Effect of Al Source and Alkali Activation on Pb and Cu
Transcript of Effect of Al Source and Alkali Activation on Pb and Cu
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Effect of Al source and alkali activation on Pb and Cu
immobilisation in fly-ash based geopolymers
J.W. Phaira,*, J.S.J. van Deventera, J.D. Smithb
aDepartment of Chemical Engineering, The University of Melbourne, 3010 Victoria, AustraliabSchool of Chemistry, The University of Melbourne, 3010 Victoria, Australia
Received 29 November 2002; accepted 30 June 2003
Editorial handling by R Fuge
Abstract
Solidification/stabilisation technologies are attracting great interest from mining and energy industries alike, to solve
their pressing waste disposal problems. Geopolymers, in particular, are becoming one of the more popular solidifi-
cation/stabilisation methods since they can be applied to a variety of waste sources at low cost, yielding added-value
products. However, the effect of Al source on the solidification/stabilisation of heavy metals within fly ash-based
Geopolymers, has received little attention. This study examines the effect of variable Al source and alkali-activator on
the final properties of fly ash-based Geopolymers as characterised by compressive strength testing, infrared and X-ray
diffraction analyses. Leaching tests were performed to determine the efficiencies of Pb and Cu immobilisation, which
were compared to the initial properties of the Al source (e.g. particle size, cation exchange capacity, total extractable
cation concentration and suspension yield stress). It was observed that Pb was generally better immobilised than Cu. Inaddition, the total extractable cation concentration of the Al source greatly affected the efficiency of Pb immobilisation
while the physical properties of the Al source (suspension yield stress and eventual compressive strength) determined
the efficiencies of Cu immobilisation. For both metals, NaOH activation was the most favourable method for metal
immobilisation, however, a clear mechanism of adsorption remains elusive.
# 2003 Elsevier Ltd. All rights reserved.
1. Introduction
Industrialised societies are producing progressively
more waste as a result of their burgeoning mining and
energy industries (Inyang and Bergeson, 1992). Thesewastes are amassing to such an extent that giga-scale
disposal is becoming a common phenomenon (Scheetz
et al., 1999). Often, giga-scale wastes such as mine tail-
ings have low-level amounts of heavy metals (e.g. Pb)
associated with them that must either be extracted or
immobilised before proper disposal can occur. If immo-
bilisation or fixation of the heavy metals within the
waste source itself does not occur, these hazardous
materials require a special landfill consisting of double
plastic and clay liners as well as a comprehensive lea-
chate collection system (Camobreco et al., 1999). Theseare mandatory requirements to ensure the waste is dis-
posed of acceptably. Such geotechnical solutions are,
however, very expensive and time-consuming to install,
requiring constant maintenance and monitoring for the
first 30 years after installation (Dijkema et al., 2000).
Given the large quantities of materials being disposed
of, it is often easier to treat the waste in situand prevent
its interaction with the environment, rather than to
separate the heavy metals from the material or install
geotechnical infrastructure. Solidification/stabilisation
presents itself as a highly practical method of immobili-
sation, since it can be applied to a wide variety of waste
0883-2927/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0883-2927(03)00151-3
Applied Geochemistry 19 (2004) 423434
www.elsevier.com/locate/apgeochem
* Corresponding author at present address: Turner-Fair-
bank Highway Research Center, FHWA, McLean VA 22101,
USA. Fax: +1-202-493-3086.
E-mail address: [email protected](J.W. Phair).
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sources containing heavy metals. Out of the various
solidification/stabilisation techniques available, fly ash-
based geopolymers are attracting significant commercial
interest for their cost-effectiveness and flexibility. For
instance, geopolymers have already been used to immo-
bilise and stabilise low-level radioactive waste of pure or
contaminated (mixed waste) forms (Mollah et al., 1992)as well as heavy metals (Van Jaarsveld et al., 1997).
When adequately fixed, the new waste form can find
subsequent service in construction and road/pavement
applications as an added-value product, closing the loop
on the material life-cycle.
The purpose of this present study is to further evalu-
ate and substantiate the wide applicability of fly ash-
based geopolymer binders, by investigating the effect of
variable Al source on the leaching properties. This will
not only establish which and why particular Al sources
work best within the geopolymers, but will help to
develop a set of criteria for predicting whether a new Alsource would be suitable for immobilisation.
