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OCR for page 3365
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3365-3371, March 1999
Colloquium Paper
This paper was presented at the National Academy of Sciences colloquium "Geology, Mineralogy, and Human' Welfare,"
held November 8-9, 1998 at the Arnold and Mabel Beckman Center in Irvine, CA.
Contaminant bioavailability in soils, sediments, and
aquatic environments
SAMUEL J. TRAINA* AND VALERIE LAPERCHE
School of Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, OH 43210
ABSTRACT The aqueous concentrations of heavy metals
in soils, sediments, and aquatic environments frequently are
controlled by the dissolution and precipitation of discrete
mineral phases. Contaminant uptake by organisms as well as
contaminant transport in natural systems typically occurs
through the solution phase. Thus, the thermodynamic solu-
bility of contaminant-containing minerals in these environ-
ments can directly influence the chemical reactivity, trans-
port, and ecotoxicity of their constituent ions. In many cases,
Pb-contaminated soils and sediments contain the minerals
anglesite (PbSO4), cerussite (PbCO3), and various lead oxides
(e.g., litharge, PbO) as well as Pb2+ adsorbed to Fe and Mn
Hydroxides. Whereas adsorbed Pb can be comparatively
inert, the lead oxides, sulfates, and carbonates are all highly
soluble in acidic to circumneutral environments, and soil Pb
in these forms can pose a significant environmental risk. In
contrast, the lead phosphates [e.g., pyromorphite,
Pb5(PO4~3Cll are much less soluble and geochemically stable
over a wide pH range. Application of soluble or solid-phase
phosphates (i.e., apatites) to contaminated soils and sedi-
ments induces the dissolution of the "native" Pb minerals, the
Resorption of Pb adsorbed by hydrous metal oxides, and the
subsequent formation of pyromorphites in situ. This process
results in decreases in the chemical lability and bioavailability
of the Pb without its removal from the contaminated media.
This and analogous approaches may be useful strategies for
remediating contaminated soils and sediments.
The Earth's surface is dominated by the elements O. H. Si, Al,
Fe, Ca, Na, K, Mg, Ti, and P. As oxides, these elements account
for ~96~o of the total mass of the continental crust (1~. Many
of the remaining elements in the periodic table (together with
C) are essential for life. Many trace elements are toxic to a wide
range of organisms when concentrated and some are toxic to
most even at very low concentration. These latter contami-
nants are natural substances (with the exception of the tran-
suranics) and life on Earth evolved in their presence. Human
activity has altered the distribution and forms of these ele-
ments, locally increasing their relative toxicities and the fre-
quency with which they are encountered by living organisms.
At present, various elements are listed as priority pollutants
by the U.S. Environmental Protection Agency. Their concen-
trations in soils, as well as in surface and ground water,
typically are regulated based on total concentration. This
system provides a convenient regulatory framework for estab-
lishing acceptable levels of contaminant metals and oxoanions
in environmental media. However, the environmental science
community recognizes that total concentration is not an
accurate predictor of the bioavailability or chemical lability of
a given contaminant in soils, sediments, or aquatic environ-
ments. Rather, the toxicity of a substance, be it an element, an
PNAS is available online at www.pnas.org.
ion, or a molecule typically is controlled by its chemical and
physical state, or speciation (2-10~. Thus, regulations based on
absolute concentration may be convenient, but their scientific
validity may be in question.
Knowledge of the link between chemical speciation and
bioavailability is not new, nor did it originate with concerns of
contaminant toxicity or environmental pollution. The fields of
soil chemistry and soil fertility were, in part, created by the
recognition that total soil concentration is a poor predictor of
the bioavailability of essential nutrients required for plant
growth and food production. This recognition has led to
extensive research on the identity and form of nutrient ele-
ments in soils and fertilizers with emphasis on predicting their
bioavailability. An example is the attention paid to the chem-
istry and mineralogy of P in soils and fertilizers (11-13~.
Phosphorous is often a limiting nutrient, and supplementa-
tion of soil P through the addition of P-containing amend-
ments has been practiced at least as early as 287 BC (13~. The
apatite mineral family is the most ubiquitous form of P in the
Earth's crust, as well as the most geochemically stable one in
neutral to alkaline environments (14~. Although used exten-
sively as fertilizer, its intrinsically low solubilities makes apatite
a very poor choice. Instead, more soluble forms of P commonly
are used as amendments to P-deficient agricultural soils. Thus,
when serving as a nutrient, the total concentration of P in the
agricultural amendment is not as important as the form or
availability of the P in the amendment material.
