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Atmospheric transport of pesticides adsorbed on aerosols I. Photodegradation in simulated atmosphere
Chemosphere, Vol. 30, No. 1, pp. 21-29, 1995
Pergamon
0045-6535(94)00372-6
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/95 $9.50+0.00
Atmospheric transport of pesticides adsorbed on aerosols I. Photodegradation in simulated atmosphere D. BOSSAN*, H. WORTHAM** and P. MASCLET* *Laboratoire d'Etudes des Syst~mes Atmosph(riques Multiphasiques, Universit6 de Savoie, 73326 Le Bourget, FRANCE **Laboratoire de Chimie H~t~og~e, Universit6 de Strasbourg, 68000 Strasbourg, FRANCE
(Received in Germany 4 July 1994; accepted 20 September 1994)
Abstract Eight pesticides (Alachlor, Pendimethalin, Trifluralin, Malathion, Terbuthylazine, Atrazine, Isoproturon and Fenithrothion), adsorbed on kaolin and fly ash are submitted to irradiation (wavelength above 290 nm) for 40 or 80 min. The first four compounds photodegrade when they are adsorbed on fly ash. The presence of metals and metal oxides in the fly ash seem to be responsible for this rapid degradation (up to 70% in 40 min). They do not degrade, however, when they are adsorbed on kaolin. Terbuthylazine and Atrazine do not degrade significantly, at least during this short irradiation time. Weakening of the pesticide-support bonds after irradiation, promoting the extraction by organic solvents, is important for several pesticides, particularly for Alachlor and Atrazin~ Isoproturon and Fenithrothion do not adsorb significantly on these two atmospheric supports and cannot be successfully submitted to irradiation. These results are used to predict the possibility of the meso-scale atmospheric transport of the pesticides from agricultural regions to non polluted areas, like alpine regions.
Introduction Pesticides make up a significant fraction of the atmospheric aerosols of anthropic origin. These compounds arise from industrial and agricultural uses. Pesticides are widely used throughout the world (500 000 T for USA alone in 1990). These compounds volatilize after spreading. Some of them are volatile enough to be found in the atmospheric gas phase, while others are less volatile ( p < 10-4 to 20°C) and can be adsorbed on atmospheric aerosols. The rate of volatilization is very variable from one pesticide to another since it is affected by the humidity of the soil, the saturated vapour pressure of the compound and even by the way it is spread. The fraction volatilized can reach 90% in 7 days for Trifluraline (1). Some pesticides (volatile or less volatile) can travel over very long distances (2) to remote areas and to non-polluted zones like ocean and polar atmospheres. Important amounts of Toxaphene (3), Chlordane, Hexachlorocyclohexanes
21
22
(HCI-I) and others chlorinated derivatives have been early found in Canada (4), in the Grat Lakes (5), the North Pacific (6) and in the Arctic polar regions (7). After passing into the atmosphere, then possible adsorption onto atmospheric particles, the pesticides can undergo physicochemical transformations under the effect of light, oxidizing agents and/or radicals in the atmosphere These phenomena lead to the elimination of the pesticides from the atmospheric medium. The atmosphere is therefore both a transport vector and also a medium which favours the degradation of these compounds. Nevertheless, the photochemical reactions sometimes lead to chemical compounds which are more toxic than the parent compounds, as is the case of hydrocarbons and Polycyclic Aromatic Hydrocarbons (PAH) (8). Given their structure, one can perhaps expect pesticides to behave rather like PAIl (9). However, contrary to PAH, pestcides are generally water-soluble. They are several modes of scavenging and therefore pesticide transport will most often be limited to meso-scale transport. But this does not exclude the possibility of finding pesticides far from the sources. Because of their high toxicity, certain pesticides are no longer used in Europe or in the USA, but they are still in less developed countries. They are replaced by products which are less toxic and for which the degradation processes in water are fairly well known, but the atmospheric degradation processes (mechanisms and kinetics) are poorly investigated. Studies performed in aqueous media have shown that catalytic photodegradation can be useful tool to destroy pesticides (10,11,12,13,14) WlNER et al.(15) have shown that pesticides can, at least in the gaseous form, degrade under the effects of atmospheric agents like ozone, or hydroxyl and nitrate radicals. For organophosphorus compounds the atmospheric life-time is about 1-50 h for OH, several days for NO3 and at least I00 days for 03 (11).This behaviour is likely the same than those of most organic compounds (16). However, it is known that rate constants, measured in the homogenous phase, are not in agreement with heterogenous phase results. In adsorbed phase, few results are available: MI~ALLIER et al. have shown that carbetamid adsorbed on particles can photodegrade significantly (17,18). The aim of this study is to investigate the atmospheric processes involving pesticides adsorbed on atmospheric aerosols of different types and, consequently, the possibilities either of a meso-scale or a long range transport or of elimination of pesticides in the atmosphere itself. The choice of the pesticides is difficult because 400 compounds are currently used in the world and any choice may appear rather arbitrary. We chose 8 pesticides often used in France. In order to simulate atmospheric reactions, we have used a dynamic flow reactor, already used to investigate the heterogenous reactions of PAH (19), to stir the aerosols, of predetermined size, on which the pesticides are adsorbed. These aerosols are irradiated with a sun-lamp in the presence or absence of oxidizing gases. The supports are characteristic of aerosols commonly encountered in the atmosphere (aluminosilicates as crustal aerosols, ammonium sulfate as marine aerosols and fly ash as anthropogenic aerosols).
