Sedimentary Catalysis of Radiolytic Hydrogen Production

Abstract

Hydrogen (H2) and oxidants (O2 and H2O2) are naturally produced by radiolysis of water in any environment where water is bombarded by α, β, and γ radiation generated during radioactive decay. The production of radiolytic H2 in aqueous solutions and in some monomineral-water mixtures has been extensively studied. However, yields of radiolytic products in natural materials remain largely unexplored. Quantification of radiolytic production in common geological materials is critical to assess the importance of water radiolysis as source of microbial reductants and oxidants in water-containing subsurface environments. Knowledge of radiolytic production is also fundamental for the nuclear industry, as maintenance and development of nuclear reactors, long-term disposal of radioactive waste and management of mixed-waste storage tanks are intricately associated with radiolysis products. We experimentally quantified H2 yields for α- and γ-irradiation of pure water, seawater, and slurries of marine sediment, montmorillonite, and two natural zeolites (mordenite and clinoptilolite) widely used in the nuclear industry. The sediment samples include the dominant types found in the global ocean (abyssal clay, nannofossil-bearing clay [marl], clay-bearing diatom ooze, and nannofossil ooze). These experiments demonstrate that all common types of marine sediment and both zeolites catalyze radiolytic H2 production. Hydrogen yields [G(H2)] from water radiolysis differ from one geological material to another. They range between 3.43 and 37.54 molecules H2 100eV-1 for α-particles and and 0.27 and 1.96 molecules H2 100eV-1 for γ-rays. Abyssal clay, earth’s most widespread marine sediment type, exhibits the highest yield amplification when exposed to α-particles with an average factor increase of 18 relative to pure water. Siliceous ooze and abyssal clay exhibit the highest H2 yields when exposed to γ-rays, increasing production by factors of up to 8 and 4, respectively. Calcareous ooze (factor 5 amplification) and lithogeneous sediment (17% amplification) exhibit the smallest yield amplification under α-particle and γ-rays irradiation, respectively. Zeolite mineral slurries increase G(H2) for α- and γ-irradiation by factors of 13 and 4, respectively (similar to abyssal clay). Our results show that substrate chemistry and specific surface area are the main factors that control radiolytic H2 production. The mineral-catalysis of radiolytic H2 production has significant implications for: (i) sustenance of Earth’s subsurface microbial ecosystems (ii) habitability of other planetary bodies, and (iii) nuclear industrial activities. In electron equivalents per unit area, radiolytic H2 production in marine sediment locally produces as much electron donor (food) as photosynthetic carbon fixation in the ocean. Although small relative to global