Evaluation of the Stability of Dioxins and Furans in Solidified
Fly Ash Produced from Municipal Waste Incinerators
Paper No. 445
Tsung-Hsien Yu and Hsing-Cheng Hsi
Department of Safety, Health, and Environmental Engineering, National Kaohsiung First
University of Science and Technology, No.2, Juoyue Rd., Nantsu, Kaohsiung 811, Taiwan, R.O.C.
ABSTRACT
Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are highly toxic organic
pollutants generated from municipal waste incinerators (MWIs). A major fraction of the PCDD/Fs
(> 80%) generated from MWIs was captured by fly ash and the injected adsorbents for
multipollutant control. In Taiwan, the mixture of fly ash and adsorbents collected by fabric filters
was typically solidified with cement and subsequently treated by landfill. However, the
conventional solidification/stabilization (S/S) process aims at inhibiting the leaching of toxic
metals; no attention is devoted to attenuate the release of PCDD/Fs from solidified fly ash. As the
solidified fly ash is dumped to landfill sites, the consequence of PCDD/Fs leached from fly ash by
rain or solvents is concerned.
This study investigated the leachability of PCDD/Fs from raw and solidified fly ash with selected
solvents, including acetic acid, simulated acid rain, humic acid, linear alkylbenzene sulfonate
(LAS) and n-hexane. High-chlorinated PCDD/F congeners were observed in all leachates of raw
fly ash samples, with the largest concentration (up to 70 pg/L) by treating humic acid.
Highly-toxic, low-chlorinated congeners can only be leached with n-hexane. S/S processes
decreased the leachability of PCDD/Fs by up to 90% with acetic acid, simulated acid rain, humic
acid, and LAS as solvents. The decrease in leachability of PCDD/Fs of solidified fly ash can be
further enhanced by adding a 5 wt% sulfur-containing chelating agent. However, S/S processes
increased the leachability of PCDD/Fs with n-hexane. These results suggest that conventional S/S
processes may restrain the release of PCDD/Fs as the fly ash is leached with rain water or nature
organic compounds (e.g., humic acid), but may has a deteriorated effect as organic solvents (e.g.,
n-hexane) coexisting in the landfill sites.
INTRODUCTION
In Taiwan, a major fraction of municipal solid wastes is treated by combustion. The flue gases of
municipal waste incinerators (MWIs), however, contain dibenzo-p-dioxins and dibenzofurans
(PCDD/Fs) that are highly concerned by the public due to their severe health effects. About 20% of
the PCDD/Fs generated from MWIs remain in the gas phase, the other 80% are adsorbed by fly ash
and injected adsorbents, for which activated carbon is typically used. The current air pollution
control act of Taiwan regulates the PCDD/Fs emission standard to be < 0.1 ng-TEQ/Nm3. Less
attention is paid to the PCDD/Fs contained in fly ash.
Fly ash and slag of about 2,000 tons per day were produced from MWIs in Taiwan. Although fly
ash generated from MWIs has been regulated to be solidified before it can be disposed of in
1
sanitary landfill sites, the primary purpose of solidification is to prevent trace toxic metal leaching
from the fly ash surface. Little attention has been designed to attenuate PCDD/Fs in fly ash. The
main reason is that PCDD/Fs possess strong hydrophobicity that they are generally difficult to
leach into the environment. Nevertheless, many studies have shown that the leachability of
PCDD/Fs was different depending on their chlorination degree. Carsch et al. found that only highly
chlorinated PCDD/Fs leached by stirring 1 kg of fly ash with 10 L of distilled water for 2 weeks.1
Fischer et al. asserted that only highly chlorinated congeners were detected in the solution obtained
from leaching experiments following the German DIN38414 testing procedure.2 Schramm et al.
showed that leaching experiments with fly ash and soil by fire-extinguishing water resulted in
significant amounts of PCDD/Fs, especially highly chlorinated congeners in the leachate.3 These
studies suggested that the highly chlorinated congeners were leached more easily than the low
chlorinated tetra- and penta-congeners.
The leaching behavior of PCDD/Fs in fly ash with various solvents has also been presented.
