Photocatalytic Water Disinfection Research Articles

Photocatalytic water disinfection on oxide semiconductors: Part 1 – basic concepts of TiO2 photocatalysis

Water disinfection (removal of microbial agents) using sunlight is an emerging technology, which has the capacity to address the global shortage of drinking water. Therefore, intensive investigations in many laboratories aim to develop photocatalyst for water disinfection. The research is focused on titanium dioxide (TiO2), which is the most promising candidate for high performance photocatalyst able to address the commercial requirements. The present work (Part 1) considers the effect of defect disorder on semiconducting and photocatalytic properties of TiO2 (rutile) in water disinfection using solar energy. It is shown that photocatalytic properties of TiO2 in water are closely related to the light induced reactivity of TiO2 with water leading to the formation of active species, such as OH*, H2O2 and , which have the capacity to oxidise microorganisms. It is also shown that the ability of TiO2 to form the active radicals is closely associated with the presence of point defects in the TiO2 lattice and the related semiconducting properties. Therefore, photocatalytic properties of TiO2 may be modified in a controlled manner by changes in its defect disorder. Consequently, defect chemistry may be used as the framework in the development of TiO2 with controlled properties that are desired for solar water disinfection. The following work (Part 2) considers the structure of bacteria and their reactivity/photoreactivity with TiO2 in aqueous environments. Both Part 1 and 2 bring together the concepts of TiO2 photocatalysis and the concepts of microbiology in order to derive the theoretical models that are needed for the development of high performance photocatalysts for solar water disinfection.

Photocatalytic water disinfection on oxide semiconductors: Part 2 – structure, functional properties and reactivity of microbial agents

Worldwide, waterborne diseases resulting from ingestion of contaminated drinking water carrying infectious microbial agents kill some 2 million people annually. Disinfection of drinking water aimed at the removal and inactivation of microbiological agents typically requires the consumption of energy, which to date is mainly generated from fossil fuels. The titanium dioxide (TiO2) based photo-induced oxidising system (POS) is one of the most promising candidates for the disinfection of water using solar energy. Progress in photocatalytic water disinfection requires understanding both concepts of TiO2 semiconductors as well the biochemistry of microorganisms. The aim of the present study is to introduce workers in the solid state sciences to relevant basic concepts and terminology of microbial biochemistry. We review the structure of the bacterial cell envelope, which is the major target of the TiO2 based POS. We outline how attack by the POS leads to the production of additional reactive species commonly referred to in the biological literature as reactive oxygen species both at the surface and the inside of the cell and develop a unifying model of the molecular mechanisms underlying the biocidal activity of the TiO2 POS.

Performance of Ag-TiO2Photocatalysts towards the Photocatalytic Disinfection of Water under Interior-Lighting and Solar-Simulated Light Irradiations

This paper reports the characterization and photoactivity of Ag-TiO2materials using different amounts of silver during the hydrothermal synthesis. Photocatalysts were characterized by means of TEM, XPS, XRD, DRS, and N2sorption isotherms to determine the textural properties. The photocatalyst's configuration was observed to be as anatase-brookite mixed phase particles with Ag partially oxidized aggregates on the TiO2surface, which increased visible light absorption of the material. Moreover, photoproduction of singlet oxygen was followed by EPR analysis under visible light irradiations following the formation of TEMPOL. Such photoproduction was totally decreased by using the singlet oxygen scavenger DABCO. Photocatalysts were tested towards the photocatalytic disinfection of water suing a solar light simulator and an interior-light irradiation setup. Results evidenced an increase in the photooxidative effect of TiO2, while dark processes evidenced that part of the inactivation process is due to the Ag-TiO2surface bactericidal effect and possible lixiviated Ag+.