In addition to being an essential nutrient, P is an important
environmental contaminant. Excessive influx of P into fresh
water can lead to increases in primary productivity (photo-
synthesis) and accelerated sedimentation (15~. This process,
cultural eutrophication, can result in the growth of deleterious
species of algae, depletions of available O2, and production of
toxic metabolic products by a number of phytoplankton.
Numerous studies of fresh-water systems have shown the
inseparable link between the chemical form or species of P
entering lakes and the potential for P-induced eutrophication
(16~. Indeed, bioassay measurements indicated that P present
as apatites is much less bioavailable to planktonic algae than
dissolved phosphate (16~. Presumably, the lower bioavailability
of apatites is a result of their low solubility in neutral and
alkaline environments (as also in similar agricultural environ-
ments). That apatite as well as other solid-phase forms of P are
less bioavailable than dissolved P strongly influenced the
institutional controls implemented for the protection of the
Great Lakes of North America. Initial efforts were focused on
the removal of dissolved P from wastewater discharges fol-
lowed by a later effort to reduce sediment loads from agricul-
ture. The latter is dominated by particulate P (16).
Abbreviations: XRD, x-ray diffraction; SEM, scanning electron mi-
croscopy; AFM, atomic force microscopy.
*To whom reprint requests should be addressed. e-mail: Traina.1@
osu.edu.
3365
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3366 Colloquium Paper: Traina and Laperche
These examples illustrate two important points relevant to
contaminant chemistry and ecotoxicology. First, a substance
beneficial in one environment (e.g., bioavailable P in an
agricultural soil) may be deleterious in another (e.g., bioavail-
able P in lake waters). Second, the specific form of a substance
has a profound impact on its bioavailability with solid-phase
forms (including sorbed species) generally controlling its bio-
availability in natural environments. It is clear that dissolved
substances are generally more labile and bioavailable than
solids, but can one generalize about the relative bioavailabili-
ties of two or more different solids, each containing the same
element of interest? As discussed below, this question may be
assessed by considering the solubility products of the solids.
The relationship between solubility and bioavailability of
contaminants has important ramifications for environmental
risk assessment and environmental remediation. Adoption of
an environmental impact or bioavailability mode of contam-
inant regulation can on be accomplished if a formal assessment
indicates that some parameter other than total concentration
(e.g., solubility) controls the bioavailability and ecotoxicology
of that substance. Concomitantly, recognition that bioavail-
ability can be tied to solubility rather than total concentration
allows one to consider remediation strategies based on in situ
reductions solubility of a contaminant, rather than its complete
removal.
Solubility and the Solubility Product
The assertion that the bioavailability of a given element in soils
or sediments is controlled by its solid-phase form rests on the
assumption that uptake of a contaminant by a target organism
happens through the solution phase. This is a safe assumption
to make when one considers the uptake of an ion by plants. It
may be less appropriate for the uptake of contaminants by
fauna where ingestion or inhalation of particulate material can
represent a significant mechanism of contaminant exposure.
Nevertheless, with the possible exception of radiological dam-
age, toxic responses to a contaminant generally require ab-
sorption by biological tissue. Even for ingestion, the relative
bioavailabilities and toxicities of different mineral forms of a
given element are subject to their relative solubility (17, 18~.
This phenomenon may in part be caused by the linkage
between intrinsic solubility and relative dissolution rates, as
discussed below. In any event, the equilibrium solubility of a
given mineral and its dissolution kinetics profoundly affect the
bioavailability and chemical lability of its constituent ions.
For a given solid, MxL,, a general dissolution reaction is:
H20
MxLy~s) ~ xMY+(aq) + yLX-aqj, [11
where MY+(aq) and LX-(aq) are the aqueous metal and ligand
ions M and L, respectively. An equilibrium constant for this
reaction is defined as:
K l MY ~ ILL is
fMx~]
[21
where ~ ~ denote activities. A solubility product is then defined
as:
K
K
[3]
If the solid MXLy is in its standard state then [MXIg,] becomes
unity and Ksp becomes:
KSp = tMY+]X ~LX-]y
.
