Kinetics of nitroaromatic reduction on granular iron in recirculating batch experiments

Article abstract of DOI:10.1021/es970896g

Granular iron has been determined to be a potentially useful reductant for the removal of common organic contaminants from groundwater. This research is aimed at improving our understanding of the processes that control the reactivity and longevity of the iron particles when they are used for groundwater treatment. A suite of nitroaromatic compounds (NACs) including 4-chloronitrobenzene (4CINB), 4-acetylnitrobenzene (4AcNB), nitrobenzene, 2-methylnitrobenzene (2MeNB), and 2,4,6-trinitrotoluene (TNT) was used to investigate granular iron reactivity in anoxic pH 10, 0.008 M KNO₃ solution. Master Builder's brand of granular iron with a surface area of about 1 m²/g was used in all experiments. The NACs were reduced rapidly to anilines that were found to sorb reasonably strongly to the solid particles and to interfere with the reduction of NACs. The granular iron was found to lose reactivity quite rapidly over the first few days of exposure and then more slowly over the next several months. Reactivity loss due to reversibly sorbed products was minimized by flushing the system with background electrolyte between experiments. Competition experiments with binary mixtures of 4CINB and each one of the other NACs were performed to investigate relative affinities of these compounds for the solid surface. Despite the overall loss in reactivity observed for the granular iron, the relative rate constants in the competition experiments appeared to remain constant in time. Granular iron has been determined to be a potentially useful reductant for the removal of common organic contaminants from groundwater. This research is aimed at improving our understanding of the processes that control the reactivity and longevity of the iron particles when they are used for groundwater treatment. A suite of nitroaromatic compounds (NACs) including 4-chloronitrobenzene (4CINB), 4-acetylnitrobenzene (4AcNB), nitrobenzene, 2-methylnitrobenzene (2MeNB), and 2,4,6-trinitrotoluene (TNT) was used to investigate granular iron reactivity in anoxic pH 10, 0.008 M KNO₃ solution. Master Builder's brand of granular iron with a surface area of about 1 m²/g was used in all experiments. The NACs were reduced rapidly to anilines that were found to sorb reasonably strongly to the solid particles and to interfere with the reduction of NACs. The granular iron was found to lose reactivity quite rapidly over the first few days of exposure and then more slowly over the next several months. Reactivity loss due to reversibly sorbed products was minimized by flushing the system with background electrolyte between experiments. Competition experiments with binary mixtures of 4CINB and each one of the other NACs were performed to investigate relative affinities of these compounds for the solid surface. Despite the overall loss in reactivity observed for the granular iron, the relative rate constants in the competition experiments appeared to remain constant in time.

Full text of DOI:10.1021/es970896g

Environ. Sci. Technol. 1998, 32, 1941-1947
often quite recalcitrant and considered toxic to humans.
Kinetics of Nitroaromatic Reduction
on Granular Iron in Recirculating
Batch Experiments
Preliminary field and laboratory studies of the performance
of zero-valent iron in removing such compounds from
groundwater were extremely encouraging (8). These studies
have triggered research into the fundamental understanding
of the reactions occurring at the iron/ water interface.
However, to date, the mechanisms of these reduction
reactions have not yet been completely elucidated although
progress is rapidly being made. There is compelling evidence
to suggest that contact between the organic solute and the
solid surface is required (9) and that sorption of chlorinated
solvents (TCE and PCE) to iron does in fact occur (10). The
reductions themselves may occur, at least partly, by two
electron transfers (11, 12) resulting in the oxidative dissolution
,
†
J O H N F . D E V L I N , *
J O¨ R G K L A U S E N ,
‡
, §
A N D
‡
R E N EÄ P . S C H W A R Z E N B A C H
Department of Earth Sciences, University of Waterloo,
Waterloo, Ontario, Canada, N2L 3G1, and Swiss Federal
Institute for Environmental Science and Technology (EAWAG)
and Swiss Federal Institute of Technology (ETH),
CH-8600 D u¨ bendorf, Switzerland
(
corrosion) of the iron (13). Reaction rates have been found
to be dependent on both iron surface area and solute
concentration (7, 14), although pseudo-first-order kinetics
are usually observed. It has been suggested that the reaction
kinetics are diffusion controlled (13, 15) on the basis of batch
tests involving acid-washed granular iron. However, recent
electrochemical evidence suggests that, in the case of carbon
tetrachloride, transport times through the diffusion layer
surrounding an oxide-free metal surface are minimal (16).
