The role of particulate carbon in the de-novo synthesis of polychlorinated dibenzodioxins and-furans in fly-ash

Chemosphere, Vol.20, Nos.lO-12, Printed in Great Britain

pp 1953-1958,

1990

0 0 4 5 - 6 5 3 5 / 9 0 $3.00 + .OO Pergamon Press DIc

THE ROLE OF P A R T I C U L A T E CARBON IN THE DE-NOVO S Y N T H E S I S OF P O L Y C H L O R I N A T E D DIBENZODIOX I N S AND - F U R A N S IN F L Y - A S H

L. Stieglitz*, G. Zwick, J. Beck, H. Bautz, W. Roth N u c l e a r Research Center, P.O. Box 3640 7500 K a r l s r u h e , FRG

ABSTRACT The influence of the surface a r e a of carbonaceous p a r t i c u l a t e s and of w a t e r vapor on the de-novos y n t h e s i s o f p o l y c h l o r i n a t e d dibenzodioxins and -furans was studied at 300 °C. No direct correlation between surface a r e a and PCDD/PCDF yield was found. By the presence of water vapor in the gas phase the y i e l d expecially of PCDD, and the pattern of congener groups and of the i n d i v i d u a l isomers is influenced.

KEYWORDS Polychlorodibenzodioxins, polychlorodibenzofurans, de-novo-synthesis, fly-ash, m u n i c i p a l waste i n c i n e r a t i o n , p a r t i c u l a t e carbon

INTRODUCTION In p r e v i o u s studies it was shown t h a t a variety of aromatic chlorocompounds is formed at m o d e r a t e t e m p e r a t u r e s from the reaction of particulate carbon with inorganic chloride, catalyzed by the presence of Cu2 +[1, 2]. This reaction m a y be considered as an i m p o r t a n t source for the production of p o l y c h l o r i n a t e d dibenzodioxins (PCDD) and -furans (PCDF) and their emission in a n u m b e r of processes: E n h a n c e d formation of these compound classes was observed to occur in cooler postcombustion zones of m u n i c i p a l waste incinerators and at the electrostatic precipitator out-let [3, 4, 5, 6]. Also the release of PCDD/PCDF and other chloroaromatics from scrap metal recovery and r e f i n i n g processes could be a consequence of such metal catalyzed conversion of carbonaceous m a t e r i a l in the presence of chloride [7, 8]. Investigations in our laboratory were continued with the objective to describe the influence of surface structure and a r e a of the carbon p a r t i c u l a t e s and of w a t e r vapor on the yield and ratio of PCDD/PCDF, congener distribution and isomer pattern.

1953

1954

EXPERIMENTAL For the i n v e s t i g a t i o n of the mentioned objectives different m a t e r i a l s and approaches were applied: A)

F l y ash from a m u n i c i p a l waste incinerator (carbon content 4.4 % wt., 800 ppm Cu) was soxhlet e x t r a c t e d with toluene for 24 hrs., the residue dried in vaccuo at 100 °C and then heated at 300 °C in a i r (50 ml/min with 150 mg/1 water vapor) for 1 hr. corresponding to the e x p e r i m e n t a l d e t a i l s given in [1], extracted with toluene and analyzed

for PCDD/PCDF [1]. W i t h the e x t r a c t e d

residue the a n n e a l i n g and a n a l y t i c a l procedure was performed five times.

B)

Carbon-free fly ash was fortified with 4 % wt. carbon of different origin and surface a r e a s (BET): active coke 22 m2/g, D55/2 769 m2/g, D50/3 533 m2/g, A35/2 1090 m2/g. The a n n e a l i n g (300 °C, 2 hrs.) was performed in air (without water vapor) and in air with 150 mg/l water.

C)

For the f u r t h e r investigation of the influence of water vapor on the formation of i n d i v i d u a l P C I ) F isomers carbon-free fly-ash was mixed with 4 % active charcoal (surface a r e a 760 m2/g) and also a n n e a l e d (300 °C, 2 hrs.) in air (A) and in air containing 150 mg/t water (B).

