Advanced oxidation of Arsenic(III) to Arsenic(V) using ozone nanobubbles under high salinity

Arsenic, a highly toxic element, is present in various water resources as As(III) and/or As(V). The removal of As(V) using adsorption is considered to be easier than the removal of As(III). In this work, we report the oxidation of As(III) to As(V) using ozone nanobubbles. Ozone has been widely used in the advanced oxidation process (AOP) to remove organics and inorganics in wastewater. The major advantage of ozone nanobubbles is their enhanced half-life, which leads to higher ozone solubility in water. In addition, the rate of mass transfer by using nanobubbles can be enhanced drastically. The effect of ozone flow rates, initial As(III) concentrations, pH levels, and the presence of salt is systematically studied in this work. The present results revealed that ozone nanobubbles positively impacted the oxidation of As(III) to As(V). AOP process based on O3 NBs (nanobubbles) was most efficient with 99% oxidation rates of 1 ppm of As(III) within 20 minutes at 6.5 ppm ozone concentrations. An acidic pH of 3-7 promoted quick oxidation due to the high mass transfer coefficient of ozone nanobubbles. About ~70% degradation of As(III) to As(V) was achieved with 0.10854 min-1 rate constant at acidic (pH = 3) compared to ~39% degradation with 0.046 min-1 rate constant at pH =11. The presence of salt (~50 mM) in the solution not only hindered the ozone mass transfer coefficient but also reduced the stability of nanobubbles. The detection of reactive oxygen species, particularly hydroxyl radical was unravelled by utilizing 2-propanol as a scavenger.