[4]
For a fixed activity of LX-, the solid with the smallest numerical
value of Ksp will support the smallest equilibrium activity of
Proc. Natl. Acad. Sci. USA 96 (1999)
MY+. Commonly, the toxicity of metal M is directly propor-
tional to the activity of the free metal ion, MY+, regardless of
whether MY+ or some hydrolytic species, or complex ion-pair
is the most toxic form of M. This relationship results from the
direct relationship between the activity of MY+ and all other
species of dissolved M (including complex ion-pairs and hy-
drolytic species). Therefore, the least toxic solid form of M will
have the smallest aqueous equilibrium activity of MY+. Anal-
ogously, the most toxic solid will be that which supports the
largest aqueous equilibrium activity of MY+.
This so-called solubility product model of contaminant
availability has several limitations. First, organisms represent
intrinsically dynamic systems, and their reaction with the
surrounding environment is typically far from equilibrium.
Second, adsorption reactions and mass transfer constraints
may lower the aqueous activity of M below that supported by
any known phase of M. The formation of solid solutions also
may lower the aqueous activity of M below that supported by
any known pure phase of M. Finally, the solubility product of
MXL:, is expressed in terms of its dissolution into its constitutive
ions. It does not account for additional side reactions that
could lead to consumption of L. Such side reactions could alter
the apparent relative solubilities of different M-containing
solids. With the exception of the first proviso (local equilib-
rium) the other limitations of the solubility product model can
be addressed by substitution of the appropriate equilibrium
constants to account for desorption from particulate surfaces,
dissolution of solid solutions, and/or incongruent dissolution
reactions. What then, can one do about the assumption of local
equilibrium?
The law of detailed balancing (19) indicates that to a first
approximation the relative dissolution rates of a series of solids
with identical specific surface areas, each containing metal M,
will be inversely proportional to their solubility products,
(when corrected for side reactions with the surrounding solu-
tion). Obviously, in natural systems solids of different chemical
composition rarely have identical specific surface areas. Ad-
ditionally, some solids may undergo surface controlled disso-
lution under the same conditions that promote diffusion
controlled dissolution reactions for other solids containing M.
Nevertheless, wide differences in mineral solubilities do pro-
duce differential dissolution rates among multiple solids, each
containing the metal M. This assertion is true for virtually all
aqueous solutions, including surface and ground waters and
soil solutions, as well as gastrointestinal tracts. The net out-
come is that the solids with greater solubilities generally can be
characterized as having greater dissolution rates, resulting in
greater bioavailabilities and chemical [abilities of their con-
stituent elements.
Recognition that the solubility product serves as a relative
constraint on the reactivity and potential toxicity of metals and
oxoanions in surficial environments provides a strategy for
environmental treatment. Often, it is not possible to remove
toxic elements from contaminated soils and sediments. In
these cases, inducing changes in the mineralogy of a contam-
inant (e.g., conversion of a metal-carbonate to a metal-
phosphate) may allow one to significantly lower its solubility
and its corresponding ecotoxicity. The remainder of this paper
will examine the feasibility of this approach with emphasis on
the formation of stable Pb precipitates in contaminated soils.
Pb-Phosphates
The orthophosphate ion forms sparingly soluble solids with
several toxic metals, including Cd, Zn, Pb, and several of the
actinides. Much research has explored the utility of using
phosphates to reduce the mobility and bioavailability of these
metals in contaminated environments as well as in a number
of waste forms. A full discussion of all of these metals and all
of these applications is beyond the scope of this paper. For
OCR for page 3367
Colloquium Paper: Traina and Laperche
brevity we focus on the formation and bioavailability of
Pb-phosphates.
The Pb-phosphates are some of the most insoluble Pb(II)-
solids known to form under surficial geochemical conditions
(Table 1~. At standard state, the Pb-phosphates are at least 44
orders of magnitude less soluble than galena (PbS), anglesite
(PbSO~), cerussite (PbCO3), litharge (PbO), and crocoite
(PbCrO4), Pb-solids common to soils contaminated by mining
and smelting activities and by paint (20, 21~. Nriagu (22-24),
Santillan-Medrano and Jurinak (25), and Sauve et al. (26)
suggest that Pb-phosphate phases may control the solubility of
Pb in noncalcareous soils. Indeed, in oxidized, noncalcareous
environments, Pb-phosphates should form at the expense of
other Pb solids if sufficient P is present Natural Pb-phosphate
minerals have been identified in soils impacted by the weath-
ering of Pb ores (17, 20, 27, 28) as well as in roadside soils
presumably contaminated by automobile emissions, and in
urban soils (29~. Often these natural Pb-phosphates are
present as Pb-Ca solid solutions (17, 27-29) as predicted by
Nriagu (24~. However, essentially pure Pb-phosphates also
have been identified in contaminated soils (20~.
