&EPA
United States
Environmental Protection
Agency
Research and Development
Environmental Research
Laboratory
Athens GA 30605-2720
EPA/600/S-93/002 April 1993
ENVIRONMENTAL
RESEARCH BRIEF
Oxidation-Reduction Mechanisms in Iron-Bearing Phyllosilicates
Joseph W. Stucki1, George W. Bailey2, AND Huamin Gan1
Abstract
Oxidation-reduction reactions of structural iron in smectite clay
minerals were studied with respect to reduction reaction mecha-
nisms and effects on physical-chemical properties of the clay.
The purpose of these studies was to assess the implications of
redox chemistry at mineral surfaces on transport, transforma-
tion, and bioavailability of redox-sensitive metal pollutants. Re-
ducing agents selected were dithionite (S2O42-), sutfide (S2-),
thiosulfate (S2O 2'), hydrazine (NLH4), ascorbic acid (C Hpj,
hydroquinone (C6H6O2), and sodium oxalate (Na2C2O4). Clay
samples were prepared as aqueous suspensions of <2-u.m
particle-size fractions of Na-saturated, freeze-dried ferruginous
smectite. The reductive strength of each reducing agent was
determined by measuring the resultant level of Fe(ll) in the clay
crystal using either a photo-colorimetric method or Mossbauer
spectroscopy. Heats of reaction were used with S2O42, S2-, and
S O32- to further determine the extent of redox reaction. The
role of free radicals in the reduction process was measured
using electron spin resonance (ESR) spectroscopy.
Results revealed that the order of reduction potential for the
various agents was S2O42 > S22' > C6H8O6> S2O32 > C HO »
C2H2O4. Free radical activity was found only in the reaction with
dithionite and was assumed to be due to the sulphoxylate (SO2
•) free radical. The free radical was labile in pure solution, but
its lifetime increased at least two-fold in the presence of the
clay. The signal from the clay-S O42 suspension may, in fact,
be only partially attributable to SO,,-. Reduction by this highly
reactive agent may induce electron hopping within the clay
'crystal itself, which could also be reflected in the persistent
signal from the clay. When free radicals or unpaired electrons
1 Department of Agronomy, University of Illinois, Urbana IL 61801
2 Environmental Research Laboratory, U.S. Environmental Protection Agency,
Athens GA 30605-2720
are involved in the reducing processes, the following steps
may occur: a) the active free radical (e.g., SO,,-) approaches
the clay surface and transfers an electron to structural Fe(lll),
reducing it to Fe(ll); b) because of this initial reduction, the
crystalline structure is energetically destabilized by an excess
negative charge, causing partial dehydroxylation, which, in
turn, energetically activates point defects, such as tetrahedral
aluminum (Al) sites, within the clay crystal; c) excess electrons
at point defects may pass to structural Fe(lll), reducing it.
These processes continue until all structural Fe(lll) is reduced.
At least two types of reducing agent were identified based on
their reducing mechanism, namely, those with and those with-
out free radical activity. The number of steps involved in the
reduction process depends on the reduction potential of the
reducing agent. Measurements of rheological characteristics of
oxidized and reduced clay suspensions indicated that struc-
tural Fe(ll) increases the attractive bond energies between clay
particles. The type of bonding between particles is uncertain;
hydrogen bonding may make an important contribution. The
effects of microbial reduction and interactions with clay miner-
als were reviewed.
Background
The risk that metal cations pose to the global ecosystem
depends largely on their activity in porous media, which can be
calculated only if the true exposure levels of the cations to
biota are known. Exposure is determined by the transport,
transformation, and bioavailability characteristics of the metal
cation, which in turn depends on speciation. The fate of redox-
active pollutants in porous media are governed by solubility
and adsorption processes, but modifications also occur due to
redox reactions in the solid-liquid interface. These effects are
governed by the thermodynamic energy of redox couples and
< X'fx Printed on Recycled Papei
-------
by the rates at which reactions proceed. Solid surfaces in the
media create a unique chemical environment that influences
both the energetics and the kinetics of reactions among chemi-
cal components.
