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1. Indian Journal of Air Pollution Control Vol. V No. II September 2005 pp 6-10
Photo-Catalytic Disinfection Of Bio-pollutants-A Review
P.M. Nithia Priya and A.Navaneetha Gopalakrishnan
Centre For Environmental Studies, Anna University, Chennai-600025 (T.N.), India.
(e-mail: nithiling@yahoomail.com)
Abstract
Heterogeneous Photo-catalysis has attracted great attention as alternative treatment method for air and water
purification process. It has been found that among a large variety of semi-conductors, Titanium Dioxide (TiO2) under
UV irradiation with light wavelengths lower than 385 nm, is the most efficient photocatalyst for environmental cleaning
and decontamination. This article reviews the work that has been published on disinfections and the killing of bio-
aerosols by photo-catalysis using titanium dioxide (TiO2).
Key words: Photo-catalyst, Semi-conductor, Titanium Dioxide, Bio-pollutants, Disinfection
Introduction
Research work on TiO2 photo-catalytic killing of cells has been intensively conducted on a wide spectrum of
organisms including viruses, bacterial, fungi, algae and cancer cells and proves to be the most promising
after the discovery of Fujishima et.al (1972). The first order reaction kinetics and several modified forms
have been proposed for the kinetics of bactericidal reaction of TiO2 photo-catalyst (Bekbolet et al 1996,
Stevenson et al 1997). Among the different improvements of TiO2 activity, apart from its large increase in
the external surface area, coupling of semi-conductors seems to be a promising way (Di Paola 2000, Kakuta
et al 1985).
The necessary condition of a semi-conductor to become photoactive is that the energy of light
radiation should exceed the energy of the band gap of the semi-conductor. Under that condition, holes (h+
)
and electrons (e-
) are created in the valence and conduction bands respectively of the semi-conductors. The
holes can react with surface bound hydroxyl groups (OH-
) to generate hydroxyl radicals and electrons (e-
)
react with oxygen to generate super-oxide ion radicals. The hydroxyl radicals are powerful oxidants and have
more oxidation potential than chlorine, hydrogen peroxide and ozone by 2.05,1.58 and 1.35 times
respectively (Zhu et al 1995). Size of the catalyst, however, plays a very important role in disinfections. As
the size of the catalyst decreases, the efficiency of disinfection increases. If titanium dioxide particles are of
small size, they may penetrate into the cell and these processes can occur in the interior. It has been found
that photo-catalytic sterilization by illumination of UV light with a wavelength of 254nm provides high rate
of sterilization at room temperature. The possibility for enhanced or unique photochemistry resulting from
the irradiation of the microbe while it is adsorbed on an oxide surface, has also been observed for various
other molecules (Kamat, 1993). Relative sizes of TiO2 particles and target organisms are given in Table 1
(Daniel et al 1999).
Table 1: Relative sizes of TiO2 particles and target organisms
Species Size (microns)
Benzene molecule 0.00043
TiO2 cystalite (Degussa P25) 0.03
TiO2 Agglomerate (In water) 1-3
Virus 0.01-0.3
E.coli (rod shape) 1x3
Yeast cell 1-5 x 5-30
Protozoa 1-2000
Atmospheric Dust 0.001-13
Tobacco smoke 0.01-1

2. Applications of photo-catalysis using TiO2
Areas of activity in titanium dioxide photo-catalysis include fog proof, self cleaning glass, anti – bacterial,
anti – viral, fungicidal, anti soiling, self cleaning, deodorizing, air purification, water treatment, and
decomposition of organic compounds. Photo-catalytic oxidation of a very wide range of organic compounds
has been observed. Therefore it, is not surprising that cellular molecules, such as carbohydrates, lipids,
proteins and nucleic acids can be damaged leading subsequently to cell death. TiO2 has shown a pronounced
activity in the adsorption of basic L-amino acids such as L-lysine and L-arginine in aqueous solutions
(Okazaki et al 1981).
