Influence of aluminum doping on ferrihydrite nanoparticle reactivity

Article abstract of DOI:10.1021/jp060957+

The composition of natural ferrihydrite varies considerably, especially in terms of aluminum and silicon substitution. This work examines the influence of aluminum content on the redox reactivity of ferrihydrite nanoparticles as determined by kinetic studies of the reductive dissolution of the particles by hydroquinone. Transition state theory applied to variable-temperature experiments is used to measure the activation enthalpy and entropy. The presence of small amounts of aluminum (0-2.1 mol % substitution) causes a decrease in ΔH^? and an increase in ΔS^?, resulting in an overall increase in reactivity.

Full text of DOI:10.1021/jp060957+

11746
J. Phys. Chem. B 2006, 110, 11746-11750
Influence of Aluminum Doping on Ferrihydrite Nanoparticle Reactivity
Teresa L. Jentzsch and R. Lee Penn*
Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455
ReceiVed: February 14, 2006; In Final Form: May 1, 2006
The composition of natural ferrihydrite varies considerably, especially in terms of aluminum and silicon
substitution. This work examines the influence of aluminum content on the redox reactivity of ferrihydrite
nanoparticles as determined by kinetic studies of the reductive dissolution of the particles by hydroquinone.
Transition state theory applied to variable-temperature experiments is used to measure the activation enthalpy
and entropy. The presence of small amounts of aluminum (0-2.1 mol % substitution) causes a decrease in
q
q
∆
H and an increase in ∆S , resulting in an overall increase in reactivity.
Introduction
greater amounts of substitution inhibited reaction. To our
knowledge, no studies have been performed on aluminum-
substituted ferrihydrites. This paper specifically examines the
abiotic reduction of well-characterized ferrihydrites with alu-
minum doping ranging from 0 to 2.1 mol %.
Iron oxides are commonly found at and near the earth’s
surface as nanoparticles in the 3-10 nm size range and can be
formed through a variety of mechanisms, including precipitation,
redox, and weathering processes.1 Because of their abundant
presence, redox reactivity, and surface activity (e.g., adsorption
affinity), iron oxide nanoparticles play important roles in the
transformation and mobility of environmentally relevant species,
including heavy metals, pollutants, and even naturally occurring
chemical species. Arsenic, for example, has been shown to be
transported through and subsequently released into groundwater
by phase transformations and dissolution of iron oxide nano-
,2
To quantitatively compare the reactivities of ferrihydrite
samples with various amounts of aluminum doping, the rate of
reductive dissolution of ferrihydrite by hydroquinone was
monitored at several temperatures. Hydroquinone was selected
as the reducing agent for its environmental relevance and the
ease with which aqueous concentrations of this compound and
its oxidation product can be characterized by analytical chem-
1
3,14
istry techniques.
Quinone functional groups are known to
3
,4
particles. Finally, some bacteria couple the reduction of Fe-
III) with the metabolism of organic materials, which can include
act as electron acceptors when bacteria reduce humic substances,
which may subsequently reduce iron(III)-bearing minerals.15 It
is also believed that bacteria may secrete quinones to serve as
electron shuttles during the reduction of insoluble metal oxides,
(
anthropogenic contaminants, or simply use iron oxides as
5
electron sinks during respiration.
1
Ferrihydrite (Fe5HO8‚4H2O) is typically considered a meta-
16
including ferrihydrite. Transition state theory was then applied
to the resulting kinetic data in order to quantify the effect of
aluminum substitution on chemical reactivity. Finally, extensive
materials characterization was performed so as to ensure that
the differences observed reflect the influence of aluminum
doping as opposed to the influence of physical differences, such
as particle size or morphology.
stable iron oxide that can act as a precursor to the more stable
iron oxides such as goethite and hematite.1 It forms as small
,6
(generally 4-6 nm), poorly crystalline particles and is classified
according to the number of lines, usually two or six, observed
in the X-ray powder diffraction pattern. Natural ferrihydrites
7
are commonly found in spring- and groundwater and as
components of soils where water lies stagnant, such as drainage
8
sites. The high surface area and low crystallinity of these
Experimental Methods
particles contribute to their enhanced reactivity relative to other
9
iron oxides and, therefore, their potential to influence envi-
Synthesis of Al-Doped Ferrihydrites. Ferrihydrite nano-
ronmental processes.
particles were prepared using a method adapted from Burleson
1
7
The soils and waters in which ferrihydrites often form are
commonly rich in aluminum. Indeed, many environmental
samples of iron oxides contain large amounts of aluminum
and Penn. Four samples of ferrihydrite with various amounts
of aluminum doping were precipitated from aqueous solutions
of Fe(NO3)3‚9H2O and Al(NO3)3‚9H2O (ACS grade, Fisher).