It has also been observed that hydrogen is evolved shortly
Granular iron has been determined to be a potentially
useful reductant for the removal of common organic
contaminants from groundwater. This research is aimed
at improving our understanding of the processes that control
the reactivity and longevity of the iron particles when
they are used for groundwater treatment. A suite of ni-
troaromatic compounds (NACs) including 4-chloroni-
trobenzene (4ClNB), 4-acetylnitrobenzene (4AcNB), nitroben-
zene, 2-methylnitrobenzene (2MeNB), and 2,4,6-trinitrotoluene
0
after water (at neutral or slightly acid pH) contacts the Fe
(17). Hydrogenolysis of PCE on palladium has been dem-
onstrated in the laboratory and shown to be faster than PCE
reductive dechlorination in the presence of iron (18). Thus,
with hydrogen present, the possibility that organic reductions
occur by hydrogenolysis cannot be ruled out.
As has been demonstrated in earlier work (19-22),
nitroaromatic compounds (NACs) are very useful probe
compounds for evaluating redox reactions in homogeneous
and heterogeneous aqueous systems. Generally, NACs are
reduced to well-defined, easily detectable products, thus
allowing one to establish mass and electron balances.
Furthermore, the transfer of the first electron to a given NAC
(TNT) was used to investigate granular iron reactivity in
anoxic pH 10, 0.008 M KNO3 solution. Master Builder’s brand
2
of granular iron with a surface area of about 1 m /g was
used in all experiments. The NACs were reduced rapidly
to anilines that were found to sorb reasonably strongly
to the solid particles and to interfere with the reduction of
NACs. The granular iron was found to lose reactivity
quite rapidly over the first few days of exposure and then
more slowly over the next several months. Reactivity
loss due to reversibly sorbed products was minimized by
flushing the system with background electrolyte between
experiments. Competition experiments with binary mixtures
of 4ClNB and each one of the other NACs were performed
to investigate relative affinities of these compounds for
the solid surface. Despite the overall loss in reactivity
observed for the granular iron, the relative rate constants
in the competition experiments appeared to remain constant
in time.
is reversible. Therefore, the corresponding one-electron
1
h
reduction potential, E ′ (ArNO
2
) (eq 1), can be determined
relatively easily and is available for a variety of environ-
mentally relevant NACs (23).
-
•-
1
h
ArNO + e ) ArNO2
E ′ (ArNO )
(1)
2
2
Using 10 monosubstituted nitrobenzenes (NBs) as model
compounds, Klausen et al. (20) and Heijman et al. (21) have
shown that, for both batch and column systems and using
single compounds as well as binary mixtures, plots of the
1
h
logarithms of reduction rate data versus E ′ values yielded
Introduction
important information on the factors that determine the
reduction kinetics of NACs by surface-bound Fe(II) species.
As shown by Agrawal and Tratnyek (15), NACs were readily
reduced by zero-valent iron in carbonate-buffered batch
systems. In that work, attempts were made to elucidate the
factors controlling the reduction kinetics by comparing
pseudo-first-order rate constants for several substituted
NACs, including TNT and parathion, as a function of one-
electron reduction potentials. No correlations were observed,
leading the authors to suggest that mass transport was rate
limiting. They cited as additional evidence for this an
observed dependence of the reaction kinetics on the stirring
rate of the solution (and iron particles) in their batch systems.
