ῐ
J. Jpn. Soc. Soil Phys.
ῐ῎῍ῑῒ῏
No. 30, p. -ῌ+* ῍,**.῎

E#ects of Structural Fe Oxidation State on Physical-chemical
Properties of Smectites : Evidence from Infrared Spectroscopy
Dongfang HUO*, Claire-Isabelle FIALIPS*, Laibin YAN* and Joseph W. STUCKI** University of Illinois, College of Agricultural, Consumer, and Environmental Sciences,
W--,+ Turner Hall, ++*, South Goodwin Avenue, Urbana, IL 0+2*+ USA
Abstract
Redox reactions involving structural Fe in smectite clay minerals contribute signiﬁcantly tothe chemistry of soils and sediments. Changes in oxidation state from Fe (III) to Fe (II) alters theclay structure, which in turn modiﬁes the chemical nature of the smectite basal surfaces. Thesechanges in structure are readily observed by infrared spectroscopy. Structural OH groups areprogressively lost as the level of Fe (II) increases, and large shifts occur in the Si-O stretching andbending modes of the tetrahedral sheet. The communication between octahedral and tetrahedral
sheets alters the conﬁguration of basal oxygens, as well as changing the charge, pH, and reduction
potential of the surface. Infrared spectra also provide evidence for the formation of trioctahedral
domains as a result of extensive Fe (III) reduction.
Key words : pH, redox, charge, iron, clay.

+.

Introduction

The physical-chemical properties of smectites have great importance in agriculture, industry, and the environment because of theirhigh speciﬁc surface area and reactivity withplant nutrients, water, pesticides, organic pollutants, and other minerals. The smectite clay
minerals are largely responsible for properties
governing soil structure, porosity, and permeability. Every smectite contains some iron (Fe)in the octahedral sheet of its crystal structure(usually in the -ῌ valence state), and somesmectites also contain Fe (III) in the tetrahedral
sheet. Octahedral Fe (III) is susceptible to reduction and reoxidation (redox) processes,
which are common in nature due to bacterial
activity and are often associated with wetting
and drying events. The redox state of the clay
can also be readily manipulated in the laboratory using chemical reductants such as sodium
dithionite.
Redox reactions are signiﬁcant becausechanges in the oxidation state of the Fe areaccompanied by changes in the chemical properties and physical behavior of the clay.
A#ected properties and behaviors include
layer charge, cation exchange capacity, cation
ﬁxation (Stucki et al., +32. b ; Stucki and Roth,+311 ; Lear and Stucki, +32/ ; Gates et al., +33- ;Kostka et al., +333), swelling (Kostka et al., +333 ;Gates et al., +33- ; Stucki et al., +32. a), surface
area (Lear and Stucki, +323 ; Kostka et al., +333),
hydraulic conductivity (Shen et al., +33,), magnetization (Lear and Stucki, +321), surface pH(Cervini-Silva et al., ,*** b), capacity to degrade
organic compounds (Tor et al., ,*** ; Xu et al.,,**+ ; Cervini-Silva et al., ,*** a ; Yan and Bailey, ,**+ ; Hofstetter et al., ,**-), and reactivity
with redox-sensitive ions in solution (Taylor etal., ,*** ; Gan et al., +330 ; Ernstsen et al., +332).
The mechanism(s) by which a change in thevalence state of Fe controls these properties is
not well deﬁned, but certain ancillary changes
in structure during redox events and redox
cycles reveal certain clues from which hypotheses can be formulated. Structural changes
have been characterized by X-ray powder῏

4

di#raction (XRD) (Manceau et al., ,***),
Mössbauer spectroscopy (Lear and Stucki,+321), infrared spectroscopy (FTIR) (Stucki andRoth, +310 ; Fialips et al., ,**, a, b), uv-visible
spectroscopy (UV-VIS) (Anderson and Stucki,+313 ; Komadel et al., +33*), high-resolution
electron microscopy (HRTEM) (Stucki andTessier, +33+ ; Gates et al., +332 ; Kim et al., ,**-),
polarized extended X-ray absorption ﬁne structure spectroscopy (P-EXAFS) (Manceau et al.,,***), and X-ray absorption near-edge spectroscopy (XANES) (Manceau et al., ,***). Presented
here is evidence obtained primarily from FTIR
spectroscopy regarding changes in clay structure due to redox transformations.
,.

