ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26
1119
2010 © The Japan Society for Analytical Chemistry
The Best Paper in Bunseki Kagaku, 2009
Chlorination Mechanism of Carbon during Dioxin Formation
Using Cl-K Near-edge X-ray-absorption Fine Structure
Takashi FUJIMORI,† Yuta TANINO, Masaki TAKAOKA, and Shinsuke MORISAWA
Department of Urban and Environmental Engineering, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo, Kyoto 615–8540, Japan
Many environmental organic chemicals have chloride in their structure. Thus, researching the chlorination mechanism of
carbon is of interest. Dioxins are typically concentrated in fly ash collected from the post-combustion zone during the
operation of municipal solid waste incinerators. In this study, we report the application of Cl-K near-edge X-rayabsorption fine structure (NEXAFS) in determining the chlorination mechanism of carbon in fly ash. The separation of
a chloride–carbon (C–Cl) bond was readily recognizable as a peak in the Cl-K NEXAFS spectrum. Chlorination effects
could be estimated using Cl K-edge NEXAFS with no dependence on metal species. Analysis of Cl K-edge NEXAFS
spectra showed the reduction of copper(II) chloride at 300°
C and oxidation of iron(III) chloride at 400°
C in connection
with the chlorination of carbon.
Introduction
Environmental organic chemicals that persist in the atmosphere,
soil, and water environment have attracted worldwide attention.
Trace environmental organic chemicals often have chlorine in
their chemical structure, such as dioxins, which can be generated
by thermal processes such as in municipal solid waste
incinerators (MSWIs). To decrease the discharge of dioxins to
environment as much as possible, we need to understand the
formation mechanism(s) of dioxins in thermal processes, so as
to be able to take countermeasures. Some chlorine-focused
studies have been conducted because dioxin formation was
thought to involve the chlorination of carbon.
Some studies have tried to determine the formation path of
dioxins from gaseous HCl, analyzing distribution patterns in
depth from the viewpoint of chlorine number bonded with
aromatic carbon,1 calculation by simulation of the influence of
the Cl radical (Cl•) in the gas phase,2 discussions about C–Cl
bonds in carbon source as origin of dioxins structure by means
of 37Cl isotopic labeling,3 and stoichiometric explanations of the
chlorination of carbon by cupric chloride, measuring the amount
of organic chlorine, represented as C–Cl.4 Although many such
studies have concluded that a relationship exists between the
amount of chlorine source as input and the amount of dioxins as
output, the behavior of the Cl atom has not been studied directly.
Thus, the chlorination mechanism of the carbon structure itself
has remained unknown.
To whom correspondence should be addressed.
Present address: Research Center for Material Cycles and Waste
Management, National Institute for Environmental Studies, 16-2
Onogawa, Tsukuba, Ibaraki 305–8506, Japan.
E-mail: [email protected]
This is an English edition of the paper which won the Best Paper
Award in Bunseki Kagaku, 2009 [Bunseki Kagaku, 2009, 58(4),
221].
†
Cl-K near-edge X-ray-absorption fine structure (NEXAFS)5
can provide chemical information about the environment
surrounding a Cl atom in samples with relatively low Cl content
and of complex composition. Myneni showed changes in
Cl-bonding elements, measuring parts of plants and many soils
using Cl K-edge NEXAFS to describe the fate of Cl in the
environment.6 Another study researched the chemical form of
Cl in ash collected from the post-combustion zone of MSWIs
(fly ash).7 However, few recent studies have used Cl K-edge
NEXAFS.
Cl K-edge NEXAFS has the potential to provide direct
evidence of the chlorination of carbon in terms of changes in the
environment surrounding the Cl atom. To our knowledge, no
previous report has examined the behavior of Cl during dioxin
formation in fly ash in MSWIs using Cl K-edge NEXAFS. In
the present study, we sought to determine the mechanism of the
chlorination of carbon in fly ash by describing the environment
surrounding the Cl atom using Cl K-edge NEXAFS. Specifically,
we measured the amount of dioxins and other aromatic chlorides
in fly ash collected from a MSWI using Cl K-edge NEXAFS.
Preparing model samples using the actual fly ash results, we
studied chlorination mechanisms of carbon with heavy metal
chlorides using Cl K-edge NEXAFS.
