- Email: [email protected]
Fate of pendimethalin, carbofuran and diazinon under abiotic and biotic conditions
The Science of the Total Environment, 132 (1993) 361-369 Elsevier Science Publishers B.V., Amsterdam
361
Fate of pendimethalin, carbofuran and diazinon under abiotic and biotic conditions I. Scheunert a, M. Mansour b, U. D6rfler a and R. Schroll a aGSF-Forschungszentrum f~r Umwelt und Gesundheit GmbH, lnstitut fzir Bodeniikologie, Ingolstiidter Landstrasse 1, D( W)-8042 Neuherberg, Germany bGSF-Forschungszenl'rumf~r Umwelt und Gesundheit GmbH, Institut f~r Okologische Chemie, Sc~)ulstrasse 10, D(W)-8050 Freising-Attaching, Germany ABSTRACT The insecticides carbofuran and diazinon as well as the herbicide pendimethalin were irradiated with UV light of different wavelengths in water or water/soil suspensions under various conditions. As compared to pure distilled water, photodegradation was increased in the presence of titanium dioxide, hydrogen peroxide or ozone, or by using natural river or lake water. In a water/soil suspension, diazinon was converted, besides to other products, to the isomeric isodiazinon. When subjected to various direct or indirect photolysis conditions, pendimetbalin was transformed to various products resulting from dealkylation and reduction. [14C]Pendimethalin was applied to two natural sandy soils in lysimeters under outdoor conditions. [.eachate collected at 1 m depth was analysed for radioactivity for 300 days. Radioactive products were detected from the third week onwards and continued to be leached during the whole experimental period. The radioactive oroducts wer~ n,;kh~, the parent compound nor catrbon dioxide nor carbonate but water-soluble organic conversion products.
Key words: pendimethalin; carbofuran; diazinon; photodegradation; lysimeter; leaching
INTRODUCTION
In order to evaluate the fate of pesticides in the environment, the influences of both abiotic and biotic factors should be taken into account. Among the abiotic chemical factors affecting the behaviour of pesticides, photochemical reactions are most important. In aquatic systems, they play a dominant role in the conversion and degradation of pes6cides. In soil, photochemical reactions are significant only at the surface. When the surface is moist, photochemical reactions may occur both with the adsorbed portion and with the portion dissolved in water. Whereas model reactions in aqueous solution simulate photoreactions of chemicals dissolved in the aqueous phase of soil, those in water/soil suspensions include also potential catalytic effects 0048-9697/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved
362
!. $CHEUNERT ET AL.
of adsorption. Therefore, both reaction conditions were studied by model experiments. In the soil layers below the surface, biotic reactions in general are more important than abiotic ones. Both abiotic and biotic reaction products in soil are subjected to various physical processes such as transport in soil and leaching from soil into percolate w~ter, Since the conversion products may differ from the parent compounds in physico-chemical properties such as water solubility and partition coefficients, they are also quite different in their tendency to be leached into groundwater. Thus, interactions between abiotic and biotic conversion reactions and physical processes govern the residue behaviour of the pesticides in the environment. In this paper, some experiments concerning abiotic conversion of pesticides as well as the leaching of a pesticide and of its conversion products from soil will be presented. Photodegradation experiments in water and water/soil suspensions were carried out with pendimethalin, carbofuran and diazinon. Additionally, for pendimethalin the leaching behaviour in soil under outdoor conditions was studied. EXPERIMENTAL
Laboratory experiments t
The photochemical experiments were performed in an apparatus suitable for both irradiation of the chemical in the liquid phase and as a suspension. Ultraviolet light was derived from a HPK 125 W high pressure mercury vapor lamp and passed through a pyrex filter to block wavelengths shorter than 290 nm or through a quartz filter for ~ < 290 rim. The light intensity determinations were usually carried out using potassium ferrioxalate actiw~meters. The decomposition and disappearance of the parent compounds were followed by gas chromatography, liquid chromatography, UV or other analytical techniques. Irradiations were performed in aqueous solutions, either in pure distilled water o~ after the addition of various sensitizing compounds, or in water/soil suspensions after adding 25 g soil (alfisol) to 100 ml of distilled water. The composition of the soil was as follows. Particle size distribution: sand 12.9%, silt 64.3%, clay 19.6%, coarse matter 3.2%. Organic carbon 0.76%; carbonate as CaCO3 0.75%; pH (1:1, H20) 7.45. In order to study the influence of natural water constituents on photodegradation, river water was collected fi'om the Rhine near Frankfurt and from the Bar River near Munich. Finally we chose water from a lake called the Ammersee. All water samples were stored at 4°C and again filtered before use (cellulose-acetate filters, pore size 0.2 ~m, from Sartorius, G6ttingen, FRG). The initial concentrations of these compounds were between 50 and 70 mM. Each solution in a conical pyrex flask (500 ml) was exposed directly
363
FATE OF PF.NDIMETHALIN. CARBOFURAN AND DIAZINON
TABLE 1 Quality characteristicsof water used in gtudy Body of water Period (1986)
pH
(02) (mg/l)
(OH) (mol/! X 10 "16)
Isar fiver Rhine fiver Ammer~e
7.8 8.1 7.5
12.80 14.70 I 1.30
6.51 12.45 10.75
May-June April-May June-July
to sunlight in April and May, 1986. The temperature was maintained constanfly (25°C) by a flowing fresh-water bath. Samples were removed after various time intervals and analysed by HPLC or GC after direct-injection. Dark controls were maintained for each experiment [1]. Some properties of the waters used are presented in Table !.