2. Background
Geopolymers are a diverse group of ceramic-like
materials formed by a geosynthetic reaction of alumi-
nosilicate minerals in the presence of an alkali solution
at low temperatures (30% Al). Initially in
geopolymer synthesis, this was restricted to metakaolin
but has since been extended to include kaolin (Van
Jaarsveld et al., 1997), feldspar (Xu and Van Deventer,
2000b) and stilbite (Xu et al., 2001). A typical fly ash-
based geopolymer mix now consists of approximately
60% mass dry fly ash and approximately 12% mass dry
Al source (Phair and Van Deventer, 2002a; Swanepoel
and Strydom, 2002; Van Jaarsveld et al., 1998a). Therest of the mix is the alkali silicate mixing solution
although in most large-scale commercial operations this
amount is substantially reduced.
It has previously been reported how small variations
in compositional factors such as the pH of the alkali
activator and the nature of the setting additive (Ca rich
source), greatly affect the efficiency of metal immobili-
sation within fly ash-based geopolymers (Phair and Van
Deventer, 2001, 2002b). However, the role of the Al
source in optimising metal immobilisation and other
material properties of fly ash-based geopolymers, has
received little attention. The present work, therefore,
aims to determine the effect of variable Al source (kao-
linite, metakaolinite, K-feldspar and fly ash), on the
immobilisation of heavy metals (Pb and Cu) within fly
ash-based geopolymers. Through examining the mate-
Fig. 1. Schematic of geopolymerisation reactions according toDavidovits et al. (1991).
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rial properties of the starting materials (eg. surface area,
cation-exchange capacity, mineralogy, particle size and
suspension properties), explanations for the final mate-
rial and leaching properties of the geopolymers can be
given as a function of Al source.
An important aspect of this work will also be to
evaluate the effect of the Al source on the mechanism ofimmobilisation of heavy metals in comparison to other
compositional variables and established theories. First
discussions on the mechanism of immobilisation by
geopolymers relied upon comparisons to zeolitic mate-
rials for adsorbing/binding heavy metals (Comrie et al.,
1988; Khalil and Merz, 1991). Immobilisation of heavy
metals within Geopolymers were subsequently proposed
to include a mechanism whereby the species were locked
within the geopolymeric matrix (Van Jaarsveld et al.,
1998a). Kinetic studies, which examined the effect of
pore and particle size of crushed fly-ash based geopolymers
on leaching, continued to speculate that immobilisation ofheavy metals occurs via adsorption onto a zeolite-like
backbone (Van Jaarsveld and Van Deventer, 1999; Van
Jaarsveld et al., 1997, 1998a). However, given the anionic
speciation of the metals at high pH, the existence of a strict
electrostatic adsorption mechanism is debatable.
Nonetheless, it may still be possible for some form of
adsorption to occur to a foreign substrate if both surface
and solution reactions are included in its description. By
adding heavy metals to the Al source (typical heavy metal
adsorbents) prior to wet mixing, it will be possible to focus
attention on the role of adsorption in the immobilisation
of heavy metals within fly ash-based geopolymers.
3. Experimental
3.1. Materials
Sodium silicate (Vitrosol N(N40), P.Q. Australia Pty.
Ltd, Dandenong South, Victoria) with weight%
SiO2=28.7, weight ratio SiO2 : Na2O=3.22 ([SiO2]=
6.62 M) and NaOH (AR, Ajax Chemicals Australia,
Sydney, NSW) were used for the alkali-activating solu-
tions. All solutions were diluted daily from stock solu-tion using distilled water. Fly ash used in the synthesis
of all Geopolymer matrices was of coal origin and
obtained from Port Augusta, South Australia. Kaolinite
(HR1 Grade) and K-Feldspar were obtained from
Commercial Minerals, Sydney, Australia. Metakaolinite
was obtained by calcining kaolinite at 700 C for 6 h.
Oxide compositions of the starting minerals (listed in
Table 1) were determined on a Siemens SRS3000 sequen-
tial X-ray fluorescence spectrophotometer after fusing the
samples with lithium borate. Distilled water was used
throughout and all other chemical reagents were of AR
grade unless otherwise stated.
3.2. Characterisation methods and sample preparation
BET surface area was determined for the Al source
materials used in the synthesis of the fly ash-based
Geopolymers, using a Micromeritics Flowsorb ASAP
2000 with a 30/70 ratio of N2and He. Particle size of the
materials was measured using a Coulter LS 130 opticalparticle size analyser and the density was determined
using a pycnometer. Determination of cation exchange
capacity and quantities of the total extractable cations
for the various Al sources was conducted at pH 7
(ammonium acetate buffered) (Chapman, 1965). The
method was chosen since none of the solids present were
acidic. Ammonium (N) concentration was determined
using an Orion ammonium electrode and voltmeter.