P r i o r to the a n n e a l i n g the systems A, B, and C used for the investigations were a n a l y z e d for the presence of P C D D / P C D F [1]. In all eases the PCDD/PCDF concentrations of the b l a n k m a t e r i a l s were below the d e t e r m i n a t i o n limit of 0.1 ng/g. All a n n e a l i n g e x p e r i m e n t s were performed in duplicate, the m a t e r i a l from the two e x p e r i m e n t s mixed to

m i n i m i z e e x p e r i m e n t a l v a r i a t i o n s and analyzed for PCDD/PCDF [1]. The results of the

e x p e r i m e n t s were found to be reproducible with v a r i a t i o n s s m a l l e r t h a n +_ 25 %. A n a l y s i s of the p a r t i c u l a t e organic carbon was performed according to a modified procedure described in [9]. RESULTS AND DISCUSSION D a t a on the formation of PCDD/PCDF in the repeated a n n e a l i n g - e x t r a c t i o n cycles are p r e s e n t e d in table 1 for the five cycles. The total yield of PCDD (Tetra- to Octa-) is between 1863 and 2510 ng/g, of P C D F between 1644 and 2335 ng/g. This slight variation lies within e x p e r i m e n t a l and a n a l y t i c a l errors, from this practically a constant yield through the e x p e r i m e n t a l cycles is obtained. In the same sense the s l i g h t decrease of the PCDD/PCDF concentration ratios from 1.05 to 0.66 is p r o b a b l y w i t h i n the e x p e r i m e n t a l errors or at most means only an insignificant contribution of surface changes to the yield d u r i n g the reaction cycles. Over the five cycles also a constant congener d i s t r i b u t i o n is o b t a i n e d with concentration m a x i m a with P5CDD/H6CDD and with T4CDF/P5CDF. In table 1 also the mole fraction in percent of the i n d i v i d u a l congener groups is shown to be r e m a r k a b l y constant. The mole fractions as a v e r a g e v a l u e of five cycles +_ s t a n d a r d deviation are for T4CDD 21 + 3 , P5CDD 35 +_5, H6CDD 31 +_6, H7CDD 9 +_2, and for the corresponding furans: T4CDF 43 +_6, P5CDF 35 + 2 , H 6 C D F 18 _+4, and H 7 C D F 2.8 +0.8. These results prove t h a t for a given carbon species and defined e x p e r i m e n t a l conditions the de-novo-synthesis proceeds reproducibly with respect to the congeners and the ratio of PCDD/PCDF. Since prior to each a n n e a l i n g step the fly ash had been t r e a t e d by toluene and so extractable compounds such as chlorobenzenes and -phenoles been removed, it is e v i d e n t t h a t these compound classes cannot be considered as p r i m a r y precursors for the discussed de-

1955

novo-synthesis as postulated [10, 11]. The formation reaction is accompanied by a decrease of the carbon content from 4.4 % to 1.4 % through the five cycles, part of the carbon being converted to a variety of other volatile and non-volatile organochloro compounds as described earlier ill. The fraction of carbon transformed to PCDD is 10-5, and 10 4 to PCDF. The results achieved in the study of the influence of carbon and surface area on PCDD/PCDF formation are shown in table 2 for fly ash specimen with different carbon types. The surface area of the carbon within the four samples varies by a factor of 50, ranging from 22 to 1090 m2/g. Independent

Table hPCDD/PCDF formation in fly ash in repeated annealing-extraction cycles initial ng/g T4CDD P5CDD H6CDO HTCDD OCDD

10 30 45 72 82

cycle Nr. 1 nglg fraction mole % 265 16 520 28 820 41 290 13 40 2

PCDD T4CDF PSCDF H6CDF HTCDF OCFG

239 100 160 92 50 11

1935 640 710 560 95 20

PCDF ratio PCDF PCDD carbon content(%)

413

2025

1.73

1.05

4.4

35 35 25 4 1

cycle Nr. 2 nglg fraction mole % 370 670 600 200 23

19 31 25 8 1

510 950 780 240 30

23 39 29 8 1

520 925 770 240 30

24 38 29 8 1

420 870 750 250 30

21 39 30 9 1

1863 800 780 430 80 5

42 37 18 3 0.1

2510 890 890 470 80 5

42 37 18 3 0.1

2485 700 590 300 50 4

47 35 16 2 0.2

23OO 7OO 520 260 40 <4

50 33 15 2 <0.1

2095 1.15

3.2 ± 0.2

cycle Nr. 3 nglg fraction mole %

1644

2335

1.12

1.03

0.93

2.3 ± 0.2

cycle Nr. 4 nglg fraction mole %

0.97

0.66

1.8 ± 0.2

cycle Nr. 5 nglg fraction mole %

1524 0.66

0.73

1.4 ± 0.2

0.74

1.5 ± 0.2

Table 2:Influence of surface area of carbon in fly ash on PCDD/PCDF formation at 300 °C (2 hrs). A (air without water vapor) B (air with 150 mg/1 water vapor)

