Titanium Dioxide and UV

National Center For Environmental Research

Final Report: Titanium Dioxide and Ultra-violet Light: A Sustainable and Inexpensive Solution for Addressing Drinking Water Quality Issues in the Developing Countries

EPA Grant Number: SU831833
Title: Titanium Dioxide and Ultra-violet Light: A Sustainable and Inexpensive Solution for Addressing Drinking Water Quality Issues in the Developing Countries
Investigators: Desu, S. , Long, S. , Reckhow, David A.
Institution: University of Massachusetts – Amherst
EPA Project Officer: Nolt-Helms, Cynthia
Project Period: October 1, 2004 through May 30, 2005
Project Amount: $10,000
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2004)
Research Category: P3 Challenge Area – Water , Pollution Prevention/Sustainable Development

Description:

Objective:

The objective of this research is to develop oxide semiconductor based photo-catalysis system with a view to provide a self-sustaining and inexpensive reactor that would detoxify the naturally available water by removing a wile variety of contaminants and pathogens and rendering it suitable for the human consumption. The oxide semiconductors photo-catalysis might prove to be a useful component in removing the disinfecting contaminants from naturally available water supplies and provide inexpensive drinking water solutions for the third world nations where the drinking water problem is acute and present day solutions are expensive. This research is focused on the application of semiconductor oxide; TiO2 based water purification process and aims to design, develop and test contamination-testing methods, implementing TiO2 as a catalyst in combination with UV-A radiation. In recent years, semiconductor photo-catalysis has received an increasing attention for water purification. In this process, low-energy ultraviolet light is used to generate holes and electrons, which oxidizes toxic organic pollutants and/or reduces toxic metal ions. Simultaneous removal of organic compounds and metal ions makes this process unique over other process. Application of semiconducting oxides like ZrO2 or ZnO is also promising in terms of increasing the efficiency of solar disinfection process. These photo catalysts are also capable of generating strong oxidant radicals that can neutralize several aqueous organic pollutants, including textile dyes and phenols, as well as other impurities. Application of semiconductor photo-catalysis based on appropriate technology design of the reactor will be ideal for water purification. It would require no special training methodology, and will be user friendly in any language or skill level to the citizen of the world. Moreover, such water purification technique will be more beneficial to the health of the environment than the traditional process as it will not add any chemical pollutants into the environment or waste energy resources.

Summary/Accomplishments (Outputs/Outcomes):

The Ti02 based purification system reactor was built and tested by various diagnostic techniques for its efficacy in detoxification of water against organic and biological matter. Initial experiments were done with ultraviolet lamp as excitation source for photo-catalysis. Substrate immobilized nano-porous TiO coated over glass and ceramic tiles were developed as catalysts. Various compositions of binders and TiO2 loading were tried and evaluated for optimized performance. This mitigates the problem of removing ultra fine semiconductor particle suspension from the purified water. To develop self purifying storage vessels for water, we developed and tested TiO2 sputter processed coatings, which can be embedded over vessel surface. This approach has potential to realize stand-alone reactors that would require nb electricity or chemical additives and provide clean water on demand at any location. In various tests we have conducted clean water was spiked with varying concentrations of methyl blue dye.
Various TiO2 embedded discs were suspended for varying periods in a reactor filled with spiked water. The TiO2 catalyst was very successful in speeding up the oxidation process of methyl blue and this was verified visually and more quantitatively by using spectrometric determinations. The microbial tests were performed on heterotropic bacteria that are naturally present in stagnant water such as po4ds. Utilizing Ti02 catalyst plates, water samples drawn from the reactor at various times showed remarkable decrease in the concentration of bacterial colonies. In about 4 hours of photo-catalysis, complete eradication of microbial matter from the water was realized.

