Experimental reduction of aqueous sulphate by hydrogen under hydrothermal conditions: Implication for the nuclear waste storage Laurent Truche a,b, * , Gilles Berger c , Christine Destrigneville b , Alain Pages b , Damien Guillaume b , Eric Giffaut a , Emmanuel Jacquot d a ANDRA, 1/7 rue Jean Monnet, 92290 Cha ˆtenay-Malabry, France b Universite ´ Toulouse, LMTG,14 av. E. Belin, 31400 Toulouse, France c CNRS, LMTG, 14 av. E Belin, 31400 Toulouse, France d GEOKEMEX, 18 rue Alain Savary, 25000 Besanc ßon, France Received 6 January 2009; accepted in revised form 8 May 2009; available online 20 May 2009 Abstract 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 H 2 partial pressure in hydrothermal-vessels. The cal- culated activation energy is 131 kJ/mol and the half-life of aqueous sulphate in the presence of hydrogen and elemental sul- phur ranges from 210,000 to 2.7  10 9 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 H 2 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 H 2 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 (HSO 4  at low pH), and we propose a three steps reaction, one for each intermediate-valence sulphur species, the first one requiring H 2 S as elec- tron donor rather than H 2 . 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. Ó 2009 Elsevier Ltd. All rights reserved. 1. INTRODUCTION The Callovo-Oxfordian argillite is under investigation at the Andra (National French Agency for Nuclear Waste Management) Underground Research Laboratory in Meuse/Haute-Marne (France), as a potential host rock type for a high-level radioactive waste storage in similar argillite. Radionuclide migration through the argillite formation de- pends among others factors, on the porewater chemistry which is still under investigation (Gaucher et al., 2004, 2006). In deep geological repositories, hydrogen is pro- duced both by radiolysis of organic matter contained in some waste packages and by anaerobic corrosion of the iron contained in the steel container protecting the nuclear waste (Andra, 2005). The degree and extent of hydrogen pressure buildup with time represents a risk of gas leakage through the surrounding engineered and geological barriers (Ortiz et al., 2002; Lassabate `re et al., 2004), and may also cause unexpected chemical reactions. For the long-term post-closure repository safety, many experimental and modelling studies were performed to evaluate gas genera- tion rates assuming different scenarios, as well as site-spe- cific conditions (Galle, 2000; De Combarieu et al., 2007; Martin et al., 2008; Xu et al., 2008). To see the long-term pressure evolution, simulations using different steel 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.05.043 * Corresponding author. Address: CNRS, LMTG, 14 av. E Belin, 31400 Toulouse, France. Fax: +33 5 61 33 25 60. E-mail address: truche@lmtg.obs-mip.fr (L. Truche). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 4824–4835