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Enhanced photocatalytic disinfection of indoor air
Amit Vohra *, D.Y. Goswami, D.A. Deshpande, S.S. Block
Solar Energy and Energy Conversion Laboratory, Department of Mechanical and Aerospace Engineering,
University of Florida, Gainesville, FL 32611, USA
Received 18 May 2005; received in revised form 19 September 2005; accepted 24 October 2005
Available online 15 December 2005
Abstract
A silver ion doped TiO2 based photocatalyst, with improved destruction of airborne microbes, has been developed. The performance of the
silver ion doped photocatalyst is demonstrated using a catalyst coated filter in a recirculating air experimental facility. Bacillus cereus,
Staphylococcus aureus, Escherichia coli, Aspergillus niger, and MS2 Bacteriophage have been used as indexes to demonstrate the high disinfection
efficiency of the enhanced photocatalysis process. The microbial destruction performance of the enhanced photocatalyst is found to be an order of
magnitude higher than that of a conventional TiO2 photocatalyst. The process of enhanced photocatalysis can thus be used effectively against high
concentrations of airborne microorganisms, making it an attractive option as a defense against bio-terrorism.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis; Disinfection; Indoor air; Airborne microorganisms; Silver ions
www.elsevier.com/locate/apcatb
Applied Catalysis B: Environmental 65 (2006) 57–65
1. Introduction
Air Filtration and Purification World Markets predicted that
the world market for air filters would rise to US $5 billion in
2005. This rise can be attributed to the security efforts to
counter chemical and biological terrorism, as well as the
increased awareness of the people towards environment and
environmental pollution. In 2000, a document published by the
World Health Organization (WHO) stressed that it is our human
right to breathe healthy indoor air [1]. It further emphasizes that
ensuring acceptable indoor air quality is the responsibility of all
concerned. A study done by the US Environmental Protection
Agency (EPA) in 1987 concluded that indoor air pollution
poses a greater risk than outdoor air pollution [2]. This indoor
air pollution is estimated to be the cause of several health
related issues and reduced work productivity among people.
Allergies and diseases such as asthma and sick building
syndrome (SBS) have increased considerably over the last few
decades. A recently concluded European survey of around
140,000 individuals in 22 countries shows that this increase is
dependent on the environment and the lifestyle of the
individuals. Because most of the Americans spend a substantial
* Corresponding author. Tel.: +1 352 392 2328; fax: +1 352 846 1630.
E-mail address: amitvohr@ufl.edu (A. Vohra).
0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2005.10.025
amount of time indoors, indoor air contamination poses a
serious threat to them. Microbial agents in indoor air are
considered a serious health hazard and therefore microbial
contamination of indoor air has been the major topic of
attention in recent times [3]. One of the biggest disease
outbreaks due to microbial contamination of indoor air was the
Legionnaires’ disease outbreak in Philadelphia in 1976 [4].
Also, the recent bio-terrorism threat due to anthrax has fueled
huge interest in new technologies for indoor air disinfection.
Advanced oxidation technologies, in particular the photo-
catalytic technology offers several environmental and practical
advantages over conventional biological or physical disinfec-
tion processes. The huge interest generated by photocatalysis
has motivated several researchers to look into the basic mode of
action of TiO2. TiO2 is a semiconductor with a band gap close to
3.2 eV. UV light with wavelengths shorter than �380 nm
photoactivates TiO2 by providing the band gap energy needed
by an electron to jump from the valence band to the conduction
band. This implies that when photons of UV light are absorbed
on TiO2, they generate excited pairs of electrons and holes. The
photogenerated holes react with the water to produce hydroxyl
radicals (�OH), while the photogenerated electrons react with
molecular oxygen to give superoxide radical anions (�O2�).
These radicals so produced are highly reactive and they work
together to completely oxidize the organic species. The attack
by the �OH radical, in the presence of oxygen, thus initiates a
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–6558
Nomenclature
cfu colony forming units
CCi averaged initial colony count
CCt averaged colony count after a specific time inter-
val ‘t’
eV electron volt
EPA Environmental Protection Agency
EHS Environmental Health and Safety�OH hydroxyl radical�O2� superoxide radical
t time of exposure
TSA Trypticase Soy agar
TSB Trypticase Soy broth
UV ultraviolet radiation
complex cascade of oxidative reactions. The mechanism of the
photocatalytic process has been extensively studied in the
literature and several complex reaction pathways have been
reported [5–8]. Although the exact mechanism of the process
and the reaction pathways are still not clear, the practical
applications that these processes offer have fueled enormous
commercial interest. Recent review articles provide a
comprehensive coverage of the application of TiO2 photo-
catalysis to disinfection [9–12].
