Degradation of benomyl, picloram, and dicamba in a conical apparatus by zero-valent iron powder

Article abstract of DOI:10.1016/S0045-6535(00)00184-3

Reduction of some pesticides (benomyl, picloram, and dicamba) was studied in an aerobic batch conical pilot system to investigate the disappearance of these pesticides on contact with iron powder (20 g/l, 325-mesh). Aqueous buffered solutions of the compounds were added to the system followed by zero-valent iron powder (ZVIP), and the decline in the pesticide concentrations was monitored over time. HPLC analyses show a complete disappearance of picloram (1.20 mg/l) after 20 min of reaction. Benomyl (1.00 mg/l) and dicamba (1.25 mg/l) disappear after 25 and 40 min, respectively. The t₅₀ values ranged from 3 to 5.5 min, and were about slightly less than the t_1/2 values reported when the log of the relative HPLC peak area was plotted versus time, where the relative peak area was calculated by dividing the measured peak area by the initial peak area. Pathways for the degradation of the studied pesticides by ZVIP are proposed.

Full text of DOI:10.1016/S0045-6535(00)00184-3

Chemosphere 43 %2001) 1109±1117
Degradation of benomyl, picloram, and dicamba in a conical
apparatus by zero-valent iron powder
*
Antoine Ghauch
ESIGEC ± Laboratory of Chemistry and Environmental Engineering LCIE, University of Savoie, Scienti®c Campus, Savoie Technolac,
73376 Le Bourget du Lac Cedex, France
Received 13 March 2000; accepted 17 May 2000
Abstract
Reduction of some pesticides %benomyl, picloram, and dicamba) was studied in an aerobic batch conical pilot system
to investigate the disappearance of these pesticides on contact with iron powder %20 g/l, 325-mesh). Aqueous bu€ered
solutions of the compounds were added to the system followed by zero-valent iron powder %ZVIP), and the decline in
the pesticide concentrations was monitored over time. HPLC analyses show a complete disappearance of picloram %1.20
mg/l) after 20 min of reaction. Benomyl %1.00 mg/l) and dicamba %1.25 mg/l) disappear after 25 and 40 min, respectively.
The t50 values ranged from 3 to 5.5 min, and were about slightly less than the t1=2 values reported when the log of the
relative HPLC peak area was plotted versus time, where the relative peak area was calculated by dividing the measured
peak area by the initial peak area. Pathways for the degradation of the studied pesticides by ZVIP are pro-
posed. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Degradation; Iron powder; Pesticides; Reduction
1. Introduction
procedures that was applied since 1993 %Gillham and
Burris, 1997).
Pesticides are prevalent groundwater contaminants
and are signi®cant components of hazardous waste and
land®ll sites. The production and the use of pesticides
and their apparent hazard to human health have
prompted investigations concerning their fate in sub-
surface waters, and in treatment facilities. In most of
these environments, photolysis, ozonation, and adsorp-
tion on activated carbon %De Laat et al., 1995) are not
important processes that can be applied at a low cost.
However, a novel technology including the use of zero-
valent iron %ZVI) becomes one of the encouraging in situ
Picloram is a systemic herbicide used for control of
woody plants and a wide range of broad-leaved weeds.
Most grasses are resistant to picloram, so it is used in
range management programs. Picloram is formulated
either as an acid %technical product), a potassium or
triisopropanolamine salt, or an isooctyl ester, and is
available as either soluble concentrates, pellets, or
granular formulations. Picloram is moderately to highly
persistent in the soil environment, with reported ®eld
half-lives from 20 to 300 days and an estimated average
of 90 days. Degradation by microorganisms is mainly
aerobic. In a laboratory study, sunlight readily degraded
picloram in water, with a half-life of 2.6 days. It is sol-
uble in water, and therefore may be mobile. These
properties, combined with its persistence, may indicate a
risk of groundwater contamination. Picloram has been
detected in the groundwater of 11 states in the USA at
* Tel.: +33-6-8680-8991; fax: +33-4-7975-8843.
E-mail address: antoine.ghauch@univ-savoie.fr %A.
Ghauch).
0045-6535/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 4 5 - 6 5 3 5 % 0 0 ) 0 0 1 8 4 - 3

