Degradation of C.I. Reactive Red 2 (RR2) using ozone-based systems: Comparisons of decolorization efficiency and power consumption

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Available online at www.sciencedirect.com

Journal of Hazardous Materials 152 (2008) 120–127

Degradation of C.I. Reactive Red 2 (RR2) using ozone-based systems:

Comparisons of decolorization efficiency and power consumption

Chung-Hsin Wu

a

,

∗

, How-Yong Ng

b

a_{Department of Environmental Engineering, Da-Yeh University, 112, Shan-Jiau Road, Da-Tsuen, Chang-Hua, Taiwan, ROC} b_{Division of Environmental science and Engineering, National University of Singapore, Singapore}

Received 14 April 2007; received in revised form 21 June 2007; accepted 21 June 2007 Available online 28 June 2007

Abstract

This study investigated the decolorization efficiency of C.I. Reactive Red 2 (RR2) in O

3

, O

3

/H

2

O

2

, O

3

/Fe

3+

, O

3

/H

2

O

2

/Fe

3+

, UV/O

3

, UV/O

3

/Fe

3+

,

UV/O

3

/H

2

O

2

and UV/O

3

/H

2

O

2

/Fe

3+

systems at various pHs. The effective energy consumption constants and the electrical energy per order of

pollutant removal (EE/O) were also determined. The experimental results indicated that the energy efficiency was highest at [H

2

O

2

]

0

= 1000 mg/l

and [Fe

3+

_]

0

= 25 mg/l. Accordingly, the H

2

O

2

and Fe

3+

doses in the hybrid ozone- and UV/ozone-based systems were controlled at these values.

This work suggests that the dominant reactant in O

3

, O

3

/Fe

3+

and O

3

/H

2

O

2

systems was O

3

and that in the O

3

/H

2

O

2

/Fe

3+

system was H

2

O

2

/Fe

3+

.

The experimental results revealed that the combinations of Fe

3+

_{or H}

2

O

2

/Fe

3+

with O

3

at pH 4 and of H

2

O

2

or H

2

O

2

/Fe

3+

with UV/O

3

at pH 4

or 7 yielded a higher decolorization rate than O

3

and UV/O

3

, respectively. At pH 4, the EE/O results demonstrated that the UV/O

3

/H

2

O

2

/Fe

3+

system reduced 85% of the energy consumption compared with the UV/O

3

system. Moreover, the O

3

/H

2

O

2

/Fe

3+

system reduced 62% of the energy

consumption compared with the O

3

system. At pH 7, the EE/O results revealed that the UV/O

3

/H

2

O

2

/Fe

3+

system consumed half the energy of the

UV/O

3

system.

© 2007 Elsevier B.V. All rights reserved.

Keywords: C.I. Reactive Red 2; Ozone; Ferric; Hydrogen peroxide; Decolorization; Power consumption

1. Introduction

The textile industry utilizes numerous dyes and pigments.

Among these, azo dyes represent the largest and the most

impor-tant class of commercial dyes. Most commercial dyes are not

directly toxic. Colored wastewater is subject to strict

environ-mental legislation because they have a negative effect on the

photosynthetic activity in Taiwan. Accordingly, decolorization

of dye effluents has attracted increased attention. The C.I.

Reac-tive Red 2 (RR2), dye with the most commonly used anchor – the

dichlorotriazine group – was selected as the parent compound

in this study. Conventional treatment cannot efficiently remove

dyes from textile wastewater, because they are stable against

light and biological degradation. Treatments such as adsorption,

flotation and coagulation only perform the phase transfer of

pol-lutants but do not destroy them. Hence, further treatments are

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E-mail address:[email protected](C.-H. Wu).

required. Advanced oxidation processes (AOPs) are alternative

methods for decolorizing and reducing recalcitrant wastewater

loads from textile companies. AOPs are based on the

genera-tion of hydroxyl radicals in water, which are highly reactive

and nonselective oxidants that can oxidize organic compounds.

Hydroxyl radicals have an oxidation potential that exceeds that

of ozone and H

2

O

2

– 2.80 V for hydroxyl radicals, 2.07 V for

ozone and 1.78 V for H

2

O

2

. Ozone may either react directly with

organic compounds or decompose highly reactive species, such

as hydroxyl radicals. Ozonation has potential in decolorization

for the following reasons: (1) no sludge remains; (2) danger is

minimal; (3) decolorization and degradation occur in one step;

(4) it is easily performed; (5) little space is required, and (6)

all residual ozone can be easily decomposed to oxygen and

water

[1]

. Accordingly, the ozone-based systems are feasible

for decolorizing azo dyes.

