Environ. Sci. Technol. 1994, 28, 204-289
Mechanisms of Chromium( I I I ) Sorption on Silica. 1. Cr( 1I I ) Surface
Structure Derived by Extended X-ray Absorption Fine Structure
Spectroscopy
Scott E. Fendorf,’lt Gerry M. Lamble,* Michael G. Stapleton,$ Michael J. Kelley,ii and Donald L. Sparks5
Soil Science Division, University of Idaho, Moscow, Idaho 83844, Brookhaven National Laboratory, Upton, New York 11973,
Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 197 17, and Engineering Technology
Laboratory, DuPont Company, P.O. Box 80304, Wilmington, Delaware 19880-0304
Metal ion reactions at the solid/solution interface are
important in an array of disciplines and are of environmental significance as such reactions can greatly affect
the risk imposed by metals. The structural environment
of metals at the solid/water interface determines their
potential for remobilization to the aqueous environment
and the physical/chemical modifications of the sorbent.
In this study, extended X-ray absorption fine structure
(EXAFS) spectroscopy was used to discern the local
structural environment of Cr(II1) sorbed on silica. Chromium(II1)formed a monodentate surface complexon silica,
with a Cr-Si distance of 3.39 A. At the surface coverages
investigated, a polynuclear chromium hydroxide surface
phase occurred with Cr-Cr distances of 2.99 A, indicative
of edge-sharingCr octahedra. Crystallographicparameters
resulting from the measured atomic distances dictate that
the surface phase was most likely of the y-CrOOH-type
local structure. Environmental considerations of Cr(II1)
remobilization must therefore consider the chemical/
physical properties of the monodentate surface-complexed
Cr(II1) and surface-nucleated chromium hydroxide.
Introduction
Inorganic compounds are potential pollutants that can
be particularly problematic due to their stability in the
environment (i.e., lack of degradation). Chromium is an
environmentally significant metal used in various industrial processes: tanneries, plating and alloying industries,
and as a cooling water anticorrosion agent. Chromium
primarily enters soils and waters at hazardous levels from
industrial wastes or spills. Two oxidation states of Cr are
stable under surficial conditions, Cr(II1) and Cr(V1).
Chromium(V1) is much more hazardous than Cr(II1)
because it is mobile through plant and animal membranes
and is toxic to cells due to its strong oxidizing nature. In
addition, anionic Cr(V1) species are very mobile in the
environment. Due to the toxicity and mobility of Cr (VI),
the maximum allowable concentration of Cr in drinking
water is
M (1). It is therefore important to determine
reactions that affect the oxidation state of Cr. Chromium(111)oxidation is an important process as the rather benign
trivalent species is transformed into the hazardous Cr(VI) species. Fortunately, the only known naturally
occurring oxidants of Cr(II1) are manganese oxides (2).
The oxidation of Cr(II1) by manganese oxides is dependent upon the formation of a Cr(III)-Mn02 complex;
* To whom correspondence should be addressed.
University of Idaho.
Brookhaven National Laboratory.
f University of Delaware.
11 DuPont Company.
t
t
284
Envlron. Scl. Technoi., Voi. 28, No. 2, 1994
thus, reactions which limit the formation of this complex
may decrease the potential for Cr(V1) production. One
possible mechanism for retarding Cr(II1) oxidation is its
retention by other nonredox reactive sorbents present in
soils or waters. This process prevents Cr(II1)complexation
with manganese oxides and, consequently, limits its
oxidation. An array of studies have investigated the
retention of metals by soils and soil components. Although
permanently charged aluminosilicate clay minerals often
are the dominant sorbents in soils, hydrous oxides can
play an important role in the environmental behavior of
metal ions. The influence of hydrous oxides, e.g., silica
(SiOz), is enhanced by the formation of high surface area
particles and surface coatings on other soil materials.
Moreover, sorption reactions regulate the risk of metal
ions in the environment since the metals are removed from
the aqueous phase. Hence, to determine the hazard and
risk of Cr in the environment, knowledgeof Cr(II1)sorption
on nonredox reactive solids is necessary.
Mechanistic interpretations and models have been
developed for metal sorption reactions at the oxidelwater
interface; however, they have largely been based on
macroscopic data without direct atomic-level evidence.
Although the heterogeneous nature of hydrous oxide
surfaces has been recognized (31, surface complexation
models have predominantly been based on the conception
of a homogeneous surface with a single surface site, which
is composed of a hydroxyl species bound to a central cation,
that can undergo protonation (forming a surface water
group) or deprotonation (forming an oxo group) reactions.
