Environ. Sci. Technol. 1998, 32, 1398-1403
Speciation of Cr in Leachates of a
MSWI Bottom Ash Landfill
M I C H A E L K E R S T E N , * ,†
BURKHARD SCHULZ-DOBRICK,†
THOMAS LICHTENSTEIGER,‡ AND
C. ANNETTE JOHNSON‡
Geosciences Institute, Johannes Gutenberg-University,
55099 Mainz, Germany, and Swiss Federal Institute for
Environmental Science and Technology (EAWAG S+E),
8600 Dübendorf, Switzerland
Cr concentrations and speciation were determined in
leachate from a municipal solid waste incinerator bottom
ash landfill both experimentally and by thermodynamic
model calculations. Total dissolved Cr concentrations of 0.2
mmol L-1 were determined by GFAAS. Two orders of
magnitude lower values were determined upon preconcentration by an in-situ solid-phase extraction technique
based on the 8-HQ cation exchanger that is specific for
Cr(III) but unspecific for Cr(VI). This suggests that chromate
dominates the dissolved Cr concentrations in the leachates
but was up to 5 orders of magnitude undersaturated with
respect to the solubility of CaCrO4 or BaCrO4. Chromate
adsorption by oxyhydroxides is less efficient in the highly
alkaline environment, but coprecipitation and solid-solution
formation with BaSO4 can explain the low chromate
concentrations in the leachates. This model assumption
was verified by EPMA/WDX measurement of Cr in
secondary barite precipitates found in aged bottom ash.
Scavenging by this secondary weathering product in
landfilled MSWI ash can thus cause an efficient immobilization
of the toxic chromate.
Introduction
The environmental impact of municipal solid waste incineration (MSWI) has increasingly become the subject of public
debate. The main goal of incineration is to develop a
sustainable waste management by reducing the volume of
nonavoidable and nonrecyclable municipal waste to be
disposed and to reduce its postdepositional reactivity due to
its organic matter inventory. While energy utilization is
increasingly being discussed as merely a secondary effect,
the extensive reduction and controllability of potential longterm emissions are the primary reason for the increasing
role of MSWI in integrated waste management systems. A
next generation of thermal treatment plants without relying
on grate systems is currently being developed. These new
systems are designed to separate more efficiently and thus
to produce more inert ash qualities for construction-related
applications (1). Even though the bottom ash can be utilized
already with conventional incinerators based on the grate
system, a major portion of these residues are still landfilled.
A bottom ash landfill can be regarded as a “heterogeneous
* To whom correspondence should be addressed. e-mail:
[email protected].
† Gutenberg-University.
‡ EAWAG.
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998
fixed bed reactor”, where fast and slow acid/base reactions
occur and continue for a long term, with a yet unknown end
point (2). Major cation and anion concentrations observed
in aqueous extracts and leachates reflect the advance of those
primarily inorganic reactions (3, 4). Priority pollutants are
trace metals enriched in MSWI products (5, 6).
Oxyanion-forming metals such as As, Se, Sb, Mo, and Cr
deserve special attention due to their toxic behavior. While
some information on the behavior of the first four metals are
available from recently published work (7, 8), the behavior
of Cr in MSWI bottom ash deposits is not yet well understood.
In this paper, Cr measurements will be reported for MSWI
landfill leachate. The resulting concentration vs pH relationships for the leachates will be evaluated by geochemical
modeling to verify the dominant speciation and mechanism
that control the dissolved Cr concentrations.