Experimental
Samples
We used real fly ash collected from a municipal solid waste
incinerator and various model fly ashes. We selected real fly
ash that had not been sprayed with activated carbon (to reduce
NOx, Hg, and dioxins) or lime hydrate (to absorb HCl and SOx)
because we wanted to measure the amount of dioxins without
any additives. Elements in the real fly ash were 1.5% carbon
(detected by a total organic carbon analyzer; TOC-V, Shimadzu),
15% chlorine (measured by X-ray fluorescence analysis;
XRF-1700, Shimadzu), and 0.3% Cu, 0.5% Fe, 1.4% Zn, 6.7%
K, 6.0% Na, and 10% Ca (analyzed by inductively coupled
1120
plasma spectroscopy; ICPS-8000, Shimadzu).
The model fly ashes were powders, prepared by grinding a
mixture of activated carbon as the carbon source, inorganic
chloride, heavy metal chloride, and base material for ca. 10 min
in a mortar. Quantitative measurement of dioxins and Cl K-edge
NEXAFS analysis were made with different model fly ashes.
In preparing the model fly ashes, activated carbon (granular
Shirasagi, C2C 20/48, palm shell base, Takeda Pharmaceutical)
was preheated at 500°
C for 60 min under a nitrogen stream
(100 mL/min) to decompose and eliminate organics adsorbed in
the activated carbon. We selected three metal chlorides,
copper(II) chloride (CuCl2·2H2O, 97 – 98%), iron(III) chloride
(FeCl3, 97%), and zinc(II) chloride (ZnCl2, 98.0%); they were
purchased from Nacalai Tesque Inc. Copper(II) and iron(III)
chloride have been reported to promote the generation of dioxins
in fly ash.8,9 Zinc(II) chloride was also thought to have potential
to promote dioxin formation because the concentration of zinc
correlated with that of dioxins in fly ash.10
Measurement of chlorinated aromatic compounds
We measured polychlorinated dibenzo-p-dioxins (PCDDs),
furans (PCDFs), and biphenyls (PCBs) as representative dioxins
that are toxic and highly concentrated in MSWI fly ash.
Additionally, we measured chlorobenzenes (CBzs), which are
known dioxin precursors as well as being toxic themselves.
Here, model fly ash was used for the measurement of chlorinated
aromatic compounds. We measured dioxins in real fly ash and
model fly ashes to evaluate the influence of the metal chlorides.
In real fly ash, PCDDs, PCDFs, PCBs, and CBzs were measured.
The content of PCDDs and PCDFs correlated with that of PCBs
and CBzs (Table 1). Thus, in model fly ash, we quantified only
PCBs and CBzs as representative chlorinated aromatic
compounds. All model fly ash samples contained activated
carbon (3%) and silicon dioxide as a matrix (SiO2, special
grade; Nacalai Tesque, Inc.). We mixed potassium chloride
(KCl) or sodium chloride (NaCl) at 10% Cl as the source of
inorganic chlorine. We used five types of model fly ash, which
contained, in addition to activated carbon and SiO2, the
following: KCl, NaCl, CuCl2 (0.2% Cu) + KCl, FeCl3 (0.5% Fe)
+ KCl, and ZnCl2 (2.0% Zn) + KCl, referred to as [K], [Na],
[Cu], [Fe], and [Zn] fly ashes, respectively. In the cases of
[Cu], [Fe], and [Zn], KCl was added as a chlorine source.
Concentrations of metal chlorides reflected those in real fly ash
because we wanted to examine the influence of trace metals on
dioxin formation. We measured chlorinated organic compounds
in residual ash and in toluene extracts after heating at 300 or
400°
C for 30 min under 10% O2 (N2 balance, 50 mL/min) by
gas chromatograph/mass spectrometry (GC/MS). Details of the
heating experimental procedure and analyses were described
previously.10
Cl K-edge NEXAFS
To understand the chemical state of chlorine in fly ash,
NEXAFS spectra were measured between 2810 and 2860 eV,
near the Cl K edge (2820 eV), using beamlines BL-11B and
BL-9A at the Photon Factory (PF), Tsukuba, Japan. BL-11B
required keeping a vacuum at the sample point. In contrast,
BL-9A did not need a vacuum, and we measured NEXAFS
under atmospheric pressure, filled with He gas. Thus, we
measured deliquescent and sublimation samples at BL-9A.