Additionally, the sorbing species is depicted as one that
binds only at isolated sites (3-7). Only a few models have
considered surface precipitation reactions (8, 9).
Recent experimental evidence has indicated that surface
nucleation of metal hydroxides occurs much more frequently than previously believed ( I 0-1 5 ) . In this paper,
we use the term surface nucleation to denote the general
process that marks the onset of a three-dimensional growth
mechanism. Polymerization is used to denote the formation of small multinuclear species, such as dimers or
trimers, which may lead to nucleation. When a precipitate
grows away from the surface before distributing over it,
we use the connotation ‘surfacecluster’or ‘island structure’;
those which are distributed across the surface are simply
termed ‘surface precipitates’. Multinuclear metal hydroxides of Pb, Co, and Cr(II1) on oxides and aluminosilicate minerals have been discerned with extended X-ray
absorption fine structure (EXAFS) spectroscopy (10-13,
16). The number of cations present in the polymerized
moiety was observed to vary depending on the sorbent. At
surface loadings of 1.1-1.2 pmol m-2, Co-hydroxide nucleation was greater on y-Al203 than on rutile or kaolinite
(11, 12). Copper(I1) sorption on aluminum oxides was
0013-936X/94/0928-0284$04.50/0
0 1994 American Chemical Society
investigated with electron paramagnetic resonance (EPR)
spectroscopy (14,15) and was found to progress from an
isolated site-binding mechanism to Cu(OH)2 surface
nucleation at fractional surface coverage. Only surface
functional groups singly coordinated to A1 were observed
to be reactive with Cu2+.
Advances in understanding metal ion retention mechanisms have been made with the employment of atomic
resolution experimentation, but many sorption mechanisms and reaction factors influencing these mechanisms
(e.g., pH, ionic strength, competing ions, etc.) remain
unresolved. It is important for researchers to utilize
techniques, or preferably a multitude of techniques, which
give direct evidence for reaction mechanisms. The results
of such studies should enhance the development of
physically accurate surface complexation models.
EXAFS spectroscopy can yield direct information on
the local chemical and structural environment of an
element. This makes EXAFS a useful method for studying
metals ions sorbed on oxides. One should recognize that
EXAFS probes the average state of an element in a sample,
and thus if the sorbed ion resides in a spectrum of structural
environments within a sample it may not be possible to
resolve these states. The objective of this study was to
determine the structural environment of Cr(II1) on silica
using EXAFS spectroscopy.
Materials and Methods
Batch Studies. The silica used in this study was a
Huber Zeothix 265 amorphous Si02 colloid, synthesized
as described by Wason (17). The oxide was washed in pH
3.5 HN03and then dialyzed in doubly-distilled deionized
water unit a stable conductivity resulted for 24 h. The
surface area was 221 m2/g, as determined by the ethylene
glycolmonoethyl ether (EGME)method (18). The particle
size of the oxide was less than 2.0 pm.
Batch studies were performed to determine the amount
of Cr(II1) sorbed on Si02 as a function of pH and initial
Cr(II1) concentration ([Crl,). A pH range of 3-7 was
investigated with initial concentrations of 100, 200,400,
and 5 X 103 pM Cr(II1). For the batch studies, 0.5 g of
Si02 was dispersed in reaction vessels with 2 L of 0.1 M
NaNO3. A 10mM Cr(II1)stock solution was used to obtain
the desired Cr(II1) concentrations. The stock solutions
were made from ACS reagent-grade Cr(N03)~9H20,
with
acidified deionized water (pH I2) and were never allowed
to age more than 5 days to limit potential polymerization.
The oxide was allowed to hydrate for 48 h prior to
reaction. After the hydration period, the pH was adjusted,
the desired amount of Cr(II1) was added, and the final
volume was brought to 2 L, yielding a suspension density
of 0.25 g/L. The pH was held constant with a pH-stat
system; upon reaching a steady pH, the vessels were placed
in a water bath reciprocating shaker. After 48 h, the
samples were filtered through a 0.22-pm pore membrane,
and the solution was analyzed for Cr with a JY-70 ICP
spectrophotometer. The solids were also digested with 12
N "03
and analyzed for total Cr. The sorbed quantities
are reported from the loss of Cr(II1)in solution (i.e., changes
in Cr solution concentration induced by the silica). The
sorption data and structural information gleaned in this
study are representative of the reaction after a 48-h time
period and, therefore, may not be indicative of sorption
at equilibrium (19).
