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Hydroxyapatite-supported Ag–TiO2 as Escherichia colidisinfection photocatalyst$

M. Pratap Reddy, A. Venugopal, M. Subrahmanyam�

Catalysis and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India

a r t i c l e i n f o

Article history:

Received 26 October 2005

Received in revised form

6 September 2006

Accepted 20 September 2006

Available online 29 November 2006

Keywords:

Hydroxyapatite (HAP)

Ag–TiO2

Ag–TiO2/HAP composite catalyst

Escherichia coli (E. coli)

Photocatalytic bactericidal activity

nt matter & 2006 Elsevie.2006.09.018

ation no. 060305.thor. Tel.: +91 40 27193165subrahmanyam@iict.res.i

A B S T R A C T

A series of hydroxyapatite (HAP), 1 wt% Ag–TiO2 (AT1), 1 wt% Ag-HAP and 5 wt% AT1/HAP

composite catalysts were prepared by incipient wetness and mechanical mixing methods.

They were characterized by X-ray diffraction (XRD), FT-IR, SEM and ESCA analyses and their

photocatalytic bactericidal activities were measured in suspension using Escherichia coli (E.

coli), a water pollutant indicator. The surface analysis revealed that the Ag/Ti ratio is found

to be ca. 0.0273 and also it indicated that the titania is present in the form of Ti4+ and Ag is

present as metallic silver. Both the XRD and ESCA analyses confirmed the phase of metallic

Ag particles, which played a significant role on the bactericidal activity of the Ag doped TiO2

catalysts. The FT-IR analysis of HAP revealed that the peak intensity is due to the

absorbance of surface PO43� group centered at wave number 1030 cm�1 and is drastically

decreased upon exposure to UV for 1 h. The HAP displayed high amount of bacteria

adsorption, ca. 80% during the dark experiments compared to other catalytic systems

tested. The cumulative photocatalytic properties of AT1/HAP catalytic system resulted in

100% E. coli bacteria reduction within 2 min.

& 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Application of photocatalysis as a remedy to the environ-

mental problems has increased tremendously in the recent

past (Blake et al., 1999; Ljubas, 2005; Dunlop et al., 2002).

Presence of bacterial pathogens in drinking water is one of

the perennial problems. Generally chlorine has been widely

used for the disinfection of water, resulting in chloroorganic

compounds during the treatment which are highly carcino-

genic (Fujishima et al., 2000). Photocatalysis is a suitable

method for disinfection of pathogenic bacteria present in

drinking water. There are some reports concerning the

photocatalytic removal of organic, inorganic and microbial

pollutants (RincOn and Pulgarin, 2005) for the purification of

water and wastewater treatment. The TiO2 catalyst has been

found to be a widely used component in various photocata-

r Ltd. All rights reserved.

; fax: +91 40 27160921.n (M. Subrahmanyam).

lytic applications. The bactericidal activity upon addition of

Ag to TiO2 tremendously enhanced (Vamathevan et al., 2004;

Zhang et al., 2005). Titania immobilization on different

supports like glass matrix, optical fibers, pumice stone and

stainless-steel plate were studied extensively (Xu et al., 1999;

Noorjahan et al., 2003; Subba Rao et al., 2004).

In this investigation we have prepared hydroxyapatite

(HAP), a novel material and characterized for its photocata-

lytic application. HAP is the major inorganic component in

natural bones and can be synthesized by chemical precipita-

tion, solid-state reaction, hydrothermal synthesis, sol–gel

route and other routes (Brooks, 1981). It is used extensively

as a matrix for the purification (Nonami et al., 2004) and

fractionation of an array of biochemical substances, including

enzymes, nucleic acids, hormones, and viruses (Brooks, 1981).

The high bacterial adsorption of different bacteria onto HAP

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WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 3 7 9 – 3 8 6380

material was described by Berry and Siragusa (1997). The

Escherichia coli (E. coli) is a popular bacterial pollutant indicator

in water and its presence makes water polluted with good

number of pathogenic bacteria and its complete absence

reckons no pathogenic bacteria and hence it is used as a

treatment efficiency substrate (Baker et al., 2000). To date,

there is no report describing utilization of Ag–TiO2 supported

over HAP. It can be used as an easy and efficient system for

the complete removal of bacterial pathogens in drinking

water.

Laminar Air Flow Hood

Shaking Unit

Petri Plate with

catalyst and E. coli

suspension

UV Light

250 W

Fig. 1 – Illustration of experimental setup.

