REV . CHIM. (Bucharest) 62 No. 12 2011 http:www.revistadechimie.ro 1195Discoloration of Waters Containing Azo Dye Congo Red [600206]

REV . CHIM. (Bucharest) ♦ 62♦ No. 12 ♦ 2011 http://www.revistadechimie.ro 1195Discoloration of Waters Containing Azo Dye Congo Red
by Fenton Oxidation Process
Estimation of activation parameters
MADALINA DRAGOI1, ADRIANA SAMIDE1*, ANCA MOANTA1
1University of Craiova , Faculty of Chemistry, 107i Calea București,200478, Craiova, Romania
The discoloration of waters containing Congo red using Fenton reagent was studied at different pH values
and different temperatures. The results reported a significant discoloration after 5 min, at pH = 3.50 and
temperature of 55 oC. In these conditions, it was obtained a color removal (CR) equal to 75.6 ± 5 and a
relative discoloration rate of 14.12 % / min. Apparent activation energy was determined using the Arrheniusequation. The value of E
a was determined as being equal to 85.9 kJ/mol. The activation enthalpy ( ΔH0) and
the activation entropy ( ΔS0) were calculated from the diagram of the transition state. The value of the
enthalpy (83.44 kJ/mol) indicates an endothermic reaction. Positive values of entropy (30.78 J/mol) suggestthat entropy increases upon achieving the transition state, which often indicates a dissociating mechanism.
Keywords: Congo red; Fenton reagent; discoloration process; UV-Vis spectra
* email: [anonimizat]; Tel: +40251-597048Azo dyes are widespread environmental pollutants
related to many important industries such as textile,printing, and cosmetic manufacturing. With the increasing
production of these dyes, the discharge of dye effluents at
high concentration and strong colour has caused seriousenvironmental pollution because many of these dyes are
toxic and recalcitrant to biodegradation [1]. Many different
approaches have been proposed to remove dyes fromaqueous solution such as adsorption and precipitation,
chemical and electrochemical oxidation, chemical
coagulation and biological anaerobic/aerobicdecomposition [2-10]. Recently, advanced oxidation
processes have been proposed as offering promise for
wastewater treatment because these processes are ableto oxidise a wide range of compounds that are otherwise
difficult to degrade. It is one of the potential alternatives to
decolorize and to reduce recalcitrant wastewater loadsfrom textile dyeing and finishing effluents. Among these
processes, the oxidation using Fenton’s reagent has proven
to be a promising and attractive treatment method for theeffective discoloration and degradation of dyes [11]. There
are several studies related to using of Fenton oxidation
process for the treatment of azo dyes wastewaters [12–15].
The Fenton system uses ferrous ions to react with
hydrogen peroxide, producing hydroxyl radicals withpowerful oxidizing abilities to degrade certain toxic
contaminants [16]. The discoloration efficiency of dyes
was mainly depended on their chemical characteristics,the generation rate and concentration of
•OH in the
process.
In this study, research was carried out to investigate the
discoloration of an azo dye Congo red by Fenton oxidation
process. Therefore, the aim of this work is to investigate
the influence of some important operating parameterssuch as pH value of solutions and temperature on the
discoloration of Congo red in aqueous solution. The optimal
reacting conditions were evaluated. In addition, the UV–Vis spectral changes of Congo red in aqueous solution
during Fenton oxidation process were also examined. The
results can provide fundamental knowledge for thetreatment of wastewater containing Congo red by Fenton
oxidation process.
Experimental part
Materialiale and methods
The azo dye, Congo red, was provided Fluka and used
as received. UV-Vis absorption spectrum and molecularstructure of Congo red are illustrated in figure 1. Hydrogen
peroxide (30% w/w), ferrous sulfate (FeSO
4•7H2O),
sulfuric acid, were purchased from Fluka or Aldrich. All thechemicals were analytical grade and used directly without
any further purification. All sample solutions were prepared
with deionized water.
