Adsorption of Hydrogen Gas and Redox Processes in Clays

Abstract

In order to assess the adsorption properties of hydrogen gas and reactivity of adsorbed hydrogen, we measured H(2)(g) adsorption on Na synthetic montmorillonite-type clays and Callovo-Oxfordian (COx) clayrock using gas chromatography. Synthetic montmorillonites with increasing structural Fe(III) substitution (0 wt %, 3.2 wt %, and 6.4 wt % Fe) were used. Fe in the synthetic montmorillonites is principally present as structural Fe(III) ions. We studied the concomitant reduction of structural Fe(III) in the clays using (57)Fe Mössbauer spectrometry. The COx, which mainly contains smectite/illite and calcite minerals, is also studied together with the pure clay fraction of this clayrock. Experiments were performed with dry clay samples which were reacted with hydrogen gas at 90 and 120 °C for 30 to 45 days at a hydrogen partial pressure close to 0.45 bar. Results indicate that up to 0.11 wt % of hydrogen is adsorbed on the clays at 90 °C under 0.45 bar of relative pressure. (57)Fe Mössbauer spectrometry shows that up to 6% of the total structural Fe(III) initially present in these synthetic clays is reduced upon adsorption of hydrogen gas. No reduction is observed with the COx sample in the present experimental conditions.

Method development for evaluating the redox state of Callovo-Oxfordian clayrock and synthetic montmorillonite for nuclear waste management

“…This structural Fe(III) reduction is confirmed by the absence of magnetic components in the Mössbauer spectra. No reduction occurs during the reaction with H 2 (g) in dry conditions, as highlighted by Didier et al (2012). We can notice that pyrite content is slightly higher for COx sample after H 2 (g) reduction in dry conditions.…”

“…Two pure synthetic Na-montmorillonite samples used in this study, SM1 and SM2, were synthesized and characterized following the work of Reinholdt and collaborators (Reinholdt et al, 2001) and contain 3.2 wt.% and 6.4 wt.% of Fe(III) respectively. All synthetic minerals used in this study had been previously characterized by different techniques (Bardelli et al, 2014;Didier et al, 2012).…”

“…The reduction of structural Fe(III) by H 2 (g) is thermodynamically possible but requires fairly harsh reaction conditions (Addison and Sharp, 1963). The reduction by hydrogen gas of a clay suspension was compared to the reduction by H 2 (g) of dried clay samples, already reported by Didier et al (2012). The initial clay suspension (4.5 g L À1 ) is prepared with V = 200 mL of NaCl(aq) (40 mM) and m = 0.9 g of clay sample.…”

“…3. The corresponding Mössbauer spectra for Namontmorillonite after reduction with H 2 (g) in dry reduction are available in Didier et al (2012). The other spectra are displayed in SD with the corresponding hyperfine parameters.…”

“…In a first step, the oxygen trapped after the disposal closure will oxidize iron from clays and organic carbon, then anaerobic conditions will develop and let to a reducing period due to hydrogen gas which can be produced by steel corrosion under these developed anoxic conditions or by micro-organisms processes in the underground nuclear waste repository. This gas can then reduce a part of the structural Fe(III) present in the surrounding geological formation (Didier et al, 2012). However, H 2 (g) is not easy to handle and remains a less powerful reactant compared to sodium dithionite (Na 2 S 2 O 4 ) (Mayhew, 1978;Schwarzenbach et al, 2003).…”