Reaction of sulphate radical anion SO4?- with cyanuric acid: A potential reaction for its degradation?

Article abstract of DOI:10.1246/cl.2002.74

A novel reaction between sulfate radical anion (SO₄^?-) and cyanuric acid (CA), a non-degradable end product of the oxidative degradation of the triazine based herbicide, atrazine, is presented using laser flash photolysis and steady state radiolysis techniques at pH 5. A second order rate constant of 1.9 × 10⁷ dm³ mol^-1 s^-11 has been determined and the transient intermediate (λ_max = 330 nm) is assigned to a radical cation of CA (CA^?+). The degradation profile indicated that about 76% of CA have been decomposed after an absorbed γ-radiation dose of 18 kGy. It is therefore proposed that the reaction of SO₄^?- could be utilised for the degradation of CA in aqueous medium which is normally stable to any Advanced Oxidation Processes.

Full text of DOI:10.1246/cl.2002.74

74
Chemistry Letters 2002
Reaction of Sulphate Radical Anion (SO4ꢀÀ) with Cyanuric Acid:
A Potential Reaction for Its Degradation?
P. Manoj, R. Varghese, V. M. Manoj, and C. T. Aravindakumarꢀ
School of Chemical Sciences, Mahatma Gandhi University, Kottayam. 686 560, India
(Received August 27, 2001;CL-010839)
A novel reaction between sulfate radical anion (SO4ꢁꢂ) and
cyanuric acid (CA), a non-degradable end product of the
oxidative degradation of the triazine based herbicide, atrazine,
is presented using laser flash photolysis and steady state radiolysis
techniques at pH 5.
s
A
second order rate constant of
1:9 ꢃ 107 dm3 molꢂ1
ꢂ1 has been determined and the transient
intermediate (ꢀmax ¼ 330 nm) is assigned to a radical cation of
CA (CAꢁþ). The degradation profile indicated that about 76% of
CA have been decomposed after an absorbed ꢁ-radiation dose of
18 kGy. It is therefore proposed that the reaction of SO4ꢁꢂ could
be utilised for the degradation of CA in aqueous medium which is
normally stable to any Advanced Oxidation Processes.
A rapid, safe and inexpensive method for the removal of
triazine derivatives which have extensively been used as
herbicides, from ground and surface water forms an important
part of water purification research based on Advanced Oxidation
Processes (AOPs) that make use of the powerful oxidant,
hydroxyl radical (ꢁOH).1 Atrazine is one among the important
components of triazine based herbicides and, therefore, many
studies using AOPs were reported with this compound.2{7 The
product analysis of all these studies reveal that cyanuric acid (CA)
is themajor end product andit remains stable even after prolonged
treatment with ꢁOH.5{8 In the present communication we report a
novel reaction which involve the degradation of CA by sulphate
radical anion (SO4ꢁꢂ) produced by both photolytic and radiolytic
techniques. To our knowledge this is the first report on the radical
mediated degradation of CA and it is expected that this reaction
could be explored for the complete degradation of CA in aqueous
medium.
Figure 1. Transient absorption spectra obtained from the
ꢁꢂ
reaction of SO4 with cyanuric acid at (ꢃ) 1 ꢂs, (O) 10 ꢂs
and ꢁ(ꢂs) 40 ꢂs after the pulse at pH 5. Inset: A. Decay traces of
SO4
in the absence (a) and in the presence of CA (5ꢃ
ꢁꢂ
10ꢂ4 mol dmꢂ3) (b);B. The pseudo first-order decay of SO
(kobs) versus concentration of CA.
4
The time resolved absorption spectra have shown an
evolution of a spectrum characterized by its ꢀmax around
330 nm at 10 ꢂs after the laser flash from the initial spectrum
at 1 ꢂs (corresponds to SO4 with ꢀmax at 460 nm10). No
ꢁꢂ
significant change in the spectral properties was observed when
the spectra were recorded in presence of O2. The degradation of
ꢁꢂ
CA after its reaction with SO4 was also monitored using ꢁ-
radiolysis. An exponential decay of CA with respect to the
absorbed dose was observed (Figure 2) and a pseudo first order
Laser flash photolysis experiments were carried out using a
SP-Quanta ray GCR-2(10) Nd-YAG laser (266 nm with a pulse
width of 5 ns and energy 50 mJ/pulse) connected with an optical
absorption detection set-up.9 K2S2O8 can be selectively excited
inpresence of CA at266 nmtoget SO4ꢁꢂ asCA has practically no
absorption at this wavelength. Photolyticꢁaꢂnd radiolytic (using a
60Co-gamma source) production of SO4 (from S2O82ꢂ) and
ꢁOH in aqueous medium are shown in reactions 1-5.
Figure 2. The ꢁdegradation profile of cyanuric acid induced by
