Methane production and carbon assimilation in nontronite at 25 °C

Methane (CH ₄) is a clean source of energy, thus current large-scale efforts aim to produce this hydrocarbon. Little is known on how the surface reactivity of clays contributes to the in situ production of CH ₄. This paper reports on the natural production of CH ₄ after reacting four nontronites, iron-rich dioctahedral smectites [Fe ₂O ₃ ca. 30% and Al ₂O ₃ < 12%], against bicarbonate under strict, free-oxygen conditions at 25 °C. The analytical techniques used included X-ray fluorescence, X-ray diffraction, elemental analysis, gas chromatography, high-resolution scanning and transmission electron microscopy, and energy dispersive X-ray spectroscopy, and nano-diffraction. The production of CH ₄ reached 5700 ppbv. By way of comparison, registered concentrations for CH ₄ in the aqueous nontronite dispersions surpassed the average concentration of CH ₄ in the Earth`s atmosphere (1.774 ppbv), and the planetary budget of CH ₄ for the Mars surface (10–250 ppbv). Fitting functions for the production of CH ₄ were calculated. The highest production of CH ₄ was registered in dispersions containing nontronites with lower contents of Fe (%Fe). The cumulative production of CH ₄ (ΣCH ₄ after 60 d) varying with %Fe according to: y = −171.5( x) + 7348, r ² = 0.98 ( n = 4). Increases in proton activity favored the production of CH ₄, in lieu with the notion that corrosion of highly-reduced, Fe nanodomains that occur naturally on nontronite surfaces render protons that hydrogenate carbon centres. Meanwhile, bicarbonate acted not as spectator anion, for which it acted as carbon source. On the other hand, microscopic observations identified the presence of biosignatures in nontronite, denoting asexual and sexual reproduction of bacterial iron-oxidizers, proper of a lithotrophic environment. In summary, both CH ₄ production and carbon assimilation were best explained because the reduction of carbonate by labile, highly-reactive Fe nanodomains present in nontronite under environmental conditions.