Rapid reductive dechlorination of atrazine by zero-valent iron under acidic conditions

Article abstract of DOI:10.1016/S0269-7491(00)00033-6

The dechlorination of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) via reaction with metallic iron under low-oxygen conditions was studied using reaction mixture pH values of 2.0, 3.0, and 3.8. The pH control was achieved through addit

Full text of DOI:10.1016/S0269-7491(00)00033-6

Environmental Pollution 111 (2001) 21±27
www.elsevier.com/locate/envpol
Rapid reductive dechlorination of atrazine by zero-valent iron under
acidic conditions
T. Dombek, E. Dolan, J. Schultz, D. Klarup *
Department of Chemistry, Eastern Illinois University, Charleston, IL 61920, USA
Received 27 July 1999; accepted 18 December 1999
``Capsule'': The e€ect of pH on atrazine dechlorination via metallic iron may be due to triazine ring protonation.
Abstract
The dechlorination of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) via reaction with metallic iron under low-
oxygen conditions was studied using reaction mixture pH values of 2.0, 3.0, and 3.8. The pH control was achieved through addition
of sulfuric acid throughout the duration of the reaction. The lower the pH of the reaction mixture, the faster the degradation of
atrazine. The surface area of the sulfuric acid-treated iron particles was 0.31 (‹0.01) m2 g 1 and the surface area normalized initial
pseudo-®rst order rate constants (kSA, where rate=kSAÂ(surface area/l)Â[Atrazine]) at pH values of 2.0, 3.0, and 3.8 were equal to,
respectively, 3.0 (‹0.4)Â10 3 min 1 m 2 l, 5 (‹3)Â10 4 min 1 m 2 l, and 1 (‹1)Â10 4 min 1 m 2 l. The observed products of the
degradation reaction were dechlorinated atrazine (2-ethylamino-4-isopropylamino-1,3,5-triazine) and possibly hydroxyatrazine (2-
ethylamino-4-isopropylamino-6-hydroxy-s-triazine). Triazine ring protonation may account, at least in part, for the observed e€ect
of pH on atrazine dechlorination via metallic iron. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Atrazine; Iron; Dechlorination; Herbicide; Degradation
1. Introduction
compounds, including chlorinated solvents. Solvent
reductive dechlorination using zero valent metals has
been studied extensively in recent years (Gillham and
O'Hannesin, 1994; Matheson and Tratnyek, 1994; John-
son et al., 1996; Orth and Gillham, 1996; Roberts et al.,
1996; Allen-King et al., 1997). Although the mechanisms
of these reactions are not well elucidated, it appears that,
generally, a two-electron transfer occurs either directly at
the iron surface or through some intermediary (Weber,
1996), oxidizing the iron to iron(II) ions:
The herbicide atrazine (2-chloro-4-ethylamino-6-iso-
propylamino-1,3,5-triazine) has been used extensively in
corn, sorghum, and sugarcane ®elds for the last 30 years
(Bridges, 1998). Although atrazine usage per acre has
dropped in recent years, 80 million pounds of atrazine
still are applied annually in the USA alone. This heavy
usage, combined with atrazine's relatively long half-life
in the environment (up to 1 year) (Wackett, 1999) means
that agricultural ®elds remain as signi®cant sources of
atrazine to surface and groundwater systems. Concerns
over drinking water contamination by atrazine, com-
bined with the uncertainty of atrazine's carcinogenic and
toxicological e€ects (and that of its metabolites), have
spurred interest in techniques that might more rapidly
degrade atrazine and its metabolites.
‡
R Cl ‡ Feꢀꢁs† ‡ H ! Fe2‡‡R H ‡ Cl
ꢁ1†
The pH of the reaction solution is important in these
reactions, probably because a low pH allows more of
the iron surface to remain available for reaction with
the halogenated molecule via:
One method to degrade atrazine is via its reaction
with metallic iron. This was ®rst reported by Sweeny
(1979), who also noted the dechlorination of many other
‡
Feꢁs† ‡ 2H ꢁaq† ! Fe2‡ꢁaq† ‡ H2ꢁg†
ꢁ2†
or at least assists in the corrosion of the iron by the
halogenated molecule [Eq. (1)]. At higher pH values
oxide and hydroxide coatings undoubtedly develop
which hinder access to the Feo surface. Pretreatment of
* Corresponding author. Tel.: +1-217-581-6227; fax: +1-217-581-
6613.
