Reactions of dibenzofuran and 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin on municipal waste incinerator fly ash

Article abstract of DOI:10.1021/es950267b

Dibenzofuran (DF) on fly ash can be converted to polychlorinated dibenzofurans (PCDF)in a N₂/O₂/HCl atmosphere, yielding especially 2,3,7,8- substituted congeners. This is consistent with an electrophilic aromatic substitution mechanism. 1,2,3,4,7,8-Hexa-chlorodibenzo-p-dioxin (1,2,3,4,7,8- H₆CDD) on fly ash can be chlorinated by HCl both in N₂ and O₂ atmospheres. Dechlorination and decomposition reactions are not important under these conditions, and isomerization reactions do not take place either. 1,2,3,4,7,8-H₆CDD dechlorinates or decomposes on fly ash when no HCl is present. Only a limited number of dechlorination products are formed. Chlorination and dechlorination are separate processes, not occurring simultaneously.

Full text of DOI:10.1021/es950267b

Environ. Sci. Technol. 1996, 30, 833-836
of chlorination is 2 f 8 f 3 f 7 f 1 f 4 f 6 f 9 (9). This
Reactions of Dibenzofuran and
1,2,3,4,7,8-Hexachlorodibenzo-p-
dioxin on Municipal Waste
Incinerator Fly Ash
order of chlorination is consistent with an electrophilic
aromatic substitution mechanism. Born investigated chlo-
rination ofDF on fly ash, but found only formation ofmono-
and dichloro congeners (5). We wanted to know whether
the electrophilic mechanism also holds for the flyash surface
and therefore carried out experiments with DF in a N2/
O2/ HCl atmosphere.
OCDD formed from pentachlorophenol starts to dechlo-
rinate on fly ash after 5 min at 300 °C, indicating that
formation and destruction/ dechlorination reactions are
parallel pathways occurring simultaneously (10). To learn
more about these processes, we chose to study chlorination
and dechlorination of 1,2,3,4,7,8-H6CDD. With a hexachlo-
rodibenzodioxin, simultaneous chlorination (resulting in
H7CDD-OCDD) and dechlorination (resulting in lower
chlorinated congeners) can be measured.
R U U D A D D I N K , * , † M I S C H A A N T O N I O L I ,
K E E S O L I E , A N D H A R R I E A . J . G O V E R S
Department of Environmental and Toxicological Chemistry,
Amsterdam Research Institute for Substances in Ecosystems,
University of Amsterdam, Nieuwe Achtergracht 166,
1018 WV Amsterdam, The Netherlands
Dibenzofuran (DF) on fly ash can be converted to
Experimental Section
polychlorinated dibenzofurans (PCDF) in a N2/O /HCl
2
Materials. The chemicals used in our cleanup have been
described before (11). Only the chemicals used in our
experiments are reported here: dibenzofuran (98%, Aldrich
Chemie, Steinheim, FRG); 1,2,3,4,7,8-hexachlorodibenzo-
p-dioxin (99%, Amchro, Sulzbach/ Taunus, FRG); fly ash
from the municipal waste incinerator in Zaanstad, The
Netherlands; hydrogen chloride (gas, 4.0 grade, UCAR,
Nieuw-Vennep, The Netherlands); nitrogen (5.0 grade,
Hoekloos, Schiedam, The Netherlands); oxygen (4.5 grade,
Hoekloos).
atmosphere, yielding especially 2,3,7,8-substituted
congeners. This is consistent with an electrophilic
aromatic substitution mechanism. 1,2,3,4,7,8-Hexa-
chlorodibenzo-p-dioxin (1,2,3,4,7,8-H CDD) on fly ash
6
can be chlorinated by HCl both in N2 and O
2
atmospheres. Dechlorination and decomposition reac-
tions are not important under these conditions, and
isomerization reactions do not take place either.
1,2,3,4,7,8-H CDD dechlorinates or decomposes on fly
6
Experim ental Apparatus. From the fly ash, all organic
material was removed by heating at 550 °C for 90 min under
a stream of air saturated with water. The fly ash was mixed
with either DF or 1,2,3,4,7,8-H6CDD as a solution in hexane.
