Article

Degradation of DEET by ozonation in aqueous solution

Environmental Research Group, Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia.
Chemosphere (Impact Factor: 3.34). 07/2009; 76(9):1296-302. DOI: 10.1016/j.chemosphere.2009.06.007
Source: PubMed
ABSTRACT
This study was undertaken in order to understand the factors affecting the degradation of an insect repellent, N,N-diethyl-m-toluamide (DEET) by ozonation. Kinetic studies on DEET degradation were carried out under different operating conditions, such as varied ozone doses, pH values of solution, initial concentrations of DEET, and solution temperatures. The degradation of DEET by ozonation follows the pseudo-first-order kinetic model. The rate of DEET degradation increased exponentially with temperature in the range studied (20-50 degrees C) and in proportion with the dosage of ozone applied. The ozonation of DEET under different pH conditions in the presence of phosphate buffer occurred in two stages. During the first stage, the rate constant, k(obs), increased with increasing pH, whereas in the second stage, the rate constant, k(obs2), increased from pH 2.3 up to 9.9, however, it decreased when the pH value exceeded 9.9. In the case where buffers were not employed, the k(obs) were found to increase exponentially with pH from 2.5 to 9.2 and the ozonation was observed to occur in one stage. The rate of degradation decreased exponentially with the initial concentration of DEET. GC/MS analysis of the by-products from DEET degradation were identified to be N,N-diethyl-formamide, N,N-diethyl-4-methylpent-2-enamide, 4-methylhex-2-enedioic acid, N-ethyl-m-toluamide, N,N-diethyl-o-toluamide, N-acetyl-N-ethyl-m-toluamide, N-acetyl-N-ethyl-m-toluamide 2-(diethylamino)-1-m-tolylethanone and 2-(diethylcarbamoyl)-4-methylhex-2-enedioic acid. These by-products resulted from ozonation of the aliphatic chain as well as the aromatic ring of DEET during the degradation process.

Full-text

Available from: Mhd. Radzi Bin Abas
Technical Note
Degradation of DEET by ozonation in aqueous solution
Kheng Soo Tay, Noorsaadah Abd. Rahman, Mhd. Radzi Bin Abas
*
Environmental Research Group, Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
article info
Article history:
Received 25 November 2008
Received in revised form 1 June 2009
Accepted 3 June 2009
Available online 30 June 2009
Keywords:
Mechanism
N,N-diethyl-m-toluamide
Radical reaction
Ring-opening reaction
By-products
abstract
This study was undertaken in order to understand the factors affecting the degradation of an insect repel-
lent, N,N-diethyl-m-toluamide (DEET) by ozonation. Kinetic studies on DEET degradation were carried
out under different operating conditions, such as varied ozone doses, pH values of solution, initial concen-
trations of DEET, and solution temperatures. The degradation of DEET by ozonation follows the pseudo-
first-order kinetic model. The rate of DEET degradation increased exponentially with temperature in the
range studied (20–50 °C) and in proportion with the dosage of ozone applied. The ozonation of DEET
under different pH conditions in the presence of phosphate buffer occurred in two stages. During the first
stage, the rate constant, k
obs
, increased with increasing pH, whereas in the second stage, the rate constant,
k
obs2
, increased from pH 2.3 up to 9.9, however, it decreased when the pH value exceeded 9.9. In the case
where buffers were not employed, the k
obs
were found to increase exponentially with pH from 2.5 to 9.2
and the ozonation was observed to occur in one stage. The rate of degradation decreased exponentially
with the initial concentration of DEET.
GC/MS analysis of the by-products from DEET degradation were identified to be N,N-diethyl-formam-
ide, N,N-diethyl-4-methylpent-2-enamide, 4-methylhex-2-enedioic acid, N-ethyl-m-toluamide, N,N-
diethyl-o-toluamide, N-acetyl-N-ethyl-m-toluamide, N-acetyl-N-ethyl-m-toluamide 2-(diethylamino)-1-
m-tolylethanone and 2-(diethylcarbamoyl)-4-methylhex-2-enedioic acid. These by-products resulted
from ozonation of the aliphatic chain as well as the aromatic ring of DEET during the degradation process.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
DEET (N,N-diethyl-m-toluamide), an active compound in insect
repellents was first introduced by the US Army in 1946 for protec-
tion against insect bites has been in the market for more than
50 years (USEPA, 1998). Contaminations of DEET have been studied
and reported in various aquatic environments, such as groundwa-
ter (Barnes et al., 2004; Costanzo et al., 2007), streams (Kolpin
et al., 2004), seawater, effluents from sewage plant (Weigel et al.,
2004), and even drinking water treated by conventional water-
treatment systems (Stackelberg et al., 2004).