Carbon type surface area m2/g (BET)

active coke 22

O 55/2

D 50/3

A 35/2

769 533 concentration of PCDD/PCDF (ng/g)

1090

T4CDD P5CDD H6CDD H7CDD OCDD PCDD T4CDF P5CDF H6CDF H7CDF 0CDF

A 4 7 60 02 37 190 280 350 300 200 180

B 73 90 115 40 3 321 370 280 150 20 10

A 2 4 15 40 20 61 55 100 110 110 130

B 110 180 200 65 3 558 620 440 190 25 < 1

A < 1 < 1 26 53 28 107 90 250 240 280 220

B 40 50 40 10 2 142 310 220 100 12 < 1

A < I < I 5 17 < I 22 30 50 60 70 50

B 50 90 120 40 < I 300 430 340 200 30 < 1

PCDF

1310

830

505

1275

1080

642

300

970

PCDF PCDD

6.7

2.6

6.2

2.3

10.1

4.5

13.6

3,2

1956

on the presence of w a t e r vapor no direct correlation between the surface a r e a of the carbon and the P C D D / P C D F yield is observed. F r o m this fact it is concluded t h a t a direct interaction c a r b o n - g a s phase such as with chlorine from a Deacon type reaction is not the y i e l d - d e t e r m i n i n g step. On the o t h e r h a n d the influence of absence and presence of water vapor is quite distinct, depending, however, on carbon type: For the PCDD the yield is higher with all samples in the presence of w a t e r vapor; the increase v a r i e s between only 40 % and a factor of 14, depending on carbon type. In dry air the h i g h e r c h l o r i n a t e d species such as the HeptaCDD are d o m i n a n t in all cases. In the presence of w a t e r v a p o r the concentration m a x i m u m is shifted to the hexa- and pentachlorocompounds as reported e a r l i e r [4]. For the formation of P C D F the relation between yield and water vapor is less pronounced. For the s p e c i m e n s with active coke and carbon D50/3 in total more PCDF are produced in the dry system t h a n in the presence of w a t e r vapor. For samples with carbon D55/2 and A35/2 increased formation is noted in wet a t m o s p h e r e . Common to all samples is the a l r e a d y mentioned shift of the concentration m a x i m a to T 4 C D F in the presence of water vapor. The ratio of PCDF/PCDD can be described by n u m e r i c a l v a l u e s r a n g i n g from 6 - 13.6 for dry air, and from 2 - 4.5 in moist atmosphere. The results can be e x p l a i n e d on the assumption t h a t the carbon structure and its changes in the presence of a i r / w a t e r v a p o r m a y influence the preferred formation of dioxins versus furans. On the p a r t i c u l a t e carbon a series of reactions is involved: The interaction of metal ions (Cu2 + ) with thc a r o m a t i c macro s t r u c t u r e leads to the formation of radical cations as is known for several aromatic molecules such as toluene, anisol to proceed on silicates containing Cu2 +, Fe3 +. [12, 13] The cation r a d i c a l s m a y react e i t h e r with H20, O H , Cl- or C12 to yield hydroxyl- and chlorocompounds, w h e r e b y also an o x i d a t i v e d e g r a d a t i o n of the carbon into smaller, t h e r m o d y n a m i c a l l y stable compounds such as chlorobenzenes, - n a p h t h a l e n e s and -biphenyles besides PCDD/PCDF is involved. G e n e r a l l y the production of C12 in these r e a c t i o n s was proven by G u l l e t t et al. [14]. Following the formation of high c h l o r i n a t e d s t r u c t u r e s a d e c h l o r i n a t i o n / hydrogenation occurs to yield lower chlorinated species as reported e a r l i e r [15, 16]. W i t h i n the congener groups the water vapor was found to play an i m p o r t a n t role in the formation of d i f f e r e n t isomers of dioxins and furans. In this investigation furan compounds were considered since due to specific s y m m e t r y properties of furans versus dioxins, transition from i n d i v i d u a l isomers by d e c h l o r i n a t i o n m a y be described in a more clearer way. The mass c h r o m a t o g r a m s of T 4 C D F to H 7 C D F are presented in fig. 1 as obtained in experiments in the presence (B) and in the absence of w a t e r v a p o r (A). In the system with water vapor the typical isomer p a t t e r n within the congener group is o b t a i n e d (trace B). In contrast to this i n d i v i d u a l isomers are found in d o m i n a n t c o n c e n t r a t i o n s in the d r y system. These are for the H7CDF: 1,2,3,4,6,7,9; 1,2,3,4,6,8,9 and 1,2,3,4,7,8,9 isomers, for the H6CDF: 1,2,3,4,6,7, 1,2,3,4,6,9 or 1,2,3,6,8,9, 1,2,3,4,8,9, 2,3,4,6,7,8; for the P5CDF the 2,3,4,6,7 and f i n a l l y the 3,4,6,7 T4CDF. The formation of lower chlorinated congeners m a y be the r e s u l t of d e c h l o r i n a t i o n reactions such as

