Indian Journal of Chemical Technology
Vol. 18, May 2011, pp. 169-176
Solar photocatalytic detoxification of cyanide with bacterial disinfection by
oxide ceramics
C Karunakaran*, P Gomathisankar & G Manikandan
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
Received 8 June 2010; accepted 1 March 2011
Under natural sunlight (975±25 Wm-2), CeO2 catalyzes the oxidation of cyanide to cyanate in alkaline medium and the
photokinetic behaviour of the reaction is similar to those on TiO2 and ZnO surfaces; the oxidation follows the LangmuirHinshelwood kinetics on cyanide ion, and depends on the area of the catalyst bed and dissolved oxygen. However, the
adsorption of cyanide on the oxides in dark is too small to be measured analytically and detected spectroscopically.
Although the photocatalytic efficiencies under identical solar irradiance are of the order: CeO2 < ZnO < TiO2, which is in
accordance with the charge-transfer resistance and capacitance of the oxides, CeO2 and ZnO show bactericidal activity as
well. At a loading of 0.8 gL-1 they inactivate 40 and 44% of 2.5 × 1012 CFU mL-1 E. coli in 30 min even without direct light.
Keywords: Photocatalysis, Semiconductor, Charge-transfer resistance, Capacitance, Bactericidal activity
Cyanide ion is a highly toxic pollutant and originates
from electroplating and metal finishing shops.
Although dissolved cyanide can be removed by
physical, chemical and biological methods, the
physical methods involve only transfer cyanide to
another phase. The very slow biodegradation is
limited to low concentration of cyanide and
encounters the difficulty of disposal of activated
sludge. Alkaline chlorination is the best available
chemical method but the limitations are the formation
of highly toxic cyanogen chloride gas and the leftover
complex metal cyanide in sludge. A promising
method of detoxification of cyanide is a
photocatalytic one and reports on TiO2 or ZnOcatalyzed oxidation of cyanide with artificial UV light
are many1-5. Illumination of nanocrystalline
semiconductor with light of energy not less than the
band gap generates electron-hole pairs, electrons in
the conduction band and holes in the valence band6,7.
Some of these pairs diffuse to the crystal surface and
react with the adsorbed electron donors and acceptors
resulting in photocatalysis. The hole oxidizes the
substrates and the adsorbed oxygen molecule accepts
the electron and transforms into highly active
superoxide radical (O2•–). In presence of moisture,
O2•– generates reactive species like HO•, HO2•, and
H2O2, which act as oxidants. Water is adsorbed on the
semiconductor surface, molecularly and dissociatively.
——————
*Corresponding author (E-mail: [email protected])
Hole-trapping by either the surface hydroxyl groups or
the adsorbed water molecules produces short-lived HO•
radicals, which are the primary oxidizing agents.
Natural sunlight is free of cost and tapping the same for
detoxification is the objective of semiconductorcatalyzed environmental remediation. Studies on
semiconductor-catalysis with natural sunlight are rare,
probably due to the fluctuation of solar irradiance. The
problem of fluctuation of sunlight intensity during the
experiment has been overcome by exposing the
semiconductors to identical solar irradiance by carrying
out the experiments simultaneously, side by side. The
simple experimental set-up and the easy separation of
the particulate semiconductors make the technique
highly adoptable for the electroplating and metal
finishing shops. Further, microbial contamination and
growth in water are potential health hazards
necessitating disinfection. In recent years, the use of
inorganic antimicrobial agents, compared to the
organics, has attracted interest due to their improved
safety and stability8,9; ceramics with inherent
antimicrobial activity are convenient to employ as they
are insoluble. The present results reveal that CeO2,
besides ZnO, exhibits antimicrobial activity; unlike
ZnO, CeO2 is stable in acidic and basic media10. Hence,
it is a two-in-one advantage, photocatalyzed
detoxification of cyanide with bacterial disinfection;
the bactericidal activity in this study has been
assessed using Escherichia coli (E. coli) bacteria as
the index.
