Implications of water chemistry on ozone nanobubble-based advanced oxidation processes

Ozone nanobubbles (O₃-NBs) are increasingly promoted to enhance advanced oxidation processes (AOPs) through improved gas dissolution and interfacial reactivity, yet their behavior in realistic waters remains poorly resolved. Here we evaluate O₃-NBs generation, stability, and reactivity across ultra-pure water, synthetic and real freshwater, and synthetic and real seawater. The ζ-potential measurements showed strong matrix dependence, shifting from -25 mV in ultra-pure water to +30 mV in real freshwater due to ionic and organic species adsorption. Salt presence during bubble nucleation, combined with matrix-induced alterations in surface charge, governed interfacial properties and ultimately controlled O₃ reactivity. Across all systems, NBs delivered mass-transfer coefficients more than an order of magnitude higher than macrobubbles (MBs), confirming superior O₃ delivery (k_La= 1.12-1.90 min^-1 vs. 0.13-0.22 min^-1). However, O₃ persistence varied sharply with matrix composition. Decay was minimal in ultra-pure water (k_d = 0.003 min^-1), but ionic and organic components in freshwater accelerated interfacial decomposition (k_d up to 0.160 min^-1), demonstrating that NBs do not inherently extend O₃ lifetime. In seawater, high ionic strength intensified NBs-driven reactions, producing rapid O₃ depletion (decay over the first minute, Δ₀₁= 1.6 mg L^-1 min^-1) and near-complete bromide oxidation to bromate. MBs formed mostly hypobromous acid, which interfered with O₃ measurements. These processes strongly influenced treatment performance. Despite higher O₃ exposure, O₃-NBs favored interface-driven and selective oxidation rather than sustained bulk mineralization, resulting in limited DOC removal in both real freshwater and seawater. Across matrices, O₃-NBs preferentially oxidized humic- and fulvic-like fractions while largely preserving protein-like and microbial by-product-like components. These mechanistic insights highlight the importance of water chemistry and interfacial processes for optimizing O₃-based treatment, particularly in saline environments.