REV . CHIM. (București) 60 Nr. 9 2009 httpwww.revistadechimie.ro 957Kinetic Study of Methylorange Oxidation Process [601896]

REV . CHIM. (București) ♦ 60♦ Nr. 9 ♦ 2009 http/www.revistadechimie.ro 957Kinetic Study of Methylorange Oxidation Process
from Aqueous Solutions
MADALINA DUMITRU1, ADRIANA SAMIDE*1, MIRCEA PREDA1, ANCA MOANTA1
1University of Craiova, Faculty of Chemistry,107i Calea București, 200478, Craiova, Romania
The degradation of Methylorange (MO) azo dye in aqueous solution by the Fenton oxidation process
has been investigated. The effect of temperature at pH=3.5 on the oxidative degradation of Methylorange
have been assessed. The UV-Vis spectral changes of Methylorange in aqueous solution during Fentontreatment process were studied
. It was easier to destruct the azo group (-N=N-) of Methylorange by
Fenton oxidation. The experimental results showed that the Fenton oxidation process was an effective
process for the degradation of azo dye Methylorange at low H2O2 and Fe2+ concentrations.
Keywords: Methylorange; degradation; Fenton oxidation; kinetic study
* email: [anonimizat]; Tel.: (+40)0251 – 597048Synthetic dyes are used extensively in many industries,
of which: textile dyes (60%), paper (10%), plastic materials
(10%). Estimates indicate that approximately 10-15% ofthe synthetic textile dyes used are lost in rivers during
manufacturing or processing operations [1]. Y early, 800,000
tonnes of dyes are produced in the world and about 50% ofthem are azo dyes. Development of the appropriate
techniques for treatment of dye wastewater is important
for the protection of natural resources. To eliminate dyes
from aqueous coloured effluents and reduce their
ecological consequences, several biological, physical
and chemical techniques have been proposed: catalyticoxidation [2], biological treatment [3], physical or
chemical flocculation, electrofilteration, membrane
filtration, electrokinetic coagulation [4], adsorption and
precipitation [5] and oxidative/reductive chemical and
photochemical processes [6]. The advanced oxidationprocesses that use hydroxyl radicals , a very strong
oxidant, provide a convenient method for the treatment
of organic pollutants as dyes. Oxidation using: ozon,H
2O2, ozon/UV , UV/H2O2, UV/TiO2, or UV/H2O2/Fe3+ [7-
13] and Fenton reagent [14-16] showed their
effectiveness to eliminate organic compounds dissolvedor dispersed in aqueous media.

The paper aims to study the kinetics of Methylorange
degradation with Fenton reagent in aqueous solution usingUV-VIS spectrophotometry.
Experimental part
Experimental determinations were performed for
aqueous solutions of MO, with initial concentrations of
7.58·10-5 mol·L-1. Hydrogen peroxide concentration was
1.76 ·10-3 mol·L-1, and added ferrous sulfate concentration
in the system was 4.17 ·10-5 mol·L-1; thus, the
concentration rate between hydrogen peroxide and MO
was 23.2 : 1. Experiments were conducted at a pH of 3.5and at different temperatures: 25, 35, 45, 55 and 650C. pH
value was adjusted using a H2SO4 5% solution and
checked with a pH -meter (Consort C533 model). Foreach experiment, Erlenmeyer glass, equipped with
refrigerator bottom, which contained 100 mL solution of
ferrous sulphate and MO subjected to degradation wereplaced in a water bath thermostat at temperatures
mentioned in the experimental data. When the
temperature reached
the desired value, reaction was
initiated by adding hydrogen peroxide. Periodically, solution
samples were taken with a pipette and analyzed by
spectrophotometry . The UV-Vis spectrum of the dye during
the degradation process with Fenton reagent was
registered between 200 and 800 nm using an UV-Vis
spectrophotometer (V arian Cary 50 Bio). From the
spectrum it resulted the wavelength corresponding to
the maximum absorbance value, λmax (of the -N=N-
bond), at 487 nm. Thus, the dye concentration in thereaction mixture at different reaction times: 1- 5 min
was determined by measuring the absorbance value at
λ
max=487 nm. We mention that after 6 minutes, for the
samples tested at 55 and 65oC, the scan reports didn’t
notice any absorbance value at λmax= 487 nm, which
involves a complete degradation of methyl-orange.Thus, for the kinetic evaluation, only results obtained
during the first 5 min were taken into account.
Results and discussions
To observe the effect of temperature on azo dye
degradation in aqueous solution with Fenton reagent atdifferent temperatures: 25, 35, 45, 55 and 65°C, spectral
changes have been assessed for 5 minutes. Samples were
taken each minute for analysis at UV-Vis spectro-photometer.
UV-Vis spectra obtained during the
degradation of MO at different temperatures and pH = 3.5
are shown in figure 1 .
Fig. 1. UV-Vis spectra obtained
during the chemical degradation of
MO in the presence of Fenton
reagent at different temperatures:
a – 25oC, b – 65oCab

