Applied Geochemistry 23 (2008) 3699–3711
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Applied Geochemistry
journal homepage: www.elsevier.com/locate/apgeochem
Leaching of lead metallurgical slags and pollutant mobility
far from equilibrium conditions
Nicolas Seignez a, Arnaud Gauthier a,*, David Bulteel b, Denis Damidot b, Jean-Luc Potdevina
a
b
Université de Lille 1, UMR CNRS 8110, ‘‘Processus et Bilans des Domaines Sédimentaires”, Bâtiment SN5, 59655 Villeneuve d’Ascq Cedex, France
Ecole des Mines de Douai, Département Génie Civil, 941, rue Charles Bourseul, BP 838, 59508 Douai, France
a r t i c l e
i n f o
Article history:
Received 17 October 2007
Accepted 15 September 2008
Available online 2 October 2008
Editorial handling by R.B. Wanty
a b s t r a c t
Lead metallurgical slags are partially vitrified materials containing residual amounts of Zn,
Pb, Cr, Cd and As. These hazardous materials are generally buried on heaps exposed to
weathering. In this study, leaching behavior of lead blast furnace slags has been tested
using pure water and open flow experiments. It appears that in such far from equilibrium
and slightly acidic conditions, the main phase to be altered is the vitreous phase. As for
lunar, basaltic and nuclear glasses, alkalis/proton exchanges prevail and lead to the formation of a non-protective altered layer enriched in Si, Fe and Al. The composition of the
altered layer is quite constant except for Si whose concentration decreases towards the
leachate interface. Owing to their sizes, micrometric Pb droplets are not always totally dissolved at the slag surface. Nevertheless, nanometric Pb droplets are instantaneously dissolved while a surrounding altered layer is formed. This leads to high Pb releases in
open flow systems. Leachate chemistry and dissolution rates of the vitreous phase are closely comparable to previous leaching tests with basaltic and nuclear glasses in conditions
far from equilibrium. Moreover, this study confirms that Fe is a stable element in such
conditions.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Lead blast furnace (LBF) and imperial smelting furnace
(ISF), both used in non-ferrous metallurgy, generate huge
amounts of primary smelting slags (Verguts, 2005). In the
industrial basin of Nord-Pas-de-Calais (northern France),
4 million tons of such hazardous materials have been landfilled on a heap in the vicinity of the Deûle river (Sobanska
et al., 2000). LBF slags are a granulated material equivalent
to a glass-ceramic charged with Zn (10–12 wt%), Pb (0.5–
3 wt%) and other elements such as Cr, Cd and As (Seignez
et al., 2006).
Considering hazards that slags and dust emitted by the
furnace can represent, some valuable studies have already
contributed to the understanding of their leaching behavior: in landfill conditions (Mahé-le-Carlier et al., 2000), in
soils or soil-like environments (Sobanska et al., 2000; Ettler
* Corresponding author. Tel.: +33 320 434 114; fax: +33 320 434 910.
E-mail address: [email protected] (A. Gauthier).
0883-2927/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apgeochem.2008.09.009
et al., 2004) and in others various batch (Ettler et al., 2002;
Ettler et al., 2003) and open flow experiments (Seignez
et al., 2006).
Spinels and wuestite contained in LBF slags are generally stable in batch and open flow experiments. In contrast,
fayalite, melilite, sulphides, glass matrix and metallic Pb
droplets are often partially or totally dissolved (Ettler
et al., 2002). Calcite (CaCO3), cerussite (PbCO3), hydrocerussite (Pb3(CO3)2(OH)2), Al-rich hydroxides and Hydrous
Ferric Oxides (HFO) are secondary phases commonly
formed in batch and natural conditions.
Exposed to weathering, the Fe–silica–lime glass matrix
is progressively depleted in alkali and alkali-earth elements, e.g. Ca, Na and K. Others elements (e.g. Pb, Zn)
can also be noticeably released (Ettler et al., 2001). In oxidizing conditions, Fe(II) contained in the glass matrix is
usually transformed into Fe(III) and remains in place
(Mahé-le-Carlier et al., 2000). Generally, the altered layers
are relatively enriched in Si, Fe and Al.
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N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
Apparent steady-state and decreasing dissolution rate
are the consequences of: (1) the formation of a protective
gel whose development and preservation depends on the
experimental conditions, (2) decreasing chemical affinity
with increasing element concentration (Gauthier et al.,
2002) or (3) nucleophilic condensation reactions at the
glass surface leading to the formation of relatively stable
complexes such as ˜Si–O–Si(OH)3 sites at the glass surface
(Abraitis et al., 2000).
