Characterization of an alkali activated lagoon ash and its application for
heavy metal retention q
P.K. Kolay, D.N. Singh*
Department of Civil Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai 400076, India
Abstract
The wet disposal of ash, from the coal-®red thermal power plants, involves its mixing with water and its impoundment in the ash ponds or
lagoons. This causes the interaction of ash and the alkalies present in it with water over a period and the formation of ash zeolites (i.e.
zeolitization of the ash) takes place. In order to simulate such ash±water interactions, alkali activation of a typical lagoon ash, from India, has
been conducted. Investigations have been conducted to identify the effect of zeolitization of the ash on its physical, chemical and mineralogical characteristics. Such studies are essential to explore the possibility of application of the lagoon ash, and the zeolitized ash, for
various environmental applications, viz. retention and removal of heavy metals from the industrial sludge.
Keywords: Lagoon ash; Characterization; Zeolitization; Heavy metal retention and removal
1. Introduction
Ef®cient utilization of the ash, being disposed of at the
coal-®red thermal power plants, is becoming a challenging
task. This calls for a complete characterization, i.e. physical,
chemical and mineralogical properties of the ash, before it
can be utilized for various purposes, viz. as a stabilizer of
sub-grade [1,2] and sub-bases in pavement construction
[3,4], as a ®ller material and ®lling of mines [5±8], as a
constituent of cement and concrete [9±11], as a treatment
of polluted water [12±15] and treatment of soil for agriculture purposes [16,17]. However, these applications do not
take into account the quality of the ash, and the subsequent
changes it may undergo in the long run.
In this context, some researchers have demonstrated, and
critically evaluated, the in¯uence of methods adopted for
handling, collection, storage and disposal (viz. wet or dry)
of the ash. It has also been demonstrated that the physical,
chemical and mineralogical properties of the ash change, to
a great extent, when `wet disposal' or `slurry disposal'
method of the ash is adopted [18]. During this process, the
ash is mixed with water to make slurry and is disposed off in
ash ponds or lagoons. As such, the ash interacts with water,
and alkalies present in it (mainly Na2O and K2O) react with
major constituents of the ash (SiO2 and Al2O3) leading to the
formation of ash zeolites. This process is termed as `zeolitization' of the ash, i.e. formation of zeolites, which alters
the overall properties of the ash. In the recent past, some
efforts have been made [19±24] to synthesize zeolites, from
the ¯y ash. These zeolites have been used for preventing
degradation of the environment as well as for different
industrial applications, such as retention and removal of
heavy metals from the industrial sludge and ¯ue gas, ammonia removal, replacement of phosphate in detergent,
removal of radioactive waste, etc.
However, studies related to the zeolitization of the lagoon
ash [25] and its effect on overall properties of the ash and its
use in various industrial applications is unexplored and
needs a proper attention. With this in view, an effort has
been made in this paper to characterize a typical lagoon ash,
from India, based on its physical, chemical and mineralogical
properties. As such, the ash±water interaction and the changes
undergone by the ash, impounded in the lagoons, over a period
of time has been simulated under controlled laboratory conditions. Studies are also conducted to demonstrate application
of the lagoon ash, and the zeolitized lagoon ash, for heavy
metal retention or removal, from the industrial sludge.
2. Experimental investigation
A lagoon ash sample, from Koradi Thermal Power Plant,
484
Nagpur, Maharashtra, India is chosen for the present
study. Ash sampling is done randomly, in four batches
during a span of seven days, to minimize the effect of
heterogeneity. Later, these batches were mixed together to
prepare a representative sample of the ash. The oven-dried
sample of this ash is designated as the original lagoon ash
(OLA) sample.
The mineralogical composition of the ash is determined
by conducting X-ray diffraction (XRD) spectrometer
studies, and with the help of JCPDS search match data
®les [26]. Thus, minerals present in the ash are identi®ed.
The morphology of the ash is studied with the help of a
scanning electron microscope (SEM). The chemical composition of the ash, for major oxides present in it, is obtained
with the help of an X-ray ¯uorescence (XRF) set-up. The
presence of trace elements in the ash sample is detected with
the help of an inductively coupled plasma (ICP) unit. To
obtain particle size distribution of the ash, sieving and
hydrometer analyses are conducted [27]. Soft imaging
system is also used to determine the size of various ash
particles [28]. The speci®c surface area of the ash sample
is determined with the help of a Blaine's apparatus and
using Portland cement as a benchmark material [29]. The
cation exchange capacity (CEC) [30] and speci®c gravity
[31] of the ash are determined to characterize it.
