Visible light plays a significant role during bacterial inactivation by the photo-fenton process, even at sub-critical light intensities

The aim of this research is to clarify the contribution of sunlight wavelengths, irradiance and Fe2+/H2O2 during bacterial disinfection by the photo-Fenton process in clear surface waters. We considered different solar spectrum distributions (visible, UVA-Visible), sub-critical irradiances (0–400 W/m2), focusing on the action modes of E. coli inactivation by the constituents involved in the composite process, at low μM reactants concentration (Fe2+/H2O2) in in ultrapure (MQ) water. We report that solar disinfection improved with Fenton reagents (photo-Fenton process) is a reality from very low light irradiance values (200 W/m2), and made possible even without the presence of UVA radiation, even when using low quantities of the Fenton reagents (0.5 mg/L Fe2+, 5 mg/L H2O2). Under light exposure, H2O2 was found to augment the intracellular Fenton process and Fe2+ to initiate further, distinct oxidative actions. Finally, validation was performed in Lake Geneva water over a wider irradiance range, where the photo-Fenton process was found to be reagent-dependent in low irradiance, shifting to light-driven in the higher values. [Display omitted] •The effects of light distribution in differentiating bacterial disinfection pathways was assessed.•Vis-only and UVA-Vis light-mediated inactivation of E. coli was considered, with/without Fe/H2O2.•The addition of H2O2 or Fe initiates different pathways under Vis or UVA-Vis illumination.•The photo-Fenton process manifests above a defined threshold of irradiance and reagents concentration.•Photo-Fenton is reagent-driven in low light and shifts to light-driven in high irradiance values.