Environmental Remediation
by Crystallization
of Solid Solutions
Manuel Prieto1, José Manuel Astilleros2,3,
and Lurdes Fernández-Díaz2,3
1811-5209/13/0009-195$2.50
F
DOI: 10.2113/gselements.9.3.195
nuclear waste management (Bruno
et al. 2007), yet many geochemical
models rely primarily on surface
adsorption to describe the interaction of sparingly soluble minerals
and dissolved metals. Adsorption
and surface coprecipitation are not
incompatible, and there is often
a connection between the two
mechanisms of uptake. However,
whereas adsorption is a surfaceaccumulation process that can be
KEYWORDS : solid solutions, ion partitioning, coprecipitation, contaminant ions,
quickly reversed within hours, a
remediation, surface passivation, epitaxy
contaminant incorporated into the
mineral structure by coprecipitation cannot be released back to
the water unless the host mineral
INTRODUCTION
dissolves. The rate of mineral dissolution is in general much
In Earth-surface environments, the primary crystallization
slower than the desorption of a surface species; therefore,
of minerals often leads to the formation of nonstoichiosolid solution formation appears to be a more durable
metric solids that can exhibit wide compositional ranges,
immobilizer for a wide spectrum of harmful ions than
that is, solid solutions. Moreover, interactions between
other sorption mechanisms.
preexisting minerals and water frequently lead to surface
precipitation and dissolution–recrystallization processes
A THERMODYNAMICALLY DRIVEN PROCESS
in which a number of substituting ions (major, minor, or
trace) are redistributed between the solid and the aqueous
In order to illustrate how cocrystallization processes work,
phase in order to adapt to new physicochemical conditions. we can examine a grain of barite (BaSO4 ) after interacThe resulting solid solutions are single crystalline phases
tion with an aqueous solution containing CrO 42- ions.
in which ions of different types substitute for each other The grain is covered by numerous white spots, which at
in equivalent structural positions. Their environmental
a higher magnification can be identified as crystals of
relevance arises from the fact that the solubility in water of
Ba(CrO4,SO4) solid solution (FIG. 1). Chromate and sulfate
a minor constituent in a solid solution can be significantly ions have identical configurations, and hashemite (BaCrO4)
reduced in comparison with the solubility of its equivais isostructural with barite. Therefore, it is not surprising
lent pure solid. Such a reduction of solubility controls the
that a complete solid solution between BaSO4 and BaCrO4
bioavailability and mobility of metals in the environment
exists and that the precipitate shown in the images in
and can be exploited as a remediation strategy to remove
FIGURE 1 occurred in a nonrandom, epitaxial orientation
heavy metals and other harmful ions from polluted waters.
with strict parallelism between the crystallographic directions of the substrate and the overgrowth. The interaction
Although the incorporation of trace elements into the
involves barite dissolution and the reaction of the released
structure of minerals has been known for a long time,
Ba 2+ and SO42- ions with the dissolved CrO42- to form solid
Tesoriero and Pankow (1996) nonetheless commented on
solution nuclei on the barite substrate.
the fact that solid solution partitioning of metal ions into
mineral phases was not widely considered in environThe images in FIGURE 1 provide a physical picture of how
mental assessments of metal-ion behavior. The role of solid
cocrystallization with barite has significant potential
solutions as sequestering phases is now widely recognized in as a method for removing Cr(VI) from polluted waters.