However, increasing surface reactivity with increasing stirring
rate could also be due to increasing abrasion of the agitated
particles (24). Therefore, one objective of this work was to
reexamine nitroaromatic reductions on granular iron, this
0
Zero-valent iron (Fe ) is capable of reducing a variety of
important organic pollutants (1-7), and as a result, its
application in in situ reactive barriers as well as in above-
ground treatment systems has recently drawn considerable
interest, particularly with respect to the reductive treatment
of polyhalogenated compounds (8). For example, chlorinated
solvents, including trichloroethene (TCE), tetrachloroethene
(
PCE), and carbon tetrachloride (CT), comprise a class of
chemicals that are common groundwater contaminants,
*
Corresponding author e-mail: jfdevlin@sciborg.uwaterloo.ca.
University of Waterloo.
EAWAG/ ETH.
†
‡
§
Present address: Department of Geography and Environmental
Engineering, The Johns Hopkins University, Baltimore, MD, 21218-
686.
2
S0013-936X(97)00896-1 CCC: $15.00
Published on Web 05/16/1998

1998 Am erican Chem ical Society
VOL. 32, NO. 13, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1 9 4 1

TABLE 1. Diffusion Coefficients in Water, Log Octanol-Water Partition Coefficients, and One-Electron Reduction Potentials of
the Compounds Studied
D (cm s-1)
a
2
log (Kow)
E ′ (ArNO2) (V)
1
h
b
compound
abbreviation
molecular formula
-
-
-
-
-
6
6
6
6
6
1.86c
1.49d
-0.30e
-0.36
-0.45
-0.49
-0.59
2
4
4
,4,6-trinitrotoluene
-acetylnitrobenzene
-chloronitrobenzene
TNT
C7H5N3O6
C8H7NO3
C6H4NO2Cl
C6H7NO2
C7H7NO2
6.1 × 10
6.8 × 10
7.4 × 10
8.2 × 10
7.4 × 10
4AcNB
4ClNB
HNB
d
2.40
nitrobenzene
-m ethylnitrobenzene
1.84c
2.30c
2
2MeNB
a
Diffusion coefficients in water at 22 °C calculated according to the m ethod of Hayduk and Laudie as described in ref 37. From ref 19. c From
b
ref 38. d From ref 39. e From ref 23.
time with batch experiments in which the solid particles were
held stationary. A recirculating batch design was adopted
with the advantages of an immobilized solid phase with a
travel path length short enough to prevent the formation of
zones of different reactivities within the solid matrix. Another
important objective was to contribute to the investigation of
the long-term behavior of commercial zero-valent iron and
to assess whether changes would occur that might lead to
preferential reactivity toward variously substituted com-
pounds. To this end, the relative rates of reduction of
substituted NACs, present as single compounds and as binary
mixtures, were studied as a function of age of the iron
particles.
FIGURE 1. Recirculating batch test apparatus. The reservoir volume
was 300 mL, the column diameter was 5 cm, and the path length
through the granular iron was 0.9 cm.
Materials and Methods
Chemicals. All chemicals including nitrobenzene (NB),
and double junction. PEEK tubing with 0.1-0.5 mm i.d. was
used throughout the assembly. All aqueous solutions were
2
-methylnitrobenzene (2MeNB), 4-acetylnitrobenzene(4-
2
carefully deaerated by bubbling N or Ar gas prior to use.
AcNB), and 4-chloronitrobenzene (4ClNB), 2,4,6-trinitro-
toluene (TNT), and the corresponding anilines (except 2,4,6-
triaminotoluene, purchased from Promochem, GmbH, Wesel,
Germany) were obtained in the highest purity available from
Fluka (Buchs, Switzerland) and were used as received without
additional purification (cf. Table 1). The NACs were selected
for two reasons: first, they span a wide range of one-electron
reduction potentials; second, these same compounds had
been used previously in experiments investigating electron
transfer in heterogeneous systems (20, 23).
However, during the experiments, the gas stream was
suspended in order to avoid evaporative losses of the organic
compounds.
The column was initially saturated by circulating 282 mL
3
of 0.008 M KNO at near-neutral pH through the system.
