pH Dependence on reduction rate of 4-Cl-nitrobenzene by Fe(II)/montmorillonite systems

Article abstract of DOI:10.1021/es990931e

The pseudo-first-order reduction of 4-Cl-nitrobenzene by Fe(II) in aqueous systems containing montmorillonite clays is investigated over the pH range 6.00-8.00. Silica and alumina is also investigated as simple analogues to aluminosilicate mineral surfaces. At pH 7.25, montmorillonite clays were found to be as much as 1000 times less effective than ferric oxides at mediating the reaction when expressed on a surface area basis. Reaction rates increase dramatically as the pH rises and at pHs above 7.5 approach those previously reported for surface bound Fe(II) on ferric oxides at pH 7.22. This increase in reactivity is attributed to both an increase in concentration of the FeOH⁺ ion and to the increased sorption of Fe(II) at high pH. Sorption isotherms for Fe(II) to montmorillonite clays at pH 7.00 are reported. Two surface sites are suggested on clay minerals and incorporated into a kinetics model for the pH dependence of the reaction. The overall reaction is modeled as the sum of the reactions between 4-Cl-NB and three reductants; FeOH⁺ and Fe(II) bound to the two surface sites. FeOH⁺ is found to be the most effective reductant in our systems. Intrinsic rate constants for both surface sites and FeOH⁺ are presented. Although the minerals investigated are much less effective at mediating the reaction than ferric oxides, the rates are sufficiently fast to be of importance to environmental processes. At neutral pHs, half-lives are less than a week and decrease to the scale of hours above pH 7.5. This is quite rapid in the context of groundwater systems in which residence times can be months or years. The pseudo-first-order reduction of 4-Cl-nitrobenzene by Fe(II) in aqueous systems containing montmorillonite clays is investigated over the pH range 6.00-8.00. Silica and alumina is also investigated as simple analogues to aluminosilicate mineral surfaces. At pH 7.25, montmorillonite clays were found to be as much as 1000 times less effective than ferric oxides at mediating the reaction when expressed on a surface area basis. Reaction rates increase dramatically as the pH rises and at pHs above 7.5 approach those previously reported for surface bound Fe(II) on ferric oxides at pH 7.22. This increase in reactivity is attributed to both an increase in concentration of the FeOH⁺ ion and to the increased sorption of Fe(II) at high pH. Sorption isotherms for Fe(II) to montmorillonite clays at pH 7.00 are reported. Two surface sites are suggested on clay minerals and incorporated into a kinetics model for the pH dependence of the reaction. The overall reaction is modeled as the sum of the reactions between 4-Cl-NB and three reductants; FeOH⁺ and Fe(II) bound to the two surface sites. FeOH⁺ is found to be the most effective reductant in our systems. Intrinsic rate constants for both surface sites and FeOH⁺ are presented. Although the minerals investigated are much less effective at mediating the reaction than ferric oxides, the rates are sufficiently fast to be of importance to environmental processes. At neutral pHs, half-lives are less than a week and decrease to the scale of hours above pH 7.5. This is quite rapid in the context of groundwater systems in which residence times can be months or years. Reduction of 4-chloronitrobenzene (CNB) by divalent iron was studied in aqueous systems containing montmorillonite clays over the 6-8 pH range. The clays proved as much as 1000 times less effective than ferric oxides at pH 7.25 at mediating the reaction. Reaction rates increased significantly as pH increased, and sorption isotherms for Fe(II) to montmorillonite clays at pH 7 are reported. The silica and alumina minerals studied were much less effective at mediating the reaction than ferric oxides but the rates were sufficiently fast to be of importance to environmental processes.

Full text of DOI:10.1021/es990931e

Environ. Sci. Technol. 2000, 34, 3641-3648
in the environment. Proposals for the remediation of NACs
pH Dependence on Reduction Rate
of 4-Cl-Nitrobenzene by Fe(II)/
Montmorillonite Systems
include phytoremediation, reaction with solid reductants,
and reaction with aqueous reducing agents. Schnoor (4) has
proposed that TNT and other explosives can be remediated
by hybrid poplar trees, and other researchers have developed
a tobacco plant that degrades TNT (5). There are several
methods of reducing NACs to their aniline analogues by
reaction with zerovalent iron (e.g. ref 6). A body of work from
the Schwarzenbach group (7-10) has investigated the
reaction between NACs and various forms of ferrous iron
complexed with both organic ligands and mineral surfaces.
This method of (P)NAC degradation has the advantage of
using a mobile reductant (aqueous Fe(II)) which can be
introduced at a distance from the potentially explosive
contamination site.
C H R I S T O P H E R A . S C H U L T Z A N D
T I M O T H Y J . G R U N D L *
Geosciences Department and Great Lakes WATER Institute,
University of WisconsinsMilwaukee,
Milwaukee, Wisconsin 53211
The pseudo-first-order reduction of 4-Cl-nitrobenzene by
Fe(II) in aqueous systems containing montmorillonite clays
is investigated over the pH range 6.00-8.00. Silica and
alumina is also investigated as simple analogues to
aluminosilicate mineral surfaces. At pH 7.25, montmorillonite
clays were found to be as much as 1000 times less
effective than ferric oxides at mediating the reaction
when expressed on a surface area basis. Reaction rates
increase dramatically as the pH rises and at pHs above 7.5
approach those previously reported for surface bound Fe-
This body of work has established that Fe(II) can reduce
many NACs if complexed to iron oxide minerals at near
neutral pH conditions. Reaction rates are largely independent
of the iron oxide mineral itself indicating that the reactivity
is due to the formation of an inner-sphere complex between
the generic iron oxide surface and Fe(II) (9, 11, 12). This can
be readily understood in terms of electron density being
-
donated to the Fe(II) atom by the adjoining OH ligands (13,
1
4). Complexed Fe(II) is thereby provided with extra reducing
power.