REV . CHIM. (București) ♦ 60♦ Nr. 9 ♦ 2009 http/www.revistadechimie.ro 958For other temperature values, similar spectra were
obtained .
In figure 1 is seen that the absorbance decreases in
time at the same temperature, which demonstrates the
degradation and discoloring of MO . With increasing
temperature , the absorbance values at the same reaction
times presents a significant decrease, hence a higher rate
of degradation. This may be shown by determining the
color removal (CR) using the equation 1.1, at time values
when the spectral recordings were performed. The
dependence of color removal by time and temperature isshown in figure 2. Thus, the color removal at 5 min increases
from 27.9 calculated at 25
oC to the value of 85.5% at 65oC.
CR = (1 – A / A0 )·100 (1.1)
where: A0 and A represents the initial absorbance and
absorbance in the moment of spectrum registering.
Setting the mechanism and reaction order
A simple mechanism has been proposed, which
assumes that MO degradation in aqueous solution in the
presence of Fenton reagent occurs in a chained process inthree stages: initiation, propagation and termination.
Initiation reaction:
(1)
Propagation reactions:
Termination reactions:
where:
P•= radical species possibly obtained as a result of
discoloring;
PF= final reaction product.
Fig.2. Dependence of colour removal on time and temperature
(2)
(4)
(5)
(6)
(7)
(8)
(9)(3)Reaction rates corresponding to the stages mentioned
in the mechanism were calculated using the following
equations:
v1=k1[Fe2+][H2O2] (I.1)
v2=k2[•OH][Fe2+] (I.2)
v3=k3[•OH][H2O2] (I.3)
v4=k4[Fe3+][•OOH] (I.4)
v5=k5[•OH][dye] (I.5)
v6=k6[P•][H2O2] (I.6)
v7=k7[P•]2 (I.7)
v8=k8[•OH][•OOH] (I.8)
v9=k9[•OH][•OH]=k9[•OH]2 (I.9)
The initiation reaction consists in the generation of
hydroxyl radicals capable to start the MO degradationprocess. The processes following after the initiation –
propagation and termination – take place while new free
radicals appear in the reaction mixture. Thus, thepropagation is a very rapid sequence of reactions, with the
regeneration, each time, of the cationic active center (Fe
2+)
and of the OH• radical, and the termination with theregeneration of a certain percentage of hydrogen peroxide,
leading to the Fe
2+/ H2O2 system recovery, capable to
reinitiate the process of degradation. The net forming rateof hydroxyl radical in time will be determined with the
following relationship:
(I.10)
P• – type radicals have a very short lifetime; practically,
their forming rate (v5) is equal to their consumption rate
(v6).
Simplified, relation (I.10) can be written as follows :
To test the proposed mechanism it is necessary to
demonstrate that it leads to an experimentally observedlaw rate.For this purpose, the approximate steady-state
will be applied, which implies certain simplifications; thus,
it is assumed that the change rate of the hydroxyl radicalconcentration is negligible: d[
.OH] / dt = 0; the relation
(I.11) becomes:
(I.12)(I.11)