Outdoor lysimeter experiments Six lysimeters (depth I m, 60 cm in diameter) were filled with natural soils, three with a sandy forest soil and three with a sandy agricultural soil. The natural soil horizons were dug out separately and placed into the lysimeters according to their natural sequence and density. ~4C-Ring-labelled pendimethalin was applied to the surface as in agricultural practice (52 mg per lysimeter). In the case of forest soil, the surface consisted of organic litter and humus; the Ah horizon below had the following composition. Particle size distribution: sand > 99%, silt and clay < 1%. Organic carbon 4.93%, inorganic carbon < 0.001%, pH (CaCI2) 3.0. In the case of agricultural soil, pendimethalin was applied on the Ap horizon which had the following composition. Particle size distribution: sand 85%, silt 10%, clay 5%. Organic carbon 1.03%; pH (CaCI2) 4.9. Grass was grown in the lysimeters with forest soil, and winter wheat in those with agricultural soil. The percolation water at the base of the lysimeters was collected for 300 days and assayed for radioactivity by liquid scintillation counting. For characterization of the chemical identity of the leached 14C, the water was subjected to liquidliquid extraction with dichloromethane. Both sobrent and aqueous phases were analyzed for the parent compound and metabolites by thin layer chromatography after concentration in a rotary evaporator. The potential presence of ~4CO2 or [14C]carbonate resulting from mineralization of the pesticide was excluded by acidifying the concentrated water samples and by trapping any CO2 evolved in a scit~tillation liquid containing an organic base, followed by liquid scintillation counting.
3~.
!. SCHEUNERT ET AL
% Remaining
ll\~
o
~
40
20
1 2 3 4 5
3
Carbofuran alone Carbo. + TiO2(160mg/I)+N? Carbo. + H=02 (5.10-3mo111) Carbo. + Ti02 (160mg11)+02 Carbo. + 03 (lO-3mol/llmin)
4 0
50
100
I
I
I
150
200
250 Time (minutes) /
Fig. I. Photodegradation (X > 290 nm) of carbofuran in the presence of hydro~;en peroxide, titanium dioxide and ozone in water.
% Remaining IO0
80
60
40
0
0
I
I
I
I
I
I
I
20
40
60
80
100
120
140
I
160 180 Time(hours)
Fig. 2. Photolysis of carbofuran in a water/soil suspension ()~ > 290 nm): 25 g alfisol/10() ml H20; carbofuran, 22 ppm.
365
FATE OF PENDIMETHALIN, CARBOFURAN AND DIAZINON
RESULTS AND DISCUSSION
Laboratory experiments The kinetics of photodegradation of the insecticides carbofuran and diazincn in aqueous media under various conditions are shown in Figs 1-3. The ohotochemical degradation of carbofuran in pure aqueous solution, as well as after the addition of various oxygen-containing substances, is presented in Fig. ~. The figure reveals that photodegradation is very slow when carbofuran is irradiated alone with UV light ~ > 290 nm. However, in the presence of TiO2, H.:~2 or 03, photodegradation is accelerated considerably. The highest photodegradation rate is achieved when, in addition to TiO2, free oxygen is supplied to the aqueous solution. This points to oxidative mechanisms of photodegradation reactions of carbofuran. Figure 2 presents the photodegradation of carbofuran in a water/soil suspension. It demonstrates that soil particles or compounds present in soil moisture are also effective in promoting photodegradation reactions. In order to elucidate the potential role of natural water constituents on photodegradation rates of diazinon under natural conditions, diazinon was dissolved in distilled water, in humic acid aqueous solution and in various natural water samples and exposed to sunlight; for comparison, additional % Remaining
loo
-~,,~
80-
~
~] 0 S HsC2__O~ tt
60
HsCz__O/ p -
-
CHa I C -- CH3
0 .,~N~
|/
I!