Aqueous phase complexing with hydroxyl ligands (i.e.
(II) on ferric oxides at pH 7.22. This increase in reactivity
hydrolysis of Fe2 to FeOH ) should also provide an increase
in reducing power. This can be understood in terms of a
solution phase complex between an electron-donating ligand
+
+
is attributed to both an increase in concentration of the
+
FeOH ion and to the increased sorption of Fe(II) at
high pH. Sorption isotherms for Fe(II) to montmorillonite
clays at pH 7.00 are reported. Two surface sites are suggested
on clay minerals and incorporated into a kinetics model
for the pH dependence of the reaction. The overall reaction
is modeled as the sum of the reactions between 4-Cl-NB
-
2+
(
OH ) and a metal ion (Fe ) donating electron density to
the complex and stabilizing the oxidized state of the metal
(13, 14). Both processes make the complex a more powerful
2
+
reductant than the metal ion alone. In the case of Fe
hydroxide donates electron density through both the σ and
π orbitals (14), and this results in the reduction rate of O by
,
+
and three reductants; FeOH and Fe(II) bound to the two
2
+
6
2+
+
2
FeOH that is 10 times faster than O reduction by Fe (15).
surface sites. FeOH is found to be the most effective
The extra reducing power of an Fe(II) ) goethite surface
reductant in our systems. Intrinsic rate constants for both
+
complex and FeOH is essentially equivalent as indicated by
+
surface sites and FeOH are presented. Although the
the nearly identical calculated one electron redox potential
minerals investigated are much less effective at mediating
the reaction than ferric oxides, the rates are sufficiently
fast to be of importance to environmental processes. At
neutral pHs, half-lives are less than a week and decrease
to the scale of hours above pH 7.5. This is quite rapid in the
context of groundwater systems in which residence
times can be months or years.
at pH 7 (E
A similar effect is observed for Fe(II) in iron-rich silicate
minerals. The oxidation by O of structural Fe(II) at the surface
h
°) of 0.36 and 0.34 V, respectively (13, 15).
2
of augite and hornblende is faster than the equivalent
oxidation of Fe(II) in solution (16-18). White and Yee (16)
h
estimate the E ° of these minerals to be within the range
0.33-0.52 V. The redox lability of iron contained in smectite
clay minerals is also well established. Iron rich montmoril-
lonites are rapidly and reversibly reduced by chemical
reductants (19) and by bacterial action (20, 21). Bacteria are
also capable of reducing the ferric iron contained in iron
oxide minerals (22).
Work by Schwarzenbach (23) and Stumm (13, 24) de-
termined the single electron reduction potentials for a
number of ferrous iron reducing agents and for chlorinated
and nitrated pollutants. It has been shown that (P)NACs are
reduced by iron porphyrin, ferrous iron complexes of quinone
Introduction
Nitroaromatic (NAC) and polynitroaromatic (PNAC) com-
pounds are of interest to environmental scientists because
they are dangerous and widely distributed in the environment
(
1, 2). NACs are used by agriculture as herbicides and
insecticides, by industry as chemical feedstock and dyes,
and by the military as explosives (1, 3). The extensive use of
these compounds has resulted in the intentional and
accidental introduction into the environment, both due to
their mode of use and as a result of improper disposal
methods. Concern about the NACs as environmental con-
taminants has led to extensive and varied approaches to the
remediation of these materials.
(
7), and Streptomyces sp. exudates under iron reducing
conditions (25). Ferrous iron has also been shown to reduce
NACs in the presence of iron oxides (8, 9, 26). The overall
reduction reaction consists of a six-electron reduction of the
nitroaromatic to the corresponding aminoaromatic through
two stable intermediates, the nitroso- and hydroxylamino-
analogues (Figure 1a). The reaction of aqueous ferrous iron
itself (Fe2 ) with NACs is not thermodynamically favorable;
the ferrous iron must form an inner-sphere complex that is
capable of transferring the first electron. The iron complexed
A considerable body of literature has developed concern-
ing various methods of degrading this class of compounds
+
*
Corresponding author phone: (414)229-4765; fax: (414)229-5452;
e-mail: grundl@uwm.edu.
1
0.1021/es990931e CCC: $19.00

2000 Am erican Chem ical Society
VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 6 4 1
Published on Web 07/26/2000

FIGURE 1. (a) Overall reduction reaction of 4-Cl-nitrobenzene to 4-Cl-aniline. (b) Proposed mechanistic pathway for the first reductive
step of 4-Cl-nitrobenzene to 4-Cl-nitrosobenzene.
to the radical intermediate may stabilize it, further favoring
the reaction (Figure 1b).
filtered through a 0.22 µm filter into concentrated hydro-
chloric acid (1 mL). This procedure eliminated any trace
colloids potentially present in the ferrous chloride tetrahy-
drate solid. Subsequent dilutions of this acidic stock solution
to more basic experimental vials were done in the anaerobic
chamber.