DEET has been reported to have potential carcinogenic properties
in human nasal mucosal cells (Tisch et al., 2002). Ingestion of low
doses of DEET in children has been reported to result in coma and sei-
zures (Petrucci and Sardini, 2000). Currently, there is no legislation
controlling the amounts of both usage and discharge of DEET into
the environment. Thus, greater attention should be paid to removing
this chemical, especially in the production of drinking water.
Ozonation is a water-treatment technology which has been pro-
jected to be the fastest growing water-disinfection method in the
market (Sowmya, 2008). During ozonation, organic pollutants un-
dergo a series of oxidation processes, and in some cases, highly
toxic by-products can be produced (Ikehata et al., 2006). These
undesired degradation by-products may become new chemical
entities in the environment following the discharge of incom-
pletely treated effluents into the environment. Thus, evaluation
and determination of by-products from ozonation are important
considerations for environmental protection purposes.
The objectives of this work are (i) to study the influences of sev-
eral operating parameters on the ozone degradation of DEET, such
as temperature, pH, initial concentration, and ozone dose (ii) to
identify major by-products of DEET degradation, and (iii) to pro-
pose the reaction mechanism for DEET degradation by ozonation.
Earlier studies have confirmed the presence of DEET at trace
levels in the environment. The concentrations of DEET used in this
experiment are higher than those found in aquatic environments
and may not be directly relevant to the environment. This method
will enable observation of DEET reduction over at least an order of
magnitude without extensive sample preparation procedures.
2. Materials and methods
2.1. Chemicals
DEET was obtained from Aldrich, USA, and used without further
purification. All the stock solutions were prepared by dissolving
DEET in ultrapure deionized water (Elga, USA). All solvents were
0045-6535/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2009.06.007
* Corresponding author. Tel.: +60 379674273; fax: +60 379674193.
E-mail address: radzi@um.edu.my (M.R.B. Abas).
Chemosphere 76 (2009) 1296–1302
Contents lists available at ScienceDirect
Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere
Page 1
of high-performance liquid chromatography (HPLC) grade and
were used without purification. HPLC grade solvents, sulfuric acid
and hydrochloric acid were purchased from Merck (Germany).
Phosphate buffer (0.5 M) was prepared using sodium dihydro-
gen-phosphate (Aldrich, USA) and/or disodium hydrogen-phos-
phate (Riedel-de Haën, Germany) and the pH was adjusted using
either phosphoric acid (Merck, Germany) or sodium hydroxide
(Fluka, Germany) solutions. Tert-butanol was purchased from Rie-
del-de Haën (Germany).
2.2. Degradation of DEET by ozonation
Ozonation experiments were performed in a 1000 mL cylindri-
cal jacketed beaker. Ozone was continuously bubbled into stirred
DEET solution through a gas-dispersion tube placed at the bottom
of the reactor. Ozone was produced from purified oxygen (99.8%)
by an OZX03 K-model ozone generator (Enaly Trade Co. Ltd., Can-
ada). Ozone doses were determined by the iodometric method.
Reaction temperatures were maintained at the desired value
±0.1 °C using a circulating water bath. Samples were withdrawn
at defined time intervals and nitrogen gas was used to remove
the residual ozone. The degradation of DEET was studied under
various initial concentrations of DEET ranging from 5 to 25 mg L
1
,
pH values ranging from 2.3 to 11.7 (pH was adjusted using 50 mM
phosphate buffer), reaction temperatures ranging from 20 to 50 °C,
and ozone dosages ranging from 0.51 to 0.76 g h
1
(Supplementary
Material (SM), Table SM-1). The pH value of the DEET solutions
without pH adjustment was 6.5. Values for the unbuffered solu-
tions were adjusted with sulfuric acid to pH 6 6.5 and with sodium
hydroxide to pH > 6.5. Some runs were carried out in the presence
of t-butanol (50 mM) as the hydroxyl radical scavenger.
2.3. Degradation by-products study
Initial DEET concentration of 10 mg L
1
was selected for identi-
fication of the by-products of DEET degradation. Ozonation was
carried out at 25 °C at an ozone dosage of 0.51 g h
1
and without
pH adjustment. A small sample (20 mL) was withdrawn every
2 min throughout a 1 h period and nitrogen gas was used to re-
move the residual ozone. Sample extraction was carried out on a
Lichrolut vacuum manifold (Merck, Germany) using 200 mg
Lichrolut EN SPE (solid-phase extraction) cartridges (Merck, Ger-
many). SPE cartridges were serially conditioned with 3 mL of ethyl
acetate and 3 mL of dichloromethane, followed by 9 mL of acidified
deionized water (pH 2). Samples were adjusted to pH 2 and then
loaded onto the SPE cartridges. The flow rate was adjusted to
1 mL min
1
. The cartridges were dried under a stream of purified
nitrogen (99.999%), and the retained organic components were
eluted with three 0.3 mL aliquots of ethyl acetate. The extract
was dried by evaporation and then dissolved in 20
l
L of ethyl ace-
tate; a 1.2
l
L aliquot of the solution was injected into GC/MS.
