2003_k.vinodgopal_hydroxyl radical-mediated advanced oxidation processes for textile dyes

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Res. Chem. Intermed., Vol. 29, No. 3, pp. 307316 (2003) VSP 2003.Also available online - www.vsppub.com

Hydroxyl radical-mediated advanced oxidation processes

for textile dyes: a comparison of the radiolytic and

sonolytic degradation of the monoazo dye Acid Orange 7

K. VINODGOPAL and JULIE PELLER

Department of Chemistry, Indiana University Northwest, Gary, IN 46408, USA

Received 18 January 2003; accepted 22 January 2003

AbstractAcid Orange 7, a textile azo dye, has been partially mineralized and degraded using

-radiolysis and sonolysis. These two different advanced oxidation processes (AOP) are effective in

producing .OH radicals and cause complete destruction of the chromophore. The reaction mechanism

of dye degradation has been probed by analyzing the reaction products with HPLC. In both cases,

the nal end products of degradation are oxalate and formate ions. The intermediates observed are

all similar. While hydroxybenzenesulfonic acid is the major observed reaction intermediate in theoxidation processes, the pulse radiolysis studies indicate that the OH radical initiated pathway for

attack occursvia the initial formation of 1,2-naphthaquinone and subsequent breakdown into oxalic

acid.

Keywords: Sonolysis; radiolysis; azo dyes; hydroxyl radical; advanced oxidation.

INTRODUCTION

Advanced oxidation processes (AOPs) using hydroxide radicals as the primaryoxidant have emerged as a promising new technology for the degradation of organic

pollutants. These AOPs include semiconductor-based photocatalysis, sonolysis and

-radiolysis, and in recent years serious consideration has been given to these

methods for treating textile dye wastes. Azo dyes are by far the single largest

group of dyes produced worldwide. They are ubiquitous commercial chemicals

that present unique environmental problems arising from their ability to resist

degradation under most conditions [1 3]. The largest discharge of these colorants

into the environment occurs via the dye efuents from textile mills and otherindustries that use them. It is quite likely that signicant restrictions on the discharge

of such colorant material into municipal wastewater streams will be passed in the

To whom correspondenceshould be addressed. E-mail: [email protected]


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308 K. Vinodgopal and J. Peller

near future, making effective treatment process prior to discharge imperative. A

signicant regulatory concern regarding any treatment process is the necessity to

identify any intermediates so that no toxic byproducts are generated during such

a process. Therefore, it is important that a comparison of the relative efciencies

of the AOPs mentioned above vis--vis this prototypical azo dye, Acid Orange 7

(AO7), include a clear identication of the intermediates generated and the extentof mineralization achieved with each method.

Sonochemical degradation methods involve irradiating aqueous solutions contain-

ing the organic pollutant with ultrasound [49]. Propagation of an ultrasound wave

in aqueous solution leads to the formation of cavitation bubbles; a prerequisite for

these bubbles is the presence of a dissolved gas [10]. The collapse of these bubbles

spawns extreme conditions such as very high temperatures and pressures, which

in turn leads to the dissociation of H2O and the production of radical species such

as OH., HOO

.etc. In recent years, evidence has accumulated indicating that higher

ultrasound frequencies (at approx. 400 kHz) are more favorable for the production

of OH [8, 11]. Several recent studies have also focused on the aspect of understand-

ing chemical reactivity of these radicals in a sonolytic reaction [1216].

Radiolytically-generated radicals are also very effective in degrading organic

compounds [17 22]. Free radicals are also formed when water is irradiated with

ionizing radiation such as -rays or a high-energy electron beam. In the absence

of specic scavengers, hydroxyl radicals (.OH) and hydrated electrons (eaq) are

the major reactive species produced in a neutral or alkaline aqueous solution. By

scavenging aqueous electrons with suitable scavengers such as N2O one can induce

oxidative degradation of the organic substrates with. OH radicals.

While the commercial application of ionizing radiation to treat routine industrial

waste is still far from marketability, transient ionizing radiation methods such as

pulse radiolysis provide unique ways to deduce mechanisms of radical attack on

such organic substrates. We have utilized product identication and pulse radiolytic

methods as a tool to elucidate kinetics and reaction mechanism of the free radical

initiated oxidation processes. A systematic comparison of the sonochemical and

radiolytic degradation of the textile azo dye AO7 is presented in this paper. Bycomparing the intermediates formed and decay mechanisms in the two different

methods of the mineralization of the azo dye AO7, we can hopefully provide new

insights into the development of new advanced oxidation processes for the treatment

of such industrial waste.