I.
cN3 40 =
~i~__.~_.~..4
~..~....~
2o _l
0 0
1
2
3
4
5
6
7
8
9
10
11
12
13
I 14
Time(days)
Fig. 3. Degradation of diazinon in various aqueous solutions. Exposure to dark in: distilled water (1~, river water (O); exposure to sunlight in: humic acid aqueous ~ol,tion (e), river water (Isar) (!1), river water (Rhine) (A), and lake water (Ammmersee) (×).
I. SCHEUNERTEl" AL.
366
samples were kept in the dark. The results are presented in Fig. 3. The figure shows that even in the dark, river water has a higher degradation capacity for diazinon than has distilled water. This fact is attributed, in addition to other factors, to the oxygen and hydroxyl content of river water (Table l). The degradation capacity of river water is further increased by exposure to sunlight. The highest degradation capacity was observed for Rhine river water having the highest content in 02 and (OH) and the highest pH-value. Dissolved humic acids exhibit a lower effect on photodegradation of diazinon than does the sum of constituents present in natural river or lake water. The chemical identity of some photodegradation products of pesticides is presented in Figs 4 and S. When diazinon was irradiated in a water/soil suspension, it was isomerized to a product containing a - S - ( P = O)- group instead of the original - O - ( P = O)- group (Fig. 4). Other products formed were diazoxon and hydroxy diazinon [2]. Figure 5 shows some products of direct and indirect photolysis of the herbicide pendimethalin (l) in the presence of soil particles. Direct photolysis in water by wavelengths X _~ 290 nm results in the dealkylation of the amino group. The resulting product (3) has been reported to be formed after irradiation of pend:imethalin in methanol [3,4] as well as by the soil bacterium Azotobacter [5], by soil fungi [6] and in soil [7]. Indirect photolysis with wavelengths X ~ 290 nm, as promoted by dissolved humic acids, proceeds very slowly to yield a reduced product containing an amino group instead of one of the nitro groups (4). This product, also, is formed from pendimethalin after irradiation in methanol [3] and by soil fungi [6]. When acetone is added as a photosensitizer, an alteration of the side chain is observed (2). The same process was observed upon irradiation of the herbicide in various solvents and on soil surfaces [8].
Outdoor lysimeter studies After application of [14C]pendimethalin (52 rag/0.32 m 2) to various soils in lysimeters under outdoor conditions, ~4C-labelled products were detected
CH3~ N
SII
h/,)(.,~< 290nm)
CH=\
O1[
.,o,.o..
CH~
CH3
Fig. 4. Conversion of diazinon after UV-light irradiation in a water/soil suspension.
367
FATE OF PENDIMETHALIN,CARBOFURANAND DIAZINON H--N-- CH2-- CH:--CH3
O:~NO: "CH3
CH3 2
l
.~, ~ 290rim
H=OlaCetone
NO2
--CH--C2Hs
H - - ~'~1
O'~I~,~NH,
CHa..~__~..NHCH(CH=CH 3 ~ )= j~,~) 290nm h.a.l'~-~" CH3
C=Hs
t~,~CH3
NO:
CH= ./1,~ 29Ohm H=O
.
NH=
O=N~NO= CH= a Fig. 5. Degradation pathways of l~ndimetha|in in the presence of simulated light and soil; h.a., humic acids.
200
Leaching (pg pendimethalin-equivalents)
/ ,~o
San dy fo rest SOl"1
/
100
50 --'-L O~ 0
~
~ 50
100
150
.. I __ 200
i 250
300
Time (days) Fig. 6. '4C-Leaching after application of [14C]pendimethalin to sandy foresl~ so~i (52 mg/0.32 m 2) in lysimeters (l-m depth) under outdoor conditions (~g-equivalents of ~dimethalin).