Although much research has been devoted to glass and
slag leaching, only few deal with slag behavior in open flow
systems (i.e. closed to the heap conditions). This study presents a detailed description of slag alteration in far-fromequilibrium conditions. Two open flow leaching tests were
conducted: (i) a column test supplied with a high flow rate
of pure water and (ii) a polished section test with rather
similar leaching conditions. Using focused ion beam (FIB)
cuttings and transmission electron microscope (TEM),
fresh glass matrix and altered layers have been precisely
characterized and compared. It shows the link with leachate chemistry, and thus a better description of the alteration mechanisms and pollutant release is revealed.
2. Materials and methods
2.1. LBF slags
Slags used here come from a significant mass of sample
(around 300 kg) collected at the exit of the melting workshop. Their size distribution is lognormal (Gaussian distribution of logarithmic diameters) with a 500 lm dominant
class (Sobanska et al., 2000) and a diameter of 5 mm at the
most. Their density is around 3.81 g cm3. Geometric surface area (SA) can reach 5.25 cm2 g1 for grains whose
diameter is greater than 1 mm.
Recent studies have shown that Fe-rich crystallized
phases represent at least 28 vol.% of the waste (Seignez
et al., 2007). They are (1) micrometric wuestite crystals
(Fe0.85x ZnxO with 0.085 < x < 0.170) (Degterov et al.,
2001; Mansfeldt and Dohrmann, 2004), (2) varied solid
solutions of spinels with magnesiochromite (MgCr2O4),
franklinite (ZnFe2O4) and magnetite ðFe2þ Fe3þ 2 O4 Þ as
end-members and (3) Fe-rich nano-phases (50–200 nm)
such as Zn substituted magnetite. Lead is contained in
metallic droplets occupying 2 vol.% of the waste. All these
phases are embedded in a vitreous phase which represents
up to 70 vol.%. Percentages of the different phases were
calculated by image analysis of SEM-BSE observations of
unaltered materials and by the effective density of each
individual phase. Although the composition of this vitreous
phase is highly heterogeneous at the sub lm scale, an estimation of its mean composition has previously been given
(Seignez et al., 2006).
2.2. Leaching methods
2.2.1. Column test
In this test, only grains of the 2–2.5 mm class were
used. To avoid dust release in the first hours of experiments, dust was removed from slag surfaces by six successive ultra-sonic baths in alcohol.
The column is a 220 mm high TeflonÒ cylinder with an
internal diameter (D) of 25 mm. According to other studies
(Tsotsas and Schlünder, 1988; Han et al., 1985), a diameter/length ratio of 10 for the column is needed to achieve
a homogenous water flux. The homogeneity of the flux
was checked by tracing experiments. At the column inlet,
a Teflon piece was placed to homogenize the flux. The column was connected to a pump and a pure water reservoir
(Fig. 1).
Fig. 1. Simplified scheme of the column test.
N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
The column contained 157 g of slag. The total slag surface was 1100 cm2 (geometric SA). The experimental device and especially the water tank were maintained at
298 K. The pump provides a constant flow at 60 mL h1
with a residence time of 1 h. Due to the high angularity
of slag grains, the intergranular porosity reaches 50% and
the pore velocity is equal to 24 mL h1. S/V ratio is approximately 20 cm1. At the column effluent, a fraction collector was driven by a computer to sample the leachate with a
high frequency (e.g. 1 sample/h).
2.2.2. Polished section tests
Polished sections (60 20 mm2) were placed in a leaching container supplied with pure water by a peristaltic
pump with a constant flow of 60 mL h1. Slags were only located on the upper face of the sections and occupied 40%
(500 mm2) of it, while the volume of water in the container
was approximately 4 cm3 (S/V 1.2 cm1). Some containers
used in this study allowed the alteration of three sections
simultaneously. In such a case, there was little change in
the S/V ratio. Darcy velocity in the container was about
1 m h1. The sections were observed after 15 and 26 days
by scanning electron microscope. Chemical analyses
(SEM–EDS) were obtained initially and after 26 days.
During the polished section preparation, contact with
water was reduced to a minimum to avoid any early alteration of the more reactive phases like the Pb droplets. The
different polishing steps consist of a first lapping and successive polishing using essentially diamond paste. An alcohol-based lubricant liquid was used to avoid contact with
water. After each step the sections were immediately dried
with alcohol.
2.3. Solution analysis
In the column test, a micro-cell was introduced before
the fraction collector to determine pH, Eh (Metrohm pH
Meter 781) and conductivity (Metrohm Conductometer
712) for all samples. Leachate chemistry was determined
using inductively coupled plasma atomic emission spectroscopy (ICP-AES Thermo iCAP 6000, Chemistry department, Ecole des Mines of Douai, France) equipped with an
ultra-sonic nebulizer. Before each use, a calibration was
performed with a minimum of three standards. During
the analytical process, quality assurance and quality control
(QA/QC) samples such as laboratory blanks, sample spikes,
sample duplicates, and calibration check samples were performed. The following elements were quantified: Al, As, Ba,
Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Si, Ti and Zn. The
detection level is 10 lg L1 for the majority of the analyzed
elements except for Cu, Pb and Zn (20 lg L1), Si (50 lg L1)
and Ca and K (200 lg L1). Ion chromatography was used to
2
quantify anions in solution, i.e. Cl, PO2
4 and SO4 . Detection level is around 0.5 mg L1.