Inset: First-order nature of the decay profile induced by SO4ꢁꢂ. C0
is the initial concentration of CA and C is that at time t. (Dose rate
of the gamma source was 100 Gy/min).
ꢂ1
ꢁꢂ
A second-order rate constant of 1:9 ꢃ 107 dm3 molꢂ1
s
was determined by monitoring the decay of absorption of SO4
ꢁꢂ
SO4 (ꢁ) and OH (Ã) measured by HPLC after ꢁ-radiolysis.
at 460 nm (pseudo first-order) as a function of varying
concentrations of CA (Figure 1), using laser flash photolysis.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
75
rate constant was calculated as 0.011 minꢂ1. The G(-CA) value
(which is the micromolar consumption of CA per joule of
radiation energy), was calculated from the slope of the initial
decay of CA as 0.1 ꢂmol Jꢂ1. No significant change in
concentration of CA was observed on its reaction with ꢁOH
(except that at higher time of irradiation, there was a slow
decrease in the concentration) under this condition (Figure 2)
indicating a much lower efficiency of ꢁOH in degrading CA.
SO4ꢁꢂ is a powerful oxidant (E0ðSO4ꢁꢂ=SO42ꢂÞ ¼ 2:43 V),
previous reports5;7;8 has made clear that even the well known
powerful oxidant, OH, is inefficient in the degradation of CA.
Undoubtedly, this difference in the efficiency of degradation is
due to the difference in their mode of reactions.
ꢁ
In conclusion, the possibility of radical mediated degradation
of CA is reported for the first time and this reaction could possibly
be used for the complete degradation of CA in aqueous medium. It
ꢁꢂ
is also interesting to note that the high efficiency of SO4 over
ꢁOH mediated degradation is a result of a different reaction
mechanism compared to ꢁOH. A detailed mechanistic study using
a variety of substituted triazines are currently in progress.
We thank Prof. P. Natarajan and Dr. P. Ramamurthy, the
National Centre for Ultrafast Processes (NCUFP), Chennai, for
providing the laser flash photolysis facility. We also thank the
Rubber Research Institute of India (RRII), Kottayam for the ꢁ-
radiolysis experiments. RV is thankful to CSIR, New Delhi, for a
fellowship.
ꢁ
ꢁꢂ
similar to OH. The SO4 is proposed to undergo an electron
transfer reaction with CA forming a radical cation (CAꢁþ) in the
initial step based on the earlier knowledge on its reaction with
pyrimidines and substituted benzenes.10{14 The possibility of
deprotonation of CAꢁþ like in the case of pyrimidine der-
ivatives,14 may be ruled out keeping in view of its structure where
there is no favorable proton for elimination (reaction 6). The
spectral features and the results with O2 (stability of the spectra in
the presence of O2) did not support the possible hydrolysis of
CAꢁþ leading to OH-adduct as the carbon centered OH-adduct
radicals of 2,4,6–trimethoxy–1,3,5–triazine (TMT) and 1,3,5–
triazine (T) were reported to have different spectral
characteristics2 and the fact that generally O2 has a high reactivity
with any carbon centered aromatic neutral radicals (diffusion
controlled) as well as with nitrogen centered radicals
(ꢄ107 dm3 molꢂ1 s1) forming peroxyl radicals.15 This leads to
the conclusion that the intermediate having the absorption spectra
(with ꢀmax at 330 nm) possesses a radical site neither at carbon
nor at nitrogen. The existence of a SO4ꢁꢂ-CA adduct (precursor of
CAꢁþ) is also ruled out because such an adduct would be a carbon
centered radical and it can have high reactivity towards O2.
Therefore, it is proposed that the species existing at pH 5 with
ꢀmax at 330 nm is a radical cation, CAꢁþ (reaction 6) and its
stability in the microsecond time scale is likely due to the
presence of three hydroxy groups substituted in the triazine ring.
The radical cations are, in general, known to react with O2 only
very slowly.16 Relatively longer lived (t1=2 ꢅ 1 ꢂs) radical
cations are reported to be formed with a number of aromatic
compounds in solution such as methyl substituted thymines.14
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The decay of CA (Figure 2) is obviously initiated by a
bimolecular reaction of the intermediate species, most likely the
radical cation. It is not clear from the laser flash photolysis studies
that this radical cation may undergo hydrolysis at a later stage
(> 200 ꢂs). On the other hand, the consumption of CA (almost
76% after 18 kGy, Figure 2) demonstrates the potential useful-
ness of this reaction. This study (see Figure 2) as well as from