Decolorization of Procion Red MX-5B in electrocoagulation (EC), UV/TiO2 and ozone related systems

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Decolorization of Procion Red MX-5B in electrocoagulation (EC),

UV/TiO

2

and ozone-related systems

Chung-Hsin Wu

a

, Chung-Liang Chang

b

, Chao-Yin Kuo

c,

*

a

Department of Environmental Engineering, Da-Yeh University, 112, Shan-Jiau Road, Da-Tsuen, Chang-Hua, Taiwan, ROC

b_{Department of Environmental Engineering and Health, Yuanpei University of Science and Technology, 306 Yuanpei Street, Hsinchu, Taiwan, ROC} c_{Department of Safety Health and Environmental Engineering, National Yunlin University of Science and Technology,}

123, Sec. 3, University Road, Touliu, Yunlin 640, Taiwan, ROC

Received 17 March 2006; received in revised form 23 May 2006; accepted 16 August 2006 Available online 11 October 2006

Abstract

This investigation assessed the decolorization efficiency of Procion Red MX-5B in electrocoagulation (EC), UV/TiO2and ozone-related sys-tems. The effectiveness of energy input was also determined. The decolorization rate constants of these EC, UV/TiO2and ozone-related systems fitted pseudo-first-order kinetics and the values were in the order O3(24 W) > O3 (16 W) > O3(16 W)/EC (8 W) > UV/TiO2/O3 (8 W)/EC (8 W) > O3 (10 W) > UV/O3 (8 W)/EC (8 W) > UV/O3 (8 W) > O3 (8 W) > UV/TiO2/O3 (8 W) > O3 (8 W)/EC (8 W) > O3 (4 W)/EC (4 W) > UV/TiO2/EC (8 W) > UV/TiO2> UV/EC (8 W) > EC (8 W). The decolorization rate constants increased with the total power input. Ad-ditionally, the decolorization efficiency could be promoted by combining UV with O3, UV with EC, EC with UV/TiO2and EC with UV/O3. This study reveals that combining EC with UV/TiO2or UV/O3can trigger a Fenton or Fenton-like reaction, which accelerates the rate of decoloriza-tion. The solution pH of O3, UV/O3and UV/TiO2systems declined during decolorization; in contrast, the pH increased to 7.4 in the UV/EC sys-tem. The effective energy consumption constant did not increase with the total power input and reached maximum at a total power input of approximately 10e16 W.

Keywords: Decolorization; Energy consumption; Procion Red MX-5B; Electrocoagulation; UV; TiO2; O3

1. Introduction

Wastewater from the textile dyeing industry has a high or low pH, high temperature and a high concentration of coloring material. Numerous dyes represent environmental hazards ow-ing to their toxicity. Azo dyes are the most extensively utilized dyes and are normally major pollutants in dye effluents. Treat-ment costs are very large for most textile factories, explaining the need to develop more efficient and economic methods, which consume less chemical and energy. Conventional treatments of dye effluents include biological oxidation and

adsorption. Although less expensive than other approaches, biological treatment is ineffective for decolorization because the dyes are toxic. Adsorption onto activated carbon transfers most of the contaminant from the wastewater to the solid phase. This method therefore requires further disposal of the sludge. Electrocoagulation (EC) is regarded as a potentially effective method for treating textile wastewater with high decolorization efficiency and with the formation of relatively little sludge. Several researchers have reported treatments of dye wastewater based on the EC method [1e5]. EC applies an electric current to produce metal ions in solution. Metal ions can react with the OHions formed at the cathode during the evolution of hydrogen gas, to yield insoluble hydroxides that sorb pollutants out of the solution. The EC process can be summarized as follows (Eqs.(1) and (2))[1].

* Corresponding author. Fax:þ886 5 5334958. E-mail address:kuocyr@ms35.hinet.net(C.-Y. Kuo).

Dyes and Pigments 76 (2008) 187e194

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At the anode,

M / Mnþ þ ne _ð1Þ

At the cathode,

nH2Oþ ne/n=2 H2 þ nOH ð2Þ

where M is the anode material andn is the number of electrons that are involved in the reaction. The insoluble metal hydrox-ides react with pollutants by surface complexation, coagulation or electrostatic attraction. Previous studies had established that the decolorization efficiency is proportional to the current den-sity[1e5], the number of electrodes[4]and the concentration of the electrolyte[2,4]but not to the gap between the electrodes [1,2]. Kim et al.[1]demonstrated that the power consumption of EC increased with the current density and the concentration of the electrolyte but fell as the number of pairs of electrodes increased.

