Application of a Semiconductor Oxide-Based Catalyst in Heterogeneous Wastewater Treatment: A Green Technology Approach
The use of chlorine bleaching in study and pulp industry has
resulted in chlorinated phenol contaminated wastewater from that industry. A
new wastewater treatment process, sonophotodegradation of 2,4,6-trichlorophenol
(TCP) in a heterogeneous aqueous system was investigated and reported in this
study. TCP was degraded with ultrasonic or ultraviolet irradiation or a combination
of both, in the presence of titanium dioxide semiconductor catalyst (anatase
and/or rutile), in order to study the effectiveness of sonocatalysis, photocatalysis
and sonophotocatalysis oxidation in a batch sonophotoreactor system. Preliminary
studies presented in this study suggested that rutile worked well under sonocatalysis,
anatase was the preference for photocatalysis and sonophotocatalysis benefited
from a combined rutile and anatase mixture catalyst. Sonophotocatalysis oxidation
of TCP demonstrated a degradation that was higher than sonocatalysis or photocatalysis
individually while the first-order kinetics rate constants indicated that sonophotocatalysis
degradation of TCP was synergistic with a positive value of 0.0203 in the presence
of the mixture catalyst.
Received: February 23, 2012;
Accepted: August 29, 2012;
Published: September 08, 2012
Chlorinated phenolic compounds are widely used synthetic organic compounds,
which are used as either synthesis intermediates in pesticides or as bleaching
agent. These organic compounds are also commonly found in industrial waste water
and are frequently detected in polluted water (Czaplicka,
2004; Gao et al., 2008). Chlorinated phenolic
compounds or chlorophenols such as 2,4,6-trichlorophenol (TCP) is a derivative
of the chlorinated phenolic family. The TCP molecule contains a chlorine atom
each at the carbon position of the 2nd, 4th and 6th on the phenolic ring. These
compounds are polar compounds and as such, their polarity decreases with an
increase in Cl substitution on the benzene ring. The solubility of the TCP compound
has been reported to be 0.434 g/L at 25°C (Czaplicka,
2004) but dissolves very slowly in distilled water, typically taking two
days (with continuous stirring) to dissolve 100 mg L-1. TCP has been
used directly or indirectly in paint, pharmaceutical, pesticide, solvent, wood,
paper and pulp industries since the 1930s and is very commonly detected in those
industrial wastewater or effluent. This chemical is also used as a fungicide,
herbicide, insecticide, antiseptic, defoliant and as a glue preservative. Waste
disposal and water treatment methods such as incineration of municipal waste
and disinfection of water with chlorine, also generates TCP. Most uses of TCP
have been discontinued due to its toxicity, mutagenic and xenobiotic properties,
but several fungicides still require the use of TCP for synthesis purposes (USEPA,
2000; Chaliha and Bhattacharyya, 2008; Radhika
and Palanivelu, 2006). TCP, a weak acid, is classified as a probable human
carcinogen by the by the United States Environmental Protection Agency (University
of Berkeley, 2007; Gao and Wang, 2007; USEPA,
2000; Chaliha and Bhattacharyya, 2008; Radhika
and Palanivelu, 2006). TCP has xenobiotic characteristics, which makes it
resistant to biodegradation. Therefore, it has to be decomposed before being
discharged into rivers and lakes to prevent biomagnified toxicity to aquatic
life, especially, its accumulation via the food chain. The toxicity and recalcitrant
of TCP in the environment is because of its C-Cl bond, which is very stable
and also the relative location of the chlorine atoms to the hydroxyl group in
the TCP molecule (Tzou et al., 2008). The conventional
technologies used in the treatment of chlorophenols consist of three categories:
physical treatment, such as reverse osmosis, activated carbon adsorption (Joseph
et al., 2009a), biosorption (Sharain-Liew et
al., 2011), solvent extraction and hyperfiltration; chemical treatments,
such as chemical degradation, chemical oxidization, incineration, wet oxidation,
hypercritical oxidation, high-pressure impulsive discharge and low temperature
plasma; biological treatment, such as activated sludge, membrane separation
technique and aerobic/anaerobic methods. Adsorption, which is a common physical
treatment process, is nothing more than a phase transfer technique. The pollutant
is transferred from the liquid state to the solid state (instead of being mineralized