EXPERIMENTAL

AO7, also known as Orange II, was obtained from Aldrich and puried byrecrystallization and column chromatography as per details described elsewhere.

Steady-state -radiolysis was carried out in sealed vials at 296 K. The samples

vials were irradiated with a 60Co source and each vial was withdrawn from the

core at different intervals. The dose rate as determined by Fricke dosimetry was


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Radiolytic and sonolytic degradation of Acid Orange 7 309

88 Gy / min. For pulse radiolysis experiments, irradiation was performed with

electron pulses (5 ns, approx. 1017 eV g1 pulse) from the Notre Dame 7-MeV

ARCO-LP-7 linear accelerator. Sonolysis experiments were carried out with a

640 kHz sonolysis setup (Ultrasonic Energy Systems, Panama City, FL, USA). A

specially designed glass vessel was attached to the transducer with silicon rubber.

Ultrasound irradiation of the solution was carried out with oxygen gas constantly

bubbling through the solution.

Samples of the dye solution at appropriate time intervals during the sonolysis, or

radiolysis experiments were taken out for analysis. The intermediates formed during

the degradation process were monitored using a HPLC (Waters Model 600 solvent

delivery system with a 996 Photodiode array). The mobile phase was an ammonium

phosphate buffer, which was maintained at 100% initially for 2 min and then

combined with methanol in a linear gradient from 0% to 100% methanol in 28 min.

Analysis of total oganic carbon (TOC) was carried out using a Shimadzu Model500 TOC analyzer. As in the HPLC analyses, samples were taken out at appropriate

intervals for TOC analyses during all three degradation methods. Oxalate, formate

and other inorganic ions such as SO24 were determined by ion chromatography

using a Dionex model 500 DX ion chromatograph. It was equipped with a 20-cm

Ion Pac As12ASC column protected by a Ion Pac Ag12A guard column. The eluent

generally consisted of an aqueous solution containing a mixture of NaHCO3 and

Na2CO3 at concentrations of 0.3 mM and 2.7 mM, respectively. The ow rate of

the eluent was generally 1.5 ml / min. Identication of ionic products was achieved

with a conductivity detector and comparison with standards.

RESULTS AND DISCUSSION

Sonolysis

Acoustic cavitation is the single most step that inuences the sonochemical process.

The nonlinear acoustic process that is controlled by the nucleation, growth and

implosive collapse of bubbles produce enormous local temperatures (10 000 K) and

pressure (up to 10 000 atm). Under these extreme conditions the water molecule is

cleaved to form H. and.OH radical species.

H2O ))))! H.C

.OH (1)

Transient radicals would then recombine or react with other chemical species

present in the medium. It has been shown that the reactivity of these radicals

is strongly dependent on the frequency of the sonolysis [8, 23, 24]. At high

frequencies, such as the one employed in the present experiments, the collapse ofthe bubble occurs quickly. Thus, the probability of ejection of H. and .OH radicals

before they undergo recombination is signicantly enhanced at higher frequency.

In the present experiments we have utilized the sonolytically produced .OH

radicals to react with the dye AO7. Sonolysis of the aqueous AO7 solution


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Figure 1. Absorption spectra of an AO7 solution (82 M) recorded at different time intervalsfollowing sonolysis in oxygen-saturated solution at high frequency. The absorption spectra were

recorded at time intervals of (a) 0, (b) 15, (c) 30, (d) 90, (e) 180 and (f) 360 min.

(82 M) under oxygen-saturated conditions at 620 kHz leads to rapid decol-

orization as illustrated in Fig. 1 Hydroxybenzenesulfonic acid (HBS) was deter-

mined by HPLC analysis as the major reaction intermediate with trace amounts

of 1,2-naphthaquinone also being detected. In fact, by the rst 15 min, the AO7 as

well as the major intermediates had disappeared. While the HPLC gradient methods

that were employed indicated the complete disappearance of AO7, it also indicated

the buildup of a fast eluting component, which was most likely a low molecular

weight carboxylate anion, such as oxalate and / or formate. The formation of such

low molecular weight acids is consistent with the decrease in pH that accompanies

the degradation of the dye. The pH of the aqueous dye solution changes from 4.3 to

2.9 following 6 h of sonication. In order to verify the nature of this nal product, we

have carried out ion chromatographic analysis. The IC enables us to unambiguously

identify the nal end product of sonication as oxalate and formate ions. Figure 2shows the kinetics of the disappearance of AO7 along with the formation of HBS,

oxalate and formate ions.

