Measurement of OH* Generation by Pulverized Minerals Using Electron Spin Resonance Spectroscopy and Implications for the Reactivity of Planetary Regolith

by Hendrix, Donald A.; Port, Sara T.; Hurowitz, Joel A.; Schoonen, Martin A.

Mineral analogs to silicate phases common to planetary regolith, including olivine; the pyroxenes augite and diopside; the plagioclase feldspars labradorite, bytownite, and albite; the Johnson Space Center-1A lunar regolith simulant; as well as quartz (used as a reference), were subjected to mechanical pulverization by laboratory milling for times ranging from 5 to 45 min. Pulverized minerals were then incubated in an aqueous solution containing the free radical spin trapping compound 5,5-Dimethyl-1-Pyrroline-N-Oxide for times ranging from 5 to 30 min. These slurries were then analyzed by Electron Paramagnetic Resonance spectroscopy to quantify the amount of hydroxyl radical (the neutral charge form of the hydroxide ion, denoted as OH*) formed in solution. We find that all tested materials generate an Electron Paramagnetic Resonance spectrum indicating the formation of OH* with concentrations ranging between 0.1 and 1.5 mu M. We also find that, in general, mineral pulverization time is inversely correlated to OH* generation, while OH* generation is positively correlated to mineral fluid incubation time for phases that have iron in their nominal chemical formulae, suggesting the possible action of Fenton reaction as a cofactor in increasing the reactivity of these phases. Our results add to a body of literature that indicates that the finely comminuted minerals and rocks present in planetary regolith are capable of generating highly reactive and highly oxidizing radical species in solution. The results provide the foundation for further in vitro and in vivo toxicological studies to evaluate the possible health risks that future explorers visiting the surfaces of planetary bodies may face from these reactive regolith materials. Plain Language Summary Future missions to the Moon will face many challenges and obstacles. Exposure to lunar dust will pose health risks to humans due to the reactive nature of the dust. We have assessed the reactivity of various mineral phases analogous to those present in lunar dust. Iron-rich mineral phases generate higher concentrations of hydroxyl radical relative to mineral phases with low amounts of iron. Humans working in the lunar mare may experience relatively more detrimental health effects related to regolith dust inhalation relative to those working in the lunar highlands.

Journal
Geohealth
Volume
3
Issue
1
Year
2019
Start Page
28-42
URL
https://dx.doi.org/10.1029/2018gh000175
ISBN/ISSN
2471-1403
DOI
10.1029/2018gh000175