Experimental Section
Landfill and Leachate Sampling Design. Leachate samples
were taken from the bottom ash monofill Im Lostorf at the
MSW incinerator in Buchs, Kt. Aargau, Switzerland, between
December 1993 and July 1994. Bottom ash from this
incinerator is collected in a water quench and is not mixed
with electrostatic precipitator dust. Prior to deposition, the
ash is screened for unburned bulky goods and treated
magnetically to remove excess ferrous material. About 40 000
m3 has been deposited into an abandoned gravel pit since
the landfill establishment in autumn 1991. The landfill is
equipped with a clay bottom liner supporting a gravel
drainage system between geotextile liners for leachate
collection. The upper porous geotextile membrane prevents
clogging of the installed HDPE pipe system. The final depth
is 6 m to which the ash is being successively filled from east
to west in discrete stages upon aging for several weeks on
a separate open dump site. This pre-aging and the relatively
low depth prevents a buildup of excessive heat production
in the landfill due to the oxide hydratation reactions. The
landfill is not covered, but the ash is being compacted by a
roller truck. The leachate drains via the HDPE pipelines
into a passable concrete outflow well situated at an edge of
the landfill, from which it is occasionally pumped for
discharge into the sewerage. The end of the drainage pipe
was equipped with a PVC sampling water tap mounted before
the flow meter, which allowed for fresh leachate sampling.
Batch samples have been poured into acid-washed 100-mL
HDPE bottles prefilled with argon gas and 1 mL of HNO3.
The aerobic leachate (1-9 mg of O2 L-1) is characterized
by its relatively high alkalinity and salinity. The pH varies
between roughly 9 and 11 and is inversely proportional to
the discharge: the higher the discharge, the lower is the pH,
which is probably an effect of carbonation due to admixture
of fresh precipitate (8-10). Sampling was performed occasionally at different discharge regimes to represent the full
pH range. Temperature varied in a narrow range of 15 ( 2
°C. Major cations are the alkali and earth alkali elements
(8-10). Care was taken to exclude CO2 contamination of
the fresh leachate during sampling, otherwise calcite is rapidly
precipitated from the alkaline water samples. Batch samples
were not filtered prior to acidification, because suspended
matter contents were usually very low (<0.1 mg L-1, consisting
mainly of pure calcite). Comparison of filtered and unfiltered
samples revealed no significant decrease in trace metal
concentrations by filtration of the leachate with a 0.2-µm
membrane filter (10).
Analytical Methods. Ba and Cr concentrations were
measured by GFAAS (PE Zeeman 3030 system) using
S0013-936X(97)00422-7 CCC: $15.00
 1998 American Chemical Society
Published on Web 04/14/1998
FIGURE 1. SEM photography of a thin section of a MSWI slag showing a pore vein partially encrusted with crystalline barite grains (white)
and filled with embedding resin (black). The length of the white bar is 100 µm.
operational parameters optimized for these elements (11).
Precision was better than 5% relative standard deviation for
triplicate measurements. Standard solutions were diluted
daily from 10 mg L-1 single-element stock solutions with 1%
nitric acid purified by subboiling point distillation of Merck
suprapure acid and diluted by Milli-Q water.
An alternative approach to contaminant sampling in
combination with preconcentration in the field is to use solidphase extraction (SPE) from the leachate solution. In this
case, a column filled with a contaminant-specific adsorbent
is used to bring about preconcentration and cleanup of the
sample prior to analysis. An apparatus for in-situ preconcentration of trace metals and organic contaminants based
on this approach was first introduced in seawater analysis
(12). A membrane filter holder, a SPE column, a flow meter,
and a battery-driven pump are coupled in sequence to a
compact closed in-line system applicable to field work. A
commercially available version of that system, the Axys
Infiltrex sampler, was used in this study. The PTFE columns
of this sampler were not packed but only half filled with the
red 8-HQ exchanger beads. These beads of about 0.5 mm
diameter have a slightly higher specific mass than water,
which allow them to float freely in the sample water pumped
(or rather sucked, to keep the sequence in mind) continuously
through the system and to behave in a manner similar to
that in a fluidized bed. The fluidized-bed concept yields
high flow rates of up to 150 cm3 min-1 due to the low-pressure
drop over the column. Moreover, any colloidal material and
carbonate sludge can pass through the column without being
retained and causing contamination of the exchanger bed
(13). The closed preconcentration system offers the possibility of sampling the extremely low levels of dissolved
metals in the leachates, without contamination risk by the
rugged and dusty field conditions of an ash landfill, by
connecting the inflow PFE tube of the SPE apparatus via a
silicone tube to the water tap. The exchanger was subsequently eluted with acid solution (2 N HCl/0.2 N HNO3, Merck
suprapure quality diluted by Milli-Q water). A preconcentration factor of 50-200 was achieved depending on the
amount of leachate pumped (10-40 L). The eluents were
analyzed directly by routine GFAAS analysis. The major
benefit of using this technique is that the 8-HQ exchanger
is specific for Cr(III) species but not specific for Cr(VI) species
(13). It is thus possible to assess the presence and proportion
of the latter species by comparison with total dissolved Cr
concentrations measured directly by GFAAS in parallel
samples. The Cr(VI) concentrations thus deduced were below
the detection limit of the conventional spectrophotometric
method using diphenylcarbazide, which is also hampered
by the 2 orders of magnitude higher Mo(VI) concentrations
in the leachates.
