Near-quantitative mineralization of two refractory triazines under hydrothermal-supercritical aqueous conditions assisted by ozone and UV/ozone

Article abstract of DOI:10.1039/b211046f

Refractory atrazine and cyanuric acid were degraded under hydrothermal and supercritical aqueous media (HY-SC) conditions, as well as in the presence of ozone (HY-SC/O₃) and UV-illuminated ozone (HY-SC/UV/O₃) to assess whether the efficacy of the decomposition process could be enhanced using a single-pass flow-through treatment device under a constant pressure of 23 MPa. The progress of the degradation was evidenced by the extent of removal of total organic carbon (TOC) in solution, by UV absorption spectroscopy (opening of the atrazine and cyanuric acid heterorings), and by the extent of deamination (formation of NH₄⁺) and dechlorination (release of Cl^- ions) of the two compounds. Loss of atrazine was confirmed by LC-MSD techniques in the positive ion mode. Formation of various intermediates from the degradation of atrazine was substantiated by positive and negative ion mode MSD analyses. Dechlorination of atrazine occurred around 100°C under hydrothermal conditions for the HY-SC and HY-SC/O₃ processes but not for HY-SC/UV/O₃; it did increase rapidly at higher temperatures (beginning at ca. 220-230°C) for all three methods: HY-SC through HY-SC/UV/O₃. Deamination, removal of TOC, and loss of atrazine mass spectral features began around 260-280°C in hydrothermal aqueous media. Degradation of cyanuric acid showed a similar behavior. For the treated effluent solution in the collector reservoir, ozonation enhanced somewhat both dechlorination and mineralization, but had no significant effect on the deamination of either atrazine or cyanuric acid.

Full text of DOI:10.1039/b211046f

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Near-quantitative mineralization of two refractory triazines under
hydrothermal-supercritical aqueous conditions assisted by ozone
and UV/ozone
a
Satoshi Horikoshi, Yoshinori Wada, Natsuko Watanabe, Hisao Hidaka* and Nick Serpone*
a
a
a
b
a
Frontier Research Center for the Global Environment Protection (EPFC), Meisei University,
-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan. E-mail: hidaka@epfc.meisei-u.ac.jp
Department of Chemistry and Biochemistry, Concordia University, 1455 de Maisonneuve Blvd.
West, Montr e´ al, PQ H3G 1M8, Canada. E-mail: serpone@vax2.concordia.ca
2
b
Received (in Montpellier, France) 8th November 2002, Accepted 10th March 2003
First published as an Advance Article on the web 26th June 2003
Refractory atrazine and cyanuric acid were degraded under hydrothermal and supercritical aqueous media
(
HY-SC) conditions, as well as in the presence of ozone (HY-SC/O ) and UV-illuminated ozone (HY-SC/UV/
3
3
O ) to assess whether the efficacy of the decomposition process could be enhanced using a single-pass flow-
through treatment device under a constant pressure of 23 MPa. The progress of the degradation was evidenced
by the extent of removal of total organic carbon (TOC) in solution, by UV absorption spectroscopy (opening of
+
the atrazine and cyanuric acid heterorings), and by the extent of deamination (formation of NH
ꢀ
4
) and
dechlorination (release of Cl ions) of the two compounds. Loss of atrazine was confirmed by LC-MSD
techniques in the positive ion mode. Formation of various intermediates from the degradation of atrazine was
substantiated by positive and negative ion mode MSD analyses. Dechlorination of atrazine occurred around
1 processes but not for HY-SC/UV/O
did increase rapidly at higher temperatures (beginning at ca. 220–230 C) for all three methods: HY-SC through
ꢁ
00 C under hydrothermal conditions for the HY-SC and HY-SC/O
3
3
; it
ꢁ
HY-SC/UV/O . Deamination, removal of TOC, and loss of atrazine mass spectral features began around
3
ꢁ
2
60–280 C in hydrothermal aqueous media. Degradation of cyanuric acid showed a similar behavior. For
the treated effluent solution in the collector reservoir, ozonation enhanced somewhat both dechlorination
and mineralization, but had no significant effect on the deamination of either atrazine or cyanuric acid.
risk for impaired sexual development. Not surprisingly, seven
European countries have banned the use of atrazine and any
pesticide that tends to occur in drinking water at levels greater
Introduction
Water pollutants originating from usage of agrochemicals,
such as herbicides and pesticides, tend to resist bacteriological
degradation. Accordingly, accumulation of these agrochem-
5
than 0.1 ppb.