Equilibrium suspensions obtained for each Al source
were analysed for elemental concentrations after cen-
trifuging, filtering and diluting with 5% conc. HCl, by
an ICP-OES PerkinElmer 3000. Yield stress measure-ments of concentrated suspensions of the Al sources in
diluted Na-silicate were measured on a vane rheometer
as described earlier (de Kretser et al., 1998). The
experimental apparatus consists of a small 4-bladed
vane attached to the spring-driving motor of a Haake
RV-3 viscometer. Yield stress experiments were con-
ducted on 40% solid (mass) suspensions in solutions of
2:1 ratio H2O to concentrated Na silicate. The physical
and chemical characteristics of the starting materials are
provided inTables 2 and 3 respectively.
The geopolymer samples were synthesised as descri-
bed previously (Phair and Van Deventer, 2001, 2002a;
Van Jaarsveld et al., 1998b). The samples were cast in
50mm cubes, vibrated for 2 min and set at 23 C for 7
days and thereafter stored at room temperature. Dis-
solved Pb(NO3)2or Cu(NO3)2was added to the reaction
mixture. The mixes were activated with NaOH (Orica
Table 1
X-ray fluorescence fusion analysis of the oxide compositions of
Al source materials used in the synthesis of fly ash-based geo-
polymers
Chemical
composition
(wt.%)
Kaolinite K-Feldspar Metakaolinite Fly
ash
SiO2 52.4 67.1 59.6 48.5
Al2O3 28.6 17.6 33.9 29.6
CaO 0.1 0.2 0.2 6.1
Fe2O3 1.2 0.2 1.2 4.6
MgO 0.2 0 0.3 2.3
K2O 0.2 10.6 0.2 0.9
Na2O 0.1
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Aust. Pty. Ltd.), Na-silicate (PQ Aust. Pty. Ltd) or a
combination of both. The SiO2: Al2O3ratios used in the
mixes followed that of previous work for kaolinite and
metakaolinite (Van Jaarsveld et al., 1998b). Composi-
tions of the synthesised fly ash-based geopolymers
are provided in Table 4. From Table 4 it is clear that
the proportion of Al source in the matrices was held
constant throughout the samples. The % mass of Na-
silicate, NaOH or Na-silicate and NaOH was also held
constant.
After weighing three-50 mm cubes for each sample,
compressive strength testing was performed according
to AS 1012.9 and the average results recorded. All sam-
ples were tested after 7 days using an Amsler FM 2750
compressive strength testing apparatus. For optical
microscopy, samples were taken for analysis after being
cured for 7 days. Thin sections were mounted on glass
slides and were polished, coated with epoxy resin and
cover plate before being placed under the microscope.
Sample thin sections were around 30 mm thick when
they were analysed. An Olympus AX70 optical micro-
scope was used and photographs were taken with a 35
mm Olympus camera.
The Infrared spectra of the ground samples were
recorded using the KBr pellet technique on a Bio-RadFTS 165 FTIR spectrometer. X-ray powder diffraction
traces were obtained using a Phillips PW 1800 dif-
fractometer with CuKa
radiation generated at 20 mA
and 40 Kv. Specimens were step scanned as random
powder mounts from 570 2y at 0.02 2 steps inte-
grated at the rate of 1.2 s per step.
3.3. TCLP leaching tests
The Toxicity Characteristic Leaching Procedure
(TCLP, 1990) protocol is designed to simulate the
environmental conditions experienced by landfill, inorder to determine whether wastes are suitable for
landfill disposal. Usually, these tests are conducted over
a relatively short period of time (
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maintained at 21 C. All samples were stirred using
overhead impellars for 18 h instead of tumblers. Tum-
blers are normally used to simulate the action of water
seeping through waste in a landfill. However, impellars
can be used to simulate a more aggressive leaching
environment. Sampling of the leachate solution was
conducted by syringe. The extracted sample solutionswere centrifuged and filtered using a 0.2 mm nylon mil-
lipore filter before dilution with acid (5% vol. Conc.
HCl) and analysis of metal concentrations using a
Perkin Elmer Optima 3000 ICP-AES.
