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Author(s): Nadège Durban, Pierre Albina, Alexandra Bertron, Achim Albrecht, Jean- Charles Robinet, Benjamin Erable
Paper category: Proceedings
Book title: Proceedings of the Final Conference of RILEM 253-MCI Microorganisms-Cementitious Materials Interactions Volume II
Editor(s): Alexandra Bertron and Henk Jonkers
ISBN: 978-2-35158-207-7 (Set)
ISBN: 978-2-35158-210-7 (Volume 2)
e-ISBN: 978-2-35158-206-0
Publisher: RILEM Publications SARL
Publication year: 2018
Pages: 571-573
Total Pages: 308
Language : English

Abstract: In situ tests in underground rock laboratories in France and Switzerland, for the radioactive waste storage prospection, highlighted the presence of anaerobic microbial activity, in particular sulphate reduction, as soon as space and free water were available [1–3]. This microbial activity affects the host rock porewater chemistry (pH and Eh) but has no significant impact on the mobility of radionuclides released by the wastes [4]. Nonetheless, microbial activity could catalyse the oxidation of organic matter and the reduction of oxyanions, such as sulphate, naturally present and released by some radioactive waste forms. The biological oxyanion reduction can promote the oxidation of some organic matter that may otherwise form complexes with radionuclide; their degradation would thus lower the risk of complexation- related enhanced mobility. The pH of the porewater where microbial activities were detected was between 6.8 and 7.7. But the leaching of concrete present in the engineered barrier system of cells for intermediate, long-lived waste will cause an alkaline environment and lead to pH locally well above 9.0. There is very little information about the sulphate reduction coupled with organic matter oxidation in such environments. The objectives of our work are to determine experimentally the pH limit up to which microbial sulphate reduction can occur. Propionate was used as a model organic component because it can be easily oxidised by sulphate-reducing bacteria [5–7] and because of its relevance (i.e. cellulose/ISA degradation product [8]); an organic component very likely present in cells containing organic-rich wastes. A microbial consortium collected in a wastewater treatment plant was acclimated to a cementitious environment in fed-batch bioreactors containing cement leachate supplemented by sulphate (10 mM) and propionate (5.7 mM) with a pH initially close to 8.0 and gradually increased to 9.3. This approach allowed quantifying sulphate reduction and propionate oxidation rate for several pH values. In complementary tests, solid cement pastes artificially aged were added in order to investigate the possible interaction between the microbial consortium and the cement paste surface. The propionate oxidation rate decreased when the pH increased from 8.0 to 9.3 similar to the sulphate reduction rate; the latter though to a lesser degree. (Table 1). The presence of the cement paste in the reactors had no apparent negative impact on the sulphate reduction or propionate oxidation rates, except of course for the related pH increase (Table 1). For all the experiments, the propionate was oxidised to acetate that accumulated in the system in accordance with Equation 1 considering that the sulphate has been completely reduced to the hydrogen sulphide (H2S).
C3H6O2 + 0,75 SO4 2- + 0,5 H+ → 0,75 H2S + HCO3 - + 1 C2H4O2 Equation 1
Several millimetres thick of the cement paste surface gradually blackened in contact with the liquid medium. The surface appears to have reacted with some chemical compounds in the solution but has not yet been clearly identified.
These preliminary results do not permit a more precise indication of the reaction at the origin of this colouring; calcite precipitates were observed by scanning electron microscopy inside the fracture area induced by the acid attack. Additional SEM observations coupled with EDS analysis of a sample cross section are planned.

Online publication : 2018
Publication type : full_text
Public price (Euros) : 0.00

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