Decoloration and degradation of an anthraquinone dye aqueous solution in flow system using an electron accelerator

Radiat. Phys. Chem. 1979, Vol. 13, pp. 107-113 Pergamon Press Ltd., Printed in Great Britain

DECOLORATION AND DEGRADATION OF AN ANTHRAQUINONE DYE AQUEOUS SOLUTION IN FLOW SYSTEM USING AN ELECTRON ACCELERATOR SHOJI HASHIMOTO, TEIJIRO MIYATA, NOBUTAKE SUZUKI and WAICHIRO KAWAKAM! Japan Atomic Energy Research Institute, Takasaki Radiation Chemistry Research Establishment, Takasaki, Gunma, Japan 370-12

(Received 12 October 1978) Abstraet--A study on the decoloration and degradation of a commercial anthraquinone dye (Acid Blue 40) was carried out by electron-beam irradiation. Experiments were done in a flow system using a five-stage, dual-tube bubbling column reactor. The oxygen contents of the gas bubbled into the inner tubes of the columns were varied from 0 to 100~. The inlet dye concentration, the solution feed rate and the dose rate were also varied from 50 to 100ppm, !.5 to 101/rain and 0.1 to 0.15 Mrad/s, respectively. The degree of decoloration and degradation of aromatic rings increased with the oxygen content and became close to those for pure oxygen bubbling system at about 25% of oxygen content. The amount of degraded aromatic rings was proportional to that of consumed oxygen. The rate expression of the decoloration and degradation of the dye and the oxygen consumption were derived according to a reaction scheme.

1. I N T R O D U C T I O N

They reported that, in the presence of dissolved oxygen, the decoloration was promoted and the skeleton of the dye molecule was also degraded. Therefore, the multi-state, dual-tube bubbling column reactor is favorable for the treatment of this type of dye by high dose rate irradiation. In this work, the experiments were carried out over a wide range of the oxygen concentration in the bubbling gas, and the rate of decoloration and degradation of Acid Blue 40 and oxygen consumption in aqueous solutions were discussed to obtain technological informations on the reactor design for electron-beam oxidation treatment of waste waters.

ALTHOUGH COLORED w a s t e waters from d y e o r textile industries are not easily degraded by ordinary treatment processes, ionizing radiations would be effective for the treatment of such waste waters. "-~) For the radiation treatment process, an electron accelerator is a promising radiation source from the view-points of safety in handling and of large energy output. (4) U n d e r such a high dose rate irradiation as that by the electron accelerator, however, the oxygen dissolving often becomes a rate determinant factor in the oxidation treatment. (5) In previous works, (6"7) the electron-beam decoloration of the azo dye (Acid Red 265, C.I. 18192) aqueous solution was studied using a fivestage, dual-tube bubbling column reactor. It was shown that the decoioration of the dye solution was successfully attained. Although the reactions were performed under high dose rate irradiation, the concentration of dissolved oxygen (DO) was maintained sufficiently high. We have studied the decoloration of Acid Blue 40 (anthraquinone dye, C.I. 62125) aqueous solution as a series studies on the electron-beam treatment of waste waters. The ~/-ray induced decoioration and degradation of this dye in aqueous solutions were studied by Suzuki et aL (s~) R P C Vol. 13, No. 3/4-.-.C

2. E X P E R I M E N T A L The experimental apparatus and procedures are almost the same as those in the previous paper. ~6) The reactor used was a five-stage, dual-tube bubbling column, and was 201 in total volume. Oxygen gas was bubbled into the inner tube from the bottom of each column to feed oxygen necessary for the reaction and to produce a lift force to make the solution circulating in the tube. The surface portion of the solution, the penetration range of the electron-beam, is the reaction zone and the portion under this range is the oxygen absorption zone. The solution circulates between the two zones. The accelerator used was an electron-beam generator from the General Electric Co., Ltd. The energy of the electron was 2 MeV maximum. The penetration range of the elec107

