26
Chapter 2
LITERATURE REVIEW
Dyestuffs and other commercial colourants have emerged as a focus of
environmental remediation efforts [Nasr et al., 1996]. These efforts have largely
been targeted at removing colourants from the effluents of textile processes, kraft
mills, dye manufacturing units. These coloured dye effluents create severe
environmental pollution problems by releasing toxic and potential carcinogenic
substances into the aqua sphere. Since the increased public concern over these
pollutants, international environmental standards are becoming more stringent;
therefore new treatment methods are required for the removal of persistent organic
chemicals or converting them to harmless compounds in water. The conventional
technologies currently used to degrade the colour of the dye contaminated water
includes primary (adsorption, flocculation), secondary (biological methods), and
chemical processes (chlorination, ozonization) [More et al., 1989; Patil and Shinde et
al., 1988]. However these techniques are non-destructive, since they only transfer
the non-biodegradable matter into sludge, giving rise to new type of pollution, which
needs further treatment [Arslan et al., 2000; Chaudhuri and Sur, 2000; Stock et al.,
2000]. The AOP technique has drawn considerable attention from various quarters
of scientific community as it is easy to handle and produces significantly less
residuals as compared to the classical approaches. Amongst the many techniques
employed in the AOP approach are the UV photolytic technique [Elmorsi et al., 2010;
AlHamedi et al., 2009 ; Gul and YildIrIm, 2009], Fenton process [Bouasla et al., 2010;
27
Masarwa et al., 2005; Chamarro et al., 2001], photo-Fenton process[Abdessalem et
al., 2010; Monteagudo et al., 2010; Modirshahla et al., 2007], ozonation process
[Bagha et al., 2010], sonolysis [Merouani et al., 2010; Ghodbane and
Hamdaoui,2009; Wang et al., 2008; Song et al., 2007], photocatalytic approach [Rauf
et al., 2010; Xu et al., 2010; Zhang et al., 2009; Bukallah et al., 2007; Habibi and
Talebian, 2007] and the radiation induced degradation of dyes [Vahdat et al., 2010;
Mohamed et al., 2009; Chen et al., 2008; Dajka et al., 2003].
2.1 Evolution of the Field of Heterogeneous Photocatalytic
Treatment of Organic Dyes
The first instance for observing dye instability in the presence of an illuminated
inorganic semiconductor appears to be when the photocatalytic reduction of
Methylene Blue (a thiazine dye) to the leuco form in presence of CdSb and TiO2 was
reported by Yoneyama et al., (1972) andPamfilov et al., (1969) respectively.
Consequently, N-dealkylation of dyes such as Rhodamine B and Methylene Blue in
aqueous suspensions of CdS was reported by Takizawa et al., (1980). Deliberate
attempts to photochemically destroy the organic dye appear to be instigated when
Brown and Darwent, (1984a & b) reported the photoreduction of Methyl Orange to a
hydrazine derivative in presence of colloidal TiO2 and photoinduced electron transfer
from TiO2 to Methyl Orange (a monoazo dye) resulting in bleaching of the dye (λmax
= 470 nm). Dye bleaching was not observed in the absence of TiO2 or visible radiation
alone indicating that the photocatalysis process involved initial light absorption (λmax
= 310 nm) with the simultaneous generation of e−h+ pairs in the semiconductor.
These early studies were accompanied by findings in other laboratories which
showed that many organic compound could be decomposed in aqueous media with
28
a combination of TiO2 and near-UV light [Fujishima et al., 2000; Tryk et al., 2000;
Litter, 1999; Mills and LeHunte, 1997; Linsebigler et al., 1995;
Hoffmann et al.,
1995; Legrini et al., 1993;Kamat, 1993; Mills et al., 1993a]. However the field
legitimately “took off” only in the late 1990s. Fundamental and applied research on
this subject has been performed extensively during the last twenty years all over the
world as documented by more than 2000 publications [Blake, 2001]. In 1998, the US
EPA (Environmental Protection Agency) made an inventory of more than 800
molecules (Chlorinated solvents, Non-chlorinated solvents, Insecticides, Pesticides,
Dyes and Detergents) that can be degraded by this process [Robert and Malato,
2002].
2.2 Photocatalytic Degradation of Synthetic Dyes
The photocatalytic degradation has emerged as a potentially powerful and versatile
method for dealing with the problem of wastewaters containing different dyes. A
number of research groups have dealt with the photocatalytic decomposition of this
class of materials in the presence of UV-A or visible light with very encouraging
results [Zhang et al., 1998; Vinodgopal et al., 1996; Davis et al., 1994].
2.3 Semiconductor Photocatalysts
2.3.1 Titanium Dioxide
On laboratory scale, the most popular photocatalyst configuration has been in the
form of powder suspensions. Commercially available samples of TiO2 (Degussa P-25,
Millenium PC500, Aldrich, Hombikat UV 100, Tranox A-K-1, Mikroanatas IF 9308/18
and DuPont R-900) have been used in many studies [Rajeshwar et al., 2008].
Degussa P25 TiO2 works the best for photoreactivity in environmental applications,
although TiO2 produced by Sachtleben, Germany show comparable reactivity (Martin
29
et al., 1994a & b). Degussa P25 TiO2 has effectively become a research standard
because it has (i) reasonably well defined nature (i.e. 70:30% anatase-to-rutile
mixture with a BET surface area of 55±15 m2g-,l typically nonporous and crystallite
sizes of 30 nm in 0.1 μm diameter aggregates and (ii) a substantially higher
photocatalytic activity than most other readily available samples of TiO2. Sivakumar
and Shanthi, (2001) reported the decolourization of reactive textile dyes namely
Procion Brilliant Orange M-2R (PBO), Procion Brilliant Magenta M-B (PBM), and
Procion Brilliant yellow M-4G (PBY) using different grades of TiO2 semiconductor
(CDH,CERAC and DEGUSSA) as catalyst under sunlight illumination and concluded
that TiO2 (Degussa P-25) was superior to any other grade. Saquib and Muneer,
(2003) compared the photocatalytic efficiency of several samples of TiO2 (Degussa
P25, UV100 and PC500) for the photocatalytic degradation of triphenylmethane dye
and found that Degussa P25 was the most efficient photocatalyst. It was explained
on the basis of the slow electron-hole recombination in case of Degussa P25.
