A review of natural hydrogen generation from iron-rich rocks: Mechanisms, influencing factors, techniques, and knowledge gaps

Natural hydrogen has emerged as a promising clean energy resource with significant potential for large-scale subsurface production. Among the various geological sources, water–rock interactions involving iron-bearing rocks are among the most extensively studied pathways for natural hydrogen generation. This study provides a comprehensive review of hydrogen production from iron-rich lithologies, including mafic and ultramafic rocks, iron oxides, iron carbonates, and peralkaline granites. The fundamental mechanism involves the reduction of water coupled with the oxidation of ferrous iron to ferric iron under anoxic conditions. Key processes, including serpentinization, magnetite alteration, and siderite decomposition, are critically evaluated using experimental and modelling data. Integration of experimental findings from representative mineral phases indicates that Fe-rich olivine exhibits the highest hydrogen yield, reaching 0.417 mmol g -1 at 230 °C and 350 bar after 3500 h. Ferroan brucite produces up to 0.22 mmol g -1 , whereas magnetite, arfvedsonite-bearing granite, and siderite yield 0.052, 0.036, and 0.0242 mmol g -1 , respectively, under varying experimental conditions. The influence of temperature, pressure, pH, grain size, salinity, water-to-rock ratio, and iron partitioning is systematically assessed, with temperature, alkalinity, and reactive surface area identified as the dominant controls on hydrogen generation.