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Leaching characteristics of heavy metals in fly ash from a Chinese coal-fired power plant.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2010; 5: 330–336
Published online 31 March 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.273
Research Article
Leaching characteristics of heavy metals in fly ash from
a Chinese coal-fired power plant
Xun Gong, Hong Yao, Dan Zhang, Yu Qiao, Lin Li, and Minghou Xu*
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology Wuhan, P. R.China
Received 20 August 2008; Revised 27 Decembr 2008; Accepted 5 January 2009
ABSTRACT: China is the largest coal ash producer in the world. Hydraulic ash transport systems are used in most
coal-fired power plants, which lead to serious water pollution due to leaching of trace elements. The investigation on
the leaching behavior of trace contaminants from coal ash is critical to environmental risk assessments. Batch leaching
tests have been performed on the fly ash collected from each field of the electrostatic precipitator (ESP) of a coal-fired
power plant to study the leaching characteristics of Cd, Cr, Pb and V. Leaching solutions included HCl solution of
initial pH = 4 and NaOH solution of pH = 10. The liquid/solid (L/S) ratio was about 4 : 1 in all leaching tests.
Fourteen leaching time intervals were selected, ranging from 15 min to 7 days. The results show that under studied
experimental conditions, Cr has a relatively higher leachability in the acid-leaching solution, while Pb has a higher
leachability in the alkaline solution. With the increase of leaching time, the leachability of Cr in each ash sample
increases obviously. Within the same time interval, Cr in the ash sample from the last field of ESP has the highest
leachability. The concentration of Cd in FA3 is the highest, but the leachability of Cd for FA3 is not the highest among
the three ash samples. The concentration of V in FA1 is the highest; no increased trend with leaching time has been
found in the experiment.  2009 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: fly ash; leaching; trace elements
INTRODUCTION
Currently, more than 70% of the electricity in China
is generated from coal. The production of fly ash is
approximately 300 million tons every year. Such a large
quantity of coal ash leads to a number of environmental
problems, among which is the leaching of heavy metals
from fly ash. The adverse impacts of leached heavy
metals result from their changing form and not easily
breaking down into harmless substances. Human being,
livestock and other animals can be poisoned through
water and the food chain because the heavy metals
can be transformed into toxic metal compounds by
biological accumulation. Therefore, the study of the
leaching behavior of heavy metals in coal fly ash
is critical to its environmental risk assessments and
comprehensive utilization.
Genersally, leaching experiments in the laboratory
mainly use the batch leaching and column leaching
methods. Choosing a suitable batch leaching condition
can help us quickly obtain the kinetic parameters of
leaching tests.[1] Batch extraction method is the most
economical and fast method. Only long-term leaching
test for the concentration trend can provide scientific
and useful information because 18 h are not enough to
reach an equilibrium.[2,3] Metal recovery is considered
as the most effective method to reduce leaching of
heavy metals. But, at present, it is difficult for wide
application because of huge cost.[4] In contrast, methods
of cement and vitrification have the advantage of lower
cost, they will become more widespread. Most coalfired power plants use the electrostatic precipitator
(ESP) to collect fly ash. The fly ash from different ESP
fields differs remarkably in its physical and chemical
characteristics. And this will lead to a distinctive
disposal in utilization.
This paper aims to elucidate the leaching characteristics of heavy metals in fly ash from different ESP fields
of a coal-fired power plant through long-time batch
leaching tests. Fourteen different shaking periods were
used and the longest period is 168 h. The considered
heavy metals involve Cd, Cr, Pb and V.
MATERIALS AND METHODS
Materials
*Correspondence to: Minghou Xu, State Key Laboratory of Coal
Combustion, Huazhong University of Science and Technology,
Wuhan, P. R.China. E-mail: mhxu@mail.hust.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Ash samples were collected from each hopper of the
three-field ESP of a boiler burning an Inner Mongolia
Asia-Pacific Journal of Chemical Engineering
LEACHING CHARACTERISTICS OF HEAVY METALS IN FLY ASH
lignite, and represented as FA1, FA2 and FA3 in the
flow direction of flue gas. After sampling, fly ash is
treated with standard dry-reduction and sealed and then
preserved in cool dry container.
