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OCR for page 3350
Proc. Natl. Acad. Sci. USA
Vol. 96, pp. 3350-3357, 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.
Characterization of complex mineral assemblages: Implications
for contaminant transport and environmental remediation
PAUL M. BERTSCH* AND JOHN C. SEAMAN
Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory, The University of Georgia, Drawer E, Aiken, SC 29802
ABSTRACT Surface reactive phases of soils and aquifers,
comprised of phyllosilicate and metal oxohydroxide minerals
along with humic substances, play a critical role in the
regulation of contaminant fate and transport. Much of our
knowledge concerning contaminant-mineral interactions at
the molecular level, however, is derived from extensive exper-
imentation on model mineral systems. Although these inves-
tigations have provided a foundation for understanding reac-
tive surface functional groups on individual mineral phases,
the information cannot be readily extrapolated to complex
mineral assemblages in natural systems. Recent studies have
elucidated the role of less abundant mineral and organic
substrates as important surface chemical modifiers and have
demonstrated complex coupling of reactivity between perma-
nent-charge phyllosilicates and variable-charge Fe-oxohy-
droxide phases. Surface chemical modifiers were observed to
control colloid generation and transport processes in surface
and subsurface environments as well as the transport of
solutes and ionic tracers. The surface charging mechanisms
operative in the complex mineral assemblages cannot be
predicted based on bulk mineralogy or by considering surface
reactivity of less abundant mineral phases based on results
from model systems. The fragile nature of mineral assem-
blages isolated from natural systems requires novel tech-
niques and experimental approaches for investigating their
surface chemistry and reactivity free of artifacts. A complete
understanding of the surface chemistry of complex mineral
assemblages is prerequisite to accurately assessing environ-
mental and human health risks of contaminants or in design-
ing environmentally sound, cost-effective chemical and bio-
logical remediation strategies.
The transport and fate of contaminants in soils and ground-
water are highly coupled to the nature and relative abundance
of the reactive mineral phases. Clay and oxide minerals, along
with humified organic matter, comprise the surface reactive
phases that are the primary controllers of sorption processes
in soils, thus serving as important regulators of contaminant
transport. Major challenges in understanding the processes
controlling contaminant behavior in the environment include
the complexity of the soil and aquifer matrix and the enormous
spatial scales over which these processes occur.
Although it is well established that a fundamental under-
standing of molecular-level interactions is required to explain
the underlying mechanisms controlling the fate and transport
of solutes and contaminants in soils and subsurface environ-
ments, there has been limited success in translating molecular-
level information to observations made at the larger scales.
Although several explanations for this conundrum can be
advanced, a prominent one is that much of our knowledge
concerning the surface chemistry of clay and oxide minerals
PNAS is available online at www.pnas.org.
primarily is derived from experiments conducted on model
mineral phases. These studies have established boundary
conditions defining sorbate/mineral surface interactions and
have identified the surface functional groups involved in
surface complexation reactions, but they have produced little
information that can be readily extrapolated to complex
mineral assemblages typically present in heterogeneous soil
and aquifer materials (1~. Thus, utilization of bulk mineralog-
ical data to represent predominant reactive phases in complex
natural systems often has failed to reliably predict solute and
contaminant behavior.
Reactive Mineral Phases in Soils: An Historical View
The pioneering work on reactive mineral phases in soils, which
focused primarily on adsorption of group IA and IIA cations,
has been comprehensively reviewed (2, 3) as has more recent
work on specific sorption of metals and metalloids (1, 4~.
The concept that surface reactive phases in soils are colloi-
dal and comprised of Al(OH)3, Fe(OH)3, and SiO2 hydrogels
was proposed over a century ago (5~. By the mid-1920s, a
comprehensive understanding of the surface chemistry of Al,
Fe, and Si colloids and their role in cation sorption was
emerging, largely based on extensive investigations of Mattson
(6-8~. Mattson viewed reactive phases in soils as mixtures of
Al2O3, Fe2O3, and SiO2 colloids. Based on the observation that
soils with a high SiO2/Al2O3 + Fe2O3 ratio had higher cation
exchange capacities (CEC) and that soils with a low SiO2/
Al2O3 + Fe2O3 ratios had high anion exchange capacities at
low pH and higher CEC at high pH, Mattson concluded that
the SiO2 colloids were primarily responsible for the CEC of a
soil and that the Al203 and Fe2O3 colloids were amphoteric in
nature. Mattson's compelling evidence for mixtures of posi-
tively and negatively charged colloids, based on careful cation/
anion sorption experiments and electrophoresis, was largely
disregarded as attention shifted to a new, rapidly emerging
paradigm of reactive mineral phases predicated on the notion
that soil clays were comprised primarily of crystalline phases.