Zielinska et al., (2003, 2001) reported less activity of Tytanpol A11 than Degussa P25
for the photocatalytic decomposition of various organic dyes (Reactive Red 198, Acid
Black, Acid Blue 7, Direct Green 99 and Reactive Black 5) due to different physical
properties of two titania materials. Bouanimba et al., (2011) compared the efficiency
of P25-Degussa and PC500-Millennium photocatalysts and reported the higher
efficiency of TiO2 P25 than TiO2 from Millennium for the degradation of
Bromophenol blue.
Martin et al., (1994c) and Mills et al., (1993a & b) claim that rutile TiO2 is catalytically
inactive whereas Karakitsou and Verykios, (1993); Ohtani and Nishimoto, (1993)
reported its less or selective activity. However, Domenech, (1993) has shown that
30
TiO2 in the rutile form is a substantially better photocatalyst for the oxidation of CNthan anatase form. Devi and Krishnaiah, (1999) investigated the photocatalytic
degradation of azo dyes (p-Amino-azo-benzene and p-Hydroxy-azo-benzene) using
heat treated TiO2 as the photocatalyst. Anatase form (TiO2 annealed at 600οC-650οC)
has proved to be efficient catalyst as compared to rutile form (annealed above 700
ο
C) and also to Degussa P-25 sample for degradation of these dyes. The anatase form
of titania is reported to give the best combination of photoactivity and photo
stability [Zeltner and Tompkin, 2005]. Habibi et al., (2005) investigated
photocatalytic degradation of three commercial textile diazo dyes, called (C.I. Direct
80, 3BL, C.I. Direct Blue 160, RL and C.I. Reactive Yellow 2, X6G) using commercial
TiO2 in aqueous solution under 400W high-pressure mercury lamp irradiation and
concluded that the employment of efficient photocatalyst and selection of optimal
operational parameters may lead to complete decolorization and substantial
decrease of the chemical oxygen demand (COD) of dye solutions.
Zhao, (2000) examined the direct photocatalytic degradation of dye pollutant
Sulphorhodamine B (SRB) in aqueous TiO2 dispersions and compared to the
photosenstization process. Wang, (2000) investigated the photocatalytic degradation
of eight commercial dyes with different structures and containing different
substituent groups using TiO2 as photocatalyst in aqueous suspension under solar
irradiation. Neppolian et al., (2002b) investigated the photocatalytic degradation of
three commercial textile dyes with different structures using TiO2 (Degussa P25)
photocatalyst in aqueous solution under solar irradiation as a function of COD
reduction. Experiments were conducted to optimize various parameters like amount
of catalyst, concentration of dye, pH and solar light intensity. Qamar et al., (2005)
31
investigated the photocatalytic degradation of two selected dye derivatives
Chromotrope 2B and Amido Black 10B in aqueous suspensions of TiO2 under a
variety of conditions.Xie and Yuan, (2003a) reported that TiO2 sol nanoparticles have
better interfacial adsorption capability and higher photoactivity than P25 TiO2 in
suspension system andcan be applied to recycle use for many times while keeping
high photoactivity on X-3B degradation reaction.
2.3.2 Other Semiconductors
Widespread use of TiO2 is uneconomical for large scale water treatment; thereby
interest has been drawn towards the search for suitable alternatives to TiO2. Many
attempts have been made to study photocatalytic activity of different
semiconductors such as SnO2, ZrO2, CdS, MoS2, Fe2O3, WO3 and ZnO [Konstantinou et
al., 2002].
CdS is an example of a highly active semiconductor photosensitizer, which has the
highly desirable feature that can be activated using visible light (thus sunlight could
be used). But as is typical for visible light absorbing semiconductors, it is liable to
photo-anodic corrosion and this feature renders it unacceptable as a photocatalyst
for water purification [Karunakan and Senthivelon, 2005; Revtergardh and
Iangphasuk, 1997]. Flowerlike cadmium sulfide (CdS) nanostructure was prepared via
the controllable solvent thermal method by using ethylenediamine as the structure
directing template. The synthesized CdS was characterized by X-ray diffraction (XRD)
and scanning electron microscopy (SEM). The photocatalytic activity of CdS was
investigated for the decolorization of methyl orange (MeO) under high-pressure
mercury lamp illumination. It was observed that the flowerlike CdS showed better
results as compared to other CdS materials [Di et al., 2009].
32
WO3 films were prepared by cathodic electrodeposition [Hepel and Luo, 2001; Luo
and Hepel, 2001] or by RF magnetron sputtering deposited on glass substrates
[Wang et al., 1998]. A high Photocatalytic activity was observed for the
electrodeposited WO3 film electrodes than for TiO2 nanoparticulate films for the
degradation of Naphthol Blue Black dye [Luo and Hepel, 2001]. Anodic nanoporous
WO3 films have showngreater photocatalytic activity for Methylene Blue than their
cathodically electrosynthesized counterparts [Watcherenwong et al., 2008]. A crucial
advantage with WO3 (relative to TiO2) is that its lower optical bandgap (2.5-2.8 eV)
results in a much greater utilization of the solar spectrum for solar photocatalysis
applications.
