Study on the Photolytic Mechanisms of Red 141 Dye Wastewaters with an 185nm Vacuum-UV lamp

The decomposition of Reactive Red 141 dye wastewaters by photolysis and VUV/H2O2 process with a 185nm Vacuum-UV lamp in a batch photoreactor was studied under various initial concentrations of organics, solution pH values, dosages of H2O2, and purging gases (N2, O2, and air). The photolytic properties of Red 141 were found to be highly dependent on the solution pH. For the VUV/H2O2 system, the individual contribution to the decomposition of Red 141 by direct photolysis, and free hydroxyl radicals destruction generated from the excitement of O2, H2O, and H2O2 by an 185nm VUV lamp, respectively was differentiated by the proposed assumption. Experimental results for the VUV-only system revealed that photolytic rates of organics by purging O2 were apparently larger than those by purging N2 and the removal of Red 141 was found to be above 90%. For the VUV/H2O2 process, the reaction rates were significantly raised compared with those by direct photolysis. The individual contribution on the decomposition of Red 141 by OH. destruction generated from the excitement of H2O2 molecules was found to be higher than 50% at low pH range (pH=3) in VUV/H2O2 system, however, only 30% at high pH range (pH=11) probably because of the production of hydroxyl radicals from the H2O2 excitement was hampered by the alkaline catalytic reaction between the molecules of H2O2 and HO2 -.

Study on the Photolytic Mechanisms of Red 141 Dye Wastewaters with an 185nm Vacuum-UV lamp

Yung-Shuen Shen

Department of Environmental Engineering, Da-Yeh University, 112 Shan-Jeau Rd., Chang-Hwa, 515, Taiwan, Republic of China, Tel : 886-4-851-1888 ext : 2363, Fax : 886-4-851-1330, (E-mail : ysshen@mail.dyu.edu.tw)

ABSTRACT

The decomposition of Reactive Red 141 dye wastewaters by photolysis and VUV/H2O2 process with a 185nm Vacuum-UV lamp in a batch photoreactor was studied under various initial concentrations of organics, solution pH values, dosages of H2O2, and purging gases (N2, O2, and air) The photolytic properties of Red 141 were found to be highly dependent on the solution pH For the VUV/H2O2 system, the individual contribution to the decomposition of Red 141 by direct photolysis, and free hydroxyl radicals destruction generated from the excitement of O2, H2O, and H2O2 by an 185nm VUV lamp, respectively was differentiated by the proposed assumption Experimental results for the VUV-only system revealed that photolytic rates of organics by purging O2 were apparently larger than those by purging N2 and the removal of Red 141 was found to be above 90% For the VUV/H2O2 process, the reaction rates were significantly raised compared with those by direct photolysis The individual

contribution on the decomposition of Red 141 by OH destruction generated from the excitement of H2O2 molecules was found to be higher than 50% at low pH range (pH=3) in VUV/H2O2 system, however, only 30% at high pH range (pH=11) probably because of the production of hydroxyl radicals from the H2O2 excitement was hampered by the alkaline catalytic reaction between the molecules of H2O2 and HO2-

Keywords

Red 141, Photolysis, VUV/H 2 O 2 Process, Vacuum-UV lamp

INTRODUCTION

Dyeing and finishing of textile goods is a major concern to the environmentalist because of large quantities of color, chemical oxygen demand (COD), nonbiodegradable organics, and other hazardous chemicals into the process effluents Due to the large degree of aromatics present in these molecules and the stability of modern dyes, conventional biological treatment methods are ineffective for decolorization and degradation (Ganesh et al., 1994)

The development status of light-induced Advanced oxidation processes (AOPs) for water and wastewaters treatment has gained industrial scales for the UV-oxidation of various refractory and hazardous organics in the presence of oxidants like hydrogen peroxide and/or ozone (UV/H2O2, UV/O3, UV/ O3/H2O2) (Chemviron Carbon, 1997) The basic concept behind these technologies relies on the photolysis of the added oxidants with powerful medium-pressure mercury lamps to generate very powerful oxidizing species, well known as hydroxyl radicals, then to decompose and even mineralize organic compounds Formation of OH. radicals by the decomposition of hydrogen peroxide can be initiated by ultraviolet (UV) light irradiation and ozone (O3) The decomposition

of various organic pollutants using UV/H2O2 oxidation process has been proved to be very effective (Stefan et al., 1996) On the other hand, UV-disinfection (von Sonntag, 1987) of water usually is performed by its direct irradiation with low-pressure mercury lamps at a single wavelengthλof 253.7 nm The use of solar is accessible to photocatalytic methods of water