3701
System (EDS) probe is about 1 lm3. The accuracy of the analyses is 0.5% (weight) in high vacuum. To be reused the polished sections were not C coated and were inserted in the
ESEM chamber in low vacuum mode. The pressure in ESEM
chamber was kept relatively low (60.45 Torr). However, the
analysis accuracy is relatively reduced, especially for light
elements, due to the development of artifacts. These artifacts are directly related to the presence of the environmental gas. The unavoidable gas scattering, both elastic and
inelastic, of a fraction of the primary beam electrons has a
significant and frequently severe impact on both qualitative
and quantitative Si-EDS X-ray microanalysis in the ESEM.
Then, only global trends can be considered. X-ray maps presented here have been filtered with PhotoshopÒ software to
reduce the background noise. In the case of column experiments, regular sampling was applied (with a spacing of
1 cm) from the bottom to the top of the column. Observations of the altered slags were made directly on the grains
and on polished sections embedded in epoxy resin.
For the precise study of altered zones first observed in
ESEM, vertical thin sections were cut from the non-altered
and altered polished sections with a Focused Ion Beam (FEI
Strata DB 235; Institut d’Electronique de Microélectronique
et de Nanotechnologies, University of Lille). Cutting dimensions are 20 30 0.1 lm which allows investigations
with a transmission electron microscope (TEM). The TEM
used here was a CM30 (Laboratoire de Structure et Propriétés de l’Etat Solide, University of Lille). It is equipped with
an EDS probe whose spatial resolution is about 105 lm3.
2.5. Alteration modelling
The numerical reactive transport code HYTEC was used
to determine the degree of saturation of the leachates in
the column with respect to the solid phases. The numerical
approach of the HYTEC code is described by Van der Lee
et al. (2003) and Trotignon et al. (2005). To derive a chemical model, it is possible to use a system of algebraic and
partial derivative equations to express the thermodynamic
equilibriums and/or the kinetic controls supposed to dominate in the studied system. An equation also allows clear
definition of species transport in the column. To explain
the simulation conditions chosen in this study, the main
equations governing the code are given below.
These equations are obtained in the framework of the
physicochemistry of a non-deformable porous medium.
The chemical concentrations are taken into account with
a classical approach (Trotignon et al., 2005). Concerning
the chemical approach, the total concentration of a component is considered as the sum of its mobile fraction (total of
the soluble species of this component) and its immobile
fraction (total concentration of the solid species or precipitates containing this component).
2.4. Solid analysis
3. Results
Altered and unaltered slags were studied with an environmental scanning electron microscope (ESEM, FEI Quanta
200) at Lille University. Its W electron source is used at
20 kV. The spatial resolution of its X-ray energy dispersive
3.1. Leachate Chemistry
Fig. 2a shows that concentrations of Si and Ca in
solution are close to 500 lg L1 and 12 103 lg L1,
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N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
about 230 lg L1 for Mg and Ba. Aluminium starting concentration is lower (near 100 lg L1). These 3 elements
show the same global tendencies as Si and Ca (i.e. abrupt
decrease and relative stabilization), but Mn was not detected at the beginning. All these elements reach very
low values: from 15 to 25 lg L1 depending on the elements. After t1138 h an increase of Ba, Mn and Mg concentrations is observed. At t2300 h Mg concentration is 1/3
greater while Mn and Ba concentrations double compared
to their values at t1138 h.
and SO2
were detected in the leachate.
No Cl, PO2
4
4
Such a lack does not implicate a charge deficit. It just implies that their concentrations are lower than the detection
level. Such very low concentrations are not surprising from
the cation concentrations and the previous batch experiments (Seignez et al., 2006).
In accordance with Seignez et al. (2006) relatively high
pH values (near 8) may occur in the first hours of the column experiment. It is linked to the release peak during
the first hours. Afterwards, pH values decrease to near 6
and at the same time Si and Ca concentrations also decrease. In this study, regular pH measurements confirm
that in the second part of the experiment (i.e. after the
concentration decreases) pH values oscillate closely
around 6.
respectively, at the beginning of the experiment. Concentrations of these elements decrease very abruptly to reach
a minimum value at t50 h for Si (near 15 lg L1) and low
values at t150 h for Ca (between 400 and 500 lg L1).
In the second part of the experiment, a progressive increase of Si concentration is observed. Indeed, at t2300 h Si
concentration is 1/3 greater than its value at t1138 h.
During the same period, the same behavior is observed
for Ca concentrations. Nevertheless, no Fe was detected
in spite of its high content in the waste; the concentration of this element was permanently below the detection
limit.
At the beginning of the experiment, the Pb concentration (Fig. 2b) is close to 110 lg L1 for 10 h and rapidly decreases between t12 h and t50 h to reach 50 lg L1.