E-mail address: cfdgk@eiu.edu (D. Klarup).
0269-7491/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0269-7491(00)00033-6

22
T. Dombek et al. / Environmental Pollution 111 (2001) 21±27
iron with mineral acids, or increasing the available iron
surface area, generally increases the rate of metal±
solvent dechlorination reactions (Johnson et al., 1996;
Su and Puls, 1999), con®rming the importance of iron
surface availability.
studied at pH 2.00 (‹0.3), pH 3.00 (‹0.3) and pH 3.8
(‹0.3) in a 500 ml round bottom three-neck ¯ask under
N2. One port was covered with a septum, the middle
port was used with a ground glass adapter and glass
stirring rod, and a pH electrode was clamped in the
third port and the opening sealed with Para®lm1.
The iron was prepared by stirring 10 g in 50 ml of 0.5
M H2SO4 for 10 min at 250 rpm, removing the wash
solution and repeating with a second wash. After
removal of the second wash solution, 200 ml of degas-
sed 45.9 ppm atrazine solution was added to the reac-
tion ¯ask and the pH adjusted with 3 M H2SO4. For the
pH comparative experiments, the reaction was con-
ducted at a stir rate of 750 rpm and the solution pH
continually adjusted by adding 3 M H2SO4 acid with a
syringe through the septum.
The duration of each reaction was approximately 2 h
during which 20.00-ml aliquots of the atrazine solution
periodically were removed from the reaction ¯ask (with a
syringe) for analysis. The reaction also was conducted
using deuterium oxide (Cambridge Isotope Laboratories,
Andover, MA, USA) at a measured pH of 2.0 (adjusted
with H2SO4) to con®rm reactant origins via comparative
gas chromatography±mass spectrometry (GC±MS).
Solution pH has also been found to be important in the
dechlorination of atrazine. Pulgarin et al. (1996) found a
low pH to greatly assist atrazine dechlorination under
photo-oxidizing conditions, attributing the e€ect to
enhanced corrosion of the iron and Fenton-like reac-
tions. Balmer and Sulzberger (1999) also found photo-
Fenton atrazine degradation in the presence of oxalate to
depend on solution pH. In this case the pH controlled the
iron speciation in the solution which in turn controlled
the photolysis eciency. These aerobic mechanisms,
however, are quite di€erent from the abiotic, anaerobic,
reductive dechlorination mechanism. Light and O2 are
central to the photo-oxidizing process where an hydro-
xide radical is responsible for the degradation.
Although no studies have been reported on the e€ect
of solution pH on the reductive dechlorination of atra-
zine with iron, investigators have found that under
ambient (environmentally relevant) pH conditions atra-
zine either degrades only slightly or sorbs onto the iron
surface. Monson et al. (1998), found dechlorination of
30% of introduced atrazine in the dark, although the
time span of their experiments was very long (25 days),
and they used a very high iron to atrazine ratio (10 g to
2.2. Aliquot treatment
For the pH comparative studies, solid phase extraction
cartridges (3 ml AccuBond1, 40-mm particle size, 60 A
pore size, J&W Scienti®c, Folsom, CA, USA) were used
to concentrate and purify the reaction mixture species for
analysis. The cartridges were prepared by pulling 20 ml
of deionized (DI) water, 20 ml of methanol, and 5 ml of
DI water through the cartridges by vacuum ®ltration.