The pioneering work in the field of photocatalytic
disinfection of indoor air was done by Goswami et al. when
they developed a technology to completely destroy biological
contaminants in indoor air. A recirculating duct facility was
used, wherein the bacteria (Serratia marcescens) was shown to
be completely destroyed [13,14]. Subsequent research con-
ducted by Goswami et al., with improved reactor designs,
demonstrated 100% destruction of Serratia marcescens
bacteria in a much reduced time [15]. Their group even
reported inactivation of dust mite antigens by photocatalytic
oxidation. Der p II was selected and its fast destruction
demonstrated the ability of the photocatalytic technology to
control allergies and diseases in the population [16].
The photomineralization of bacteria on a photocatalytic
surface in air was first shown by Jacoby et al. They studied
photooxidation of Escherichia coli and found 54% miner-
alization in 75 h by measuring the carbon dioxide released [17].
Also in their most recent studies, Wolfrum et al. demonstrated
complete mineralization of E. coli, Micrococcus luteus,
Bacillus cereus (bacterial cells and spores), and Aspergillus
niger spores by photocatalytic oxidation. They based their
results on kinetic data and carbon mass balance [18].
In 1999, Masaki and coworkers reported destruction of
bacteria and foul odor in air using stainless steel plates coated
with thin films of TiO2 [19]. Photocatalytic disinfection in gas
phase was further illustrated by Lopez and Jacoby in 2002.
They showed destruction of E. coli in a contaminated air stream
as it was passed though a self-cleaning metal microfibrous mesh
filter coated with TiO2 [20]. Greist et al. demonstrated the
capability of photocatalytic oxidation to destroy B. anthracis
(Anthrax) through the successful destruction of B. subtilis
spores [21]. Following the recent attack of SARS virus,
Howells studied a system, based on photocatalytic disinfection,
to control the spread of infectious microorganisms such as
SARS virus on flights [22]. Most recently, Lin and Li
investigated the disinfection effectiveness of commercial
titanium dioxide coated filters for airborne microbes. They
studied the destruction of nebulized E. coli (gram negative
bacteria), B. subtilis (bacterial spores), Candida famata (yeast),
and Penicillium cetrinum (fungal spores) in a laboratory setup
and concluded that the process was effective against airborne
microorganisms [23].
The intent of this study was to enhance the overall rate of
destruction of the photocatalytic process, and to make it
commercially more attractive as a defense against bio-terrorism
in indoor air environments. An efficient way to improve the
kinetics of photocatalysis is the addition of transition metals to
TiO2 [6]. Introduction of metal ions in the lattice of TiO2 has
shown significant enhancement in the photcatalytic activity of
TiO2 for the degradation of various organics. Iron(III) doped
TiO2 [24,25], platinized TiO2 [26,27], lanthanide metal ion
doped TiO2 [28], chromium doped, manganese doped and
cobalt doped TiO2 [29], and silver doped TiO2 [30–32] have all
been successfully demonstrated as photocatalysts leading to an
increased rate of destruction of organics.
Although there is extensive literature on the use of silver ion
doped TiO2 for photocatalytic degradation of organics, its
application for photocatalytic disinfection in gas phase has not
been studied much. It is widely recognized that Ag+ ions
possess anti-microbial properties and the work done by Sokmen
et al. demonstrated the enhancement in inactivation of E. coli in
liquid phase using Ag–TiO2/UV system [33].
The main aim of this study is to demonstrate the
effectiveness of silver ion doped TiO2 photocatalyst for fast
inactivation of a wide range of airborne microorganisms in a
recirculating air experimental facility with the enhanced
photocatalyst, and compare it with the performanace of
conventional Degussa P25 TiO2 photocatalyst. The Ag+ ions
from the dopant act as an electron trap in the photocatalysis
process. This reduces the recombination between electrons and
holes, and thus results in an increased availability of holes. The
synergistic effect of the doped Ag+ ions, and highly oxidizing
radicals generated by TiO2 photocatalysis process, may lead to
a highly enhanced rate of microbial destruction.