Nitrate was chosen as the electrolyte in order to maintain a
constant ionic strength and because it is a common
groundwater constituent. Recent results indicate that nitrate
may also be reduced by zero-valent iron (26, 27), although
the reported rates strongly decreased with increasing pH.
Within 30 min of circulation, the pH had risen to 9.3, and
after several hours it reached 9.9, where it eventually
stabilized. On the basis of this pseudo-steady-state pH, all
subsequent experiments were conducted with an initial pH
of 10, which is also expected to develop in typical groundwater
passing through a permeable reactive barrier (7). Adjust-
ments of pH were made with the addition of either KOH or
Granular Iron. Master Builder’s brand granular iron was
used in all experiments without pretreatment. The material
was dry sieved prior to use to remove the size fraction below
0
.3 mm diameter, minimizing the possibility of fine solids
circulating through the experimental apparatus during the
tests. X-ray photoelectron spectroscopy (XPS) analysis of
the material showed that the surface iron was entirely Fe(III)
and resulted in a spectrum nearly identical to the one
obtained with the commercially available magnetite from
Macherey-Nagel (D u¨ ren, Germany, No. 14439, 97%). BET
analysis of a sample of the granular iron indicated a surface
HNO
Kinetic experiments were conducted by weighing about
50 mL of 0.008 M KNO solution into the round-bottom
3
.
2
3
flask, adjusting the pH to 10, deaerating the solution by
bubbling Ar gas through it while stirring for at least 1 h, and
finally spiking it with the nitrobenzene(s) of interest to bring
the initial concentration of the organic compound to about
100 µM. Initial samples of 200-250 µL were then collected
with a 1 mL polypropylene syringe and immediately dis-
pensed into an HPLC sample vial that was subsequently
capped and stored in a refrigerator at 4 °C. The pump was
2
-1
area of 0.964 m
g . The elemental analysis of Master
Builder’s granular iron is given elsewhere (17, 25).
Recirculating Batch Experim ents. The iron filings were
packed in a 5 cm diameter glass column with fritted glass/
PTFE end-plates. The column was packed with 52.0 g of
iron to a length of 0.9 cm. The relatively short travel path
was desirable to prevent the development of zones of
geochemically altered iron along the flow path. For the
purposes of this work, it was preferred that the iron
throughout the column react as uniformly as possible.
-
1
then turned on at a flow rate of between 3 and 4 mL min
,
and additional samples were collected at increasing time
intervals for about 10 h. All samples were collected and stored
as described above. Between each kinetic experiment, the
granular iron was washed by flushing the column (to waste)
with about 1.5 L (corresponding to about 85 pore volumes)
The column was connected on the outlet side to a 300 mL
round-bottom flask and on the inlet side to a single-piston
HPLC pump (Figure 1). The pump and the flask were also
connected to complete a closed loop. The flask was equipped
with two openings: the first was fitted with a rubber stopper
through which tubing to and from the column and to a
sampling syringe was passed. The second was used as a
port for a combined glass pH electrode with Ag/ AgCl reference
of deaerated 0.008 M KNO
of about 12 h.
3
solution at pH 10 over a period
The design of the kinetic experiments, i.e., a reactive
porous medium and a well-mixed reservoir connected by
advective links (see Figure 1), necessitated the use of a
1
9 4 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 13, 1998

FIGURE 2. Effect of background 4-chloroaniline on the kinetics of
-chloronitrobenzene reduction.
FIGURE 3. Isotherm of 4ClAn sorption from solution to Master
Builder’s granular iron. Data were collected at about 22 °C in 3-day
batch tests with minimal particle abrasion.
4
transport model to calculate a pseudo-first-order rate
constant for the reaction of the nitroaromatic compounds
in the column using concentration data obtained from the
reservoir. Using the modeling tool AQUASIM (28, 29), the
system was described by the one-dimensional advection-
dispersion equation with a pseudo-first-order reaction term,
where the calculated concentration at the end of the column
served as the input for a mixed reservoir and vice versa.