Application for water treatment and as disinfectant
Adam et al (1993) had conducted studies on Photo assisted oxidation of oil and organic spills on water and
found that n–TiO2 photo assisted oxidation eliminated crude oil aromatic components including the
polycyclic aromatics. Photo-catalytic inactivation of Clostridium perfringens and coliphages in water was
studied by Guimaraes et al (2003). TiO2 (titanium dioxide) coated self – cleaning roof tiles for homes and
buildings activated by the UV light of the sun, are being widely employed in Japan and many other European
countries today. Since bacteria are destroyed by TiO2 coated ceramic tiles (Niwa et al, 2000), making such
tiles has become especially attractive for hospital operating rooms.
Malato et al (1998) developed particulate suspensions of TiO2 irradiated with natural solar light in a
large experimental plant to catalyze the oxidation of a typical organic contaminant: Pentachlorophenol
(PCP). TiO2 degrades PCP in the presence of solar energy. Fernandez et al (2004) developed the technology
of discoloration and mineralization of the azo dye orange II in a photo reactor using TiO2 – coated glass rings
in the form of immobilized photo-catalyst. Immobilized TiO2 solves the problem of separation, recovery and
repeated usage of suspension mode TiO2. Biljana et al (2004) investigated the kinetics of photo-catalytic
degradation of 3-amino-2-chloropyridine, as a model compound for pyridine containing pesticides, in UV
illuminated aqueous suspensions of TiO2 and found that mineralization to CO2, water, chlorine, ammonia and
nitrate take place during the process.
Photo-catalytic disinfection of Bio-pollutants in Air
Matsunaga et al (1985) and his coworkers were the first to work on these photo-catalytic dis-infections and
reported that Lactobacillus, acidophilus, Saccharomyces cerevisiae and E.coli were completely sterilized
when incubated with platinum loaded TiO2 particles under metal halide lamp irradiation for 60-120 minutes.
Most work was done in aqueous phase. An extensive study in this respect, was conducted by Wei and Gall
(1994) on solar assisted water disinfection system using TiO2 photo-catalyst. It corroborated the findings of
Matsunaga et.al (1985) and Ireland et al (1998) on the bactericidal activity, and established that irradiation of
suspensions of E.coli and TiO2 with UV-visible light of wavelength longer than 380 nm resulted in the
complete killing of the bacteria within 3 minutes. Similar studies were reported by Goswami et al (1997b)
and Trivedi (1994) also.
The usual procedure is to grow the organisms, separate them from the culture medium, wash them, and
re-suspend them to a known concentration in buffer saline solution or in de-ionized water. Destruction of
microbial population in air also follows the same method. In air phase known quantity of bacterial load is
made as suspension in saline water and convert it to aerosol by use of an atomizer or nebulizer (Goswami
1997). Work on TiO2 has been extended to disinfect moulds, yeasts, viruses, tumour cells etc. E.coli has
been the most studied organism. Organisms tested for photo-catalytic destruction using Titanium dioxide
under light conditions are given in Table 2 (Daniel 1999).
Valerie et al (2000) carried out innovative work on bactericidal indoor air decontamination on TiO2
based photo-catalysts. The photo-catalytic reactor was a vigreux-like pyrese tubular reactor (length 300 mm,
diameter 5 cm), allowing a better contact between the catalysts and the flowing bacteria to be obtained. The
photo-catalyst is coated on the internal wall of the reactor and illuminated with four external commercial 8W
UV-light tubes. A possible mechanism of bactericidal effect through cell attack and destruction was
obtained.

3. Table 2: Organisms Treated by Photo-catalytic disinfections using Titanium Dioxide
Organism Gram +/- Shape/Size (m)
Bacteria
Escherichia coli - rod/1x3
Pseudomonas stutzeri - rod/0.5-1 x 1.5-4
Seratia marcescens - rod/0.5-0.8 x 0.9 – 2.0
Staphyloccus aureus + spherical/0.5-2.0
Clostridium perfringens spores Oval, subterminal
Salmonella typhimurium - Rod/0.7-1.5 x 2.0 2.0 – 5.0
Streptococcus mutans + Spherical/0.5-2.0
Lactobacillus acidophilus + Rod/0.6-0.9 x 1.5-6.0
Streptococcus cricetus + Spherical/<2
Streptococcus rattus + Spherical/<2
Bascillus pumilus + Rod/0.6 x 2-3
Streptococcus sobrinus + Spherical /<2
Bacillus subtilis + Rod/0.7-0.8 x 2-3
Yeast, Fungi
Saccharomyces cerevisiae Oval/3.5-7.0 x 3.5-9.0
Candida albicans Oval/~7
Hyphomonas polymorpha Multiple shapes
Cancer Cells
Hela 14-16
T24 30 30
U937 14-20
Mouse lymphomaL5178Y 11-12
Viruses
Phage Q 0.028
Phage MS-2 0.024
Poliovirus 1 0.028
Lactobacillus phage PL-1 0.05
Other
Human skin fibroblasts 16-18
Alveolar macrophage 20-30
Chinese hamster CHL/IU cells 14-16
Chinese hamster CHL/IU cells 14-16
Indoor environmental quality on bio-aerosols especially the prevalence of Aspergillus versicolor in
residential and commercial buildings with history of water intrusion was investigated by Samimi (1994).