108

SHOJi

The degree of decoloration, 11, is defined as

tron in the column and the mean dose rate in the range were 0.65--0.70gJcm2 and 0.038-0.053 Mrad/s/mA, respectively. The overall oxygen transfer coefficients in the inner tubes were determined experimentally for the same conditions as in the irradiation experiments, and the mean value of the five columns was 1.43 min -~ at the gas bubbling rate of 2.0 I/rain. The solution was prepared by dissolving Acid Blue 40 (Kayanol Blue N2G) from Nippon Kayaku Co., Ltd., without further purification in ion-exchanged water from which organics were not detected by TOC measurements. The commercial dye contained sodium sulfate. The purity of the dye was determined by measuring the optical density of the dye solution before and after refining and was 80%. The refining was done by extraction using aceton as a solvent. In this study, oxygennitrogen mixed gases were also used for bubbling. For the mixed gas and nitrogen bubbling experiments, the nitrogen saturated solution was fed into the reactor, and the oxygen saturated solution for the oxygen bubbling experiments. The solution was sampled out from the hot cell through the tube connected at the bottom of each column. The absorption spectra of the sampled solutions were measured with a Shimazu spectrophotometer UV200. The concentration of DO was determined by "Fieldlab" from Toshiba-Beckman Co., Ltd. The pH of the solution was measured by a TOA HM-18B pH meter, which was corrected with the standard buffer solutions (pH 6.86 and 4.01 at 25°C).

aono

where a is the optical density of the solution at 610nm and subscript 0 means the reactor inlet. The absorption band at 250 nm is attributable to substituted aromatic rings, " ° ' ' ) and the relative reduction of the optical density of this band, ~, defined by the next equation will be a measure of degradation of the dye.

~J=

1.0

.

1

G >

1 2

d ¢X:

3

.

.

.

0% lO 2s

0.5

1.0

,

•

•

-

UNIRRAD|ATED OXYGENCONTENT

5

Z UJ Q

bo- b bo

where b is the optical density at 250 nm. Figures 2 and 3 show 11 and ~ for various oxygen contents in the bubbling gas. In these figures, the values of total dose, 0.030, 0.073, 0.115, 0.156 and 0.185 Mrad, are the doses absorbed between the inlet and each column. ,? at the 5th column is 0.595 in nitrogen bubbling system and 0.871 in oxygen. In the previous work using Acid Red 265 as an example of azo dye, (6) the decoloration was only slightly affected by oxygen bubbling. But in the case of the anthraquinone dye, the degree of decoloration increases with the oxygen content, and becomes close to that in the pure oxygen bubbling system at about 25% of oxygen content. This indicates that the oxygen supply is sufficient above 25% for the experimental conditions. The optical density at 250rim does not decrease so much as that at 610 nm does in the nitrogen bubbling system, but increases with the oxygen content. In Fig. 4, DO concentrations are plotted against dose. Marks (&) and (V) show the

3. R E S U L T S The absorption spectra of the irradiated dye solutions for various contents of oxygen in the bubbling gas are shown in Fig. 1 together with the spectrum of the unirradiated solution. Those solutions were sampled out from the last column. It was shown that the absorption bands at 610 and 250 nm disappeared w i t h increase of oxygen content. A new absorption band appeared at 460490 nm in nitrogen bubbling system. This was discussed in detail by Suzuki et al. (s) 2-0

et al.

HASHIMOTO

1 / ~

/

\

100

~

0

0

0 200

300

400 WAVE LENGTH (nm)

500

600

700

800

FIG. 1. Absorption spectra of irradiated solutions for various oxygen contents in bubbling gas. Dye concentration, 50 ppm; liquid flow rate, 5 l/min; gas flow rate, 2 l/min; irradiation current, 2 mA.

Decoloration and degradation of an anthraquinone dye aqueous solution 1.0

,

,

.

,

.

!

.

.

.

'

OXYGEN CONTENT

i

o

s

'

i

,

i

,

,

j

t

/.f//v""

o

a.

q (~5 8uJ

•

./~

Z

•

'

.,,.~"

0 "/.

,,

'

109

.