Solutions of the salts were prepared such that the total
concentration of metal cations was 0.40 M; the percentage of
aluminum ion present was between 0 and 30 mol %. To each
solution, an equal volume of 0.48 M (for 0%, 5%, and 20% Al
samples) or 0.64 M (for a 30% Al sample) NaHCO3 (ACS
grade, Mallinckrodt) was added dropwise, with stirring, over a
period of 23 ( 0.3 min, resulting in an approximate final
solution pH of 2.5. The deep-red suspension that resulted was
heated by microwave for three 40 s intervals with intermediate
shaking until boiling occurred and was then immediately placed
into an ice bath. The suspension was dialyzed against purified
water (Milli-Q) over a period of 4 days, with the water replaced
1,10
impurity. Studies have shown that this aluminum content may
1
be as high as 32 mol % substitution. The incorporation of a
redox inactive species such as aluminum is expected to affect
the chemical properties of iron oxide nanoparticles. Therefore,
to better understand the behavior of natural iron oxides, the
effects of cation substitution must be examined. Previously,
researchers have studied the effects of varying amounts of
aluminum on the reactivity of other environmentally relevant
iron oxides such as goethite.1 These studies showed that small
amounts of aluminum appeared to enhance reactivity, while
1,12
*
penn@chem.umn.edu.
1
0.1021/jp060957+ CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/26/2006

Al Doping on Ferrihydrite Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11747
nine times with a minimum of 3 h between each water change.
After dialysis, the suspension was poured into shallow dishes
to dry. The shiny, deep-red solid that resulted was ground into
a fine reddish-brown powder and stored in a glass vial.
Materials Characterization. The materials were character-
ized using inductively coupled plasma mass spectrometry (ICP-
MS), X-ray diffraction (XRD), and transmission electron
microscopy (TEM).
ICP-MS samples were prepared by dissolving the solid
particles in a solution of 4.8 M HCl and 3.2 M HNO3, followed
by dilution in 0.1 M HNO3. The diluted solutions were analyzed
for Fe, Al, As, and Na cation content using a ThermoElemental
PQ ExCell quadrupole ICP-MS. Each sample was analyzed in
duplicate, and concentrations were determined using a four-
point calibration curve for each cation.
Figure 1. XRD patterns of the 0%, 0.4%, 1.4%, and 2.1% aluminum-
substituted ferrihydrites. Diffraction patterns are shown in black.
Difference plots, which are determined by subtracting the 0% Al pattern
from each Al-substituted pattern, serve to highlight small shifts in peak
positions and are shown in gray beneath each pattern. All patterns are
consistent with the reference pattern for 6-line ferrihydrite, shown as
solid lines (s). No evidence of goethite was observed in the patterns,
and the reference peak for the strongest goethite reflection is shown as
a dashed line (- - -).
X-ray diffraction was performed using a PANalytical X’pert
PRO MPD X-ray diffractometer equipped with an X’Celerator
detector and cobalt source. Powdered samples were back-loaded
into XRD sample holders to create flat surfaces. Data was
collected over the range of 10-100° 2θ at a scan rate of 1°/
min. The diffraction patterns were compared to the reference
powder diffraction file (PDF) for 6-line ferrihydrite (PDF #29-
TABLE 1: Aluminum Content (mol % Substitution) and
Concentrations of Possible Contaminants (ppm) as
Determined Using ICP-MS
0
712) and the PDFs for the other iron oxides. Differential plots
were determined by subtracting the XRD patterns for the 0%
Al ferrihydrite from the Al-doped materials in order to assess
changes in lattice parameters as a result of the aluminum doping.