The present study is concerned with determining if
montmorillonite clay minerals are capable of mediating the
reduction of NACs to their aniline analogues by forming
similar complexes with ferrous iron. Montmorillonite clays
2
exhibit very large surface areas (approximately 700 m / g)
Analytical Procedures. 4-Cl-NB and 4-Cl-An were ana-
lyzed using the method of Klausen (8, 26). Analysis was
performed by HPLC using an Econosphere C-18 3U column
(Alltech Associates, Deerfield, IL), Waters model 501 HPLC
Pump (Waters Inc., Milford, MA), and Gilson model 112 UV/
visible detection at a wavelength of 254 nm (Gilson, Middle-
ton, WI). The mobile phase was 10 mM, pH 6 hydroxylamine
hydrochloride in methanol/ water, 3/ 2 v/ v. Aqueous ferrous
iron concentrations were measured spectrophotometrically
using ferrozine (27, 28) at a wavelength of 562 nm on a
Beckman model DU 7400 spectrophotometer (Beckman,
Fullerton, CA). All experiments were buffered using sulfonic
acid buffers. The buffers and pH ranges used are as follows:
MES, pH 6.00-6.50; MOPS, pH 6.75-7.50; and EPPS, 7.75-
8.00. All procedures were carried out in an anaerobic chamber
and a correspondingly large cation exchange capacity (80-
1
50 mequiv/ 100 g) (34). Montmorillonites are ubiquitous,
forming from weathering of primary silicate minerals under
neutral to alkaline conditions. Montmorillonites have several
attributes that make them desirable mediators for this
reaction; they have a strong ability to form surface complexes
with metals, and, in addition, iron within the lattice may
facilitate and promote the reaction. Silica gel and alumina
were also examined, because the surface of a clay consists
of individually exposed silicon and aluminum hydroxides,
thus these phases are the simplest analogues to the surface
of a clay. Silica and alumina can also be considered simple
analogues for silicate and aluminosilicate minerals that, in
general, form the bulk of the solid phases encountered in the
environment. This study has measured the rate of reaction
for the reduction of NACs by ferrous iron when mediated by
montmorillonite clays and has developed a mechanistic and
kinetic model for the reaction.
2 2
under an atmosphere of H / N (10:90) with a palladium
catalyst scrubber. All experiments were carried out in the
dark, and the temperature was maintained at 23 ( 2 °C.
Measurem ent of Ferrous Iron K . A 25 mL borosilicate
d
glass vial was loaded with 240 mg of mineral, 22 mL of
deoxygenated water, and 2 mL of buffer. Sufficient ferrous
chloride (1.0 M stabilized with 0.1 M HCl) was added to obtain
a desired initial concentration. The pH was checked and
adjusted with sodium hydroxide (0.1 M). The volume added
Experimental Section
Chem icals. 4-Chloronitrobenzene (4-Cl-NB); 4-chloroaniline
(
(
4-Cl-An); N-2-hydroxyethylpiperazinepropanesulfonic acid
EPPS, pK 8.00); ferrous chloride tetrahydrate; and ferrozine
a
were purchased from Aldrich Chemical (Milwaukee, WI).
2
was generally 110% of the volume of FeCl added.
3
2
pK
-(N-morpholino)propanesulfonic acid (MOPS, pK
-(N-morpholino)ethanesulfonic acid monohydrate (MES,
6.15); silica gel; alumina; and hydroxylamine hydro-
a
7.20);
Samples were shaken overnight on a shaker table. An
aliquot was removed, and ferrous chloride (1.0 M) and sodium
hydroxide (0.1 M) were added to obtain a new ferrous iron
concentration. The oxide minerals settled out of suspension
rapidly, and the supernatant (100 µL) was added directly to
ferrozine (1 mL) for analysis. The clays remained in suspen-
sion and required centrifugation for 15 min prior to ferrozine
analysis.
Preparation of Kinetics Experim ents. 4-Cl-NB was
chosen as a representative nitroaromatic compound. The
kinetics of this reaction were measured using batch experi-
ments for each mineral and the blank (no minerals added)
at pHs from 6 to 8. Borosilicate glass serum bottles were
a
chloride were purchased from Fisher Scientific (Pittsburgh,
PA). Gas was obtained from Praxair (Oak Brook, IL). All
chemicals were analytical grade and were used without
further purification. SWa-1 nontronite clay was obtained from
the Clay Minerals Society, and Wyoming montmorillonite
was obtained commercially. Both clay minerals were Na-
saturated prior to use. Ferrous chloride solution (1.0 M) was
prepared using a modified procedure of Klausen (8), wherein
ferrous chloride tetrahydrate solid was transferred to an
anaerobic hood, dissolved in deaerated water (10 mL), and
3
6 4 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000

loaded with dry mineral (10 g/ L), transferred into an
anaerobic hood where deaerated water and the buffer were
added, sealed with a black butyl-rubber stopper, and shaken
overnight to suspend and hydrate the mineral. Sulfonic acid-
based buffer (1.0 M), FeCl
2
(1.0 M), and NaOH (0.1 M) were
used to provide the desired final concentrations (buffer 50
2
+
mM and Fe 500 µM after sorption to the mineral) and pH.
The addition of FeCl and NaOH were made in smaller
quantities over several days to prevent the precipitation of
Fe(OH) (s). The final solution was shaken overnight to ensure
2
2
sorptive equilibrium. pH levels remained unchanged through-
out the course of the experiment.
To start a kinetics experiment, 4-Cl-NB (0.1 M in methanol)
was added by syringe to yield an initial concentration of 50
µM. The experiment was periodically sampled by syringe for
measurement of the ferrous iron concentration and analysis
of the organics. The reaction was quenched by adding 100
µL of hydroxylamine (2.16 M, pH 6) to 1 mL of solution. At
high pH clay experiments, the use of hydroxylamine was
ineffectual. In these cases, the experimental solution was
sampled, centrifuged, and directly injected into the HPLC.
The time assigned to these samples was the time of injection.
FIGURE 2. Sorption isotherm for ferrous iron to SWa-1montmoril-
lonite at pH 7.00. Solution was 50 mM MOPS buffer and 10 g/L clay.