Afterwards, Pb concentration increases rapidly to oscillate
around 120 lg L1 with an amplitude of 50 lg L1. In contrast to Pb, Zn is not released at the beginning (Fig. 2b). Its
concentration regularly increases to reach 100 lg L1 at
t72 h. Zinc concentration is not as stable as Pb. It oscillates
between 61 and 174 lg L1. However from t1138 h to
t2300 h Zn has globally the same behavior as Si and Ca. Its
concentration generally increases.
Four other elements were detected in relatively low
concentrations: Mg, Mn, Al and Ba. Their first value is
Concentration (in µg L-1)
800
Si
Ca
700
600
500
400
300
200
100
0
0
500
1000
Time in hours
1500
2000
Concentration (in µg .lL-1)
160
120
80
40
Pb
Zn
0
0
500
1000
1500
2000
Time in hours
Fig. 2. Concentrations (lg L1) versus time of the elements liberated by the slag grains in pure water at 293 K.
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N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
The morphology of the slag surface rapidly changes during leaching with the creation of micro-canyons in the
glass. In a same glass field, glass matrix can be non-uniformly leached (Fig. 5). Close to apparently unchanged
glass surfaces (top right), some zones are highly altered
(bottom left). These zones are fractured and are localized
in the bottom of the micro-canyons. Dendritic Fe oxides
appear to be very stable phases since they are protuberant
objects at the glass surface.
In SEM backscattering mode (BSE), altered areas are
darker than the unaltered glass (Fig. 5). EDS-maps show
that in such micro-canyons Ca is totally depleted and Si
partially so (see corresponding EDS-maps). It leads to a relatively high enrichment in Fe in the altered layer. The darkness of the glass at the bottom of the micro-canyons in BSE
mode has been interpreted as a decrease of the altered
glass density caused by the element losses (Seignez et al.,
2007).
In the column experiment, SEM-EDS analyses often
show relative enrichment of Fe contents due to Ca and
Pb/Si
Zn/Si
Ca/Si
Elt / Si ratio (in moles)
0.5
occasionally Si losses (Fig. 6). However, the nearer the inlet
slags to the fresh water source, the more affected they are
by such alteration. For slag grains situated higher in the
column (i.e. further from the column inlet) the vitreous
phase composition is less altered.
FIB cuttings have been made from polished sections to
better describe the altered glass layer in TEM (Fig. 7). Considering very close points (several lm at the very most),
the thickness of the altered layer at their base can vary
strongly. TEM images show that the layer thickness rarely
exceeds several lm if it exists at all. The altered glass is
lighter than the fresh glass in transmission mode. This confirms that the glass density decreases significantly in the
altered layer due to element losses.
The chemical characteristics of the altered layer are
shown in Fig. 8. It corresponds to the line scan profile localized in Fig. 7. The elements considered are the three major
constituents of the glass and the one main pollutant: Zn.
The profile is graduated from 0 at the glass/water interface
to 2150 nm in the fresh glass.
The chemical profile of Ca clearly shows the near total
loss of Ca in the altered layer. There is only a background
nPb/nSi in the fresh waste = 0 028
nZn/nSi in the fresh glass =
0 055
nCa/nSi in the fresh glass =0 573
5.0
0.4
4.0
0.3
3.0
0.2
2.0
0.1
1.0
Ca / Si ratio (in moles)
3.2. Glass dissolution
0
Ba/Si
Mg/Si
Al/Si
Ca/Si
0.5
Elt / Si ratio (in moles)
500
0.4
1000
1500
Tme in hoursi
2000
nBa/nSi in the fresh glass = 0.006
5.0
nMg/nSi in the fresh glass = 0.054
nAl/nSi in the fresh glass = 0.17
4.0
nCa/nSi in the fresh glass = 0.573
0.3
3.0
0.2
2.0
0.1
1.0
Ca / Si ratio (in moles)
0
0
0
500
1000
1500
2000
Time in hours
Fig. 3. Molar ratios (nElt/nSi) versus time of the elements liberated by the slag grains in pure water at 293 K. Molar ratio in the fresh glass or in the fresh
waste (depending on the element) are given for comparison.
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N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
0.9
NL(Ca)
NL(Pb)
NL(Si)
NL(Mg)
0.8
0.7
NL (g.m-2)
0.6
0.5
0.4
0.3
0.2
0.1
0
0
500
1000
1500
2000
1500
2000
Time in hours
0.4
NL(Al)
NL(Mn)
NL(Zn)
NL (g.m-2)
0.32
0.24
0.16
0.08
0
0
500
1000
Time in hours
Fig. 4. Normalized mass losses (gm2) versus time of the elements liberated by the slag grains in pure water at 293 K.
noise of Ca in the altered layer: around 200 cts. Calcium
values immediately increase in the fresh glass near
1800 nm to reach 5000 cts. It indicates that Ca is nearly totally removed from the glass structures while in contact
with water and not contained in the altered layer.
