Kinetic modelling of hydrogen generation via subsurface iron mineral transformations

Natural hydrogen is receiving increasing interest as a potential low-carbon fuel. There are various mechanisms for natural hydrogen generation but the reduction of water during oxidation of iron in minerals is recognised to be the major source of naturally generated H2. While the overall reaction is well known, the identity and nature of the key rate limiting steps is less understood. This study investigates the dominant reaction pathways through the use of kinetic modelling. The modelling results suggest there are a number of conditions required for effective H2 production from iron minerals. These include the presence of ultramafic minerals that are particularly high in Fe rather than Mg content, pH in the range of 8 to 10, solution temperatures in the 200 to 300oC range, and strongly reducing conditions. High reaction surface area is key and this could be achieved by the presence of finely deposited material and/or assemblages of high porosity or with mineral assemblages with surface sites that are accessible to water. Finally, conditions favouring the co-deposition of Ni together with FeO/Fe(OH)2-containing minerals such as brucite (and, possibly, magnetite) could enhance H2 generation