Pyroxene control of H2 production and carbon storage during water-peridotite-CO2 hydrothermal reactions

Abstract

Strategies of H2 production and CO2 mineralization were combined through olivine [(Mg,Fe)2SiO4] serpentinization and carbonation in a CO2-rich hydrothermal system. However, natural mantle peridotites commonly contain not only olivine but also orthopyroxene and/or clinopyroxene, which have effects that are not well understood. The present study investigated the reactions in H2O-olivine/orthopyroxene-CO2 systems by performing hydrothermal experiments in 0.5 M NaHCO3 solutions at 300 °C and 10 MPa.The yields of H2 and HCOOH initially were first suppressed in the presence of orthopyroxene; however, after orthopyroxene consumption, the rate of H2 production increased significantly. H2 yield increased to 348.3 mmol/kgmineral in 120 h with the presence of 20 wt% orthopyroxene at the beginning of the reaction. The initial suppression of H2 generation was due to incorporation of more Fe(II) into serpentine [(Mg,Fe)3Si2O5(OH)4] in the high SiO2(aq) concentration system. The presence of orthopyroxene also dramatically accelerated serpentine formation. In contrast, magnesite [(Mg,Fe)CO3] formation was inhibited upon addition of orthopyroxene, which also contributed to the release of Fe(II). Therefore, peridotite containing ≤20 wt% of pyroxenes is more suitable for long-term H2 production than pure olivine. When considering the reaction output of a water-peridotite-CO2 system, controlling the percentage of pyroxenes in the starting mineral may be more important than expected.