Results
Sorption Experim ents. In our study, it was discovered that
the sorption of ferrous iron to clay minerals was significant.
Despite the recognized importance of ferrous iron to
environmental redox processes in terms of its ability to
h
transfer electrons and poise the E of the system, we have
found only two papers which explicitly address the sorption
of ferrous iron to mineral surfaces (12, 29). Haderlein and
Pecher (12) report minimal sorption of ferrous iron onto
d
several ferric oxides (K s from 0.25 to 2.3 at pH 7.25).
To have a constant aqueous Fe(II) concentration at the
start of each kinetics experiment, the sorption of Fe(II) to
each mineral over the entire pH range had to be determined.
Sorption isotherms for ferrous iron in a background of
sulfonic acid buffer (50 mM), sodium, and chloride were
determined for each of the minerals over the pH range of
FIGURE 3. Amount of ferrous iron bound to SWa-1montmorillonite
when solution contains 500 µM ferrous iron. Amount of iron on
surface site 1 (S1) is measured below pH 6.75. Above pH 6.75, S1
is set to the average of the values from 6.00 to 6.75. Amount of iron
on surface site 2 (S2) is the difference between the measured value
and the value assigned to S1.
6
-8. Sodium originated from the clay as iron was exchanged
to the surface, and chloride came from the ferrous chloride
solution, which was stabilized using hydrochloric acid (0.1
M). The ionic strength of the various experiments ranged
from 9 to 35 mM at the beginning and increased to a range
of 12-50 mM at the end of the experiment; the ranges result
from differences in the loading of the iron and the extent of
ionization of the buffer. These measurements were performed
in batch experiments by successive additions of ferrous iron
followed by equilibration for 24 h. The additions of Fe(II)
were made in increments small enough to prevent precipi-
to a change in the manner in which iron is bound to the
surface. It was assumed that the consistent extent of sorption
from pH 6.00 to 6.75 was due to a cation exchange type
process on the clays and that the concentrations of these
exchange sites remained unchanged at all pHs. The increase
in sorption at pHs above 6.75 was assumed to be from a
second set of sorption sites. The actual nature of these sites
has not been determined. Although the concentration of Fe-
(II) in solution is always well below the nominal solubility
2
+
-
tation of ferrous hydroxide (Fe(OH)
2
(s) ) Fe + 2OH , log
2
of Fe(OH) (less than one-quarter of the calculated solubility
Ksp ) -14.5 (30)). These experiments were performed for the
at pH 8 and well below that at all other pHs measured),
precipitation in microenvironments or the formation of mixed
valent precipitates cannot be ruled out particularly at high
pHs (12, 31).
purpose of determining the quantity of ferrous chloride which
must be added to obtain a specific concentration of Fe(II)
in the water (500 µM); therefore, no competition or revers-
ibility experiments were run. An example isotherm for the
sorption of ferrous iron to SWa-1 at pH 7.00 is presented in
Figure 2. The data follow a Freundlich isotherm with a
distribution coefficient of 21.2 and an exponent of 0.38 (Figure
Regardless of the type of surface sites involved, the
geochemical implication of this sorption data is that in
reduced sediments, especially those containing more than
a few percent clay, the pool of sorbed, redox-labile ferrous
iron can be vast. Our data indicates that at pHs as low as
6.00, the concentration of ferrous iron sorbed to clays is
considerably higher than the concentration of iron in solution.
Therefore the poising capacity of similar sediment systems
can be large, and the addition of an oxidizing agent may
have less effect on the redox potential than would be expected
from the redox species that are measurable in the aqueous
phase.
2
). Wyoming montmorillonite behaves in a similar manner
with a distribution coefficient of 36.7 and an exponent of
.33 (data not shown). A more complete study of Fe(II)
0
sorption to clays is currently underway.
From these experiments the amount of Fe(II) needed to
obtain an aqueous concentration of 500 µM was determined
(
Figure 3). Figure 3 indicates that the necessary iron loading
was constant up to pH 6.75 and then rose rapidly. It was also
noted that the color of the samples changed from tan to
green over the pH range from 6 to 7 and to dark green at pH
For the purposes of kinetics modeling, the sorbed iron on
clays was interpreted to exist on two independent sorption
sites of surface bound iron, S1 and S2 (Figure 3). The S1 site
8
as the samples were loaded. These changes were attributed
VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 6 4 3

because 4-Cl-NOB is probably rapidly reduced to 4-Cl-HAB.
The pseudo-first-order nature (with respect to NB) of this
TABLE 1. Concentration of Surface Bound Ferrous Iron
µmol/g) for S1 and S2 over the pH Range 6-8
(
o
reaction is evident from the plot of ln(C/ C ) versus time
(
Figure 4b). The kobs values for the blank (no added minerals)
pH
6.00
6.50
7.00
7.50
8.00
and for each mineral are listed in Table 2at every half pH
unit from 6 to 8.
Mechanistic Model. Previous studies of this reaction have
reported that the rate of reaction in the absence of a mediating
surface at neutral conditions is slow and not of interest (8,
Surface Site 1 (S1)
SWa-1
m ontm orillonite
silica
175
158
4
164
149
7
169
156
16
169
156
31
169
156
120
N/Aa
alum ina
1
11
20
69
9
, 26). The present study is concerned with longer time
Surface Site 2 (S2)
periods, and as a result the blank was found to have significant
reactivity, especially at higher pHs. The reactivity of Fe(II) in
the absence of a mediating surface was found to be very low
at pHs below 7 but became significant above pH 7 (Figure
SWa-1
m ontm orillonite
59
231
594
681
674
112
a
N/A m eans not available.