Two classic papers by Pauling (9, 10) figured prominently in
this paradigm shift. Shortly thereafter, soil chemists applied
x-ray diffraction to soil clays and discovered the existence of,
and delineated the structures for, the major classes of phyllo-
silicate clays commonly found in soils (11, 12~. Soon after it was
demonstrated that most phyllosilicates in soils had a perma-
nent negative charge resulting from substitution of lower
valence cations in both the tetrahedral and octahedral layers
(13~. For decades the surface chemistry of reactive phases in
soils would be interpreted primarily according to this para-
digm, i.e., that predominant reactive phases in soil were
Abbreviations: pznc, point of zero net charge; EM, electron micros-
copy.
*To whom reprint requests should be addressed.
3350
OCR for page 3351
Colloquium Paper: Bertsch and Seaman
crystalline and comprised of negatively charged minerals of the
phyllosilicate class.
Three rather fortuitous circumstances solidified this view of
reactive mineral phases. First, free Fe oxides and organic
macromolecules were removed from soil clay fractions via
pretreatment to improve x-ray diffraction patterns by mini-
mizing background scatter and improving preferred orienta-
tion of the phyllosilicate clay minerals. Second, most of the
active soil mineralogy groups emerging during this period were
limited to geographical areas characterized by young circum-
neutral soils having clay fractions dominated by 2:1 phyllosili-
cates; albeit, on a worldwide basis these soils were more of an
exception. Finally, much of the experimentation during this
period continued to involve the adsorption/exchange of class
IA and IIA cations, both of which are relatively weakly bound
and present at relatively high concentrations (an exception is
K+, whose chemistry is controlled by a unique combination of
cation size, low hydration energy, and structural properties of
micaceous minerals and their weathering products). Thus,
much of the data generated under these conditions was
consistent with the phyllosilicate model, and the distribution of
phyllosilicates within a given soil clay fraction generally could
be used to predict observed cation exchange behavior for this
limited range of extensively studied soils.
There continued to be prominent exceptions to this model
that could be better interpreted according to a Mattson-like
model of surface reactive phases (14, 15~. Evidence for anion
adsorption to soil clays having low SiO2/Al2O3 + Fe203 ratios
and slightly acidic pH continued to appear. Evidence for
positively charged regions (edge sites) on phyllosilicate clays in
slightly acidic suspensions appeared during this time (16-19~.
This model also was used to interpret anion adsorption and
complex flocculation/dispersion behavior of kaolinite suspen-
sions (20~. Clearly, the phyllosilicate model of reactive mineral
phases based largely on 2:1 minerals in soils of circumneutral
pH was limited in its extent of applicability.
As mineralogical techniques improved and experimental
approaches evolved, another very important body of literature
on hybrid phyllosilicate-Al/Fe oxohydroxides emerged. The
discovery (21) that 2:1 minerals in soils weathered from parent
materials rich in mica schist were interlayered with nonex-
changeable, positively charged hydroxo-Al polynuclear com-
ponents stimulated a significant body of research that contin-
ues to this day and includes investigations on an important class
of zeolite-like clay catalysts (22~. Although this finding ex-
plained a number of properties related to the surface chemistry
of many 2:1 soil clays, the research emphasis on the hydroxy-
interlayered minerals largely focused on explaining the unique
adsorption behavior of large weakly hydrated monovalent
cations, such as K+, NH4+, and Cs+, with less emphasis on
anion sorption. Only many years later would the role of this
complex mineral assemblage in the specific sorption of tran-
sition metals be considered (22~.