170
INDIAN J. CHEM TECHNOL., MAY 2011
Experimental Procedure
Characterization
The oxides used were supplied by Merck and their
powder XRDs were recorded using a Siemens D-5000
XRD employing Cu-Kα X-rays of wavelength 1.5406 Å
under a scan range of 5-60° with a scan speed of
0.2° s-1 or a Bruker D8 system using Cu-Kα radiation
of 1.5406 Å in a 2θ range of 5-60° at a scan speed of
0.050° s-1 or with a PANalytical X’Pert PRO
diffractometer using Cu-Kα rays at 1.5406 Å in a
2θ range of 15-75º with a scan rate of 0.020º s-1. The
specific surface areas of the oxides were determined
by the nitrogen-adsorption desorption method using
the Brunauer-Emmett-Teller (BET) equation. A
Perkin-Elmer Lambda 35 spectrophotometer was
employed to record the UV-visible diffuse reflectance
spectra of the oxides. The impedance spectra of the
oxides were obtained using a HP 4284A Precision
LCR meter over the frequency range of 1 MHz to
20 Hz at room temperature in air; the disk area was
0.5024 cm2 and the thicknesses of CeO2, ZnO and
TiO2 pellets were 3.8, 1.87 and 2.98 mm, respectively.
An Avatar 330FT-IR spectrometer was used to record
the Fourier-transform infra-red (FTIR) spectra of the
semiconductors.
Photocatalytic studies
The semiconductor-catalyzed solar photooxidation
was carried out with AM 1 sunlight under clear sky in
summer (March-July). The sunlight intensity was
measured using a Daystar solarmeter (USA). Fresh
solutions of KCN (25 mL) of required ppm of pH
12.5 (using NaOH) were air-saturated and taken in
wide cylindrical glass vessels of uniform diameter.
The entire bottom of the vessel was covered with the
catalyst powder forming the catalyst bed which was
not disturbed till the completion of the experiment.
The dissolved oxygen was determined using an
Elico dissolved oxygen analyzer PE 135 and the
cyanide ion was estimated argentometrically using
p-dimethylaminobenzylidene rhodanine as the
indicator11,
before
and
after
illumination.
Spectrophotometric determination of cyanide ion at
590 nm by complexing it with ninhydrin in alkaline
medium12 also provided identical results. Cyanide ion
analysis using cyanide ion selective electrode with
Ag/AgCl electrode as the reference was also in
agreement with the other two methods. Cyanate was
analyzed spectrophotometrically5. The decrease of
cyanide concentration for a finite time of illumination
provided the oxidation rate and under identical solar
irradiance the rates were reproducible to ±5%.
Calcinations were made in a muffle furnace fitted
with PID temperature controller and the heating rate
was set at 10°C min-1 uniformly for all the oxides.
Further, all the samples were calcined at the desired
temperatures for 2 h. Acid treated TiO2 were prepared
by stirring continuously 2 g of TiO2 in 50 mL of 5 N
sulfuric or hydrochloric or nitric or phosphoric or
acetic acid for 24 h, separating the oxide and
calcining at 150 or 450°C.
Bactericidal study
A nutrient broth culture medium of pH 7.4 was
prepared by dissolving 13.0 g nutrient broth (5.0 g
peptone, 5.0 g NaCl, 2.0 g yeast extract, 1.0 g beef
extract) in 1 L distilled water followed by sterilization
in an autoclave at 121°C. MacConkey agar plates
were prepared separately by dissolving 55 g
MacConkey agar (20 g peptic digest of animal tissue,
10 g lactose, 5 g sodium taurocholate, 0.04 g neutral
red, 20 g agar) in 1 L boiling distilled water followed
by sterilization in an autoclave at 121°C and poured
into Petri dish. E. coli bacteria were inoculated in 10
mL of a nutrient broth and incubated for 24 h at 37°C.
The cultured bacteria were centrifuged at 3500 rpm
for 5 min before washing with an autoclaved 0.9%
NaCl solution twice and suspended in 50 mL of an
autoclaved 0.9% NaCl solution. For the counting of
E. coli colonies in CFU mL-1, the bacterial solution
was successively diluted to109 times using 0.9% NaCl
solution in order to achieve about 100 to 200 colonies
on the Petri dish; using a loop, 10 µL of the diluted
E. coli was streaked on the MacConkey agar plate and
incubated at 37°C for 24 h. The CFU was counted by
a viable count method. The bactericidal studies were
made as follows. 20 mg of the oxide was added to
25 mL of E. coli solution taken in a 150 mL bottle and
shaken well continuously without any illumination. At
the required time interval, a finite volume of E. coli
solution was removed from the oxide, diluted stepwise and enumerated as already stated.