After evaporation of the solvent, 1.0-2.0 g of the mixture
was placed in a cylindrical sample basket and coupled with
a glass inlet tube for introduction of a gas flow through the
fly ash bed. The sample basket and inlet tube were fit into
a horizontal pyrex glass reactor and put in a furnace (Lenton
CSC 1100 split tube furnace with PID 808 temperature
controller, Leicestershire, U.K.). Experiments lasted for
0-50 min, preceded by 10-20 min of heating under a gas
flow (identical with the reaction gas) in order for the sample
basket, inlet tube, and reactor to reach the set point
temperature (218-348 °C, accuracy (7 °C). A gas stream
(N2/ O2/ HCl; N2/ HCl; O2/ HCl; N2) was introduced. The flow
was controlled by Series 840 Side)Trak mass flow control-
lers (Sierra Instruments, Monterey, CA). The flow was
checked before and after experiments with a flow meter.
N2 and O2 were mixed in a mixing chamber (V ) 800 mL)
containing ceramic pellets. HCl was introduced into the
gas stream after the mixing chamber. Products evaporating
from the fly ash surface were collected using a cold trap (60
mL of toluene cooled with ice). After the experiment, the
fly ash bed was taken out of the furnace immediately and
cooled to room temperature. Fly ash fractions and cold
traps were generally combined after the experiment and
analyzed for PCDD/ F together. However, sometimes cold
traps were analyzed separately to provide information on
the amount of PCDD/ F that evaporated from the fly ash
surface during the experiment.
ash when no HCl is present. Only a limited number
of dechlorination products are formed. Chlorination and
dechlorination are separate processes, not occurring
simultaneously.
Introduction
Polychlorinated dibenzo-p-dioxins (PCDD) and polychlo-
rinated dibenzofurans (PCDF) were detected in emissions
of municipal waste incinerators in 1977 (1). Formation
occurs in the post-combustion zone of the incinerator (T
< 600 °C), rather than in the oven during the actual
incineration process (2). These formation reactions are
catalyzed by residue particles (fly ash) that are carried into
the post-combustion zone together with the off-gas of the
incineration process. Metal ions present in fly ash (Cu and
Fe) convert macromolecular carbon to PCDD/ PCDF (de
novo synthesis) (3). The carbon content of fly ash can be
up to ca. 7% (4). Other pathways yielding these toxic
compounds include formation from chlorophenols (5) and
propene (6), both on fly ash. Such small volatile organic
molecules (precursors) can be adsorbed on the fly ash
surface from the gas phase and are subsequently converted
to PCDD/ PCDF. Both NaCl (7) and HCl (8) act as a chlorine
source during the formation reactions of PCDD/ PCDF.
Chlorination reactions of DF on Al2O3-SiO2 with HCl at
300 °C yield mainly 2,3,7,8-substituted congeners; the route
* Corresponding author telephone: (518)276-6377; fax: (518)276-
4030.
† Present address: Isermann Department ofChemicalEngineering,
Cleanup and Analysis. These have been described
before (11). Only the T4CDD-OCDD and T4CDF-OCDF
Rensselaer Polytechnic Institute, Troy, NY 12180.
9
0013-936X/96/0930-0833$12.00/0
1996 Am erican Chem ical Society
VOL. 30, NO. 3, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 8 3 3

TABLE 1
Yields of PCDD/F (in nmol/g of Fly Ash)a
experiment
no.
reactant
atmosphere ∑PCDD
∑PCDF
1b
2c
3d
DF
N2, O2, HCl 0.09 ( 0.02 0.98 ( 0.03
1,2,3,4,7,8-H6CDD N2, HCl
1,2,3,4,7,8-H6CDD O2,HCl
2.37 ( 0.66 0.01 ( 0.01
3.02 ( 0.03 0.003
a
b
All experim ents in duplicate, m ean value ( range is given. 2.0 g
of fly ash with 33 nm ol/g DF; T ) 348 ( 7 °C; 50-m in reaction; flow: N2
c
112 ( 2 m L/m in, O2 12 m L/m in, HCl 6.5 ( 2.2 m L/m in. 1.0-1.1 g of fly
ash with 3.19 (s ) 0.37) nm ol/g 1,2,3,4,7,8-H6CDD; T ) 248 ( 7 °C; 15-
d
m in reaction; flow: N2 59 ( 1 m L/m in, HCl 5.5 m L/m in. Sam e as under
footnote c, but with flow of O2 57 ( 1 m L/m in, HCl 5.5 m L/m in.
