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Photocatalytic degradation of reactive orange 84(RO 84) in dye-house effluent using single pass reactor
T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved
1045
Photocatalytic degradation of Reactive Orange 84(RO 84) in dye-house effluent using single pass reactor N.N. Rao *+ and S. Dube Central Salt & Marine Chemicals Research Institute, BHAVANAGAR- 364 002 (INDIA) ABSTRACT Photocatalytic degradation of Reactive Orange 84 (RO 84) in UV radiation, released by a dye manufacturing unit has been examined using TiO2 catalyst as slurry in aqueous dye solution. Experiments were conducted both in batch and single pass reactors. Complete color bleaching and significant reduction (> 90%) in chemical oxygen demand (COD) could be attained in 4-6 h of UV irradiation. The effect of flow rate on COD removal has suggested the need for upgradation of the catalyst.
Keywords: Reactive Orange 84 Degradation Photocatalysis Single-pass reactor. 1.
INTRODUCTION
, Techniques like dissolved air-floatation, coagulation, ion-exchange, reverse osmosis, adsorption and oxidation with peroxide or ozone are usually applied for the removal/destruction of dyes in waste water [1]. More recently, the TiOz/UV based degradation of various organic pollutants present in waste water is recognized to have the potential to emerge as an alternative waste water treatment technology [2-4]. This technique has been shown to be useful for color removal and TOC/COD reduction originating from dyestuffs belonging to triarylmethyl, azo, heterocyclic, anthraquinoid and phthalein classes [5-8]. In this paper, we present the results of TiO2 catalyzed degradation of Reactive Orange 84 in a specially designed single pass reactor, also supplemented with the data collected using a batch reactor. 2.
EXPERIMENTAL
2.1
Titaniumdioxide catalysts
Powder titanium dioxide from Degussa (P-25, predominantly anatase, surface area 50 + 15 mZg-l, 21 nm) as well as Indian commercial TiO2 powder TiO2 (SD) (S.D. Fine Chem., 99%+) were used. Since the as-received TiOz(SD) was ineffective as photocatalyst, it was reduced in H2 at 670 K for 8-1 Oh, then the powder was sieved (300 mesh) to remove particles of size larger than 50~t. The resulting TiO: powder with surface area about 10.45. m2g1 has exhibited significant photoactivity. For catalyst upgradation experiments, the reduced catalyst
1046 powder was sieved using ASTM 100, 140, 270 meshes successively in order to get three size fractions: 105-150, 53-105 and <53kt.
2.2
Reactive Orange 84 dyestuff This was obtained from M/s Atlas Dye Chem. Ltd., Ahmedabad, in the form of concentrated effluent ('filtrate' from filter press). This effluent contained benzidine based chlorotriazine bis azo dye (see Structure) in addition to significant amounts of dyeing bath salts such as NaC1 and KC1. The as-received dye effluent was used for preparing a stock solution of known COD which in turn was used for preparing dye solutions of different initial COD levels. CI
CI
|
HO3S SO3H
o
SO3H
D
OH
N
=
~NI-12 SO3H
REACTIVE ORANGE 84 (RO 84)
2.3 2.3.1
Photocatalytic experiments Batch reactor- rate data collection Aqueous aerated solution of dye (500 ml of RO 84) containing 0.25g P-25 TiO2 or TiO2 (SD) catalyst were irradiated with UV light (400 W medium pressure Hg lamp, immersion type), the catalyst suspensions were stirred magnetically and equilibrated with air supplied using an aerator pump. Rate measurements were made at different initial COD levels of the dye solutions and estimating COD as a function of irradiation time. All estimations were made in duplicate after removing the catalyst from suspensions (samples). The formation of CO2, SO42", NO3" and NH4+ was identified by gas chromatographic / spectrophotometric/and ion selective electrode methods. 2.3.2
Single pass reactor The single pass reactor fabricated mainly using perspex hose (dia. 4.5") and PVC block, houses a 400 W water cooled Hg lamp, has facilities for stirring, aerating and irradiating the contents of the reactor, a reservoir of the dye stock solution (influent) which is admitted into the reactor at a chosen rate, a UF membrane located in a PVC block assembly for filtering the catalyst and a stopcock attached to it for collecting the catalyst-free treated
1047 water (referred as effluent from the reactor). Typically, 1.7 1 of dye stock solution of known COD or distilled water was taken into the reactor and added 2g TiO2 catalyst. The color and COD reductions were monitored over a period of 4-6 h at any particular flow rate. If the reactor is initially filled with dye solution of known COD, this was brought down to a particular level before commencing the influent and effluent flows. In case, distilled water was used, then the influent and effluent flows were commenced after allowing the lamp to glow at full intensity. 3.
