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WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 2 7 6 – 1 2 8 6
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Treatment of textile dyehouse wastewater byTiO2 photocatalysis
Pantelis A. Pekakis, Nikolaos P. Xekoukoulotakis, Dionissios Mantzavinos�
Department of Environmental Engineering, Technical University of Crete, Polytechneioupolis, GR-73100 Chania, Greece
a r t i c l e i n f o
Article history:
Received 24 September 2005
Received in revised form
13 December 2005
Accepted 16 January 2006
Keywords:
Textile wastewater
Photocatalysis
TiO2
Catalyst reuse
Ecotoxicity
nt matter & 2006 Elsevie.2006.01.019
uthor. Tel.: +30 28210 [email protected] (D.
A B S T R A C T
The oxidative degradation of an actual textile dyehouse wastewater was investigated by
means of photocatalysis in the presence of TiO2. The UV-A-induced photocatalytic
oxidation over TiO2 suspensions was capable of decolorizing the effluent completely, as
well as reducing chemical oxygen demand (COD) sufficiently (COD reduction generally
varied between about 40% and 90% depending on the operating conditions) after 4 h of
treatment. Two crystalline forms of TiO2, viz. anatase and rutile, were tested for their
photocatalytic activity and anatase was found to be more active than rutile. The extent of
photocatalytic degradation was found to increase with increasing TiO2 concentration up to
0.5 g/L TiO2, above which degradation remained practically constant, reaching a plateau.
Furthermore, textile effluent degradation was enhanced at acidic conditions (i.e. pH ¼ 3)
and in the presence of hydrogen peroxide. To assess catalyst activity on repeated use,
experiments were performed where the catalyst was recovered and reused; after three
successive uses, TiO2 had sufficiently retained its photocatalytic activity. Finally, the
luminescent marine bacteria Vibrio fischeri was used to assess the acute ecotoxicity of
samples prior to and after the photocatalytic treatment and it was found that ecotoxicity
was fully eliminated following photocatalytic oxidation.
& 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Textile manufacturing involves several processes (e.g. sizing
of fibers, scouring, de-sizing, bleaching, rinsing, mercerizing,
dyeing and finishing) which generate large quantities of
wastewaters. These effluents are highly variable in composi-
tion with relatively low biological oxygen demand (BOD) and
high chemical oxygen demand (COD) contents (Mantzavinos
and Psillakis, 2004). The most typical characteristic of textile
wastewaters is their strong color due to residual dyes. It is
estimated that approximately 15% of the total production of
colorants is lost during synthesis and processing and the
main source of this loss is to be found in wastewaters due to
incomplete exhaustion. In addition, a little more than half of
the worldwide production of organic colorants is textile dyes
r Ltd. All rights reserved.
97; fax: +30 28210 37846.Mantzavinos).
(Zollinger, 2003). Another characteristic of textile dyehouse
wastewaters is their recalcitrance due to the presence of
compounds such as dyes, surfactants and sizing agents as
well as their high salinity, high temperature and variable pH
(Mantzavinos and Psillakis, 2004).
Dye molecules often receive the largest attention due to
their color, as well as the toxicity of some of the raw materials
used to synthesize dyes (e.g. certain aromatic amines),
although dyes are often not the largest contributor to the
wastewater (Reife and Freeman, 1996). Dyes concentration in
wastewaters is usually lower than any other chemical found
in these wastewaters, but due to their strong color they are
visible even at very low concentrations, thus causing serious
aesthetic and pollution problems in wastewater disposal
(Zollinger, 2003). Therefore, methods for decoloration of
ARTICLE IN PRESS
Table 1 – Composition of the textile wastewater used inthis study
Component Concentration (mg/L)
Remazol Black B 99
Remazol Red RB 23
Remazol Golden Yellow RNL 13
Cibacron Black WNN 59
Drimaren Red K-8B 4
Drimaren Scarlet K-2G 6
Drimaren Yellow K-2R 8
Drimaren Navy K-BNN 4
Drimaren Yellow K-4G 8
Other reactive dyes (eight in total) 4
Organic auxiliary chemicals 1350
Na2SO4 5500
Na2CO3 444
NaOH 111
COD 404
WAT E R R E S E A R C H 40 (2006) 1276– 1286 1277
textile wastewaters have received considerable attention in
recent years.