368
1000
L SCHEUNERT ET AL
Leaching (IJg pendimethalin-equivalents)
800
//
Sandy agricultural soil
./
6OO
Jy
400
~/// /
"
/
/ /
200
.....~ ~
0
. ~
~
L1
50
100
150
200
250
300
Time ( d a y s ) Fig. 7. 14C-Leaching after application of [14C]pendimethalin to sandy agricultural soil (52 mg/0.32 m 2) in lysimeters (l-m depth) under outdoor conditions (~g-equivalents of pendimethalin).
in the leachate from the third week after application onwards. Leaching continued during the experimental time of 300 days (Figs 6 and 7). The amounts of leached m4C were much higher in the agricultural soil than in the forest soil. The ~4C-labelled compounds were neither unchanged pendimethalin, nor carbon dioxide nor carbonate resulting from the mineralization of the ring of the pesticide molecule. They were highly polar conversion products formed in the soil. Their identity with some of the products shown in Fig. 5 was not demonstrated thus far. Biotic dealkylation and reduction are wellknown processes in soil [9]. The formation of products (3) and (4) in Fig. 5 by soil microorganisms has already been demonstrated. However, the high polarity of the conversion products in leachate points to the presence of hydroxyl groups, or of COOH groups. Conversion products having such structures were identified in soil [7]. Another possibility is the association of pendimethalin or of its conversion products with non-dialyzable organic soil molecules of high molecular weight, as observed by Nelson [10]; these associates could have high polarity and thus be leached easily from soil. CONCLUSION
In aqueous solutions as well as in water/soil suspensions, various abiotic conversion, and degradation reactions occur whose rates depend on en-
FATE OF PENDIMETHALIN. CARBOFURAN AND DIAZINON
369
vironmental conditions and on the pregcnce of other compounds. In natural soil, biotic conversions prevail; some of them lead to the same products as abiotic reactions. Highly hydrophilic conversion products are leached from soil into the percolate water. Degradative reactions are desirable processes if they result in a removal of the pesticide ol ~ reduction of its concentration in the environment. However, if they lead to new products having different physico-chemical properties and thus an increased tendency to be either bioa¢cumulated in organisms or leached into groundwater, they are an additional hazard for environmental quality. ACKNOWLEDGEMENT
We thank the German Federal Mir~istry of Research and Technology for financial support of parts of these studies within the framework of the project 02WT89137. REFERENCES 1 M. Mansour, E. Feicht and P. M6allier, Improvement of the photostability of selected substances in aqueous medium. Toxicol. Environ. Chem., 20,21 (1989) 139-147. 2 G. Pestlin, I. Scheunert, M. Mansour, D. Wabner and A. Kettrup, Photochemical behaviour of diazinon in water and in a soil-water suspension, in Seventh International Congress of Pesticide Chemistry, Book of Abstracts, Vol. 111, IUPAC, Frankfurt 1990, p. 131. 3 P. Dureja and S. Walia, Photodecomposition of pendimethalin. Pestic. Sci., 25 (1989) 105-114. 4 S. Pal, P.N. Moza and A. Kettrup, Photochemistry of pendimethalin. J. Agric. Food Chem., 39 (1°~91) 797-g~. 5 J. Saha, A. Chowdhury and S. Chaudhuri, Stimulation of l,~'terotrcphic dinitrogen fixatioa in barley root association by the herbicide pendimethalin and its metabolic transformation by Azotobacter spp. Soil Biol. Biochem., 23 (1991) 569-573. 6 A.S. Barua, J. Saha, S. Chaudhuri, A. Chowdhury and N. Adityachaudhury, Degradation of pendimethalin by soil fungi. Pestic. Sci., 29 (1990) 419-425. 7 J. Zulalian, CL92,553-metabolism V. Fate of carbon-14 labelled CL92,553 (PROWL herbicide) in soil. Progress Report, Project 2-463, American Cyanamid Company, Princeton, N J, 1973. 8 P. Halder, A.S. Barua, P. Raha, B. Biswas, S. Pal, A. Bhattacharya, S. Bedi and A. Chowdhury, Studies on the photodegradation of pendimethalin in solvents and in Kalyani soil. Chemosphere, 18 (1989) 1611- 16! 9. 9 I. Scheunert, Transformation and degradation of pesticides in soil, in W. Ebing (Ed.), Chemistry of Plant Protection, Vol. 8, Springer-Verlag, Berlin-Heidelberg, 1992. 10 J.E. Nelson, Residues of Pendimethalin [N-(I-ethylpropyl)-3,4--dimetbyl-2,6dinitrobenzenaminel, trifuralin (¢x,ot,¢-trifluoro-2,6-dinitro-N, Nodipropyl-p-toluidine), and Oryzalin (3,5-dinitro-N4,N4,dipropylsulfanilamide) in Soil Organic Matter, PhD Thesis, Michigan State University, 1979.