Powdery samples were applied to carbon tape (Nisshin EM
Corp.) and used for measurement of Cl K-edge NEXAFS by
total electron yield (TEY) at BL-11B and conversion electron
yield (CEY) at BL-9A. We corrected the energy position of
Cl K-edge NEXAFS using the absorption edge (maximum-peak
ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26
Table 1 Concentrations of chlorinated organic compounds in
fly ash (ng/g)
rta
300°
C
400°
C
CBzs
D2
T3
T4
P5
H6
Total
19
11
32
34
28
120
320
880
2200
2300
870
6600
5200
4300
4400
3100
1200
18000
PCBs
D2
T3
T4
P5
H6
H7
O8
Total
1.3
1.7
2.6
1.6
1.3
0.9
0.7
10
12
15
21
19
16
11
7.8
100
670
650
340
220
110
44
24
2100
PCDDs
T4
P5
H6
H7
O8
Total
2.3
3.0
5.3
6.7
10
27
190
240
390
160
76
1100
570
410
440
180
110
1700
PCDFs
T4
P5
H6
H7
O8
Total
17
16
13
8.1
2.7
57
71
110
120
69
26
400
420
390
290
130
31
1300
a. Room temperature.
D2 – O8: Chlorine number of each chlorinated organic compounds.
position) of KCl at 2822.8 eV.6
We prepared pre- and post-heated real fly ash and model fly
ash. We sealed real fly ash samples at room temperature (rt)
and 300°
C for 30 min under 10% O2 (50 mL/min) in reagent
bottles and submitted them to the Photon Factory for analyses.
The model fly ashes here contained activated carbon (5%) and
boron nitride as the matrix (BN, special grade; Wako Pure
Chemical Industries, Ltd.). CuCl2·2H2O, FeCl3, and ZnCl2 were
mixed in each model fly ash at 2.1% Cl, 3.8% Cl, and 2.6% Cl,
respectively (we did not mix inorganic chloride). The model fly
ashes are referred to as {Cu}, {Fe}, and {Zn} fly ashes. When
we selected SiO2 as the base matrix for Cl K-edge NEXAFS
measurements, the baseline of the spectrum rose and negatively
affected the quality of spectra. Si K-edge absorption was
thought to be the cause. The K edge derived from Si was at
1838 eV, a lower energy position than 2820 eV of the Cl Kedge. So, here, we used inert BN as the base matrix. We
previously confirmed there was no significant difference in the
formation of dioxins or distribution patterns of PCDDs and
PCDFs between BN and SiO2.11 KCl was not mixed in these
model fly ashes, because the Cl K-edge NEXAFS spectrum of
inorganic chloride in real fly ash hardly changed, and we wanted
to examine only the behavior of chlorine with the metal. We
also measured these model fly ashes that were heated at 300 and
400°
C and then immediately sealed.
We measured Cl K-edge NEXAFS spectra of organic and
inorganic compounds having chlorine in their structures as
standards to assess the chemical state of chlorine. The selected
organic compounds were polyvinyl chloride (PVC) as an
aliphatic carbon and three types of chlorobenzene (1,3,5-,
1,2,4,5-: Nacalai Tesque, Inc., penta-: Tokyo Chemical Industry
ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26
1121
Fig. 1 Cl K-edge NEXAFS spectra of standard Cl compounds in
inorganic and organic molecules.
CP, Chlorophenol; CBz,
chlorobenzene; PVC, polyvinyl chloride.
Fig. 2 Cl K-edge NEXAFS spectra of real fly ash at room
temperature (rt) and heated at 300°
C for 30 min.
Co., Ltd.) and six types of chlorophenol (2,3-, 2,4-, 2,6-, 3,4-:
Nacalai Tesque Inc., 2,3,6-, 2,3,4,6-: Tokyo Chemical Industry
Co., Ltd.) as example aromatic carbons. We measured two
copper chlorides, two iron chlorides, one zinc chloride, and two
inorganic chlorides (NaCl and KCl) as inorganic compounds.
Cl K-edge NEXAFS spectra of the major standard compounds
are shown in Fig. 1. Unknown NEXAFS spectra were fitted by
a linear combination of known NEXAFS spectra of standard
compounds using analytical software (REX2000, Ver. 2.5.5;
Rigaku Corp.). Then, we calculated the existence ratio of the Cl
chemical form in the unknown fly ashes. Criteria for linear
combination fitting depended on the fitting error factor,
R = Σ(Xm – Xc)2/ΣXc2 (Xm, measured NEXAFS spectrum; Xc,
calculated NEXAFS spectrum), plus the value of the fitting
weight and whether the chemical form of Cl seemed plausible.