Given the complex and bioresistant character of textile
effluents, their effective treatment and particularly their
decoloration, usually requires a combination of various
physical, chemical and biological technologies. Among these
techniques, those of practical interest are: (i) adsorption on
activated carbon, (ii) coagulation and flocculation followed by
sedimentation or dissolved air flotation, (iii) reductive clea-
vage of dyes containing azo or other reducible groups, (iv)
oxidative degradation by chlorine and by ozone, (v) electro-
chemical oxidation, and (vi) aerobic and anaerobic biological
treatment (Reife and Freeman, 1996).
Over the past several years, heterogeneous semiconductor
photocatalysis using TiO2 as the photocatalyst has received
considerable attention for water and wastewater treatment
(Parsons, 2004; Oppenlander, 2003). TiO2 photocatalysis is an
emerging wastewater treatment technology with key advan-
tages including the lack of mass transfer limitations, opera-
tion at ambient conditions and the possible use of solar
irradiation. The catalyst itself is inexpensive, commercially
available in various crystalline forms and particle character-
istics, non-toxic and photochemically stable. The process can
easily decolorize and reduce considerably the organic load of
textile wastewaters and similar effluents (Konstantinou and
Albanis, 2004; Parsons, 2004).
It is interesting to note that most of the studies reported in
the literature deal with simulated textile effluents or solu-
tions of specific textile dyes from different chemical classes
and there are very few literature reports studying actual
textile wastewater degradation by photocatalysis (Balcioglu
and Arslan, 1998; Peralta-Zamora et al., 1998; de Moraes et al.,
2000; Alaton et al., 2002).
The aim of the present work was to investigate the main
factors affecting the photocatalytic treatment of an actual
textile dyehouse wastewater. Emphasis was given on the
photocatalytic efficiency of different crystalline forms of TiO2,
effect of catalyst concentration, solution pH, air sparging and
addition of hydrogen peroxide. The possibility of recycling the
photocatalyst was also evaluated. Acute ecotoxicity of treated
textile effluents was assessed using the marine bacteria Vibrio
fischeri in order to investigate if toxic by-products were formed
during the photocatalytic process.
2. Materials and methods
2.1. Materials
The textile wastewater used in the present study was kindly
provided by EPILEKTOS SA, a textile manufacturing industry
located in the region of Sterea, Central Greece and it was used
as received without any pretreatment. The composition of the
textile effluent is given in Table 1. The dyeing process through
which the effluent was generated is as follows: the cotton
fiber is first mercerized and bleached using NaOH, H2O2 and
other bleaching agents and then rinsed with water prior to
dyeing. The dyestuff consists of about 200 kg of 17 dyes in
30 m3 water to dye 3 tons of cotton fibers and large amounts of
inorganic salts are also added. Following dyeing, the fiber is
washed several times with water and detergents in consecu-
tive baths and finally undergoes fixation and softening. Waste
streams from the various baths are collected in an equaliza-
tion tank where the effluent used in this study was taken
from. It consists of residual dyes, sodium hydroxide, inor-
ganic salts and various organic components such as deter-
gents, softening, dispersing and fixing agents (collectively
referred to as organic auxiliary chemicals in Table 1). The
effluent is highly alkaline (pH ¼ 9.8), with a dark green–blue
color (its absorbance at lmax ¼ 584 nm was 0.39) and partially
ecotoxic (EC50 ¼ 75%).
Titanium dioxide, TiO2, used in the present study was
kindly provided by Kerr–McGee Chemicals (Hamilton, USA), in
the following crystalline forms: (i) Tronox A-K-1 TiO2 in
anatase form, 97% purity, with specific surface area of 90 m2/g
and predominant particle size of approximately 20 nm; (ii)
Tronox TR-HP-2 TiO2 in rutile form, 99.7% purity, with specific
surface area of 7 m2/g. The catalysts were used as received,
while hydrogen peroxide, as a 35% (w/w) solution, was
supplied by Fluka.
2.2. Photocatalytic reactor
UV-A artificial irradiation was provided by a 400 W, high-
pressure mercury lamp (Osram, HQL, MBF-U). Photocatalytic
experiments with artificial irradiation were conducted in an
immersion well, batch-type, laboratory-scale photoreactor,
purchased from Ace Glass (Vineland, NJ, USA). It is a three-
compartment apparatus and consists of an inner, double-
walled, borosilicate glass housing the lamp and an external
cylindrical reaction vessel joined together with an internally
threaded connection with the aid of a nylon bushing
connector and an O-ring. The reaction mixture was placed
in the external cylindrical reaction vessel (compartment 1)
and the inner double-walled borosilicate glass was immersed
inside the reaction mixture. The lamp was placed inside
the inner borosilicate glass (compartment 3) and was
effectively cooled by a water circulation stream through the
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WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 2 7 6 – 1 2 8 61278
double-walled compartment (compartment 2), acting as a
cooling water jacket. During photocatalytic experiments,
temperature was maintained between 30 and 33 1C. The
external reaction vessel was covered with aluminum foil to
reflect irradiation exerting the outer wall of the reaction
vessel. This reaction geometry is ideal for full exploitation of
UV-A irradiation emitted from the lamp.
