5:15 PM - 5:30 PM
[SCG64-15] Abiotic formation of methane by oxidation of sulfur species under hudrythermal conditions
Keywords:hydrothermal fluid vent, abiotic methane, CO2 reduction
In recent years, abiotic reactions have been considered as one of the potential mechanisms for the formation of reduced carbon species (i.e., CH4, ..) in hydrothermal systems at seafloor. Because the fluid flux through deep-sea hot-springs represents a potentially significant source of carbon and energy to support microbial activities in surface and sub-surface habitats, the possibility that abiotic processes may influence the speciation of carbon in vent fluids has direct implications for the maintenance of life in present-day hydrothermal environments. Although aqueous carbon compounds have significant role in broad spectrum of geochemical and biological processes, reactions to produce abundant aqueous hydrocarbons at seafloor hydrothermal environment are poorly understood.
Abiotic synthesis of hydrocarbons in hydrothermal environments is attributed to Fischer-Tropsch type processes, which are characterized by the reduction of CO2 or CO by H2 on catalytic mineral surfaces including magnetite (Anderson, 1984). These reactions are also thought to occur in association with serpentinization of mantle peridotites, which produces H2 and Fe3O4. Previous experimental studies under hydrothermal conditions (e.g., Foustoukos et al. 2004) succeeded in production of H2 and abiotic CH4. For example, Foustoukos et al. (2004) reported the production of 208 mmol/kg of H2 and 39 μmol/kg of CH4 by the olivine hydration over 1000 hours; however, the concentration of CH4 gas was quite low than those observed in natural ultra mafic ?hosted hydrothermal vent fluid, for example, 0.13 ~ 2.2 mmol/kg of CH4 from the hydrothermal vent at the Lost City.
In this study, we focused on sulfur species as reducing agent, based on Putri et al. (2011), which reported high H2 generation rate (64.3 mmol/kg in an hour) in the system of H2S and H2O. We conducted a series of hydrothermal experiments with H2S to generate H2 by reduction of H2O. We used Na2S•9H2O for H2S species, NaHCO3 for CO2 species, and Fe3O4 for catalyst of Fischer-Tropsch type CH4 synthesis. The initial concentration of H2S and CO2 species were set to be 10 mmol/kg and 40 mmol/kg, as analogue of hydrothermal vent fluids. The experiments were conducted at 300 degree C , and initial pH was controlled at 9.9~10.0 with NaOH. After 168 hours experiment, the concentration of H2 gas was 39.7 mmol/kg, which means almost H2S species was consumed by the reduction of H2O. The generated H2 gas was used for the second reaction CH4 gas. The CH4 gas concentration was 30.3 μmol/kg in 168 hours, 6.3 times higher than that from serpentinization experiment (Foustoukos et al., 2004). In the same condition except for absence of Fe3O4, the gas concentration of H2 were 40.14 mmol/kg and 4.91 μmol/kg, respectively. The experiment without Fe3O4 generated CH4 gas and the concentration of CH4 was quite lower than the experiment using Fe3O4, that indicates Fe3O4 takes the role of catalyst in the formation of CH4, while other catalytic effect should be considered in the system.
References
Anderson, R.B., The Fischer-Tropsch reaction, 1984.
Foustoukos, D.I. and Seyfried, W.E., Science, 2004, 304, 1002.
Putri S., Javier V., Watanabe N., Kishita A., Tsuchiya N., International Journal of Hydrogen Energy, 2011, 36, 10674.
Abiotic synthesis of hydrocarbons in hydrothermal environments is attributed to Fischer-Tropsch type processes, which are characterized by the reduction of CO2 or CO by H2 on catalytic mineral surfaces including magnetite (Anderson, 1984). These reactions are also thought to occur in association with serpentinization of mantle peridotites, which produces H2 and Fe3O4. Previous experimental studies under hydrothermal conditions (e.g., Foustoukos et al. 2004) succeeded in production of H2 and abiotic CH4. For example, Foustoukos et al. (2004) reported the production of 208 mmol/kg of H2 and 39 μmol/kg of CH4 by the olivine hydration over 1000 hours; however, the concentration of CH4 gas was quite low than those observed in natural ultra mafic ?hosted hydrothermal vent fluid, for example, 0.13 ~ 2.2 mmol/kg of CH4 from the hydrothermal vent at the Lost City.
In this study, we focused on sulfur species as reducing agent, based on Putri et al. (2011), which reported high H2 generation rate (64.3 mmol/kg in an hour) in the system of H2S and H2O. We conducted a series of hydrothermal experiments with H2S to generate H2 by reduction of H2O. We used Na2S•9H2O for H2S species, NaHCO3 for CO2 species, and Fe3O4 for catalyst of Fischer-Tropsch type CH4 synthesis. The initial concentration of H2S and CO2 species were set to be 10 mmol/kg and 40 mmol/kg, as analogue of hydrothermal vent fluids. The experiments were conducted at 300 degree C , and initial pH was controlled at 9.9~10.0 with NaOH. After 168 hours experiment, the concentration of H2 gas was 39.7 mmol/kg, which means almost H2S species was consumed by the reduction of H2O. The generated H2 gas was used for the second reaction CH4 gas. The CH4 gas concentration was 30.3 μmol/kg in 168 hours, 6.3 times higher than that from serpentinization experiment (Foustoukos et al., 2004). In the same condition except for absence of Fe3O4, the gas concentration of H2 were 40.14 mmol/kg and 4.91 μmol/kg, respectively. The experiment without Fe3O4 generated CH4 gas and the concentration of CH4 was quite lower than the experiment using Fe3O4, that indicates Fe3O4 takes the role of catalyst in the formation of CH4, while other catalytic effect should be considered in the system.
References
Anderson, R.B., The Fischer-Tropsch reaction, 1984.
Foustoukos, D.I. and Seyfried, W.E., Science, 2004, 304, 1002.
Putri S., Javier V., Watanabe N., Kishita A., Tsuchiya N., International Journal of Hydrogen Energy, 2011, 36, 10674.