The optimisation of plutonium separation

Soil erosion processes present the greatest risk to land degradation worldwide and, due to fertile soil being an essential resource, there is increasing concern around the world regarding accelerated soil erosion, particularly in developing countries.

The analysis of plutonium (Pu) in soil samples can inform the understanding of soil erosion processes globally. However, there are specific challenges associated with such analysis in tropical soils, so an optimal analytical methodology that ensures the best sensitivity is critical.

Why use plutonium?

Due to their long retention times and minimal spatial variability, Pu isotopes have proven useful as an alternative fallout radionuclide tracer for determining soil erosion rates. To utilise Pu as an effective soil erosion tracer in the southern hemisphere, separation techniques and analyses need to be optimised to establish a robust analytical method for the determination of ultra-trace level Pu isotopes. This method must also have sufficient sensitivity for African soil samples, which typically have very low Pu concentrations compared to the northern hemisphere.

This research aimed to accurately establish fallout Pu activity concentrations in tropical soils in order to determine soil erosion rates with an improved separation and analysis method for ultra-trace Pu determination. To achieve this aim we had to:

• adapt and optimise a separation method using trialkyl methylammonium nitrate (TEVA) cartridges to remove matrix interferences with pre-concentration of ultra-trace Pu isotopes (this reduced waste and increased throughput)
• establish a robust analytical method for the determination of ultra-trace level Pu isotopes with sufficient sensitivity for African soil samples using oxygen as a reaction gas for inductively coupled plasma mass spectrometry (ICP-MS)

The development of robust analytical methods to determine rates of soil erosion and its effect on land degradation is vital to advise mitigation strategies, ultimately ensuring the future sustainability of soils.

Sophia Dowell, PhD student at BGS.

Where does the plutonium come from?

Pu is present in the environment primarily because of nuclear weapons testing. Between 1945 and 1980, 520 atmospheric tests were conducted worldwide; however, only 10 per cent of these experiments were conducted in the southern hemisphere. This resulted in significantly less fallout in the tropics than in the mid-latitudes of the northern hemisphere, which makes the analysis of ultra-trace Pu isotopes in tropical soils challenging.

The challenge of plutonium analysis

Due to their long retention time and minimal spatial variability, Pu isotopes have recently been used as an alternative fallout radionuclide tracer for determining soil erosion rates. As a result of the long half-lives of ²³⁹Pu and ²⁴⁰Pu (24 110 and 6561 years, respectively), approximately 99 per cent of the original activity remains in soils. This means they are suitable as stable, long-term tracers compared to, for example, 137 caesium (Cs), despite Cs’s significantly higher activity in the environment, as Cs only has a half-life of 30 years. Additionally, more than six times as many atoms of ²³⁹Pu and ²⁴⁰Pu were initially dispersed compared to ¹³⁷Cs. This combination of long half-life and higher atom content makes mass spectrometry (MS) techniques better suited to Pu isotopes, whereas radiometric decay counting techniques are more appropriate for the higher specific activity ¹³⁷Cs.

Consequently, recent developments in mass spectrometry techniques have the potential to increase the sensitivity of Pu isotope quantification and subsequently the availability of analytical methods applicable to tropical soils. This raises the potential of using Pu as a soil erosion tracer in the tropics, where the risk of soil degradation is increasing due to extreme weather patterns.

A powerful tool

This method presents a simple, cost-effective, robust sequence with reduced laboratory waste disposal, which is vital to ensure the separation method is applicable to low-resource laboratories. Along with the low detection limits that are comparable to alternative MS methods, this outcome makes the method applicable to the detection of ultra-trace fallout Pu in African soils.

Due to increasing concern regarding accelerated soil erosion and its impact on sustainable intensification of agriculture in developing countries, this work provides advancements in the detection of Pu. The new method is also a powerful tool for the analysis of ultra-trace Pu in African soils, ultimately improving the determination of soil erosion rates in tropical soils to better inform mitigation strategies.

This method has the potential to improve access to advanced soil erosion measurements that could be produced faster than traditional laboratory techniques to enable analyses at scale, yet with greater accuracy than machine learning predictions based on remote sensing data in developing countries which are most at risk to land degradation.

Sophia Dowell, PhD student at BGS.

Funding

BGS led the research in conjunction with the University of Plymouth and the University of Eldoret in Kenya.

Sophia’s PhD was supported by the NERC funded ARIES doctoral training programme (grant number NE/S007334/1), and from the NERC International National Capability grants to BGS (NE/R000069/1 and NE/X006255/1), Royal Society International Collaboration grant (ICA/R1/191077), British Academy (WW21100104) and BGS University Funding Initiative (GA/19S/017).

More information

The full research paper is available: Optimisation of plutonium separations using TEVA cartridges and ICP-MS/MS analysis for applicability to large-scale studies in tropical soils.