Experimental determinations were performed for
aqueous solutions of Congo red, with initial concentrations
of 3.558×10
-5 mol L–1. The added ferrous sulfate
concentration in the system was 4.17×10-5 mol L–1 and
the h ydrogen peroxide concentration was 1.76×10-3 mol
L–1. Experiments were analysed at different initial values
of pH such as: 2.51, 3.02, 3.50, 3.93 and at different
temperatures: 25, 35, 45, 55°C . For each experiment,
Erlenmeyer glass, equipped with refrigerator bottom, whichcontained 100 mL solution of ferrous sulphate and Congo
red subject to degradation were placed in a thermostat
water bath with constant temperature and stirred by amagnetic stirrer. The pH value of each reaction solution
was adjusted to the desired value by using the prepared
5% sulfuric acid solution, and was measured with a modelConsort C533 pH meter. The reactions were initiated by
adding calculated amounts of hydrogen peroxide to the
reactor. Samples were taken out from the beakerperiodically using a pipette and were immediately analyzed
by spectrophotometry and then returned back to the
beaker.
Analytical methods
The pH value of the solutions was measured by using a
digital pH meter. Before the measurement, the pH meter
was calibrated with standard buffers of 4.0, 7.0 and 10.0
(25°C). The UV–Vis spectra of Congo red were recordedfrom 200 nm to 800 nm using a UV–Vis spectrophotometer

REV . CHIM. (Bucharest) ♦ 62♦ No. 12 ♦ 2011 http://www.revistadechimie.ro 1196(V arian Cary 50 Bio) with a 1 cm path length spectrometric
quartz cell. The absorption spectrum of Congo red in
aqueous solution was recorded and it was found that themaximum wavelength was at 497.
Results and discussion
UV-Vis spectrum recorded before the addition of
hydrogen peroxide is shown in figure 1. It can be observed
the presence of two maximum absorption: one in thevisible region at a wavelength of 497 nm and the other at
318 nm. The peak at 318 nm was attributed to the
absorption of the π→ π * transition related to the aromatic
rings bonded to the azo group in the dye molecule.
Absorbance at 497 nm is due to the color of dye solution
(n→ π * transition from –N=N– group) and is used in order
to monitor the compound discoloration. Therefore, the
discoloration of azo compound at different reaction times
was monitored by registering absorbance atλ
max=497 nm.The color removal (CR) was determined using the
following formula:
CR (%) = ( 1 – Art / Ar0 ) · 100 (2)
where Ar0 and Art represents the initial real absorbance and
respectively the real absorbance at the moment of
spectrum registering.
CR variation in time is shown in figure 4. From the
inserted graph in figure 4, which represents the CR variation
as function of pH, at 130 min, it can be observed that CR
reaches the value of 70.6% at pH = 3.50. Thus, this pH
value can be considered optimal for the discoloration
process of waters containing Congo red at temperature of
25oC.
Fig. 1. UV-Vis spectrum of dye Congo red registered before the
addition of hydrogen peroxide
Effect of pH value on the discoloration process
The effect of pH on the discoloration of waters containing
azo compound Congo red in the Fenton oxidation processwas studied at different values: 2.51; 3.02; 3.50; 3.93 and
at temperature of 25
oC. The UV-Vis spectra were recorded
for 130 min. The UV-Vis spectrum obtained for thediscoloration process at the temperature of 25
oC and pH =
3.50 is shown in figure 2. For other pH values, similar
spectra were obtained.
Fig. 2. UV-Vis spectrum recorded during the discoloration process
of waters containing azo dye Congo red at pH = 3.50 and
temperature of 25oC
In all cases, it was observed that the absorbance
decreased in time, the most pronounced decrease beingrecorded at a pH value of 3.50, after 130 min from the
initiation of the experiment (fig.3). In order to determine
the real absorbance, the baseline was considered to beat a value of 0.45 and the following formula it was used:
Art = At – 0.45 (1)
where:
Art is the real absorbance;
At is the absorbance registered at reaction time “t”.
