Microbes for managing waste7 November 2017
Professor Katherine Morris and her collaborators at The University of Manchester have developed new analysis of biogeochemical reactions affecting the long term environmental stability of radioactive elements. Her work using synchrotron X-ray absorption spectroscopy affects the safety case for geological disposal of nuclear waste and offers potential new biotechnological remediation solutions.
Over the last 60 years, the UK has accumulated over 4.77 million cubic metres of radioactive waste and spent nuclear fuel. More than 90% is low or very low-level waste, 9% is intermediate level waste, and only 0.03% is high level waste and spent fuel. Discharged spent fuel is stored under water, which provides an effective radiation shield and conducts away excess heat. But it can’t stay there forever: higher activity waste contains long-lived radionuclides of technetium, neptunium and plutonium formed in the reactor, and uranium-238 (VI) from fuel, all of which remain radiotoxic for tens to hundreds of thousands of years – and, in the case of U-238, billions. Ultimately, the spent fuel and intermediate level wastes from handling it are destined for deep geological disposal. The plan is to grout intermediate level waste into steel canisters and bury it hundreds of metres underground in impermeable rocks such as argillaceous or plutonic rocks, before being backfilled with cementitious materials. This deep geological disposal facility (GDF) will be designed to keep waste encapsulated for thousands of years before the engineered barrier degrades, the waste gradually penetrates and integrates back into the environment.
Professor Katherine Morris from The University of Manchester is Diamond Light Source’s top science user on synchrotron- based radioactive research. She is the nuclear environment and waste lead for the university’s Dalton Nuclear Institute. Her work focuses on the boundary between radiochemistry, mineralogy and microbiology. Working with collaborators including Jon Lloyd, Professor of Geomicrobiology, she is illuminating the environmental fate of anthropogenic radionuclides and informing our contaminated land strategy.
It is crucial to understand how radionuclides behave in the inorganic and organic environment. The rates of and mechanisms for their interactions are affected by a host of factors including canister degradation rates, rock porosity and swelling capacity, groundwater supply and, perhaps surprisingly, microbial processes.
Morris’s work has illustrated not only how waste will affect the environment, but also how the environment will affect waste.
“The groundwater will definitely encounter radionuclides over time,” says Morris’s colleague Dr Laura Newsome. “Eventually the groundwater will dissolve the cement to form an alkaline plume around the repository.”
It was this high pH environment that inspired the BIGRAD (BIogeochemical Gradient and RADionuclide transport)
project funded by NERC. A consortium project spanning seven universities and five years (2010-2015), BIGRAD aimed to explore the untapped territory of biogeochemical processes under high pH conditions reflective of the alkaline gradient surrounding the repository.
BIGRAD researchers predicted that radioactive elements including TcO4-, UO22+, NpO2+ would transform and be scavenged onto sediments under biologically reducing conditions. Results confirmed the microbially catalysed reduction of radioactive elements under alkaline conditions, and their association with iron-based minerals. The presence of iron could be an important factor: metal-reducing organisms are able to conserve energy by reducing Fe(III) to Fe(II); this may even have been an early form of respiration on Earth. There will be iron species present naturally in the rock aound the repository and more may be leached from the corrosion of the steel canisters.
Harnessing these environments and their biogeochemical processes could unlock new biotechnological remediation solutions and provide an additional “biological barrier” to radionuclide migration.
Indigenous alkaline-tolerant microbes
Morris and her collaborators went on to explore the indigenous microbial communities found in naturally high pH environments. BIGRAD PhD student Adam Williamson collected near surface samples from a pH 11-12 lime working site near Buxton and incubated U(VI) and Np(V) anoxically for 210 days within pH 10-10.5 Some experiments were enriched with Fe(III) as ferrihydrite to explore the impact on biogeochemistry. In uranium experiments, the pH was held constant at pH 10.5; this was not possible for neptunium experiments due to the higher radiotoxicity of neptunium, and experiments buffered to below pH 8. This was attributed to microbially-driven reactions generating carbon dioxide and organic acids.
Samples were investigated using biogeochemical, spectroscopic and radiochemical techniques.
The unaltered sediment consisted of quartz and ankerite incorporated calcite, hosting 0.27 g/kg bioavailable iron before ferrihydrite addition. Microbiological communities were characterised via pyrosequencing, revealing 2709 reads affiliated to 13 bacterial phyla. Gram-positive species such as Firmicutes (16.6%) were found to dominate, rather than the gram-negative species that typically dominate metal-reducing microbial communities in contaminated lands at neutral pH. Microbial metabolism functioned to pH 11, performing bioreduction to transform the provided materials into energy.
X-ray absorption spectroscopy (XAS)
Morris and Williamson used X-ray absorption spectroscopy (XAS) to see how the radionuclides reacted in these experiments.