Materials and Methods

The smectites studied included Garﬁeld nontronite (sample API ---a from Wards Natural
Science Establishment, Rochester, New York,
USA) and ferruginous smectite (sample SWa-+
from the Source Clays Repository of The Clay
Minerals Society, USA). Structural Fe was
reduced either chemically using the methods
described by Stucki et al. (+32. c) or bacteriallyusing the method described by Kostka et al.
(+330). Structural Fe (II) and total Fe were determined quantitatively by the +,+*-phenanthroline method (Komadel and Stucki, +322)
and semi-quantitatively by Mössbauer spectroscopy.
Infrared spectra were obtained from driedﬁlms or from powders mixed with KBr using a
Midac Model ,*** FTIR spectrometer, andMössbauer spectra were obtained with a constant acceleration spectrometer operating witha triangle wave form over /+, data channels
using a /1Co (+*ῌ in Rh matrix) as the (-ray)source. Mössbauer spectra were deconvoluted
using a least-squares ﬁtting procedure, usuallyconstraining the component peaks to be ofLorentzian line shape.
All sample preparations, manipulations, andanalyses were performed using inert-atmosphere techniques. These included a controlledatmosphere liquid exchanger for replacing

30

,**.

and/or removing entrained salt solutions in
reduced suspensions, septum-sealed screw caps
on reaction vessels, and a glove box in which
the clay was dried either as a ﬁlm on a ﬁlter
membrane or as a freeze-dried powder.
-.

Results

The FTIR spectrum of smectite from .** to.,*** cm+ encompasses four principal spectral
regions in which signiﬁcant changes occur
during redox reactions. These regions are :Region I. -,.** to -,0** cm+, associated with
the O-H stretching vibrations of structural hydroxyls and adsorbed water ; Region II. +,***
to +,+/* cm+, associated with the Si-O vibrations ; Region III. 1/* to 3/* cm+, associatedwith the M-O-H bending modes (MFe, Al, orMg) ; and, Region IV. .**ῌ1** cm+, associatedwith various other modes involving Fe and Si.
Structural changes due to the reduction of octahedral Fe are readily monitored by changes
in all four of these regions of the spectrum.
Region I
The O-H stretching band in oxidized Fe (III)-rich, dioctahedral smectite is found at -,/1*
cm+ (Fig. +), characteristic of an [Fe (III)], OH
environment, and systematically shifts tolower wavelength to about -,/-/ cm+ as Fe (III)ions all become completely reduced to Fe (II).
The O-H stretching band also diminishes inintensity and two other features increase ; one
being a very broad band centered at about-,.** cm+ and the other being a sharp, small
band at -,0,, cm+. The latter small band islikely due to a portion of the Fe migrating to
adjacent vacant sites within the octahedral
sheet to form some trioctahedral [Fe (II)]- OHarrangements (Manceau et al., ,***). The position of this band is similar to the O-H stretching band in biotite, a trioctahedral mica where
the Fe (II) is clearly coordinated to OH from all
three octahedral sites. The relative intensity ofthis band is expected to be low because thesymmetrical electric ﬁeld due to uniform distribution of Fe (II) causes the OH dipole moment
to be oriented normal to the plane of the clay῎῍ : E#ects of Structural Fe Oxidation State on Physical-chemical Properties of Smectites : Evidence from Infrared Spectroscopy

Fig. +

5

FTIR spectra of O-H stretching region (Region I, see text) of Garﬁeld nontronite reduced tovarious levels by citrate-bicarbonate bu#ered dithionite suspension.