361
Fate of pendimethalin, carbofuran and diazinon under abiotic and biotic conditions I. Scheunert a, M. Mansour b, U. D6rfler a and R. Schroll a aGSF-Forschungszentrum f~r Umwelt und Gesundheit GmbH, lnstitut fzir Bodeniikologie, Ingolstiidter Landstrasse 1, D( W)-8042 Neuherberg, Germany bGSF-Forschungszenl'rumf~r Umwelt und Gesundheit GmbH, Institut f~r Okologische Chemie, Sc~)ulstrasse 10, D(W)-8050 Freising-Attaching, Germany ABSTRACT The insecticides carbofuran and diazinon as well as the herbicide pendimethalin were irradiated with UV light of different wavelengths in water or water/soil suspensions under various conditions. As compared to pure distilled water, photodegradation was increased in the presence of titanium dioxide, hydrogen peroxide or ozone, or by using natural river or lake water. In a water/soil suspension, diazinon was converted, besides to other products, to the isomeric isodiazinon. When subjected to various direct or indirect photolysis conditions, pendimetbalin was transformed to various products resulting from dealkylation and reduction. [14C]Pendimethalin was applied to two natural sandy soils in lysimeters under outdoor conditions. [.eachate collected at 1 m depth was analysed for radioactivity for 300 days. Radioactive products were detected from the third week onwards and continued to be leached during the whole experimental period. The radioactive oroducts wer~ n,;kh~, the parent compound nor catrbon dioxide nor carbonate but water-soluble organic conversion products.
Key words: pendimethalin; carbofuran; diazinon; photodegradation; lysimeter; leaching
INTRODUCTION
In order to evaluate the fate of pesticides in the environment, the influences of both abiotic and biotic factors should be taken into account. Among the abiotic chemical factors affecting the behaviour of pesticides, photochemical reactions are most important. In aquatic systems, they play a dominant role in the conversion and degradation of pes6cides. In soil, photochemical reactions are significant only at the surface. When the surface is moist, photochemical reactions may occur both with the adsorbed portion and with the portion dissolved in water. Whereas model reactions in aqueous solution simulate photoreactions of chemicals dissolved in the aqueous phase of soil, those in water/soil suspensions include also potential catalytic effects 0048-9697/93/$06.00
© 1993 Elsevier Science Publishers B.V. All rights reserved
362
!. $CHEUNERT ET AL.
of adsorption. Therefore, both reaction conditions were studied by model experiments. In the soil layers below the surface, biotic reactions in general are more important than abiotic ones. Both abiotic and biotic reaction products in soil are subjected to various physical processes such as transport in soil and leaching from soil into percolate w~ter, Since the conversion products may differ from the parent compounds in physico-chemical properties such as water solubility and partition coefficients, they are also quite different in their tendency to be leached into groundwater. Thus, interactions between abiotic and biotic conversion reactions and physical processes govern the residue behaviour of the pesticides in the environment. In this paper, some experiments concerning abiotic conversion of pesticides as well as the leaching of a pesticide and of its conversion products from soil will be presented. Photodegradation experiments in water and water/soil suspensions were carried out with pendimethalin, carbofuran and diazinon. Additionally, for pendimethalin the leaching behaviour in soil under outdoor conditions was studied. EXPERIMENTAL
Laboratory experiments t
The photochemical experiments were performed in an apparatus suitable for both irradiation of the chemical in the liquid phase and as a suspension. Ultraviolet light was derived from a HPK 125 W high pressure mercury vapor lamp and passed through a pyrex filter to block wavelengths shorter than 290 nm or through a quartz filter for ~ < 290 rim. The light intensity determinations were usually carried out using potassium ferrioxalate actiw~meters. The decomposition and disappearance of the parent compounds were followed by gas chromatography, liquid chromatography, UV or other analytical techniques. Irradiations were performed in aqueous solutions, either in pure distilled water o~ after the addition of various sensitizing compounds, or in water/soil suspensions after adding 25 g soil (alfisol) to 100 ml of distilled water. The composition of the soil was as follows. Particle size distribution: sand 12.9%, silt 64.3%, clay 19.6%, coarse matter 3.2%. Organic carbon 0.76%; carbonate as CaCO3 0.75%; pH (1:1, H20) 7.45. In order to study the influence of natural water constituents on photodegradation, river water was collected fi'om the Rhine near Frankfurt and from the Bar River near Munich. Finally we chose water from a lake called the Ammersee. All water samples were stored at 4°C and again filtered before use (cellulose-acetate filters, pore size 0.2 ~m, from Sartorius, G6ttingen, FRG). The initial concentrations of these compounds were between 50 and 70 mM. Each solution in a conical pyrex flask (500 ml) was exposed directly
363
FATE OF PF.NDIMETHALIN. CARBOFURAN AND DIAZINON
TABLE 1 Quality characteristicsof water used in gtudy Body of water Period (1986)
pH
(02) (mg/l)
(OH) (mol/! X 10 "16)
Isar fiver Rhine fiver Ammer~e
7.8 8.1 7.5
12.80 14.70 I 1.30
6.51 12.45 10.75
May-June April-May June-July
to sunlight in April and May, 1986. The temperature was maintained constanfly (25°C) by a flowing fresh-water bath. Samples were removed after various time intervals and analysed by HPLC or GC after direct-injection. Dark controls were maintained for each experiment [1]. Some properties of the waters used are presented in Table !.
