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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