Nanobubbles by hybrid electro-membrane method: ROS quantification and utilization in complex wastewater treatment

Nanobubbles (NBs) possess unique interfacial properties that enhance gas-liquid reactions and radical generation, making them promising for intensifying advanced oxidation processes (AOPs). However, the mechanistic understanding of reactive oxygen species (ROS) generation across different NB generation strategies and its impact on treatment performance remains limited. In this study, major NB generation methods were systematically compared in terms of NB characteristics, gas-liquid mass transfer, and ROS production using fluorescence-based diagnostics. Electrochemically reactive nanobubbles (ERNBs) exhibited the highest ROS activity, generating hydroxyl radicals (OH⋅), superoxide radicals (O₂^⋅-), and hydrogen peroxide (H₂O₂), while membrane-derived CO₂ NBs produced moderate OH⋅ and oxygen-, nitrogen-, and air-based NBs yielded detectable H₂O₂. While ERNBs showed high intrinsic ROS productivity, their mass-transfer capability was limited, whereas membrane-based NBs provided high mass-transfer with lower ROS yields. To integrate these complementary advantages, a hybrid electro-membrane nanobubble (HEM-NB) system was engineered, achieving higher NB density, enhanced mass transfer, and increased ROS availability. Application studies using tannic acid (TA) as a representative polyphenolic contaminant in single and complex textile-like matrices demonstrated improved degradation kinetics, with synergy factors of ∼1.33 and ∼1.23 compared to ERNBs-only treatment. Overall, this work establishes mechanistic-performance linkages and demonstrates the oxidative robustness of hybrid NB technology under chemically complex effluent treatment.