Hydrogen generation by subsurface iron mineral transformations

Natural (geogenic) hydrogen has gained increasing interest as a possible new “carbon-free” energy source. If found in commercially exploitable quantities, there is potential for this new energy source to significantly support the global transition to a low carbon economy. The many origins of natural hydrogen in our geologic system make exploration challenging. A recognised production pathway is hydrogen generated in subsurface environments through a variety of water-rock interactions, with oxidation of reduced iron minerals one of the most important. Of the various iron-mediated pathways, the hydrolysis of magnesium and iron rich (ultramafic) minerals such as olivine to serpentine (a process known as “serpentinization”), brucite, magnetite and H2 is considered to be particularly important: Mg1.82Fe0.18SiO4 + wH2O → 0.5(Mg,FeII,FeIII)3(Si,FeIII)2O5(OH)4 + x(Mg,Fe)(OH)2 + yFe3O4 + zH2 Olivine Serpentine Brucite Magnetite H2 generation via this process is severely limited both by the slow reaction kinetics and partitioning of Fe(II) into brucite with the rate of H2 production determined by the oxidation of Fe(II) associated with this mineral. While magnetite should be the end product of this oxidation process, laboratory studies have shown that the magnetite so formed typically contains elemental iron. This suggests that the mechanism of H2 generation may involve disproportionation of Fe(II) to Fe(III) and Fe(0) with the Fe(0) produced expected to induce water reduction as shown below. 4Fe(OH)2 → Fe(0) + Fe3O4 + 4 H2O Fe(0) + 2H2O → Fe(OH)2 + H2 Laboratory studies of this process reveal that pH, temperature and the presence of other elements such as nickel have a dramatic effect on rate of H2 generation. We present a range of additional studies that could be undertaken to both confirm this hypothesized mechanism and, potentially, enhance the natural generation of H2.