We have looked into scientific concepts governing the use of oxide semiconductors for toxic water remediation. More important of these is inefficient separation of the charges generated by the UV light irradiation which translates into low quantum efficiency of the process. Therefore mechanistic aspects of photo-catalytic oxidation property of wide band gap semiconductor materials such as TiO2 and ZnO to degrade organic wastes in the water was investigated to find ways to improve the catalyzing efficiency and to develop novel materials for optimum results. The mechanism we visualize is the two-step electron reduction process. The conduction electron first produces 0H radical, which injects hole in the semiconductor valence band enhancing the luminescence. 0H radicals are produced by oxidation of 4iethanol and water. These data are suggestive of alternative charge exchange processes at the semiconductor interface with the toxic substances. More investigations are however needed to establish the processes. Such knowledge is relevant for improving the quantum efficiency of the photo-catalytic process.

Conclusions:

In order to develop inexpensive drinking water systems for the third world nations, titanium dioxide (TiO2) has been found to be a useful component in removing the disinfecting contaminants from naturally available water supplies. TiO2 based water purification process focus of this research project is designed to develop and test contamination testing methods, implementing TiO2 as a catalyst in combination with UV-A radiation. Evaluation methods used in this project show affectivity of immobilized nano-porous TiO2 substrates to perform photo-catalysis and in eliminating water pollutants such as harmful pathogens, organic and inorganic compounds.

The challenge for future research is to discontinue use of electrically operated UV source and utilize available solar radiation on the earth surface. This requires innovative approaches for developing new materials and structures that not only absorb visible solar radiation but also have energy for holes and electrons to dive the catalysis process. Shifting the absorbance of TiO2 by suitable doping is an area of future work. Enhancing the photo-catalytic ability of oxide semiconductors using innovative concepts such as solar cells to drive photoeoelectrochemical process and develc4 multi-layer structures to carryout electron and hole processes in different regions of the semiconductor have considerable technology potential.

Supplemental Keywords:

POLLUTANTS/TOXICS, Water, TREATMENT/CONTROL, Sustainable Industry/Business, Scientific Discipline, RFA, Arsenic, Technology for Sustainable Environment, Sustainable Environment, Drinking Water, Chemical Engineering, Technology, Water Pollutants, Environmental Chemistry, New/Innovative technologies, drinking water system, drinking water contaminants, treatment, environmental sustainability, UV light emitting diodes, clean technologies, arsenic removal, green chemistry, drinking water distribution system, photocatalyst, activated carbons, adsorption, detoxification, other – risk assessment, drinking water treatment, green engineering
Progress and Final Reports:
Original Abstract


Water Purification by a Combination of Sunlight, Titanium Dioxide and Alum

Water purification was attempted using sunlight as the germicidal source, TiO2 as a photocatalyzing agent, aluminum sulphate (alum) as a coagulant and Escherichia coli as the microbiological indicator. Sunlight possesses a potential water disinfection property but still suffers shortcomings, mainly the length of exposure required. The efficiency of sunlight disinfection was significantly improved by the presence of one g.1−1 powdered TiO2 to produce an enhancement ratio of 1.64. This enhancement was attributed to the aggregation of TiO2 particles and bacterial cells, and the generation of hydroxyl free radicals (OH, a highly reactive species) by the photoactivation of TiO2. The presence of powdered TiO2 in water imposed a serious technical problem, as it was very difficult to remove by conventional means, and even after leaving the sample to settle for a reasonably long time. This turbidity problem was overcome by the addition of alum at 80 mg.1−1. The latter resolved the problem by clarifying the water in about 5 hours. However, when alum was present during light exposure, further aggregation of bacterial cells and TiO2 particles was detected despite the fact that light exposure gradually decomposes aggregates. This effect augmented the photocatalytic action of TiO2 to about 1.53 times that seen with TiO2 alone increasing the total enhancement ratio to a value of 2.51 of that without any added agent. Such aggregation possibly increases the local concentration of OH generated thus creating a better chance for the OH radicals to produce more cell damage. It is worthwhile noting, however, that the presence of alum alone did not change the solar disinfection capacity.

