Photocatalytic Discoloration of Methyl Orange and Indigo Carmine on TiO2 (P25) Deposited on Conducting Substrates: Effect of H2O2 and S2O82-
Limam Moctar Bawa
The photocatalytic degradation of Methyl Orange (Mo) has been investigated in aqueous solutions using thin layers of TiO2 deposited on stainless steel by electrophoresis. The effects of photocatalyst support, pH, the addition of hydrogen peroxide and the addition of peroxodisulfate ions have been investigated. The results suggest that conducting glass was the best photocatalyst support but due to its cost, we privilege the use of the system TiO2/stainless steel. Hydrogen peroxide and peroxodisulfate ions have a positive effect on reaction rate. S2O82- was the best oxidant which allows in a few minutes (~30 min) the total disappearance of MO. Using the best condition for MO degradation; we studied the photocatalytic degradation of Indigo Carmine, an indigoid dye.
to cite this article:
Tomkouani Kodom, Etsri Amouzou, Gbandi Djaneye-Boundjou and Limam Moctar Bawa, 2012. Photocatalytic Discoloration of Methyl Orange and Indigo Carmine on TiO2 (P25) Deposited on Conducting Substrates: Effect of H2O2 and S2O82-. International Journal of Chemical Technology, 4: 45-56.
January 12, 2012; Accepted: April 03, 2012;
Published: June 02, 2012
Dyes are a large group of organic compounds used in different fields such as
food, textile, cosmetic and chemical processes. Much waste water is produced
during these processes. A sizable fraction of synthetic or natural organic dyes
is lost during the dying process and is released in the effluent water streams
from the above industries or laboratories (Rajeshwar et
al., 2008). Of the synthetic dyes manufactured today, azo compounds
are dominant (50-70). Methyl orange belongs to one of the most important classes
of commercial dyes and it has a very short excited-state life and is stable
in visible and near UV light (Nam et al., 2002).
Methyl orange is an anionic azo dye with good stability and special color characteristics.
It is widely used in the printing, textile and photographic industries (Guo
et al., 2011). Methyl orange is also much used in chemical laboratories
as a color indicator. Azo dyes are known to be carcinogens because of their
decomposition to aromatic amines (Guivarch, 2004). Therefore,
discoloration and detoxification of azo dye effluents have an increasingly important
environmental significance in recent years.
Indigo carmine is considered a highly toxic indigoid dye. Contact with it can
cause skin and eye irritation. It can also cause permanent injury to the cornea
and conjunctiva (Barka et al., 2008). Many common
methods such as adsorption on activated carbon, ultrafiltration, reverse osmosis,
etc., are used for the removal of dye pollutants. However, they are non-destructive
and merely transfer pollutants from one phase (for example, aqueous) to another
(for example, adsorbent) (Maleki et al., 2006).
For some azo dyes removal in water, biodegradation method is slow and inefficient
(for example, acid orange 7). Chlorination and ozonation are also relatively
inefficient and have high operating cost (Rajeshwar et
al., 2008). Due to their synthetic origin and the presence of complex
aromatic structure, many textile dyes are difficult to degraded by these methods
(Atmani et al., 2009; Pasukphun
et al., 2010). New methods for water treatment, as well as improvements
in the existing processes, are required to protect our environment. Advanced
Oxidation Processes (AOPs) such as heterogeneous photocatalysis using titanium
dioxide have gained much attention today. In this process, when TiO2
is illuminated by photons of energy equal to or greater than its band gap energy,
the promotion of electrons from the valence band to the conduction band occurs.
While electrons are attracted by their acceptors such as oxygen, metal cations
and photoproduced holes react with electron donors (here organic compounds).
The mechanism can be represented by the following steps (Ahmed
et al., 2010):
In this reaction, h+ and e- are powerful oxidizing and reducing agents, respectively. The decomposition of organic steps are expressed as:
In this study, we report the photo-oxidation of Methyl Orange in aqueous media
using thin layers of titanium dioxide in order to avoid the expensive step of
filtration when using TiO2 in suspension. We used two different supports
and their effect on the photooxidation process will be discussed. H2O2
and S2O82- have been reported as electron acceptors
having a higher activity and efficiency in this role than oxygen for titania
conduction-band electrons (Fernandez et al., 2004;
Konstantinou and Albanis, 2004) as shown below:
These reactions inhibit electron-hole recombination at the semi-conductor surface.