2.3. Photocatalytic degradation experiments
Photocatalytic experiments with artificial irradiation were
performed as follows: 350 mL of textile effluent were intro-
duced in the reaction vessel and the appropriate amount of
TiO2 was added to achieve the desirable concentration. The
resulting TiO2 suspension in the textile wastewater was
magnetically stirred for 30 min in the dark to ensure complete
equilibration of adsorption/desorption of organic compounds
on the catalyst surface. After that period of time, the UV-A
lamp was turned on, while (unless otherwise stated) CO2-free
air was continuously sparged in the liquid and the reaction
mixture was continuously stirred. In most cases, experiments
were performed at ambient pH which was 9.8 and left
uncontrolled during the reaction. In those cases where runs
were carried out at near-neutral or acidic conditions, the
initial pH was adjusted adding the appropriate amount of
HCl. In those cases where experiments were performed in the
presence of hydrogen peroxide, the appropriate amount of
35% (w/w) solution of H2O2 was added to achieve the desirable
final concentration of H2O2. Samples periodically drawn from
the vessel were centrifuged at 12,800 rpm to remove catalyst
particles and then analyzed with respect to their residual
color and COD content, as well as sample ecotoxicity.
Experiments with recycled catalyst were performed as
follows: at the end of the run with the fresh catalyst, the
reaction mixture was centrifuged at 3500 rpm and the super-
natant liquid was carefully decanted. A new batch of the
textile effluent was then added to the vessel and the used
TiO2 was slurried again under vigorous stirring.
2.4. Analytical measurements
The extent of decoloration that had occurred during photo-
catalytic treatment was assessed measuring sample absor-
bance at 584 nm, which is the wavelength that corresponds to
the maximum absorbance in the visible region, on a
Shimadzu UV 1240 spectrophotometer. Percent decoloration
was calculated relative to the initial absorbance of the
untreated textile effluent. COD was determined colometricaly
at 620 nm, using digestion solution for COD in the range
0–1500 mg/L on a DR 2010 photometer (HACH Company,
Loveland, USA). Percent COD reduction was calculated
relative to the initial COD value of the untreated textile
effluent.
2.5. Evaluation of acute toxicity
The luminescent marine bacteria Vibrio fischeri was used to
assess the acute ecotoxicity of textile wastewater samples
prior to and after photocatalytic treatment. The inhibition of
bioluminescense of Vibrio fischeri exposed to untreated and
treated dyehouse wastewater samples for 15 min at 15 1C was
measured using a LUMIStox analyser (Dr Lange, Germany)
and the results were compared to an aqueous control with
color correction. Toxicity is expressed as EC50, which is the
effective concentration of a toxicant causing 50% inhibition of
light output during the designated time intervals at 15 1C. For
each sample, its EC50 value was determined by applying
several dilutions. More details regarding the test can be found
elsewhere (luminescent bacteria test DIN/EN/ISO 11348-2,
Edition 99/01, Dr Lange, Germany).
3. Results and discussion
3.1. Screening photocatalytic experiments
Preliminary runs were performed to assess the extent of
textile effluent degradation under photocatalytic conditions.
Fig. 1 shows the decoloration–time profile of the textile
effluent irradiated without TiO2, irradiated in the presence
of 0.5 g/L anatase TiO2 and irradiated in the presence of 0.5 g/L
rutile TiO2. Fig. 2 shows the corresponding COD reduction
after 4 h of reaction, while Fig. 3 shows the UV–Visible spectra
of the initial untreated textile effluent, of the sample after 4 h
irradiation without TiO2 and the sample after 4 h irradiation
in the presence of 0.5 g/L anatase TiO2. All runs were
conducted at effluent ambient pH.
As can be seen from the UV–Visible spectra shown in Fig. 3,
upon UV-A irradiation of the initial untreated sample in the
absence of TiO2 the peak at 584 nm progressively disappeared
and the sample irradiated without TiO2 exhibited a contin-
uous band without any distinct maximum. Moreover, the
solution changed color from deep green–blue to yellow–-
orange.
