Photoinduced reactivity of titanium dioxide

Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental applications

Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends

The photocatalytic properties of titanium dioxide are well known and have many applications including the removal of organic contaminants and production of self-cleaning glass. There is an increasing interest in the application of the photocatalytic properties of TiO2 for disinfection of surfaces, air and water. Reviews of the applications of photocatalysis in disinfection (Gamage and Zhang 2010; Chong et al., Wat Res 44(10):2997–3027, 2010) and of modelling of TiO2 action have recently been published (Dalrymple et al. , Appl Catal B 98(1–2):27–38, 2010). In this review, we give an overview of the effects of photoactivated TiO2 on microorganisms. The activity has been shown to be capable of killing a wide range of Gram-negative and Gram-positive bacteria, filamentous and unicellular fungi, algae, protozoa, mammalian viruses and bacteriophage. Resting stages, particularly bacterial endospores, fungal spores and protozoan cysts, are generally more resistant than the vegetative forms, possibly due to the increased cell wall thickness. The killing mechanism involves degradation of the cell wall and cytoplasmic membrane due to the production of reactive oxygen species such as hydroxyl radicals and hydrogen peroxide. This initially leads to leakage of cellular contents then cell lysis and may be followed by complete mineralisation of the organism. Killing is most efficient when there is close contact between the organisms and the TiO2 catalyst. The killing activity is enhanced by the presence of other antimicrobial agents such as Cu and Ag.

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Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity

Influence of parameters on the heterogeneous photocatalytic degradation of pesticides and phenolic contaminants in wastewater: A short review

Photocatalytic inactivation of E. coli in surface water using immobilised nanoparticle TiO2 films

Pathogens in drinking water supplies can be removed by sand filtration followed by chlorine or ozone disinfection. These processes reduce the possibility of any pathogens entering the drinking water distribution network. However, there is doubt about the ability of these methods to remove chlorine resistant microorganisms including protozoan oocysts. Concern has also been raised about the production of disinfection by-products following the chlorination process. Titanium dioxide (TiO 2 ) photocatalysis is a possible alternative/complementary drinking water treatment method. TiO 2 electrodes were prepared by the electrophoretic immobilisation of TiO 2 powder (Aldrich and Degussa P25). These electrodes were tested for their photocatalytic bactericidal efficiency. E. coli K12 was used as a model test organism. The rate of disinfection was greater for the P25 electrode compared to the Aldrich electrode under open circuit conditions. The application of an electrical bias to the working electrode increased the rate of disinfection by ∼40% for the P25 electrode and ∼80% for the Aldrich electrode. The effect of applied potential was more pronounced under conditions of high initial bacterial cell loading and high light intensities. Bacterial recovery did not occur up to 48 h after disinfection.

The photocatalytic removal of bacterial pollutants from drinking water

The photocatalytic removal of atrazine from water was investigated using immobilised TiO2 films in a stirred tank reactor designed to maximise mass transfer. The degradation of atrazine was demonstrated with a number of breakdown products identified including the stable end product cyanuric acid. The process was monitored using high performance liquid chromatography (HPLC), total organic carbon analysis (TOC) and liquid chromatography-mass spectrometry (LC-MS). A decrease in the TOC was observed and attributed to the oxidative degradation of atrazine side chains. Intermediates identified included 2-chloro-4-acetamido-6-isopropylamino-1,3,5-tiiazine, 2-chloro-4-ethylamino-6-(1methyl-1-ethanol)amino-1,3,5-triazine, 2-chloro-4-ethylamino-6-(2-propanol)amino-1,3,5-triazine, 2-hydroxyatrazine, desethylatrazine, deisopropylatrazine, 2-hydroxydesethyl atrazine and cyanuric acid. Operational parameters such as catalyst loading, oxygen concentration, initial pollutant concentration and UV source were investigated. Atrazine removal followed first order kinetics and the rate was dependent upon catalyst loading up to an optimum loading (above which a decrease in the degradation rate was observed). No difference in the rate was observed when either air and 02 sparging was used. The rate was directly proportional to initial concentration in the rangestudied. The use of UVB irradiation did not appear to increase the rate of degradation in comparison with UVA irradiation. However, the maximum apparent quantum yield for the photocatalytic degradation was higher under UVB (0.59%) compared to UVA (0.34%).

