Decolorization of Reactive Blue 19 Dye from Textile Wastewater by the UV/H2O2 Process
Mohammad Taghi Ghaneian,
Sayed Jamalodin Hashemian,
Photo-oxidation of dyes is a new concern among researchers since it offers an attractive method for decoloration of dyes and breaks them into simple mineral forms. An advanced oxidation process, UV/H2O2, was investigated in a laboratory scale photoreactor for decolorization of the Reactive blue 19 (RB19) dye from synthetic textile wastewater. The effects of operating parameters such as hydrogen peroxide dosage, pH, initial dye concentration and UV dosage, on decolorization have been evaluated. The RB19 solution was completely decolorized under optimal hydrogen peroxide dosage of 2.5 mmol L-1 and low-pressure mercury UV-C lamps (55 w) in less than 30 min. The decolorization rate followed pseudo-first order kinetics with respect to the dye concentration. The rate increased linearly with volumetric UV dosage and nonlinearly with increasing initial hydrogen peroxide concentration. It has been found that the degradation rate increased until an optimum of hydrogen peroxide dosage, beyond which the reagent exerted an inhibitory effect. From the experimental results, the UV/H2O2 process was an effective technology for RB19 dye treatment in wastewater.
In the textile industry, the process of dyeing product of large amounts
of wastewater exhibiting intense coloration that has to be eliminated
before release into environment (Mohorcic et al., 2006). Wastewaters
from the textile industries contain different types of synthetic dyes,
which are mostly toxic, mutagenic and carcinogenic. Moreover, they are
very stable to light, temperature and microbial attack, making them recalcitrant
compounds (Kokol et al., 2007). The discharge of the wastewaters
into receiving streams not only affects the aesthetic nature but also
interferes with transmission of sunlight into streams and therefore reduces
photosynthetic activity (Çiçek et al., 2007). Reactive
dyes are extensively used in the textile industry, fundamentally due to
the capacity of their reactive groups to bind on textile fibers by covalent
bonds formation. This characteristic facilitates the interaction with
the fiber and reduces energy consumption. The dyes are mainly used for
dyeing cellulosic fibers, such as cotton and rayon, but are also used
for silk, wool, nylon and leather (Yang and Mc Garrahan, 2005). The Reactive
Blue 19, also known as Remazol brilliant blue, is very resistant to chemical
oxidation due to its aromatic anthraquinone structure highly stabilized
by resonance. In the particular case of the RB19, the relatively low fixation
efficiency (75-80%) is due to the competition between the formation of
the reactive form and the hydrolysis reactions (Pelegrini et al.,
1999; Lizama et al., 2002). Various physical, chemical and biological
treatment methods have been used for the treatment of these textile effluents
(Çiçek et al., 2007; Daneshvar et al., 2005).
Advance oxidation processes are widely used both in industrial preparations
and in environmental treatments. In the textile industry, these processes
are used for degrading and removing color from dye baths, which allow
wastewater reuse (Gemeay et al., 2007). These processes are based
on the production of very reactive hydroxyl radicals with an oxidation
potential of 2.8 V as primary oxidizing species (Aleboyeh et al.,
2003; Galindo et al., 2000; Mandal et al., 2004). UV/H2O2
process has been recognized that the efficiency of the oxidation process
strongly depends on experimental conditions (García Einschlag et
al., 2003). The reactions of dye removal under UV, H2O2
and UV/H2O2 processes can be presented by:
The aim of this study is to analyze the feasibility of discoloration
of RB19 using a efficient advanced oxidation process, i.e., UV/H2O2.
The influence of the pH solution and the concentration of the dyes on
the decolorization were studied.
MATERIALS AND METHODS
Dye: The RB19 was purchased from the Dystar (Germany) and used
without further purification. Solutions were prepared with the dye using
distilled deionized water. The molecular structure of the dye is shown
in Fig. 1 (Kurbus et al., 2002). The pH of solutions
was adjusted with NaOH and H2SO4. All other reagents
were analytical grade.
Photoreactor: All experiments were carried out in a mixed batch
photoreactor of 2.5 L in volume. Mixing was provided by circulating the
reaction solution with a peristaltic pump 5001 (Heidolph, Germany) at
a rate of 250 cm3 min-1. The radiation source was
low-pressure mercury UV-C lamps (15 and 55 w, Philips, Holland). The UV
lamps were inserted into a walled quartz immersion well located at the
center of reactor. For the current decolorization study, 2.5 L of the
colored water was initially placed in a UV/H2O2
photoreactor. Reactions were performed at ambient temperature (25°C).
Experiments were carried out with three different pHs of nominal 3, 7
and 10, corresponding to initial color intensities of nominal 25 and 100,
respectively. Different initial H2O2 concentrations
(0-250 mmol L-1) were also investigated. Control experiments
without UV irradiation (i.e., H2O2 oxidation only)
were conducted at 2.5 mmol L-1 H2O2 concentration
at the pHs. Blank experiments without H2O2 dosage
(i.e., UV photolysis only) were also carried out at the pH levels.