In light of their intrinsically low solubilities and their natural
occurrence in some contaminated soils, effort has been given
to inducing the formation of Pb-phosphates in Pb-contami-
nated soils and soil materials through the addition of P. The
treatment of Pb-contaminated soils with additions of highly
soluble forms of orthophosphate as (Na2HPO4 or KH2PO4)
can reduce the bioavailability of Pb as assessed by an in v~tro
gastrointestinal assay (30, 31) as well as induce the formation
of Pb-phosphate particles (32~. Unfortunately, treatment with
highly soluble P increases the risk of offsite P migration (29, 33)
and eutrophication of surrounding surface waters. An alter-
nate approach is to use a lower solubility source of P such as
apatite. These Ca-phosphates are prevalent as accessory min-
erals in igneous rocks and as low-temperature precipitates in
soils and sedimentary environments.
Apatite Chemistry
The hexagonal (P63/m) crystals of the apatite group
tCas(PO4~3X] comprise three dominant end-members, where
X = 0H in hydroxylapatite, F in fluorapatite, and Cl in
chlorapatite. The solubility product constants of the most
common hydroxyl and fluoro end members are 10-3 ~ and
10-25, respectively. Apatites of geologic origin are dominated
by the fluorapatites, exhibiting inhomogeneous solid solution
with Cl, OH, and CO3 (11, 34, 35~. The Ca in apatites resides
in two distinct crystal sites (Fig. 1~. The Ca(1) site is coordi-
nated to nine oxygens. The Ca(2) site is comprised of CaO5X
octahedron. In fluor- and hydroxylapatites an additional weak
bond to O exists (0.15 valence units), resulting in CaOsX(O)
polyhedra (34~.
Natural apatites exhibit extensive substitution with the in-
corporation of K, Na, Mn, Ni, Cu. Co, Zn, Sr, Ba, Cd, Sn, Y.
and rare earth elements in the Ca sites (refs. 11, 34, and 36, and
Table 1. Solubility products of selected Pb minerals
Mineral Formula Log Ksp*
Litharge
Anglesite
Cerussite
Pyromorphite
Hydroxypyromorphite
Fluoropyromorphite
Bromopyromorphite
Corkite
Hindsalite
Plumbogummite
PbO
PbSO4
PbCO3
Pbs(PO4~3C1
Pb5(PO4~30H
Pbs(PO4~3F
Pbs(PO4~3Br
PbFe3(PO4) (SO4)(OH)6
PbAl3(PO4)(SO4)(OH)6
PbAl3(PO4)2(0H)s H20
*Data from ref. 20 and references cited therein.
Proc. Natl. Acad. Sci. USA 96 (1999J 3367
Catl)
Cat2)
FIG. 1. Projection of the hydroxylapatite structure down the c axis,
as well as the two cation sites in hydroxylapatite. Structural data from
ref. 52. Atom sizes are not to scale.
references therein). Substitution occurs at both Ca sites,
depending on the ionic radius of the ion in question.
Reaction of Dissolved Pb with Apatites
Apatites have been examined extensively for use in the removal
of toxic metal ions from wastewater and aquatic solutions (33,
37-51~. Mechanisms of metal uptake vary with identity of the
sorbate, the sorbent, and the solution conditions. From
changes in solution composition, powder x-ray diffraction
(XRD) patterns, and scanning electron micrographs, Ma et al.
(33) described the reaction of dissolved Pb2+ with hydroxy-
lapatite by the sequential dissolution and precipitation reac-
tions:
Cas(PO4~30H + 7H+ ~ SCa2+ + 3H2PO4 + H2O [5]
sPb2+ + 3H2PO4- + H2O ~ Pb5(PO4~30H + 7H+, [6]
where Pbs(PO4~30H is the mineral hydroxypyromorphite.