environmental studies (Godelitsas and Astilleros 2010) and
However, any evaluation of the suitability of a remediation method should also consider the reaction kinetics,
because reaching the “end-point” to which the system
tends can take a long time. In fact, the timescale may be
1 Department of Geology, University of Oviedo
too long for a typical remediation program. Obviously,
33005 Oviedo, Spain
such an “end-point” is thermodynamic equilibrium. The
E-mail: [email protected]
equilibrium scenario is completely different from that
2 Department of Crystallography and Mineralogy
shown in FIGURE 1, where two solid phases, substrate and
Complutense University of Madrid
overgrowth, coexist. True equilibrium requires complete
28040 Madrid, Spain
dissolution of barite and subsequent cocrystallization of
3 Institute of Geosciences (CSIC, UCM)
BaSO4 with CrO42- to form a single solid solution phase of
oreign ions can be incorporated into minerals during mineral growth and
mineral–water interactions, resulting in solid phases with substitutional
impurities in their structure. These “cocrystallization” processes control
the mobility of minor elements in the environment and can be exploited as
a remediation strategy to remove toxic metals from polluted waters and in
the design of engineered barriers for the retention of metals, radionuclides,
and other inorganic wastes generated by industry. The effectiveness of such
remediation tools relies on thermodynamic and mechanistic factors that
operate at different scales in space and time.
28040 Madrid, Spain
E LEMENTS , V OL . 9,
PP.
195–201
195
J UNE 2013
B
B
C
C
[10
0]
A
10 μm
100 μm
(A) Scanning electron micrograph of a barite grain
after interaction with a chromate-bearing aqueous
solution in a porous medium. (B) Selected region at higher magnification. (C) Detail of the barite surface showing an oriented precipitate of Ba(CrO 4,SO 4) crystals. The arrows indicate parallel
crystallographic directions of substrate and overgrowth. FROM PRIETO
ET AL . (2002), WITH PERMISSION FROM E LSEVIER
FIGURE 1
homogeneous composition. For example, the interaction
of 1 g of barite with 100 cm3 of a 0.1 millimolar aqueous
solution of CrO42- would ultimately yield a solid solution
with a molar fraction of barium chromate of about 0.0016.
Such a solid solution would be in equilibrium with a ∼0.01
millimolar CrO42- aqueous solution, which means a reduction of one order of magnitude in the aqueous concentration of chromate. In contrast, if we disregard the solid
solution effect and simply consider equilibrium of the
aqueous solution with a mixture of pure barite and pure
hashemite, the final concentration of aqueous CrO42- would
be ∼0.08 millimolar, i.e. eight times higher!
The previous calculations, performed using the hydrochemical modeling program PHREEQC (Parkhurst and
Appelo 1999), demonstrate how the solid solution effect
reduces significantly the chromate solubility. The fi nal
assembly of phases calculated with this program obeys
the traditional condition given by the equilibrium distribution coefficient,
1 μm
aqueous concentration of chromium to the parts per billion
(ppb) level. A frequently disregarded peculiarity of these
types of systems is that the equilibrium end-point not only
depends on the solubility and degree of ideality of the solid
solution but also on the specific initial amounts of solid
and liquid (BOX 1). Thus, by increasing the initial amount
of barite, we can strongly decrease the “target” concentration of dissolved chromium. This effect occurs in all solid
solution systems and is crucial for the “natural” attenuation
of contamination in many water-saturated porous media,
where the high solid/water ratios can favor the decrease of
contaminant concentration by orders of magnitude.
The effectiveness of the uptake by coprecipitation increases
significantly when the solubility of the host mineral is
higher than the solubility of the equivalent end-member
of the guest ion. Radiobarite [(Ra,Ba)SO4 ] is an interesting
example that has received sustained attention from the
early times of radiochemistry. Barite is an important phase
in many nuclear waste repositories, where the mobility
of radium (Ra) is strongly decreased by interaction with
this mineral (Curti et al. 2010). Due to the slightly lower
solubility of the RaSO 4 end-member, radium partitions
preferentially towards the solid phase, with Deq (Ra) ≈ 1.8
for trace concentrations. As a result, starting from the same
initial conditions as in the previous chromate example (1 g
of barite and 100 cm3 of a 0.1 millimolar aqueous solution
, (1)
where XHas and XBrt are the mole fractions of the chromate
and sulfate components in the solid phase, and the activities of the uncomplexed CrO42- and SO42- aqueous ions are
given in curly brackets. At equilibrium (eq), the distribution
coefficient can be seen as a product of two terms. The fi rst
one is the quotient of the solubility products (KBrt and KHas)
of the end-members and is therefore a thermodynamic
constant. The second term accounts for the nonideality
of the solid solution (γ Brt and γ Has are solid-phase activity
coefficients used to correct for nonideal behavior) and
depends on the solid composition4. In our example, Deq (Cr)
≈ 0.5. The fact that this value is less than 1 means that
sulfate incorporates preferentially into the solid phase.
