A review of heterogeneous photocatalysis for water and surface disinfection

Abstract

Photo-excitation of certain semiconductors can lead to the production of reactive oxygen species that can inactivate microorganisms. The mechanisms involved are reviewed, along with two important applications. The first is the use of photocatalysis to enhance the solar disinfection of water. It is estimated that 750 million people do not have accessed to an improved source for drinking and many more rely on sources that are not safe. If one can utilize photocatalysis to enhance the solar disinfection of water and provide an inexpensive, simple method of water disinfection, then it could help reduce the risk of waterborne disease. The second application is the use of photocatalytic coatings to combat healthcare associated infections. Two challenges are considered, i.e., the use of photocatalytic coatings to give "self-disinfecting" surfaces to reduce the risk of transmission of infection via environmental surfaces, and the use of photocatalytic coatings for the decontamination and disinfection of medical devices. In the final section, the development of novel photocatalytic materials for use in disinfection applications is reviewed, taking account of materials, developed for other photocatalytic applications, but which may be transferable for disinfection purposes.

Figure 1

Schematic showing the basic mechanism of heterogeneous photocatalysis.

Figure 2

Schematic of photocatalytic mechanism on a titanium dioxide particle leading to the production of reactive oxygen species.

Figure 3

(a–c) Schematic illustration of the process of E. coli inactivation on photo-excited TiO₂. In the lower row, the part of cell envelope is magnified. (Reproduced from Sunada, K.; Watanabe, T.; Hashimoto, K. Studies on photokilling of bacteria on TiO₂ thin film. J. Photoch. Photobio A 2003, 156, 227–233 [27]).

Figure 4

Photographs showing the double tube configuration with internal tube cap and the valve for external tube (a) and the solar photocatalytic reactor with and without CPC during disinfection tests (b). Schematic cross-section representation of the different reactor configurations tested in the solar reactor (c), (1)/(7) uncoated single tube without/with CPC; (2)/(8) coated single tube without/with CPC; (3)/(9) coated double tube without/with CPC; (4)/(10) coated external–uncoated internal without/with CPC; (5)/(11) coated internal–uncoated external without/with CPC; (6)/(12) uncoated double tube without/with CPC (reproduced from Alrousan, D.M.A.; Polo-López, M.I.; Dunlop, P.S.M.; Fernández-Ibáñez, P.; Byrne, J.A. Solar photocatalytic disinfection of water with immobilized titanium dioxide in re-circulating flow CPC reactors. Appl. Catal. B 2012, 128, 126–134 [54]).

Figure 5

(a) TiO₂-GO aggregate before photoreduction; (b) TiO₂-RGO after UV assisted photoreduction and (c) E. coli inactivation at several TiO₂-RGO concentrations. Figure inserts shows efficiency of TiO₂-RGO and TiO₂-P25 on the E. coli inactivation (reproduced from Fernández-Ibáñez, P.; Polo-López, M.I.; Malato, S.; Wadhwa, S.; Hamilton, J.W.J.; Dunlop, P.S.M.; D’Sa, R.; Magee, E.; O’Shea, K.; Dionysiou, D.D.; Byrne, J.A. Solar Photocatalytic Disinfection of Water using Titanium Dioxide Graphene Composites. Chem. Eng. J. 2015, 261, 36–44 [20]).

Figure 6

Solar bag commercially available from Puralytics, which utilizes photocatalysis (from Puralytics website [61]).

Figure 7

(a) Images of front view of the solar 60 L-CPC reactor at PSA facilities (4.5 m² of collector mirrors) with air injection points indicated; (b) Enhanced SODIS batch reactor filled with 100 NTU turbid water; (c) Schematic of the sequential batch system.

Figure 8

Band gap energies and band edge potentials for different photocatalytic materials with respect to the water splitting couples (with permission from Maeda, K.; Domen, K. New Non-Oxide Photocatalysts Designed for Overall Water Splitting under Visible Light. J. Phys. Chem. C 2007, 111, 7851–7861 [113]).