RESULTS AND DISCUSSIONS
The photodegradation of RO 84 in terms of decrease in COD levels and its dependence on initial COD levels is displayed in Fig. 1. The rates, rate constants and halfvalue periods are given in Table 1. 200
150 [COD]. = 100 i [COD], = 2001 100
50
0
100
200
Illumination
300
400
Time,min.
Fig. 1: Photodegradation of RO 84 in terms of decrease in COD levels and it's dependence on initial COD levels (batch reactor). Table 1: Rates, rate constants and half-value periods for photocatalytk degradation of RO 84 COD, ppm 28 100 200
Rate, ppm min "1
Rate constant, 102 min l
tl/2, min.
0.50 1.33 1.66
2.90 2.80 0.86
24.0 25.0 80.5
The rate constants are independent of initial concentration of the dye, atleast upto 100 ppm. Thus, the photodegradation of RO 84 follows pseudo-first order kinetics at initial COD levels <100 ppm. The significant decrease in the rate constant at higher initial COD levels (>200 ppm) may be attributed to strongly inhibited direct excitation of TiO2 semiconductor due to diminished penetration depth of UV light in these highly colored solutions.
1048
3.1
Single-pass reactor data The use of the single pass catalytic reactor for removing COD and to maintain it at a lower level is depicted in Fig.2. 180
\ /.,93
-.
o (J
90
"'~",~"
9,',,,
ppm
/
3.3 ml rain "1 _
r
_1" 1
2
3
4
5
Time,
6
7
8
9
h
Fig. 2: Photodegradation of RO 84 9 Removal of COD and its maintenance at a lower level using single pass reactor. In the absence of TiO2 catalyst and UV illumination, the colored dye influent is recovered unaltered in respect to its color and COD. Thus, the membrane employed in the design of the single pass reactor itself does not remove the dye. In Fig. 2, the traces "a' and "b' represent the situation when the reactor is initially filled with 158 ppm COD dye solution, which was subsequently brought down to 60-65 ppm before influent having 493 ppm COD is admitted into the reactor. The constancy of COD beyond 4h implies that the reactor is effective in controlling the COD by the degradation of RO 84 dye. In a similar way, a slurry of TiO2 catalyst in distilled water is also equally effective (traces "c' and "d'). The bar diagram in Fig. 3 illustrates that at higher flow rates effluent containing a little higher COD is resulted as expected. 800
d 0
400
m
Flow rate, ml/min Fig. 3: Influence of fow-rate on the efficiency of COD removal using a single pass reactor.
1049 At the highest flow rate (5.4 ml min l ) about 1.3 1 treated water is collected in 4h having slight color and 84% lesser COD compared to that of influent. At flow rates < 3.5 ml min l the dye molecule appears to have sufficient residence time in contact with the catalyst and therefore lead to complete color removal and significant COD reduction. It may be noted that the applied flow rates are not satisfactory for any practical application. There is need to enhance the rate of COD removal in the single pass reactor so as to be compatible with the desirable flow rates for collecting the treated water. 3.2 Attempted catalyst upgradation and rate enhancement Although the catalyst upgradation involves a series of steps to characterize the reaction dependencies, it initially calls for knowing whether the catalyst is diffusion-rate or reactionrate limited. Using TiO2 (SD) powder catalyst, ground and sieved into three size fractions, the rates of COD removal due to degradation of RO 84 dye in a batch reactor were estimated and plotted against increasing fineness of the sieves (Fig. 4). 1.5 ,=,i
E m,,
./ ~ 0.5 O
0 0
i00 200 ASTM Sieve Mesh Number
300
Fig. 4: Rate of photodegradation of RO 84 using TiO2 powder catalysts having different fineness (ASTM Sieve Number). It is found that the rate is only feebly dependent on the sieve mesh number. This is unlike a steep rise in rate expected for diffusion-rate limited situation (in this case, the rate can be improved by modifying catalyst's physical features) or independent as expected for reaction-rate limited situation (rates can be improved by changing chemical features of the catalyst). In the present case with TiO2 (SD) there is need to optimize both chemical and physical features of the catalyst to a certain extent. Further work in this direction is currently in progress.