Photocatalysis: Light energy conversion for the oxidation, disinfection, and decomposition of water

The results of many-year studies of the relationship between the physical properties and photocatalytic activity of TiO2 and Pt/TiO2 in photocatalytic purification and disinfection of air and water and water photodecomposition with oxygen evolution are presented. Recommendations are given as to finding the optimal method for platinum supporting on TiO2 to achieve the highest possible catalytic activity. Multisite kinetic models of the gas-phase oxidation of simple organic substances are considered. Methods for regenerating the photocatalyst after its deactivation in the oxidation of sulfur-containing organic substances are suggested. New data are discussed on the acceleration of air purification by the combination of photocatalytic oxidation with atmospheric electric discharges, the addition of gaseous hydrogen peroxide, and oxidation on photocatalysts existing in the aerosol state. As compared to pure TiO2, platinated titanium dioxide has a higher capability for disinfection and complete mineralization of microorganisms. Two promising methods for production of hydrogen from water using solar light are presented.

Fe-modified TiO 2 photocatalysts for the oxidative degradation of recalcitrant water contaminants

Fe–TiO 2 with enhanced photocatalytic oxidation activity was synthesized by hydrolysis of titanium butoxide. Sol–gel process leads to less active Fe–TiO 2 particles than bare TiO 2; meanwhile, the hydrothermal synthesis of Fe–TiO 2 seems to promote the stabilization of Fe 3+ in the TiO 2 structure promoting the photo-oxidation activity of TiO 2 towards the degradation of azo-dyes and the inactivation of Escherichia coli ( E. coli) in water. Photocatalysts were characterized by X-ray diffraction (XRD), diffusive reflectance spectrophotometry (DRS), and N 2 adsorption–desorption. The TiO 2 synthesized by the hydrothermal synthesis reaches the TiO 2 Degussa P-25 photo-oxidation activity towards the degradation of orange II. Furthermore, Fe loading of TiO 2 in hydrothermal synthesis improves remarkably the photoactivity of HT. The comparative performance of the photocatalysts towards the direct photo-oxidation of iodide, the photocatalytic degradation of orange II and the photocatalytic disinfection of water infected with E. coli suggested different mechanisms of oxidation: a surface-hole-oxidation mode for hydrothermally synthesized TiO 2 particles and a homogeneous phase oxidation mode promoted by OH radicals produced in the TiO 2 P25 surface.

Inorganic materials for photocatalytic water disinfection

Traditional water disinfection methods such as chlorination and ozonation inevitably form harmful disinfection by-products (DBPs). UV irradiation is a safe alternative but it is very energy intensive. It makes perfect sense to enhance the utilization of photons by integrating engineered photocatalytic nanostructures in the treatment system. UV-light active photocatalytic inorganic nanomaterials, such as titania, are capable of inactivating various bacteria and viruses. They work by generating powerful but short-lived oxygen-based radicals upon irradiation. Unlike conventional chemical disinfectants, the antimicrobial nanomaterials are not consumed in the process. Photocatalytic water disinfection is a green technology because the same materials can be used over and over again. This paper reviews the recent progress in the fabrication of inorganic nanomaterials for photocatalytic water disinfection. A variety of UV and even visible-light driven water disinfection systems are introduced. Their advantages and limitations as well as the antimicrobial mechanisms are discussed.

Photocatalytic disinfection of natural well water contaminated by Fusarium solani using TiO 2 slurry in solar CPC photo-reactors

Phytopathogenic infection of food crops with fungi such as Fusarium spp. is a major problem faced by the farming industry and it has a major economic impact through the reduction in crop yields and through the need to use chemical pesticides to control the problem. As fungi are resistant to many disinfecting agents, which may also be phytotoxic or create environmental problems, more and more effort is being invested in the development of new control techniques for phytopathogenic fungi. It has recently been demonstrated on a small scale that the Fusarium, and within it, Fusarium solani, a plant and human pathogen, is susceptible to solar photocatalytic disinfection with TiO 2 in distilled water.We report on disinfection of water contaminated by F. solani spores using a solar bottle reactor and a 14-L CPC solar photoreactor with distilled and also with natural well water. It was possible to reach high disinfection yields with different TiO 2 concentrations (from 10 to 500 mg L −1) and state flow conditions (20 L/min), at these volumes, not only in distilled, but also in natural well water. We also give the inactivation rate constant for the first stage of photocatalysis under the various experimental conditions, for selection of the best parameters for disinfecting water with phytopathogenic fungi in a future pilot treatment plant.