--

•

z

¢3

with

irradiation OXy en •

~

v

15

L

=

2s

L

o

,00

without

irradiation

•

J-

•

15

•

P, MJ h= ee O W r~

0

i

i

I

I

'

'

'

0.1 TOTAL DOSE(Mrad)

0

0.2

0 i

i

I

INLET 1

I

2 3 STAGE

i

i

4

5

0

0.1 TOTAL DOSE ( Mrad )

i

PIG. 2. Degree of decoloration for various oxygen contents in bubbling gas. Experimental conditions are the same as in Fig. I.

I

i

INLET 1

I

2 3 STAGE

0.2

I

I

4

5

FIG. 4. Dissolved oxygen concentration with and without irradiation. Experimental conditions are the same as in

Fig. 1. "7

0.7

,

,

~

0.L,

,

i

,

OXYGENCONTENT ii°1

"0"60

O5

•

° • •

~3

,

,

•

~ ~

/"

" 1

~

~

-

~

-

liquid circulates between the irradiation and oxygen absorption zones, the DO concentration is maintained high for pure oxygen. Figure 5 shows the variation of pH with dose. The pH decreases with dose and the decrement is larger for the higher oxygen content. s

7

.

.

.

t

0.1 TOTAL DOSE (Mrad) !

i

I

0.2

I

.

.

.

.

.

.

"

•

"I- 6 " 0

.

~

D. 5

I

INLET 1

2 3 4 5 STAGE FIG. 3. Reduction in the optical density at 250 nm for various oxygen contents in bubbling gas. Experimental

4 t 0

,

, . . . . 0.1 TOTAL DOSE ( Mrad )

conditions are the same as in Fig. 1.

DO concentrations at each column without irradiation for 5 and 15% of the oxygen content, respectively. (A) and (V) show the DO concentration under irradiation for the two cases. The DO concentration decreases by irradiation, for example, 8.43-2.71 ppm at the last column for 15%. As the

|

|

INLET 1

,

i

i

i

0.2

i

2 3 4 5 STAGE Fro. 5. Variation of pH for various oxygen contents in bubbling gas. Experimental conditions are the same as in Fig. 1. O, oxygen content 0%; A, 5%; 1-1, 10%; V, 15%; @, 25%; A, 100~.

110

SHOrt HASHIMOTO et al.

4. DISCUSSION 4.1 S c h e m e o f decoloration

When an aqueous solution is irradiated by an ionizing radiation, active species, such as OH radical, solvated electron, etc. are formed from water. The main active species concerned with the decoloration of Acid Red 265 (azo dye) in the aqueous solution was the OH radical. (~ One dye molecule was not necessarily decolored by the attack of an OH radical, and the probability of decoloration was about 0.3. ~6) The G-value of the decoloration of Acid Blue 40 at the earlier stage of the reaction is calculated from the experimental data and is 0.35. As the value is smaller than that of formation of the active species, an attack of the species does not necessarily cause the decoloration as in the case of Acid Red 265. The decoloration of Acid Red 265 was not affected by DO. °~) On the other hand, the decoloration of Acid Blue 40 is promoted by DO as shown in Fig. 2. This suggests that the intermediate dye radical, formed by the reaction of dye and active species, may be caused structural change to decoloration by the reaction with oxygen. Then, the scheme of decoloration of the anthraquinone dye is assumed as follows: Initiation

where A, B, X, and Y represent the overall colored, decolored materials, active species, and oxygen, respectively, and, those in the rate expression mean their concentration. I is dose rate and k, to ks are apparent rate constants. M is an inert molecule such as solvent, and the concentration is assumed to be constant. K~ and Ks are apparent rate constants including the concentration of the inert. A. and B- are dye radicals. G represents the specific rate of formation of the primary active species, and the value is 1.04 x 10-3G~ (mol/i/Mrad) for the aqueous system, where G~ is the G-value of active species formation. 4.2 Decoloration o f the dye in nitrogen bubbling system

In the nitrogen bubbling system, the oxygen concentration Y is equal to zero. Assume a steady state as to X, A. and B., then the rate of decoloration, r~, is derived as follows; (I0)

rA = k 2 A X = pq~GI

where aA P = ~,A + (1 - a ) ( A o - A ) k, ql = kl + k2 kj + kz

rate (1)

H20 ~

X

GI.

r, = kl + k2 + k3

Attack of X on dye molecules (2)

A +X

(3)

A+X

(4)

B +X

kl k2 kS

, A"

klAX

, B"

k2AX

~ B"

k3BX.

p is the probability that the active species attack colored materials, and ql, the probability that the decoloration occurs in this case. From the mass balances of the dye in the dualtube bubbling column discussed in the previous work ~6) and equation (10), the next equation is obtained for each column.