Ash particle size distributions are shown in Figs. 1
and 2. FA1’s particle size is significantly larger than the
other two, the volume average particle sizes of FA1,
FA2 and FA3 are 87.79 µm, 18.11 µm and 9.32 µm
respectively. Generally, ash particle size distributions
have important impact on the ash leaching characteristics because it can affect the leaching characteristics of
trace element in the surface of the particles. Fly ash’s
surface area analysis uses the US Mike’s surface area
analyzer (ASAP2020). Surface area and carbon content test results in Table 1 indicate that the BET surface
area of FA1 is significantly greater than FA2 and FA3.
FA3’s test result is slightly bigger than that of FA2 due
to the small size of the ash which has larger surface
area. FA1’s test result is contradictive to the common
understanding that smaller size makes larger surface by
reason of FA1’s higher carbon content and the limit of
the surface area analyzer. ASAP2020 measures the surface area by nitrogen adsorption and it cannot detect the
micropore which is smaller than 0.02 µm.
The US EDAX’s EAGLE III–XRF and the US thermoelectric’s Tracescan Advantage inductively coupled
plasma atomic emission spectrometry (ICP-AES) are
used for elemental analysis. Digestion process uses the
US CEM’s MARS5 microwave chemical reaction system. Element analysis results of original fly ash are
shown in Table 2. The data show that Si and Al are
dominant elements in these samples. With particle size
decreases, the contents of Al, Mg, Ni and Cr have followed the trend of decrease, but the contents of S, Ca,
Cd, Pb, Cu and Zn have significantly increased. The reason may be that Ca and S are easy to form CaSO4 in the
incineration process. It is easy to enrich in small particle
surface, which is similar to trace metal elements.
Figure 2.
Particle size accumulative distribution of
fly ash. This figure is available in colour online at
www.apjChemEng.com.
Table 1. Surface area and carbon content test results.
2
BET(m /g)
Carbon content(wt%)
FA1
FA2
FA3
11.02
3.25
3.73
1.31
4.10
0.87
Table 2. Element analysis of original fly ash (mg/kg).
Element
Al
Ca
Cd
Cr
Cu
Fe
K
Mg
Mn
Na
Ni
Pb
S
Si
Ti
V
Zn
FA1
FA2
FA3
206 400
72 450
4
86
34
133 100
20 650
12 450
913.0
6 167
648
143
4 000
531 300
11 750
1 821
111
191 500
76 550
9
77
48
79 450
25 650
11 570
752.8
6 782
501
159
8 000
568 200
12 200
2 430
223
176 200
104 100
11
71
67
79 350
23 750
5 686
1 076
4 774
266
197
10 550
554 300
11 750
2 330
265
Leaching experiments
Figure 1. Particle size distribution of fly ash.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
The leaching solutions were prepared in the laboratory
prior to leaching tests. Bulk solutions of pH 4 and 10
were mixed in 5-l polyethylene bottles with ultrapure
Milli-Q water, adjusted to acidic conditions (pH = 4)
by adding 0.01 M HCl, and to alkaline conditions
(pH = 10) by adding 0.01 M NaOH. The concentrations
Asia-Pac. J. Chem. Eng. 2010; 5: 330–336
DOI: 10.1002/apj
331
332
X. GONG ET AL.
Asia-Pacific Journal of Chemical Engineering
of trace elements in the Milli-Q water in duplicate
pH solutions are less than 1 µg/l. These concentrations
are well below the analytical data obtained from the
leaching test and do not affect the values obtained from
the leaching tests.