The degradation of triazine-type herbicides has been investi-
gated by advanced oxidation technologies (AOTs) such as the
photocatalytic method, by Fenton chemistry, by UV/ozone
ꢀ
1
icals present at ppm (mg L ) levels in agricultural runoffs
can cause considerable problems to aquatic ecosystems (the
drinking water standard level for atrazine set by the U.S.
Environmental Protection Agency is currently 3 ppb, i.e.,
6
,7
8
9
ꢀ
ꢂ
radicals produced by
oxidation, and by oxidation by SO
4
1
0
flash photolysis and radiolysis techniques. The radiolytic
method caused 76% of cyanuric acid to be decomposed after
an applied dose of 18 kGy. Relevant studies have demon-
strated that degradation of atrazine generates cyanuric acid
ꢀ
1
3
mg L ). To the extent that most of these agrochemicals
are classified as endocrine disruptors, treatment of their waste-
waters poses substantial challenges in the remediation of the
ecosystems involved. Degradation of these disruptors through
usage of strong oxidizing agents necessitates the development
of practical devices to achieve an acceptable treatment meth-
odology for such waste chemical products. Disposal of refrac-
tory, non-biodegradable chemicals is an important aspect of
green chemistry technologies.
1
1,12
as the final product. Atrazine could not be mineralized
further under the AOT conditions used, even though the oxi-
dative reactions involved the highly oxidizing OH radicals.
6
,7
ꢂ
Consequently, it is relevant to examine other treatment meth-
odologies (e.g., hydrothermal and supercritical water) to
degrade the refractory atrazine and cyanuric acid. The degra-
dation efficiency is remarkably enhanced by the cooperative
combination of the photocatalytic method in hydrothermal
Pesticides bearing a triazine skeleton are widely used world-
wide to treat soils for agriculture purposes. Consequently, tria-
zines accumulate in nature for long periods of time without
noticeable (bio)degradation. As an example of the damage
such triazines cause, we need only note that the reproduction
of alligators, plankton and fish is highly affected by the herbi-
1
3
and supercritical water media.
The goals of the present study were twofold. (1) Develop
and establish a possible treatment methodology to degrade
atrazine and cyanuric acid in a hydrothermal and supercritical
water environment using a single-pass flow-through reactor
system. Typically, cyanuric acid is converted to the acid anhy-
1
–3
4
cide atrazine. As a specific example, Hayes and coworkers
found that when male tadpoles were exposed to levels of atra-
zine greater than 0.1 ppb, this triazine induced aromatase and
promoted the conversion of testosterone to estrogen, thereby
effectively demasculinizing male frogs. It is not unlikely that
other amphibian species exposed to such triazines are also at
ꢁ
dride (HONC) upon heating at 100 C, followed by further
conversion to cyanic acid after increasing the temperature to
ꢁ
300 C. However, cyanic acid tends to polymerize back to cya-
nuric acid upon cooling. Accordingly, it was relevant to assess
1
216
New J. Chem., 2003, 27, 1216–1223
DOI: 10.1039/b211046f
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003

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the degradation of atrazine and cyanuric acid by the methodol-
ogies herein examined. (2) Assess and improve the efficacy of
the hydrothermal (HY) and supercritical (SC) water methodol-
ogies to decompose the two refractory products using the com-
bination HY-SC, and also study combinations in which ozone
3
is introduced (HY-SC/O ) and is subsequently irradiated with
UV radiation (HY-SC/UV/O ) from a suitable light source.
3
Fig. 2 Temperature rise of the solution and variation of pressure
against flow time.
Experimental
ꢀ
2
sapphire windows. The light irradiance was ca. 6 mW cm
at 250 nm and ca. 7.2 mW cm at 360 nm.
ꢀ2
Materials and procedures
The quantity of TOC remaining in solution (reservoir; see
Fig. 1) was determined by a Shimadzu TOC-5000A analyzer.
Temporal variations in the concentrations of both refractory
products were monitored though UV absorption spectroscopy
utilizing a JASCO UV/VIS/NIR V-560V spectrophotometer.
