Preparation and characterization

Preparation and characterization of sorbents prepared from ash (waste
material) for sulfur dioxide (SO2) removal

J Mater Cycles Waste Manag (2005) 7:16–23 © Springer-Verlag 2005
DOI 10.1007/s10163-004-0121-2
Keat Teong Lee · Subhash Bhatia
Abdul Rahman Mohamed
Preparation and characterization of sorbents prepared from ash (waste
material) for sulfur dioxide (SO2) removal
ORIGINAL ARTICLE
K.T. Lee · S. Bhatia · A.R. Mohamed (*)
School of Chemical Engineering, Engineering Campus, Universiti
Sains Malaysia, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang,
Malaysia
Tel.
604-5941012; Fax
604-5941013
e-mail: chrahman@eng.usm.my
Received: September 9, 2003 / Accepted: March 15, 2004

Abstract Sorbents synthesized from various types of ash
(coal fly ash, coal bottom ash, oil palm ash, and incinerator
ash) for flue gas desulfurization were investigated. The sorbents
were prepared by mixing the ashes with calcium oxide
and calcium sulfate using the water hydration method. The
effects of various sorbent preparation variables, such as the
hydration period, the ratio of calcium oxide to ash, and
the amount of calcium sulfate, on the Brunauer-Emmett-
Teller (BET)-specific surface area of the resulting sorbent
were studied using a two-level full factorial design. The
surface area of the sorbents obtained range from 15.4 to
122.1m2/g. Regression models were developed to correlate
the significant variables to the surface area of the sorbents.
An analysis of variance (ANOVA) showed that the model
was significant at a confidence level of 95%. It was found
that apart from all the individual variables studied, interactions
between variables also exerted a significant influence
on the surface area of the sorbent. From the activity test
results, it was found that sorbents prepared from coal fly ash
and oil palm ash have the highest SO2 absorption capacity.
Scanning electron microscope (SEM) analysis showed that
the sorbent was composed of a compound with a high structural
porosity, while an X-ray diffraction spectrum showed
that calcium aluminum silicate hydrate compounds are the
main products of the hydration reaction.
Key words Ash · Desulfurization · Sorbent · Statistical
design · Surface area
Introduction
Ash is a waste product left over after the burning of many
combustible substances such as paper, rubbish, coal, and
many others. Some of the main industries in the world that
are producing ash in large quantities are coal-fired power
plants (burning coal to generate electricity), palm oil mills
(disposing of the remains of the fruit), and incinerators
(incinerating domestic rubbish). For instance, a 1400-MW
power station burning coal produces 1.3 million tonnes of
coal ash per year, which must be disposed of or used in other
areas. Energy and environmental forecasts for the coming
years point to a greater use of coal as a source of electricity.
As more utilities are forced to change from gas and oil
to coal as a source of fuel, the availability of quantities of
ash (coal ash) will definitely increase.1
Ash primarily consists of silica (SiO2) and alumina
(Al2O3). Secondary ingredients are carbon and oxides of
iron, calcium, magnesium, and potassium. Although there
are many well-developed technologies for ash utilization
(particularly coal fly ash) throughout the world (the manufacture
of cement, concrete, building bricks, thermal insulation
bricks, aggregates in pavements, soil stabilization, and
soil conditioning),1 most of the ash produced still has to be
disposed of, in either landfills or ash ponds. For instance, in
the USA (in 2000), only 30% of the 57 millions tonnes of
ash produced annually is being utilized, while the rest of it
is disposed of in landfills. This method of ash disposal
requires a lot of land, which is not easily available in urban
areas. Apart from that, the utilization of other types of ash,
such as oil palm ash and incinerator ash, are still limited.
In 1987, Jozewicz et al.2 reported that when silica and/or
alumina (eluted from coal fly ash) is mixed with calcium
hydroxide (Ca(OH)2) or calcium oxide (CaO), a sorbent
with a high sulfur dioxide (SO2) sorption capacity can be
attained. This is due to the pozzolanic reaction between
Ca(OH)2 or CaO and silica and/or alumina that produces
highly hydrated compounds (calcium aluminum silicate
hydrate compounds) that have a large surface area. These
large surface area compounds have a high SO2 capture
capacity. The pozzolanic reaction starts with the elution of
silica and/or alumina from the ash by alkaline water, and
this step is believed to be rate-limiting. The silica and/or
alumina then reacts with Ca(OH)2 or CaO to form calcium
aluminum silicate hydrate compounds. It has also been
shown that the addition of calcium sulfate (CaSO4) to the
preparation mixture can further promote the formation of
the hydrated products.3 This phenomena is because of the
role played by CaSO4, which promotes the formation of
hydrated products by suppressing the crystal growth of
Ca(OH)2 to force the reactivity of the Ca(OH)2 to produce
hydrated products.
This method of SO2 removal using a sorbent is known as
the dry desulfurization process. This process offers an
alternative technology to the wet desulfurization process,
which is currently being used in many industries. In the
wet process, SO2 is removed by a limestone–gypsum
method. Although the wet desulfurization process is
suitable for large-scale boilers such as those installed in
coal- or oil-fired power plants, it has many disadvantages.
Among some of them are the large space required, for
installation, the large volume of water required, and the
high capital and operating expenses. Owing to the limitations
of the wet process, the dry process is a more attractive
alternative since it overcomes all the limitations of the wet
process.
Apart from that, the use of ash as a source of silica and/or
alumina for synthesizing the sorbent for the dry desulfurization
process is attractive both economically and environmentally,
as ash is the most voluminous by-product from
combustion systems.4 After its reaction with sulfur dioxide,
the sorbent is converted into an eco-friendly product which
can be disposed off easily owing to its multifunctional uses.
This includes its application as a fertilizer, a coagulating
agent, and a deodorizer for refrigerators, shoes, and even
pet excretions.Thus, the preparation of sorbents for flue gas
desulfurization using coal fly ash has recently been studied
extensively.5–12 It is well known that the reactivity of the
hydrated sorbents depends strongly on their surface area.
Fernandez et al.4 reported that the SO2 capture activity of
these sorbents generally increases with the higher specific
surface area of the sorbent. In 2003, Lin et al.13 reported that
the use of calcium in the sorbent increased linearly with the
increasing specific surface area of the sorbent. However,
most of the sorbents reported so far have been prepared
from coal fly ash.
In this study,sorbents prepared from different types of ash
(coal fly ash, coal bottom ash, oil palm ash, and incinerator
ash) using water hydration were investigated. The significance
of the influence of various sorbent preparation variables,
such as the hydration period (x1), the ratio of calcium
oxide (CaO) to ash (x2), and the amount of calcium sulfate
(CaSO4) (x3), on the Brunauer-Emmett-Teller (BET)
surface area of the sorbent was studied using a two-level full
factorial statistical design. Data from the experimental
design were then used to develop a multiple linear regression
model that correlates the significant experimental variables
with the BET specific surface area of the sorbent.The validity
of the model was verified using the Analysis of Variance
(ANOVA). X-ray diffraction (XRD) was used to detect the
various phases present in the sorbent, while scanning electron
microscope (SEM) analysis was used to observe the
macrostructural properties of the sorbent. The sorbent was
also tested for its efficiency in the removal of SO2.