The ash sample, denoted as OLA, is activated by alkali
treatment to simulate its interaction with water and the
changes it undergoes in the process of wet or slurry disposal.
Table 1
Designation of the ash sample representing its alkali activation
Alkali
Alkali
strength (M)
Time (h)
12
24
36
48
NaOH
0.5
1.0
2.0
3.5
ALA1
ALA2
ALA3
ALA4
ALA5
ALA6
ALA7
ALA8
ALA9
ALA10
ALA11
ALA12
ALA13
ALA14
ALA15
ALA16
KOH
0.5
1.0
2.0
3.5
ALA17
ALA18
ALA19
ALA20
ALA21
ALA22
ALA23
ALA24
ALA25
ALA26
ALA27
ALA28
ALA29
ALA30
ALA31
ALA32
In an open system, 160 ml alkali solution (NaOH and KOH)
of 0.5, 1.0, 2.0 and 3.5 M is used for activating the ash
sample, maintaining a solid±liquid ratio of 0.125 g/l. The
activation temperature of approximately 1008C is maintained by using a water-bath and a re¯ux system. The
process time is then varied from 12, 24, 36 and 48 h, respectively. At the end of these activation periods, the ash
sample is ®ltered and repeatedly washed with distilled
water.
Table 1 presents details of the alkali activated lagoon ash
(ALA) samples as a result of alkali activation (with a given
strength of NaOH and KOH for a particular time duration)
of the OLA.
Fig. 1. XRD patterns of the OLA and the ALA6 samples.
485
Table 2
X-ray crystalline minerals present in the OLA and ALA samples (treated with NaOH) (PD: predominant; P: present; NP: not present)
Mineral
OLA
ALA
1
ALA
2
Quartz
Mullite
NaP1 zeolite
Hydroxy-sodalite
zeolite
ALA
3
ALA
4
ALA
5
PD
P
P
P
NP
NP
ALA
6
ALA
7
ALA
8
ALA
9
ALA
10
PD
P
Results of various experiments conducted on OLA and
ALA samples are presented in the following.
3.1. Mineralogical characteristics
The XRD pattern for the OLA sample is presented in
Fig. 1. It can be noticed from the ®gure that Quartz and
Mullite are the most predominant minerals present in the
OLA sample. To establish effect of alkali activation (with
NaOH and KOH) on the OLA sample, the XRD of different
ALAs (ALA1 to ALA32) is also conducted. With the help of
JCPDS data ®les, predominant minerals present in the ash
samples are identi®ed and the same are presented in Tables
2 and 3. From Table 2, it can be noticed that NaP1 zeolite is
formed in the samples treated with NaOH. In addition to the
formation of NaP1 zeolite, sample ALA6 indicates formation of hydroxy-sodalite zeolite also. For the sake of easy
comparison, XRD pattern for the ALA6 sample is also
presented in Fig. 1. On the contrary, as depicted in Table
3, KOH activation of the ash does not result in the formation of any zeolites. As such, further characterization of
KOH activated samples (ALA17 to ALA32) is not
conducted.
3.2. Chemical characteristics
The chemical composition (by % weight) of the OLA and
ALA samples (ALA1 to ALA16) is presented in Table 4. It
can be observed from the table that the major constituents of
these samples are; silica (SiO2), alumina (Al2O3) and ferric
oxide (Fe2O3). Minor quantities of calcium oxide (CaO),
magnesium oxide (MgO), sodium oxide (Na2O), potassium
ALA
12
ALA
13
ALA
14
ALA
15
ALA
16
PD
P
P
P
3. Results and discussion
ALA
11
NP
oxide (K2O), sulphur oxide (SO3) and other compounds are
also observed to be present. The loss on ignition (LOI) of the
OLA sample is 2.4%. These results indicate that the OLA
sample belongs to Class `F' [32]. However, it can be noticed
from the data presented in Table 4 that an increased alkali
activation causes a decrease in the silica content of the OLA
sample. In other words, the alkali activation causes dissolution of silica.