The Hastelloy C276 single-pass flow-through reactor is illu-
strated in Fig. 1. It consists of a heated chamber fitted with
quartz and sapphire windows for illuminating the solution
with UV light (Hg lamp), a high-pressure pump, a cooling
jacket, an ozone generator, and a backpressure regulator
connected to a PC computer. An aqueous solution of either
atrazine (0.1 mM; C H ClN ) or cyanuric acid (0.1 mM;
ꢀ
+
ꢀ
Formation of Cl , NH
4
and NO ions was assayed with a
3
JASCO HPLC chromatograph, equipped with a CD-5 conduc-
tivity detector using either a Y-521 cationic column or an I-524
anionic column. Decrease of the quantities of atrazine was also
followed using an Agilent Technologies HP1100 LC-MSD
apparatus (liquid chromatograph coupled to an electrospray
mass spectral detector operated in both negative and positive
ion modes). The eluent was a solution of methanol and water
8
14
5
3 3 3 3
C H N O ), supplied by Kanto Kagaku Co. and Wako Pure
Chem. Industries Ltd., respectively, was introduced into the
reactor using a JASCO high-pressure pump. The temperature
ꢁ
of the solution was allowed to rise to ca. 400 C at a rate of
ꢁ
ꢀ1
2
C min . Control of the pressure at 23 MPa in the reactor
was achieved with a JASCO backpressure regulator (BPR).
The temporal dependence of temperature and pressure on
the time of flow of the solution is illustrated in Fig. 2. The flow
{
1:1 v/v; the ion-exchanged water used throughout was ultra-
ꢀ
1
pure to < 5 mg L . TOC and electrical conductivity was < 0.1
mS cm ). The LC-MSD system was equipped with an Agilent
ꢀ1
ꢀ1
rate of the pump was 0.5 mL min ; heating of the solution
was begun simultaneously with the start of the pumping. Sam-
ples of the solution were collected from the reservoir after
rapid cooling of the solution through the cooling jacket. The
decomposition of atrazine and cyanuric acid was examined
using the following three combined techniques: (a) hydrother-
mal and supercritical water method (HY-SC; note that water
Eclipse XDB-C column. The mass spectra from the API-ES
ionization mode were recorded under conditions whereby the
fragmenter was set at 100 V, the capillary voltage was 2200
8
ꢁ
V and the dry gas temperature was 300 C. The intermediates
generated from the degradation of atrazine were also identified
with the HP1100 LC-MSD using direct injection into the mass
spectral detector with methanol–water as the eluent. The set-
tings of the mass spectral detector were identical to those used
for atrazine.
ꢁ
goes supercritical at 374 C and 22 MPa), (b) an integrated
HY-SC method in a solution that contained dissolved ozone
(
HY-SC/O ), and (c) a combination in which the HY-SC/O
3 3
system was irradiated by UV light (HY-SC/UV/O
was introduced into the aqueous solution from the appropriate
Tokyokaken OZM-01D ozone generator in which O was gen-
erated from pure oxygen gas in about 60% yield, as stated in
the manual supplied by Tokyokaken Co. Ltd. The concentra-
tion of ozone in the aqueous atrazine solution was monitored
and maintained at ca. 0.2 ppm with a Lab-ozone detector
3
). Ozone
Ozone under UV radiation
3
ꢂ
Formation of OH radicals from UV-irradiated ozone takes
place through the processes summarized by eqns. (1)–(4).
1
4
ꢂ
O3 þ H2O þ hn ! f2 HO g ! H2O2
ð1Þ
H2O2 ꢃ HO2ꢀ þ H
þ
ð2aÞ
ð2bÞ
ð3Þ
(
3
Asahi Technoglass Co. Ltd.). In the HY-SC/UV/O com-
bined method, a 250 W mercury lamp supplied the UV radia-
tion impinging upon the solution through the quartz and
ꢀ
ꢂ
ꢀꢂ
HO2 þ O3 þ hn ! HOO þ O3
ꢀ
ꢂ
þ
ꢂ
ꢂ
O3 þ H ! HO3 ! HO þ O2
ꢀ
ꢂ
ꢂ
ꢀ
O3 þ H2O ! HO þ HO þ O2
They typically involve degradation of ozone. Detection of the
ð4Þ
ꢂ
OH radicals by electron spin resonance (ESR) methods tends
to be somewhat difficult because of the rather short lifetime of
these radicals (a few microseconds). Accordingly, when needed
ESR measurements were conducted at liquid nitrogen tem-
peratures or at ambient temperatures using the DMPO
(
5,5-dimethyl-1-pyrroline-N-oxide) spin trap agent. It was
ꢂ
not possible to detect formation of OH radicals at tempera-
tures much above ambient because of the thermal decomposi-
tion of DMPO. Substantiation of the formation of O
radicals (see Fig. 3) was achieved in the presence of DMPO
under ambient conditions with a JEOL JES-TE200 ESR
ꢀ
ꢂ
3
Fig. 1 Experimental setup of the single-pass flow-through reactor
(60 mL) equipped with a pump and a backpressure regulator (BPR).