In contrast, Si values regularly increase from 800 to
4500 cts from the interface to the fresh glass limit. This
could be due to technical reasons. Indeed, top border of
FIB cuttings may have been bevelled during the cut. The
counts could increase because of progressive thickening
of the glass. Nevertheless, this hypothesis is excluded because of the chemical profile of Fe. Considering the Fe
immobility during the leaching, if beveling occurs during
the cut, Fe counts would have shown a tendency identical
to Si. This is not the case, the Si behavior is due only to
alteration.
Even if the Fe counts are globally the same in the altered and fresh glass, there are occasionally important
variations in Fe contents. Along the profile, the locations
of the Fe peaks are: 0–150, 300–600, 1000–1100 and
1300 nm. In comparison, Si peaks are localized approximately at 200, 700, 950, 1150 and 1400 nm. No correlation could be made between these two elements in the
altered layer.
Regarding the pollutants, Zn shows approximately the
same behavior as Ca. There is also a background noise
around 200 counts. Zinc counts reach approximately
1350 in the fresh glass. Nevertheless, small peaks of Zn appear at 150, 400, 600, 1000 and 1300 nm. They globally
correspond to Fe peaks suggesting a Zn incorporation in
nanometric Fe-rich entities.
In Fig. 7, rounded shapes are observable both in fresh
glass and in the altered layer. They correspond to droplet
sites. In fresh glass, they appear dark while they become
white in the altered glass. These very light grey-levels indicate a nearly complete transparency in transmission mode.
Then, it can be assumed that Pb nano-droplets have been
totally leached. In Fig. 7, just at the limit between altered
N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
3705
Fig. 5. SEM micrograph showing the slag surface after alteration of a polished section (over 26 days) and corresponding EDS element maps.
and fresh glass (dotted line) a nano-droplet has been completely removed while its surface in contact with the altered layer is relatively reduced. The altered layer front is
highly curved in this zone. The droplet dissolution may
have hindered the advance of the front in this area.
4. Discussion
4.1. Alteration mechanisms
Molar concentrations at the outlet have been normalized to Si concentration which is considered the most rep-
resentative element of glass alteration (Gauthier et al.,
2000). Thus it could be possible using these ratios to determine if the dissolution was stoichiometric or not (Chardon
et al., 2006). Fig 3a and b give the evolution of the corresponding molar ratios during the run. At the beginning of
the experiment, ratio values are very high. Regarding Ca,
its ratio is approximately 50 while ratios reach 0.54 for
Mg, 0.2 for Al and 0.1 for Ba. All these ratios abruptly decrease to reach a plateau. Molar ratios in the fresh glass
have been estimated considering the mean glass composition given in Table 1. In the second part of the experiment,
nCa/nSi ratio is still relatively high (near 1) compared to the
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N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
Fig. 6. Ternary diagram (in mass%) showing FeO, CaO and SiO2 contents in fresh glass and after the leaching test.
Fig. 7. TEM micrograph of a FIB cutting showing the limit between the altered glass layer and the underlying pristine (fresh) glass.
molar ratio measured in the fresh glass (about 0.57). For
Mg, the ratio is twice that of the fresh glass (about 0.1
against 0.054). Barium shows the same behavior as Si. Its
ratio is twice as high in the leached glass as in the fresh
glass. In the contrast, the nAl/nSi ratio is lower in the leached glass than in the fresh glass. Its value varies between
0.037 and 0.077 in the second part of the experiment.
Considering the Pb ratio, its evolution during the tests is
similar to Ba, Mg and Ca. Nevertheless, its starting value is
relatively low (0.1). After a brief decrease, the values oscillate between 0.04 and 0.08. Then, nPb/nSi is relatively high-
er in the leached than in the fresh glass. The case of nZn/nSi
is different. Indeed, its starting value is very low and increases abruptly to reach 0.3. Afterwards, the value is not
very stable and oscillates between 0.1 and 0.2. The ratio value in the leached glass is twice as high as the value in the
fresh glass at minimum, suggesting a non-congruent dissolution mechanism.
The data shown in Fig. 4 have been integrated using a
trapezoid rule (mathematical formula for numerical integration) for each measured element. The total mass loss
is approximately 1.48 101 g. Considering each element,
N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
3707
Fig. 8. TEM-EDS profiles through the altered glass layer and the pristine glass.
the releases proceeds in the following sequence (in g): Ca
(6.92 102) > Si (3.88 102) > Zn (1.52 102) > Pb
(1.52 102) > Mg (3.74 103) > Ba
(1.94 103) > Mn (9.70 104).
(2.70 103) > Al
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N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
Table 1
Average composition determined by EMPA on 40 analyses of the vitreous phase.