Concurrent with these exciting developments, a new para-
digm of surface reactive mineral phases was emerging. The
structural aspects of important functional groups on oxide
minerals were being unraveled (4, 23J. The notion of surface
structural hydroxyl groups having acid/base properties that
could quantitatively explain the observed amphoteric behavior
of oxides became firmly established (24~. Thus, an accurate
model of surface functional groups that could explain Matt-
son's original observations was emerging, and a number of
studies on anion adsorption to metal oxide surfaces followed
quickly as did spectroscopic evidence for the proposed reactive
surface hydroxyls (4~. It was now established that solutes could
interact with charged metal oxide surfaces via electrostatic
(outer sphere) reactions or through specific ligand exchange
reactions with the surface functional groups (inner sphere).
The conceptual model of surface complexation to describe
nonspecific and specific adsorption of anions and cations was
Proc. Natl. Acad. Sci. USA 96 (1999J 3351
advanced shortly thereafter by the classic work of Schindler
and Gamsjager (25) and Stumm et al. (26~. The surface
complexation model has remained the basic framework for
research on metal and anion sorption to metal oxide surfaces
to the present time (1, 4), and many studies have demonstrated
the importance of metal oxides as resident phases for a variety
of metals and metalloids (1, 27~.
Although extensive modeling efforts have demonstrated
reasonable success for predicting metal and metalloid sorption
to model monomineralic metal oxide phases, applications to
natural systems have been less than satisfying (1~. A major
challenge in extending such results to complex mineral assem-
blages typically found in nature has been the identification and
quantification of the primary reactive phase and associated
surface functional groups. High surface area, low abundance
metal oxohydroxide phases, and organic materials can be
coassociated with more prominent mineral grains as armoring
agents or as surface coatings. The term surface coating as used
here does not imply the presence of a uniform gel-like phase
as is often envisioned. Rather, it is used to describe domains
of crystalline or noncrystalline components coassociated with
well-defined mineral grains. The complex nature of the elec-
trostatic and van der Waals interactions between fine-grained
crystalline and poorly ordered phases with mixed surface-
charge properties has hampered the development of suitable
models to represent surface reactive functional groups in
mixed mineral assemblages. Adsorption studies using binary
mixtures of model mineral phases have demonstrated remark-
able complexity, with adsorption generally being very poorly
predicted by considering a weighted sum of individual mineral
components (1, 28~.
Mixed Mineral Assemblages in Natural Systems
It is becoming increasingly clear that many natural mineral
phases possess different surface chemical properties than their
model mineral analogues. Zachara and others (29-31) have
provided compelling evidence suggesting that the small crys-
tallite size of soil smectites enhances the importance of edge
site aluminol (Al-OH) functional groups imparting an oxide-
like behavior compared with the widely used Source Clay
Repository, SWy-1 montmorillonite. Other studies have sug-
gested that organic or metal oxide minerals may be the primary
reactive phases in soils and sediments even at relatively low
abundance (32-37~. A major theme that emerges from these
investigations is that surface modifiers in the form of organic/
metal oxohydroxide armoring agents or coatings, rather than
bulk mineralogical composition per se, control the surface
chemistry of reactive phases in soils and aquifers. For example,
it has been demonstrated that organic constituents coassoci-
ated with variable charge minerals significantly alter the point
of zero net charge (pznc, the pH at which the cation and anion
exchange capacities are equal), shifting the pzuc to signifi-
cantly lower pH values (32, 33, 35-37~. Conversely, Fe and A1
oxohydroxide phases coassociated with quartz, and permanent
charge phyllosilicate minerals have been found to shift the
pznc to higher pH values (38-40~.
A number of recent studies have focused on the surface
chemistry of mineral assemblages isolated from natural sys-
tems (33-35, 37, 38, 40-424. Characterizing natural mineral
assemblages is challenging, because it has been virtually im-
possible to isolate them free of artifacts. In fact, the methods
used to isolate and concentrate clay minerals involve disper-
sion of the clay fraction via treatment with harsh reagents
designed to significantly alter surface charge properties and
destroy complex mineral assemblages present in the original
material. Recently, however, collection and examination of
complex mineral assemblages has been achieved in a different
context: that dealing with the transport of colloidal phases
through porous media. The past decade has witnessed great
OCR for page 3352
3352 Colloquium Paper: Bertsch and Seaman
interest in the generation and transport of mineral colloidal
phases through natural porous media (33, 34, 40, 42-47~.
