Oxidation regulation mechanism of air nanobubbles and application in sulfide removal from industrial wastewater

Abstract

Micro–nano bubbles (MNBs) enhance the oxidative capacity of water by promoting hydroxyl radical (·OH) generation. This study investigated the regulatory effects of the gas–liquid ratio (Q_g/Q_l), aeration rate (v), and pH on ·OH generation by air MNBs generated via a pressurized gas dissolution method, with further validation in sulfur-containing wastewater (SC-W). The results indicated that a lower gas–liquid ratio favored nanobubble (NB) formation and consequently enhanced the cumulative ·OH yield. Increasing the gas–liquid ratio from 1% to 5% reduced NB concentration by a factor of 6.5 and decreased the cumulative ·OH yield by a factor of 4.4. Varying the aeration rate from 13.3 m s⁻¹ to 17.0 m s⁻¹ resulted in a non-monotonic change in NB quantity, which first increased and then decreased. Weakly alkaline conditions were most favorable for cumulative ·OH yield via NB shrinkage and collapse, while strong acidic or alkaline conditions suppressed ·OH generation by 83–96%. The optimal cumulative ·OH yield of 1.18 μmol L⁻¹ was achieved under the conditions of a 1% gas–liquid ratio, an aeration rate of 15.5 m s⁻¹, and pH 7.7. The enhancement mechanism for sulfide removal was attributed to a synergistic effect between ·OH oxidation and oxygen mass transfer, with the dominant mechanism shifting from ·OH oxidation to mass transfer enhancement as the sulfide concentration increased. Under optimal conditions, the S²⁻ removal rate reached 86.53% within 1 hour for industrial SC-W. This study establishes a foundation for optimizing the oxidative capacity of MNBs and provides technical support for the low-carbon treatment of industrial SC-W.