The above observations indicate that organic substances
present in the textile wastewater, and especially dyes, are
rather photo labile under strong UV-A irradiation and they are
transformed to more stable compounds. Furthermore, the
organic load of the sample irradiated without TiO2 did not
change drastically; for instance, a COD decrease of about 10%
was recorded after 4 h of irradiation without TiO2 probably
due to photo-oxidation of organic compounds present in the
textile wastewater (Fig. 2). Therefore, irradiation without TiO2
was not sufficient to degrade efficiently the textile effluent. It
should be borne in mind that a fraction of the COD reduction
might be due to the oxidation of [2-(sulfoxy)ethyl]sulfonyl
groups (-SO2CH2CH2OSO3�) found in the dyes present in the
textile wastewater.
When UV-A irradiation was applied to 0.5 g/L of anatase
TiO2 suspension in textile effluent, decoloration of the
irradiated solution proceeded very fast reaching values of
65% and 93% after 0.5 and 4 h, respectively. This was
accompanied by a substantial COD removal of about 40% at
the end of the experiment. The UV–Visible spectrum of the
irradiated solution in the presence of 0.5 g/L anatase TiO2
exhibited an almost flat line between 500 and 700 nm, while
at the rest of the visible region of the spectrum the
absorbance was found to be less than 0.15.
In a final preliminary experiment, rutile TiO2 was also
tested for its photocatalytic activity and the results are shown
ARTICLE IN PRESS
00 1 2 3 4
20
40
60
80
100
Time, hours
Dec
olor
atio
n, %
Fig. 1 – Decoloration–time profile of the textile wastewater at different experimental conditions under UV-A illumination: -~-
sample irradiated without TiO2; -m- sample irradiated in the presence of 0.5 g/L anatase TiO2; -K- sample irradiated
in the presence of 0.5 g/L rutile TiO2.
0
10
20
30
40
Run 1 Run 2 Run 3
CO
D re
duct
ion,
%
Fig. 2 – COD reduction of the textile wastewater at different
experimental conditions under UV-A illumination:
Run 1—sample irradiated without TiO2; Run
2—sample irradiated in the presence of 0.5 g/L
anatase TiO2; Run 3—sample irradiated in the
presence of 0.5 g/L rutile TiO2. Reaction time: 4 h.
WAT E R R E S E A R C H 40 (2006) 1276– 1286 1279
in Figs. 1 and 2. At the conditions under consideration, rutile
was found to be less active than anatase for the photo-
catalytic textile effluent treatment; for instance, the extent of
decoloration and COD reduction after 4 h of irradiation with
rutile TiO2 was about 75% and 20%, respectively, while the
corresponding values with anatase were 93% and 40%.
Therefore, all subsequent photocatalytic experiments where
performed using Tronox A-K-1 anatase TiO2.
3.2. Adsorption studies
Preliminary control experiments were performed to assess
the adsorption capacity of anatase TiO2 at various solution pH
values and catalyst loadings. Fig. 4 shows the decoloration–
time profile of the textile wastewater during the adsorption at
pH 9 and 0.5 g/L anatase TiO2, at pH 6.7 and 1.5 g/L anatase
TiO2, and at pH 3 and 0.125 g/L anatase TiO2. As seen,
decoloration due to adsorption practically occurred within
the first half an hour and then remained unchanged for the
rest of the 4 h experiment regardless the starting solution pH
and catalyst loading. Therefore, in all subsequent experi-
ments TiO2 suspensions in the textile wastewater were
magnetically stirred for 30 min in the dark to ensure complete
equilibration of adsorption/desorption of organic compounds
on the TiO2 surface. Table 2 also shows the extent of
decoloration due to adsorption after 30 min in the dark at
various conditions. As seen from Fig. 4 and Table 2, adsorp-
tion significantly increased at acidic conditions (as will be
discussed later), while, at the conditions in question, the
catalyst loading had little effect on adsorption. These results
show that dyes and other organic molecules are adsorbed to
some extent on the catalyst surface.
3.3. Mechanism of photocatalytic degradation
The detailed mechanism of the photocatalytic process has
been discussed extensively in the literature (Konstantinou
and Albanis, 2004; Parsons, 2004; Oppenlander, 2003; Hoffman
et al., 1995) and will be only briefly summarized here.
