Tv

Ia

b

c

a

ARR1AA

KOAUFK

1

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1h

Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 49– 58

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology A:Chemistry

journa l h om epa ge: www.elsev ier .com/ locate / jphotochem

he kinetics and efficiency of UV assisted advanced oxidation ofarious types of commercial organic dyes in water

vana Grcic a,∗, Sanja Papic a, Danijela Meseca,b, Natalija Koprivanaca, Dinko Vujevic c

University of Zagreb, Faculty of Chemical Engineering and Technology, Marulicev trg 19, 10000 Zagreb, CroatiaKoncar – Switchgear Inc., Strojarska cesta 10, 10361 Sesvetski Kraljevec, CroatiaUniversity of Zagreb, Faculty of Geotechnical Engineering, Hallerova aleja 7, 42000 Varazdin, Croatia

r t i c l e i n f o

rticle history:eceived 4 July 2013eceived in revised form2 September 2013ccepted 21 September 2013vailable online 30 September 2013

eywords:rganic dyesdvanced oxidation processes (AOPs)

a b s t r a c t

Five model solutions formulated from the commercial dyes: C. I. Reactive Black 1 (RB1), C. I. Direct Red 23(DR23), C. I. Acid Blue 25 (AB25), C. I. Basic Red 1 (BR1) and C. I. Mordant Orange 1 (MO1) were treated byUV-C assisted homogeneous AOPs: UV/Fe(II), UV/H2O2 and UV/Fe(II)/H2O2 (hereafter: UV/Fenton). Dyedegradation in acidic medium (pH 3) was evaluated by the decrease in Total Organic Carbon (TOC) contentand colour, measured by the decrease in chromophore absorption bands. In comparison to photolysis (UValone), UV/Fe(II) and UV/H2O2, the highest extents were observed when UV/Fenton process was applied.

Optimal experimental conditions were taken from our previous work: 0.5 mM FeSO4·7H2O and 2.5 mMof hydrogen peroxide. Under these experimental conditions, more than 90% of TOC and 100% of colourremoval were obtained for AB25 and MO1 dye after 60 min of UV/Fenton. The lowest extents were

V-assisted AOPsenton reactioninetic modelling

observed for DR32; only 34% of TOC and 48% of colour removal.Mineralization and decolourization kinetics were modelled according to the comprehensive kinetic

model introduced in our previous study [1], taking into account crucial reactions of Fenton catalytic cycle,UV-C induced photolysis and reaction between dye/organic molecule and •OH radicals. Direct photolysiswas modelled according to semiempirical model based on Lambert–Beer’s law (LLM). The results werediscussed in respect to each dye.

. Introduction

Organic synthetic dyes are present in almost all aspects of ourveryday life and their application is continuously growing. Theyre widely used for traditional applications, textile and non-textileyeing, and as functional dyes and optical brighteners. Traditionalpplications include dyeing of all kinds of natural and syntheticbres, leather, fur, paper, hair, food, ink and other materials. In the

ast twenty years, group of dyes called functional dyes are specif-cally designed for high-technology (hi-tech) applications such ashotocopying; ink-jet, laser and thermal printing; solar cells; liquidrystal displays and biomedical applications such as photodynamicherapy. Optical brighteners are colourless or weakly colouredrganic compounds that applied to a substrate, absorb UV light300–430 nm) and reemit blue fluorescent light (400–500 nm). Pro-uction of coloured textiles is one of the basic technologies in

uman civilization. The scale and growth of the dyes industry haseen inextricably linked to that of the textile industry. The world

∗ Corresponding author. Tel.: +385 915017398.E-mail addresses: igrcic@fkit.hr, ivana.grcic.fkit@gmail.com (I. Grcic).

010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jphotochem.2013.09.009

© 2013 Elsevier B.V. All rights reserved.

production of hi-tech dyes will remain small in comparison to dyesfor traditional applications [2].

Synthetic dyes represent a relatively large group of organicchemicals, which are classified according to chemical structure ofchromophore or chromogen and by area, and method of applica-tion. These two classifications colouristic and chemical overlap,i.e. there is hardly a chemical class of dye which occurs solelyin one colouristic group, and vice versa. Both classifications areused by Colour Index, which lists all dyes used commercially forlarge-scale purposes. Some of the chemical groups of dyes are azo,anthraquinone, indigoid, polymethine, di- and triarylmethine, xan-thene, acridine, azine, oxazine and phtalocianine dyes. The azodyes are the largest chemical class of dyes. Some of the applica-tion (colouristic) classes of dyes are acid, basic, direct, disperse,mordant, reactive, pigments, solvent, sulphur and vat dyes [3].