After TiO2, ZnO perhaps is the most studied inorganic semiconductor in the dye
photocatalysis community. Roselin et al., (2002) investigated the photoctalytic
degradation of Reactive Red 22 (RR 22) dye in the presence of a thin film of ZnO
photocatalyst using a thin film flat bed flow photoreactor under solar irradiation.
Chakrabarti and Dutta, (2004) explored the potential of a common semiconductor,
ZnO as an effective catalyst for the photodegradation of two model dyes; Methylene
Blue and Eosin Y. The effects of parameters like catalyst loading, initial dye
concentration, air flow rate, UV irradiation intensity and pH on the extent of
photodegradation have been investigated. They proposed a rate equation for the
degradation based on Langmuir-Hinshelwood model. Photocatalytic activity of
various semiconductors such as titanium dioxide (TiO2), zinc oxide (ZnO), stannic
oxide (SnO2), zinc sulphide (ZnS) and cadmium sulphide (CdS) has been investigated
for Methyl Orange (MO) and Rhodamine 6G (R6G) degradation and was reported
33
that the maximum decolorization (more than 90%) of dyes occurred with ZnO
catalyst [Kansal et al., 2006].
Studies have also been done using ZnO catalysts under sunlight [Chakrabarti and
Dutta, 2004; Neppolian et al., 1998] and good results have been obtained but their
applications remain limited only by pH. Pandurangan et al., (2001) carried out the
photocatalytic degradation of textile dye, Basic Yellow Auramine O by a batch
process using ZnO as the catalyst and sunlight as the illuminant. In addition to
decolourization of dye solution, the COD was also reduced suggesting that the
fragments produced from the dye were mineralized.
Jang et al., (2006) compared the photocatalytic activity of ZnO nanoparticles and its
nano-crystalline particles using methylene blue under UV light illumination. The
photocatalytic degradation capacity of the ZnO nanoparticles was higher than that of
the ZnO nano-crystalline particles. Several authors used the design of experiments
for discolouration of wastewater [Muthukumar et al., 2004; Lizma et al., 2002;
Herrera et al., 2000; and DeGiorgi and Carpignano, 1996]. Lizma et al., (2002)
reported the photocatalytic discolouration of Reactive Blue 19 (RB-19) in aqueous
solutions containing TiO2 or ZnO as catalysts. The reactions can be described as a
function of parameters like pH, amount of catalyst and dye concentration being
modelled by the use of response surface methodology. It has been concluded that
ZnO is a more efficient catalyst than TiO2 in the colour removal of RB-19. Daneshvar
et al., 2004 reported that ZnO appears to be a suitable alternative to TiO2 since its
photodegradation mechanism has been proven to be similar to that of TiO2 for Acid
Red 27. Kansal et al., (2010) investigated the photocatalytic decolourization of
biebrich scarlet dye (BS) in a batch reactor under UV light in slurry mode using
34
different nanophotocatalysts (TiO2, ZnO, CdS and ZnS) and found ZnO to be a better
photocatalyst among others. The biggest advantage of ZnO is that it can absorb over
a larger fraction of UV spectrum and the corresponding threshold of ZnO is 425 nm
[Behnajady et al., 2006]. Efficiency of ZnO has been reported to be particularly
noticeable in the photooxidation of textile mill wastewater [Daneshvar et al., 2003].
Wang et al., (2009) reported advantages of ZnO over TiO2 in efficient
photodegradation of Acid Red B dye. Karunakaran et al., (2010) also reported
importance of ZnO as a suitable alternative to TiO2 for degradation of cyanide.
2.4 Trends in Improving the Activity of Titania
The limitations of a particular semiconductor as a photocatalyst for a particular
application can be surmounted by modifying the surface of the semiconductor. One
approach to modify TiO2 photocatalyst is to dope transition metals into TiO2 and
other is to form coupled photo catalysts [Chen et al., 2005; Anpo et al., 2000].
2.4.1. Dissolved Inorganic Ion Impurity
The influence of dissolved transition metal impurity ions on the photocatalytic
properties of TiO2 has become another interesting area of semiconductor
modification. Chen et al. (2002) investigated the effect of metal ions (Cu2+, Fe3+, Zn2+,
Al3+, and Cd2+) on the photodegradation of several dyes: Sulforhodamine B (SRB),
Alizarin Red (AR), and Malachite Green (MG) in aqueous TiO2 dispersions under
visible irradiation (λ> 420 nm) and reported that trace quantities of transition metal
ions such as Cu2+ and Fe3+ having suitable redox potentials markedly depress the
photodegradation of all three dyes under visible irradiation by blocking the
formation of reactive species(O−•, •OH). Other metal ions, such as Zn2+, Cd2+, and Al3+,
have only a slight influence on the photoreaction by altering the adsorption of dyes.
35
Paola et al. (2002) reported that photocatalytic activities of TiO2 polycrystalline
powders loaded with transition metals, such as Cr, Co, Cu, Fe, Mo, W and V, are
generally reduced due to the presence of the transition metals with the exception of
W.
Baran et al., (2003) studied the decolouration of solutions of azo, anionic (Acid
Orange 7, Reactive Red 45, Acid Yellow 23) and cationic (Basic Blue 41 and Basic
Orange 66) dyes during illumination with UV irradiation in the presence of TiO2 and
FeCl3. The process of decolouration during illumination of the solutions containing
FeCl3 underwent significant intensification in the case of anionic dyes and
unfavorable inhibition in case of cationic dyes.