The reasons are the slightly lower solubility of barite (KBrt
= 10 -9.97) when compared with that of hashemite (KHas =
10 -9.67) and the similar values of γBrt and γHas (≈1). Despite
such a low value for the distribution coefficient, the interaction of CrO42- with barite may be able to reduce the total
4 Distribution coefficients are frequently expressed using total
molalities of the aqueous ions instead of activities. In such a
case the last term in equation (1) should include two additional
correction ratios related to the activity coefficients of the
substituting ions in the aqueous phase and their complexation
(e.g. Curti 1997).
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[01
0]
196
BOX 1
Equilibrium end-points in aqueous solution–solid
solution (AQ–SS) reactions
The solubility of a binary solid solution is defined by two
mass-action equations, which for the barite–hashemite series
are given by:
and
,
and
are the activities
where
of the sulfate and chromate components in the solid solution
(X is mole fraction and γ is activity coefficient). Therefore,
checking whether a given AQ–SS system is at equilibrium
simply requires confirming that the corresponding massaction equations are simultaneously fulfilled. The prediction
of equilibrium end-points during AQ–SS reactions is, however,
not so straightforward. In contrast with pure minerals,
aqueous compositions resulting from reactions with solid
solutions depend on the specific solid/aqueous phase ratio.
Given an initial mass and composition of solid solution and
aqueous phase, the final compositions cannot be calculated
using only the mass-action equations. The calculation also
requires an equation related to the conservation of charge in
the reacted solid and two equations related to the conservation of mass of the solid solution components (Glynn at al.
1990). Moreover, the iterative derivation of solutions for this
set of equations needs independent knowledge of the
aqueous and solid-phase activity coefficients as a function of
composition. Fortunately, the corresponding algorithms are
implemented in the popular speciation program PHREEQC
(Parkhurst and Appelo 1999), and the estimations can be
easily made using this program.
J UNE 2013
B
B
AA
(A) Comparison
between the uptake
of cadmium by calcite and aragonite. (B) Atomic force microscopy
(AFM) image of cadmium-rich
(Cd,Ca)CO3 islands growing
epitaxially on a calcite (101̄4)
cleavage surface. (C) The islands
spread laterally to form a layer
that covers almost completely the
original surface (AFM image).
FROM PÉREZ-GARRIDO ET AL. (2007),
WITH PERMISSION FROM E LSEVIER .
(D) Scanning electron micrograph showing the nucleation
of (Cd,Ca)CO3 crystals on the
surface of an aragonite grain.
FROM PRIETO ET AL. (2003), WITH
PERMISSION FROM E LSEVIER
FIGURE 2
1 μm
C
C
be very effective in removing
Cd 2+ from polluted waters.
Surprisingly, if we mix 1 g
of calcite cleavage fragments
(diameter ≈ 1 mm) and 100
cm3 of a 0.1 millimolar Cd 2+
aqueous solution in a reactor,
the result is disappointing:
after an initial small decrease
of Cd 2+, its removal virtually stops. The paradox
50 μm
1 μm
becomes even more evident
if we use aragonite instead of
calcite. In this second case,
the concentration of Cd 2+
2+
2+
of Ra ), the end-point concentration of Ra would be two
decreases
dramatically
to
quickly
reach the ppb level. The
orders of magnitude smaller. As in the case of chromium,
plot
in
F
IGURE 2A compares the concentration decay in both
increasing the initial amount of barite would strongly
experiments. The difference is striking since aragonite is
decrease the fi nal concentration of dissolved radium. Such
also CaCO3. In other words, if the reactants are chemia “dilution effect” appears when the minor component is
present in much smaller amounts compared to the major cally analogous, should not the concentration decrease be
analogous too?
component and is crucial in the assessment of harmfulD
ion behavior.
