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Journal of Environmental Scienceand Health, Part B: Pesticides, FoodContaminants, and Agricultural WastesPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/lesb20

Chlorpyrifos degradation in turkish soilÜlkü Yücel a , Murat Ýlim a , Kyymet Gözek a , Charles S. Helling b

& Yüksel Sarýkaya ca Turkish Atomic Energy Authority , Ankara Nuclear Research andTraining Center , Saray, 06983, Ankara, Turkeyb US Department of Agriculture, Agricultural Research Service ,Weed Science Laboratory , Beltsville, MD, 20705–2350, USAc Faculty of Science, Chemistry Department , Ankara University ,Bepevler, 06100, Ankara, TurkeyPublished online: 21 Nov 2008.

To cite this article: Ülkü Yücel , Murat Ýlim , Kyymet Gözek , Charles S. Helling & Yüksel Sarýkaya(1999) Chlorpyrifos degradation in turkish soil, Journal of Environmental Science and Health, PartB: Pesticides, Food Contaminants, and Agricultural Wastes, 34:1, 75-95

To link to this article: http://dx.doi.org/10.1080/03601239909373185

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J. ENVIRON. SCI. HEALTH, B34(l), 75-95 (1999)

CHLORPYRIFOS DEGRADATION IN TURKISH SOIL

Key Words: Soil biodegradation, chlorpyrifos, trichloropyridinol residues,supercritical fluid extraction

Ülkü Yücel1, Murat Ýlim1, Kyymet Gözek1, Charles S. Helling2

and Yüksel Sarýkaya3

1Turkish Atomic Energy Authority, Ankara Nuclear Research and Training Center,06983 Saray, Ankara-TURKEY; 2US Department of Agriculture, AgriculturalResearch Service, Weed Science Laboratory, Beltsville, MD 20705-2350, USA;3Ankara University, Faculty of Science, Chemistry Department, 06100 Bepevler,Ankara-TURKEY

ABSRACT

Degradation of chlorpyrifos was evaluated in laboratory studies. Surface

(0-15 cm) and subsurface (40-60 cm) clay loam soils from a pesticide-untreated

field were incubated in biometer flasks for 97 days at 25°C. The treatment was 2

ug g-1 [2,6-pyridinyl-14C] chlorpyrifos, with 74 kBq radioactivity per 100 g soil

flask. Evolved 14CO2 was monitored in KOH traps throughout the experiment.

Periodically, soil subsamples were also methanol-extracted [ambient shaking, then

supercritical fluid extraction (SFE)], then analyzed by thin-layer chromatography.

Total 14C and unextractable soil-bound 14C residues were determined by

75

Copyright © 1999 by Marcel Dekker, Inc. www.dekker.com

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76 YÜCEL ET AL.

combustion. From the surface and subsurface soils, 41 and 43% of the applied

radiocarbon was evolved as 14CO2 during 3 months incubation. The time required

for 50% loss of the parent insecticide in surface and subsurface soils was about 10

days. By 97 days, chlorpyrifos residues and their relative concentration (in

surface/subsurface) as % of applied 14C were: I4CC>2 (40.6/42.6), chlorpyrifos

(13.1/12.4), soil-bound residues (11.7/11.4), and 3,5,6-trichloropyridinol (TCP)

(3.8/4.8). Chlorpyrifos was largely extracted by simple shaking with methanol,

whereas TCP was mainly removed only by SFE. The short persistence of

chlorpyrifos probably relates to the high soil pH (7.9-8.1).

INTRODUCTION

Chlorpyrifos [0,0-diethyl 0-(3,5,6-trichloro-2-pyridinyl) phosphorothioate] is a

broad-spectrum organophosphorus insecticide and is widely used throughout the

world. In Turkey, it is widely applied to control insect pests of maize (Zea mays

L.), potato (Solarium tuberosum L.), tomato (Lycopersicum esculentum Mill.), and

other vegetable crops. The use of chlorpyrifos in Turkey has increased

considerably in the amounts of 93, 94, 102, 156 and 241 tonnes for the year of

1992, 1993, 1994, 1995 and 1996, respectively (Anonymous, 1997).