VUV(λ<190nm)

treatment in the presence of semiconductors like titanium dioxide (Hoffmann et al., 1995) or to the Photo-Fenton (Bossmann et al., 1998) In practical application, a considerable disadvantage of the majority of the degradation processes introduced above is the need to add external agents into the aqueous medium In such situation, the effectiveness of the processes relies on solid-liquid and gas-liquid mass transfer that sometimes limits the process The development of novel vacuum light (VUV) sources over the last few years (Chiron et al., 2000), has opened up new possibilities for

in-situ generation of hydroxyl radicals (OH.) Hence, the vacuum-UV photolysis of water (H2O-VUV) is still a field of active research compared to other AOPs (Oppenlander and Gliese, 2000) The special requirements of the VUV photolysis of water according to Eq (1) are related to the formation of high local concentration of hydroxyl radicals (OH.) and a series of other species (Gonzalez and Braun, 1995) within a photochemical reaction zone of less than 0.1 mm (Heit and Braun, 1997) The UV light with high energy causes the homolysis of water into hydroxyl radicals and hydrogen molecules Common light sources for this process are “ozone-producing” low pressure mercury lamps (emitting at 185 nm) and the Xe excimer lamp (emitting at 172 nm) The depth of this reaction volume is defined by the high absorption cross-section of water at the wavelength

H2O H. + OH (1)

The oxygen molecules in aqueous solutions was also reported to be excited by VUV with the wavelength ranging 140 nm to 190 nm to generate hydroxyl radicals as bellows (Lenard, 1990) :

3O2 + 185nm hν → 2O3 (2)

O3 + H2O + hν → O2 + H2O2 (3)

H2O2 + hν → 2OH. (4)

The VUV irradiation of contaminated water provides a simple technique for the oxidation and mineralization of water contaminants without the addition of supplementary oxidants For example, it has being used in the treatment of ultrapure water in the semiconductor industry However, intensive basic research is still necessary to completely understand the fundamental principles of the photochemical process of VUV-induced water treatment

In this work, the treatment of dye wastewaters by the VUV-only and VUV/H2O2 process in a homogeneous liquid-phase batch photoreactors was studied under various solution pH values Reactive Red 141 is used as a surrogate chemical to represent azo dyes as shown in the following chemical structure:

In this study, a detail investigation on the reaction kinetics of the reacting species under various solution pH conditions was monitored in order to establish a conceptual model to differentiate the contributions on the decomposition of Red 141 in aqueous solutions by VUV direct photolysis,

OH. generated from the excitement of water, oxygen, and H2O2, respectively

EXPERIMENTAL

The photoreaction system employed in this work contained one batch annular photoreactor The outer tube of the annular photoreactor was made entirely of Pyrex glass with an effective volume

of 2.0 liters and was water-jacketed to maintain constant solution temperature at 25oC The low-pressure mercury 185 nm vacuum-UV lamp was inserted directly into the reactor at the center The light intensity of the UV lamp was kept constant with approximately 12 watts maximum output The solution pH value was kept manually constant at desired levels with NaOH and

H2SO4 solutions The Red 141 and H2O2 and other chemicals used were reagent grade and all experimental solutions were prepared with deionized water

The solution of Red 141 was added to the reactor with a predetermined amount of H2O2 solution Typical reaction runs lasted 60 minutes At desired time intervals, aliquots of solution were withdrawn from the sampling port, which was located at the bottom of the reactor, and analyzed for Red 141 and H2O2 concentrations Total sample volumes were kept below 2% of the total reactor volume Each run of the experiments in this work was replicated twice The standard deviations of the concentration of Red 141 was analyzed to be ±0.1 mg/l, respectively The concentration of H2O2 in the aqueous solution was determined by the KI titration method (Snell and Ettre, 1987) The UV light absorbance of reacting solutions were detected by a HITACH U-2000 UV/Visible spectrophotometer

RESULTS AND DISCUSSION

The experimental works were conducted and studied in the both systems: (1) 185nm VUV Direct photolysis system and (2) the 185nm VUV/H2O2 system, to discuss the differentiation of the contributions on the decomposition of Re 141 in aqueous solutions by UV direct photolysis, OH. generated from the excitement of water, oxygen, and H2O2, respectively