Results and Discussion
Catalyst characterization
The XRD of CeO2 matches with the standard
JCPDS pattern (898436, face centered cubic)
confirming its crystal structure. The XRD pattern of
the TiO2 used is identical with the standard pattern of
anatase (JCPDS 01-078-2486 C, tetragonal, body-
171
KARUNAKARAN et al.: SOLAR PHOTOCATALYTIC DETOXIFICATION OF CYANIDE
centered) and the rutile lines (01-089-0553 C) are
absent. This clearly reveals that the TiO2 employed in
this study is of anatase phase. The XRD of ZnO is
that of the JCPDS pattern of zincite (00-005-0664 D,
hexagonal, primitive). The specific surface areas (S),
determined by the BET nitrogen-adsorption
desorption method, are presented in Table 1. The
average particle sizes (D) of the oxides, deduced
using the relationship D = 6/ρS, where ρ is the
material density, are also listed in Table 1. The UVvisible diffuse reflectance spectra of the oxides show
that they require UV-A light for excitation (spectra
not shown). Electrochemical impedance spectroscopy
(EIS) is a relatively new and powerful tool to probe
the electrical properties of semiconductors13. It could
be used to investigate the dynamics of the mobile and
bound charges in the interfacial or bulk region of the
semiconductors. In polycrystalline materials, the
overall sample resistance may be a combination of the
intragranular or bulk crystal resistance and
intergranular or grain boundary resistance. Generally,
the impedance data are analyzed in terms of an
equivalent circuit model; an electrode interface
undergoing an electrochemical reaction is typically
analogous to an electric circuit consisting of a specific
combination of capacitors and resistors. By fitting the
EIS data to a model or an equivalent circuit the
electrical properties of the semiconductors could be
inferred. The impedance spectra (not given) show
decrease of impedance with increase of frequency
indicating the capacitance of the semiconductors
employed. The Nyquist plot, a popular format of
evaluating the impedance data, is presented in Fig. 1.
The ohmic or uncompensated resistance (RΩ)
corresponds to the grain boundary or intergranular
resistance and the polarization or electron-transfer
(charge-transfer) resistance (RP or RCT) refers to the
intragranular or bulk crystal resistance; the latter is
related to the Warburg resistance, the resistance to
mass transfer, which is controlled by the specific
conductance, σ. The constant phase element, CPE,
can be associated with a non-uniform distribution of
current due to material heterogeneity and is equivalent
to a double layer capacitance, C. The Nyquist plot of
TiO2, compared to those of ZnO and CeO2, displays a
very small charge-transfer resistance with a very large
frequency region where mass transfer is a significant
factor, thus suggesting TiO2 to be kinetically facile;
the semiconductor may be more photocatalytically
active than others. The Nyquist plots of CeO2 and
ZnO display large regions of mass-transfer and kinetic
Table 1—Band gap (Eg), BET surface area (S), particle size (D),
ohmic (RΩ) and charge-transfer (RCT) resistances, specific
conductance (σ) and capacitance (C)
Oxide
Eg
(eV)
S
(m2 g-1)
D
(nm)
RΩ
(kΩ)
RCT
(MΩ)
CeO2
ZnO
TiO2
2.89
3.15
3.18
11.0
12.2
14.7
76
87
104
754
6.13
56.8
14.9
7.26
0.0579
C
σ
(µS m-1) (pF)
5.08
5.13
1020
5.34
7.31
13.7
Fig. 1—The Nyquist plots
control at low and high frequencies, respectively.
Further, the Nyquist plots reveal the presence of
appreciable (but not large or insignificant) series
capacitance in the equivalent circuit and the Warburg
impedance is also important. Table 1 presents the
determined specific conductance, capacitance and the
ohmic and charge-transfer resistances of the oxides;
the specific conductance has been deduced from the
measured charge-transfer resistance and the
capacitance has been obtained using the equations
ωmax = 1/CRCT and ωmax = 2πf, where f is the
frequency corresponding to the top of the semicircle
of the Nyquist plot. The specific conductance and
capacitance are of the order CeO2 < ZnO < TiO2.
