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Wat. Res. Vol. 34, No. 8, pp. 2408±2411, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter

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RAPID COMMUNICATION DISCOLORATION OF AQUEOUS REACTIVE DYE SOLUTIONS IN THE UV/Fe0 SYSTEM N. DENG*, F. LUO, F. WU, M. XIAO and X. WUM Department of Environmental Science, Wuhan University, Wuhan 430072, People's Republic of China (First received 1 November 1998; accepted in revised form 1 July 1999) AbstractÐDiscoloration of aqueous reactive dyes C.I. reactive red 2, C.I. reactive blue 4, and C.I. reactive black 8 is enhanced in the UV/Fe0 system. We examined the e€ects of pH, dye concentration, and iron dosage on the discoloration of dye solutions in the UV/Fe0 system. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐdiscoloration, reactive dyes, UV irradiation, zero-valent iron (Fe0)

INTRODUCTION 0

Zero-valent iron metal (Fe ) is one of the most abundant metals on the earth. It is a very promising reactive medium due to its low-cost, easy-to-obtain, e€ectiveness and ability of degrading contaminants (Hardy and Gillham, 1996). Over the last several years, a great deal of interest has been focused on the degradation of organic compounds and groundwater remediation with new treatment methods by zero-valent iron metal. Zero-valent iron metal has recently been used to rapidly dechlorinate a wide range of halogenated organic compounds (Matheson and Tratnyek, 1994; Orth and Gillham, 1996), and reduce nitro aromatic compounds (Agrawal and Tratnyek, 1996) and aromatic azo compounds (Weber, 1996). The reactions were reductive and heterogeneous. Weber and Adams (1995) investigated the reduction of azo dye Disperse Blue 79 and showed the treatment with Fe0 and a catalytic amount of ferric chloride selectively reduced the aromatic nitro groups of the dye without breaking azo linkage. However, in the presence of Fe0, aromatic azo compounds are susceptible to reduction to produce aromatic amines (Larson and Weber, 1994; Weber, 1996). The destruction of the azo bond N1N in the chromophore of azo dyes led to discoloration of dye solutions. *Author to whom all correspondence should be addressed. Tel.: +86-27-87686550; fax: +86-27-87882661; e-mail: [email protected]

In recent years, photochemical methods for wastewater treatment have drawn much attention from researchers. Tang and Chen (1996) reported the discoloration kinetics and mechanisms of commercial dyes by an H2O2/iron powder system. Two of the three dyes they used are azo dyes, another is an anthraquinone dye. The authors attributed the discoloration of the dyes to adsorption by iron powder and oxidation by photo-Fenton's reagent, the Fe2+ of which came from the oxidizing dissolution of iron powder. We wanted to study the e€ect of UV light on discoloration of dyes by Fe0 without H2O2 in the system. The purpose of this work was to investigate the feasibility of discoloration of the dyes by the UV/ Fe0 system and the e€ects of experimental factors such as pH, dye concentration, and iron dosage on the discoloration eciencies.

MATERIALS AND PROCEDURES

The dyes and their analyses are the same as those displayed in the previous paper (Deng et al., 1996). Purity of zero-valent iron powder (<100 mesh) is over 98%. All the reagents are analytical grade. The water used is distilled. Experiments were performed in an open batch system (Fig. 1). Dye solutions were stirred with an agitator to keep the iron powder suspended. Either HCl or NaOH was used to adjust the pH of the solutions to the desired levels. Samples were withdrawn at intervals for a few minutes and centrifuged for 5 min at 4000 rpm. The absorbancy of the solutions was detected. UV-Vis spectra from 190 to 800 nm were recorded with a UV-1601 spectrophotometer (Shimadzu). The soluble Fe in the solutions was detected with an AAS (WF-5, Guizhou, China).