Schramm et al. measured the leachability of PCDD/F in soil and fly ash column eluted with pure
water and linear alkylbenzene sulfonate (LAS) solution. 4 Results showed that the leachability was
significantly increased with LAS as compared to that with water. Kim et al. showed that the
dissolved humic matter (DHM) had a great effect on the leachability and mobility of the highly
chlorinated congeners.5 The other study conducted by the same research group also found an
increase in the leachability of PCDD/Fs as the DHM concentration increased, in all tested pH
values.6
The results addressed above reveal that the types of solvents strongly affect the leachability and
mobility of PCDD/Fs from fly ash and contaminant soil. Thus, it is critical to gain a better
understanding of the leaching behavior of PCDD/Fs from fly ash in landfill sites, in which many
organic compounds may be abundant in leachate or landfill layers. This study aims at investigating
the leachability of PCDD/Fs from fly ash with five solvents, including acetic acid, simulated acid
rain, humic acid, LAS and n-hexane. The effects of the solidification with cement and chelating
agent on the leachability of PCDD/Fs were also examined and further compared the results to those
from raw fly ash samples. The feasibility of S/S processes on inhibition of the release of PCDD/Fs
into the environment is then evaluated.
MATERIALS AND METHODS
Fly Ash Sample
Fly ash was sampled from a well-operated municipal waste treatment plant located in Kaohsiung
City, Taiwan. A 100 kg aliquot of fly ash was obtained from the hoppers below the compartments
of pulse jet fabric filters. Powdered activated carbon was injected upstream of the fabric filters at
the MWIs, resulting in the fly ash sample a mixture of incineration residues and adsorbents. The
collected fly ash was further stirred in a barrel sufficiently to attain a homogeneous sample, and
then sieved with a 200-mesh Tyler Standard Screen. These two samples were designated Ash-A
(with particle size < 200 mesh) and Ash-B (with particle size > 200 mesh) and stored in a dry
environment prior to sample characterization, solidification, and leaching tests.
2
Sample Characterization
The elemental analyses of fly ash sample were performed based on the standard ASTM method
with minor modification; a 0.5-g Li2B4O7 dosage was used to melt the fly ash sample of 1.0 g at
1100 C, followed by using 150 ml, 5 vol% HCl(aq) to completely solve the sample. The resulting
solution was diluted, and subsequently analyzed with an inductively coupled plasma (ICP)
spectrophotometer (ARL Fisons model 380B). Characterization of crystalline substances was
completed with X-ray diffraction (XRD) instrument (SIEMENS model D5000P) at the High Value
Instrument Center, National Sun Yat-Sen University, Taiwan. The database of the Joint
Committee on Powder Diffraction Systems (JCPDS) was used to identify the crystal phases in
samples.7
Solidification/Stabilization (S/S) Treatment
The Ash-A was mixed with 0−30 wt% cement and/or with a 0−5 wt% sulfur-containing agent to
prepare the solidified samples. The cement and agent used in this study were also provided by the
municipal waste treatment plant mentioned earlier. The cement contained 21.50 wt% SiO2, 5.56
wt% Al2O3, 2.54 wt% Fe2O3, 63.65 wt% CaO, 1.64 wt% MgO, and 2.36 wt% SO3. The detailed
composition of the sulfur-containing agent used in this study is unknown. Deionized water was
added at a fixed 0.3 volume to fly ash mass ratio. After stirring the mixture of fly ash, cement,
agent, and water to become homogenous slurry by a mechanical mixer, the sample was poured into
a 2-in. i.d., 4-in. deep PVC-based cylinder mould. The samples were then stored at 100% relative
humidity, 1 atm, and 35 C for 7 days. The resulting samples were designated Ax-Sy: A for Ash-A,
x for x wt% cement added, S for sulfur-containing agent, and y for y wt% agent added.
Unconfined single axial compression strength tests (SHIMADZU model AG-10TG) were
performed on the solidified fly ash samples according to the NIEA R207.21T method. The purpose
was to evaluate the effects of cement and agent contents on samples’ strength (kg/cm2). After the
compression strength tests, the crushed samples were collected and stored for the subsequent
leaching tests.