Figure 2 presents the truncated FTIR spectra of the
semiconductors as such, i.e., before use, exposed to
the atmosphere only. TiO2 and ZnO show strong
absorption around 3430 cm-1 indicating abundant
adsorption of water molecules on the oxide surface.
However, the corresponding spectrum of CeO2 does
not exhibit such absorbance implying insignificant
adsorption of water molecules on its surface.
Cyanide photodetoxification
CeO2, like TiO2 and ZnO, catalyzes the oxidation
of cyanide ion under natural sunlight at pH 12.5 (vide
172
INDIAN J. CHEM TECHNOL., MAY 2011
Fig. 2—The FTIR spectra
infra for the experimental conditions). The pKa of
HCN is 9.3 and to avoid its liberation into the
atmosphere the detoxification was carried out in a
highly alkaline medium. Under identical conditions
cyanide ion is not oxidized in the absence of any of
the oxides. Spectrophotometric analysis of cyanate in
the cyanide solution illuminated with CeO2 or ZnO or
TiO2 reveals formation of cyanate at the expense of
cyanide and conforms to the carbon balance ([CN–]0 =
[CN–] + [OCN–]). This is in agreement with the earlier
studies using TiO2; the nitrate generation due to
further oxidation of the cyanate formed is
insignificant till the cyanide is completely oxidized to
cyanate4,5. It has been stated that the adsorption sites
of cyanide and cyanate are the same and cyanide gets
adsorbed in preference to cyanate resulting in the
commencement of the cyanate oxidation only after the
near complete oxidation of cyanide.
Even under clear sky in summer the sunlight
intensity fluctuates during the course of experiment
(975±25 Wm-2). Also, it varies from day-to-day. To
obtain analyzable solar results the solar irradiance in a
set of experiments was kept identical by carrying out
the experiments simultaneously, side by side. The
solar results obtained with CeO2 or ZnO or TiO2 are
consistent (vide infra for the experimental conditions).
For each catalyst, a couple of experiments under
identical
reaction
conditions
carried
out
simultaneously with sunlight yield results within
±5%. Carrying out a control experiment along with
others under the required experimental conditions
simultaneously makes possible the comparison of the
solar results.
Fig. 3—Cyanide photooxidation on CeO 2, ZnO and TiO2, The
Langmuir-Hinshelwood kinetics. Catalyst bed = 16.7 cm2, catalyst
loading = 0.50 g, pH = 12.5, [O2]dissolved = 26.8 ppm, cyanide solution
= 25 mL. Each catalysis corresponds to identical solar irradiance
Photokinetic characteristics
The photokinetic characteristics of the CeO2, ZnO
and TiO2-catalyzed oxidation were deduced
individually. The solar irradiance in each study was
kept identical by carrying out a set of experiments
simultaneously. Under identical solar irradiance, the
cyanide oxidation rates increase with its concentration
as shown by Fig. 3. The variation of cyanide-removal
rates with its concentration is characteristic of the
Langmuir-Hinshelwood kinetics14. However, the
adsorption of cyanide ion on CeO2, ZnO and TiO2 in
dark at pH 12.5, even at the highest concentration of
cyanide ion employed, is too small to be measured
analytically. Also, the infrared spectra of the dried
catalysts, prior to illumination but after allowed to
attain equilibrium with cyanide solution, fail to show
characteristic absorbance of cyanide (~2250 cm-1)
indicating insignificant adsorption of cyanide ion on
the semiconductors (spectra not shown). The removal
of cyanide in 2½ h from 25 ppm solution, under
the conditions given in Fig. 3, with CeO2, ZnO and
TiO2 are 24%, 52% and 68%, respectively. The
photooxidation rates show linear dependence on the
apparent surface area of the catalyst bed (Fig. 4). This
is also in accordance with the Langmuir-Hinshelwood
model. In 2½ h, under the conditions stated in Fig. 4,
the percentages of cyanide oxidation with 50.0 cm2bed of CeO2, ZnO and TiO2 are 48, 60 and 92,
respectively. CeO2, ZnO and TiO2 show sustainable
KARUNAKARAN et al.: SOLAR PHOTOCATALYTIC DETOXIFICATION OF CYANIDE
Fig. 4—Cyanide photooxidation as a function of catalyst bed
area, Catalyst loading = 0.50 g, pH = 12.5, [CN–]0 = 100 ppm,
[O2]dissolved = 26.8 ppm, cyanide solution = 25 mL. Each catalysis