2. Materials and methods
2.1. Experimental facility
The recirculating experimental setup designed for this study
is illustrated in Fig. 1. The apparatus consists of a recirculating
duct, a reactor section and a blower with a belt driven motor to
circulate air through the duct facility. The upper portion of the
recirculating duct is rectangular in cross section while the lower
duct portion is circular. The whole duct is designed to ensure
uniform air flow through the duct with minimum separation,
which would otherwise lead to dead spaces in the duct where
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–65 59
Fig. 1. Recirculating air experimental facility.
microorganisms could accumulate and multiply. The rectan-
gular upper portion of the duct consists of a sample injection
section, a reactor section, and a sample collection section. The
sample injection section is 0.89 m (35 in.) long and has a cross
sectional area of about 0.05 m2 (84 in.2). This section has a port
attached to a Hudson RCI nebulizer for injecting the microbial
culture into the duct facility.The reactor section houses the UV
lamp assembly and the catalytic filter. It has an inner cross
sectional area of about 0.06 m2 (100 in.2). The UV lamp
assembly is as illustrated in Fig. 2. The assembly consists of a
bank of 6 equally spaced RPR 3500 UV lamps (Southern New
England Ultraviolet Company). Each lamp has a nominal
power rating of 14 W, and emits approximately 1.5 W of UV
radiation, predominately at 350 nm.1 The UV light intensity on
the catalytic filter surface was 100 W/m2. The UV light
intensity was measured with an Eppley radiometer (model
TUVR).
Downstream of the reactor section is the sample collection
section. Samples were collected on two culture plates securely
placed on a rack about 6 in. downstream of the reactor section.
The lower portion of the recirculating duct consists of a circular
duct with a diameter of 0.3 m (1 foot). The circular cross
section duct and the rectangular cross section duct are
connected though a Dayton (model 2C887) blower. The blower
has a Dayton 1/4 HP motor (model 5K9070), with a belt drive
and stepped pulley, to vary the speed and adjust the air flow
inside the recirculating duct.
The duct is instrumented with Vaisala (model HMD6OY)
humidity and temperature transmitters upstream and down-
stream of the reactor section to measure the relative humidity
and temperature of the airstream inside the duct. The pressure
drop across the reactor section is measured by a Dwyer Mark II
model 25 manometer. An annubar flowmeter is attached to the
circular cross section duct to determine the air flow rate. The
whole duct assembly is additionally sealed using duct tape at
every seam. A smoke test was conducted prior to experimenta-
tion to ensure that there is no leakage during air circulation.
A 0.1 m (4 in.) round flexible duct is attached to the sample
injection section to flush out and disinfect the recirculating duct
1 The spectrum of the black light used is a bell shaped curve centered at
350 nm and extends from 300 to 400 nm.
after each experimental run. A formalin vapor evacuation
system, as approved by University of Florida Environmental
Health and Safety (EHS) Department, is connected to the duct
assembly to remove the formalin vapors used to sterilize the
duct. The evacuation system consists of a roof mounted
dedicated air blower which vents out the formalin vapor at
10.1 m/s (2000 fpm) to a location 1.83 m (6 ft) above the
highest point of the laboratory roof.
2.2. Preparation of the photocatalytic filter
Titanium dioxide used in this study was Degussa P25. It had
an approximate composition of 75% anatase and 25% rutile
forms of TiO2, a BET surface area of 50 m2/g and a primary
particle size of 20 nm (Degussa). The enhanced photocatalyst
was prepared by doping TiO2 P25 with silver ions by a
proprietary process. A 2% by weight slurry of enhanced
photocatalyst was coated onto the fabric filter. The exact filter
dimensions were 0.34 m � 0.23 m � 0.05 m (13(1/
2) � 9 in. � 1(7/8) in.).
2.3. Culture media
Different culture media were used for the growth of various
microorganisms. Standard procedures for the preparation of
growth media are described below.
2.3.1. Preparation of BBLTM
TrypticaseTM
Soy agar
BBLTM
TrypticaseTM
Soy agar is a Soybean-Casein digest
agar and was used as the plate count agar for gram-positive (S.
aureus) and gram-negative bacteria (E. coli), bacterial spores of
B. cereus, and MS2 bacteriophage. For preparation of
Trypticase Soy agar, 40 g of the powder was suspended in
1 L of sterilized water and mixed thoroughly. The suspension
was heated with frequent agitation and boiled for 1 min to
completely dissolve the powder. The solution was then
autoclaved at 121 8C for 15 min without overheating.
