Flower Pollen-Based Photosensitization Process for Enhanced Solar Disinfection of Drinking Water: Reactor Design and Inactivation Mechanisms

Abstract

Solar disinfection (SODIS) is a chemical-free, low-cost, and user-friendly approach to achieving Sustainable Development Goal No. 6 (SDG 6). Herein, a natural flower pollen-based photosensitizer was fabricated in a facile way and used to enhance the conventional SODIS process. Complete inactivation of five biohazards (Escherichia coli K-12, Spingopyxis sp. BM1-1, Bacillus subtilis spores, Enterococcus faecalis, and tetracycline-resistant E. coli) at their culturable or viable but nonculturable states was achieved by both batch reactors and a flow-through reactor with a standing separation–recycling system. Herein, microorganisms adhered to the photosensitizer’s surface due to electrostatic attraction and topological interactions. The photosensitizer induced the localized heating effect under solar irradiation, and a portion of microorganisms was inactivated by the photothermal effect. Meanwhile, abundant reactive species (•OH and 1O2) were generated and contributed to the photocatalytic inactivation of microorganisms. Synergism between the photothermal and photocatalytic processes enhanced the destruction of cell membranes and the penetration of reactive species. A comprehensive inactivation pathway was proposed and verified. The findings suggested that the natural flower pollen-based photosensitization process is promising to enhance the SODIS for effective disinfection, which assists rural and underdeveloped areas in achieving SDG 6.

Reactive Nitrogen Species Mediated Inactivation of Pathogenic Microorganisms during UVA Photolysis of Nitrite at Surface Water Levels

“…The critical role of reactive species on cell membrane destruction and inactivation was further confirmed by the results obtained from a fluorescence microscope (see detailed principles of the method in Text S14) (Figure d–f). It is known that the SYTO 9 dye can diffuse into the cell membrane, combine with the cells’ DNA/RNA, and gives green fluorescence. , In contrast, the PI dye could not freely diffuse into cells unless the cell membrane was broken. , If the PI dye diffuses into cells, it combines with DNA/RNA and emits red fluorescence. , As shown in Figure d–f, the untreated E. coli K-12 or the treated E.…”

“…Conventional disinfection technologies, including chlorination, ozonation, and ultraviolet (UV) irradiation, have a few limitations. For instance, chlorine or ozone disinfection processes are prone to form toxic chlorinated disinfection byproducts (Cl-DBPs) or inorganic carcinogens (e.g., bromate). − A number of pathogens (e.g., adenovirus, bacteriophage MS2, and Bacillus subtilis spores) are resistant to UV disinfection, , and the microorganisms may stay at the viable but noncultural (VBNC) state after UV disinfection. − To overcome the shortcomings of conventional disinfection methods, advanced disinfection processes (ADPs) based on photolytic and photocatalytic processes are increasingly being developed. ,,− The UV-based ADPs are promising due to the easy installation of the UV lamps into the existing water treatment units and the high disinfection efficiency of the reactive species generated from UV photolysis of oxidant precursors (e.g., H 2 O 2 , chlorine, peroxydisulfate, peracetic acids). ,,,, On the other hand, practical concerns associated with the reported UV-ADPs include the use of UVC lamps, which are energy intensive, and the need for additional dosing and quenching of the oxidant precursors. − Researchers and engineers are thus pursuing ways for the in situ utilization of the water matrix components without dosing additional chemicals and the shifting from the UVC lamps to more energy-efficient UVA lamps or solar light for water advanced disinfection.…”

“…It is known that the SYTO 9 dye can diffuse into the cell membrane, combine with the cells' DNA/RNA, and gives green fluorescence. 6,9 In contrast, the PI dye could not freely diffuse into cells unless the cell membrane was broken. 6,9 If the PI dye diffuses into cells, it combines with DNA/RNA and emits red fluorescence.…”

“…6,9 In contrast, the PI dye could not freely diffuse into cells unless the cell membrane was broken. 6,9 If the PI dye diffuses into cells, it combines with DNA/RNA and emits red fluorescence. 6,9 As shown in Figure 4d−f, the untreated E. coli K-12 or the treated E. coli K-12 by direct UV 365 photolysis only gave a strong green fluorescence, suggesting that the cell membranes were not destructed.…”

“…6,9 If the PI dye diffuses into cells, it combines with DNA/RNA and emits red fluorescence. 6,9 As shown in Figure 4d−f, the untreated E. coli K-12 or the treated E. coli K-12 by direct UV 365 photolysis only gave a strong green fluorescence, suggesting that the cell membranes were not destructed. However, strong red fluorescence was observed after the treatment by the coexposure of UV 365 photolysis and NO 2 − .…”