In mass%
SiO2
CaO
FeO
Al2O3
MgO
MnO
Na2O
K2O
TiO2
ZnO
PbO
LBF glass
43.09
23.05
14.99
6.22
1.58
0.55
4.92
0.60
0.73
4.01
–
The cumulative normalized mass loss (NL: Normalized
mass Loss) has been calculated for each element using
the following general equation (Techer et al., 2001):
NLðiÞt ¼ NLðiÞt1
C it þ C it1 2C b
þ
/ðt ðt 1ÞÞ þ ðVðC it C it1 ÞÞ
2
Rc
ð1Þ
%ðiÞS
where Cb and C it are, respectively, the concentration (in
g L1) of the element i in the initial solution at t (time in
days), u the flow rate (in L day1), V the vector volume
(L), Rc the oxide/element mass conversion ratio, %(i) the
mass% of the oxide i and S the slag surface area. Considering that the average amount and general composition of
the glass matrix is just an approximation, %(i) refers to
the mean glass composition. NL values are expressed in
g m2. At t2270 h, the cumulative normalized losses proceed
in the following sequence (in g m2): Ca (8.48 101) > Pb
(4.00 101) > Mg (4.34 101) > Zn (3.81 101) > Mn
(1.75 101) > Si (1.55 101) > Al (4.21 102).
The release rates (r) of each element i have also been
calculated between each sampling point using Eq. (2)
(Techer et al., 2001). The main results are given in Table 2.
rðiÞtþðtþ1Þ ¼
2
NLðiÞtþ1 NLðiÞt
ðt þ 1Þ t
ð2Þ
In these experiments, pH is around 6 even for polished section tests. In such conditions, alkali and alkaline-earth metal exchanges prevail. Indeed, Ca, Mg, Na, Mn, Ba, Pb and Zn
are preferentially released. As for basalts (Daux et al.,
1997; Techer et al., 2001), Si, Al and Fe are the main elements constituting the altered glass layer. In contrast,
according to Ettler et al. (2004), neither soluble Fe nor Fe
precipitation (e.g. HFO) have been detected. Indeed, Fe(III)
is often very stable due to a very low solubility constant
while Fe(II) is rapidly oxidized to Fe(III) in the oxidizing
conditions during slag alteration ( Mahé-le-Carlier et al.,
2000).
Table 2
Releasing rate (r) of the element i during the column experiment. Meantotal
and meanII are respectively the average values of the releasing rates during
the whole experiment and during the second part of the experiment, i.e.,
when an apparent steady-state is reached (from t92 h to t2270 h).
R(i) in
g m2 d1
Min
Max
Meantotal
Mean(II)
Al
Ca
Mg
Mn
Pb
Si
Zn
–
1.30 104
1.35 104
–
1.77 105
7.50 105
1.16 104
3.34 102
8.96 101
2.93 101
2.42 102
9.06 102
2.97 102
3.45 102
1.47 103
7.85 102
2.37 102
3.42 103
1.35 102
4.68 103
6.81 103
6.47 104
5.12 103
5.16 103
2.56 103
6.48 103
2.53 103
8.27 103
This altered layer is less dense than the fresh glass. Line
scan profiles suggest that density decreases towards the
glass/leachate interface within the altered glass layer. It
is essentially caused by a decrease of Si content towards
the altered layer, caused by a high solution renewal. The
flow rate tends to maintain leachate chemistry with rather
low concentrations. Conditions far from equilibrium are
maintained during all these leaching experiments. Such
conditions are not favorable to the formation of a protective gel. Percolation channels may be formed in the altered
layer by the preferential diffusion of modifier cations
(Greaves and Ngai, 1995; Calas et al., 2003). It can be assumed that the layer thickness can keep increasing while
acidic conditions are maintained. Indeed, in such environments, persistent selective dissolutions are observed (Vernaz et al., 2001). However, the greater the layer thickness,
the lower the releasing rates, because alkali metals diffuse
from a greater depth (Eick et al., 1996). This can lead to the
reduction of the release rate in the place where the altered
layer is developed. In parallel, water diffusion into the glass
is relatively high at low pH compared to alkaline conditions (Grambow and Müller, 2001).
By maintaining pH near 6 and low concentrations in
solution, conditions are not favorable to the formations
of secondary phases such as clay, calcite and Pb carbonates.
This is quite common in high flow rate experiments (Daux
et al., 1997). In far from equilibrium conditions, only few
Pb carbonates and sorption phenomena have been
observed.
In this study, dissolution rates have been normalized
considering a mean glass composition. Even if vitreous
phase amount and composition are an approximation, it
offers an efficient tool to compare LBF glass dissolution
to other glasses. Table 2 shows that rates range approximately from 1.47 103 to 7.85 102 g m2 d1 (see
meantotal) while for the second part of the experiments
(meanII), values range from 6.47 104 to 8.27 103
g m2 d1. At the beginning of the column test, rates are
very high (except for Zn): their values can reach 1 101
and 1 102 g m2 d1 order of magnitude.