The overall reaction:
Cas(PO4~0H + sPb2+ ~ Pbs(PO4~30H + SCa2+ [7]
12.9
-7.7
- 12.8
-84.4
-76.8
-71.6
-78.1
112.6
-99.1
-99.3
is exergonic with a standard state Gibbs energy change of
-137.08 kJ mol-i (51~. The pyromorphite group also includes
the mineral pyromorphite tPbs(PO4~3Cl], fluoropyromorphite
tPbs(PO4~3F], bromopyromorphite tPbs(PO4~3Br], and various
arsenate and vanadate analogs. The solubility of the pyromor-
phites decreases fluoropyromorphite > hydroxypyromor-
phite ~ bromopyromorphite > pyromorphite, with log Ksp of
-72, -77, -78, and -84, respectively. The pyromorphites are
isostructural with the apatites.
Discrete products of Pb reactions with hydroxylapatite are
detectable by powder XRD and by scanning electron micros-
copy (SEM) in model aqueous systems when the initial aque-
ous Pb concentrations are >5 mg liter -~ and the initial pH
ranges from 3 to 7 (33~. Under these conditions, discrete
Pb-bearing solids (identified by XRD as hydroxypyromor-
phite) form in less than 10 min. They have different crystal
OCR for page 3368
3368 Colloquium Paper: Traina and Laperche
habits than the original hydroxlyapatites and do not contain Ca
(within the detection limits of energy dispersive x-ray analysis).
Apparently they form according to the sequential reactions
described in Eqs. 5 and 6 and not from ion substitution of Pb
for the Ca in the apatite particles (334. Fourier transform IR
spectroscopy and x-ray absorption fine structure spectroscopy
indicate that the reaction of <1 mg Pb literal with hydroxy-
lapatite still results in the formation of hydroxypyromorphite
(52~.
Ex situ and in situ atomic force microscopy (AFM) studies of
Pb-reacted hydroxylapatite indicate that when initial condi-
tions are far from equilibrium (>1 mg Pb liter-i, pH = 6),
pyromorphite can nucleate homogeneously as a result of
interactions between dissolved Pb and phosphate (50, 51~. In
situ, AFM measurements of Pb solutions reacting with hy-
droxylapatite particles, showed only "clean" apatite surfaces
without coatings of pyromorphite crystals (51~. Nevertheless,
pyromorphite crystals were found in the outflow from the
AFM liquid cell, suggesting that homogeneous nucleation had
occurred. Additionally, the presence of pyromorphite needles
atop the AFM cantilever (Fig. 2) was consistent with precip-
itation of the Pb-phosphates in solution and not on the surfaces
of the hydroxylapatite (514.
The initial composition of the solution phase influences the
interactions of dissolved Pb with apatites. Ma et al. (42)
examined the effects of NO3, Cl, F. S04' and CO3 on the
immobilization of aqueous Pb by hydroxylapatite. Pb concen
FIG. 2. Pyromorphite crystals formed from the reaction of dis-
solved Pb with hydroxylapatite. (A) An ex site tapping mode AFM
image of effluent from the AFM fluid cell. Scan size = 661 rim on a
side. (B) SEM image of pyromorphite crystals deposited atop the AFM
cantilever after reaction of dissolved Pb with hydroxylapatite in an
AFM liquid cell. [Reproduced with permission from ref. 54 {f:onvrirrht
1998, Elsevier Science).]
Proc. Natl. Acad. Sci. USA 96 (1999)
"rations were reduced from an initial 5-100 mg liter-i to <15
,ugliter-~ (the Environmental Protection Agency's drinking
water limit for Pb) except at very high concentrations of CO3.
Hydroxylapatite was transformed to hydroxypyromorphite in
the presence of NO3, S04, and CO3, to pyromorphite after
reaction with PbCl2, and to fluoropyromorphite after reaction
with PbF2. These reaction products were identified by XRD
and SEM.
Ma et al. (53) explored the interactions of dissolved Pb with
natural fluorapatites and carbonated fluorapatites. These sol-
ids varied in their capacity to remove aqueous Pb (from 39%
to 100%~. The fraction of Pb removed was not related to the
initial surface areas of the apatite particles, but rather to their
dissolution rates. Fluoropyromorphite and hydrocerussite
were the principal Pb phases formed in these experiments.