Then the 20.00-ml reaction aliquot was pulled through
the cartridge followed by 5 ml of DI water. A standard
sample was prepared by extracting a 20.00-ml aliquot of
45.9 ppm atrazine solution in the same manner. Two
milliliters of methanol [high performance liquid chro-
motography (HPLC) grade, Mallinckrodt, St. Louis,
MO, USA] were added to the cartridge and allowed to
gravitate through the cartridge. The methanol solutions
were then ®ltered using Millex1 Ð GV13 0.22 mm, 13 mm
millipore ®lter units and stored over 3A molecular sieve
until analysis with GC, GC±MS, or HPLC.
For quantitative GC analysis of the samples, 1 ml of
each sample was combined with 0.25 ml of 780 ppm 3-
nitro-o-xylene (Chem Service, West Chester, PA, USA)
which served as an internal standard. The system used
for analysis was a Hewlett Packard 5890 GC equipped
with manual injection, ¯ame ionization detection, and a
DB1 30 mÂ0.53 mm ID, 1.5-mm ®lm thickness capillary
column (J&W Scienti®c). Injector and detector tem-
peratures were held at 250ꢀC and the oven programmed
to start at 50ꢀC for 3 min, then ramped at 20ꢀ/min up to
250ꢀ and held at this temperature for 2 min.
1
30 ml of 1000 mg l atrazine). The pH of their solution
rose to approximately 10 during the reaction due to
water dissociation and H2 generation. Davenport (1996)
concluded that atrazine had adsorbed to the surface of
the iron, not dechlorinated, although dechlorination
of alachlor and metolachlor proceeded quite readily in
non-acidi®ed media (Eykholt and Davenport, 1998).
Similarly, sorption of atrazine onto iron was observed,
using an initial pH of 6.1, by Singh et al. (1998). These
reported results indicate that at ambient pH values the
dechlorination of atrazine with iron is relatively dicult
to achieve compared to other pesticides and chlorinated
solvents. Davenport (1996) suggested that the reduction
potential for the dechlorination of atrazine might be too
high for iron to be e€ective.
The objective of the study reported here was to deter-
mine how acidi®ed media a€ects the reductive dechlor-
ination of atrazine with metallic iron. To accomplish this
we measured the rate of the reaction as a function of pH.
2. Materials and methods
2.1. Reaction conditions
The reaction of atrazine (Pfaltz & Bauer INC,
Waterbury, CT, USA) with iron powder (100-mesh,
Fisher Scienti®c Company, St. Louis, MO, USA) was

T. Dombek et al. / Environmental Pollution 111 (2001) 21±27
23
GC±MS was conducted with a Hewlett Packard 5890
GC equipped with a HP-5 M.S. (Crosslinked 5% Ph Me
Silicone), 30 MÂ0.25 mmÂ0.25 mm ®lm thickness col-
umn and a Hewlett Packard Model 5971 Mass Selective
Detector. A similar temperature program was used as
with GC alone.
System (Quantachrome Corp., Boynton Beach, FL,
USA). In order to minimize exposure to oxygen, H2SO4
treatments were carried out directly within the BET
analysis cell and degassed under N2.
HPLC was conducted on the ®nal reaction solution of
the pH 2.00 run. This apparatus consisted of a Beckman
model 112 HPLC pump equipped with a 20-ml sample
loop (Beckman Coulter Inc., Fullerton, CA, USA), a
Waters Model 450 variable wavelength detector (Waters
Corp., Milford, MA, USA) (set to 222 nm) and a Shi-
madzu Chromatopac C-RIB Data Processor (Shimadzu
Scienti®c Instruments Inc., Columbia, MD, USA). The
column used was a 4.6Â250 mm reverse-phase What-
man PARTISPHERE1 C-18 column equipped with a
removable guard cartridge assembly (Whatman Inc.,
Clifton, NJ, USA). The mobile phase used was degassed
HPLC grade 40:30:30 methanol±acetonitrile±water,
bu€ered to pH 7 with 0.01 M ammonium acetate.
3. Results and discussion
3.1. Surface area
The sulfuric acid treatment of the iron particles did
not drastically alter the measured BET surface area of
the iron. Prior to acid treatment, the measured surface
area was 0.30 (‹0.01) m2
g
1. After treatment, the
measured surface area was 0.31 (‹0.01) m2 g 1. This
value is in good agreement with the surface area repor-
ted by Su and Puls (1999) for hydrochloric acid-treated
Fisher 100-mesh iron.