The effects of various inorganic anions on the photodegradation of dye pollutants
under UV light irradiation have been examined [Guillard et al., 2003; Epling and Lin,
2002a; Sökmen and Özkan, 2002]. Wang et al. (2004a) investigated the effects of
various inorganic anions (NO3−, Cl−, SO42−, HCO3−, H2PO4−) on the photodegradation of
Acid Orange 7 (AO7) under UV or visible light irradiation in the presence of TiO2
particles. In TiO2/UV system, inorganic anions inhibited the photodegradation of the
dye by trapping hydroxyl radicals. In TiO2/Vis system, the observed inhibition effects
of inorganic anions can be interpreted by competitive adsorption. In addition, the
results indicated that the photodegradation of AO7 took place mainly in the bulk
solution under UV light irradiation, while under visible light irradiation, the reaction
occurred on the catalyst surface.Effects of acidity and inorganic ions on the
photocatalytic degradation of azo dyes, Procion Red MX-5B (MX-5B) and Cationic
Blue X-GRL (CBX), have been investigated in UV illuminated TiO2 dispersions [Hu et
al., 2003c].These results demonstrated that inorganic anions affect the
36
photodegradation of dyes by their adsorption onto the surface of TiO2 and trapping
positive hole (h+) and •OH.
2.4.2 Photocatalyst Preparation
The major practices involve catalyst modification by doping, metal coating, surface
sensitization, and increase in surface area by design and development of secondary
titania photocatalyst. Surface sensitization has been intensively reviewed by Carp et
al., (2004).TiO2 has three natural phases—anatase, rutile, and brookite. Modification
with certain metals such as Ni, Fe, Th, Cu, V and Mo, Co, Sn and Ag [Barakat et al.,
2005; Sen et al., 2005; Mahanty et al., 2004; Riyas et al.,2002; Reddy et al., 1994;
Sankar et al., 1991] may alter the phase transformation of TiO2 from active anatase
to inactive rutile by lowering the activation energy. The activation energy is further
affected by metal dosage and method of preparation. On the other hand, metals
such as Mg, Ba, Mn, Tb, Eu, Sm, La, Sc, Nb [Ahmad et al., 2008; Arroyo et al., 2008;
Parida and Sahu, 2008; Saif and Mottaleb, 2007; Venkatachalam et al., 2007] have
been reported to inhibit phase transformation.
2.4.2.1 Metal Doped TiO2
The benefit of transition metal doping species is the improved trapping of electrons
to inhibit electron-hole recombination during illumination. With Fe3+ doping of TiO2,
an increase in Ti3+ intensity was observed by ESR upon photoirradiation due to
trapped electrons [Gratzel and Howe, 1990]. Only certain transition metals such as
Fe3+ [Butler and Davis, 1993] and Cu2+ [Fujihira et al., 1982] actually inhibit electronhole recombination. The concentration of the beneficial transition metal dopants is
very small and large concentrations are detrimental. Wong et al., (2004) synthesized
copper-doped TiO2 nanocatalysts by photo-deposition and sol–gel methods.
37
The catalysts’ activity was evaluated using 0.2mM Orange II. The results indicated
that 1% Cu-doped TiO2 nanocatalysts prepared by the photo-deposition method
showed enhanced photocatalytic activity.
Other transition metal dopants such as Cr3+ [Herrmann et al., 1984] create sites,
which increase electron-hole recombination. It is believed that these transition
metals create acceptor and donor centers where direct recombination occurs.
Negative effects of doping [Luo and Gao, 1992] have been noted for Mo and V in
TiO2, while Gratzel and Howe, (1990) noted an inhibition of the electron/hole
recombination with the same dopants. Karakitsou and Verykios, (1993) reported that
doping TiO2 with cations of higher valency than that of Ti (IV) resulted in enhanced
photoreactivity, while Mu et al., (1989) noted that doping with trivalent and
pentavalent cations was actually detrimental to the photoreactivity of TiO2.
Chromium, manganese and cobalt-doped titanium dioxide photocatalysts containing
0.2, 0.5 or 1% of metal-dopant were investigated by UV–Vis, FT-IR, near-IR and
electron paramagnetic resonance (EPR) spectroscopic techniques. The presence of
the doping ions in the titania structure caused significant absorption shift to the
visible region compared to pure TiO2 powder (P25 Degussa) [Dvoranova et al., 2002].
Cr-doped anatase TiO2 was prepared by the combination of sol–gel and
hydrothermal methods. Cr doping improved photocatalytic activity for the
degradation of XRG dye [Zhu et al., 2006].Bouras et al., (2007) reported that under
UV–Vis excitation anatase–rutile transformation increased with increasing dopant
concentration inFe3+-, Cr3+- and Co2+-doped nano-crystalline TiO2. Iron-doped
anatase titanium (IV) dioxide (TiO2) samples were prepared by hydrothermal
hydrolysis and were crystallizedn in octanol-water solution.The catalyst doped with
38
optimal content of iron showed maximum degradation of active yellow XRG dye
diluted in water under UV and visible light irradiation [Zhu et al., 2004] Doping of
Fe3+ ion improved the photodegradation performance of TiO2 coated surfaces
[Asiltürk et al., 2009]. V-doped TiO2 Photocatalyst prepared by modified sol–gel
method showed high-photocatalytic activity in the degradation of crystal violet and
methylene blue under visible light [Wu and Chen, 2004].
Anatase TiO2 was doped with divalent transition metal ions like Mn2+, Ni2+ and Zn2+.
The photocatalytic activities of these catalysts were evaluated in the degradation of
Aniline Blue (AB) under UV/solar light. Among the photocatalysts, Mn2+ (0.06%)–TiO2
showed enhanced activity, which is attributed to the synergistic effect in the
bicrystalline framework of anatase and rutile [Devi et al., 2010]. The dye (20 ppm)
was found to degrade about 88% after illumination for 10 h. Zn2+-doped TiO2
prepared by sol–gel and solid phase reaction methods show significant enhancement
of the photoactivity when evaluated with Rhodamine B [Liu et al., 2005]. The
photocatalytic degradation of Crystal Violet, a triphenyl methane dye (also known as
Basic Violet 3) in aqueous solution was investigated with Ag+ ion doped TiO2 under
UV and simulated solar light by Sahoo et al., (2005).
