ReviewIrradiation treatment of azo dye containing wastewater: An overview
Introduction
Azo dyes are frequently used for dyeing fabric therefore they are expected to be adherent, long lasting, and resistant to sunshine and chemical processes, in the case of dyed fabric it is required that the dyes should not fade through oxidation in the normal washing process. However, it is of special importance that these dyes could be removed from industrial effluents. For this purpose, either sorption (Aksu, 2005; Solpan and Kölge, 2006) or degradation, e.g., by treating with one-electron oxidants have been employed. In the so-called advanced oxidation processes (AOP) for the degradation of organic pollutants, highly reactive species, mainly hydroxyl radicals are used as primary oxidants. Radiation treatment belongs to the class of AOP.
Radiation processing is extensively used in industry to produce a wide range of products. Use of radiolysis in the environmental remediation of wastewater, contaminated soil and sediment is a promising treatment technology; the chemistry behind these technologies is under extensive investigation. Together with the removal of target chemical contaminant, one should also concern the elimination of a series of intermediates of progressively higher oxygen-to-carbon ratios that are involved in the conversion of an organic molecule to CO2. Therefore, it is essential to improve substantially our basic understanding of the radiation chemistry of dyes (reactions, pathways, and rates) in various systems.
The purpose of this work is to make a review on the results obtained both at our laboratory and at other laboratories on the degradation of azo dyes with special emphasizes on radiation degradation of H-acid containing azo dyes. For better understanding of the mechanism of degradation of the large dye molecules our knowledge on the reactions of smaller constituents of the dye molecules are also summarized. Although we mainly refer to radiolysis results, the undergoing processes, the reaction mechanisms are also relevant to other AOP. Some of the AOP are summarized in Fig. 1 and Table 1. The issues involved here are: (1) establishing the mechanisms of free radical reactions such as those applicable to remediation processes and (2) improving the overall efficiency of remediation by controlling the reactivity and the generation of free radicals.
Radiation treatment of polluted waters belongs to the class of AOP. Other frequently used methods employed to the oxidation of organic compounds are UV-peroxide, ozonation, the photo-Fenton process, photocatalysis, and sonolysis (see for example, Legrini et al., 1993; Pera-Titus et al., 2004; Forgacs et al., 2004) (Fig. 1 and Table 1). These methods are based on generation, and use of highly reactive intermediates, e.g., hydroxyl radicals as the primary oxidant for the decomposition of organic pollutants. A desirable goal may be the conversion of a pollutant into an easily degradable, non-toxic product (e.g., fuel or polymer) or mineralization.
In order to allow optimization of conditions, provide desired versatility and commercial competitiveness, it is necessary to understand the basic mechanisms of AOP: it is particularly important establishing the mechanisms of the free radical reactions in the remediation processes. The overall efficiency of remediation can be improved by controlling the reactivity and generation of free radicals. Pulse radiolysis (PR) is a unique tool for the generation of •OH radicals and other powerful oxidants in aqueous solution and to study their reactions. Elegant mechanistic model studies were made based on this technique and the results can lead to a better understanding of AOP in complex environmental systems.
In dilute dye solutions the hydrated electron (eaq−), hydroxyl radical (OH) and hydrogen atom (H) reactive intermediates of water radiolysis induce the decomposition of the solute:H2O↝eaq−, OH, H G(eaq−)=0.28; G(OH)=0.28; G(H)=0.06.
G-values are the yields of primary intermediates in μmol J−1 units.
By combining appropriately selected experimental conditions there are possibilities to reduce the kinds of primary reacting radicals, and therefore to obtain some information about the mechanism of undergoing reactions, or at least about the main possible reaction pathways (Spinks and Woods, 1990; Buxton et al., 1988; Woods and Pikaev, 1994):
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In N2 or Ar saturated solutions between pH 3 and 11 the reaction takes place with OH, H and eaq−.
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Below pH 2 in N2 or Ar saturated solutions in nearly equal amounts OH radicals and H atoms are the primary reactive species. In the acidic pH range, eaq− is converted to H in reaction with hydroxonium ions: eaq−+H3O+ →H+H2O.
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Reactions of H atoms are usually investigated in N2 or Ar saturated solutions containing 0.2–1 mol dm−3 tert-butanol below pH 2. The reaction between H and tert-butanol is slow: H+(CH3)3COH→CH2(CH3)2COH+H2. In this system, the relatively unreactive radicals formed from tert-butanol are also present. The radicals form in the reaction: OH+(CH3)3COH→CH2(CH3)2COH+H2O.
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Reactions of eaq− are studied in N2 or Ar saturated solutions above pH 3 and in the presence of 0.2–1 mol dm−3 tert-butanol (there is a small contribution from the H atom reactions). Radicals formed from tert-butanol are also present.