Significant losses occur during the manufacture and processingof dyes and these dyes and other chemicals are discharged inthe wastewaters. These wastewaters have adverse effect on theenvironment if not treated properly [4]. Colour is the first con-

taminant to be recognized, and environmental regulations in mostof the countries (EU directive 91/271) have made it mandatory todecolourize the dye wastewater prior to discharge [5]. Dyes canhave toxic effects (acute and/or chronic) on exposed organisms

5 nd Ph

dtmnifFctAcrmolwafiir

F

ior

H

F

Ir

pf

toedtiaaFdowltdmt

2

2

2MTas

0 I. Grcic et al. / Journal of Photochemistry a

epending on the exposure time and dye concentrations [6]. Syn-hetic dyes are difficult to degrade due to their complex aromatic

olecular structure and xenobiotic properties [5,7]. Therefore, it isecessary to find an effective method of dye wastewater treatment,

n terms of limited water recourses management and the needor nature preservation. Advanced Oxidation Processes (AOPs), likeenton and photo-Fenton processes, appear to have the capacity toompletely decolourize and partially mineralize the textile indus-ry dyes in short reaction time and with a low treatment cost [8,9].OPs although making use of different reacting systems, are allharacterized by the same chemical feature: production of •OHadicals. •OH are extraordinarily reactive species; they attack theost part of organic molecules with rate constants usually in the

rder of 106–109 dm3 mol−1 s−1. They are also characterized by aittle selectivity, which is a useful attribute for an oxidant used in

astewater treatment [9]. Homogeneous Fenton process is a cat-lytic oxidation process using a mixture of hydrogen peroxide anderrous ions in aqueous solution. In an acidic media, the ferrouson initiates and catalyses the decomposition of the H2O2, result-ng in the generation of hydroxyl radicals, •OH [10,11]. The overalleaction is (Eq. (1)):

e2+ + H2O2 → Fe3+ + •OH + OH− (1)

The rate of organic pollutant degradation could be increased byrradiation with UV light (UV/Fenton process). UV light leads notnly to the formation of additional hydroxyl radicals but also toecycling of ferrous catalyst by reduction of Fe3+ (Eqs. (2) and (3)):

2O2 + h� → •OH + •OH (2)

e3+ + H2O + h� → •OH + Fe2+ + H+ (3)

n this way, the concentration of Fe(II) is increased and the overalleaction is accelerated.

The oxidation using UV/Fenton process has been found to be aromising treatment method for the effective degradation of dif-erent pollutants including organic synthetic dyes [12–17].

The textile wastewaters from dyehouses as well as other indus-rial dyeing wastewaters never contain a single dye but several dyesf different chemical structures [18]. Recent studies found in lit-rature mostly deal with the optimal process conditions for theegradation of a particular dye. The main objective of this study iso analyze and to compare the efficiency and kinetics of decolour-zation and mineralization of five dyes from different chemical andpplication classes by UV-assisted AOPs at the same conditions forll studied dyes. The optimal process conditions (0.5 mmol dm−3

e2+, 2.5 mmol dm−3 H2O2 and pH = 3) were determined for theegradation of representative dye pollutant C. I. Reactive Blue 2 inur previously published papers [19,20]. Dye degradation kineticsas studied for all the applied processes, applying previously estab-

ished comprehensive kinetic model [1]. The used model assumedhe preferred degradation of dye molecules and organic interme-iates via •OH attack. The compliance of experimental results withodel predictions for dye degradation was checked. In conclusion,

he chemistry of each studied dye was discussed.

. Experimental

.1. Chemicals

The commercial dyes C. I. Reactive Black 1 (RB1), C. I. Direct Red3 (DR23), C. I. Acid Blue 25 (AB25), C. I. Basic Red 1 (BR1) and C. I.

ordant Orange 1 (MO1) were used without further purification.

he chemical structures, manufacturers and applications of dyesre listed in Table 1. Hydrogen peroxide (H2O2 30%, w/w), ferrousulphate (FeSO4·7H2O) and sulphuric acid (H2SO4) were obtained

otobiology A: Chemistry 273 (2014) 49– 58

from Kemika (Croatia). Aqueous solutions containing 100 mg L−1 ofdye were prepared with distilled water. The characteristics of thesolutions of different dyes used in the experiments were presentedin Table 2.

2.2. Reactor and experimental procedures

All experiments were performed in a batch water-jacketed pho-toreactor of total capacity volume of 0.8 dm3. A quartz tube withmercury lamp (9.5 W, emission peak at 254 nm, UVP-Ultra Vio-let Products, Cambridge, UK) was placed vertically in the middleof the photoreactor. The value of incident photon flux by reactorvolume unit at 254 nm, I0 = 3.42 × 10−6 Einstein dm−3 s−1, was cal-culated based on the ferrioxalate actinometry measurements [21].Experiments of UV/Fenton oxidation were carried out as follows:0.07 g of FeSO4·7H2O was added into the 0.5 dm3 of prepared dyesolution to obtain 0.5 mmol dm−3 concentration of Fe(II). After pHadjustment (using 1 mol dm−3 H2SO4), a 0.14 cm3 of H2O2 (30%)was added to obtain 2.5 mmol concentration of H2O2. The pH valuewas set at pH 3.0 in all the experiments and slightly decreased dur-ing reaction (at most to 2.7). Simultaneously, UV lamp was turnedon. The solution was continuously mixed with magnetic stirring barfor 60 min; the temperature of the reaction mixture was kept con-stant at 23 ± 1 ◦C by recirculation of cooling water. At the specifictime intervals, 1 cm3 of the treated solution was taken and analyzedimmediately to determine the decolourization and mineralizationextents.

A Perkin-Elmer Lambda EZ 201 UV–vis spectrophotometer wasused for decolourization monitoring at �max while mineralizationwas determined on the basis of total organic carbon (TOC) mea-surements performed by TOC-VCPN 5000 A analyzer, Shimadzu.Experiments including UV, UV/Fe(II) and UV/H2O2 oxidation wereperformed in the absence of both reagents, H2O2 and FeSO4·7H2O,respectively. The concentration of ferrous ions and hydrogen per-oxide in the bulk was determined by colorimetric methods as givenin our previous publication [1]. All experiments were performed intriplicate.