It should be noted that in oxygenated solutions, sonication of water can lead

to the formation of hydroperoxy radicals and hydrogen peroxide as well. These

secondary radicals can initiate degradation of the dye substrate. The primary role of

OH radicals in the oxidative degradation of AO7 can be determined by employing

a .OH radical scavenger. When the sonolysis of AO7 was carried out in a aqueous

solution containing 2.5%t-butanol, the degradation of the dye was inhibited. Sincet-butanol is an excellent scavenger of hydroxide radicals, it preferentially competes

with AO7 for reacting with.OH radicals. It is therefore reasonable to conclude that

the initial step in the degradation mechanism is attack by the.OH radical on the dye

solute particle at or near the surface of the cavitation bubble.


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Figure 2. Kinetics of the sonochemical degradation of AO7 in an oxygen-saturated solution. Also

shown are the growth and decay curves of the intermediates HBS, oxalate and formate ions.

-Radiolysis

As discussed in the Introduction,-radiolysis of water produces a variety of radicals

which can react with the organic substrate present in the solution. The selectivity

of the reactive conditions can be improved by using appropriate scavengers. For

example, oxidizing conditions are achieved by saturating the solution with N2O.

Hydrated electrons formed by the radiolysis of water are readily scavenged by N2O

to generate.OH radicals (reactions 3 and 4):

H2O e!.OH, eaq,

. H (2)

eaq C N2O C H2O ! N2C. OH C OH (3)

Degradation of the dye is observed when N2O-saturated aqueous solutions of the

AO7 was subjected to-radiolysis.

AO7C. OH ! Products (4)

The samples taken out at different time intervals were analyzed with HPLC. The

only identiable product formed as a result of .OH radical attack was once again

HBS. Figure 3 shows the decay trace of AO7 and the formation of HBS at the end

of radiolysis experiment.

We have also compared the extent of mineralization achieved in the two processes

by monitoring the total organic carbon (TOC) content of the solution. The decreasein the TOC content of the dye solution with time for the two different AOPs is

plotted in Fig. 4. What is clear however in the sonochemical degradation is that

the residual TOC present after 6 h of sonication can be attributed to the oxalate and

formate ions present in solution.


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Figure 3. Kinetics of the radiolytic degradation of AO7 in an oxygen-saturatedsolution. Also shown

are the growth and decay curves of the intermediates HBS, oxalate and formate ions.

Figure 4. Plot of the degradation of AO7 by sonochemical and radiolytic methods monitored as a

function of the Total Organic Content (TOC)versustime.

Reaction pathways

There are a number of reports in literature on the oxidative degradation of AO7,

ranging from ozonation to biodegradation and noncatalytic oxidations. The inter-

mediates and end products identied in all of these studies are quite similar. In the

cases of the photocatalytically induced OH radical initiated degradation of AO7,

Zeppet al. identify only HBS as an intermediate [25], whereas Ranganathan et al.

report the presence of both 1,2-naphthaquinone and HBS as intermediates during

the microbial oxidation of AO7 [26]. Given the structure of the AO7 it would be

reasonable to assume that the initial OH-AO7 radical adduct formed is centered onthe electron rich naphthalene ring. On the other hand, the detection of HBS as an

intermediate in all of the AOPs under discussion here, suggests that initial radical

attack could be occurring on the benzene moiety. However, the inability to detect

naphthaquinone as an intermediate could be due to rapid oxidation to end products


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Figure 5. Transient absorption spectra of the pulse radiolytically-generated OH radical adduct of

AO7. The spectrum was recorded at 13 (f) and 362 s (F) following the radiolysis of a N2O-saturated solution of AO7 at pH 7.

and the lack of a substantial steady state concentration. To resolve the mechanism

of .OH radical-initiated attack, we have used pulse-radiolysis methods to further

probe the oxidation of AO7 in aqueous solution.

Time-resolved transient absorption spectra were recorded following the pulse

radiolysis of AO7 at pH 7 in N2O saturated aqueous solutions in the absence and

presence of 0.1 M azide ion (Figs 5 and 6, respectively). In the absence of azide

radicals, the observed spectrum should be that of the .OH radical adduct. The. OH

radical adduct of AO7 produced during this radiolytic process exhibits characteristic

absorption peaks at 350 and 560 nm.

These pulse-radiolysis studies characterize the initial chemical events between the.OH radical and AO7. The question then arises how this .OH radical adduct further

leads to the rupture of azo bond and, thus, initiates the oxidative degradation of the

dye. The decay of the transient absorbance following pulse radiolysis of the N2O

saturated AO7 is quite slow. Therefore, we have examined the decay of the radi-

cal adducts at longer times. While the 350 nm absorption band remains unaffectedeven at long time scales, the transient absorbance, centered at 550 nm decreases

after 360s.