The environmental fate of chlorpyrifos was the subject of an extensive

review (Racke, 1993). The pathway of chlorpyrifos degradation in soil involves

both chemical and microbial processes. Major hydrolysis products of degradation

have been identified as 3,5,6-trichloro-2-pyridinol (TCP), the secondary metabolite

3,5,6-trichloro-2-methoxy-pyridine (TCMP), and eventually CO2 resulting from

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CHLORPYRIFOS DEGRADATION 77

mineralization of the heterocyclic ring common in both metabolites. Degradation

and the half-life of chlorpyrifos in soil will vary depending on the soil type. It can

be influenced greatly by environmental factors such as moisture, pH, organic

carbon content and pesticide formulation, (Racke, 1993; Racke et al., 1990 and

1996). Enhanced biodégradation of this insecticide in soil does not occur (Racke et

al., 1990).

Cl. .CI Clv p

M'o

Chlorpyrifos TCMP TCP

In a previous study, potato, tomato and maize plants were grown in

lysimeters under outdoor conditions at the Ankara Nuclear Research and Training

Center (ANAEM), applying [14C]chlorpyrifos as in practice in order to determine

chlorpyrifos residues in the crops (Yücel, Ü., Ylim, M, and Gözek, K.,

unpublished). Although the focus of that study was on plant uptake and

transformation of the insecticide, it was observed that soil also contained

radioactivity which decreased with depth. No residues of chlorpyrifos were

detected as indicated by gas Chromatographie analysis. Presumably, the residues

detected in soil occurred from direct contamination during application, since there

is little likelihood of soil contamination due to translocation and exudation of this

pesticide (Racke, 1993). Chlorpyrifos itself is immobile in soil, but the principal

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78 YÜCEL ET AL.

metabolite TCP has some potential to leach, especially in more alkaline soils

(Racke, 1993; Somasundaram et al., 1991). The purpose of the present study was

to determine the degradation rate and products of chlorpyrifos, as a function of soil

depth, using the same type of soil in which the aforementioned crops were grown.

Such information on the fate of chlorpyrifos in Turkish agricultural soils has not

previously been reported.

MATERIALS AND METHODS

Chemicals

The specific activity of original radiolabelled [2,6-pyridinyl-[uC]]

chlorpyrifos (supplied by IAEA, Vienna) was 1090 MBq g"1. Radiolabelled and

unlabelled chlorpyrifos (Greyhound, chlorpyrifos ethyl, 99% purity) were mixed to

yield a specific activity of 370 MBq g'1. A reference standard of trichloropyridinol

(99% pure) was supplied from Dow Elanco, Midland, Michigan, USA. The

solution for liquid scintillation counting was obtained from Rotizsint Eco Plus

(Roth Lab. Chem. GmbH, Karlsruhe, Germany). The cocktail used for trapping

14CO2 from combusted samples was purchased from R. J. Harvey Instrument Corp.

(Hillsdale, New Jersey, USA). All other chemicals used were analytical reagent

grade.

Soils

Surface (0-15 cm) and subsurface (40-60 cm) clay loam soils used for the

studies were collected in November 1994 (Expt. I) and November 1995 (Expt. II)

from a pesticide - untreated wheat field at the ANAEM, in Anatolia, Turkey.

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CHLORPYRIFOS DEGRADATION 79

TABLE 1

Properties of Soils used for Degradation Studies

SoilLocation

Surface

Subsurface

PH

8.1

7.9

Organiccarbon

(%)

0.8

0.8

Particle sizedistribution (%)

sand silt Clay

31 24 45

35 21 44

Water contentatFMC

(gg1)

0.343

0.318

No. ofmicroorganisms

(cfug1)

3.8 x 107

4.0 x 107

Soils were sieved (< 2 mm) to remove stones and debris. Soil properties are given

in Table 1. All soil data are expressed on a dry weight basis. Mineralogical analyses

of clay fractions in the soils were done qualitatively by X-ray diffraction (Wittig

and Allardice, 1986). Montmorillinite was predominant in both soils; illite and

kaolinite occured in lesser amounts. Field moisture capacity (FMC) was

determined by free drainage of a saturated soil column, for 24 h; this is

approximately equivalent to the soil moisture content at -33 kPa potential. Total

microbial population in soil was estimated as colony forming units (cfu) by the

dilution method; plate count agar (Oxoid) and phosphate buffered saline was used.

Except for FMC, data in Table I are for the soil used in Expt. II; however,

physicochemical properties are expected to be nearly identical for soils used in both

experiments.

Soil Treatment and Incubation

For determination of the rate of chlorpyrifos degradation, a general

approach involving incubation in biometer flasks was used (Bartha and Pramer,

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80 YÜCELETAL.