corresponds to identical solar irradiance
photocatalytic activities. Recycling the catalysts
without any pre-treatment provides identical results
(conditions as in Fig. 3 with 100 ppm cyanide
solution). Cyanide detoxification requires dissolved
oxygen. The photooxidation of cyanide does not
occur in nitrogen-purged solution (conditions as in
Fig. 3 but with 100 ppm cyanide and 1.8 ppm of
dissolved O2). Initial addition of acryl amide (10%) to
the cyanide solution exposed to sunlight along with
CeO2 or ZnO or TiO2 leads to polymer formation
indicating the involvement of free radical in the
photooxidation process. Nitrate (0.01M), sulfate
(0.01M) and phosphate (0.01M) do not interfere
in the semiconductor-catalyzed cyanide oxidation
process. In the case of reducing ions, sulfite (0.01M)
inhibits the photooxidation whereas nitrite (0.01M)
does not. Also, the photooxidation is suppressed
by citrate (0.01M) but not by oxalate (0.01M). Azide
ion (0.01M) also slows down the photooxidation; the
reaction conditions for all the stated experiments
are as given in Fig. 3 with 100 ppm cyanide.
Generally, sonication modifies the surface and
particle size of the catalyst and the photocatalytic
activity is susceptible to the surface and size
modification of semiconductor particles15. But,
pre-sonication for 10 min at 37±3 kHz and 100 W
does not influence the solar detoxification of cyanide
by all the three semiconductors studied; the
experimental conditions remain the same. However,
the degradation of salicylic acid on Hombikat TiO2
under UV light enhances on sonication16. Generally,
the photocatalytic activity of the semiconductor
173
depends on its sintering temperature17. But,
calcination of the oxides studied does not enhance the
photodetoxification of cyanide. While the catalytic
efficiencies of ZnO and CeO2 calcined at 300, 400,
500, 600, 700, 800 and 900°C and that of TiO2 at 300,
400, 500, 600 and 700°C are not different from those
of the uncalcined samples TiO2 calcined at 800
and 900°C show lower catalytic efficiencies, the
former by about 70% and the latter by about 50%
of the uncalcined sample (16.7 cm2-catalyst bed,
0.10 g-catalyst loading, 12.5 pH, 26.8 ppm-dissolved O2,
25 mL 250 ppm-cyanide solution, 2½ h-sunshine); it is
known that at these temperatures photocatalytically
active anatase transforms into less active rutile.
Sulfuric or nitric or hydrochloric or phosphoric or
acetic acid-treatment of TiO2 does not modify the
photocatalytic activity (conditions remain the same).
Mixed semiconductors enable a vectorial transfer of
electrons and holes from one semiconductor to
another and hence show enhanced activity18,19. But,
the 1:1 (w/w) mixture of TiO2 and CeO2 or TiO2 and
ZnO or ZnO and CeO2 does not exhibit improved
photocatalysis under the conditions stated already.
The experiments show that the adsorption of
cyanide ion in dark on CeO2, ZnO and TiO2 is too
small to be determined analytically or detected
spectroscopically. In alkaline medium adsorption of
HO– on semiconductor results in negatively charged
surface which is the reason for the insignificant
adsorption of cyanide ion on the semiconductors. But,
the title study exhibits Langmuir-Hinshelwood
kinetics (Fig. 3) which is based on the adsorption of
cyanide ion on the oxides. Hence, it is possible that
cyanide ion gets adsorbed significantly on the
illuminated
photoactive
surfaces
of
the
semiconductors. However, the fact that acryl amide
gets polymerized during the semiconductor-catalyzed
oxidation of cyanide in aqueous alkaline solution
indicates the involvement of radicals such as HO• in
the cyanide oxidation; the detailed mechanism of
cyanide photooxidation on semiconductor surface has
been discussed elsewhere3,5. A possible explanation
for the inhibition of cyanide oxidation (i) by sulfite
ion but not by nitrite ion and (ii) by citrate ion but not
by oxalate ion is the site of adsorption. Sulfite and
citrate ions may compete with cyanide ion for
adsorption at the same site on the illuminated oxides
while the adsorption sites of oxalate and nitrite ions
could be different. Probing the adsorption of citrate
and oxalate on the catalysts supports the proposition.