New J. Chem., 2003, 27, 1216–1223
1217

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higher temperatures. Reaction times for complete conversion
of atrazine under the low-temperature HY conditions are much
longer.
The relevant UV absorption spectral patterns are displayed
in Fig. 5, whereas Fig. 6 displays the decreases in the concen-
tration of atrazine monitored by UV absorption (wavelength,
2
22 nm) during decomposition by the HY-SC, HY-SC/O
3
,
and HY-SC/UV/O methods. We note that the quantity of
3
atrazine, and thus its UV absorption band, during its degrada-
ꢁ
tion by the HY-SC method decreased rapidly at ca. 259 C;
that is, at the same temperature as the temperature of disap-
pearance of atrazine monitored by the LC-MSD technique
(
see below). These observations suggest, at least in part, that
the initial degradation of atrazine takes place through ring
opening in competition with formation of aromatic inter-
mediates.
Fig. 3 ESR spectrum of 5,5-dimethyl-2-ketopyrrolidone-N-oxyl
DMPOX) for the O -containing aqueous solution under UV irradia-
tion (ambient temperature and pressure).
The corresponding temperatures for the loss of UV absorp-
tion of atrazine at 222 nm during its decomposition by the
HY-SC/O and HY-SC/O /UV methods are very similar to
(
3
3
3
those of the HY-SC method. Nonetheless, there is a percepti-
ble difference, however slight, in the temperature region of
spectrometer. The ESR spectrum accorded with the spectrum
obtained from the JEOL-supplied database for 5,5-dimethyl-
ꢁ
2
50–300 C. Moreover, we observed no new UV absorption
features and no band shifts in the UV absorption spectra of
atrazine at 222 nm (Fig. 5). Also, we were unable to observe
UV absorption features of cyanuric acid (expected absorption
2
-pyrrolidone-N-oxyl (DMPOX).
ꢁ
Results and discussion
band at 193 nm) in the entire temperature range of 22–400 C.
The temperature-dependent evolutions of the NH4 and Cl
+
ꢀ
Degradation of atrazine
ions formed during the decomposition of atrazine are depicted
in Fig. 7(a) and Fig. 7(b), respectively. Under supercritical con-
ditions, the mineralization yield of the five nitrogen atoms in
the atrazine structure was 100% for degradation by all three
HY-SC, HY-SC/O and HY-SC/UV/O methods. The onset
3 3
temperature at which we observed the initial formation of
The thermally dependent decrease of the concentration of atra-
zine occurring during its decomposition by the HY-SC method
and monitored by LC-MSD techniques is summarized in
Fig. 4. Decomposition was slow at first from ambient to about
ꢁ
2
59 C, following which further heating caused rapid degrada-
+
ꢁ
ꢁ
NH4 ions was 225 ꢄ 10 C for the HY-SC and HY-SC/
tion of the atrazine up to about 364 C, at which point the LC-
MSD signal of atrazine (molecular weight of 217), seen at the
mass/charge ratio (m/z) of 216 in the positive ion mode, was
no longer observable at near-supercritical conditions. The data
of Fig. 4 were obtained from the water-soluble samples col-
lected from the reservoir. It is worth noting that under super-
critical conditions water behaves like an organic solvent, since
both its dielectric constant and the ion-product of water
decrease above the critical points of temperature and pres-
UV/O3 methods. In contrast, the onset for the HY-SC/O
ꢁ
3
method was 164 C [barely perceptible in Fig. 7(a)]. The nitro-
gen atoms in atrazine were only negligibly converted to NO
ꢀ
3
ꢁ
ions at 300 C by the HY-SC and HY-SC/O
nitrate ions were detected for the HY-SC/UV/O
3
methods. No
3
combination
under otherwise identical conditions. We deduce that the
ꢀ
initial formation of NO
of the nitrogen atoms in the substituent groups of the atrazine
3
ions originates from the conversion
1
5
structure, and not from the triazine skeleton. The negligible
sure. Evidently, the solubility of atrazine and of the degrada-
tion intermediates in supercritical water is different from the
solubility of these products usually encountered at ambient
temperatures and atmospheric pressure. In addition, the degra-
dation of this triazine is most efficient in supercritical water
ꢀ
quantity of NO
ozone to the HY-SC combination.