Bi/Co and Fe/Co codoped TiO2 were prepared by stearic acid gel method and the
absorbance of prepared samples in the visible light region followed the order: Bi/Co
codoped TiO2> Co monodoped TiO2> Fe/Co codoped TiO2. The photoactivity was
evaluated by the photodegradation of Rhodamine B solution under visible light. The
results showed that Fe (0.1%)/Co (0.4%) codoped TiO2 had the highest photoactivity
among all, indicating that the photoactivity not only benefits from absorbance but
also relates to the cooperative effect of the two dopants [Wang et al., 2009].
39
Ce-doped mesophorous anatase-TiO2 exhibited higher photocatalytic activity than
commercial Degussa P-25 as doping inhibited mesophores collapse and anatase–
rutile phase transformation [Xiao et al., 2006]. Xie and Yuan (2003b) prepared
cerium ion (Ce4+) modified titania sol and nanocrystallites by chemical
coprecipitation–peptization and hydrothermal synthesis methods and reported that
Ce4+-TiO2 sol has shown higher photocatalytic efficiency for degradation of reactive
brilliant red dye (X-3B) than nanocrystallites. Nd3+–TiO2 sol prepared by
coprecipitation had anatase crystalline structure and showed higher photocatalytic
activity with reactive dye X-3B than titania under visible light illumination [Xie et al.,
2005]. Pr3+-TiO2 had better adsorption capacities and photocatalytic abilities for dye
removal than that of pure TiO2 [Liang et al., 2009]. El-Bahy et al., (2009) successfully
synthesized Lanthanide ions (La3+, Nd3+, Sm3+, Eu3+, Gd3+, and Yb3+)/doped TiO2
nanoparticles by sol–gel method and found that Gd3+/TiO2 is the most effective
photocatalyst.
2.4.2.2 Non Metal Doped TiO2
Nitrogen doped into substitutional sites of TiO2 has proven very efficient for
photocatalysis [Asahi et al., 2001]. N-doped TiO2 has been prepared by Kumar et al.,
(2005) which showed extended absorption in visible region. The addition of PdCl2
further extended absorption to near IR with high-photocatalytic activity.Yin et al.,
(2006) prepared TiO2−xNy by low-temperature process involving mechanical doping
and oxygen plasma treatment. Kryukova et al., (2007) prepared Sulphur-doped TiO2
from titanyl sulfate solution by reacting with ammonia at 85 °C and controlled pH
(thermal hydrolysis) and evaluated its activity for photooxidation of Acid Orange
7.N–F codoped TiO2 was prepared by solvothermal method using tetrabutyl titanate
40
precursor [Huang et al., 2006]. The photocatalyst showed very high activity towards
visible light induced p-Chlorophenol photooxidative degradation.Morikawa et al.
(2006) prepared Nitrogen-doped TiO2 (TiO2-xNx) photocatalysts loaded with various
transition metal ions, including Cu, Pt, Ni, Zn and La using a wet impregnation
method and found that photocatalytic activity of TiO2-xNx was markedly enhanced by
Cu or Pt loading, while Ni, Zn or La loaded TiO2-xNx showed similar photodegradation
rate to the bare TiO2-xNx. Among them, the enhancement effect of Cu ion was found
highest.
2.4.2.3 Metal Coating
Arabatzis et al., (2003) modified the surface of rough, high-surface area,
nanocrystalline titania thin-film photocatalysts by gold deposition (Au/TiO2) via
electron beam evaporation.
2.4.3 Coupled Semiconductors
Instead of using TiO2 alone, a “coupled semiconductor” configuration [Tacconi et al.,
2003] improves charge separation in many cases because of “vectorial” electron
transfer [Smotkin et al., 1986].
Various composites formed by TiO2 and other inorganic oxides or sulfides have been
reported [Jiang et al., 2008]. The p-ZnO/TiO2 was prepared by ball milling of TiO2 in
H2O solution doped with p-ZnO and its photocatalytic activity for oxidation of
methyl orange was reported [Shifu et al., 2008].
Sun et al., (2002) have prepared titanium dioxide/bentonite clay nanocomposite by
acid-catalyzed sol–gel method and used as a photocatalyst for cationic azo dye
decomposition in water. Zhang et al., (2004) studied the photocatalytic activity of
nanocoupled oxides (ZnO-SnO2) using Methyl Orange as a pollutant. Experimental
41
results showed that the nanometer coupled oxides mainly consist of nanometer ZnO
and SnO2, and they have the same excellent photocatalytic activity as Degussa P25
TiO2 for the degradation of methyl orange. Nanosized coupled ZnO/SnO2
photocatalysts with different Sn contents were prepared using the coprecipitation
method and enhanced photocatalytic activity of the coupled ZnO/SnO2 photocatalyst
was reported as compared to ZnO and SnO2 by Wang et al. (2004b). Liu et al., (2010)
prepared mesoporous ZnO/SnO2 composite nanofibers via the electrospinning
technique and found that the photocatalytic activity of the mesoporous ZnO/SnO2
composite nanofibers was dependant on the surface area, light utilization efficiency,
and the separation of photogenerated electron/hole pair.
TiO2/SnO2 photocatalyst prepared by ball milling through doping of TiO2 using H2O as
disperser showed new crystal faces with no change in crystal faces of TiO2 [Shifu et
al., 2006]. Korosi et al., (2004) prepared SnO2/clay nanocomposites having Sn
content varying between 15 and 50 percentage by weight. The photooxidative
efficiency of SnO2/clay photocatalysts was investigated and established that the
catalytic effect is mainly due to the presence of nanosized tin dioxide particles.