Leaching Test
Leaching tests with selected solvents were carried out using a sample to solvent ratio of 1:20, as
suggested by the standard Toxicity Characteristic Leaching Procedure (TCLP) based on NIEA
R201.11C. PCDD/Fs are semivolatile organic compounds (SVOCs); as a result, a zero-headspace
extractor (ZHE) was used to perform the PCDD/Fs extraction. The main purpose of using ZHE
was to prevent undesired volatilization of PCDD/Fs into the gaseous phase during the extracting
period.
Five solvents, including acetic acid, simulated acid rain, humic acid, LAS, and n-hexane, were
chosen to evaluate the leachability of PCDD/Fs from raw and solidified fly ash samples. Acetic
acid solution was prepared by dissolving 5.7 ml, 99 vol% acetic acid in 1 liter deionized water, and
then adjusted the pH to 2.88 as suggested by the TCLP standard method. Simulated acid rain was
prepared using 150 μeq/L H2SO4 and 30 μeq/L HNO3. The pH of resulting solution was then
adjusted to 4.3 with 0.1 M NaOH and 0.1 M HCl. Humic acid solution was prepared by dissolving
10 mg humic acid (43.45 wt% C, 3.75 wt% H, and 3.87 wt% N) in a 30 ml, 0.1 M NaOH solution,
and then diluted to 1 L with deionized water. The concentration of final solution was 10 mg/L (5.45
3
mg-OC/L). LAS solution was prepared by dissolving 2 g sodium dodecylbenzene sulfonate
(SDBS) in 1 L deionized water without adjustment of pH and ionic strength. The pH of the solution
is about 4.1. Reagent-graded n-hexane (99.5%) was commercially obtained.
PCDD/Fs Analysis
Analyses of seventeen 2,3,7,8-substituted PCDD/Fs congeners were carried out by the Super
Micro Mass Research and Technology Center in Cheng-Shiu University, which was the only
accredited laboratory in Taiwan for PCDD/Fs analysis during the testing period. Contents of the
PCDD/Fs of fly ash and leachate were analyzed according to the US EPA Reference Method
1613B and TAIEPA NIEA M801.10B. All PCDD/Fs standards in this study were obtained from
Wellington Laboratories Inc. (Wellington, CT); solvents and reagents were pesticide grade/high
purity. The extraction and clean-up procedures, as well as the analytical determination of PCDD/Fs
can be found elsewhere.8 Brief description was provided here for clarity. The PCDD/Fs were
analyzed by high-resolution gas chromatography/high-resolution mass spectrometry
(HRGC/HRMS) using a gas chromatography (HP model 5890) connected to a Micromass
AutoSpec-Ultima mass spectrometry at a resolving power of 10,000. A DB-5 MS column (J&W,
length 60 m, i.d. 0.25mm, film thickness 0.25 μm, carrier gas helium) was employed with the
following temperature program: initial 150 C /min (1 min), 30 C/min to 220 C (12 min), 1.5
C/min to 240 C (5 min), 15 C/min to 310 C (20 min). Two masses were recorded for each
analyte and each isotope-labeled standard. Quantification of analytes was achieved by using the
isotope dilution relative internal standard method referring to the peak areas of the specific
13
C12-labeled surrogate for each 2,3,7,8-PCDD/F analyte.
Toxic equivalent quantity of PCDD/Fs is given by:
n
I  TEQ   X i I i
(1)
i 1
where I-TEQ is the international toxic equivalent quantity (pg-TEQ/L), Xi is the concentration of
PCDD/Fs congeners (pg/L), and Ii is international toxic equivalent factor of each PCDD/Fs
congeners (I-TEF)
RESULTS AND DISCUSSION
Characteristics of Fly Ash
The elemental analyses showed that both Ash-A and Ash-B mainly composed of Al2O3, CaO,
Fe2O3, K2O, MgO, Na2O, SiO2, SO3, and other oxidized metals in trace amounts. Ash-A contained
a larger amount of SiO2 (37.4 wt%), CaO (34.6 wt%), K2O (8.9 wt%), Al2O3 (7.1 wt%), Fe2O3 (6.4
wt%), and MgO (5.4 wt%). Ash-B contained a larger amount of CaO (32.0 wt%), SiO2 (22.8 wt%),
Fe2O3 (12.1 wt%), K2O (5.8%), and SO3 (5.8 wt%). These results demonstrate the difference
between the composition of fine and coarse fly ash particles.
