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

Application of ultraviolet light-emitting diodes (UV-LEDs) for water disinfection: A review

This paper is a review of the fundamental disinfection mechanisms of photocatalysis and the models used to fit the disinfection process. Photocatalysis is an attractive technology for water treatment largely due to its potential to utilize solar energy directly and achieve both disinfection and chemical detoxification. Many papers have been published on photocatalytic disinfection, but there is still considerable debate on disinfection mechanisms and a general lack of mechanistic models for the process. The fundamental photocatalytic disinfection mechanisms as they relate to the inactivation of bacteria are comprehensively surveyed here. The process of lipid peroxidation of membrane fatty acids, particularly polyunsaturated fatty acids, is gaining momentum in the literature. In recent papers, an increasing number of researchers are paying close attention to the products of lipid peroxidation. The mathematical models, empirical and mechanistic, used to fit the disinfection process have also been thoroughly reviewed. In this regard, most of the proposed models are empirical in nature and rooted in traditional chemical disinfection principles, which are often not representative of the heterogeneous photocatalytic process. The theoretical development of a mechanistic model for photocatalytic disinfection based on lipid peroxidation is explored with due consideration to the interaction between microbes and photocatalyst particles. The extensive literature on autooxidation of lipids in such fields as biology and medicine is informative to the development of the model.

A review of the mechanisms and modeling of photocatalytic disinfection

In view of the situation that environmental issues become more serious day by day, recent studies on practical applications of TiO 2 photocatalysis for environmental purification are reviewed. Although the fundamental aspects and the mechanisms of TiO 2 photocatalysis have recently become quite well understood, effective photocatalytic environmental purifier, especially water purifier, could not be developed to the stage of real industrial technology. The removal rate of gaseous or aqueous contaminants is influenced by numerous parameters; UV light intensity, substrate concentration, O 2 partial pressure, humidity, substrate type, and so on. Moreover, TiO 2 photocatalyst essentially has a difficulty in decomposition of large amount of contaminants or refractory chemicals. As the solutions of these problems, combination with other processes such as advanced oxidation processes and improvement of the design of photocatalytic environmental purification systems are described. During the past several years, the strategies for effective design of the system are well discussed and evaluated. The reactor design for air- or water-purification can be classified into two main strategies: (1) enlargement of reactive surface area and (2) improvement of mass transfer. Based on these insights, very recent achievements for development of photocatalytic environmental purification system with our contribution in each aspect and future research directions are reviewed.

Photoelectrochemical properties of TiO2 photocatalyst and its applications for environmental purification

Photocatalytic water treatment using nanocrystalline titanium dioxide (NTO) is a well-known advanced oxidation process (AOP) for environmental remediation. With the in situ generation of electron-hole pairs upon irradiation with light, NTO can mineralize a wide range of organic compounds into harmless end products such as carbon dioxide, water, and inorganic ions. Photocatalytic degradation kinetics of pollutants by NTO is a topic of debate and the mostly reporting Langmuir-Hinshelwood kinetics must accompanied with proper experimental evidences. Different NTO morphologies or surface treatments on NTO can increase the photocatalytic efficiency in degradation reactions. Wisely designed photocatalytic reactors can decrease energy consumption or can avoid post-separation stages in photocatalytic water treatment processes. Doping NTO with metals or non-metals can reduce the band gap of the doped catalyst, enabling light absorption in the visible region. Coupling NTO photocatalysis with other water-treatment technologies can be more beneficial, especially in large-scale treatments. This review describes recent developments in the field of photocatalytic water treatment using NTO.

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Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates

Photocatalytic inactivation of two different faecal contamination indicator microorganisms, the Gram-negative Escherichia coli and the Gram-positive Enterococcus faecalis , has been studied using TiO 2 in suspension and immobilized onto the reactor wall. The effect of the main variables of the photocatalytic process on the disinfection efficiency has been analyzed using deionized water and a simulated effluent of wastewater treatment plant (WTP). Noticeable differences were observed between both types of bacteria during photolytic experiments without TiO 2 in deionized water, probably due to the higher sensibility of E. coli to the osmotic stress, which leads to a higher cell membrane permeability and consequently a lower amount of hydroxyl radical attacks required to overcome the inactivation threshold. In contrast, despite their structural differences, Gram-positive and Gram-negative bacteria seem to follow the same inactivation mechanism, showing no significant differences in the experiments carried out with TiO 2 in suspension either in deionized water or in WTP simulated effluent, with similar responses to changes in the concentration of catalysts and irradiation power (both variables involved in the hydroxyl radical generation). Similar results are observed using immobilized TiO 2 in deionized water, although disinfection experiments of WTP simulated effluent using immobilized TiO 2 showed much longer initial delays before the beginning of the inactivation for E. faecalis , suggesting a critical effect of the water composition of the bacteria-catalyst interaction. In any case, the irradiation time required to achieved the inactivation below the experimental bacterial detection limit is similar for both microorganisms, and experiments with mixtures of E. faecalis and E. coli in WTP simulated effluent show no significant differences. Therefore, it can be concluded that the results of photocatalytic disinfection experiments using E. coli as model bacteria could be reasonably extrapolated to other types of bacteria or bacterial mixtures.

Comparison between the photocatalytic inactivation of Gram-positive E. faecalis and Gram-negative E. coli faecal contamination indicator microorganisms

Incorporation of silver to titanium dioxide is of great interest for photocatalytic disinfection applications since in addition to the enhancement of the electron–hole separation and interfacial charge transfer and the increase in the visible light response, silver compounds present a strong bactericidal effect. Ag/TiO 2 materials used in suspension and immobilized in two different configurations (catalytic wall and fixed-bed reactors) have been prepared, characterized and tested using Escherichia coli as model microorganism. Although the incorporation of silver to powdered Degussa P25 TiO 2 increases the activity, the thermal treatment required for the stabilization of the supported metal particles reduces the global efficiency. The comparison with experiments of dye photodegradation indicates that the activity of Ag/TiO 2 is mainly due to the bactericidal role of silver and not to the enhancement of the photocatalytic mechanism. The best tested system has been proved to be the Ag/TiO 2 catalytic wall reactor with a 0.6 wt.% of Ag loading, showing a high activity both in relative (per gram of TiO 2 ) and absolute terms, an optimal use of the radiation source, and a good stability of the film with negligible silver lixiviation, allowing the continuous treatment of water.

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Cristina Pablos

Papers

Comparison between the photocatalytic inactivation of Gram-positive E. faecalis and Gram-negative E. coli faecal contamination indicator microorganisms

Photocatalytic inactivation of bacteria in water using suspended and immobilized silver-TiO2

Comparison of the photocatalytic disinfection of E. coli suspensions in slurry, wall and fixed-bed reactors

Mechanism of photocatalytic disinfection using titania-graphenecomposites under UV and visible irradiation

Understanding the effect of morphology on the photocatalytic activity of TiO2 nanotube array electrodes