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Enregistrement scientifique n° :2127Symposium n°: 38Présentation : poster

Mobilité de s-triazines dans le solMobility of s-triazine in soil

MOUNTACER Hafida (1), FERHAT Mohamed (2), MANSOUR Mohamed (3)

(1) Université Hassan I, Faculté des Sciences et Techniques de Settat, Département dechimie appliquée et d´environnement, B.P. 577 Settat - Maroc(2) Université MohammedV, Faculté des Sciences, Département de chimie, Laboratoirede Chimie Physique Générale- Av. Ibn Batouta ,BP. 1014- Rabat Maroc.(3) GSF-National Research Center for Environment and Health, Institut fürÖkologische Chemie. Ingolstadter Landstrasse 1. D-85764 Neuherberg - Germany

ABSTRACT

S-triazine herbicides are widely used for preemergence and postemergence weedcontrol. In order to understand the behaviour of s-triazines herbicides in soil, it isnecessary to know the processes which govern the mobility and the persistence ofthis compounds.Persitence of some S-Triazines molecules was studied in Chaouia soil . Their half-timeestimated from first order kinetic model ranged from 90 to 200 days . 80% and 68%of terbuthylazine is localised in the first 5 cm respectively in Moroccan soil andinGerman soil. Distribution , since more than 90% of the radioactivity applied wasrecovered in the soil. After 7 days duration of the test , 85 % of the radioactivity inthe soil was extractable .In lysimeter, terbuthylazine was essentially (90% of theapplied ) located in the upper layers of the soil (0 -10 cm ) , 244 days afterapplication . The extractable amount accounteded for

INTRODUCTION

Triazine herbicides family are widely used in agricultural production all over theworld. The atrazine ( 2-chloro-4-isopropylamino-6-ethylamino-1,3,5-triazine) is themost important compound of this family. In Mediterranean region atrazine isfrequently found in many environmental compartments, like surface water4, groundwater 5 and soil 6 ,7 . The herbicide tends to be replaced by terbuthylazine ( 2-chloro-4-tertio-butylamino-6-ethylamino-1,3,5-triazine) in many contries like Germany 8 .Many factors could influence the mobility and the persistence of terbuthylazine in soil9 . To make the contribution in the development of the use of terbuthylazine inMorocco, the aim of this study is the mobility of terbuthylazine in Moroccan andGerman soil and the persistence of some s-triazine ( including terbuthylazine) in aMoroccan soil from Chaouia region.

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MATERIALS AND METHODS

The terbuthylazine in the present work is a pestanal of Reidel de Haën at purity 99%;melting point: 177-179°C, molecular weight: 229.72 g, solubility in water: 8.5 mg/l,toxicity: DL50 2190mg/Kg, CL50 in 96h is 4.6mg/Kg , EC50 in 48h is 21.1 mg/l.The soil is sampled at a depth of 0 to 20cm, air-dried, homogenised, sieved to pass a2mm mesh and stored in the refrigerator. Its physico-chemical properties are presented intable 110.

Table 1 : Physico - chemical properties of soils.

Origin of soil pH Organicmatter(%)

Clay(%)

Silt(%) Sand

(%)Chaouia 7.7 5.2 2.7 11.3 65Germany 6.5 3.8 8.9 7.1 88.7

The chemicals used in this study and their physico-chemical properties are presentedin table 211.

Table 2 : Physico-chemical properties of some s-triazine

Compound Molecular weight

(g)

water solubility

(mg/l at 20°C)

Melting point

(°C)

Atrazine 215.7 30 175 - 177

Terbuthylazine229.7 8.5 177 - 179

Simasine 201.7 5 225 - 227

Propazine 229.7 5 212 - 214

Cyanazine 240.7 171 166 - 167

Mobility of terbuthylazine in soil

a) Laboratory study

The technique consist of filling a column with soil. The size of columns is 6.5 cm ofdiameter and 30 cm of height. We prepared 4 columns at differents heights of soil : 5,10, 15 and 20 cm.-The herbicide is deposit at the surface soil and equilibrate with a soil for 24 hours.