Experimental
Sorbent preparation
The sorbents were prepared from calcium oxide (CaO),
calcium sulfate (CaSO4), and various types of ash. The
calcium oxide (laboratory grade) and calcium sulfate
(reagent grade) used were supplied by BDH Laboratory
Supplies, UK. The coal fly ash and coal bottom ash were
supplied by the Kapar Power Plant, Malaysia, of Tenaga
Nasional Berhad, while the oil palm ash was supplied by the
United Palm Oil Mill, Malaysia, and the incinerator ash was
supplied by the Department of Environmental Engineering,
Kyoto University, Japan, from one of the incinerator plants
operating in Japan. The BET surface areas of these raw
materials were determined using an Autosorb 1C Quantachrome
analyzer, and the results are given in Table 1.The
chemical compositions of the various ashes were analyzed
using a Rigaku X-ray Spectrometer RIX 3000, and the
results are given in Table 2.
The procedure used to prepare the sorbent is as follows.12
To prepare 13g sorbent, 5g calcium oxide is added into
100ml water at 65°C. After stirring, the temperature of the
slurry will eventually increase to about 80°C, and 5g ash and
3g calcium sulfate are then added to the slurry simultaneously.
The slurry is then heated to about 95°C for 10 h in
order for the hydration process to occur. After the hydration
process, the resulting slurry is filtered and dried at
200°C for 2 h. The sorbent, in powder form, is then made
into pellets and subsequently crushed and sieved into the
required particle size range (250–300μm).
Physical and chemical analysis
The sorbents were analyzed for their BET-specific surface
area (calculated using the BET standard method) and
pore-size distribution [calculated using the Barrett-Johner-
Halenda (BJH) method] using an Autosorb 1C Quantachrome
analyzer. The XRD spectra of the powdered
samples were recorded on a Philips PW 1820 system with
Cu–Kα radiation in a diffraction angle (2θ) range of 5°–90°
at a sweep rate of 3 deg/min. SEM image were taken with a
Leica Cambridge S360 camera with 15kV of accelerating
voltage. The chemical composition was determined using a
Rigaku X-ray Spectrometer RIX 3000.

