Thermodynamic Modulation by Micro-Nanobubbles Stimulate Hydroxyl Radical Production during O₂ Reduction by Aqueous...

Ferrous iron (Fe(II)) species are prevalent in reductive subsurface environments, where some active Fe(II) species can generate hydroxyl radical (·OH) during oxidation under O₂ perturbation, facilitating pollutant attenuation. However, aqueous Fe(II), a common form of ferrous iron, can be rapidly oxidized but scarcely contributes to ·OH production. Herein, this study proposes a thermodynamic control strategy by introducing micro-nanobubbles (MNBs) into an aqueous Fe(II)/O₂ system to stimulate its potential. Experimental results indicate that MNBs alter the electron transfer pathway from aqueous Fe(II) to O₂, shifting the predominant mechanism from a one-electron transfer pathway to a two-electron transfer pathway; this can reduce energy loss and improve the overall electron utilization efficiency. Under optimal conditions, ·OH production and pollutant degradation reached 20 μM and 15%, respectively, compared to almost 0 in the system without MNBs. Thermodynamic analysis reveals that MNBs lower the oxidation-reduction potential (ORP) of the system and reduce the Gibbs free energy of key reaction steps, enabling efficient ·OH production. The aqueous Fe(II)/MNBs/O₂ system demonstrates a broad applicability for the degradation of various pollutants with removal rates ranging from 25% to 36%, highlighting its potential as a promising approach for green and sustainable groundwater remediation.