A Camscan SEM was used to depict morphology and major
composition of secondary barite in pore spaces of bottom
ash samples (Figure 1). Though the ash sample used is
derived from another bottom ash monofill nearby supplied
by the same type of MSWI (Riet, Winterthur), both landfills
are comparable with respect to slag and leachate composition
(14). The sample was recovered from 5 m depth in 1989,
which represented an aging time of about 5 years at that
time. A subsample was embedded in resin, immediately
VOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
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FIGURE 2. EMPA/WDX spectra of barite and chromite, respectively, superimposed to each other to show resolution of the Ba-Lγ and
Cr-Kr peaks: left with LiF monochromator; right with PET monochromator. Note that the PET intensity scale (arbitrary but identical units)
is twice as high.
after recovering and drying, from which thin sections were
made for microanalysis. The major problem with X-ray
microanalysis of Cr traces in a Ba-rich matrix is the overlap
of the Ba-Lγ1 peak on the Cr-KR1,2 line, which renders the
conventional SEM/EDX setup unsuitable to analyze this
element couple. The spectral interference can only be
resolved by wavelength dispersive electron microprobe X-ray
microanalysis (EPMA/WDX). Two types of monochromators
are used in EPMA/WDX: LiF and PET with 2-d spacings of
0.4027 and 0.875 nm, respectively. LiF exhibits better
resolution of this peak overlap but leads to poor counting
rates. The opposite situation holds for the PET monochromator. A Jeol JXA8900RL EPMA equipped with both WDX
options was used to resolve this problem. Figure 2 shows
that the peak overlap could be resolved satisfactorily by both
monochromators. No peak overlap corrections were necessary, and even the offsets on either side of the Cr peak for
the background measurement were accessible. The PET
option was thus used to analyze the Cr, S, and Ba contents
in the precipitates at a beam of 15 kV/12 nA and 30 and 15
s for the peak and background counting time, respectively.
Pure barite and chromite minerals were used as standard
reference materials.
Results and Discussion
Cr and Ba Concentrations and Speciation in the Leachate.