3
ions produced decreased on addition of
The nitrogen atoms in atrazine were nearly completely con-
verted to NH
+
4
ions under supercritical water conditions. Evi-
dently, each of the degradation method was able to induce
cleavage of the triazine skeleton in the atrazine structure. The
ꢁ ꢁ
(
temperature hydrothermal conditions (temperature range:
see results from ca. 370 C to 400 C) than under low
ꢁ
initiation of dechlorination by the HY-SC and HY-SC/O
3
ambient to 259 C), however, hydrothermal degradation in
ꢁ
ꢁ
methods occurred at temperatures slightly below 100 C, with
cleavage of the chlorine substituent to yield Cl ions being
the temperature range 260–370 C is evidently fast at the
ꢀ
nearly complete under supercritical conditions. Although
the initial formation of Cl ions for the HY-SC/UV/O3
ꢀ
ꢁ
method occurred at ca. 164 C, complete dechlorination did
not occur; only about 75–80% chloride ions were formed.
It is evident that the preferred method to dechlorinate atrazine
is the HY-SC/O method [Fig. 7(b)].
3
The dependence of the concentration of TOC on tempera-
ture assayed during the degradation of atrazine is depicted in
Fig. 8. Using the HY-SC method, the TOC increased some-
what relative to its initial concentration of ca. 9.5 ppm at tem-
ꢁ
peratures less than 100 C. The near 40% increase in TOC
ꢁ
under hydrothermal conditions at 84 C was likely due to the
high pressure (23 MPa) to which the solution was subjected
in the reactor. This may have led to concentrating atrazine
and any of the intermediates formed as a result of the sudden
pressure drop from 23 MPa to ambient pressure in the reser-
voir (Fig. 1). At temperatures from ambient to ca. 280 C, this
was followed by a slow decrease subsequent to which the TOC
Fig. 4 Decrease in atrazine concentration under hydrothermal (HY)
and supercritical (SC) aqueous conditions as evidenced by LC-MSD
detection.
ꢁ
1
218
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Fig. 5 Temperature dependent changes of the UV spectral patterns at selected temperatures during the degradation of an aqueous solution of
3 3
atrazine (0.1 mM) under different conditions: (a) HY-SC, (b) HY-SC/O and (c) HY-SC/UV/O .
ꢁ
ꢁ
decreased rapidly upon further heating. In contrast, the
ꢁ
possible intermediates obtained at 196, 259 and 343 C (posi-
tive ion mode, left column) and at 83.5, 133, and 364 C (nega-
increase of TOC below 100 C was not as significant when
employing the HY-SC/O
3
and HY-SC/UV/O
3
methods for
tive ion mode, right column). Molecular peaks of atrazine
appear at m/z 216 and 238 (positive ion mode, M + H and
M + Na peaks, respectively) and at m/z 214 (negative ion
mode, M ꢀ H peak) in accord with a mono-chlorinated species
(note the Cl-35 and Cl-37 isotopes). The positive peak of atra-
zine at m/z 216 is in accord with the database of standards
from the LC-MSD manufacturer Agilent Technologies. The
formation of cyanuric acid during the degradation of atrazine
by the HY-SC combination was not observed at m/z 128
(negative mode) by mass spectral methods at all temperatures
without the use of the liquid chromatographic technique. The
1
6
reasons that remain somewhat elusive. Nonetheless, we can-
not preclude variations in the hydrophilic/hydrophobic char-
acteristics of the intermediates formed, and to some extent,
any influence of the added ozone gas. The rate of decrease of
3
TOC by the HY-SC/O method was only slightly lower than
the rate by either the HY-SC or HY-SC/UV/O methods.