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Discoloration of aqueous reactive dye solutions

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Fig. 1. Laboratory set-up and analyses line. RESULTS AND DISCUSSION 0

The e€ect of the UV/Fe system on the discoloration of dye solutions To examine the e€ect of the UV/Fe0 system, control experiments for discoloration of C.I. reactive red 2 were conducted under di€erent conditions: (1) UV, (2) Fe0, (3) UV/Fe0. Figure 2 shows that the self-photolytic discoloration of the dye solution was not signi®cant under UV irradiation. When iron powder was added, the discoloration of the dye solutions occurred. Under the UV/Fe0 system, although the discoloration eciency (DE) of the dye solutions was not obviously di€erent during the ®rst 40 min compared to the Fe0 system, the rate gradually increased with time, and the gap between them became greater. The UV-Vis absorption spectra of the dye solutions before and after treatment are displayed in Fig. 3. The absorption of the dye solutions reduced over the whole UV-Vis spectrum range.

pH e€ect To investigate pH e€ect on the discoloration of dye solutions under the UV/Fe0 system, pH 3.0, 3.5, 4.0, 5.0 and 6.0 were chosen to conduct discoloration experiments. A representative dye C.I. reactive red 2 was used for discoloration. At the same time, the control experiments without UV irradiation were also performed. Figure 4 shows that the discoloration rate increased with decreasing pH. Table 1 gives a comparison of the discoloration eciencies of C.I. reactive red 2 under UV/Fe0 and Fe0 systems. The discoloration is enhanced when UV light is introduced into the Fe0 system. Over the pH range of 3.0±4.0 especially, the portion of discoloration eciency caused by the UV light is greater, which is expressed as DDE, with a maximum value of 10.1 at pH 3.5. In this work, in order to show discoloration eciencies of dyes in the UV/ Fe0 system, we undertook the following experiments at pH 3.5. Zero-valent iron dosage e€ect The iron concentration is an important factor. To examine its e€ect on the dye discoloration, the di€erent iron dosages 0.5, 1.5, 2.0, 2.5, 3.0, 4.0 and 5.0 mg/l were plugged into C.I. reactive red 2 sol-

Fig. 2. UV/Fe0 system e€ect on the discoloration of C.I. reactive red 2 [dye]=20 mg/l, [Fe0]=3.0 g/l, pH=5.0.

Fig. 3. The UV-Vis absorption spectra of dye (C.I. reactive red 2) solutions before (a) and after treatment under (b) Fe0 system and (c) UV/FE0 system.

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Fig. 4. The pH e€ect on discoloration rate of C.I. reactive red 2 [dye]=20 mg/l, [Fe0]=2.5 g/l.

Fig. 5. Iron powder concentration e€ect on the discoloration of the dye solutions [dye]=200 mg/l, pH=3.5.

Table 1. Comparison of discoloration eciency of C.I. reactive red 2 after 100 min radiation Discoloration eciency (%)

pH=3.0

pH=3.5

pH=4.0

pH=5.0

pH=6.0

64.6 58.8 5.8

49.0 38.9 10.1

23.1 15.0 8.1

13.0 9.4 3.6

12.3 9.4 2.9

DEUV=Fe0 DEFe0 DDE

utions and irradiated by UV light. The discoloration rate increased with iron powder concentration increasing over the range of 0.5±4.0 g/l. More iron powder addition caused no signi®cant increase in discoloration (see Fig. 5). The soluble Fe in the solutions detected with AAS included Fe(II) and Fe(III). Its concentration increased from 3.1l to 7.6 mg/l with iron powder concentration increasing from 0.5 to 5.0 g/l in the solution at pH 3.5. The soluble Fe reached a constant value after treatment for the ®rst 30 min, and after that it virtually stayed the same with time. E€ect of initial dye concentration Five dye solutions containing 10, 20, 30, 40 and

50 mg/l of C.I. reactive 2, respectively, were treated in the UV/Fe0 system. Figure 6 shows that the higher the initial concentration, the smaller the discoloration rate.

Discoloration kinetics of dyes in aqueous solutions Reactive dyes C.I. reactive red 2, C.I. reactive blue 4 and C.I. reactive black 8 can be decolorized under the UV/Fe0 system. From these results (Fig. 7), it can be seen that the molecular structure of dyes containing an azo bond or anthraquinone structure is an important factor for discoloration. Correlation analyses showed that the discoloration of dyes C.I. reactive red 2, C.I. reactive black 8 and

Fig. 6. The e€ect of initial concentration of the dye solutions [Fe0]=2.5 g/l, pH=3.5.

Discoloration of aqueous reactive dye solutions

2411 REFERENCES

Fig.

7.

Photodegradation rates of [Fe0]=2.5 g/l, pH=3.5.

dye

solutions

C.I. reactive blue 4 were pseudo-zero order reactions. AcknowledgementsÐWe thank the anonymous reviewers for their contributions to this work. This work is ®nanced by the Natural Science Foundation (No. 97j084) of Hubei Province, and Wuhan University Ziqiang Foundation, China.

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