4. Results and discussion
4.1. Characterisation of the Al source
The Al sources examined all varied in their chemical
structure and properties. Metakaolinite contains 4 co-ordinated Al while kaolinite and K-feldspar contain 6-
co-ordinated Al, although kaolinite is hydrophilic. Fly
ash contains 4 co-ordinated Al in a mostly amorphous
structure. Mineralogical compositions of the starting
materials are presented in Table 1. Metakaolinite has
the highest Al content and K-feldspar the lowest.
The physical characteristics of the materials provided
in Table 2 demonstrate consistent trends between the
samples. Generally, the minerals with the largest surface
area produced a suspension with the highest yield stress.
An explanation for this is that since kaolinite and
metakaolinite have a plate-like structure and hydro-
philic surface, their interparticle interactions and
resulting suspension yield stress are higher (Van Olphen,
1977).
The chemical characteristics of the Al source are dis-
played inTable 3. Measured cation exchange capacities
(CEC, units of meq./100 g) compare favourably with
literature values (Sharma and Lewis, 1994) with kaoli-
nite clearly having the largest value followed by meta-
kaolinite then K-feldspar. On the other hand, fly ash
had the highest extractable Ca and total cation concen-
tration followed by kaolinite, then K-feldspar. These
values may provide an indication of the extent to which
a material may be able to undergo rapid superficialreactions or precipitation.
4.2. Characterisation of fly ash-based geopolymers
Optical micrographs of geopolymeric matrices U1
and K1 are provided in Figs. 2 and 3. It is clear from
Figs. 2 and 3 just how heterogeneous the particle size
distribution is within these matrices. Fly ash particle
sizes, for instance, range from 1 to 40 mm in diameter.
Infrared spectra were recorded for all of the geopoly-
mer samples. Spectra of NaOH/Na-silicate activated
matrices of varying Al source are presented in Fig. 4,
while the peak assignments and their respective fre-
quencies are provided in Table 5. No observable peak
could be attributed to the presence of Pb or Cu. The
peaks around 1009 and 1030 cm1 have been attributed
to asymmetric stretching of AlO and SiO bonds (v1)
while the peaks at 550 cm1 have been attributed to
octahedrally co-ordinated Al (Palomo and Glasser,1992) (v2). Peaks at approximately 460 cm
1 are
assigned to in-plane bending of AlO and SiO linkages
(v3).
Only in K (kaolinite) matrices were the two peaks at
1009 and 1030 cm1 observed suggesting that the kaoli-
nite still maintains a structural role in the new matrix.
Of further interest is the carbonate peak at 875 cm1
(Yousuf et al., 1993) which increases with the concen-
tration of Na-silicate added. An additional carbonate
peak at 1450 cm1 on the other hand, maintains con-
sistent intensity throughout.
The study of gepolymers by XRD is complicated bythe extensive broad peaks associated with amorphous
(fly ash) structure, seen between 20 and 40 degrees 2in
Fig. 5 (Van Jaarsveld et al., 1998a). Sharp peaks are
normally associated with un-reacted starting materials
such as the heamatite, quartz and mullite present in the
fly ash.
4.3. Compressive strength
Compressive strength tests were performed on the
geopolymers to indirectly link the bulk material prop-
erties of the matrix to its efficiency of metal immobili-
sation. Generally, there is accepted to be a strong link
between material porosity and compressive strength. If
immobilisation efficiencies decrease with decreasing
compressive strength, then a direct correlation with
porosity can exist. If immobilisation efficiency does not
vary with compressive strength, this suggests that other
factors are affecting the immobilisation efficiency rather
than pore size/distribution alone.
The compressive strength and measured densities of the
geopolymers are presented inTable 6. All samples had a
similar density of approximately 1.6 g/cm3, however, U1
Table 5The peak assignments for the FTIR spectra of fly ash-based
geopolymers synthesised according toTable 4
Matrix n1 n2 n3 v2CO32
K1 1031.3 541.2 469
1008.5
L1 983.8 558.4 458.8 874.6
T1 984.6 564.2 458.5
U1 985.2 559.5 453.4
U2 1028.89 560.3 461.5 875.1
U3 986.4 558.5 454.4
Fly ash 1026.3 550.5 461.1
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seemed slightly denser which could be attributed to the
extensive hydration of fly ash in the absence of any
other Al source. This would cause more water to be
retained within the matrix and increase the observed
density. Geopolymers containing kaolinite and meta-
kaolinite were found to be the strongest under com-
pressive strength testing while fly ash lacked any
considerable strength alone.