3. Kinetic modelling

The kinetics of decolourization (initial dye molecule cleavage)and mineralization (TOC reduction) was described with a compre-hensive kinetic model introduced in previous studies [1,22,23]. Themodel includes the •OH chemistry in both Fenton catalytic cycleand H2O2 photolysis induced by applied UV-C irradiation. In addi-tion, it was assumed that dyes (and corresponding total organicmatter – TOC) are primarily degraded by hydroxyl radicals, Eq. (4)and (5).

dye + •OHk1,dye−→ colourless products (4)

TOC + •OHk1,TOC−→ CO2 + H2O (5)

Accordingly, oxidation kinetics is given as a disappearance ratein batch reactor, thus can be expressed as, Eq. (6):

rX = −d[X]dt

= k1[X][•OH] (6)

where k1 (dm3 mol−1 s−1) represent the rate constant for the reac-tion between dye molecule, k1,dye, or TOC (k1,TOC) and •OH radicals(hereafter: •OH-reaction rate constants). Note that X stands foreither dye or TOC (hereafter). If mineralization was studied, rX wasthe rate of mineralization rTOC. Similarly, in case of decolourization

kinetic study, rX represented the rate of dye degradation rdye.

The crucial reactions of the Fenton catalytic cycle were sum-marized by Wu et al. [22], highlighting the most important onesin terms of an accurate kinetic modelling: Eq. (1) with designated

I. G

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Journal of

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and Photobiology

A:

Chemistry

273 (2014) 49– 5851

Table 1The chemical structures, manufacturers and applications of the dyes used in the experiments.

C. I. Generic Name(Constitution number)Commercial name(Manufacturer)Chemical class

Chemical structure and application UV–vis spectra of initial solution

C. I. Reactive Black 1 (17 916)Cibacron Black BG (Ciba-Geigy)Monoazo; metal complex(the metal is a mixture of Cr andCo)

N=NNaO3S

O2N

O OMe

NH

Cl

NH2

NaO3S

N

N

N

Dyeing cellulose, naylon, silk.

Printi ng silk, wool.0

0,5

1

1,5

2

2,5

200 30 0 40 0 50 0 60 0 70 0 80 0

λ/nm

A

C. I. Direct Red 23 (29 160)Pergasol Scarlet RS(Ciba-Geigy)Disazo

NH CO CH3. .

N=N

OH

NaO3S NH CO HN. .

OH

SO3Na

N=N

Dyeing cellulose, sil k, wool.

Printi ng cell ulose, silk.

Non-textile usage: dyeing lea ther, paper.0

0,5

1

1,5

2

2,5

200 30 0 40 0 50 0 60 0 70 0 80 0

λ/nm

A

52I.

Grcic

et al.

/ Journal

of Photochem

istry and

Photobiology A

: Chem

istry 273 (2014) 49– 58

Table 1 (Continued)

C. I. Generic Name(Constitution number)Commercial name(Manufacturer)Chemical class

Chemical structure and application UV–vis spectra of initial solution

C. I. Acid Blue 25 (62 055)Sellacid Blue GRL (Ciba-Geigy)Anthraquinone

NH2

SO3Na

NH

O

O

Dyeing wool, n ylon, sil k.

Printi ng wool, silk, acetate.

Non-textile usage: dyeing soap, bat h salts, leather, anod ised al uminium.

0

0,5

1

1,5

2

200 30 0 40 0 50 0 60 0 70 0 80 0

λ/nm

A

C. I. Basic. Red 1 (45 160)Basazol Red 4OL (BASF)Xanthene

O

C

N+H(C2H5) Cl-(C5H2)HN

CH3H3C

COOC2H5

Dyeing silk.

Non-textile usage_dyeing paper, leather.

0

0,5

1

1,5

2

2,5

3

3,5

200 30 0 40 0 50 0 60 0 70 0 80 0

λ/nm

A

C. I. Mordant Orange 1 (14 030)Alizarine Orange G (SandozLtd)Monoazo

O2N N=N

COOH

OH

Dyeing wool, acetate, viscose, nylon, silk.

Printing wool, silk.

Non-textile usage: dyeing anodised al uminium, biological stai n, leat her, paper.

Use as indicat or.

λ/nm

0

0,5

1

1,5

200 300 400 500 600 700 800

A

I. Grcic et al. / Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 49– 58 53

Table 2The characteristics of the aqueous solutions of different dyes used in the experiments.

Dye � (mg dm−3) TOC (mg dm−3) Purity (%) �max (nm) A�max ε�max (dm3 mol−1 cm−1) Initial pH

RB1 100 26.49 >70 590 0.979 6909 6.65DR23 100 18.55 36 500 2.258 18,358 7.33AB25 100 15.42 30 625 1.084 4517 6.56BR1 100 74.63 >99 500 3.149 15,139 6.51MO1 100 19.20 35 370 1.400 4023 6.92

Table 3Estimated kinetic data: coefficients �1 and �2, •OH-reaction rate constants and corresponding half-life for decolourization and mineralization of dye solutions by UV/Fentonprocess.