Parameters obtained by nonlinear least-squares fitting of
the model to the experimental data included the pseudo-
first-order rate constant for the reduction of the NAC in the
column and the initial NAC concentration in the reservoir
0
3
.008 M KNO solution were weighed into a 60 mL glass vial
and immediately spiked with a stock solution of 4-chloroa-
niline (4ClAn) to bring the final dissolved concentration to
between 0 and 600 µM. The vials were prepared in duplicate,
sealed with Viton stoppers, and stored at room temperature
in the dark for a minimum of 3 days prior to reopening them
for analysis. The vials were gently rotated once per day over
the 3 day test to minimize transport limitations without
causing excessive abrasion of the particles. Stock solutions
were prepared in methanol, and final methanol concentra-
tions in the vials never exceeded 1 vol %. Good agreement
between the duplicates within any particular experiment and
experiments performed on different days (with different
concentration ranges) indicated that the protocol was an
acceptable one. For example, combined data from three
experiments are presented in Figure 3 for the sorption of
(
see Figure 2 for a representative example) (28). The upper
limit of rate constants that can be determined in this system
is a function of the volumetric flow. It is asymptotically
approached when the concentration of the NAC leaving the
column compartment approaches zero (full conversion). The
flow rate was similar in all experiments and was chosen so
that the NAC was never fully converted in the column
compartment. Possible mass-transport limitations, at the
grain scale, were not accounted for in this treatment. Only
data of the first 2-4 h of a kinetic experiment were included
in the fit since pronounced deviations from pseudo-first-
order kinetics (possibly due to product accumulation) became
apparent at later times.
4
ClAn.
Analytical Methods. Substituted NBs and the corre-
sponding anilines were determined by HPLC analysis on an
RP-18 or RP-8 reversed-phase column (125 ×4 mm) equipped
with a precolumn (4 × 4 mm; both LiChroCart stainless steel
cartridge, 5 µm spheres; Merck AG, Darmstadt, Germany)
connected to a pumping system (Jasco 980-PU; Jasco
Spectroscopic Co., Ltd., Tokyo, Japan; flow rate ca. 1 mL
min ) supplemented with a autosampler (Gina 50; Gynkotek,
Germering b.M., Germany; injected volume 20 µL) and a
variable-wavelength UV/ vis detector (Jasco 875-UV) set to
2
To facilitate comparison between experiments conducted
with different single compounds, pseudo-first-order rate
constants were re-expressed as relative rate constants,
-1
k
X-NB (relative) according to
28-246 nm. The mobile phase was typically CH
3 2
OH/ H O
(
60/ 40 v/ v). 2,4,6-Triaminotoluene (TAT) was determined
kX-NB
using 5 mM phosphate, pH 7, buffer/ acetonitrile (99/ 1 v/ v)
with the detector set to 220 nm. The analysis of TNT and
its reaction products (except for TAT) was performed on a
Supelcosil LC-18 column (250 × 4.6 mm; Supelco, Buchs,
Switzerland) with LC-18 precolumn (20 × 4.6 mm) using
kX-NB (relative)
)
(2)
k4ClNB
where kX-NB refers to the rate constant for an X-substituted
NB and k4ClNB is the rate constant for 4ClNB, both determined
from single-compound experiments performed close together
in time. Competition experiments were conducted with
2-propanol/ acetonitrile/ H O (20/ 10/ 70 v/ v/ v) with the de-
2
tector set to 220 nm.
4
ClNB as one of the competing compounds, so relative rate
constants, QX-NB, in these cases were calculated from
Results and Discussion
Reaction Kinetics and Product Inhibition. The disappear-
ance of any single NB did not strictly follow pseudo-first-
order kinetics. Rates of reaction often declined as the reaction
progressed. A series of experiments was performed with
4-chloronitrobenzene (4ClNB) to examine the effect of
4-chloroaniline (4ClAn) accumulation on the reaction rate
(cf. Figure 2). To this end, the reservoir was spiked with
about 100 µM 4ClNB, and the disappearance was monitored
until 4ClNB was no longer detectable. The reservoir was
subsequently respiked three more times, each time with about
100 µM 4ClNB and, before the last experiment only, an
kX-NB (binary)
QX-NB
)
(3)
k4ClNB (binary)
where the kX-NB (binary) and k4ClNB (binary) refer to the rate
constants measured for the two compounds reacting together
in competition.