Different trace elements and their concentration (in ppm),
present in the OLA and ALA samples, are presented in
Table 5. It can be noticed from the table that no appreciable
change occurs in the concentration of the trace elements,
due to alkali activation of the ash.
The CEC of the OLA and ALA samples are presented in
Fig. 2. It can be noticed from the ®gure that in general, CEC
of the OLA sample increases for different activation periods
and different strength of NaOH. This may be attributed to
the formation of NaP1 zeolites. However, ALA6 sample
exhibits maximum increase ( ˆ 156.84%) in CEC of the
OLA sample. As mentioned earlier, this may be attributed
to the formation of hydroxy-sodalite zeolite, in addition to
the NaP1 zeolite.
3.3. Physical characteristics
The average speci®c gravity of the OLA sample is found
to be 2.14. To examine the in¯uence of alkali activation on
speci®c gravity of the ash sample, speci®c gravity of ALA
samples is also obtained, as depicted in Fig. 3. It can be
noticed from the ®gure that in general, the speci®c gravity
of the OLA sample increases with an increased activation
time period and NaOH strength. However, with the same
strength of NaOH and different activation time periods,
Table 3
X-ray crystalline minerals present in the OLA and ALA samples (treated with KOH) (PD: predominant; P: present; NP: not present)
Mineral
Quartz
Mullite
NaP1 zeolite
Hydroxy-sodalite
zeolite
OLA
ALA
17
ALA
18
ALA
19
ALA
20
ALA
21
ALA
22
ALA
23
ALA
24
PD
P
NP
ALA
25
ALA
26
ALA
27
ALA
28
ALA
29
ALA
30
ALA
31
ALA
32
486
Table 4
Chemical composition (by % weight) of the OLA and ALA samples
Oxide
OLA
ALA
1
ALA
2
ALA
3
ALA
4
ALA
5
ALA
6
ALA
7
ALA
8
SiO2
Al2O3
Fe2O3
CaO
SO3
MgO
Na2O
K2 O
TiO2
P2O5
58.33
27.10
5.02
1.12
0.12
0.53
0.22
0.89
1.63
0.24
64.20
26.77
5.00
1.19
0.09
0.51
0.73
0.83
1.55
0.14
61.53
30.84
5.06
1.24
0.09
0.59
1.82
0.71
1.66
0.13
58.01
30.90
5.14
1.23
0.08
0.56
2.39
0.49
1.73
0.04
48.76 62.29
30.99 29.27
5.44
4.19
1.27
1.20
0.08
0.09
0.62
0.57
4.10
1.13
0.19
0.80
1.90
1.61
0.001 0.17
58.94
31.12
4.49
1.27
0.09
0.59
2.35
0.63
1.72
0.09
56.98
32.16
5.14
1.25
0.08
0.60
3.07
0.44
1.81
0.02
43.27
59.78
32.29
30.87
6.05
3.07
1.28
1.28
0.08
0.11
0.63
0.60
4.94
1.92
0.26
0.79
2.00
1.72
0.0001 0.07
the change in the speci®c gravity of the OLA sample is
approximately 2±4%. While, for the same activation time,
with an increase in the strength of NaOH, the speci®c
gravity of the OLA sample increases by approximately 8±
11.5%. These trends can be attributed to the fact that due to
the alkali activation, silica of the ash particles gets `etched'
and due to which the air entrapped in the cenospheres/plerospheres escapes. However, to con®rm the alkali action on
the OLA sample, it is boiled with distilled water for 24 h and
is allowed to cool, and its speci®c gravity determined. The
obtained speci®c gravity is 2.18, which is almost equal to
the average speci®c gravity of the OLA sample. This
con®rms the role of alkali activation in altering the speci®c
gravity of the ash sample.