Dye �1 (s−1)a �2 (s−1) k1,TOC (dm3 mol−1 s−1) t1/2,TOC (min)b k1,dye (dm3 mol−1 s−1) t1/2,dye (min)

RB1 (1.60 ± 0.01) × 10−4 (1.5 ± 0.1) × 10−3 (1.08 ± 0.01) × 106 18 (6.40 ± 0.03) × 106 5DR23 (3.10 ± 0.02) × 10−4 (6.2 ± 0.5) × 10−4 (3.30 ± 0.01) × 105 >60 (4.50 ± 0.02) × 105 >60AB25 (1.60 ± 0.01) × 10−4 (5.5 ± 0.1) × 10−4 (2.30 ± 0.01) × 106 7 (2.20 ± 0.01) × 107 30 sBR1 (1.60 ± 0.03) × 10−4 (5.5 ± 0.1) × 10−4 (7.70 ± 0.03) × 105 30 k1.1 = (1.82 ± 0.02) × 107

k1.2 = (1.35 ± 0.01) × 10715

MO1 (1.60 ± 0.01) × 10−4 (5.5 ± 0.1) × 10−4 (1.80 ± 0.02) × 106 9 (2.65 ± 0.02) × 106 5

licatet

rr

H

F

Crf

r

wctectt

(at[tkpii(

a The values are shown as mean ± S.D. Note that the experiments are done in triphree estimated values of coefficient or constant.

b Average value.

2 and k2 = 170 dm3 mol−1 s−1, Eqs. (7) and (8) with correspondingate constants [22]:

2O2 + •OHr3−→H2O + •HO2, k3 = 3.3 × 107 dm3 mol−1 s−1 (7)

e2+ + •OHr4−→Fe3+ + OH−, k4 = 3.2 × 108 dm3 mol−1 s−1 (8)

onsequently, final equation for mineralization or dye oxidationate by Fenton process in batch reactor is written in the followingorm, Eq. (9) [22,23].

X = −d[X]dt

= k1[X][•OH] = k1k2[Fe2+]0[H2O2]0e−�1te−�2t

k1[X] + k3[H2O2]0 + k4[Fe2+]0

[X]

(9)

here �1 (s−1) represent the rate coefficient of the overall H2O2oncentration change in a particular system during the studied Fen-on process. The decrease in concentration of Fe(II) ions follows thexponential law with the estimated coefficient, �2 (s−1) [1]. Theseoefficients were determined by monitoring the respective concen-rations during the reaction time span in all experiments and fittinghe first order rate equation (Table 3).

In the adopted reaction scheme and resulting rate equation (Eq.9)), the reaction between Fe(II) and H2O2 (Eq. (1)) is considereds a main source of •OH radicals. In addition, under UV-C irradia-ion, •OH radicals are also generated due to the photolysis of H2O224], Eq. (10), while certain number of •OH radicals is lost dueo recombination, Eq. (11). These reactions influence the overallinetics and their portion must be included in the final solution for

•

hoto-Fenton kinetic model. Furthermore, rate of OH generations described using the expression for the rate of photolysis accord-ng to the semiempirical model based on Lambert–Beer’s law (LLM)Eq. (12)).

rX = −d[X]dt

= k1

⎛⎝−(k1[X] + k3[H2O2]0 + k4[Fe2+]0)

2k6

+

⎛⎝√

(k1[X] + k3[H2O2]0 + k4[Fe2+]0)2 + 4k6(k2[Fe2+]0[H2O2]

2k6

; each set of experimental data was fitted in the model and S.D. was calculated for

H2O2 + h�r5−→2•OH (254 nm) (10)

2 •OHr6−→H2O2, k6 = 5.5 × 109 dm3 mol−1 s−1 (11)

r5 = −dCi

dt= Fi˚iI0 (1 − e−2.303·L·

∑εiCi ) (12)

Note that Fi represent the fraction of absorbed radiation by specie i(Eq. (13)), I0 and L represent the intensity of incident irradiation andeffective path of radiation through the photoreactor, respectively,and ˚i and εi are quantum yield and molar absorption coefficient ofa specie i at the wavelength of the irradiation source, respectively.

Fi = εiCi∑ni=1εiCi

(13)

Kinetic rate expression was further extended to include theeffect of r5 and r6 on either mineralization or dye oxidation rate(labelled as X). Accordingly, generation rate of •OH radicals is givenby Eq. (14).

r•OH = d[•OH]dt

= r2 − r3 − r4 + r5 − r6 − rX = k2[Fe2+][H2O2]

− k3[•OH][H2O2] − k4[Fe2+][•OH]

+ FH2O2 ˚H2O2 I0(1 − e−2.303LA254 nm ) − k6[•OH][•OH] − k1[X][•OH]

(14)

Following the pseudo-steady state assumption (d[•OH]/dt = 0)[22,23], the quadratic equation for concentration of •OH radicals,[•OH]t, is obtained (see Supplementary materials for further expla-nation – part A). After introducing the solution for [•OH]t in dyeoxidation kinetic model, final expression for photo-Fenton kineticsis obtained, Eq. (15).