At this stage, it is also possible to make a correlation
between the viscosity of the Al source and the final
strength of the matrix with the more viscous clay sus-
pensions yielding higher compressive strengths. The
increased strength may not be attributable to the visc-
osity directly, but since the viscosity is related to the
clays physical structure and morphology, it may indir-
ectly indicate the Geopolymers compressive strength.
4.4. Leaching
Leaching tests are probably the single-most important
measure of the efficiency of heavy metal immobilisation
within fly ash-based Geopolymers. Accordingly, the
Fig. 2. Optical micrograph of geopolymeric matrix U1 (fromTable 4).
Fig. 3. Optical micrograph of geopolymeric matrix K1 (fromTable 4).
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Toxicity Characteristic Leaching Procedure (TCLP) of
the EPA was the leaching method of choice due to its
wide recognition and established use for evaluating
solidification/stabilisation technologies. Not only were
equilibrium concentrations of heavy metals measured
after the leaching, but equilibrium concentrations of Si
and Al were also determined. The TCLP is typically
designed for assessing whether a particular waste is safe
for disposal by landfill. However, more aggressive
leaching conditions were used to assess the suitability of
the stabilised waste form for other applications (e.g.
roads), which have more rigorous leaching standards.
Fig. 4. FTIR spectra of NaOH/Na-silicate activated matrices.
Fig. 5. The XRD spectra for fly ash and fly ash based Geopolymer matrices (U1, U2, U3) defined inTable 4.
Table 6
The compressive strength and density of fly ash-based geo-
polymers synthesised according toTable 4
Geopolymer
matrix
Compressive
strength (MPa)
Density
(g/cm3)
K1 32.7 1.6
L1 26.8 1.61
T1 13.9 1.58
U1 7.7 1.63
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The final solution concentrations of Pb after the
leaching tests are shown inFig. 6. It is readily apparent
that the alkali activator had a consistent effect on the
efficiency of immobilisation (minimum concentration of
Pb in equilibrium solution) for all Al sources. NaOH
was determined to be the most effective activator while
Na-silicate was determined to be the least effective.
Nevertheless, all matrices were generally found to be
highly efficient in retaining Pb within the matrix with the
following order of effectiveness established: fly
ash>kaolinite>K-feldspar>metakaolinite. This trend
can be directly correlated with the liberation of Si and
Al from fly ash-based Geopolymers according to the
data in Figs. 7 and 8. No correlations, however, could
Fig. 6. Equilibrium concentrations of Pb in leachate after leaching of fly ash-based geopolymers (defined inTable 4)for 18 h.
Fig. 7. Equilibrium concentrations of Si in leachate after leaching of fly ash-based geopolymers (defined inTable 4) for 18 h.
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be observed between the efficiency of Pb immobilisation
and the compressive strength (porosity) or the viscosity
of the matrices.
In contrast to NaOH being the most effective acti-
vator for minimising the quantity of Pb leached out,
Na-silicate was determined to be the most effective
activator for retaining Al and Si. The fact that less
Al and Si is leached out in the presence of silicate
activator alone is most likely due to the fact that it is
less alkaline than NaOH and, therefore, does not
dissolve the mineral ingredients as much. This result
suggests that immobilisation of Pb requires reaction
with species dissolved from the starting materials in
order to form a new phase. Its immobilisation effi-
ciency is lowest when Na-silicate alone is used as the
activator. The new phase in which Pb is immobilised
requires Al, Si and possibly other minerals and can-
not simply rely upon the excesses of Si made avail-
able using Na-silicate.
The total extractable cation content of the mineralsource was found to be the single chemical characteristic
that correlated best with Pb immobilisation data. This
suggests that the higher the amount of available cations
on the surface, the greater the ability for insoluble metal
hydroxide/silicate compounds to undergo further reac-
tions. These subsequent reactions result in new alumi-
nosilicate phase formation, which immobilises Pb more
effectively. No correlation was observed between the
cation exchange capacity of the various mineral samples
and the efficiency of immobilisation indicating that
direct electrostatic adsorption does not occur. This is
most likely due to the high pH (>13) used in geopoly-
merisation that creates negatively charged Pb species
and reduces the adsorbent characteristics of the Al
source. As a consequence, it is suggested that Pb is
immobilised within a new phase rather than being
adsorbed onto the surface of the mineral.