2.4. Analytical methods
The concentration of DEET was monitored using a HPLC (Ther-
mo Finnigan, Spectra System P2000) equipped with a UV detector
(UV 2000), degasser (SCM1000) and a chromolith RP-18
(100 4.6 mm) monolithic column (Merck, Germany). The mobile
phase used was a mixture of acetonitrile (Solvent A) and deionized
water (Solvent B). The flow rate was maintained at 1.0 mL min
1
for all runs. Gradient elution was carried out as follows: the initial
mobile phase was a mixture of 20A:80B (v/v); subsequently, it was
increased to 40A:60B (v/v) over a period of 10 min. A 5 min
reequilibration using 20A:80B (v/v) was carried out between sam-
ple injections. The detection wavelength was 210 nm.
The analysis of degradation by-products was carried out using
a Hewlett-Packard Model 6890 GC, with a HP-5 (5% phen-
ylmethylpolysiloxane) column of dimensions 30 m 0.25 mm
0.25
l
m. Helium (purity 99.999%) was used as the carrier gas,
with an average velocity of 40 cm s
1
, and the temperature for
the GC oven was programmed for an initial temperature of
60 °C maintained for 2 min, which was further increased to
280 °C at the rate of 6 °C min
1
and was maintained at this tem-
perature for 2 min. The temperatures of the injection port and
the transfer line were maintained at 290 and 300 °C, respectively.
The data for quantitative analysis was acquired in the electron
impact mode (70 eV), scanning in the range of 50–550 amu at
1.5 s scan
1
.
3. Results and discussion
3.1. Rate of degradation and kinetic study
In this study, degradation of DEET by ozonation involving a gas–
liquid heterogeneous system followed the pseudo-first-order ki-
netic model. The pseudo-first-order kinetic plots are shown in
Fig. 1 and the rate constants obtained are presented in Table SM-1.
3.1.1. Influence of temperature
This study shows that the rate of DEET degradation increases
exponentially with temperature (Fig. SM-1a). Temperature is an
important factor that influences the solubility of ozone. According
to Song et al. (2007), the solubility of ozone in aqueous solution de-
creases with increase in the temperature of the solution; however,
from our observation, the rate of degradation of DEET increases
with temperature. This may indicate that the solubility of ozone
is not a major factor that influences the decomposition of DEET
and that the removal of DEET from water can be accelerated by
increasing the reaction temperature.
3.1.2. Influence of initial DEET concentration
The rate of DEET degradation is found to decrease exponentially
(Fig. SM-1b) with an increase in the initial concentration of DEET.
Because ozone inlet concentration and flow rate are maintained
constant throughout the experiment, the amount of ozone intro-
duced into the aqueous phase and the amount of hydroxyl radical
produced are presumed to be constant. Higher concentration of
degradation by-products were produced initially, when the con-
centration of DEET was high. However, due to the nonselective
behavior of hydroxyl radicals, there is competition between the
degradation by-products and DEET for the reaction with hydroxyl
radicals, thereby reducing the rate of DEET degradation later on.
Thus, as reported by Wu et al. (1998) in the treatment of reac-
tive-dye wastewater, the concentration of degradation products
produced during ozonation is a major factor in reducing the rate
of DEET degradation.
3.1.3. Influence of pH
Experiments were carried out in both buffered and unbuffered
conditions. For buffered condition, the pH did not change (de-
crease) more than ±0.1 pH units throughout the experiments.
However, for those without buffer, the pH was found to decrease
during the ozonation process except for the initial pH of 2.5 and
11.4 (Fig. SM-2).