0e−�1te−�2t + FH2O2 ˚H2O2 I0(1 − e−2.303LA254 nm ))⎞⎠⎞⎠ [X] (15)

5 nd Ph

1t(awlAAwg

F

N

pambSdepp

r −(k1

e−�1t

kr

r

2 + 4

Ie

r k3[H2

dpds7

i

4

t(dAtw

4 I. Grcic et al. / Journal of Photochemistry a

The values of ˚H2O2 and FH2O2 are found in literature: 0.5 and9.6, respectively [24]. However, in order to get the accurate solu-ion to the rate equation, fraction of absorbed irradiation by H2O2FH2O2 ) has to be well estimated in a particular system. Therefore,bsorbance of light at 254 nm by model solution/reaction system inhole was monitored during the experiments (A254 nm). The estab-

ished mean values for A254 nm are given in Table 3. Apparently,254 nm remained constant during some experiments. In case when254 nm changed significantly, exact values in each time intervalsere incorporated into a model. FH2O2 at certain periods of time is

iven in Eq. (16) [1].

H2O2 = εH2O2 [H2O2]t

A254 nm/1 cm= εH2O2 e−�1t

A254 nm/1 cm(16)

ote that A254 nm/1 cm corresponds with (�εici)254 nm.Additional experiments were also performed to check the

hotolysis of dyes. The rate of photolysis (rUV-C) was modelledccording to LLM (Eq. (12)). As a result, quantum yields were esti-ated for each dye (˚dye), while a fraction of absorbed irradiation

y a dye (Fdye) was estimated in the same manner as FH2O2 (seeupplementary materials for more details – part B). If dye degra-ation extents achieved by photolysis exceeded 5%, overall kineticxpression was composed from both photolysis and •OH-reactionart, Eq. (17). This is only applied for dye oxidation, since directhotolysis had no influence on mineralization kinetics.

dye = rUV-C + k1[dye][•OH] = Fdye˚dyeI0(1 − e−2.303LA254 nm ) + k1

⎛⎝

+

√(k1[dye] + k3[H2O2]0 + k4[Fe2+]0)

2 + 4k6(k2[Fe2+]0[H2O2]0

2k6

Finally, kinetics of photo-Fenton process was compared with theinetics of UV/H2O2 process. In case of UV/H2O2, rate equation iseduced to Eq. (18) [1].

X = −d[X]dt

= k1

⎛⎝− (k1[X] + k3[H2O2]0)

2k6+

√(k1[X] + k3[H2O2]0)

n case when dyes degrades also via direct photolysis, the followingquation was applied to describe decolourization kinetics, Eq. (19).

dye = Fdye˚dyeI0(1 − e−2.303LA254 nm ) + k1

(− (k1[dye] + k3[H2O2]0)

2k6+

√(k1[dye] +

The comparison between the experimental data and model pre-ictions of corresponding kinetic rate expressions of photo-Fentonrocess, UV/H2O2 and photolysis serves as a validation of fullyeveloped model (Eqs. (15) and (17)). Differential equations wereolved using the backward differentiation method in Mathematica.0, coupled with a nonlinear fit to obtain respective constants.

A special case of BR1 decolourization modelling was explainedn Supplementary materials (part C).

. Results and discussion

In the frame of this work, five model solutions formulated fromhe commercial dyes: C. I. Reactive Black 1 (RB1), C. I. Direct Red 23DR23), C. I. Acid Blue 25 (AB25), C. I. Basic Red 1 (BR1) and C. I. Mor-

ant Orange 1 (MO1) were treated by UV-C assisted homogeneousOPs: UV/Fe(II), UV/H2O2 and UV/Fe(II)/H2O2 (UV/Fenton) under

he same operating conditions. Decolourization and mineralizationas monitored in order to assess the oxidation kinetics of different

otobiology A: Chemistry 273 (2014) 49– 58

[dye] + k3[H2O2]0 + k4[Fe2+]0)/2k6

e−�2t + FH2O2 ˚H2O2 I0(1 − e−2.303LA254 nm ))⎞⎠ [dye] (17)

k6FH2O2 ˚H2O2 I0(1 − e−2.303LA254 nm )

2k6

⎞⎠ [X] (18)

O2]0)2 + 4k6FH2O2 ˚H2O2 I0(1 − e−2.303LA254 nm )

2k6

)[dye] (19)

dyes via •OH attack. Decolourization and mineralization kineticsvia photo-assisted and Fenton processes and corresponding modeldevelopment have been elaborated in the previous section.

Decolourization efficiencies achieved by photolysis and UV-assisted oxidation processes (UV/Fe(II), UV/H2O2 and UV/Fenton)are shown in Fig. 1. The efficiencies are obtained in the follow-ing order: UV/Fenton > UV/H2O2 > UV/Fe(II) > UV. Photolysis aloneappeared as negligible in case of DR32 and BR1. Decolourizationextents achieved by UV/Fe(II) are in average 5–10% higher thanthose achieved by UV, due to UV-assisted iron redox cycling. Inacidic aqueous solution, the oxidation of Fe(II) ions occurs by thedissolved oxygen, Eq. (20) [25], and •OH are generated under arti-ficial irradiation, Eq. (21) [26].

4Fe2+ + 4H+ + O2 → 4Fe3+ + 2H2O (20)

Fe(III)-OHh�−→ Fe2+ + •OH (21)

Note that Fe(III)-OH denotes the Fe(III)-hydroxy complexes formedin aqueous solution as governed by the hydrolysis equilibria inthe pH range ≤5; Eq. (21) corresponds with Eq. (3). Several otherreactions yield with •OH, Eqs. (1), (22) and (23):

Fe2+ + O2 → Fe3+ + O2•− (22)

2O2•− + 2H+ → H2O2 + O2 (23)

H2O2 can be generated in situ (Eq. (23)), serving as an oxidant fora typical Fenton oxidation (Eq. (1)).

