Abiotic nitrate reduction induced by carbon steel and hydrogen: Implications for environmental processes in waste repositories

Reducing conditions induced by steel canister corrosion and associated H2 generation are expected in nuclear waste repositories. Aqueous NO3- present in the aquifers will become thermodynamically unstable and may potentially be converted to N2 and/or NH4+. However, NO3- reduction by H2, in the absence of bio-mediators, is generally thought to be kinetically hindered at low temperature, although the reaction may be promoted by the concomitant oxidation of Fe. In this study the reduction rate of aqueous NO3- is quantified in the presence of H2 and carbon steel surfaces from waste canisters and construction materials, as well as magnetite as their possible corrosion by-products. A parametric study (0 < P(H2) < 10 bar, 0.1 < [NO3-] < 10 mM, 90 < T° < 180 °C, 4 < pHin situ < 9) reveals that even at 90 °C the reaction can occur within hours or days and leads to the formation of NH4+ and pH increase. Different mechanisms may be potentially involved. It is shown that NO3- reduction in the presence of carbon steel does not require H2, since steel constitutes an electron donor by itself, as does metallic Fe. The reaction rate is strongly pH-dependent. Activation energy in the 90–180 °C range is found to be 45 kJ/mol. Magnetite is the main corrosion by-product and specific experimental runs demonstrate that it can serve as a catalyst for the NO3-–H2 reaction. Hydrogen alone, without the presence of steel, is not sufficient to reduce NO3- under the temperature and pressure conditions used in this study.

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► We evaluate the consequences of carbon steels on the reduction of aqueous nitrate in the presence of hydrogen.
► Carbon steel constitutes an electron donor by itself but hydrogen can enhance the reaction rate under certain conditions.
► Magnetite is the main corrosion by-product and it is also a catalyst by itself.
► Hydrogen alone is not sufficient by itself to reduce nitrate in the 90–200 °C temperature range.