XRD results indicated that the main crystalline phases in both Ash-A and Ash-B were calcium
halide and calcium oxide, including CaClOH, CaCl2, Ca(OH)2, and Ca(ClO)2 (Figure 1). Other
minor phases including NaCl, NaOH, KCl, CaCO3, CaSO4, and SiO2 were also suspected to form
according to the XRD spectrum. These results are expected because the flue gases generated from
4
waste incineration contain appreciable HCl and SOx. Flue gas desulfurization using lime slurry
was employed upstream the sampling position for control of acid gases, which resulted in the
formation of oxides and halides of calcium mentioned above.
Figure 1: XRD Examination of Raw Fly Ash Sample (Ash-A)
The total contents of PCDD/Fs congeners for Ash-A and Ash-B are shown in Table 1. The
PCDD/Fs concentration in Ash-A and Ash-B was 24,042 pg/g and 29,052 pg/g, respectively,
which were equivalent to 1,118 pg- TEQ/g and 1,376 pg-TEQ/g. Results reveal that fly ash
samples with a different particle size possess similar total contents of PCDD/Fs.
Results shown in Table 1 also indicated that the concentration of high-chlorinated congeners (e.g.,
1,2,3,4,6,7,8-HpCDD and OCDF) are remarkably higher than those of low-chlorinated ones (e.g.,
2,3,7,8-TCDD and 1,2,3,7,8-PeCDF). The amounts of high-chlorinated congeners were > 90% of
the total for both ash samples. It was explained that lower-chlorinated congeners may have
favorable desorbing reactions and escaped from fly ash more readily than higher-chlorinated ones.9
Figure 2 shows the compression strength of solidified samples. Result indicated that compression
strength can be enhanced with cement added, but it did not affected by adding the agent.
Furthermore, only two samples, A30-0 and A30-5, could meet the compression strength regulation
in Taiwan ( 10 kg/cm2). This result may be inferred that too much water entered the solidified
samples in the curing period, and then produced bobbles and chinks in the solidified samples.
Therefore, it causes the compression strength of solidified samples to be relatively low.
Leaching Test of Raw and Solidified Ash Samples
Raw Ash
Results obtained from the leaching tests of raw fly ash (Ash-A) with selected solvents are shown in
Figure 3. For here, the empty spaces corresponding to each PCDD/Fs congener versus solvent
indicated that the leaching concentration were undetectable. The total leaching concentration of
PCDD/Fs from the raw ash with acetic acid and simulated acid rain, is relatively low (3.25 and 6.79
pg/L, respectively), and high-chlorinated congeners were mainly observed in the leachates.
