Reaction patterns in photooxidative degradation of two herbicides

~

Chemosphere, Vol. 36, No. 7, pp. 1485-1492, 1998

Pergamon

© 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0045-6535/98 $19.00+0.00

P I h S0045-6535(97) 10047-9

REACTION PATTERNS IN PHOTOOXIDATIVE DEGRADATION OF TWO HERBICIDES

L. M u s z k a t ~ l, L. Feigelson l, L. Bir 1 and IC A. M u s z k a t 2

Agricultural Research Organization, Volcani Center, Department of Chemical Agroecology Bet-Dagan 50250 Israel 2 Weizmann Institute of Science, Rehovot 76100, Israel (Received in Germany 12 March 1997; accepted 4 September 1997)

ABSTRACT Different reaction patterns of photooxidation has been witnessed for the herbicides metribuzin, (4-amino6-tert-butyl-4,5-dihydro-3-methylthio-l,2,4-triazine-5-one)

and

bromacil

(5-bromo-3-sec-butyl-6-

methyluracil). In the first case oxygen has a pronounced effect on the rate of photooxidation, while the influence of hydrogen peroxide is quite moderate. The photolytic process in this case would apparently start via a reaction of the excited herbicide molecule with hydrogen peroxide or with oxygen.

In

another case, exemplified by bromacil, oxygen does not have a pronounced effect on the rate of photooxidation, which however is considerably enhanced by hydrogen peroxide. The reaction is initiated by hydroxyl radicals generated by hydrogen peroxide photolysis. These conclusions are supported by the different effects ofisopropanol inhibition. ©1998Elsevier ScienceLtd INTRODUCTION The wide use of pesticides gives rise to serious ecological problems, due to their negative environmental effects [ 1,2]. The relatively slow rate of decomposition, together with sufficient water solubility lead to their distribution in the environment and the migration of some of them into ground-water [3, 4]. The problem is even more complicated given that the toxic effects of pesticides and of their degradation products may not have been yet fully elucidated [5]. The detoxification of waters contaminated with pesticides residues, by the use of conventional purification techniques may be slow and inefficient. Thus in the-e,.x4sting processes of water treatment biodegradation is known to be problematic at the sub-ppm levels, and it occurs at rates which are slower by orders of magnitude than those expected from the Monod kinetics [6]. The application

*Author for correspondence 1485

1486 of TiO2 photocatalytic oxidation to water treatment has been widely studied during the last decade [7-9]. It could be shown that TiO2 suspensions, irradiated by solar or artificial near UV, may serve as a powerful oxidation system, capable of mineralizing organic micropollutants. The elimination of pesticides from polluted waters by short UV irradiation, with or without hydrogen peroxide was shown to be widely applicable [9-12]. In our recent studies we examined the solar photocatalytic treatment of groundwater from a polluted well. We could show that a broad range of pesticides underwent efficient degradation, to levels lower than 0.1 microgram/liter. Also, the suitability of the method has been demonstrated in the treatment of rinse waters from agricultural spray containers [12-13]. In the present study we examined several parallel non-catalytic photooxidation steps of pesticides encountered in our previous studies. These steps involve photooxidation in the presence of hydrogen peroxide or oxygen, but in the absence of TiO2. The concentration range of pesticides covered in the present study (< 100ppm) is just the range observed in rinse waters of land and airborn agricultural sprayers.

MATERIALS and METHODS

Experimental details on solar photocatalysis are described in previous studies [12-13]. UV irradiation was carried out with a compact arc mercury lamp (HBO 200W, Osram) in Duran glassware, transparent above 310nm (Fig. 1). Irradiation through Duran provides the possibility of comparing kinetics of photooxidation by HO' generated by H202 photolysis in the case of bromacil, with photooxidation by substrate excitation in the case of metribuzin (see Fig. 2 for structure). The possible formation of intermediates was checked by GC and GC/MS measurements. Kinetic studies were carried out using a Cary 1E spectrophotometer. These measurements, when carried out till 200 nm allow to follow the complete course of the photooxidation. In the case of bromacil and metribuzin the primary oxidation products are cyclic (and aromatic when protonated) absorbing at around 200 nm In the present study only the initial part of the photooxidation is considered.