or destroyed) and further treatment is require prior to disposal. This physical
method is useful for fast removal of the pollutant from a specific location
but requires further treatment elsewhere. In chemical treatments, high costs
are normally required and in addition, reaction by-products and other chemicals
are released into the environment as a result of the treatment process. The
toxicity of TCP has rendered biological treatments to be ineffective in decomposing
TCP and other chlorophenols. This has resulted in biodegradation treatment be
ineffective (Wang et al., 2000; Krishnaiah,
2003; Jung et al., 2001; Aksu
and Yener, 2001). As these systems and technologies do not destroy or degrade
the pollutant molecules but merely transfers the liquid pollutant into a solid
one, which has to be treated before burying in landfills while the use of chemicals
in wastewater treatment causes secondary reactions, which may require further
treatment, there is a need to develop a cheap, innovative and regenerative method
which uses Green Technology to completely degrade water based organic pollutants
to harmless molecules in situ. Advanced oxidation process has been successfully
employed to treat wastewater contaminated with chlorophenols in recent years
(Joseph et al., 2009a; Li
Puma et al., 2008). Previously, using a combined batch-annular sonophotolysis
reactor, the feasibility of using ultrasonic sound waves and ultraviolet irradiation
to degrade the TCP molecules (Joseph et al., 2011)
in a non-catalytically reaction were successfully demonstrated. In that paper,
the intensities of the ultrasonic sound waves and ultraviolet irradiation were
investigated and documented. In this paper, using only a batch type reactor,
we present the heterogeneous sonophotocatalysis degradation reaction in batch
mode using two types of crystalline structured titania.
SEMICONDUCTOR -BASED CATALYST
The chemical and physical composition of TiO2 or titania is a very
well known and well researched material due to the stability of its chemical
structure, biocompatibility, physical, optical and electrical properties.
||Crystalline structure of, (a) Rutile, (b) Anatase and (c)
It exists in four mineral forms that are, anatase, rutile, brookite and Titanium
dioxide (B) or TiO2(B). Anatase type TiO2 has a crystalline
structure (Fig. 1) that corresponds to the tetragonal system
(with dipyramidal habit) and is use mainly as a photocatalyst under UV irradiation.
Rutile type TiO2 also has a tetragonal crystal structure (with prismatic
habit) (Fig. 1). This type of titania is mainly use as white
pigment in paint. Brookite type TiO2 has an orthorhombic crystalline
structure (Fig. 1). TiO2(B) is a monoclinic mineral
and is a relatively newcomer to the titania family. TiO2 is, therefore,
a versatile material that is use in various applications such as paint pigments,
sunscreen lotions, electrochemical electrodes, capacitors, solar cells and even
as a food coloring agent (Meacock et al., 1997)
including toothpastes. The preparation of TiO2 films has even demonstrated
futuristic application use such as thin dielectrics in Dynamic Random Access
Memory (DRAM) storage capacitors (Campbell et al.,
1999). In the last decade, however, TiO2 has been developed and
used as a photocatalyst for indoor and outdoor air purification and for the
purification and remediation of contaminated waters loaded with low concentrations
of toxic organic pollutants (Ollis and Al-Elkabi, 1993).
MATERIALS AND METHODS
Materials: 2,4,6-Trichlorophenol (98%) obtained from Aldrich was used without further purification. Anatase (powder, 99.8% trace metals basis) and rutile (powder, 99.995% trace metals basis) were obtained from Aldrich.
Procedure: A commercial 1 L jacketed glass vessel (Fig.
2a) was used as a batch-type sonophotoreactor with a diameter of 10 cm and
the depth of 20 cm. The sonophotoreactor was thermostated by a water jacket
and kept at a constant 30°C for all experiments by means of a temperature
controller (Thermo Haake K10). A modified syringe (Fig. 2b)
was used for sample extraction from point X (Fig. 2a). An
ultrasonic horn, 20 kHz, 750 W direct- immersion horn sonicator (Sonics and
Materials 750 W Vibra CellTM) was use to generate ultrasonic sound
waves in the sonophotoreactor.
|| Schematic diagram of the batch-type (a) sonophotoreactor
(b) Modified syringe used for sample extraction
The tip of the horn has a radiating surface area of 4.91 cm2, which
is sufficient to radiate solutions with a volume capacity of 500 to 1000 mL.