1 Development of the proposed conceptual model

The driving forces for the decomposition of organics in aqueous solutions in a 185nm VUV Direct photolysis and VUV/H2O2 systems are conceptually proposed as shown in Figure 1 In a nitrogen-purging VUV system (VUV/N2), the contribution on the decomposition of reactants can

be attributed to VUV direct photolysis and OH· indirect oxidation generated from the excitement

of H2O The pseudo-first order reaction rate constants of the two driving forces are referred to be

kuv.only and kOH·H2O The two reaction rates can be skillfully differentiated by adding some OH scavengers (e.g t-butanol ect.) (Hiraku et al., 1998) in the reaction systems and supposed to be the linear summation (Shen et al., 1995), thus:

kUV/N2 = kuvonly + kOH·H2O (5) where kUV/N2 : pseudo-first order rate constant of pollutants in a VUV/N2 system

In the oxygen-purging VUV system (VUV/O2), the contribution on the decomposition of reactants can be attributed to VUV direct photolysis, and OH· indirect oxidation both generated from the excitement of H2O and oxygen molecules The pseudo-first order reaction rate constants of the three driving forces are referred to kuv.only, k OH·H2O, and kOH·O2 and also supposed to be the linear summation, thus:

kUV/O2 = kuvonly + kOH·H2O + kOH·O2 =kUV/N2 + kOH.O2 (6)

VUV

direct

photolysis

system

VUV/H2O2

system

where kUV/O2 : pseudo-first order rate constant of pollutants in a VUV/O2 system

Similarly, in the VUV/H2O2 system, the contribution on the decomposition of reactants can be

attributed to VUV direct photolysis, and the three sources of OH· indirect oxidation – those are

generated from the excitement of H2O, oxygen molecules and H2O2 The pseudo-first order

reaction rate constants of the four driving forces are also supposed to be the linear summation,

thus:

kUV/H2O2 = kuvonly + kOH·H2O + kOH·O2 + kOH·H2O2 = kUV.N2 + kOH·O2 + kOH·H2O2 (7) where kUV/H2O2 : pseudo-first order rate constant of pollutants in a VUV/H2O2 system

VUV direct photolysis

OH· indirect oxidation generated from the excitement of H2O

VUV direct photolysis

OH· indirect oxidation generated from the excitement of O2

OH·

OH· indirect oxidation generated from the excitement of H2O OH· indirect oxidation generated from the excitement of O2

OH· OH· indirect oxidation generated from the excitement of H2O OH· indirect oxidation generated from the excitement of H2O2

Figure 1 The driving forces for the decomposition of organics in aqueous solutions in a 185nm

UV Direct photolysis and VUV/H2O2 systems

In VUV/O2 system, the contributions on the decomposition of organic pollutants by OH· indirect

oxidation generated from the excitement of O2 (η1), OH· indirect oxidation generated from the

excitement of H2O (η2), and VUV direct photolysis (η3) are defined as bellows:

η1 ＝ kO2/OH·/ kUV.O2 x 100％, η2 ＝ kH2O/OH / kUV/O2 x 100％,

η3 ＝ kuvonly / kUV/O2 x 100％, η1 +η2+η3 = 1

In VUV/H2O2 system, the contributions on the decomposition of organic pollutants by OH·

indirect oxidation generated from the excitement of O2, H2O, and H2O2 are θ1, θ2, θ4 and VUV

direct photolysis (θ3) are defined as bellows:

θ1 ＝ kO2/OH·/ kUV.H2O2 x 100％, θ2 ＝ kH2O/OH / kUV/H2O2 x 100％,

θ3 ＝ kuvonly / kUV/H2O2 x 100％, θ4 ＝ kH2O2/OH / kUV/H2O2 x 100％, θ1 + θ2 + θ3 + θ4 = 1

N 2

O 2

k uv.only

k OH·H2O

k uv.only

k OH·H2O

k OH·O2

kUV.N2

kUV.O2

H 2 O 2

k OH·O2

k OH·H2O

k OH·H2O

kUV.H2O2

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time(min)

