Water Production by Microwaving Lunar Simulants with Hydrogen

Water production by microwaving lunar simulants with hydrogen. A. D. Morse1 S. Lim2, J. D. Cole1, and M. Anand1, 1The Open University, School of Physical Sciences, R.Hooke building, Walton Hall, Milton Keynes MK7 6AA, UK. (andrew.m[email protected]k), 2Surrey Space Centre, University of Surrey, Arthur C Clarke Building, Stag Hill, University Campus, Guildford GU2 7XH, UK Introduction: The availability of energy on the Moon is a constraining factor in lunar ISRU for constructing a lunar habitat and producing water. Microwave energy has been shown as an efficient method of volumetrically heating mare lunar simulant to the melting point [1,2] for use in habitat construction. There are many techniques for extracting oxygen from lunar simulants, of which hydrogen reduction of ilmenite-rich regolith is a favourite. It requires a relatively low temperature of ~900 ºC and is comparatively simple, and as a result it has been demonstrated to the highest TRL 5 [3]. However, it is limited to ilmenite-rich samples and has a low oxygen yield. In this study, we investigate the possibility of combining the microwave heating of simulants for construction purposes and simultaneously extracting oxygen from lunar simulants. The rate of the hydrogen reaction is increased by higher temperatures [4]. However, the reaction rate decreases as hydrogen must diffuse into the bulk sample while water diffuses out. Method: Experiments were conducted on 50g samples of simulant placed in a vacuum microwave chamber [5]. JSC-1A simulant was selected as it represents a low titanium lunar mare regolith and has been used in previous microwave experiments. Ilmenite was added to give a range of simulants with 0 to 20 wt.% ilmenite. The microwave cavity was evacuated and flushed several times until a vacuum pressure below 1x10-3 mbar was attained; it was then filled with ~0.2 bar hydrogen. The sample was heated by 1000 W microwaves for an hour. The gas composition in the chamber was sampled by a quadrupole mass spectrometer every 30 seconds with a mass range from 1 to 100 amu. A cold trap in liquid nitrogen was used to collect water. The gas pressure and composition, sample surface temperature, and sample appearance were monitored throughout the heating. At the end of the microwave heating, the excess gas was evacuated through the cold trap. The cold trap was then heated to 100 ºC and the released water vapour was trapped by liquid nitrogen onto a smaller cold finger. The amount of water produced was determined by weighing the cold finger before being transferred by pipette to a 2 mL vial. Blanks were determined by repeating the experiment with helium instead of hydrogen and by conducting an experiment without a sample. Results: A typical result for the gas composition during microwave heating is shown in Figure 1. Initially, the hydrogen intensity and gas pressure decrease as the cryo-trap is cooled. At about 12 minutes, the sample was observed to melt, there was a rapid decrease in hydrogen intensity, and an increase in m/z 18 (water), m/z 28 (probably CO) and m/z 44 (CO2). There was a slight increase in m/z 77 and m/z 78 when the sample melted indicating the presence of benzene. Figure 1. Gas composition during microwave heating of 46g JSC-1A with 4g Ilmenite simulant. The water collected for various concentrations of ilmenite is shown in Figure 2. The quantity of water collected increased with increasing ilmenite concentration by 0.01 g per 1 wt% ilmenite. The oxygen yield from the sample is 0.018 % per wt% ilmenite. Figure 2. Water collected from JSC-1A/Ilmenite samples. Ilmenite concentration 0, 4, 8, 12 and 16 wt.% (left to right). Discussion: The theoretical yield of oxygen from pure ilmenite is 10.3%, indicating that approximately 18% of the ilmenite in the samples was reduced by the hydrogen. The reaction is rapid, completing in about 10 minutes compared to typical reaction times of 4 hours at lower temperatures [4]. However, the reaction halted after about 10 minutes either because hydrogen was unable to diffuse further into the molten sample or because water was unable to diffuse out. The technique of hydrogen reduction would be most efficient where the liquid thickness is about 10 mm or less. The presence of benzene indicates that chemical reactions are occurring in the high-energy environment of the microwave chamber. The simulant and ilmenite should not contain carbon; however, the microwave chamber has been used for mine waste experiments and carbon contamination is unsurprising. Lunar samples do contain carbon ~ 100 ppm so contamination of the water by organics produced during the reaction is likely. Summary: Microwave heating with hydrogen of lunar simulants containing ilmenite produced water with 18% of the ilmenite reacting. This could be a useful ISRU technique where ilmenite-rich regolith is melted for construction resulting in water being produced as a useful by-product. References: [1] Lim S. et al. (2021) Sci. Rep. 11(1), 2133. [2] Lim S. et al. (2023) Sci. Rep. 13 (1), 1804. [3] Sanders G.B. and Larson W.E. (2013) J. Aer. Eng 26(1), 5-17. [4] Sargeant H. M. et al. (2020) Planet. Space. Sci. 180, 104751. [5] Cole J. D. et al. (2025 Planet. Space. Sci., 255, p.106011