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-The columns are saturated with deionized water from low to heigh using peristaticpump (Abimed MP3) with 1 ml/min.

-At each column we added 250 ml of distillated water with a flow rate of 4 ml/hour .Under each column we putan erlenmeyer flasck for collecting the leachate. For eachpercolat we extracted the herbicide by 50 ml of n-hexane (3 times) and the extractscollected were analysed by gas chromatograph ( Carlo-Erba HRGC 5300 MegaSeries) equiped with a DB35ms column (30 m x 0.25 mm i.d.) and FID detector.

b) Lysimeter study

Mobility in the soil was performed in a lysimeter ; It consists in a round lysimeter witha depth of 1m and 0.4m diameter, made with stainless steel . a wircoloth and 3 metalplates wich had been punched before resulting in a " hole area " of about 50 % of thetotal lysimeter area in the bottom of the lysimeter allowed the leachate to run out ofthe soil column. The German soil from different horizon was brought to thelaboratory in plastic bags and put into the lysimeter and pressed to the same density asin the natural soil .

Persistence in soil

The soil samples were stored in the dark in erlenmeyer flashes with cotton-wool plugsto allow air exchange at temperature 23°C . Soil humidity was 40% . The initialconcentration of each herbicide was 10 µg /g of soil . Water evaporation losses werecompensated twice a week by adding destilled water. After 15 weeks, samples (20g ofsoil) were extracted with acetone in soxhlet ( 50 ml ) apparatus and the extracts wereanalysed by gaz chromatograph ( Carlo-Erba HRGC 5300 Mega Series) equiped witha DB 35ms column (30 m x 0.25 mm i.d.) and FID detector.Two replicates were madefor each soil pesticide combination .

RESULTS AND DISCUSSION

Mobility study

The dynamic of herbicides in soil is gouverned by many factors as interaction withorganic matter , chemical and biological transformation of active matter , solubility inwater of herbicide and adsorption capacity on solid13. In the laboratory study thedistribution of pesticide in the leachates is represented by figure 1. For both soils themajor quantity of herbicide is localised in the first 5 cm of soil layer. this amount ismaximum for Moroccan soil because it's more rich in organic matter like humicsubstances.

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Figure 1:Distribution of terbuthylazine in German soil (serie 1) and Moroccan soil(serie 2)

0

10

20

30

40

50

60

70

80

5 10 15 20

soil layer (cm)

% o

f ter

buth

ylaz

ine

in s

oil

Série1

Série2

The distribution of radioactivity in the first 20 cm of the soil in the lysimeter is shownby the figure 2.

Figure 2 : Distribution of the radioactivity

0

5

10

15

20

25

30

35

40

45

50

5 10 15 20soil layer (cm)

To

tal 1

4C (

%)

Nearly 90% of total radioactivity recorved in this layer was located in the first 10 cmof the soil after a test period of 244 days. The percentage radioactivity found in adepth between 10 and 15 cm was 11.6%, and only 1.3% was in the layer between 15and 20 cm. After this test period only 41.15% of the total radioactivity in the first 5cm of the soil was extractable. Of the extractable residues 11.19% could be identifiedas tebuthylazine (Rf 0.71); 87.21%were highly polar metabolites (Rf 0.00) and only1.60% were less polar metabolites (Rf 0.30).

Persistence in Chaouia soil (Morocco)

Persistence of the s-triazine tested in this study was shown to follow first order kineticprocess . Half times (t1/2 ) estimated were 150 and 190 days for atrazine and

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terbuthylazine respectively (table 3) ; Comparison with t1/2 deduced from aqueoushydrolysis, shows that chemical hydrolysis is not the main degradation process for theall the pesticides tested14,15. However for terbuthylazine this process seems to beprevalent (close half-time).