Adsorption Isotherm s. Batch experiments were per-
formed to evaluate the sorptive properties of the aniline
compounds. A typical batch sample was prepared as
follows: about 8 g of granular iron and 45 g of deaerated
VOL. 32, NO. 13, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1 9 4 3

TABLE 2. Langmuir Isotherm Parameters Determined from Individual and Competition Sorption Experiments
Cmax (µmol g-
1 a
)
KL (µM )
-1 a
compound
abbreviation
n
Individual Sorption Experiments
4
4
2
-chloroaniline
-acetylaniline
-m ethylaniline
4ClAn
4AcAn
2MeAn
44
18
17
0.926 ( 0.028
1.260 ( 0.063
0.779 ( 0.031
0.014 ( 0.002
0.005 ( 0.001
0.009 ( 0.001
Competition Sorption Experiments
4
4
-chloroaniline
-acetylaniline
4ClAn
4AcAn
17
17
0.482 ( 0.024
0.881 ( 0.026
0.014 ( 0.003
0.011 ( 0.002
4
2
-chloroaniline
-m ethylaniline
4ClAn
2MeAn
18
18
0.940 ( 0.038
0.668 ( 0.033
0.011 ( 0.002
0.006 ( 0.001
a
Errors shown are 95% confidence intervals.
additional 100 µM 4ClAn. This resulted in increasing initial
ClAn concentrations but constant initial 4ClNB concentra-
tions in the first four experiments. The reduction rates for
ClNB decreased significantly with increasing initial 4ClAn
4
4
concentration (cf. Figure 2; the first, third, and fourth
experiments are shown), indicating blocking of reactive
surface sites by the aniline. This may also explain the
deviation from pseudo-first-order behavior at longer reaction
times during an experiment, because the number of available
reactive sites may have decreased during the experiment
under these conditions. Notice that, even though 4ClNB
appeared to be converted completely, stoichiometric amounts
of 4ClAn were not observed in solution, indicating either
sorption of 4ClAn to the iron particles or further reaction.
Three-day static batch experiments confirmed that various
anilines (including 4ClAn, 4AcAn, and 2MeAn) sorbed
significantly to the granular iron at pH 10 (Figure 3). The
isotherms produced were nonlinear over the range tested
FIGURE 4. Effect of removal of 4ClAn from the granular iron on the
kinetics of 4ClNB reduction. Before flush: last of a series of
experiments conducted in the course of ca. 4 weeks. After flush:
experiment conducted immediately after removal of 4-chloroaniline
from the column.
(
0-600 µM in the water) and could be well described by a
-
8
-1
Langmuir model with a constant of K
L
) 1.4 × 10
M
and
-
1
a saturation concentration of Cmax ) 0.9 µmol g Fe.
However, Cmax was found to depend on the range over which
the isotherm was generated, increasing steadily as the range
was expanded. Thus, the sorption interaction does not
appear to be truly Langmuirian in nature. Extractions of the
drained iron particles with acetonitrile, following one ex-
periment, were performed on a set of 12 bottles containing
of the iron was reproducible between experiments over short
time intervals (several days to 2 weeks) and was only slowly
decreasing.
Aging and Substituent Effects. The reactivity of the
column was investigated for a total of 8 months. Over this
extended time period, pseudo-first-order rate constants for
individual compounds were found to decline with time,
indicating a continuous loss of reactivity of the solids despite
the flushing procedure (Figure 5). As a result, relative rate
constants from these experiments were only calculated for
X-NB and 4ClNB experiments conducted close together in
time. Note that over this same time, relative rate constants
4
ClAn at concentrations in the range of 100-500 µM. This
procedure yielded mass recoveries exceeding 90% in all
bottles, indicating that the 4ClAn was reversibly sorbed to
the granular iron and not further transformed. Experiments
were also performed to test the dependence of the sorption
interaction on the type of anion and cation prevailing in the
solution at constant ionic strength (I ) 0.008). Substitution
of calcium for potassium and sulfate for nitrate had no effect
on the isotherms. However, when pairs of anilines were
tested together in the batch sorption experiments, competi-
tion for sorption sites between the various compounds was
observed with 4AcAn > 4ClAn > 2MeAn being the order of
affinity (cf. Table 2).