Analytical methods: Samples were collected at different times
and immediately analyzed for color intensity and pH. Since color removal
was the principal objective of this study, color intensity was the main
parameter monitored. In this regard, the color without filtering or centrifuging
the samples was measured spectrophotometrically. UV-Vis spectra have been
acquired (200-800 nm) with a Unico spectrophotometer UV2100 (Fig.
||Absorption spectra of the RB19 dye
Dye concentrations were calculated from the calibration curve prepared
from the dye concentration and the measured absorbance at λmax
(592 nm) (Rajkumar et al., 2007). The percentage of decolorization
was calculated as follows:
where, C0 is initial dye concentration and C is final dye
concentration (Sayan, 2006).
RESULTS AND DISCUSSION
Different H2O2 concentrations were dosed to determine
the effect of H2O2 concentration on the decolorization
rate by UV/H2O2. Blank experiments in the absence
of H2O2 (i.e., UV only) showed a quicker decolorization
at a higher pH, although decolorization rates were slower than those for
UV/H2O2 systems. Control experiments with H2O2
only achieved negligible decolorization (Fig. 3). The
observation in the blank experiments was consistent with the results by
Ince et al. (1997), who reported that the chemical bond of azo
compounds could be directly but slowly photolyzed by UV irradiation and
that the process could be substantially improved by the addition of H2O2.
The excess H2O2 dose can reduce the oxidation rate
by acting as a •OH radical scavenger itself
as follows (Ku et al., 1998).
||Effects of UV-C Irradiation, H2O2
and UV-C/H2O2 process on the dye residual fractions,
(C0 = 25 mg L-1, pH 7, L:55W)
||Effect of pH on the dye residual fractions during UV-C/H2O2
process, (C0 = 100 mg L-1, H2O2
= 2.5 mmol L-1, L:15W)
Wang et al. (2000) reported decreased humic acid oxidation rates
with the increase of H2O2 concentration greater
than 0.01%. Huling et al. (2000) also found a decreased oxidation
of adsorbed 2-chlorophenol to granular activated carbon containing fixed
iron oxide at the high H2O2 concentration due to
increased scavenging by excess H2O2. Also, the excess
H2O2 can absorb most of the light. The efficiency
of photo-oxidation processes strongly depend upon the pH of the reaction
solution. In general, the best UV/H2O2 decolorization
was achieved at the pH of 3.0 (Azbar et al., 2004), whereas a relatively
less effective decolorization was observed in the solutions of pH 7.0
(Fig. 4). The effect of initial color intensity on decolorization
efficiency was investigated. The decolorization rates were compare at
pH 7.0 and different initial color intensity (Fig. 5).
For the concentration 25 mg L-1 decomposition was almost complete
after 30 min of illumination, but for the higher concentrations the photo-oxidation
process runs with some lower efficiency, however better results can be
obtained by extension of reaction time. The large amount of dye inhibits
the reaction of dye molecules with decrease of hydroxyl radicals. A pseudo
first-order rate constant was calculated by simple least-squares regression
of the natural log of color intensities versus time (Fig.
6). UV irradiation has a great influence to increase the photo-oxidation
of dyes. The wastewater with high strength color is generally regarded
as to be treated. Especially by the UV-irradiation technology, the strong
strength of wastewater absorbs UV resulting deduction of photo-efficiency,
which irradiates hydrogen peroxide molecules into less free radical. Hence,
the oxidation in UV/H2O2 process is promoted positively
by increasing UV power in order to overcome the treatment of color wastewater.
The higher UV lamp power produced more and faster formation of OH•
free radicals so as to improve the decolorization rate.
||Effects of initial color intensity on the dye residual
fractions, (H2O2 = 2.5 mmol L-1,
pH 7, L:55 W)
||First order plot of dye photochemical degradation with
time, (C0 = 100 mg L-1, pH 7, L:55W, H2O2
= 2.5 mmol L-1)
The experimental results showed that the UV/H2O2
process can be a suitable treatment method for decolorization of RB19
dye wastewater from textile industries under the optimal operating conditions.
The decolorization rates of RB19 were determined and affected significantly
by the pH, UV dosage, hydrogen peroxide dosage and initial dye concentrations.
The UV/H2O2 treatments were capable of decolorizing
the colored wastewater at the pHs investigated (3.0, 7.0 and 10) within
feasible treatment duration of less than 3 h. Enhanced decolorization
was achieved with an increase in H2O2 dosage and
decrease in pH. An increase in H2O2 concentration
leads to a faster degradation up to a critical value; at a higher ratio
the degradation process becomes slower. The only exception to this trend
was at pH 10 where less decolorization was observed with an increase in
H2O2 dosage. Efficient decolorization and use of
H2O2 were observed at pH 3.0 conditions. It is recommended
that the colored effluents be acidified prior to a decolorization step
by UV/H2O2 advanced oxidation. Present results prove
that the pseudo-first order kinetic model is in good agreement with the
experimental data. Although the decolorization rate will depend on the
particular type of the dye, the technique seems to be applicable to any
industrial wastewater containing dyes. These results can be taken as a
starting step to establish the economical feasibility of the method, defining
previously the degree of destruction and mineralization desirable for
the next application of the wastewater.
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