Experimental reduction of aqueous sulphate by hydrogen under hydrothermal conditions: Implication for the nuclear waste storage

Sulphate reduction by hydrogen, likely to occur in deep geological nuclear waste storage sites, was studied experimentally in a two-phase system (water + gas) at 250–300 °C and under 4–16 bars H2 partial pressure in hydrothermal-vessels. The calculated activation energy is 131 kJ/mol and the half-life of aqueous sulphate in the presence of hydrogen and elemental sulphur ranges from 210,000 to 2.7 × 109 years at respective temperatures of 90 °C, the thermal peak in the site and 25 °C, the ambient temperature far from the site. The features and rate of the sulphate reduction by H2 are close to those established for TSR in oil fields. The experiments also show that the rate of sulphate reduction is not significantly affected in the H2 pressure range of 4–16 bars and in the pH range of 2–5, whereas a strong increase is measured at pH below 2. We suggest that the condition for the reaction to occur is the speciation of sulphate dominated by non symmetric species (HSO4- at low pH), and we propose a three steps reaction, one for each intermediate-valence sulphur species, the first one requiring H2S as electron donor rather than H2. We distinguish two possible reaction pathways for the first step, depending on pH: reduction of sulphate into sulphur dioxide below pH 2 or into thiosulphate or sulphite + elemental sulphur in the pH range 2–5.