Results and Discussion
Quantity of dioxins and chemical state of chlorine in real fly ash
Compared with the concentrations of dioxins in the real fly
ash at room temperature, concentrations after heating to 300 or
400°
C were higher by one or two orders of magnitude. We
thought that this high concentration of dioxins was generated as
a result of reactions between unburned carbon and chlorine
sources in the fly ash along with oxygen gas in the surrounding
gas phase, triggered by the reheating. Table 1 shows the
concentrations of CBzs, PCBs, PCDDs, and PCDFs with
homologs in the real fly ash at each temperature. The formation
concentrations of dioxins were highest at 400°
C among the
three temperature conditions examined. It has been reported
C was the temperature window of maximum
that 300 – 400°
dioxins formation in fly ash.12 The real fly ash used here
showed this same maximum temperature window. The order of
concentrations of chlorinated aromatic compounds was
CBzs > PCDDs, PCDFs > PCBs, consistent with a previous
study.13
We studied the chemical form of chlorine in the real fly ash
because the chlorine source was thought to be a key factor in
generating dioxins by reheating. Figure 2 shows the Cl K-edge
NEXAFS spectra from real fly ash. The spectrum shape at
room temperature (rt) was hardly changed after heating to
300°
C and had major features of the two standard compounds,
NaCl (solid line in Fig. 2) and KCl (dashed line). Linearly
combining standard NEXAFS spectra in comparison with the
unknown spectrum, we identified the chemical form of chlorine
in the unknown real fly ash. All standard compounds described
in the Experimental part were used for linear combination fitting
because real fly ash has many possible Cl forms. We found that
the chemical forms of chlorine were primarily KCl (ca. 30%)
and NaCl (ca. 60%). Thus, we concluded that inorganic
chlorides were the great majority of Cl forms in the real fly ash.
Apart from NaCl and KCl, CaCl2 has been reported to exist in
similar fly ash, without any spraying of additive chemicals.7
However, CaCl2 was not identified in the present study. K and
Na levels in the real fly ash of this study were more than twice
the amounts in the real fly ash of a previous study, and the Ca
level was less than half. Although CaCl2 might be present, we
believe that any low concentration of Ca was not detected by
NEXAFS measurements because of the high level of chlorine
bonding with K and Na. It is also possible that Ca was present
in a chemical form not bonded with Cl.
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ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26
Table 2 Concentrations of chlorobenzenes and PCBs in model fly ashes (ng/g)
[Na]
300°
C 400°
C
CBzs
PCBs
[K]
300°
C
400°
C
[Cu]
300°
C 400°
C
[Fe]
300°
C 400°
C
[Zn]
300°
C 400°
C
D2
T3
T4
P5
H6
Total
16
0.80
0.27
0.82
1.1
19
18
2.2
1.2
1.0
0.67
23
22
1.0
0.33
0.32
1.1
25
39
5.9
4.9
4.8
4.0
58
840
1200
6700
5800
2900
18000
280
850
2400
2300
3600
9400
360
1300
4100
5600
3900
15000
840
4000
14000
6900
2500
28000
280
120
98
150
170
820
1700
1200
480
120
11
3500
Residue
Impinger
Impinger/Residue
14
5.1
0.35
11
12
1.0
20
4.5
0.22
46
12
0.25
15000
2900
0.19
3900
5500
1.4
12000
3200
0.27
3600
25000
6.9
370
450
1.2
550
3000
5.5
2.2
6.8
8.8
2.0
1.9
nd
nd
22
0.34
5.6
6.2
0.97
1.5
nd
nd
15
0.94
7.4
7.5
1.2
1.8
nd
nd
19
1.8
30
29
4.1
2.1
nd
nd
67
5.5
41
55
128
176
318
427
1200
5.2
4.0
8.3
6.9
6.9
17
32
80
12
35
36
14
33
36
49
210
11
24
22
32
81
120
110
400
3.2
5.7
4.5
1.5
0.27
0.71
0.015
16
3.0
13
11
4.7
4.0
0.14
nd
36
19
2.6
0.14
14
0.70
0.050
18
0.77
0.043
65
2.5
0.038
1150
4
0.0030
69
11
0.16
210
4
0.019
338
57
0.17
14
2
0.11
31
4
0.14
D2
T3
T4
P5
H6
H7
O8
Total
Residue
Impinger
Impinger/Residue
nd: Not detected.