Activity test
The activity test was carried out in a fixed-bed stainless steel
absorber (13 cm outer diameter) under isothermal conditions
at 100°C.This reaction temperature was used because
at a reaction temperature of 100°C or above, the hydrated
water trapped in the sorbent can be released, thus giving
moisture and new pores to the sorbent. The generation of
new pores will give an additional surface area for the reaction
between the sorbent and SO2 to occur. The sorbent
(0.7 g) was packed into the center of the absorber and supported
by glass wool.The particle size of the sorbent was in
the range 250–300μm. A gas stream of 2000 p.p.m. of SO2,
with N2 as the balance, was passed through the sorbent.14
Before that, the gas stream was passed through a humidification
system where the gas was saturated with water vapor.
The total flow rate of the gas stream was controlled at
150ml/min using a mass flow controller. The concentration
of SO2 in the flue gas was measured using the portable flue
gas analyzer Enerac 2000E before and after the absorption
process. The concentration of SO2 was recorded continuously
until it reached a steady state. A schematic diagram
of the experimental setup used for the activity test is given
in Fig. 1.
Statistical experimental design
A two-level full factorial experimental design was used to
evaluate the significance of three sorbent preparation variables
on the BET-specific surface area of the sorbent.Table
3 lists the range and levels of the three independent variables
studied in terms of coded and actual values. The relation
between the coded independent variables and the
actual values are given in Eq. 1.

(1)
where xi is the coded value of the independent variable i,
and Ai, A0, and ΔA are the actual value, the actual value at
the center point, and the step change of variable i, respectively.
For this, a 23 full factorial design with four replicates
at the central point was employed, which means that 12
experiments are required for each type of ash for this procedure.
The complete design matrix and results are shown

in Table 4. All the experimental runs were carried out in
random order to minimize personal bias. Data from the
design were subjected to a multiple linear regression model
(in coded form) as in Eq. 2 using Design Expert 6.0.6
software.
(2)
where Y is the measured surface area in m2/g,A0 is the intercept
term, and A1 to A7 are the coefficients of the effects of
variables xi, xixj, and xixjxk, respectively. The variable xixjxk
represent the first-order interaction between the variables
studied.
Results and discussion
A two-level full factorial design was employed to evaluate
the influence of three experimental variables on the surface
area of a sorbent prepared from four types of ash.The complete
design matrix and results are shown in Table 4. Experimental
runs 9-I to 9-IV were performed at the center point
of the experimental design in order to determine the experimental
error. As the results of these four runs were consistent,
only a single replicate experiment was needed for
this study. In relation to the results of the BET-specific
surface areas given in Table 4, it was found that the value
obtained ranged from 24.2 to 122.1m2/g, from 21.3 to 42.2
m2/g, from 15.4 to 60.4m2/g, and from 15.7 to 31.2m2/g for
sorbents prepared from coal fly ash, coal bottom ash, oil
palm ash, and incinerator ash, respectively. This range is
higher than the surface area range of the starting materials,
as shown in Table 1 (1.46–5.62m2/g). It was also observed
that the optimum BET surface area for the sorbents prepared
using the various types of ash was obtained with dif-ferent preparation conditions. This might be a result of the
varying composition of the various ash used. From the four
types of ash used, it was found that the sorbent prepared
from coal fly ash gives the highest BET surface area, at a
value of 122.1m2/g, using preparation conditions of a hydration
period of 10 h, a CaO to ash ratio of 2 :1, and 3g of
CaSO4.
The results presented in Table 4 were subjected to analysis
using Design Expert 6.0.6 software. Regression analysis
was used to determine the coefficient A1–A7 in Eq. 2, while
the significance of the coefficient was determined by using
ANOVA. An analysis of variance is the most effective
analysis technique in factorial-designed experiments.15 At a
confidence level of 95%, the variance test showed that some
of the coefficients were not statistically significant. Therefore,
coefficients that were not significant were dropped
from the equation leaving only the significant terms. The
finalized regression equations (in coded form) that correlate
the significant sorbent preparation variables to the
sorbent BET surface areas for sorbent prepared from coal
fly ash, coal bottom ash, oil palm ash, and incinerator ash
are given in Eqs. 3, 4, 5, and 6, respectively.