In UV/H2O2 and UV/Fenton, photolysis of H2O2 and reactionbetween Fe(II) and H2O2 (if applicable) are considered as mainsources of •OH (see previous section). However, in UV/Fe(II), the•OH yield depends indirectly on the amount of dissolved O2. Sincethere is no additional bubbling of air/oxygen in reactor, concen-tration of O2 reduces as reaction takes its course. Consecutively,decolourization rates slow down. A deviation from observed resultsis noticed in case of DR32, where a higher decolourization extentsare achieved by UV/Fe(II) (31%) than by UV/H2O2 (23%) after 60 min.Also, a rate of decolourization seems roughly constant in the entire

time span. These interesting observations for DR32 will be dis-cussed later.

Results for decolourization kinetics for UV, UV/H2O2 andUV/Fenton are shown in Fig. 2; model predictions follow the trend

I. Grcic et al. / Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 49– 58 55

0 2 5 10 20 30 40 50 60

UV

UV/Fe(II)

UV/peroxide

UV/Fenton

0

20

40

60

80

100

res

idu

al c

olo

r (%

)

time ( min)

(a)

0 2 5 10 20 30 40 50 60

UV

UV/Fe(II)

UV/peroxide

UV/Fenton

0

20

40

60

80

100

res

idu

al c

olo

r (%

)

time (min)

(b)

0 2 5 10 20 30 40 50 60

UV

UV/Fe(II)

UV/peroxide

UV/Fenton

0

20

40

60

80

100

res

idu

al c

olo

r (%

)

time ( min)

(c)

0 2 5 10 20 30 40 50 60

UV

UV/Fe(II)

UV/peroxide

UV/Fenton

0

20

40

60

80

100

resid

ual c

olo

r (%

)time (min)

(d)

0 2 5 10 20 30 40 50 60

UV

UV/Fe(II)

UV/peroxide

UV/Fenton

0

20

40

60

80

100

res

idu

al c

olo

r (%

)

time (min)

(e)

F )/A�ma

D

ordBcBorsiBNatSsto(pda(

B

those of dyes.Among studied dyes, DR32 appears as the most resistant

towards •OH-attack. Slow decolourization is observed for bothUV/H2O2 (Fig. 2b) and UV/Fenton process (Fig. 2c). There are two

Table 4Absorption data and estimated quantum yields for dye photolysis by UV-Cirradiation.

Dye A254 nm (mean ± S.D.) ε254 nm,t = 0 (dm3 mol−1 cm−1) ˚254 nm

RB1 1.71 ± 0.44 9520 0.00022

ig. 1. Decolourization efficiencies achieved expressed as residual colour (A�max(tR32, (c) AB25, (d) BR1 and (e) MO1 model solution.

f experimental data. Corresponding parameters and estimatedate constants are given in Tables 3 and 4. It must be noted thatye BR1 behaves differently from other dyes. It was observed thatR1 degraded in two different kinetic regimes (Fig. 2c – apparentoncentration shown). Considering the structure of BR1 (Table 1),R1 is a typical xanthene dye with N-ethyl groups at either sidef the xanthene ring and side alkyl chains at attached benzeneing. Moreover, BR1 is an analogue of Rhodamine B; a widelytudied model refractory organic dye pollutant due to its ecotox-city and stability in environment [27–31]. Similar to Rhodamine, under the attack of •OH generated in Fenton catalytic cycle,-de-ethylation, chromophore cleavage, ring-opening and miner-lization were involved in the degradation of BR1. It is well knownhat Rhodamine B decompose due to •OH attack at higher rates.ince the only difference between BR1 and Rhodamine B are theide alkyl chains on the attached benzene ring, it can be concludedhat these chains act as scavengers for •OH in the first 10–15 minf the process, leading to the formation of coloured intermediatesBR-CI) with an unaffected chromophore and similar absorptionroperties as initial BR1. Simultaneously, chromophore is being

•

egraded by OH-attack on BR1-CI, resulting with accelerated over-ll decolourization rates. These observation are summed in Eq. (24)see Supplementary materials for detailed kinetic study – part C).

R1 + •OHk1.1,dye−→ BR1 − CI + •OH

k1.2,dye−→ colourless products (24)

x(0)) during photolysis (UV), UV/Fe(II), UV/H2O2 and UV/Fe(II)/H2O2; (a) RB1, (b)

In addition, decolourization occurs in two kinetic regimes forUV/H2O2 as well (Fig. 2b). However, it was observed that k1.2,dyeis 40 times lower (k1.2,dye (in UV/H2O2) = 3.37 × 105 dm3 mol−1 s−1)than the one listed in Table 3. Having in mind that only H2O2 photo-lysis is responsible for •OH generation, there must be a competitivereaction in progress. Regarding the chemical structure, it must benoted that benzoyl etanoate is attached to xanthene ring. An acid-catalyzed reaction can occur with the carboxyl group acting as anucleophile, as depicted in Fig. 3a. In Fenton process, this competi-tive reaction can be neglected since the reaction between Fe(II) andH2O2 is predominant due to a higher reagent concentration than

