Hydrogen from Aqueous Fluid Inclusions: An Untapped Source of Clean Energy
Hydrogen is one of the most abundant elements in the universe, and it has a range of potential applications in energy production and transportation. Hydrogen is also crucial for a zero-carbon economy as it has the potential to replace fossil fuels in many applications due to its high specific energy capacity as compared to natural gas. Recently, a study in the UK showed that potassium-bearing rocks enriched with water could generate measurable amounts of hydrogen through radiolysis. This article discusses the data and methodology used in the study, the results obtained, and the challenges involved in this promising method of producing hydrogen through rocks.
Introduction
Water is the most commonly found liquid in fluid inclusions and it is typically modified from its original composition due to various factors such as interaction with the host mineral or crystallisation of dissolved salts. However in some circumstances, due to radiolysis caused by irradiation, the water may be broken down into hydrogen and oxygen as seen in highly radioactive uranium ores. Radiolysis is a chemical reaction that occurs when ionising radiation (such as gamma rays, X-rays, or high-energy particles) interacts with matter resulting in the dissociation of existing chemical bonds and the formation of new chemical species. This process can occur in any material when exposed to ionising radiations, including liquids, gases, and solids (D. Féron, 2012).
The radiolysis of rocks was first observed in the 1950s when scientists discovered that certain rocks contained high levels of hydrogen gas. These rocks were granite and other types of felsic rocks, which are composed mainly of silica and feldspar minerals. These minerals contain trace amounts of radioactive isotopes such as uranium, thorium, and potassium, which emit alpha, beta, and gamma radiations. The potential of radiolysis of rocks as a source of hydrogen has not been extensively studied, and much remains unknown about the process. However, recent research has shown that the radiolysis of rocks may have the potential to produce hydrogen gas at rates that could be commercially viable. In one study, researchers found that the radiolysis of granite could produce up to 42 micromoles of hydrogen gas per kilogram of rock per hour under certain conditions.
Potassium-bearing rocks, especially the mineral sylvite, which contains high levels of both potassium and water could generate measurable amounts of hydrogen through radiolysis. This potential for hydrogen generation is relevant in potash ore deposits where sylvite is abundant. To test whether these rocks could generate hydrogen in the subsurface, sylvite and halite samples were analysed from an underground potash deposit in the UK using a cold crush technique which has been proven effective for analysing fluid inclusions in ore deposits and geothermal systems. The results of this study may have implications for subsurface microbial communities as hydrogen generated through radiolysis could serve as an electron donor.
Data and Methodology
Mineral samples used in this study was obtained from the Boulby potash mine in Yorkshire, UK, as well as from a nearby borehole called Eskdale No. 2 in Aislaby (Parnell J. & Blamey N., 2017). The mine is known for extracting marine evaporites of Upper Permian age, including thick beds of sylvite, a potassium chloride mineral.
Samples of sylvite and halite were collected from the underground stockpiles of the mine which consisted of relatively pure salt deposits. The Boulby mine serves as a testing laboratory for astrobiology and the investigation of potential habitats for life. To prepare the samples for analysis, the sylvite and halite were cleaned with isopropanol to remove surface organics, air-dried, and vacuumed overnight to remove any gas between the crystals. The samples, made up of several small mineral pieces, were incrementally crushed using a specialised valve and mass spectrometer setup. The crushing produced 5 to 12 successive gas bursts, which were analysed using two Pfeiffer Prisma quadrupole mass spectrometers operating in a special mode. The instrument was calibrated using gas mixtures and verified using fluid inclusion gas standards, and the gas content was calculated using matrix multiplication to provide quantitative results. The volatile gases were reported in mol percentage, and the 3-sigma detection limit for gases was about 0.3 ppm. The accuracy of the method was demonstrated using capillary tubes filled with gas mixtures, which clustered close to the global atmospheric gas content.
Results and Discussion
The presence of hydrogen in all the sylvite samples is unusual compared to other rocks and minerals analysed in the same laboratory, as hydrogen was not detected in most halite samples from the same location. The anomalous hydrogen content in sylvite is consistent with the prediction of hydrogen generation due to irradiation-induced radiolysis. The oxygen content in sylvite and halite samples does not show a clear trend as it can come from various sources. The highest helium contents in the samples are associated with the highest hydrogen contents, which is also observed in other uranium-rich rocks. Uranium, which is present in the iron oxide in sylvite, can yield helium through alpha irradiation over geological timescales. In contrast, halite does not contain iron oxide and, therefore, does not yield helium although some halite samples may contain limited helium that migrated from adjacent sylvite or another source of alpha irradiation. Two-phase inclusions in the sylvite imply some post-depositional fluid migration.
The method of analysing fluid inclusions for volatile substances has effectively identified small amounts of hydrogen in sylvite mineral samples. All 13 sylvite samples were found to contain hydrogen, while only a minimal amount or none at all was found in any of the 10 halite samples from the same location. This evidence supports the theory that radiation from potassium in sylvite causes the breakdown of water in fluid inclusions, resulting in the formation of hydrogen. These findings suggest that hydrogen can be produced in the subsurface through this process, providing a potential energy source for a chemoautotrophic deep biosphere.
Conclusion
While the potential for using radiolysis of rocks to produce hydrogen is promising, there are also several challenges that need to be addressed. One major challenge is the low efficiency of the process, as the amount of hydrogen gas produced is relatively small compared to the amount of radiation required. Another challenge is the difficulty in extracting the hydrogen gas from the rock matrix as the gas is tightly bound to the rock surface and can be difficult to access.
Despite these challenges, the radiolysis of rocks remains an intriguing area of research for hydrogen production. Further studies are needed to explore the potential of this process and to develop methods for improving its efficiency and scalability. If successful, the radiolysis of rocks could provide a new and sustainable source of hydrogen gas, helping to reduce our dependence on fossil fuels and advance the transition to a low-carbon economy.
References
D. Féron, (2012), Overview of nuclear materials and nuclear corrosion science and engineering, Editor(s): Damien Féron, In Woodhead Publishing Series in Energy, Nuclear Corrosion Science and Engineering, Woodhead Publishing, Pages 31-56.
Parnell, John & Blamey, Nigel. (2017). Hydrogen from Radiolysis of Aqueous Fluid Inclusions during Diagenesis. Minerals. 7. 130. 10.3390/min7080130.
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