Decolorization of C.I. Reactive Red 2 in O3, Fenton-like and O3/Fenton-like Hybrid systems

(1)

Decolorization of C.I. Reactive Red 2 in O

3

, Fenton-like

and O

3

/Fenton-like hybrid systems

Chung-Hsin Wu

*

Department of Environmental Engineering, Da-Yeh University, 112, Shan-Jiau Road, Da-Tsuen, Chang-Hua 515, Taiwan, ROC Received 26 December 2006; received in revised form 24 February 2007; accepted 2 March 2007

Available online 12 March 2007

Abstract

The decolorization of C.I. Reactive Red 2 using ozone-related, Fenton-like-related and ozone/Fenton-like hybrid systems was studied. The UV,

H

2

O

2

, UV/Fe

3þ

and H

2

O

2

/Fe

3þ

systems did not achieve significant decolorization at pH 7; decolorization rates followed the order: pH 10 > pH

7 > pH 4 in the O

3

and UV/O

3

systems and followed the opposite order in the case of the UV/H

2

O

2

system. Decolorization rate constants were consistent

with pseudo-first-order kinetics. In the O

3

system, decolorization rate declined as dye concentration increased. The decolorization rate constants at pH

7$followed the order UV/O

3

/H

2

O

2

/Fe

3þ

(5.78 h

1

) > UV/O

3

/H

2

O

2

(5.33 h

1

) > UV/H

2

O

2

/Fe

3þ

(4.59 h

1

) > UV/H

2

O

2

(4.08 h

1

) > UV/O

3

(2.91 h

1

)

¼ O

3

(2.91 h

1

)

UV/O

3

/Fe

3þ

(2.90 h

1

) > O

3

/Fe

3þ

(2.42 h

1

) > O

3

/H

2

O

2

/Fe

3þ

(2.03 h

1

) > O

3

/H

2

O

2

(1.42 h

1

).

Ó 2007 Elsevier Ltd. All rights reserved.

Keywords: Advanced oxidation processes; Decolorization; C.I. Reactive Red 2; UV; O3; Fenton-like

1. Introduction

Azo dyes are by far the largest of all classes of dyes. As the

wastewater from textile dyeing typically contains high

concentra-tions of colorants, effective decolorization methods are urgently

required. Although numerous physical/chemical schemes,

including coagulation, flocculation, adsorption and membrane

filtration, have been used to decolorize textile effluents, these

techniques suffer disadvantages of sludge generation, adsorbent

regeneration and membrane fouling.

Advanced oxidation processes (AOPs) are alternative methods

for decolorizing and reducing recalcitrant wastewater loads from

the textile dyeing industry. Among the AOPs, treatment with

ozone

[1e5]

or Fenton-type

[4e6]

processes have yielded very

good results. The Fenton-type process combines an iron

com-pound with hydrogen peroxide to produce hydroxyl radicals.

Hy-drogen peroxide

[7,8]

, OH

[3]

, UV light

[3,9]

and reduced

transition metals

[1,7,10,11]

can activate the decomposition of

ozone to hydroxyl radicals. The effects of dye concentration

[3,12]

, ozone dosage

[12,13]

, pH

[1,3,12,14]

, presence or absence

of UV

[3,9]

and UV intensity

[9]

in ozone-related systems have

been evaluated. The general mechanism involves Fenton reagent

that utilizes Fe

2þ

(Fenton) or Fe

3þ

(Fenton-like) ions as catalysts

to decompose hydrogen peroxide. Ultraviolet irradiation

(photo-Fenton and photo-(photo-Fenton-like reactions)

[15,16]

promotes

degra-dation of organic compounds in Fenton and Fenton-like reactions.

Several studies have investigated the effects of iron salt dosage

[16e19]

, H

2

O

2

dosage

[6,16e19]

, the nature of the iron salt

[6,17]

, the presence or absence of UV

[16,17,20]

and UV intensity

[16]

in Fenton-type processes.

Various researchers have explored synergic effects in the

de-colorization of dyes using hybrid systems, including Fe

0

/H

2

O

2

[21]

, O

3

/H

2

O

2

[7,8,21,22]

, O

3

/Fe

2þ

[23,24]

, UV/O

3

/H

2

O

2

[7,22,25]

, O

3

/H

2

O

2

/Fe

2þ

[7]

, UV/O

3

/Fe

2þ

[11,23,24]

, UV/O

3

/

Fe

3þ

[10]

, UV/TiO

2

/O

3

[2,22]

, UV/TiO

2

/H

2

O

2

[20]

, UV/O

3

/

H

2

O

2

/Fe

3þ

[22]

, UV/O

3

/Fe

2þ

/Cu

2þ

[11,24]

, UV/O

3

/H

2

O

2

/

Fe

2þ

[7]

and UV/O

3

/TiO

2

/SnO

2

[3]

. Several studies have

analyzed the factors that influence dye decolorization in H

2

O

2

-and ozone-related individual systems. Most hybrid H

2

O

2

-related

processes, when used under acidic conditions, achieved a higher

decolorization efficiency than that obtained under neutral or

alkaline conditions; conversely, hybrid ozone-related processes

* Fax:þ886 5 5334958.