Hydrogen peroxide also increases OH° radical production when absorbing light
during photolysis (Saquib et al., 2003; So
et al., 2002) according to the equation below:
Considering these reported reactions, H2O2 or S2O82- addition seems beneficial to the photooxidation of dyes. The effect of H2O2 and S2O82- ions will be discussed.
The objectives of this research were to determine (i) How the system (UV/TiO2/SS) differs from the system UV/TiO2/SnO2: F and (ii) How these photocatalytic degradation pathways differ from the corresponding system in the presence of oxidants such as H2O2 and S2O82¯.
MATERIALS AND METHODS
Materials: Titanium dioxide P-25 from Degussa is used in this study as purchased. For thin layer preparation, acetylaceton and polyethylen glycol tert-octylphenyl ether (Triton X -100) from Fluka and distillated water are used to prepare a first suspension which was then diluted with analytical grade methanol. TiO2 P-25 was deposited on three conducting supports such as stainless steel 304 L and conducting glass from Solems France. Methyl orange C14H14N3SO3Na, used as model pollutant was obtained from Acros organic without any purification. Indigo Carmine (indigo disulphonic-5, 5 acid) was supplied by Acros Organics and was used as purchased. The molecular schemes of the two dyes are presented below.
Thin layer preparation: Thin layers were prepared by electrophoresis method. The conducting glass, stainless steel or aluminum used as substrates was first cleaned using a solution of sulfuric acid (75%) and oxygen peroxide (25%) and washed several times with distilled water. A DC voltage of 10 V was applied between a stainless steel anode and a cathode (conducting glass SnO2: F or stainless steel), immersed in a colloidal suspension (50 g L-1 of photocatalyst in methanol). The distance between the electrodes was 1 cm and the deposition time used was 20 sec. Layers so obtained were dried at room temperature and then treated in an oven at 450°C (5°C min-1) for one hour.
Photocatalytic reactor and photodegradation studies: Photocatalysis experiments were performed in a flow loop reactor open to air, provided by a 240 cm2 surface of thin photocatalyst layer, volume of treatment 0.9 L solution.
A UV-A Black light Blue Lamp, (15 W, 365 nm) was used as light source under continuous flow conditions. The optical path through the solution was 1 cm and the temperature was kept at ca. 29-31°C. A New Jet pump was used for solution circulation during all the experiment.
The disappearance of the MO was measured using a Digitron Elvi 675 spectrophotometer. All experiments were performed at room temperature and atmospheric pressure. The photocatalytic degradation experiments were carried out by loading 900 mL of dye solution in the reactor. The scheme of the reactor is shown in Fig. 1.
Langmuir-Hinshelwood model: For azo dye photooxidation using AOPs, it
is well known that the discoloration rate obeys the Langmuir-Hinshelwood (L-H)
kinetic model (Tang and An, 1995):
where, r is the oxidation rate of the reactant, C the concentration of the reactant, t the illumination time, k the reaction rate constant and K is the adsorption coefficient of the reactant. When the chemical concentration Co is a millimolar solution (Co small) the equation can be simplified to an apparent first-order equation:
where, A0 and A are the initial absorption and absorption at illumination
time t, kapp equals the apparent first-order rate constant (Tchatchueng
et al., 2009).
||(a) Schematic diagram of photocatalytic flow loop reactor,
(b) Photograph of real reactor (c), Initial solution of MO (1 mM) and the
resulting solution after photodegradation (t = 60 min) using S2O82-
For comparison of different conditions, this kinetic model is used to depict
the pseudo-first order, kapp, that corresponds to the slope of the
straight line of the curve Ln(C0/C) vs. time, t corresponding to
50% discoloration. Also, we used the discoloration percentage calculated as
follows Sarnthima and Khammuang 2008:
Photodegradation of Methyl Orange: effect of photocatalyst support:
Most studies related to such photodegradation reactions have been carried out
using suspensions of powdered TiO2 (usually Degussa P-25) in a polluted
aqueous solution. However, from a practical point of view it may not be possible
to use catalyst suspensions in slurry photoreactors because of the filtration
problems linked to the small size of the titania particles (Fernandez
et al., 1995). For this reason, attempts have been made to immobilize
the catalyst on rigid supports (Acevedo-Pena et al.,
2009; Villarreal et al., 2004; Wagner
et al., 2008). In this study, we used two metallic substrates (stainless
steel and conducting glass SnO2: F) to deposit TiO2 by
electrophoresis. The discoloration of MO 0.01 mM on TiO2 deposited
on these substrates was performed at free pH 6.1. Figure 2
shows the photodegradation rate in these conditions. The rate of photodegradation
was greater when TiO2 was supported on conducting glass.
||Kinetics of the disappearance of methyl orange (1 mM), UV
irradiation with or without TiO2/SS or TiO2/SnO2:
|| Absorbance of methyl orange (1 mM) versus wavelength, effect
The rate constants were 0.00315 and 0.00508 min-1, respectively
with TiO2/SS and TiO2/SnO2: F. We noted also
that there was no significant photooxidation of MO using only UV light (15 W).