Interest in this subject has paralleled evidence that colloidal
minerals are important vectors for facilitating the transport of
contaminants in certain environments (43, 47-50~.
Mobile colloids can be generated by a number of mecha-
nisms, including precipitation of colloidal size phases, disso-
lution of cementation agents composed of fine-grained crys-
talline and poorly crystalline secondary minerals, and release
from soil and aquifer materials via physicochemically con-
trolled dispersion processes. Transport of the mineral colloids
also depends on a number of factors, including fluid flow rate,
electrostatic and van der Waals forces between colloids and
between colloids and matrix minerals, and physical factors
related to the relative size of the colloids and pores and pore
throats. Recent evidence has indicated that mobile colloids
comprised of minerals and complex mineral assemblages can
be generated via dispersion processes and transported through
many soils and groundwater systems with relatively minor
changes in solute chemistry of the invading fluid, thereby
avoiding the serious artifacts typically encountered in the
isolation of complex mineral assemblages.
Mineralogy and Surface Chemistry of Complex Mineral
Assemblages Isolated From Soils and Aquifer Materials
Although several studies have demonstrated the enhanced
mobility of contaminants in the presence of mobile colloids, far
fewer have focused on characterizing the mineralogical com-
position and surface charge properties of the mineral and
organic-mineral assemblages comprising the mobile phase.
Over the past decade, our investigations have focused on
providing evidence for the facilitated transport of contami-
nants associated with mobile colloidal phases and in defining
the mechanisms leading to the generation and transport of
mobile colloidal phases and solutes (33, 34, 40, 46, 49, 51-53~.
These studies have examined surface chemical controls on
colloid generation and of colloid and solute migration in
surface and subsurface highly weathered oxide-rich systems
having similar bulk clay mineralogy.
The samples examined are coarse textured (~85~o sand;
<9% clay), have varying quantities of Fe-oxide and organic
carbon, a predominance of exchangeable Al, low pH, low pore
water ionic strength, and similar bulk clay mineralogies (Table
1~. They are also poorly structured with little or no evidence
for meso/macropore development, thus minimizing compli-
cations involving preferential flow. They both have silt and
sand fractions composed entirely of quartz, thus minimizing
artifacts resulting from dissolution of less stable minerals that
contribute solutes and complicate solution chemistry. The
properties of these highly weathered soils and aquifer materials
have important commonalities with those widely distributed in
humid tropics, which play an important role in global geo-
chemical cycles (41~.
Table 1. Chemical and mineralogical characteristics of a surface
soil (Orangeburg Series) and three subsurface sediments
representative of the Tobacco Road (TR) formation from the
Atlantic Coastal Plain
Orangeburg
series TR1
TR2 TR3
5.33 5.18
pH
Extractable Fe (ma kg-1)
CDB*
NH4-oxalate
Clay mineralogy
4.61 5.37
15.0 73.5 111.0 359.4
3.1 1.4 1.2 2.3
k, HIV, gibb k, m, g k, m, g g, k, m
k, kaolinite; HIV, hydroxy-interlayered vermiculite; m, mica (illite);
gibb, gibbsite; g, goethite.
*Citrate dithionite extraction.
Proc. Natl. Acad. Sci. USA 96 (1999)
Our results have demonstrated that colloids mobilized from
surface soils have high negative electrophoretic mobilities
~-2.5 to -3.5 ,um cm s-~ V-~), inconsistent with mineral
OCR for page 3353
Proc. Natl. Acad. Sci. USA 96 (1999) 3353
-
,~ 30
z
-
~ 20
m
G
. ,
2
~ 3~
-
-
~_
O 2
c'
. _
o 1
s
a
it_
tile! o
C
Q
z
4
Colloquium Paper: Bertsch and Seaman
~D a ~
0.001 NCaCI2(pH3.0) ~
top 0
10
0-:
6
a b ~
0.001 NCaCI' ~ ~ I,'
i
1
NaCI~ O it, lo, ~ ~ ~ Job ~ 0 ~ ~ · · .