aluminum sodium arsenic
content
content content
synthesis
(mol %)
(ppm)
(ppm)
Samples for transmission electron microscopy (TEM) were
prepared by diluting a small amount of the suspended particles
by ∼1000 times using Milli-Q water, sonicating the diluted
suspension, and placing one drop onto a 3 mm, 200 mesh holey
carbon-coated copper grid (SPI). The samples were then allowed
to air-dry. Images were collected using an FEI Tecnai T12 TEM
operated at 120 kV. All images were collected using a charge-
coupled device (CCD) camera and were analyzed using Gatan
Digital Micrograph 3.3.1. The surface area of the samples was
determined from length and width measurements of ap-
proximately 500 individual particles, which were modeled as
ellipsoids. Using the standard equation for the volume of an
ellipsoid and the equation for the surface area of a cigar-shaped
ellipsoid, the average surface area per average volume for the
particles was calculated. From the surface area per volume and
ferrihydrite, 0.48 M base
0.0
0.4
1.4
2.1
136
163
307
80
16
8
7
ferrihydrite w/5% Al, 0.48 M base
ferrihydrite w/20% Al, 0.48 M base
ferrihydrite w/30% Al, 0.60 M base
6
filters and into amber vials. The solutions were clear following
filtration, indicating that the solid was removed from the reaction
mixture. The filtered solutions were immediately analyzed using
high-performance liquid chromatography (HPLC), as described
below. The temperature was recorded at the beginning and end
of each reaction to ensure that it had remained stable, and each
reaction was performed in duplicate. Blanks were prepared in
an identical manner, but omitting the solid ferrihydrite sample.
HPLC Method. Filtrates were analyzed using an Agilent
HPLC equipped with a Zorbax C18 column. The mobile phase
was a 65:35 mixture of acetate buffer and acetonitrile, and a
flow rate of 0.75 mL/min and a 10 µL injection volume were
used. Hydroquinone eluted at 2.3 min and benzoquinone at 3.4
min. The absorbance at 235 nm was monitored, and eight-point
calibration curves, ranging from 5 to 1000 µM for hydroquinone
and 0.5 to 100 µM for benzoquinone, were used to determine
concentrations.
3
1
the density of ferrihydrite (3.96 g/cm ), the specific surface
area was calculated for each sample. TEM was used exclusively
for surface area determinations, as previous BET measurements
on ferrihydrites have demonstrated that such measurements are
unreliable, at best, for these materials.9
Kinetic Reactions. Each reaction used 50 mg of solid sample,
which was massed into a 60 mL amber glass vial (for heated
reactions) or a 30 mL Nalgene bottle (for cooled reactions) and
was placed in an anaerobic chamber (<100 ppm O2). Following
the addition of 5 mL of deoxygenated acetate buffer (40 mM,
pH 3.75, acetic acid (Mellinckrodt) and sodium hydroxide
Results and Discussion
ICP-MS. The composition of each ferrihydrite sample was
determined using ICP-MS (See Table 1). The amount of
aluminum present in each of the samples was substantially lower
than the percentage of aluminum in the original solutions. This
is most likely the result of the relatively high solubility of
aluminum, certainly as compared to ferric iron solubility, under
the conditions of the ferrihydrite synthesis. The concentrations
of possible contaminants (e.g., Na and As) were also measured
and found to be lower than parts per thousand.
XRD. Powder X-ray diffraction (XRD) patterns show that
the materials prepared were consistent with the reference pattern
for 6-line ferrihydrite (Figure 1). No evidence for the presence
of other iron oxides was detected. In addition, no significant
(Titristar) in Milli-Q water), samples were sonicated, vortexed,
and stirred overnight to further reduce aggregation of the
particles. Nalgene bottles were wrapped in aluminum foil to
block light since benzoquinone is known to photodegrade. Prior
to reaction, an additional 19.5 mL of acetate buffer was added,
and the samples were placed into an ice bath or a silicon oil
bath heated to 30, 40, or 50 °C. After the temperature had
equilibrated, 0.5 mL of 10 mM hydroquinone (Sigma), which
was prepared using the deoxygenated acetate buffer, was added
to initiate the reaction and bring the total suspension volume to
1
2
5 mL. At regular time intervals, 1 mL aliquots of the reaction
mixture were quenched by filtration through 0.2 µm Acrodisc

11748 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Jentzsch and Penn
TABLE 2: Average Particle Dimensions and Specific
Surface Areas for Aluminum-Doped Ferrihydrite
Nanoparticles as Determined Using TEMa
particle
particle
surface
2
sample
ferrihydrite
length (nm) width (nm) area (m /g)
4.6 ( 0.4
4.5 ( 0.5
4.6 ( 0.5
4.5 ( 0.5
3.7 ( 0.3
3.6 ( 0.5
3.6 ( 0.4
3.5 ( 0.5
374 ( 30
375 ( 50
373 ( 43
385 ( 48
0.4% Al-subst ferrihydrite
1.4% Al-subst ferrihydrite
2.1% Al-subst ferrihydrite
a
The errors reported are standard errors.