Illumination of an aqueous TiO2 suspension with irradiation
energy greater than the band gap energy (Ebg) of the
semiconductor (hn4Ebg ¼ 3.2 eV in the case of anatase TiO2)
generates valence band holes ðhþvbÞ and conduction band
electrons ðe�cbÞ, (Eq. (1)):
TiO2 þ hn �!lo400 nm
e�cb þ hþnb. (1)
Due to this wide band gap energy, TiO2 in the anatase form
can be activated only by ultraviolet irradiation of wavelengths
below 385 nm (Parsons, 2004), i.e. in the UV-A region of the
electromagnetic spectrum (320olo400). The photogenerated
valence band holes ðhþvbÞ and conduction band electrons ðe�cbÞ
can either recombine to liberate heat, or make their separate
ways to the surface of TiO2, where they can react with species
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00 1 2 3 4
20
40
60
80
100
Time, hours
Dec
olor
atio
n, %
Fig. 4 – Decoloration–time profile of the textile wastewater at different experimental conditions in the dark: -~- adsorption at
pH 9 and 0.5 g/L anatase TiO2; -’- adsorption at pH 6.7 and 1.5 g/L anatase TiO2; -m- adsorption at pH 3 and 0.125 g/L
anatase TiO2.
0
0.4
0.8
1.2
1.6
2
300 400 500 600 700λ, nm
Abs
orba
nce
Fig. 3 – UV–Visible spectra of the textile wastewater at different experimental conditions: -~- initial untreated textile effluent;
-’- sample after 4 h irradiation without TiO2; -m- sample after 4 h irradiation in the presence of 0.5 g/L anatase TiO2.
Table 2 – Percent decoloration after 30 min in the dark inthe presence of anatase TiO2
TiO2 concentration (g/L) pH Decoloration (%)
0.25 9.8 23
0.5 9.8 26
1.0 9.8 32
1.5 9.8 27
3.0 9.8 37
0.5 6.7 29
1.0 6.7 29
1.5 6.7 25
0.125 3 61
0.25 3 58
0.5 3 51
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 2 7 6 – 1 2 8 61280
adsorbed on the catalyst surface according to Eqs. (2)–(7):
hþvb þH2O! HO� þHþ, (2)
hþvb þH2O! HO�, (3)
organic moleculeþ hþvb ! oxidation products; (4)
e�cb þO2 ! O��2 , (5)
O��2 þHþ ! HO�2, (6)
organic moleculeþ e�cb ! reduction products: (7)
Hydroxyl radical HOd, along with perhydroxyl radical HO2d
can oxidize most of the organic compounds to mineral end
products, thus reducing the organic load of the wastewater
(Eq. (8)):
ARTICLE IN PRESS
WAT E R R E S E A R C H 40 (2006) 1276– 1286 1281
radicals ðHO�; HO�2Þ þ organic compounds
! degradation products: ð8Þ
Enhanced photocatalytic activity of anatase compared to
rutile is in accordance with previous literature reports stating
that anatase has usually a better photocatalytic activity than
rutile (Carp et al., 2004; Kaneko and Okura, 2002; Sclafani and
Herrmann, 1996; Tanaka et al., 1993). The different behavior
of anatase and rutile has been attributed to the different
position of the conduction band (more positive for rutile), to
the higher recombination velocity of electron–hole pairs
photoproduced in rutile and to the higher capacity of anatase
to adsorb oxygen, due to higher density of superficial
hydroxyl groups (Sclafani and Herrmann, 1996).
Furthermore, in the present case an additional explanation
may be given in terms of the specific surface area of the
catalysts employed. It has been stated that in cases where the
surface reaction rates of conduction band electrons e�cb and
valence band holes hþvb with various substrates are faster than
the electron–hole recombination, the photocatalytic activity
increases with increasing specific surface area (Kaneko and
Okura, 2002). Anatase TiO2 used in the present study has
larger specific surface area than rutile. If surface reaction
rates of photogenerated electrons and holes with various
organic compounds present in the textile wastewater are
faster than the electron–hole recombination, then the en-
hanced photocatalytic activity of anatase compared to rutile
can be readily explained in terms of the larger specific surface
area of anatase.
3.4. Effect of catalyst concentration
TiO2 loading in slurry photocatalytic processes is an impor-
tant factor that can strongly influence textile wastewater
degradation. Experiments were performed using catalyst
loadings up to 3 g/L and the extent of textile wastewater
decoloration and COD reduction after 4 h of irradiation at
effluent ambient pH are shown in Fig. 5. It is evident that
decoloration was sufficiently high (82%) even for the lower
TiO2 loading tested (0.25 g/L) and increased over 90% with
00 0.25 0.5
20
40
60
80
100
TiO2 con
Con
vers
ion,
%
Fig. 5 – Effect of TiO2 concentration on textile wastewater decolo
irradiation.
increasing TiO2 concentration to 0.5 g/L; above this, catalyst
concentration decoloration remained practically constant. At
3 g/L TiO2, which is the higher loading tested, decoloration
was found to be almost 100% resulting in a completely
colorless solution. COD reduction remained practically un-
changed at a value of about 40% and only in the case of very
high catalyst loading was it found to increase to 55%.