The FTIR spectral studies reveal adsorption of oxalate
174
INDIAN J. CHEM TECHNOL., MAY 2011
and citrate ions on the surfaces of CeO2, ZnO and
TiO2. The IR spectra of CeO2, ZnO and TiO2 (not
shown), recovered from citrate solution and dried
(0.50 g catalyst from 25 mL 0.01 M solution), show
strong absorption at 1579, 1589 and 1587 cm-1,
respectively; the corresponding frequencies for the
catalysts recovered from oxalate solution are 1639,
1641 and 1649 cm-1. While the adsorption of oxalate
on CeO2 is insignificant that on TiO2 as well as ZnO
is 40%; citrate adsorbs on all the three oxides to 54%.
Lack of inhibition by nitrate, sulfate and phosphate
suggest that these ions do not compete with cyanide
for adsorption on the catalyst surface. Absence of
enhanced photocatalysis in semiconductor mixtures
may be due to strong adsorption of water molecules
on the semiconductor surfaces. The primary sheath of
water molecules around the semiconductor particles
could prevent physical contact between adjacent
semiconductor particles thus hindering inter-particle
charge transfer.
The Langmuir-Hinshelwood kinetic model is
applicable to semiconductor-photocatalysis and the
corresponding kinetic equation is20:
rate = kK1K2IS[CN–][O2]/(1 + K1[CN–])(1 + K2[O2])
where K1 and K2 are the adsorption coefficients of
cyanide ion and molecular oxygen on the illuminated
surface of the catalyst, k is the surface pseudo-firstorder rate constant, S is the surface area of the catalyst
and I is the light intensity in Einstein m-2 h-1. The
cyanide solutions are oxygen-saturated and the
dissolved oxygen concentration remains almost
constant during the photooxidation. Since K2 is a
constant for a given catalysis K2[O2]/(1 + K2[O2])
turns to be a constant for a given catalyzed oxidation.
For a set of experiments conducted simultaneously I
is a constant. The Langmuir-Hinshelwood kinetic
model holds good as seen from the data fit to the
curve governed by the kinetic equation and drawn
using a computer program. The kinetic parameters
deduced from the fit are presented in Table 2.
Physicochemical properties and photodetoxification
Experiments carried out simultaneously, side by
side, under identical conditions and solar irradiance
reveal the relative cyanide oxidation rates on TiO2,
Table 2—The kinetic parameters
Kinetic parameter
103K1 (ppm-1)
kK2I/(1 + K2[O2]) (m-2 h-1)
CeO2
ZnO
TiO2
4.5
487
11.6
462
11.9
692
ZnO and CeO2 surfaces as 1.0, 0.7 and 0.4,
respectively. The photocatalytic efficiency depends
on the crystal size; smaller the size, higher is the
reactivity. In smaller crystals the photoinduced holes
and electrons have to diffuse shorter distance to reach
the interface. Consequently, the holes and electrons
can be effectively captured by the substrates in the
solution. However, in the title study the particle size is
of the order CeO2 < ZnO < TiO2 and so is the
photocatalytic activity. This indicates that some other
factor overrules this effect. The solar detoxification
has been carried out by temporarily immobilizing the
semiconductor powder in the form of a catalyst bed
and the observed photocatalytic efficiencies are not in
strict compliance with the BET surface area. While
the relative photocatalytic activities of TiO2, ZnO and
CeO2 under identical solar irradiance are 1.0, 0.7 and
0.4, respectively, the corresponding relative BET
surface areas are 1.0, 0.83 and 0.75. Hence, some
other factor influences strongly the photocatalytic
activities of the oxides. The solar detoxification has
been carried out in highly alkaline medium (pH 12.5)
and hence the point of zero charge (PZC) of the
semiconductor is of no significance; the PZC of
CeO221, and ZnO and TiO222 are 8.1, 8.8 and 5.8,
respectively. Adsorption of water molecules may also
be a reason for the observed low catalytic activity of
CeO2; the FTIR spectra of the oxides show
insignificant adsorption of water molecules on CeO2.
