Coupled reduction of bicarbonate to methane and generation/ sequestration of carbon dioxide in natural basaltic- and clay- matrices at 25 °C ≤ T ≤ 100 °C

Abstract In nature, methane (CH4) is produced via both biotic and abiotic processes, yet little is known on the abiotic mechanism(s) on the hydrogenation of abiotic carbon (C) to produce CH4 at low temperatures. This study reports on the abiotic generation of CH4via Fischer-Tropsch type reactions at room temperature. Natural sources of mineral Fe2+ were reacted with aqueous bicarbonate under sterile and non-sterile conditions. The selected sources of mineral Fe2+ were well-characterized Ediacaran basalts and green clay veins in basalts from the Volyn-Brest continental flood basalt province and hectorite SHCa-1 from Hector, California. Quantitative X-ray powder diffraction results and the Mossbauer characteristics of these materials from the literature were supplemented with specific surface area values determined by N2 physisorption, high-resolution transmission electron microscopy and nanodiffraction analysis. The headspace concentration of the gases in the dispersions was related to the presence of bicarbonate and the mineral composition. Evidence presented herein confirmed that wustite occurs naturally in SHCa-1, which is unprecedented. Of all mineral dispersions those containing wustite (basalt MIB and SHCa-1) showed the highest concentration of CH4 and CO2, ranging from 1457 to 6329 μmol CH4 g solid−1 and from 2100 to 8771 μmol CO2 g solid−1, respectively. The production of CO2 was also registered in dispersions containing the sources of mineral Fe(II) chlorite and saponite, which ranged from 375 to 8180 μmol CO2 g solid−1 and from 480 to 1230 μmol CO2 g solid−1, respectively. Registered mineral transformations i) wustite → hematite and ii) chlorite or saponite → hematite, and iii) wustite → pyrite → jarosite were explained because bicarbonate and wustite (chlorite or saponite) reduction and oxidation to CH4 and hematite; and wustite dissolution releasing Fe2+ that reacted with sulfide ions.