2. Experimental

2.1. Materials and chemicals

Titanium dioxide (P-25, 80% anatase and 20% rutile, of a

specific area 50 m2/g) was from the Degussa Corporation. The

active precursor salts, viz. calcium nitrate tetrahydrate,

diammonium hydrogen phosphate, ammonium hydroxide,

silver nitrate, ethanol and sodium chloride are from s.d. Fine-

Chem are of analytical grade quality. The E. coli broth (Luria-

Bertani) and E. coli agar (Luria-Bertani) are from Sigma-

Aldrich and the E. coli bacteria used was supplied by the

Microbial Type Culture Collection (MTCC), Institute of Micro-

bial Technology (IMTECH), India and sterile distilled water

was used in all the experimental studies.

2.2. Preparation of catalysts

Ca10 (PO4)6 (OH)2 (HAP) denoted as HAP was prepared by the

precipitation method (Venugopal and Scurrell, 2003). The TiO2

used was the commercial Degussa (P-25) and 1 wt% Ag–TiO2

(AT1) and 1 wt% Ag–HAP are made by impregnation techni-

que. The 5 wt% TiO2/HAP and 5 wt% AT1/HAP catalysts were

obtained by mechanical mixing method.

2.3. Characterization of catalysts

X-ray diffraction (XRD) patterns of fresh TiO2, HAP, 5 wt%

TiO2/HAP, AT1, 1 wt% Ag–HAP and 5 wt% AT1 loaded on HAP

support catalyst systems were obtained using Rigaku Miniflex

diffractometer with Ni filtered Cu-Ka radiation. The FT-IR

spectra of all the fresh catalysts used were recorded on a

Nicolet 740 FT-IR spectrometer using the KBr self-supported

pellet technique in the frequency range of 400–4000 cm�1.

Electron spectroscopy for chemical analysis (ESCA) was

carried out with KRATOS AXIS 165 photoelectron spectro-

scopy using the Mg Ka (150 W) anode. The catalysts used for

the chemical state and surface compositions were TiO2, AT1,

HAP, 5 wt% TiO2/HAP and 5 wt% AT1/HAP. The Ag/Ti atomic

ratios of the AT1 and 5 wt% AT1/HAP photocatalysts were

determined by the intensities of Ti2p and Ag3d. The scanning

electron microscopic (SEM) images were analyzed using

model JEOL-JSM 5600 instrument.

2.4. Preparation of E. coli culture

E. coli was inoculated into fresh sterilized autoclaved E. coli

broth of 10 ml in a 50 ml capacity conical flask from stock agar

slants and they were grown overnight at 37 1C by constant

agitation (100 rpm) under aerobic conditions. The bacteria

was subcultured from 50 to 500 ml flask having 250 ml broth

and incubated aerobically (37 1C, 100 rpm) upto getting a

maximum OD of 0.8 at 600 nm by UV-DRS. At exponential

growth phase, bacterial cells were collected by centrifugation

at 4000 rpm (10 min, 4 1C) and the bacterial pellet was washed

three times with saline water (0.9% NaCl solution) in order to

remove the culture media components. Finally the resulting

pellet was resuspended in sterile saline water and diluted to

cell density of 107 colony forming units (CFU)/ml by serial

dilution method using sterile saline water. This culture

solution was stored at 4 1C for 1 h and was used for further

experiments in the entire study. The CFU counts/ml were

performed with serial dilution and spread plate method using

E. coli agar medium and the obtained counts were multiplied

with a dilution factor.

2.5. Photocatalytic experiments

The photocatalytic experimental setup was kept in laminar

airflow hood after proper sterilization. It consisted of a

shaking unit with petriplates of capacity 50 ml, about 0.75 g/l

catalyst and 25 ml of bacterial suspension were taken into

each petriplate. The optimum catalyst concentration ob-

tained with TiO2 for bactericidal activity in our recent study

was about 0.75 g/l (Pratap Reddy and Subrahmanyam, 2006).