Comparison of formation concentrations among heavy metal
chlorides
The results of measurement of the real fly ash revealed that
chlorine existed largely in inorganic forms. We measured
dioxins in [Na] and [K] model fly ashes admixed with only
NaCl and KCl, respectively, after heating; 300 and 400°
C-heated
[Na] showed 19 and 23 ng/g for CBzs and 22 and 15 ng/g for
PCBs, which were several orders of magnitude lower than CBzs
(6600 and 18000 ng/g) and PCBs (100 and 2100 ng/g) in the
real fly ash. Thus, it appeared that most of the chlorine in the
real fly ash had a low potential to promote dioxin formation.14
[K] model fly ash after heating at 300 and 400°
C also showed
much lower concentrations of CBzs (25 and 58 ng/g) and PCBs
(19 and 67 ng/g) than were present in the real fly ash.
Concentrations of CBzs and PCBs in [K] model fly ash heated
at 400°
C were 2 and 4.5 fold higher than that in [Na] model fly
ash at 400°
C. Thus, we concluded that KCl also had a low
potential to promote dioxin formation. As explained above, not
only inorganic chlorine, such as KCl and NaCl, but also other
causative agents need to be considered as the chlorine source
involved in generating dioxin in fly ash.
Heavy metal chlorides have been thought to be causative
chlorine sources in promoting the formation of dioxins. We
prepared [Cu], [Fe], and [Zn] model fly ashes admixed with
representative metal chlorides, copper, iron, and zinc, in fly ash,
respectively. Additional amounts of metals were added to
reflect the amount of each heavy metal in the original real fly
ash, as described in the Experimental section. When heated to
300 and 400°
C, these model fly ashes showed concentrations of
chlorinated aromatic compounds that were several orders of
magnitude higher, compared with the case of adding inorganic
chloride such as KCl and NaCl. Indeed, Table 2 shows that at
300°
C, the order of concentrations of CBzs and PCBs was [Cu]
(CBzs, 18000; PCBs, 1200 ng/g) > [Fe] (15000, 210) > real fly
ash (6600, 100) > [Zn] (820, 16) > [K] (25, 19) = [Na] (19, 22).
Furthermore, at 400°
C, the order was [Fe] (28000 ng/g) > real
fly ash (18000) > [Cu] (9400) > [Zn] (3500) > [K] (58) > [Na]
(23) for CBzs and real fly ash (2100 ng/g) > [Fe] (400) >> [Cu]
(80) > [K] (67) > [Zn] (36) > [Na] (15) for PCBs. These data
indicate that copper and iron chlorides were strong factors
promoting the formation of dioxins because the concentrations
in heated [Cu] and [Fe] model fly ashes were higher than those
of heated real fly ash. Although zinc chloride was also thought
to have the potential to promote the formation of dioxins, we
found that the promoting effect by zinc chloride was less than
that of copper or iron chloride in real fly ash. The temperatures
of maximum concentrations of CBzs and PCBs were 300°
C in
[Cu] and 400°
C in [Fe] and [Zn] model fly ashes. Thus, the
temperature of maximum generation of dioxins in real fly ash
would be expected to be related to the difference in the
maximum temperatures of CBzs and PCBs generation,
depending on the metal chlorides. The ratio of organic
component trapped in toluene in the impinger to that in the
residue after heating (impinger/residue) tended to increase from
300 to 400°
C. One reason for this may be that volatile amounts
of CBzs and PCBs increased with rising temperature. However,
CBzs and PCBs trapped on the impinger in the case of [Fe]
model fly ash after heating at 400°
C generated higher
concentrations than the other conditions. It is possible that there
is a gas-phase formation path for dioxins after volatilization of
organic components from the solid phase.
Cl K-edge NEXAFS of organics and inorganics
Here, we explain the difference of spectra among the
chlorinated compounds shown in Fig. 1 before discussing
results of the chemical forms in the model fly ashes by analysis
of Cl K-edge NEXAFS. Focusing on the energy position of the
maximum peak, this position was different depending on
whether chlorine was bonded with organic or inorganic
compound. Figure 3 shows the grouping of maximum peak
positions for inorganic chlorine (inorg Cl) and chlorine bonded
with aromatic and aliphatic carbon (aromatic Cl and aliphatic
Cl, respectively). Maximum peaks of inorg Cl had energy
intervals due to the dependence on oxidation number. Compared
ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26
Fig. 3 Energy of Cl K-edge NEXAFS maximum peak.