Bessekhouad et al., (2006) investigated the activity of TiO2, CdS and coupled
CdS/TiO2 powders. He concluded that under visible light, CdS/TiO2 exhibit faster
degradation rate than either of the photocatalysts.
Other studies report “coupled” semiconductor powders (TiO2/ZnO, TiO2/SnO2, ZnO/
SnO2) [Wu and Chang, 2006; Wu, 2004] but it is not clear whether the two
semiconductor particles are truly coupled in these cases.
42
2.4.4 Ternary Hybrid Photocatalysts
Recently three-component junction systems have attracted considerable interest
since these junction systems can result in higher photocatalytic activity and peculiar
characteristics in comparison with pure TiO2 or two-component junction systems
(i.e., noble metal/TiO2 or semiconductor/TiO2 systems). At appropriate Ag and V
dopings, the visible and UV-light photocatalytic activity of the Ag/V–TiO2
nanocomposites towards dyes Rhodamine B (RB) and Coomassie Brilliant Blue G-250
(CBB), degradation outperformed Degussa P25, as-prepared pure TiO2, and singledoped Ag/TiO2 or V–TiO2 systems [Yang et al., 2010]. Ag and InVO4 codoped TiO2
composite thin film with 1% Ag doping exhibited higher visible-light photocatalytic
activity for decomposition of aqueous methyl orange compared with TiO2 or InVO4–
TiO2 [Ge et al., 2006].
2.4.5 Catalyst in Immobilized Form
Vinodgopal and Kamat, (1995) employed thin film semiconductor to enhance the
rate of azo dye photocatalytic destruction. Gopalkrishnan and Mohan, (1997) carried
out photodegradation of few textile dyes using TiO2 in fixed mode i.e. by coating TiO2
on sand and hollow glass beads.
TiO2 has been affixed to a variety of surfaces like glass, silica, alumina, clay, metals,
composite inert oxide containing either Ru or Ln metal, ceramics, polymers etc.
[Ibhadon et al., 2008; Feng et al., 2004; Fernandez at al., 2004; Otsuka and Ueda
2004; Rao et al., 2003; Liu et al., 2000; Heung and Anderson, 1996; Lei et al., 1999;
Miller et al., 1999; Shchukin et al., 1999; Sirisuk et al., 1999; Gopidas and Kamat,
1989].
43
Subrahmanyam et al., (1998) studied the photocatalytic degradation of varios dyes
such as Vat Blue, Fast Orange GC Base, Drimarine Yellow, 3GLI and Bromothymol
Blue using batch reactor. They used TiO2 based catalysts immobilized on ceramic
beads and SiO2.
Surface bound-conjugated TiO2/SiO2 was prepared by means of impregnation
method for photocatalysis of azo dyes by Chun et al., (2001). This TiO2 fixed on silica
gel showed three times higher photo-activity for the degradation of reactive dye and
was characterized by XRD, FTIR, XPS and BET measurements. The Pt-modified TiO2–
SiO2 catalysts synthesized by the photo-reduction method showed a high
photocatalytic activity for the degradation of methyl orange in the visible-light range
[Zhang et al., 2007].
Guillard et al., (2003) studied the influence of chemical structure of dyes, pH and
inorganic salt on their photocatalytic degradation using TiO2 as photocatalyst.
Comparison of the efficiency of powder and supported TiO2 by using anionic
(Alizarin S, azo-Methyl Red, Congo Red and Orange G) and cationic (Methylene Blue )
dyes either individually or in mixtures was done. The photocatalytic deficiency of
TiO2 coated on glass by sol-gel method was found comparable to that of TiO2
powder. Noorjahan et al., (2003) reported the photocatalytic degradation of H-acid,
a dye intermediate in TiO2 suspensions and TiO2 thin film fixed bed reactor (TFFBR).
The immobilization method used was simple and did not require thermal treatment
of the catalysts at high temperatures and it was concluded that this type of
treatment method might be used for the photocatalytic treatment of effluents at
higher scale.
44
Bouras et al., (2004) demonstrated the photodegradation of Basic Blue by highly
efficient nanoctystalline titania films. Transparent nanocrystalline titania films were
deposited on glass slides by using sol-gel procedures carried out in the presence of
surfactants, Triton X -100. Films were calcined at 550 °C to ensure destruction of all
organic residues. These films were found to be very efficient for photodegradation of
Basic Blue dye, in aqueous solutions.
Photocatalytic degradation of a few dyes have been successfully done using
immobilized ZnO films (Comparelli et al., 2004; Roselin et al., 2002; Yoshida et al.,
2002; Sakthivel et al., 2001). Comparelli et al., (2005) immobilized ZnO nanocrystals
with different surface organic coatings and commercial ZnO powder onto
transparent substrates and comparatively examined as photocatalyst for the UV
induced degradation of two dyes, Methyl Red and Methyl Orange in water.
Photocatalyst of TiO2/ZnO in a film state has also been prepared by different
methods, but no detailed study has been conducted on the relationship between the
characteristics and photocatalytic activity of the film [Tian et al. 2009; Liao et al.,
2008].However the drawback of immobilization has been reported, as the active
surface area of the photocatalyst is dramatically reduced, in turn leading to a
relevant decrease in the performances [Arabatzis et al., 2002; Rachel et al., 2002;].
Behnajady et al., (2007) studied the photocatalytic degradation of C.I. Acid Red 27
(AR27), an anionic monoazo dye of acid class, in aqueous solutions in a tubular
continuous-flow photoreactor with immobilized TiO2 on glass plates under UV light.