Outdoor lysimeter experiments Six lysimeters (depth I m, 60 cm in diameter) were filled with natural soils, three with a sandy forest soil and three with a sandy agricultural soil. The natural soil horizons were dug out separately and placed into the lysimeters according to their natural sequence and density. ~4C-Ring-labelled pendimethalin was applied to the surface as in agricultural practice (52 mg per lysimeter). In the case of forest soil, the surface consisted of organic litter and humus; the Ah horizon below had the following composition. Particle size distribution: sand > 99%, silt and clay < 1%. Organic carbon 4.93%, inorganic carbon < 0.001%, pH (CaCI2) 3.0. In the case of agricultural soil, pendimethalin was applied on the Ap horizon which had the following composition. Particle size distribution: sand 85%, silt 10%, clay 5%. Organic carbon 1.03%; pH (CaCI2) 4.9. Grass was grown in the lysimeters with forest soil, and winter wheat in those with agricultural soil. The percolation water at the base of the lysimeters was collected for 300 days and assayed for radioactivity by liquid scintillation counting. For characterization of the chemical identity of the leached 14C, the water was subjected to liquidliquid extraction with dichloromethane. Both sobrent and aqueous phases were analyzed for the parent compound and metabolites by thin layer chromatography after concentration in a rotary evaporator. The potential presence of ~4CO2 or [14C]carbonate resulting from mineralization of the pesticide was excluded by acidifying the concentrated water samples and by trapping any CO2 evolved in a scit~tillation liquid containing an organic base, followed by liquid scintillation counting.
3~.
!. SCHEUNERT ET AL
% Remaining
ll\~
o
~
40
20
1 2 3 4 5
3
Carbofuran alone Carbo. + TiO2(160mg/I)+N? Carbo. + H=02 (5.10-3mo111) Carbo. + Ti02 (160mg11)+02 Carbo. + 03 (lO-3mol/llmin)
4 0
50
100
I
I
I
150
200
250 Time (minutes) /
Fig. I. Photodegradation (X > 290 nm) of carbofuran in the presence of hydro~;en peroxide, titanium dioxide and ozone in water.
% Remaining IO0
80
60
40
0
0
I
I
I
I
I
I
I
20
40
60
80
100
120
140
I
160 180 Time(hours)
Fig. 2. Photolysis of carbofuran in a water/soil suspension ()~ > 290 nm): 25 g alfisol/10() ml H20; carbofuran, 22 ppm.
365
FATE OF PENDIMETHALIN, CARBOFURAN AND DIAZINON
RESULTS AND DISCUSSION
Laboratory experiments The kinetics of photodegradation of the insecticides carbofuran and diazincn in aqueous media under various conditions are shown in Figs 1-3. The ohotochemical degradation of carbofuran in pure aqueous solution, as well as after the addition of various oxygen-containing substances, is presented in Fig. ~. The figure reveals that photodegradation is very slow when carbofuran is irradiated alone with UV light ~ > 290 nm. However, in the presence of TiO2, H.:~2 or 03, photodegradation is accelerated considerably. The highest photodegradation rate is achieved when, in addition to TiO2, free oxygen is supplied to the aqueous solution. This points to oxidative mechanisms of photodegradation reactions of carbofuran. Figure 2 presents the photodegradation of carbofuran in a water/soil suspension. It demonstrates that soil particles or compounds present in soil moisture are also effective in promoting photodegradation reactions. In order to elucidate the potential role of natural water constituents on photodegradation rates of diazinon under natural conditions, diazinon was dissolved in distilled water, in humic acid aqueous solution and in various natural water samples and exposed to sunlight; for comparison, additional % Remaining
loo
-~,,~
80-
~
~] 0 S HsC2__O~ tt
60
HsCz__O/ p -
-
CHa I C -- CH3
0 .,~N~
|/
I!