DR23 1.65 ± 0.31 10,931 ≈0AB25 1.44 ± 0.08 6242 0.00085BR1 2.25 ± 0.04 10,529 ≈0MO1 1.69 ± 0.48 2075 0.00170

56 I. Grcic et al. / Journal of Photochemistry and Ph

(a)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 10 20 30 40 50 60

t (min)

[dye] t/[

dye] 0

RB1

DR32

AB25

BR1

MO1

model

(b)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 10 20 30 40 50 60

t (mi n)

[dy

e] t/[

dy

e] 0

mode l

RB1

DR32

AB25

BR1

MO1

(c)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 10 20 30 40 50 60

t (mi n)

[dy

e] t/[

dy

e] 0

mod el

RB1

DR32

AB25

BR1

MO1

Fig. 2. Decolourization kinetics in dye model solutions by applied processes;experimental data vs. model predictions; (a) dye decomposition due to photoly-sis (� = 254 nm); a LLM was applied (Eq. (12)), (b) UV/H2O2 (Eq. (18) or (19)) and (c)UV/Fe(II)/H2O2 (Eq. (15) or (17)).

pBftawnttfatmt

ossible explanations: (i) side group attracts H2O2 (as explained forR1; Fig. 3a), (ii) urea-bridge, NH CO NH , act as a scavenger

or H2O2. It is well known that urea (NH2)2CO and H2O2 cocrys-allize from aqueous solution as a stable urea-hydrogen peroxidedduct (commonly known as UHP or percarbamide). Similarly, dyeith urea-bridge can form adducts with H2O2 whereas N is con-ected with H via hydrogen bonds (Fig. 3b.). The energy of thisype of hydrogen bonds has been discussed [32]. The formation ofhis adducts can further explain the interesting observations madeor DR32 decolourization by UV/Fe(II) (Fig. 1.). Namely, DR32-H2O2

dducts are formed from in situ generated H2O2 (Eq. (23)). Similaro UHP, DR32-H2O2 adducts can catalyze the oxidation of organic

olecules present in reaction mixture whereas H2O2 decomposeso H2O and O2. The additional portion of O2 can further enhance the

otobiology A: Chemistry 273 (2014) 49– 58

formation of •OH due to iron redox cycling, Eqs. (20)–(23). Addi-tionally, Fenton and Fenton-like reactions may occur in UV/Fe(II)process. Urea group can serve as an active site for reactions betweenFe(II) (Eq. (1)) or Fe(III) (Eq. (25)) and in situ generated H2O2. In aFenton-like reaction, HO2

• radicals are formed; additional portionof O2 emerges due to reduction of Fe(III) by HO2

•, Eq. (26) and H2O2can be generated again, Eq. (27).

Fe3+ + H2O2 → Fe2+ + HO2• + H+ (25)

Fe3+ + HO2• → Fe2+ + H2O + O2 (26)

Fe2+ + HO2• → Fe3+ + H2O2 (27)

Results for mineralization kinetics obtained by the comprehen-sive model (Eq. (15)) were compared to experimental data (Fig. 4.).It can be observed that model predictions follow the trend ofexperimental results. The mineralization is fast and efficient in thesolution of AB25 and MO1 dyes. Namely, an extent of more than90% is obtained after 60 min by the applied photo-Fenton process.Furthermore, approximately 70% of TOC is removed in RB1 modelsolution after 60 min. While mineralization of other dyes, DR32 andBR1, is slower, resulting with mineralization extents of 34 and 63%,respectively. Having in mind that MO1 is a simple azo dye witha single chromophore and that AB25 is a simple anthraquinonedye, the high mineralization extents were expected [17,33]. More-over, those dyes do not contain any refractory reactive centre, suchas triazine group. In contrary, RB1 is a reactive dye with a singleazo chromophore, naphthalene rings and triazine reactive group.According to UV–vis spectra obtained during reaction (data notshown) there is a substantial decrease in absorption at other wave-lengths of interest. Decrease of absorption at 310 nm lead towardsconclusion that naphthalene rings opened due to advanced oxi-dation. Simultaneously, decrease in absorption was observed inrange from 240 to 280 nm; simple aromatic compounds were fur-ther decomposed. RB1 degradation is fast but not as efficient as theone observed for other azo dye, MO1, which can be ascribed to thepresence of naphthalene rings, i.e. more complex structure in gen-eral. Additionally, in RB1 solution Fe(II) can be attracted to triazinegroup leading to formation of triazine-based Fe(II) complexes [34].Consequently, less Fe(II) is available for Fenton reaction.