23
Exverimental section Reactor
The reactor consists of two parts: the reactor as such and the irradiation system. The former consists of a cylindrical quartz tube (12 cm diameter and 80 cm long; i.e. 9000 cm3). The irradiated volume is 5000 cm3. The tube is closed at both ends by fritted disks of 1.2/am porosity. The reactor is continuously flushed with a stream of ultrapure nitrogen by means of a system of eleadcally operated valves. The stream is directed upwards and downwards, so as to maintain the particles in a state of constant agitation. The time of opening depends on the particle size. The irradiating system consists of a sun lamp (Hannovia 679A-0360) whose spectrum ranges from 260 nm to 700 nm. This lamp is placed in a water-cooled quartz sheath maintaining a temperature less than 30°C in order to prevent volatilization of adsorbed pesticides, under irradiation. This sheath cuts off the light below 290 nm. Two gas traps (PUF and XAD2) are set at the exit of the reactor, in order to control possible direct desorptions during the experiment. Homogenous irradiation is obtained by placing the reactor and the lamp at two foci of an ellipsoid made of highly reflecting metal foil. Pesticides
and
substrates
Eight pesticides and three supports were studied. The pesticides are volatile (V) or involatile (NV) if their vapor pressure at 20°C is less than l0 -4 Pa:. Two insecticides:
- Malathion (V) and Fenithrothion (NV)
Six herbicides:
-Trifluarine (V),
Alachlor (V),
Pendlmethaline (V), Atrazine (NV),
Tarbutylazine (NV) and Isoproturon (NV). The following substrates three used: - Substrate
Size
Specific
- kaolinite
10-20 lam
250 m2.g-1
- fly ash
20-30/am
50 m2.g-l
- ammonium sulfate
20-30 lam
unknown
Ammonium sulfate is v ~
area
hygroscopic. It sticks rapidly the wall of the reactor and the
experimental time cannot exceed 15 ran. For this reason the experience is very difficult and the results relative to pesticides adsorbed on this substrate are too doubtful to be published. Sample
preparation
Samples were prepared by adsorption of pesticides from an organic liquid phase. Solutions containing known amounts of pesticides in appropriate solvents were stirred for 12 h with the substrate previously cleaned by two Soxhlet extraction with a mixture of cyclohexane and dichloromethane (1/2). The particles were removed and dried for 2 h. The pesticides were then partially adsorbed on the
24 substrate (adsorption yield - see below). For each support and each kind of pesticide (group V and group NV) five grammes of particles, divided into 16 fractions were prepared in this way. Two fractions were used as a control in order to determine the initial pesticide concentrations [Co]. The others fractions were introduced into the reactor and submitted to irradiation for various times ( 5, 10, 15, 20 40 or 80 mn, depending of the assay), then extracted after irradiation. Reverse extractions were carried out with the same solvent mixture for the non-irradiated samples and the irradiated ones. Each experiment was performed two times. Experiments
1
Fly ash
Atrazine, Terbuthylazine
2
Kaolin
Atrazine, Terbuthylazine
3
Fly ash
Alachlor, Malathion, Trifluralin Peadimethalin
4
Kaolin
Alachlor, Malathion, Trifluralin, Pendimethalin
Analysis Before analysis each sample was spiked with a 6.65 ng.mL-1 of lindane (y-HCH) as internal standard. Pesticides were analysed by GC with electron capture detection (ECD) on a Carlo Erba 8000 fitted with a Ni 63 radioactive source. A split mode (0.5 jaL) and SPB-5 Supelco capillary column, 30 m length) were used. The make-up
gas and the
carrier gas were nitrogen. All these analysis
conditions are often used in the litterature (20). For each pesticide the concentration range was linear for concentrations from 5 to 200 ng.mL-1, which correspond to environmentaly observed concentrations.
Results
and discussion
Adsorption yields For each pesticide-substrate, the adsorption yield is calculated as follows: Mass of oesticide extracted after adsoration oer _re'amof substrate Mass of pesticide present in the solution per gram of solution The adsorption yields are presented in the Table 1 for the 6 following pesticides Pendimethalin, Atrazine, Malathion, Terbuthylazine, Alachlor and Trifluralin) Table 1 Extraction yield
Pesticide
Kaolin
Fly Ash
Pesticide
Kaolin
Fly Ash
Alachlor
nd 0,23 0,04
nd 0,27 0,04
Terbuthylazine
0,34
0,31
0,09
0,08
0,12
0,18
Isoproturon Malathion Trifluralin
0,14
0,15
Atrazine Fenithrothion Pendimethalin
* nd : not determinated
0
0
25 They range from 0.08 and 0.34 for fly ash and kaolin. Adsorption is thendore far from complete but this does not raise any problems, since all results are expressed as relative amounts of pesticide, before and after irradiation. For Isoproturon and Fenitln~hion, the adsorption yields are less than 0.04. This small value means that the preparation method is not suitable for these compounds and/or they do not adsorb on atmospheric aerosols. In this last case they cannot be transported in the atmosphere, adsorbed on aerosols. Photodegradation and bond weakening The mass ratio is unity for zero irradiation time. It can be grester than unity if the amount exlracted after irradiation is greater than the amount e~tracted before irradiation.