REV . CHIM. (București) ♦ 60♦ Nr. 9 ♦ 2009 http/www.revistadechimie.ro 959The concentration of hydroxyl radicals can be
determined from the relation (I.12) as follows:
(I.13)
The consumption rate of dye means the degradation
rate with the formation of possible organic / inorganic
species and corresponds to equation (I.14).
(I.14)
where [dye] is MO concentration.
By replacing the [•OH] obtained with relation (I.13), in
ecuation (I.14) it results:
(I.15)
In this study, the ratio of [H2O2] / [dye] = 23.2 / 1. It is
noted that H2O2 is sufficient to form a large amount of
hydroxyl radicals, but, at the same time, there are generatedmany species in which the
•OH radical may be consumed.
(I.16)
where:
and represen ts pseudo-first-
order reaction rate, in the case of MO degradation in
aqueous solution in the presence of Fenton reagent.
By adaptation the relation (I.16) it results:

(I.17)
By integrating the relation (I.17) it results:

Because [dye]o ≈ Ao and [dye] ≈ A we can write:

(I.22)
where: [dye]0 and [dye] represent the initial concentration
and concentration at a time “t” respectively, of MO inaqueous solution; A
0 and A represent the initial absorbance
and absorbance at a given “t” time corresponding to the
experimental data, respectively .
From the ln (A0 / A) = f (t) graph, straight lines result
which pass through the origin and whose slopes are equal
to kap.
Verification of first order kinetics
a) Absorbance variation in time
The absorbance is directly proportional with the
concentration of MO degraded in the presence of Fenton
reagent. Thus, the variation of absorbances resulted in thegiven experimental conditions, in time and as a function of
the temperatures of the degradation process, will be
evaluated.
Graphs Absorbance = f (time) are shown in figure 3.
It is noted that, for the same temperature, absorbance
decreases exponentially with time, according to A = A
0exp (-kt) type relationship, which corresponds to a first
order kinetics, related to MO degradation in aqueous
solution in the presence of Fenton reagent. A0 and A
represent the initial absorbance and absorbance at a given
“t” time corresponding to the experimental data,
respectively. From figures 3 and 5, it can be observed that
apparent rate constants increase with the increasing of
temperature and have values ranging from 0.0787 min-1 to
0.2842 min-1 .
b) Integrated equation rate verification. Determination of
rate constants
The curves ln (A0 / A) = f (time) obtained from
experimental data during the process of degradation of
MO are shown in figure 4. (I.18)
(I.19)
(I.20)
(I.21)
Fig.3. Absorbances variation during MO degradation in aqueous
medium, in the presence of Fenton reagent, at different
temperatures
Fig.4. Diagrame corresponding with the first order kinetics in case
of MO degradation, in aqueous medium, in the presence of Fenton
reagent, at different temperatures
From figure 4, it can be noticed that straight lines were
obtained, passing through the origin and whose slope dy/
dx = k; this proves that the degradation of MO in aqueousmedium in the presence of Fenton reagent respects the
first order kinetics. Rate constants (k) increases with the
increasing of temperature and have values ranging from0.0972 min
-1 to 0.2606 min-1 (fig. 5). From figure 5, it can be
observed that from the cases “a” and “b” are obtained
values of apparent rate constants approximatelyequal”.According to experimental data it can be concluded

