Photochemical Reduction of the Soluble Radioactive Pertechnate Ion to Insoluble TcO2.

[NNadir]

The paper I'll discuss in this post is this one: Efficient Photocatalytic Reduction of Aqueous Perrhenate and Pertechnetate (Shi et al, Environ. Sci. Technol. 2019, 53, 18, 10917-10925)

Technetium is a synthetic element - the element in the periodic table with the lowest atomic number for which no stable isotopes exist - that is often regarded as so called "nuclear waste," something which is true in the paper I'm about to discuss. (I personally argue that there is no such thing as "nuclear waste" in the absence of stupidity, fear and ignorance, but that's my opinion. Fear and ignorance are far more popular and far more powerful than any of my opinions will ever be.)

The most common use for technetium is in medicine, a short lived nuclear isomer Tc-99m is the workhorse of medical imaging as well as some treatment modalities. It decays to the same isotope as is found in used nuclear fuel, Tc-99. People who have undergone medical testing and medical treatment with Tc-99m generally piss the resultant Tc-99 decay product away, because in general, it is in the form of the highly soluble TcO4- anion, known as the pertechnetate ion. In addition, unlike other soluble radioactive fission products such as isotopes of cesium and strontium (although strontium sulfate and carbonate are insoluble its nitrate is quite soluble) the pertechnetate ion has a fairly low affinity for adhesion to minerals. It migrates quite readily.

Historically fission product technetium from commercial nuclear reprocessing has been dumped into the ocean. This was true at both Sellafield in the UK and at La Hague in France, which is unfortunate, not because there is an incredible risk to the environment because of this practice, but because the potentially valuable element was not recovered.

Technetium metal has many interesting properties, both as a surrogate or potential replacement for the relatively rare and expensive element rhenium which is essential to modern technology. In other ways in which it is actually superior to rhenium, for example in dehydrogenation reactions for alcohols, chemistry which conceivably might play a role in the elimination of the mining of dangerous petroleum - with all the observed tragedy that represents - for the production of polymers: (cf. Theoretical design of a technetium-like alloy and its catalytic properties Koyama and Xie, Chem. Sci., 2019, 10, 5461-5469. The authors of this paper claim, without much justification, that technetium is "too dangerous" to use and therefore attempt to duplicate its electronic structure by alloying other metals.)

The pertechnetate ion is an excellent corrosion inhibitor, and personally I have been extremely interested in technetium alloys, some of which have extremely valuable properties. The hardness of technetium tetraboride is exceeded only by its rhenium analogue.

I'm not necessarily a big fan of nitric acid dissolution of used nuclear fuels - I think there are better approaches to performing this essential task - but the reality is that this has historically and is probably still the most prevalent way the valuable materials in them are recovered. In nitric acid type dissolutions, the chemical form of technetium is generally the pertechnetate ion. This is, for example, how it is found in the Hanford tanks that dumb anti-nukes always carry on about, even though they are spectacularly disinterested in the 7 million air pollution deaths that occur each year because we don't have more technetium.

The recovery of technetium for the exploitation of its many useful properties, now that it is available to humanity, will therefore require facile methods for its removal from aqueous solutions of pertechnetate, which is why this paper caught my eye.

From the introduction, covering some of what I've just said and some things I didn't say:

Technetium-99 (99Tc), a ?-emitting isotope (?–max = 293.7 keV), is generated from thermal-neutron-induced fission of uranium-235 (235U) and spontaneous fission of 238U in the earth’s crust.(1,2)99Tc is also formed from the decay of the medical radioisotope 99mTc with a half-life of only 6.0 h.(3) The most common chemical form is pertechnetate 99TcO4–, which is of particular environmental concern due to the long half-life of 99Tc (2.13 × 105 years)(1) and the resistance to adsorption on mineral surfaces and sediments that results in migration with potential ecosystem risks.(4?7)