5
).
The reaction rate for the blank is modeled by attributing
+
the reaction to reduction by FeOH . The E
h
° for this species
is 0.34 V SHE, similar to that of surface bound Fe(II) species
(E ° ) 0.36 V (bidentate) or 0.34 V (monodentate) vs the
h
standard hydrogen electrode (SHE)) (13, 16), but it has been
ignored in the past because of its low concentration at neutral
+
pH. At pH 7, the concentration of FeOH represents only
0
.08% of the total concentration of Fe(II) in solution (32).
The first electron transfer, the rate-limiting step, for the
+
reaction between FeOH and 4-Cl-NB is shown in eq 1.
+
-
rate ) kFeOH+ * [FeOH ] * [4-Cl-NB] * [OH ]
(1)
FIGURE 4. (a) Reduction of 50 µM 4-Cl-NB (∆), in growth of the
intermediate 4-Cl-HAB ()) and the final product, 4-Cl-An (0) as
mediated by SWa-1at pH 7.75. Mass balance is indicated by the
sum of all three species (- - -). (b) Natural log plot of the 4-Cl-NB
data contained in (a). Solid line is the linear regression through the
data.
The best-fit value for kFeOH+ is 6.3 × 10 min -1 M-3 and was
obtained from experiments conducted with no minerals
present (Figure 5).
+13
Ferric oxyhydroxide precipitates are formed during the
course of this reaction, and these precipitates are a source
of mediating surfaces that could potentially control the
reaction. If ferric precipitates controlled the rate of reaction,
the expected kinetic behavior in any given experiment would
be an ever-increasing rate over time as the concentration of
ferric precipitates increased. In contrast to this, the observed
behavior is pseudo-first-order in ferrous iron for many half-
lives (Figure 4b) as is expected for a homogeneous reaction
is the pool of iron sorbed predominately on exchange sites
which is evident below pH 6.75. These sites are considered
to remain active within the larger pool of iron sorbed at pHs
above 6.75. The extent of sorption attributed to S2 sites at
pHs above 6.75 is the difference between iron sorbed to S1
sites (the average of the loadings from pH 6 to 6.75) and total
sorption. Sorption of ferrous iron to the oxide minerals did
not exhibit any distinction between sites 1 and 2; therefore,
oxide minerals are modeled with only one surface site (listed
as S1). The values for the sorption of ferrous iron attributed
to S1 and S2 sites (on clays) are listed in Table 1for each
mineral from pH 6 to 8.
+
with FeOH
pattern of rate increases is observed as the pH moves from
6 to 8 that exactly correlates to the increase in FeOH . Both
as the reductant. Furthermore, a characteristic
+
of these observations indicate that ferric precipitates do not
control the reactivity in the blank experiments.
Aqueous complexes formed between ferrous iron and
(aq)) are also known to be quite reactive
in the oxidation of ferrous iron by oxygen (38), and a similar
reactivity might occur in the oxidation of NB. Our experiments
2
were carbonate free and were carried out under a strict N /
3
(aq) was present to serve
as a control on the reactivity of the system.
The observed reaction rates for surface-mediated reac-
tions are the result of three competing reactions using FeOH
0
Kinetics Experim ents. The measurement of the kinetics
of 4-Cl-NB reduction was performed in batch experiments
by reaction of 4-Cl-NB with Fe(II) (500 µM) in the presence
of each mineral every quarter pH unit for the blank and the
clays (SWa-1 and Wyoming montmorillonite) and every half
pH unit for silica gel and alumina from pH 6 to 8. The
concentrations of 4-Cl-NB, 4-Cl-An, and one of the inter-
mediates were measured and plotted versus time (Figure
carbonate (FeCO
3
0
H
2
atmosphere; therefore, no FeCO
+
4
a). The presence of the intermediate was based on the
and the two types of surface bound iron, S1 and S2, as
discussed previously. The equations for the reactions between
the surface species and 4-Cl-NB along with the associated
rate laws are given in eqs 2 and 3.
measurement of an HPLC peak that grew in early in the
experiment and subsequently decayed to zero. The com-
pound was not identified but was presumed to be 4-Cl-HAB
3
6 4 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000

TABLE 2. Observed Pseudo-First-Order Rate Constants (1/min) for Each Mineral at Each pHa
pH
6.00
6.50
7.00
7.50
8.00
blank
Swa-1
m ontm orillonite
silica
alum ina
6.31 ((6.33) E-6
1.94 ((2.34) E-6
7.71 ((3.93) E-6
3.10 ((0.94) E-6
1.17 ((1.27) E-6
9.87 ((9.19) E-6
9.58 ((2.28) E-6
7.63 ((8.61) E-6
8.43 ((0.74) E-6
7.19 ((0.62) E-6
6.07 ((0.64) E-4
1.06 ((0.17) E-4
2.70 ((0.36) E-4
1.86 ((0.22) E-4
2.24 ((0.08) E-4
9.20 ((0.76) E-4
4.59 ((0.68) E-3
1.45 ((0.15) E-3
6.57 ((0.80) E-3
2.01 ((0.11) E-3
1.24 ((0.12) E-2
2.16 ((0.20) E-2
1.35 ((0.22) E-2
3.00 ((0.21) E-2
N/Ab
a
Errors are (1 SD. b N/A m eans not available.
FIGURE 5. pH dependence of observed rate of 4-Cl-NB reduction by ferrous iron in the absence of a mediating mineral. Solid squares
are the measured data. Error ((1 SD) is within the size of the data symbol, except where shown. Solid line is the modeled fit to the data.
Inset is the natural log plot of the same data.