Cr concentrations were measured at various leachate flow
and pH values in the range typically encountered throughout
the year at this MSWI bottom ash monofill. Cr concentrations
were below the detection limit of the SPE/GFAAS method,
which lies just at the solubility of chromium(III) oxide (Cr2O3)
in that environment (2 nmol L-1). However, the total
dissolved Cr values determined by direct GFAAS were 2 orders
of magnitude higher (0.2 mmol L-1). This discrepancy
suggests that the speciation of this metal is dominated by
Cr(VI) rather than by Cr(III) in the leachate samples. Ba
concentrations measured by GFAAS were in the same order
of magnitude (Table 1). Moreover, the concentrations of
both elements were essentially constant throughout the
measured pH range, suggesting that BaCO3 and Cr2O3 were
not the solubility-controlling solids. Crocoite (PbCrO4, log
K ) -13.7; 15) is thus also not able to explain the chromate
concentration, because of the low dissolved Pb concentrations (in the lower nanomolar range 10) and the significant
pH dependence of its solubility in the alkaline range. A similar
concentration range and pH behavior of both dissolved Cr
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998
TABLE 1. Leachate Chemistry for 10 Samples from Bottom
Ash Landfill Im Lostorf of the MSWI Buchs (Kt. Aargau,
Schwitzerland)a
component (concn level)
GFAAS
SPE/GFAAS
L-1)
Na (mmol
Cl- (mmol L-1)
SO42- (mmol L-1)
CO32- (mmol L-1)
OH- (mmol L-1)
Ca (mmol L-1)
Ba (µmol L-1)
Cr (µmol L-1)
0.5 ( 0.3
0.21 ( 0.02
ref 10
81 ( 13
80 ( 30
20 ( 4
0.8 ( 0.4
1.5 ( 0.5
13 ( 8
<0.002
a
Total dissolved Cr concentrations measured by GFAAS were
compared with data measured by the Cr(III)-selective solid-phase
extraction preconcentration technique.
and Ba was reported by Fruchter et al. (16), who found also
Cr(VI) rather than Cr(III) in their coal fly ash leachate.
The actual source of the Cr(VI) in MSWI bottom ash is yet
unknown. The occurrence of chromate in high-temperature
combustion products is a known common problem, e.g.,
from cementitious clinker materials. However, occurrence
of chromate in the leachates would be arguable if a significant
proportion of the high iron content in the bottom ash was
ferrous iron, which is known to effectively reduce Cr(VI)
species (17-20). Significant Fe(II) in the leachate is unlikely
to occur at the ambient oxygen fugacity and the very low Fe
concentrations in the leachate (<1 µmol L-1) (31). Reduction
rates in laboratory experiments at acidic to near-neutral pH
were reported in the order of hours for reaction with Fe(II)-bearing oxides (19, 20) that would be fast enough to reach
steady-state in a landfill at those pH conditions. However,
the surfaces of any Fe(II)-bearing oxide may have been rapidly
oxidized and passivated under the actually alkaline conditions, which was observed to occur for Cr(VI) reacting even
at neutral pH with synthetic magnetite in laboratory experiments (21). Magnetite weathered under oxidizing vadose
conditions thus shows only minimum reactivity toward
chromate ions (20).
The measured concentration-pH relationships should
enable conclusions on the main release-controlling mechanisms of the chromate based on the assumption of
equilibrium processes. The geochemical program ChemEQL
[22, an enhanced version of the original MICROQL program
developed by Westall (23) adapted to the Mac-OS] was used
TABLE 2. Thermodynamic Formation Constants for the
Hydrolysis, Complexation, and Dissolution Equilibria
Considered To Model the Cr(VI) Speciation (Consistent with
PHREEQE and MINTEQA2 Database as Cited in 15)
aq species
formation reaction (25 °C, I ) 0)
log K
OHCO32HCO3H2CO3
HSO4SO42SO42BaSO40
CaSO40
BaCO30
CaCO30
NaSO4NaCrO4CrO42CrO42-
H2O - H+
CaCO3(s) - Ca2+
CO32- + H+
CO32- + 2H+
SO42- + H+
CaSO4‚2H2O(s) - Ca2+
BaSO4(s) - Ba2+
SO42- + Ba2+
SO42- + Ca2+
Ba2+ + CO32Ca2+ + CO32SO42- + Na+
CrO42- + Na+
CaCrO4(s) - Ca2+
BaCrO4(s) - Ba2+
-14.00
-8.48
10.33
16.68
1.99
-4.58
-9.97
2.70
2.30
2.71
3.22
0.70
0.70
-2.26
-9.67
to evaluate the equilibrium between the leachates and
potential solubility-controlling solid phases including adsorption on charged surfaces in the MSWI ash. CaCrO4 and
BaCrO4 were selected on the basis of their likely presence or
formation in that environment. The initial assumption is
that the leachate chemistry is controlled by thermodynamic
equilibrium throughout the entire pH range rather than
kinetics due to the moderate solubilities of these minerals.