1050 ACKNOWLEDGMENTS Prof. P. Natarajan (Director, CSMCRI, Bhavanagar) and Dr. S.N.Kaul (Head, WWTD, NEERI, Nagpur) are thanked for kindly encouraging to perform this work as well as for permitting to use computer facilities. Mr. Prakash Ingle is also thanked for kind assistance on computer. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
Kirk-Othmer's "Encyclopedia of Chemical Technology" Fourth Edition Vol 8 (1993), 753-763. O. Legrini,. E. Oliveros & A.M. Braun, Chem. Rev., 93 (1993) 671. M.R. Hoffman, S. T. Martin, W. Choi, & D.W. Bahnemann, Chem. Rev., 95 (1995) 69. N.N. Rao & P. Natarajan, Curt. Sci., 66 (1995) 742. K. Vinodgopal, I. Bedja, S. Hotchandani & P.V. Kamat, Langmuir, 10 (1994) 1767. R.F.P. Nagueira & W.F. Jardim, J Chem. Educ., 70 (1993) 861. P. Reeves, R. Ohlhausen, D. Solan, K. Pamplin & T. Scoggins, Sol. Energy, 48 (1992) 413. G. Ruppert, R. Bauar & G. Heisler, Chemosphere, 28 (1994) 1447.
1045
Photocatalytic degradation of Reactive Orange 84(RO 84) in dye-house effluent using single pass reactor N.N. Rao *+ and S. Dube Central Salt & Marine Chemicals Research Institute, BHAVANAGAR- 364 002 (INDIA) ABSTRACT Photocatalytic degradation of Reactive Orange 84 (RO 84) in UV radiation, released by a dye manufacturing unit has been examined using TiO2 catalyst as slurry in aqueous dye solution. Experiments were conducted both in batch and single pass reactors. Complete color bleaching and significant reduction (> 90%) in chemical oxygen demand (COD) could be attained in 4-6 h of UV irradiation. The effect of flow rate on COD removal has suggested the need for upgradation of the catalyst.
Keywords: Reactive Orange 84 Degradation Photocatalysis Single-pass reactor. 1.
INTRODUCTION
, Techniques like dissolved air-floatation, coagulation, ion-exchange, reverse osmosis, adsorption and oxidation with peroxide or ozone are usually applied for the removal/destruction of dyes in waste water [1]. More recently, the TiOz/UV based degradation of various organic pollutants present in waste water is recognized to have the potential to emerge as an alternative waste water treatment technology [2-4]. This technique has been shown to be useful for color removal and TOC/COD reduction originating from dyestuffs belonging to triarylmethyl, azo, heterocyclic, anthraquinoid and phthalein classes [5-8]. In this paper, we present the results of TiO2 catalyzed degradation of Reactive Orange 84 in a specially designed single pass reactor, also supplemented with the data collected using a batch reactor. 2.