Figure 2. Representative TEM images of aluminum-substituted
ferrihydrites: (a) 0% Al; (b) 0.4% Al (c) 1.4% Al; (d) 2.1% Al.
Figure 3. Mass-normalized concentration of benzoquinone as a
function of time. Solid lines represent the linear regression and have
been extended with a dashed line for clarity: (b) ferrihydrite (fh); (O)
differences in the degree of crystallinity, which would be
detected by changes in peak broadening and relative intensity,
were apparent between samples. The most notable difference
between the patterns was a slight peak shift toward higher angles
in the aluminum-substituted samples, which is highlighted by
the differential plots shown beneath each pattern for the Al-
substituted ferrihydrites. This shift indicates a small but
significant decrease in lattice spacings with aluminum substitu-
tion and supports the idea that aluminum was incorporated into
the crystal structure of the particles. The peak shifts become
more evident as the amount of aluminum substitution increases;
this is consistent with previous work that showed a relationship
between increasing aluminum content and a decrease in the
0
.4% Al-subst fh; (9) 1.4% Al-subst fh; (0) 2.1% Al-subst fh.
18
reaction, as determined using the method of initial rates, and
9
the literature values for the orders with respect to hydroquinone
and the particle surface (S) according to the rate law for the
reaction
d[Q]
dt
m
n
)
k[QH ] [S]
2
Here, k is the rate constant, and m and n are the empirical orders
with respect to hydroquinone and the particle surface, respec-
tively. It is assumed that these orders are independent of
aluminum content, and this assumption was verified for the 1.4%
aluminum-substituted sample (results not shown). In addition,
the concentration of surface sites is assumed to be proportional
to the surface area of the particles and, therefore, to the mass
of the material used for a constant particle size. It should be
noted that the proton concentration has been incorporated into
the rate constant, as all reactions were performed at a constant
pH of 3.75.
The rate of each reaction was determined by fitting the initial,
linear portion of the data for the rate of formation of benzo-
quinone (Figure 3). Data points were included in the linear fit
as long as their inclusion did not cause a deviation from linearity.
This resulted in four to eight data points from each trial (8-16
points for each sample) being included in each fit. When making
comparisons between samples, reaction rates were mass normal-
ized, since the specific surface areas are assumed to be similar
for the materials studied, as previously discussed. The formation
of benzoquinone in particle-free blanks was also monitored, and
in all cases, the spontaneous conversion of hydroquinone to
benzoquinone was one or more orders of magnitude slower than
reaction rates (results not shown).
1
atomic-plane spacing for several different iron oxide structures.
TEM. A representative TEM image for each ferrihydrite
sample is shown in Figure 2. Particle size measurements
demonstrate that all samples were similar in particle size (4.4-
4
.5 by 3.5-3.7 nm) and aspect ratio (1.2). Average particle
dimensions and calculated specific surface areas for each sample
are shown in Table 2. The particle size distributions suggest
that there is no statistical difference between the particle
dimensions of the various samples, and the aggregation states
of the samples appear similar. Thus, we believe that, at the low
levels of Al substitution of this study, it can be assumed that
the specific and accessible surface area is independent of
aluminum substitution.
9
Kinetics. The expected half and overall reactions between
ferrihydrite and hydroquinone (QH2) to produce benzoquinone
(Q), aqueous Fe(II), and water can be expressed as
-
+
2
Fe HO ‚4H O(s) + 10e + 30H (aq) f
5 8 2
2+
10Fe (aq) + 24H O(l)
2
-
+
5
QH (aq) f 5Q(aq) + 10e + 10H (aq)
2
+
2
Fe HO ‚4H O(s) + 5QH (aq) + 20H (aq) f
5
8
2
2
The rate of hydroquinone oxidation clearly increases with
aluminum content (Figure 3), and this increase was more
pronounced at higher temperatures. Transition state theory19 was
employed in order to determine the nature of the enhancement
2+
1
0Fe (aq) + 5Q(aq) + 24H O(l)
2
Rate constants were calculated using the measured rate of

Al Doping on Ferrihydrite Nanoparticles
J. Phys. Chem. B, Vol. 110, No. 24, 2006 11749
Figure 4. Application of the Eyring equation to temperature series
for 1.4% Al-substituted ferrihydrite. The line shown is the linear fit of
the data.
in reactivity. The natural log of the Eyring equation produces a
linear relationship between ln(k/T) and 1/T
q
q
k
T
kB
∆S
R
∆H
ln ) ln
+
-
h
RT
Here, k is the rate constant for the reaction, T is absolute
temperature, kB is Boltzmann’s constant, h is Planck’s constant,
q
q
R is the gas constant, and ∆S and ∆H are activation entropy
and enthalpy, respectively. The slope of the line obtained using
this equation is proportional to the activation enthalpy, while
the activation entropy can be extracted from the intercept (Figure
4). By performing each reaction at a series of four temperatures
and using the empirical rate law to determine the rate constant
at each temperature, activation entropy and enthalpy can be
calculated. The reaction mechanisms, and thus empirical orders,
are not expected to change within the temperature range studied.
