The biogeochemical behaviour of plutonium and americium in contaminated soils

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Abstract

The biogeochemical behaviour of plutonium and americium was investigated incontaminated soils from the UK to help determine possible remediation andmanagement options. Stimulating anoxic sediments from Aldermaston, through theaddition of a carbon substrate (glucose), induced reducing conditions resulting in anegligible change in Pu mobility. This was despite a substantial shift in the bacterialprofile from a diverse community to one dominated by fermentative Betaproteobacteriaand Clostridia. The latter group also includes organisms associated with metal reduction,such as close relatives to Clostridium species, reported previously to facilitate thereduction of Pu(IV) to Pu(III). A sequential extraction was performed on soils fromAldermaston and the Esk Estuary to identify which selected fractions the Pu and Am aremost strongly associated with. The majority of Pu was associated with the “residualfraction”: 63.8 – 85.5 % and 91.9 – 94.5 % in the Aldermaston and Esk Estuary soilsrespectively. Metals associated with this fraction are highly recalcitrant and are unlikelyto be released into solution over a significant time span under most geologicalconditions. The Am was more evenly distributed with the “organic fraction” being themost dominant. Degradation of organic matter under oxidising conditions may result inmobilization of metals associated with this fraction. The Aldermaston soil was alsosubjected to bioleaching using a sulfuric acid producing microbial community, whichresulted in a maximum 0.18 % of Pu released into solution. However, up to 12.5 % ofAm was found in solution suggesting Am is more susceptible to mobilization than Pu.The potential for Pu mobilization through abiotic oxidative leaching was investigatedusing permanganate. Even when carbonate was added to act as a potential complexantfor the Pu, less than 1 % of the Pu was leached. Greater success was observed whenleaching was attempted using citric acid; an estimated 25 – 30 % of Pu was releasedinto solution offering a potential route for remediation of Pu-contaminated soils. Thesedata would suggest that the Pu is highly recalcitrant, and may exist in a small particulateform in the Aldermaston soils, possibly in the oxide form, and is unlikely to mobilizeunder natural biogeochemical conditions.

Keyword(s)

biogeochemistry; bioremediation; contaminated; radionuclides

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