Chemical oxidative degradation of acridine orange dye in aqueous solution by Fenton's reagent

Article abstract of DOI:10.1002/jccs.200900165

Degradation of acridine orange (AO) in aqueous solution by Fenton's reagent (Fe²⁺ and H₂O₂) was investigated. The effects of different reaction parameters such as initial AO concentration, pH value of solution, ferrous con

Full text of DOI:10.1002/jccs.200900165

Journal of the Chinese Chemical Society, 2009, 56, 1147-1155
1147
Chemical Oxidative Degradation of Acridine Orange Dye in Aqueous
Solution by Fenton’s Reagent
Chiing-Chang Chena (
Yi-You Tzengb (
), Ren-Jang Wub (
),
) and Chung-Shin Luc* (
)
aDepartment of Science Application and Dissemination, National Taichung University,
Taichung 403, Taiwan, R.O.C.
bDepartment of Applied Chemistry, Providence University, Taichung 433, Taiwan, R.O.C.
cDepartment of General Education, National Taichung Nursing College, Taichung 403, Taiwan, R.O.C.
Degradation of acridine orange (AO) in aqueous solution by Fenton’s reagent (Fe2+ and H2O2) was in-
vestigated. The effects of different reaction parameters such as initial AO concentration, pH value of solu-
tion, ferrous concentration, hydrogen peroxide concentration, and the presence of chloride ion on the oxi-
dative degradation of AO were investigated. Under optimum conditions, 2 mM H2O2, 0.4 mM Fe2+ and pH
3.0, the initial 0.2 mM AO solution was reduced by 95.8% within 10 min. The primary intermediates of the
degradation reaction of AO were identified. The analytical results indicated that the N-de-methylation
degradation of AO dye took place in a stepwise manner to yield mono-, di-, tri-, and tetra-N-de-methylated
AO species generated during the Fenton process. The probable degradation pathways were proposed and
discussed.
Keywords: Fenton’s reagent; Acridine orange; Hydroxyl radical; N-de-methylation.
INTRODUCTION
Acridine orange (AO) is a heterocyclic dye contain-
pounds that are toxic for the microorganisms. The disposal
of sludge formed during biological treatment can create ad-
ditional expenses and environmental problems.5
ing nitrogen atoms which is widely used in the fields of
printing and dyeing, leather, printing ink, and lithography.1
It has also been used extensively in biological stains. Toxi-
cological investigations indicate that aminoacridine has
mutagenic potential.2 The release of this colored wastewater
in the ecosystem is a dramatic source of water pollution,
eutrophication, and perturbation in aquatic life.3 Therefore,
a method of treating the wastewater containing AO is highly
desirable now and in the near future.
In recent years “advanced oxidation processes” (AOPs)
have emerged as an alternative to conventional methods.
AOPs are based on the generation of very reactive species
such as hydroxyl radicals, which oxidize a broad range of
organic pollutants quickly and non-selectively.6 Among the
various AOPs that have been developed the application of
Fenton’s reagent for the destruction of water contaminants
is one of the promising technologies because of its power-
ful oxidizing potential and comparatively low cost.7 Fenton
oxidation is achieved from the reaction between H2O2 and a
ferrous salt under acidic conditions according to the fol-
lowing reaction:8,9
Conventional wastewater treatment methods such as
precipitation, adsorption, air stripping, flocculation, re-
verse osmosis, and ultrafiltration can be used for color re-
moval from dye-contaminated effluents.4 However, these
methods are non-destructive, since they only transfer the
contamination from one phase to another, causing second-
ary pollution and requiring further treatment. Biological
treatment is highly effective for the removal of most con-
taminants. Despite their success and cost effectiveness,
biodegradation processes are inherently slow, do not allow
for high degrees of removal, and are not suitable for com-
Fe2+ + H2O2 ® Fe3+ + OH- + •OH
(1)
Because of their high oxidation power, hydroxyl radi-
cals are capable of oxidizing many organic pollutants to
lower molecular weight compounds and eventually can
lead to the complete mineralization, converting them to
* Corresponding author. Tel: +886-4-2219-6999; Fax: +886-4-2219-4990; E-mail: cslu6@ntcnc.edu.tw

