Presentation Information
[U05-04]Hydrogen Production Induced by Degree of Serpentinization: Hydrothermal Experiment on Natural Ultramafic Samples.
*Nomin Tumurkhuu1, Otgonbayar Dandar1, Manzshir Bayarbold1, Kazuki Yoshida2, Jiajie Wang1, Noriaki Watanabe1, Yasuhiro Niwa2,3, Masao Kimura2,3, Atsushi Okamoto1 (1.Graduate School of Environmental Studies, Tohoku University, 2.High Energy Accelerator Research Organization (KEK), 3.The Graduate University for Advanced Studies)
Keywords:
Natural Hydrogen,Serpentinization,Water-Rock interaction,Hydrothermal Experiment
Serpentinization of ultramafic rocks at mid-ocean ridges generates substantial amounts of hydrogen H2 and methane CH4, creating conditions that sustain specialized biological communities Schrenk et al., 2011 and highlighting its potential as a source of green energy. Hydrogen is produced abiotically through the reduction of water during the oxidation of ferrous iron in primary minerals such as olivine and pyroxene. Consequently, the total amount of hydrogen generated depends directly on the abundance and oxidation state of iron incorporated into secondary minerals, principally magnetite and serpentine. Several experimental studies have investigated hydrogen production using peridotite and olivine to determine the controlling mechanisms and reaction pathways Klein et al., 2009, 2013; Andreani et al., 2023; Malvoisin et al., 2013; McCollom et al., 2020. However, hydrothermal experiments conducted on natural ultramafic rocks with varying degrees of serpentinization remain limited. In this study, we investigated natural hydrogen production through hydrothermal stirring experiments conducted at 300 C and 10 MPa for 72 h using peridotite samples from the Iwanaidake and the Horoman peridotite body, Hokkaido. In each experimental run, 5.0 g of powdered mineral sample <62 um was reacted with 100.0 mL of MilliQ water. Six groups of ultramafic rock samples were analyzed, including four from the Iwanaidake peridotite body G1 to G4 and two from the Horoman peridotite body G5 and G6. The samples were classified according to their degree of serpentinization confirmed by thermogravimetry measurement TG. Sample G1 consists of dominantly olivine TG loss 1 wt percent, with minor orthopyroxene Opx and clinopyroxene Cpx. Samples G2 TG loss 5.2 wt percent and G3 TG loss 9 wt percent contain variable amounts of serpentine in addition to Opx and Cpx. Sample G4 TG loss 15.2 wt percent is composed predominantly of serpentine and brucite, with no primary minerals remaining. Among the Horoman samples, G5 TG loss 1.13 wt percent has a composition similar to G1 and is dominated by primary minerals, whereas G6 TG loss 7 wt percent is partially serpentinized. After the stirring experiment, the released gas is measured by gas chromatography, and G1 most fresh samples released the highest amount of H2 15.85 mmol per kg rock whereas G4 serpentinite released the lowest amount of H2 5.5 mmol per kg rock. In range of H2 amount from G1 and G4, G5 and G6 yields 10 to 12 mmol per kg rock which is higher than those released from G2 and G3 samples 8 to 9 mmol per kg rock.
To estimate the hydrogen that had already been released naturally before the experiment, we calculated hydrogen production based on bulk rock Fe3 plus over Fetotal ratios determined by X ray absorption fine structure XAFS analysis. The results indicate that G1 and G5, which are less serpentinized and contain abundant olivine, have the lowest estimated natural hydrogen generation 5.6 and 16.3 mmol per kg rock, respectively. In contrast, G2, G3, and G6, which contain significant serpentine, are inferred to have already released substantial hydrogen during natural serpentinization, with estimated values ranging from 146 to 196.3 mmol per kg rock.
Our experimental results using natural ultramafic rocks at 300 C demonstrate a linear relationship between the degree of serpentinization and hydrogen production. This correlation may be applied to estimate the hydrogen generation potential of naturally serpentinized ultramafic rocks.
To estimate the hydrogen that had already been released naturally before the experiment, we calculated hydrogen production based on bulk rock Fe3 plus over Fetotal ratios determined by X ray absorption fine structure XAFS analysis. The results indicate that G1 and G5, which are less serpentinized and contain abundant olivine, have the lowest estimated natural hydrogen generation 5.6 and 16.3 mmol per kg rock, respectively. In contrast, G2, G3, and G6, which contain significant serpentine, are inferred to have already released substantial hydrogen during natural serpentinization, with estimated values ranging from 146 to 196.3 mmol per kg rock.
Our experimental results using natural ultramafic rocks at 300 C demonstrate a linear relationship between the degree of serpentinization and hydrogen production. This correlation may be applied to estimate the hydrogen generation potential of naturally serpentinized ultramafic rocks.