of HRe04 was added until it was just acidic after boiling. The solution was filtered and concentrated to half its initial volume under vacuum at room temperature and AR acetone (50 ml/g of Re) added. It was filtered to remove a trace of insoluble material and AR ethyl ether added (200 ml/g of Re). After 2 hr at 0 the white precipitate was collected, washed with anhydrous ether, and dried at 70 under vacuum for 2 days. Yield was 80-90% of theory. Analysis2 for Re gave 68.2% (calculated for NaRe04, 68.16). The analysis only shows the compound to be essentially pure; it would not indicate the presence of eatalytically important impurities. In order to clarify this point, NaRe04 was prepared through recrystallized AgRe0418 by reaction with the stoichiometric amount of AR NaCl. After removal of the AgCl, the solution was evaporated until crystallization occurred and the product was removed and dried under vacuum. The two preparations of NaRe04 showed identical (1%) rates of oxygen exchange in both the acidic and basic regions. LiCl, NaCl, and LiN03 stock solutions were prepared from AR solids and the filtered solutions analyzed for content by standard methods. The CsC1-H20 precipitating solution was made by adding ca. 25 g of AR CsCl to 50 ml of filtered solution.

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