5
Table 1. Contents of PCDD/Fs Congeners in Raw Fly Ash
Ash-A
(pg/g)
Congeners
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
OCDD
Total
I-TEF (pg-TEQ/g)
Ash-B
(pg/g)
290.135
480.057
759.697
891.898
972.656
1261.886
45.831
4237.763
626.181
2789.077
62.359
232.983
213.144
387.242
370.533
3296.594
7124.313
24042.349
351.756
580.348
888.668
998.003
1129.790
1517.644
59.117
4881.269
771.330
3554.863
81.638
333.249
292.247
531.914
550.716
4266.954
8262.080
29051.586
1117.553
1375.745
Figure 2: Compression Strength of Solidified Fly Ash Samples
12
compress strength (kg/cm2)
10
8
6
4
2
0
A30-S5
A30-S3
A30-S1
A20-S5
A20-S3
A20-S1
A10-S5
A10-S3
A10-S1
A0-S5
A0-S3
A0-S1
A30-0
A20-0
A10-0
A0-0
6
Figure 3: Congeners Distribution in the Leachate of Raw Fly Ash Extracted with Selected
Solvents
60
50
40
o
Leaching c
ncentration
(pg/L)
70
30
n-hexane
LAS
20
humic acid
10
acid rain
HAc
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
OCDD
0
With humic acid as the solvent, high-chlorinated congeners were still the major PCDD/Fs existing
in the leachates. The total leaching concentration of PCDD/Fs increased to 73.5 pg/L (~0.1
pg-TEQ/L), which is 22 times larger than that with acetic acid. The total PCDD/Fs concentration in
Ash-A was 24,042 pg/g (Table 1), indicating that the portion of PCDD/Fs leached from the fly ash
surface with humic acid is extremely small (0.006%). These results suggest that PCDD/Fs may
stably retain on the fly ash surface as the fly ash undergo rainfall or is leached by humic acid
containing water. However, it is important to note that the standard TCLP is an 18-hr test, the
long-term leachability of PCDD/Fs from fly ash with simulated rain and humic acid needs to be
further examined.
LAS is a substance extensively containing in washing agents and can exist in municipal waste with
a complex composition. LAS may remarkably increase the leaching concentration of PCDD/Fs as
the remaining concentration of LAS in the leachate exceeded its critical micelle concentration
(CMC), which was reported to be 0.5 g/L.10 The total leaching concentration of PCDD/Fs from the
raw ash with LAS and another organic solvent, n-hexane, was also shown in Figure 3. For LAS and
n-hexane, the leaching concentrations of PCDD/Fs were 22.2 pg/L and 26.6 pg/L, respectively.
These values were about 3 times smaller than that obtained from humic acid leaching test (73.5
pg/L). Nevertheless, the leachates from LAS and n-hexane contained appreciable amounts of
low-chlorinated congeners, especially when n-hexane was used as the solvent. The toxic
equivalent quantities for LAS and n-hexane leachate were calculated to be 0.3 pg-TEQ/L and 1.5
pg-TEQ/L, respectively, which were up to 15 times larger than that for humic acid leachate. These
7
results suggest that low-chlorinated PCDD/Fs were easier to be leached from the fly ash surface
with organic solvents and surface-active agents, both of which can be found in landfill sites.
Solidified Ash
The leaching concentrations of PCDD/Fs congeners from the solidified ash with acetic acid were
shown in Figure 4. In general, the leaching concentrations of PCDD/Fs from all solidified samples
were 0.5−8.8 pg/L, in toxic equivalent quantity of 0.001−0.059 pg-TEQ/L. Compared the results
with those obtained from raw ash, it was found that the leaching concentrations of PCDD/Fs from
solidified samples were comparable with those of raw ash. A significant improvement was not
statistically revealed as cement and/or agent was added into the fly ash. We also tested the contents
of PCDD/Fs in the residues of raw and solidified fly ash after leaching test, and found that more
than 80% of PCDD/Fs still remained on the surface of fly ash. These results, again, indicate that
PCDD/Fs on the fly ash surface are highly stable in an acetic-acid containing environment.
Figure 4: Congeners Distribution in the Leachate of Solidified Fly Ash Treated by Acetic Acid for
(a) 0 %, (b) 1 wt%, (c) 3 wt% and (d) 5 wt% sulfur-containing agent added
(a)
(b)
6
3.5
(pg/L)
1.5
A30-0
1.0
A20-0
0.5
4
3
Leaching co
2.0
A10-0
A30-S1
2
A20-S1
1
A10-S1
A0-S1
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,6,7,8-HpCDD
0
A0-0
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
0.0
5
ncentration
2.5
Leaching co
ncentration
(pg/L)
3.0
(d)
1.8
1.8
1.6
(pg/L)
2.0
ncentration
1.6
ncentration
Leaching co
1.4
1.2
1.0
0.8
A30-S3
0.6
A20-S3
0.4
A10-S3
0.2
1.4
1.2
1.0
0.8
A30-S5
0.6
A10-S5
0.2
A0-S5
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
A0-S3
A20-S5
0.4
0.0
1,2,3,4,6,7,8-HpCDD
0.0
Leaching co
(pg/L)
(c)
The effects of cement and sulfur-containing agent were also evaluated using humic acid as the
solvent by compared the results obtained from solidified fly ash with those from the raw sample.
