Plasma-bubble-generated immobilized ZnO for plasmacatalytic degradation of SMX in water: Reactive species formation, process optimization and degradation pathways

Nanosecond-pulsed plasma bubbles were employed to generate an immobilized ZnO catalyst via in-liquid plasma activation of zinc foil and subsequently applied for the plasmacatalytic degradation of sulfamethoxazole (SMX) in water. Surface oxidation of the zinc foil to ZnO was confirmed by scanning electron microscopy (SEM), photoluminescence analyses and X-ray photoelectron spectroscopy (XPS), demonstrating the successful formation of a stable, immobilized catalyst. The presence of ZnO significantly enhanced SMX degradation and mineralization compared to plasma bubbles alone, while exerting only negligible effects on solution pH and conductivity. Process optimization revealed that increasing the plasma pre-activation time of the zinc foil up to 5 min markedly improved degradation performance, achieving 94.7% SMX degradation within 20 min. The influence of plasma gas composition and initial SMX concentration was systematically examined, highlighting strong plasma-catalyst synergy under air and oxygen atmospheres. Energy efficiency was substantially improved by ZnO addition, with electrical energy per order (E EO ) values as low as 0.4 kWh/m 3 , representing a 23% reduction compared to non-catalytic conditions. Reactive species scavenging experiments identified reactive oxygen species as the dominant contributors to SMX degradation. A total of 44 degradation intermediates were identified, enabling the proposal of comprehensive degradation pathways and demonstrating enhanced mineralization in the presence of immobilized ZnO, with total organic carbon (TOC) removal reaching 71.1% under O 2 plasma. The immobilized ZnO catalyst exhibited excellent reusability over multiple treatment cycles. Overall, the plasma-bubble-activated ZnO system constitutes an energy-efficient, robust, and sustainable approach for the removal of pharmaceutical contaminants from water.