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

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/kg_mineral 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 SiO_2(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.