The exact composition of the products formed by the
reaction of aqueous Pb with apatite depends on the solution
pH. In the regime of pH 3.1-6.2, Chen et al. (49) found that
mixtures of dissolved Pb and carbonate-containing fluorapa-
tite reacted to form fluoropyromorphite by the coupled reac-
tions:
Cas~po4~3-x~co3)xF~+x + 6H+ ~
SCa2+ + (3-x)H2PO4- + xH2CO3O + (1 + xjF- [8]
sPb2+ + 3H2PO4- + F- ~ Pb5(PO4~3F + 6H+. [9]
At pH 6.6-6.8, a mixed hydroxylated fluorapatite forms.
Ca5(po4~3-x~co3)xF~+x+ (6 - x)H ~
5Ca2+ + (3-x)H2PO4- + xHCO3- + (1 + xjF- [101
SPb2+ + 3H2PO4- + (F-,OH-) ~ Pb5(PO4~3(F,OH) + 6H+.
[111
At circumneutral pH, hydrocerussite and carbonated hvdroxvl
fluoropyromorphite form by the reactions:
Castpo4~3-x~co3)xF~+x + (6 - x)H+ ~
,, _ .,
5Ca2+ + (3 - x)H2PO4- + xHCO3- + (1 + xjF- [12]
3Pb2+ + 2HCO3- + 2H2O ~ Pb3(CO3~2(OH)2 + 4H+ [13]
5Pb2+ + 3(H2PO4-,HCO3-) + (F-,OH-) ~
Pb5(PO4,CO3~3(F,OH) + 6H+. [14]
Finally, at pH 10.7-11.9, the reaction products consist of
hydrocerrusite, hydroxypyromorphite, and lead oxide fluoride
(49~. Formation of the lead oxide fluoride is described as:
2Pb2+ + H2O+2F--= Pb2OF2 + 2H+. [15]
Competition with other metal ions also influences the reac-
tions of aqueous Pb with apatites. At initial solution concen-
trations of <20 mg liter-i, dissolved Zn, Cd, Ni, Cu. Fe(II),
and Al have no discernible effect on the immobilization of 20
mg Pbliter-i, by hydroxylapatite. Additionally, significant
quantities of these metals also are removed from solution (43~.
Cadmium and Zn also have been shown to have no influence
on influence the uptake of Pb by carbonated fluorapatite (49~.
In these latter experiments, the initial concentrations of dis-
solved Cd, Zn, and Pb all were equimolar.
When the concentration of competing metals far exceeds
that of Pb, (M/Pb = 7:1, where M = Zn, Cd, Ni, Cu, Fe(II),
or Al and dissolved Pb = 20 mg liter-~) the interactions of Pb
with hydroxylapatite can be significantly inhibited (43~. Ma et
al. (43) reported dissolved Cu was the most effective in
inhibiting Pb immobilization by hydroxylapatite, followed by
Fe(II), Cd, Zn, Al, and Ni. In all cases, hydroxypyromorphite
OCR for page 3369
Colloquium Paper: Traina and Laperche
was the only reaction product detected by XRD besides
hydroxylapatite. The intensities of hydroxypyromorphite XRD
peaks decreased with increased concentrations of competing
metals. Inhibition of hydroxypyromorphite formation was
positively correlated with the solubility of known M-
phosphates; however, no metal-phosphates other than hy-
droxylapatite could be detected with XRD or SEM (43~.
Additionally, geochemical calculations indicated that the so-
lutions were all undersaturated with respect to known Zn-,
Cd-, Ni-, Cue, Fe(II)-, or Al-phosphate phases. The nature of
this inhibition is not known, but it possible that these metal
ions passivated the surfaces of the apatite, through the for-
mation of sorbed, or surface-precipitated species.
Reaction of Apatites with Sorbed and Solid-Phase Pb
The pyromorphites are much less soluble than the other
Pb-solids commonly present in terrestrial and aquatic envi-
ronments (Table 14. Thus, it is expected that if sufficent soluble
P is present, pyromophites will form at the expense of the
"native" Pb-solids (including both adsorbed and precipitated
Pb). Ma et al. (33) observed XRD-detectable hydroxypyro-
morphites after the reaction of hydroxylapatite with Pb-
saturated cation exchange resins with hydroxylapatite (note,
the initial concentration of dissolved Pb was <1 mgliter-~.