Knowledge of the solution concentration or activity of metals
alone is insufficient to provide a complete model of exposure
because, in this interfacial region, two phenomena occur that
are absent in solution. First, the solid surfaces become an
active participant, as both reactant and product, in the chemi-
cal reactions. Second, chemical constituents in solution come
under the influence of van der Waals, electrostatic, hydration,
and possibly other forces that alter their total potential energy
or reactivity. Accurate definition of speciation thus requires an
understanding of the processes and forces that are operating
in the interface, in terms of both the participants and the rates
of reaction. When this understanding is achieved, the capability
to model and predict more successfully the behavior of perco-
lating ions in porous media will be greatly enhanced.
The purpose of this study was to characterize more fully the
physical-chemical properties and processes that occur when
iron-rich clay minerals undergo reduction and reoxidation. Re-
dox reaction mechanisms in phyllosilicate clay minerals were
studied with respect to the free radical activity, heat of reaction,
and effect on clay interlayer forces. Several reducing agents
representing a wide range of reduction potentials and free
radical activities were compared, and the redox interactions
between clays and microorganisms were reviewed. The project
also produced a publication containing a detailed review of the
thermodynamic basis of redox reactions in clays (Stucki et ai,
1992).
Methods
The <2-u,m particle-size fraction of ferruginous smectite SWa-1
(Source Clays Repository of The Clay Minerals Society, Co-
lumbia, Missouri) was Na-saturated, washed free of excess
salts to approximately 10/3 M Na, and freeze dried. Twenty-
five- to 30-mg portions of the freeze-dried clay were then
resuspended for subsequent reduction treatments generally by
mixing with 37.5 ml of high-purity H2O (18 Mohm crrr1 resistiv-
ity) and 2.5 ml of citrate-bicarbonate buffer (1 part 0.3 M
Na2C6O5-6H2O and 8 parts 1 M NaHCO3). The mixture was
shaken gently overnight. The citrate-bicarbonate (CB) buffer
was used in order to maintain near-neutral pH and thereby
minimize acid dissolution of the clay during treatment with
Na2S2O4. In some experiments, however, only high purity H2O
was used. Structural Fe in the clay was reduced at room
temperature (nominally 25°C), for time periods ranging from 1
to 57 hr, by reagent-grade reducing agents. Those used were
sodium dithionite (Na^OJ, sodium sulfide (Na.S*9H2O), so-
dium thiosulfate (Na2S2O3-5H2O), hydrazine (N2H4), ascorbic
acid (C6H8O6), hydroquinone (C6H6O2), and sodium oxalate
(NeLC2O4). Iron(ll) and total Fe were measured quantitatively
by the photo-colorimetric method of Komadel and Stucki (1988),
or semi-quantitatively by Mossbauer spectroscopy.
Electron spin resonance (ESR) spectra were obtained at room
temperature using a Bruker ESP 300 (X-band) ESR spectrom-
eter equipped with both liquid and solid sample cells. Each
reducing agent was analyzed in the solid phase and in solution
with either high purity HO or CB buffer solution. ESR spectra
of the clays also were obtained after resuspension, but in the
absence of reducing agent. Each reducing agent then was
added to the suspended clay to bring the Na concentration in
suspension to 0.01 M, and the mixture was immediately loaded
into the ESR liquid cell. ESR spectra were obtained after
various time intervals up to 57 hr.
The resonance peak position in each ESR spectrum was
expressed in terms of the value of g, which was calculated
from the magnetic field intensity (H) using the relation
(M
(1)
where h is Planck's constant, v is the microwave frequency
with which the sample was irradiated while the magnetic field
was varied (for an X-band ESR spectrometer, the value of vis
about 9 GHz), and 8 is the Bohr magneton (= 9.2741-1021 erg
gauss'1).