Under buffered conditions, the amount of hydroxyl radical re-
mained constant during the ozonation process and the ozonation
was observed to occur in two stages (Fig. 1ci). As suggested by
Chen et al. (2003), the change in k
obs
is possibly due to the de-
crease of the concentration of selected organic compounds in the
K.S. Tay et al. / Chemosphere 76 (2009) 1296–1302
1297
Page 2
solution. At pH 2.3 and 2.5, and 11.7 and 11.4 for the buffered
and unbuffered DEET solutions respectively, the pH values were
found to remain constant throughout the ozonation process and
the ozonation was also observed to occur in two stages. For
unbuffered DEET solution, at the initial pH values of 3.9, 6.5,
and 9.2, the pH drops rapidly during ozonation (Fig. SM-2).This
0102030405060
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln([DEET]/[DEET]
0
)
Time (minute)
20
o
C
25
o
C
30
o
C
40
o
C
45
o
C
50
o
C
0 102030405060
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln([DEET]/[DEET]
0
)
Time (Minute)
5 mg L
-1
10 mg L
-1
15 mg L
-1
20 mg L
-1
25 mg L
-1
0 10 20 30 40 50 60
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln([DEET]/[DEET]
0
)
Time (Minute)
pH 2.3
pH 3.9
pH 6.9
pH 9.9
pH 11.7
F
i
r
s
t s
ta
g
e
o
z
o
n
a
ti
o
n
S
e
c
o
n
d
s
t
a
g
e
o
z
o
n
a
t
i
o
n
0102030405060
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln ([DEET]/[DEET]
0
)
Time (min)
pH 2.5
pH 3.9
pH 6.5
pH 9.2
pH 11.4
0 40 80 120 160 200
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
ln ([DEET]/[DEET]
0
)
Time (min)
unbuffered at pH 6.5 with t-butanol
buffered at pH 6.9 with t-butanol
unbuffered at pH 11.8 with t-butanol
0 102030405060
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
ln([DEET]/[DEET]
0
)
Time (Minute)
0.51 g h
-1
0.63 g h
-1
0.76 g h
-1
(
a
)
(
b
)
(cii)
01234
-4
-3
-2
-1
0
(ci)
(d)
(e)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Fig. 1. Pseudo-first order reaction plots for the various ozonation conditions. (a) Effect of reaction temperature (initial concentration of DEET = 10 mg L
1
; pH = 6.5; ozone
dose = 0.76 g h
1
), (b) effect of initial concentration of DEET (pH = 6.5; temperature = 25 °C; ozone dose = 0.76 g h
1
), (c) effect of pH under (i) buffered and (ii) unbuffered
condition (initial concentration of DEET = 10 mg L
1
; temperature = 25 °C; ozone dose = 0.76 g h
1
), (d) effect of tert-butanol (initial concentration of DEET = 10 mg L
1
;
temperature = 25 °C; ozone dose = 0.76 g h
1
), and (e) effect of ozone dosages (initial DEET concentration = 10 mg L
1
; pH = 6.5; temperature = 25 °C).
1298 K.S. Tay et al. / Chemosphere 76 (2009) 1296–1302
Page 3
drop in pH suppressed the formation of hydroxyl radical (Chiang
et al., 2006) thus, resulting in only one stage of ozonation
(Fig. 1cii).
When t-butanol was added to both buffered and unbuffered
solutions, similar results were obtained (Fig. 1d). This supports
the above observation where suppression of hydroxyl radical for-
mation gave a one stage ozonation.
Under buffered conditions, k
obs
were found to increase with pH.
However, k
obs2
increases with increase of pH from 2.3 to 9.9 but de-
creases when the pH value exceeds 9.9 (Fig. SM-1ci). When buffers
were not employed, the k
obs
were found to increase exponentially
with pH of solution from 2.5 to 9.2 (Fig. SM-1cii).
Under acidic conditions, ozone itself becomes the reactive spe-
cies in the degradation process. Due to the selectivity of ozone for a
specific site of the molecule undergoing the reaction, the degrada-
tion of DEET has low efficiency under acidic conditions. Increasing
the pH of the solution will result in an increase in hydroxyl radi-
cals. Since the reactivity of most organic compounds with hydroxyl
radical is significantly higher than aqueous ozone, the rate of DEET
degradation increases with increasing pH. The reduction in the rate
of DEET degradation when pH values exceeded 9.9 could be attrib-
uted to the formation of additional free-radical scavengers, such as
carbonate and bicarbonate ions (Song et al., 2007), which reduced
the amount of hydroxyl radicals generated. Extreme pH values
such as 2.3 and 11.7 have been used to show the effect of pH on
the rate of degradation. However, these extreme conditions are
unrealistic in real natural aquatic environments.
3.1.4. Influence of ozone dosage
As shown in Fig. SM-1d, the rate of DEET degradation increases
in proportion with the ozone dosage. Increase in the gas–liquid
interface with increasing ozone dosage appears to enhance the
ozone concentration in solution (Song et al., 2007) and conse-
quently, the formation of hydroxyl radicals in the solution is
enhanced.
3.2. Ozonation by-products of DEET
GC/MS analysis of the extract from the ozonated DEET solution
indicates the generation of a variety of ozonation by-products
(Fig. 2). The mass spectra of all the identified compounds are pre-
sented in the SM. The peak for the undegraded DEET appears at a
retention time of 19.08 min. The mass spectrum of DEET shows
the molecular ion peak to be m/z 190 (36%). The other peaks can
be attributed to the fragments lost from the parent compound:
m/z 176 (M–CH
3
, 3%), m/z 162 (M–CH
2
CH
3
, 4%), m/z 119 (M–
N(CH
2
CH
3
)
2
, 100%), and m/z 91 (M–CON(CH
2
CH
3
)
2
, 41%) (Fig. SM-
3). The peak at 5.14 min in the total ion chromatogram is attrib-
uted to N,N-diethylformamide (L). This compound has been identi-
fied based on its mass in the mass spectrum (Fig. SM-4). The peaks
at 16.04 and 16.68 min represent the N,N-diethyl-4-methylpent-2-
enamide (X) and 4-methylhex-2-enedioic acid (W), respectively.