Table 1. Solution Speciation and Saturation Indices for 50
pM Cr(II1) at pH 6, As Predicted by the Computer Program
MINTEQA2 (23) Using Thermodynamic Values from Rai et
al. (24)
species
%"
SI*b
Cr3+
3
Cr(OH)2+
68
Cr(OH)Z+
17
Cr(OH)3(aq)
6
. Crz(OH)z4+
<6c
1.5
Cr(OH)s(s)
Calculated for species in 0.1 M NaN03. b SI (saturation index)
= log (IAP/K,,), where IAP and Kapare the ion activity and solubility
products, respectively. c Maximum value based on solubility data of
Rai et al. (24) and polymeric Cr(II1) speciation values of Stunzi and
Marty (32).
All reactions were carried out at 25 f 0.5 "C at 1 atm
pressure in aNz(g) environment to eliminate the influences
of C02. The initial solutions were purged with Ndg) prior
to the addition of the oxide. Thereafter, a Nz(g) stream
was maintained over the surface of the fluid when the
vessels were exposed to the surrounding environment.
Several mechanisms can be operational for the retention
of metals on oxides; the term sorption is used to describe
the general uptake of a sorptive without implying a specific
mechanism, e.g., adsorption or precipitation. Various
terms have been used to describe the amount of a metal
sorbed on an oxide. In this study the data are referenced
to the potential surface site occupancy, 4
4 = (mol of Cr sorbed)/(mol of surface sites) (1)
One should be aware that this does not imply a site
occupancy but only a potential maximum if each sorbed
Cr(II1) occupied a single site. If polymerization or
nucleation occurs, then a different amount of the surface
sites would be occupied (i.e., an amount less than 4 would
be covered). This parameter, 4, is employed because it
allows one to easily recognize the quantity of sorbed Cr
necessary for monolayer coverage; at less than potential
monolayer coverage, isolated site binding can account for
the total sorption quantity, but at 4 > 1, nucleation or
multilayer adsorption is required. An estimated reactive
surface site density of 5.5 site/nm (20)was used to calculate
the site density for silica.
For XAFS studies, samples reacted at pH 6 were
employed for analysis because this pH is common to soils
and waters and represents the complete uptake of aqueous
Cr onto the silica surface (vide infra). To minimize
potential solution polymerization or precipitation, Cr was
dispensed in stepwise additions so as to never exceed an
aqueous concentration of 50 pM. This concentration was
chosen because the solution is only slightly saturated with
respect to chromium hydroxide, solution polymerization
is minimal (Table 11, and changes in concentration at this
level are easily and accurately measured with the methods
employed in this study.
Solids were consolidated prior to XAFS analysis to
maximize the amount of sorbed material analyzed. After
48 h, the suspensions were passed through a 45-mm
diameter 0.22-pm membrane filter. The filtrate was then
rinsed with 100 mL of high-purity water to remove the
entrained electrolyte. In addition, a hydrous chromium
oxide (HCO) was precipitated by titrating 20 mM Cr(II1)
to pH 6, with pH maintained at this level for 24 h. This
procedure was similar to that used by Charlet and Manceau
Environ. Scl. Technol., Vol. 28, No. 2, 1994
285
(10). The solid material was then placed in a vial and
sealed. The samples remained moist and were never
allowed to dry.
XAFS Studies. XAFS spectroscopy was performed
at the National Synchrotron Light Source, Brookhaven
National Laboratory, under dedicated running conditions
on beam line X-11A. The electron storage ring operated
at 2.528 GeV with currents ranging from ~ 2 1 5mA
immediately after a fill to a refill current of 110 mA.
A Si(ll1) water-cooled double-crystal monochromator
was employed with a sagitally focused beam (21). The
focused beam provided an intensity greater than three
times that provided by the conventional flat monochromator arrangement over the energy range used for this
study. A 0.5-mm pre-monochromator slit width was
employed, which was readjusted as necessary to compensate for vertical motion of the stored electron beam. Higher
order harmonics were rejected by detuning 30% from the
maximum incident intensity (Io).
The incident X-ray intensity was measured with a 15cm-long N2(g) filled ionization chamber. Reference materials were run in transmission mode, and the transmitted
intensity was measured with a second ionization chamber.