Combining various AOPs commonly causes interesting

syn-ergistic effects that can markedly reduce the reaction time and

economic cost. Various studies have explored the synergistic

effects of the decolorization of dyes in ozone-based systems,

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C.-H. Wu, H.-Y. Ng / Journal of Hazardous Materials 152 (2008) 120–127 121

such as O

3

/H

2

O

2

[2–4]

, O

3

/Fe

2+

[5,6]

, UV/O

3

/H

2

O

2

[2,4,7]

,

O

3

/H

2

O

2

/Fe

2+

[2]

, UV/O

3

/Fe

2+

[5,6,8]

, UV/O

3

/Fe

3+

[9]

,

UV/TiO

2

/O

3

[4,10]

, UV/O

3

/H

2

O

2

/Fe

3+

[4]

, UV/O

3

/Fe

2+

/Cu

2+

[6,8]

, UV/O

3

/H

2

O

2

/Fe

2+

[2]

and UV/O

3

/TiO

2

/SnO

2

[11]

. Since

Fe

3+

in hybrid ozone-based systems has rarely been examined

and iron catalysts are abundant in nature, this study

incorpo-rates Fe

3+

into the hybrid ozone-based systems to evaluate the

decolorization efficiency of RR2 at various pHs.

The photodegradation of aqueous organic pollutant is an

electric-energy-intensive process, and electric energy typically

represents a major fraction of the operating costs. Simple

figures-of-merit based on electric energy consumption can therefore be

very useful. The electrical energy per order of pollutant removal

(EE/O) is a powerful scale-up parameter and a measure of the

treatment rate in a fixed volume of contaminated water as a

function of the applied specific energy dose

[12]

. The EE/O

value was adopted to compare the energy efficiency of

differ-ent systems. In the case of low-pollutant concdiffer-entrations, the

EE/O (kW h m

−3

order

−1

) can be determined from the following

equations.

EE

/O =

Pt × 1000

V × 60 × log (A

i

/A

o

)

(1)

ln

A

i

A

o

= k

a

t

(2)

where P is the power (kW) of the AOPs; t is the reaction time

(min); V is the volume (l) of the water in the reactor; A

i

and A

o

are the inflow and outflow RR2 absorbance and k

a

is the

pseudo-first-order rate constant (min

−1

) for the decay of the pollutant

in the pollutant concentration

[12,13]

. Combining Eqs.

(1)

and

(2)

yields Eq.

(3)

for EE/O.

EE

/O =

38 .4 × P

Vk

a

(3)

Most related studies compared efficiency using reaction rate

constants. Few works considered the effects of power

con-sumption

[12,13]

. Wu et al.

[14]

had plotted ln(A

i

/A

o

) against

total energy consumption and determined the effective energy

consumption constants (k

b

, kJ

−1

). Since the ozone reaction

path-ways depend strongly on the characteristics of the wastewater to

be treated, including pH, promoters and scavengers in the

solu-tion, this study simultaneously employs k

a

, EE/O and effective

energy consumption constants, as proposed by Wu et al.

[14]

,

to evaluate the decolorization efficiency and power

consump-tion of ozone-based systems at different pHs. The objectives of

this investigation are (i) to calculate the k

a

, k

b

and EE/O values

of ozone-based systems O

3

, O

3

/H

2

O

2

, O

3

/Fe

3+

, O

3

/H

2

O

2

/Fe

3+

,

UV/O

3

, UV/O

3

/Fe

3+

, UV/O

3

/H

2

O

2

and UV/O

3

/H

2

O

2

/Fe

3+

sys-tems at pH 4, 7 and 10; (ii) to clarify the effects of UV irradiation

in these ozone-based systems; (iii) to determine the synergistic

effects of different pHs and (iv) to compare the variations of k

a

,

k

b

and EE/O with pH.