C/Co

pH3

pH5

pH7

pH9

pH11

System:185UV/O2 Red141

Co, Red141=25mg/l Temp.=25.0±0.5 Do>20mg/l Rotation rate=750rpm

2 Experimental Results

(1) VUV-only system

Figure 2, 3, and 4 reveal the decomposition of Red 141 in aqueous solutions in VUV/N2/t-butanol, VUV/N2 and VUV/O2 systems, respectively at various solution pH values It was found that the photolytic rates of Red 141 in the VUV/N2/t-butanol system (Red 141 : t-butanol molar ratio = 1:150) are much less than those in the VUV/N2 and VUV/O2 systems because of OH generated from the photolysis of H2O and O2 are almost scavenged by the adding t-butanol In addition, the decomposition rates of Red 141 conducted by purging O2 were apparently larger than those by purging N2 and the removal of Red 141 was found to be above 90%

The pseudo-first order reaction rate constants

of the Red 141 in the in VUV/N2/t-butanol (kUV-only), VUV/N2 (kUV/N2) and VUV/O2

(kUV/O2) systems, are summarized in Table 1

It was found that all the decomposition rates

of Red 141 obtained in the three systems decreased with increasing solution pH values The ratio of kUV/O2/kUV/N2 determined at acidic and neutral conditions was about 2.3 which is larger that it (1.6) at alkaline conditions The rate constants determined in VUV/N2/t-butanol system (kUV-only) were very small and less than others about for

one-order

Figure 2 The decomposition of Red 141 in aqueous

solutions in VUV/N 2 /t-butanol systems at various solution

pH values

Figure 3 The decomposition of Red 141 in aqueous solutions in VUV/N 2 systems at various solution pH values

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

Time(min)

C/Co

pH3 pH5 pH7 pH9 pH11

System:185UV/N2/t-butanol

Red141

Initial conc of Red141=25mg/l

Temp.=25.0±0.5

Do<0.75mg/l

Rotation rate=750rpm

Red 141 : t-butanol=1:150

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

Time(min)

C/Co

pH3 pH5 pH7 pH9 pH11

System:185UV/N2 Red141

Initial conc of Red141=25mg/l Temp.=25.0±0.5

Do<0.75mg/l Rotation rate=750rpm

Figure 4 The decomposition of Red 141 in aqueous

solutions in VUV/O 2 systems at various solution pH values

Table 1 The pseudo-first order reaction rate constants of the phenolic compounds in VUV/N 2 and

VUV/O2 systems

k of Red 141

pH k UV-only

(min -1 )

k UV/N2

(min -1 )

k UV/O2

(min -1 )

k UV/O2

k UV/N2

Based on the proposed decomposition scheme shown in Figure 1, the calculated individual

contributions on the decomposition of Red 141 by various driving forces in the VUV/O2 system

were shown in Table 2 For the decomposition of Red 141, the contribution (about 57%) of the

destruction by OH generated from the excitement of O2 (η1) were found to be larger than it

(about 39%) from the excitement of H2O (η2) at acidic and neutral conditions, but at the alkaline

conditions the destruction by OH generated from the excitement of H2O (η2) plays a dominant

role for the Red 141 removal The contributions on the decomposition of Red 141 by VUV direct photolysis (η3) were determined to be quite trivial

Table 2 The individual contribution of various driving force to the decomposition of Red 141 in

VUV/O2 system

Red 141

(2) VUV/H2O2 system

Figure 5 shows the decomposition of Red 141 in aqueous solutions in VUV/H2O2 systems (Red

141 : H2O2 molar ratio = 1 : 80) at various solution pH values The pseudo-first order reaction rate constants of Red 141 in VUV/H2O2 systems are summarized in Table 3 It was found that the

decomposition rates of Red 141decreased with increasing solution pH values and were apparently larger than those (kUV/O2, Table 1) in VUV/O2 system about 2~3 times and Red 141 can be totally removed in the desired reaction time The decomposition of Red 141 in aqueous solutions in

VUV/H2O2 systems at various initial concentration of the pollutant is revealed in Figure 6

indicating that the decoloration rates of Red 141decreased with increasing the initial

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time(min)

C/Co

pH3

pH5

pH7

pH9

pH11

System:185UV/H2O2 Red141

Initial conc of Red141=25mg/l Temp.=25.0±0.5

Red 141:H2O2=1:80 Rotation rate=750rpm

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Time(min)

C/Co

10ppm 25ppm 50ppm 100ppm 200ppm

System:185UV/H2O2 Red141

Initial conc of Red141=25mg/l Temp.=25.0±0.5 Red 141 : H 2 O 2 =1:80 pH=7.0±0.2

concentration The calculated contributions on the decomposition of Red 141by various driving

forces in the VUV/H2O2 systems were shown in Table 4

For the decomposition of Red 141, the destruction by OH generated from the excitement of H2O2

