Visible-Light-Driven Photocatalytic Degradation of Methylene Blue Using Green-Synthesized ZnO Nanoparticles: Kinetics, Reactive Oxygen Species Mechanism, and Reusability

Zinc oxide (ZnO) nanoparticles synthesized via an Eucalyptus leaf extract-mediated green route (crystallite size 18.35 nm; optical band gap 3.25 eV; specific surface area 34.82 m²/g; zeta potential −32.40 mV) were evaluated as visible-light photocatalysts for the degradation of methylene blue (MB), a model cationic dye pollutant. Critically, irradiation was restricted to wavelengths above 420 nm using a UV cut-off filter, ensuring that the observed photocatalytic response arose specifically from visible-light activation rather than residual ultraviolet excitation, a methodological control largely absent from prior reports on plant-mediated ZnO photocatalysis. Under optimized conditions (catalyst dosage 0.75 g/L; solution pH 9.0), the synthesized nanoparticles achieved 94.57% degradation of MB within 120 min, rising to 97.80% at 180 min. The degradation process closely followed pseudo-first-order Langmuir-Hinshelwood kinetics (R² = 0.9942), with an apparent rate constant of 0.0241 min⁻¹ and a half-life of 28.76 min. Radical scavenging experiments established a clear reactive-species hierarchy, with superoxide radical anions (•O₂⁻) identified as the dominant oxidative species (70.24% suppression with benzoquinone), hydroxyl radicals (•OH) contributing as a secondary pathway (40.45% suppression with isopropyl alcohol), and photogenerated holes (h⁺) playing only a minor role (13.17% suppression with EDTA). The photocatalyst further retained 85.32% of its initial activity after five consecutive reuse cycles, with post-cycling XRD, FTIR, and SEM analyses confirming retention of the crystalline wurtzite phase and the protective organic capping layer. These findings establish Eucalyptus-mediated green-synthesized ZnO as a mechanistically well-characterized, reusable, and genuinely visible-light-active photocatalyst for organic pollutant remediation.