This
dimensionless
number does not depend on the extraction yield. In Fig. la, Ib, lc and ld, we have shown the variation of the mass ratio for 6 pesticides adsorbed on fly ash and kaolin. Because of the lack of data for Malathion on kaolin, this curve is not shown. Volatile Pesticides on Fly Ash
Volatile Pesticides on Kaolinite
I
10
A|~Phlnr
,
Malathion Pendimethalin
8
Trifl,r~lin
2
6
mr
mr 4
1
2 i
i
z
10
20
30
Non Volatile on Fly Ash
0
40 run Irradiation time
20
40
60
80
Irradiation Non Volatile on Kaolinite
Pesticides
time
Pesticides
I
I
Terbuthvlazine Atrazine
e
100 mn
o
Terbuthvlazine i trazine
2 mr
2 mr 1
1
0
i
i
!
10
20
30
0
40
Irradiation
time Figure 1
z
i
20
40
i
t
60 80 Irradiation
100 mn time
26
Volatile pesticides For the four volatile pesticides adsorbed on fly ash there are two types of behaviour. For Trifluralin and Pendimcthalin, the regular decrease in the ratio shows that these compounds photodegrade fairly, since 50% and 70% respectively are decomposed after 30 mn irradiation. These compounds, adsorbed on anthropogenic or combustion aerosols, cannot therefore be transported in the atmosphere, at least for long range transport. Of course the irradiation conditions are severe; in the atmosphere they are often less so. Nevertheless, they allow us to estimate that under favorable meteorological conditions (strong luminosity), the greater part of these pesticides will be degraded after a few days' transport. Alachlor and Malathion behave differently. For short irradiation time there is a marked increase in the mass ratio, which reaches 2 and 1.7 respectively, for these two compounds after 5 mn irradiation. Subsequently, as for the previous compounds, the mass ratio decreases fairly regularly without going below unity before 30ran. This behaviour is similar to that observed for several PAH (9). The rapid increase in the mass ratio could be due to a weakening of the bonds between the adsorbed compound and the support. Irradiation would cause rupture of the strong bond between the adsorbant and the adsorbed compound (chemisorption). Some chemical bonds are broken. Perhaps the compound is now only physisorbed. The weakening of the bond makes easier the extraction by an organic solvent. During the first step of the experiment (first 10 mn), desorption of the pesticide does no occur, since no pesticide is observed in the gas phase collected in the PIJF or the XAD2 traps Then for some pesticides and PAH adsorbed on fly ash, we observe an increase in the amount of organic compound extracted after irradiation, and a photodegradation of the chemical substances, which is fairly slow but could continue for longer irradiation times.The two phenomena are superimposed. For short irradiation, the main phenomenon is the first one; the second phenomenon becomes preponderant after 10 or 20 min irradiation. The observed curve results of superimposing these two phenomena. For volatile pesticides adsorbed on kaolin the behaviour is quite different. Trifluralin and Pendimethalin are not significantly degraded when there are adsorbed on this substrate. This result does not agree with the previous hypothesis suggesting that the degradation rate is higher for white substrate (kaolinite) than for black substrate (fly ash ) (21). The screen effect invoked for PAIl cannot explain the experimental results for pesticides. On the other hand the hypothesis of an increasing of the rate constant by the presence of catalytieal metal oxides or metals in the fly ash (22) can be retained for the four pesticides. Alachlor adsorbed on kaolin behaves specifically. It is coherent both with the observations concerning Alaclor adsorbed on fly ash and those concerning Trifluralin and Pendimethalin adsorbed on kaolin. Indeed, Alachlor is made more available to solvent extraction after irradiation, by bond weakening. This phenomenon is strongly favoured when the compound is adsorbed on kaolin. After 5rain, the mass ratio increases 5 times in this case and 2 times when it is adsorbed on fly ash. Moreover, there is no significant degradation of Alachlor adsorbed on kaolin; this behaviour is similar to Trifluralin and Pendimethalin adsorbed on kaolin.