A 4 W UV BLB (Sylvania) lamp, was used to provide a source of UV-A radiation
with a maximum peak of 365 nm. This lamp was placed inside a quartz lamp sheath
and submerged into the solution as shown in Fig. 2a. Prior
to the experiment, the lamp was switched on for 20 min (before placing into
the quartz lamp sheath). This was done in order to bring the low-pressured mercury
lamp up to working temperature. The effective radiating length of the UV tube
was 7.8 cm (with the radiating surface area equal to 37.96 cm2) after
the lamp ends were masked with Teflon tape, due to instability of UV intensity
in these regions. A radiometer (Solar Light, model PMA2100) with a UVA detector
(Solar Light, model PMA2110) was used to measure the UV-A intensity from the
lamp encased in the quartz lamp sheath. The UV lamp and US horn are located
at the sides (1 cm from the vessels walls) and together they were placed
in a triangle with the aerator. The residual concentration of TCP was determined
by using a UV-Vis spectrophotometer (Varian Cary 50 conc) at the detection wavelength
of 295 nm (Shih et al., 1990; Michizoe
et al., 2004). In this study, the sonicator was set at 50% amplitude
in a continuous wave mode which gave an acoustic intensity of 10 W cm-2
and the UV-A lamp intensity was set at 6 mW cm-2 for all experiments.
All experiments were dark experiments and repeated in triplicates with the average
value taken and plotted into graphs. Four milliliter of sample was extracted
for each sampling (every 15 min). The total amount of sample taken for each
experiment was 4% of the total solution volume (500 mL).
RESULTS AND DISCUSSION
Three grams of catalyst (anatase or rutile) and 3 grams of anatase-rutile catalyst
with the ratio of 1:1, was use to study the sonocatalysis, photocatalysis and
sonophotocatalysis degradation of 500 mL, 50 ppm TCP. Fig. 3
shows the residual concentration TCP after 1 h of degradation for (a) Sonocatalysis,
(b) Photocatalysis and (c) Sonophotocatalysis. It is clearly shown that rutile
worked well under sonocatalysis, anatase was the preference for photocatalysis
and sonophotocatalysis benefited from a combined rutile and anatase mixture
catalyst in a ratio of 1:1. After 60 min contact time, sonocatalysis reduced
the 50 ppm TCP to 24.8 ppm (anatase), 19.42 ppm (rutile) and 22.4 ppm (anatase/rutile).
Under the same contact time parameter, photocatalysis reduced the 50 ppm TCP
to 10.3 ppm (anatase), 39.6 ppm (rutile) and 28.3 ppm (anatase/rutile). While
combining sono and photodegradation reduced the 50 ppm TCP to 15.1 ppm (anatase),
30.1 ppm (rutile) and 1.2 ppm (anatase/rutile). This clearly demonstrates the
effectiveness of combining both types of semiconductor catalyst under sonophotodegradation.
Anatase-type catalyst has a higher band-gap value of 3.2 eV as compare to rutile-type
catalyst but rutile-type catalyst can absorbed radiation that is nearer to visible
light. However, photocatalysis favors the use of anatase-type catalyst over
rutile-type catalyst due to its conduction band position which allows for better
reducing power as compared to rutile-type catalyst (Li Puma
et al., 2008). Sonophotocatalysis oxidation of TCP demonstrated a
degradation that was higher than sonocatalysis or photocatalysis individually
and after 1 h of degradation, almost all of the TCP was degraded.
||Residual concentration of TCP after 1 hour of degradation
for, (a) Sonocatalysis, (b) Photocatalysis and (c) Sonophotocatalysis
In sonocatalysis, the semiconductor oxide was activated by the ultrasonic
sound waves to generate .OH radicals. It is these radicals that contributed
to the breakdown of the TCP. The exact mechanism is still under investigation.
Ultrasonic sound waves can also generate .OH radicals by means of
sonolysis process (Joseph et al., 2011).
The degradation of TCP during sonolysis is due to the oxidation of the TCP molecule by hydroxyl radicals generated during the sonication (sonolysis or sonodecomposition of water) by means of cavitation according to Eq. 1 and 2:
where, ))) denotes sonication.
||The general mechanism of the photocatalytic reaction process
on irradiated TiO2
Cavitation is also positively influenced by the presence of particulate in
the solution and by the temperature of the solution (Joseph
et al., 2011).
