Clean water scarcity intensifies the need for advanced treatment of emerging contaminants in nanofiltration (NF) concentrates. Conventional electro-Fenton (EF) systems face unstable oxygen supply and electrode wetting in bubbleless aeration, limiting selective reactive oxygen species generation in high-salinity matrices. Here we show a molecular oxygen confinement ladder strategy by developing a conductive aeration membrane cathode in which boron nitride spontaneously forms wedge-shaped structures inside carbon nanotubes. This architecture achieves kinetic antiwetting and physical oxygen confinement at the nanoscale, accelerating Fe(Ⅱ)/Fe(Ⅲ) cycling and directing the pathway to dominant singlet oxygen (¹O₂) generation. Experiments and simulations demonstrate fivefold faster degradation kinetics, complete sulfamethoxazole removal within 15 min, and sustained high efficiency over 48 h of continuous operation, including a switch to real NF concentrate. The structure-oriented gas confinement approach thus offers a robust platform for selective and stable EF treatment of complex industrial wastewater.