Batch leaching experiments with the FA1, FA2 and
FA3 were performed on 14 time points. Fly ash sample of 30 g was weighted in an acid-cleaned 250 ml
polyurethane bottles. Then, 120 ml per-prepared solution was added, and the liquid/solid (L/S) ratio was
close to 4 : 1. The sealed bottles with respective fly
ashes and solutions were shaken in a horizontal shaker
at 150 ± 10 rpm at 25 ◦ C (±0.5 ◦ C). Fourteen different
shaking periods were used: 15 min, 30 min, 1 h, 2 h,
3 h, 6 h, 12 h, 24 h, 48 h, 72 h, 96 h, 120 h, 144 h and
168 h. After corresponding leaching time, the admixture was filtrated by a rapid leaching device and a
0.45-µm synthetic resin microporous membrane was
used. A 50-ml filtrate was transferred into a volumetric flask for ICP-AES analysis. Solution was immediately acidified by using 1 ml nitrate, then reserved at
4 ◦ C after shaking in order to prevent additional chemical reactions. Another aliquot (approximately 50 ml)
of each solution was filtered into a second container
but not acidified, for determination of pH and total
alkalinity immediately after sample collection. Residual
ash was used for analysis and quality balance. A synchronous leaching blank was used to remove leachate
and other factors in each leaching time. Each leachate
was tested for three times and the standard deviations
for all the test results were smaller than 2% in ICPAES analysis. All glass containers and equipment were
soaked by using 6 M HNO3 at least 24 h before experiment.
SartoriusPB-10 pH meter was used to determinate
pH value of leachate, total alkalinity (defined as HCO3
and CO3 2− concentrations) was measured by titration
of 0.01 M HCl solution, alizarin red-methyl red mixed
indicator method.[5]
RESULTS AND DISCUSSION
pH and total alkalinity
Figure 3 shows the relationship between leachate’s pH
value and leaching time. The curve shows that the
leachate presents alkaline in both acidic (pH = 4) or
alkaline (pH = 10) conditions after 15 min. Therefore,
FA1, FA2 and FA3 are all alkaline fly ash. Besides, pH
value tended to be stable after 2 h leaching time. The
buffer capacity of alkaline fly ash plays more important
role than characteristic of solution because the L/S is
relatively smaller in the experiment. By comparison of
the results of these three fly ash, the pH value of FA3’s
leachate maintains at a relatively high level, because
FA3 contains more alkaline and alkaline earth metal,
and its particle size is smallest, which has positive effect
on the dissolving and reactions of alkaline substances.
Alkalinity is defined as the concentration of HCO3 −
or CO3 2− in the solution which is formed by dissolving
of carbonate (CaCO3 ) in fly ash. HCO3 − and CO3 2−
are conduced to exist in alkaline solution because they
are easy to form CO2 by reaction with H+ . Figure 4
Figure 3. Leachate’s pH value.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 330–336
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
LEACHING CHARACTERISTICS OF HEAVY METALS IN FLY ASH
Figure 4. Leachate’s alkalinity.
shows the relationship between leachate’s alkalinity
and leaching time. The pH value of the leaching
solutions does not have significant influence on the
alkalinity of the corresponding leachates. As for the
same leaching solution, the alkalinity of the leachates
from different ash samples increases in the order of FA1,
FA2 and FA3. Alkalinity increased most significantly
within 1 h. The decrease of alkalinity at the end of
the test is probably associated with the precipitation
of trace metal and calcium carbonates. The leachate
of FA3 has the highest alkalinity after 7 days of
leaching because FA3 contains more soluble carbonate
components. Calcium and lead carbonates are insoluble
materials, and the alkalinity of the leachate may have
some influence on their leachability. The formation of
insoluble metal carbonate may cause the decline of
alkalinity in the leaching process. The metal minerals
in fly ash will continue to dissolve for the equilibrium
of metal’s cation. At the same time more soluble
carbonate continues to dissolve as the consumption of
the HCO3 − and CO3 2− to maintain the equilibrium of
alkalinity. When the consumption rate of HCO3 − and
CO3 2− becomes smaller than the rate of production, the
alkalinity will increase.
Mass balance
The mass balance is obtained to evaluate the validity
of our leaching experiments. The mass balance constant
+ V × c1 × 100%, in which
is defined as a = m1 ×mw1 ×
w2
2
m1 (g) is the increased weight of polyethylene bottles,
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
microporous membrane and filter paper; w1 (mg/g) is
the content of element in the residual ash; V (ml) is the
volume of leachate; c1 (mg/ml) is the concentration of
element in the leachate; m2 (g) is the mass ash prior to
leaching; w2 (mg/g) is the content of element in original
fly ash.
