The effects of wavelength, metals, and reactive oxygen species on the sunlight inactivation of microorganisms: observations and applications to the solar disinfection of drinking water

Author(s): Fisher, Michael Benjamin | Advisor(s): Nelson, Kara L | Abstract: Sunlight has long been known to inactivate microorganisms in natural waters and engineered systems. However, the mechanisms of inactivation are not yet fully understood. Solar disinfection (SODIS) is a treatment technology that relies on the germicidal effects of sunlight to inactivate pathogens in drinking water at the point of use. The objective of this work was to explore the roles of wavelengths, transition metals, and reactive oxygen species in the inactivation of indicator microorganisms in water, and discuss the implications of these findings for solar water disinfection. Alternative container materials and hydrogen-peroxide-producing additives were found to accelerate the sunlight inactivation of MS2 bacteriophage as well as E. coli and Enterococcus bacteria during field trials. Furthermore, it was observed that the inactivation of E. coli and Enterococcus derived from local wastewater was significantly slower than the inactivation of laboratory-cultures of the same organisms, while the inactivation of MS2 was slowest of all. The inactivation of all organisms appeared to be heavily dependent on the UVB-transparency of the container material used. To investigate these apparent wavelength effects in more depth, sunlight action spectra were measured in clear water for bacteriophage and E. coli. Both UVA (320 - 400 nm) and UVB (280 - 320 nm) light were found to contribute to the inactivation of PRD1 bacteriophage, while only UVB inactivated MS2. The inactivation of three laboratory E. coli strains and three E. coli strains isolated from wastewater was also studied. Both UVB and UVA wavelengths contributed to the inactivation of all strains, which exhibited strong similarities in their inactivation characteristics, while E. coli naturally present in fresh wastewater was found to be less sensitive to UVA than a cultured laboratory strain. A computational model was developed for interpreting the action spectra of these viruses and bacteria with 3-nm resolution. Studies were also conducted to investigate the roles of iron and reactive oxygen species in the photoinactivation of E. coli. Mutants lacking peroxidase and superoxide dismutase enzymes were found to be more sensitive to polychromatic simulated sunlight, while cells grown with low iron concentrations were more resistant to photoinactivation. Furthermore, prior exposure to light sensitized E. coli to subsequent exposure to hydrogen peroxide in the dark, an effect which was diminished for cells grown on low-iron media. Collectively, these results provide further evidence for the involvement of both UVA and UVB wavelengths in driving E. coli photoinactivation through a mechanism that appears to be consistent with intracellular photoFenton chemistry. These findings also reinforce the critical role of UVB wavelengths in the sunlight inactivation of viruses and, to a lesser extent, wastewater-derived bacteria. The approach to measuring photoaction spectra used in this work may be applicable to investigations of a variety of photobiological and photochemical systems. Finally, the fieldwork results suggest that additives and alternative container materials may be able to greatly accelerate the photoinactivation of microorganisms in drinking water.