1110
A. Ghauch / Chemosphere 43 12001) 1109±1117
concentrations ranging from 0.01 to 49 lg/l %Howard,
1991).
This paper presents, results obtained with a conical
laboratory constructed apparatus for the treatment of
water contaminated with picloram, dicamba and beno-
myl pesticides %Fig. 1) at room temperature %22 Æ 1°C).
The estimated reductive degradation pathways are pre-
sented for each of picloram, dicamba, and benomyl. The
e€ects of pH and dissolved oxygen %DO) are not studied.
A comparison between the t50 values and the half-lives
t1=2 of all the pesticides studied is also reported.
Benomyl is a systemic, benzimidazole fungicide that
is selectively toxic to microorganisms and to inverte-
brates, especially earthworms. It is used against a wide
range of fungal diseases of ®eld crops, fruits, nuts, or-
namentals, mushrooms, and turf. Formulations include
wettable powder, dry ¯owable powder, and dispersible
granules. Benomyl is strongly bound to soil and is highly
persistent. It completely degrades to carbendazim within
several hours in acidic or neutral water. The half-life of
carbendazim is 2 months %Howard, 1991).
2. Experimental
Dicamba is a benzoic acid herbicide. It can be applied
to leaves or to soil. Dicamba controls annual and pe-
rennial broadleaf weeds in grain crops and grasslands,
and it is used to control brush and bracken in pastures.
It will kill broadleaf weeds before and after they sprout.
Legumes will be killed by dicamba. It is moderately
persistent in soil, the half-life being typically 1 to 4
weeks. Metabolism by soil microorganisms is the major
pathway of loss under most soil conditions. In water,
microbial degradation is the main route of dicamba
disappearance. Photolysis may also occur, aquatic hy-
drolysis, volatilization, adsorption to sediments, and
bioconcentration are not expected to be signi®cant
%Howard, 1991).
2.1. Instrument
HPLC analyses of pesticides were performed by a
Diode Array Controller %Waters TM996), equipped
with a diode array detector, and supplemented with a
programmable multiwavelength UV/VIS detector. 20 ll
samples taken from the pilot were injected automatically
via
a C18 non-polar column %Supelco, Discovery
250  4:6 mm, porosity ˆ 5 lm). The mobile phase %¯ow
rate ˆ 1.5 ml/min) was a mixture of acetonitrile/water
%20:80, v/v) for benomyl and dicamba, and %5:95, v/v) for
picloram.
Research into the use of ZVI metals to remediate
groundwater contaminated with mixed wastes %inor-
ganic and organic) has been growing at the US Envi-
ronmental Protection Agency since 1991. The primary
emphasis has been on inorganic constituents such as
chromate, arsenic, nitrate and sulfate, and chlorinated
organic compounds %e.g., trichloroethylene, cis-dichlo-
roethylene, and vinyl chloride). In a recent research re-
port, Rijnaarts %1998) showed results on the reductive
degradation of hexachlorocyclohexanes %HCHs) ob-
tained by a full-scale pilot plant. He found that HCHs
can be degraded by ZVI without producing benzene as
by-product and concluded that iron may be an appro-
priate reactive material for these types of compounds.
Laboratory research conducted by Ghauch et al. since
1999 conclusively demonstrated the e€ectiveness of
ZVIP to remediate water contaminated with pesticides
like atrazine and ethyl-parathion %Ghauch et al., 1999),
carbaryl %Ghauch et al., 2000; Ghauch, 2000a), and
s-triazines %Ghauch, 2000b).
2.1.1. Conical pilot
A laboratory constructed apparatus %Fig. 2) was de-
veloped to study the degradation of picloram, dicamba
and benomyl using ZVIP. Iron was treated with HCl %1
M), washed with distilled water, and then manually in-
troduced into the equipment. The apparatus is equipped
with a propeller located at middle height, allowing the
aspiration of ZVIP through a 5 mm diameter tube, from
the bottom to the top surface, for ZVIP to be propa-
gated in the whole reactive zone. Once settled, the ZVIP
is aspirated again continuously. The pilot equipment
was built to be used in batch, semi-batch, or in contin-
uous mode.
2.1.2. Reagents
Iron powder %purity > 99%, 325-mesh nitrogen ¯u-
shed) was purchased from Acros %Geel, Belgium), and
KH2PO4 and Na2HPO4 Á 12H2O from Prolabo %France).
Picloram, dicamba and benomyl were obtained from

Riedel-de-Haen %Germany) at the highest purity avail-
able. Acetonitrile was of spectroscopic grade and pur-
chased from Acros. Deionised water was used
throughout the experiments.
2.2. Experimental
All experiments were carried out at room tempera-
ture %22 Æ 1°C) in phosphate bu€ered solutions %0.025
M ; pHˆ 6.6). The ZVIP was pretreated with 200 ml 1 M
HCl for 10 min, then washed with distilled water %400
Fig. 1. Structure of pesticides.