27 It appears therefore that on this substrate, fairly typical of terrigenic aerosol, pesticides do not degrade or very little (even for severe irradiation conditions). Consequently, long range transport of pesticides in this form is possible. N o n - v o l a t i l e pesticides For Terbuthylazine and Atrazine, the two same phenomenaare found and it is logical to assume the same reasons. Nevertheless, the relative importance of bond weakening and photodegradation differ both for the range of variation of the mass ratio and for the irradiation time necessary for these two phenomena to become important. Thus, for Atrazine adsorbed on fly ash, one observes, as for Alachlor and Malathion, the weakening of the pesticide-substrate bonds, the irradiation time is longer (20 rain instead of 5 rain). Furthermore, the product appears to resist photodegradation, at least for irradiation times used. Experiments on longer irradiation times are in hand. The same result is observed for Atrazine adsorbed on kaolin. The pesticide photodegrade very slightly, but this degradation is not greater than the weakening of the bonds, since the the mass ratio remains above the unity whatevff the irradiation time. For Terbuthylazine there is a slight increase in the mass ratio for short irradiation times, whatever the substrate. This compound appears to degrade more markdely than Atrazine ( 50% and 40% on fly ash and kaolin respectively after 30 mn irradiation). Tcxtiobuthylazine and Atrazine (like Simazine which has not been studied here) can be transported during meso-scale transport. There are found in non-polluted areas like high altitude lakes and oceanic zones (23).The laboratory results appear therefore to be consistent with field observations. Conclusion This laboratory study simulating photodegradation of pesticides adsorbed on substates typical of the atmospheric aerosol allows us to forsee which compounds we can expect to find in media not directly contaminated by spreading but via atmospheric transport. The triazine family appears to resit to photodegradation and the atmospheric medium does not lead to the elimination of these compounds. For the other compounds studied the nature of the support appears to be very important. Pesticides adsorbed on kaolin (a tmigenic aerosol) can travel over long distance. In return pesticides adsorbed on fly ash (an urban aerosol) photodegrade rapidly and cannot travel. In agricultural zones the adsorption on terrigenic aerosol may be more probable. Then their transport is possible and the use of pesticides as tracers of meso scale transport is possible. The other phenomenon observed indicates that the majority of the pesticide adsorbed is often not extracted by the simple use of organic solvent. The chemisorbed compounds are strongly attached. Consequently, classical procedure (Soxhlet or Ultrasonic extraction) maybe seriously underestimates the amount of pesticides adsorbed on atmospheric aerosols (up to a factor of 5). Then this study shows that the measurement of pesticide concentrations in the atmosphere, or in other media as ice cores of water, in order to determine fluxes is may be questionnable.
28
BIBLIOGRAPHY
1. YANMAGUCHI Y., KAWANO M., KANNAN N. and TANABE S. Global monitoring of organochloride insecticides in Long Range Transport of Pesticides; KURTZ D.A. Ed., LEWIS Publ.pp. 127-143 (1990) 2. ATLAS E. Long range transport of organic compounds KNAP A.H. Ed. NATO ASI Series, Dordrecht, The Netherlands, KLUWER Acad. Publ., pp. 105135, (1990)11 3. VOLDNER E.C. and SCHROEDER W.H. Long range transport and deposition of Toxaphene in Long Range Transport of Pesticides; KURTZ D.A. Ed., LEWIS Publ.pp. 223-233, (1990) 4. BIDLEMAN T.F., PATTON G.W., HINCKLEY D.A., WALLA M.D., COTHAM W.E. and HARGRAVE B.T. Chlorinated pesticides and Polychlorobiphenyls in the atmosphere of the Canadian Arctic in Long range transport of pesticides KURTZ D.A. Ed.,, LEWIS Publ. pp. 347-373, (1990) 5. MURPHY T.J. and RZESZUTKO V.P. Precipitations inputs pf PCBs to Lake Michigan J. Great Lakes Res., 3, 305-312, (1977) 6. ATLAS E L and SCHAUFFLER S Concentration and variation of trace organic compounds in the North Pacific atmosphere in Long Range Transport of Pesticides; KURTZ D.A. Ed., LEWIS Publ.pp. 185-199, (1990) 7. MUIR D.C.G., GRIFT N.P., FORD C.A., REIGER A.W., HENDZEL M.R. and LOCKHART W.L. Evidence for long range transport of Toxaphene to remote arctic and subarctic waters from monitoring of fish tissues