The 250 W high-pressure mercury vapor lamp was provided

as an illumination source from top, so that the radiation

circumference covered all the plates under study. The lamp

emitted radiation over a wavelength range of 320–420 nm. The

experimental setup was as shown in Fig. 1. An air-cooling fan

was provided to reduce the temperature developed due to the

irradiation. The bacterial suspension (N0 ¼ 107 CFU/ml) with

the catalyst was kept shaking at 50 rpm for proper agitation at

room temperature. Experiments were conducted at room

temperature and at pH of 6.5. One weight percent of Ag was

doped onto TiO2 and HAP, as this was found to be the

minimum concentration to observe a good amount of

bacterial cell death. For the present investigation 5 wt% of

TiO2 and 5 wt% AT1 supported on HAP catalysts were taken. A

catalyst loading of 5 wt% was used since high bactericidal

activity was observed over Hb zeolite support in our recent

study (Pratap Reddy and Subrahmanyam, 2006). In view of

this the 5 wt% loading of active component was selected for

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Inte

nsi

ty (

cps)

Anatase

Ag

a

b

c

WAT ER R ES E A R C H 41 (2007) 379– 386 381

testing its bactericidal efficiency and a comparison with Hbzeolite support was made. All photocatalytic experiments

were carried out for a period of 280 min under UV light

irradiation. The various combinate catalyst systems tested for

the present investigation were TiO2, HAP, 5 wt% TiO2/HAP,

1 wt% Ag–TiO2 (AT1), 1 wt% Ag–HAP and 5 wt% AT1 loaded on

HAP support. Before performing photocatalytic experiments,

the dark experiments were performed for �4 h over all the

catalysts chosen for testing. This data gave the difference in

bacterial adherence property on various catalyst systems

under study, which is a key property that will influence the

bactericidal activity. Frequent samples of 0.1 ml were col-

lected at 10 min interval time and inoculated into sterile

0.9 ml distilled water which was serially diluted and 0.1 ml of

each dilution was inoculated into E. coli agar medium plates,

the inoculum was spread and kept for colony growth at 37 1C

for 48 h. The number of CFU/ml were noted after 48 h of time.

0 20 40

Two theta (degree)

60 80

d

e

f

Fig. 2 – XRD pattern of (a) TiO2, (b) HAP, (c) 5 wt% TiO2/HAP, (d)

ATI, (e) 1 wt% Ag–HAP and (f) 5 wt% ATI/HAP.

3. Results and discussion

3.1. Characterization of catalysts

3.1.1. XRD analysisXRD analysis revealed the reflections due to Ca10 (PO4)6 (OH)2phase in pure HAP, 1 wt% Ag–HAP, 5 wt% AT1/HAP, and 5 wt%

TiO2/HAP catalysts and both the anatase (at 2y values of 25.31,

48.01, 53.91 and 55.01) and rutile phases (at 2y values of 27.41,

36.01, 54.21 and 56.51 (ICDD no.-86-1157)) were observed where

TiO2 loadings were provided as shown in Fig. 2. The crystal-

linity of the prepared HAP was confirmed by the reflections

observed at 2y values of 31.71, 32.171, 33.01, 34.31, 46.61 and

49.51 (ICDD no.-86-0740). It is also found the metallic Ag phase

at 2y of 32.21 and 46.21 (ICDD no.-87-0720) are found over Ag-

doped catalysts.

3.1.2. Infra red spectroscopy analysisThe comparative FT-IR spectrum analyzed for fresh HAP and

after 1 h UV treatment in aqueous suspension is provided in

Fig. 3. The spectra clearly represents the absorbance intensity

at 1030 cm�1 due to PO43� group and is drastically decreased

upon exposure to UV for 1 h and a similar observation is also

reported by Nishikawa (2004).

3.1.3. ESCAESCA analysis indicated the presence of metallic silver over

the Ag-doped catalysts and the Ag/Ti ratio was found to be ca.

0.0273. This value contributes to the increase in photocata-

lytic activity as it stands in the range represented earlier,

0.0198–0.0595 (Sokmen et al., 2001). The comparative bacter-

icidal activity performance of all the catalysts and the

amount of Ag used are presented in Table 1. The relative

intensities of O1s spectra (binding energy 532.1 eV) from the

surface analysis comparison revealed that the O1 s contribu-

tion from hydroxyl radical is increased in the order of

TiO2oAT1oHAPoAg–HAPo5 wt% TiO2/HAPo5 wt% AT1/

HAP. This implies that more hydroxyl groups are present on

the surface of 5wt% AT1/HAP for trapping the holes at TiO2,

resulting in an enhanced photocatalytic activity (Milella et al.,

2001).

3.1.4. Scanning electron microscopy (SEM)SEM analysis photographs carried out for the samples of AT1,

HAP, 1 wt% Ag–HAP and 5 wt% AT1/HAP are presented in Fig.