2.3.2. Preparation of BBLTM
TrypticaseTM
Soy broth
BBLTM
TrypticaseTM
Soy broth is a Soybean-Casein digest
broth and was used as the culture media for gram-positive (S.
aureus) and gram-negative bacteria (E. coli), and MS2
bacteriophage. For preparation of Trypticase Soy broth, 30 g
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–6560
Fig. 2. UV lamp assembly.
(10 g for preparation of broth for MS2 bacteriophage) of the
powder was dissolved in 1 L of sterilized water and mixed
thoroughly. The suspension was warmed gently so that the
solution was complete. The solution was then autoclaved at
121 8C for 15 min without overheating.
2.3.3. Preparation of BBLTM
Columbia broth
BBLTM
Columbia broth was used as the culture media for B.
cereus bacterial spores. For preparation of Columbia broth,
35 g of the powder was dissolved in 1 L of sterilized water and
mixed thoroughly. The suspension was warmed gently so that
the solution was complete. The solution was then autoclaved at
121 8C for 15 min without overheating.
2.3.4. Preparation of difcoTM
sabouraud dextrose agar
DifcoTM
sabouraud dextrose agar is a base used for
cultivation of fungi and other acidic microorganisms. For
preparation of DifcoTM
sabouraud dextrose agar, 65 g of the
powder was suspended in 1 L of sterilized water and mixed
thoroughly. The suspension was heated with frequent agitation
and boiled for 1 min to completely dissolve the powder. The
solution was then autoclaved at 121 8C for 15 min without
overheating.
2.3.5. Preparation of soft agar
Soft agar is used as the top agar for the cultivation of phage
on bacterial host. Soft agar was prepared by dissolving 30 g of
BBLTM
TrypticaseTM
Soy broth and 8 g of BBLTM
TrypticaseTM
Soy agar in 1 L of sterilized water. The solution was then
autoclaved at 121 8C for 10 min without overheating.
2.4. Microorganism cultures
Stock cultures of B. cereus, E. coli, S. aureus, A. niger, and
MS2 Bacteriophage were obtained from the Department of
Microbiology at the University of Florida. Standard procedures
used for preparing cultures of various microorganisms are as
explained below.
2.4.1. Procedure for B. cereus spores
B. cereus (ATCC 2) bacterial spores were used for the
disinfection study. The spores were prepared and purified by the
method of Nicholson and Galeano [34]. For preparation of
spores, B. cereus strains were cultivated in Columbia broth at
30 8C for 12–18 h aerobically with vigorous agitation. Spores
were then harvested and purified by lysozyme treatment, and
salt and detergent washes to remove the vegetative bacterial
cells. Lysozyme (Fisher Biotech) was stored at 4 8C and
prepared fresh from powder for each use. The vegetative cells
left after the lysozyme treatment were subjected to heat
shocking in a constant temperature bath at 80 8C for 1 h.
2.4.2. Procedure for E. coli
E. coli was inoculated into BBLTM
TrypticaseTM
Soy broth
and grown overnight at 37 8C by constant agitation under
aerobic conditions. The overnight culture was then inoculated
in a fresh medium and incubated aerobically at 37 8C for 24 h.
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–65 61
Bacterial cells were then collected by centrifugation at 500 � g
for 10 min at 4 8C. The bacterial pellet was subsequently
washed three times with tryptone solution. Finally, the bacterial
pellet was resuspended in tryptone solution. Cell suspensions
were then diluted in sterilized water to the required bacteria cell
concentration.
2.4.3. Procedure for S. aureus
S. aureus was inoculated into BBLTM
TrypticaseTM
Soy broth
and grown overnight at 37 8C under aerobic conditions. The
overnight culture was then inoculated in a fresh medium and
incubated aerobically at 37 8C for 48 h. Culture suspensions
were then diluted in sterilized water to the required bacteria cell
concentration.
2.4.4. Procedure for A. niger spores
A. niger was inoculated onto DifcoTM
sabouraud dextrose
agar and grown at room temperature under aerobic conditions.
The incubation period was close to 3 days, when the fungi
micelia covered the entire agar surface. Fungal spores were
scraped away from the micelia and suspended in solution using
0.05 wt.% of sodium dodecyl sulfate (SDS). The suspension
was then diluted in sterilized water to the required fungal spore
concentration.