http://www.electrochem.org/dl/ma/203/pdfs/2730.pdf

TiO2 photocatalysis for water treatment
A. Fujishima, Y. Ohko, T. Nakashima and Y. Kubota
Department of Applied Chemistry, School of
Engineering, The University of Tokyo, 7-3-1, Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
TiO2 photocatalysis is becoming an increasingly
interesting oxidation process for the air cleaning as well
as water treatment. The advantage of photocatalysis in
water purification is the complete mineralization of
organics caused by the photogeneration of ·OH radicals
originating from water via the OH groups of the TiO2
surface. There are several reports on photocatalytic water
treatment using TiO2 photocatalysts (1,2). However, not
much attention was paid for the photocatalytic treatment
of pollutants referred to as endocrine disruptors (EDs),
which are known to be associated with abnormal sexual
development and abnormal feminizing responses of
animals. We have been interested in photocatalytic
degradation of EDs such as bisphenol A (BPA) and 17b-
estradiol (E2). These EDCs can cause abnormal growth of
wildlife (3,4). According to the Ministry of Construction
of Japan, these EDCs are not removed completely by
conventional treatment and an average of 24% of the E2
originally contained in the raw sewage remains after
treatment and is released to the environment.
Photocatalysis appears to be a promising approach to treat
such chemicals.
We have recently demonstrated the
photocatalytic degradation of EDs under UV irradiated
TiO2 suspensions. For example, an initial BPA
concentration of 175 μM in water was totally degraded to
carbon dioxide by TiO2 photocatalyzed reactions under
UV irradiation of 10 mWcm-2 (5). In case of 17b estradiol
(E2), 99% of it could be degraded after 30 min of UV
irradiation (Fig. 1) (6).
Figure 1. Changes of the mass balance of carbon atom
during photocatalytic degradation of E2 in a stirred TiO2
suspension under UV intensity of 6 mW cm-2.
Although photocatalytic water treatment is
promising, a disadvantage with regard to it is the lower
quantum efficiencies in comparison to gas-phase
photocatalysis. The principle reason for this is the smaller
availability of oxygen in liquid water and lower mass
transport of target chemicals in comparison to air.
We have developed new immobilized TiO2
system to improve the effective surface area of TiO2 as
well as the mass transport. We used PTFE mesh sheets as
supports for immobilizing the TiO2, because the PTFE
has long-term stability against photocatalytic oxidation.
The mesh sheets were attached to a single bar-rotator to
facilitate the rotation in the solution. E2 and BPA in
aqueous solutions were decomposed quickly under
relatively weak UV illumination, while the rate of
decomposition being higher with the rotation of the TiO2-
PTFE sheets. The apparent mass transfer rate of the EDCs
to the TiO2 under rotation was 4.5 times higher than that
under static conditions. Improvement and optimization of
the reactor is now in progress.
References
1. J.-M Hermann, Catal. Today, 53, 115 (1999).
2. K. Tanaka, K. Padermpole, T. Hisanaga, Wat. Res. ,
34, 327 (2000).
3. Y. Fukazawa, T. Iguchi, Zool. Sci., 16, 153 (1999).
4. E. Pfeiffer, B. Rosenberg, S. Deuschel, M. Metzler,
Mutat. Res., 21, 390 (1997).
5. Y. Ohko, I. Ando, C. Niwa, T. Tatsuma, T.
Yamamura, T. Nakashima, Y. Kubota and A.
Fujishima, Environ. Sci. Technol., 35, 2365 (2001).
6. Y. Ohko, K. Iuchi, C. Niwa, T. Tatsuma, T.
Nakashima, T. Iguchi, Y. Kubota and A. Fujishima,
Environ. Sci. Technol., 36, 4175 (2002).
E2 CO2
carbon balance
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 1 2 3 4
Amount of carbon /mmol
UV irradiation time /h

http://www.azonano.com/details.asp?ArticleId=2410

Using Nanotechnology to Improve Photocatalytic Efficiencies for Water Treatment

Dr John Byrne, Nanotechnology & Integrated BioEngineering Centre, University of Ulster at Jordanstown
Corresponding author: j.byrne@ulster.ac.uk