In the photocatalysis process, the general mechanism of the photocatalytic
reaction process on irradiated TiO2 is shown in Table
1. The process of photodegradation of pollutants by TiO2 starts
by the absorption of UV radiation equal or higher than the band gap value of
3.2 eV for anatase or 3.0 eV for rutile onto the TiO2 particles creating
a photogeneration of holes and electron pairs (Eq. 3, Table
1) in the semiconductor valence band (hole) and conduction band (electron).
It must be noted that although both anatase and rutile type TiO2
absorb UV radiation, rutile type TiO2 can also absorb radiation that
are nearer to visible light. However, anatase type TiO2 exhibits
higher photocatalytic activity than rutile type TiO2 due to its conduction
band position which demonstrates stronger reducing power as compared to rutile
type TiO2. These energized holes and electron can either recombine
(Eq. 10, Table 1) and dissipate the absorbed
energy as heat or be available for use in the redox reactions (Eq.
4-6, Table 1). As such, photocatalysis
favors anatase instead of rutile.
In sonophotocatalysis, the combined use of ultrasonic sound waves and ultraviolet A irradiation in the presence of anatase and rutile catalysts contributed to a significant degradation rate as compared to the use of ultrasonic sound waves and ultraviolet A irradiation individually. Due to the preference of anatase by ultraviolet A irradiation and rutile by ultrasonic sound waves, a catalyst mixture of anatase and rutile in the ratio of 1:1 was most effective in the degradation of TCP and contributed to the synergistic effect of this process. The synergy effect of the sonophotocatalysis process can be represented as shown in Fig. 4.
||Schematic diagram of the sonophotocatalysis synergistic effect
||Kinetic constant plots for the sonocatalysis, (a) photocatalysis,
(b) and sonophotocatalysis (c) at constant solution temperature of 30°C
Reaction kinetics study was conducted in this research work. It was determined
that the sonocatalysis, photocatalysis and sonophotocatalysis degradation reaction
schemes complied with the first order reaction as the linear regression coefficient
(R2) was more than 0.9. Figure 5 shows the first
order reaction kinetic analysis. Under sonocatalysis degradation scheme, rutile-type
catalyst gave the highest reaction rate of 0.0179 min-1, followed
by anatase/rutile catalyst which was 0.0146 min-1 and finally anatase-type
catalyst, which gave 0.0122 min-1. While under photocatalysis degradation
scheme, anatase-type catalyst gave the highest reaction rate of 0.0261 min-1,
followed by anatase/rutile catalyst which was 0.0108 min-1 and finally
rutile-type catalyst, which gave 0.0042 min-1. Combining the sono
and photodegradation scheme, anatase/rutile-type catalyst gave the highest reaction
rate of 0.0478 min-1, followed by anatase-type catalyst which was
0.0186 min-1 and finally rutile-type catalyst, which gave 0.0089
min-1.The synergism between the photocatalysis, sonocatalysis and
sonophotocatalysis was determined using of first-order rate constants, according
to Eq. 2a, which was modified from our previous paper (Joseph
et al., 2009a, b):
In this equation, if S is greater than 0, the effect is synergistic,
if S is equal to 0, the effect is additive and if S
is less than 0, the effect is antagonistic. From Fig. 5c and
by means of calculation using equation 1, it is clearly shown
that the synergism of sonophotocatalysis in this degradation scheme is at (+)
The use of sound and light energy (ultrasound and ultraviolet irradiation), which has Green Technology characteristics, to degrade 2,4,6-trichlorophenol in the presence of titania catalyst has proven to be effective in degrading TCP. Preliminary studies of the reaction kinetics in the sonophotodegradation of 2,4,6-trichlorophenol in a heterogeneous aqueous system, suggest that rutile worked well under sonocatalysis, anatase was the preference for photocatalysis and sonophotocatalysis benefited from a combined rutile and anatase mixture catalyst. Combining both types of catalyst and degradation schemes resulted in a synergistic effect, which accelerated the degradation of TCP. This paper has demonstrated the effectiveness of treating chlorinated phenol contaminated wastewater using sonophotodegradation and suitable semiconductor catalyst. This work has also provided water treatment specialist and pollution experts an effective, low-cost and innovative water treatment method with Green Technology applications. This research work has promising commercializing prospect. With the development of suitable treatment plants, this application is expected to treat high volume of wastewater in situ, in shorter treatment time and at a lower cost than current methods.
This research was supported by the Center of Research and Innovation, University Malaysia Sabah (Grant No. FRG0203-SG-1/2010) and is gratefully acknowledged.
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