congeners were analyzed. 1,2,3,4,7,8-H6CDD was checked
for contamination. A purity of 103% was found, which is
not significantly different from the 99% guaranteed by the
manufacturer. Heating a sample of carbon-free fly ash at
350 °C in a N2/ O2/ HCl mixture resulted in 0.05 nmol/ g
PCDD and 0.11 nmol/ g PCDF. The cleanup procedure,
the fly ash, and the DF were all checked for background
PCDD/ F. Results were in-between 0.011 and 0.023 nmol/ g
∑PCDD/ PCDF. These contributions are negligible when
compared with the amount of PCDD/ F formed during our
experiments.
FIGURE 1. Route of chlorination and dechlorination for 1,2,3,4,7,8-
H CDD (Cl atoms are not shown).
6
chlorination mechanism for DF holds on fly ash too and
can explain part of the PCDF formed.
The preferred formation of 2,3,7,8 congeners from DF
differs from the isomer distribution that is found in de novo
synthesis experiments under similar conditions. Typically,
at 348 °C in a N2/ O2/ HCl mixture with carbon as a reactant
on fly ash, no preference for 2,3,7,8-substitution occurs
(11), indicating that DF is not an intermediate in de novo
synthesis of PCDF from carbon.
Chlorination of 1,2,3,4,7,8-H6CDD. Chlorination ex-
periments with 1,2,3,4,7,8-H6CDD were carried out at 248
°C (experiments 2 and 3, Table 1). We chose this tem-
perature to compare the results of these experiments with
those described below (studying dechlorination and de-
composition). We used mixtures of N2/ HCl (experiment 2)
and O2/ HCl (experiment 3). This makes a comparison
possible of the influence of these gases, which are both
present in incinerator flue gas, on chlorination processes.
We chose a short reaction time (15 min) to avoid that the
chlorination process would proceed toward an almost 100%
formation of OCDD, which would eliminate useful infor-
mation regarding the chlorination pathway at the hep-
tachlorodibenzodioxin level.
In N2, a great variance was seen between duplicate
experiments, resulting in a mass balance of between 54
and 95%. With O2, we obtained a mass balance of 94-96%.
We calculated this mass balance by (∑T4CDD-OCDD/
1,2,3,4,7,8-H6CDD before experiment) × 100%. No sig-
nificant amounts of PCDF were detected, and the trans-
formation PCDD f PCDF does not occur. Our results show
that chlorination takes place in both atmospheres. Griffin
(13) has suggested that HCl passed over fly ash is converted
to Cl2 (Deacon reaction) according to:
Results and Discussion
In Table 1, the results ofexperiments with DF and 1,2,3,4,7,8-
H6CDD are reported.
Chlorination of DF. We chose to perform the experi-
ment with DF under similar conditions as in the post-
combustion chamber in an incinerator, i.e., at 348 °C, and
in a N2/ O2/ HCl mixture. To make a comparison with de
novo synthesis experiments possible, we performed the
experiment at typical de novo synthesis reaction times: 50
min.
Ca. 3% of the DF is converted to T4CDF-OCDF
congeners (experiment 1), which is somewhat higher than
the 0.3-1.0% found by Luijk for chlorination of DF on
Al2O3-SiO2 at 250-300 °C (12). The higher temperature
during experiment 1 (348 °C) might explain the difference.
Some PCDD is formed in experiment 1, 0.09 nmol/ g. As
described under Cleanup and Analysis, some background
formation of PCDD from organics left on the fly ash will
occur, and the 0.09 nmol/ g may stem partially from that
source. It does not constitute proof of the transformation
of DF to DD.