Because all technetium isotopes species are radioactive, research progress is challenging. As a result, rhenium (Re) is often used as a nonradioactive chemical analogue of 99Tc.(8?11) One of the various methods used for removal of 99TcO4–/ReO4– from aqueous solution is conventional solvent extraction.(12,13) Nevertheless, there remain shortcomings, such as utilization of large amounts of toxic and volatile organic reagents, resulting in production of secondary wastes. Alternative ion exchange methods(14?16) require high quality of raw liquid to avoid column blockage. Despite a recent breakthrough toward TcO4– elimination via molecular recognition,(17) long-term storage stability of Tc-containing materials requires further attention, and large-scale practical applications have not been demonstrated.(18) Solid waste forms for 99Tc immobilization include metals such as Tc-Zr alloys(19) and borosilicate glasses.(20) Disadvantages of the latter are oxidation and release of volatile Tc molecules during high-temperature vitrification.(1)

An appealing method to immobilize 99Tc is reduction of soluble Tc(VII) to sparingly soluble Tc(IV) with removal from aqueous solution as 99TcO2·nH2O species,(8,21) which can be separated by physical filtration and then converted to metal or other waste forms for long-term disposal.(19,20)

Common reducing agents such as SO32–, Sn2+, Fe2+,(9,22,23) and biomass(24,25) are exhausted in one cycle and not readily reused. Using Fe(0)/Fe(II) as the reductant couple, 99Tc/Re was sequestrated using a simultaneous adsorption–reduction strategy.(21,26?28) Electrochemical methods(29?31) involve toxic chemicals, and furthermore, the presence of SO42– suppressed Re(VII) reduction in aqueous solution. Although ?-radiation-induced reduction(32) via hydrated electrons might efficiently reduce and separate Re(VII), the conditions are impractical. Photochemical-induced reduction(31,32) of Re(VII) using broadband UV or laser irradiation over 6 h afforded 94.7% recovery of Re; unfortunately, the high molar absorptivity of Re(VII) limits the practical concentration of Re(VII).

Heterogeneous semiconductor-based photocatalytic reduction of heavy metal ions such as Cu2+, Hg2+, Ag+, U(VI), and Cr(VI)(33?37) has been proposed. Many photocatalysts are regarded as environmentally friendly materials because of their chemical inertness and biological compatibility in natural systems. For example, titanium dioxide (TiO2) is a good prospect for photocatalytic reduction and removal of metal ions due to its high resistance to photocorrosion, nontoxicity,(38) low environmental pollution, regeneration ability, low cost, and convenient operations.(38,39) Evans et al.(40) reported selective removal (98%) of uranium from waste liquid containing strong complexing agents using TiO2 as a photocatalyst. Wang et al.(41) prepared a TiO2/g-C3N4 heterojunction composite that facilitated rapid separation and transfer of photogenerated electrons, thus achieving efficient reduction and fixation of uranium...

...The objective of this study was to provide fundamental understanding of photocatalytic 99Tc/Re reduction and removal using TiO2 nanoparticles in the presence of HCOOH. Most of this work was still conducted using nonradioactive ReO4– as a surrogate for 99TcO4–.(8,42) Anyway, the reported 99Tc(VII/IV) redox potential (E0 = +0.74 V) is somewhat more positive than that for Re(VII/IV) (E0 = +0.51 V), which means that photocatalytic reduction of Tc(VII) should be more energetically favorable. In addition, the reduction/removal mechanism was elucidated by photoelectrochemical measurements, electron paramagnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. These results suggest an environmentally friendly photocatalytic approach for 99TcO4–/ReO4– removal and sequestration from aqueous solution.

Titanium dioxide is a very cool photocatalyst in general, love it!

The experimental light source here is in the UV range, 320 nm, which means we can't apply in a verified way the magic word on which we've bet the planetary atmosphere with poor results, "solar" although the authors are happy to apply this word, although the experiments, using a xenon lamp, were UVa radiation.

UV radiation is continuously available by downrating X-rays and gamma rays from fission products by the use of barium fluoride, so this should not represent much of a problem in a putative reprocessing industrial plant.

