Surface-mediated redox activity in the Pyrite – Nitrate/nitrite – Hydrogen system under conditions relevant for the geological disposal of bituminized waste in Boom Clay

The reactivity of a system with pyrite powder, nitrate or nitrite, and hydrogen in a 15 mM sodium bicarbonate solution was assessed over the course of 2 years in the frame of compatibility studies of nitrate-containing bituminized radioactive waste with the host rock for final disposal. A series of batch tests was performed with pyrite suspensions in bicarbonate solution to which nitrate or nitrite was added before filling the headspace of the recipient with a constant, and non-renewed, volume of 100% pure hydrogen gas (initial PH2 1.5 bara). Under anoxic conditions and at room temperature, hydrogen reacted readily with nitrate and nitrite in the presence of pyrite powder. Ammonia was formed while hydrogen was consumed. Based on the XPS analyses of the pyrite surface and the absence of dissolved pyrite oxidation products, the pyrite surface was not oxidized. Moreover, no reaction between hydrogen and nitrate or nitrite was observed in the absence of pyrite. This reaction was thus clearly mediated by the Boom Clay pyrite surface. The reducing atmosphere kept the pyrite surface intact and protected it from precipitation of carbonates from the medium, thus effectively preventing pyrite surface deactivation, previously observed under anoxic conditions in the absence of H2. Overall, only 5% of the 0.1 M of nitrate that was added to the tests, was reduced over the course of 2 years, without complete consumption of H2. Nitrite was added in a lower concentration of 0.05 M, but was more reactive: about 50% of nitrite was reduced, producing stoichiometric amounts of ammonia, and nearly depleting the H2 in the gas phase. The possible consequences of these processes for the final repository performance are also discussed.