However, due to the greater expense of the conducting glass than stainless steel,
the latter substrate was used in all experiments.
Effect of pH on photodegradation of Methyl Orange: In Chemistry, pH
is an important factor in many reactions. Particularly in photocatalysis, pH
has some influence such as: - ionization state of the surface of TiO2;
-effect on the structure of dye component by ion exchange;-reaction between
hydroxide ion and positive hole. So, it is important to know and to control
the pH in order to enhance the rate of degradation. Five pH values (1; 3; 6.1;
10; 12) were chosen for our experiment to determine their effect on photodegradation
rate of MO. Firstly, we measured the absorption of MO over the wavelength 400
to 650 nm to determine the maximum absorption peak. Figure 3
shows a plot of the absorption of MO versus wavelength.
|| Kinetics of the disappearance of methyl orange (1 mM) under
UV irradiation with or without TiO2/SS at different pH (1; 3;
6.1; 10; 12)
|| Evolution of the apparent first order kinetic constant (kapp)
with change in pH of MO solution
The maximum absorption peak depends on the pH. From natural pH to pH = 12,
the maximum peak didnt change and was 450 nm while it was 490 and 500
nm, respectively for pH = 3 and 1. For the MO photodegradation studies, we used
450 nm when the pH was higher than 6, 490 nm when the pH varied from 2 to 3
and 500 nm when the pH was under 1.
The photodegradation of MO was studied at these different pH values (Fig. 4). The rate of disappearance of MO strongly depends on pH. The rate increases when pH decreases. The evolution of the pseudo-first order constant illustrated these phenomena well (Fig. 5). pH = 1 was more favorable for the photodegradation of MO in our conditions. The adsorptions calculated were 3.3, 2.9, 0.4, 0.3 and 0.8% for pH (1; 3; 6.1; 10; 12), respectively.
Effect of H2O2 on photodegradation of MO: Electron/hole
recombination is one of the main drawbacks in the application of TiO2
photocatalysis as it causes a waste of energy. In the absence of a suitable
electron acceptor or donor, the recombination step is predominant and limits
|| Influence of H2O2 on photodegradation
rate of methyl orange (1 mM)
||Photooxidation of methyl orange (1 mM) under UV light: effect
of peroxodisulfate ions on the rate of oxidation
Thus, it is crucial to prevent electron-hole recombination to ensure efficient
photocatalysis. Molecular oxygen is generally used as an electron acceptor in
heterogeneous photocatalytic reactions (Nam et al.,
2002). Addition of external oxidant/electron acceptors to a semiconductor
suspension has been shown to improve the photocatalytic degradation of organic
contaminants by (1) Removing the electron-hole recombination by accepting the
conduction band electron; (2) Increasing the hydroxyl radical concentration
and oxidation rate of intermediate compounds and (3) Generating more radicals
and other oxidizing species to accelerate the degradation efficiency of intermediate
compounds (Ahmed et al., 2010). Figure
6 shows the discoloration curves for MO 0.01 mM for different hydrogen peroxide
concentrations. The discoloration rate increased in the presence of H2O2.
Increasing the concentration of H2O2 did not significantly
affect degradation rate.
Effect of peroxodisulfate ions S2O82- on
photodegradation of MO: Figure 7 reports the discoloration
rate of MO under UV light in the presence of persulfate at different concentrations.
S2O82- had a strong positive effect on the
rate of photodegradation of Methyl Orange. The rate of oxidation was also significant
when S2O82- was used without TiO2 but
under UV light.
|| Photolysis, adsorption and photocatalytic degradation of
Indigo Carmine (0.03 mM) under UV light, effect of hydrogen peroxide and
In all cases, peroxodisulfate ions strongly affected photodegradation rate
than did H2O2. When persulfate ions were added to the
MO solution, the pH decreased. The initial pH values were 3.6; 3.1; 2.8; 2.6;
2.5, respectively for initial concentration of S2O82-
equal to 0.02; 0.04, 0.08; 0.12; 0.2 M.
