Effect of Al Source and Alkali Activation on Pb and Cu

8/13/2019 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).
mailto:[email protected]:[email protected]://www.sciencedirect.com/http://www.sciencedirect.com/http://www.sciencedirect.com/


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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).

424 J.W. Phair et al. / Applied Geochemistry 19 (2004) 423434


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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

J.W. Phair et al. / Applied Geochemistry 19 (2004) 423434 427


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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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Effect of Al Source and Alkali Activation on Pb and Cu