The thermodynamic database of the relevant solid phases
and dissolved species was selected from a critically reviewed
compilation (15) and is summarized in Table 2. Model
predictions are presented as total element concentrations
rather than ion activity products in the leachate solutions at
each pH. This approach enables the presentation of the
analytical data in a graph of log concentration versus pH
together with the pH-dependent solubility curves of suggested
solid phases. Ionic strength corrections using the Davies
equation were made for a mean background total anion
concentration measured in the leachate samples, with 100
mmol L-1 chloride, 20 mmol L-1 sulfate, and 1 mmol L-1
total carbonate (Table 1). This ionic strength was, however,
not high enough to warrant electrolyte effect corrections on
the solubility of, for example, barite (24). No temperature
correction to 25 °C was made using the van’t Hoff equation
because of the minor deviation to the temperature range of
the leachates and the lack of correction parameters for the
chromate compounds. SI calculations indicate that the
leachate samples were close to saturated with respect to
gypsum (CaSO4‚2H2O). However, the leachate was up to 6
orders of magnitude undersaturated with respect to CaCrO4
or BaCrO4 (Figure 1). Adsorption of CrO42- on iron oxide
compounds is unlikely in the alkaline environment. Fruchter
et al. (16) suggested by saturation index calculations that
chromate coprecipitation with barium sulfate, Ba(S,Cr)O4, is
an explanation for a similar undersaturation in their coal fly
ash leaching tests. Paterson et al. (25) suggested that the
most probable Cr(VI)-bearing phase in a Cr-contaminated
soil site is a Cr(VI)-substituted gypsum, Ca(S,Cr)O4‚2H2O.
The latter option is unreliable for the present case because
the solubility of CaCrO4 is much too high to explain the Cr(VI) concentrations in the leachates by this solid-solution
option. The stability of Cr-substituted barite is much higher
and thus a more favorite option.
Solid-Solution Aqueous-Solution Equilibrium Modeling.
To check for the coprecipitation hypothesis, we may assume
a solid-solution aqueous-solution (SSAS) equilibrium using
the equations given by Stumm and Morgan (26). At
thermodynamic equilibrium, the solid-solution system
Ba(CrO4)x(SO4)1-x is described by the equivalence of the
chemical potentials of the solid- and aqueous-phase components, that is, by the appropriate mass action expressions:
{Ba2+}{CrO42-} ) KBaCrO4XBaCrO4λBaCrO4
(1)
{Ba2+}{SO42-} ) KBaCrO4XBaCrO4λBaCrO4
(2)
and
where the brackets denote the aqueous phase molal-scale
activities, XBaCrO4 and XBaSO4 are the mole fractions of the endmembers, and λBaCrO4 and λBaSO4 are the activity coefficients
of the end-members in the solid solution, respectively. The
product of the latter two parameters gives the solid-phase
activities in the solid solution. Dividing eq 1 by eq 2 and
rearranging yields the Berthelot-Nernst distribution law:
XBaCrO4/XBaSO4 ) DCr{CrO42-}/{SO42-}
(3)
where DCr, the distribution coefficient, is given by
DCr ) KBaCrO4λBaCrO4/KBaSO4λBaSO4
(4)
Equation 3 is a classical expression of a SSAS system at
thermodynamic equilibrium. In the present case of an ideal
solid solution both λBaCrO4 and λBaSO4 values will remain close
to 1.0 over the whole composition range, and eq 3 can be
combined with eq 4 and rearranged to give
{CrO42-}/{SO42-} ) XBaCrO4KBaCrO4/XBaSO4KBaSO4
(5)
The behavior of the BaSO4-BaCrO4 solid-solution system
during precipitation and dissolution has been studied
experimentally by Prieto et al. (27). They found that, due to
the similar solubilities of the isomorphous end-members in
this ideal solid-solution system, small local changes in the
aqueous-phase compositions do not result in significant
changes in the solid. Moreover, there is no preferential
partitioning or kinetic effects leading to an incongruent
reaction pathway typical for trace metal coprecipitation at
high supersaturations (27, 28). Consequently, the substituting ions incorporate into the solid nearly in the same
stoichiometric proportion as in the aqueous phase. The
stoichiometry of the aqueous phase with respect to the
components scarcely change as growth or dissolution
proceeds leading to congruent dissolution (27). The aqueous
activity coefficients of the chromate and sulfate anions do
not differ much from each other. Both the cations and anions
do not undergo protolysis reactions to an appreciable extent,
but ion-pair binding with cations may be significant.