Photodegradation of indigo carmine (IC): The decrease in the concentration
of the dyes was observed from the characteristic absorption at 610 nm for indigo
carmine in water. The adsorption and photolysis of Indigo Carmine was studied
to determine the exact effect of UV/TiO2/SS. We used the best conditions
of MO photodegradation using the oxidants H2O2 and S2O82-.
Figure 8 presents the kinetics of IC degradation under these
conditions. The initial pH and the conductivity of the IC (0.03 mM) solution
were 4.64 and 22.9 μS cm-1, respectively. After 200 min of photocatalytic
experimentation under UV without any addition, the pH decreased slightly to
4.1 while the conductivity increased to 44.7 μS cm-1. The curves
in Fig. 8 indicate that there is no disappearance of IC under
UV light without TiO2 and the adsorption of IC reached 7.8% after
200 min. A strong adsorption of Indigo Carmine has also been reported in the
literature (Barka et al., 2008; Vautier
et al., 2001). Addition of H2O2 and S2O82-
was very beneficial to the rate of the degradation but peroxodisulfate ions
had a strong effect as we ready found in the case of MO. The apparent first
order kinetic constants obtained according to Langmuir-Hinshelwoods law
of adsorption, photocatalytic degradation with C only, photocatalytic degradation
in the presence of H2O2 or S2O82-
are 3.57x10-4, 6x10-3, 17.55x10-3 and 172.52x10-3
min-1, respectively. In photocatalytic degradation without oxidant
addition, the rate of degradation of IC was two times bigger than the MO one
but the effect of S2O82- although strong, was
reduced in the case of IC.
Discussions of the interface reaction between a photocatalyst layer and a substrate,
and the effect of the interface reaction on the structure and properties of
the catalyst are important because the interface reaction could have a great
influence on the properties of the photocatalyst. According to the method of
deposition, here by electrophoresis that needs a conducting substrate, the elementary
composition of the substrate can be detrimental or beneficial to activity of
the photocatalyst. As shown in Fig. 2, the photodegradation
of MO under the same condition was greater with conducting glass (SnO2:
F) than with Stainless Steel (SS). The rate of degradation is enhanced about
1.6 times with conducting glass according to kapp (Table 1).
Our previous work showed that annealing the conducting glass at 450°C did
not affect its stability while for stainless steel the XRD patterns showed the
diffusion of iron (Fe) leading to the formation of iron oxides (Kodom,
2011). Zhu et al. (2001) also reported the
diffusion of Fe, Cr and Ni in the TiO2 surface layer during the annealing
treatment of stainless steel (Zhu et al., 2001).
The presence of these elements in the thin layer probably has a negative effect
on the activity of TiO2 resulting in less degradation of MO in the
case of TiO2/SS. These impurities may act as electron-hole recombination
centers. The photoactivity of TiO2 is more preserved when it is deposited
on conducting glass. However stainless steel has some advantages such as its
low coast and abundance.
Taking account only of the photoactivity of titanium dioxide, it could be enhanced in the presence of some oxidant or metallic cation. In this project, we report on the influence of pH, H2O2, S2O8.
pH is an important variable influencing the rate of degradation of some organic
compounds in photocatalytic processes. Analyzing our result, the rate of degradation
of MO is greater in acid medium. The result shows that high apparent first order
was obtained at pH 1 probably because of the adsorption which was also high
at this pH. One important consideration in the TiO2-photocatalyzed
reactions is the adsorption of the organic compounds on the surface of semiconductor
particles (Qourzal et al., 2006). Many investigations
have observed similar trends in the discoloration of azo dyes (Sakthivel
et al., 2003; Nam et al., 2002). However,
according to the curve of kapp versus pH, one can clearly see that
the rate tends to increase at pH 12 compared with pH 10.
All these observations can be explained: i) in terms of the location of the
point of zero charge (about pH 6.5) of the TiO2; ii) hydroxide ions
(OH-) in the solution inducing the generation of hydroxyl free radicals
(OH°), from the photooxidation of OH- by holes formed on the
surface of TiO2 particles. So, in acid media, the surface of TiO2
is electropositive (Xu et al., 2009). which favors
the adsorption of MO (negatively charged) related to degradation by photoproduced
holes. In alkaline media, negatively charged surface of TiO2 does
not favor the adsorption of MO but the high concentration of hydroxyl ions probably
favors the production of hydroxyl radicals which oxidize MO in solution.