Line intensity is an important characteristic of an ESR spec-
trum and can be a qualitative indicator of spin concentrations if
relaxation and saturation effects are absent. According to
Vedrine (1980), the spin populations directly determine the
magnetic susceptibility, \, to which the line intensity, 7, is
proportional according to the relationship
3kT
(2)
where L, S, and J (= L + S) are, respectively, the orbital,
spin, and total angular momenta of the electron; N is the
number of unpaired spins; k is Boltzmann's constant; / is the
absolute temperature; and g is the "so called" electron free-
spin g- factor. In the present study, all of the terms on the right
side of Equation 4, except Np, were assumed to be constant.
The intensity of the ESR signal thus was assumed to be
directly proportional to No and was obtained by integrating the
area under the pre-derivative spectrum.
For rheological measurements, clay suspensions were pre-
pared as above but treated only with Na2S2O4, N^SO (sodium
suit ate), or NaCI (sodium chloride). Shear stress vs. shear rate
for different treatments were made using a Fann rotational
viscometer (Model 35, Fann Instrument Corp., Houston, TX) at
25°C. During these measurements, the R1-B1 rotor-bob com-
bination was used with a spring having a constant of 0.2 or 0.5.
When the oxidation of the structural iron was a factor, the
measurement was made inside an inert-atmosphere glove box.
Enthalpy, or heat of reaction, was measured using a Calvet
microcalorimeter in the laboratory of Dr. Philip F. Low at Purdue
University, using the techniques described by Gan (1990) and
by Gan and Low (1992).
Results and Discussion
Comparison of Reducing Agents
The reduction of structural iron in ferruginous smectites by
different reducing agents has important implications for the
behavior of redox-sensitive metals in the environment. Reduc-
ing agents used were dithionite (S O42-), sulfide (S2-), thiosul-
fate (S2O2"), stannous chloride (SnCl2), hydrazine (N H4), ascor-
bic acid (C H8O6), oxalate (C2H2O4), phenol (C6H6O), and hyd-
roquinone (C6H6O2). Experiments were carried out at different
temperatures and with different contact times between clay
and reducing agent. Survey results revealed that great differ-
ences in reducing power exist among these reducing agents.
Results from three of these are compared in Figure 1. In the
inorganic group, S2O42 demonstrated very strong reducing abil-
ity, whereas S2O32 was the weakest. In the organic group, the
reducing power was in the order C6H8O6 > C?H6O2 > C2H2O4 «
C6H6O. The reduction process sometimes involves ancillary
-------
0.6 -
5"
fi
0.2
0.57
0.11
0.042
Figure 1.
Dithionite Sulfide Jhiosulfate
Reducing Agent
Reduction of structural Fe in Na smectite SWa-1 by freshly
prepared 0.01 M solutions o1Na2S3O, NaaS, and Na2SaO3 for
24hrat25°C.
reactions in the mineral structure, depending on the reducing
power of the reductant.
sisted for up to 9 hr of contact between clay and Na2S2O4,
which was more than double its lifetime in pure Na S n solu-
tion. Hence, the concentration of free radicals from the Na2S,O4
solution was preserved and enhanced if added to the clay.
Apparently, the SWa-1 initially reacted with SO/- which do-
nated an electron to the clay; but as the reaction proceeded,
unpaired electrons were produced within the clay crystal struc-
ture, giving rise to the persistent signal in Figure 3B. This
explanation is consistent with earlier studies (Stuck! et a/.,
1984b; Lear and Stucki, 1985; Stuck! and Lear, 1989) that
strongly Indicated, based on layer charge measurements, that
some of the Fe is reduced by a source of electrons (Z) within
the clay structure. Aluminum-substituted tetrahedral sites may
provide such a source of high-potential electrons due to the
lower-valent tetrahedral cation. This step would have occurred
only after sufficient Fe(lll) was reduced by SO/- to either
activate or catalyze the movement of internal electrons to
structural Fe(lll). This process would be similar to the reaction
proposed by Stucki and Lear (1989):
^)s + (m-a) Z1'1
(3)
(4)
(5)
where subscripts x and s denote clay and solution phases,
respectively; Z is an unidentified electron donor located within
the clay crystal, which may be the Al-substituted tetrahedral
Free Radical Activity
Further comparisons were made among the inorganic group of
reducing agents. Figure 1 reveals the relative reducing abilities
of S2O42', S2-, and SLO32 in ferruginous smectite (SWa-1) at
room temperature (25°C). Hydrazine's effect was similar to that
of S2\ Notice that S2O42- is peculiar and demonstrates a much
higher ability to reduce structural Fe(lll) than any of the other
compounds.