Heterogeneous photocatalysis involves the use of a semi-conducting material which can be excited by the absorption of light. The applications of photocatalysis include water treatment and purification, air treatment and purification, and ‘self-cleaning’ surfaces. Photosynthetic applications are also widely reported including photoelectrolytic water splitting, CO2 reduction and organic synthesis. There are a wide range of materials employed in photocatalytic research and applications. The important properties of these materials include the band gap energy and hence the wavelength of light required for excitation, the chemical and photochemical stability, particle size and surface area.

The use of nano-structured materials may lead to improved photocatalytic efficiencies where the reduction in particle size results in a greater surface area and possibly size quantisation effects. The former provides more active sites for reaction and the latter gives an increase in the absorption coefficient at specific wavelengths. The most commonly employed photocatalyst material for research and industrial applications is titanium dioxide (TiO2). This is because it is photostable, chemically stable, photoactive, relatively inexpensive and non-toxic.

There are various routes to producing nanostructured titania including sol gel, hydrothermal, electrochemical oxidation of titanium, chemical vapour deposition and plasma sputtering deposition. Titania has a band gap energy of around 3.2 eV and therefore is a UV absorber. When excited by UV irradiation the electron-hole pairs in the titania can react with water and dissolved oxygen to form reactive oxygen species which can attack organic (and inorganic) pollutants in the water. Effectively, each excited particle becomes a nano-electrochemical cell driving redox reactions at the interface. Dispersed nano-structured (but most likely aggregated) titania can be utilised for water treatment and purification.

Suspensions have been used in lab based research and even on large scale treatment systems, however, the catalyst must be recovered from the water prior to discharge. Alternatively, the catalyst may be immobilised as thick or thin films on a solid supporting substrate to negate the catalyst recovery stage1.

For water treatment applications, the use of immobilised films presents problems for reactor design due to mass transfer limitations2. If the catalyst is immobilised onto an electrically conducting supporting substrate, one can employ this substrate as a photoanode in a photoelectrochemical cell (PEC), either in photolytic or photogalvanic mode). For example, one application may be the solar driven photocatalytic oxidation of organics and the simultaneous reduction of dissolved metal ions in a two compartment PEC3.

Photocatalysis has been reported to be effective against a wide range of chemical pollutants including persistent organic pollutants (POPs). One interesting application investigated at Ulster was the destruction of the female hormone 17-β-oestradiol and it’s analogues. Hormones and hormone mimics are termed endocrine disrupting chemicals EDCs and pose a significant threat to the environment.

Photocatalysis is a degradative process where attack by reactive oxygen species results in the overall oxidation of an organic pollutant via intermediate products. These intermediates may be just as harmful as the parent compound. In relation to the oestrogen compounds, it is important to determine the destruction of the oestrogenic properties. This was achieved using a yeast screen bioassay which responds to the oestrogenic effect of pollutants. It was shown that photocatalysis was more effective than UVA photolysis in destroying the oestrogenic effect of 17-B-oestradiol, esterone and estriol4,5.

More recent research has demonstrated the photocatalytic destruction of pharmaceuticals in water. Given that photocatalysis generates reactive oxygen species including hydroxyl radical, superoxide radical anion and hydrogen peroxide, it is a logical step to apply the treatment towards the disinfection of water containing pathogenic microogranisms. Indeed, photocatalysis has been reported to be effective against a wide range of microoganisms including bacteria, viruses and protozoa.

E. coli is the name of a type of bacteria that lives in your intestines. Most types of E. coli are harmless. However, some types can make you sick and cause diarrhea. One type causes travelers’ diarrhea. The worst type of E. coli causes bloody diarrhea, and can sometimes cause kidney failure and even death. These problems are most likely to occur in children and in adults with weak immune systems.