Advanced oxidation processes (AOPs) are alternative ap-proaches for decolorizing and reducing recalcitrant wastewater loads from textile companies. Significant progress has been made in the development of AOPs for textile effluent in recent years, especially in TiO2[6e13]and ozone-related processes

[12e17]. AOPs involve primarily the generation of a very powerful and non-selective oxidizing agent, the hydroxyl rad-ical (OH_{), to destroy hazardous pollutants. In UV/TiO}

2

-related systems, TiO2particles absorb UV energy that is greater

than the TiO2band gap and form a pair of electron and hole in

conduction band and valence band. The positive holes can ox-idize water molecules to hydroxyl radicals and the negative electrons reduce molecular oxygen to yield superoxide radical anions (O2). The positive holes, hydroxyl radicals and

super-oxide radical anions are the dominant oxidizing species in UV/ TiO2systems. However, the rapid unfavorable recombination

of photoproduced electrons and holes in TiO2considerably

re-duces photocatalytic efficiency. The effect of the form of TiO2

[11,18], TiO2loading[7,11], UV intensity[7], UV irradiation

time [9,13], solution pH [7,10,11], substrate concentration [11,13] and the presence of different electron acceptors [9,11]in UV/TiO2-related systems has been extensively

inves-tigated. The degradation efficiency typically increases with the UV intensity and irradiation time, the concentration of TiO2

and the presence of electron acceptors, but is inhibited by an increase in the initial substrate concentration.

Ozonation may be a promising method for decolorization since ozonation can remove color and degrade organics in one step; moreover, no sludge remains in the treated effluent. Ozone oxidizes organics via two possible degradation routes: (i) at basic pH, it rapidly decomposes to yield hydroxyl and other radical species in solution according to Eqs.(3)e(5), and (ii) at acidic pH, ozone is stable and can react directly with organic substrates[19]. UV radiation can decompose ozone in water, generating highly reactive hydroxyl radicals[17]. The hydroxyl radicals are known to be the most powerful oxidizing agents and oxidize organics faster than ozone itself. Ozone has an oxidation potential of 2.07 V, whereas the OH radical has an oxidation potential of 2.80 V; notably, direct oxidation is slower than

radical oxidation. The effects of dye concentration [13,20], ozone dose[20,21], pH[12,13,20], the presence or absence of UV [13,17]and UV intensity [17]have been evaluated. The results of these works all indicate that the decolorization efficiency increases with ozone dose, pH and UV intensity, and declines with increasing dye concentration.

O3 þ OH/O3 þ OH ð3Þ

O₃_/_O _{þ O}

2 ð4Þ

O _{þ H}þ_/_OH _ð5Þ

The factors that influence dye decolorization in EC, UV/ TiO2and ozone-related systems have been investigated in

sev-eral works. However, comparisons of the amounts of energy consumed by these systems are few. Additionally, the effects of combining these methods on dye decolorization and energy consumption have not been examined. Therefore, in this study, Procion Red MX-5B was selected as a parent compound and decolorized with various total power inputs (8, 10, 16 and 24 W). The goals of this study are to compare the decolorization efficiencies and energy consumptions of EC, UV/EC, UV/TiO2,

ozone, UV/ozone, ozone/EC, UV/TiO2/ozone, UV/TiO2/EC,

UV/ozone/EC and UV/TiO2/O3/EC systems. The

decoloriza-tion rate and effective energy consumpdecoloriza-tion constants were eval-uated to identify the appropriate operating system.

2. Materials and methods 2.1. Materials

The parent compound, Procion Red MX-5B, purchased from Aldrich Chemical Company, was used without further purifi-cation. The formula, molecular weight and maximum light absorption wavelength (lmax) of Procion Red MX-5B were

C19H10Cl2N6Na2O7S2, 615 g/mol and 538 nm, respectively.

