1. Decolorization of azo dyes by modified Fenton process
In this study, decolourization of C.I. Re. Orange 127 & C.I. Re. Yellow 145 by modified-
Fenton process (Fe0/H2O2) was investigated. Experiments were carried out to determine
process’s optimal operational conditions: pH, Fe0 & H2O2 concentrations. This study
shows that modified Fenton process is an efficient process for decolourization of synthetic
textile wastewater including azo dye & polyvynil alcohol (PVA). Optimal conditions
experimentally determined was found to be initial pH = 3.5, [H2O2] = 20mg/l, [Fe0] =
80mg/l for Re. Orange 127 & pH = 4.0, [H2O2] = 20mg/l, [Fe0] = 60mg/l for Re. Yellow
145. Under optimal conditions, 92 & 80% decolorization were achieved after 60min of
reaction for Re. Orange 127 & Re. Yellow 145, respectively.
Introduction
Textile industry is one of the most important industrial sector in developing countries.
Coloured wastewater of textile industry including adjuvant chemicals such as azo dyes &
PVA is an environmental threat risk for aquatic life in receiving medium. This type of
wastewater causes colorization of aquatic life with its color, prevents sunlight to be spread
in aquatic medium, & moreover, decreases oxygenation capacity of aquatic medium. In
addition to this, azo dyes in effluents have risk with their cancerogenic & toxic structures.
In different studies, there are a lot of researches related to methods such as chemical
precipitation, adsorption, photo-catalytic oxidation, ozonation, Fenton oxidation & acoustic
cavitation for removal of dyestuffs. Among these methods, coagulation & adsorption have been
based on phase transfer of pollutants from liquid to solid phase. However, phase transfer is
not accurate solution for this problem. Another method used for removal of dyestuff is oxidation.
Recently, advanced oxidation processes (AOPs) have been used for treatment of
wastewater in textile industry & removal of dyestuffs. Fenton process, the most commonly known
AOP, has been based on production of OH• as a result of reaction between Fe+2 & H2O2
under acidic conditions. Process has been improved by using different sources of iron such as iron
powder. Reaction of Fenton process in which Fe0 is used can be carried out mainly in two ways:
Oxidation of pollutant as a result of reaction with H2O2 on surface of iron &
Oxidation of pollutant as a result of reaction of H2O2 with Fe+2 which is transferred to
liquid phase by dissolving on surface of iron.
In present study, Re. Orange 127 & Re. Yellow 145 were selected which have not been
attained in literature till now. It was aimed to remove dyestuff by Fenton type process in
which zero-valent iron was used. Optimizations of pH, dosages of Fe0 & H2O2 as well as
kinetic studies were performed for removal of both dyes.
Material & method
Re. Orange 127 & Re. Yellow 145 together with polyvinyl alcohol (PVA) were obtained
commercially. Fe0 (iron powder, 10μm) & H2O2 were purchased from Merck. Additional
purification was not carried out for chemicals used in experiments. Synthetic wastewater
was prepared by using Re. Orange 127 & Re. Yellow 145 dyestuff & PVA. This prepared
textile wastewater was included 50mg/l dyestuff & 100mg/l PVA.
Decolorization of reactive azo dyes by modified Fenton process Page | 1 of 3
Compiled by: M. Rezaul Karim Tutul

