High-Fidelity Dust Simulants for Long-Term Toxicological Assessment of Lunar Regolith to Support In-Situ Resource Utilization (ISRU)

NASA, ESA, and other space agencies are planning a new phase of Moon exploration, with the goal of establishing a permanent human presence. Human interaction with the lunar environment will expose the crew to fine (< 20 μm) lunar dust (LD) which might exhibit toxic effects on the respiratory system and other organs. The potential toxicity of LD may stem from its unique physico-chemical properties, shaped by the distinctive environmental conditions under which it forms. The lunar soil is continuously bombarded by micro-meteorites, creating freshly fractured surfaces, whose reactivity might be preserved by the water- and oxygen-free Moon environment. Moreover, the occurrence of nanophase zero-valent iron (np-Fe0), i.e., nanoclusters of metallic iron in an amorphous silicate matrix observed in LD and absent in terrestrial mineral dusts, may contribute to LD oxidative activity. Due to the limited availability of LD samples from Apollo and Luna missions, research on LD toxicity primarily depends on LD simulants (LDS). However, a high-fidelity standardized LDS, specifically designed for toxicological studies, is not yet available. We propose here a strictly controlled procedure to obtain np-Fe0-rich silicate dust in respirable size that mimic LD particle features that could be relevant in reactivity and toxicity. A synthetic np-Fe0-rich silicate glass (i.e., Simulant Moon Agglutinate, SMA) was ball milled in a non-oxidative atmosphere to i) reduce particle size in the respirable range (< 5 μm) and, ii) expose freshly fractured surfaces containing reactive iron sites, reproducing the mechanism of dust generation on the Moon. The milled SMA was analyzed for particle morphology, size distribution, specific surface area and crystallinity, and compared to the pristine material. Moreover, the surface reactivity, in terms of capacity to induce oxidative stress and cell membrane damage, was assessed. Milling in a nonoxidative environment produced respirable particles exposing nanoclusters of metallic iron at the surface. The milled SMA preserved in the inert atmosphere induced a greater oxidative stress in biomimicking fluids compared to the pristine sample. However, both pristine and milled SMA did not cause lysis of cellular membranes, indicating that possible toxic effect should primarily rely to oxidative stress mechanisms. The proposed milling procedure is relevant to produce LDS with highly reactive np-Fe0-rich surfaces that can be used to investigate the potential hazard of long-term exposure to LD.