Solar photocatalytic disinfection of water in the presence of kaolinite catalyst

In this study a kaolinite‐Ag catalyst and a kaolinite‐Zn catalyst are prepared for use in photocatalysis which uses irradiation by sodium light to damage the cell structures of bacteria in water for disinfection. The experimental results indicated that kaolinite‐Ag catalyst and kaolinite‐Zn catalyst have the best disinfection efficiency when 0.06 wt% are added and reached 90% and 85% disinfection efficiency when they were exposed to sodium light for 200 minutes. Among the individual catalysts, kaolinite‐Zn catalyst releases more intermediates at the latter stage of photocatalysis (proved by the concentration level of total organic carbon, TOC); it is easy for the intermediates to compete with the catalyst. The activation rate of target objects on the surface of the Zn catalyst, thus, slows down and results in slower spread of disinfection in the latter part of the reaction curve.

Photocatalytic disinfection of water with Ag–TiO2 nanocrystalline composite

A method of solution impregnation and calcination has been demonstrated for synthesizing nanoparticles of Ag–TiO2 composite photocatalysts for use in the disinfection of water. Only a small proportion of the TiO2 surface is covered by nano-islands of Ag corresponding to a loading of 4 wt.% of Ag; thus, most of the TiO2 surface is available for photocatalytic function. Although the primary particles of both Ag and TiO2 are in the 10- to 20-nm range, microscopic studies indicate that the primary particles of Ag are deposited on nano-agglomerates of 30- to 70-nm-sized TiO2. It is seen that the relatively small loading of Ag has not caused any UV–vis spectral shift but has enhanced the rate of photocatalytic antibacterial action of TiO2, presumably by electron trapping.

Effect of UV solar intensity and dose on the photocatalytic disinfection of bacteria and fungi

The purpose of this work was to study the dependence of solar photocatalytic and solar water disinfection on solar irradiation conditions under natural sunlight. This dependency was evaluated for solar photocatalysis with TiO 2 and solar-only disinfection of three microorganisms, a pure E. coli K-12 culture and two wild strains of the Fusarium genus, F. solani and F. anthophilum. Photocatalytic disinfection experiments were carried out with TiO 2 supported on a paper matrix around concentric tubes, in compound parabolic collectors (CPCs) or with TiO 2 as slurry in bottle reactors, under natural solar irradiation at the Plataforma Solar of Almería (Spain). The experiments were performed with different illuminated reactor surfaces, in different seasons of the year, and under changing weather conditions (i.e., cloudy and sunny days). All results show that once the minimum solar dose has been received, the photocatalytic disinfection efficacy is not particularly enhanced by any further increase. The solar-only disinfection turned out to be more susceptible to changes in solar irradiation, and therefore, only took place at higher irradiation intensities.

Effects of experimental conditions on E. coli survival during solar photocatalytic water disinfection

Results of photocatalytic disinfection of Escherichia coli K12 in water in a compound parabolic collector (CPC) solar reactor are reported. The aim of the study is to quantify the influence of operating parameters, such as flow rate, water quality and bacterial concentration, on bacterial viability in solar photocatalysis and in the dark. The catalyst used was an industrial titanium-dioxide-coated paper matrix fixed on a tubular support in the focus of the CPC. Addition of TiO 2 notably improved solar-only disinfection up to 6 logs disinfection in 90 min. Between 10 and 2 L/min, photocatalytic disinfection effectiveness tended to increase with decreasing flow rates. In dark experiments, inactivation of 99% of viable E. coli cells in distilled water was detected after 90 min of recirculation at 10 L/min in the CPC reactor. A detailed study of bacterial viability in the solar reactor in the dark was therefore performed, varying flow rates, initial concentrations and osmolarity. It was found that bacterial viability in the reactor strongly depends on all the parameters examined, so that disinfection and dark inactivation overlap when working under low-osmolarity conditions and low bacterial concentrations.

