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.     

Characteristics of hydrogen production with carbon storage by CO2-rich hydrothermal alteration of olivine in the presence of Mg–Al spinel

Artificial control of olivine alteration has potential applications for both H₂ production and CO₂ reduction (by mineralization and hydrogenation). To explore methods to overcome the still-constrained olivine alteration problem, olivine + spinel alteration experiments were performed with the addition of Mg–Al spinel in CO₂-rich (0.5 M NaHCO₃) solution under hydrothermal conditions (300 °C and 10 MPa). Mg–Al spinel enhanced olivine serpentinization significantly (more than 2 times), and generation of both H₂ and CO₂ hydrogenation products was accelerated (up to 3 times) with ≥10 wt% Mg–Al spinel especially at the latter stage of the 72 h reaction.Mineral measurements revealed that more Al released from Mg–Al spinel was incorporated into Al-serpentine by the replacement of Fe with higher Mg–Al spinel content. Both Al and Fe incorporated into Al-serpentine were released as the reaction proceeded. Thus, H₂ production was elevated with the presence of a large amount Mg–Al spinel at the latter stage of the reaction. HCO₃⁻ played an important role in the promotion of Mg–Al spinel dissolution with the release of Al, which was stored in magnesite after being utilized. This study also suggests that the presence of Mg–Al spinel (5–20 wt%) in the starting mineral does not have significant influence on the total H₂ yield from olivine alteration over the entire operation period.     

Thermodynamic assessment of the possibility of olivine interaction with deep-seated hydrogen

Olivine is one of the most widespread minerals of the Earth's upper mantle. It is widely accepted in the scientific community, that during olivine interaction with water large amounts of hydrogen are released. On the other hand, the idea of an accumulation of huge hydrogen amounts in the core and lower mantle of the Earth is becoming more and more popular now. In this paper, we attempt to assess the possible interaction of hydrogen rising from the depths with olivine in the upper mantle, which has not been done before. Program GEMS was used for modeling calculations. The influence of the hydrogen amount on the serpentinization process at T = 323.15 ÷ 593.15 K and P = 3?10⁷ ÷ 3?10⁸ Pa has been estimated. The initial composition of the system was set as 1 mol of olivine (Fe_0.1Mg_0.9SiO₄). The amount of added hydrogen was set as 1, 500, and 5000 mol. The water was not specified as a composition of the initial system. The simulation showed that hydrogen can cause the serpentinization process, which indicates in favor of the hypothesis about deep hydrogen sources, although it is not 100% proof of this.     

Fermentative hydrogen production by a new alkaliphilic Clostridium sp. (strain PROH2) isolated from a shallow submarine hydrothermal chimney in Prony Bay,New Caledonia

The hydrogen-producing strain PROH2 pertaining to the genus Clostridium was successfully isolated from a shallow submarine hydrothermal chimney (Prony Bay, New Caledonia) driven by serpentinization processes. Cell biomass and hydrogen production performances during fermentation by strain PROH2 were studied in a series of batch experiments under various conditions of pH, temperature, NaCl and glucose concentrations. The highest hydrogen yield, 2.71 mol H₂/mol glucose, was observed at initial pH 9.5, 37 °C, and glucose concentration 2 g/L, and was comparable to that reported for neutrophilic clostridial species. Hydrogen production by strain PROH2 reached the maximum production rate (0.55 mM-H₂/h) at the late exponential phase. Yeast extract was required for growth of strain PROH2 and improved significantly its hydrogen production performances. The isolate could utilize various energy sources including cellobiose, galactose, glucose, maltose, sucrose and trehalose to produce hydrogen. The pattern of end-products of metabolism was also affected by the type of energy sources and culture conditions used. These results indicate that Clostridium sp. strain PROH2 is a good candidate for producing hydrogen under alkaline and mesothermic conditions.