(ll)

Addition of oxygen to dye radicals

1

(5)

A. + Y k,, A

(6)

A" + Y

(7)

B" + Y ~ , B

k5

~B

k,A- Y ksA" Y

k~B. Y.

Deactivation of dye radicals k7

(8)

A" + M "-'-* A

k T A ' M m KTA"

kl

(9)

B" + M

~B

ksB'M m KsB"

D~

=

1

1-a

Ao 'It~ - ,1,J q,--G ~ -

1

v~n

-

where Di is the absorbed dose for one path into the reaction zone in each column. Subscripts f and e represent the column outlet and the upflow and j means the jth column. ~t~ and ~,~ are defined by ( Ao - Alj)/ Ao and ( A 0 - A,~)/ Ao, respectively. Plots of D1/Ad(v/lj- q,1) vs v~lj/(l- v111) based on equation (l I) using experimental data are shown in Fig. 6. The line in this figure is drawn by the least squares method. Values of intercept and slope are 2.1 × 103

D e c o l o r a t i o n and d e g r a d a t i o n o f an a n t h r a q u i n o n e d y e a q u e o u s s o l u t i o n ! O

1.0

i

i

111

The rate of oxygen consumption in a column for the flow system is given by

3<

r r = (Yp - YI)I¢,

(14)

where Yp, Yf and ~', are the inlet and outlet concentration of oxygen and the residence time of the liquid in the reaction zone. Substitution of equation (13) into equation (14) gives

O5

(15)

0 I

I

I

1

0

2

~ti 1 - ~Jq

(_)

FIG. 6. D/A0/(v/n -v/,j) vs 7/i/(!- v/n) for nitrogen bubbling experiments. Dye concentration, 50-100 ppm: liquid flow rate, 1.5-10 I/rain; gas flow rate, 2 I/min: irradiation current, 2 mA. and 2.9 x 103 Mrad.l/mol, respectively. Then a

= 0.42

( -

Dj y.j-

)

qºG = 4.8 x 10-4 (mollllMrad). As seen from reaction (4), the decolored materials also react with the active species, a is roughly equal to 0.5, that is, k3~ k, + k2, in the anthraquinone dye (Acid Blue 40). This suggests that the reactivity of the decolored products with the active species is almost the same as that of colored materials.

1/ y,, : G~I

K7 1 \ +k'~6 "Y-nn) "

Equation (15) means that the plots of D~/(Ypj- Yn) vs I/YIj according to experimental data should give a straight line if the reaction scheme assumed is reasonable. Figure 7 shows the relation between those terms. As seen from this figure, a linear relation is obtained, although the experimental data are rather scattered. The scattering of data may be due to difficulties of measuring the DO concentration in the column. The irradiated solutions were sampled out through the long tube from the reactor to the outside of hot cell. The concentration of DO may be affected by many factors, such as the temperature variation in the path, the existence of small bubbles in the tube, etc. These may be the main reason of the scattering. The line in this figure is also drawn by the least squares method. F r o m the values of the intercept and the slope of the straight line, G and KTIks are obtained as follows G = 2 . 6 x 10 -3 ( m o l / l l M r a d )

4.3 Decoloration of dye in the presence of dissolved oxygen The reactivity of the decolored materials with the active species is almost the same as that of colored as discussed previously (k, + k2 ~ k3). The reactivities of the colored and decolored materials toward the oxygen or the inert are assumed to be equal each other as a first approximation, that is, k4 + ks ~ ks, K7 ~- Ks. Steady states as to A. and B. are also assumed. Then, the next relation is obtained from reactions (1)-(9).

KTIk6 = 2 . 4 x 10 - s ( l l m o i ) . t~ I 0

2-0

.