Photocatalytic decontamination and disinfection of water with solar collectors

In recent years, there has been a tremendous amount of research and development in the area of photocatalysis, a process included in a special class of oxidation techniques defined as advanced oxidation processes (AOPs), all characterized by the same chemical feature, production of OH radicals. This paper reviews the use of sunlight to produce the OH radicals. The systems necessary for performing solar photocatalysis are described. The paper also summarizes most of the research related to solar photocatalytic degradation of water contaminants carried out during recent years, and how it could significantly contribute to the treatment of very persistent toxic compounds. Various solar reactors for photocatalytic water treatment based mainly on non-concentrating collectors erected during the last few years are also described in detail in this review, as well as the use of the solar photocatalytic processes to inactivate microorganisms present in water, placing special emphasis on some experimental systems erected to optimize this disinfecting technique.

Solar Photocatalytic Detoxification and Disinfection of Water: Recent Overview

During the last few years, there has been a plethora of research and development in the area of solar photocatalysis (TiO2 and photo-Fenton). This overview, of the most recent papers on the use of sunlight to produce the O∙H, comments on those most relevant to the development of the technology and summarizes most of the recent research related to the degradation of water contaminants, and how solar photocatalysis (coupled with biotreatment) could significantly contribute to the treatment of very persistent toxic compounds. Various solar reactors for photocatalytic water treatment based mainly on nonconcentrating collectors developed during the last few years are also described in detail. This review also reports the use of the photocatalytic processes (TiO2) to inactivate microorganisms present in water, placing special emphasis on those applications that make use of sunlight. Work on water disinfection mechanisms in the last decade is summarized in the last part of this overview, with attention to some experimental systems developed to optimize this disinfection technology.

Solar Photolytic and Photocatalytic Disinfection of Water at Laboratory and Field Scale. Effect of the Chemical Composition of Water and Study of the Postirradiation Events

Background. In recent years, there has been a growing interest in the development of new processes for water disinfection since the traditional processes, such as chlorination, can lead to the production of toxic disinfection by-products. Sunlight has been used as a method of water disinfection and heliophotocatalysis by titanium dioxide (TiO2) has been recently considered as a new approach to improve the conventional solar water disinfection. This paper discusses the effect of the chemical composition of water on the E. coli photo inactivation. Method of Approach. Ten types of water having a different chemical composition were contaminated by E. coli K12 and exposed to a simulated solar irradiation in the absence of TiO2 (photolysis) and in presence of TiO2 (photocatalysis). Bacteria were monitored by plate count. The durability of disinfection was assessed in terms of the effective disinfection time (EDT) in a subsequent dark period of 24h(EDT24). Natural water from the Leman Lake (LLW), milli-Q water (MQW), MQW containing a mixture of NO3−, PO4−3, SO4−2, Cl− and HCO3−, phosphate buffered saline water, water from the outlet of a biological wastewater treatment plant (WW); MQW containing a mixture of KCl-NaCl and commercial bottled drinking water (CBW) where used to suspend E. coli at laboratory scale. Field scale experiments using solar irradiation in a compound parabolic concentrator (CPC) with E. coli suspended in LLW were also carried out. Results. The sensitivity of bacteria to the phototreatment depends on the nature of the water. Moreover, experiments systematically performed under the solar simulator showed that the order of E. coli inactivation rate and the EDT24 are different for each system. In photolytic systems, E. coli solar inactivation rate is accelerated by the presence in water of NO3− and HCO3− when compared to that observed in MQW. EDT24 was reached at 3h of irradiation for only 3 (WLL, WW1, and CBW) of the ten studied waters. In the presence of TiO2, the rate of the solar disinfection generally increased. However, a negative effect of chemical substances present in water on the E. coli photocatalytic inactivation was observed in waters when compared to MQW. This effect was especially important in the presence of phosphate, and carbonate. EDT24 was less than 2h for the majority of the water types. In the presence of TiO2, a “residual disinfection effect” was observed for samples even when bacterial culturability below the detection limit was not reached after photocatalytic treatment. Solar irradiation in a CPC photoreactor indicates that the presence of TiO2 accelerates the detrimental action of sunlight. The EDT24 was reached before 3h, in photocatalytic experiments but not in those in the absence of TiO2. The influence of TiO2 surface characteristics and charge, as well as the postirradiation events observed in heliophototreated water, in an optimal growth medium are also discussed. Conclusions. The presence of NO3−, HCO3−, PO4−3, SO4−2, Cl−, and HCO3− greatly affects the photolytic and photocatalytic disinfection processes. The natural ions and organic matter affect the solar disinfection of water in the presence and absence of TiO2 and influences the post irradiation events after sunlight illumination. Antagonistic effect in several conditions or synergistic effects in others can be expected when inorganic and/or organic substances are present in phototreated water sources. EDT24 is useful tool for standardization of solar water disinfection. EDT24 values depend on parameters such as the chemical composition of water, light intensity, initial bacterial concentration, and TiO2 concentration.