3
ꢁ
The percent loss of TOC at about 400 C was 89% by the
HY-SC method, 95% by the HY-SC/O method and 89% by
3
3
the HY-SC/UV/O method.
The UV absorption features of atrazine and the quantity of
TOC did not totally disappear, even under SC conditions.
However, to the extent that some nitrogen-free and chlorine-
free intermediates were generated by the three techniques,
effluents emanating from treated wastewaters containing
atrazine are expected to be relatively non-toxic and more
biodegradable.
ꢁ
peak at m/z 196 (M ꢀ H; 83.5 C; ii-negative spectrum) is
attributed to a dechlorinated species, which displayed a major
peak last seen at 364 C (v-positive spectrum). This intermedi-
ꢁ
ate was also seen in the mass spectra at m/z ¼ 222 (positive
LC-MSD mass spectra in the positive and negative ion
modes were recorded at all temperatures used in order to
detect and substantiate the nature of the intermediates pro-
duced in the degradation of atrazine. These spectra are
depicted in Fig. 9 (spectra i-positive and i-negative) for the
initial atrazine solution, followed by selected mass spectra of
+
Fig. 6 Loss of atrazine as evidenced by UV absorption spectroscopy
in the degradation of atrazine by the HY-SC, HY-SC/O and HY-SC/
Fig. 7 Temperature dependence of the formation of (a) NH
4
and
(b) Cl ions during the degradation of 0.1 mM atrazine (circles:
ꢀ
3
UV/O methods (O concentration maintained at 0.2 ppm).
3
3
3 3
HY-SC; triangles: HY-SC/O ; squares: HY-SC/UV/O ).
New J. Chem., 2003, 27, 1216–1223
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deamination and ultimately dechlorination to yield the final
product cyanuric acid, which under the photocatalytic condi-
tions used degraded no further. The minor pathway (ꢅ10%)
involved dechlorination first, followed by dealkylation and
ultimately deamination to cyanuric acid.
On the basis of the results obtained under our conditions, we
deduce that dechlorination was the major step at low tempera-
tures in the overall process, followed by dealkylation and de-
amination (see Scheme 1). Cleavage of the atrazine heteroring
competes with these events. Accordingly, it was relevant that
we also examine the degradation of the refractory cyanuric
acid under conditions otherwise identical to those used for
atrazine.
Degradation of cyanuric acid
The UV absorption spectral patterns recorded during the
degradation of cyanuric acid at selected temperatures from
ꢁ
ambient to ca. 400 C by the HY-SC, HY-SC/O
SC/UV/O methods are illustrated in Fig. 10. For the HY-
3
and HY-
3
SC method, the initial UV absorption band of a solution of
cyanuric acid (0.1 mM; enol form) at ambient temperature
ꢁ
occurs at 193 nm. Upon heating the solution to 103 C this
band remained but an additional band at 214 nm evolved,
1
7–19
which we ascribe to a keto-enol tautomeric equilibrium
between the keto form mostly present below pH 6 and the enol
form present at pH above 6 (see Scheme 2). Interestingly, the
presence of ozone appears to affect this keto-enol equilibrium
[compare Fig. 10(b) and Fig. 10(c) with Fig. 10(a)]. At higher
temperatures, the intensity ratio of the two bands varied
because of changes in pH with increase in temperature. The
enol form was produced as a result of an increase in pH from
initial pH ¼ 5.1 at ambient temperatures to pH ¼ 7.0 at
Fig. 8 Disappearance of TOC during the degradation of atrazine by
the (a) HY-SC, (b) HY-SC/O
3
3
and (c) HY-SC/UV/O methods.
ꢁ
mode, M + Na) recorded at 238 and 259 C. We assign this
0
ꢁ
peak to the 2,4-diamino-6-hydroxy-N-ethyl-N -(1-methyl-
ethyl)-1,3,5-triazine species (I, see Scheme 1below). The second
400 C. To test this notion we also recorded absorption spec-
tra, under otherwise identical conditions, of an aqueous solu-
tion of cyanuric acid upon varying the pH from 5.1 to 7.5.