The speci®c surface area of the OLA sample is found to
be 1670 cm 2/g. To examine the in¯uence of alkali activation
on speci®c surface area of the ash, the speci®c surface area
of alkali-activated samples (ALA1 to ALA16) is also determined. The obtained results are presented in Fig. 4. It can be
noticed from the ®gure that the speci®c surface area of the
ALA
9
ALA
10
ALA
11
ALA
12
ALA
13
ALA
14
ALA
15
ALA
16
54.89
56.06
42.12
58.55
30.47
32.53
32.48
30.04
3.53
4.41
5.76
2.39
1.25
1.30
1.31
1.22
0.09
0.09
0.08
0.09
0.57
0.62
0.63
0.56
2.92
3.07
5.16
1.54
0.58
0.43
0.28
0.76
1.76
1.80
1.88
1.69
0.0001 0.0001 0.0001 0.13
57.03
32.76
2.94
1.28
0.09
0.60
3.04
0.65
1.79
0.03
53.94
40.59
34.14
32.20
3.90
5.46
1.35
1.47
0.08
0.07
0.63
0.67
3.74
5.90
0.50
0.20
1.88
2.08
0.0001 0.0001
OLA sample increases by 0.72±142.51% with an increased
activation time and NaOH strength. As stated earlier, the
increase in speci®c surface area of the ash may be attributed
to etching, caused by alkali activation, which is responsible
for exposing the inner surface of ash particles. However, this
action is much more pronounced on the cenospheres and
plerospheres, present in the ash sample, and hence, the
speci®c surface area increases even by more than 100% of
the OLA sample. Incidentally, an increase in speci®c
surface area of the OLA on alkali activation, is one of the
possible reasons for increase in its CEC value.
To study the in¯uence of alkali activation on the OLA
sample, gradation analysis is performed for the OLA and
ALA samples, and the obtained results are presented in
Table 6. It can be noticed from the data presented in the
table that alkali activation of the OLA sample results in a
decrease in its ®nes content. Although the obtained results
cannot be compared easily, the grain-size distributions for
the OLA and ALA6 samples when further analyzed, as
shown in Fig. 5, indicate that the OLA sample consists of
Table 5
Trace element present in OLA and ALA samples (ND: ,1.00 ppm)
Sample
OLA
ALA1
ALA2
ALA3
ALA4
AAA5
ALA6
ALA7
ALA8
ALA9
ALA10
ALA11
ALA12
ALA13
ALA14
ALA15
ALA16
Element (ppm)
Ba
Cu
Sr
Ni
Cr
Zn
Cd
Mo
V
Hg
Se
Pb
As
4.007
0.056
0.051
0.073
0.051
0.061
0.050
0.066
0.050
0.048
0.059
0.059
0.063
0.051
0.051
0.056
0.057
, 0.1
0.904
0.022
0.022
0.023
0.020
0.016
0.018
0.017
0.022
0.017
0.019
0.022
0.018
0.015
0.014
0.021
0.005
0.693
2.860
, 0.1
0.584
1.336
, 0.1
, 0.7
, 0.1
, 0.5
ND
ND
2.570
0.0111
0.0099
0.0178
0.0111
0.0123
0.0114
0.0127
0.0197
0.0110
0.0120
0.0098
0.0135
0.0135
0.0094
0.0135
0.0099
ND
ND
ND
ND
ND
ND
ND
ND
487
Fig. 4. Variation of speci®c surface area of the OLA sample with alkali
activation.
Fig. 2. Variation of CEC of the OLA sample with alkali activation.
55, 41.5 and about 3.5% sand-sized (,4.75 mm), silt-sized
(0.075±0.002 mm) and clay-sized (,0.002 mm) particles,
respectively. For the ALA6 sample, the silt-sized and
sand-sized particles are noticed to be 45 and 55%, respectively, with no clay-sized particles.
As the ALA6 sample exhibits maximum CEC value, a
soft imaging system is also employed to compare the gradation characteristics of this sample vis-aÁ-vis the OLA sample.
For this, 206 particles are selected randomly, and the
obtained results are presented in Fig. 6. From the ®gure, it
can be noticed for the OLA sample that the minimum and
maximum sizes of the particles are 2.64 and 45.77 mm,
respectively. However, for the ALA6 sample, the range of
particle size varies from 3.99 to 85.41 mm. It can further be
noticed from the ®gure that the number of particles in the
size ranges of 10±15, 20±30 and 50±90 mm are greater in
Fig. 3. Variation of speci®c gravity of the OLA sample with alkali
activation.
the ALA6 sample than in the OLA sample. However, for the
particles size range 30±45 mm, the number of particles
decreases in the ALA6 sample. The sieve analysis also
indicates these changes, as shown in Fig. 5. The change in
particle size can be attributed to either growth of the
surface of the lagoon ash particle or due to the agglomeration of smaller particles, of the ash, to form particles of
larger size.