XAS employs the excitation of core electrons to probe local electronic and geometric structure. Studying the absorption edges of element-characteristic wavelengths provides a precise chemical profile, detailing element- specific bonding and oxidation state. The technique is not restricted to solids, and can probe liquids, gases and amorphous materials, resolve in situ reactions to the millisecond, and map species spatially. Although the technique measures a single element at a time, the high tuneability of synchrotron radiation permits fast multi-element collection.
At Diamond’s Core XAS beamline (B18) XAS techniques include X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), resonant inelastic X-ray scattering (RIXS) and X-ray emission spectroscopy (XES). Measurements are possible in fluorescence and transmission. The beam operates across a 2.05-35 keV energy range, and the high intensity synchrotron beam provides a small spot size, making it possible to detect lower concentrations. It is this experimental set up which makes B18 an ideal beamline to study environmental materials including, under controlled conditions, analysis of radioactive samples. Complementary to B18 is the microfocus spectroscopy beamline (I18), which employs a high-brightness micron- sized X-ray beam to gather compositional, temporal and spatial information of the sample under realistic conditions. Professor Fred Mosselmans, principal beamline scientist for I18, worked with Morris to characterise the legacy spent nuclear fuel pond materials as well as the fate of differing species.
Results from the Buxton limeworking samples showed that U(VI) and Np(V) were almost completely sorbed to the mineral phase. After bioreduction, XANES spectra of uranium suggested a 50:50 mixture of U(VI) and bioreduced U(IV) developed without iron, and with added iron, U(IV) was completely reduced and ferrihydrite converted to biomagnetite. Uranium-mediated bioreduction is complex, but the work suggests that at high pH an enzymic mechanism may dominate with the Gram-positive species. Partial and gradual reoxidation and remobilisation of uranium occurs upon re-exposure to air, meaning the reduced products may well be more stable than the oxidised forms. XANES and EXAFS spectra of selected neptunium experiments confirmed reduction to Np(IV), with indirect abiotic Fe(II) catalysis the likely pathway. The Np(IV) was strongly associated with the mineral and appeared to be bonded to iron via an inner sphere relationship – again suggesting a stabilised bioreduction product.
Perhaps these results should not be a surprise; after all, the metabolic diversity of bacteria is enormous, allowing microbes to colonise the most extreme environments on Earth, extracting energy from minerals and waste that drive changes in ambient geochemistry.
“At Manchester, working with my colleague Dr Gareth Law, we are now doing experiments on long periods of oxidative and reductive cycling to model biogeochemical processes under fluctuating micronutrients and oxygen availability,” says Morris.
Another BIGRAD-affiliated project involving EPSRC PhD student Diana Brookshaw involved priming minerals for reductive scavenging of radionuclides.
This was microbial reduction of micaceous phyllosillicates including biotite and chlorite from granite rocks using the model Fe(III)-reducing microorganism Geobacter sulfurreducens. Brookshaw separated the microbes from the mineral phases
and exposed the bioreduced minerals to radionuclides under anaerobic 80:20 N2:CO2 atmosphere for 24 hours. As shown by XAS performed with Morris’ team, the bioreduced minerals proved adept at precipitating poorly soluble phases: technetium, neptunium and uranium were all significantly reduced and scavenged onto the solid phase. They formed short chains of TcO2, nanocrystalline NpO2 and UO2, and non crystalline U(IV) species. However, uranium was poorly reactive under high carbonate conditions, and much remains in solution as [UO2(CO3)2]2-.
Whilst microbes consume minerals in metabolic processes, they can also manufacture and deposit them both inside and outside cells (much as we synthesise bone); metal oxidation states are modified and the kinetics and thermodynamics of natural processes altered biogeochemically in ways that cannot simply be explained inorganically.
Wider biogeochemical processes
In another BIGRAD project led by Lloyd, Dr Thanos Rizoulis examined the limits and rates of metallic bioreduction. He identified microbial metabolism up to pH 12 and found Fe(III) cycling strongly influenced reaction rates.
Of course, the fast evolutionary rates of bacteria mean that in one or a hundred thousand years the microbiological profile around the geological disposal facility could be unrecognisable, following the expansion and diversification of alkaline-tolerant species. In these scenarios, bacteria could also help mediate the problem of over- pressurisation by consuming hydrogen gas produced by corrosion and methane from the degradation of organics in the wastes, ensuring safer long-term waste containment.
Should we build this “biobarrier” concept into waste disposal models and radioactive waste management? What future questions are there to consider?
Synchrotron resources continue to be instrumental in much of the work. Together, via BIGRAD and the follow-on project EnvRadNet (www.envradnet.co.uk), they have helped enable transport, interrogation and handling of controlled radioactive materials at Diamond including highly radiotoxic elements such as neptunium. Working with international radionuclide beamline facilities such as ANKA in Germany, SLS in Switzerland and ROBL in France, radionuclide handling skills have developed in the UK, and translated to new analyses at Diamond. These facilities have created a vibrant interdisciplinary community who together made this work possible.