Comparative evaluation of Fe3+ and TiO2 photoassisted processes in solar photocatalytic disinfection of water

A lost of culturability of bacteria Escherichia coli K12 was observed after exposition to a solar simulator (UV–vis) in a laboratory batch photoreactor. The bacterial inactivation reactions have been carried out using titanium dioxide (TiO2) P25 Degussa and FeCl3 as catalysts. At the starting of the treatment, the suspensions were at their “natural” pH. An increase in the efficiency in the water disinfection was obtained when some advanced oxidation processes such as UV–vis/TiO2, UV–vis/TiO2/H2O2, UV–vis/Fe3+/H2O2, UV–vis/H2O2 were applied. The presence of H2O2 accelerates the rate of disinfection via TiO2. The addition of Fe3+ (0.3mg/l) to photocatalytic system decreases the time required for total disinfection (<1CFU/ml), for TiO2 concentrations ranging between 0.05 and 0.5g/l. At TiO2 concentrations higher than 0.5g/l the addition of Fe3+ does not significantly increase the disinfection rate. The systems: Fenton (H2O2/Fe3+/dark), H2O2/dark, H2O2/TiO2/dark showed low disinfection rate. The effective disinfection time (EDT24) was reached after 60 and 30min of illumination for the Fe3+ and TiO2 photoassisted systems, respectively. EDT24 was not reached for the system in the absence of catalyst (UV–vis). The effect on the bacterial inactivation of different mixture of chemical substance added to natural water was studied.

05/02186 Photocatalytic disinfection of water using low cost compound parabolic collectors

05/02186 Photocatalytic disinfection of water using low cost compound parabolic collectors

Use of coaxial photocatalytic reactor (CAPHORE) in the TiO 2 photo-assisted treatment of mixed E. coli and Bacillus sp. and bacterial community present in wastewater

This paper reports the photocatalytic disinfection of water contaminated by a mixture of Escherichia coli and Bacillus sp. as well as that of wastewater containing a larger microbial community. The photocatalytic reactions were carried out in a coaxial photocatalytic reactor called CAPHORE, using TiO 2 P-25 of Degussa. E. coli is more sensitive than Bacillus sp. to photocatalytic treatment. Bacterial inactivation was dependent on organic matter and dissolved oxygen (DO) concentration. Of the bacterial community present in partially treated wastewater, E. coli appears to be more sensitive to the treatment than Enterococcus sp., coliforms (other than E. coli), and Gram-negative (other than coliforms). After photocatalytic treatment, no bacterial recovery of previous groups was observed for 24 h in the dark. However a very low bacterial inactivation rate was observed for the whole bacterial population present in wastewater and detected by non-selective media. The effective disinfection time (EDT), the time necessary for total inactivation of bacteria without re-growth in a subsequent dark period referenced at 24 h (or 48 h), was reached only for Enterococcus sp., and coliform groups. EDT 24 was not reached for the whole population.