Table 3 : The half-life in the moroccan soil compared with the hydrolysis half-lives inaqueous solution

Compound Soil half-lives(days)

Hydrolysis half- lives (days)

Atrazine 150 180

Terbuthylazine 190 200

Simazine 107 160

Cyanazine 120 200

Propazine 90 180

The stability in soil of these substances increased in the following order propazine <simazine < cyanazine< atrazine < tarbuthylazine .

CONCLUSION

The mojor amount of terbuthylazine is localised in the first 5 cm of both soils.Persitence of some s-triazines molecules was studied in Chaouia soil. Their half-timeestimated from first order kinetic model ranged from 90 to 200 days . The presence oforganic matter has an effect on the mobility and the persistence of terbuthylazine insoil. It has been found that the presence of humic acids enhenced the formation ofcomplex with terbuthylazine. Therefore, terbuthylazine has to be ranked among themore persistent herbicides having potential unwanted side effects on the environment.

REFERENCES

(1) SCHEUNERT, I., M. MANSOUR, & F. ANDREUX (1992). Binding of organicpolluants to soil organic matter. Intern. J. Environ. Anal. Chem., (46), 189-199

(2) ABBOT,D.C., J.A. BUNTING & J. THOMSON (1965)

The thin-layer chromatographie determination of atrazine herbicides in soil and water.Analyst , (90), 356-361

(3) MOUNTACER,H. M., FERHAT, M.,EL AZZOUZI & M.MANSOUR (1996).Sorption and degradation of s-triazine in Morocco soils. Proceeding of COST 66 -6thInternational Worshop, Pesticides in soil and the Environment, 135-136

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(4) BURKHARD, N. & J.A. GUTH, (1981). Chemical hydrolysis of 2-chloro-4,6-bis(alkylamino)-1,3,5-triazine herbicides and their breakdown in soil under theinfluence of adsorption . Pestic. Sci., (12), 45-52

(5) DUCHAUFOUR, Ph. (1970). Précis de pédologie . Masson & Cie, Paris

(6) GEBER, H.R., P. ZIEGLER & P. DUBACH (1970). Leaching as a tool in theevaluation of herbicides. In Proceedings 10th British Weed Control Conference, 118-125

(7) SCHIAVON, M. & F. JACQUIN (1973). Etude de la présence d'atrazine dans leseaux de drainage . C.R.Columa , 35-43

(8) SCHIAVON, M. (1980). Contribution à l'étude du mouvement et de ladégradation de l'atrazine dans deux sols agricoles drainés.Thèse Doctorat Etat ,Nancy, INPL

(9) MUIR,D.C.G. & B.E.I. BAKER (1976). Detection of atrazine and theirdegradation products in tile-drain water from fields under intensive corn (maize),production . J. Agric. Food Chem., (1), 122-125

(10) MUIR,D.C.G. & B.E.I. BAKER (1978).The disappearance and movement ofthree triazine herbicides and several of their degradation products in soil under fieldconditions . Weed Res.,(18), 111-120

(11) CIPAC, Handbook, IB, (1983)

(12) KORTE, F. (1987). Lehrbuch der ökologische chemie, G. Thieme Verlag,Stuttgart, 1987, 2° ed.

(13) SCHEUNERT,I., & F. KORTE, (1985). Interactions in the fate of chemicals interrestrial systems. Ecotoxicol. Environ. Saf., (9), 385-391

(14) SCHEUNERT, I., MANSOUR, M., and ANDREUX, F., 1992. Intern. J.Environ. Anal. Chem., (46), 189-199

(15) BROWMAN, B.T. (1989) Mobility and persistence of the herbicides atrazine,metachlor and terbutylazine in plainfield sand determined using field lysimeters.Environ. Toxicol. Chem., (8), 485-491

Mots clés: s-triazine, mobilité, substances humiques, solKey words: mobility, s-triazine,humic substances, soil