The structure of N,N-diethyl-4-methylpent-2-enamide and 4-
methylhex-2-enedioic acid have been proposed based on the frag-
mentation pattern of their mass spectra (Fig. SM-5 and SM-6).
The peak at 18.27 min represents N-ethyl-m-toluamide (H).
This compound results from the deethylation of DEET and is con-
firmed by comparing its mass spectrum to that of the synthesized
compound (Fig. SM-7). The peak at 18.54 min is attributed to an
isomer of DEET because the fragmentation pattern of this com-
pound is similar to the fragmentation pattern of DEET (Fig. SM-
8). To confirm the structure of this isomer, ortho-substituted DEET
(N,N-diethyl-o-toluamide) and para-substituted DEET (N,N-
diethyl-p-toluamide) were synthesized using the method adapted
from Knoess and Neeland (1998) and their retention times on
the GC were compared. The result confirmed that the peak at
18.54 min to be that of ortho-substituted DEET (N,N-diethyl-o-tol-
uamide), which, presumably, is produced as a result of rearrange-
ment of the DEET molecule during the ozonation process.
The peaks at 19.20 and 22.40 min represent the N-acetyl-N-
ethyl-m-toluamide (I) and 2-(diethylamino)-1-m-tolylethanone
Fig. 2. GC–MS total ion chromatogram of DEET mixture after ozonation (initial concentration of DEET = 10 mg L
1
; pH = 6.5; temperature = 25 °C; ozone dose = 0.76 g h
1
).
K.S. Tay et al. / Chemosphere 76 (2009) 1296–1302
1299
Page 4
(F), respectively. The identity of I has been confirmed by compari-
son with mass spectrum of the synthesized compound (Fig. SM-9).
Unfortunately, no standard is available for F for a direct mass-spec-
trum comparison. The structure of F is proposed exclusively based
on the fragmentation pattern of its mass spectrum (Fig. SM-10).
The molecular ion peak for F appears at m/z 204 as the (M–1)
+
and the m/z 133 peak in the mass spectrum is interpreted to be
2-oxo-2-m-tolylethan-1-ylium, which is stabilized by the presence
of various resonance hybrids.
The peak at 26.27 min is attributed to 2-(diethylcarbamoyl)-4-
methylhex-2-enedioic acid (V), which represents a product of the
ring-opening reaction, with the molecular ion peak appearing at
m/z 257. This structure is proposed based on the fragmentation
pattern in its mass spectrum (Fig. SM-11a). The peaks at m/z 156
and m/z 184 are attributed to the M–CON(CH
2
CH
3
)
2
and M–
N(CH
2
CH
3
)
2
–H fragments, respectively (Fig. S11b).
The time profiles for the major degradation by-products of DEET
plotted against ozonation time are illustrated in Fig. 3. The result
0 102030405060
0.0
2.0x10
6
4.0x10
6
6.0x10
6
8.0x10
6
1.0x10
7
Peak area
Ozonation time (min)
4-methylhex-2-enedioic acid
N
-ethyl-
m
-toluamide
N
,
N
-diethyl-
o
-toluamide
N
-acetyl-
N
-ethyl-
m
-toluamide
2-(diethylamino)-1-
m
-tolylethanone
2-(diethylcarbamoyl)-4-methylhex-2-enedioic
acid
N
,
N
-diethyl-4-methylpent-2-enamide
Fig. 3. Time profiles for major DEET degradation by-products (initial concentration of DEET = 10 mg L
1
; pH = 6.5; temperature = 25 °C; ozone dose = 0.76 g h
1
).
N
O H
H
OH
N
O
N
O
N
O
N
O
H
OH
N
H
O
N
O
O
O
O
-
O
2
N
O
O
O
N
O
H
2
C
N
O
H
OH
CH
2
N
O
O
CH
2
O
N
O
+
N
N
OH
O
N
+
C
O
N
C
O
N
HO H
further reaction
C
O
N
H
DEET
I
II
III
A
DEET
D
B
C
-
OH
E
F
G
H
I
N
O
C
H
2
K
L
N
O
+
further reaction
O
3
+ H
2
O
OH
-
OH
-
OH
-
H
CH CH
2
+
H
2
O
(a)
Fig. 4. Proposed pathway for DEET degradation during ozonation of the (a) aliphatic chain and (b) aromatic ring (initial concentration of DEET = 10 mg L
1
; pH = 6.5;
temperature = 25 °C; ozone dose = 0.76 g h
1
).
1300 K.S. Tay et al. / Chemosphere 76 (2009) 1296–1302
Page 5
showed that most of the major degradation by-products of DEET
were successfully removed after 1 h ozonation, with the exception
of F and V. Thus, it can be concluded that these two by-products are
more resistant to ozonation.