2.4.5. Procedure for MS2 Bacteriophage
Escherichia coli C-3000 was used as the bacterial host.
About 0.1–0.3 mL of Escherichia coli C-3000 was added to
sterile 13 mm tubes containing 0.1 mL of MS2 sample, and
incubated approximately for 1 min at 37 8C. Soft BBLTM
TrypticaseTM
Soy agar was then added to the tubes with bacteria
and sample, mixed and poured onto plate-count agar (BBLTM
TrypticaseTM
Soy agar) plates. The plates were then incubated
overnight (close to 6 h) at 37 8C. For preparation of phage
culture, a plaque was picked and added to a BBLTM
TrypticaseTM
Soy broth culture of E. coli that was in its early log-phase of
growth. After growth of the culture (6 h to overnight at 37 8C),
the sample was centrifuged at 10,000 rpm for 10 min. Finally,
the supernatant was filtered through a 0.45 mm or 0.25 mm
plastic filter and diluted to the required phage concentration.
2.5. Experimental procedure
2.5.1. Calibration
Prior to the microorganism destruction experiments, the
facility was calibrated to identify the various parameters which
would give measurable microbial concentration, proper
microbial dispersion, and adequate static pressure inside the
duct during the experiment. The system parameters varied
during calibration were spore concentration injected, nebuliz-
ing pressure for injection, spore collection time on culture
plates, blower RPM, and insertion of filter. The microorganism
used for calibration was B. cereus bacterial spores. The
calibration process consisted of injecting varying concentra-
tions of B. cereus into the circulating air stream using a Hudson
RCI nebulizer operated by compressed air, dispersing the
spores in the circulating air for 10 min, and then sampling the
dispersed spores on two petri dishes, containing plate count
agar, inserted at the sampling port.
Injection of 15 mL of spore culture diluted to 20 mL in
sterilized water (20 mL of pure culture for other microorgan-
isms), with an initial concentration of 5000 cfu/mL, and a 2 min
sampling time gave measurable colony counts on agar plates.
For proper dispersion of spores into the circulating air, the
nebulizer was operated by compressed air at a pressure of 138–
172 kPa (20–25 psi). Air velocity of 2.2 m/s (7.3 fps) leads to
proper dispersion of microbes inside the duct.
The microbial retention capacity of the fabric filter per air
pass was expected to be high. This was checked by injecting the
microbes in the recirculating duct upstream of the filter and
collecting them from the air stream downstream of the filter
using the sampling arrangement as described before. It was
observed that the microbial concentration downstream of the
filter was less than 2% of the initial concentration injected
within 2 min of air recirculation. The enhanced photocatalyst
coated filter thus had high microbe collection efficiency per air
pass through it. As the catalytic filter retained most of the
microbes within the initial few airpasses, it was appropriate to
collect the microorganisms from the filter surface to monitor
the destruction performance of the enhanced photocatalyst.
This led to the development of a swabbing technique for
sampling the microbes from the filter surface, and thus
monitoring the disinfection efficiency of the filter. Using
CopanTM
sterile moist swabs, a 0.025 m � 0.025 m
(1 in. � 1 in.) square area of the contaminated filter surface
was swabbed. The microbes collected by the swabs were then
transferred to 1 mL sterile water by vortexing for about 20 s.
This 1 mL water with collected microbes was plated on agar
and incubated to determine the initial microbial population on
the filter surface.
2.5.2. Microorganism destruction test
The blower was turned on and the air flow inside the duct
was allowed to stabilize for 10 min before the microbial culture
was nebulized. The air velocity inside the duct was maintained
at 2.2 m/s (7.3 fps). The temperature and RH inside the duct
were measured to be close to 24 8C and 50%, respectively. The
UV light intensity at the catalytic filter surface was around
100 W/m2. The microorganisms were injected into the duct and
circulation continued for another 10 min to ensure proper
dispersion of microorganisms.
Then the blower was stopped, a fresh catalytic filter inserted
inside the duct, UV lamps turned on and the filter exposed to the
contaminated air stream. Sampling was done on the filter
surface using the swabbing technique as described before.
Sampling was done for a set of UV exposure times. Colony
counts were then quantified, and thus microbe destruction
determined relative to initial colony count.