2
+
-
4
-Cl-NB + Fe(S1) + H O + 2OH f intermediate
2
TABLE 3. Intrinsic Rate Constants as Obtained from Kinetics
Modeling
rateFe(S1) ) kFe(S1) * [4-Cl-NB] * [Fe(S1) ] * [OH-]2 (2)
2
+
kFeOH+a
kFe(S1)b
kFe(S2)b
2
+
-
4
-Cl-NB + Fe(S2) + H O + 2OH f intermediate
blank
SWa-1
m ontm orillonite
silica
alum ina
6.3E+13
6.3E+13
6.3E+13
6.3E+13
6.3E+13
2
8.0E+17
2.0E+17
3.3E+17
3.4E+17
8.0E+16
2.0E+16
rateFe(S2) ) kFe(S2) * [4-Cl-NB] * [Fe(S2) +] * [OH-]2 (3)
2
The overall observed rate law for surface-mediated reactions
can be written as the sum of the rate laws of the three
reductants, FeOH , Fe(S1), and Fe(S2):
a
3
/(m in*M ). b 1/(m in*m ol/g*M ).
3
1
+
reaction did not proceed. This condition is unique to clays
and cannot be achieved with ferric oxides because they have
a very low affinity for ferrous iron (12). This lack of reactivity
in the presence of surface bound Fe(II) leads to two
conclusions: first, the second electron of the first reduction,
nitro- to nitroso- (Figure 1b), came from the solution, hence
the need for a consistent concentration of aqueous ferrous
iron, and second, the organic radical formed in the first
electron transfer is not mobile on the mineral surface,
otherwise it would be capable of obtaining the second
electron from a second surface-bound iron.
+
+
-
rate ) kFeOH * [FeOH ] * [NB] * [OH ] + k
* [Fe
Fe(S1)
-
2
* [Fe(S2)] * [NB] * [OH-]2
(
S1)] * [NB] * [OH ] + k
Fe(S2)
(
4)
The intrinsic rate constants kFe(S1) and kFe(S2) can be deter-
mined by fitting eq 4 to the observed reaction rates for each
mineral. Note that the model for the overall kinetics is fit to
experimental data with only two degrees of freedom, kFe(S1)
and kFe(S2). The resulting values of the intrinsic rate constants
are listed in Table 3for both Swa-1 and Wyoming montmo-
rillonite. Figure 6is the graphical representation of eqs 1
through 4 for SWa-1. The logarithmic inset in Figure 6
illustrates that the model fits the data over the entire pH
range studied. A comparable plot is obtained for Wyoming
montmorillonite (data not shown).
Discussion
The overall rates of reaction obtained in the present study
can be compared to the overall rates obtained on ferric oxides
by previous workers. After adjusting to a constant ferrous
iron concentration, the overall rate of reduction reported by
Hoffstetter (9) on goethite at pH 7.22 is equal to the rates
seen on our minerals at pH levels between 7.4 and 7.8. At
these high pHs, the primary reductant in our systems is
Further insights into this reaction on clay minerals were
obtained from kinetic experiments performed on flasks early
in the loading process. In these experiments, essentially all
of the ferrous iron added to the system was sorbed, and the
+
FeOH . It seems clear that ferrous)ferric oxide sorption
VOL. 34, NO. 17, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
3 6 4 5

FIGURE 6. pH dependence of observed rate of 4-Cl-NB reduction by ferrous iron mediated by SWa-1montmorillonite. Solid squares are
the measured data. Error ((1 SD) is within the size of the data symbol, except where shown. Modeled reaction rates of the three reductants
+
FeOH ()), Fe(S1) (0), and Fe(S2) (∆) are plotted. The dashed line is the sum of these three rates and represents the modeled fit to the
overall reaction. Inset is the natural log plot of the measured data and the modeled fit to the overall reaction.
estimated to constitute anywhere from 1% to 10% of the
TABLE 4. Observed Rate Constants at pH 7.25 (kobs), Surface
total surface area (34, 35). For the present calculations, it
Areas, and Observed Rate Constants Normalized to Mineral
was assumed that 5% of the total surface area of the clays
sa a
Surface Area (kobs
)
2
consists of edges. The total surface area was set to 720 m / g
2
kobs
1/min)
surface
area (m /g)
kobssa
(1/min*m )
(36) and 800 m / g (37) for SWa-1 and Wyoming montmo-
2
2
(
rillonite, respectively. As can be seen in Table 4, the clay
minerals as well as silica and alumina exhibit a reactivity,
when expressed on a surface area basis, that is in stark
contrast to goethite. Goethite exhibits a reactivity that is as
much as 1000 times as large. Ferrous iron that is sorbed to
the edges of the clays as well as to silica and alumina forms
ferrous)alumina oxide or ferrous)silica oxide centers, and
these centers are much less effective at mediating the
reduction reaction.
goethite
SWa-1
m ontm orillonite
silica
4.40E-02
6.21E-04
4.88E-04
9.71E-04
7.55E-04
17.5
35.5
40.0
450
3.93E-04
1.75E-06
1.22E-06
2.16E-07
4.87E-07
alum ina
155
a
Surface areas for clays represent edge areas.
complexes on the surface of ferric oxides are analogous to
A further observation that supports the conclusion that
ferrous)ferric oxide centers are an important surface reactant
is that both intrinsic rate constants (kFe(S1) and kFe(S2)) for the
reaction on SWa-1 are four times faster than for the reaction
of Wyoming montmorillonite (Table 3). SWa-1 is an iron rich
montmorillonite (a nontronite) that has ferric iron in 3 out
of 4 octahedral sites, whereas Wyoming montmorillonite
contains only one-fifth as much octahedral iron. It is possible
that the difference in the intrinsic rate constants is due to
the relative paucity of iron centers on the edge sites of
Wyoming montmorillonite. It is also possible that the
additional structural ferric iron contained in SWa-1 promotes
the precipitation of a highly reactive, mixed valent “green
rust” at the surface of the clay.