layer. This leads to a small interaction with theelectric ﬁeld vector of the incident infrared
radiation impinging on the oriented clay ﬁlm(Barshad and Kishk, +302). If the dipole orientation is normal to the clay layer, the band intensity should be pleiochroic, i.e., it should dependon the orientation of the clay ﬁlm. Conﬁrmation that this is indeed the case is given in
Fig. ,.
The broad band centered at about -,.** cmῌ+is assigned to H, O in the smectite structure. At
least two scenarios can be considered to explain the formation of this H, O band, namely,that the hydration energy of reduced smectite
is greater than oxidized smectite or that structural OH groups are converted to structural H,
O groups during reduction. Attention is drawn
to the fact that all samples were submitted tothe same dehydration process involving desiccation in vacuum and over P, O/ (Huo, +331),
which was e#ective in removing all adsorbed
H, O from the oxidized sample, as evidenced by
the absence of any signiﬁcant absorbance inthe -,,**ῌ-,.** cmῌ+ region for that sample, but
not from the reduced sample (Fig. +). The H, O
molecules responsible for the large band at

Fig. ,FTIR spectra of a 3/ῌ reduced Garﬁeld
nontronite ﬁlm oriented at 0*ῌ and 3*ῌ
relative to the incident beam, showing
pleochroic behavior of this band.

-,.** cmῌ+ in the reduced samples could, therefore, be adsorbed to the basal surfaces of thesmectite, but held to the surface with greater
energy than in the oxidized samples. Other
evidence for this scenario is found in thestudies of Yan and Stucki (+333, ,***), who6

30,**.

Fig. .
Fig. -FTIR spectra of the Si-O vibrations(Region II) of oxidized and reduced
Garﬁeld nontronite.

observed an increase in the interaction between basal Si-O groups and adsorbed H, O as
the extent of Fe reduction increased, and of
Stucki et al. (,***) who concluded that the surface hydration energy of reduced smectite is
greater than oxidized smectite.
A second scenario to explain the -,.** cm+band is that the H, O molecules are actually
located inside the clay layer as protonated OH
groups. Drits and Manceau (,***) proposed a
reduction reaction mechanism by which electron transfer to structural Fe (III) from reducing agents in solution is accompanied by protonation of structural OH groups. While theyfurther suggested that these internal H, O molecules then di#use back into solution, perhaps,
in reality, at least some of them remain in theoctahedral sheet in order to stabilize it withrespect to increased charge due to structural
Fe (II).
Region II
Changes are also observed in the Si-O
stretching region (Fig. -), including a large (upto .- cm+) downward shift of the main Si-O
stretching band of the reduced samples (Stucki

FTIR spectra of the M-O-H bending
modes (Regions III and IV) of oxidized
and reduced Garﬁeld nontronite.

and Roth, +310 ; Fialips et al., ,**, a, b ; Yanand Stucki, +333, ,***). Such a large shift indicates that the change in Fe oxidation state in
the octahedral sheet strongly a#ects the structural properties of the tetrahedral sheet, whichis also reﬂected in the alternate clock-wise,
counter-clock-wise rotations of the Si tetrahedra as measured by an increase in the crystallographic b-dimension 3.+-  to 3.,+ (Manceau et al., ,***).
These changes in the tetrahedral sheet a#ect
physical and chemical properties at the miner
al surface, such as those that are conﬁgurationally dependent. A good example of thisis found in the results of Yan and Stucki (+333,,***) who linked these changes in Si-O vibrations to the coupling of H, O molecules with thebasal oxygen surfaces. Other examples arefound in the change in pH (Cervini-Silva et al.,,*** b) and reduction potential (Cervini-Silva etal., ,*** a).
Region III
In the M-O-H deformation region (Fig. .), theprimary band is due to Fe (III)-O-H at 2,+ cm+,which shifts downward by about / cm+ withincreasing Fe reduction, then disappears alto-῏῎ : E#ects of Structural Fe Oxidation State on Physical-chemical Properties of Smectites : Evidence from Infrared Spectroscopy