1148 J. Chin. Chem. Soc., Vol. 56, No. 6, 2009
Chen et al.
CO2, H2O and inorganic ions.10 Fenton’s reagent is an at-
tractive oxidative system for wastewater treatment due to
the fact that iron is an abundant and non-toxic element, and
because hydrogen peroxide is easy to handle and can be
broken down to environmentally benign products.11
amount of AO stock solution, adding ferrous sulfate, and
adjusting the pH value with nitric acid. The reactions were
initiated by adding calculated amounts of hydrogen perox-
ide. During the experiment, samples were collected after
various reaction times and immediately quenched by add-
ing 10 mLof 1 N Na2S2O3 solution to the reaction mixtures.
The degradation efficiency of the Fenton process was
evaluated using a double beam UV/vis spectrometer (Lamb-
da 25, Perkin-Elmer). The maximum absorbance wave-
length (lmax) of AO was found at 492 nm. Throughout the
reaction process, the degradation products were never
found to interfere with the measurement of AO concentra-
tion. Therefore, the concentration of AO in the reaction
mixture at different reaction times was determined by mea-
suring the absorption intensity of solution at 492 nm and
using a calibration curve. Prior to the measurement, a cali-
bration curve was obtained by using the standard AO solu-
tion with known concentrations.
The main objective of this study is to analyze the fea-
sibility of degradation of AO by the Fenton process. The in-
fluence of different operational parameters (pH of solution,
H2O2, Fe2+, AO concentration and the presence of chloride
ion) which affect the efficiency of the Fenton reaction in
AO oxidation was investigated. Accordingly, identification
of the reaction intermediates was performed using HPLC
coupled with a photodiode array detector and electrospray
ionization mass spectrometer, which reveals the degrada-
tion pathways of AO dye in the Fenton process, and can in
turn serve as a foundation for future applications.
EXPERIMENTAL
Materials
A Waters ZQ LC/MS system–equipped with a binary
pump, a photodiode array detector, an autosampler, and a
micromass detector–was used for separation and identifi-
cation. Two different kinds of solvents were prepared in
this study. Solvent A was 25 mM aqueous ammonium ace-
tate buffer (pH 6.9) while solvent B was methanol. LC was
carried out on an AtlantisÔ dC18 column (250 mm ´ 4.6
mm i.d., dp = 5 mm). The flow rate of the mobile phase was
set at 1.0 mL/min. A linear gradient was run as follows: t =
0, A = 95, B = 5; t = 20, A = 50, B = 50; t = 35-40, A = 10, B =
90; t = 45, A = 95, B = 5. The column effluent was intro-
duced into the ESI source of the mass spectrometer. Equipped
with an ESI interface, the quadruple mass spectrometer
with heated nebulizer probe at 350 °C was used with an ion
source temperature of 120 °C. ESI was carried out with the
vaporizer at 300 °C, nitrogen as sheath (80 psi), and auxil-
iary (20 psi) gas to assist with the preliminary nebulization
and initiate the ionization process. A discharge current of 5
mA was applied. Tube lens and capillary voltages were op-
timized for the maximum response during perfusion of the
AO standard.
The acridine orange dye was obtained from Sigma-
Aldrich and used without further purification. The chemi-
cal structure of the AO dye is shown in Fig. 1. Stock solu-
tion containing 10 mM of AO dye in water was prepared,
protected from light, and stored at 4 °C. HPLC analysis was
employed to confirm the presence of the AO dye as a pure
organic compound. Ferrous sulfate and hydrogen peroxide
(purity, 30%) were obtained from Acros Organics. FeSO4
solution (100 mM) was prepared and stored at 4 °C for 1
week at most. Reagent-grade ammonium acetate, sodium
hydroxide, nitric acid, and HPLC-grade methanol were pur-
chased from Merck. De-ionized water was used throughout
this study. The water was purified with a Milli-Q water ion-
exchange system (Millipore Co.) to give a resistivity of 1.8
´ 107 W-cm.
Experimental procedure and analytical methods
The reactions were carried out in a well stirred, batch
reactor with a total volume of 100 mL. The oxidation ex-
periments were performed in darkness and at 25 ± 1 °C. Re-
action mixtures were obtained by taking an appropriate
The mineralization of AO was monitored by measur-
ing the total organic carbon (TOC) content with a Dohrmann
Phoenix 8000 Carbon Analyzer, which employs a u.v./per-
sulfate oxidation method by directly injecting the aqueous
solution. For the TOC measurements, potassium phthalate
solution was used as the calibration standard with the con-
centrations of 0, 5, 10, 15, 20 and 25 mg/L.
Fig. 1. Chemical structure of acridine orange.