I b
~0.001 N NaCI
of f
~ 0.001 N 08tl2 ~
at. _
0.001 N CaClz~pH 3.0~
Dl Water
0.001 N CaCI2
O 5 10 15
PORE VOLUME
FIG. 1. Influence of treatment solution (1 x 10-3 and 0.1 M NaCl;
5 X 10-4 M CaC12; 5 X 10-4 M CaCl2, pH 3.00) on effluent turbidity
(a measure of colloid concentration) (a), pH (b), and the electro-
phoretic mobility (c) of mobile colloids from sample TR1 (Table 1~.
Columns were leached with a given influent for 10 pore volumes at a
Darcy velocity of ~ 0.72 m d-i followed by several pore volumes of
deionized (DI) water.
concentration between ~2.5 and S pore volumes because of
destabilization of the suspended colloids as the ionic strength
of the efluent approaches that of the influent. Evidence to
support this explanation is provided in the column colloid
transport histories, which reveal a second colloid concentra-
tion maximum when deionized water was introduced as the
influent solution after 10 pore volumes of the Ca2+ solution.
To explain the complex surface charging processes observed
in this mixed mineral system, we propose a mechanism involv
ing the strong coupling between surface exchange reactions on
permanent negatively charged phyllosilicate minerals (kaolin-
ite and mica) and subsequent protonation of the variable-
charge Fe-oxohydroxide mineral components (Fig. 2~. Accord-
ing to this model, Ca2+ undergoes exchange with native cations
associated with the negatively charged phyllosilicate minerals,
of which A1 occupies ~85% of the exchange phase. Evidence
for ion exchange of Ca2+ with native cations is provided by
examining the effluent discharge concentrations of major
cations from the column, where the delay in Ca2+ transport is
accompanied by the breakthrough of Mg2+ and trace levels of
Na+ (data not shown). The model also suggests that the
observed decrease in effluent pH is a result of exchanged A1
undergoing hydrolysis reactions. Consistent with this model,
we were unable to detect elevated A1 in the effluents but could
measure a decrease in cation exchangeable A1. Dilute Na+
influent solutions were ineffective at displacing native ex-
changeable cations associated with the phyllosilicate minerals,
explaining the differences between the Na+ and Ca2+ systems
in effluent pH and colloid transport histories. This mechanism
was confirmed in transport experiments with more concen-
trated Na+ solutions (0.1 M) where native cations, including
A1, were displaced, as evidenced by elevated effluent cation
concentration and depressed pH. Mobile colloids were not
detected in these high Na+ systems, however, because of ionic
strength destabilization. Consistent with the proposed mech-
anism, introduction of an acidified CaCl2 solution enhanced
positive surface charge development as evidenced by the high
mobile colloid yields throughout the leaching event, i.e., higher
positive surface charging countered the ionic strength desta-
bilization mechanism.
Examination of the mobile colloids by transmission electron
microscopy (EM) and selected area electron diffraction re-
veals that they are comprised primarily of aggregates of
microcrystalline Al-substituted goethite along with complex
mineral assemblages of goethite-armored kaolinite and cran-
dallite (Ca Al3tPO432~0H]s.H2O) in the 200- to 300-nm size
range (Fig. 3~. Examination of mobile colloids by scanning EM
reveals that the phyllosilicates and phosphate minerals present
in the mobile phase are extensively armored in all instances
and generally fall in the 100- to 300-nm size range. Examina-
tion of isolated bulk clay reveals that the kaolinite and
crandallite are present in the bulk clay fraction in two major
size populations; the 100- to 300-nm size class as observed in
the mobile phase and the 700-nm to 1-,um size class, which is
predominant and presumably more representative of the re-
active minerals comprising the immobile matrix. The former
size class also contains the micaceous minerals found in the
bulk clay mineral fraction. Scanning EM images of the bulk
clay suggest that even these larger mineral grains of kaolinite
and micaceous minerals are partially armored along the neg-
atively charged basal surfaces with goethite crystallites and
A. Al exchange from phyliosiiicates and subsequent hydrolysis
~ ~ Al3++ 3/2Ca2+ ~ a.,. I+. All~\ 3-m .
B. pH clependent charging mechanisms of oxohydroxide phases
Net Positive Net Zero Net Negative
Charge Charge Charge
FIG. 2. Schematic representation of the coupled mechanisms
controlling effluent pH and the generation of positive surface charge
on the mobile colloids and the stationary matrix.