REV . CHIM. (București) ♦ 60♦ Nr. 9 ♦ 2009 http/www.revistadechimie.ro 960Fig.6. Arrhenius diagrame, in case of MO degradation in aqueous
medium in the presence of Fenton reagent
that the reaction of degradation of MO dye, in aqueous
medium, in the presence of Fenton reagent follows thefirst order reaction kinetics.
Determination of activation energy
Apparent activation energy was determined using the
logarithmic form of Arrhenius equation:
ln k = ln A’ – Ea / RT (I.18)
where :
k – rate constant, at temperature T (K);
A’ – preexponential factor ;
Ea – apparent activation energy ;
R – universal constant of ideal gas.
From the graphical representation of ln k = f (1/T), a
straight line with the slope d(ln k) / d(1/T) = Ea / R (fig. 6)
was obtained; thus, the value of the apparent activation
energy was determined as being equal to 22.09 kJ / mol.
This value of the activation energy shows that most
molecules acquire the amount of energy required for the
degradation of the azo bond (-N=N-); however, it is
possible, for a very small fraction of molecules, to acquirethe amount of energy required for the transformation in
stable non-aromatic species.
Conclusions
The study of MO chemical degradation in aqueous
solution with Fenton reagent revealed the following:
– the absorbance decreases in time, at the same
temperature, which demonstrates the degradation and
discoloring of MO;
– with increasing temperature, the absorbance values
at the same reaction time present a significant decrease,
so a higher degradation rate;
– the proposed mechanism supposes that the MO
degradation in aqueous solution in the presence of Fenton
reagent , undergoes a chained process, the predominant
active species being the hydroxyl radicals;
Fig.5. V ariation of rate constants with temperature, in case of MO
degradation in aqueous medium in the presence of Fenton reagent
– the process of MO degradation in aqueous medium in
the presence of Fenton reagent follows the first orderreaction kinetics;
– the activation energy value
(22.09kJ/mol) shows that
most molecules acquire the amount of energy needed for
the degradation of the azo bond (-N=N-); however, it is
possible, for a very small fraction of molecules, to acquire
the amount of energy required for the transformation instable non-aromatic species.
.
References
1.VAIDYA, A.A., DATYE, K.V ., Colourage, 14, 1982, p.3
2.AVRAMESCU, S.M., BRADU, C., NEATA, M., UDREA, I., Rev. Chim.
(Bucuresti), 56, nr. 3, 2005, p.281
3.ZILOUEI, H., GUIEYSSE, B., MATTIASSON, B., Process Biochem., 41,
2006, p.1083
4.BANDARA, J., MIELCZARSKI, J.A., KIWI, J., Appl. Catal. B: Environ.,34, 2001, p.321
5.CRINI, G., Bioresour. Technol., 97, 2006, p.1061
6.INCE, N.H., W ater Res., 33, 1999, p.1089
7.RUPPERT, G., BAUER, R., HEISLER, G., NOVALIC, S., Chemosphere,
27, 1993, p.1339
8.ROBERT, D., MALATO, S., Sci. Total Environ., 291, 2002, p.85-97
9.PERKINS, W .S., WALSH, W .K., REED, I.E., NAMBOODI, C.G., Text.
Chem. Color., 28, 1995, p.31
10.BRILLAS, E., BOYE, B., SIRES, I., GARRIDO, J.A., RODRIGUES,R.M., ARIAS, C., CABOT, P .-L., COMNINELLIS, C., Electrochim. Acta.,
49, 2004, p.325
11.KANG, Y .W ., HWANG, K.Y ., Eater Res., 34, 2000, p.4487
12.BADER, M., HOIGNE, J., W ater Res., 15, 1981, p.449
13.FAOUZI, M., CANIZARES, P ., GADRI, A., LOBATO, J., NASE, B., PAZ,
R., RODRIGO, M.A., SAEZ, C., Electrochim. Acta, 52, 2006, p. 325
14.SUN, J.-H., SUN, S.-P ., WANG, G.-L., QIAO, L.-P ., Dyes and Pigments,
74, nr. 3, 2007, p.647
15.DUTTA, K., MUKHOPADHYAY , S., BHATTACHARJEE, S., CHAUDHURI,B., J. Hazard. Mater., 84, 2001, p.57
16. LUO, W ., ABBAS, M.E., ZHU, L., DENG, K., TANG, H., Anal. Chim.
Acta, 629, 2008, p.1
Manuscript received: 14.04.2009