The standard electrode reduction potential, g**, of S2O42 is
approximately -1.12 V (Vanysek, 1992). But N H4 (or its conju-
gate acid N H * if hydrated), which generally reduces only
about 10% of the structural Fe in these same clays (Stucki et
a/., 1976; Rozenson and Heller-Kallai, 1976a, 1976b; Stucki
and Roth, 1977; Russell era/., 1979; Stucki, 1988; Stucki and
Lear, 1989), has an jf* of -0.94 V (Douglas et a/., 1983).
Obviously, the difference between these two reducing agents
must be due to something other than their standard electrode
potentials.
The ESR spectra of these same reducing agents revealed no
signal from N2H4, S2O32, or S2-, and a strong signal from S O42
(Figure 2). These results clearly indicate the presence of un-
paired electrons in S2O42' by the resonance signal centered at
about g = 2.0091 for the solid phase and shifted to g = 2.0113
for the solution phase. The signals were most intense in the
freshly prepared S2O42- solutions, then decreased in intensity
over time, disappearing completely after about 57 hr in the 1.0
Absolution, and after less than 4 hr in the 0.01 M solution.
When S2O42' was added to the clay suspension, making the
final Na concentration 0.01 M, a moderately strong ESR signal
was evident initially (Figure 3A), then after 4 hr the signal
became even stronger than that of the pure Na2SX>4 solution of
the same concentration (0.01 M) (Figure 3B). The signal per-
3.44
3.48
Field (kG)
3.52
Figure 2. ESR spectra of solid (A) and 1.0M aqueous solution (B) on
Na,S,04.
-------
g = 2.01077
3.45
3.46
3.47
Field (kG)
3.48
3.49
Figure 3. ESP spectra of 0.01 M Na2S2O4 in smectitle SWa-1 suspen-
sion: A) fresh, B) after 4 hr.
sites as suggested above; e represents the reducing agent in
the solution surrounding the clay crystal, and is believed to be
the SO,;- free radical when Na^G^ is the reducing agent; and
m, r, and a are stoichiometry coefficients. According to this
hypothesis, structural Fe is reduced partially by internal and
partially by external reducing agents, and the reduction is
accompanied by dehydroxylation and reprotonation reactions.
Lear and Stucki (1985) determined that two of the stoichiom-
etry coefficients are linearly related, viz., r = 0.32 m, but the
value of a has yet to be determined.
The point of contact between the clay and the reducing agent
is still unknown. The same arguments of coulombic repulsion
against the S2O42 anion approaching the negatively charged
basal surfaces also apply to the SO2- free radical anion,
except perhaps the high reactivity of the free radical would
overcome the coulombic repulsion energy barrier. Like-charge
collisions where one is a free radical are not unusual, however
(Neta et al., 1988). Alternatively, the free radical may react at
the edges of clay layers, setting up a conduction pathway
through the layer to internal Fe ions. Such a process is con-
ceivable in Fe-rich smectite, and may explain why Rozenson
and Heller-Kallai (1976a, 1976b) observed incomplete reduc-
tion of Fe-poor montmorillonite by Na2S2O4.
The conductivity of electrons from the layer edge through the
octahedral sheet would depend heavily on the presence of the
transition metal. But Lear and Stucki (1987), based on mea-
surements of magnetic exchange interactions and of inter/a-
lence electron transfer, concluded that structural Fe is reduced
nearly randomly within the octahedral sheet. Reduction only
from layer edges would likely create a reducing front passing
through the layer, creating homogeneous domains of Fe(ll)
and Fe(lll) with a rather constant number of Fe(ll)-Fe(lll) pairs
at the reduced-oxidized interface. Results of Lear and Stucki
(1987) clearly reject this possibility in ferruginous smectite.