Clostridium perfringens is an anaerobic, Gram-positive, sporeforming rod (anaerobic means unable to grow in the presence of free oxygen). It is widely distributed in the environment and frequently occurs in the intestines of humans and many domestic and feral animals.

Cryptosporidium parvum is one of several species that cause cryptosporidiosis, a parasitic disease of the mammalian intestinal tract.

Our work at Ulster has investigated the inactivation of E.coli as a model organism6 using photocatalysis and electrochemically assisted photocatalysis. In the latter, the process is assisted by the application of an external electrical bias. While E.coli is a pathogen in its own right and is used as an indicator for faecal contamination, it is relatively easy to kill. Therefore, it is more interesting to study the inactivation of disinfection resistant species.

We have shown that photocatalysis and electrochemically assisted photocatalysis are effective against the spores of Clostridium perfringens7. Furthermore, we have also demonstrated that photocatalysis is effective against the protozoan oocysts of Cryptosporidium parvum. This organism is a big problem for the water industry because it is resistant to conventional disinfection and causes severe diarrhoea in humans.

Ulster is a partner in the EC FP6 Sodiswater project which aims to investigate the solar disinfection of water for use in developing countries. By simply filling a transparent bottle with water (preferably glass or PET) and placing in direct sunlight, one can inactivate most pathogens in the water, therefore rendering the water safer to drink. Given that around one sixth of the World’s population do not have access to safe water, it makes sense to utilise the power of the sun in such a simple process.

While SODIS (solar disinfection) is used throughout the world by around 2 million people, the uptake for SODIS could be improved. Additionally, there is a need for improvements in SODIS efficiency and quality assurance for the end user.

To this end we have been investigating the use of photocatalysis to enhance the rate of kill of pathogens at pilot scale under real sun conditions at the Plataforma Solar de Almeria in collaboration with Pilar Fernandez, CIEMAT, Spain. Also, we have been developing sensor technologies to provide automated control and quality assurance for the end user. Our collaborators include partners in Kenya, Zimbabwe and South Africa. Nanotechnology could help save lives in the developing world.


Reference

1. Byrne J.A., Eggins B.R., Brown N.M.D., McKinney B., and Rouse M., Immobilisation of TiO2 powder for the treatment of polluted water. Applied Catalysis B: Environmental, 1998, 17, pp 25-36.
2. McMurray, T.A., Byrne, J.A., Dunlop,P.S.M., Winkelman, J.G.M., Eggins, B.R., and McAdams, E.T., “Intrinsic kinetics of photocatalytic oxidation of formic and oxalic acid on immobilised TiO2 films.” Applied Catalysis A: General, 2004, 262, 1, 105-110.
3. Byrne J.A., Eggins B.R., Byers W., and Brown N.M.D., Photoelectrochemical cell for the combined photocatalytic oxidation of organic pollutants and the recovery of metals from waste waters. Applied Catalysis B: Environmental, 1999, 20, L85.
4. Coleman H.M., Eggins B.R., Byrne J.A., Palmer F.L., and King E., Photocatalytic degradation of 17-ß-oestradiol, Appl. Catal. B Environmental, 2000, 24, L1 – L5.
5. Coleman, H.M., Routledge, E.J., Sumpter, J.P., Eggins, B.R., and Byrne, J.A., “Rapid loss of estrogenicity of steroid estrogens by UVA photolysis and photocatalysis over an immobilised titanium dioxide catalyst,” Water Research, 2004, 38, 3233-3240
6. Dunlop, P.S.M., Byrne, J.A., Manga, N., and Eggins, B.R., The photocatalytic removal of bacterial pollutants from drinking water, Journal of Photochemistry and Photobiology A: Chemistry, 2002, 148, pp 355-363.
7. Dunlop P S M, McMurray T A, Hamilton J W J, Byrne J A, “Photocatalytic inactivation of Clostridium perfringens spores on TiO2 electrodes”, Journal of Photochemistry and Photobiology A: Chemistry, 2008, 196, 113-119

Copyright AZoNano.com, Dr. John Byrne (University of Ulster)

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