Modified Indigo Method to Quantify Ozone Diffusion from Nanobubbles

The use of nanobubbles to deliver ozone (O₃) gas to natural and engineered water systems is an emerging technology. The long lifetime of nanobubbles prolongs the timeframe of O₃ mass transfer to aqueous solution, raising conflict with conventional methods used to quantify O₃. Standard Method 4500-O3B is a colorimetric method that uses indigotrisulfonate (indigo) to measure residual aqueous O₃ concentration at a discrete timepoint. This discrete method does not account for gaseous O₃ that remains in nanobubbles at the time of the indigo absorbance measurement. In this study, a modified indigo method was developed to quantify the prolonged diffusion of O₃ from nanobubbles. The cumulative O₃ gas released by nanobubbles and dissolved into water was captured by measuring changes in indigo absorbance over time. The transfer of O₃ from nanobubbles into the aqueous phase occurred primarily within the first 4 days after treatment in natural waters. The effects of indigo concentration, water quality parameters, and H₂O₂ were minimal over a week-long sample hold time. Sample storage at 4°C, consistent with the standard indigo method, is recommended in this study to prevent overcalculation of O₃ production. This is the first method developed for quantifying O₃ diffusion from nanobubbles.