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Microbially mediated reduction of Np(V) by a consortium of alkaline tolerant Fe(III)-reducing bacteria

Published online by Cambridge University Press:  02 January 2018

Adam J. Williamson ,
Katherine Morris ,
Christopher Boothman ,
Kathy Dardenne ,
Gareth T.W. Law  and
Jonathan R. Lloyd
Adam J. Williamson
Affiliation:
Research Centre for Radwaste Disposal andWilliamson Research Centre forMolecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
Katherine Morris
Affiliation:
Research Centre for Radwaste Disposal andWilliamson Research Centre forMolecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
Christopher Boothman
Affiliation:
Research Centre for Radwaste Disposal andWilliamson Research Centre forMolecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
Kathy Dardenne
Affiliation:
Karlsruhe Institute of Technology, Institut für Nukleare Entsorgung. D-76021-Karlsruhe, Germany
Gareth T.W. Law
Affiliation:
Centre for Radiochemistry Research and Research Centre for Radwaste Disposal, School of Chemistry, The University of Manchester, Manchester M13 9PL, UK
Jonathan R. Lloyd*
Affiliation:
Research Centre for Radwaste Disposal andWilliamson Research Centre forMolecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester M13 9PL, UK
*
*E-mail: jon.lloyd@manchester.ac.uk
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Abstract

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Neptunium-237 will be present in radioactive wastes over extended time periods due to its long half-life (2.13 × 106 years). Understanding its behaviour under conditions relevant to radioactive waste disposal is therefore of particular importance. Here, microcosm experiments were established using sediments from a legacy lime workings with high-pH conditions as an analogue of cementitious intermediate-level radioactive waste disposal. To probe the influence of Fe biogeochemistry on Np(V) in these systems, additional Fe(III) (as ferrihydrite) was added to select experiments. Biogeochemical changes were tracked in experiments with low levels of Np(V) (20 Bq ml–1; 3.3 μM), whilst parallel higher concentration systems (2.5 KBq ml–1; 414 μM) allowed X-ray absorption spectroscopy. As expected, microbial reduction processes developed in microbially-active systems with an initial pH of 10; however, during microbial incubations the pH dropped from 10 to ∼7, reflecting the high levels of microbial metabolism occurring in these systems. In microbially-active systems without added Fe(III), 90% sorption of Np(V) occurred within one hour with essentially complete removal by one day. In the ferrihydrite-amended systems, complete sorption of Np(V) to ferrihydrite occurred within one hour. For higher-activity sediments, X-ray absorption spectroscopy (XAS) at end points where Fe(II) ingrowth was observed confirmed that complete reductive precipitation of Np(V) to Np(IV) had occurred under similar conditions to low-level Np experiments. Finally, pre-reduced, Fe(III)-reducing sediments, with and without added Fe(III) and held at pH 10, were spiked with Np(V). These alkaline pre-reduced sediments showed significant removal of Np to sediments, and XAS confirmed partial reduction to Np(IV) with the no Fe system, and essentially complete reduction to Np(IV) in the Fe(III)-enriched systems. This suggested an indirect, Fe(II)-mediated pathway for Np(V) reduction under alkaline conditions. Microbial analyses using 16S rRNA gene pyrosequencing suggested a role for alkali-tolerant, Gram-positive Firmicutes in coupled Fe(III) reduction and Np immobilization in these experiments.

Keywords

bioreductionNp(V) reductionintermediate-level wastebiomineralizationactinide biogeochemistry
Type
Research Article
Information
Mineralogical Magazine , Volume 79 , Issue 6 , November 2015 , pp. 1287 - 1295
Creative Commons
Creative Common License - CCCreative Common License - BY
Copyright © The Mineralogical Society of Great Britain and Ireland 2015. This is an open access article, distributed under the terms of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2015

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