Photocatalytic disinfection of water using low cost compound parabolic collectors

The objective of this work was to assess the effectiveness of using near UV light to disinfect water supplies for potential applications in developing countries. A pilot scale photoreactor comprised of non-tracking compound parabolic collectors installed at Plataforma Solar de Almerı́a was examined and a comparison of disinfection efficiency using E. coli K-12 was carried out with a reactor configuration of 3 and 1 m 2 illuminated area. Tests were also carried out using suspensions of titanium dioxide (TiO 2) at concentrations ranging from 0 to 9 mg/l. The removal of E. coli K-12 from initial concentrations of 1 × 10 5 CFU/ml to below the limit of detection was achieved both with and without the use of TiO 2. Levels of inactivation in 35 l of inoculated water of up to 4-log removal were achieved in under 30 min. The addition of TiO 2 created an enhancement to the process only at the 3 mg/l dose. However, the results have also suggested the possibility that another disinfection mechanism, possibly governed by the frequency of intermittent UV light exposure, was regulating the overall inactivation kinetics of the trials.

New and emerging developments in solar energy

Solar energy can potentially play a very important role in providing most of the heating, cooling and electricity needs of the world. With the emergence of solar photocatalytic detoxification technology, solar energy also has the potential to solve our environmental problems. However, we do not see widespread commercial use of solar energy. Some of the emerging developments in solar may change that situation. This paper describes some of the new and emerging developments, with special emphasis on: (1) nanoscale antennas for direct conversion of sunlight to electricity with potential conversion efficiencies approaching 80–90%; (2) new thermodynamic cycles for solar thermal power, that have the potential to reduce capital costs by 50%; and (3) solar photocatalytic oxidation for cleanup of industrial wastewater, drinking water, soil and air. The paper describes the fundamentals of each of these developments, their potential, present status and future opportunities for research. (1) Nanoscale antenna solar energy conversion: The current photovoltaic technologies rely on the quantum nature of light and semiconductors which are fundamentally limited by the band-gap energies. A revolutionary new approach suggested by Professor Robert Bailey in 1972 revolves around the wave nature of light. Professor Bailey suggested that broadband rectifying antennas could be used for solar to d.c. conversion. These rectennas would not have the fundamental limitation of semiconductor band-gap limiting their conversion efficiencies. Rectennas for solar conversion would have dimensions of the order of the wavelengths of solar radiation which falls mostly in the sub-micron range. The challenges in actually achieving the objectives are many. This paper describes the challenges and approaches to their solution. (2) New thermodynamic cycles for solar thermal power: It is recognized that the capital costs of solar thermal power will have to be reduced by about 50% in the near future in order to make it competitive with fossil fuels (especially natural gas) based power systems. Potential exists for meeting this goal by reducing the costs and improving the thermodynamic performance of power cycles by hybridization and combined cycle approaches and by employing new and innovative ideas in thermal power cycles. This paper describes the new thermodynamic approaches with an emphasis on an innovative new thermodynamic cycle using ammonia and water mixtures as the working fluids. (3) Solar photocatalytic detoxification and disinfection of water and air: Although the potential of solar radiation for disinfection and environmental mitigation has been known for years, only recently has this technology been scientifically recognized and researched. Solar photocatalytic oxidation has been demonstrated to effectively treat groundwater, drinking water, and industrial wastewater. In some applications such as decoloration and reduction of COD it may be the only effective method of treatment. Treatment of indoor air by the photocatalytic method has been demonstrated as the most effective technology for that application. This paper describes the recent developments and identify challenges and future research opportunities.

Evaluation of photocatalytic disinfection of crude water for drinking-water production

TiO 2-photocatalysis disinfection experiments were carried out with both E. coli in distilled water and natural water samples from the Cauca River (Cali, Colombia). The experiments performed with E. coli in distilled water showed no increase in cell concentration after the treated solution was left in the dark for 24 h. However, the experiments carried out with natural water samples showed drastic culturable cell concentration increase 24 h after stopping the irradiation. This shows the lack of residual effect of TiO 2-photocatalysis.