Degradation of AO Dye by the Fenton Oxidation Process
J. Chin. Chem. Soc., Vol. 56, No. 6, 2009 1149
RESULTS AND DISCUSSION
generation in the Fenton reaction. It was discovered that if
only Fe2+ were added in the solution instead of H2O2, AO
did not show decomposition. The decolorization efficiency
increased from 57.1 to 93.2% as a consequence of increas-
ing the H2O2 dosage from 0.5 to 2.0 mM at 60 min (Fig. 3).
This can be explained by the effect of •OH radicals pro-
duced additionally. However, when the dosage rose above
2.0 mM, the decolorization of AO was not improved but
dropped. This could be due to the fact that hydroperoxyl
radicals (•OOH) were generated in the presence of excess
of H2O2. Although •OOH is an effective oxidant itself, its
oxidation potential is much lower than that of •OH.17 The
hydroperoxyl radicals are much less reactive and do not
contribute to the oxidative degradation of organic sub-
strates, which occur only by reaction with •OH.18 From the
experimental results, therefore, 2.0 mM was selected as a
suitable H2O2 dosage.
Effect of initial pH on the decolorization of AO
The effect of the initial pH value of solutions on the
decolorization of AO by the Fenton oxidation process was
studied in the pH range of 2.0-6.0, and the results are
shown in Fig. 2. The results indicated that the decoloriza-
tion of AO was significantly influenced by the pH of the so-
lution, and the best decolorization efficiency was obtained
at pH 3.0. The lower efficiency at high pH values may be
due to the precipitation of Fe(OH)3. In this form, iron de-
composes H2O2 into oxygen and water,12 and consequently
the oxidation rate decreases because less hydroxyl radicals
are available. The precipitation of Fe(OH)3 is experimen-
tally confirmed by the presence of turbidity in the experi-
mental samples carried out at pH 5~6.
When the initial pH fell from 6.0 to 3.0, the decolor-
ization efficiency of AO within 60 min increased signifi-
cantly from 56.3% to 93.2%. However, the decolorization
efficiency of AO slowed down to 44.4% with a further de-
crease of the initial pH from 3.0 to 2.0. This could be ex-
Effect of Fe2+ dosage on the decolorization of AO
Because H2O2, with an oxidation potential of 1.77 eV,
has less oxidizing power,19 AO cannot be effectively oxi-
dized by only adding H2O2 to the solution alone. Fe2+ is an-
other main parameter in the Fenton reaction that catalyti-
cally decomposes H2O2 to generate •OH. To study the ef-
fect of Fe2+ dosage on the decolorization of AO by Fenton
oxidation, a series of experiments were conducted with dif-
ferent initial concentrations which ranged from 0.05 to 0.5
mM, and the results obtained are presented in Fig. 4. The
results indicated that the decolorization efficiency of AO
increased with the increase of the initial concentration of
Fe2+ in the solution. It can be seen that the decolorization
+
plained by the formation of oxonium ion (i.e. H3O2 ), which
enhanced the stability of H2O2 and restricted the generation
of •OH at low pH conditions (pH < 3.0).13,14 In addition, the
scavenging of •OH by the excess of H+ is another reason for
the lower decolorization efficiency of AO at pH 2.0.15,16
Thus, 3.0 is recommended as a suitable initial pH for the
decolorization of AO by the Fenton oxidation process.
Effect of H2O2 dosage on the decolorization of AO
H2O2 plays a very important role as a source of •OH
Fig. 3. Effect of H2O2 dosage on the decolorization of
AO by Fenton oxidation. Experimental condi-
tions: [AO] = 0.2 mM; [Fe2+] = 0.2 mM; pH =
3.0.
Fig. 2. Effect of initial pH values on the decolorization
of AO by Fenton oxidation. Experimental con-
ditions: [AO] = 0.2 mM; [H2O2] = 2.0 mM;
[Fe2+] = 0.2 mM.