3.3. Reaction mechanism
Both the aliphatic chain and the aromatic ring of DEET are found
to react with hydroxyl radicals, and the proposed mechanisms are
presented in Fig. 4a and b. For the reaction between the aliphatic
chain of DEET and the hydroxyl radical, the ozonation by-products
identified were generated based on A (Fig. 4a). This radical was
formed during the reaction between the aliphatic chain of DEET
and the hydroxyl radical, as the case of N,N-dimethylformamide
which reacted with hydroxyl radical (Yonezawa et al., 1969). Ozon-
ation by-products F, H, I, and L were generated based on both the
rearrangement of A and the reaction of A with water and ozone.
In this case, the degradation pathway of DEET was proposed to oc-
cur in three pathways. Pathways I and III involve the rearrangement
of A. In addition to abstraction of hydrogen from DEET to form A,
hydrogen abstraction can also occur on the methyl group that is di-
rectly attached to the aromatic ring of DEET to form D (Durme et al.,
2007). For pathway I, rearrangement of A produced B and the carb-
anion of diethylamine, C. B can also react with DEET to form radicals
D and E. E could then couple with the radical of diethylamine to
yield F in the termination step. Pathway II involves the oxidation
of A to form the keto analogue of DEET, I, and the deethylation of
A forms radical G, which could further react with water to form H.
For pathway III, rearrangement of A formed the benzyl radical, J,
and carbanion K. K then reacted with water to generate L.
Zhang and Lemley, (2006) have reported the existence of
hydroxylated DEET (such as monohydroxylated and dihydroxylated
N
OOH
HO
N
O
HO
OH
HO
H
N
O
HO
OH
HO H
OH
N
O
HO
O
H
OH
N
O
HO
O
OH
H
N
OO
HO
H
HO
N
OO
HO
H
HO
N
OO
HO
HO
HO
N
OO
HO
HO
OH
N
O
DEET
M
N
O
O
O
O
N
OO
O
O
H
N
OO
HHO
P
R
Q
S
T
U
H
2
O
H
2
O
6
5
4
3
2
1
HO
O
N
OO
HO
V
O
HO
HO
O
N
O H
H
OH
O
HO
HO
O
N
O
O
HO
HO
O
N
O
HOH
W
O
HO
HO
O
OH
C
HO
O
N
OO
HO
H
H
HO
C
N
OO
O
O
HO
H
H
H
2
O
N
OO
HO
O
O
N
OO
O
CO
2
H
N
O
X
HO
N
OO
O
HO H
OH
CO
2
O
2
+
(b)
Fig 4. (continued)
K.S. Tay et al. / Chemosphere 76 (2009) 1296–1302
1301
Page 6
DEET) as a product of the anodic Fenton treatment of DEET which
involved hydroxyl radical oxidation process. However, no hydrox-
ylated DEET was observed in our study. In this study, ozone was
introduced continuously into the DEET solution. Presumably, the
instability of hydroxylated DEET under such strong oxidation con-
ditions may be the main reason why these compounds were not ob-
served. However, we postulated that these hydroxylated DEET to be
present as an intermediate in the formation of V (Fig. 4b).
The initial attack of ozone on DEET through electrophilic substi-
tution on the aromatic ring led to the formation of monohydroxy-
lated DEET (M). M then reacted with the hydroxyl radical to form
dihydroxylated (N) and trihydroxylated (O) DEET which further re-
acted with a hydroxyl radical to form the radical P. Intra-molecular
rearrangement of radical P led to the formation of aromatic ring-
opening by-product through the cleavage of C1–C6 bond to form
the radical R which then rearranged into the radical T. Radical T re-
acted with a hydroxyl radical to form the initial ring-opening by-
product U, an enol with the hydroxyl groups attached to a carbon
with C–C double bond. According to Hornback (2005), the enol
group is an unstable species which tautomerised rapidly to its keto
form. The aromatic ring-opened by-product detected in this study
is suggested to be V, instead of U. The presence of W and X, a break-
down product of V, further confirmed the aromatic ring-opening
reaction during ozonation. Presumably by-products from ozona-
tion can further react with the highly reactive hydroxyl radical
and ozone present in the reaction mixture to form other products
that are not identified in this study.
Acknowledgements
This research was financially supported by the Malaysia Toray
Science Foundation (MTSF) and University of Malaya (UM-RU
Grant SF025-2007A). We thank Emeritus Professor Bernd R.T. Sim-
oneit (Oregon State University) for his constructive comments on
the proposed reaction pathways.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.chemosphere.2009.06.007.
References
Barnes, K.K., Christenson, S.C., Kolpin, D.W., Focazio, M.J., Furlong, E.T., Zaugg, S.D.,
2004. Pharmaceuticals and other organic waste water contaminants within a
municipal landfill. Ground Water Monit. Remediat. 24, 119–126.