in Long range transport of pesticides KURTZ D.A. Ed.,, LEWIS Publ. pp. 329-347, (1990) 8. PITI'S J. N. Jr, VAN CAUWENBERGHE K.A., GROSJEAN D., SCHIMD J.P., FITZ D.R, BELSER W.L., KNUDSON G.B. and HYNDS P.M. Facile formation of mutagenic nitro derivatives Science, 202, 515, (1978) 9. MASCLET P., PISTIKOPOULOS P., BEYNE S. and MOUVIER G. Long range transport and gas/particle distribution of Polycyclic Aromatic Hydrocarbons at a remote site in the Mediterranean Sea Atmos. Environ., 4, 639-645, (1988) 10. GALE., AIRES P., CHAMRRO E. and ESPLUGAS S. Photochemical degradation of Parathion in aqueous solution Wat. Res. 26, 911-915, (1992) 11. LU M.C., ROAM G. D., CHEN J.N. and HUANG C.P. Mierotox Bioassay of photodegradation products from photocatalytie oxidation of pesticides Chemosphere 27, 1637-1647, (1993) 12. PELIZZETYI E., MAURINO V., MINERO C., CARLIN V., PRAMAURO E., ZERBINATII O. and TOSATO M. Photocatalytic degradation of Atrazine and others s-Triazine Herbicides Environ. Sci. Technol., 24, 1559-1565, (1990)
29
13. RAHA P. and DAS A.K. Photodegradation of Carbofuran Chemosphexe 21,99-106, (1990) 14. PELIT~ETrI E., MAURINO V., MINERO C., ZERBINATI O. and BORGARELLO E. Photocatalytic degradation of Bentazon by TiO2 particles Chcmosphere 18, 1437-1445, (1989) 15. WINER A.M. and ATK1NSON R. Atmospheric reaction pathways and lifetimes for Organophosphorus Compounds in Long range transport of pesticides; KURTZ D.A. Ed., LEWIS Publ. pp.115-127, (1990) 16. ATKINSON R. Structure reactivity relationship for the estimation of rate constants for the gas phase reactions of OH radicals with organic compounds Int. J. Chem. Kinet., 19, 799-828, (1987) 17. EMMELIN C., GUITONNEAU S., LAMARTINE R. and IV~ALLIER P. Photodegradation of pesticides on adsorbed phases - Photodegradation of Carbetamid Chemosphere 27, 757-763, (1993) 18. IV~ALLIER P., MAMOUNI A. and MANSOUR M., Chemosphere 21,913-917, (1990) 19. WORTHAM H., BON NGUYEN E., MASCLET P. and MOUVIER G. Study of hetea'ogenous reactions of Polycyclic Aromatic Hydrocarbons. I. Weakening of PAil-Support bonds und~ photonic irradiation Sci. Tot. Environ., 128, 1-I 1, (1993) 20. PETRICK G., SCHULTZ D.E. and DUINKER J.C. Clean-up of environmental samples by I-IPLC for analysis of organoch'lorine compounds by GC with elearon capture detection J. Chromatogr., 435, 241-248, (1988) 21. BUTLER J.D. and CROSLEY P. Reactivity of Polycyclic Aromatic Hydrocarbons adsorbed on soot particles Atmos. Environ., 15, 91-96, (1981) 22. GUILLARD C., DELPRAT H., HOANG-AN C. and PICHAT P. Laboratory study of rates and products of the phototransformations of Naphthalene adsorbed on samples of Titanium dioxyde, Ferric oxide, Muscovite and Fly ash J. of Atmos. Chem., 16, 47-59, (1993) 23. PETIT V. and MASCLET P. Presence of pesticides in alpine atmosphere and in mountain lakes submitted to Atmos. Environ.
Pergamon
0045-6535(94)00372-6
Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0045-6535/95 $9.50+0.00
Atmospheric transport of pesticides adsorbed on aerosols I. Photodegradation in simulated atmosphere D. BOSSAN*, H. WORTHAM** and P. MASCLET* *Laboratoire d'Etudes des Syst~mes Atmosph(riques Multiphasiques, Universit6 de Savoie, 73326 Le Bourget, FRANCE **Laboratoire de Chimie H~t~og~e, Universit6 de Strasbourg, 68000 Strasbourg, FRANCE
(Received in Germany 4 July 1994; accepted 20 September 1994)
Abstract Eight pesticides (Alachlor, Pendimethalin, Trifluralin, Malathion, Terbuthylazine, Atrazine, Isoproturon and Fenithrothion), adsorbed on kaolin and fly ash are submitted to irradiation (wavelength above 290 nm) for 40 or 80 min. The first four compounds photodegrade when they are adsorbed on fly ash. The presence of metals and metal oxides in the fly ash seem to be responsible for this rapid degradation (up to 70% in 40 min). They do not degrade, however, when they are adsorbed on kaolin. Terbuthylazine and Atrazine do not degrade significantly, at least during this short irradiation time. Weakening of the pesticide-support bonds after irradiation, promoting the extraction by organic solvents, is important for several pesticides, particularly for Alachlor and Atrazin~ Isoproturon and Fenithrothion do not adsorb significantly on these two atmospheric supports and cannot be successfully submitted to irradiation. These results are used to predict the possibility of the meso-scale atmospheric transport of the pesticides from agricultural regions to non polluted areas, like alpine regions.