When setting ∑T4CDF-OCDF ) 100%, ca. 70% of the
PCDF formed is OCDF. Under the reaction conditions used,
where HCl is present as a chlorinating agent, dechlorination
of PCDF formed is obviously not a predominant pathway.
The isomer distributions within the various PCDF
homologues show a distinct trend toward 2,3,7,8-substitu-
tion. The order of chlorination 2,3,7,8 (ca. 25%) f 1,2,3,7,8
(ca. 34%) f 1,2,3,4,7,8 (ca. 53%) f 1,2,3,4,6,7,8 (ca. 91%)
is clearly observed. The relative contribution of these
isomers within their homologues is given in parentheses.
The increase from 25% for 2,3,7,8-T4CDF to 91% for
1,2,3,4,6,7,8-H7CDF can be explained by the conversion of
structurally related isomers. Within the T4CDF group,
isomers other than 2,3,7,8, e.g., 1,2,3,7 and 1,2,7,8, which
can also be converted to 1,2,3,7,8, are present at percentages
of ca. 10%. Thus, the 1,2,3,7,8-P5CDF isomer is formed at
higher percentages than 2,3,7,8-T4CDF, because it can be
formed from more than one T4 isomer. The electrophilic
2HCl + 1/ 2O2 f H2O + Cl2
Since in experiment 2 with N2 no O2 was present, this
pathway should not be operative. Therefore other chlo-
rination routes must exist. The Deacon reaction may be
partially responsible for the chlorination of 1,2,3,4,7,8-H6-
CDD in experiment 3.
Cold traps were not analyzed separately, and the amount
of 1,2,3,4,7,8-H6CDD evaporating from the fly ash surface
9
8 3 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 3, 1996

TABLE 2
Yields of PCDD Homologues during Dechlorination/Decomposition of 1,2,3,4,7,8- H CDD (in nmol/g of fly ash)a
6
T ) 218 °C
exp 6,
12 min
exp 4,
0 min
exp 5,
6 min
exp 7,
18 min
exp 8,
24 min
exp 9,
30 min
T4CDD
P5CDD
H6CDD
H7CDD
OCDD
0.01 (0)
0.18 (0.02)
2.65 (0.20)
0.02 (0)
0 (0)
0.03 (0.03)
0.31 (0.14)
1.87 (0.44)
0.02 (0.01)
0 (0)
0.06 (0.03)
0.43 (0.04)
1.24 (0.26)
0.01 (0.01)
0 (0)
0.07 (0.01)
0.42 (0.02)
1.12 (0.04)
0.02 (0)
0.05 (0.04)
0.34 (0.05)
1.25 (0.53)
0.02 (0.01)
0 (0)
0.08 (0.02)
0.47 (0.05)
1.04 (0.07)
0.02 (0)
0 (0)
0 (0)
total
2.86 (0.23)
2.23 (0.29)
1.75 (0.28)
1.63 (0.04)
1.66 (0.47)
1.60 (0.09)
T ) 248 °C
exp 12,
12 min
exp 10,
0 min
exp 11,
6 min
exp 13,
18 min
exp 14,
24 min
exp 15,
30 min
T4CDD
P5CDD
H6CDD
H7CDD
OCDD
0.04 (0.01)
0.28 (0.02)
0.82 (0.20)
0.02 (0)
0.05 (0.01)
0.31 (0.05)
0.90 (0.30)
0.02 (0.01)
0 (0)
0.11 (0.04)
0.53 (0.16)
0.81 (0.03)
0.02 (0)
0.15 (0.08)
0.56 (0.06)
0.80 (0.14)
0.01 (0)
0.07 (0.01)
0.39 (0.06)
0.71 (0.15)
0.02 (0)
0.10 (0.02)
0.47 (0.02)
0.45 (0.02)
0.01 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
total
1.16 (0.23)
1.27 (0.23)
1.46 (0.18)
1.52 (0.21)
1.20 (0.16)
1.03 (0.04)
a
All experim ents carried out three tim es; m ean concentration is shown (s) in parentheses; conditions: 1.0 g of fly ash m ixed with 3.19 (s ) 0.37)
nm ol/g 1,2,3,4,7,8-H6CDD; T ) 218 or 248 °C; reaction tim e: 20 m in warm ing up + 0-30 m in reaction; N2 ) 59 ( 3 m L/m in.