I.
cN3 40 =
~i~__.~_.~..4
~..~....~
2o _l
0 0
1
2
3
4
5
6
7
8
9
10
11
12
13
I 14
Time(days)
Fig. 3. Degradation of diazinon in various aqueous solutions. Exposure to dark in: distilled water (1~, river water (O); exposure to sunlight in: humic acid aqueous ~ol,tion (e), river water (Isar) (!1), river water (Rhine) (A), and lake water (Ammmersee) (×).
I. SCHEUNERTEl" AL.
366
samples were kept in the dark. The results are presented in Fig. 3. The figure shows that even in the dark, river water has a higher degradation capacity for diazinon than has distilled water. This fact is attributed, in addition to other factors, to the oxygen and hydroxyl content of river water (Table l). The degradation capacity of river water is further increased by exposure to sunlight. The highest degradation capacity was observed for Rhine river water having the highest content in 02 and (OH) and the highest pH-value. Dissolved humic acids exhibit a lower effect on photodegradation of diazinon than does the sum of constituents present in natural river or lake water. The chemical identity of some photodegradation products of pesticides is presented in Figs 4 and S. When diazinon was irradiated in a water/soil suspension, it was isomerized to a product containing a - S - ( P = O)- group instead of the original - O - ( P = O)- group (Fig. 4). Other products formed were diazoxon and hydroxy diazinon [2]. Figure 5 shows some products of direct and indirect photolysis of the herbicide pendimethalin (l) in the presence of soil particles. Direct photolysis in water by wavelengths X _~ 290 nm results in the dealkylation of the amino group. The resulting product (3) has been reported to be formed after irradiation of pend:imethalin in methanol [3,4] as well as by the soil bacterium Azotobacter [5], by soil fungi [6] and in soil [7]. Indirect photolysis with wavelengths X ~ 290 nm, as promoted by dissolved humic acids, proceeds very slowly to yield a reduced product containing an amino group instead of one of the nitro groups (4). This product, also, is formed from pendimethalin after irradiation in methanol [3] and by soil fungi [6]. When acetone is added as a photosensitizer, an alteration of the side chain is observed (2). The same process was observed upon irradiation of the herbicide in various solvents and on soil surfaces [8].
Outdoor lysimeter studies After application of [14C]pendimethalin (52 rag/0.32 m 2) to various soils in lysimeters under outdoor conditions, ~4C-labelled products were detected
CH3~ N
SII
h/,)(.,~< 290nm)
CH=\
O1[
.,o,.o..
CH~
CH3
Fig. 4. Conversion of diazinon after UV-light irradiation in a water/soil suspension.
367
FATE OF PENDIMETHALIN,CARBOFURANAND DIAZINON H--N-- CH2-- CH:--CH3
O:~NO: "CH3
CH3 2
l
.~, ~ 290rim
H=OlaCetone
NO2
--CH--C2Hs
H - - ~'~1
O'~I~,~NH,
CHa..~__~..NHCH(CH=CH 3 ~ )= j~,~) 290nm h.a.l'~-~" CH3
C=Hs
t~,~CH3
NO:
CH= ./1,~ 29Ohm H=O
.
NH=
O=N~NO= CH= a Fig. 5. Degradation pathways of l~ndimetha|in in the presence of simulated light and soil; h.a., humic acids.
200
Leaching (pg pendimethalin-equivalents)
/ ,~o
San dy fo rest SOl"1
/
100
50 --'-L O~ 0
~
~ 50
100
150
.. I __ 200
i 250
300
Time (days) Fig. 6. '4C-Leaching after application of [14C]pendimethalin to sandy foresl~ so~i (52 mg/0.32 m 2) in lysimeters (l-m depth) under outdoor conditions (~g-equivalents of ~dimethalin).
