Microbial Catalysis of Redox Reactions in Concrete Cells of Nuclear Waste Repositories: A Review and Introduction

Abstract

In order to be able to simulate the behaviour of radionuclides (RN) in waste repositories in space and time it is important to know their chemical speciation. ¹⁴C in its reduced form (CH₄) does not have the same behaviour as in its oxidised form (CO₂, CO ⁻₃ ). Similarly, tritium in the reduced gaseous form, HT, does not at all behave as its oxidised form (liquid water, HTO). For other RN such as U, Se, Tc, Np and Pu the impact is less striking as the change in redox state does not generate a phase change but a change in the sorption behaviour. As a rule of thumb the oxidised form is more mobile than the reduced form. Nuclear waste repositories for both low and intermediate level wastes are characterised by the presence of cementitious phases and zero-valent metals as part of waste, waste containers or engineered materials; organic matter is also likely present in both waste and engineered barrier. Hydrogen gas can be formed either via radiolysis or anaerobic corrosion. We therefore have two main electron donors to participate in redox reactions within an “unnaturally” high pH environment. Oxygen, present during the exploitation phase, is quickly consumed and not considered to diffuse significantly into deep or near-surface repositories. Nitrate, Fe(III) or Mn(IV) are only in specific cases present in significant quantities. Consequently, H⁺ and C⁴⁺ present in water and carbonate will become the main electron acceptors in redox reactions after reduction of sulphates, present in some wastes, concrete and host rocks; H⁺ and C⁴⁺ are likely to control in fine the overall redox potential and the speciation of RN. There is more and more evidence for the microbial control of reactions implying electron transfer within H and C species [Hoehler TM (2005) Biogeochemistry of dihydrogen (H2). In: Sigel A, Sigel H, Sigel RKO (eds) Metal ions in biological systems. Taylor & Francis, Boca Raton, FL, pp 9–48]. Furthermore, the impact of microbial activity on the degradation of complex organic matter (i.e. polymers) adds to the need to evaluate their catalytic impact on waste cell redox potential [Askarieh MM, Chambers AV, Daniel FBD, FitzGerald PL, Holtom GJ, Pilkington NJ, Reesb JH (2000) The chemical and microbial degradation of cellulose in the near field of a repository for radioactive wastes. Waste Manag 20: 93–106]. Quantification of reaction dynamics in alkaline systems involving Fe(0), H₂ or organic matter as electron donors and nitrates/sulphates (if present) as well as carbonates or water (and RN in their possibly oxidised form) as electron acceptors will have to consider a microbial catalysis. There are many analogues for testing simulation approaches for microbial catalysis of related redox reactions, but few are in alkaline systems. With H₂ almost omnipresent as an energy source, essential and trace nutrients most likely present in the heterogeneous waste cell environment, with space and water available depending on depth, architecture and re-saturation, the high pH may become the most critical parameter controlling microbial activity in space and time. In this chapter, we will review the importance of oxyanions in the nuclear industry and their impact together with concrete, steel and organic matter on the redox state in the near field of a waste storage cell. Particular consideration will be given to the knowledge in relation to alcaliphilic microbial activity in some cases derived from existing natural analogues. Case studies will consider specific redox-sensitive radionuclides in both near surface and deep storage settings. This information will serve as input to two ongoing experimental endeavours dealing with the specific reaction of nitrate reduction by organic matter and/or H₂ in the concrete cells for bituminous waste disposal.