This assumption is based on kinetic studies of hydroquinone
oxidation by ferrihydrite that indicated that although the system
becomes more complicated at temperatures above 65 °C, there
is no evidence to support a change in mechanism at temperatures
Figure 5. Plots of (a) activation free energy, (b) activation entropy,
and (c) activation enthalpy as a functions of increasing aluminum
substitution. Lines represent the linear regression. Error bars represent
the standard deviation obtained from the duplicate experiments
performed at four temperatures.
2
0
lower than 60 °C.
bilization of the bonds and allowing the ferrihydrite nanoparticle
to more readily dissolve. The aluminum cation is therefore
serving as a nonreactive species that destabilizes the reactive
sites on the surface because of the slight differences in bond
angles and lengths that would occur as a result of the different
The results of these calculations demonstrate that there is an
q
inverse relationship between the activation free energy (∆G )
and the amount of aluminum present (see Figure 5a). As the
amount of aluminum substitution increases, an increase in the
q
activation entropy (∆S ) (Figure 5b) and a decrease in the
q
ionic radii. The resulting decrease in ∆H combines with the
q
activation enthalpy (∆H ) (Figure 5c) are observed. An increase
q
q
increase in ∆S to cause a decrease in ∆G as a function of
aluminum substitution and enhance the reactivity of ferrihydrite
particles with small amounts of aluminum substitution. This
effect on reactivity is likely to also be applicable to other, less
environmentally relevant, cation substitutions.
q
in ∆S likely arises from the introduction of the aluminum
cation, which serves to greater diversify the transition states
q
available during the reaction. The positive value of ∆S indicates
that the transition state is more disordered than the reactants,
and the increase in magnitude suggests that the transition states
for the particles containing aluminum are relatively more
disordered than those of the pure ferrihydrite particles. This
increase in activation entropy signifies that there is a greater
probability that a collision between reactants will result in the
formation of products. Therefore, the introduction of a small
amount of aluminum, which will only slightly reduce the number
of reactive sites, is expected to greatly influence reactivity. This
trend is consistent with kinetic data presented by Gonzalez,
Ballesteros, and Rueda that showed an increase in fre-
quency factor with increasing aluminum content for substituted
Conclusions
q
The results of this work show an increase in ∆S , a decrease
q
q
in ∆H , a decrease in ∆G , and, therefore, an overall increase
in the reactivity of ferrihydrite particles with increasing
aluminum substitution in the range of 0-2.1 mol %. These
results are consistent with those of other iron oxides with small
11
amounts of substitution (i.e., goethite ). The enhanced reactivity
displayed by aluminum-bearing iron oxides may allow them to
participate in the cycling of other species in the environment
more efficiently than pure iron oxides. These results provide
additional understanding of the solid-state materials present in
groundwater and soil systems. It is expected that as aluminum
content increases further, there will be a level at which the
substitution eliminates a significant number of reactive iron sites,
causing a decrease in the frequency factor and thus a reversal
in the reactivity trend, as has been observed in the goethite
1
1
goethites.
The decrease in ∆Hq with increasing aluminum content
suggests that the aluminum substitution destabilizes the particle
reactant. The substitution of an aluminum cation for an iron
1
cation results in the contraction of the lattice parameter, as
supported by the XRD data. This mismatch of ionic radii would
introduce strain into the crystal lattice, resulting in the desta-

11750 J. Phys. Chem. B, Vol. 110, No. 24, 2006
Jentzsch and Penn
system.11 Future work will focus on increasing the aluminum
content in ferrihydrite samples without substantially changing
size and morphology to determine whether there is a reversal
in reactivity.
(5) Lovley, D. R. Microbiol. ReV. 1991, 55, 259.
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(
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University of Minnesota, and the National Science Foundation
(11) Gonzalez, E.; Ballesteros, M. C.; Rueda, E. H. Clays Clay Miner.
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(
funding. We thank Rick Knurr for the ICP-MS analyses.
(
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