8
The PCDD/Fs congener distribution is shown in Figure 5. The leaching concentration of PCDD/Fs
are 6.246 pg/L (0.016 pg-TEQ/L), 1.105 pg/L (0.003 pg-TEQ/L), and 1.294 pg/L (0.001
pg-TEQ/L), respectively, for A30-0, A0-S5 and A30-S5. Compared to those from raw ash, it was
found that solidification suppressed the release of PCDD/Fs from the fly ash. The decreases in
leaching quantity were 91.5, 98.5 and 98.2%, respectively. Furthermore, the decrease in
leachability of PCDD/Fs of solidified fly ash can be further enhanced by adding a 5 wt%
sulfur-containing agent (Figure 5).
Figure 5: PCDD/Fs Distribution in the Humic Acid Leachate of Raw and Solidified Samples
70
(pg/L)
ncentration
Leaching co
60
50
40
30
A30-S5
20
A0-S5
10
Ash-A
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
0
A30-0
The decrease in leachability of solidified fly ash was also shown using LAS as the solvent (Figure
6). For A30-0, A0-S5, and A30-S5 samples, the leaching concentration were 4.574 pg/L (0.14
pg-TEQ/L), 1.105 pg/L (0.002 pg-TEQ/L), and 1.435 pg/L (0.0014 pg-TEQ/L), respectively, as
compared to those of raw ash (22.2 pg/L; 0.3 pg-TEQ/L). It is important to note that adding cement
and sulfur-containing agent decreased not only the total leaching concentration but also the toxic
equivalent quantity, due to the inhibition of low-chlorinated congeners release (Figure 6). These
results suggest that the adding cement and agent significantly decreases both the leachability of
PCDD/Fs and the toxicity of leachates.
9
Figure 6: PCDD/Fs Distribution in the LAS Leachate of Raw and Solidified Samples
12
(pg
ncentration
Leaching co
/L)
14
10
8
6
A30-S5
4
A0-S5
2
Ash-A
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
0
A30-0
In contrast to results obtained from acetic acid, humic acid, and LAS, S/S processes enhanced the
leachability of PCDD/Fs from fly ash with n-hexane as the solvent (Figure 8). The PCDD/Fs
leaching concentrations of 204.05 pg/L (8.119 pg-TEQ/L), 531.2 pg/L (22.841 pg-TEQ/L), and
137.1 pg/L (8.538 pg-TEQ/L) for A30-0, A0-S5 and A30-S5, respectively, were detected in the
leachates. These leaching quantities of PCDD/Fs corresponded to 0.022, 0.043 and 0.019% of total
PCDD/Fs content in the fly ash sample. They also corresponded to 6.7, 19 and 4.2 times increases
in the leachability as compared to those of untreated fly ash. Results also showed that no only the
amounts of high-chlorinated PCDD/Fs congeners increased due to the S/S processes, the
low-chlorinated, highly toxic congeners significantly leached from the fly ash surface. The detailed
mechanism pertaining to the enhancement of leachability of PCDD/Fs within S/S processes is still
not clearly understood at this moment. However, more attention may be needed to devote in the
future to attenuate the increasing release of PCDD/Fs from solidified fly ash, if organic solvents
exist in the leachate of landfill sites.