Heats of Reaction
The driving force for any chemical reaction is reflected totally in
the partial molar Gibbs free energy, AG, of the reaction^which
is given by the sum of the changes in enthalpy, AR, and
entropy, AS, viz.,
AG=AH-TAS
(6)
where T is the absolute temperature. A more detailed thermo-
dynamic treatment is provided by Stucki et al. (1992).
Enthalpy changes were measured for the reaction of ferrugi-
nous smectite with reducing agents S2O42-, S2, and S2O32. As
noted above, the strength of these three reducing agents is in
the order S2O42 > S2 > S2O 2\ The value _pf AG", therefore,
should follow the same trend. The value of AR is known as the
heat of reaction and can be measured experimentally with a
microcalorimetejr. If AS is constant in all three reactions, AR
should reflect AG.
The heat of reduction reaction was found to follow the order
S2O42 > S2O 2 > S2. Compared to the levels of Fe(ll) achieved,
the results for S2O32 and S2' are reversed, suggesting that
entropy changes are greater in the S2' treatment. Further ex-
planation of this phenomenon will require more detailed under-
standing of the S2-clay reaction. These results also indicate
that, in addition to considering AR, one must not overlook the
AS term.
Recall that the reducing power of S2O42- deteriorates with the
age of the solution because of the lability of the sulphoxylate
(SO?-») free radical, but S2- and S2O 2- solutions are unaffected
by time. Measurements of heats of reduction reaction using
variably aged solutions revealed that AR decreased with in-
creasing age only with the S2O42- solutions. These results,
therefore, are consistent with the free radical studies that found
that reducing power of S2O42 is labile.
Microbial Reduction of Clay Fe
Microorganisms apparently also reduce structural Fe in clay
minerals (Komadel et al., 1987; Stucki et al., 1987; Wu et al.,
1988). The mechanisms for microbial reduction have yet to be
identified, and many questions arise as to the precise role of
microorganisms in Fe redox reactions. For instance, does the
reduction occur because of a membrane-bound process requir-
ing intimate contact between clay mineral and organism, or is it
due to an extra-cellular or exudate compound from the organ-
ism? Is it an aerobic or anaerobic process? What are the
metabolic sequences responsible for reduction? Which organ-
isms are most efficient? How does microbial reduction affect
physical-chemical properties of the minerals and their effect on
metal fate, transport, and bioavailability characteristics? Stud-
ies to answer these questions are currently underway (e.g.,
Gates era/., 1991, 1992).
Reduction Process Schematic
The reduction mechanism(s) for Fe(lll) smectites depend on
the nature of the reducing agent and medium in which reduc-
-------
tion takes place. Laboratory methods may or may not replicate
actual redox processes that occur in nature but should provide
insights as to the potential range of reactions that can occur.
Standardization of techniques, natural variations among
samples, and handling of air-sensitive clays all are factors that
must be considered before valid comparisons between studies
can be made. In Figure 4 is a conceptual model illustrating how
the reduction potential or energy of the external reducing elec-
tron may determine the extent of Fe(ll) produced and the type
of ancillary processes that occur, such as those represented in
Equations 3 through 5. A small amount of Fe(lll) can be
reduced to Fe(ll) in the clay crystal by a number of reducing
agents having only modest reductive capability (having energy
» E,), as indicated by the small energy barrier A. The principal
effect of these agents is simply the reduction of Fe(lll) to Fe(ll),
with a concomitant increase in layer charge and probably a
decrease in the crystal lattice stabilization energy because the
dioctahedral structure naturally prefers trivalent octahedral cat-
ions.
Further reduction by agents having greater reductive capabili-
ties (having energies > E, or E3), such as the SO2'»free radical,
invokes a dehydroxylation process as indicated by energy
barrier B. Dehydroxylation was reported by Roth and Tullock
(1973) and Stuck! and Roth (1976), based on infrared spectral
information, and by Lear and Stucki (1985) based on tritium
exchange between structural OH in the clay and H2O in the
surrounding solution. These changes in the clay crystal, com-
bined with the high electron energy of the free radical, eventu-
ally surmount energy barrier C, which mobilizes electrons al-
ready present in the clay crystal into molecular or metallic-like
orbitals or into semiconductor-like conductivity bands. The de-
UJ
c
UJ
I
Electron
Delocalization
Figure 4.
