azodye degradarion by high energy irradiation

8/14/2019 azodye degradarion by high energy irradiation

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NUKLEONIKA 2007;52(2):6975 ORIGINAL PAPER

Introduction

Azo dyes are produced in very large quantities and usedas reactive dyes by the cotton industry. They are verystable and resistant to chemical processes. Therefore,their removal from industrial effluents by conventionalmethods (mechanical screening, sedimentation,biological treatment, biofilters) alone is usually not veryeffective. This problem can be solved by combining theconventional techniques with other methods. Radiationchemistry is one of the most promising advancedoxidation processes (AOPs) [3, 8, 9] for environmentalremediation. In AOPs hydroxyl radicals are the main

oxidants for the degradation of organic pollutants [13].The disappearance of the dye is generally followed byspectrophotometry at the absorbance maximum between450 and 600 nm.

The dye disappearance curves are very oftendescribed by applying some kinetic equations [4, 6, 7,10, 15, 1719] in order to describe the dependence ofthe destruction on such parameters as dose, dose rate,additives, etc. in radiolysis experiments, illuminationintensity, illumination time, etc. in photo oxidationexperiments [15].

For the present mechanistic and kinetic studies, theexperimental results obtained during radiolysis of

Apollofix Red (AR-28) and Reactive Black 5 (RB-5)are discussed. Some of the experimental results werealready published [16, 20, 21].

Azo dye degradation by high-energyirradiation: kinetics and mechanism

of destruction

Erzsbet Takcs,Lszl Wojnrovits,

Tams Plfi

E. Takcs, L. Wojnrovits, T. PlfiInstitute of Isotopes, Hungarian Academy of Sciences,H-1525 Budapest, P. O. Box 77, Hungary,Tel./Fax: +36 1 392 2548,E-mail: [email protected]

Received: 11 January 2007Accepted: 2 April 2007

Abstract. The kinetics and mechanism of dye destruction is discussed on the example of Apollofix Red (AR-28) andReactive Black 5 (RB-5) radiolysis in dilute aqueous solution. The dose dependence of colour disappearance is linear

when the reactive intermediate reacts with the colour bearing part of the molecule causing destruction of the conjugationhere with nearly 100% efficiency. In this case, spectrophotometry can be used to follow-up dye decomposition. Sucha linear dependence was observed when hydrated electrons or hydrogen atoms reacted with the dye. However, inthe case of hydroxyl radical reactions some coloured products form with absorption spectra very similar to those of thestarting dye molecules. For that reason, spectrophotometric measurements give false results about the concentration ofintact dye molecules. Analysis made by the HPLC (high-pressure liquid chromatography) method reveals logarithmictime dependence in agreement with a theoretical model developed.

Key words: azo dyes decolouration reaction mechanism kinetics Apollofix Red Reactive Black 5


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70 E. Takcs, L. Wojnrovits, T. Plfi

The colours of these dyes are determined by theconjugated phenylsulphonic acid-azo group-H-acidparts of the molecules [1] (see Scheme 1). Any changein these parts leads to change of colour, i.e. change ofthe absorption spectrum in the visible range. Destructionof the extensive conjugation leads to loss of colour, i.e.disappearance of absorption in the visible range.

Experimental

AR-28 was obtained from Taiheung Corporation

(Kyunggido, South Korea), RB-5 was purchased fromAldrich, the dyes were purified by recrystallization frommethanol/ethanol mixtures. Perchloric acid was usedto set pH of the solutions when the H atom reactions

were studied. Methanol (HPLC grade) and tetrabutyl-ammonium-hydrogen sulphate (TBAHS) were obtainedfrom Sigma-Aldrich, tert-butanol from Spektrum-3D.

The gamma radiolytic experiments were carried outusing a 60Co irradiation facility with a dose rate of3.0 kGyh1 as determined by Fricke dosimetry (G(Fe2+)= 15.6 (100 eV)1). The solutions were bubbled withN2 or N2O for 5 min prior to irradiation in 5 ml Pyrexglass ampoules. 1 cm Suprasil quartz optical cells were

used for taking the UV-VIS absorption spectra beforeand after the reaction by using a Jasco 550 UV-VISspectrophotometer.

The chromatographic system consisted of a JascoPU-2089Plus quaternary gradient pump, a Jasco MD-2015Plus diode-array Multiwavelenght Detector anda Nucleosil 100 C18 column (Teknokroma) (pore size5 m, length 15 cm, diameter 0.4 cm). Separations weremade using 50 mmoldm3 aqueous solution of TBAHSat pH 6.1 and methanol. All mobile phases were filteredusing a Millipore 0.45 m filter and were degassed byultrasonication before use. The separations were madeat room temperature using injection volume of 10 l,and flow-rate of 0.8 ml/min.