The photocatalytic degradation of atrazine on nanoparticulate TiO2 films

Titanium dioxide (TiO2) photocatalysis is a possible alternative/complementary technology to conventional water treatment methods. The TiO2 catalyst may be used as slurry or it may be immobilised onto a supporting substrate. With immobilised TiO2 films mass transfer problems occur in most photocatalytic reactors, which results in a reduction of reactor efficiency and in the accuracy of measured catalyst efficiency and kinetics. In order to determine the real intrinsic kinetics of photocatalytic reactions on immobilised TiO2 films a stirred tank reactor (STR) was used. The reactor incorporated a propeller and a baffle, thus providing good mixing and efficient mass transfer to the TiO2 film. Degussa P25 was immobilised onto borosilicate glass by a dip coating method and the kinetics of the photocatalytic degradation of the model pollutants, formic acid and oxalic acid were investigated as a function of catalyst loading, initial pollutant concentration and propeller rotation speed. The rate of degradation. of either acid. was not mass transfer limited at propeller speeds greater than 1000 rpm. The rate of formic acid degradation was dependent upon catalyst loading up to a maximum loading above which a decrease in the degradation rate was observed. The apparent quantum yield for the photocatalytic degradation was 5% for oxalic acid and 10% for formic acid. This compares very well with usual reported apparent quantum efficiencies for photocatalysis which are similar to1%. The photocatalytic oxidation of both acids could be described using a Langmuir-Hinshelwood kinetic model. (C) 2003 Elsevier B.V. All rights reserved.

Intrinsic kinetics of photocatalytic oxidation of formic and oxalic acid on immobilised TiO2 films

Novel disinfection methods are being sought to provide additional means of protection in a number of areas where disease outbreaks could lead to illness or fatalities For example, the risk of contamination arising from contact with surfaces and medical devices has received much attention due to the rise in incidence of healthcare acquired infections It is possible that reducing bio-burden on these sites may supplement the disinfection protocols currently in place and help reduce risk of infection Photocatalytic surfaces offer promise as innovative and cost-effective biocidal engineering solutions which address these specific problems whilst maintaining stringent health and safety controls A method was developed to assess the disinfection efficiency of photocatalytic surfaces allowing (a) determination of pathogen viability as a function of treatment time; (b) assessment of the surface for viable surface bound organisms following disinfection; (c) measurement of the re-growth potential of inactivated organisms This method was used to demonstrate the inactivation of extended-spectrum beta-lactamase Escherichia coli , methicillin resistant Staphylococcus aureus, Pseudomonas aeruginosa and Clostridium difficile spores using immobilised films of commercial titania nanoparticles 999% reduction in viability (a 3-log kill) was observed for all bacterial cells within 80 min photocatalytic treatment Complete surface inactivation was demonstrated and bacterial re-growth following photocatalytic treatment was not observed Greater than 99% inactivation (26-log reduction) was observed when the photocatalytic surfaces were challenged with C difficile spores The efficacy of photocatalytic disinfection to inactivate Staphyloccocus epidermidis cells within a biofilm was also demonstrated, with 3 h treatment rendering 965% ± 6 of the biofilm cells on the TiO 2 coated substrate non-viable Disinfection of cells throughout the 3–4 μm thick biofilm was observed

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P. S. M. Dunlop

Papers

Photocatalytic inactivation of E. coli in surface water using immobilised nanoparticle TiO2 films

The photocatalytic removal of bacterial pollutants from drinking water

The photocatalytic degradation of atrazine on nanoparticulate TiO2 films

Intrinsic kinetics of photocatalytic oxidation of formic and oxalic acid on immobilised TiO2 films

Inactivation of clinically relevant pathogens by photocatalytic coatings