A. Ghauch / Chemosphere 43 12001) 1109±1117
1111
carried out in a non-deoxygenated solution at 8 mg/l of
DO. E€ect of DO is not studied here, but the conse-
quence of the DO on the degradation rate of pesticides
should be non-negligible. This phenomenon was studied
by Siantar et al. %1996) who found that increasing the
amount of DO from 0 to 41.6 mg/l %saturation concen-
tration of O2 in water at 22°C) decreases linearly the
pseudo ®rst-order rate reduction of 1,2-dibromo-3-
chloropropane to propane from 0:28 Æ 0:03 to
0:07 Æ 0:02 min. They concluded that this observation
may indicate that the DBCP competes with O2 for iron
surface active sites and/or that the O2 deactivates the
surface by forming non-reactive iron oxide coatings. In
the O2 concentration range relevant for deionised water
%under the conditions of the current experiment, 8 mg/l),
the decrease in pesticide transformation rate should
match with that obtained for 1,2-dibromo-3-chloropro-
pane by a maximum of 17%.
3.1. Benomyl
Fig. 3 shows chromatograms of benomyl %1.00 mg/l)
before and after treatment with ZVIP. As can be seen,
after 25 min of contact with ZVIP, a complete disap-
pearance of benomyl was observed. Under aerobic
conditions %8 mg/l DO), benomyl was reduced by ZVIP
within few minutes of half-life t1=2. On the other hand,
the appearance of some by-products was detected after 5
min of contact with ZVIP. The by-product at retention
time Rt ˆ 7.54 min presents a UV absorption spectrum
with a maximum at 223.8 nm. However, the by-product
at Rt ˆ 3.34 min shows a shift to longer wavelength
%299.3 nm) for the second peak of the parent product
which is situated at 285.1 nm. These by-products dis-
appeared completely after 25 min and are converted to a
®nal product having Rt ˆ 1.04 min and a maximum of
absorbency at 270.9 nm. This fact can be explained by a
major transformation in the structure of benomyl pes-
ticide and then, the molecular weight of the ®nal product
is to be very lowest than the parent compound. It could
be a reductive dealkylation of the amino alkylated group
followed by the nitrogen ring opening, shown in Fig. 4
and the formation of o-phenylenediamine as ®nal by-
product. This phenomenon was observed by Sweeny
%1981) who treated hexachlorocyclopentadiene by ZVIP.
Reduction led to the production of non-chlorinated
branched chain alkanes, evidence of dechlorination and
ring opening. The concentration in the reactive zone of
the pilot showed what appeared to be an exponential
decline in concentration to about undetectable amount
at the last part of the experiment %40 min). The log of
relative HPLC peak area versus time, where relative
peak area was calculated by dividing the measured peak
area by the initial peak area, is included in Fig. 5 with a
Fig. 2. Schematic representation of the conical pilot. Capacity:
3.75 l, material: clear PVC, weight: 1 kg.
ml) four times to remove residual HCl and Fe2‡. Pesti-
cide stocks solutions were prepared in aqueous phos-
phate bu€ered solution as follows: picloram %20 mg/l),
benomyl %4 mg/l) and dicamba %6 mg/l). Working solu-
tions were prepared by an appropriate dilution giving
1.00 mg/l for benomyl, 1.20 mg/l for picloram and 1.25
mg/l for dicamba. 75 g of acid-washed ZVIP were
manually added to the pilot containing 3.75 l of pesticide
solution. At intervals, 20 ll were taken with a syringe
from the reactive media to carry out HPLC analyses
during the reductive process.
3. Results and discussion
To understand the global reductive device, we pro-
ceeded to study the degradation of each pesticide by
using HPLC to separate by-products and to monitor the
disappearance of the starting compounds and the ap-
pearance of the new and ®nal by-products. All tests were
®rst-order decay model. The half-life obtained %t1=2
from the model was 4.32 min, with a coecient of
)