where the first term in each equation is the average BET
surface area for sorbents with a solid code of 1–8. A positive
sign in front of the terms indicates a synergistic effect,while a negative sign indicates an antagonistic effect. The
coefficients of determination (R2) for Eqs. 3–6 were determined
as 0.999, 0.845, 0.988, and 0.946, respectively, and the
absolute average percentage deviation (AAPD) between
calculated and experimental data were found to be 0.1%,
7.3%, 4.5%, and 4.2%, respectively.
The high values for the coefficients of determination for
all four regression equations indicate that the equations
developed are reliable in predicting the sorbent surface
area. Also, the low value of the absolute average percentage
deviation obtained for the four equations further supports
the finding that the model is highly reliable. From
these statistical tests, it can be concluded that the regression
model equations developed have successfully captured the
relation between the significant experimental variables and
the surface area of the sorbent.
Regression Eqs. 3–5 clearly show that the coefficient for
hydration period (x1) is the highest of all the variables.Thus,
it is possible to conclude that the effect of this variable on
the surface area of sorbents prepared from coal fly ash, coal
bottom ash, and oil palm ash is the strongest. It has been
reported that the dissolution rate of silica and/or alumina
present in the ash is the rate-limiting step for the formation
of calcium aluminum silicate hydrate compounds.15 This is
in agreement with our results because as the hydration
period proceeds, more silica and/or alumina will elute from
the ash to react with CaO to form calcium aluminum silicate
hydrate compounds and thus have a positive effect on
the sorbent surface area.
On the effect of the CaO to ash ratio on the surface area
of the sorbent, only sorbent prepared from coal fly ash
(from Eq. 3) has a significant positive effect, while for
sorbent prepared from coal bottom ash (from Eq. 4) and oil
palm ash (from Eq. 5), the effect of this variable is negligible.
The ratio of CaO to ash generally determines the
amount of CaO present in the preparation mixture. It is well
known that the amount of CaO and silica and/or alumina
dissolved during the hydration reaction determines the
amount of active species formed during the pozzolanic reactions.
Looking at the composition of the ashes, it was found
that coal fly ash contains the highest percentage of silica and
alumina at 80%, while coal bottom ash and oil palm ash
contain only 57% and 51.3%, respectively. In the case of
sorbent prepared from coal fly ash, owing to the high
content of silica and alumina in the ash, more CaO is
required to form the active species. Thus, a higher ratio of
CaO in the preparation mixture will result in the formation
of more active species. However, owing to the lower content
of silica and alumina in the other two ashes, increasing the
amount of CaO in the preparation mixture is not crucial as
there is already sufficient CaO to react with the silica and
alumina in the preparation mixture even at a CaO to ash
ratio of 1 :1.
The addition of CaSO4 during the preparation step had
a positive effect on the surface area of sorbent prepared
form coal fly ash (from Eq. 3), while it had a negative effect
on the sorbent prepared from coal bottom ash (from Eq. 4)
and oil palm ash (from Eq. 5).The positive effect of CaSO4
on the surface area of sorbent prepared from coal fly ash
was also reported by Ishizuka et al.3 However, in his work,
Ca(OH)2 is used as the raw material instead of CaO. It was
reported that this phenomenon is due to the role played by
CaSO4, since it promotes the formation of calcium silicate
by suppressing the crystal growth of Ca(OH)2, and thus
forces the reactivity of the Ca(OH)2 to produce hydrated
products.Thus, it can be concluded that CaSO4 has the same
effect on CaO as it has on Ca(OH)2 for sorbent prepared
from coal fly ash. However, this phenomenon was not found
to be true for sorbents prepared using the other two types
of ash. This is most probably due to the different forms of
silica and/or alumina present in the ashes.
Unlike the regression models developed for sorbents
prepared from coal fly ash, coal bottom ash, and oil palm
ash, Eq. 6 shows that there is no significant effect of individual
variables on the surface area of sorbent prepared
from incinerator ash. However, it is only the interactional
effects between the variables that exerts significant effects
on the surface area of the sorbent. In another words, if one
of the variables is changed with respect to another one, it
will have a considerable effect on the total surface area of
the sorbent. Interactional effects between the variables is
also significant in the sorbent prepared from coal fly ash and
oil palm ash, as shown in the regression models in Eqs. 3
and 5.
Sorbents with the highest surface area for each type of
ash were subjected to an SO2-removal activity test in the
experimental rig shown in Fig. 1. The breakthrough curves
(the ratio of the SO2 concentration to the initial SO2 concentration,
C/C0) for the prepared sorbents and inert silica
sand are shown in Fig. 2. Breakthrough curves for inert silica
sand were used as controls. From this figure, it is seen that
the sorbents prepared from coal fly ash and oil palm ash
give the highest SO2 absorption activity, and SO2 was completely
removed from the system in the first 10min. From
that point, the concentration of SO2 gradually increases
until there is no more SO2 absorption activity in the sorbent
(i.e., when the SO2 concentration in the outlet flue gas is the
same as the inlet concentration). Although the surface area
of sorbent prepared from oil palm ash O6 (60.4m2/g) is only
about half the surface area of sorbent prepared from coal
fly ash F4 (122.1m2/g), it was found that both of the sorbents
exhibited similar desulfurization activity. An XRD spectrum
and a SEM micrograph (shown below) revealed that

