Modified Indigo Method to Quantify Ozone Diffusion from Nanobubbles

Abstract

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.