.

•

.

*

.

,

.

,

,

,

,

,

i

|

,

,

.

,

x

"0

I[ 1.0

!

(12) • d(A. + B.)ldt = GI - k6(A. + B . ) Y - kT(A. + B.) =0.

m-o

0.5 llY~i

The rate of oxygen consumption, ry, is given by (13) ry

= --d

Y / d t = ks(A" + B . ) Y = ks Y G l l ( k s Y + K , ) .

(llmol)

|

1.0 x10 -5

FIG. 7. D/(Y~-Yli) vs I/YIe Dye concentration, 50100 ppm; liquid flow rate, 5 I/min: gas flow rate, 2 I/min, oxygen content in the gas 5-25%: irradiation current, 2 mA.

112

SHOJ!

HASHIMOTO

The rate of decoloration in the presence of DO is given by equation (16) based on reactions (3) and (6) (16)

rA

=

k2AX +

ksA" Y.

From the mass balances with respect to the active species X and the colored dye radical A., equation (16) is rewritten as

A

rA = q, -~oGl + (1

(17)

A

q,)q2-~orrGY

-

q2 = ks~ks where q, is the probability that the decoloration occurs when the oxygen molecule reacts with the dye radical A-. The next equation is obtained for each column by using mass balances of dye and oxygen, equations (10), (14) and (17).

Ao(~l/ ~?,/)= q,O +(1 - q,)q, Ye/D/ YI/ Dj(1 - - ,~n)

(18)

Figure 8 shows the plots of Ao(nfj - ~,j)/DJ(I - ~n) against (Ypj - YIj)IDj based on equation (18). From the intercept and the slope of the straight line

qtG = 2.4 x 10-4 (mol/l/Mrad) (1 -- q|)q2 "---0.28

hence, q, = 9.2 x 10 -2

(- )

q , = 0.31

( - ).

et

al.

electrons on the dye in the oxygen free system. ~m The G-value of active species formation is calculated, and the value is 2.5 in the presence of dissolved oxygen. This is nearly equal to that of OH radical formation. And the G-value of oxygen consumption calculated from the experimental data is 3.0. Since the OH radical reacts with aromatic rings rapidly"2~ and the dye radical seems to react with one oxygen molecule, the decoloration may be induced by the attack of the OH radicals. The contribution of other species, such as hydrated electrons, may be very small in the oxygen contained gas bubbling system. On the other hand, in the nitrogen bubbling system, the G-value of active species formation calculated ,using q, of 9.2 x 10-2 is 5.0, and is larger than that in the oxygen bubbling system. The difference may be attributable to the contribution of the hydrated electrons. 4.4 Degradation of the dye As shown in Fig. 3, the absorption band near 250 nm does not decrease with dose so much as that at 610nm in the nitrogen bubbling system. While in the presence of DO, the reduction in the optical density at 250nm is large, and increase with the oxygen content. Since this absorption band is attributable to substituted aromatic rings, the reduction suggests that the aromatic rings are destroyed by irradiation in presence of DO. In other words, the oxygen is necessary to the degradation of the dye. In Fig. 9, 6A0 is plotted against the amount of consumed oxygen per unit volume of the solution

The decoloration of Acid Blue 40 is attributable to the attacks of the OH radicals and of hydrated

./o

u~ O

2 x

x ~

oo.

v E

O

O

='2

I

0

...~--'/

0

0

S ° O

I

~'~-o

a

0

I

1

(Vpj-V~))/Oj

t

I

2

|

I

0

t

3

4

2 ~" Q ( Ypj-Yfj ) I F

!

!

I

4 (moll|)

6 x 10 4

(molll/Mrad) xlO3

FIG. S. A0(WIj- W,~)I/~/(I- Wn) vs (Yp~- Yn)IDj. Experimental conditions are the same as in Fig. 7.

FIG. 9. Reduction of the optical density at 250 nm as a function of the amount of consumed oxygen. Experimental conditions are the same as in Fig. 7.

Decoloration and degradation of an anthraquinone dye aqueous solution which is calculated

by

k Q(

Ypj

,jl

-

Yfj)/F.

