REV . CHIM. (Bucharest) 66 No. 9 2015 http:www.revistadechimie.ro 1273Removal of Some Triphenylmethane Dyes from Aqueous [602864]

REV . CHIM. (Bucharest) ♦ 66 ♦ No. 9 ♦ 2015 http://www.revistadechimie.ro 1273Removal of Some Triphenylmethane Dyes from Aqueous
Solutions by Fenton Reagent. I
MADALINA DRAGOI*, ANCA MOANTA, CRISTIAN TIGAE, MARIUS DRAGOI
University of Craiova, Faculty of Mathematics and Natural Sciences, Department of Chemistry, 107 Calea Bucuresti Str., 200512,
Craiova, Romania
The oxidative discoloration of three triphenylmethane dyes, Methyl Blue, Eriochrome Cyanine R and Phenol
Red in aqueous solution has been comparatively studied using the Fenton process. The effects of different
reaction parameters such as initial pH and temperature on the dyes degradation have been assessed. TheUV-Vis spectral changes of triphenylmethane dyes during Fenton treatment process have been studied. The
decay kinetics has also been investigated. In order to determine the concentration of unknown samples of
studied dyes, the calibration curves have been achieved by measuring the absorbances at differentconcentration values of dyes.
Keywords: Methyl Blue, Eriochrome Cyanine R, Phenol Red, Fenton process, kinetic study
* email: [anonimizat]; Tel.: +40251-597048A large amount of wastewater containing dyestuffs with
intensive colour and toxicity is introduced into the aquaticsystems during dye production and textile manufacturing
processes. Even a small quantity of dye in the water (e.g.,
10-20 mg L
–1) is highly visible and the water transparency
and the gas solubility of water body are affected as well
[1].
Traditional treatments involving biological and
coagulation/flocculation methods are generally ineffective
for total colour removal. Advanced oxidation processes
(AOPs) are treatment processes based on the generationof radicals that are highly reactive and nonselective species
[2]. Therefore, AOPs have often been used for the treatment
of wastewaters containing a wide range of organicpollutants. Among the AOPs, Fenton oxidation has shown
pollutant removal efficiencies >90 % [3-6]. The Fenton-
type process combines iron (Fe
2+ or Fe3+) with hydrogen
peroxide to produce hydroxyl radicals. The general
mechanism using Fenton reagent is a number of cyclic
reactions which utilize Fe2+ or Fe3+ ions as a catalyst to
decompose the hydrogen peroxide [2].
Azo dyes degradation study by different methods has
been reported by many researchers [1, 7-15].Triphenylmethane dyes that belong to the group of
synthetic colorants are also used extensively in textile
industries. Some of them such as, Aniline Blue, EriochromeCyanine R, Phenol Red, Methyl Blue, etc., were removed
from wastewaters by using AOPs [16-21].
Methyl Blue, MB also known as Acid Blue 93 (C.I. 42780)
is an anionic triphenylmethane dye used for dyeing cotton,
cotton based fibers and leather. It is also used as a biological
and bacteriological stain. It is harmful if swallowed andcauses skin, eye and respiratory tract irritation. As evident
from the literature, much research has not been done on
the degradation of this dye [20].
The Eriochrome Cyanine R, ECR (C.I. 43820), frequently
used in chemical analysis as an indicator for
complexometry and particularly for Cu
2+ ion detection [22],
has two carboxylic groups, one hydroxyl, one carbonyl and
one sulphonic group which ensure compatibility with polar
solvents [23].Phenol Red, PR (also known as phenolsulfonphtalein or
PSP) is a water-soluble dye used as a pH indicator dye in
various medical and cell biology tests. The photo-Fentondegradation of Phenol Red triphenylmethane dye was
studied by A. Jain et al. [18].
The objective of this work was to investigate the
application of Fenton oxidation process in the treatment of
waters containing three triphenylmethane dyes: Methyl
Blue, Eriochrome Cyanine R and Phenol Red. The influenceof temperature and pH on dyes degradation was
investigated. This study presents effective informations
about the optimized reaction conditions for thediscoloration of the solutions containing the mentioned
compounds.
Experimental part
Materials and methods
Triphenylmethane dyes (Methyl Blue, Eriochrome
Cyanine R, Phenol Red), hydrogen peroxide (30 % w/w),
ferrous sulfate (FeSO
4•7H2O) and sulfuric acid were all
purchased from Fluka or Aldrich. All reagents were ofanalytical grade and used without any further purification.
Distilled water was used to prepare the colored solutions.
Chemical structures and characteristics of the testedtriphenylmethane dyes are presented in table 1.