Introduction Pesticides make up a significant fraction of the atmospheric aerosols of anthropic origin. These compounds arise from industrial and agricultural uses. Pesticides are widely used throughout the world (500 000 T for USA alone in 1990). These compounds volatilize after spreading. Some of them are volatile enough to be found in the atmospheric gas phase, while others are less volatile ( p < 10-4 to 20°C) and can be adsorbed on atmospheric aerosols. The rate of volatilization is very variable from one pesticide to another since it is affected by the humidity of the soil, the saturated vapour pressure of the compound and even by the way it is spread. The fraction volatilized can reach 90% in 7 days for Trifluraline (1). Some pesticides (volatile or less volatile) can travel over very long distances (2) to remote areas and to non-polluted zones like ocean and polar atmospheres. Important amounts of Toxaphene (3), Chlordane, Hexachlorocyclohexanes
21
22
(HCI-I) and others chlorinated derivatives have been early found in Canada (4), in the Grat Lakes (5), the North Pacific (6) and in the Arctic polar regions (7). After passing into the atmosphere, then possible adsorption onto atmospheric particles, the pesticides can undergo physicochemical transformations under the effect of light, oxidizing agents and/or radicals in the atmosphere These phenomena lead to the elimination of the pesticides from the atmospheric medium. The atmosphere is therefore both a transport vector and also a medium which favours the degradation of these compounds. Nevertheless, the photochemical reactions sometimes lead to chemical compounds which are more toxic than the parent compounds, as is the case of hydrocarbons and Polycyclic Aromatic Hydrocarbons (PAH) (8). Given their structure, one can perhaps expect pesticides to behave rather like PAIl (9). However, contrary to PAH, pestcides are generally water-soluble. They are several modes of scavenging and therefore pesticide transport will most often be limited to meso-scale transport. But this does not exclude the possibility of finding pesticides far from the sources. Because of their high toxicity, certain pesticides are no longer used in Europe or in the USA, but they are still in less developed countries. They are replaced by products which are less toxic and for which the degradation processes in water are fairly well known, but the atmospheric degradation processes (mechanisms and kinetics) are poorly investigated. Studies performed in aqueous media have shown that catalytic photodegradation can be useful tool to destroy pesticides (10,11,12,13,14) WlNER et al.(15) have shown that pesticides can, at least in the gaseous form, degrade under the effects of atmospheric agents like ozone, or hydroxyl and nitrate radicals. For organophosphorus compounds the atmospheric life-time is about 1-50 h for OH, several days for NO3 and at least I00 days for 03 (11).This behaviour is likely the same than those of most organic compounds (16). However, it is known that rate constants, measured in the homogenous phase, are not in agreement with heterogenous phase results. In adsorbed phase, few results are available: MI~ALLIER et al. have shown that carbetamid adsorbed on particles can photodegrade significantly (17,18). The aim of this study is to investigate the atmospheric processes involving pesticides adsorbed on atmospheric aerosols of different types and, consequently, the possibilities either of a meso-scale or a long range transport or of elimination of pesticides in the atmosphere itself. The choice of the pesticides is difficult because 400 compounds are currently used in the world and any choice may appear rather arbitrary. We chose 8 pesticides often used in France. In order to simulate atmospheric reactions, we have used a dynamic flow reactor, already used to investigate the heterogenous reactions of PAH (19), to stir the aerosols, of predetermined size, on which the pesticides are adsorbed. These aerosols are irradiated with a sun-lamp in the presence or absence of oxidizing gases. The supports are characteristic of aerosols commonly encountered in the atmosphere (aluminosilicates as crustal aerosols, ammonium sulfate as marine aerosols and fly ash as anthropogenic aerosols).
23
Exverimental section Reactor
The reactor consists of two parts: the reactor as such and the irradiation system. The former consists of a cylindrical quartz tube (12 cm diameter and 80 cm long; i.e. 9000 cm3). The irradiated volume is 5000 cm3. The tube is closed at both ends by fritted disks of 1.2/am porosity. The reactor is continuously flushed with a stream of ultrapure nitrogen by means of a system of eleadcally operated valves. The stream is directed upwards and downwards, so as to maintain the particles in a state of constant agitation. The time of opening depends on the particle size. The irradiating system consists of a sun lamp (Hannovia 679A-0360) whose spectrum ranges from 260 nm to 700 nm. This lamp is placed in a water-cooled quartz sheath maintaining a temperature less than 30°C in order to prevent volatilization of adsorbed pesticides, under irradiation. This sheath cuts off the light below 290 nm. Two gas traps (PUF and XAD2) are set at the exit of the reactor, in order to control possible direct desorptions during the experiment. Homogenous irradiation is obtained by placing the reactor and the lamp at two foci of an ellipsoid made of highly reflecting metal foil. Pesticides
