Spatially-confined bubble plasma oxidation induced by microchannel discharge to boost energy efficiency in water treatment

Water pollutants, in particular emerging contaminants, pose severe risks to aquatic ecosystems and public health due to their high persistence and bioaccumulation potential. While advanced oxidation processes (AOPs) offer sustainable solutions for pollutant degradation, plasma-based AOPs often suffer from low energy efficiency and slow mass transfer of active species. Here we report a spatially-confined plasma AOP, generated in situ by electrical discharge within microchannel during bubble formation. By reducing the aperture from 1.0 to 0.1 mm, we achieve an energy yield of 4.68 g/kWh for the degradation of diclofenac, with an extremely low specific energy input (6.75 kJ/L), breaking through the energy efficiency limit of existing plasma reactors (e.g., pulsed corona discharge, dielectric barrier discharge, and atmospheric pressure plasma jet) by a factor of two, which is also highly competitive among current AOPs. This is accomplished by the elevated electron temperature (from 0.80 to 1.35 eV) by decreasing aperture size, enhancing the excitation and dissociation of oxygen molecules within the confined space. Furthermore, the intensified discharge breaks gas bubbles into micrometer-scale or even smaller ones within microsecond period, which effectively enhances the mass transfer of the short life-time species through gas-liquid interface. Mechanistic studies verify ·OH as the primary oxidant, enabling efficient degradation via decarboxylation and hydroxylation pathways. This study sheds light on the energy-effective abatement of water pollutants, providing a promising water purification paradigm.