Methods
All experimental determinations were performed foraqueous solutions of dyes, with initial concentrations of
6.5×10
–5 mol L–1. The added ferrous sulfate concentration
in the system was 4×10–5 mol L–1 and the hydrogen peroxide
concentration was 2×10–3 mol L–1.
For each experiment, an Erlenmeyer glass equipped with
a refrigerator, containing 100 mL solution of ferrous sulphateand the dye subjected to degradation was placed in a water
bath with constant temperature. The studied solution was
constantly omogenized by a magnetic stirrer. The pH valueof each reaction solution was measured with a model
Consort C533 pH-meter and was adjusted at the desired
value by using a 0.05 M H
2SO4 solution.
The reactions were initiated by adding calculated amounts
of hydrogen peroxide to the reactor. The kinetics of
oxidation was followed by taking samples at regular time

http://www.revistadechimie.ro REV . CHIM. (Bucharest) ♦ 66 ♦ No. 9 ♦ 2015 1274intervals. The discoloration of dyes was monitored by
registering the absorbance using an UV-Vis V arian Cary 50
Bio spectrophotometer at the maximum absorptionwavelength for each considered dye. The color removal
was determined using equation 1.
(1)
where A0 and A are the absorbances at initial time ant time
t, respectively.
Results and discussions
Influence of pH
The pH affects directly the mechanism of dye oxidation,
because a change in pH of the solution involves a variation
of the concentration of Fe2+ ions and therefore, the rate of
production of •OH radicals responsible for dye oxidation
will be restricted [24]. The effect of pH on Fenton
discoloration of triphenylmethane dyes was investigated
at different pH values such as: 3, 4, 5 and 6. The
temperature was maintained at 25oC. The results which
are represented by the colour removal according to pH in
figure 1 indicate that the discoloration pattern of the studieddyes is different and each dye presents a behaviour
according to its chemical structure and substituents.
As it is shown in figure 1, color removal decreases with
pH increase from 3 to 6, this being also supported byprevious studies [25]. At pH = 3, colour removals of MB,
ECR and PR obtained after 9 min were 93.06 , 51.56 and70.92 %, respectively. At pH = 6, colour removals of 87.13,
27.03 and 5.96 % were obtained for MB, ECR and PR,
respectively, after the same time. Phenol Red reached >99% colour removal within only 14 min at pH = 3 (table 2),
while at pH = 6, for the same dye, the discoloration process
has been studied for about 9 h, the color removal obtainedafter this time interval being approximately 90 %.
Table 1
CHEMICAL STRUCTURES AND
CHARACTERISTICS OF
TRIPHENYLMETHANE DYES
Fig. 1. Effect of pH on triphenylmethane dyes discoloration by
Fenton process. Experimental conditions: [dye]0 = 6.5×10–5 mol L–1;
[Fe2+]0 = 4×10–5 mol L–1; [H2O2] 0 = 2×10–3 mol L–1; temperature =
25oC; reaction time = 9 min.
Table 2
TIME REQUIRED FOR > 99% COLOUR REMOVALS IN CASE OF
TRIPHENYLMETHANE DYES DISCOLORATION AT DIFFERENT
INITIAL pH VALUES AND T = 25OC
Influence of temperature
Because of the fact that, from the above study, the
optimal pH value was observed as being 3, this value was
used for all discoloration processes of this study.Experiments were performed for different temperature
values: 25 , 35 , 45 and 55
oC. Figure 2 illustrates the effect
of temperature on the Fenton oxidation of dyes.
Fig. 2. Effect of temperature on triphenylmethane dyes
discoloration by Fenton process. Experimental conditions: [dye] 0
= 6.5×10–5 mol L–1; [Fe2+]0 = 4×10–5 mol L–1; [H2O2] 0 =2×10–3 mol L–1;
pH = 3; reaction time = 1 min.Colour

REV . CHIM. (Bucharest) ♦ 66 ♦ No. 9 ♦ 2015 http://www.revistadechimie.ro 1275From figure 2 it can be observed that the temperature
has a great effect on the rate of Methyl Blue and EriochromeCyanine R discoloration. With increasing temperature, the
colour removal values at the same reaction time present a
significant increase, hence a higher rate of discoloration.Color removals of 55.6 , 60 and 31.6 % were observed for
MB, ECR and PR, respectively, at the temperature of 55
oC
within 1 min, while 38.93 , 4.88 and 30.08 % of colorremovals were obtained for MB, ECR and PR, respectively,
at the temperature of 25
oC within the same time interval.
Methyl Blue reached >99 % colour removal within 3
min at the temperature of 55oC. In the case of Eriochrome
Cyanine R, >99 % colour removals were obtained within 3
min and 1.5 min at 45 and 55oC, respectively (table 3).