4. The SEM images of the catalysts illustrate the presence of

particles of varying size and some pockets of agglomeration

as seen from Fig. 4(a)1–(d)1. The HAP, Ag–HAP and AT1/HAP

sample photographs (Fig. 4(b)2–(d)2) after 1 h dark adsorption

period showed significant bacteria adsorbed on the catalyst

surface. Upon exposure to UV for a period of 1 h, there is a

drastic reduction in the existence of bacteria over HAP,

Ag–HAP and AT1/HAP catalysts.

3.2. E. coli adsorption studies

E. coli adsorption studies were carried out in dark condition

for AT1, HAP, 1 wt% Ag–HAP, 5 wt% TiO2/HAP and 5 wt% AT1/

HAP systems and they were monitored in terms of bacterial

counts as CFU/ml as shown in Fig. 5(a). Furthermore, the

pictorial inspection of SEM photographs for fresh catalysts as

seen in Fig. 4(a)1–(d)1 and the systems suspended in bacterial

suspension in dark for 1 h as provided in Fig. 4(a)2–(d)2supplements the above relative differences in the adsorption

amounts over various catalyst samples.

From Fig. 5(a) it is observed that 80% and 20% of total

bacterial count is adsorbed over bare HAP and Hb supports,

respectively. In case of titania-supported HAP 55% of bacteria

is adsorbed on to HAP surface within 80 min. It remains

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7.6

7

6

5

4

3

2

1

0

4000 3000 2000 1500

Wavenumber/cm-1

1000 500 400

After UV

Before UV

Abso

rban

ce

Fig. 3 – FTIR spectra of HAP before and after 1 h UV exposure.

Table 1 – Characteristics of the catalysts used for the photocatalytic disinfection of water

Catalyst XRD phase Timea (min) Ag/Tib Percent contribution from OHgroups in O1s spectra of ESCA

TiO2 Anatase and rutile 65 — 11.4

HAP Ca10 (PO4)6 (OH)2 180 — 40.1

1 wt% Ag–HAP Ag, Ca10 (PO4)6 (OH)2 20 0.0274 46.0

1 wt% Ag–TiO2 (AT1) Ag, TiO2 16 0.0274 19.8

5 wt% TiO2–HAP TiO2, Ca10 (PO4)6 (OH)2 30 — 71.0

5 wt% AT1/HAP Ag, TiO2, Ca10 (PO4)6 (OH)2 02 0.0273 78.0

a Time taken for the complete removal of bacteria under UV irradiation (N0 ¼ 107 CFU/ml).b Ag/Ti atomic ratios from XPS analyses.

WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 3 7 9 – 3 8 6382

constant even after 280 min. No adsorption is observed on

pure titania even after 280 min duration with a continuous

stirring in dark whereas 10% bacterial adsorption is observed

on titania-supported Hb. This observation is consistent with

our recent report (Pratap Reddy and Subrahmanyam, 2006). In

case of Ag-loaded catalysts of AT1, Ag–HAP, and AT1/HAP as

seen in Fig. 5(b) 100% bacteria removal in suspension is

observed within 90, 60 and 70 min, respectively.

The observed bacterial adsorption trend is HAP4TiO2/

HAP4Hb 4TiO2/Hb and this may be due to the higher

adsorption capacity of HAP than that of Hb. The adsorption

of bacteria over HAP is due to nonspecific Van Der Walls and

electrostatic attraction between positive calcium atoms of

HAP and negatively charged surface of E. coli bacteria (Berry

and Siragusa, 1997).

In case of Ag-loaded catalysts the complete inactivation of

bacteria from the solution may be due to two factors. It is the

bactericidal activity attributed by the release of Ag+ ions into

solution (Matsumura et al., 2003) and the other is due to

adsorption property of HAP. However, in case of HAP,

titania supported on Hb and on HAP catalysts, the inactiva-

tion of bacteria was due to adsorption and no bactericidal

activity could be detected. The amount of Ag present in

Ag–HAP is more than AT1/HAP due to which more reduction

of bacteria was achieved in Ag–HAP. Between Ag–TiO2 and

Ag–HAP, higher inactivation of bacteria is observed in

Ag–HAP, due to the Ag chemical activity and bacterial

adherence property.