Data obtained in this study have been compared to previous studies of glass alteration (Table 3). Glasses have
been chosen because of their composition or experimental
protocol. Considering temperature, pH and surface area,
glasses E and G seem to be the more comparable with this
study. Their dissolution rates (RateSi) are both around
1 103 g m2 d1 while the concentrations in the leachate are under 1 mg L1 for all the considered elements. In
basic pH alteration (samples A, B and C; Table 3), Si alteration rate values are often one or two orders of magnitude
higher (especially for glass A and B) except for glass D
which does not contain any alkali and alkaline-earth elements. Concentrations in the altered solution are a little
higher (e.g. >1 mg L1 for Si) in these conditions.
3709
N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
Table 3
Dissolution parameters and results of open flow leaching tests found in the literature (initial solutions are free from any inhibiting elements).
Glass type
S (cm2)
V (cm3)
m
Temp (K)
Flow rate (mL h1)
Outlet pH
Concentration in lg L1
Si
Fe
Nuclear glass
10.69
Ca
Db
9400
60
–
9.7
5.5
313
363
4.63
61.8
9.74
7.39
381
1430
7
–
Basaltic glass
495
Ac
Bd
547
e
E
1206
7975
Ff
506
Gg
120
55
300
250
300
5.5
5.5
5.3
3
4
363
363
303
298
298
72
37.8
52.8
51.6
55.2
8.1
7.62
5.35
2.98
4
3700
1100
59
1040
120
9.52
–
–
–
–
a
b
c
d
e
f
g
Rate(Si)
Rate(elt)
Na
(g m2 d1)
–
–
158
–
–
4.3 103
1.6 101
–
570
490
–
–
–
–
390
–
–
–
4.5 101
–
6.4 103
1.6 103
3.1 103
3.6 102
1.2 101
–
–
–
Ca
Abraitis et al., 2000.
Leturcq et al., 1999.
Daux et al., 1997.
Techer et al., 2001.
Gislason and Oelkers, 2003.
Oelkers and Gislason, 2001.
Wolff-Boenisch et al., 2004.
Both alteration rates at the beginning and in the second
part of the column test are close to results summarized in
Table 3. In Daux et al. (1997), Rate(Si) is high at the beginning when pH is slightly basic. It decreases by 2 orders of
magnitude at best when pH becomes acidic. Indeed, in
slightly acidic near neutral pH, glass dissolution rates are
always very low compared to extreme pH values (Abraitis
et al., 2000; Oelkers and Gislason, 2001).
The saturation indices (SI) of selected solubility-controlling phases are reported in Fig. 9. All the phases studied
exhibit negative SI all through the column. In the first centimeters of the column, an increase of the SI is observed.
After 5 cm, no variation was detected. These results shows
that no phase is susceptible to formation in the column
experiments, except for cerrussite or Al(OH)3 which are
close to equilibrium. Moreover this simulation demonstrates that there is no variation of the chemistry of the
slags along the column from the second quarter to the output as previously observed (Fig. 6).
In closed systems, LBF slag releases Pb in higher concentrations in the leachate (e.g. ½Catend ¼ 25 mg L1 ; ½Sitend
¼ 5 mg L1 ) than in the column. Dissolution rates are
strongly higher in the batch than in the second part of
the column test. Nevertheless, Zn and Pb concentrations
remain quite similar ½Zntend ¼ 0:23 mg L1 ; ½Pbtend ¼ 0:21
mg L1 .
One would expect that the pronounced Pb mobility is
induced by Pb repartition in slags, i.e. only contained in
Pb droplets and not in the vitreous phase where it would
have been less labile. The Pb source is hard to determinate
precisely. Either dissolution of micrometric Pb droplets or
glass alteration could predominantly control Pb concentration in the leachate. Indeed, dissolution of the nanometric
Pb droplets depends on the thickening of the altered layer,
Fig. 9. Degree of saturation of the leachates with respect to selected solubility-controlling phases as calculated by HYTEC.
3710
N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
while Pb droplets localized at the slag/leachate interface
are partially dissolved during open flow tests.
In the column test, Pb concentration can also be controlled by pollutant sorption in the altered glass layer or
Pb carbonate precipitation. However, at slightly acidic
pH, Pb carbonates are relatively rare and sorption could
prevail (Dimitrova and Mehandgiev, 1998; Dimitrova,
2002). Indeed, EDS-maps indicate that Pb sorption occurs
everywhere altered glass is observed. These two immobilisation phenomena can be considered as negligible regarding Pb concentrations in the leachate. This leads to
relatively high pollutant releases using such a flow rate.
No Zn immobilization phenomenon has been detected
(Fig. 9).