The preceding examples show that the choice of suitable
host minerals for remediation strategies primarily requires
determining the equilibrium behavior in the specific
system. The main difficulty in this task is the lack of
thermodynamic data for nonideal solid solutions. The
experimental determination of nonideality parameters is
complex and tedious, but the use of atomistic simulations
is providing results that can convincingly compete with
experimental measurements (Kulik et al. 2010). Anyway,
independently of how reliable the equilibrium model is,
numerous phenomena can dramatically affect the removal
behavior. Aqueous solution–solid solution systems may
need a long time to approach equilibrium, and in practice
thermodynamic models simply provide a reference state in
modeling actual reaction pathways.
SURFACE PASSIVATION
Cadmium is a highly toxic metal whose concentration
in water and soils occasionally reaches dangerous levels
due to anthropogenic activities. Calcium and cadmium
have similar ionic radii, and the mineral otavite (CdCO3)
is isostructural with calcite (CaCO3). It is, therefore, not
surprising that the uptake of Cd 2+ by coprecipitation on
calcite surfaces has traditionally been considered as a
potential remediation tool (Stipp et al. 1992). Otavite and
calcite form an almost ideal solid solution ( γOta ≈ γCal ≈ 1),
and otavite (KOta = 10 -12.1) has very low solubility
compared to calcite (KCal = 10 -8.48 ). The consequence is
a strong tendency for cadmium to partition into calcite
[Deq (Cd) ≈ KCal/KOta ≈ 4230], suggesting that calcite should
E LEMENTS
In fact, the plot in FIGURE 2A is a glaring example of how
thermodynamic models may be insufficient to account for
laboratory and field observations. Micro- and nanoscale
phenomena are often the key to understanding macroscopic measurements. The scanning electron micrograph
in FIGURE 2D shows the surface of an aragonite grain after
its interaction with a Cd-bearing aqueous solution. The
surface is covered by rhombohedral crystals with sizes of
∼20 μm and with compositions in the (Cd,Ca)CO3 solid
solution. The scenario is in some way similar to that in
FIGURE 1, but in this case substrate and precipitate are not
isostructural and there is no epitaxial orientation. The
aqueous solution has permanent access to the aragonite
surface, and the dissolution process can continue until
completion. In contrast, the uptake of cadmium by calcite
occurs by epitaxial nucleation of solid solution islands
that spread laterally, eventually covering the surface with
a layer of nanometric thickness that “protects” the substrate
from further dissolution (FIG. 2B, C). As a consequence, the
process stops when only a small amount of cadmium has
been removed from the water. In the example (FIG. 2B, C),
the islands are thin, but they are actually three-dimensional, with a constant thickness of ∼2.9 nm, corresponding
to 10 monolayers. Generally, the thickness depends on the
composition of the islands, which determines the degree
of structural matching between substrate and overgrowth.
The term surface passivation may be used to describe this
blocking of the surface by an unreactive coating. Under
certain conditions, the overgrowth can consist of one
monolayer, which quickly spreads on the substrate and
blocks the process, at least temporarily. The inhibitory effect
of a newly formed surface on subsequent growth was fi rst
197
J UNE 2013
observed during the interaction of calcite with Mn-bearing
aqueous solutions in an AFM fluid cell (Astilleros et al.
2006 and references therein). The phenomenon becomes
evident when monitoring the original etch pits on the
calcite surface. During growth, the etch pits are rapidly
fi lled up, and the new surface behaves as a barrier that
hinders subsequent growth on it. Consequently, the area
is surrounded by the next advancing layer, which leads
to an almost perfect reproduction of the topography of
the original surface (FIG. 3). This “template” effect is not
exclusive of calcite and is probably a consequence of the
mismatch at the borders between the original and the
newly formed surface.
with respect to the underlying mineral. In the long term,
however, partial equilibrium tends to evolve towards “true
equilibrium” via dissolution–recrystallization processes.