The SEM photographs provided for bare (Fig. 4(a)1–(d)1) and

after 1 h dark soaking period in bacterial suspension over the

catalyst systems AT1, HAP, Ag/HAP and AT1/HAP represents

(Fig. 4(a)2–(d)2) clearly that bare catalysts show titania and Ag-

doped titania dispersions on HAP crystals show Ag atoms

dispersion on titania as well as HAP. Ag atoms are clearly

observed on the catalyst surface of Ag–HAP and AT1/HAP (Fig.

4(c)2 and (d)2) and also these showed concentrated bacterial

HAP surface during dark period. There is no bacteria adsorbed

on AT1 and more or less equal adsorptions are observed on

HAP, Ag–HAP and AT1/HAP catalysts after 1 h dark adsorption

period.

3.3. Photocatalytic bactericidal activity performance study

From Fig. 6(a) it is observed that under UV irradiation titania-

loaded HAP and Hb take 30 and 40 min duration for complete

inactivation of bacteria whereas 180 and 65 min for HAP and

titania, respectively. In order to improve the photocatalytic

activity of titania-loaded HAP, Ag is doped to TiO2 and the

same is supported on HAP. The results obtained in Fig. 6(b)

show that the time taken for complete inactivation of bacteria

is found to be only 16, 20 and 2 min for AT1, Ag–HAP and AT1/

HAP, respectively (see Table 1).

ARTICLE IN PRESS

Fig. 4 – Scanning electron microscopy photographs of (a)1–3 ¼ AT1, (b)1–3 ¼ HAP, (c)1–3 ¼ 1 wt% Ag–HAP and (d)1–3 ¼ 5 wt% ATI/

HAP. Arrow (-) on the photographs pin point the E. coli bacteria.

WAT ER R ES E A R C H 41 (2007) 379– 386 383

The data in Fig. 6(a) seems to be due to the photocatalytic

property exhibited by HAP under UV exposure where vacan-

cies are formed on the surface of PO43� group of HAP.

This property is due to photoinduced electronic excitation

and it is similar to the phenomena of the formation of

electron trapped on oxygen vacancy in plasma-treated TiO2

(Nakamura et al., 2000). In the present case, the activation of

oxygen is due to the electron trapped on the vacancy of

HAP that occurs and it is followed by the formation of O2d�

radical formation that is important for the photocatalytic

oxidation of the bacteria adsorbed on HAP (Nishikawa, 2004;

Pratap Reddy et al., 2006). The pictorial representation of HAP

under UV illumination and the plausible mechanism for

photocatalytic behavior is shown in Fig. 7(a). The same

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0 20 40 60 80 100

Time/min

0 50 100 150 200 250 300101

102

103

104

105

106

107

101

102

103

104

105

106

107

Bac

teri

al s

urv

ival

(C

FU

/ml)

Bac

teri

al s

urv

ival

(C

FU

/ml)

Time/min

a

b

Fig. 5 – Adsorption of E. coli in dark condition over catalyst

systems. (A) (’) TiO2, (K) hydroxyapatite (HAP), (m) 5 wt.%

TiO2/Hb, (.) 5 wt.% TiO2/HAP and (B) (’) ATI, (K) l wt.%

Ag–HAP, (m) 5 wt.% ATI/HAP. Photocatalyst ¼ 0.75 g/l.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Bac

teri

al s

urv

ival

(C

FU

/ml)

Time/min

0 50 100 150 200 250 300

Bac

teri

al s

urv

ival

(C

FU

/ml)

Time/min

101

102

103

104

105

106

107

101

102

103

104

105

106

107

a

b

Fig. 6 – Inactivation of E. coli under UV illumination over

catalyst systems. (A) (’) TiO2, (K) HAP, (m) 5 wt.% TiO2/Hb,

(.) 5 wt.% TiO2/HAP and (B) (’) ATI, (K) 1 wt.% Ag–HAP, (m)

5 wt.% ATI/HAP. Photocatalyst ¼ 0.75 g/l.

WA T E R R E S E A R C H 4 1 ( 2 0 0 7 ) 3 7 9 – 3 8 6384

does not exist with Hb zeolite support. Thus adsorption

and photocatalytic property of HAP (called as sense

and shoot approach) for the bactericidal effect is found

to be high. Therefore it is clearly seen the bactericidal

activity is enhanced in HAP and TiO2-supported HAP

combination.