The absorption curves of bromacil and metribuzin without hydrogen peroxide and in its

presence are given in Fig. 3. It can be noticed that the absorption due to hydrogen peroxide can interfere with the measurement of the concentrations of substrates, especially at the higher H202 levels. Corrections for

this effect were applied at the higher hydrogen peroxide concentrations. Hydrogen peroxide

concentrations were determined spectrophotometrically by the titanyl sulfate method [ 15].

1487

O. Metribuzin

I00

f

0 t-BuCN-NH 2 N~N~SMe

80 o_ •~ E

60

.T: 4O ._~ I

20

b. 8romacil ol ~O

I

2OO

J

I

I

5OO 4OO 50O

H

I

Light wavelength (nm)

Fig. 1.Transmission spectrum

Fig. 2. Structure of

of Duran glass, 4mm thickness.

a. metribuzin b. bromacil.

Br-4~N-- Sec-Bu

RESULTS and DISCUSSION The two molecules investigated (metribuzin and bromacil) differ in several important respects: a. Long wavelength position of the absorption spectrum in the case of metribuzin, which prevents direct excitation of

H202 at X >275 nm (Fig. 3A). In bromacil (Fig. 3B) direct excitation of H202, and

photogeneration of hydroxyl radicals is possible at k > 305nm b. Metribuzin shows clearcut evidence for photooxidation in the presence of oxygen (in the absence of hydrogen peroxide), as shown by results in Fig. 4A and Table 1A. This process is very inefficient in the case of bromacil (Fig. 4B). The oxygen/nitrogen half life ratios R( 02/N2) are: for metribuzin, R = 17; for bromacil, R = 1.6. However, the rate of bromacil photolysis is considerably enhanced by the addition of hydrogen peroxide (Table 1A vs. Table 1B and 1C; Fig 4B vs. Fig. 4C). c. UV-irradiation through Duran glass permits selective excitation of metribuzin. In the presence of hydrogen peroxide ( and the absence of oxygen, Table 113 ) the observed photodegradation of metribuzin is due to photosensitization by excited substrate of H202 splitting to O H . This process has previously been identified in benzoic acid photodegradation in the presence of H202 [17]. In the present case this process is inefficient for various reasons, as seen from its falling offat [H202] = 0.01M.

1488

'\ \ -\'\H20 z O.OIM \H202 O.IM \c I

\b

d. Isopropanol as scavenger [15] (Table 2) exerts a

Fig. 3

significant effect on the H202 Metrlbuzln 0.125mM

.

bromacil

(1 l-fold

decrease

[isopropanol]=lxl02 •,,.\

o

\

photooxidation of in

rate

at

M; 4-fold decrease at

A

[isopropanol]=2xl03 M). The strong quenching of B m

== O

0

t

photooxidation by isopropanol in the case of

~.

B

"\.H20g O,OIM \HE02 0.IM \.c ~b \ \,

bromacil is thus

a good

indication for the

predominant role of hydroxyl radicals in the Bromacil 0.112mM

photodegradation of this molecule. The much more moderate quenching observed ~br metribuzin is nevertheless an indication that hydroxyl radicals play

200

250

300 Wavelength (nm)

3~0

as well a

role (but

albeit smaller) in the

photodegradation of this molecule.

Fig. 3. Absorption spectra A, Metribuzin:

a metribuzin (0.125mM ),

b hydrogen peroxide 0.1M (aqueous),

e hydrogen peroxide 0.01M. B. Bromacil:

a bromacil (0.112mM),

b hydrogen peroxide 0.1M (aqueous),

¢ hydrogen

peroxide 0.01M

e. Clearcut evidence is available for efficient metribuzin photooxidation by oxygen (in the absence of hydrogen peroxide, Fig. 4A, Table 1A). The mechanism of this process is unclear at the present stage, though several possibilities are indicated in the available literature, see, e.g. [ 18].