Fig. 3. Absorbances variation in time for the discoloration process
of waters containing azo dye Congo red at 25 oC and different pH
values: 2.51; 3.02; 3.50; 3.93
Fig. 4. Color removal variation of waters containing Congo red
depending on time at different pH values: 2.51; 3.02; 3.50; 3.93
Determination of discoloration rate
In order to determine the relative discoloration rate (DR)
the values obtained for CR in the time intervals between 0min – 40 min and 40 min – 130 min were linearized. Thus,
from the slopes of straight lines [dy/dx = d(CR)/dt]
obtained, the discoloration rates (DR) were determined asbeing the percentage of degraded Congo red in a minute
(% / min). Relative discoloration rates were obtained from
the arithmetic average of the rates values indicated byequations inserted in figure 5. From figure 6a it can be seen
that, within the first 40 min, the discoloration rate has the
highest value of 0.9529 % / min, at pH = 3.50 and the
lowest of 0.455 % / min, at pH = 2.51. At higher time
intervals (fig. 6b), a decrease in DR compared with the
phytate at the time interval between 0 and 40 min isobserved. Therefore, the average values of DR, that satisfy
both time intervals were determined (table 1).
Using obtained values for DR, it was calculated the
theoretical colour removal at 130 min, obtaining relatively

REV . CHIM. (Bucharest) ♦ 62♦ No. 12 ♦ 2011 http://www.revistadechimie.ro 1197appropriate values to the experimental results. In order to
fit the results in a margin of error accepted in both cases,
there were determined the average values of CR (table 1).
Determination of the activation parameters: (Ea); (ΔH0);
(ΔS0)
The UV-Vis spectra were recorded for the discoloration
process of waters containing azo dye Congo red, at different
temperatures and pH = 3.50 (fig.6). It was found that with
the incresing temperature, the time at which it is obtaineda value of 75 % for CR, decreases significantly from 130
min (at 25
oC) to 5 min (at 55 oC).
The discoloration rates (DR % / min), whose values are
presented in table 1, were also determined. It is noted that
DR increases exponentially with the temperature (fig. 7a).
Thus, the apparent activation energy ( Ea) was
determined using the logarithmic form of Arrhenius
equation:
ln DR = ln A – Ea / RT (3)
where:
DR – relative discoloration rate (% / min);
A – preexponential factor;
Ea – apparent activation energy (kJ/mol);
R – universal constant of ideal gas ( 8.31 J/mol.K).
From the graphical representation of ln DR = f (1/T), a
straight line with the slope d(ln DR) / d(1/T) = Ea / R was
obtained (fig.7b ); thus, the value of the apparent activation
energy was determined as being equal to 85.9 kJ/mol.
This value of activation energy is much greater than other
azo dyes oxidized with Fenton reagent. For example, in
case of metilorange, in similar experimental conditions,
an activation energy value equal to 22.09 kJ/mol has beenreported [13]. Hence, it is concluded that the oxidation
process of Congo red in the presence of Fenton reagentwith difficulty occurs at low temperatures. However, the
method can be more effective by raising the temperature
to 55 °C, when the discoloration time decreases of 26 times.
Fig. 5. Fitting of experimental data for the DR determination (% / min): a – for the time interval
between 0 and 40 min; b – for the time interval between 40 and 130 min
Fig. 6. UV-Vis spectra recorded for the discoloration process, at pH
= 3.50 and the different temperatures: a – 35oC; b – 45oC; c – 55oC
Table 1
THE COLOR REMOVALS AND THE
RELATIVE DISCOLORATION RATES
OBTAINED AT DIFFERENT pH VALUES
AND DIFFERENT TEMPERATURES

REV . CHIM. (Bucharest) ♦ 62♦ No. 12 ♦ 2011 http://www.revistadechimie.ro 1198The activation enthalpy ( ΔH0) and the activation entropy
(ΔS0) were calculated from the diagram of the transition
state using the following equation:
(4)
On the other hand, the plots of ln (DR/T) against 1/ T Eqn
(4) also gave straight lines, as shown in figure 8.
The slopes of these lines represent ΔH0/ R and the
intercepts are [ln R/NAh + ΔS0/ R], from which the values
of ΔH0 and ΔS0 were calculated. It can be noticed that the
reaction enthalpy has a great value equal to 83.44 kJ/mol.
The sign of the enthalpy indicates an endothermic
reaction. Positive values of entropy (30.78 J/mol) suggest
that entropy increases upon achieving the transition state,which often indicates a dissociating mechanism.