It has been reported in several studies that dye degradation
increases with increasing TiO2 concentration up to a value,
above which dye conversion remains practically unchanged,
reaching a plateau (Herrmann, 2005; Konstantinou and
Albanis, 2004). This crucial TiO2 loading depends on several
factors like reactor geometry, operating conditions of the
photoreactor, wavelength and intensity of the light source,
and corresponds to the point where the entire catalytic
surface is fully illuminated. In the present study, catalyst
loadings greater than 0.5 g/L had little effect on decoloration
and COD removal and, therefore, all subsequent experiments
were performed at this TiO2 concentration.
3.5. Effect of solution pH
The efficiency of photocatalytic processes strongly depends
upon solution pH usually in a complex manner. All photo-
catalytic experiments described so far were performed at
ambient pH which was 9.8 due to the addition of NaOH during
the dying process. Additional photocatalytic experiments
were conducted at pH values of 6.7 and 3 at a TiO2
concentration of 0.5 g/L and the results are shown in Fig. 6.
The degree of decoloration and COD reduction at pH 6.7
were nearly equal to those recorded at ambient wastewater
pH of 9.8. A drastically different behavior was observed at pH
3. Decoloration was 100% resulting in a completely colorless
solution, while COD reduction was found to be 92% indicating
that almost complete mineralization of the organic com-
pounds present in the wastewater had occurred. Further-
more, at pH 3, photocatalytic efficiency was sufficiently high
even at lower TiO2 loadings as depicted in Fig. 7. At a catalyst
loading as low as 0.125 g/L, decoloration was 100%, while COD
1 1.5 32centration, g/L
ration (black bars) and COD reduction (hatched bars) after 4 h
ARTICLE IN PRESS
0
20
9.8 6.7 3
40
60
80
100
pH
Con
vers
ion,
%
Fig. 6 – Effect of solution pH on textile wastewater decoloration (black bars) and COD reduction (hatched bars) after 4 h
irradiation. TiO2 concentration 0.5 g/L.
0
20
40
60
80
100
0.125 0.25 0.5TiO2 concentration, g/L
Con
vers
ion,
%
Fig. 7 – Effect of TiO2 concentration on textile wastewater decoloration (black bars) and COD reduction (hatched bars) after 4 h
irradiation at pH 3.
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 2 7 6 – 1 2 8 61282
reduction reached 80%. These findings indicate that at pH 3
catalyst concentration can be remarkably reduced without
affecting photocatalytic efficiency.
The above experimental observations can be readily
explained in terms of the amphoteric behavior of TiO2.
Solution pH influences the ionization state of the TiO2 surface
according to the following reactions:
Ti OH + H Ti OH2(9)
Ti OH + HO Ti O + H2O (10)
In Eqs. (9) and (10) 4TiOH represents the primary hydrated
surface functionality of TiO2 (Hoffman et al., 1995). At pH
values lower than about 6.5, which is the point of zero charge
for TiO2 (Grzechulska and Morawski, 2002), the TiO2 surface
becomes positively charged, while at pH values greater than
about 6.5 it becomes negatively charged. The majority of the
reactive dyes found in the wastewater under study have
sulfonic (�SO3�) and [2-(sulfoxy)ethyl]sulfonyl groups
(�SO2CH2CH2OSO3�) as water solubilizing and reactive groups
which are negatively charged. Therefore, acidic conditions
would favor the electrostatic attraction between the positively
charged TiO2 surface and the reactive dyes which would
result in increased absorption and consequently in increased
degradation of dyes.
