(56c) Inducing Nanobubble Collapse Via Dynamic Stimuli to Generate ·OH for Contaminant Degradation

Due to their enhanced lifetime and stability, nanobubbles (NBs)—gaseous particles with diameters of 100 to 200 nm—have captured the attention of researchers for applications involving lake and pond remediation, horticulture, aquaculture, dissolved air flotation, scouring biofilms, and medical imaging. The enhanced properties afforded by NBs versus larger bubbles arise from: (1) movement of NBs is governed by Brownian motion (versus the buoyancy force that impacts the lifetime of larger bubble species); and (2) coalescence is hindered due to electrostatic repulsion between NBs, which have a negative zeta-potential.

Because of their fast reaction kinetics, strong oxidation potential, and ability to react with and mineralize organic contaminates, hydroxyl radicals (^·OH) represent a promising green oxidant for the removal of organic contaminants of emerging concern (CECs), pharmaceutical and personal care products (PPCPs), dyes, explosives, bacteria, and viruses during water and wastewater treatment. To date, the majority of literature on NBs contends that collapsing NBs would have insufficient energy to generate ^·OH. To realize the full potential of NBs derived from stable and readily available gas species as a source of ^·OH, NBs must undergo coalescence and subsequently collapse.

By altering the chemical conditions (i.e., ionic strength, pH, and gas species) as well as applying a dynamic stimulus (e.g., potential), ^·OH is produced in a controllable manner via the coalescence/collapse mechanism. For example, after the addition of 100 mM NaCl to air NBs, the relative size distribution determined by dynamic light scattering (DLS) indicated NB coalescence as evidenced by a change from a unimodal relative size distribution (black, mean diameter of 120 nm) to a bimodal distribution (red, mean diameters of around 100 nm and 450 nm). Meanwhile, Figure 1b depicts the synergetic effects of combining 100 mM NaCl with an applied potential (±250 mV, 5 min). While there is a minimal difference between the NB sample and DI samples after the application of +250 mV, the fluorometric tracer for ^·OH demonstrated a twofold increase for the NB sample versus the DI control after the application of-250 mV. The elucidation of the underlying conditions and mechanisms that govern the controlled coalescence and collapse will be critical for realizing the full potential of NBs as a source of ^·OH for sustainable environmental remediation efforts in the areas of wastewater and water treatment.