The results in Fig. 6(b) indicate that photocatalytic activity

of titania is enhanced by doping with Ag. This effect is due to

the enhancement of OH radicals which are produced by the

presence of Ag atom on TiO2 (Herrmann et al., 1988; Arabatzis

et al., 2003). The pictorial representation of AT1/HAP-

supported system and the plausible mechanism for bacter-

icidal mode of activity is shown in Fig. 7(b). The actual

mechanism of Ag present on titania under UV light is due to

higher Fermi level position of titania over Ag. The electron

transfer from TiO2 to the metallic Ag particles coated on TiO2

results in a space charge layer at the boundaries between Ag

and TiO2. Thus Ag can help the electron–hole separation by

attracting the photoelectrons:

ðTiO2Þ þ hn! e� þ pþ,

ðAgÞ þ e�2e�Ag.

This enables the valency band photogenerated holes that

are free to react with OH� adsorbed on to the TiO2 to create

hydroxyl radicals (dOH), which are able to degrade the

surrounding adsorbed bacteria:

OH� þ pþ ! OHd,

OHdþ E:coli ðcultivableÞ ! E:coli ðnoncultivableÞ.

In the case of Ag, it can also improve the quantum yield by

accelerating the removal and transfer of electrons from the

catalyst particles to the molecular oxygen to form superoxide

radicals. The superoxide radicals undergo further reactions to

form hydroxyl radicals via the formation of hydrogen

peroxide that is used for the oxidation of bacterial pollutants.

(Vamathevan et al., 2004; Zhang et al., 2005). In case of AT1/

HAP there is an increase in the availability of bacteria for the

photocatalytic activity of AT1 in view of the adsorption

capacity of HAP and also due to the photocatalytic activity

of HAP. The vacancies formed on the surface of excited PO43�

group in UV illumination will lead to the formation of O2d� and

attack the surrounding bacteria adsorbed on HAP (Nishikawa,

2004; Nakamura et al., 2000; Teraoka et al., 2000). The

cumulative bactericidal activity is more in the case of AT1/

HAP where complete 100% inactivation is achived within

2 min. But, in the case of Ag–HAP the bactericidal activity is

not present in view of the absence of TiO2 and hence the

overall performance of photocatalytic bactericidal is found to

be less and the inactivation rate observed is within 20 min.

Thus, AT1/HAP is found to be a highly efficient photocatalyst

ARTICLE IN PRESS

Photocatalytic behaviorE. coli

Ag

TiO2

PO43-

PO43- O2

O2•-

O2•-

O2•-

OH•

OH-

h+ e-

e-

e-

Vacant Oxygen

UV

Hydroxyapatite Hydroxyapatite

a b

Fig. 7 – The pictorial representation of UV-illuminated (a) HAP for photocatalytic behavior and (b) 5 wt% ATI supported HAP for

bactericidal mode of activity.

WAT ER R ES E A R C H 41 (2007) 379– 386 385

and it can be easily prepared and it is a maintenance-free

adsorbent that can be applied in the inactivation of E. coli

bacteria from drinking water. The work is in progress to

further develop the practical application of the present

photocatalyst containing sense and shoot property during

water treatment.

4. Conclusions

HAP was prepared by co-precipitation and 1 wt% Ag–titania

(AT1), 1 wt% Ag–HAP and 5 wt% AT1/HAP catalysts were

prepared by wet impregnation method. The results of XRD

and ESCA reveal that titania is present in the form of Ti4+ and

silver as Ag0 in all the catalysts prepared. IR analysis of UV-

irradiated HAP confirmed the photocatalytic activity contri-

bution is due to the changes of surface PO43� group. ESCA

analysis reported that the Ag/Ti ratio is 0.0273 in AT1 and

5 wt% AT1 loaded HAP showed improved photocatalytic

activity. The SEM analysis of the fresh, dark and UV-exposed

samples revealed that Ag–TiO2 supported on hydroxyapatite

catalyst seems to be promising for the complete inactivation

of E. coli. In SEM photographs a good amount of bacterial

adsorption on HAP, Ag–HAP, AT1/HAP and no adsorption over

Ag–TiO2 are observed. It is concluded that photocatalytic

bactericidal activity is achieved in the combination system

AT1/HAP due to (i) Ag effect (ii) synergistic effect of Ag–TiO2,

and (iii) sense and shoot property of HAP.

Acknowledgments

One of the authors (MPR) acknowledge CSIR-New Delhi for

‘‘SRF’’ grant. The authors thank Dr. P.N. Sarma, Biochemical

and Environmental Engineering Center (BEEC) for extending

the laboratory facilities.

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