Although the band gap energies of TiO2 and ZnO do
not differ significantly the former is more
photocatalytically active than the latter. The
adsorption of water and hydroxyl groups may also be
an explanation; anatase TiO2 actively adsorbs water
and hydroxide ion23 and the photocatalytic activity
depends on the surface-adsorbed water and hydroxyl
group. However, with xenon lamp ZnO is reported to
be more efficient than TiO224.
The observed solar photocatalytic activities of the
oxides are in accordance with the determined chargetransfer resistance, capacitance and the specific
conductance of the semiconductors; while the
electron-transfer resistance is of the order CeO2 >
ZnO > TiO2, the capacitance and the specific
conductance are as follows: CeO2 < ZnO < TiO2
(Table 1). A recent EIS study on the Ag/TiO2photocatalyzed degradation of rhodamine B also
reveals a similar relationship between the
photocatalytic activity and the charge-transfer
resistance as well as the capacitance25. Of the 0.5% to
7% Ag-doped TiO2 studied, the most active 3% Ag-
KARUNAKARAN et al.: SOLAR PHOTOCATALYTIC DETOXIFICATION OF CYANIDE
doped TiO2 shows the lowest charge-transfer
resistance and highest capacitance. A similar study on
the photodegradation of acid red 44 also supports
such
relationships26.
While
the
observed
photocatalytic activity is of the order TiO2
nanospheres < TiO2 nanorods < Ag-TiO2 nanorods,
the measured charge-transfer resistance is as follows:
TiO2 nanospheres > TiO2 nanorods > Ag-TiO2
nanorods; the determined capacitance is in the order
of observed photocatalytic activities.
Table 2 shows that the adsorption of cyanide ion on
the illuminated surfaces of TiO2 and ZnO are not
significantly different but that on CeO2 is much less.
This may be one of the reasons for the slow
photodetoxification of cyanide on CeO2. The solar
irradiances, monitored during the course of cyanide
detoxification, on CeO2, ZnO and TiO2 do not differ
remarkably and hence, the variation in the kK2I/(1 +
K2[O2]) values of TiO2 and ZnO may be taken as that
of kK2/(1 + K2[O2]); the dissolved O2-concentration
remains unaltered for all the three photocatalysis.
Since the kK2I/(1 + K2[O2])-value of TiO2 is larger
than that of ZnO the combined processes of (i) O2adsorption on the illuminated surface, and (ii) the
cyanide oxidation on the surface are more feasible
with TiO2 than with ZnO. The kK2I/(1 + K2[O2])values of CeO2 and ZnO do not differ significantly.
Hence, the combined processes of adsorption of O2
and the cyanide oxidation on the illuminated surface
on ZnO and CeO2 are equally feasible.
Bactericidal activity
Figure 5 shows the E. coli disinfection by CeO2
and ZnO in aqueous suspension without any
illumination. The antibacterial efficiencies of the
oxides in a population of 2.5 × 1012 CFU mL-1 in
30 min are 40 and 44%, respectively. In absence of
the oxides the E. coli population remains unaffected
during the experimental period revealing the
Fig. 5—E. coli inactivation by CeO2 and ZnO without illumination in
30 min, oxide loading = 0.8 g L-1, pH = 7.4
175
bactericidal activity of ZnO and CeO2. However,
experiments with TiO2 under identical conditions do
not show any significant killing of the bacteria. E. coli
bacteria in 0.9% saline were used for the evaluation of
the bactericidal activity. The cell population was
determined by a viable count method on MacConkey
agar plates after proper dilution of the culture. The
antibacterial efficiency is the ratio of the decrease of
E. coil population at a given time to its initial
population; antibacterial efficiency = 100 (C0 – Ct)/C0,
where Ct and C0 are the E. coli population at time t
and zero, respectively.