Progression of Reduction Process
Conceptual model of relationships among energy of reducing
electrons, structural Fe(ll) content, and progression of the
reduction process in ferruginous smectites.
localized electrons then are captured by Fe(lll), thus effecting
further Fe reduction.
Because these latter electrons were initially present in the clay,
the change in Fe(ll) content at this point is not reflected in the
total layer charge of the clay, which explains the discrepancy
between the predicted and the observed layer charge de-
scribed by Stucki et al. (1984b), Lear and Stucki (1985), and
Khaled and Stucki (1991). The energy barrier configuration
may vary depending on the total Fe content of the clay be-
cause a low-Fe smectite, such as montmorillonite, may have a
much lower metallic character than a high-Fe smectite, which
would diminish the probability for the electron delocalization
process represented by barrier C (Figure 4).
Experimental results presented herein may explain some as-
pects of why Na2S2O4 reduces more Fe(lll) in the clay structure
than other reducing agents that have similar electrode reduc-
tion potentials. When unpaired electrons are involved in the
reducing processes, the following steps may take place: a) the
active free radicals (e.g., SO2-) approach the clay surface and
transfer electrons to structural Fe(lll), reducing it to Fe(ll); b)
because of this initial reduction, the crystalline structure is
energetically destabilized by an excess negative charge, caus-
ing partial dehydroxylation, which, in turn, energetically acti-
vates point defects, such as tetrahedral Al sites, within the clay
crystal; c) excess electrons at point defects may pass to struc-
tural Fe(lll), reducing it. The processes continue until all struc-
tural Fe(lll) is reduced. At least two types of reducing agent are
identified based on their reducing mechanism, namely, those
with and those without free radical activity.
Effects of Reduction on Interlayer Forces
Studies by Stucki era/. (1984c), Chen et al. (1987), Khaled and
Stucki (1991), and Stucki and Tessier (1991) indicated that Fe
reduction increased attractive forces between clay layers, but
the exact nature of these is still unknown. Shear stress mea-
surements in the present study indicate that hydrogen bonding
may be an important factor. Oxidized and reduced clay sus-
pensions were placed in a rotational viscometer, which mea-
sured the shear stress (s) as a function of the shear rate (o).
Results were then plotted according to the relation
s = rjpio + 9
(7)
where the slope, TJ , is the plastic viscosity of the system, and
the y-intercept, 6, is the Bingham yield stress. 6 is a measure
of the number of interacting clay particles and the strength of
bonds or links that occur between them, as given by the
relation
(8)
where EA is the energy of the interparticle bond, N is the
number of particles, and K1 is a constant.
Flow curves of reduced and unaltered 1% suspensions of
ferruginous smectite SWa-1 are shown in Figure 5. Notice that
the slope (TJ ) and y-axis intercept (9) are higher for the re-
duced than for the unaltered sample. According to Stucki and
Tessier (1991), the particles are larger in reduced compared to
oxidized smectites, so for the same mass the number of par-
ticles, N, must be less in the reduced clay. Hence, by Equation
(8), the particle interaction energy,
duced clay.
E is stronger in the re-
-------
— 6- Reduced
Oxidized
^ 4
Figure 5.
400 800
Shear Rate (1/sec)
Flow curves of oxidized and reduced 1% suspensions ofNa-
nontronite (SWa-1) at 1Q3 M A/a* concentration
strong flocculation decreases the particle interaction energy.
(4) Reduction of structural Fe(lll) increases the interparticle
forces as indicated by increases in 6. And, (5) the possible
forces controlling the interaction energy between particles are
hydrogen bonding and cation bridging.
Environmental Implications
Soil serves many functions in the environment. It acts as a
geologic filter scavenging undesirable solutes from a percolat-
ing solution, or it may release previously sorbed substances
back into the surrounding media depending on climatic and
other conditions. The surfaces of soil minerals provide unique
chemical environments that often facilitate the transformation
of some chemical species to very different forms, some of
which may be less desirable and more mobile, or vice-versa.