Chen, J.H., Hsu, Y.C., Yang, H.C., Hsu, C.H., 2003. Ozonation treatment of 2-
nitrophenolic wastewater using a new gas-inducing reactor. Chem. Eng.
Commun. 190, 1541–1561.
Chiang, Y.P., Liang, Y.Y., Chang, C.N., Chao, A.C., 2006. Differentiating ozone direct
and indirect reactions on decomposition of humic substances. Chemosphere 65,
2395–2400.
Costanzo, S.D., Watkinson, A.J., Murby, E.J., Kolpin, D.W., Sandstrom, M.W., 2007. Is
there a risk associated with the insect repellent DEET (N,N-diethyl-m-
toluamide) commonly found in aquatic environments? Sci. Total Environ. 384,
214–220.
Durme, J.V., Dewulf, J., Sysmans, W., Leys, C., Langenhove, H.V., 2007. Abatement
and degradation pathways of toluene in indoor air by positive corona discharge.
Chemosphere 68, 1821–1829.
Hornback, J.M., 2005. Organic Chemistry, second ed. Thompson Brooks/Cole,
Belmont.
Ikehata, K., Naghashkar, N.J., El-Din, M.G., 2006. Degradation of aqueous
pharmaceuticals by ozonation and advanced oxidation process a review.
Ozone Sci. Eng. 28, 353–414.
Knoess, H.P., Neeland, E.G., 1998. A modified synthesis of the insect repellent DEET.
J. Chem. Educ. 75, 1267–1268.
Kolpin, D.W., Skopec, M., Meyer, M.T., Furlong, E.T., Zaugg, S.D., 2004. Urban
contribution of pharmaceuticals and other organic wastewater contaminants to
streams during differing flow conditions. Sci. Total Environ. 328, 119–130.
Petrucci, N., Sardini, S., 2000. Severe neurotoxic reaction associated with oral
ingestion of low-dose diethyltoluamide-containing insect repellent in a child.
Pediatr. Emerg. Care 16, 341–342.
Song, S., Xia, M., He, Z., Ying, H., Lü, B., Chen, J., 2007. Degradation of p-nitrotoluene
in aqueous solution by ozonation combined with sonolysis. J. Harzard. Mater.
144, 532–537.
Sowmya, J., 2008. European Emerging Trends and Technologies Include UV, Ozone.
<http://ww.pennnet.com/display_article/329727/20/ARTCL/none/none/1/
European-Emerging-Trends-&-Technologies-include-UV,-Ozone/>.
Stackelberg, P.E., Furlong, E.T., Meyer, M.T., Zaugg, S.D., Henderson, A.K., Reissman,
D.B., 2004. Persistence of pharmaceutical compounds and other organic
wastewater contaminants in a conventional drinking-water-treatment plant.
Sci. Total Environ. 329, 99–113.
Tisch, M., Schmezer, P., Faulde, M., Groh, A., Maier, H., 2002. Genotoxicity studies on
permehtrin, DEET and diazinon in primary human nasal mucosal cells. Eur.
Arch. Oto-rhino-l. 259, 150–153.
USEPA, 1998. Registration eligibility decision (RED) DEET (EPA738-R-98-010).
Office of Pesticides and Toxic Substances, United State Environmental
Protection Agency, Washington DC.
Weigel, S., Berger, U., Jensen, E., Kallenborn, R., Thoresen, H., Hühnerfuss, H., 2004.
Determination of selected pharmaceuticals and caffeine in sewage and
seawater from Tromsø/Norway with emphasis on ibuprofen and its
metabolites. Chemosphere 56, 583–592.
Wu, J.N., Eiteman, M.A., Law, S.E., 1998. Evaluation of membrane filtration and
ozonation processes for treatment of reactive-dye wastewater. J. Environ. Eng.
ASCE 124, 272–277.
Yonezawa, T., Noda, I., Kawamura, T., 1969. Electron spin resonance study of
radicals formed by abstracting hydrogen from amides. Bull. Chem. Soc. Jpn. 42,
650–657.
Zhang, H., Lemley, A.T., 2006. Reaction mechanism and kinetic modeling of DEET
degradation by flow-through anodic fenton treatment (FAFT). Environ. Sci.
Technol. 40, 4488–4494.
