CONSTRAINTS ON THE BIOPERSISTENCE OF OLIVINE PARTICULATES FROM EXPERIMENTAL DISSOLUTION IN SIMULATED LUNG AND GASTRIC FLUID

Abstract:

With the Artemis mission set to launch in 2024, there are many safety issues that must be addressed, including those related to the long- and short-term effects of exposure to lunar dust, which is the fine-grained, respirable fraction of the lunar regolith. Previous work demonstrated that finely pulverized lunar regolith analogue minerals are reactive in the short-term, generating nano- to micromolar quantities of reactive oxygen species (ROS) upon initial contact with liquid media. Among these phases, olivine consistently generates the highest concentrations of ROS. Less well understood is the long-term biopersistence of such minerals in the human body, e.g., in the lung or stomach. In an effort to constrain the lifetime of fine particulate matter in contact with liquid media in the human body, we conducted dissolution experiments in simulated lung fluid (SLF) and 0.1 M HCl (proxy for gastric fluid) in a flow-through reactor at human body temperature (37^oC). Effluent samples were collected and analyzed for Mg, Fe, and Si concentrations. Dissolution rates were calculated from effluent concentrations derived from 5 separate experiments in HCl and 6 in SLF (Table 1). These rate calculations enable us to determine that a 1 µm olivine particle would dissolve after ~8 years in lung fluid; that same particle would dissolve in ~7 days in gastric fluid. Solution chemical data from our experiments were used to determine fluid saturation state (PHREEQC v3.5). Model results indicate that interaction between SLF and olivine produces fluids saturated with respect to sepiolite, talc, and chrysotile. These minerals dissolve more slowly than olivine and have fibrous crystal structures, posing an additional hazard if formed during the course of long-term exposure. In HCl, our models indicate that the major precipitate is amorphous silica, which was observed in XRD and SEM analysis of samples recovered from our experiments. Our results constrain the lifetime of olivine particles in parts of the human body that are likely to be exposed to fine-grained, reactive regolith during lunar exploration and provide insight into additional reactions that can occur between body fluids and minerals after they are respired or ingested.