and
substrates
Eight pesticides and three supports were studied. The pesticides are volatile (V) or involatile (NV) if their vapor pressure at 20°C is less than l0 -4 Pa:. Two insecticides:
- Malathion (V) and Fenithrothion (NV)
Six herbicides:
-Trifluarine (V),
Alachlor (V),
Pendlmethaline (V), Atrazine (NV),
Tarbutylazine (NV) and Isoproturon (NV). The following substrates three used: - Substrate
Size
Specific
- kaolinite
10-20 lam
250 m2.g-1
- fly ash
20-30/am
50 m2.g-l
- ammonium sulfate
20-30 lam
unknown
Ammonium sulfate is v ~
area
hygroscopic. It sticks rapidly the wall of the reactor and the
experimental time cannot exceed 15 ran. For this reason the experience is very difficult and the results relative to pesticides adsorbed on this substrate are too doubtful to be published. Sample
preparation
Samples were prepared by adsorption of pesticides from an organic liquid phase. Solutions containing known amounts of pesticides in appropriate solvents were stirred for 12 h with the substrate previously cleaned by two Soxhlet extraction with a mixture of cyclohexane and dichloromethane (1/2). The particles were removed and dried for 2 h. The pesticides were then partially adsorbed on the
24 substrate (adsorption yield - see below). For each support and each kind of pesticide (group V and group NV) five grammes of particles, divided into 16 fractions were prepared in this way. Two fractions were used as a control in order to determine the initial pesticide concentrations [Co]. The others fractions were introduced into the reactor and submitted to irradiation for various times ( 5, 10, 15, 20 40 or 80 mn, depending of the assay), then extracted after irradiation. Reverse extractions were carried out with the same solvent mixture for the non-irradiated samples and the irradiated ones. Each experiment was performed two times. Experiments
1
Fly ash
Atrazine, Terbuthylazine
2
Kaolin
Atrazine, Terbuthylazine
3
Fly ash
Alachlor, Malathion, Trifluralin Peadimethalin
4
Kaolin
Alachlor, Malathion, Trifluralin, Pendimethalin
Analysis Before analysis each sample was spiked with a 6.65 ng.mL-1 of lindane (y-HCH) as internal standard. Pesticides were analysed by GC with electron capture detection (ECD) on a Carlo Erba 8000 fitted with a Ni 63 radioactive source. A split mode (0.5 jaL) and SPB-5 Supelco capillary column, 30 m length) were used. The make-up
gas and the
carrier gas were nitrogen. All these analysis
conditions are often used in the litterature (20). For each pesticide the concentration range was linear for concentrations from 5 to 200 ng.mL-1, which correspond to environmentaly observed concentrations.
Results
and discussion
Adsorption yields For each pesticide-substrate, the adsorption yield is calculated as follows: Mass of oesticide extracted after adsoration oer _re'amof substrate Mass of pesticide present in the solution per gram of solution The adsorption yields are presented in the Table 1 for the 6 following pesticides Pendimethalin, Atrazine, Malathion, Terbuthylazine, Alachlor and Trifluralin) Table 1 Extraction yield
Pesticide
Kaolin
Fly Ash
Pesticide
Kaolin
Fly Ash
Alachlor
nd 0,23 0,04
nd 0,27 0,04
Terbuthylazine
0,34
0,31
0,09
0,08
0,12
0,18
Isoproturon Malathion Trifluralin
0,14
0,15
Atrazine Fenithrothion Pendimethalin
* nd : not determinated
0
0
25 They range from 0.08 and 0.34 for fly ash and kaolin. Adsorption is thendore far from complete but this does not raise any problems, since all results are expressed as relative amounts of pesticide, before and after irradiation. For Isoproturon and Fenitln~hion, the adsorption yields are less than 0.04. This small value means that the preparation method is not suitable for these compounds and/or they do not adsorb on atmospheric aerosols. In this last case they cannot be transported in the atmosphere, adsorbed on aerosols. Photodegradation and bond weakening The mass ratio is unity for zero irradiation time. It can be grester than unity if the amount exlracted after irradiation is greater than the amount e~tracted before irradiation.
This
dimensionless
number does not depend on the extraction yield. In Fig. la, Ib, lc and ld, we have shown the variation of the mass ratio for 6 pesticides adsorbed on fly ash and kaolin. Because of the lack of data for Malathion on kaolin, this curve is not shown. Volatile Pesticides on Fly Ash
Volatile Pesticides on Kaolinite
I
10
A|~Phlnr
,
Malathion Pendimethalin
8
Trifl,r~lin
2
6
mr
mr 4
1
2 i
i
z
10
20
30
Non Volatile on Fly Ash
0
40 run Irradiation time
20
40
60
80
Irradiation Non Volatile on Kaolinite
Pesticides
time
Pesticides
I
I
Terbuthvlazine Atrazine
e
100 mn
o
Terbuthvlazine i trazine
2 mr
2 mr 1
1
0
i
i
!