1302 K.S. Tay et al. / Chemosphere 76 (2009) 1296–1302
Page 7
  • Source
    • "[5,6] DEET is toxic and resistant to biodegradation, so it cannot be degraded through water self-purification and traditional treatment methods (e.g., activated sludge treatment), and its removal efficiencies are usually poor (average D »50%). [7] DEET can be abated in advanced oxidation processes (AOPs), such as ozone treatment, [8,9] Fenton oxidation, [10] and chemical oxidation. [11] However, little attention has been paid to the DEET degradation by hydroxyl radicals (OH) generated in electrochemical advanced oxidation processes (EAOPs). "
    [Show abstract] [Hide abstract] ABSTRACT: This study investigates the electrochemical degradation of N,N-diethyl-m-toluamide (DEET) on PbO2 and Bi-PbO2 anodes. The difference in electrode crystalline structure was responsible for the better DEET degradation and TOC removal on PbO2 than on Bi-PbO2. In 1 M Na2SO4, the degradation efficiency and apparent rate constant (kapp) of DEET oxidation on PbO2 increased with the increase in current density or temperature (activation energy = 24.4 kJ mol(-1)). The kapp values in DEET-spiked environmental matrixes (municipal wastewater treatment plant secondary effluent (MWTPSE), groundwater (GW), and river water (RW)) were the same (6.05 × 10(-4) s(-1)), but significantly smaller than that in 1 M Na2SO4 (2.23 × 10(-3) s(-1)). The TOC removal efficiency was better in MWTPSE than in RW and GW; however, the mineralization current efficiencies in MWTPSE and RW were similar but higher than that in GW. During electrolysis, the aromaticity was lower in GW than in RW.
    Full-text · Article · Jul 2015 · Journal of Environmental Science and Health Part A Toxic/Hazardous Substances & Environmental Engineering
  • Source
    • "It can be seen that the degradation becomes faster until pH 9, and then slows down at pH 11. This decline in the degradation could be attributed to the formation of recalcitrant intermediaries and HO scavengers (Li et al., 2008; Tay et al., 2009). The COD removal mainly occurs when decarboxylation reactions take place, i.e. when the organic matter is already substantially oxidized (Nö the et al., 2009). "
    [Show abstract] [Hide abstract] ABSTRACT: This study investigates the degradation of the β-blockers in hospital wastewater by direct ozonation and Fe2+/ozonation with a focus on measurements at different initial pHs and Fe2+ concentrations, and the determination of kinetic constants. The results showed that these 'emerging contaminants' were completely degraded, when the removal rate of organic matter reached 30.6% and 49.1% for ozonation and Fe2+/ozonation, respectively. Likewise, the aromaticity removal rates were 63.4% and 77.9% for ozonation and Fe2+/ozonation, respectively. The experimental design showed that pH was the variable which had the greatest effect on the Fe2+/ozonation. The kinetic constants of atenolol, metoprolol and propranolol degradation by direct ozonation complied with pseudo-first-order conditions, while Fe2+/ozonation was suited to a biphasic degradation model. The k obs tended to rise when the pH increases; propranolol showed high k obs, which can be attributed to the naphthalene group (an electron-rich moiety). The identification of degradation products was carried out in aqueous solution using HPLC-MS2, followed by a suggestion of degradation pathways by means of ozonation. The degradation products proved to be dependent on the initial pH, and followed pathways that are based on direct ozonolysis and free radicals.
    Full-text · Article · Jan 2014 · Water Research
  • Source
    • "Its extensive usage has been related to the total protection it provides against a broad spectrum of biting insects [3]. In the last few years, contamination of DEET has been widely reported and it has been detected in various aquatic environments, including rivers, groundwater, seawater, wastewaters and even in drinking water treated by conventional water-treatment systems [1,456 . DEET can enter the environment mainly through municipal wastewater and is considered to be persistent in hydrolysis [1]. "
    [Show abstract] [Hide abstract] ABSTRACT: Analytical and electron paramagnetic resonance (EPR) spectroscopic methods were systematically used for the kinetic and mechanistic investigation of the photocatalytic degradation of N,N-diethyl-m-toluamide (DEET), in aqueous TiO2 suspensions under simulated solar light. The degradation of DEET followed first-order kinetics while enhanced reduction (>85%) of total organic carbon (TOC) and stoichiometric transformation of nitrogen to nitrate and ammonium ions took place after 240 min of irradiation. Numerous different structures of transformation products (TPs), with at least one isomer for the majority of them, were identified with high resolution accurate mass liquid chromatography (HR-LC–MS) and gas chromatography mass spectrometry (GC–MS). Low temperature EPR spectroscopy was used to study the photoinduced radicals created during the initial events of the photocatalytic oxidation. Two kinds of aromatic ring carbon-centered radicals i.e. hydroxy-methylcyclohexadienyl radicals have been resolved at 77 K. The second-transient conformation of the radicals is maximized after 5 min. of irradiation and then slowly decays. On the basis of identified products and radicals, a proposed pathway of photocatalytic degradation of DEET is presented, involving mono- and polyhydroxylation and/or oxidation, dealkylation and continuously the opening of the aromatic ring. Scavenging experiments indicated the contribution of �OH as the main species in the DEET oxidation while O�� 2 contributes also to the degradation in a lesser extend after the initial steps of the reaction. Finally, toxicity studies based on luminescence of Vibrio fischeri bacteria before and after the photocatalytic treatment were also performed.
    Full-text · Article · Sep 2013 · The Chemical Engineering Journal
Show more