1123
●, Average.
with inorg Cl (n = 7), the lower energy position of the maximum
peak derived for aromatic Cl (n = 9) was at 2821.1 ± 0.1 eV.
We also distinguished aliphatic Cl (only PVC) from the others
because of its lower maximum peak position, 2820.4 eV. This
feature of Cl K-edge NEXAFS was confirmed in a previous
study.6 When compared with inorg Cl, Cl bonded with carbon
atoms in organic compounds indicated lower energy maximum
peaks, corresponding to the 1s → π* and σ* electronic
transitions of the C–Cl bonds. Thus, in principle, the required
transition energy of weak-binding electrons in Cl connected to
organic carbon was lower than that of binding electrons in Cl
connected to inorganic elements. Cl K-edge NEXAFS is a
simple and useful analytical method to distinguish whether the
Cl is organic (i.e., aromatic and aliphatic) or inorganic depending
on the maximum-peak position. We conclude that Cl K-edge
NEXAFS spectra were useful for directly revealing “carbon–
chlorine” bonds (C–Cl bonds) as found in the structures of dioxins.
Evidence of C–Cl bonds
We measured Cl K-edge NEXAFS of the model fly ashes to
examine the behavior of Cl. Measured spectra are shown in
Fig. 4. Dashed lines denoted by “a” and “b” in Fig. 4 indicate
maximum peak positions derived from Cl connected to aliphatic
and aromatic carbon, respectively. We could clearly judge the
visible peak derived from aromatic Cl in {Cu} model fly ash
after heating at 300°
C and {Fe} model fly ash at 400°
C (asterisk,
*, in Fig. 4). From these two spectra, we directly observed the
C–Cl bond, which was generated in these model fly ashes.
Additionally, this indicated the occurrence of direct chlorination
of carbon by the Cl bonded with Cu and Fe because no KCl was
added as a chlorine source in these model fly ashes. The
absence of any additional effect of direct chlorination by Cu and
Fe was thought to be one reason for the low contribution by
[Na] and [K] model fly ashes compared with real fly ash in
forming CBzs and PCBs. The analytical procedure of Cl K-edge
NEXAFS used to assess these model fly ashes was quite easy
because the elemental composition of these model fly ashes was
simple, and the NEXAFS spectra of standards used for analysis
were limited. For example, in the case of {Cu} model fly ash,
we used only CuCl2 and CuCl as inorganic Cl and ten types of
organic Cl. Figures 5A and 5B show optimum analytical results
of NEXAFS and derivative NEXAFS for {Cu} model fly ash
heated at 300°
C. An experimental spectrum was well simulated
by a combination of four standard spectra. We performed this
Fig. 4 Cl K-edge NEXAFS spectra of model fly ashes. (a) and (b)
are the maximum peaks of aliphatic and aromatic Cl.
optimum fitting under an energy range from ca. 2814 eV to
ca. 2824 eV and calculated simulated NEXAFS spectra by
combination of four components. Reasons for these fitting
conditions are discussed below. Using five or more components,
fitting weights of a few standards were calculated as minus
values for several energy ranges. Based on these, we selected
the number of components to be four or less. On the other
hand, an optimum combination result using three components is
shown in Fig. 5C. The R-value, an indicator of combination
fitting, was higher (0.039) with three components than with four
components (0.016). Some parts of the spectrum calculated by
combining three components did not fit the measured spectrum
(Fig. 5C, arrows). Thus, we determined that the number of
components should be four. The maximum peaks were
concentrated in the range 2820 – 2824 eV (Fig. 3), and pre-edge
peaks of NEXAFS derived from Cl connected to Cu existed in
the range 2814 – 2820 eV (Fig. 1). Thus, we performed linear
combination fitting from ca. 2814 eV to ca. 2824 eV. In fact,
when the fitting energy range was extended to 2835 eV, the
optimal number of components was still four, and the existence
ratios of each component had the same values. The applied
energy range (2814 – 2824 eV) was suitable for the linear
combination fitting.
According to the analytical procedure described above, the
fitting results of Cl K-edge NEXAFS are shown in Fig. 6. C–Cl
was represented by the summation ratio of Cl bonded with
aromatic and aliphatic carbon. Also, inorg Cl indicated the total
ratio of Cl connected to inorganic elements. Analytical results
showed that the highest C–Cl ratios were {Cu} model fly ash at
300°
C (ca. 50%) and {Fe} model fly ash at 400°
C (ca. 40%).