OCR for page 3354
3354 Colloquium Paper: Bertsch and Seaman
Proc. Natl. Acad. Sci. USA 96 (1999J
E. goethite aggregate _ c
6. ~ to,, . ~_ ~
3:.
. ~
it_
l
_
I_
__ _ ~
_~-
Amp'
_k ~
`~100nm ~
_
' °~ ~G I
lf°1
~.e
-1. ~. . . . ~
0 1 2 3 4 5 ~7
died
..__..,--._..-: ~
\
\
-2 -1 0 1 2 3 4
Electrophoretic Mobility [(pm~sy(V/cm)l
FIG. 3. Transmission EM and selected-area electron diffraction pattern (Inset) for (A) crandallite and (B) kaolinite armored with fine-grained
goethite and (E) goethite aggregates generated during dynamic transport experiments with 5 x 10-4 M CaCl2 solutions. Transmission EM image
of crandallite (C) and kaolinite (D) from bulk clay with Fe-oxides removed by dithionite extraction. Electrophoretic mobility behavior of (F)
complex mineral aggregates generated during reactive transport experiments when repeatedly analyzed by laser doppler velocimetry: (a) initial
mobility distribution, (b) third consecutive, (c) fifth consecutive, (d) sixth consecutive mobility distribution, respectively, and (e) typical mobility
distribution observed after sample relaxation (t ~ 5 min). (G) Average electrophoretic mobility of colloidal suspension as a function of consecutive
analyses.
that these assemblages often are coassociated with larger
quartz grains as has been observed in at least one other natural
subsurface system (38~.
Thus, in these oxide-rich subsurface systems, Fe-
oxohydroxide surface modifiers increase the pznc of the
complex mineral assemblages, resulting in surface reactivity
OCR for page 3355
Colloquium Paper: Bertsch and Seaman
it is controlled by the development of a net positive surface
arge. Based on electrophoretic mobility measurements of
able colloidal suspensions generated in the column transport
periments we conclude that the complex mineral assem-
ages observed in the EM images are not artifacts of sample
elation and preparation, i.e., surface armoring of negatively
barged basal surfaces with Fe-oxohydroxide crystallites in the
)- to 30-nm size range is required to fully explain the observed
gh surface positive charge of the mineral aggregates. Further
evidence for complex mineral aggregates comprised of a
hyllosilicate/phosphate mineral core and a Fe-oxohydroxide
neer is provided on longer-term exposure of the assemblages
~ fluctuating electric fields. During consecutive electro-
horetic mobility measurements we observed evidence that
he complex mineral assemblages were disaggregating in the
lectric field, resulting in a suspension having two primary
barged populations, one positively charged and one negatively
barged (Fig. 3~. The strong bias for enhanced scattering
ntensity observed for the negatively charged colloid popula-
ion can be explained by the relative size and thus scattering by
he 100-300 rim phyllosilicate and phosphate minerals domi-
~ates over the 10-30 rim goethite crystallites. When the
lisaggregated mineral mixture is allowed to reequilibrate in
the absence of an electric field the system is found relax to the
original state, i.e., the mixed mineral assemblage again is
formed and the net positive charge reestablished.
Other evidence of the rather fragile nature of the complex
mineral assemblages in these natural systems was found in an
attempt to conduct conventional batch flocculation/
dispersion and critical coagulation concentration experiments.
On either air drying or physical disturbance of the sample, the
surface chemistry of the mineral assemblages was found to be
highly biased toward the permanently negatively charged
mineral components in the mineral mixture, an observation
consistent with several previous studies (56, 57~. Even drying
minimally disturbed columns via purging with Ar before
conducting the dynamic transport experiments resulted in the
complete loss of colloid generation and transport as observed
for identically prepared columns that were maintained in the
field moist condition.
These observations have significant ramifications for how
experiments are designed to examine surface chemical prop-
erties of mixed mineral assemblages. Furthermore, they pro-
vide some insight into the electrostatic forces involved in the
stabilization of the complex mineral assemblages, although
major challenges remain for developing methods for isolating,
examining, and quantitatively describing the electrostatic/van
der Waals forces involved in their stabilization.