An important group of processes

involves sensitization of oxygen to singlet state by triplets of dyes followed by addition of IO2, to give initially hydroperoxide, dioxetane, or epoxide products. The formation of these is very well documented. In our case, sensitization of oxygen could be obtained by energy transfer from triplet states of substrates. All these products could react in subsequent photooxidation steps, or undergo

further photooxidation by

hydroxyl radicals in the presence of H2Oz Singlet oxygen, formed by triplet dye sensitization can give hydrogen abstraction products [16]. In the study of the singlet oxygen reaction with bromacil such free radicals were found to give products of disproportionation, dimerization and rearrangement [16]. In the present case these would react further with hydroxyl radicals originating from H202 splitting.

Another

mechanism of oxygen photooxidation would involve reaction of excited (triplet) substrate molecule with ground state oxygen [18]. These are yet early conclusions which deserve further studies. However, in the environmental context, the difference between metribuzin and bromacil deserves special attention. Bromacil oxidative photodegradation

1489 clearly requires the presence of H202. On the

Fig. 4 Metrlbuzln / 02 saluraled a. before Irrad. ----b. after 15 rain. Irrad. -. . . . c. after 30 rain. Irrad.

.~. / ~ \. )~' \ \ \. \'\

other

hand,

metribuzin

can

undergo

photooxidation by oxygen even in the absence of hydrogen peroxide. Still, hydrogen peroxide

o

increases the rate of this process. Thus the present study shows that photodegradation processes need to be tailored to the exact type of major water contaminants. >, .

--

.

.

.

.

.

B r o m a e l l / O2 n l u r a l o d a . boforo "lrrea. -----b. after 30 rain. Irrad. c. after 60 rain. Irred. ..... d. after 120 mln. Irrad.

Fig. 4. Absorption spectra A. Metribuzin, irradiation in oxygensaturated aqueous solution:

I

a before irradiation,

b after 15min., c

after 30min. irradiation.

I

B. Bromacil, irradiated aqueous solution, -~"~

-----.....

B r o m a c l l I H202 a. before Irred. b. after 15 rain. Irrad. c. after 30 mln. Irred.

"\N

,

200

saturated with oxygen: a before irradiation, b after 30min., c after 60rain., d after 120 min. irradiation.

" ~ :2: 5_0~

.....

_"\x. 300

3~o

Wavelength ( n m )

A. Photooxidation with oxygen m the absence of HzOQ~ half life Metribuzin 25 ppm ( 0. lmM) 189 min N2 11.8 rain air 11 min 02 Bromacil 30ppm ( 0. lmM) 475 min N2 400 min air 281 min 02 B. Photooxidation m the presence of H Oz_Q z_( m the absence of Oz} Metribuzin 25 ppm ( 0. lmM) I-I202 0.1M, N2 H202 0.01M, N2 Bromacil 30ppm ( 0. lmM) H2020.1M, N2 H202 0.01M, N2

half life 6 min 76 rain 3 min 3.4 min

1490

Table 1 (continued)

C. Photooxidation by tt20_2 03 Metribuzin

half life

H202 0.1M, 02 H202 0.01M, 02 H202 0.1M, air H202 0.01M, air Bromacil

8.5 rain 7.6 rain 7.1 rain 7.4min

H202 0.1M, O~ H202 0.01M, 02

2.5 min 3.1 min 2.5 min 3.1 rain

H202 0.1 M, air H202 0.01M, air

Table 2. QUENCHING by SCAVENGER*

Isopropanol concentration

Half life (rain) Metribuzin Bromacil 72.5

3.67

2x10 -3 M

114.8

12.4

lxl0-2M

156.7

409

*In the absence of oxygen, H202 0.01M

ACKNOWLEDGEMENTS The authors kindly acknowledge the sopport of the EC AVICENNA FoundatioE Project No. CT93AVI2074.

1491 REFERENCES

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4.

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18.

Matsuura T., Photosensitized oxygenation, inPhotosetts'ilizedReaclionsPart II (Edited by Koizumi, M. Kato, S. Mataga, N. Matsuura, T. and Usui, Y.) Chap. 12, Kagakudojin Publishers, Kyoto (1978).