Most of the work was performed using a rhenium surrogate for technetium, although ultimately technetium was directly utilized:

Tc Removal

99Tc was obtained as a 2% HNO3 stock solution of potassium pertechnetate (KTcO4) from China Institute of Atomic Energy. The 99Tc experiments were performed in a special radiological laboratory. In accordance with the above experimental protocol for Re, the corresponding 99TcO4– solution was illuminated for 150 min under the identified optimal Re(VII) reduction/removal conditions. Residual concentration of 99Tc was analyzed by a liquid scintillation counter (Tri-Carb, PerkinElmer). Aliquots of 0.5 mL were periodically collected during light irradiation and filtered through 0.2 ?m Millipore membranes before analysis. 0.2 mL of the filtrate was then mixed with 5 mL of liquid scintillation cocktail (ULTIMA Gold, PerkinElmer) and held in a 6 mL plastic scintillation vial for measurements. The reacted suspension was stirred in air to observe the reoxidation and release of reduced Tc.

Some pictures from the text:

Figure 1. (A) Removal of Re(VII), for no TiO2 and 0.4 g L–1 TiO2 in different conditions; pH = 3, [HCOOH] = 1%, [Re(VII)] = 5 mg L–1. Removal of Re(VII) for different concentrations of HCOOH, for (B) no light and (C) UV–visible irradiation; pH = 2, [Re(VII)] = 10 mg L–1. (D) Removal of Re(VII) with different organic additives under light irradiation; pH = 3, [organic additive] = 1%, [Re(VII)] = 5 mg L–1. V = 50 mL, T = 298 K throughout.

Figure 2. (A) First-derivative EPR spectra of DMPO spin adducts. In the dark: TiO2, HCOOH, and TiO2/HCOOH/Re(VII); under light: TiO2, HCOOH, TiO2/HCOOH, and TiO2/HCOOH/Re(VII). (B) TiO2 current–potential measurements: (black ■ Idark with 0.1 mol L–1 Na2SO4 + 0.1% HCOOH + 5 mg L–1 Re(VII); (blue ● Iphoto with 0.1 mol L–1 Na2SO4; (red ▲ Iphoto with 0.1 mol L–1 Na2SO4 + 0.1% HCOOH; (green ▼ Idark with 0.1 mol L–1 Na2SO4 + 0.1% HCOOH + 5 mg L–1 Re(VII).

Figure 3. Time profiles of Re(VII) reduction during the irradiation of TiO2 suspensions with N2 bubbling, V = 50 mL, T = 298 K. (A) Various dosages of TiO2, [HCOOH] = 1%, [Re(VII)] = 10 mg L–1, pH = 2. (B) Effects of initial Re(VII) concentration, [HCOOH] = 1%, 0.2 g L–1 TiO2, pH = 2. (C) Influence of NO3– concentration, [HCOOH] = 1%, [Re(VII)] = 10 mg L–1, 0.2 g L–1 TiO2, pH = 2. (D) Solution pH values, [HCOOH] = 0.2%, [Re(VII)] = 10 mg L–1, 0.4 g L–1 TiO2.

Figure 4. (A) Cycling runs of TiO2 for photocatalytic reduction of Re(VII). Time profiles of Re(VII) reduction during the irradiation of 0.4 g L–1 TiO2 suspensions at pH = 3, with N2 bubbling, [HCOOH] = 1%, [Re(VII)] = 5 mg L–1, V = 50 mL, T = 298 K. (B) Color change of both solid and solution before and after photocatalysis.

Figure 5. Time profiles of 99Tc(VII) and Re(VII) reduction during the irradiation of 0.4 g L–1 aqueous TiO2 suspensions at pH = 3, with N2 bubbling, [HCOOH] = 1%, [99Tc(VII)] or [Re(VII)] = 0.05 mmol L–1, [NO3–] = 20 mmol L–1, V = 50 mL, T = 298 K.

I'm not convinced this process is necessarily worthy of industrialization. The text suggests that nitrate is a problem.

I think it's time to move past the workhorse Purex type solvent extraction process and there are many other approaches to the recovery of technetium for use, but one can imagine this process being of some utility in some places, for example, in extant situations where pertechnetate is migrating in the environment.

I trust you're having a nice afternoon.

[gibraltar72]

1. Took the words right out of my mouth.