In the slightly acidic media (pH ¼ 5.1), the spectrum displayed
only the band at ꢅ193 nm, whereas in the pH range 6.0–7.5 the
band at 214 nm also appeared, in exact accord with the recent
intermediate with m/z 126 (M + H) was first observed at
ꢁ
133 C (see iii-negative mode spectrum), which we attribute
to the unsaturated 2,4-diamino-6-hydroxy-1,3,5-triazine spe-
cies (II). An intermediate with m/z 102 (M + H) first appeared
ꢁ
ꢁ
17
at 238 C and was still observable at 343 C (see e.g. the
iii-positive spectrum). It is likely due to the unsaturated hydro-
study of Cantu and coworkers (see their Fig. 3, left panel)
and identical to our observations illustrated in Fig. 10. Cya-
ꢁ
xylamine species CH –N=CH–N=CH–NHOH. At higher
nuric acid has two ionization constants (25 C), K ¼ 6.31 ꢆ
3
a
1
ꢁ
ꢀ8
ꢀ12
20
temperatures, another species was first detected at 322 C
ꢁ
10
Only in strongly alkaline solutions are these two ionized forms
(pK 1
a
¼ 7.2) and Ka2 ¼ 7.94 ꢆ 10
(pK 2
a
¼ 11.1).
(
m/z ¼ 129; M + H) and was prominent at 395 C. It is attrib-
1
9
uted to the intermediate species 2-amino-4,6-dihydroxy-1,3,5-
triazine (III; see also e.g. the v-positive spectrum). The chlor-
ine-containing intermediate with m/z ¼ 169 (positive ion
of any relevance to the absorption spectra.
Perusal of the spectral features in Fig. 10 show that the 214
nm band disappeared almost completely at temperatures
ꢁ
mode; M + Na) was detected in mass spectra taken at 322,
greater than ca. 300 C for all three methods. Concomitantly,
the 193 nm band also decreased considerably, but did not
completely disappear under these conditions.
ꢁ
343, 364 and 400 C (e.g., iv- and v-positive mode spectra).
The peak is assigned to the intermediate 6-chloro-2-amino-4-
ꢁ
dihydroxy-1,3,5-triazine (IV). The signal at m/z 170 (positive
ꢁ
The relative loss of cyanuric acid at 400 C by the HY-SC,
ion mode; M + Na; iv-positive spectrum) observed at 343 C
is due to the 6-chloro-2,4-dihydroxy-1,3,5- triazine species (V).
For the HY-SC/O and HY-SC/UV/O combinations, the
HY-SC/O
scopically was 87%, 78% and 76%, respectively. Clearly, the
presence of O and UV/O had little effect on the decrease
3 3
and HY-SC/UV/O methods evidenced spectro-
3
3
3
3
ꢁ
initial formation of cyanuric acid occurred around 100 C.
Undoubtedly this difference in temperature must be due to
differences in the degradation processes. Nonetheless, the
quantity of cyanuric acid formed was rather small with these
two methods.
of the UV absorption features. No shift of the initial peak at
193 nm was observed for the degraded solution. Also, since
no additional UV absorption features were observed in the
decomposition of cyanuric acid (except for the 214 nm band),
we deduce that no intermediates formed that had the atrazine
skeleton. Alternatively, intermediate(s) that did form must
have degraded fairly rapidly, and in all cases the triazine ring
in cyanuric acid was cleaved.
In principle, a molecule such as atrazine can be degraded
through either of two mechanistic sequences: (1) dechlorina-
tion followed by dealkylation and then deamination; or (2)
dealkylation as the initial step followed by deamination and/
or dechlorination as the subsequent steps (see Scheme 1).
The mechanistic details of the heterogeneous photocatalyzed
+
The decrease of TOC and the increase of NH4 ions pro-
duced during the degradation of cyanuric acid are illustrated
in Fig. 11. The initial concentration (3.60 ppm) of the acid’s
ꢁ
degradation of atrazine in irradiated aqueous TiO
2
dispersions
In these earlier studies, the degrada-
TOC increased to 7.12 ppm (HY-SC) at 100 C as already
observed above for atrazine. Concomitantly, the maximal
1
1,12
15
have been reported.
ꢂ
tive oxidation process occurred via OH radical attack on the
surface of titania particles. The major pathway (ca. 90%)
involved deethylation followed by further dealkylation,
concentration of cyanuric acid for the HY-SC/UV/O
ꢁ
3
method
increased to 4.74 ppm at 206 C. In contrast, there was no
increase in all the temperature ranges examined when the
1
220
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Fig. 9 Electrospray mass spectra of atrazine solutions recorded in the positive ion mode (M + H and M + Na; left column) and in the negative ion
mode (M ꢀ H; right column) at several selected temperatures during the degradation of atrazine.