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Reactions of OH radicals are investigated in N2O saturated solution in the 3–11 pH range. In such solution, eaq− is converted to OH in the reaction eaq−+N2O+H2O→OH+OH−+N2. (At saturation the N2O concentration is 0.025 mol dm−3 at room temperature.) There is a small (∼10%) contribution from the H atom reactions.
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In air or oxygen saturated solutions the reactive species are the •OH radicals, and the O2−/HO2 superoxide radical anion/perhydroxil radical pair that form in eaq− and H scavenging reaction by O2 molecules (eaq−+O2→O2−; H+O2→HO2). There is an acid/base equilibrium between the two species with a pK of 4.8: O2−+H3O+↔HO2+H2O.
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Solutions containing the tert-butanol and saturated with O2 are used for studying the reactions of the O2−/HO2 pair.
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N2O saturated solution above pH 3 containing tert-butanol give a possibility to study the reactions of CH2(CH3)2COH radicals.
Among the dye classes which can be applied to cellulosic fibres, for which vat dyes, direct dyes and reactive dyes are the most important ones, the demand for reactive dyes is steadily growing. Their key properties are excellent wet fastness, brilliant shades, and simple application techniques.
Azo dyes are chemical compounds bearing the functional group R–NN–R′ in which R and R′ are aryl groups. Because of the electron delocalization through the NN group these compounds have vivid colours, such as red, orange, or yellow. The colour is dependent on the chromophore and the extent of conjugation. Depending on the number of azo groups there are mono-, di- and triazo dyes. Azo dyes generally are bound to the textile fibres through secondary bonds. The reactive dyes have active groups for chemically bonding to the textile. Such moieties are for instance the chlorine substituted triazine or the sulphatoethyl sulphonate groups. H-acid containing dyes belong to the class of reactive dyes.
H-acid has two acid–base transitions one of them is at low pH (pK1=3.54) and this is due to the ionization of the amino group, and the other is connected with the ionization of the hydroxyl group (pK2=8.64). We show these two possibilities in Scheme 1 (Pálfi et al., 2007).
The ionization of the –SO3H group is below pH 1. The substituents attached to H-acid slightly modify the pK-values (Bredereck and Schumacher, 1993).
As model compound in the following we often refer to Apollofix Red (AR-28, also called as CI. Reactive Red or CI. 18215, Scheme 2) a triazine and H-acid containing azo dye with intensive red colour, λmax=514, 532 nm, εmax=31400 mol−1 dm3 cm−1 (Bredereck and Schumacher, 1993). The dye undergoes an acid dissociation at the OH group with pK=11.74.
Azo dyes containing OH and azo group in neighbouring positions, like AR-28, exist in azo⇔hydrazone tautomer pair. We show an example for tautomerization, Scheme 3, Scheme 4.
Below the pK value either of the two isomers may dominate, depending on the chemical structure of the dyestuff. In the case of p-substituted dyes the azo configuration very strongly prevails. When a phenyl group is attached to the –NN– diazo group with a substituent in o-position (like for AR-28) the hydrazone tautomer dominates (Bredereck and Schumacher, 1993; Hihara et al., 2006).
In the case of H-acid containing dyes with secondary amino group (i.e. there is an H atom attached to the amino N atom), for instance AR-28, the H atom on N may also be involved in the tautomerizm (Scheme 4).
Section snippets
Experimental techniques
The degradation of high molecular mass dye compounds is rather complicated and for understanding the details of the processes highly sophisticated techniques are needed. There was a great progress during the last decade in this field. The final degradation products are investigated by UV–Vis spectroscopy, HPLC chromatography, eventually combined with such detection methods as FTIR, MS, and NMR (Solpan and Güven, 2002; Butt et al., 2005; Huang et al., 2005; Wojnárovits et al., 2005a, Wojnárovits
Radiolysis of model compounds
Our understanding of the mechanism of radiolysis of azo dyes is greatly enhanced by studies on model compounds. These model compounds, e.g., phenol, aniline, H-acid are usually the subparts of the practically applied dyes. The reactivity of the reducing species, eaq− and H, in air-free media and also that of the oxidizing species, OH, can be strongly affected by the molecular structure (e.g. by the substituents). This will be illustrated by individual compounds as examples.
Degradation of azo dyes
Decolouration curves offer a convenient means to follow the radiolysis of dyes giving information on the disappearance of the starting molecules. However, the measurements cannot be applied to identify the products formed: for this purpose separation techniques are used. These techniques are needed also when the absorption spectra of the products strongly overlap with those of the starting compounds. The complete mineralization may be followed by TOC, BOD and COD measurements. In order to
Acknowledgements
We express our thanks to Hungarian Science Foundation (OTKA K 60 096). Support of the International Atomic Energy Agency (Contact no. 302-F2-HUN-12015) is also acknowledged.
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