1112
A. Ghauch / Chemosphere 43 12001) 1109±1117
Fig. 5. Disappearance of benomyl %1.00 mg/l). System: 20 g/l of
ZVIP, bu€ered pH 6.6 phosphate 0.025 Mand 22 °C. The line
on the %reactive) benomyl data is an arbitrary polynomial ®t.
variation %r2) of 0.92. As a second index of reaction rate,
the time required for a 50% reduction in the initial
concentration %t50 ˆ 3 min) is lower than the half-life
%Table 1). The same result was observed by Gillham and
OÕHannesin %1994) on solutions containing halogenated
aliphatic compounds. These authors suggested the fol-
lowing: ``for compounds that degraded slowly, with an
apparent decline in the rate constant at late time, the
half-lives obtained by least-squares ®t of the ®rst-order
model are higher than the actual values at early time and
lower than the actual values at late time''. Based on the
generally high r2 value, but the late-time trends in the
data for those compounds that degrade slowly, it is
proposed that the reaction is indeed pseudo ®rst-order
with respect to the parent compound, but that the rate
Fig. 3. %a) B1-HPLC chromatogram and UV absorption spec-
trum of benomyl Rt ˆ 7.4 min. Maximum absorption peaks are
at 205.2 and 285.1 nm, and a shoulder at 242.6 nm; %b) B2-
HPLC chromatogram and UV absorption spectra of by-prod-
ucts at 5 min of treatment with ZVIP %20 g/l) at room tem-
perature %22 Æ 1°C); %c) B3-HPLC chromatogram and UV
absorption spectrum of the ®nal by-product at 25 min of con-
tact with ZVIP Rt ˆ 1 min. Maximum absorption peaks are at
209.8 and 270.9 nm.
Fig. 4. Degradation mechanism of benomyl.

A. Ghauch / Chemosphere 43 12001) 1109±1117
1113
Table 1
shoulder at 226 nm and the secondary peak at 275.7 nm
disappeared completely in the UV absorption spectrum
of the by-product obtained after 40 min of reaction
%Rt ˆ 3.18 nm). On the other hand, the rate of degra-
dation of dicamba seems to be similar to benomyl. From
the examination of the graph %Fig. 7), dicamba re¯ects a
declining rate constant at late time %after 12 min of
contact with ZVIP). This appeared as a slowly declining
but persistent ``tail'' in the concentration versus time
graph. Then again, the time required for a 50% reduc-
tion in the initial concentration %t50 ˆ 4.8 min) %Table 1)
is lower than the half-life of dicamba obtained by ®tting
log %S/S0) versus time %min) %t1=2 ˆ 5.57 min).
Table of compounds studied showing the calculated half-lives
%t1=2), and the regression coecients %r2) determined from ®tting
the ®rst order decay equation to the experimental data and
times for 50% loss %t50
)
r2
t50
%min)
a
b
Pesticides
Initial con-
centration
%mg/l)
t1=2
%min)
Benomyl
Picloram
Dicamba
1.00
1.20
1.25
4.32
4.20
5.57
0.92
0.99
0.97
3.1
4.0
4.8
a Calculated half-lives.
b Times for 50% loss.
To trace a scheme presenting the degradation mech-
anism of dicamba %Fig. 8), we have suggested a hy-
pothesis similar to the reduction of polyhalogenated
alkanes %Siantar et al., 1996) that results in production
of both alkanes and alkenes. Vogel et al. %1987) de-
scribed that the reduction of halogenated aliphatic
compounds depends on the formation of the alkyl rad-
ical that is a process governed by carbon±halogen bond
energies. In general, smaller carbon±halogen bond en-
ergies conductive to faster reduction and the heat of
formation of the alkyl radical is one measure of the
constant declines at the late time as a consequence of
secondary e€ects. They may include the accumulation of
reaction products, a slight increasing of the pH, or other
unidenti®ed changes in the reaction conditions.
3.2. Dicamba
The benzoic acid compound investigated in this work
has two chlorine atoms on its ring. The ®rst transfor-
mation should be the elimination of the chlorine atoms
after contact with ZVIP. The HPLC monitoring during
the reductive treatment %Fig. 6) shows a complete dis-
appearance of dicamba after 40 min. Dicamba, which
has Rt ˆ 4.79 min, presents a UV absorption spectrum
with a maximum at 205.1 nm, a shoulder at 226 nm and
a secondary peak at 275.7 nm. Only the 205.1 nm peak is
not a€ected by the reductive treatment. However, the
Fig. 7. Disappearance of dicamba %1.25 mg/l). System: 20 g/l
iron, bu€ered pH 6.6, and 22°C.
Fig. 6. %a) D1-HPLC chromatogram and UV absorption
spectrum of dicamba %1.25 mg/l) Rt ˆ 4.79 min. The UV spec-
trum shows two peaks at 205.2 and 275.7 nm and a shoulder at
226 nm; %b) D2-HPLC chromatogram and UV absorption
spectrum of the ®nal product after 40 min of treatment with
ZVIP %20 g/l) at room temperature %22 Æ 1°C).
Fig. 8. Degradation mechanism of dicamba.

