Nanobubble Applications in Graphene Adsorption of Hydrophobic Pollutants: Towards Wastewater Treatment on the International Space Station

The National Aeronautics and Space Administration (NASA) aims to improve the wastewater treatment system efficiency on the International Space Station by preventing the accumulation of dimethylsilanediol (DMSD), part of the siloxane chemical group, in the filters. Adsorption has proven to be a potentially effective approach. Meanwhile, recent literature has indicated that nanobubbles in water can improve the hydrophobic interactions between hydrophobic adsorbents like carbon and organic pollutants such as per- and polyfluoroalkyl substances (PFAS), thereby enhancing the pollutant removal efficiency. However, it is not yet established whether this nanobubble-enhanced adsorption phenomenon applies to DMSD. Because DMSD is highly volatile and difficult to measure, a proof-of-concept adsorption study was conducted with phenanthrene (PNT), a model organic hydrophobic pollutant, as a preliminary step towards DMSD adsorption studies. Adsorption tests were performed using graphene, graphene oxide, and reduced graphene oxide and different adsorption matrices viz degassed water and nanobubble water. The presence of NBs decreased the removal of phenanthrene by all three graphene derivatives. Therefore, a potential mechanism is that NBs and PNT compete for the adsorption sites on the graphene and due to the higher concentration of NBs, less PNT is adsorbed to the graphene. However, blank experiments performed without graphene demonstrated that PNT volatilization occurred in degassed and deionized water but was prevented in NB water. These data suggest a second potential mechanism: PNT has a higher affinity for NBs than graphene and their interactions alter the surface chemistry of the NBs such that they no longer adsorb to graphene. Further experiments will be conducted to determine the mechanism for interactions between PNT, NBs, and graphene and to understand why PFAS and PNT behave differently in a NB-graphene adsorption system. This research is crucial for developing technology that can manage spacecraft waste efficiently and with limited resources, allowing NASA to continue expanding its space exploration.