The product of the chlorine ratio connected to carbon (C–Cl)
or to inorganic elements (I–Cl) was analyzed by linear
1124
ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26
Fig. 6 Cl forms by analyzing Cl-K NEXAFS spectra of fly ashes.
Table 3 Concentrations of Cl, total organic Cl (TOCl), and total
inorganic Cl (TICl) in real and model fly ashes using Cl K-edge
NEXAFS
C Clb, mol% TOCl, mol% TICl, mol%
Temperaturea/°
Fly ash
{Cu}
Fig. 5 Cl K-edge NEXAFS of {Cu}, 300°
C analyzed by fitting of
standard spectra. Bold line, data; circle, calculated. (A) The best-fit
NEXAFS and (B) its derivative by four components (R = 0.016). (C)
The best-fit NEXAFS by three components (R = 0.039).
combination fitting (Fig. 6), and the total amount of Cl in fly
ash was calculated as follows:
TOCl (mol%) = C–Cl (mol%) × Cl (mol%),
TICl (mol%) = I–Cl (mol%) × Cl (mol%).
From this calculation, we estimated total organic Cl (TOCl) and
total inorganic Cl (TICl) in the solid phase. Using energy
dispersive X-ray analyzer (EDS, EX-23000BU, JEOL, Ltd.)
attached to a scanning electron microscope (SEM, JSM-5600T,
JEOL, Ltd.), we measured the total amount of Cl by analyzing
a field of view detecting the Cl Kα fluorescence X-ray spectrum.
The reported value is the average of analyses in three view
fields. The error appeared to be ca. 0.02 mol%. Table 3 shows
these results. Because the amount of Cl in real fly ash at room
temperature was ca. 11% (base weight), nearly matching the
value of 14% measured by XRF, we confirmed the validity of
the measuring method using SEM-EDS. Total Cl, TOCl, and
TICl in real fly ash before and after heating hardly changed. In
contrast, the model fly ashes consumed large amounts of
chlorine from solid phase on heating. In the model fly ashes
mixed with metal chlorides, the amount of Cl decreased to
C and to 1/5 – 1/76 on heating at
1/3 – 1/16 on heating at 300°
400°
C compared with the room-temperature condition. This
chlorine consumption suggested that some chlorine interacted
with and became connected to carbon, and then TOCl increased.
TOCls in {Cu} and {Fe} model fly ashes at 400°
C were lower
than those at 300°
C. At 400°
C, thermal destruction and
volatilization of dioxins were thought to more important than
further chlorination of carbon. These results indicated that in
{Fe}
{Zn}
rt
300
rt
300
400
rt
300
400
rt
300
400
6.6
6.5
1.4c
0.12
0.06
2.7c
0.086
< 0.02d
1.8c
0.32
0.18
0.99
1.00
na
0.058
0.019
na
0.016
< 0.007
na
0.0
0.013
5.6
5.5
na
0.065
0.044
na
0.071
< 0.013
na
0.32
0.17
a. rt: room temperature.
b. Measured by SEM-EDS.
c. Theoretical value.
d. Detection limit.
na: Not analyzed.
the chlorination of carbon mechanism chlorine consumption
was promoted by the trace metal chlorides.
TOCl of the model fly ashes and CBzs in the heated residue of
model fly ashes showed almost the same ranking, depending on
metal chloride and temperature (Tables 2 and 3). However, a
correlation between TOCl and PCBs in the residue was not
seen. TOCl indicates the total sum of all chlorinated aromatic
compounds, including PCDDs, PCDFs, PCBs, and CBzs. The
amounts of generated CBzs and PCBs in real and model fly
ashes were the highest and lowest, respectively. Thus, it
appeared that the amount of CBzs reflected TOCl, whereas the
amount of PCBs did not.
Comparison of influences among heavy metal chlorides
Details of the analytical results of Cl K-edge NEXAFS are
described in this section. We discuss the influence of dioxin
formation in real fly ash. Figure 4 shows that from the spectrum
shape of {Cu} model fly ash at 300°
C, the Cl atom clearly
connected to organic carbon was ca. 50% of chemical form of
chlorine. The remaining 50% was in the form of inorganic
compounds, CuCl2 and CuCl. In contrast, the maximum peak
C. Then, the
derived from organic Cl almost vanished at 400°
spectrum changed to a shape similar to that for CuCl, and the
ANALYTICAL SCIENCES NOVEMBER 2010, VOL. 26
1125
ratio of CuCl was maximal (ca. 70%). Thus, cupric chloride
was partly and totally reduced (to CuCl) at 300 and 400°
C,
respectively. Analysis of Cl K-edge NEXAFS showed evidence
that the previously reported reduction of cupric chloride4,15
triggered chlorination of carbon by chlorine from copper. This
also indicated that the chlorination potential was weakened by
reduction to CuCl. From Table 3 it can be seen that consumption
of chlorine over 300°
C was smaller than that at temperatures
from room temperature to 300°
C, indicating that the formation
of Cl bonded with carbon was also smaller above 300°
C.