Recrystallization requires that the aqueous phase penetrates
the external layer, which can occur through preexisting
pores and fractures or through porosity generated during
the recrystallization process due to the different solubility
and molar volume of the dissolving and precipitating
phases (Putnis and Putnis 2007). Such a “ripening” process
can be rather sluggish and hence irrelevant for the time
frame of interest in a given remediation project, but it
must be considered in the assessment of spontaneous selfattenuation of pollution in soils and aquifers.
In most cases, surface passivation can be regarded as a type
of “partial equilibrium,” where the aqueous solution is at
equilibrium with the surface coating but out of equilibrium
COPRECIPITATE COMPOSITION
A
1μm
For coprecipitation and recrystallization to occur, the
aqueous solution has to be supersaturated with respect
to the crystallizing phase. In the case of stoichiometric
minerals, like pure barite, there is a single supersaturation
(Ω) condition, namely,
2
,
1
r
r
r’
r’
t=0
B
1μm
2
1
r
r
r’
r’
t = 3’02’’
Growth sequence on a calcite (101̄4) cleavage surface
in contact with a Mn-bearing aqueous solution. The
arrows indicate growth-step advancement. The etch pits on the
original surface (A) are rapidly filled in and the newly formed
surface (B) acts as a barrier preventing subsequent growth. The
schematic profiles along the r-r’ lines illustrate this effect. AFM
IMAGES FROM A STILLEROS ET AL . (2006), WITH PERMISSION FROM E LSEVIER
FIGURE 3
E LEMENTS
(2)
where the activities of the Ba 2+ and SO42- aqueous ions
are in brackets and KBrt is the solubility product of barite.
However, in dealing with solid solutions, both solubility
and Ω are functions of the solid composition (x). In general,
there will be a continuous range of compositions fulfi lling
the condition (x) > 1; therefore, the precipitating solid
could have any such composition. Equilibrium thermodynamics and supersaturation in aqueous solution–solid
solution systems are complex issues beyond the scope of
this article. In a recent review by Prieto (2009), interested
readers can fi nd up-to-date information and references to
the pioneering works in this field. For the present purpose,
the relevant fact is that when coprecipitation occurs, the
aqueous phase is typically supersaturated with respect to
a range of solid compositions. Under such conditions, the
“effective” distribution coefficient (D eff ) usually differs
from the equilibrium value (Deq ), particularly at a high
supersaturation level and precipitation rate. In general, the
composition of the precipitating phase becomes richer in
the more soluble component, which implies an attenuation of the preferential partitioning. At a high precipitation rate, the thermodynamically driven selectivity is less
efficient and the substituting ions tend to incorporate in
the same proportion as in the aqueous phase. Thus, Deff
tends to unity.
A typical example of attenuation is found in the case of
the (Cd,Ca)CO3 system (FIG. 4). In the diagram, the molar
fraction of otavite (XOta) is represented on the ordinate
against the Cd 2+ aqueous-activity fraction (XCd,aq). The
solid line represents the equilibrium curve, whereas the
dashed lines correspond to nucleation experiments carried
out at increasing supersaturation. The degree of curvature
decreases as the supersaturation increases, which indicates
a decrease of Deff (Cd) (FIG. 4 A). Moreover, because the
substituting ions do not incorporate into the solid in the
same proportion as in the fluid, both crystal and aqueousphase compositions tend to vary during growth. This variation may result in a crystal with compositional zoning, and
the reaction pathway can be followed on an XOta –XCd,aq
plot (FIG. 4B). In the (Cd,Ca)CO3 example, the strong partitioning of cadmium into the solid usually results in a sharp
concentric zoning with a Cd-rich core encapsulated by a
rim of nearly pure calcite equilibrated with the aqueous
solution (FIG. 4B).