Surface Chemistry of Mixed Mineral Assemblages and the
Implication for Solute Transport
Surface charge reversal of reactive mineral phases according to
the proposed model should be manifested in the reactive
transport behavior of anionic solutes. This phenomenon was
examined by investigating Br- breakthrough behavior in ad-
ditional dynamic transport experiments. Consistent with the
proposed model based on colloid generation and transport
behavior, Br- displayed significant retardation when refer-
enced to the truly conservative 3H2O (Fig. 4~. Also consistent
with the proposed model was the greater retardation of Br-
with increased abundance of Fe-oxohydroxide minerals. The
fact that the surface sample displays enhanced Br- transport,
compared with the 3H2O tracer, suggests anion exclusion,
consistent with the proposed role of organic matter on surface
charge characteristics of the mineralogically similar surface
soil and, furthermore, demonstrates the sensitivity of dynamic
transport compared with batch experiments for investigating
subtle changes in surface charge characteristics of mixed
mineral systems. Also the fact that Br- transport was more
Proc. Natl. Acad. Sci. USA 96 (1999J 3355
1.0
o
~ 0.8
._
~0.6-
m
~ 0.4
-
0.2
0.0~ .
6.5 1 B 1
L ~
· ~ o
· · ·n
o o
Org. ~ TR2 O
TR1 TR3
to
5.5
5.0
-
Ul
4.5
~ em.,_ TR3
~ `~
~ Org. ~
. ~
".u
0 1
See Table 1.
0rg. = 0rangeburg
TR# = Subsurface Tobacco Rd/Barnwell Sediment
2 3
Pore Volume
4
FIG. 4. Bromide breakthrough (A) and effluent pH (B) for 10-3
M KBr tracer solutions in columns packed with materials described in
Table 1. Column pore volumes were calibrated based on tritium
breakthrough (adapted from ref. 54~.
retarded in samples with higher pH (because of the greater
abundance of Fe oxohydroxide minerals) provides additional
evidence for the coupling of the surface reactions between the
phyllosilicates and the Fe-oxohydroxides. The use of Mg2+
compared with K+ as the counter ion resulted in a greater
decrease in effluent pH and greater Br- retardation, an
observation also consistent with the proposed coupled reac-
tion model (539. As with the colloid generation and transport
behavior, disturbance of the samples for use in batch experi-
ments or air drying before dynamic transport experiments
greatly reduced or eliminated observed anion retardation (549.
Finally, dynamic transport experiments conducted over a wide
range of Br- concentrations revealed nonlinear sorption and
at high concentrations of influent Br- salts (> 0.1 M) a nearly
complete masking of the anion retardation behavior, with
breakthrough appearing to be conservative (53, 54~. This
observation is critical because column transport experiments
are commonly conducted to calibrate physical parameters for
field scale tracer experiments. The concentrations used in
these column experiments are typically 2-3 orders of magni-
tude higher than those used in the subsequent field scale tracer
experiments. Thus, sorption reactions in the field typically
would be misinterpreted as having physical significance (i.e.,
mixing, flow rate, permeability, regions of immobile/mobile
water, stratification, etc.~.
Surface Chemistry of Mixed Mineral Assemblages and
the Implication for Contaminant Transport and
Subsurface Remediation
There are a number of important implications of this work to
the modeling of contaminant transport and to the environ-
mental remediation of contaminated aquifers. In oxide-rich,
OCR for page 3356
3356 Colloquium Paper: Bertsch and Seaman
organic-poor subsurface environments, typical of many aquifer
systems, underestimation of contaminant transport distances
can result from both an overestimation of contaminant sorp-
tion to reactive mineral phases that are assumed to be related
to mineral abundance and assumed to posses a static surface
chemistry, and by a misunderstanding of the primary mecha-
nisms leading to the generation and transport of mineral
colloids when predominantly negative charge surfaces are
assumed to control the surface chemical behavior. For exam-
ple, previous modeling efforts on coarse-textured highly
weathered sediments similar to those studied here have con-
sidered quartz and kaolinite as the primary reactive phases and
predicted limited metal and actinide mobility from an acidic
plume (Sly. However, metal transport distances from the
source plume were found to be significant and the primary
mechanisms for this apparent enhanced transport were found
to be a charge reversal on the matrix mineral phases leading to
limited sorption reactions and the transport of trace levels of
actinides and other metals specifically sorbed to colloids.