E-mail address:[email protected]

Dyes and Pigments 77 (2008) 24e30

(2)

formation of hydroxyl radicals (Eq.

(1)

) and consumes

hydro-peroxyl radicals (Eq.

(6)

). The oxidation potential of hydroxyl

radicals exceeds that of hydroperoxyl radicals; however, the

generation rate of hydroxyl radicals (3.3

10

6

M

1

s

1

) is

much lower than the consumption rate of hydroperoxyl radicals

(3.1

10

5

_M

1

_s

1

₎

_[17]

_{. Therefore, adding Fe}

3þ

_{did not}

sig-nificantly promote decolorization in the UV/O

3

system.

Eqs.

(1)e(9)

present the reaction mechanisms in the UV/

H

2

O

2

/Fe

3þ

(photo-Fenton-like) system. Iron cycles between

Fe

3þ

and Fe

2þ

under light irradiation. Experimental results

in-dicated that the decolorization rate of the UV/H

2

O

2

/Fe

3þ

system

exceeded that of UV/H

2

O

2

, which is consistent with findings

ob-tained by previous studies, which showed that the reaction rates

of photo-Fenton and photo-Fenton-like systems exceed that of

the UV/H

2

O

2

system

[7,22,27]

. In the UV/O

3

/H

2

O

2

system,

re-actions described by combining Eqs.

(9), (13) and (14)

, which

hybrid system had a higher decolorization rate than the UV/

O

3

, UV/H

2

O

2

and O

3

/H

2

O

2

systems. Interestingly, H

2

O

2

/Fe

3þ

and O

3

/H

2

O

2

systems exhibited low decolorization rates;

how-ever, the hybrid systems UV/H

2

O

2

/Fe

3þ

and UV/O

3

/H

2

O

2

had

high decolorization rates, implying that UV irradiation played

an important role in these systems. Since UV irradiation can

de-compose H

2

O

2

to hydroxyl radicals (Eq.

(9)

), the scavenging

ef-fect of H

2

O

2

can be ignored (Eq.

(7)

), and the decolorization rate

is then finally increased. Given the synergic effects in the UV/

H

2

O

2

/Fe

3þ

and UV/O

3

/H

2

O

2

systems, increased decolorization

rate in the UV/O

3

/H

2

O

2

/Fe

3þ

system was expected and

reason-able. Beltran-Heredia et al.

[7]

combined O

3

with the UV/H

2

O

2

/

Fe

2þ

system to degrade

p-hydroxybenzoic acid. Dominguez

et al.

[22]

incorporated O

3

into the UV/H

2

O

2

/Fe

3þ

system to

de-colorize Acid Red 2. Both studies suggested that the hybrid O

3

/

photo-Fenton and O

3

/photo-Fenton-like systems had the highest

reaction rate among the O

3

, O

3

/H

2

O

2

, O

3

/Fe

3þ

, O

3

/Fe

2þ

, H

2

O

2

/

Fe

3þ

, H

2

O

2

/Fe

2þ

, O

3

/H

2

O

2

/Fe

3þ

, O

3

/H

2

O

2

/Fe

2þ

, UV/H

2

O

2

,

UV/H

2

O

2

/Fe

3þ

, UV/H

2

O

2

/Fe

2þ

, UV/O

3

, UV/O

3

/Fe

3þ

, UV/

O

3

/Fe

2þ

and UV/O

3

/H

2

O

2

systems.

4. Conclusions

Neither UV nor H

2

O

2

alone caused significant RR2

decol-orization. As the H

2

O

2

concentration was increased from 500

to 1000 ppm in the UV/H

2

O

2

system, decolorization efficiency

increased; however, at H

2

O

2

> 1000 ppm, no further

improve-ment occurred. Attention must be paid to the molar Fe

3þ

:H

2

O

2

ratio to prevent undesired radical scavenging reactions. The

k

values of the UV/H

2

O

2

system followed the order of pH

4 > pH 7 > pH 10; the

k values for the UV/O

3

system

ex-ceeded those obtained for O

3

and the

k values of the O

3

and

UV/O

3

systems followed the order: pH 10 > pH 7 > pH 4.

Without UV irradiation, decolorization rate constants at pH

7 followed the order: O

3

> O

3

/Fe

3þ

> O

3

/H

2

O

2

/Fe

3þ

> O

3

/

H

2

O

2

. Under UV irradiation, the decolorization rate constants

at pH 7 followed the order: UV/O

3

/H

2

O

2

/Fe

3þ

> UV/O

3

/

H

2

O

2

> UV/H

2

O

2

/Fe

3þ

> UV/H

2

O

2

> UV/O

3

UV/O

3

/Fe

3þ

.

Notably, UV irradiation was important to the synergic effects

of hybrid systems.

Acknowledgements

The authors would like to thank the National Science

Coun-cil of the Republic of China for financially supporting this

re-search under Contract No. NSC 95-2221-E-212-022.

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