198
J UNE 2013
A
B
Deq = 4230
XCd,aq=
XOta
Increasing
supersaturation
XOta
Deff (Cd)
eq
Reaction
pathway
(Eq)
Ca-rich
Cd-rich
73
42
1 μm
30
12
{Cd2+}
{Cd2+}
+
XCd,aq
{Ca2+}
(A) Experimental XOta –XCd,aq curves obtained at
increasing supersaturation (modified after
Katsikopoulos et al. 2008). All the curves plot below the equilibrium curve (solid line), indicating that Deff (Cd) decreases as the
supersaturation increases.
(B) Backscattered electron image of a (Cd,Ca)CO3 crystal with
a Cd-rich core encapsulated by a rim of nearly pure calcite.
A hypothetical reaction pathway is also shown. XOta = mole fraction
of otavite; XCd,aq = Cd2+ aqueous-activity fraction; Deq = equilibrium
distribution coefficient; Deff = “effective” distribution coefficient.
IMAGE FROM PRIETO (2009), WITH PERMISSION FROM THE M INERALOGICAL SOCIETY
OF A MERICA
Predictive Models
an atomic position in the bulk structure may have diverse
configurations when exposed at the mineral surface. The
importance of multiple surface sites for foreign-element
incorporation into calcite was studied by Reeder and
coworkers (Reeder 1996), who used synchrotron microX-ray fluorescence (μ-XRF) to obtain element-distribution
maps. Most analyses have been carried out on the (101̄4)
surface of calcite, without any doubt the most studied
surface at the nanoscale (Ruiz-Agudo and Putnis 2012).
The general idea is that large ions tend to incorporate in
steps with less-confi ned growth sites, although factors
other than size may also have an influence. As a result,
nonequivalent growth steps can adsorb foreign elements in
different proportions, giving rise to growth subsectors with
different compositions during spiral growth (an overview
of the calcite growth mechanisms is given in Teng et al.
2000). In the same way, each nonequivalent face may
yield a characteristic composition sector when its surface
chemistry is captured within the bulk crystal according to
the entrapment model.
FIGURE 4
Nonequilibrium partitioning is a common phenomenon
during mineral crystallization. Many classical works deal
with the effect of supersaturation on the partitioning of
divalent cations in calcite (Böttcher and Dietzel 2010), but
the theoretical treatment is not straightforward. Pina and
Putnis (2002) applied classical nucleation theory to solid
solutions, with the aim of explaining the experimental
evidence that the higher the nucleation rate the higher the
deviation of Deff from equilibrium. Their model qualitatively predicts this behavior, but needs further development
to improve quantitative matching with precipitation data.
The prediction of nonequilibrium partitioning during
layer-by-layer growth is a different problem and requires
separate consideration. The surface-entrapment model
proposed by Watson (2004) is probably the most widely
used to account for minor-element partitioning during
mineral growth and biomineralization. Watson postulates that, during crystal growth, foreign ions adsorbed
on surface steps can be trapped and incorporated into
the mineral volume in a proportion that depends on the
interplay between growth rate and ion mobility in the
near-surface region by diffusion. That interplay determines
the ability to purge the structure of “unwanted” amounts
of substituting ions. With increasing growth rate, the
surface layer can be buried fast enough to “capture” the
surface chemistry within the bulk crystal. Unfortunately,
as Watson pointed out, even if the principles embodied
in his model are believed to be correct, the number of
unknown parameters is an important obstacle to making
quantitative assessments.
One of the virtues of the Watson model is that it connects
adsorption and coprecipitation, which is key to understanding phenomena such as compositional sector zoning.
The notion of a single distribution coefficient for a given
element in a given host mineral fails when we consider that
E LEMENTS
Epitaxial Coprecipitation
So far we have considered a “simple” uptake mechanism
where the host mineral grows layer-by-layer incorporating
minor amounts of substituting ions. However, coprecipitation is frequently better described as an epitaxial growth
phenomenon, affected by factors such as the elastic
stress generated at the interface between substrate and
overgrowth. This stress depends in a complex way on
lattice matching, interface coherency, epitaxy thickness,
and relaxation (Shtukenberg et al. 2005). Thus, for a given
aqueous solution, the precipitate composition results from
the interplay of all these factors with the system thermodynamics. Obviously, the theoretical modeling of such a
complex behavior is an intricate task and an important
challenge for future research.