368
1000
L SCHEUNERT ET AL
Leaching (IJg pendimethalin-equivalents)
800
//
Sandy agricultural soil
./
6OO
Jy
400
~/// /
"
/
/ /
200
.....~ ~
0
. ~
~
L1
50
100
150
200
250
300
Time ( d a y s ) Fig. 7. 14C-Leaching after application of [14C]pendimethalin to sandy agricultural soil (52 mg/0.32 m 2) in lysimeters (l-m depth) under outdoor conditions (~g-equivalents of pendimethalin).
in the leachate from the third week after application onwards. Leaching continued during the experimental time of 300 days (Figs 6 and 7). The amounts of leached m4C were much higher in the agricultural soil than in the forest soil. The ~4C-labelled compounds were neither unchanged pendimethalin, nor carbon dioxide nor carbonate resulting from the mineralization of the ring of the pesticide molecule. They were highly polar conversion products formed in the soil. Their identity with some of the products shown in Fig. 5 was not demonstrated thus far. Biotic dealkylation and reduction are wellknown processes in soil [9]. The formation of products (3) and (4) in Fig. 5 by soil microorganisms has already been demonstrated. However, the high polarity of the conversion products in leachate points to the presence of hydroxyl groups, or of COOH groups. Conversion products having such structures were identified in soil [7]. Another possibility is the association of pendimethalin or of its conversion products with non-dialyzable organic soil molecules of high molecular weight, as observed by Nelson [10]; these associates could have high polarity and thus be leached easily from soil. CONCLUSION
In aqueous solutions as well as in water/soil suspensions, various abiotic conversion, and degradation reactions occur whose rates depend on en-
FATE OF PENDIMETHALIN. CARBOFURAN AND DIAZINON
369
vironmental conditions and on the pregcnce of other compounds. In natural soil, biotic conversions prevail; some of them lead to the same products as abiotic reactions. Highly hydrophilic conversion products are leached from soil into the percolate water. Degradative reactions are desirable processes if they result in a removal of the pesticide ol ~ reduction of its concentration in the environment. However, if they lead to new products having different physico-chemical properties and thus an increased tendency to be either bioa¢cumulated in organisms or leached into groundwater, they are an additional hazard for environmental quality. ACKNOWLEDGEMENT
We thank the German Federal Mir~istry of Research and Technology for financial support of parts of these studies within the framework of the project 02WT89137. REFERENCES 1 M. Mansour, E. Feicht and P. M6allier, Improvement of the photostability of selected substances in aqueous medium. Toxicol. Environ. Chem., 20,21 (1989) 139-147. 2 G. Pestlin, I. Scheunert, M. Mansour, D. Wabner and A. Kettrup, Photochemical behaviour of diazinon in water and in a soil-water suspension, in Seventh International Congress of Pesticide Chemistry, Book of Abstracts, Vol. 111, IUPAC, Frankfurt 1990, p. 131. 3 P. Dureja and S. Walia, Photodecomposition of pendimethalin. Pestic. Sci., 25 (1989) 105-114. 4 S. Pal, P.N. Moza and A. Kettrup, Photochemistry of pendimethalin. J. Agric. Food Chem., 39 (1°~91) 797-g~. 5 J. Saha, A. Chowdhury and S. Chaudhuri, Stimulation of l,~'terotrcphic dinitrogen fixatioa in barley root association by the herbicide pendimethalin and its metabolic transformation by Azotobacter spp. Soil Biol. Biochem., 23 (1991) 569-573. 6 A.S. Barua, J. Saha, S. Chaudhuri, A. Chowdhury and N. Adityachaudhury, Degradation of pendimethalin by soil fungi. Pestic. Sci., 29 (1990) 419-425. 7 J. Zulalian, CL92,553-metabolism V. Fate of carbon-14 labelled CL92,553 (PROWL herbicide) in soil. Progress Report, Project 2-463, American Cyanamid Company, Princeton, N J, 1973. 8 P. Halder, A.S. Barua, P. Raha, B. Biswas, S. Pal, A. Bhattacharya, S. Bedi and A. Chowdhury, Studies on the photodegradation of pendimethalin in solvents and in Kalyani soil. Chemosphere, 18 (1989) 1611- 16! 9. 9 I. Scheunert, Transformation and degradation of pesticides in soil, in W. Ebing (Ed.), Chemistry of Plant Protection, Vol. 8, Springer-Verlag, Berlin-Heidelberg, 1992. 10 J.E. Nelson, Residues of Pendimethalin [N-(I-ethylpropyl)-3,4--dimetbyl-2,6dinitrobenzenaminel, trifuralin (¢x,ot,¢-trifluoro-2,6-dinitro-N, Nodipropyl-p-toluidine), and Oryzalin (3,5-dinitro-N4,N4,dipropylsulfanilamide) in Soil Organic Matter, PhD Thesis, Michigan State University, 1979.