EXPERIMENTAL
2.1
Titaniumdioxide catalysts
Powder titanium dioxide from Degussa (P-25, predominantly anatase, surface area 50 + 15 mZg-l, 21 nm) as well as Indian commercial TiO2 powder TiO2 (SD) (S.D. Fine Chem., 99%+) were used. Since the as-received TiOz(SD) was ineffective as photocatalyst, it was reduced in H2 at 670 K for 8-1 Oh, then the powder was sieved (300 mesh) to remove particles of size larger than 50~t. The resulting TiO: powder with surface area about 10.45. m2g1 has exhibited significant photoactivity. For catalyst upgradation experiments, the reduced catalyst
1046 powder was sieved using ASTM 100, 140, 270 meshes successively in order to get three size fractions: 105-150, 53-105 and <53kt.
2.2
Reactive Orange 84 dyestuff This was obtained from M/s Atlas Dye Chem. Ltd., Ahmedabad, in the form of concentrated effluent ('filtrate' from filter press). This effluent contained benzidine based chlorotriazine bis azo dye (see Structure) in addition to significant amounts of dyeing bath salts such as NaC1 and KC1. The as-received dye effluent was used for preparing a stock solution of known COD which in turn was used for preparing dye solutions of different initial COD levels. CI
CI
|
HO3S SO3H
o
SO3H
D
OH
N
=
~NI-12 SO3H
REACTIVE ORANGE 84 (RO 84)
2.3 2.3.1
Photocatalytic experiments Batch reactor- rate data collection Aqueous aerated solution of dye (500 ml of RO 84) containing 0.25g P-25 TiO2 or TiO2 (SD) catalyst were irradiated with UV light (400 W medium pressure Hg lamp, immersion type), the catalyst suspensions were stirred magnetically and equilibrated with air supplied using an aerator pump. Rate measurements were made at different initial COD levels of the dye solutions and estimating COD as a function of irradiation time. All estimations were made in duplicate after removing the catalyst from suspensions (samples). The formation of CO2, SO42", NO3" and NH4+ was identified by gas chromatographic / spectrophotometric/and ion selective electrode methods. 2.3.2
Single pass reactor The single pass reactor fabricated mainly using perspex hose (dia. 4.5") and PVC block, houses a 400 W water cooled Hg lamp, has facilities for stirring, aerating and irradiating the contents of the reactor, a reservoir of the dye stock solution (influent) which is admitted into the reactor at a chosen rate, a UF membrane located in a PVC block assembly for filtering the catalyst and a stopcock attached to it for collecting the catalyst-free treated
1047 water (referred as effluent from the reactor). Typically, 1.7 1 of dye stock solution of known COD or distilled water was taken into the reactor and added 2g TiO2 catalyst. The color and COD reductions were monitored over a period of 4-6 h at any particular flow rate. If the reactor is initially filled with dye solution of known COD, this was brought down to a particular level before commencing the influent and effluent flows. In case, distilled water was used, then the influent and effluent flows were commenced after allowing the lamp to glow at full intensity. 3.
RESULTS AND DISCUSSIONS
The photodegradation of RO 84 in terms of decrease in COD levels and its dependence on initial COD levels is displayed in Fig. 1. The rates, rate constants and halfvalue periods are given in Table 1. 200
150 [COD]. = 100 i [COD], = 2001 100
50
0
100
200
Illumination
300
400
Time,min.
Fig. 1: Photodegradation of RO 84 in terms of decrease in COD levels and it's dependence on initial COD levels (batch reactor). Table 1: Rates, rate constants and half-value periods for photocatalytk degradation of RO 84 COD, ppm 28 100 200
Rate, ppm min "1
Rate constant, 102 min l
tl/2, min.
0.50 1.33 1.66
2.90 2.80 0.86
24.0 25.0 80.5
The rate constants are independent of initial concentration of the dye, atleast upto 100 ppm. Thus, the photodegradation of RO 84 follows pseudo-first order kinetics at initial COD levels <100 ppm. The significant decrease in the rate constant at higher initial COD levels (>200 ppm) may be attributed to strongly inhibited direct excitation of TiO2 semiconductor due to diminished penetration depth of UV light in these highly colored solutions.