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J UNE 2013
RELEVANT HOST MINERALS
A
A
The previous examples reveal that a mineral requires
specific characteristics to be suitable for the uptake of toxic
elements by coprecipitation. First, it needs to be sparingly
soluble, be fairly common, and form in sufficient amounts
to provide a sustainable remediation option. Unfortunately
many common silicates and oxides dissolve too slowly.
Second, the existence of an ample solid solution range with
preferential partitioning of the guest ion towards the solid
would represent an extra advantage.
BB
Calcite fulfi lls both conditions, and its interaction with
divalent metals (Sr2+, Co2+, Cd 2+, Pb2+, Zn2+, Ni2+, Mn2+,
etc.) has been widely studied. Calcite is also a potential
host for incorporating SeO42-, AsO43-, and CrO42- because its
structure has some flexibility to accommodate tetrahedral
anions substituting for the triangular CO32- groups (e.g.
Tang et al. 2007), as confi rmed by extended X-ray absorption fine structure (EXAFS) data. The incorporation of trivalent lanthanides and actinides has also gained increasing
attention, due to the common occurrence of calcite in
geological formations currently studied for nuclear waste
disposal (Bruno et al. 2007). The heterovalent substitution
of these trivalent ions for Ca 2+ benefits from the similar
ionic radii, but the charge balance requires a compensation mechanism, such as the incorporation of calcium
vacancies and/or coupled substitution by a second trace
element. The local coordination of rare earth elements and
trivalent actinides in calcite has been studied by EXAFS,
time-resolved laser fluorescence spectroscopy, and other
spectroscopic techniques, which mostly confi rm substitution into distorted calcium lattice sites (Marques Fernandes
et al. 2008; Heberling et al. 2008).
Among the phosphate minerals, the apatite group
[Ca5 (PO4) 3 (OH,F,Cl)] is considered an effective agent for
sequestering Pb2+, Cd2+, Ni2+, Zn2+, etc., in waters and soils,
where the metal cations can precipitate by substituting
for Ca 2+ in the apatite structure. An example involving
anionic substitution is the pyromorphite–mimetite
[Pb5 (PO 4,AsO 4 ) 3Cl] solid solution, which has recently
been proposed as an immobilizing phase for both Pb2+
and AsO43- in remediation methods using phosphates (Flis
et al. 2011). Considering sulfates, the effectiveness of barite
as a sequestering phase for some divalent cations (Sr2+,
Pb2+, Ra 2+, etc.) and oxyanions (CrO42-, SeO42-, etc.) has
been widely studied in the literature. Moreover, coprecipitation with some iron hydroxyl sulfates has been reported
to play an important role in reducing As(V) concentrations
in acidic mine drainage conditions (Fukushi et al. 2003).
Finally, due to their abundance and ubiquity, iron oxides
and oxyhydroxides are very important in terms of mass
balance and control of concentration of metals in natural
waters. For example, natural goethite (α-FeOOH) usually
incorporates metal cations isovalent or heterovalent to Fe3+.
EXAFS analyses demonstrate that some of these cations,
like Cr3+, substitute for iron in lattice positions (Manceau
et al. 2000). Goethite and bracewellite (α-CrOOH) are
isostructural and can form mixed α-(Fe,Cr)OOH solids,
which is not surprising given the similar ionic radii and
comparable hydrolyzing properties of Cr3+ and Fe3+. Metals
like Cu, Ni, and Zn have a lower affi nity than chromium
for iron oxides, but their incorporation, by substituting for
Fe in lattice positions, has also been observed.