1114
A. Ghauch / Chemosphere 43 12001) 1109±1117
carbon±halogen bond strength and is inversely propor-
tional to the rate of reduction %Vogel et al., 1987). For
dicamba, the C±Cl bond strength depends on the pres-
ence of other groups on the molecular ring. For exam-
ple, the presence of the methoxy group %O±CH3)
in¯uences the elimination rate of the chlorine atoms
situated on the adjacent and on the second carbon of the
benzoic acid ring. As well, carboxylic group %COOH)
may also minimize the strength of the C±Cl bond due to
its electronegative properties, so the rate of dechlorina-
tion should be higher than molecules that do not contain
attractive group on their ring. The elimination of the
chlorine atoms occurs successively %beginning from the
chlorine atom situated on the adjacent carbon of the
carboxylic function) or simultaneously followed by the
formation of the hydroxy-benzoic acid as a by-product
that will be transformed to the corresponding alcohol
%hydroxybenzoyl alcohol). The degradation mechanism
of dicamba is shown in Fig. 8.
3.3. Picloram
After 10 min of contact of picloram with ZVIP, a
complete transformation of its structure was observed
%Fig. 9). This statement is explained by the disappear-
ance of the 223.8 nm peak of its UV absorption spec-
Fig. 9. %a) P1-HPLC chromatogram and UV absorption spectrum of picloram %1.20 mg/l), Rt ˆ 10.9 min, maximum absorption peak is
at 223.8 nm and a shoulder at 252 nm; %b) P2-HPLC chromatogram and UV absorption spectra of by-products at 10 min of treatment
with ZVIP %20 g/l) at room temperature %22 Æ 1°C); %c) P3-HPLC chromatogram and UV absorption spectrum of the intermediate by-
product %Rt ˆ 1 min) after 20 min of reaction. Maximum absorption peaks are at 209.8 and 275.7 nm.

A. Ghauch / Chemosphere 43 12001) 1109±1117
trum and the appearance of a by-product at Rt ˆ 1 min
1115
presenting two maximum absorption peaks at 209.8 nm
and 270.9 nm. However, as the reaction continues, the
absorption peak at 270.9 nm shifts to longer wavelength
%275.7 nm), nevertheless the 209.8 nm peak persists. This
observation is true until 30 min of reaction because the
by-product decreases to undetectable amount after 60
min, shown in Fig. 10. This observation is probably due
to the elimination of chlorine atoms and appearance of
by-product %4-aminopyridine-2-carboxylic acid) at
Rt ˆ 1 min, followed by the transformation of the
COOH group to CH2OH group %4-amino-2-pyridyl-
carbinol) under the enhanced reductive ZVIP treatment
%disappearance of the by-product). The presence of three
chlorine atoms on the pyridine ring results in a more
rapid dechlorination of the compound. This result
matches with results obtained by Gillham and
OÕHannesin %1994) who concluded that within each
group of halogenated aliphatic compounds %methanes,
ethanes and ethenes), the rate of degradation increases
with an increase in the degree of chlorination. Under the
reducing conditions of tests, the more highly oxidized
compounds would be most susceptible to degradation. A
similar result was observed for picloram on the t50 and
the t1=2 values than for benomyl and dicamba. Table 1
Fig. 10. Graph showing the accumulation and subsequent de-
gradation of the by-product during the degradation of picloram
%1.2 mg/l and 20 g/l of ZVIP). Bu€ered solution pH 6.6 in 0.025
Mphosphate.
shows that t50 closes to 4.00 min, however the t1=2 of
picloram closes to 4.20 min when log %S/S0) is plotted
versus time %graph not shown). The dechlorination of
picloram can occur at di€erent carbon of the pyridine
Fig. 11. Degradation mechanism of picloram.

1116
A. Ghauch / Chemosphere 43 12001) 1109±1117
ring. The breakdown of the C±Cl bond neighboring to
the nitrogen atom should arise before the other C±Cl
bonds because of the electrophile property of the ni-
trogen atom in an acidic media. However, we are not
interested here if the elimination of the ®rst chlorine
atom occurs on one carbon before another, but we can
conclude that the dechlorination takes place in the
presence of ZVIP in the media. A degradation map
showing the dechlorination of picloram is presented in
Fig. 11.
Acknowledgements
The author acknowledges Prof. Rima of the Leba-
nese University %Lebanon), Prof. Martin-Bouyer of the
LCIE laboratory, and Dr. Gallet of the laboratory of
Dynamics of Altitude Ecosystems, University of Savoie
%France) for their helpful cooperation. I also gratefully
acknowledge Joel Suptil for his tireless e€orts in con-
structing the treatment apparatus.
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