Results show that a linear relation exists between pseudo-first-order reaction rate
constant and reciprocal of volumetric flow rate.Fathinia et al., (2010) compared
45
photocatalytic degradation of an anionic and a cationic dye with different molecular
structures using glass plates as a support of TiO2 nanoparticles.
2.5Photoreaction Intermediatesand Mineralization Products
Rajeshwar et al., (2008) reviewed the formation of reaction intermediates during the
heterogeneous photocatalytic degradation of azo dyes (AO7, Acid Orange 20, Acid
Orange 52, Reactive Black 5, Reactive Yellow 145,Congo Red, Disperse Orange 1,
Disperse Red 1 and Disperse Red 13 etc.) and non-azo dyes (Methylene
Blue/thiazine, Rhodamine B, Eosin, Sulphorhodamine/xanthenes, Alizarine Red, Acid
Blue 80, Reactive Blue 4/Anthraquinone etc.). Hu et al., (2003a) reported the
formation of more aliphatic amines and/or amide by-products, which were further
transformed to NH4+.
2.6 Analytical Techniques for Identification of Intermediates
Various analytical techniques such as high performance liquid chromatography
(HPLC) [Baiocchi et al., 2002 ; Galindo et al., 2000], gas chromatography–mass
spectrometry (GC–MS) [Sajjad et al., 2010; Daneshvar et al., 2003; Galindo et al.,
2000; Stylidi et al., 2003; Liu et al., 1999], 1H NMR [Galindo et al., 2000; Liu et al.,
1999 ; Vinodgopal et al., 1994a], diffuse reflectance FTIR [Stylidi et al., 2003; Hu et
al., 2003b; Lucarelli et al., 2000; Vinodgopal et al., 1994b;] and electron spin
resonance (ESR) [Liu et al., 1999] were used for the determination of organic
intermediates.
2.7 Degradation Pathway
The kinetic and mechanistic aspects of dye degradation have been investigated and
reported in literature [Galindo et al., 2002; Bauer et al., 2001; Houas et al., 2001]. Liu
and Zhao, (2000) examined the mineralization extent of Sulphorhodamine B (SRB)
46
photocatalytic degradation in presence of TiO2, the formation of intermediates and
final products were monitored to assess the degradation pathways. A TiO2-mediated
photodegradation mechanism for the evolution of aliphatic organic acids during the
amaranth photocatalytic degradation was proposed. [Karkmaz et al., 2004].
Konstantinou and Albanis, (2004) reviewed the photocatalytic degradation of azo
dyes containing different functionalities using TiO2 as photocatalyst in aqueous
solution under solar and UV irradiation and concluded that the mechanism of the
photodegradation depends on the radiation used. Charge injection mechanism takes
place under visible radiation whereas charge separation occurred under UV light
radiation. The study also focused on the determination of the principal organic
intermediates (hydroxylated derivatives, aromatic amines, naphthoquinone,
phenolic compounds and several organic acids) as well as on the degradation
pathways for two azo dyes, Acid Orange 7 and Acid Orange 52 followed during the
process.
Hasnat et al., (2005) compare photocatalytic degradation of Methylene Blue, a
cationic dye and Procion Red, an anionic dye in TiO2 dispersions under visible light
and discussed the extent of degradation in terms of Langmuir-Hinshelwood model.
The degradation pathway of Procion Red was found to be somewhat different from
Methylene Blue. Stylidi et al., 2003 identified reaction intermediates for phocatalytic
degradation of Acid Orange 7 (AO7) by GC/MS. Reaction mechanism and
degradation pathway of AO7 removal by three-dimensional electrode reactor has
been investigated by using HPLC, FTIR and GC/MS [Zhao et al., 2010].
A detailed degradation pathway of methylene blue has been proposed by a careful
identification of intermediate products, in particular aromatics, whose successive
47
hydroxylations lead to the aromatic ring opening and complete mineralization of
carbon, nitrogen and sulfur heteroatoms into CO2, NH4+, NO3− and SO42−respectively.
[Houas et al., 2001].Marci et al., (2003) identified three main transient products still
maintaining the chromophoric azo group of methyl-orange prior to their
transformation into other photocatalytic degradation products by HPLCMS.Holcapek et al., 2001 recommended liquid chromatographic technique for the
analysis of mixtures of sulfonated azo dyes in water and wastewater.
McCallum et al., (2000) analyzed four degradation products of Reactive Blue 19 by
using NMR, LC-MS and Raman spectroscopy.The formation of highly oxidized
products formed during ozonation of Reactive Yellow 84 such as salts (NO3-, SO42-, Cl) and short chained carboxylic acids (oxalic acid, formic acid, etc.) were observed
using HPIC [Schulz et al., 1992]. With the help of LC -mass spectrometry polar
components in surface water were reported on the basis of accurate masses
[Hogenboom et al., 1999]. Degradation products of CI Reactive Orange 16 were
identified by GC/MS [Bilgi and Demir, 2005].The HPLC–PDA–ESI-MS technique was
used to obtain the mechanistic details of this TiO2-assisted photodegradation of the
MG dye with UV irradiation [Chen et al., 2006]. Under acidic conditions, the results
indicated that the photodegradation mechanism is favourable to cleavage of the
whole conjugated chromophore structure of the MG dye. Under basic conditions,
the results showed that the photodegradation mechanism is favourable to a
formation of a series of N-de-methylated intermediates of the MG dye. Liquid
chromatography/mass spectrometry (LC/MS) analysis confirmed that methyl orange
was completely degraded after 30 min using 0.1% Zn-doped TiO2 nanocrystals [Chen
et al., 2008].