1150 J. Chin. Chem. Soc., Vol. 56, No. 6, 2009
Chen et al.
was limited at 0.05 mM of Fe2+, and only 15.6% of AO was
degraded within 60 min of reaction. In the presence of 0.1,
0.2, 0.3, 0.4 and 0.5 mM of Fe2+, a great improvement of
the decolorization of AO could be observed, and the decol-
orization efficiencies within 60 min of reaction achieved
were 76.6%, 93.2%, 97.9%, 98.3% and 98.4%, respec-
tively. The fact that higher decolorization efficiency was
achieved at high Fe2+ dosages was mainly attributable to
the higher production of •OH with more Fe2+ in the Fenton
reaction.13
Table 1. Kinetic parameters (rate constants and linear regression
coefficients R2) for Fenton oxidation of acridine orange
at various initial concentrations
Initial concentration (M)
kapp (min-1)
R2
4.0 ´ 10-4
6.0 ´ 10-4
1.0 ´ 10-3
0.0113
0.0052
0.0031
0.990
0.942
0.917
were shown in Table 1. Degradation rate constants, kapp (in
min-1), were determined from the slope of ln(C0/C) = kapp
t
plots, where C0 and C are the concentration of AO at times
0 and t. As illustrated in Table 1, the degradation rate con-
stants increased with decreasing initial concentrations, in-
dicating a faster reaction rate at lower initial concentration.
Effect of AO concentration on the decolorization of
AO
It is important from an application point of view to
study the dependence of decolorization efficiency on the
initial concentration of the AO dye. The decolorization of
different concentration of AO was studied, and the results
are shown in Fig. 5. It can be seen that the decolorization
efficiency of AO fell as AO concentration rose. As AO con-
centration increased from 0.1 to 1.0 mM, the decolorization
efficiency of AO within 60 min of reaction fell from 98.6%
to 19.0%. This is because a relatively lower concentration
of •OH results from the increasing concentration of AO
while the dosage of H2O2 and Fe2+ remains the same, which
leads to a decreasing of the decolorization efficiency of
AO.
Effect of chloride ion on the decolorization of AO
Salts (sodium chloride, specifically) are important in
using many types of dyes, and they co-exist with dyes in the
effluent, a fact which could affect the treatment of waste-
water.13 In the presence of inorganic ions the rate for the re-
action of H2O2 with ferrous ion is different.22 In our study,
the effect of the presence of chloride ion (0.05 to 0.25 M)
on the decolorization of AO was investigated, and the re-
sults are shown in Fig. 6. It can be seen that chloride ion
had a negative impact on the decolorization of AO by
Fenton oxidation. The decolorization efficiency within 60
min of reaction decreased from 93.2% to 35.1% as a conse-
quence of increasing the concentration of chloride ions
from 0 to 0.25 M, and therefore about 58% of the decol-
orization efficiency was lost. The inhibitive effect of chlo-
ride ions on the decolorization of AO can be explained by
Several investigators have found that the Fenton re-
action for dye degradation follows pseudo-first-order ki-
netics.20,21 Regression analysis based on the pseudo-first-
order reaction kinetics for the decolorization of AO in
Fenton oxidation process was conducted and the results
Fig. 4. Effect of Fe2+ dosage on the decolorization of
AO by Fenton oxidation. Experimental condi-
tions: [AO] = 0.2 mM; [H2O2] = 2.0 mM; pH =
3.0.
Fig. 5. Effect of AO concentration on the decoloriza-
tion of AO by Fenton oxidation. Experimental
conditions: [H2O2] = 2.0 mM; [Fe2+] = 0.4 mM;
pH = 3.0.

Degradation of AO Dye by the Fenton Oxidation Process
J. Chin. Chem. Soc., Vol. 56, No. 6, 2009 1151
the scavenging effect of chloride ion on •OH, and the
chemical reactions are shown below (Eq. (2) and Eq.
(3)).13,23
seen, AO degradation is much higher than TOC removal in
the Fenton process. Complete mineralization of AO was
not achieved after 6 h of oxidation although AO disap-
peared after 20 min. The great difference between degrada-
tion efficiency and mineralization efficiency implied that
the products of AO oxidation mostly stayed at the interme-
diate product stage under the present experimental condi-
tions.
Cl- + •OH ® ClOH•-
ClOH•- + Fe2+ ® Cl- + OH- + Fe3+
(2)
(3)
Mineralization study
In order to assess the degree of mineralization reached
during AOPs, the decrease in total organic carbon (TOC) is
generally estimated. To investigate the mineralization de-
gree of AO in the Fenton process, the experiments of AO
degradation were conducted at an initial AO concentration
of 0.2 mM, and the results are shown in Fig. 7. As could be
Separation and identification of the intermediates
In order to understand the degradation pathway of the
dye during the Fenton process, we identified the intermedi-
ates of the degradation of AO with HPLC, coupled with a
photodiode array detector and electrospray ionization mass
spectrometer. Fig. 8 displays the chromatogram of the re-
acted solution after 30 min of reaction. At least six com-
pounds were identified at retention times of less than 45
min. One of the peaks was the initial AO; the other five new
peaks were those of the intermediates formed. We denoted
the AO dye and its related intermediates as species I-VI.
Except for the initial AO dye (peak I), the intensities of the
other peaks increased at first and subsequently decreased,
indicating the formation and transformation of the interme-
diates.
Fig. 9 displays the absorption spectra of each inter-
mediate in the visible spectral region formed after 30 min
of reaction. The absorption spectra of intermediate prod-
ucts were recorded from 200 to 700 nm using a HPLC
equipped with a photodiode array detector. In order to com-
pare the maximum absorbance wavelength (lmax) of the
spectral bands, the absorption spectra of intermediate prod-
ucts were overlapped and adjusted into the similar absorb-
ance. The absorption maximum of the spectral bands shifts
hypsochromically from 489.1 (Fig. 9, Spectrum I) to 458.7
nm (Fig. 9, Spectrum VI). This hypsochromic shift of the
absorption band was presumed to result from the stepwise
formation of a series of N-de-methylated intermediates
(i.e., methyl groups were removed one by one as confirmed
Fig. 6. Effect of chloride ion on the decolorization of
AO by Fenton oxidation. Experimental condi-
tions: [AO] = 0.2 mM; [H2O2] = 2.0 mM; [Fe2+]
= 0.2 mM; pH = 3.0.
Fig. 7. Depletion in TOC measured as a function of re-
action time for an aqueous solution of AO (0.2
mM) in the Fenton process. Experimental con-
ditions: (a) [H2O2] = 2.0 mM, [Fe2+] = 0.4 mM,
pH = 3.0; (b) [H2O2] = 20 mM, [Fe2+] = 2.0 mM,
pH = 3.0.
Fig. 8. HPLC chromatogram of the reacted solution af-
ter 30 min of Fenton reaction.