Table 7 presents the leaching data for Cu doped fly
ash-based geopolymers and it is clear from the outset,
that the efficiency of Cu immobilisation is much lower
than it is for Pb. A strong correlation can be observed
between the immobilisation efficiency and the compres-
sive strength/viscosity of the Al source. This trend was
not apparent for the immobilisation of Pb, which was
determined to be dependent upon the total extractable
cation content of the Al source.
As was observed for the immobilisation of Pb (See
Fig. 6), the most effective immobilisation of Cu occur-
red when NaOH was used as the activator. With respect
to the leachate concentrations of Al and Si in Table 7, it
is clear that they are both at a maximum for NaOH
activation, followed by Na-silicate/NaOH and are at aminimum for Na-silicate activation. On the other hand,
Na-silicate/NaOH activation was determined to pro-
duce the highest concentrations of Al and Si for Pb
doped geopolymers (SeeFigs. 7 and 8). Nonetheless, the
effect of the Al source on the leachate concentrations of
Al were similar for Cu doped matrices as they were for
Pb doped matrices with the following descending order
of concentrations established: metakaolinite>fly
ash>kaolinite>K-feldspar. This trend essentially fol-
lows the Al2O3compositions of the minerals as provided
in Table 1. A similar trend for Si leachate concentra-
tions was observed for both Cu and Pb doped matrices
Fig. 8. Equilibrium concentrations of Al in leachate after leaching of fly ash-based Geopolymers (defined inTable 4) for 18 h.
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with metakaolinite and kaolinite providing the highest
concentrations followed by K-feldspar and fly ash.
4.5. Mechanism of immobilisation of heavy metals
within fly ash-based geopolymers
Sodium silicate based technology has widely been
used for remediation processes by industry. Portland
cement/soluble silicate remediation systems are based on
the reaction of polyvalent metals with soluble silicates to
form a 3-D polymer matrix with a structure similar to
that of a natural pyroxene (Conner, 1990). In geopoly-
merisation, Na silicate activation alone does not attain
the highest efficiency in the immobilisation of Pb and
Cu. Sodium silicate activation does, however, allow for
a considerable reduction in the leaching of Al and Si
from the matrices compared to NaOH. This is most
likely due to the fact that Na silicate is less alkaline than
NaOH and, therefore, does not liberate such species as
readily during the hydration of the starting materials.
This process is to be distinguished from Na silicate
causing an apparent improvement in the susceptibility
of fly ash based-geopolymers to leaching conditions,thus reducing the quantities of Al and Si liberated.
NaOH activation alone was determined to provide the
most effective environment for immobilising both Pb
and Cu.
This immediately indicates the advantages of geopoly-
mers for heavy metal immobilisation compared to tech-
niques relying on Na silicate as the heavy metal binding
agent alone. Adding more alkali is expected to increase
the solubility of the metals since Cu and Pb hydroxides
are more labile than a bulky polysilicate Pb or Cu pre-
cipitate, but this is offset by the advantages associated
with the degradation of the mineral starting materials at
higher pH. Only by liberating more Al, Si, Ca etc. spe-
cies in situ, is it possible to provide the right environ-
ment to form a new phase that will adequately bind and
encapsulate the heavy metal into a new, more insoluble
form.
While alkali activation has a consistent effect on the
immobilisation of both Cu and Pb within fly ash-based
geopolymers, it is readily apparent that the Al source
affects the immobilisation efficiencies in separate ways
for both Cu and Pb. While Pb immobilisation was
demonstrated to be highly dependent upon the total
extractable cation concentration of the Al source, Cu
immobilisation efficiencies depended more upon the
overall physical characteristics of the Al source and its
contribution to the compressive strength of the final
material.
This result can be explained by the fact that Pb pre-
cipitates are larger and less labile than Cu precipitates.
Therefore, they are more susceptible to subsequent
reactions and may be influenced by the presence of extra
cations (from the Al source), which may stabilise the
formation of new amorphous aluminosilicate phases.
Furthermore, the possibility of Pb being adsorbed ontothe surface of the Al through a series of complex reac-
tions is not excluded.
The immediate consequence of Cu precipitates being
more labile is that their immobilisation efficiencies are
considerably less than for those of Pb. Thus, the rela-
tive efficiencies of Cu immobilisation are dominated by
the bulk physical properties of the matrix, which con-
trol the migration of Cu precipitates. Properties that
have been shown to influence the efficiency of Cu
immobilisation include the eventual compressive
strength of the matrix and the suspension viscosity of
the Al source.
