hydroxyapatite-supported ag–tio2 as escherichia coli
TRANSCRIPT
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WAT E R R E S E A R C H 4 1 ( 2 0 0 7 ) 3 7 9 – 3 8 6
0043-1354/$ - see frodoi:10.1016/j.watres
$IICT Communic�Corresponding auE-mail address:
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Hydroxyapatite-supported Ag–TiO2 as Escherichia colidisinfection photocatalyst$
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 [email protected]
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).
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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
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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.
R E F E R E N C E S
Arabatzis, I.M., Stergiopoulos, T., Bernard, M.C., Labou, D.,Neophytides, S.G., Falaras, P., 2003. Silver-modified titanium
dioxide thin films for efficient photodegradation of methylorange. Appl. Catal. B: Environ. 42, 187–201.
Baker, K.H., Troy, A.M., Herson, D.S., 2000. Detection andoccurrence of indicator organisms and pathogens. WaterEnviron. Res. 88, 46–133.
Berry, E.D., Siragusa, G.R., 1997. Hydroxyapatite adherence as ameans to concentrate bacteria. Appl. Environ. Microbiol. 63,4069–4074.
Blake, D.M., Maness, P.C., Huang, Z., Wolfrum, E.J., Huang, J.,Jacoby, W.A., 1999. Application of the photocatalytic chemistryof titanium dioxide to disinfection and the killing of cancercells. Sep. Purif. Methods 28, 1–50.
Brooks, T.L., 1981. Hydroxylapatite. Calbiochem Brand Biochem-icals, San Diego, CA.
Dunlop, P.S.M., Byrne, J.A., Manga, N., Eggins, B.R., 2002. Thephotocatalytic removal of bacterial pollutants from drinkingwater. J. Photochem. Photobiol. A: Chem. 148, 355–363.
Fujishima, A., Rao, T.N., Tryk, D.A., 2000. Titanium dioxidephotocatalysis. J. Photochem. Photobiol. C: Photochem. Rev.1, 1–21.
Herrmann, J.M., Disdier, J., Pichat, P., 1988. Photocatalytic deposi-tion of silver on powder titania: Consequences for the recoveryof silver. J. Catal. 113, 72–81.
Ljubas, D., 2005. Solar photocatalysis—a possible step in drinkingwater treatment. Energy 30, 1699–1710.
Matsumura, Y., Yoshikata, K., Kunisaki, S., Tsuchido, T., 2003.Mode of bactericidal action of silver zeolite and its comparisonwith that of silver nitrate. Appl. Environ. Microbiol. 69,4278–4281.
Milella, E., Cosentino, F., Licciulli, A., Massaro, C., 2001. Prepara-tion and characterisation of titania/hydroxyapatite compositecoatings obtained by sol–gel process. Biomaterials 22,1425–1431.
Nakamura, I., Negishi, N., Kutsuna, S., Ihara, T., Sugihara, S.,Takeuchi, K., 2000. Role of oxygen vacancy in the plasma-treated TiO2 photocatalyst with visible light activity for NOremoval. J. Mol. Catal. A: Chem. 161, 205–212.
Nishikawa, H., 2004. A high active type of hydroxyapatite forphotocatalytic decomposition of dimethyl sulfide under UVirradiation. J. Mol. Catal. A: Chem. 207, 147–151 and referencestherein.
Nonami, T., Hase, H., Funakoshi, K., 2004. Apatite-coated titaniumdioxide photocatalyst for air purification. Catal. Today 96,113–118.
Noorjahan, M., Pratap Reddy, M., Durga Kumari, V., Lavedrine, B.,Boule, P., Subrahmanyam, M., 2003. Photocatalytic degradation
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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 6386
of H-acid over a novel TiO2 thin film fixed bed reactor and inaqueous suspensions. J. Photochem. Photobiol. A: Chem. 156,179–187.
Pratap Reddy, M., Subrahmanyam, M., 2006. Photocatalyticdisinfection of E. coli over titanium (IV) oxide supported onHb zeolite. Catal. Lett., in press.
Pratap Reddy, M., Venugopal, A., Subrahmanyam, M., 2006.Hydroxyapatite photocatalytic degradation of calmagite (anazo dye) in aqueous suspension. Appl. Catal. B: Environ. 69,164–170.
RincOn, A.G., Pulgarin, C., 2005. Use of coaxial photocatalyticreactor (CAPHORE) in the TiO2 photo-assisted treatment ofmixed E. coli and Bacillus sp. and bacterial community presentin wastewater. Catal. Today 101, 331–344 and referencestherein.
Sokmen, M., Candan, F., Sumer, Z., 2001. Disinfection of E. coli bythe Ag–TiO2/UV system: lipidperoxidation. J. Photochem.Photobiol. A: Chem. 143, 241–244.
Subba Rao, K.V., Subrahmanyam, M., Boule, P., 2004. ImmobilizedTiO2 photocatalyst during long-term use: decrease of itsactivity. Appl. Catal. B: Environ. 49, 239–249.
Teraoka, K., Nonami, T., Yokogawa, Y., Taoda, H., Kameyama, T.,2000. Preparation of TiO2-coated hydroxyapatite single crys-tals. J. Mater. Res. 15, 1243–1244.
Vamathevan, V., Amal, R., Beydoun, D., Low, G., McEvoy, S., 2004.Silver metallisation of titania particles: effects on photoactiv-ity for the oxidation of organics. Chem. Eng. J. 98, 127–139.
Venugopal, A., Scurrell, M.S., 2003. Hydroxyapatite as a novelsupport for gold and ruthenium catalysts behavior in thewater gas shift reaction. Appl. Catal. A: Gen. 245, 137–147.
Xu, Y., Zheng, W., Liu, W., 1999. Enhanced photocatalytic activityof supported TiO2: dispersing effect of SiO2. J. Photochem.Photobiol. A: Chem. 122, 57–60.
Zhang, X., Zhou, M., Lei, L., 2005. Preparation of an Ag–TiO2
photocatalyst coated on activated carbon by MOCVD meter.Chem. Phys. 91, 73–79.