Although there are a few reports on the inactivation
of E. coli by ZnO nanoparticles and bulk particles in
absence of any illumination the mechanism of action
still remains obscure8,9,27-29. In spite of the detected
release of Zn2+ from ZnO suspension8,27,29
examination of E. coli inactivation by added Zn2+ ions
separately shows that the Zn2+ ion release from the
oxide is not the reason for the E. coli disinfection8,29.
Some workers have also detected the generation of
H2O2 by ZnO in absence of light27,29 and have
suggested the same as a main factor for the bacterial
activity27-29. But the observed extent of inactivation of
E. coli population in the present study (vide supra)
makes the proposition unviable. As the antibacterial
activities of some synthesized ZnO nanorods do not
correlate with their photoluminescence features
corresponding to surface defects (green emission) it
has been concluded that the surface defects do not
play any significant role in the antibacterial activity of
ZnO27. The SEM and TEM images displayed in a few
reports reveal the binding of ZnO nanoparticles to the
surface of E. coli bacteria8,27,28. There are several
possible attachment mechanisms of the oxide to the
bacterial surface: electrostatic forces, van der Waals
forces and receptor-ligand interactions8. The zeta
potential of ZnO suspended with E. coli at pH ~ 7 is
approximately +24 mV28. The overall E. coli surface
is negatively charged at neutral pH due to
the polysaccharides of lipopolysaccharide, which
predominate over the amide28, and its potential is 7.2 mV at pH 6.58. Hence, the interaction of ZnO with
E. coli is favored by the electrostatic forces. The
receptor-ligand interactions may dominate in the ZnO
bound E. coli bacteria; carboxyl, amide, phosphate,
hydroxyl groups and carbohydrate-related moieties in
the bacterial cell wall may provide sites for the
molecular-scale interactions with the oxide8.
Operation of similar attachment mechanism is likely
176
INDIAN J. CHEM TECHNOL., MAY 2011
with CeO2. It has been reported that ZnO causes
membrane damage, which leads to leakage of cell
contents and cell death27. Further, only a few E. coli
cells show internalization of ZnO nanoparticles while
a large number of them do not exhibit such thing
revealing that internalization is not a general
occurrence27. The TEM displays also shrinkage of
cytoplasmic material inside the cell wall of some of
the E. coli cells27. The interaction of oxide
nanoparticles with the bacterial cell walls and possible
permeation of the nanoparticulate oxide into the
bacterial cells are still not clearly understood8. It has
been proposed that the induction of intracellular
oxidative stress seems to be a key event of the toxicity
mechanisms of many nanomaterials9. Once inside the
cell, nanomaterials may induce intracellular oxidative
stress by disturbing the balance between oxidant and
anti-oxidant processes. On one hand, the oxidative
stress induced by exposure to nanomaterials may
stimulate an increase of the cytosolic calcium
concentration or may cause the translocation of
transcription factors to the nucleus, which regulate
pro-inflammatory genes. Alternatively, exceeding
oxidative stress may also modify proteins, lipids and
nucleic acids, which further stimulates the antioxidant defense system or even leads to cell death.
Conclusions
With natural sunlight, CeO2 catalyzes the oxidation
of cyanide ion to cyanate ion and its photocatalytic
behaviour is similar to those of ZnO and TiO2
anatase; follows the Langmuir-Hinshelwood kinetics
on cyanide, and depends on the area of the catalyst
bed and dissolved O2. However, the adsorption of
cyanide ion on CeO2, ZnO and TiO2 in dark is
insignificant. Although the solar photocatalytic
efficiencies are of the order: CeO2 < ZnO < TiO2,
which is in accordance with charge-transfer resistance
and capacitance, CeO2 and ZnO exhibit bactericidal
activity in absence of direct light itself.
Acknowledgement
The authors thank the Council of Scientific and
Industrial Research (CSIR), New Delhi, for the
financial support through research grant no.
01(2031)/06/EMR-II and one of the authors (PG) is
grateful to CSIR for Junior Research Fellowship.
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