is not known. In anycase, our results show that chlorination
ofthe starting compound and possibly desorption is favored
under these conditions rather than dechlorination and
decomposition. Especially in O2 these latter two reactions
are almost negligible. This is corroborated by the homo-
logue distributions found. With ∑T4CDD-OCDD ) 100%,
in N2 86 ( 2% and in O2 94 ( 1% of the starting 1,2,3,4,7,8-
H6CDD is recovered. The amount of H7CDD + OCDD is
13 ( 3% in N2 and 5 ( 1% in O2. T4CDD and P5CDD
congeners are present at <1%.
The isomer distribution found for the H6CDD homologue
shows that isomerization of the starting compound is
limited. With ∑H6CDD ) 100%, the amount of the other
isomers is <1%. Results of the two H7CDD conge-
nerss1,2,3,4,6,7,9 and 1,2,3,4,6,7,8sshow that some dechlo-
rination may play a role in experiment 2 with N2. In O2 the
1,2,3,4,6,7,9 isomer is formed at ca. 2%, compared to ca.
98% for 1,2,3,4,6,7,8. The route of formation is depicted in
Figure 1. Only the latter isomer can be formed directly
from 1,2,3,4,7,8-H6CDD. In N2 the percentage of 1,2,3,4,6,7,9
is higher, ca. 21%and ca. 79%for 1,2,3,4,6,7,8. Isomerization
of 1,2,3,4,7,8-H6CDD, followed by chlorination, might
explain the formation of 1,2,3,4,6,7,9 but is unlikely since
extensive isomerization of the starting compound is not
observed. Dechlorination of OCDD to 1,2,3,4,6,7,9 is a
plausible alternative.
Due to the variance between duplicate experiments with
N2, it is difficult to draw conclusions regarding differences
in mass balance between N2 and O2. Homologue distribu-
tions show that in N2 more chlorination occurs than in O2
and some dechlorination from OCDD to 1,2,3,4,6,7,9-H7-
CDD takes place in N2. This dechlorination is not followed
by a H7CDD f H6CDD step. O2 may adsorb better on the
fly ash surface than the inert N2, thus blocking active sites
of the fly ash for reactions. Hence, a higher percentage of
1,2,3,4,7,8-H6CDD remains intact.
Dechlorination and Decom position of 1,2,3,4,7,8-
H6CDD. Experiments to study the dechlorination and
decomposition were carried out at 218 and 248 °C. These
lower temperatures were chosen to avoid desorption of the
starting compound from the fly ash surface, which is
essential for the study of dechlorination/ decomposition
kinetics and applying first-order kinetics to this process. In
all experiments the desorption of the starting material was
<7%. However, the scatter in the data does not allow for
the calculation of rate constants. Since dechlorination
reactions of OCDD and OCDF proceed quickly in an inert
atmosphere and on a short time scale, (14), we chose N2
as a carrier gas and reaction times of 0-30 min.
Results of our experiments (4-15) are shown in Table
2. Both at 218 and 248 °C dechlorination/ decomposition
of the starting compounds occurs. As only the T4CDD-
OCDD homologues were measured, the mono-trichloro-
dibenzodioxins escape detection. The 1,2,3,4,7,8-H6CDD
lacking in the mass balance can either be present as DD
and mono-trichlorodibenzodioxin or be decomposed.
These processes also take place during the 20 min needed
for the sample basket to reach the set point temperature,
and therefore the mass balance [calculated by (∑T4CDD-
OCDD/ 1,2,3,4,7,8-H6CDD before experiment) × 100%] is
<100% at t ) 0 min. At 218 °C the mass balance decreases
from ca. 90% at t ) 0 min to ca. 50% at t ) 30 min. At 248
°C there is no significant decrease, due to the variance
between the triplicate experiments, and the mass balance
fluctuates between 30 and 50%.