3.4. Heavy metal retention studies
Studies are conducted to investigate heavy metal retention capabilities of the OLA sample and the in¯uence of
alkali activation on these properties. As ALA6 sample exhibits maximum CEC, the same is used to simulate response
of alkali activation of the OLA, on its heavy metal retention
Table 6
Gradation characteristics of different ash samples
Sample
Clay-sized
(%)
Silt-sized
(%)
Sand-sized
(%)
OLA
ALA1
ALA2
ALA3
ALA4
ALA5
ALA6
ALA7
ALA8
ALA9
ALA10
ALA11
ALA12
ALA13
ALA14
ALA15
ALA16
3.50
7.00
6.00
6.04
5.20
5.00
0.00
4.19
6.05
4.10
6.00
7.20
5.60
4.36
4.30
6.49
7.00
41.50
64.00
49.09
40.27
44.83
66.76
45.00
69.83
48.59
66.82
41.30
42.70
49.55
68.41
39.98
41.06
57.16
55.00
29.00
44.91
53.69
49.97
28.24
55.00
25.98
45.36
29.08
52.70
50.10
44.85
27.23
55.72
52.45
35.84
488
Fig. 5. Gradation characteristics of the OLA and the ALA6 samples.
or removal properties. As such, Pb(NO3)2 solution, with
concentration equal to 3885 ppm and pH varying from
2±8, is used to simulate industrial sludge. The contact
time for the ash sample and the Pb(NO3)2 solution is taken
as 6, 12 and 24 h. The difference between the initial concentration of Pb and the concentration of Pb present in the
solution, after Pb(NO3)2 solution has attained an equilibrium
with the ash, indicates the concentration of Pb, retained by
the ash samples as presented in Fig. 7. The results presented
in the ®gure indicate that in general, for OLA and ALA6
samples, the amount of Pb retained (and hence the percentage Pb retention) increases as the pH of the solution and
contact time increases. It can further be noticed that for pH
equal to 2 and 4, the ALA6 sample retains almost three
times Pb, as compared to the OLA sample. However,
when pH of the solution is 6, the percentage Pb retention
Fig. 6. Grain-size distributions of the OLA and the ALA6 samples using a
soft imaging system.
Fig. 7. Pb retention characteristics of the OLA and the ALA6 samples.
by the OLA and ALA6 samples varies from 64.58±94.71 to
89.78±100%, respectively. It must be noticed that at pH
equal to 8, the Pb gets precipitated. It can also been noticed
from Fig. 7 that for ALA6 sample, a contact time of 12 h is
suf®cient to achieve 100% Pb retention, even before the Pb
gets precipitated.
The effect of different strengths of Pb(NO3)2 solution on
the retention capacity of the ALA6 sample with different pH
values, has also been studied and the same is being
presented in Fig. 8. It can be noticed from the ®gure that
as the concentration of Pb(NO3)2 solution decreases from
0.05 to 0.00005 M, a higher amount of Pb is retained by the
Fig. 8. Effect of Pb(NO3)2 strength and pH on Pb retention characteristics of
the ALA6 sample (equilibrium time ˆ 12 h).
489
ALA6 sample, for a given pH value. However, it must be
appreciated that 0.0005 M strength of Pb(NO3)2 solution is
suf®cient for almost 100% Pb retention and, as such, further
dilution of the sludge is not recommended.
4. Conclusions
The ash±water interaction, which occurs when the ash
is disposed off using wet or slurry-disposal method, by
imparting hydrothermal activation (alkali activation) to a
Class F lagoon ash has been simulated, under the controlled
laboratory conditions. Based on the experimental investigations conducted in the present study, on the OLA and ALA
samples, the following generalized conclusions can be
drawn:
1. Activation of the ash with 1 M NaOH, leads to the formation of Zeolites NaP1 and Hydroxy-sodalite, when the
activation times are 12 and 24 h, respectively. However,
activation with KOH does not lead to zeolitization of
the ash.
2. It has been noticed that the alkali activation is responsible
for etching (and hence dissolution) of the silica present in
the ash.
3. The alkali activation of the OLA sample results in an
increase in its speci®c gravity, speci®c surface area and
its CEC.
4. It is observed that the ALA is much more ef®cient in
retaining the heavy metals, even at low pH value. The
amount of Pb retained by the ash sample increases as the
pH of the solution and contact time increases.
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