Methane hydrothermal

Ifrah, S., Kaddouri, A., Gelin, P. and Leonard, D. (2007) Conventional hydrothermal process versus microwave-assisted hydrothermal synthesis of La3 xAgxMn03 + s (x = 0, 0.2) perovskites used in methane combustion. Comptes Rendus Chimie, 10, 1216-1226. 

Horita and Berndt (1999) studied the abiogenic formation of methane under conditions present at hydrothermal vents. Solutions of bicarbonate (HCO3 ) were subjected to temperatures of 470-670 K and a pressure of 40 MPa. Under these conditions, CO2 was reduced only very slowly to methane. Addition of a nickel-iron alloy, which corresponds closely to the minerals in the Earth s crust, led to a clear increase in the reaction rate of methane synthesis. The following reaction is assumed to occur ... 

Fig. 22.5. Concentrations of components (sulfate, sulfide, carbonate, methane, and acetate) and species (O2 and H2) that make up redox couples, plotted against temperature, during a model of the mixing of fluid from a hot subsea hydrothermal vent with cold seawater. Model assumes redox couples remain in chemical disequilibrium, except between 02(aq) and H2(aq). As the mixture cools past about 38 °C, the last of the dihydrogen from the vent fluid is consumed by reaction with dioxygen in the seawater. At this point the anoxic mixture becomes oxic as dioxygen begins to accumulate.

Horita J, Berndt ME. 1999. Abiotic methane formation and isotopic fractionation under hydrothermal conditions. Science 285 1055-7. 

Some gases have subsurfece sources that are related to physical phenomena, such as inputs from the introduction of hydrothermal fluids in bottom waters or release from warming sediments. The latter is a source of methane, which can occur in sediments in a solid phase called a clathrate hydrate. Biogeochemical reactions in sediments can also produce gases that diffuse from the pore waters into the deep sea. 

Anaerobic methane oxidation (methanotrophy) CH4 S04 OH4 CH4 + S04 HC03 + HS- + H2O Consortia of methanogenic archaea and SRBs at seeps and in hydrothermally active sediments... 

The hydrothermal chemistry of methane also provides another buffering control on the global biogeochemical carbon cycle by serving as the site of reactions that act as sources and sinks of methane. Examples of source reactions are... 

Steeper, R.R., Methane and methanol oxidation in supercritical water chemical kinetics and hydrothermal flame studies, Sandia Rep., Sand96-8208.UC-1409, 1-150, 1996. 

The same phenomenology must be important locally on Earth, too, where thick evaporite deposits of hydrated salts and local thick beds of methane clathrate in permafrost or seafloor sediments should influence the thermal environment of the crust. The predicted control on the crust s thermal state by hydrate deposits should have consequences for the localization of hydrothermal springs around and within evaporite basins, hydrothermal metamorphism... 

The permeation of hydrogen and methane through the membrane was investigated, revealing increased permeability for hydrogen on raising the temperature, while the methane permeability remained at a low level. Between 100 and 525 °C, the separation factor (see Section 2.6.3) increased from 7.5 to 31. The hydrothermal stability of the membrane was verified at 525 °C for 8 h for a feed composed of 18% hydrogen, 18% methane and 74% steam. It revealed a decrease of the separation factor from 31 to 26 [51]. 

COl" > N03 > eland that the stabilities of the catalysts during reaction in a CO + 3H2 mixture at 500 °C fell in the same order. It was concluded that the main effects of the anions were manifested during the calcination of the coprecipitates. It was also observed that the hydrothermally aged materials gave reduced samples with very similar methanation activities to those derived from the freshly prepared materials. 

Figure 10.1 Hydrothermal vents, so-called black smokers, carry high concentration of sulphur compounds with traces of methane and hydrogen together with suspended sulphide particles. (Picture by courtesy of Professor Verena Tunnidiffe, University of Victoria, Canada)...

This requirement is fulfilled for electric discharges in a reduced atmosphere containing methane, ammonia, and water, as in the original Miller experiment. It has also been observed for atmospheres based on N2 and CO or CO2 on the condition that H2 or methane is also present in snfflcient amonnts (19). A neutral atmosphere (based on N2, CO2, and water) wonld produce much lower yields of organics (by several orders of magnitude). In the absence of other species to be oxidized, the rednction of CO2 reqnires the concomitant thermodynamically nnfavorable conversion of water into O2 (as in photosynthesis). However, even if the atmosphere was nentral when life arose, as nsnaUy believed, the Earth was not nniform with respect to redox state simply becanse the rednced state of the mantle and the high volcanic activity favored the occnrrence of locally rednced environments (for instance, in hydrothermal vents in the oceans). Then, a preservation of the hydrogen content of the early atmosphere or the diversity of environments on the early Earth is likely to have made amino acid formation possible, at least at specific places. 

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