It is relatively difficult to keep the mass balance in
leaching experiment because of the process of filtration
and the test precision of instrument for trace elements.
The mass balance constants on the condition of pH =
4, 360 min leaching are showed in Table 3. The mass
balance above has an error of ±25% except Cr in FA1.
Leachability of selected trace elements
To study migration of trace elements in fly ash from
different electrostatic precipitator, the authors define Lx
−6
x × 10
as the leachability of elements: Lx = V ×mC×
;
wx
in which V (ml) is the volume of leachate, Cx (mg/l)
is the concentration of x of element in the leachate,
m (g) is the quality of the original fly ash; wx is the
Table 3. The mass balance constant on the condition
of pH = 4 360 minutes leaching.
FA1
FA2
FA3
Cr (%)
V (%)
Cd (%)
Pb (%)
137
98
104
95
81
81
85
75
75
90
116
102
Asia-Pac. J. Chem. Eng. 2010; 5: 330–336
DOI: 10.1002/apj
333
334
X. GONG ET AL.
Asia-Pacific Journal of Chemical Engineering
mass concentration of x element in original fly ash. For
example, the leachability of Cr in FA1 is represented as
LCr1 .
Cadmium
Figure 5 shows the relationship between leachability of
Cd and leaching time. The leachabilities of three fly
ash samples are not stable in the leaching process and
have no significant difference in leaching on different
solution. The general trend of Cd’s leachability for
three fly ash samples is LCd2 > LCd3 ≥ LCd1 , the highest
values are 14.36%, 11.80%, 7.84%, respectively. Some
researchers[6] believe that Cd is rich in the surface
of fly ash particles so it is easy to leach out in fly
ash. On the other hand, Cd can combine with Zn
oxides and Fe oxides (e.g. FeS2 or ZnS) in the coal.
These sulphides can form spinel which is insoluble (as
ZnFe2 O4 ) in coal combustion process. Partial Cd could
be included in these minerals, which leads to leaching
incompletely. Besides, Cd can combine with CO3 2−
to form insoluble CdCO3 2− ; this may also affect the
leaching characteristics of Cd. The concentration of Cd
in FA3 is the highest, and the content of Zn is also
higher than the other two. This leads to the increase of
Cd form as spinel, so the leachability of Cd for FA3 is
not the highest among the three ash samples.
Chromium
Figure 6 shows the relationship between leachability of
Cr and leaching time. Whether the fly ash leaching
with alkaline (pH = 10) or acidic (pH = 4) solution,
the leachability of Cr rises significantly with the time.
The balance cannot be kept at the end of leaching
experiment. The leachability is a little higher with the
role of acidic solution (pH = 4) than alkaline (pH = 10)
solution. The relationship of leachability of three fly
ash is LCr3 > LCr2 > LCr1 (7.7%, 21.7%, 36.9%). So the
size of fly ash particle may be the main factor effecting
the leaching characteristic of Cr. 50–90% of chromium
is enriched in the organic matter in coal, and 10–50%
exists in clay minerals which is illite mainly. Only a
small amount of chromium occurs in sulfides.[7] Cr is
likely to concentrate on the particle surface through the
combustion process. The dissolution of main minerals
will affect the leaching characteristics of Cr, especially
the element adsorbed on the surface of the particle. In
addition, Cr can exist in Al and Fe oxides and is possible
to concentrate in illite clay. Cr can also be enriched in
other minerals by the effect of conversion of organic
material in the coal.[6]
Lead
Figure 7 shows the relationship between leachability of
Pb and leaching time. The leachability of Pb fluctuates
over time, which still shows an increasing trend on
the whole. The leachability does not reach stability in
the leaching process. The leachability of Pb is slightly
higher when the fly ash is mixed with the alkaline
solution (pH = 10). The highest leachability approaches
4.21%, 2.70% and 3.16%, respectively. Pb is easy
to enrich in the surface of fly ash particles. Some
researchers simulate the Pb leaching characteristics
with complex surface model for fly ash particles,[8]
the simulation results are in line with the experiment
Figure 5. Leachability of Cd.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 330–336
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
LEACHING CHARACTERISTICS OF HEAVY METALS IN FLY ASH
Figure 6. Leachability of Cr.