A linear relation is obtained. This indicates that the amount of degraded aromatic rings is proportional to that of consumed oxygen. The experimental results on the decoloration and degradation mentioned above show that it is necessary to maintain oxygen at a high concentration for the treatment of the dye aqueous solution, and that the bubbling column type of reactor can reply to this demand even under high dose rate irradiation. 4.5 Change of pH The drop in the pH is larger for the higher oxygen content as seen from Fig. 5. In the presence of oxygen, the dye molecules after decoloration are degraded to lower molecular weight compounds, mainly to organic acids.“” and finally to carbon dioxide.‘9.“.‘5’ In Fig. 10, the hydrogen-ion concentrations in the irradiated solutions are plotted as a function of the amount of consumed oxygen. Although the optical density at 250 nm decreases

0

2 10 (Ypj-Yfj l/F

4

6

(molll) x lo4

FIG. 10. Variation of hydrogen-ion concentrations as a function of amount of consumed oxygen. Dye concentration, 50 ppm; liquid flow rate, 5 Umin; gas flow rate, 2 I/min: oxygen content in the gas, O-25%: irradiation current. 2-3 mA.

113

linearly with the amount of consumed oxygen as mentioned above, the hydrogen-ion concentration is nearly equal to zero until the amount of consumed oxygen to about 1.5 x lo-’ mol/l, and then increases rapidly. This induction period may indicate that a certain amount of oxygen is necessary for formation of organic acid from a dye molecule by oxidation. Acknowledgemenfs-We are indebted to Mr. M. Washino for his valuable discussion, and Nippon Kayaku Co., Ltd., for providing Acid Blue 40 (Kayanol Blue N2G).

REFERENCES 1. A. I. MYTELKAand R. J. MANGANELLI, J. Water Pollut. Control. Fed. 1968,40, 260. 2. T. F. CRAFT and G. G. EICHHOLZ,ht. J. Appl. Radiar. lsoropes 1971, 22, 543. 3. N. SUZUKI. T. NAGAI. H. HOT~Aand M. WASHINO. Bull. Cheml Sot. Japan 1975,48,2158. 4. J. G. TRUMP,K. A. WRIGHT,E. W. MERU, A. J. SINSKY. D. SHALK and S. SOMMER. IAEA-SM 1941503, Int. Symp. on the Use of High Level Radiation in Waste Treatment. Munich, Germany, 1975. 5. W. KAWAKAMI and S. HASHIMOTO, Preprint presented at the 8th Autumn Meeting of rhe Sot. of Chem. Engrs, Japan, B 308, 1974. 6. W. KAWAKAMI,S. HASHIMOTO,K. NISHIMURA.T. MIYATAand N. SUZUKI,Environ. Sci. Technol. 1978. 12, 189. 7. N. SUZUKI,T. MIYATA,A. SAKUMOTO, S. HASHIMOTO and W. KAWAKAMI,Int. J. Appl. Radiar. Isotopes 1978, 29. 103. 8. N. SUZUKI,T. NAGAI. H. HUITA and M. WASHINO, Bull. Chem. Sot. Japan 1976,49,600. 9. T. NAGN and N. SUZUKI.Ink J. Appl. Radiat. Isotopes 1976.27.699. 10. A. E. CILLANand E. S. STERN,An ZnrroducGon to Electronic Absorption Specfroscopy in Oganic Cheniistry. Arnold, London. 1960. Il. J. R. DYER,Applicafion on Absorption Spectroscopy of Organic Compound. Prentice-Hall, New Jersey, 1965. 12. M. ANBAR and P. NETA. Znt. J. Appl. Radiat. Isotopes 1967. 18,493. 13. K. SHIMADA,N. SUZUKI,N. ITATANIand H. HIXTA. Bull. Chem. Sot. Japan 1964,37, 1143. 14. J. W. T. SPINKS and R. T. WOODS.An Introduction to Radiation Chemistry, Chap. 8. Wiley. New York. 1964. IS. A. 0. ALLEN, The Radiation Chemistry of Water and Aqueous Solutions. Van Nostrand. New Jersey,