Slower mineralization of dyes DR32 and BR1 can be justifiedby considering their structure. DR32 is a direct dye with two azochromophores and naphthalene rings linked with urea bridge. Ingeneral, urea bridge contributes to stability of a dye moleculetowards •OH radicals. However, unlike thiourea, urea is poor ascavenger for •OH radicals. This conclusion was drawn from sev-eral studies examining the scavenging role of urea-derivatives inbiological systems [35–37]. Thus, the stability can be ascribed toa formation of DR32-H2O2 adducts, as elaborated previously. Therole of side alkyl chains in BR1 as for decolourization inhibition hasalready been discussed. Distinct to decolourization (Fig. 2.), min-eralization of BR1 model solution can be described with a singlekinetic regime (Fig. 4.), which is expected since both BR1 and BR1-CIdegrades at similar rates. The rate constant for BR1 mineralization islower than those calculated for RB1, AB25 and MO1 (Table 3), point-ing at a recalcitrant nature of xanthene dye BR1. However, relatedto previously discussed decolourization results, a simpler analogueof BR1 (Rhodamine B) efficiently degrades via •OH-based oxida-tion processes. It can be concluded that xanthene chromophoreis not more resistant to •OH radical attack than the other chro-mophores in studied dyes, but a lower degradation extents occur

due to H2O2 consumption via competitive acid-catalyzed reactionon side carbonyl groups (Fig. 3a).

Estimated values of k1,TOC and k1,dye (dm3 mol−1 s−1) and corre-sponding t1/2 (min) are summarized in Table 3. In general, obtained

I. Grcic et al. / Journal of Photochemistry and Photobiology A: Chemistry 273 (2014) 49– 58 57

OEt

OH OH2

+

OEt

O+

H

C+

OEt

OH

H2O2

organic peroxidesOEt

OH OH2

+

OEt

O+

H

C+

OEt

OH

H2O2

organic peroxides

(a)

(b)NH CO CH3. .

N=N

OH

NaO3S NH CO HN. .

OH

SO3Na

N=N

NH CO CH3. .N=N

OH

NaO3S NH CO HN. .

OH

SO3Na

N=N

H-O-O-H

H-O-O-H

NH CO CH3. .N=N

OH

NaO3S NH CO HN. .

OH

SO3Na

N=N

NH CO CH3. .N=N

OH

NaO3S NH CO HN. .

OH

SO3Na

N=N

H-O-O-H

H-O-O-H

Fig. 3. (a) Tentative pathway of an acid-catalyzed reaction between carboxyl group and H2O2 and (b) examples of possible adducts between initial DR32 molecule and H2O2.

rcrwcbaiiekhoidt

Fc

esults justify the applicability of a comprehensive model andorresponding assumptions. The possible influence of all ongoingeactions on Fe(II) and H2O2 concentration in a reaction mixtureas considered through the experimentally determined reagent

onsumption rates (coefficients �1 and �2), including the possi-le influence of irradiation or temperature oscillation; �1 and �2re preferably called coefficients as they moderately vary depend-ng on the experimental conditions. Values for �1 and �2 are givenn Table 3; the variation in reagent consumption rates acknowl-dges the latter discussion on dye chemistry influence on overallinetics. Namely, typical value of �1 is 1.6 × 10−4 s−1, but twiceigher in DR32 solution (3.1 × 10−4 s−1), favouring the discussionn DR32-H2O2 adducts formation. Similarly, �2 is slightly higher

n DR32 solution corresponding to the enhanced iron redox cyclingue to O2 generation. Finally, �2 is significantly higher in RB1 solu-ion, providing the evidence for triazine-based Fe(II) complexes

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 10 20 30 40 50 60

t (min )

TO

Ct/T

OC

0

model

RB1

DR32

AB25

BR1

MO1

ig. 4. Mineralization kinetics in dye model solutions by applied photo-Fenton pro-ess; experimental data vs. model predictions (Eq. (15)).

formation. Reaction between side carbonyl groups and H2O2 seemto have no influence on overall H2O2 consumption rate.

5. Conclusions

The oxidation of five model solutions formulated from thecommercial dyes RB1, DR23, AB25, BR1 and MO1 was studied inUV-C assisted homogeneous AOPs. In comparison to photolysis (UValone), UV/Fe(II) and UV/H2O2, the highest extents were observedwhen UV/Fenton process was applied.

Under the similar experimental conditions, more than 90% ofTOC and 100% of colour removal were obtained for AB25 and MO1dye after 60 min of UV/Fenton. The lowest extents were observedfor DR32; only 34% of TOC and 48% of colour removal.

Mineralization and decolourization kinetics were modelledaccording to the comprehensive kinetic model, while a direct pho-tolysis was modelled according to semiempirical LLM. Developedmodels follows the trend of observed experimental results, justi-fying the assumption that reaction between Fe2+ and H2O2, andH2O2 photolysis are main sources of •OH radicals. An omission ofsome reactions of Fe(II) and Fe (III) in aqueous solution (as shownimportant for describing the UV/Fe(II) process) during model devel-opment did not influenced the applicability of the model.

DR32 contains a urea-bridge in its structure and it is the mostrecalcitrant model pollutant among the studied dyes. Formationof DR32-H2O2 adducts was discussed, including their role on theoverall chemical changes.

Following the observation for dyes with a side carbonyl group,it was concluded that it acted as a nucleophile in acidic media viaformation of a carbocation. In the absence of Fe(II) ions, carboca-tions scavenge H2O2, resulting with the lowest degradation extents

observed in UV/H2O2 process.

Fe(II) were consumed at higher rates during the treatment of atriazine-containing dye (RB1), pointing at a possible formation oftriazine-based Fe(II) complexes.

5 nd Ph

A

i2

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

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[

[

[

[

[

[

[

[

[

8 I. Grcic et al. / Journal of Photochemistry a

ppendix A. Supplementary data

Supplementary material related to this article can be found,n the online version, at http://dx.doi.org/10.1016/j.jphotochem.013.09.009.

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