SEM showed that the spherical particles of the exchange resin
were coated with hexagonal hydroxypyromorphite needles
after they were reacted with apatite. Precipitates were not
detectable on the surfaces of the reacted apatite particles.
Zhang et al. (54) observed the formation of pyromorphite
when hydroxylapatite particles were reacted with goethite
suspensions containing sorbed Pb. In these experiments, the
Pb-treated goethites and the apatite particles were separated
by dialysis membranes. Hexagonal pyromorphite crystals
formed on the inside surfaces of the dialysis membranes,
indicating that nucleation on the apatite surface is not required
for pyromorphite precipitation as was also suggested by Lower
and coworkers (50, 51~.
Consistent with thermodynamic predictions hydroxypyro-
morphite forms at the expense of more soluble Pb-minerals
when they are exposed to apatites. Laperche et al. (21) found
XRD- and SEM-detectable hydroxypyromorphite when pure
systems of PbO and cerussite each were reacted with hydroxy-
lapatite at pH 5, 6, and 7. Decreased pH caused more rapid
reaction rates with greater loss of the parent phases and
increased formation of hydroxypyromorphite (Fig. 3~. Appar-
ently dissolution of the apatite and/or the original Pb-phases
was rate limiting.
In all of these cases, initial precipitation of pyromorphite or
hydroxypyromorphite quickly reduced the concentration of
dissolved Pb. Growth of the Pb-phosphate crystals required
continued dissolution of the apatite particles, desorption of
adsorbed Pb from the surfaces of the exchange resin and the
goethite, and dissolution of the PbO and cerussite phases. The
driving force for these dissolution and desorption reactions was
formation of the Pb-phosphates.
Reactions of Apatites with Pb-Contaminated Soils
The interactions of apatites with soil Pb are similar to those
observed in model solutions and laboratory mixtures of pure
solids. When added to a soil slurry from an automobile battery
cracking facility, hydroxylapatite caused a 99% reduction in
dissolved Pb (from 3,370 to 36 ~g liter-~) over a 5-h period
(334. Similarly, amendment of Pb-contaminated soils with
natural fluorapatites reduced the leachability of Pb in soil
columns (53) and decreased its extractability by neutral salts
(MgCl2) and weak acids (Na-acetate) (55, 56~.
Laperche et al. (21) used direct physical methods to show
apatite-induced formation of pyromorphites in soil slurries.
Proc. Natl. Acad. Sci. USA 96 (1999) 3369
FIG. 3. SEM images of the reaction products of hydroxylapatite
and solid-phase forms of Pb (cerussite) at pH 5 (A) and pH 7 (B).
"Reproduced with permission from ref. 21 (Copyright 1996, American
Chemical Society).]
When synthetic hydroxylapatite was added to a soil contami-
nated by paint residues, these investigators observed decreases
in the intensity of the XRD peaks associated with the "native"
Pb (cerussite) and appearance of peaks attributed to pyro-
morphite. The rate and magnitude of changes in the XRD
peaks was greater at pH 5 than 7, presumably because of the
more rapid dissolution of the cerussite and the hydroxylapa-
tite.
Effects of Apatites on the Bioavailability of Pb in
Contaminated Soils
The conversion of soil or sedimentary Pb from highly reactive,
chemically labile forms to less reactive solids should result in
concomitant decreases in bioavailability. Chlopeka and Adri-
ano (57) evaluated this hypothesis by adding natural apatite
from North Carolina to a soil amended with varying amounts
of Pb-containing flue dust. These materials then were used as
potting mixes in glass-house experiments that produced a
single crop of maize (Zea mays, var. Pioneer 3165) followed by
single crop of barley (Hordeum vulgare, var. Boone). Apatite
amendment resulted in decreases in extractable Pb from the
soil as well as decreases in tissue Pb concentrations in both
crops.