All Cr(III)-Si02 systems were analyzed by fluorescence
detection. The Cr K edge (5989 eV) was used for analysis.
For fluorescence detection, the samples were placed at
a 45O angle to the incident beam, and a wide angle collector
with an ionization chamber, a Lytle detector (EXAFS Co.),
was located 45O off the sample (Le., normal to the incident
beam). The Lytle detector was filled with Ar(g) for Cr K a
fluorescence detection. A V filter and Soller slits were
placed between the sample and the detector to reduce
elastically scattered incident X-rays from entering the
fluorescence detector. Samples were analyzed at ambient
and Nz(1) temperatures (298 and 77 K); no structural
differences were discerned between these temperatures,
and consequently, the 77 K spectra are reported due to
the reduction in thermal disorder.
Fluorescence samples were mounted in a 4 X 6 X 25 mm
slot cut in an A1 block, which provided good thermal
conductance necessary for temperature variation of the
samples. These dimensions provided 1mm on each side
of the beam and a depth in excessof four adsorption lengths
when mounted at 45O to the incident beam. The sample
cell was sealed with 0.0005-in. thick Kapton polymide film
(CHR Industries, type K-104) to prevent moisture loss
while minimizing X-ray absorption. The samples were
mounted on a Cu cold finger that was connected to an
Nz(1) reservoir. To minimize the heat transfer imposed
on the cold finger, samples were precooled by immersion
into Nz(1) for several minutes prior to analysis.
All samples were run at least in triplicate. Data analyses
were accomplished by optimizing the fit of the predicted
spectra with Fourier filtered experimental spectra using
the program EXCURVE (22). Phase shifts for 0, Si, and
Cr were determined by first-principlecalculations and then
refined by using model compounds: Cr using a Cr metal
and a-Cr203,O with a-Cr203, and Si with Nisi. Amplitude
parameters were defined based on a-Cr203. Successive
shells were isolated in the Fourier transformed spectra,
back-transformed, and the interatomic distances (r's),
coordination numbers (CNs), and Debye-Waller factors
( 2 ~ 2 )varied until the best fit was obtained between the
predicted and the experimental curves. After obtaining
the structural information for each successive shell out to
288
Envlron. Scl. Technol., Vol. 28, No. 2, 1994
100
Initial Aqueous Cr(I1I)
90-1
A
rA
A
100pM
0 200pM
400 pM
0 0 0
b
0
9
0
0
70
G.
1UJ
OYU S & + d
Results
The sorption of Cr(II1) as a function of pH at various
initial aqueous Cr(II1) concentrations is presented in
Figure 1. With all the initial concentrations, complete
(100%)Cr(II1) sorption occurred by pH 6. Thus, at pH
6 the aqueous concentrations of 100,200,400, and 5 X 103
pM Cr(II1)correspond to potential surface site occupancies
(4) of 0.20, 0.40, 0.79, and 9.9, respectively. In addition
to surface coverages, the speciation of Cr(II1) in solution
can play an important role in affecting the sorption
mechanism. In order to equate the solution effects of the
different surface loadings investigated and, therefore,
isolate the influences induced by the surface, the solution
concentrations never exceeded 50 pM Cr(II1). This upper
concentration limit was imposed to diminish the potential
for solution polymerization or precipitation. Table 1gives
the predictedspeciation of 50pM Cr(II1)at pH 6 computed
with the program MINTEQA2 (23) using thermodynamic
values form Rai et al. (24). These reaction conditions were
used for further structural analysis.
The extended portion of the XAFS (EXAFS) was
utilized to discern the local structural environment of Cr(111)sorbed on silica under reaction conditions conducive
for complete Cr(II1)sorption, Le., pH 6. The experimental
spectra for the Cr-Si02 systems are shown in Figure 2
along with that of hydrous chromium oxide (HC0)-Cr(OH)3.nH20.Due to possible influencesof self-absorption,
spectra of the highest surface loading (4 = 9.9) are not
reported. The structural parameters derived from EXAFS
analysis are summaried in Table 2. In all of the Cr
specimens, approximately six 0 coordinated the Cr at 1.99
A. In the HCO, a second shell composed of two Cr at 2.99
A (edge-sharing CrOe octahedra) and a third shell of 1.5
Cr at 3.98 A (corner-sharing octahedra) were discerned.
Thus, the HCO had the y-CrOOH local structure. These
results are in good agreement with the structural parameters determined by others for y-CrOOH (16, 25).
