Removal mechanisms of geosmin and MIB by oxygen nanobubbles during water treatment

Nanobubbles (NBs) have emerged for water/wastewater treatment, removal of pollutants from sediments and soils, and other environmental and biomedical applications. In this study, the application of NB technology was investigated to control extracellular geosmin and 2-methylisoborneol (MIB) which are naturally occurring terpenoid alcohols produced by cyanobacteria (blue-green algae) and actinomycetes (bacteria) found in surface waters. In addition, their removal mechanisms by oxygen NBs were systematically examined. Volatilization was the dominant mechanism for the removal of geosmin (∼40%) and MIB (∼20%) from distilled and deionized water, while oxidation by reactive oxygen species (ROS) brought additional removals up to ∼ 15%. The formation of hydroxyl (•OH) radicals was promoted when NBs were mixed with microbubbles (MBs), while there was no evidence of geosmin/MIB oxidation by either oxygen NBs or MBs alone. Alkalinity decreased the removal rates of both geosmin and MIB by scavenging •OH radicals, while elevated temperature increased the removal of geosmin and MIB due to enhanced volatilization and oxidation. Natural organic matter (NOM) and chloride ion slightly decreased the removal of geosmin and MIB. The effect of pH in the range of 3–10 was found insignificant. When oxygen NBs were applied to selected natural waters with a wide range of water characteristics, the oxidative pathway was significantly suppressed by ambient alkalinity, NOM, and chloride by which •OH radicals can be quenched. These findings will guide the applications of the oxygen NBs technology in the water treatment, and the further investigations for the improvement of its oxidation potential.