Standard radiation chemical techniques were

applied to separate the water radiolysis intermediatesand follow their reactions with the dye. The radiolysisof water supplies hydrated electron (eaq), hydroxylradical (OH) and hydrogen atom (H) reactive inter-mediates with G values (species (100 eV)1) shown inEq. (1). In order to reduce the number of reactiveintermediates, the reactions of OH radicals werestudied in N2O saturated solutions (pH 56), in suchsolutions the hydrated electrons are converted to OHradicals (Eq. (2)). When the eaq reactions wereinvestigated the solutions were bubbled with N2 fordeoxygenation and contained 0.5 moldm3tert-butanolin order to transform the OH radicals to less reactive

CH2(CH3)2COH radicals (Eq. (3)) (pH 56). H

atomsreact with a low rate coefficient with tert-butanol(Eq. (4)). The H reactions were studied in t-butanol

Scheme 1. Structures of Reactive Black 5 (RB-5) and Apollofix Red (AR-28) dyes.


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71Azo dye degradation by high-energy irradiation: kinetics and mechanism of destruction

containing N2 purged pH 1.1 solutions, where e

aq

was converted to H atom (Eq. (5)) [2].

(1) H2O /\/ e

aq,OH, H

G(eaq) = 2.6; G(OH) = 2.7; G(H) = 0.6

(2) e

aq + N2O + H2O N2 + OH

+

OHk2 = 9.1 109 mol1dm3s1

(3) OH + (CH3)3COH H2O +CH2(CH3)2COH

k3 = 6 108 mol1dm3s1

(4) H + (CH3)3COH H2O +CH2(CH3)2COH

k4 = 1 106 mol1dm3s1

(5) eaq + H+H k5 = 2.3 10

10 mol1dm3s1

Results and discussion

Degradation of AR-28 and RB-5

Figure 1 shows the comparison of the decolouration ofAR-28 and RB-5 in the reaction with OH (1A, 1B)

Fig. 1. Decolouration plots of AR-28 (0.085 mmoldm3) and RB-5 (0.1 mmoldm3) obtained when the hydroxyl radicalsreacted with the dye (A, B), or when the hydrated electrons were the reaction partners (C, D).


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72 E. Takcs, L. Wojnrovits, T. Plfi

and eaq (1C, 1D), respectively. WhenOH radicals

reacted with the dye, during the decolouration the shapeof the spectra in the visible range changed: the visiblebands shifted to longer wavelengths. After separationof the main product formed from AR-28 by HPLC, wefound that its spectrum is very similar to the spectrum ofthe starting dye. The only difference is the shift of the

visible band to longer wavelength (Fig. 2) and this causesthe change in the shape of the UV-VIS spectra (Fig. 1A).The initial G values (extrapolated to zero dose) of dyedisappearance were 2.92 for RB 5 and 2.5 for AR-28.

When eaq reacted with the dyes (Figs. 1C and 1D),

the intensity of the visible absorption decreased with theradiation dose without any major change in the shapeof the spectrum in the visible range for AR-28. In thecase of RB-5 there is no change in the spectrum aboveca. 550 nm. The initial G values (extrapolated to zerodose) of dye disappearance were 1.91 for RB 5 and 2.6for AR-28.

Mechanism of degradation

As it was discussed in our former papers [20, 21] OHradical has very high reactivity with the aromatic ring

and with the azo group subparts of the molecule [12,14]. The addition ofOH radical to an aromatic ring onthe conjugated part of the molecule results in a cyclo-hexadienyl type radical. This radical may disappear inradical-radical combination reaction where dimermolecule is produced (in this reaction the conjugation

may be destroyed). Radical disproportionation leadsto compounds that contain cyclohexadiene ring andanother compound with extensive conjugation inthe molecule characteristic of the dye. However, thelatter molecule contains an extra OH group inthe conjugated part of the molecule (see Scheme 2).For that reason, its absorption spectrum is slightlydifferent from that of the starting molecule. This shiftin the spectrum certainly affects the molar extinctioncoefficient, and, therefore, any kinetic analysis basedon the absorbance measurements should be handled

with a certain caution. The correct analysis should bebased on chemical separation and individual compoundanalysis (most conveniently by the HPLC method).

The hydrated electron (and also the H atom) issuggested to react with the azo group (with rate coeffi-cients close to the diffusion controlled limit [5, 11])destroying the conjugation with nearly 100% efficiency[20, 21]. The other parts of the molecule and the productsformed during decay have smaller reactivity with eaqand H atom than the azo group.

Kinetic analysis

Under steady state conditions, the OH, H, eaqintermediates that destroy the dye molecules may react

practically exclusively with the intact dye moleculesdecolourizing them. The time dependence of dyedisappearance can be described by the differentialequation:

(6)

In the equation [Dye] represents the dye concentration,[I] is the intermediate radical concentration, and k isthe second order rate coefficient.

During steady-state radiolysis, the intermediateradicals form with a constant rate (rI) and they decay

with the same rate (k[I] [Dye]):

d[Dye][I][Dye]

dk

t =

Fig. 2. Comparison of the absorption spectra of AR-28 andone of its main transformation products formed in OH radicalreaction. The spectra were recorded by using diode-arraydetection after HPLC separation.

Scheme 2. Degradation of AR-28 by OH radical.