The transient spectrum generated by pulse radiolysis of N2O-saturated AO7 in

the presence of azide ions is shown in Fig. 6. Azide ions scavenge hydroxide

radicals to produce azide radicals. These radiolytically generated azide radicals

can now participate in direct electron transfer with a substrate such as AO7, thereby

providing a convenient method for the generation of the cation radical of AO7 as

per equations (5)(7):

H2O !.OH C eaq (5)

N

3 C .OH ! .N3 C OH

(6)

AO7 C .N3 !.AO7C C N

3 (7)


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Figure 6. Transient absorption spectra of the pulse radiolytically generated cation radical of AO7.

The spectrum was recorded at 13

s (f

) and 362

s (F

) following the radiolysis of a N2O-saturatedsolution of AO7 containing 0.1 M sodium azide.

The oxidation of the AO7 in the AOPs under discussion could proceed as in the

case of azide ions by direct electron transfer via the formation of the cation radical

of AO7 and its subsequent decay. While the transient spectra in Figs 5 and 6 show

identical absorption maxima at 350 nm, the broad absorbance at 550 nm seen for the

pure OH radical adduct in Fig. 5 is absent in Fig. 6 in the presence of azide radicals.

The transient spectrum in the presence of azide radicals remains the same even at

very long time scales, suggesting that the cation radical of the AO7 is quite stable

under the conditions of pulse radiolysis.

A variety of groups have studied the reaction of .OH radicals with azobenzene

[9, 27] and related azo dyes. They conclude that the addition of .OH radicals to

the azo double bond is the rst step in the sequence of reactions that leads to the

eventual oxidation of the substrate. In their transient studies on the reaction of.OH radicals with azobenzene, Panjakar and Mohan have observed two primary

transients: one resulting from the attack of the .OH on the benzene ring with maxat 330 nm and the other transient arising from the attack on the azo bond with

max at 420 nm [27]. In the presence of the electron-rich naphthalene ring, the

initial .OH attack should not occur on the azo bond as suggested by these previous

authors. Comparison with earlier studies by Schuleret al. on p-uorophenol [28]

and 1-naphthol [29] strongly suggest that the transient at 550 nm observed in the.OH radical adduct spectra can be attributed to the naphthaquinone .OH radical. The

pulse radiolysis studies therefore seem to suggest that initial radical attack occurs

on the naphthalene ring followed by formation of naphthaquinone and the rupture of

the orange chromophore. Although our HPLC methods identify HBS as the majorreaction intermediate with traces of 1,2-naphthaquinone, the pulse radiolysis studies

indicate that 1,2-naphthaquinone could be undergoing rapid oxidation to another

end product, thus denying the chance to observe this intermediate in concentrations

comparable to that of HBS.


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Figure 7. Reaction pathway for the hydroxide radical-induceddegradation of AO7.

The similarity of the reaction pathway in the degradation approaches, viz.,

radiolysis and sonolysis is evident from the single identiable intermediate, HBS,

produced in the experiments discussed above. It should be noted that .OH radical

is the primary oxidizing species produced by the two AOPs. While the method of

production of the hydroxide radicals is different, the eventual initiation of oxidation

begins with them in each of these methods. It is reasonable to conclude that

oxidation of the dye starts with the formation of a hydroxyl radical adduct followed

by the formation of 1,2-naphthaquinone, the rupture of the azo bond leading to

the formation of HBS and eventually the breakup of the aromatic rings. The .OH

radical-mediated reaction pathway is summarized in the scheme shown in Fig. 7.

The results presented above show that hydroxyl radical mediated oxidation using

any of the AOPs discussed above is an effective method for degrading textile azodyes.

Acknowledgements

K. V. acknowledges the support of Indiana University Northwest through a Grant-in-

Aid. K. V. also wishes to acknowledge NSF Grant CHE-9512052 for the purchase

of an HPLC and the Notre Dame Radiation Laboratory for use of their facilities.

REFERENCES

1. A. A. Vaidya and K. V. Datye,Colourage14, 3 (1982).

2. H. Zollinger, Color Chemistry: Synthesis, Properties and Applications of Organic Dyes and

Pigments. VCH Publishers, New York, NY (1987).


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2003_k.vinodgopal_hydroxyl radical-mediated advanced oxidation processes for textile dyes

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