UV-Vis spectral changes
To study the discoloration of triphenylmethane dyes
solutions, UV-Vis absorption spectra of dyes solutions were
recorded before and during the Fenton process at pH = 3
as optimal value at different times (fig. 3).
Before the treatment, the UV-Vis spectrum of MB
consisted in two main characteristic bands (fig. 3a). In theUV region, a shoulder around 213 nm and a band at 312
nm were observed. These wavelengths were ascribed to
π-π* transitions corresponding to the conjugated aromatic
system. In the visible region, an absorbance peak was
observed at 600 nm, which corresponded to the absorption
of the n-π* transition related to the quinone structure and
which was used in order to monitor the compound
discoloration. It can be observed that not only A
600 (quinone
chromophore) significantly and rapidly decreased, but alsothe UV band absorption at 200-400 nm (aromatic
intermediates) in the mentioned experimental conditions
decreased in time.
It can be seen from the figure 3 (b and c) and from the
table 1 that the maximum absorptions for ECR and PR
dyes in the visible region were at 580 nm and 430 nm,respectively. Absorbance in the visible region decreases in
time for each considered dye which demonstrates their
discoloration. The ECR almost complete discoloration wasobtained after 26 min of treatment, whereas about 11 min
were necessary to obtain the same efficiency for MB. The
peak at 430 nm corresponding to the PR dye has almosttotally disappeared after 14 min, which was in agreement
with the discoloration results. These results can be also
observed from tables 2 and 3, respectively.Kinetic studies
The kinetics of triphenylmethane dyes degradation by
Fenton oxidation process under various reaction conditions
have been investigated. For discoloration of dyes by Fenton
reagent, first order kinetic model has been suggested [26].Hence, the kinetic data of first 5 min were fitted into the
following equation:
(2)
where:
A0 and A – absorbances of the dyes at initial time and at
time t, respectively;
k – pseudo-first-order rate contant in min–1;
t – time in minutes.
All of the values for the pseudo-first-order rate constant,
were calculated from the linear regression of the pseudo-first-order kinetic model. The effects of pH and temperature
on the kinetic rate constants, k, for the used dyes
degradation will be separately discussed in the followingsections.
The role of pH
The effect of pH on the kinetic rate constants, k, for
triphenylmethane dyes degradation was studied in the
range of 3-6, at the experimental conditions of [dye]
0 = 6.5
. 10–5 mol L–1; [Fe2+]0 = 4 . 10–5 mol L–1; [H2O2] 0 =2 . 10–3 mol
L–1 and t = 25 oC. The plots of ln(A0/A) = f(t) at different pH
values in case of ECR discoloration were shown in figure 4.It can be seen the pseudo-first-order kinetic model is
applicable to the ECR degradation at pH = 3-5 (R
2 > 0.99).
Table 3
TIME REQUIRED FOR >99 % COLOUR REMOVALS IN
CASE OF TRIPHENYLMETHANE DYES DISCOLORATION
AT DIFFERENT TEMPERATURE VALUES AND pH = 3
Fig. 3. Spectral changes of triphenylmethane
dyes at t = 25oC and pH = 3: a. MB; b. ECR; c. PR
Fig. 4. Plot of ln(A0/A) = f(t) in case of ECR discoloration at
different initial pH values and t = 25oC

http://www.revistadechimie.ro REV . CHIM. (Bucharest) ♦ 66 ♦ No. 9 ♦ 2015 1276The rate constants and correlation coefficients values
are shown in table 4. Also, to have a better knowledge on
the discoloration process, the time necessary to reduce to
50 % the initial concentration of dyes (the half-life time) ispresented. As it can be seen from the table 4, at constant
temperature, there is an increasing trend in k values (from
0.3037 to 0.4461 for MB, from 0.0346 to 0.0539 for ECR andfrom 0.0108 to 0.1413 for PR) with decrease in initial pH
from 6 to 3. The differences in the values of the rate
constants presumably reflect the relative levels of available
•OH radicals provided by each of the treatment processes
[27]. It can also be observed that in case of MB degradation
at pH = 3, R2 is approximately equal to 1 and this proves
that the MB discoloration follows the first order reaction
kinetic. The smallest half-life obtained with MB at pH = 3
(1.553 min) indicates the fastest discoloration. A first orderreaction kinetics was also reported for the electrochemical
degradation of Methyl Blue from synthetic solutions
containing SO
4
2- and Cl– anions [28].