10
20
30
0
40
Irradiation
time Figure 1
z
i
20
40
i
t
60 80 Irradiation
100 mn time
26
Volatile pesticides For the four volatile pesticides adsorbed on fly ash there are two types of behaviour. For Trifluralin and Pendimcthalin, the regular decrease in the ratio shows that these compounds photodegrade fairly, since 50% and 70% respectively are decomposed after 30 mn irradiation. These compounds, adsorbed on anthropogenic or combustion aerosols, cannot therefore be transported in the atmosphere, at least for long range transport. Of course the irradiation conditions are severe; in the atmosphere they are often less so. Nevertheless, they allow us to estimate that under favorable meteorological conditions (strong luminosity), the greater part of these pesticides will be degraded after a few days' transport. Alachlor and Malathion behave differently. For short irradiation time there is a marked increase in the mass ratio, which reaches 2 and 1.7 respectively, for these two compounds after 5 mn irradiation. Subsequently, as for the previous compounds, the mass ratio decreases fairly regularly without going below unity before 30ran. This behaviour is similar to that observed for several PAH (9). The rapid increase in the mass ratio could be due to a weakening of the bonds between the adsorbed compound and the support. Irradiation would cause rupture of the strong bond between the adsorbant and the adsorbed compound (chemisorption). Some chemical bonds are broken. Perhaps the compound is now only physisorbed. The weakening of the bond makes easier the extraction by an organic solvent. During the first step of the experiment (first 10 mn), desorption of the pesticide does no occur, since no pesticide is observed in the gas phase collected in the PIJF or the XAD2 traps Then for some pesticides and PAH adsorbed on fly ash, we observe an increase in the amount of organic compound extracted after irradiation, and a photodegradation of the chemical substances, which is fairly slow but could continue for longer irradiation times.The two phenomena are superimposed. For short irradiation, the main phenomenon is the first one; the second phenomenon becomes preponderant after 10 or 20 min irradiation. The observed curve results of superimposing these two phenomena. For volatile pesticides adsorbed on kaolin the behaviour is quite different. Trifluralin and Pendimethalin are not significantly degraded when there are adsorbed on this substrate. This result does not agree with the previous hypothesis suggesting that the degradation rate is higher for white substrate (kaolinite) than for black substrate (fly ash ) (21). The screen effect invoked for PAIl cannot explain the experimental results for pesticides. On the other hand the hypothesis of an increasing of the rate constant by the presence of catalytieal metal oxides or metals in the fly ash (22) can be retained for the four pesticides. Alachlor adsorbed on kaolin behaves specifically. It is coherent both with the observations concerning Alaclor adsorbed on fly ash and those concerning Trifluralin and Pendimethalin adsorbed on kaolin. Indeed, Alachlor is made more available to solvent extraction after irradiation, by bond weakening. This phenomenon is strongly favoured when the compound is adsorbed on kaolin. After 5rain, the mass ratio increases 5 times in this case and 2 times when it is adsorbed on fly ash. Moreover, there is no significant degradation of Alachlor adsorbed on kaolin; this behaviour is similar to Trifluralin and Pendimethalin adsorbed on kaolin.
27 It appears therefore that on this substrate, fairly typical of terrigenic aerosol, pesticides do not degrade or very little (even for severe irradiation conditions). Consequently, long range transport of pesticides in this form is possible. N o n - v o l a t i l e pesticides For Terbuthylazine and Atrazine, the two same phenomenaare found and it is logical to assume the same reasons. Nevertheless, the relative importance of bond weakening and photodegradation differ both for the range of variation of the mass ratio and for the irradiation time necessary for these two phenomena to become important. Thus, for Atrazine adsorbed on fly ash, one observes, as for Alachlor and Malathion, the weakening of the pesticide-substrate bonds, the irradiation time is longer (20 rain instead of 5 rain). Furthermore, the product appears to resist photodegradation, at least for irradiation times used. Experiments on longer irradiation times are in hand. The same result is observed for Atrazine adsorbed on kaolin. The pesticide photodegrade very slightly, but this degradation is not greater than the weakening of the bonds, since the the mass ratio remains above the unity whatevff the irradiation time. For Terbuthylazine there is a slight increase in the mass ratio for short irradiation times, whatever the substrate. This compound appears to degrade more markdely than Atrazine ( 50% and 40% on fly ash and kaolin respectively after 30 mn irradiation). Tcxtiobuthylazine and Atrazine (like Simazine which has not been studied here) can be transported during meso-scale transport. There are found in non-polluted areas like high altitude lakes and oceanic zones (23).The laboratory results appear therefore to be consistent with field observations. Conclusion This laboratory study simulating photodegradation of pesticides adsorbed on substates typical of the atmospheric aerosol allows us to forsee which compounds we can expect to find in media not directly contaminated by spreading but via atmospheric transport. The triazine family appears to resit to photodegradation and the atmospheric medium does not lead to the elimination of these compounds. For the other compounds studied the nature of the support appears to be very important. Pesticides adsorbed on kaolin (a tmigenic aerosol) can travel over long distance. In return pesticides adsorbed on fly ash (an urban aerosol) photodegrade rapidly and cannot travel. In agricultural zones the adsorption on terrigenic aerosol may be more probable. Then their transport is possible and the use of pesticides as tracers of meso scale transport is possible. The other phenomenon observed indicates that the majority of the pesticide adsorbed is often not extracted by the simple use of organic solvent. The chemisorbed compounds are strongly attached. Consequently, classical procedure (Soxhlet or Ultrasonic extraction) maybe seriously underestimates the amount of pesticides adsorbed on atmospheric aerosols (up to a factor of 5). Then this study shows that the measurement of pesticide concentrations in the atmosphere, or in other media as ice cores of water, in order to determine fluxes is may be questionnable.
28
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