3.2. Atrazine degradation rates
2.3. Surface area determination
Fig. 1 shows how the rate of atrazine degradation
varied with pH. No evidence of atrazine sorption to the
iron (by analysis of metal wash solutions) was observed
under these acidic conditions, and the absence of light
did not a€ect our results. At a pH of 2.0 the atrazine
The Brunaver-Emmett-Teller (BET) surface areas of
the iron particles both before and after H2SO4 treat-
ment were determined using a Quantasorb Sorption
Fig. 1. Representative runs of the degradation of atrazine via reaction with metallic iron as a function of pH. pH control (to within ‹0.3 pH units)
was achieved by continual monitoring and addition of 3 M sulfuric acid as required. Uncertainty estimates (error bars) re¯ect standard error of
exponentially ®tted results of multiple runs using identical starting conditions at pH 3.0 (‹.3). The surface area normalized initial pseudo-®rst order
rate constants (kSA, where rate=kSAÂ(surface area/l)Â[Atrazine]) at pH solution values of 2.0, 3.0, and 3.8 were equal to, respectively, 3.0
3
2
4
1
2
4
1
2
(‹0.4)Â10 min 1 m l, 5 (‹3)Â10 min
m
l, and 1 (‹1)Â10 min
m
l.

24
T. Dombek et al. / Environmental Pollution 111 (2001) 21±27
essentially was completely degraded by 100 min. In
contrast, only about 50% of the atrazine was degraded
within this time span at a pH of 3.8. The surface area
our quantitative data do not show if the degraded atra-
zine ®rst formed DCA, then HA, or if HA was an
additional product of atrazine degradation. Neither of
the major microbial metabolites of atrazine, de-ethyla-
trazine and de-isopropylatrazine, was detected.
normalized initial pseudo-®rst order rate constants (kSA
,
where rate=kSAÂ(surface area/l)Â[Atrazine]) at pH
values of 2.0, 3.0, and 3.8 were equal to, respectively, 3.0
3
Fig. 3A shows the GC±MS chromatogram of a sam-
ple collected from a pH 2.0 solution at 90 min. The mass
spectrum of the peak at 10.7 min, shown in Fig. 3B, is
DCA. The parent ion of this molecule has a mass/charge
ratio of 181, corresponding to the replacement of the
chlorine of atrazine with hydrogen (215 35+1= 181).
The lack of an appropriate isotopic ratio of the parent
ion con®rms that this molecule does not contain chlorine.
Fig. 4A shows the GC±MS chromatogram of a sample
collected during a run using D2O as the solvent. The
mass spectrum of the DCA produced during this reaction
(Fig. 4B) proves that the proton replacing the chlorine
of atrazine came from the solvent. The parent ion in this
mass spectrum has a mass/charge ratio of 182, one more
than when the reaction is conducted in water.
1
2
4
1
2
(‹0.4)Â10 min
m
l, 5 (‹3)Â10 min
m
l,
l. Although these values
4
1
2
and 1 (‹1)Â10 min
m
essentially are the same as the overall pseudo ®rst-order
rate constants, it is the initial rates which best re¯ect
known experimental conditions. As the reaction pro-
ceeds, the solution:iron ratio changes due to sample
removal and the available active iron surface is likely to
change due to solution conditions.
Within the relatively short time-span of these ex-
periments, the only product identi®ed via GC, and
con®rmed with GC±MS, was dechlorinated atrazine
(DCA, 2-ethylamino-4-isopropylamino-1,3,5-triazine).
Fig. 2 shows the gas chromatograms of samples col-
lected during a pH 2.0 degradation reaction. With this
instrumentation the retention time of atrazine was 11.6
min and that of DCA was 10.3 min. The concentration
of DCA did, at all pH values, initially grow as the atra-
zine concentration decreased. However, after its initial
growth, the concentration of DCA often began to
decrease and/or show concentration ¯uctuations. Clearly
the DCA can react further. HPLC analysis of the ®nal
(pH 2.0) reaction solution (which contained no atrazine)
showed the presence of what was possibly hydroxy-
atrazine (HA) (based on retention time comparison).