Figure 5 showed that the pseudo-first-order rate constant
of dyes degradation was influenced by the pH value of
solutions and the optimal solution pH was observed at pH
= 3. Rate constants (k) decrease with the increasing of
pH for all dyes. The poor dyes degradation at high pH values
may be caused by the formation of ferrous and ferrichydroxide complexes [29] with much lower catalytic
capability then Fe
2+.
The role of temperature
Temperature affects the reaction between H2O2 and Fe2+
and therefore, it should influence the kinetics of dyesdegradation. Because of the fact that discoloration processin case of MB and ECR took less than 5 min for temperature
values of 45 and 55
oC, we have taken into account onlyresults obtained in case of PR degradation process. The
effect of temperature on the kinetic rate constants for PR
discoloration was studied in the range of 25 – 55oC at the
experiment conditions of [dye]0 = 6.5 . 10–5 mol L–1; [Fe2+]0= 4 . 10–5 mol L–1; [H2O2] 0 =2 . 10–3 mol L–1 and pH = 3.
The curves of ln(A0/A) = f(t) obtained from experimental
data during the degradation process of PR at different
temperature values are shown in figure 6. It is obvious that
the line is liniar at t = 55oC (R2 > 0.99). Therefore, it can be
deduced that the PR degradation fits the pseudo-first-order
kinetic model.
From the data shown in the table 5 and from figure 7 it
can be seen the rate constant (k) increases with the
increasing of temperature and has values ranging from
0.1413 min-1 to 0.2692 min-1.
Apparent activation energy was determined using the
logarithmic form of Arrhenius equation:
RTEaA k / 'ln ln −= (3)
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) in
case of PR degradation, a straight line with the slope d(lnk)
/ d(1/T) = Ea / R (fig. 8) was obtained; thus, the value ofthe apparent activation energy was determined as being
equal to 18.29 kJ/mol.
In order to determine the concentration of some
unknown samples of studied dyes (MB, ECR and PR), the
absorbances were measured at different concentration
values: 1 . 10
–5 mol L–1, 3 . 10–5 mol L–1, 5 . 10–5 mol L–1, 7 . 10–
5 mol L–1 and 9 . 10–5 mol L–1, respectively. The calibration
curves resulted by plotting the absorbance vs. dyes
concentration (fig. 9). Equations obtained for the three dyes
Table 4
KINETIC COEFFICIENTS AND HALF-
LIFE TIMES (t1/2) FOR A FIRST
ORDER DEGRADATION REACTION
OF TRIPHENYLMETHANE DYES AT
DIFFERENT INITIAL pH VALUES
AND t = 25oC.
Fig. 5. V ariation of rate constants with pH in case of dyes
degradation in aqueous medium in the presence
of Fenton reagent at t = 25oC
Fig. 6. Plot of ln(A0/A) = f(t) in case of PR discoloration at different
temperature values and pH= 3
Table 5
KINETIC COEFFICIENTS AND HALF-LIFE
TIMES (t1/2) FOR A FIRST ORDER
DEGRADATION REACTION OF PR AT
DIFFERENT TEMPERATURE VALUES
AND pH = 3

REV . CHIM. (Bucharest) ♦ 66 ♦ No. 9 ♦ 2015 http://www.revistadechimie.ro 1277are shown in figure 9. From these equations, the
concentration of each dye was calculated as being: 6.5 .10
–5 mol L–1, 4,1 . 10–5 mol L–1 and 7.5 . 10–5 mol L–1 for PR,
ECR and MB, respectively.
Conclusions
The present study demonstrated that triphenylmethane
dyes, Methyl Blue, Eriochrome Cyanine R and Phenol Red,could be degraded effectively by Fenton process.
The Fenton oxidation of dyes was influenced by the initial
pH value and the optimal solution pH was observed at pH
= 3, the colour removals obtained in this case being 93.06
, 51.56 and 70.92 % for MB, ECR and PR, respectively, within
9 min.
The increase in temperature could greatly accelerate
the discoloration. Colour removals increased from 38.93 %
to 55.6 % for MB, from 4.88 to 60 % for ECR and from 30.08to 31.6 % for PR, with increase of temperature from 25 to
55
oC within 1 min of reaction.
Kintec studies suggest that the pseudo-first-order kinetic
model is applicable to the ECR and MB degradation under
the pH = 3-5 and at pH = 3, respectively. Also, PR
degradation fits the pseudo-first-order kinetic model at thetemperature of 55
oC.
From calibration curves obtained for the three dyes, the
concentrations of unknown samples of dyes have beendetermined. The concentration values were: 6.5 . 10
–5 mol
L–1, 4,1 . 10–5 mol L–1 and 7.5 . 10–5 mol L–1 for PR, ECR and
MB, respectively.
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Manuscript received: 17.07.2014