(θ4) were found to be the largest contribution (above 50%) among the driving forces and

decreased with increasing pH values possibly due to the catalytic decomposition reaction of H2O2

with the deprotonated species HO2

ions and the scavenging effect of HO2

to hydroxyl radicals (Shen et al., 1995) at alkaline conditions The contributions on the decomposition of Red 141 by

VUV direct photolysis (θ3) were also determined to be quite trivial as the same as VUV/O2

system The contribution (θ1) on the decomposition of Red 141 by OH· indirect oxidation

generated from the excitement of O2 was determined to be larger than that (θ2) from H2O

molecules at acidic and neutral pH conditions

Table 3 The pseudo-first order rate constants of Red 141 in VUV/H2O2 system

pH 3 5 7 9 11

kVUV/H2O2, min-1 0.0916 0.0900 0.0884 0.0461 0.0335

CONCLUSION

The results obtained have shown that the VUV photolysis and VUV/H2O2 processes were capable

of efficiently decomposing Red 141 dye wastewaters For the VUV/H2O2 system, the reaction

rates of Red 141 were significantly raised compared with those by VUV direct photolysis The

individual contribution to the decomposition of Red 141 by direct photolysis, and free hydroxyl

Figure 5 The decomposition of Red 141 in aqueous

solutions in VUV/H 2 O 2 systems at various solution pH

values

Figure 6 The decomposition of Red 141 in aqueous solutions in VUV/H 2 O 2 systems at various initial concentration of the pollutant

radicals destruction generated from the excitement of O2, H2O, and H2O2, respectively was dexterously differentiated by the proposed conceptual model in the VUV/O2 and VUV/H2O2

systems The relative contribution on the decomposition of Red 141 by OH· indirect oxidation generated from the photolysis of various oxidants (O2, H2O, and H2O2) was found to be dependent

on the solution pH values

Table 4 The individual contribution of various driving force to the decomposition of Red 141 in VUV/H2O2 system

Red 141

REFERENCE

Bossmann, S.H., Oliveros, E., Gob, S., Siegwart, S., Dahlen, E.P., Payawan, Jr., L., Straub, M.,

Worrner, M., Braun, A.M (1998) New evidence against hydroxyl radicals as reactive

intermediates in the thermal and photochemically enhanced Fenton reactions, J Phys Chem.,

A 102, 5542-5550

Chemviron Carbon (1997) The AOT Handbook, Advanced Oxidation Technologies, Brussels

Chiron, S., Fernandez-Alba, Rodriguez, A., and Garcia-Calvo, E (2000) Pesticide chemical

oxidation: state-of –the-art, Wat Res., 34, 2, 366-377

Ganesh R., Boardman G G and Michelsen D (1994) “Fate of azo dyes in sludges” Water

Research, 28 (6) : 1367-1376

Gonzalez, M.C and Braun, A.M (1995) VUV photolysis of aqueous solutions of nitrate and

nitrite Res Chem Intermed 21, 837-859

Heit, G and Braun, A.M (1997) VUV photolysis of aqueous systems: spatial differentiation

between volumes of primary and secondary reaction Wat Sci Tech., 35, 25-30

Hiraku,Y., Yamasaki, M., and Kawanishi, S (1998) Oxidative DNA damage induced by

homogentisic acid, a tyrosine metabolite, FEBS Letters, 432, 13-16

Hoffmann, M.R., Martin, S.T., Choi, W Bahnemann, D.W (1995) Environmental applications of semiconductors photocatalysis, Chem Rev 95, 69-96

Lenard, P (1990) Ueber Wirkugen des Ultravioletten Lichtes auf Gasformige Korper, Annalen fur Physik, 1, 486

Oppenlander, T and Gliese, S (2000) Mineralization of organic micropollutants (homologous

alcohols and phenols) in water by vacuum-UV-oxidation (H2O-VUV) with an incoherent

xenon-excimer lamp at 172nm, Chemosphere, 40, 15-21

Shen, Y S., Ku Y., and Lee C C (1995), The Effect of Light Absorbance on the Decomposition of Chlorophenols by Ultraviolet Radiation and UV/H2O2 Process, Wat Res., 29, 3 , 907-914 von Sonntag (1987) Disinfection with UV-radiation In: Stucki, S (Ed.), Process Techonoliges for Water Treatment, Plenum Press, New York, 159-177