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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 -.
Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 19 - 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/H 2 O 2 process with a 185nm Vacuum-UV lamp in a batch photoreactor was studied under various initial concentrations of organics, solution pH values, dosages of H 2 O 2 , and purging gases (N 2 , O 2 , and air). The photolytic properties of Red 141 were found to be highly dependent on the solution pH. For the VUV/H 2 O 2 system, the individual contribution to the decomposition of Red 141 by direct photolysis, and free hydroxyl radicals destruction generated from the excitement of O 2 , H 2 O, and H 2 O 2 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 O 2 were apparently larger than those by purging N 2 and the removal of Red 141 was found to be above 90%. For the VUV/H 2 O 2 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 H 2 O 2 molecules was found to be higher than 50% at low pH range (pH=3) in VUV/H 2 O 2 system, however, only 30% at high pH range (pH=11) probably because of the production of hydroxyl radicals from the H 2 O 2 excitement was hampered by the alkaline catalytic reaction between the molecules of H 2 O 2 and HO 2 - . 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/H 2 O 2 , UV/O 3 , UV/ O 3 /H 2 O 2 ) (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 (O 3 ). The decomposition of various organic pollutants using UV/H 2 O 2 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 wavelength λ of 253.7 nm. The use of solar is accessible to photocatalytic methods of water Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 20 - 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 (H 2 O-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. H 2 O 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) : 3O 2 + 185nm hν → 2O 3 (2) O 3 + H 2 O + hν → O 2 + H 2 O 2 (3) H 2 O 2 + 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/H 2 O 2 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 H 2 O 2 , respectively. Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 21 - 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 25 o C. 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 H 2 SO 4 solutions. The Red 141 and H 2 O 2 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 H 2 O 2 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 H 2 O 2 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 H 2 O 2 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/H 2 O 2 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 H 2 O 2 , 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/H 2 O 2 systems are conceptually proposed as shown in Figure 1. In a nitrogen-purging VUV system (VUV/N 2 ), the contribution on the decomposition of reactants can be attributed to VUV direct photolysis and OH· indirect oxidation generated from the excitement of H 2 O. The pseudo-first order reaction rate constants of the two driving forces are referred to be k uv.only and k OH·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: k UV/N2 = k uvonly + k OH·H2O (5) where k UV/N2 : pseudo-first order rate constant of pollutants in a VUV/N 2 system In the oxygen-purging VUV system (VUV/O 2 ), the contribution on the decomposition of reactants can be attributed to VUV direct photolysis, and OH· indirect oxidation both generated from the excitement of H 2 O and oxygen molecules. The pseudo-first order reaction rate constants of the three driving forces are referred to k uv.only , k OH·H2O , and k OH·O2 and also supposed to be the linear summation, thus: k UV/O2 = k uvonly + k OH·H2O + k OH·O2 = k UV/N2 + k OH.O2 (6) Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 22 - VUV direct photolysis system VUV/ H 2 O 2 system where k UV/O2 : pseudo-first order rate constant of pollutants in a VUV/O 2 system Similarly, in the VUV/H 2 O 2 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 H 2 O, oxygen molecules and H 2 O 2 . The pseudo-first order reaction rate constants of the four driving forces are also supposed to be the linear summation, thus: k UV/H2O2 = k uvonly + k OH·H2O + k OH·O2 + k OH·H2O2 = k UV.N2 + k OH·O2 + k OH·H2O2 (7) where k UV/H2O2 : pseudo-first order rate constant of pollutants in a VUV/H 2 O 2 system VUV direct photolysis OH· indirect oxidation generated from the excitement of H 2 O VUV direct photolysis OH· indirect oxidation generated from the excitement of O 2 OH· OH· indirect oxidation generated from the excitement of H 2 O OH· indirect oxidation generated from the excitement of O 2 OH· OH· indirect oxidation generated from the excitement of H 2 O OH· indirect oxidation generated from the excitement of H 2 O 2 Figure 1. The driving forces for the decomposition of organics in aqueous solutions in a 185nm UV Direct photolysis and VUV/H 2 O 2 systems In VUV/O 2 system, the contributions on the decomposition of organic pollutants by OH· indirect oxidation generated from the excitement of O 2 (η 1 ), OH· indirect oxidation generated from the excitement of H 2 O (η 2 ), and VUV direct photolysis (η 3 ) are defined as bellows: η 1 ＝ k O2/OH· / k UV.O2 x 100％, η 2 ＝ k H2O/OH. / k UV/O2 x 100％, η 3 ＝ k uvonly / k UV/O2 x 100％, η 1 +η 2 +η 3 = 1 In VUV/H 2 O 2 system, the contributions on the decomposition of organic pollutants by OH· indirect oxidation generated from the excitement of O 2 , H 2 O, and H 2 O 2 are θ 1 , θ 2 , θ 4 and VUV direct photolysis (θ 3 ) are defined as bellows: θ 1 ＝ k O2/OH· / k UV.H2O2 x 100％, θ 2 ＝ k H2O/OH. / k UV/H2O2 x 100％, θ 3 ＝ k uvonly / k UV/H2O2 x 100％, θ 4 ＝ k H2O2/OH. / k UV/H2O2 x 100％, θ 