In the case of {Fe} model fly ash, ca. 80% of the chlorine was
C. The quantitative results
still in the form of FeCl3 at 300°
showed that [Fe] model fly ash generated greater amounts of
CBzs and PCBs than did real fly ash at 300°
C. Calculation of
the thermodynamic equilibrium using the FactSage software
indicated a spontaneous reaction from FeCl3 and gaseous O2 to
Fe2O3 and gaseous Cl2, consistent with a report of experimental
results.16 Thus, we suggest a possible formation path of dioxins
by Cl2 generated from the reaction of FeCl3 and gaseous O2. At
400°
C, the maximum peak due to organic Cl appeared clearly,
and ca. 40% of Cl was in the form of C–Cl (Fig. 4). Then,
FeCl3 and FeCl2 represented ca. 40% and ca. 20% of the
chlorine form. Because oxidation of ferric chloride would
proceed as the temperature rises, formation of dioxins might be
expected to increase. Consumption of chlorine by ferric chloride
showed the highest value of the three metals (Table 3).
Cl content decreased to 1/16 and 1/76 at 300 and 400°
C,
respectively, compared with at room temperature. Ferric
chloride had a more volatile character. Although dioxins were
volatilized from the solid phase, gas-phase formation of dioxins
was also thought to be promoted by volatilized ferric chloride.
This proposed formation path is consistent by the large amount
of CBzs and PCBs generated in the gas phase by heating [Fe]
model fly ash (Table 2).
The spectrum of {Zn} model fly ash heated at 300 and 400°
C
hardly changed. By analysis, ca. 10% of the Cl form was
organic Cl at 400°
C. Part of the zinc chloride was possibly
oxidized, as with ferric chloride. Although [Zn] model fly ash
showed lower formation of dioxins than [Cu] and [Fe] model fly
ashes, [Zn] model fly ash still showed higher potential than [Na]
and [K]. This result is consistent with previous reports,10,17
indicating that zinc was a factor in dioxins formation in real fly
ash.
Although all {Cu}, {Fe}, and {Zn} model fly ashes showed
C, actual formation of
oxidation and/or reduction at 300 – 400°
dioxins differed greatly. We thought that dioxins were generated
by a synergetic effect of proposed catalytic chlorination18–20 by
cupric and ferric chlorides, apart from for direct chlorination.
By quality and quantity, the potential for chlorination exists in
the following order:
surrounding Cl atoms, as detected by Cl K-edge NEXAFS, to
determine the chlorination mechanism of carbon during dioxin
formation. Compared with most of the chlorine existing as
inorganic chloride in real fly ash, mixing trace metal chlorides
showed powerful chlorination effects by qualitative and
quantitative experiments. In particular, we demonstrated the
possibility that Cl K-edge NEXAFS analysis may be independent
of metal species. This study provides important information
regarding phenomena in real fly ash. Cl K-edge NEXAFS was
a useful tool to readily judge whether the state of chlorine was
organic or inorganic, not only in MSWI fly ash used here, but
also in more general environmental samples.
CuCl2 > FeCl3 >> ZnCl2 >> KCl > NaCl,
where “>>” indicates orders of magnitude. Dioxins formation
C and in FeCl3 and ZnCl2
was great in the case of CuCl2 at 300°
at 400°
C, which was, at least in part, caused by reduction of
CuCl2 and oxidation of FeCl3, as analyzed by Cl K-edge
NEXAFS. These results suggest that both direct effects by
metals and the catalytic behavior of copper and iron promoted
C.
maximum formation at 300 – 400°
Conclusions
In the present study, we examined changes in the environment
Acknowledgements
We thank A. Shiono, K. Oshita, K. Shiota, and N. Takeda for
supporting this study; Y. Kitajima (BL-11B) and Y. Inada
(BL-9A) for helping with Cl K-edge NEXAFS measurement at
Photon Factory (Proposal No. 2007G069).
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