1152 J. Chin. Chem. Soc., Vol. 56, No. 6, 2009
Chen et al.
Orange (Fig. 10, mass spectra AO-D); V, m/z = 224.03,
N,N,N¢-de-trimethyl-Acridine Orange (Fig. 10, mass spec-
tra AO-DM); and VI, m/z = 210.00, N,N,N¢,N¢-de-tetra-
methyl-Acridine Orange (Fig. 10, mass spectra AO-DD).
Table 2 presents the absorption maximum and the
mass peaks of the N-de-methylated intermediates and the
corresponding compounds identified by interpretation of
their mass spectra. Two species had protonated molecules
of m/z = 238 eluted at retention times of 30.95 min (com-
pound IV) and 31.60 min (compound III) during LC/MS,
suggesting the formation of di-N-de-methylated products
of AO. Both intermediates display similar HPLC-ESI-MS
characteristics. One of them, AO-MM, was formed by the
removal of a methyl group from two different amino groups
of the AO molecule. Loosening two methyl groups from
the same amino group of the AO dye produced the other
one, AO-D. The LC chromatogram revealed a shorter re-
tention time for compound IV than for compound III, sug-
gesting compound IV was more polar. Considering that the
polarity of the AO-D species is greater than that of the
AO-MM intermediate, we expected the latter to be eluted
after the AO-D species. As well, to the extent that two
N-methyl groups are stronger auxochromic moieties than
the N,N-dimethyl or amino groups are, the maximal absorp-
tion of the AO-D intermediate was anticipated to occur at a
wavelength shorter than the band position of the AO-MM
species.
Fig. 9. Absorption spectra of the N-de-methylated in-
termediates formed during the Fenton process
of the AO dye corresponding to the peaks in the
HPLC chromatogram of Fig. 8. Spectra were
recorded using the photodiode array detector.
Spectra are denoted I-VI and correspond to the
peaks I-VI in Fig. 8, respectively.
by the gradual peak wavelength shifts toward the blue re-
gion). The N-de-methylation of the AO has the wavelength
position of its major absorption band moved toward the
blue region, lmax, AO, 489.1 nm; AO-M, 478.2 nm; AO-
MM, 472.1 nm; AO-D, 466.0 nm; AO-DM, 464.8 nm; AO-
DD, 458.7 nm.
The N-de-methylated intermediates were further iden-
tified using the HPLC-ESI-MS, and the relevant mass spec-
tra are illustrated in Fig. 10. The molecular ion peaks ap-
peared to be in the acid forms of the intermediates. From
the results of mass spectral analysis, we confirmed that the
component I, m/z = 266.05, in liquid chromatogram was
AO (Fig. 10, mass spectra AO). The other components
were II, m/z = 252.07, N-de-mono-methyl-Acridine Or-
ange (Fig. 10, mass spectra AO-M); III, m/z = 238.00,
N,N¢-de-dimethyl-Acridine Orange (Fig. 10, mass spectra
AO-MM); IV, m/z = 238.08, N,N-de-dimethyl-Acridine
Degradation pathways of AO dye in the Fenton pro-
cess
Fenton’s reagent is a mixture of hydrogen peroxide
and ferrous salt. Hydrogen peroxide decomposes catalyti-
cally in the presence of ferrous ions and generates radicals
such as the hydroxyl (•OH) and hydroperoxyl (•OOH) radi-
cals.24 In the process, the major oxidant was the •OH, since
Table 2. Identification of the N-de-methylation intermediates of the AO dye by HPLC-ESI-MS
HPLC
Peaks
ESI-MS peaks
Absorption
N-de-methylation intermediates
Abbreviation
(m/z)
maximum (nm)
I
Acridine Orange
AO
266.05
252.07
238.00
238.08
224.03
489.1
478.2
472.1
466.0
464.8
II
N-de-mono-methyl-Acridine Orange
N,N¢-de-dimethyl-Acridine Orange
N,N-de-dimethyl-Acridine Orange
N,N,N¢-de-trimethyl-Acridine
Orange
AO-M
AO-MM
AO-D
AO-DM
III
IV
V
VI
N,N,N¢,N¢-de-tetramethyl-Acridine
Orange
AO-DD
210.00
458.7

