Environmental Science: Nano Blog

Significant quantities of radioactive compounds are contaminating our environment. Due to nuclear weapon production during the Cold War, large quantities of radioactive Technetium-99 (⁹⁹Tc) has been released into the environment. Although the Cold War ended 25 years ago, the total Tc inventory continues to rise as it is a major product during the generation of nuclear power resulting from the nuclear fission of uranium and plutonium.

Radioactive Tc is long lived and predominantly found in nuclear waste, in the highly mobile form of the water-soluble pertechnetate anion TcO₄^–. The high mobility possessed by TcO₄^– causes concerns regarding its distribution within the environment and makes storage impracticable within cement-based materials, a widely employed medium for the storage of nuclear waste.

Managing nuclear waste
Reduction of TcO₄^– from Tc(VII) to Tc(IV)  is one possible solution. This is a challenging solution as Tc(IV) tends to convert back to TcO₄^– if exposed to oxygen. The reduction process requires a proton source to proceed and is disfavoured under the strongly alkaline conditions found in the waste media. The nuclear waste tanks also contain a large amount of both spectator ions, which make TcO₄^– more difficult to reduce, and high-valent metal species such as Cr(VI). This can interfere with the reduction process by consuming the available Sn(II) and/or by converting any formed Tc(IV) back to TcO₄^–.

To overcome these obstacles, Eric D. Walter and colleagues have devised a novel Tin(Sn)-based material which displays selective and efficient removal of Tc under conditions similar to those found in nuclear waste tanks. The approach taken relies on the inclusion of both Sn(II) and Sn(IV) within an inert aluminophosphate matrix. The low-valent form of tin is known for its ability of reducing Tc(VII) to Tc(IV), whereas the high-valent Sn(IV) was chosen for its ability to form SnO₂ phases capable of accommodating the resulting Tc(IV) and thus facilitate its sequestration.

Representative SEM images of the Sn–Al–PO4 composite (A) before and (B) after exposure to TcO4 − .

The material labelled Sn-Al-PO₄ could be easily made from common laboratory chemicals and in this study was characterised by a number of analytical techniques investigating both the bulk and surface properties. It consists of two distinct phases:

1. An amorphous Sn-based matrix containing the majority of the aluminium.  This material has a large surface area and contains predominantly Sn(II), a combination of properties expected to yield efficient reduction of pertechnetate from the solution medium.
2. Small embedded fibres of crystalline material with the composition dominated by Sn and phosphate.

The full article is free to access for a limited time only:

Inorganic tin aluminophosphate nanocomposite for reductive separation of pertechnetate Environ. Sci.: Nano, 2016, Advance Article
DOI: 10.1039/C6EN00130K

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About the webwriter

Dan Mercea is a PhD student in the Fuchter group at Imperial College London. He is working on developing enantioselective FLP catalysis.

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Graphical Abstract

The precious element gold can exist in various forms, the most thought of probably being the shiny solid treasured for its aesthetic qualities. However, a number of other relevant forms exist not just in the chemistry laboratory but also in nature. Gold in its common oxidation states (I and III) can be found in water sources in the environment at appreciable concentrations. The coexistence of natural organic matter (NOM) and gold dissolved in surface water opens up the possibility for photoproduction of gold nanoparticles (AuNP), a form of gold which displays toxicity towards aquatic organisms and is prone to becoming distributed through the food chain upon ingestion.

In this Environmental Science: Nano paper entitled Aqueous photoproduction of Au nanoparticles by natural organic matter: effect of NaBH₄ reduction the authors’ attention is focused on investigating what NOM functionality is involved in AuNP formation, and which mechanism is being facilitated by its presence. Samples of NOM from various sources were included in the study and characterised using their content of carbonyl, quinone and aliphatic functionality. Samples with a large percentage of the former two showed an enhanced ability for AuNP production under simulated sunlight conditions, whereas a large content of the latter afforded inferior performance in this respect. In order to test whether this oxygen-containing functionality was responsibile for the facile production of AuNPs a selective chemical reducing agent, sodium borohydride (NaBH₄), was chosen to remove it from the NOM samples. This modification led to Au³⁺ reduction by NOM being observed, albeit at a much slower rate, which confirmed the importance of the carbonyl functionality and suggested that other functionality in the NOM was ultimately responsible for performing reduction.

One way in which the carbonyl functionality could accelerate the photoreduction process is by acting as a convenient electron source following the absorption of light, with the molecular oxygen/superoxide radical pair acting as an electron shuffle between the NOM and the gold cations. Formation of the superoxide radical on irradiation of NOM solutions was followed using electron paramagnetic resonance both before and after NaBH₄ treatment. Elimination of the carbonyl functionality hindered formation of the superoxide radical, however a correlation between superoxide concentration and AuNP formation rate could not be established. To test whether the superoxide radical was involved in the nanoparticle formation process, the enzyme superoxide dysmutase was added. The enzyme efficiently removed the superoxide radical, which led to an expected decrease in rate. These observations helped support the operation of the indirect reduction mechanism, however not to the dismissal of alternatives. Indeed another mechanism which is believed to operate in parallel is based on the charge transfer from the NOM directly to the gold cations in complexes formed between the two (the charge transfer mechanism).

Experiments carried out with model substrates of aromatic ketones and quinones further supported the role that had been ascribed to the carbonyl functionality. Aromatic ketones proved superior over the quinoid model compounds in the generation of nanoparticles, as faster reduction was achieved even at much lower substrate concentration and indicated that NOM quinones played only a secondary role in the production of AuNPs. Noteworthy was the decrease in photoreduction rate observed upon NaBH₄ treatment of quinones, which indicated that the resulting phenolic functionality was not responsible for photoreduction as had been previously believed.

Jiahai Ma and co-authors from the School of Chemistry and Chemical Engineering have shaped our understanding for the central role that the aromatic ketone functionality in natural organic matter plays in the photoproduction of gold nanoparticles in the aquatic environment.

Read the full article for free*:

Aqueous photoproduction of Au nanoparticles by natural organi matter: effect of NaBH₄ reduction
Zilu Liu, Pengfei Xie and Jiahai Ma
Environ. Sci.: Nano, 2016, Advance Article
DOI: 10.1039/C6EN00126B, Paper

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About the webwriter

Dan Mercea is a PhD student in the Fuchter group at Imperial College London. He is working on developing enantioselective FLP catalysis.

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*Access is free until 24/08/2016 through a registed RSC account – register here