Conclusions
In case of Congo red oxidation with Fenton reagent, the
optimum pH was 3.50; the colour removal is aprox. 75%,
at temperature of 55
oC, the discoloration time being of 5
min.
The discoloration rate has the higher value at pH = 3.50
and increases exponentially with temperature.
The apparent activation energy value is equal to 85.9
kJ/mol. This value of activation energy is much higher than
other azo dyes oxidized with Fenton reagent. Thus, thediscoloration process of Congo red, in the presence of
Fenton reagent, occurs with difficulty at low temperatures.
However, the method can be more effective by raising thetemperature to 55°C, when the discoloration time
decreases of 26 times.
The activation enthalpy ( ΔH
0) and the activation entropy
(ΔS0) were calculated from the diagram of the transition
state. The value of the enthalpy (83.44 kJ/mol) indicates
an endothermic reaction. Positive values of entropy (30.78J/mol) suggest that entropy increases upon achieving the
transition state, which often indicates a dissociating
mechanism.Fig. 8. Transition state plots of the azo dye Congo
red Fenton oxidation at pH = 3.50.Fig. 7. The variation of DR with temperature (a) and the Arrhenius
diagrame for the discoloration process of waters containing Congo
red (b)
References
1. GUPTA, A.K., PAL, A., SAHOO, C., Dyes Pigm., 69, 2006, p. 224.
2. ZHU, M.X., LEE, L., WANG, H.H., WANG, Z.,J. Hazard. Mater. , 149,
2007, p. 735
3. RIO, S., LE COQ, L., FAUR, C., LE CLOIREC, P ., W ater Sci. Technol.,
53, 2006, p. 237
4. CRINI, G., Bioresour. Technol., 97, 2006, p. 1061
5. CARNEIRO, A.P ., PUPO NOGUEIRA, F.R., VALNICE ZANONI, B.M.,
Dyes Pigm., 74, 2007, p. 127
6. PLATON, N., SIMINICEANU, I., NISTOR, I.D., MIRON, N.D., MUNTIANU,
G., MARES, A.M., Rev. Chim. (Bucharest) , 62, no. 6, 2011, p. 676.
7. MURUGANANDHAM, M., SOBANA, N., SWAMINATHAN, M., J. Hazard.Mater. , 137, 2006, p. 1371
8..SUN, S.P ., LI, C.J., SUN, J.H., SHI, S.H., FAN, M.H., ZHOU, Q., J.
Hazard. Mater. , 161, 2009, p. 1052
9..MATOS, C.T., VELIZAROV , S., CRESPO, J.G., REIS, M.A.M., Water
Res., 40, 2006, p. 231
10..ZILOUEI, H., GUIEYSSE, B., MATTIASSON, B., Process Biochem. ,
41, 2006, p. 1083
11..LU, M.C., CHEN, J.N., CHANG, C.P ., J. Hazard. Mater.B, 65, 1999,
p. 277
12..HSUEH, C.L., HUANG, Y.H., WANG, C.C., CHEN, C.Y ., Chemosphere ,
58, 2005, p. 1409
13..DUMITRU, M., SAMIDE, A., PREDA, M., MOANTA, A., Rev. Chim.
(Bucharest) , 60, no. 10, 2009, p. 957
14. SUN, J.H., SUN, S.P ., WANG, G.L., QIAO, L.P ., Dyes Pigments , 74,
2007, p. 647
15..SAMIDE, A., DUMITRU, M., CIUCIU, A., TUTUNARU, B., PREDA, M.,
Studia Universitatis, 54, 2009, p. 157
16..NEYENS, E., BAEYENS, J., J. Hazard. Mater. , 98, 2003, p. 33
17..KWON, B.G., LEE, D.S., KANG, N., YOON, J., W ater Res. , 33, 1999,
p. 211018..NEAMTU, M., YEDILER, A., SIMINICEANU, I., KETTRUP , A., J
Photochem Photobiol A Chem , 161, 2003, p. 87
19..HASSAN, M.M., HAWKYARD, C.J., J Chem. Technol. Biotechnol.,77, 2002, p. 834
Manuscript received: 25.10.2010