Degradation of AO Dye by the Fenton Oxidation Process
J. Chin. Chem. Soc., Vol. 56, No. 6, 2009 1153
Fig. 10. ESI mass spectra of the N-de-methylated intermediates formed during the Fenton process of the AO dye after they
were separated by HPLC-ESI-MS method.
•OOH and its conjugate base O2• were much less reactive
as hydroxyl radicals.25,26
fragmentation of the alkoxy radical produces a de-methyl-
ated product. The mono-de-methylated species, AO-M,
can also be attacked by •OH species and be implicated in
other similar events (H-atom abstraction, oxygen attack,
and the bimolecular Russell mechanism) to yield the bi-
de-methylated intermediates, AO-MM and AO-D. The N-
de-methylation process as described above continues until
formation of the completely N-de-methylated species, AO-
DD. The mechanism of AO degradation by TiO2 photoca-
talysis was discussed in our previous study.29 The mecha-
nism of the Fenton process, generating •OH radical to oxi-
dize organic compounds, is similar to that of TiO2 photo-
The N-de-methylation of the AO occurs mostly by the
attack of the •OH species on the N,N-dimethyl groups of the
AO, as shown in Scheme I. Hydroxyl radicals yield car-
bon-centered radicals upon the H-atom abstraction from
the methyl group, or they react with the lone-pair electron
on the N atom to generate cationic radicals, which subse-
quently convert into carbon-centered radicals.27 The car-
bon-centered radicals react rapidly with O2 to produce
peroxy radicals that subsequently transform into alkoxy
radicals through the bimolecular Russell mechanism.28 The

1154 J. Chin. Chem. Soc., Vol. 56, No. 6, 2009
Chen et al.
Scheme I A proposed reaction pathway for acridine orange degradation in acidic aqueous medium in the Fenton process

Degradation of AO Dye by the Fenton Oxidation Process
catalysis.
J. Chin. Chem. Soc., Vol. 56, No. 6, 2009 1155
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CONCLUSIONS
The Fenton process appears to have the capacity to
completely decolourize and partially mineralize the acri-
dine orange dye in aqueous solution in a short reaction
time. A direct influence of initial pH on the decolorization
of AO could be observed, and the best efficiency was ob-
tained at pH of 3.0. At a constant AO concentration (0.2
mM), decolorization efficiency increased with increasing
H2O2 and Fe2+ concentrations up to a certain level above
which decolorization efficiency decreased due to the scav-
enging effects of H2O2 on hydroxyl radicals. A suitable op-
erating condition for the Fenton oxidation of AO was se-
lected as: [H2O2] = 2.0 mM, [Fe2+] = 0.4 mM and pH = 3.0.
In the given conditions, more than 95.8% of decolorization
efficiency was achieved within 10 min of reaction. How-
ever, low TOC removal or mineralization yield and high
AO removal indicated the formation of intermediate prod-
ucts. The N-de-methylation degradation of the AO dye
takes place in a stepwise manner with the various N-de-
methylated intermediate AO species. The methyl groups
are removed one by one as confirmed by the gradual wave-
length shifts of the maximum-peaks toward the blue re-
gion. The process continues until formation of the com-
pletely N-de-methylated dye.
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