Changing the surface properties of the minerals through oxida-
tion or reduction of structural Fe will likely have a great impact
on the capability of the clay interlayers to trap or sequester
metal cations, which in turn will affect the bioavailability of the
metal.
The chemical nature of mineral surfaces is widely recognized
as one of the most significant factors contributing to the fate
and behavior of contaminants, but these properties are gener-
ally considered to be unchanging during the time of contami-
nant exposure and reaction. The aforementioned studies clearly
demonstrate, however, that mineral surface chemistry is greatly
modified by changes in the oxidation state of Fe in the mineral
crystal structure.
The redox activity of mineral surfaces also affect the oxidation
state, and consequently the speciation and chemical behavior,
Figure 6 shows the flow behavior of reduced and unaltered
SWa-1 clay in 3% suspension, and compares the effects of
three different anions, namely, S2O42- (reducing agent), SO42-,
and Cr. The Na* concentration was the same in all three
samples. Notice that both S2O42 and SO 2 have greater effects
than Ch The difference between the S2O42 and SO42 may be
attributed to structural Fe reduction, but the difference between
the sulfate and Cr must be due to different anion effects. The
ability of Cr to break hydrogen bonds is well known, so the
smaller interaction energy between particles in the presence of
Cr may be due to fewer or weaker H bonds between the
particles.
The increase of particle interaction indicated by increasing 6
and T) clearly reveals that the reduction of structural Fe(lll)
increases the attractive forces between particles. This, in turn,
likely alters the particle arrangement in the system. This sup-
ports the hypothesis that reduction of structural Fe(lll) in-
creases the proportion of collapsed layers, decreases the swell-
ing (Stuck! et al., 1984a; Stuck! and Lear, 1989; Wu et al.,
1989; Gates et al., 1991), and changes the hydraulic conduc-
tivity of the system (Shen et al., 1992).
These and other results from this study help us understand the
colloidal properties of the oxidized and reduced samples, and
lead to the following general conclusions: (1) The interaction
between clay particles in aqueous suspension increases as the
solid concentration increases; the sol to gel transition is found
to occur at a solid to liquid concentration of 2%. (2) Stronger
interaction was detected for the intermediate particle-size frac-
tion <2 and >0.5 u,m. (3) Interaction between particles in-
creases as the Na* concentration increases up to 0.01 M, then
50 -
0 '•
400 800
Shear Rate (1/sec)
Figure 6. Flow curves of oxidized and reduced 3% suspensions of Na-
nontronite (SWa-1) at 0.5 M A/a* concentration.
-------
of redox-sensitive metal ions in the surrounding solution. The
application of such insight can be seen in the case of Cr(VI), a
major environmental pollutant. Chromium may exist as Cr(VI)
anion in solution (CrO42-, Cr,O72-)- A reduced Fe clay may react
with the Cr(VI) species, reducing it to Cr(lll), which will either
be cationic or precipitated as the oxide. By this process, Cr will
be less mobile and less hazardous in the soil environment.
Geochemical models, e.g., MINTED (Allison et al., 1991), for
predicting the fate and behavior of metals in the vadose zone,
therefore, must account for redox transformations of the metal
ion at mineral surfaces.
The exposure of soil minerals to redox-sensitive sorbates,
solvents, humic materials, varying redox environments, and
microorganisms may invoke surface chemical changes in situ
over short time periods and thereby produce vital differences in
the speciation and reactivity of all components in the soil-water
system.
Acknowledgments
Financial support of this project by the U.S. Environmental
Protection Agency, under Cooperative Agreement CR816780-
01-2, is gratefully acknowledged. Mention of trade names or
commercial products does not constitute endorsement or rec-
ommendation by the U.S. Environmental Protection Agency.
The authors also thank Dr. Lelia Coyne for her interest and
numerous suggestions regarding the reduction mechanism of
structural Fe.
References Cited
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