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(7)

At the beginning of the process, when the dye concen-tration is high, the intermediate radicals quickly react

with the solute molecules, and therefore, theintermediate radical concentration is low. At the endof the process, [Dye] is low andk[I] is high. Substituting[I] rI/k [Dye] into Eq. (6), after integration, we obtainlinear time dependence (since the irradiation is usuallycarried with a constant dose rate, there is linear dosedependence):

(8)

Such a linear dependence was observed several timesduring decolouration at the beginning part of theprocess. We found linear initial dose dependence, whenin radiolysis the hydratedelectrons or the hydrogenatoms reacted with the dyes (Figs. 3 and 4). As wementioned before, these intermediates were found toreact principally with the azo group causing destructionof the intensive conjugation in the molecule throughthe azo bond. In AR-28 there is only one azo group

with high reactivity, and here the linearity is maintainedtill about 80% destruction. In the RB-5 there are two

azo groups. When eaq or H destroys one of them, dueto the decreased conjugation, the longest wavelengthabsorption disappears. However, the other azo groupmay also react with high rate coefficient with eaq orH and, therefore, the linear range is shorter. Theformation rates of reactive intermediates at a dose rateof 3 kGyh1, calculated from the slopes of the straightlines arerI,measured (2.02.5) 10

7 mols1 for both eaqand H. The value is just slightly lower than the valuecalculated based on the radiation chemical yield of theintermediates: rI,calculated 2.8 10

7 mols1.At higher doses, however, the dye is depleted;

therefore, the eaq and H intermediates of water

radiolysis react mainly with the products of radiolysisand only a small part of the intermediates reacts with theazo moiety causing decolouration. For that reason,

the time dependence curve at higher depletion deviates

from linearity.The reaction ofOH radical with the dye is different

from the reactions of eaq and H. The OH radical reacts

with practically diffusion limited rate coefficient (k1010 mol1dm3s1) with the aromatic rings and probablyalso with the N=N double bond. The dye moleculesgenerally have several places for the attack, e.g. AR-28has five aromatic rings and one N=N double bond.The reaction with the N=N double bond or with therings involved in the extensive conjugation leadsto the destruction of the conjugation, so may decolourizethe molecule. However, the new products have alsohigh, practically the same reactivity towards the OH

radicals as the starting compound. That is probably truefor the product of the product, i.e. for the secondaryproduct, tertiary product, etc. For that reason, the rateequations should be written in the following form:

(9)

(10)

Since as it was mentioned before the reaction ofOH with both the dye and the products is practically

diffusion limited; we use for both reactions the same krate coefficient. Applying the simplification:

(11) [Product] [Dye]0 [Dye]

also taking [OH]rOH/k[Dye]0 expressed from formulae(6) and (7) (I = OH), the differential equation has theform:

(12)

After integration, we obtain logarithmic time depend-ence (Eq. (13)) with rate parameter ofkobs =rOH/[Dye]0.

(13)

Fig. 3. Dose dependence of decolouration of AR-28 andRB-5 caused by hydrated electron.

Id[I] [I][Dye] 0d

r kt

=

Iobs

0 0

[Dye]1 1[Dye] [Dye]

r

t k t= =

Fig. 4. Dose dependence of decolouration of AR-28 andRB-5 caused by H atom.

d[Dye][ OH][Dye]

dk

t

=

OH

d[ OH][Dye][ OH]

d

[Product][ OH] 0

r kt

k

=

OHobs

0 0

[Dye]ln[Dye] [Dye]

rt k t= =

OH

0

[Dye]d[Dye]

d [Dye]

r

t =


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14. Roder M, Wojnrovits L, Fldik G, Emmi SS, BeggiatoG, DAngelantonio M (1999) Addition and eliminationkinetics in OH radical induced oxidation of phenol andcresols in acidic and alkaline solutions. Radiat Phys Chem54:475479

15. Shu H-Y, Chang M-C (2006) Development of rateexpression for predicting decolorization of C.I. Acid Black

1 in a UV/H2O2 process. Dyes Pigments 70:313716. Solpan D, Gven O, Takcs E, Wojnrovits L, Dajka K

(2003) High-energy irradiation treatment of aqueoussolutions of azo dyes: steady state gamma radiolysisexperiments. Radiat Phys Chem 67:531534

17. Tezcanli-Guyer G, Ince NH (2003) Degradation andtoxicity reduction of textile dyestuff by ultrasound.Ultrason Sonochem 10:235240

18. Tezcanli-Guyer G, Ince NH (2004) Individual andcombined effects of ultrasound, ozone and UVirradiation: a case study with textile dyes. UltrasonSonochem 42:603609

19. Wang M, Yang R, Wang W, Shen Z, Bian S, Zhu Z (2006)Radiation-induced decomposition and discoloration ofreactive dyes in the presence of H2O2. Radiat Phys Chem

75:28629120. Wojnrovits L, Plfi T, Takcs E (2005) Reactions of azo

dyes in aqueous solution: Apollofix Red. Res ChemIntermed 31:679690

21. Wojnrovits L, Plfi T, Takcs E, Emmi SS (2005)Reactivity differences of hydroxyl radicals and hydratedelectrons in distracting azo dyes. Radiat Phys Chem74:239246

azodye degradarion by high energy irradiation

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