3.6. Effect of air sparging and addition of hydrogenperoxide
It has already been mentioned that the photogenerated
electrons ðe�cbÞ react with adsorbed molecular oxygen on the
TiO2 surface, leading to the formation of perhydroxyl radical
(Eqs. (5)–(6)). In addition, perhydroxy radical can generate
hydrogen peroxide (Eq. (11)), which in turn gives rise to
hydroxyl radicals (Eqs. (12)–(14)), thus avoiding electron–hole
ARTICLE IN PRESS
WAT E R R E S E A R C H 40 (2006) 1276– 1286 1283
recombination which is a major cause of low TiO2 photo-
catalytic quantum yield (Konstantinou and Albanis, 2004):
HO�2 þHO�2 ! H2OþO2, (11)
H2O2 þ e�cd ! HO� þHO�, (12)
H2O2 þO��2 ! HO� þHO� þO2, (13)
H2O2�!hn
2HO�. (14)
Therefore, the presence of excess molecular oxygen, as well
as the addition of hydrogen peroxide is expected to influence
photocatalytic efficiency (Velegraki et al., 2006).
An additional run without air sparging was performed at
0.5 g/L TiO2 and ambient pH; lack of air was found to decrease
only marginally the extent of color removal; after 4 h of
irradiation the extent of decoloration was about 80% and 93%,
respectively, without and with aeration. Furthermore, the
corresponding values of COD reduction were 23% and 40%.
Therefore, the presence of excess molecular oxygen positively
influenced photocatalytic degradation.
The effect of H2O2 addition was also studied. Fig. 8 shows
the decoloration–time profile of the textile effluent when (i)
H2O2 was added at a concentration of 0.025 M in the dark and
without TiO2, (ii) H2O2 was added at a concentration of 0.025 M
in the dark and in the presence of 0.5 g/L TiO2, (iii) the effluent
was irradiated without TiO2 in the presence of H2O2 at a
concentration of 0.025 M, (iv) the effluent was irradiated
without H2O2 in the presence of 0.5 g/L TiO2, and (v) the
effluent was irradiated in the presence of H2O2 at a
concentration of 0.025 M and 0.5 g/L TiO2. Addition of H2O2
in the textile effluent at a concentration of 0.025 M gave about
25% decoloration presumably due to bleaching. This was
achieved in the first half an hour and then remained
practically constant, possibly due to H2O2 consumption.
0
20
0 0.5
40
60
80
100
Tim
Dec
olor
atio
n, %
Fig. 8 – Effect of hydrogen peroxide addition on decoloration of t
and without TiO2 at a concentration of 0.025M; -’- addition of H
the presence of 0.5 g/L TiO2; -m- irradiation without TiO2 in the
irradiation without H2O2 in the presence of 0.5 g/L TiO2; -K- irrad
0.025 M and 0.5 g/L TiO2.
In addition, when H2O2 was added in the dark in the
presence of 0.5 g/L TiO2 a significant degree of decoloration
was observed, at about 73%, possibly due to the combined
effect of bleaching and adsorption of dyes on the TiO2 surface.
Furthermore, upon irradiation without TiO2 and in the
presence of 0.025 M H2O2, about 65% decoloration occurred
and this can be attributed to bleaching, as well as color
degradation due to photogenerated hydroxyl radicals accord-
ing to Eq. (14).
After 2 h of irradiation in the presence of 0.5 g/L TiO2 and
0.025 M H2O2 the extent of decoloration was 96%, with the
respective value for the photocatalytic run without hydrogen
peroxide being 85%. Furthermore, the extent of COD reduc-
tion after 2 h of photocatalytic treatment with and without
0.025 M H2O2 was 52% and 25%, respectively (Fig. 9). The above
results clearly demonstrate the beneficial effect of H2O2
addition on photocatalytic efficiency.
Nonetheless, it is well documented that, depending on the
reaction conditions and system in question, there is an
optimum H2O2 concentration above which H2O2 acts as
electron and radical scavenger, thus leading to reduced
degradation (Velegraki et al., 2006; Konstantinou and Albanis,
2004; Poulios et al., 2003). In light of this, we studied the effect
of H2O2 concentration in the range 0–0.05 M and the results
are shown in Fig. 9. It was observed that decoloration and
COD reduction increased with increasing H2O2 concentration
up to 0.025 M, after which textile degradation remained
practically constant.
3.7. Photocatalyst reuse
The possibility of catalyst recovery and reuse in photocata-
lytic processes has received considerable attention (Doll and
Frimmel, 2005; Fernandez-Ibanez et al., 2003; Reutergardh
1 1.5 2e, hours
extile wastewater at pH 9.8: -~- addition of H2O2 in the dark
2O2 at a concentration of 0.025 M in the dark and in
presence of H2O2 at a concentration of 0.025 M; -J-
iation in the presence of H2O2 at a concentration of
ARTICLE IN PRESS
00 0.0250.01 0.05
20
40
60
80
100
H2O2 concentration, M
Con
vers
ion,
%
Fig. 9 – Effect of H2O2 concentration on textile wastewater decoloration (black bars) and COD reduction (hatched bars) after 2 h
irradiation at pH 9.8. TiO2 concentration: 0.5 g/L.