+
FeOH complexes in the aqueous phase. This idea is
+
supported by the near equivalence in the E
h
°s of FeOH and
ferrous)ferric oxide centers (13, 15). Ferrous)alumina oxide
or ferrous)silica oxide centers are much less effective at
mediating this reaction. A similar phenomenon is seen in
the mineral mediation of the reduction of Cr(VI) by aqueous
Fe(II). Ferrous)ferric oxide centers (supplied by R- and
γ-FeOOH) increase the rate of Cr(VI) reduction by factors of
2
0-40, respectively, whereas ferrous)silica centers (supplied
by SiO and iron deficient montmorillonite) increase the rate
by factors of 1.2-4, respectively (39). Ferrous)alumina
centers (supplied by Al ) increased the rate by a factor of
(39).
The difference in the behavior between ferric oxides and
2
2
O
3
2
the oxides of aluminum and silicon in our system is clearly
seen by comparison of the observed overall rates on a surface
area basis. Table 4lists the overall rate constants for each
mineral at pH 7.25 divided by the respective surface areas.
Goethite data were extracted from information contained in
Hoffstetter (9). Surface areas for alumina and silica were
specified by the supplier. Basal plane surfaces, which account
for the vast majority of the total surface area, do not have
a surface charge sufficient to dehydrate Fe(II) ions. Interlayer
ions that remain hydrated occur close to the central plane
of the interlayer space and are not inner spherically com-
plexed to the surface (33). As a result, edge surfaces are the
only place where ferrous ions inner spherically sorb to the
surface of a clay and therefore are the active surfaces on a
clay mineral. Edge surface areas of smectite clays have been
Although the minerals investigated in this study are much
less effective at mediating the reduction of 4-Cl-NB than
ferric oxides, the rates of reaction are sufficiently fast to be
of importance to environmental processes. Extrapolation of
our results to other nitro-compounds can be accomplished
by reference to previously defined relationships between
h
degradation rate and the E ° (8, 9). Most NACs and (P)NACs
react more rapidly than 4-Cl-NB, and no previously measured
compound reacts more slowly than 1/ 4 of the 4-Cl-NB rate.
In our systems, at neutral pHs and at environmentally
reasonable Fe(II) concentrations (500 µM), measured 4-Cl-
NB half-lives are less than a week. This is quite rapid in the
context of groundwater systems in which residence times
can be months to years. Furthermore, a rise in pH to 7.5 or
more will decrease half-lives to the scale of hours. This same
3
6 4 6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 17, 2000

rise in pH also increases the relative importance of the
aqueous reactant FeOH with respect to heterogeneous
reductants.
Compounds; Hutzinger, O., Ed.; Springer-Verlag: Berlin, 1991;
Part G, pp 195-237.
+
(
4) Thompson, P. L.; Ramer, L. A.; Schnoor J. L. Environ. Sci. Technol.
1
998, 32, 975-980.
A comparison can be made between the reactivity of Fe-
(
(
5) Fountain, H. New York Times May 18, 1999.
(
II) and NB and the reactivity with other common oxidants.
6) Agrawal, A.; Tratnyek, P. G. Environ. Sci. Technol. 1996, 30, 153-
Kinetic studies exist for the reduction by Fe(II) of Cr(VI) and
molecular oxygen (39, 40-43). A direct comparison of the
intrinsic reaction between all three oxidants can be made by
using published intrinsic rate constants (40, 43) and setting
160.
(7) Schwarzenbach, R. P.; Stierli, R.; Lanz, K.; Zeyer, J. Environ. Sci.
Technol. 1990, 24, 1566-1574.
8) Klausen, J.; Tr o¨ ber, S. P.; Haderlein, S. B.; Schwarzenbach, R.
P. Environ. Sci. Technol. 1995, 29, 2396-2404.
(
2
the pH to 7.2, the concentration of oxidant (NB, Cr(VI), O )
(
9) Hoffstetter, T. B.; Heijman, C. G.; Haderlein, S. B.; Holliger, C.;
Schwarzenbach, R. P. Environ. Sci. Technol. 1999, 33, 1479-
to 50 µM, and the concentration of Fe(II) to 500 µM. Under
these conditions, the relative rates of reaction (rate )
1
487.
-
d[oxidant]/ dt) are 1:8.4:0.1 for NB:Cr(VI):O
2
, respectively.
(10) Heijman, C. G.; Grieder, E.; Holliger, C.; Schwarzenbach, R. P.
These rates show that NB is intermediate in reactivity between
oxygen and Cr(VI); however, this applies only to the specified
conditions and is not directly applicable to natural environ-
ments where pH and ionic strength variations, complexing
ligands, mineral surfaces, and bacteria are present. Changes
in pH, ionic strength, and the presence of Fe(II)-ligand
complexes are all known to affect reaction rates (30, 38, 41,
Environ. Sci. Technol. 1995, 29, 775-783.
(
(
11) R u¨ gge, K.; Hofstetter, T. B.; Haderlein, S. B.; Bjerg, P. L.; Knudsen,
S.; Zraunig, C.; Mosbæk, H.; Christensen, T. H. Environ. Sci.
Technol. 1998, 32, 23-31.
12) Haderlein, S. B.; Pecher, K. In Mineral-Water Interfacial Reac-
tions; Sparks, D. L., Grundl, T. J., Eds.; American Chemical
Society: Washington, DC, 1998; Chapter 17.
