The formation of polychlorinated dibenzo-p-dioxins/dibenzofurans from carbon model mixtures containing ferrous chloride

Article abstract of DOI:10.1016/S0045-6535(99)00346-X

The potential to form polychlorinated dibenzo-p-dioxins/furans (PCDD/F) was investigated in carbon model systems containing ferrous chloride tetrahydrate and a matrix representative of that found in particle emission from the catalytic extraction process (CEP) for wastes. Various types of carbons were used resulting in different PCDD/F yields but, with one exception, similar homologue distributions. Due to the similarity between the turbostratic structure of the carbon in the representative CEP dusts and the carbon blacks used in the model system, experiments were performed using two carbon blacks (termed CBA and CBB). On a mass basis, CBB was more reactive over the temperature range of 275-325°C and reaction times of 20-60 min in the formation of PCDD/F; as well as more adsorptive in terms of the desorption of PCDD/F. On a volume basis, the reactivities and adsorptivities were similar. A maximum in PCDD/F formation occurred at an oxygen concentration of 2% in nitrogen. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0045-6535(99)00346-X

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Chemosphere 40 (2000) 1009±1014
The formation of polychlorinated dibenzo-p-dioxins/
dibenzofurans from carbon model mixtures containing ferrous
chloride
*
Shawn P. Ryan , Elmar R. Altwicker
Howard P. Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA
Abstract
The potential to form polychlorinated dibenzo-p-dioxins/furans (PCDD/F) was investigated in carbon model sys-
tems containing ferrous chloride tetrahydrate and a matrix representative of that found in particle emission from the
catalytic extraction process (CEP) for wastes. Various types of carbons were used resulting in di€erent PCDD/F yields
but, with one exception, similar homologue distributions. Due to the similarity between the turbostratic structure of the
carbon in the representative CEP dusts and the carbon blacks used in the model system, experiments were performed
using two carbon blacks (termed CBA and CBB). On a mass basis, CBB was more reactive over the temperature range
of 275±325°C and reaction times of 20±60 min in the formation of PCDD/F; as well as more adsorptive in terms of the
desorption of PCDD/F. On a volume basis, the reactivities and adsorptivities were similar. A maximum in PCDD/F
formation occurred at an oxygen concentration of 2% in nitrogen. Ó 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Carbon block; De novo synthesis; Iron chloride; PCDD/F
1. Introduction
A laboratory study using model compounds based on
ferrous chloride tetrahydrate was initiated. In order to
produce measurable quantities of PCDD/F from these
Processes other than combustion/incineration and
metals other than copper have the potential for the
formation of polychlorinated dibenzo-p-dioxins/furans
(PCDD/F). Such processes include a variety of metal-
lurgical and related processes (many of these operate
under conditions of a low oxygen concentration in the
gas phase) and metals such as iron and its compounds.
In one such process, the so-called catalytic extraction
process (CEP), a molten metal bath is used to transform
wastes into potentially useful products (Nagle et al.,
1996). This process typically operates under oxygen
3
model mixtures, oxygen partial pressures of 10 atm or
greater had to be used.
2. Materials and methods
Five carbons were obtained from various sources:
activated (100 mesh, Aldrich), graphic powder (Aldrich),
two carbon blacks (Cabot) and Spherocarb (100 mesh,
Phase Separations). These carbons were Soxhlet ex-
tracted in toluene for 24 h, vacuum dried and used in
model mixtures containing ferrous chloride tetrahydrate
(99.995%, Aldrich) and a support matrix of aluminum,
calcium and silicon oxides (12%, 48% and 40%, respec-
tively) (Al:Ca:SiOx powder). No additional information
pertaining to the Al:Ca:SiOx powder is currently avail-
able. The ferrous chloride tetrahydrate crystals were
15
9
partial pressures between 10
and 10 atm. Fine
particles released contain iron compounds, carbon, and
inorganic salts, including oxides. The nature of the
carbons depends somewhat on the reactor conditions.
* Corresponding author.
0045-6535/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 ( 9 9 ) 0 0 3 4 6 - X