TiO2 was obtained from Degussa P-25 and utilized directly

without treatment. The water was deionized and doubly dis-tilled with MINIQ. One pair of Fe plates (outer diame-ter¼ 13.7 cm and inner diameter ¼ 4.2 cm) was used as electrodes in EC-related systems and the electrodes were con-nected to a DC power supply. The total effective electrode area was 133.6 cm2; the gap between the anode and the cathode was set to 0.4 cm, and the current density was maintained at 1.5 mA/cm2. In ozone-related systems, a dielectric barrier dis-charge (DBD) reactor was adopted to generate ozone. A stain-less steel wire (5.0 mm diameter) was suspended as an inner electrode along the axis of a Pyrex-glass tube (inner diameter 20.0 mm). The effective length of the DBD reactor was 137 mm. Glass pellets with a diameter of 5 mm were used as packing material, and placed in the plasma region between the two electrodes. A high voltage was applied to the inner elec-trodes. The DBD reactor consumed various powers (8, 10, 16 and 24 W) in pure oxygen at a flow rate of 500 mL/min. The schematic diagram of the ozone generator and the photoreactor presented herein is same as that of Wu and Chang[13].

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with pseudo-first-order kinetics (Table 1). This investigation proposes that a higherkbvalue corresponds to more efficient

en-ergy consumption during decolorization. The effective enen-ergy consumption constants follow the order O3 (8 W) > UV/O3

(8 W) > UV/TiO2/O3 (8 W) > UV/TiO2/O3 (8 W)/EC (8 W)

SUV/O3 (8 W)/EC (8 W) > O3 (8 W)/EC (8 W) (Table 1).

Interestingly, the effective energy consumption constants did not increase in proportion to the total energy input. The results imply that not all of the input energy is used in decolorization. For simplicity, this research focused on ozone-related systems to evaluate the relationships among the reaction rate constant, the effective energy consumption constant and the total power input (Fig. 7). At total power inputs of 8, 10, 16 and 24 W, the average reaction rate constants of ozone-related systems were 10.4, 19.8, 21.2 and 25.7 h1and the effective energy consump-tion constants were 18.0, 30.6, 21.8 and 16.5 k/J, respectively. The mean reaction rate constants increased with the total power input; however, the effective energy consumption constants initially increased with total power input, and then declined. A favorable power input is suggested to maximize the efficiency of energy consumption. Although the reaction rate constant was not maximal, this work suggests that a total power input of 10e16 W can maximize the efficiency of energy consumption.

This investigation verified that the decolorization rate could be promoted by coupling UV with O3, UV with EC, EC with

UV/TiO2and EC with UV/O3; however, all of the effective

en-ergy consumption constants of these systems dropped after cou-pling. The most effective total power input was 10e16 W, so this study suggests that UV/O3(8 W) and O3(16 W) systems were

more effective and economic for dye decolorization than other single or coupled systems.

4. Conclusion

This work compares the decolorization efficiency and en-ergy consumption of EC, UV/EC, UV/TiO2, ozone, UV/

ozone, ozone/EC, UV/TiO2/ozone, UV/TiO2/EC, UV/ozone/

EC and UV/TiO2/O3/EC systems in terms of reaction rate

constants and effective energy consumption constants. The ka value of UV/O3 (8 W) (17.94 h1) exceeded that of O3

(8 W) (15.92 h1). The reaction rate constants of UV/TiO2

and EC-related systems followed the order UV/TiO2/EC

(0.89 h1) > UV/TiO2 (0.35 h1) > UV/EC (0.26 h1) S EC

(0.22 h1). The synergy coefficients of UV/O3 (8 W), UV/

EC (8 W), UV/TiO2/EC (8 W), UV/O3(8 W)/EC (8 W) and

UV/TiO2/O3(8 W)/EC (8 W) systems were 0.11, 0.15, 0.31,

0.04 and 0.07, respectively. This study proposes that combin-ing EC with UV/TiO2or UV/O3may induce a Fenton or

Fen-ton-like reaction and accelerate decolorization. At a total power input of 8, 10, 16 and 24 W, the mean reaction rate constants of the ozone-related systems were 10.4, 19.8, 21.2 and 25.7 h1and the effective energy consumption con-stants were 18.0, 30.6, 21.8 and 16.5 k/J, respectively. This investigation suggests that a total power input of 10e16 W maximize the effectiveness of energy consumption, making the operation maximally economic.

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 93-2621-Z-264-001.

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