A.versicolor is known as a toxigenic fungus able to survive and grow in relatively low moisture conditions.
Air samples were collected, using an Andersen N-6 sampler and malt dextract culture plates, prior to and
after cutting the affected walls for inspection. A.versicolor fungus was present in 87% of air samples
collected prior to disturbances of indoor surfaces. Characterization of fungal bio-aerosols during sick
building investigations, was carried out by Hipakka et al (1994). Fungal air samples were collected with an
Anderson N-6 air sampler. Airborne fungal concentrations ranged from 85 to 6175 CFU's per cubic meter of
sampled air. These results demonstrated the critical role of an ongoing HVAC maintenance program for
prejudicing reservoirs of fungal organisms in indoor work environments. Various pathogenic
microorganisms such as fungi, bacteria, viruses, protozoa can be found in the indoor environment, causing
contagious microorganism related diseases like tuberculosis and influenza as reported by Kirk et al(1999).

4. Greist et al (2002) worked on photo-catalytic disinfection by using scanning electron microscopy to
visualize photo-catalytic mineralization of airborne microorganisms. Serratia marcescens showed the most
significant changes in cell morphology when exposed for 11.75 hours and more than 99% destruction when
exposed for 36 hours. Purified Bacillus subtilis and Aspergillus niger spores began showing changes when
exposed for 11.75 hours and significant reduction in spores when exposed for 36 hours.
A number of studies on sick building syndrome have concluded that microorganisms are responsible
for allergies and sickness of people in the buildings. Application of photo-catalytic technology for
disinfection of indoor air provides one of the most viable solutions to this problem (Goswami 1997). Initial
scientific investigations of using this technology were performed by Goswami (1995). In order to study the
effectiveness of photo-catalytic technology, experiments in a re-circulating duct using bacteria, Serratia
marcescens in air were conducted. These experiments showed 100% destruction in re-circulating loop in
600 minutes. Blake (1995), Hoffmann (1995), Kamat (1993), Legrini (1993), Linsebigler (1995) described
detailed engineering aspects of the photocatalytic process. Goswami et al (1997a) established Universal Air
Technology (UAT) using photo-catalytic system to destroy bio-aerosols in Air. This Photo-catalytic
technology could be successfully employed for the 100% destruction of bio-aerosol in indoor air. The time
for 100% destruction was later found to reduce to less than 3 minutes (Goswami and Xu 1999)
Clean - up of contaminated indoor air using photo-catalytic technology was an innovative work done
by Sanjeev et.al (2000). A number of sick building syndrome studies have concluded that microorganisms
are responsible for allergies and respiratory problems. A mass balance between the VOC destruction and
CO2 production has shown that photo-catalytic technology is completely effective. Another study has shown
that the technology is very effective in destroying bio-aerosols, dust mites and allergen contaminants in
indoor air (DeBlay 1998, Goswami and Hingorani 1999).
Conclusion
The work described in this article reviews the potential of photo-catalytic disinfection using TiO2 as a means
of killing bacteria and other bio-pollutants in air. It has been seen that treatment of indoor air by photo-
catalytic method has a lot of commercial interest. Development of photo-catalytic indoor air cleaner as the
most effective air-cleaning device is the most desirable commercial product. The cost of the technology,
however, needs to be reduced by reduction in the catalyst cost, commercial development of engineering scale
systems and performance studies. Control of bio-aerosols by a photo-catalytic process is only a beginning. It
remains to be seen if the efficiency and selectivity required by potential applications can be technically
achieved.
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