4.2. Alteration intensity
Low in the column, where the fresh water enters, the
vitreous phase is more highly altered, compared to the
upper parts of the column where alteration is much less intense. From the second quarter to the top of the column
LBF glass seems to be relatively protected and shows only
rare observable alteration textures. In this part of the column, the glass matrix seems to be relatively safe from
alteration. Yet, solution chemistry is always highly under-saturated (Fig. 9). Then, an inhibiting effect plays an
important role. For example, it might be due to the development of a protective altered layer. Protective effects of
an altered layer can be related to an orthosilicic acid concentration limit. Authors like Vernaz et al. (2001) indicate
that this apparent saturation concentration is an indicator
of a protective gel formation rather than a real saturation
concentration. Then, such conditions are comparable to
the column top. Nevertheless, what is happening in the
column is not really consistent with the development of
a protective gel.
Another explanation may be the formation of stable Si
sites at the glass surface. For example, at high pH, dissolution rates of silica glasses are nearly proportional to the
density of surface species like ˜Si–O sites (Brady and
House, 1996). Their deprotonation is closely linked to the
pH (point zero net proton charge) above which ˜Si–OH
are increasingly deprotonated. Glass alteration abilities
can be strongly reduced thanks to the reaction between
the orthosilicic acid and ˜Si–O which promotes the formation of stable ˜Si–O–Si(OH)3 groups at the glass surface
(Bunker et al., 1988). Other authors have indicated that
glass dissolution can be partially inhibited by the presence
of Pb, Zn or Al (Hudson and Bacon, 1958). Buckwalter and
Pederson (1982) proposed the formation of ˜Si–O–Pb–O
at the glass surface. Then, negative charges repel OH ions
in solution. Even if these phenomena occur in leaching
conditions which do not completely correspond to those
encountered in this study, such mechanisms may prevail
in the upper part of the column and tend to inhibit glass
dissolution rates. Moreover, evidence shows that Pb sorption occurs on LBF slags.
It is not the first time that a study has tried to explain
why in certain alteration conditions, no clear alteration
layer is formed (at the top of the column in this experiment)
and the rate of glass dissolution is very low (Leturcq et al.,
1999). The assumptions previously evoked to explain this
phenomenon in batch tests were (1) the formation of a very
thin protective altered layer and (2) low chemical affinity of
the glass dissolution. Such hypotheses are not so different
from those evoked in this study and concerning an open
flow test. However, the low chemical affinity is not explained by the same phenomenon as proposed here.
As described by Seignez et al. (2007), some rare secondary Pb carbonates have been observed around big Pb droplets. Lead immobilization (i.e. precipitation or adsorption)
exists too. Indeed, EDS-maps (Fig. 5) indicate this occurs
in altered glass regions characterized by high Ca and Si removal and Fe enrichment. The high correlation between Pb
and Fe concentration may be explained by a possible binding of Pb to some newly formed hydrous ferric oxides as
previously observed by Ponthieu et al. (2006) and Bibby
and Webster-Brown (2006). Observations tend to show
that Pb sorption occurs whatever the distance to Pb droplets. These phenomena may not predominate regarding the
residence time of the leachate in the experimental device
(1 h) and the high Pb concentration measured at the column output.
Conversely to Pb, there is no Zn enrichment in the altered glass matrix (Fig. 5). Indeed, Zn repartition is quite
similar to the Si EDS-map. It suggests a partial removal of
Zn, which is in accordance with the TEM-EDS profile
through the altered layer. The residual amounts of Zn observed on the EDS-map can be explained by the stability
of Zn in the Fe-rich entities such as nanometric magnetite
and wuestite.
5. Conclusions
LBF slags have been altered using two different far from
equilibrium leaching experiments. The aim was to better
understand alteration mechanisms and pollutant releases
inherent to this type of material when placed under landfill
conditions.
1. In far from equilibrium conditions, only glass matrix
and metallic Pb droplets are altered. In acidic pH,
alkali and alkaline-earth elements are preferentially
released. High solution renewal allows the formation of an altered glass layer with non-protective
properties.
2. Releasing rates of the LBF glass (calculated from Si
concentrations) are similar to nuclear and basaltic
glasses leached in such open flow conditions.
3. No immobilization phenomenon has been observed
for Zn. In spite of some minor immobilization phenomena, i.e. Pb sorption and formation of rare Pb
carbonates, Pb concentrations are as high as results
obtained in some batch experiments. Then, Pb is
strongly released in open flow conditions.
Acknowledgements
This work is supported by the Nord-Pas-de-Calais Region Council (PRC Sites et Sols Pollués) and the French
N. Seignez et al. / Applied Geochemistry 23 (2008) 3699–3711
Environmental Protection Agency (ADEME). We would like
to thank Laurent Dewindt (Ecole des Mines de Paris) and
Emmanuel Tertre (CEA - Gif sur Yvette) for their constructive discussions about glass alteration. We are also grateful
to Patrick Degrugilliers (Ecole des Mines de Douai) for
sharing data with us and to every Engineer who has helped
us: Sophie Gouy and Philippe Recourt, Bruno Malet and
Jean-François Barthe.
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