Hydrogen peroxide concentration, the intensity of UV irradiation, pH, dye structure
and dye bath composition are the key factors influencing photodegradation rate
when H2O2 is used. Here, we focused our research on the
effect of hydrogen peroxide concentration using TiO2/SS illuminated
by UV light (15 W). As shown in Fig. 6 and 7,
the discoloration rate increased significantly in the presence of H2O2.
Regarding the apparent first order constant, the rate of degradation increased
about 3 times in the presence of H2O2 (20 mM). This might
be because hydrogen peroxide was reduced by photo-electrons in the conduction
band of TiO2 as shown in Eq. 6. Also, hydrogen
peroxide can be activated by UV light resulting in the formation hydroxyl radicals
Eq. 9. (Fatimah et al., 2009).
From the rate constant values in Table 1 it may be noted that
the reaction rate doesnt increase significantly when the concentration
of H2O2 was increased from 0.02 to 0.2 M. Also, the reaction
rate began to decrease with the concentration of 0.2 M H2O2.
This may be because the excess H2O2 becomes a scavenger
of photo-electrons in the TiO2 conduction band (Daneshvar
et al., 2003). Without TiO2, the 0.08 M hydrogen peroxide
induces the degradation of MO. H2O2 is decomposed by UV
light to produce hydroxyl radicals Eq. 9. which oxidize the
dye molecule in solution. The rate of oxidation with H2O2
is similar to rate obtained with TiO2/SS indicating the efficiency
of this advanced oxidation (H2O2/UV).
|| Apparent first order constant, kapp, depicted
according to Langmuir-Hinshelwood law
The presence of hydrogen peroxide decreases the pH of the solution and favors
the rate of degradation of MO in the presence of TiO2.
As for hydrogen peroxide, peroxodisulfate ions in solution are a strong oxidant
species which can accelerate the oxidation of organic dyes. Figure
7 very well shows its action which is much better than the action of H2O2.
In the presence of S2O82- (0.08 M), 1 h is
sufficient to obtain the total discoloration of Methyl Orange whereas in this
time only 45% discoloration is obtained with H2O2 (0.08
M). Augugliaro et al. (2002) have also reported
on the fast discoloration of MO with the system (MO+S2O82-+UV)/TiO2.
We remark also that without a photocatalyst, the action of peroxodisulfate ions
was still strong (Fig. 7). But in this case Augugliaro
et al. (2002) observed that there is no mineralization. In the presence
of S2O82- pH decreases in the course of the
reaction to 3 for all the runs due to the formation of hydrogen ions according
to Eq. 8. So, the fast degradation is attributed both to the
pH and the action of the oxidant.
Most of the organic pollutants in water can be completely decomposed and mineralized at the surface of UV-excited TiO2 photocatalysts. However, many factors have been monitored during the photodegradation of organic compounds using TiO2. Nevertheless, the discussion about the group of compounds is rarely mentioned in the literature. Here, we transpose the best condition of Methyl Orange, an azo-dye, photodegradation to Indigo Carmine, an indigoid dye Fig. 8. The result shows that the addition of H2O2 or S2O82- has a strong positive effect on the rate of IC disappearance. However, under the same condition H2O2 gives a positive effect during IC degradation than it does in the case of MO degradation. Also, we observe that the presence of S2O82- in the same concentration increases the rate of MO than IC degradation. The mechanisms of photodegradation of these dyes are significantly different probably because of their chromophore group. Also, due to their structure, the reactions of the hydroxyl radical (HO°) or photoproduced holes (h+) are different. In addition, the adsorption, negligible for MO, was about 7.8% for IC. This factor is important in photocatalysis considering direct mechanism reaction (adsorbed organic compound attacked by holes) which will be enhanced in the case of IC. Comparing the apparent first order kinetic constants, one clearly sees that the rate of IC with the TiO2/UV system is two times greater because adsorption is better.
The results of our study have shown that the degradation of Methyl Orange dye was successfully carried out using coated TiO2 on stainless steel. pH, H2O2 and S2O82- have a strong effect on the rate of MO degradation. The major degradation of the dye was achieved for pH less than the free pH of MO (1 mM), but the maximum rate was obtained for pH 1. A complete discoloration was obtained in a few minutes (~30 min) in the presence of peroxodisulfate in MO or IC solution. H2O2 has a positive effect on the rate of degradation of the two dyes but its influence is less significant than with S2O82- ions, the strongest oxidant that increases reaction rate.
This work was financially supported by IFS (International Foundation of Science) and also the AquaTiSol Projet of University of Poitiers.
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