Unlike pure water or an inert electrolyte for which these
calculations are valid, in the leachates the sulfate concentration is determined by the much higher solubility of gypsum
rather than by barite (9). In the specific case of congruent
dissolution occurring in an aqueous phase initially free of
the respective ions, the aqueous activity ratio of the Ba2+ and
SO42- ions can be considered equal to the stoichiometric
ratio of 1:1 in the solid due to the electroneutrality requirement. However, this assumption does not hold if the total
activity of the common anion is controlled by the much higher
solubility of gypsum (29). This complex heterogeneous
equilibrium lowers the activity of the Ba2+ cation by 3 orders
of magnitude:
log{Ba2+} ) -pKs0(BaSO4) + p{SO42-} ≈
0.5pKs0(CaSO4‚2H2O) - pKs0(BaSO4) (6)
VOL. 32, NO. 10, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Composition of 17 Barite Grains Analyzed by
EPMA/WDX in a MSWI Samplea
FIGURE 3. Total dissolved concentrations of Cr as a function of
leachate pH. The model curves (solid and dashed lines) were
calculated by assuming equilibrium between the leachates and a
variety of potential solubility-controlling solid phases in the MSWI
bottom ash.
Ion-pair binding with sulfate contributes to about half of
the cation activity for both Ba2+ and Ca2+, but carbonate
complexation or hydrolysis can be neglected. Total dissolved
Ba concentrations of 0.1 mmol L-1 are predicted by this
calculation, which is in the same order of magnitude as the
measured aqueous Ba concentrations (Table 1). It is thus
reasonable to assume that barite in fact controls the Ba2+
concentrations in the leachate of the bottom ash.
The nearly constant anion concentration ratio of {CrO42-}/
{SO42-} ) 4 × 10-5 ( 1 × 10-5 found in the leachates
throughout the entire pH range will determine the composition of the solid solution according to eq 5:
XBaCrO4 ) {CrO42-}KBaSO4XBaSO4/{SO42-}KBaCrO4
(7)
By assuming XBaSO4 ≈ 1, it is possible to predict ultimately a
value for the mole fraction of XBaCrO4 ) 8 × 10-5 ( 3 × 10-5
or about 100 ( 40 ppm Cr in barite for the dissolved chromate
concentrations measured in the leachate samples (Figure 3).
An analogue calculation for a Ca(S,Cr)O4‚2H2O solid solution
would yield in a XCaCrO4 ) 4.7 × 10-8, which is irrelevant. The
Ba(S,Cr)O4 solid-solution composition thus calculated deviates from that suggested by Fruchter and co-workers (XBaCrO4
) 0.1; 16) in their coal fly ash leachate studies. Since they
had not given any detailed discussion of their hypothesis,
the difference cannot be explained. It seems, however, that
they have not taken into account the common sulfate effect
that decreases the Ba but increases accordingly the chromate
concentration in equilibrium with the solid-solution phase.
