Surface and bacterial reduction of nitrate at alkaline pH: Conditions comparable to a nuclear waste repository

Abstract

International audienceThis study investigates the reactivity of nitrates in abiotic and biotic conditions at alkaline pH in the context of a repository for long-lived inter-mediate-level radioactive waste. The work, carried out under environmental conditions comparable to those prevailing in the repository: alkaline pH, no oxygen, solid materials (cementitious material, steel), aims to identify the by-products of the nitrate reduction and to evaluate reaction kinetics of these reactions with and without the presence of denitrifying microorganisms. This paper demonstrates that, even at the high pH characteristic of nuclear waste repositories, nitrate reduction may be a likely scenario, with biotic catalysis in the presence of microorganisms and surface catalysis in the presence of steel and/or corrosion products

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“…organic acids released by bitumen, such as acetic or oxalic acids, H 2 , the various metallic phases in the repository, etc.) in the reactions [129][130][131]. The environment under consideration is complex, multicomponent and multi-phase.…”

“…The environment under consideration is complex, multicomponent and multi-phase. A step-wise approach was thus adopted, starting with simplified decoupled model systems (model media studied in batch conditions) [129,130,132], the complexity of which was progressively increased to move to close-to-real systems using an experimental pilot specifically developed for the study [131].…”

“…At pH up to about 11.5, nitrates can be reduced in the presence of C-steel, but the reaction rates strongly decrease when pH increases. In this case, carbon steel is likely to act as the electron donor [121,131,147], visible signs of corrosion being observed at the steel surface. Stainless steel, also present in the repository, could be involved in nitrate reduction, up to pH 12, in the presence of H 2 gas as an electron donor.…”

“…notably at high pH (in the range of pH 9-11) and anoxic conditions (conditions that will prevail in the storage in the long term). Works carried out to explore the biologically induced reduction of nitrates in such conditions have considered selected pure strains and wild consortia (using natural analogs) [120,129,131,[151][152][153][154]. The first step of such studies was therefore to identify microorganisms capable of (i) growing in alkaline conditions, (ii) using nitrate as electron acceptor and (iii) using one or more electron donors present in the repository.…”

“…The first step of such studies was therefore to identify microorganisms capable of (i) growing in alkaline conditions, (ii) using nitrate as electron acceptor and (iii) using one or more electron donors present in the repository. Two types of electron donors in the repository are explored in the literature: organic acids [129,131,[152][153][154] and H 2 [120,153]. Bacteria using organic carbon (organic acids here) as substrates and energy source are called heterotophic, whereas bacteria using CO 2 as carbon source and chemical compounds such as H 2 as electron donor are said to be autotrophic.…”