1048
3.1
Single-pass reactor data The use of the single pass catalytic reactor for removing COD and to maintain it at a lower level is depicted in Fig.2. 180
\ /.,93
-.
o (J
90
"'~",~"
9,',,,
ppm
/
3.3 ml rain "1 _
r
_1" 1
2
3
4
5
Time,
6
7
8
9
h
Fig. 2: Photodegradation of RO 84 9 Removal of COD and its maintenance at a lower level using single pass reactor. In the absence of TiO2 catalyst and UV illumination, the colored dye influent is recovered unaltered in respect to its color and COD. Thus, the membrane employed in the design of the single pass reactor itself does not remove the dye. In Fig. 2, the traces "a' and "b' represent the situation when the reactor is initially filled with 158 ppm COD dye solution, which was subsequently brought down to 60-65 ppm before influent having 493 ppm COD is admitted into the reactor. The constancy of COD beyond 4h implies that the reactor is effective in controlling the COD by the degradation of RO 84 dye. In a similar way, a slurry of TiO2 catalyst in distilled water is also equally effective (traces "c' and "d'). The bar diagram in Fig. 3 illustrates that at higher flow rates effluent containing a little higher COD is resulted as expected. 800
d 0
400
m
Flow rate, ml/min Fig. 3: Influence of fow-rate on the efficiency of COD removal using a single pass reactor.
1049 At the highest flow rate (5.4 ml min l ) about 1.3 1 treated water is collected in 4h having slight color and 84% lesser COD compared to that of influent. At flow rates < 3.5 ml min l the dye molecule appears to have sufficient residence time in contact with the catalyst and therefore lead to complete color removal and significant COD reduction. It may be noted that the applied flow rates are not satisfactory for any practical application. There is need to enhance the rate of COD removal in the single pass reactor so as to be compatible with the desirable flow rates for collecting the treated water. 3.2 Attempted catalyst upgradation and rate enhancement Although the catalyst upgradation involves a series of steps to characterize the reaction dependencies, it initially calls for knowing whether the catalyst is diffusion-rate or reactionrate limited. Using TiO2 (SD) powder catalyst, ground and sieved into three size fractions, the rates of COD removal due to degradation of RO 84 dye in a batch reactor were estimated and plotted against increasing fineness of the sieves (Fig. 4). 1.5 ,=,i
E m,,
./ ~ 0.5 O
0 0
i00 200 ASTM Sieve Mesh Number
300
Fig. 4: Rate of photodegradation of RO 84 using TiO2 powder catalysts having different fineness (ASTM Sieve Number). It is found that the rate is only feebly dependent on the sieve mesh number. This is unlike a steep rise in rate expected for diffusion-rate limited situation (in this case, the rate can be improved by modifying catalyst's physical features) or independent as expected for reaction-rate limited situation (rates can be improved by changing chemical features of the catalyst). In the present case with TiO2 (SD) there is need to optimize both chemical and physical features of the catalyst to a certain extent. Further work in this direction is currently in progress.
1050 ACKNOWLEDGMENTS Prof. P. Natarajan (Director, CSMCRI, Bhavanagar) and Dr. S.N.Kaul (Head, WWTD, NEERI, Nagpur) are thanked for kindly encouraging to perform this work as well as for permitting to use computer facilities. Mr. Prakash Ingle is also thanked for kind assistance on computer. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8.
Kirk-Othmer's "Encyclopedia of Chemical Technology" Fourth Edition Vol 8 (1993), 753-763. O. Legrini,. E. Oliveros & A.M. Braun, Chem. Rev., 93 (1993) 671. M.R. Hoffman, S. T. Martin, W. Choi, & D.W. Bahnemann, Chem. Rev., 95 (1995) 69. N.N. Rao & P. Natarajan, Curt. Sci., 66 (1995) 742. K. Vinodgopal, I. Bedja, S. Hotchandani & P.V. Kamat, Langmuir, 10 (1994) 1767. R.F.P. Nagueira & W.F. Jardim, J Chem. Educ., 70 (1993) 861. P. Reeves, R. Ohlhausen, D. Solan, K. Pamplin & T. Scoggins, Sol. Energy, 48 (1992) 413. G. Ruppert, R. Bauar & G. Heisler, Chemosphere, 28 (1994) 1447.