(
Q
2MeNB) for 2MeNB obtained from competition experiments
in binary mixtures with 4ClNB remained essentially constant
Figure 5), indicating that both compounds were similarly
affected by the apparent loss in reactivity of the iron material
(
(
see below).
Kinetic experiments involving single compounds revealed
As described above, the column was flushed between
experiments in order to desorb the anilines from the iron
surface. In some experiments, aniline desorption curves were
recorded which confirmed that the anilines had deposited
on the solid surfaces as expected from the static batch
adsorption experiments. After the first flush, only partial
recovery of the iron reactivity was observed (Figure 4). This
may indicate irreversible blocking or loss of a fraction of the
reactive sites as a consequence of either the initial hydration
of the iron particles or the reduction of the nitroaromatic
compound. Although a complete recovery of the initial
reactivity was not achieved, subsequent experiments per-
formed in succession, separated only by a flushing step,
exhibited similar reaction rates, indicating that the reactivity
only small differences between the rate constants of 4ClNB,
2MeNB, and 4AcNB, even though these compounds exhibit
very different one-electron reduction potentials. Little or
no correlation was observed between log(kX-NB (relative)) and
the corresponding one-electron reduction potentials (Figure
6A). This result is consistent with previously reported data
for various NACs reacting on a different iron that had been
pretreated with hydrochloric acid (15). This finding indicates
that the (hydr)oxide layer present on the surface of Master
Builder’s iron has little effect on the relative reaction rates
of variously substituted NACs and suggests that the actual
electron transfer is not the rate-determining process in these
reactions. Thus, other rate-controlling steps have to be
considered, including the formation of the precursor complex
1
9 4 4
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 13, 1998

FIGURE 7. Initial rates, νini ) k4ClNB × [4ClNB]ini, and rate constants
for 4ClNB reduction as a function of initial 4ClNB concentration.
Rates began to slow when the initial concentrations exceeded
FIGURE 5. Reactivity changes with time for 4ClNB in single-
compound kinetic experiments (circles) and in competition experi-
ments with 2MeNB (triangles). While absolute reactivity was lost,
relative reactivity in competition experiments remained constant
over time.
0.125 mM.
netite-ferrous iron systems (Figure 6A). Given that the solids
used in this study comprise a zero-valent iron core sur-
rounded by a magnetite and/ or maghemite (30-32) coating
that may be several micrometers thick (33) and given that
the corrosion reaction is thought to release ferrous iron in
the first step, a pattern very similar to that reported by Klausen
et al. (20) might have been expected in the granular iron
columns. However, the comparability of the two systems is
limited since the zero-valent iron core may exert a consider-
able and sustained influence, through the semiconductive
(
hydr)oxide coating on the redox potential at the particle-
water interface (32, 34, 35). Thus, even though the surface
coatings of the iron particles may be similar to the magnetite
used by Klausen et al., this distinction could result in (a) a
much more negative redox potential of the iron particles,
giving rise to different electron-transfer characteristics, and/
or (b) different tendencies to sorb either ferrous iron or
nitroaromatics.
Com petition Experim ents. Unlike single-compound
experiments, experiments with binary mixtures of com-
pounds offer the opportunity to study the relative affinities
of the reacting species for the solid surface and gain further
insight into the nature of the interactions between the
nitroaromatics and the granular iron. Prior to initiating these
tests, a series of experiments was performed to evaluate the
concentration above which saturation kinetics would apply.
