Plasma-photocatalytic gCeO2 for selective Fe(ii) and Mn(ii) oxidation: charge-carrier-driven reactive oxygen species pathways

Abstract

Plasma-based water treatment offers a green approach for oxidizing dissolved metals in groundwater, yet Mn(II) removal remains challenging under near-neutral pH due to its high redox potential requirement. Here, we introduce a plasma-photocatalysis (PPC) platform that integrates air dielectric-barrier-discharge (air-DBD) plasma with reusable granular CeO₂ immobilized on activated alumina (gCeO₂) under LED irradiation for efficient simultaneous removal of Fe(II) and Mn(II). While air-DBD alone effectively oxidizes Fe(II) but removes only 10–13% of Mn(II), PPC achieves 97% Mn(II) removal within 20 min and maintains synergistic performance over repeated cycles (f_syn = 3.4 initially; average 2.3). Kinetic results reveal a strong linear correlation between the Mn(II) removal and H₂O₂ decomposition rate, identifying plasma-derived O₃/H₂O₂ as key precursors of highly oxidative species. Scavenger tests, ESR spin trapping, and in situ XPS/surface-enhanced Raman spectroscopy demonstrate that PPC enhanced reactive oxygen species production via photoexcited electron/hole-driven activation of O₃/H₂O₂ on oxygen-vacancy-rich CeO₂, whereas coupled Ce³⁺/Ce⁴⁺–Fe²⁺/Fe³⁺–Mn³⁺/Mn⁴⁺ interfacial redox cycling sustains active sites despite metal deposition. Photoelectrochemical measurements indicate PPC-induced bandgap narrowing and improved charge transfer after cycling. In continuous-flow treatment, 400 nm irradiation suppresses long-term re-elution observed at 365 nm, enabling stable compliance with guideline limitations. PPC demonstrates superior energy efficiency for Mn(II) removal (2663 mg kWh⁻¹), outperforming sole plasma (660 mg kWh⁻¹) and ozonation (527 mg kWh⁻¹). This study establishes a scalable granular PPC strategy for the simultaneous removal of Fe/Mn in groundwater and acid mine drainage, providing an energy-efficient pathway that couples plasma oxidant generation with vacancy-engineered photocatalytic activation.