Because of the inability of GC to detect this compound,
3.3. The role of solution pH
It is clear from these results that pH plays a role in the
rate of reductive dechlorination of atrazine via metallic
iron. The rate of the process increases as the solution pH
decreases. Enhanced corrosion of the iron at lower
pH values is probably one of the reasons for the
observed pH dependence, but the general success of
iron-mediated reductive dechlorination reactions at
ambient (environmental) pH values reported by other
Fig. 2. Gas chromatograms of aliquots withdrawn from a pH 2.0 experimental run at various times. The 3-nitro-o-xylene was used as an internal
standard. Note the initial formation and the degradation of the dechlorinated atrazine as the reaction proceeded.

T. Dombek et al. / Environmental Pollution 111 (2001) 21±27
25
Fig. 3. (A) The total ion current gas chromatogram from a typical 90-min sample of a pH 2 reaction. The peak at 12.0 min is atrazine and that at 10.7 min
is dechlorinated atrazine (DCA). (B) The mass spectrum associated with the chromatographic peak at 10.7 min, clearly identifying this peak as DCA.
investigators suggests reactive iron sites are available
even at near neutral pH values. Given the fact we did
not observe atrazine dechlorination at ambient pH
values (although the time span of our experiments was
relatively short), it seems reasonable to suspect that
there are factors other than surface corrosion which
account for the pH e€ects presented here.
The pKa of (protonated) atrazine is 1.7, so approxi-
mately 33% of the atrazine is protonated at a pH of 2,
5% at a pH of 3, and 1% at a pH of 4. The correlation
between the rate of degradation and the degree of atra-
zine protonation certainly is suggestive. The protona-
tion of the triazine ring would draw electron density
from the C Cl bond, weakening the bond and better
allowing the dechlorination reaction to proceed. In
e€ect, reducing the magnitude of the reduction potential
to a point where dechlorination can occur readily. Pos-
pisil et al. (1995) determined that electrochemical
reductive dechlorination of atrazine at a mercury elec-
trode required acidic media, and they concluded the
reactive species was the protonated atrazine. It seems
probable that protonated atrazine also is the actual
species which undergoes reductive dechlorination via
metallic iron:
‡
‰H Atr ClŠ ‡ Feꢁs† ! Atr H ‡ Fe2‡ ‡ Cl
ꢁ3†
where [H Atr Cl]+ represents the protonated atraz-
ine and Atr H the dechlorinated product. The lower
the pH, the higher the equilibrium concentration of the
protonated atrazine and the faster the reaction rate.
3.4. Atrazine hydrolysis
Solution pH also plays a signi®cant role in the acid-
catalyzed hydrolysis of atrazine. Plust et al. (1981)

26
T. Dombek et al. / Environmental Pollution 111 (2001) 21±27
Fig. 4. (A) The total ion current gas chromatogram obtained when carrying out the reaction in D2O. (B) The mass spectrum associated with the
dechlorinated atrazine (DCA) which elutes at 10.7 min. The parent ion and fragmentation pattern con®rm that the substituted deuterium atom
comes from the solvent.
found that although atrazine was quite stable in water
at neutral pH, it does undergo acid hydrolysis at lower
pH values. The hydrolysis reaction occurs with the pro-
tonated atrazine species. Although this con®rms the
reactivity of the protonated species versus the neutral
species (which is a general observation for triazines; see
Joule et al., 1995) the hydrolysis reaction, even at a pH
of 2.0, is not rapid enough to account for the degrada-
tion of atrazine we observed. Indeed, we saw no meas-
urable atrazine degradation (within the time-span of our
experiments) in the absence of iron.