(13) Stumm, W. Chemistry of the Solid-Water Interface; John Wiley
Sons: New York, 1992; Chapter 9.
&
4
2, 44-49). Mineral surfaces also mediate the oxidation of
(
14) Luther, G. W. III. In Aquatic Chemical Kinetics; Stumm, W., Ed.;
John Wiley & Sons: New York, 1990; Chapter 6.
15) Wehrli, B. In Aquatic Chemical Kinetics; Stumm, W., Ed.; John
Wiley & Sons: New York, 1990; Chapter 11.
(16) White, A. F.; Yee, A. Geochim. Cosmochim. Acta 1985, 49, 1263-
1275.
(17) White, A. F. Rev. Minerology 1990, 23, 467-509.
18) White, A. F. In Mineral-Water Interface Geochemistry, Reviews
in Mineralogy, Volume 23; Hochella, M. E., Jr., White, A. F., Eds.;
Mineralogical Society of America: Washington, DC, 1990;
Chapter 12.
19) Komadel, P.; Madejova, J.; Stucki, J. W. Clays Clay Miner. 1995,
43, 105-110.
of Fe(II) (2, 12, 39), and bacterial action can accelerate Fe(II)
oxidation by several orders of magnitude (50).
(
The ability to reduce nitrated pollutants without the need
for solid-phase reductants may be useful to the design of
engineered or in-situ remediation systems. The primary
2
+
conditions that need to be achieved for Fe mediated
reduction to proceed are anaerobiosis, the presence of
(
2
+
aqueous Fe , and either ferrous)ferric oxide centers (het-
erogeneous reaction) or pH levels of 7.5-8.0 (homogeneous
+
reaction). The use of a soluble, mobile reductant (FeOH ) in
(
the homogeneous case would allow in-situ remediation to
proceed even with highly sorbed, immobile contaminants.
This may prove to be an advantage over the heterogeneous
case in which the contaminant must move to the site of
reaction.
(20) Stucki, J. W.; Komadel, P.; Wilkinson, H. T. Soil Sci. Soc. Am. J.
987, 51, 1663-1665.
1
(
21) Kostka, J. E.; Stucki, J. W.; Nealson, K. H.; Wu, J. Clays Clay
Miner. 1996, 44, 522-529.
22) Kostka, J. E.; Nealson, K. H. Environ. Sci. Technol. 1995, 29,
(
The fully reduced (poly)aromatic anilines that result from
Fe2 mediated pathways are carcinogenic compounds. These
anilines are stable under reducing conditions (11); however,
under aerobic conditions they are subject to a variety of
oxidative, sorptive, and polymerization reactions that either
degrade or immobilize the compound (2). The need for
aerobic conditions to fully degrade or immobilize the aniline
forms must be kept in mind when designing any remediation
scheme.
2
535-2540.
+
(23) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environ.
Organic Chemistry; John Wiley & Sons: New York, 1993; Chapter
1
2.
(
(
24) Stumm, W.; Sulzberger, B. Geochim. Cosmochim. Acta 1992, 56,
3
233-3257.
25) Glaus, M. A.; Heijman, C. G.; Schwarzenbach, R. P.; Zeyer, J.
Appl. Environ. Microbiol. 1992, 58, 1945-1951.
(26) Klausen, J. Ph.D. Dissertation, Swiss Federal Institute of
Technology.
27) Gibbs, C. R. Anal. Chem. 1976, 48(8), 1197-1201.
28) Stookey, L. L. Anal. Chem. 1970, 42, 779-781.
29) Mimides, T. M. Sci. Total Environ. 1987, 66, 245-261.
30) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; John Wiley
(
(
(
(
Although the data presented here pertains only to a
mononitrated aromatic, the complete reduction of di- and
trinitrotoluenes (TNT) on ferric oxides has been demonstrated
(
9). Work on the homogeneous reduction of PNACs is
&
Sons: New York, 1996.
progressing in our laboratory. This provides a much simpler
pathway for the remediation of polynitrated explosive
residues than straight biodegradation that requires mi-
croaerobic or alternating anaerobic/ aerobic conditions, often
requires carbon amendment, and may result in incomplete
degradation (51).
(
31) Sorensen, J.; Thorling, L. Geochim. Cosmochim. Acta 1991, 55,
1289-1294.
(32) Langmuir, D. Aqueous Environmental Geochemistry; Prentice-
Hall: New York, 1992; Chapter 12.
(
33) Skipper, N. T.; Refson, K.; McConnell, J. D. C. In Geochemistry
of Clay-Pore Fluid Interactions; Manning, D. C. A., Hall, P. L.,
Hughes, C. R., Eds.; Chapman & Hall: London, 1993; Chapter
2
.
Acknowledgments
(34) Goldman, L. J.; Greenfield, L. I.; Damie, A. S.; Kingsbury, G. L.;
Northeim, C. M.; Truesdale, R. S. Clay Liners for Waste
Management Facilities; Noyes Data Corp.: Park Ridge, NJ, 1990.
Financial support for this project was provided partly through
a UWsMilwaukee Graduate School Fellowship to C.S.
Additionally the authors would like to thank Pat Anderson
for assistance in portions of the analytical work.
(
(
(
(
(
35) Greene-Kelly, R. Clay Min. Bull. 1964, 5, 392-400.
36) Lear, P. R.; Stucki, J. W. Clays Clay Miner. 1989, 37, 547-552.
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39) Buerge, I. J.; Hug, S. J. Environ. Sci. Technol. 1999, 33, 4285-
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ES990931E
3
6 4 8
9
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