Dechlorination of the starting compound to T4CDD-
P5CDD congeners occurs readily, and after 30 min at 218
°C (with ∑T4CDD-OCDD ) 100%), ca. 65% is 1,2,3,4,7,8-
H6CDD, ca. 29% is P5CDD, and ca. 5% is T4CDD. Chlo-
rination hardly takes place, and H7CDD is only 1%. No
PCDF were detected in any of these experiments. At 248
°C after 30 min, ca. 55% of ∑T4CDD-OCDD is present as
T4CDD or P5CDD.
In Figure 1, the dechlorination route of 1,2,3,4,7,8-H6-
CDD is shown. Again no isomerization of the starting
compound is observed. Both the 1,2,4,7,8 (loss of Cl at the
3-position) and the 1,2,3,7,8 isomer (loss of Cl at the
4-position) are formed in a ratio of 30-40% and 60-70%,
indicating that loss of Cl at the 4-position is preferred. Other
isomers formed have a negligible contribution. The isomer
patterns do not depend on the reaction time and temper-
ature. Of the T4CDD isomers, only the 1,3,7,8, 2,3,7,8, and
9
VOL. 30, NO. 3, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 8 3 5

1,2,7,8 are formed in a ratio of 25-35%:35-65%:0-20%.
These percentages do change with reaction time or tem-
perature, but without clear trend. Van Berkel et al. found
that 2,3,7,8-substituted congeners are the most stable on
fly ash under dechlorination and destruction conditions
(15). This is also seen in our experiments as the 2,3,7,8 and
1,2,3,7,8 isomers are the most important within the T4CDD
and P5CDD homologues, respectively. Furthermore, the
three T4CDD isomers formed all have two Cl atoms in each
ring rather than [3+1] or [4+0]. Similar results were found
by Hagenmaier and co-workers for the dechlorination of
OCDD on copper-powder: Cl loss occurs in the most
substituted ring, yielding congeners with equal numbers
of Cl atoms in each ring (16).
Other groups have reported on PCDD formation from
unchlorinated starting material on fly ash without HCl, e.g.,
from carbon (4). The chlorination of the DD structure must
in these cases occur at an early stage ofthe reaction, likewise
before oxidative breakdown of the carbon macromolecule.
Acknowledgments
We would like to thank Dr. M. H. Schoonenboom for reading
the manuscript and P. Serne´ for technical assistance.
Support of this work by the Technology Foundation
(Stichting voor de Technische Wetenschappen), Utrecht,
The Netherlands, under Grant ACH03.2183 is gratefully
acknowledged.
Literature Cited
Conclusions
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follows an electrophilic aromatic substitution mechanism.
DF appears not to be an intermediate in PCDF formation
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(2) Shaub, W. M.; Tsang, W. Environ. Sci. Technol. 1983, 17 (12),
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place both in N2 and O2. This shows that the Deacon
reaction cannot be the exclusive chlorination pathway on
fly ash, since this reaction requires the presence of oxygen.
In fact, in O2, less chlorination is observed than in N2, which
can be explained by assuming a better adsorption of O2 on
the fly ash surface, thus reducing the number of active sites
available for reactions. Dechlorination and decomposition
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(12) Luijk, R. Ph.D. Thesis, University of Amsterdam, Amsterdam,
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Without the presence of HCl, 1,2,3,4,7,8-H6CDD readily
dechlorinates or decomposes. Only a limited set of T4CDD
and P5CDD isomers is formed through stepwise dechlo-
rination. No chlorination occurs under these circum-
stances.
Our experiments show that chlorination and dechlori-
nation are separate processes. PCDD present on fly ash
will dechlorinate or decompose if no strong gaseous
chlorinating agent is present. The flyash itselfis not capable
of chlorinating PCDD. On the other hand, if such an agent
is present, dechlorination should make a negligible con-
tribution.
(13) Griffin, R. D. Chemosphere 1986, 15 (9-12), 1987-1990.
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Received for review April 18, 1995. Revised manuscript re-
ceived September 27, 1995. Accepted November 1, 1995.X
ES950267B
X Abstract published in Advance ACS Abstracts, January 15, 1996.
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