Thermodynamic Modulation by Micro-Nanobubbles Stimulate Hydroxyl Radical Production during O2 Reduction by Aqueous Fe(II) for Sustainable Water Remediation

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.