0
Initial concentrations (C ) of 4ClNB were varied from 8 to
3
87 µM, and depletion curves were compared. As shown in
Figure 7, pseudo-first-order reaction rate constants decreased
significantly with increasing initial 4ClNB concentration while
initial reaction rates began to plateau at higher concentra-
tions, indicating increasing saturation of reactive sites. On
this basis, it was decided that meaningful competition
experiments could be conducted with total nitrobenzene
concentrations of 200 µM or more. While the single-
compound experiments did not reveal any pronounced
differences in the reaction rates of the various NACs tested,
these differences were clearly evident in the competition
experiments (Figure 6B), even though the effect was not as
pronounced as in similar experiments conducted previously
(20). In that study, the relative rates (QX-NB) of meta- and
para-substituted NBs in the magnetite-ferrous iron system
correlated strongly with the one-electron reduction potentials
in a fashion very similar to that observed in other ferrous
iron-containing systems (19, 21). It was suggested that such
a correlation could serve as a probe for the reactive species
(namely, the ferrous-ferric redox couple) involved in these
nitro reductions. Thus, a similarly strong correlation in this
work would be suggestive of Fe(II) involvement in the rate-
FIGURE 6. (A) Relative rate constants as a function of one electron
reduction potentials for single-compound kinetic experiments. (B)
Relative rate constants as a function of one-electron reduction
potentials for competition experiments. Circles represent meta-
and para-substituted compounds, and squares represent ortho-
substituted compounds. Filled symbols represent data generated in
this work while open symbols are from ref 20. The open diamond
is from ref 23. 4ClNB was used as the reference compound in
generating this plot (eqs 2 and 3).
between the organic compound and the solid surface, the
release of the aniline product from the successor complex,
physical transport limitations in delivering the nitrobenzenes
to the solid surface, and electron transport from the zero-
valent iron core through the surface coating(s) of the iron
particles to the NAC in question (directly or possibly via
species such as hydrogen gas, in a hydrogenation reaction,
or ferrous ion).
In contrast to the results obtained here, Klausen et al.
(
20) reported a somewhat stronger correlation for log(kX-NB
1
(
relative)) vs E ′/ 0.059 V for nitroaromatic reductions in mag-
h
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determining step of nitroaromatic reduction by zero-valent
iron. In fact, competition between the various nitrobenzene
compounds was observed but was considerably weaker than
what has been reported for the other systems mentioned
above (Figure 6B). This finding indicates that the granular
state while the reaction is still relatively rapid. Additional
studies are also needed to assess the effects of various
background geochemistries on these passivation rates.
Acknowledgments
This research was supported by the Solvents in Groundwater
Consortium of the University of Waterloo and the Swiss
Federal Institute for Environmental Science and Technology
“
zero-valent” iron particles interact in a quantitatively
different fashion with NACs than the iron(II)/ iron(III) (hydr)-
oxide systems previously studied, probably due to the
underlying zero-valent metal core. Remarkably, the order
of affinity of the compounds tested was the same in all
studies: TNT > 4AcNB > 4ClNB > nitrobenzene, 2MeNB. It
is also of interest to note that the orders of affinity of the
nitrobenzenes to Master Builder’s granular iron, as inferred
from their relative reaction rates (QX-NB), and those of the
corresponding anilines, as determined in the static batch
adsorption tests, are identical. This raises questions about
possible similarities in the ways the two classes of compounds
interact with the solid surface. Furthermore, these rankings
do not match the order expected on the basis of either
hydrophobicity or diffusion alone, but do match the order
of the NAC reduction potentials (Table 1). This argues against
either hydrophic sorption or diffusional transport limitations
governing the reaction kinetics. Further investigation of the
mechanisms of interaction of these types of compounds with
(
EAWAG). The authors are grateful to Thomas Hofstetter for
assistance with the HPLC analysis; to Klaus Pecher, Stefan
Haderlein, Werner Angst, John C. Westall, and Thomas Suter
for valuable discussions; and to John Cherry and Bob Gillham,
who initiated the EAWAG-Waterloo collaboration.
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