1 + θ 2 + θ 3 + θ 4 = 1 N 2 O 2 k uv.only k OH·H2O k uv.only k OH·H2O k OH·O2 k UV.N2 k UV.O2 H 2 O 2 k OH·O2 k OH·H2O k OH·H2O k UV.H2O2 Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 23 - 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 102030405060 Time(min) C/Co pH3 pH5 pH7 pH9 pH11 System:185UV/O 2 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/N 2 /t-butanol, VUV/N 2 and VUV/O 2 systems, respectively at various solution pH values. It was found that the photolytic rates of Red 141 in the VUV/N 2 /t-butanol system (Red 141 : t-butanol molar ratio = 1:150) are much less than those in the VUV/N 2 and VUV/O 2 systems because of OH. generated from the photolysis of H 2 O and O 2 are almost scavenged by the adding t-butanol. In addition, the decomposition rates of Red 141 conducted by purging O 2 were apparently larger than those by purging N 2 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/N 2 /t-butanol (k UV-only ), VUV/N 2 (k UV/N2 ) and VUV/O 2 (k UV/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 k UV/O2 /k UV/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/N 2 /t-butanol system (k UV-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 102030405060 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 102030405060 Time(min) C/Co pH3 pH5 pH7 pH9 pH11 System:185UV/N 2 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 pHvalues Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 24 - Table 1. The pseudo-first order reaction rate constants of the phenolic compounds in VUV/N 2 and VUV/O 2 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 3 0.0023 0.0197 0.0454 2.3 5 0.0014 0.0167 0.0397 2.4 7 0.0014 0.0166 0.0383 2.3 9 0.0013 0.0149 0.0245 1.6 11 0.0013 0.0140 0.0236 1.7 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/O 2 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 O 2 (η 1 ) were found to be larger than it (about 39%) from the excitement of H 2 O (η 2 ) at acidic and neutral conditions, but at the alkaline conditions the destruction by OH. generated from the excitement of H 2 O (η 2 ) plays a dominant role for the Red 141 removal. The contributions on the decomposition of Red 141 by VUV direct photolysis (η 3 ) were determined to be quite trivial. Table 2. The individual contribution of various driving force to the decomposition of Red 141 in VUV/O 2 system Red 141 pH η 1 η 2 η 3 3 56.61% 38.32% 5.07% 5 57.93% 38.54% 3.53% 7 56.65% 39.68% 3.66% 9 39.18% 55.51% 5.31% 11 40.68% 53.81% 5.51% (2) VUV/H 2 O 2 system Figure 5 shows the decomposition of Red 141 in aqueous solutions in VUV/H 2 O 2 systems (Red 141 : H 2 O 2 molar ratio = 1 : 80) at various solution pH values. The pseudo-first order reaction rate constants of Red 141 in VUV/H 2 O 2 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 (k UV/O2 , Table 1) in VUV/O 2 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/H 2 O 2 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 Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 25 - 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 102030405060 Time(min) C/Co pH3 pH5 pH7 pH9 pH11 System:185UV/H 2 O 2 Red141 Initial conc. of Red141=25mg/l Temp.=25.0±0.5 Red 141:H 2 O 2 =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 0 102030405060 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/H 2 O 2 systems were shown in Table 4. For the decomposition of Red 141, the destruction by OH. generated from the excitement of H 2 O 2 ( θ 4 ) 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 H 2 O 2 with the deprotonated species HO 2 - ions and the scavenging effect of HO 2 - to hydroxyl radicals (Shen et al., 1995) at alkaline conditions. The contributions on the decomposition of Red 141 by VUV direct photolysis (θ 3 ) were also determined to be quite trivial as the same as VUV/O 2 system. The contribution (θ 1 ) on the decomposition of Red 141 by OH· indirect oxidation generated from the excitement of O 2 was determined to be larger than that (θ 2 ) from H 2 O molecules at acidic and neutral pH conditions. Table 3. The pseudo-first order rate constants of Red 141 in VUV/H 2 O 2 system pH 3 5 7 9 11 k VUV/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/H 2 O 2 processes were capable of efficiently decomposing Red 141 dye wastewaters. For the VUV/H 2 O 2 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 Journal of Water and Environment Technology, Vol.3, No.1, 2005 - 26 - radicals destruction generated from the excitement of O 2 , H 2 O, and H 2 O 2 , respectively was dexterously differentiated by the proposed conceptual model in the VUV/O 2 and VUV/H 2 O 2 systems. The relative contribution on the decomposition of Red 141 by OH· indirect oxidation generated from the photolysis of various oxidants (O 2 , H 2 O, and H 2 O 2 ) 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/H 2 O 2 system Red 141 pH θ 1 θ 2 θ 3 θ 4 3 28.06% 18.99% 2.51% 50.44% 5 25.56% 17.00% 1.56% 55.88% 7 24.55% 17.19% 1.58% 56.68% 9 20.83% 29.51% 2.82% 46.85% 11 28.65% 37.91% 3.88% 29.55% 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. 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S., Ku Y., and Lee C. C. (1995), The Effect of Light Absorbance on the Decomposition of Chlorophenols by Ultraviolet Radiation and UV/H 2 O 2 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. . system Red 141 pH η 1 η 2 η 3 3 56.61% 38 .32 % 5.07% 5 57. 93% 38 .54% 3. 53% 7 56.65% 39 .68% 3. 66% 9 39 .18% 55.51% 5 .31 % 11 40.68% 53. 81% 5.51% (2) VUV/H 2 O. UV/O2 k UV/N2 3 0.00 23 0. 0197 0.0454 2 .3 5 0.0014 0.0167 0. 039 7 2.4 7 0.0014 0.0166 0. 038 3 2 .3 9 0.00 13 0.0149 0.0245 1.6 11 0.00 13 0.0140 0.0 236 1.7 Based