The first two shells (i.e., six 0 at 1.99 A and Cr at 2.99
A) in all the Cr(III)-Si02 samples were similar to those of
- experimental
.... predicted
n
'
.y,
x
n
l
ry,
x
'
,
'
\
'
,
'
,
'
,
- experimental
.'.. predicted
I
I
4
6
,
8
I
I
10
12
Figure 2. Experimental EXAFS spectra of Cr(II1) sorbed on silica.
m
Table 2. Structural Information Derived from EXAFS
Analysis: Interatomic Distances (r,A), Coordination
Numbers (CN), and Debye-Waller Factors (2u2,# ) a
Cr-0
r ( A ) CN
7-CrOOH
6 = 0.7gb
6 = 0.40b
6 = O.2Ob
1.99
1.99
1.99
1.98
5.7
6.2
6.0
6.2
Edge: Cr-Cr
2a2 r ( A ) CN
0.008
0.005
0.005
0.005
2.99
3.00
2.98
3.01
1.5
2.3
2.5
2.9
Si-Cr
2a2 r ( A ) CN
n
ry,
x
2a2
0.006
0.010 3.38 1.1 0.009
0.010 3.40 1.0 0.008
0.011 3.39 0.71 0.009
a The 0, edge-sharing Cr octahedra, and Si coordination shell of
a central Cr are reported. Based on standards and internal deviation,
the reported values are accurate to within r f 0.03 A, CNc,.o f 20%,
and CNc,-c, and CNsixr f40%. Potential surface site coverage for
Cr(II1) sorbed on silica (mol of Cr/mol of surface sites).
'
I
6
I
,
I
10
8
7
-
7
12
A-1
the chromium hydroxide, with higher CN values. However, the fit between the predicted and the experimental
curves was greatly improved with the inclusion of a Si
atom. Thus, a different third shell resided in these
materials: a single Si atom at 3.39 8, (Figure 3). The SiCr distance of 3.39 A represents a monodentate complex,
in agreement with the CN, and would necessitate a 150'
Cr-0-Si bond angle. Consolidating the structural parameters derived from the isolated shells resulted in good
agreement between the predicted and the experimental
functions for the samples (Figure 4). Beyond the Si shell,
the EXAFS intensity is much weaker than the inner-shells,
and therefore, we could not place confidence in the derived
structural values. Nonetheless, further structural information was obtained based on crystallographic considerations and the measured atomic distances. With a Cr0-Si bond angle of 150°, adjacent Cr atoms would
necessitate a Cr-Cr distance of approximately 3.9 A,
assuming the silica surface to be composed of alternating
Si tetrahedra. These values indicate that the surfacenucleated chromium hydroxide phase had the local
y-CrOOH-type structure-similar to that of the homogeneous HCO precipitate. In the second part of this sereis
(23,infrared spectra are presented which confirm the
Figure 4. EXAFS function for the Fourier back-transformed spectra.
The theoreticalline was derivedwith parametersobtainedfrom analysis
of the isolated shells.
presence of a y-CrOOH-type phase-as noted by the CrOH-Cr deformation mode.
Although chromium hydroxide nucleation was observed
in all of the Cr-Si02 systems, the extent of nucleation
varied depending on 4. Figure 5 gives the relative
intensities of the atomic shells in the Fourier transformed
spectra (the radial structure function, RSF). The second
peak (centered at approximately 3 A) composed of Si and
edge-sharing Cr octahedra is apparent in all of the RSFs.
Figure 6 illustrates the chromium silica surface structure
derived from EXAFS analysis and crystallographic considerations. For alternating silicon tetrahedra linkages
(261,the distances arising between apical 0 would produce
favorable distances for hydroxyl bridging of surfacecomplexed Cr, provided the Cr are angled toward each
other, as depicted in Figure 6. Thus, it appears that
chromium(II1) hydroxide nucleation occurs on silica with
a y-CrOOH-type local structure, similar to that observed
for precipitation on 6-MnOz (27,28) and homogeneous
solution precipitation (i.e., HCO). The second paper of
Environ. Scl. Technol., Vol. 28. No. 2, 1994
287
-
....
1
I
I
2
3
,
4
experimental
predicted
I
5
A
Flgure 5. Fourier transformed spectra resulting in the radial structure
function (RSF) of the inner 4 &shells: HCO and Cr sorbed on silica
are shown. The first peak results from six 0 at 1.99 A; the second
from Cr at 2.99 8.In the presence of Si, the second peak incorporates
SI at 3.39 A.