Only the pH 2 phosphoric acid bu€er held promise for
pH control, although even here the results were not
consistent with the non-bu€ered pH 2 experiments and
were dicult to interpret (data not shown). When the
other bu€er systems were attempted for pH control,
either the reaction did not occur, or the atrazine was
degraded while in solution, perhaps via a speci®c-acid±
general-base sequence (Plust et al., 1981).
In addition to sulfuric acid, nitric and hydrochloric
acid were tested in carrying out the reaction. Prelimin-
ary results show that although degradation occurs with
all of these acids, the rates vary: RateH SO
>
4
2
3.5. In¯uence of bu€ers and acidic species
RateHCl > RateHNO (data not shown). Further experi-
3
ments to determine the reasons behind the rate depen-
dence on acid species are required. It is possible for
sulfate to act as an oxidant under our reaction condi-
tions, although atrazine oxidation usually results in
The pH control of the reaction solutions was dicult.
Phosphoric, formic, sulfurous, and acetic acid bu€ering
systems were employed in an attempt to control pH.

T. Dombek et al. / Environmental Pollution 111 (2001) 21±27
27
initial products which are dealkylated or oxidized (Lar-
son et al., 1991), none of which was observed in our
experiments.
Gillham, R.W., O'Hannesin, S.F., 1994. Enhanced degradation of
halogenated aliphatics by zero-valent iron. Ground Water 32 (6),
958±967.
Johnson, T.L., Scherer, M.M., Tratnyek, P.G., 1996. Kinetics of
halogenated organic compound degradation by iron metal. Envir-
onmental Science and Technology 30, 2634±2640.
4. Conclusions
Joule, J.A., Mills, K., Smith, G.F., 1995. Heterocyclic Chemistry, 3rd
Edition. Chapman & Hall, New York.
Larson, R.A., Schlauch, M.B., Marley, K.A., 1991. Ferric ion pro-
moted photodecomposition of triazines. Journal of Agricultural and
Food Chemistry 39, 2057±2062.
Compared to other chlorinated compounds which
have been reductively dechlorinated via reaction with
metallic iron under anaerobic conditions, atrazine
dechlorination at environmental pH occurs very slowly
and with great diculty. At low pH, however, the reac-
tion proceeds readily, producing dechlorinated atrazine
and possibly hydroxyatrazine. The in¯uence of pH on
the degradation rate is probably the result of both
maintaining the available iron surface area for reaction
and protonation of the triazine ring.
Matheson, L.J., Tratnyek, P.G., 1994. Reductive dehalogenation of
chlorinated methanes by iron metal. Environmental Science and
Technology 28 (12), 2045±2053.
Monson, S.J., Ma, Li., Cassada, D.A., Spalding, R.F., 1998. Con-
®rmation and method development for dechlorinated atrazine from
reductive dehalogenation of atrazine with Feꢀ. Analytica Chimica
Acta 373, 153±160.
Orth, W.S., Gillham, R.W., 1996. Dechlorination of trichloroethene in
aqueous solution using Feꢀ. Environmental Science and Technology
30, 66±71.
Plust, S.J., Loehe, J.R., Feher, F.J., Benedict, J.H., Herbrandson,
H.F., 1981. Kinetics and mechanism of hydrolysis of chloro-1,3,5-
triazines. Atrazine. Journal of Organic Chemistry 46 (18), 3661±
3665.
Acknowledgements
Pospisil, L., Trskova, R., Fuoco, R., Colombini, M.P., 1995. Electro-
chemistry of s-triazine herbicides: reduction of atrazine and terbu-
tylazine in aqueous solutions. Journal of Electroanalytical
Chemistry 395, 189±193.
The authors would like to thank the Council for
Faculty Research at Eastern Illinois University for
®nancial support and Mr. Tim Davis for laboratory
assistance.
Pulgarin, C.O., Schwitzguebel, J.-P., Peringer, P.A., Pajonk, G.M.,
Bandara, J.S., Kiwi, J.T., 1996. Abiotic oxidative degradation of
atrazine on zero-valent iron activated by visible light. J. Adv. Oxid.
Technol. 1, 94±101.
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