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S.P. Ryan, E.R. Altwicker / Chemosphere 40 (2000) 1009±1014
ground into a ®ne powder under nitrogen in a glove box
due to its hygroscopic nature. This ferrous chloride was
physically mixed with the carbon and Al:Ca:SiOx pow-
der in speci®c concentrations, as noted in Section 3. The
sample mixture was then placed in an inverted glass
thimble on a coarse glass frit and held in place with a
glass wool plug. The reactor, a 152.4 cm long Pyrex tube
of 3 cm ID, was heated using three ceramic heating el-
ements while 0.9 l/min of nitrogen was passed through.
The temperature at the center of the tube was controlled
by three temperature controllers located within each
heating section. When the reactor was within 50°C of the
desired reaction temperature, the thimble with the
sample mixture was inserted at the top of the reactor and
heated to the desired reaction temperature. This heat-up
time was kept constant at 30 min. Thereafter, the air
¯ow corresponding to the desired concentration of
oxygen was added to the ¯ow of nitrogen; thus bringing
the total ¯ow through the reactor to 1 l/min. The outlet
gas of the reactor exited through a side arm at the top to
an ice cooled gas trap containing 250±300 ml of di-
chloromethane.
The gas phase and solid phase samples were puri®ed
separately by fractionation on alumina, as described
elsewhere (Addink and Altwicker, 1997). A sulfuric acid
wash of the samples was done prior to this procedure to
remove any inorganic ferrous compounds present. The
T4CDD-O8CDD and T4CDF-O8CDF were quanti®ed
via gas chromatography/mass selective detection.
3. Results and discussion
Initial experiments were performed with samples
consisting of 21% ferrous chloride tetrahydrate on the
support matrix and ®ve di€erent carbons (cf. Fig. 1).
With the exception of Spherocarb, O8CDF was the
dominant product under these experimental conditions.
For the two carbon blacks (labeled CBA and CBB) and
activated carbon, O8CDF accounted for approximately
85% of the total T4-O8CDD/F formed; the remainder
being H7CDF and H7- and O8CDD. A slightly greater
amount of H6- and H7CDF was formed with graphitic
carbon, however, O8CDF was still the predominate
congener. The formation of moderately chlorinated
PCDF (P5-H7) was favored when Spherocarb was used
as the carbon source. However, the [PCDD]:[PCDF]
ratio of 0.05 was in agreement with the other carbon
sources used.
After the desired reaction time, the thimble was
removed from the reactor and the sample was spiked
with an isotopically labeled PCDD/F internal standard
(CIL) and Soxhlet extracted for 24 h with toluene. This
toluene extract was then labeled the solid phase sam-
ple. The dichloromethane in the gas trap was also
spiked with internal standard and labeled the gas phase
sample.
While the congener distribution appeared to be in-
dependent of the carbon source, the amounts of PCDD/
F produced were not. Activated carbon was the most
Fig. 1. Comparison of PCDD/F Congener yields as a function of carbon source.

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S.P. Ryan, E.R. Altwicker / Chemosphere 40 (2000) 1009±1014
1011
reactive of these carbons by a factor of 1.85 over gra-
phitic carbon. The reactivities of CBB and Spherocarb
were very similar, while carbon black A exhibited little
reactivity. These results indicate that the morphology of
the carbon could be important in the formation of
PCDD/F.
observed with increasing ferrous chloride concentration
between 1% and 21% at 275°C. Increasing the concen-
tration to 33% at 300°C had no further e€ect on the
PCDD/F concentration. In the absence of ferrous
chloride, no PCDD/F formation occurred in this system,
suggesting that ferrous chloride is both a catalyst and
chlorinating agent. The congener distribution was not a
function of ferrous chloride concentration, nor was the
[PCDD]:[PCDF] or the desorption of PCDD/F.
The relative abundance of O8CDF produced in these
experiments suggests that ferrous chloride can pre-
chlorinate carbon, as suggested by the investigations of
(Stieglitz et al., 1996). The occurrence of congeners other
than octa-, namely hepta- and hexa-PCDD/F, can be
explained by dechlorination of O8CDD/F on the 12%
alumina support (Schoonenboom et al., 1995). The ab-
sence of lower chlorinated congeners such as T4CDD/F
can then be explained by destruction on the calcium
oxide support (Gullett et al., 1997).
The sponsor of this work reported that the processing
of some wastes yields carbons that resemble the
turbostratic structure of carbon blacks in certain ways.
Hence, the two carbon blacks were selected for further
experiments. The e€ect of varying the reaction temper-
ature on the de novo synthesis of PCDD/F with CBA
and CBB in a model system at a reaction time of 60 min
is summarized in Fig. 2. While a peak in solid phase
PCDD/F was observed at ca. 300°C with CBA, no dis-
tinct peak occurred up to 325°C for CBB. A phase shift
The e€ect of the ferrous chloride concentration was
investigated using activated carbon. A linear increase in
the formation of PCDD/F on the model mixture was
Fig. 2. Solid (SP) and gas (GP) PCDD/F yields as a function of temperature with CBA and CBB.
Fig. 3. Solid (SP) and gas (GP) Phase PCDD/F yield as a function of time with CBA and CBB.

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S.P. Ryan, E.R. Altwicker / Chemosphere 40 (2000) 1009±1014
occurred between 300°C and 325°C with CBA, indicat-
ing that desorption becomes important above this tem-
perature. Although the gas phase to solid phase ratio did
increase with increasing temperature for CBB, the ratio
climbed no higher than 0.26. This indicates that the
adsorptivity of CBB is greater than that of CBA, thus
limiting the role of desorption with CBB. The greater
overall reactivity of CBB over CBA is apparent.
CBA is observed at 60 min. Since the solid phase yield of
PCDD/F remained relatively constant for all times
studied, destruction of the gas phase PCDD/F became
important after 60 min at 325°C, while the desorption
rate increased up until this time. With CBB, no optimum
occurred within 90 min. In both the temperature and
time study, the PCDD/F congener patterns were unaf-
fected. For the data in Fig. 3, the model system con-
sisting of one gram samples of 8 wt% CBB or CBA, 21
wt% ferrous chloride tetrahydrate and the remainder
A comparison of the e€ect of reaction time at 325°C
is presented in Fig. 3. A peak in overall formation with
Fig. 4. Rastering Electron Microscopy (REM) of (a) CBA and (b) CBB surfaces.
Table 1
Comparison of PCDD/F yield on a per ml carbon basis
PCDD/F (ng/g sample)
PCDD/F (ng/ml Carbon)
R Â n Temp (°C)
CBA
CBB
CBA
CBB
275
300
325
26
110
363
57
544
650
221
935
249
2380
2844
3086
Fig. 5. E€ect of CBB Concentration on PCDD/F yield with ferrous chloride.