ꢀ
HY-SC/O method was employed. Once again, the TOC level
3
decreased rapidly around 260 C. Under supercritical condi-
of cyanuric acid, 0.1 mM). Formation of NO ions was rather
3
ꢁ
ꢁ
insignificant up to ca. 300 C by the HY-SC and HY-SC/O
3
tions, the quantity of cyanuric acid remaining in the reservoir
solution was 1.21 ppm for the HY-SC method, 0.29 ppm for
the HY-SC/O method and 0.37 ppm for the HY-SC/UV/
methods. Addition of UV radiation in the presence of ozone
produced no nitrate ions at all the temperatures examined.
Evidently, the nitrogen atoms in cyanuric acid are transformed
3
+
O
3
method.
Formation of NH4 ions was fairly rapid around 200 C,
almost entirely into NH
4
ions, with the mineralization yield
+
ꢁ
of all nitrogens for all three sets of conditions being nearly
quantitative. In all cases, no cyanuric acid was detected at
+
reaching the expected maximal quantity of NH ions at ca.
ꢁ
00 C for all three methods employed (initial concentration
4
ꢁ
3
temperatures greater than 300 C by LC-MSD analyses.
Scheme 1 Intermediates produced in the degradation of atrazine.
New J. Chem., 2003, 27, 1216–1223
1221

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Fig. 10 UV spectral patterns recorded at several selected temperatures during the degradation of cyanuric acid by the (a) HY-SC, (b) HY-SC/O
and (c) HY-SC/UV/O methods.
3
3
Concluding remarks
The present study has shown that two refractory products,
namely atrazine and cyanuric acid, can be degraded to some
extent under hydrothermal conditions at a pressure of 23
MPa and almost completely in supercritical water media.
Addition of ozone enhanced somewhat the dechlorination of
atrazine. The decomposition of intermediates formed in the
degradative process was also effective as observed from the
decreases of TOC. On the basis of the TOC data, addition of
ozone and irradiation with UV light had some positive but
small effects on the efficiency of degradation. UV absorption
Scheme 2 Relevant structures showing the keto-enol tautomeric
equilibrium between the different species of cyanuric acid.
3 3
results show that adding O or UV/O to the HY-SC method
had little effect on the cleavage of the triazine structure. None-
theless, the present study demonstrates that cleavage of the
triazine ring, dechlorination, deamination and mineralization
are significantly different mechanistically in supercritical aqu-
eous media from what transpires in the photocatalytic AOT
1
1,12
methodologies
and by the flash photolytic and radiolytic
1
0
methods. Unlike the hydrothermal/supercritical water meth-
ologies examined herein, however, these last two methods are
not very practical to degrade large quantities of these two
refractory substrates. In contrast, atrazine and cyanuric acid
can be mineralized almost quantitatively by the HY-SC meth-
ology, a feat that the photocatalytic method alone does not
achieve for atrazine and which hardly affects cyanuric acid
for reasons that are still not well understood. The nearly quan-
titative conversion of the nitrogens into NH ions and the
+
4
ꢀ
negligible (if any) quantities of NO
of both atrazine and cyanuric acid are noteworthy.
3
ions in the degradation
Acknowledgements
We are grateful to the Frontier Research Promotion Founda-
tion sponsored by the Japan Ministry of Education, Sports,
Culture, Science and Technology (to H.H.) and to the Natural
Sciences and Engineering Research Council of Canada (to
N.S.) for financial support of this work and for the fruitful
collaboration between our two respective laboratories. One
of us (N.S.) thanks Prof. A. Albini of the University of Pavia
for his hospitality during the winter semester 2003.
+
Fig. 11 Temperature dependence of the evolution of NH
4
ions and
the decrease of TOC in the degradation of cyanuric acid by the HY-
SC (circles), HY-SC/O
methods.
3 3
(triangles) and HY-SC/UV/O (squares)
1
222
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1
5
J. W. Tester, H. R. Holgate, F. J. Armellini, P. A. Webley, W. R.
Killilea, G. T. Hong and H. E. Barner, ACS Symp. Ser., 1993,
518, 35.
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