gether, along with all other features in thisRegion, with complete Fe reduction. A lesser
band at 2.- cm῍+, which appears as a shoulder
on the principal peak, is assigned to an Fe-OH
bending mode, where the OH is taken as asingle unit. This assignment was originallygiven by Stucki and Roth (+310), but has notbeen widely accepted. Nevertheless, no better
assignment has been found. These major modiﬁcations clearly indicate that changes occurred within the clay crystal far beyond
merely a change in Fe oxidation state. Partial
dehydroxylation and redistribution of Fe, andperhaps Al, cations occurred upon reduction,changing the structural properties of the tetrahedral and octahedral sheets.
The loss of structural O-H stretching andbending vibrations is direct evidence that the
OH groups are intimately involved in the reduction mechanism of the smectites. In nontronite, the ratio of Fe to OH in the clay structure is about + : +. If electron transfer is accompanied by protonation of the structural OH(Drits and Manceau, ,***), one would expect
complete reduction to eliminate all structuralO-H vibrational modes and replace them witheither H-O-H modes or nothing, depending on
the extent to which the newly formed H-O-H
di#uses out of the smectite structure. Thisappears in fact to be the case (Figs. + and .).
Region IV
The bands in Region IV (Fig. .) seem to havesome correlations with the bands in Region III,
but these relationships are only poorly established at this point. The attribution of the /23
cm῍+ band is uncertain but it may be due to a
combined Si-O-Fe (III) bending mode (Stubicanand Roy, +30+ a, b). Upon reduction, this band
completely disappears and a new band emerges at 0/- cm῍+. No apparent connection isobserved between this new band and any ofthe disappearing ones in Region III, but a band
located in the same region is generally observed for [Fe (II)]- OH deformation modes intalcs (Wilkins and Ito, +310). The band at 0/cm῍+ in the fully reduced sample could, there-7

fore, be due to [Fe (II)]- OH deformation, which
supports the hypothesis of trioctahedral domain formation upon reduction. The high intensity of this band, however, seems to be outof proportion to the possible number of trioctahedral groupings, so an additional contribution to this band may exist. A possible
candidate is the band located at 01, cm῍+ in theunaltered nontronite, which is attributed to
Fe-O out-of-plane vibrations (Farmer, +31.).
This band is expected to shift to lower wavenumber upon Fe (III) reduction, but no evidence of such a shift was observed (Fialips etal., ,**, b).
Perhaps it does not change
signiﬁcantly upon reduction and could still bepresent in the spectrum of the fully reduced
sample, in which case the 0/- cm῍+ band observed for this sample could be generated by
the overlap of one band at 01, cm῍+ and one
band due to [Fe (II)]- OH deformation at lower
wavenumber.
..

Conclusions

Infrared spectra reveal signiﬁcant di#erences in structural vibrational modes in Ferich smectites when structural Fe (III) is
reduced to Fe (II). These di#erences clearly
indicate that structural OH groups are being
eliminated as a part of the reduction process,either directly by dehydroxylation or by protonation. The latter appears to be the better
explanation. A portion of the protonated hydroxyl groups evidently remain in the smectite
structure after complete reduction. Those sites
from which structural OH groups are lost
evoke a migration of the structural Fe from theoriginal site to an adjacent vacant site, thereby
creating a trioctahedral domain and a defect inthe octahedral sheet. The relative number ofsuch domains has yet to be determined.
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8

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30

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~N2 Q?A ORP wxE*
7, ORP  Fe # U*0
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Journal of Soil Science. /- : +1/῍+2-.
Komadel, P. and J.W. Stucki. (+322) : Quantitative
assay of minerals for Fe,¹ and Fe-¹ using +,+*
-phenanthroline : III. A rapid photochemical
method. Clays and Clay Minerals. -0 : -13῍-2+.
Kostka, J.E., D.D. Dalton, H. Skelton, S. Dollhophand J.W. Stucki. (,**,) : Growth of iron (III)reducing bacteria on clay minerals as the soleelectron acceptor and comparison of growthyields on a variety of oxidized iron forms.
Applied and Environmental Microbiology. 02 :0,/0῍0,0,.
Xu, J.C., G.K. Sims, J.E. Kostka, J. Wu and J.W.
Stucki. (,**+) : Fate of atrazine and alachlor inredox treated ferruginous smectite. Environmental Toxicology and Chemistry. ,* : ,1+1῍,1,..