/*'
Flgure 6. Depiction of the surface structure derived by EXAFS analysis
(a) showing the interatomic distances for Cr sorbed on silica. The
y-CrOOH-type structure is shown forming in panel a, and in panel b
the interatomic distances of this phase are given.
this series (25) gives further evidence for the 7-CrOOHtype local structure structure on silica.
Discussion
EXAFS analysis indicates that Cr(II1) formed an innersphere monodentate surface complex on silica. A Si-Cr
288
Environ. Sci. Technoi., Vol. 28, No. 2, 1994
distance of 3.39 A was determined; a linear Cr-0-Si
arrangement would result in a distance of 3.59A, and thus,
a bond angle of 150' is necessary to produce the observed
distance. This Cr-Si distance and angle are similar to
that observed for Mn in MnSiOs (29). Monodentate
coordination with a bond angle of 150' implies that the
reactive surface 0 atoms are singly coordinated by Si. This
complexation structure agrees with the multisite surface
complexation (MUSIC) model (30, 3I), which indicates
that only the singly-coordinated 0 of Si02 are reactive in
the pH range of 2-10. McBride et al. (15) and McBride
(14) also observed that Cu2+ sorbed only to singlycoordinated 0 groups on aluminum oxides.
A t coverages equal to and exceeding 0.20, surface
nucleation of the y-CrOOH-type structure occurred.
Chromium hydroxide nucleation on goethite was also
observed (IO). However, a deviation from the yCrOOH
local structure occurred at the goethite interface (IO);
surface-nucleated Cr(II1) formed the a-CrOOH-type structure ( I O ) . As surface coverageincreased, nucleation growth
expanded over the surface before expanding outward (away
from the surface). Further increases in Cr(II1) levels
resulted in nucleation progressing outward from the
a-FeOOH surface, and there was a phase transition from
the a- to the y-CrOOH-type structure. It appears that
Cr(II1) sorption on silica is similar to that on goethite in
that surface precipitation occurred prior to monolayer
coverageand under conditions where solution precipitation
was not observed (25). In contrast to sorption on goethite,
however, a monodentate surface complex formed on silica,
and the nucleation of y-CrOOH was not extensively
distributed over the surface but appears to have formed
surface clusters-island structures (25).
Determining the sorption mechanism of metal ions at
the oxide/solution interface is necessary for evaluating
their hazard in the environment. The accuracy of mechanistic models utilized for predicting the fate of metals in
colloidalsystems will be enhanced by incorporating results
from atomic level experimentation. The surface structure
of sorbed metals determines their potential for remobilization. The strength of retention will differ depending
on whether nucleation of a metal hydroxide has occurred,
and it will depend on the structure of the metal hydroxide
(i.e.,different polymorphs often have different solubilities).
Furthermore, the sorbent will be modified by the sorption
process and the resulting chemical/physical properties
determined by the sorption mechanism. For Cr(II1)
sorption on silica, at potential surface coverages greater
than 20 % ,surface nucleation of chromium hydroxide with
the y-CrOOH-type local structure occurred in addition to
a monodentate surface complex; the final conglomerated
colloid would exhibit chemical/physical properties of the
surface clusters and the unreacted silica. One should be
aware, however, that since XAFS probes the average state
of an element there is the potential for small quantities
of Cr(II1) to reside in other surface structures. Additionally, the surface clusters may underto a structural
rearrangement with time. Nevertheless, the dominant
processes influencing Cr(II1) sorption on silica can be
modeling considering both monodentate and surface
nucleation sorption mechanisms.
In this portion of the study, the surface structure of
Cr(II1) on silica was discerned. In the second part of this
series (25),the reaction factors (degree of surface coverage,
pH, and initial aqueous Cr concentration) are investigated
as to their effects on the sorption mechanism and the
resulting surface structure.
Acknowledgments
We would like to acknowledge the US.Department of
Energy, Division of Materials Sciences, under Contract
DE-FG05-89ER45384,
for its role in the operation and
the development of beam line X-11A at the National
Synchrotron Light Source. The NSLS is supported by
the Department of Energy, Division of Materials Sciences
and Division of Chemical Sciences, under Contract DEAC02-76CH00016.
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Received for review April 19, 1993. Revised manuscript received September 10, 1993. Accepted October 20, 1993.'
Abstract published in Advance ACS Abstracts, December 1,
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