48
Hu et al., (2003b) investigated the photocatalytic degradation products of triazinecontaining azo dyes, Procion Red MX-5B and Reactive Brilliant Red K-2G, in aqueous
TiO2 dispersions by using ion chromatography (IC), HPLC, UV-Vis, FTIR and GC-MS
analysis.
2.8 Into the Real World: Textile Effluents
Real wastewater often contains high levels of suspended solids as well as other
additives including salts. Wastes from dye rinse baths having different colours were
compared and it was found that blue-coloured rinse degrade faster than pink,
orange, or yellow coloured dye rinse baths [Gopalakrishnan and Mohan, 1997].
Balcioglu et al., (1999) also compared raw, coagulated and biologically pre-treated
textile effluent. Aguedach et al., (2005) suggested that UV-irradiated TiO2 coated on
non-woven paper may be considered as an adequate process for the discolouration
and detoxification of the treatment of diluted coloured textile wastewateravoiding
the tedious filtration step. Lachheb et al., (2002) suggested that TiO2/UV
photocatalysis may be envisaged as a method for treatment of diluted coloured
wastewaters not only for decoluorization, but also for detoxification, in particular in
textile industries in semi-arid countries.
It was suggested that the utilization of combining photocatalysis and solar
technologies may be developed to a useful process for the reduction of water
pollution caused by dying compounds [Li and Zhang, 1996; Minero et al., 1996;
Lindner et al., 1995; Daoxin et al., 1994; Minero et al., 1993]. Kuo and Ho (2006) and
Cangcang and Dewan (1998) reported the photocatalytic degradation of various dyes
using solar light and immobilized TiO2
decolourization of wastewater.
as potential application for the
49
The photocatalytic degradation of few organic dyes in aqueous solution with TiO2 as
photocatalyst
in
slurry
form
have
been
investigated
under
sunlight
[Muruganandham and Swaminathan, 2004b; Augugliaro et al., 2002; Neppolian et
al., 2002a; Sauer et al., 2002; Sivakumar and Shanthi, 2001]. It was concluded that
solar light induced degradation of textile dye in wastewater is a viable technique for
wastewater treatment. Stylidi et al., (2003) and Saquib and Muneer (2002) studied
the photocatalytic degradation of an aqueous solution of dyes in TiO2 suspension
with the use of a solar light simulating source and the photo degradation pathway
was proposed on the basis of quantitative and qualitative detection of intermediate
compounds. Prieto et al., (2005) reported that solar photocatalysis is very efficient to
decolour the textile effluent of the dyes investigated.
Daneshwar et al., (2005) reported the photocatalytic decolourization of the azo dye
C.I. Direct Red 23, under the experimental conditions present in real wastewater
sample. The photocatalytic decolourization efficiency of the dye in the real sample
under the optimized conditions were raised to 80% at the irradiation time of 3 h.
The photocatalytic degradation of various dyes (Orange II, Orange G, Congo Red,
Indigo Carmine, Crystal Violet, Malachite Green, Remazol Blue and Methyl Yellow)
has been studied, using P25 Degussa as catalyst and was concluded that the process
was found to be effective for the decolourization of textile wastewater [Hachem et
al., 2001]. The solar photocatalytic degradation of various dyes has been studied
over combustion synthesized nano TiO2 and the activity was compared with that of
commercial Degussa P-25 TiO2 under similar conditions and the effect of various
parameters was also studied [Nagaveni et al., 2004] and concluded higher
50
degradation rate with combustion synthesized nano TiO2 as compared to Degussa P25.
Neppolian et al., (2002b) investigated the photocatalytic degradation of three
commercial textile dyes with different structure using different photocatalysts in
aqueous solution under solar irradiation. Experiments were conducted to optimize
various parameters like amount of catalyst, concentration of dye, pH and solar light
intensity. It was reported that TiO2 (Degussa P25) is the best catalyst in comparison
with other commercial photocatalysts (TiO2 (Merck), ZnO, ZrO2, WO3 and CdS)
photocatalysis under solar irradiation may be a viable technique for the safe disposal
of textile wastewater into the water streams.
The feasibility of photocatalytic decolourization of real textile dyeing rinse
wastewaters (RWW’s) collected from the low salt cotton textile dyeing industry was
studied by Kanmani and Thanasekaran, (2003) using two grades of titanium dioxide
(TiO2) under ultraviolet and solar light sources. Their colour measurements were
done at multiple wavelengths of 436, 525 and 620 nm as the RWW’s contained more
than one dye.
2.9 Combined Operations
Wastewater containing dyes can be treated biochemically in combination with
photocatalytic, chemical, thermochemical and physico-chemical treatment methods
[Gopalkrishnan and Mohan, 1997; Zhang et al., 1997; Nasr et al., 1996;
Subrahmanyam, 1996; Lakshmi et al., 1995; Vinodgopal and Kamat, 1995; Zang et al.,
1995; Davis et al., 1994; Vinodgopal et al., 1994; Subrahmanyam, 1987]. Scott and
Ollis (2006); Ledakowicz et al., (2001) reviewed the studies which used the
combination of chemical and biological degradation of organic contaminants in
51
water.Sarria et al., (2002) reviewed work in coupling advanced oxidation processes
(AOP’s) and biological system for wastewater treatment and confirmed the
beneficial effects of such two-step treatment at laboratory scale.Reddy, (2003)
studied the decolourization and mineralization of common industry effluents with
TiO2 photocatalyst using solar light illumination and reported the colour and COD
removal by the photocatalytic treatment is 74% and 62%, respectively (treated under
sunlight for 40 h). Whereas the COD removal in biological treatment is only 18%
after treating for 120 h using Up-flow Anaerobic Sludge Blanket (UASB) reactor and
the samples treated after photocatalytic method are further subjected to biological
treatment resulted in improved levels of COD removal of 72%.