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 NO 3 - present in the aquifers will become thermodynamically unstable and may potentially be converted to N2 and/or NH 4 + . However, NO 3 - 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 NO 3 - 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 < [ NO 3 - ] < 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 NH 4 + and pH increase. Different mechanisms may be potentially involved. It is shown that NO 3 - 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 NO 3 - – H 2 reaction. Hydrogen alone, without the presence of steel, is not sufficient to reduce NO 3 - under the temperature and pressure conditions used in this study.