Laperche et al. (58) investigated the use of apatite minerals
to induce in situ formation of Pb-phosphates in contaminated
soil and determined the impact of apatites on Pb uptake by
plants. Subsamples of a Pb-contaminated soil (containing
37,026 mg Pb kg-i soil from paint residues) were mixed with
sufficient quantities of either synthetic hydroxylapatite or
natural fluorapatite to convert 33~o, 66%, 100~o, and 150~o of
the native soil Pb to either pyromorphite or hydroxypyromor-
phite. These materials then were planted with sudax grass
(Sorghum bicolor L. Moench) a hybrid of sorgum (Sorghum
vulgare L. Moench) and sudan grass (`Sorghum vulgare var.
sudanese). In all cases, hydroxylapatite amendments decreased
the concentrations of Pb in the above-ground biomass (shoots)
OCR for page 3370
3370 Colloquium Paper: Traina and Laperche
(by 92-98%) relative to the unamended soil. Fluorapatite-
induced reductions in the concentration of Pb in the shoots
ranged from 87% to 96%. The effect of apatite amendments
on the Pb concentration in the roots was much different. For
both apatites, the lowest concentrations of Pb within the root
tissue were associated with the smallest levels of apatite
amendments. Increased levels of apatite addition corre-
sponded to increases in the quantity of Pb associated with the
roots. At the greatest levels of apatite amendment, the root Pb
actually exceeded that found in the unamended soil.
Examination of the root surfaces with SEM, energy disper-
sive x-ray analysis, and XRD analysis of root-associated par-
ticles indicated the presence of Ca-substituted pyromorphites
on those plants grown in the apatite-treated soils (Fig. 44.
Similar particles were not found on the root surfaces of sudax
grown in the unamended soils; nor could similar particles be
found in the bulk soil after reaction with apatite. Apparently,
addition of apatite to the contaminated soil resulted in pre-
cipitation of pyromorphite particles on the exterior of the root
surfaces. Local acidity within the rhizosphere may have en-
hanced the local dissolution of apatite grains, facilitating
pyromorphite precipitation. Cotter-Howells and Caporn (32)
also observed the precipitation of Ca-substituted pyromor-
phites on plant roots (Agrostis capillaris) grown on Pb-
contaminated soils. In this case pyromorphite precipitation
may have resulted from root-exudate phosphatase, causing
increased local concentrations of phosphate in the rhizo-
sphere. In any event, the formation of pyromorphite likely
decreased the bioavailability of Pb.
Soils, surface sediments, and surficial aquatic environments
are open, dynamic systems best characterized as mixtures of
meta-stable solids. It is safe to say that these systems never
attain absolute thermodynamic equilibrium, and one can ex-
pect to find multiple forms of a given element present. Thus,
the bioavailability and chemical lability of toxic elements in
these systems are transient properties that are controlled by
the reaction dynamics and the total quantity of the most
reactive forms of these elements present. Ideally, treatment
technologies would facilitate the complete conversion of a
toxic element (e.g., Pb) from all pre-existing forms to the most
geochemically stable phase; but as we know, kinetic constraints
will prevent this from happening. Fortunately, decreases in the
chemical reactivity of and bioavailability of a given element
(e.g., Pb) can be accomplished by elimination of the most
reactive forms (58~.
Conclusions
Human activity on this planet has altered the distribution and
form of various elements in the periodic table. Often these
activities have converted many potentially toxic metals from
~_ ~_
"' - ~ ~43~.:
FIG. 4. SEM micrograph of sudax root grown in Pb-contaminated 22.
soil m~xed with hydroxylapatite. Note the pyromorphite crystals on the 23.
root. 24.
Proc. Natl. Acad. Sci. USA 96 (1999J
nonreactive, geochemically stable solids into forms that are
more soluble and bioavailable, increasing their effective tox-
icity. Fortunately, it is often possible to reverse this process,
transforming reactive forms of toxic metals to less labile
species through appropriate precipitation reactions. Although
such an approach does not remove the element in question
from the biosphere, it can significantly reduce its bioavailabil-
ity. In many instances, this approach may be a practical
alternative to more invasive methods of environmental resto-
ration (e.g., excavation and removal of contaminated materi-
als). In the event that more extensive treatment is chosen,
geochemical stabilization may still be desirable as a rapid
response treatment or to increase the chemical stability of
excavated materials in landfill environments.
This approach is not limited to Pb, nor is it only possible to
induce precipitation of phosphates. Extensive research has
been conducted on various treatment technologies designed to
remove toxic ions from contaminated waters or to stabilize
these elements in soil waste materials or contaminated soils
and sediments (5~. Many of these efforts involve the formation
of geochemically stable solids through precipitation and/or
adsorption reactions. In essence, these methods attempt to
close the circle, converting labile forms of toxic elements into
less reactive solids more consistent with long-term geochemi-
cal equilibrium.
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Representative terms from entire chapter:
solubility product