the good desulfurization activity by the sorbent prepared
from oil palm ash was probably due to the needle-like
macrostructure of the potassium calcium aluminum silicate
hydrate (K2Ca2(Al2Si)16O32·13.5H2O) in the sorbent. In this
needle-like macrostructure, it is believed that the calcium
(Ca) ions in the compound are arranged in such a way that
it is easier for an excess of SO2 molecules.Thus, the sorbent
prepared from oil palm ash can still give very good desulfurization
activity even though its surface area is only half
that of sorbent prepared from coal fly ash. From Fig. 2, it
can also be concluded that the sorbents prepared from coal
bottom ash and incinerator ash also have the capacity to
absorb SO2, but their absorption capacity is not as high as
that of the sorbents prepared from coal fly ash and oil palm
ash. This is probably due to the low surface area of these
sorbents.
The XRD spectra for sorbent prepared from coal fly ash
(F4) before and after it was subjected to the activity test
are shown in Fig. 3a and b, respectively. From Figure 3a, it
can be deduced that calcium aluminum silicate hydrate
(Ca–Al2Si4O12·2H2O), calcium carbonate (CaCO3), and
calcium sulfate (CaSO4) are the main phases present in
sorbent F4 (prepared from coal fly ash). The formation of
Ca–Al2Si4O12·2H2O and CaCO3 occurs in the hydration
process, while CaSO4 is the raw material which has not
reacted.The presence of the Ca–Al2Si4O12·2H2O compound
in the sorbent (from the pozzolanic reaction of the raw
materials) which has a high surface area is believed to be
the main contributor to the high SO2 absorption capacity of
that sorbent. Although the sorbent consists of a complex
hydrated compound, it is believed that the active species in
the sorbent that reacts with SO2 is the calcium (Ca) ions
only. The role of the high-surface-area hydrated compound
is basically to make the Ca ions contained in the sorbent
more accessible to SO2 during the desulfurization reaction.
The absence of CaO in sorbent F4 shows that it reacts completely
to form Ca–Al2Si4O12·2H2O. Figure 3b shows the
XRD spectrum of sorbent F4 after it was subjected to the
activity test in the experimental rig shown in Fig. 1. Only
two phases are detected in this spectrum, i.e., aluminum
silicate hydrate (Al4Si2O10·H2O) and CaSO4. This shows
that all the calcium ions present in the sorbent have reacted