Table 7
Leaching data for fly ash-based geopolymers containing Cu as a function of Al source and alkali activator (element concentrations are
in units of mg/l)
Element Matrix Al Source Na-Silicate/NaOH Na-Silicate NaOH
Cu K Kaolinite 85.7 128 81.6
L Metakaolinite 101 120 91.6T K-feldspar 116 122 70.1
U Fly Ash 113 149 79.3
Al K Kaolinite 1180 469 1740
L Metakaolinite 1800 401 2240
T K-Feldspar 1210 229 1430
U Fly Ash 1620 202 1920
Si K Kaolinite 842 539 1070
L Metakaolinite 853 491 997
T K-Feldspar 679 245 849
U Fly Ash 333 235 979
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4.6. The role of the Al source in fly ash-based
Geopolymers
The present work demonstrated that it was possible
to utilise variable Al sources to successfully synthesise
fly ash-based Geopolymers for the purposes of heavy
metal immobilisation. While the study was restricted todirectly comparing only four Al sources, a list of all the
Al sources that may be used in fly ash-based Geopoly-
mers is far from complete.
So far, it is possible to define three main requirements
if a particular Al source is to be successfully included
within a fly ash-based Geopolymer. Firstly, the Al
source must provide enough Al and in a form useful for
Geopolymerisation reactions (Van Jaarsveld et al.,
1997). This may also require the presence of minimum
quantities of Si, Ca, K etc. Secondly, the Al source must
be an additive that optimises (or at least does not
detract) the engineering properties of the Geopolymericmatrix eg. shrinkage, compressive strength, brittleness
etc. (Davidovits et al., 1990). Thirdly, if the Al source
has the ability to act as an adsorptive/reactive surface
when added to a wet mix containing heavy metals, then
it will minimise the leaching of heavy metals or other
contaminants from the waste.
Special care must be given when utilising clay based
materials as the Al source, since an increase in the
swelling capacity and water adsorption of the mix often
accompanies better metal adsorption properties. This
may subsequently affect the density and strength of the
final product (Guler et al., 1995).
5. Conclusions
Fly-ash based geopolymers are emerging as a viable
alternative for the solidification/stabilisation of bulk
industrial wastes contaminated with heavy metals. By
fixing heavy metalsin situ, it is possible to create added-
value products, which do not necessarily have to be
disposed of immediately. Examination of varying the Al
source within various fly ash-based geopolymers has
confirmed that the geopolymers generally retain useful
immobilisation properties no matter which Al source isutilised.
The immobilisation of Pb in geopolymers involves a
chemical immobilisation process that depends on the
total extractable cation concentration of the Al source
and the type of alkali activator used. Only NaOH acti-
vated matrices consistently produced a TCLP leachate
within the EPA limit of 5 mg/l for Pb. K-feldspar and
kaolinite exclusively, produced a TCLP leachate within
the EPA guideline under NaOH/Na silicate activation.
Immobilisation of Cu within fly ash-based geopolymers
was not as effective due to the increased lability of Cu
precipitates. Nevertheless, alkali activation with NaOH
still produced the most favourable environment for Cu
immobilisation.
It is concluded that the mechanism of immobilisation
of Pb and Cu not only involves a physical encapsulation
mechanism, but the formation of a new phase through
the reaction of the insoluble Pb or Cu compounds with
Al- and Si-rich species dissolved from the Al source.Moreover, the immobilisation mechanism of Pb is
directly affected by the extractable alkali cations from
the Al source while the physical characteristics (suspen-
sion viscosity, final material compressive strength) of the
Al source largely control the immobilisation of Cu. New
aluminosilicate phase formation is a necessity for the
most efficient immobilisation of Pb and Cu and activa-
tion based on Na-silicate alone, is insufficient.
At this stage, for increased efficiency of immobilisation
it is suggested that the metal waste be pre-treated with the
Al source/clay before being added to the geopolymer
mix. This would maximise the sorptive capacities of theAl source. More work is required to improve the Cu
immobilisation properties of fly ash-based geopolymers
before they can be applied to the field.
Acknowledgements
JWP acknowledges the support of a Melbourne Uni-
versity Research Scholarship (MRS) Award. Financial
contributions from the Australian Research Council
and Particulate Fluids Processing Center are also grate-
fully recognised.
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