Other studies also have demonstrated that the surface chem-
ical properties of aquifer materials are dominated by high
surface area phases of relatively low abundance (38~. Greater
emphasis must be placed on the identification and surface
chemical characterization of complex mineral assemblages in
natural systems to accurately define the mechanisms control-
ling solute and contaminant transport.
An additional implication of these results relates to reme-
diation of contaminated oxide-rich, organic-poor subsurface
environments. Understanding the surface chemical controls of
the reactive mineral phases has facilitated the development of
an enhanced groundwater remediation technology, which is
predicated on the selective mobilization through surface chem-
ical manipulation of the highly reactive Fe-oxohydroxide
phases and Fe-oxohydroxide armored minerals, which are the
primary resident phase for both inorganic and organic con-
taminants (58~.
Conclusions
Investigations of complex mineral assemblages in a highly
weathered coarse-textured system have demonstrated that the
surface chemistry of these assemblages is influenced by com-
plex physicochemical interactions between natural organic
constituents, phyllosilicate, and Fe-oxohydroxide phases. In
surface soil environments containing as little as 1% organic
matter, the surface chemistry was found to be controlled by
organic constituents coating the phyllosilicate and Al- and
Fe-oxohydroxide clay minerals, resulting in a much higher
negative charge and a pznc shifted to much lower values than
predicted based on bulk mineralogical composition. The sur-
face charge modification by organic constituents was found to
control the flocculation/dispersion processes of clay mineral
assemblages in the surface soils, as well as the transport of
mineral colloidal phases through soils.
The complex mineral assemblages isolated from subsurface
environments were found to be comprised of Fe-oxohydroxide
phases partially or totally armoring the more abundant phyl-
losilicate minerals present. Unperturbed, these systems appear
to be near the pznc; however, minor changes in solute chem-
istry can induce surface charge reversal (from slightly net
negative to strongly net positive) through an elaborate coupled
reaction between the permanent negatively charged and vari-
able charged surfaces leading to the dispersion and transport
of Fe-rich colloidal mineral phases. The surface charge rever-
sal of these systems also is manifested by a significant retention
of anions, such as Br-, which traditionally have been consid-
ered as conservative tracers in hydrological investigations of
these systems.
Defining the surface chemical properties of these complex
mineral assemblages has a number of important implications
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Proc. Natl. Acad. Sci. USA 96 (1999)
for solute and contaminant transport. The development of
robust predictive models describing solute and contaminant
transport requires a thorough understanding of physical trans-
port parameters (generally derived from solute tracer exper-
iments) and solute/contaminant-mineral surface interactions.
Most reactive transport modeling efforts define reactive min-
eral phases based on the relative abundance of the clay
minerals present and surface chemistry defined by studies with
model minerals. That the surface chemistry of reactive phases
in aquifers may be controlled by minerals of relatively low
abundance and more importantly, by complex physicochemical
interactions occurring between individual components in com-
plex mineral assemblages, suggests that we need less emphasis
on studies of model minerals and more research on mineral
assemblages isolated from natural systems. Clearly, the results
of this and other investigations demonstrate that information
based entirely on mineral abundance is insufficient for pre-
dicting the surface charge characteristics of natural mixed
mineral systems.
An important finding of these investigations is that the
surface charge behavior defined in dynamic transport exper-
iments could not be reproduced with conventional batch
experiments. Both air drying or significant physical manipu-
lation of the aquifer materials apparently results in a disruption
of the complex mineral assemblages that appear to be primarily
composed of a phyllosilicate core with a partial or total
Fe-oxohydroxide veneer. Batch experiments were found to
produce results strongly biased toward the permanent nega-
tive-charged components in the complex mineral assemblages
and could not be used to predict solute transport or floccu-
lation/dispersion behavior observed in both column and field
scale transport experiments. These observations raise impor-
tant questions concerning methods used for determining sur-
face chemistry of complex mineral assemblages and pose
significant challenges for designing isolation techniques so that
complex mineral assemblages can be investigated in a mean-
ingful way. It is clear that a detailed understanding of reactive
mineral phases in soils and aquifers is necessary to accurately
evaluate environmental and human health risks associated
with contaminants and to design technologies for the protec-
tion or remediation of soil and water resources.
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Representative terms from entire chapter:
complex mineral