CONCLUSION
Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) releasing into the environment
are highly concerned by the public, due to their high toxicity and bioaccumulation effects. In this
study, the leachability of PCDD/Fs from raw and solidified fly ash with selected solvents,
including acetic acid, simulated acid rain, humic acid, linear alkylbenzene sulfonate (LAS) and
10
Figure 7: PCDD/Fs Distribution in the n-Hexane Leachate of Raw and Solidified Samples
180
140
120
Leaching co
ncentration
(pg/L)
160
100
80
A30-S5
60
A0-S5
40
A30-0
20
Ash-A
2,3,7,8-TeCDF
1,2,3,7,8-PeCDF
2,3,4,7,8-PeCDF
1,2,3,4,7,8-HxCDF
2,3,4,6,7,8-HxCDF
2,3,4,6,7,8-HxCDF
1,2,3,7,8,9-HxCDF
1,2,3,4,6,7,8-HpCDF
1,2,3,4,7,8,9-HpCDF
OCDF
2,3,7,8-TeCDD
1,2,3,7,8-PeCDD
1,2,3,4,7,8-HxCDD
1,2,3,6,7,8-HxCDD
1,2,3,7,8,9-HxCDD
1,2,3,4,6,7,8-HpCDD
0
n-hexane was examined. High-chlorinated PCDD/F congeners (e.g., 1,2,3,4,6.7,8-HpCDD and
OCDF) were relatively easier to be extracted, and can be observed in all leachates of raw fly ash
samples. The largest total leaching concentration up to 73.5 pg/L (~0.1 pg-TEQ/L) was detected
with humic acid as the solvent. Highly-toxic, low-chlorinated congeners, however, can only found
in the leachate with n-hexane as the solvent. Solidification/stabilization (S/S) processes decreased
the leachability of PCDD/Fs by up to 90% with acetic acid, simulated acid rain, humic acid, and
LAS as solvents. The decrease in leachability of PCDD/Fs of solidified fly ash can be further
enhanced by adding a 5 wt% sulfur-containing chelating agent. However, S/S processes increased
the leachability of PCDD/Fs with n-hexane as the solvent. These results suggest that conventional
S/S processes may restrain the release of PCDD/Fs as the fly ash is leached by rain or nature
organic compounds (e.g., humic acid), but may has a deteriorated effect as organic solvents (e.g.,
n-hexane) existing in the landfill sites.
The study presented here was conducted by standard TCLP tests with an 18-hr leaching duration.
The TCLP tests shown here can be referred to as short-term examination on the leachability of
PCDD/Fs from raw and solidified fly ash. Conventionally, fly ash dumped into the landfill sites
would not be dug out in years. Long-term evaluation, such as multiple TCLP or column tests, may
provide a better understanding of the actual leaching behavior of PCDD/Fs and may generate a
better solution to attenuate the release of PCDD/Fs from raw and solidified fly ash.
11
ACKNOWLEDGEMENTS
Financial support by the National Science Council, Taiwan, R.O.C. under grant No. NSC
92-2621-Z-327-001 is appreciated. The authors also thank Dr. Gou-Ping Chang-Chien and the
Super Micro Mass Research and Technology Center in Cheng-Shiu University for provided
PCDD/Fs analysis assistance.
REFERENCES
1. Carsch, S.; Thoma, H.; Hutzinger, O. Chemosphere 1986, 15, 1927-1930.
2. Fischer, J.; Lorenz, W.; Bahadir, M. Chemosphere 1992, 25, 543-552.
3. Schramm, K.-W.; Merk, M.; Henkelmann, B.; Kettrup, A. Chemosphere 1995, 30, 2249-2257.
4. Schramm, K.-W.; Wu, W.Z.; Henkelmann, B.; Merk, M.; Xu, Y.; Zhang, Y.Y.; Kettrup, A.
Chemosphere 1995, 31, 3445-3453.
5. Kim, Y.J.; Ohsako, M.; Lee, D.H. J. Jpn. Soc. Waste Manage 1999, 10, 214-223.
6. Kim, Y.J.; Lee, D.H.; Osako, M. Chemosphere 2002, 47, 599-605.
7. Mineral Powder Diffraction File Data Book, Sets 1-42. International Centre for Diffraction
Data, Pennsylvania, PA, 1993.
8. Wang, L.C.; Lee, W.J.; Lee, W.S; Chang-Chien, G.P; Tsai, P. J. Environ. Sci. Technol. 2003,
37, 62-67.
9. Chang, M.-B.; Huang, T.-F. Chemosphere 1999, 39, 2671-2680.
10. Osako, M.; Kim, Y.J. Chemosphere 2004, 54, 105-116.
12