1234679 H7CDF

~

123467 H6CDF 123469 H6CDF

1234689 H7CDF

~

1234789 H7CDF

~

123469 H6CDF 123489 H6CDF 123489 H6CDF

-.

23467 P5CDF

--, 3467 T4CDF

I

~ 2367 T4CDF

1957

;~

le.

T4CPF

~z9 mu

~

B

1 ~

'

~o'

'

'

":

. -

:

.

~

. . . .

t,t

'

'

"

~

~-'~

1000~

:7

I~1~....

~

,

!oi

'

A jjl

,~

,

.

.

.

.

.

T

,

Time

TimeImin.I

H6CDF

Iminl H7COF

B A o~..~...~...3~...~,'

' ',' TimeImin.)

' ~'~-~

-~

~ . . . . . .

,~, TimeIminl

, ,~,

....

Fig. 1 Comparison of the isomer distribution from H7CDF to T4CDF in thermal experiments with fly ash in the absence (A) and in the presence of water vapor (B)

CONCLUSION The conversion of particulate carbon in fly ash to PCDD/PCDF depends on the type of carbon used. The physico-chemical surface area has, however, only minor influence on the reaction yield. From this fact it is concluded that the yield-determining reaction is a solid-solid interaction rather than a gassolid reaction. Solid-solid interactions may be the formation of metal ion complexes with the aromatic structures of the particulate carbon followed by oxychlorination and partial conversion to CO2. By the presence of water vapor the formation of dioxins is preferred against the synthesis of furans. The ratio of PCDD/PCDF depends on the carbon specimen i.e. the structure and the water vapor. Additionally water vapor favors the dechlorination of individual isomers.

1958

ACKNOWLEDGEMENTS The authors express their thank for the financial support given by the project 'W~asser, Abfall, Boden" of the state of Baden-W(irttemberg. REFERENCES [1] Stieglitz, L.,G. Zwick, J. Beck, W. Roth, H. Vogg, ChemospherelS, 1219-1226(1989) [2] Stieglitz, L., G. Zwick, J. Beck, W. Bautz, W. Roth, Chemosphere 19,283-290 (1989) [3] Environment Canada: The National Incinerator Testing and Evaluation Program, Two Stage Combustion Prince Edward Island, Summary Report EPS 3/VPI, Sept. 1985 [4] Vogg,H., M. Metzger, L. Stieglitz, Waste Management and Research (1987) 5, 285-294 [5] Nottrodt, A., M. Duwel, K. Ballschmiter, Chemosphere 19_9,309-316 (1989) [6] Yamamoto, T., S. Inoue, M. Sawachi, Chemosphere 19,271-276 (1989) [7] Tysklind, M., G. S6derstrom, C. Rappe, L. E. Hagerstedt, F. Burgstrom, Chemosphere 19, 705710 (1989) [8] Oberg, T., G. Allhammar, Chemosphere 19,711-716 (1989) [9] Metzger, M., Z. Analyt. Chem. in press [10] Dickson, L. C., F. W. Karasek, J. Chromatogr. 389, 127-137 [11] Dickson, L. C., D. Lenoir, O. Hutzinger, Chemosphere 19,277-282 (1989) [12] Doner, H. L., M. M. Mortland, Science 166, 1406 (1969) [13] Mortland, M. M., T. It. Pinnavaia, Nature Phys. Sci. 227, 75-77 (1971) [14] Gullett, B. K., K. R. Bruce, L. O. Beach, Proceedings of the International Conference on Municipal Waste Combustion, Apr. 11-14, 1989, Hollywood, Florida, Vol. 2, 8C1-8C25 [15] Vogg, H., L. Stieglitz, Chemosphere 15,373 (1986) [16] Hagenmaier, H. P., M. Kraft, H. Brunner, R. Haag, Environ. Sci-Technol. 21, 1880 (1987)