Percentage destruction ¼ 100ðCCi � CCtÞ
CCi
where CCi is the averaged initial colony count and CCt is the
averaged colony count after a time interval ‘t’.
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–6562
The microorganisms used for experimentation were B.
cereus bacterial spores, E. coli, S. aureus, A. niger fungal
spores, and MS2 Bacteriophage virus. Control tests were done
with a conventional TiO2 coated filter to compare it with the
enhanced photocatalyst coated filter. Baseline dark control
experiments were performed for the enhanced photocatalyst as
well as the conventional TiO2 coated filters. No effort was
made to study the effect of UV-A radiation on microbial
destruction in this study based on the literature and our
previous research. The resistance of microbial species to UV-A
radiation as a disinfectant has been reported in several studies
[35–37]. Recently, our own studies showed that UV-A
photolysis was ineffective in complete destruction of B.
cereus spores even after 24 h of exposure to UV-A radiation
[38].
Also each of the experiments was repeated at least three
times to confirm the validity of the results. The averaged
percentage photocatalytic destruction was plotted against UV
exposure times for each of these microbes.
2.5.3. Duct sterilization
Formalin was necessary to ensure effective sterilization of
the duct after each experiment run with B. cereus bacterial
spores and A. niger fungal spores. Duct sterilization consisted
of nebulizing 20 mL. of 4% (v/w) formalin into the duct,
circulating the formalin vapor for 4 h and then venting the duct
for 12 h. 70% alcohol/volume isopropyl alcohol was effective
in sterilizing the duct for E. coli, S. aureus, and MS2
Bacteriophage.
3. Results and discussion
A comprehensive study of the effectiveness of the enhanced
photocatalysis was conducted in a recirculating air facility.
Calibration of the duct facility was performed to identify
optimum experimental conditions. The effectiveness of the
enhanced photocatalysis was investigated for destruction of B.
cereus (bacterial spores), E. coli (gram-negative bacteria), S.
Fig. 3. Comparison of enhanced photocatalytic, conventional photocatalytic and d
conventional photocatalysis (b) enhanced photocatalysis.
aureus (gram-positive bacteria), A. niger (fungal spores), and
MS2 Bacteriophage (virus).
3.1. Destruction of B. cereus
Initial concentration of the stock culture of B. cereus spores
was 104 cfu/mL. 15 mL of the stock spore suspension diluted
to 20 mL in sterilized water was injected inside the duct
facility.
Fig. 3 shows the comparative spore destruction by
conventional TiO2 photocatalysis and enhanced photocatalysis.
The destruction under dark conditions for conventional and
enhanced photocatalyst is also illustrated in the figure. The
error bars in the figure depict standard deviations. No
significant bacterial spore destruction was observed under
dark conditions over the entire experiment duration. Under UV
exposure, close to 85% destruction was seen in the first 4 h,
which then leveled off to 99% in 24 h of UV exposure with
conventional TiO2� photocatalysis. The dark control tests with
the enhanced photocatalyst showed close to 77% spore
destruction in 10 min. This dark control destruction can be
attributed to the disinfecting ability of the doped Ag+ ions. A
leveling off trend in dark control destruction with enhanced
photocatalyst was observed. There could be several possible
explanations for this trend, most common of which is that some
microorganisms are intrinsically more resistant than other
microorganisms [39]. There maybe several other factors such as
potential protection of microbial species due to dead microbes
or the products of destruction, oxygen depletion at the sites or
the inactivation of the disinfectant.
For enhanced photocatalysis with UVexposure, phenomenal
reduction in destruction times were observed with almost 99%
spore destruction in 2 min and complete spore destruction in
10 min with UV exposure. This extremely fast destruction
result can be explained due to the additive effect of the
disinfecting ability of doped Ag+ ions, and electron trapping by
the doped Ag+ ions leading to reduced recombination, thus
leading to enhanced photocatalysis. Experiments with repeated
ark destruction of Bacillus cereus spores in the recirculating duct facility (a)
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–65 63
Fig. 4. Comparison of enhanced photocatalytic, conventional photocatalytic and dark destruction of Escherichia coli in the recirculating duct facility.
use of the filter with enhanced photocatalyst showed no
discernible reduction in the photocatalytic activity.
3.2. Destruction of E. coli
Escherichia coli K12 strain was used for this study. The
initial concentration of the stock culture was 109 cfu/mL.
20 mL of pure E. coli suspension was injected inside the duct.