2. Materials and methods

2.1. Materials

The parent compound, RR2, obtained from Aldrich

Chem-ical Company, was employed without further purification. The

formula, molecular weight and maximum light absorption

wave-length (

λ

max

) of RR2 were C

19

H

10

Cl

2

N

6

Na

2

O

7

S

2

, 615 g/mol

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126 C.-H. Wu, H.-Y. Ng / Journal of Hazardous Materials 152 (2008) 120–127

Table 2

Effective energy consumption constants (kb, kJ−1) and electrical energy per order (EE/O, kW h m−3order−1) of various ozone-based systems

pH 4 pH 7 pH 10

kb R2 _EE/O _kb _R2 _EE/O _kb _R2 _EE/O

Non-UV systems O3 0.058 0.983 3.684 0.109 0.970 2.111 0.199 0.996 1.070 O3/Fe3+ 0.086 0.999 2.473 0.084 0.948 2.541 0.150 0.988 1.332 O3/H2O2 0.023 0.987 9.225 0.049 0.994 4.321 0.060 0.990 3.568 O3/H2O2/Fe3+ 0.154 0.956 1.384 0.073 0.990 3.030 0.075 0.986 2.968 With-UV systems UV/O3 0.036 0.974 6.420 0.051 0.975 4.223 0.118 0.994 1.865 UV/O3/Fe3+ _0.025 _0.998 _8.533 _0.050 _0.958 _4.240 _0.068 _0.989 _2.909 UV/O3/H2O2 0.105 0.935 2.256 0.093 0.972 2.306 0.064 0.977 3.141

UV/O3/H2O2/Fe3+ _0.234 _0.968 _0.979 _0.108 _0.968 _2.133 _0.072 _0.984 _2.972

UV/O

3

and O

3

systems were satisfactory for decolorizing RR2

at pH 10. Gutowska et al.

[36]

indicated that ozonation was more

effective for C.I. Reactive Orange 113 degradation than for

Fen-ton’s process. However, Jozwiak et al.

[37]

demonstrated that

Fenton’s process was more effective than ozonation for C.I. Acid

Brown 159. Additionally, the EE/O values were found to depend

Fig. 7. Relationship between decolorization ratio and total energy consump-tion for ozone- and UV/ozone-based systems (a) pH 4, (b) pH 7 and (c) pH 10 (RR2 = 40 mg/l, ozone flow rate = 500 ml/min, H2O2= 1000 mg/l, Fe3+_{= 25 mg/l and T = 25}◦_C).

on the concentration of oxidant, the concentration and the basic

structure of the dye

[13]

. Hence, this investigation suggests that

the optimal conditions (both for decolorization efficiency and

effective energy consumption) varied among the dyes, revealing

that the development of a general decolorization method for a

mixture of dyes would be very difficult.

4. Conclusion

The decolorization rate constants, effective energy

consump-tion constants and electrical energy per order of pollutant

removal in O

3

, O

3

/H

2

O

2

, O

3

/Fe

3+

, O

3

/H

2

O

2

/Fe

3+

, UV/O

3

,

UV/O

3

/Fe

3+

, UV/O

3

/H

2

O

2

and UV/O

3

/H

2

O

2

/Fe

3+

systems

were determined at pH 4, 7 and 10. The effect of Fe

3+

dose

on dye decolorization was similar to that of H

2

O

2

; the

reac-tion rate constants initially increased to a critical value and

then declined. The k

a

values of O

3

, O

3

/Fe

3+

, O

3

/H

2

O

2

, UV/O

3

and UV/O

3

/Fe

3+

systems were larger under alkaline than under

acidic conditions. However, O

3

/H

2

O

2

/Fe

3+

, UV/O

3

/H

2

O

2

and

UV/O

3

/H

2

O

2

/Fe

3+

systems varied oppositely. The experimental

results indicated that the combination of Fe

3+

or H

2

O

2

/Fe

3+

into

O

3

at pH 4 and H

2

O

2

or H

2

O

2

/Fe

3+

with UV/O

3

at pH 4 and

7 could yields a higher decolorization rate than O

3

and UV/O

3

,

respectively. The EE/O and k

b

values followed the same order for

both ozone- and UV/ozone-based systems. Based on the

anal-yses of decolorization efficiency and power consumption, this

study suggests that the UV/O

3

/H

2

O

2

/Fe

3+

system was an

appro-priate method for decolorizing RR2 at pH 4 and 7. Moreover,

UV/O

3

and O

3

systems are acceptable for decolorizing RR2 at

pH 10.

Acknowledgement

The authors would like to thank the National Science Council

of the Republic of China for financially supporting this research

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

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