with SO2 in the desulfurization reaction.The Al4Si2O10·H2O
could be from the Ca–Al2Si4O12·2H2O after it has reacted
with SO2.
Table 5 lists the various phases detected in the fresh
sorbent (before the desulfurization reaction) prepared from
coal bottom ash, oil palm ash, and incinerator ash using
XRD analysis.The main phases detected in the sorbent prepared
from coal bottom ash and incinerator ash were found
to be similar. These phases were identified as calcium silicate
hydrate, calcium hydroxide, and calcium sulfate. This
result shows that part of the calcium oxide used in the
preparation of the sorbent is converted into calcium
hydroxide only and not into calcium aluminum silicate
hydrate compounds. This is most probably due to the low
contents of silica and alumina in coal bottom ash and incinerator
ash that limit the formation of calcium aluminum silicate
hydrate compounds. Tsuchiai et al.16 reported that the
efficiency of SO2 capture by Ca(OH)2 is very low due to its
low surface area. This explains the low absorption capacity
of these two sorbents towards SO2. Although Ca(OH)2 is
also detected in the sorbent prepared from oil palm ash, this
sorbent still manages to show good desulfurization activity.
Therefore, it is believed that the good desulfurization activity
of the sorbent is due to the presence of a potassium
calcium aluminum silicate hydrate phase.
SEM micrographs of coal fly ash and various sorbents
are shown in Fig. 4. The SEM micrograph of coal fly ash

shows that it consists mainly of spherical particles of different
sizes with smooth surfaces. However, it was observed
that in the hydrated coal fly ash (F4), compounds with a
higher structural porosity were obtained, as shown in Fig.
4b. This suggests that the spherical coal fly ash reacted
extensively with calcium oxide, so that not only the surface
layers of the spherical particles but also the inside of the
particles could not retain their original shapes.9 On the
other hand, after reacting with SO2 (Fig. 4c), the porous
structure of the sorbent was no longer seen as it was covered
by a layer of product, believed to be calcium sulfate. Comparing
the SEM micrographs of the sorbents prepared from
coal fly ash (F4) and oil palm ash (O6), it was found that in
the sorbent prepared using oil palm ash (Fig. 4d), a compound
with a needle-like structure was formed. It was concluded
that the high SO2 absorption for sorbent O6 was dueto the presence of potassium calcium aluminum silicate
hydrate, which has a needle-like structure.
The pore-size distribution of sorbent F4 before and after
the desulfurization reaction is shown in Fig. 5. It has been
reported in the literature that pore sizes between 2 and
100nm have been identified as the effective zone for the
sulfation reaction between SO2 and a sorbent prepared
from Ca(OH)2/coal fly ash,6 and therefore special attention
has been paid to this region. The BJH procedure, which
permits a better characterization of mesoporosity, was
applied to obtain the pore-size distribution from nitrogen
desorption data. The pore-size distribution is represented
by the derivative d(Vp)/d(dp) as a function of pore diameter,
where, Vp is the pore volume and dp is the pore diameter.
For fresh sorbent, mesopores with an average pore size
of 38.9 nm appeared to be the major contributor to the total
pore volume. After desulfurization, a dramatic decrease in
the mesopore volume (from 10 to 100nm) was observed,
indicating significant pore filling by the reaction product. It
can be concluded that the surface of the spent sorbent was
covered by a layer of the reaction product, thus reducing its
porosity.
Conclusions
This study has shown that sorbents prepared from coal fly
ash, coal bottom ash, oil palm ash, and incinerator ash have
the capacity to absorb SO2. Experimental evidence and
mathematical analyses showed that all the variables studied,
i.e., hydration period, weight ratio of CaO to ash, andamount of CaSO4, had a significant influence on the surface
area of the sorbents. Multiple linear regression models,
developed to predict the surface areas of the sorbents prepared
using the various ashes, were found to be successful,
with high values of the coefficients of determination (R2).
The reactivity of sorbents prepared from coal fly ash and oil
palm ash was found to be much higher than that from those
prepared from coal bottom ash and incinerator ash. The
XRD spectra and SEM micrographs showed that the differences
in reactivity of the various sorbents are due to the
different phases present in the sorbent, and are also due to
its macrostructural properties.
Acknowledgments The authors thank the ASEAN University
Network/Southeast Asia Engineering Education Development
Network (AUN/SEED-Net), the JSPS–VCC (Program on Environmental
Science, Engineering and Ethics), the Ministry of Science,Technology,
and Environment (Project No. 08-02-05-2040EA001), and the
Universiti Sains Malaysia (USM short-term grant) for the funding and
support of this project.We also express our deepest gratitude to Kyoto
University, Japan, for supplying the incinerator ash.
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