The comparative destruction trends with conventional
photocatalysis and enhanced photocatalysis are shown in
Fig. 4.
E. coli, being gram-negative bacteria, is much easier to
inactivate than B. cereus bacterial spores. This fact was
demonstrated by their fast destruction even with conventional
photocatalysis. In the first 15 min, 75% of the bacteria were
destroyed and this destruction trend continued with 100%
destruction in about 60 min. Dark control with conventional
photocatalysis did not lead to any appreciable destruction as
expected. Close to 72% destruction was evident in the dark with
the enhanced photocatalyst though, thus confirming the action
Fig. 5. Comparison of enhanced photocatalytic, conventional photocatalytic and
of doped Ag+ ions. UV irradiated enhanced photocatalysis on
the other hand lead to complete destruction of E. coli in about
5 min thus improving the destruction rate almost 12-fold over
conventional photocatalysis.
3.3. Destruction of S. aureus
S. aureus was the next microorganism to be tested in the
recirculating duct facility. Stock culture of S. aureus had an
initial concentration of around 109 cfu/mL. 20 mL of this pure
bacterial suspension was selected as the control to be injected
inside the duct facility.
As is evident from the comparative destruction Fig. 5, S.
aureus bacterial cells were inactivated completely in 5 min of
UV irradiation with enhanced photocatalysis as compared to
complete inactivation in 45 min with conventional TiO2
photocatalysis. Dark control experiments demonstrated no
appreciable reduction in colony counts with TiO2 photocata-
lysis. However, silver ion doped titania photocatalyst showed
close to 65% destruction in 10 min even under dark conditions.
dark destruction of Staphylococcus aureus in the recirculating duct facility.
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–6564
Fig. 6. Comparison of enhanced photocatalytic, conventional photocatalytic and dark destruction of Aspergillus niger in the recirculating duct facility (a)
conventional photocatalysis (b) enhanced photocatalysis.
3.4. Destruction of A. niger
The stock culture of fungal spores used had an initial
concentration of about 105 cfu/mL. The experimentation
involved injecting 20 mL of pure fungal spore suspension
inside the recirculating duct facility.
The destruction of A. niger spores with conventional
photocatalysis and enhanced photocatalysis is depicted in
Fig. 6. The rate of photocatalytic destruction of fungal spores is
extremely slow with less than 78% destruction observed after
24 h. The destruction increases to around 91% in 36 h and as
much as 48 h are required to achieve complete inactivation of A.
niger spores. This result was expected because of the extreme
resistance of this microbe to disinfectants. Also, dark control
experiments with TiO2 did not report any fungal spore
destruction.
An appreciable improvement in percentage destruction of
fungal spores is obtained with enhanced photocatalysis. The
time required for inactivation of A. niger is reduced
dramatically with 74% destruction observed in close to
Fig. 7. Comparison of enhanced photocatalytic, conventional photocatalytic an
45 min under dark conditions, and complete inactivation
achieved at 45 min of UV exposure. Enhanced photocatalysis
thus was extremely effective in the overall destruction of
resistant A. niger fungal spores.
3.5. Destruction of MS2 Bacteriophage
MS2 Bacteriophage with E. coli C3000 as the host was the
final microbe tested to complete this study. 20 mL of stock
phage culture with an initial concentration of 1013 cfu/mL was
injected inside the duct for experimentation.
Bacteriophage had the fastest destruction rate among all
microbes tested inside the recirculating duct with both
conventional photocatalysis as well as enhanced photocatalysis
(Fig. 7). Almost 95% phage destruction was observed in 10 min
even under dark conditions for the enhanced photocatalyst.
Complete inactivation was observed in close to 15 min with
conventional TiO2 photocatalysis and the destruction time with
enhanced photocatalysis was less than 2 min for 100%
destruction.
d dark destruction of MS2 Bacteriophage in the recirculating duct facility.
A. Vohra et al. / Applied Catalysis B: Environmental 65 (2006) 57–65 65
4. Conclusions
The enhanced photocatalysis process for indoor air
applications was presented and its effectiveness was success-
fully demonstrated for a range of microorganisms. The study
reported complete inactivation of microbes with the Ag+ ion
doped TiO2 photocatalyst. Control experiments involving
conventional TiO2 photocatalytic, and dark destruction of
microbes showed that silver ion doped TiO2 photocatalysis
results in much faster destruction kinetics.
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