01 2 3
20
40
60
80
100
Run
Con
vers
ion,
%
Fig. 10 – Effect of TiO2 reuse on textile wastewater decoloration (black bars) and COD reduction (hatched bars) after 2 h
irradiation at pH 9.8. TiO2 concentration: 3 g/L. Run 1—fresh TiO2; Run 2—first reuse; Run 3—second reuse.
WAT E R R E S E A R C H 4 0 ( 2 0 0 6 ) 1 2 7 6 – 1 2 8 61284
and Iangphasuk, 1997) since it can contribute significantly to
lowering the operational cost of the process, which is an
important parameter in the applicability of photocatalysis as
a method for wastewater purification. In order to evaluate
reused photocatalyst efficiency, a series of experiments were
performed at the maximum catalyst loading of 3 g/L TiO2 to
avoid influence of minor losses in the amount of catalyst.
Results are depicted in Fig. 10. At the conditions in question,
although photocatalytic efficiency marginally deteriorated on
repeated use, it still remained sufficiently high both in terms
of color and COD conversion. The recorded catalyst deactiva-
tion could be, to a certain degree at least, explained by the
fact that upon irradiation anatase TiO2 is partly transformed
to its rutile counterpart (Domınguez et al., 1998). Given that,
at the conditions under consideration, rutile was less active
than anatase, this could explain the decreased photocatalytic
activity after repeated use. An additional explanation
of catalyst deactivation may be given in terms of poisoning
of the catalyst surface due to the deleterious effect of
intermediates containing N or S strongly adsorbed on TiO2
surface.
3.8. Effect of photocatalytic treatment on ecotoxicity
The possibility of generating photocatalytic degradation by-
products that are more toxic than the initial untreated
samples has to be taken into account and toxicity measure-
ments are necessary to exclude this possibility. In light of this,
the effect of photocatalytic treatment on acute ecotoxicity
was assessed using the marine bacteria Vibrio fischeri and the
results are shown in Fig. 11. As can be seen, the original
effluent was only partially ecotoxic with its EC50 value being
75%. Nonetheless, photocatalytic treatment under various
operating conditions for 4 h was capable of eliminating
toxicity fully, thus indicating that residual organics and
degradation by-products are less ecotoxic than the original
effluent.
ARTICLE IN PRESS
01 2 3 4 5 6
20
40
60
80
100
Run
EC
50, %
Fig. 11 – Acute ecotoxicity to Vibrio fischeri of samples prior to and after photocatalytic degradation. Run 1—initial untreated
textile wastewater; Run 2—irradiated with 1.5 g/L TiO2 at pH 9.8; Run 3—irradiated with 2 g/L TiO2 at pH 9.8; Run
4—irradiated with 3 g/L TiO2 at pH 9.8; Run 5—irradiated with 0.5 g/L TiO2 at pH 3; Run 6—irradiated with 0.5 g/L TiO2
at pH 6.8. Irradiation time: 4 h.
WAT E R R E S E A R C H 40 (2006) 1276– 1286 1285
4. Conclusions
The conclusions drawn from this study can be summarized as
follows:
(1)
TiO2-mediated UV-A photocatalysis was found capable ofcompletely decolorizing and sufficiently reducing the
organic load of an actual textile wastewater consisting of
reactive dyes, inorganic compounds and other organic
auxiliary chemicals. The extent of conversion depends on
the operating conditions employed such as the type and
concentration of TiO2, solution pH, dissolved oxygen and
the presence of extra hydroxyl radical sources.
(2)
Catalyst recycle and reuse need to be taken into accountsince it can render the treatment process (for full-scale
applications) more attractive from an economic point of
view.
(3)
Photocatalytic oxidation of the textile wastewater reducedthe acute ecotoxicity to marine bacteria Vibrio fischeri. In
this context, photocatalytic oxidation may be used as an
efficient pre-treatment step to biological post-treatment
of textile effluents since it does not generate toxic by-
products.
Acknowledgements
The authors wish to thank G. Balagouras of EPILEKTOS SA for
kindly providing the textile effluent used in this study. They
also thank A. Coz for his involvement with toxicity tests.
Financial support for NPX was provided by the Hellenic
Ministry of National Education & Religious Affairs under the
‘‘PYTHAGORAS’’ program.
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