In total, the solubility of Cr(VI) is still greatly reduced as
compared to the pure end-member BaCrO4. The variability
of Cr(VI) concentrations as a function of pH can be explained
under the assumption that Cr becomes a minor constituent
in solid solution with barite.
Verification and Implications of the SSAS Model. Clearly
any success with SSAS calculations is unsatisfying as long as
the solid-solution composition fitted to the measured aqueous composition has not been verified by any direct analytical
methods. Particles with Ba-S composition were found by
SEM/EDX analysis of a bottom ash sample. These phases
occur as crystalline pore vein fillings, which indicates their
secondary formation during weathering of the ash particles
(Figure 1). EPMA/WDX analyses of 17 mineral grains in a
MSWI sample yield Ba and S concentrations that indicate
barite as the most probable compound (Table 3). The Cr
concentrations of 280 ( 300 ppm found in these grains are
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 10, 1998
a
no.
BaO
SO2
Cr2O3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
73.3
70.3
70.7
72.6
73.6
73.0
72.3
71.0
70.6
70.8
71.5
73.5
70.9
75.1
74.7
73.2
74.3
26.7
29.7
29.3
27.4
26.3
27.0
27.7
29.0
29.3
29.1
28.5
26.5
29.1
24.9
25.3
26.8
25.7
<100b
440
280
<100
480
<100
230
<100
700
1230
230
<100
280
<100
<100
110
<100
wt % normalized to 100%; Cr2O3 in ppm.
b
Detection limit.
quite variable but at least in the order of magnitude predicted
by the thermodynamic SSAS equilibrium model. Though
both the leachate and the solid samples were not taken from
the identical site, the analyses indicate that this solid solution
may in fact occur with the hypothesized composition in aged
MSWI slags. We may thus conclude that the relative kinetics
of solid solute precipitation vs Cr(VI) reduction and subsequent chromium(III) oxide precipitation may favor the Cr(VI)-substituted barite formation under the highly alkaline
conditions of bottom ash monofills, which likely controls
the chromate mobility at the relatively low levels found in
the leachate.
Solid-solution partitioning to barite can cause a strong
retardation in the mobility of the toxic chromate species. A
precipitation/dissolution equilibrium should imply that there
is no relationship between the aqueous concentration and
the solid content as for adsorption reaction. As shown for
the case of the solid solution of Cr in BaSO4, the aqueous
solutions can achieve considerable undersaturation with
respect to the pure phase BaCrO4. The degree of undersaturation depends linearly on the concentration of the
impurity in dilute solid solutions. The SSAS equilibrium
implies therefore that there is in fact a relationship between
the aqueous Cr concentration and the solid Cr content
similary to adsorption reactions. Identification of leaching
mechanisms from leaching tests would thus yield misleading
results (30). Test data (expressed in µmol L-1) (leachate
concentrations) would be equal at different liquid-to-solid
ratios as long as the solid composition does not change and
would thus indicate solubility control, albeit no pure solid
phase could be readily identified to control the low leachate
concentrations. Although little is known about the actual
phase composition and trace element inventory of the
secondary weathering products formed in aged MSWI bottom
ash, our results suggest that these phases may effectively
retard metals. This in fact raises the question as to what
extent short-term laboratory leachability tests of fresh ash
are a representative scenario for trace metals mobility in
landfills if the solid/solution interaction processes are
changing with time.
Acknowledgments
The experimental work on this paper was performed while
M.K. was on leave at EAWAG. He is grateful to Peter Baccini
for his hospitality and encouragement and to the German
Science Foundation for the fellowship. Dr. Baumann, Pollution Control Department of the Aargau Swiss Federal State,
und Mr. Suter, manager of the MSWI Buchs AG, provided
logistical support for the sampling campaigns. Sandro
Brandenberger helped with anion analyses. Rainer Bahlo from
IOW Warnemünde made the SEM photograph.
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Received for review May 14, 1997. Revised manuscript received December 10, 1997. Accepted March 4, 1998.
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