Addition of Hydrogen Peroxide to Groundwater with Natural Iron Induces Water Disinfection by Photo‐Fenton at Circumneutral pH and other Photochemical Events

Samples of natural groundwater (with low turbidity, neutral pH and 0.3 mg L−1 iron concentration) inoculated with Escherichia coli K‐12 were exposed to simulated solar light both in the presence and in the absence 10 mg L−1 of H2O2. Results demonstrated that the viability of E. coli (by DVC–FISH) was grounded to zero after 360 min of irradiation. This abatement could be caused by the oxidative stress induced by ·OH radicals or another photo‐induced reactive oxygen species. Two 23 factorial experimental designs enabled the evaluation of the effects of chemical factors on the inactivation of E. coli. The first experimental design considered the pH, iron and H2O2, while the second evaluated the ions fluoride, carbonate and phosphate found in groundwater. pH was found to play a key role in the inactivation of E. coli. The best reduction in viability was obtained at the lower pH (6.75), while a nonsignificant effect was observed when iron or H2O2 concentrations were raised. At higher concentrations, anions, such as carbonate and phosphate, negatively affected the E. coli abatement. However, a higher concentration of fluoride accelerated it. In all experiments, the pH was observed to rise to values higher than 8.0 units after 360 min of treatment. It is possible to inactivate E. coli cells in natural groundwater samples that already contain iron by the simple addition of 10 mg L−1 of H2O2 and subsequent irradiation with simulated solar light. Different photochemical processes leading to the production of reactive oxidative species (ROS) could be involved as follows: circumneutral photo‐Fenton by dissolved or colloidal iron, photocatalysis by metallic oxides or iron (hydr)oxides, and photochemical by dissolved organic matter excitation, H2O2 photolysis and nitrate/nitrites light‐absorption. Fluoride used at 1.2 mg L−1 exhibited a positive effect. Furthermore, CO32− could react with ·OH radicals producing CO3−· which could be toxic to microorganisms.