Bulk nanobubbles do not spontaneously generate radicals but function as field-assisted energy-reducing catalysts

The fundamental capacity of bulk nanobubbles (NBs) to spontaneously generate reactive oxygen species (ROS) remains a subject of intense mechanistic debate, hindering their rational application in environmental engineering. This study provides insights into the mechanisms underlying ROS generation by combining experimental observations with multi-scale molecular simulations. Atomistic molecular dynamics (MD) simulations demonstrate that despite transient localized pressure exceeding 24,124 atm during NB collapse, the interfacial solvent temperature remained thermally moderate (below 162 °C), effectively indicating that the "hot spot" effect is inapplicable to free radical generation during the free collapse of NBs under the modeled nanoscale conditions. Instead, ab initio MD (AIMD) identified an interfacial electric field-induced electrochemical pathway wherein the vertical ionization energy of adsorbed hydroxide ions is lowered by ∼6 kcal/mol. This electronic destabilization designates the NB interface as a kinetically-primed arena that narrows the activation barrier for radical generation, although this field effect remains thermodynamically insufficient to trigger spontaneous electron detachment without exogenous energy. Consistent with these findings, experimental assays indicate that bulk NBs function primarily as field-assisted energy-reducing catalysts rather than intrinsic oxidants, synergistically enhancing UV-induced Rhodamine B degradation to 53.7% compared to < 8% in NB-alone controls. Furthermore, optical simulations yielded a sub-unity photon flux enhancement ratio (0.84-0.86), suggesting that physical light focusing does not act as the primary contributing driver. By providing critical mechanistic insights into NB reactivity, this study establishes a theoretical framework for designing energy-efficient, NB-potentiated advanced oxidation processes.