Figure 7. Leachability of Pb.
results. The leaching characteristic of Pb is closely
associated with minerals characteristic of dissolving.
Pb3 (CO3 )2 (OH)2 , PbSO4 and PbCO3 were considered
to be the major minerals that will affect the leachability
of Pb.[6,8] FA1, FA2 and FA3 are all alkaline fly ash. The
leachate is always maintained in alkaline environment.
The compounds of Pb have low solubility. These factors
may lead to the low leachability of Pb in the experiment.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Vanadium
Although the toxicity of metal vanadium is very low,
its compounds are toxic to plants and animals. The
toxicity of vanadium will increase with the increase
of valence, and the V (V) is the most poisonous
state in vanadium. Figure 8 shows the relationship
between leachability of V and leaching time. The
leachability of V is the smallest in all the trace elements
Asia-Pac. J. Chem. Eng. 2010; 5: 330–336
DOI: 10.1002/apj
335
336
X. GONG ET AL.
Asia-Pacific Journal of Chemical Engineering
Figure 8. Leachability of V.
studied, which is below 1E-3. There are remarkable
differences among the leachability of FA1, FA2 and
FA3, which is LV1 > LV3 > LV2 . This can be attributed
to the fact that FA1 contains more soluble oxides
of V (III) and V (V). And this part of the soluble
vanadium compounds’ leaching had already started
shortly, the remains are insoluble compounds. Therefore
no increased trend with leaching time has been shown
in the experiment.
CONCLUSIONS
a) Fly ashes collected from different electrical fields
have different amounts of metallic elements. In
accordance with FA1,FA2 and FA3, the contents
of S, Ca, Cd, Pb, Cu and Zn have significantly
increased
b) The three experimental ashes are alkaline fly ash.
The pH of leachate has few difference for the
experimental conditions is either acidic (pH = 4)
or alkaline (pH = 10). The leachate of FA3 has
relatively high pH value which is stable at around
11.0, the other two are around 10.0.
c) The leachability of trace elements in fly ash is
affected by the time of leaching. The leachability of
Cd, Cr and Pb is still increasing when experiment
has been carried on for a week.
d) The leachability of three fly ash collected by
different electrostatic precipitator has difference.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
The obvious results are LCr3 > LCr2 > LCr1 , LV1 >
LV3 > LV2 . The leachability of Cr is higher with
the leaching in acidic solution (pH = 4), and leachability of Pb is higher with the leaching in alkaline
(pH = 10).
Acknowledgments
The authors acknowledge the financial support from
the National Natural Science Foundation of China
(90610017, 50721005), and the Ministry of Education
of China (20050487014). The authors would also like
to acknowledge the support of the Analytical and
Testing Center at Huazhong University of Science &
Technology.
REFERENCES
[1] H.A. van der Sloot. Waste Manage. Res., 1990; 8, 215–228.
[2] D.J. Hassett. Fuel Process. Technol., 1994; 39, 445–459.
[3] J. Jankowski, C.R. Ward, D. French, S. Groves. Fuel, 2006; 85,
243–256.
[4] D. Dermatas, X. Meng. Eng. Geol., 2003; 70, 377–394.
[5] L. Xiao. Dev. Chem. Ind., 2004; 6, 43–44.
[6] M. Seferinoglu, M. Paul, A. Sandstrom, A. Koker, S. Toprak,
J. Paul. Fuel, 2003; 82, 1721–1734.
[7] F.E. Huggins, N. Shah, G.P. Huffman, A. Kolker, S. Crowley,
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[8] J.J. Dijkstra, H.A. van der Sloot, R.N.J. Comans. Appl.
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Asia-Pac. J. Chem. Eng. 2010; 5: 330–336
DOI: 10.1002/apj
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