Use of Biominerals
The previous examples of minerals that can be suitable for
the uptake of toxic elements are illustrative. Of course, there
are many other relevant harmful ions and host minerals,
but an exhaustive review is beyond the scope of this
E LEMENTS
C
C
100 μm
Images of Apatite II TM (PIMS NW, Inc.), a biogenically
precipitated apatite material derived from fish bones.
Apatite II is especially suitable for phosphate-induced metal stabilization (PIMS TM ) due to its low crystallinity, carbonate content, and
high internal porosity. (A) Bulk material. (B) Scanning electron
micrograph showing the porosity of the internal bone structure. (C)
Porosity of the capillaries in the walls of the bone. I MAGES FROM
CONCA AND WRIGHT (2006) WITH PERMISSION FROM ELSEVIER
FIGURE 5
short article and would shift the focus from fundamental
issues, such as passivation. Passivation can be avoided by
increasing the surface roughness of the host mineral. For
this reason, the use of biogenic material in remediation
strategies is growing. As explained above, the uptake of
Cd 2+ by calcite cleavage fragments is rather unsuccessful
due to the development of thin epitaxial coatings of a
Cd-rich precipitate on the calcite surface (FIG. 2). The effectiveness can, however, be improved using materials where
the coatings are inefficient in passivating the dissolving
surface. As such, the use of ground shells, even when they
consist of calcite, greatly enhances the removal of Cd 2+
and other metals from wastewaters (Köhler et al. 2007).
Similarly, biogenic hydroxylapatite (F IG. 5) in various
forms (fishbone, bone meal combustion residues, etc.) is
an efficient and cheaper substitute for inorganic apatite in
coprecipitating lead and other metals from water (Sneddon
et al. 2006; Conca and Wright 2006).
COMPLICATIONS AND CHALLENGES
Recent years have seen increasing development of new
remediation methods based on coprecipitation. We have
highlighted several examples and the main difficulties
in reliably assessing the effectiveness of these methods.
There are, however, further complications that deserve
comment. Even understanding the system thermodynamics may be a challenge. While the thermodynamic
properties of the host mineral are usually well known,
the isostructural end-member of the guest ion may not
exist. For example, the incorporation of trace amounts of
Sr2+ in calcite involves considering a “fictitious” calcitetype end-member because SrCO3 has an aragonite-type
structure. The problem is still more complex in the case
of heterovalent substitutions. How can the solubility
products of these fictitious end-members be estimated in
order to apply equation (1)? A traditional way to solve this
200
J UNE 2013
problem is to interpolate from free-energy or solubility
correlations obtained for isostructural families, but ab initio
atomistic simulations appear to be more promising (Kulik
et al. 2010). Considering nonequilibrium partitioning,
factors like competitive substitution in the presence of
additional foreign ions, the cation/anion ratio, the effect
of background electrolytes and organic molecules, etc.,
have to be considered (Putnis 2010). Combining improved
simulation methods with state-of-the-art techniques and
macroscopic measurements is no doubt on the agenda for
future research.
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E LEMENTS
ACKNOWLEDGMENTS
The authors thank Christine Putnis and Encarnación
Ruiz-Agudo for the opportunity to contribute to this
issue and Enzo Curti and Andrew Putnis for their valuable
comments on the manuscript. Insightful suggestions by
Principal Editor Patricia Dove and Copy Editor Thomas
Clark greatly helped to improve the original manuscript.
We thank Judith Wright and James Conca for kindly
providing the Apatite II images.
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J UNE 2013
A History of Growth
FEI’s Automated Mineralogy technology is helping mining
companies understand ore variability in terms of mineralogy
and textures. This knowledge leads to improved metallurgical
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Learn more about this image:
FEI-natural-resources.com/elements-june
The distinctive sub-rounded particles in this
bauxite sample are known as pisoliths, which
form at the Earth’s surface as a result of
breakdown of a previous rock, typically basalt.
Extraction of the aluminium from the ore
involves complex chemical digestion techniques,
and the efficiency of this process is significantly
reduced if impurities other than aluminium
hydroxide (green) are present in the ore, such
as quartz (pink), clays (brown) and iron oxides
(orange).