2. Oxidation experiments with iron powder were carried out in jar-test equipment (Velp,
FC6S) at room temperature. First of all, pH was adjusted to desired value & then it was
accepted that oxidation reaction was started by addition of Fe0 & H2O2 in desired amounts.
At end of 1-hour reaction period, pH was adjusted to 7.5 & solution was recreated for
30mins under steady-state conditions for precipitation of iron flocks. After that, 25ml of
samples were taken for color analysis.
MnO2 was used for quenching of unreacted H2O2 in samples. Being no residual H2O2 in
sample was confirmed with Merckoquant H2O2 test strips. Then samples were filtered
through 0.45um membrane filter papers & removal of MnO 2 & ferric hydroxyl
compounds was provided. In colour analysis, λmax for Re. Yellow 145 & Re. Orange 127
was measured as 480 & 497nm, respectively. concentration of residual dyestuff in filtrates
was determined by spectrophotometer at maximum wavelength.
Result & discussion:
Effect of initial pH on decolourization:
pH of solution affects iron concentration in Fenton type process where Fe 0 is used &
therefore, also affects amount of OH• produced & oxidation efficiency of system. In this
study, pH optimization was performed in range of 2.0-4.5. Results are shown in Fig-1.
When initial pH was lower than 2.5, efficiency decreased in accordance with Eq. 1 due to
radical scavenging effect of H+ ions.
Moreover, H2O2 was stabilized as H3O2+
(Eq-2) at low pH values & its reaction
with Fe+2 in solution was retarded. At
pH 4.5, efficiency decreased since
solubility of iron in solution also decreased.
Optimum pH values for Re. Orange
127 & Re. Yellow 145 were determined as
3.5 & 4.0, respectively.
 OH  H   e   H 2 O (1)

 H 2 O 2  H   H 3O 2 (2)
Fig-1: Effect of initial pH on decolourization
Effect of mixing speed on decolourization:
Mixing speed of solution affects
solubility speed of iron & rate of
reaction. Optimization of mixing
speed was carried out in range of
50-200rpm. Decolourization
increased for Re. Orange 127
with increasing of mixing speed
till 200rpm, while no significant
changes were being observed for
Re. Yellow 145.
Fig-2: Effect of mixing speed on decolourization
Decolorization of reactive azo dyes by modified Fenton process Page | 2 of 3
Compiled by: M. Rezaul Karim Tutul

3. Effect of Fe0 concentration on decolourization
Optimization of Fe0 which is used as a catalyst in Fenton process is very important in terms
of process efficiency. When excess iron is used, amount of sludge & cost of treatment
increase. Fe0 optimization was performed in range of 5-100mg/l. While decolourization
efficiency increased with increase in iron dosage until 80mg/l for Re. Orange 127 in
accordance with Eq-1, for Re.
Yellow 145, decolourization
efficiency increased until
60mg/l. Negligible change
was observed in decolorization
for both dyes with increasing
iron concentration.
Optimum Fe0 dosages were
selected as 80 & 60mg/l for Re.
Orange 127 & Re. Yellow 145.
.
Fig-3: Effect of iron powder (Fe0) dosage on decolourization.
Effect of H2O2 concentration on decolourization
Optimization for concentration of H2O2 is important in decolourization process since H2O2
is source of OH- which is produced in Fenton process. Its optimization study was carried
out in range of 5–30mg/l & decolourization efficiencies of both dyes increased with increasing
dosage until 20mg/l in accordance with Eq-3. With increasing high dosages, on other hand,
efficiency decreased because of radical scavenging effect of excess H2O2 for Re. Orange 127 as
shown in Eq-4, while a negligible
increase in efficiency was observed
for Re. Yellow 145. Optimum
concentration was selected as 20mg/l
for decolourization of both dyes.
2
 Fe  H 2 O 2  Fe 3  OH   OH  (3)
 
 H 2O 2  OH  HO2  H 2O (4)
Fig-4: Effect of H2O2 dosage on decolorization.
Conclusion
In this present study, decolourization of Re. Orange 127 & Re. Yellow 145 from aqueous
solutions was investigated. Optimum conditions for Re. Orange 127 were determined as
90rpm at pH 3.5 with 80mg/l Fe0 & 20mg/l H2O2 & they were determined as 200rpm at pH
4.0 with 60mg/l Fe0 & 20mg/L H2O2 for Re. Yellow 145. According to this, optimum molar
ratios [Fe0]/ [H2O2] were determined as 2.4/1 & 1.8/1 for Re. Orange 127 & Re. Yellow
145, respectively. It was also concluded that Fenton type process in which zero-valent iron
was used was an effective process for decolourization of both dyes.
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Compiled by: M. Rezaul Karim Tutul