1965). Triplicate, moist soil samples (100 g, oven-dried equivalent) from both

depths were weighed into individual 250-ml glass biometer incubation flasks.

Moisture contents of surface and subsurface soils were determined. The soil in

each flask was treated with 200 ^g [74 kBq (2 nCi)] of [14C]chlorpyrifos in

hexane, yielding a soil concentration of 2 ng g'1 chlorpyrifos. After mixing the soil

thoroughly, air was blown in to remove hexane and distilled water was added to

raise the soil moisture to 75% of FMC. Untreated control soil flasks were also

included in the experiment. The sidearm of each biometer flask was filled with 10

ml of 0.1 M KOH to serve as a CO2 trap and the flasks were incubated aerobically

at 25°C, in the dark (IAEA Laboratory Training Manual, 1983).

Methanol Solvent Extraction

In Expt. II, the KOH traps were sampled for evolved 14CO2 daily in the first

week, semiweekly until Day 28, then weekly thereafter. After 1-4, 6, 8, 14, 21, 28

and 97 days of incubation, soil subsamples (5 g) were extracted three times by

shaking with methanol (10 ml). Extracts were combined and filtered through

Whatman No. 1 filter paper, then evaporated to dryness. The residues were

redissolved in acetone (4 ml). Expt. I was conducted similarly, differing only in the

exact sampling frequency.

Supercritical Fluid Extraction (SFE't

Solvent-extracted soils (1 g), from Expt. II, were subjected to SFE with

methanol (Capriel et al., 1986). Methanol, compressed by a high-pressure liquid

chromatography pump (Waters 600E) up to 15.2 MPa, passed through a pre-

heated, stainless steel capillary [0.5 mm (i.d.) x 2 m] into a 5-ml extraction vessel

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CHLORPYRIFOS DEGRADATION 81

containing the soil sample. Pre-heating of this capillary and the extraction vessel

was carried out in an oven (Elektromag 3025) maintained at 250°C and purged

continuously with nitrogen. The extracts were passed through a cooling system

[water-cooled stainless-steel tube, 3.2 mm (i.d.) x 1 m] and a regulating valve

(Whitey SS-31RS4-A), finally being collected in a 100-ml measuring cylinder. The

flow rate was adjusted to 1-1.5 ml min"1 in order to maintain the optimum pressure.

The extraction was carried out until 100 ml was collected. From this extract, an

aliquot was removed for liquid scintillation counting (LSC); the remainder was

then evaporated in a Buchi rotary evaporator at 35°C under vacuum, and the

residues were redissolved in acetone (2 ml).

Determination of Bound Residues

Total 14C, and unextractable soil-bound I4C residues (after conventional

and supercritical fluid extractions), were determined by combustion of soil samples

(0.15-0.25 g) to 14CO2 in a Harvey Biological Oxidizer OX600. Radiocarbon in

KOH traps, soil extracts, and combustion traps was counted by LSC (Packard

Tricarb 1500).

Thin-Laver Chromatography

Qualitative and quantitative determinations of the 14C residues in soil

extracts were done by using thin-layer chromatography. The 20 x 20 cm silica gel

F2M TLC plates (Merck, 250 u.m layer thickness) were developed with a

toluene+methanol+hexane (18+1+1, by volume) solvent system. Chlorpyrifos and

TCP standards, and soil extracts (50-150 [i\) in acetone, applied as spots to the

TLC plates, were initially visualized by their quenching under UV illumination. The

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82 YÜCEL ET AL.

Rf values for chlorpyrifos and TCP were 0.76 and 0.24, respectively. Bands on the

plates were then scraped in 1-cm increments and analyzed directly by LSC.

The distribution of I4C in incubated soil samples was expressed as percent

14C recovered relative to initially applied [14C]chlorpyrifos.

RESULTS AND DISCUSSION

Mineralization of 14C-ring carbon, an indicator of microbial catabolism,

indicated that in Expt. I, from surface and subsurface soils up to 46.3 ± 2% and

51.6 ± 2% of the applied radiocarbon, respectively was evolved as 14CO2 during

the 130-day incubation (Fig. 1). After a very slight lag phase in surface soil (and

none, with subsurface soil), the evolution of 14CO2 was rapid and linear to ca. 25

days. Mineralization was only slightly less complete and slower in Expt. II. It was

observed that up to 40.6 ± 1% and 42.6 ± 1% of applied radiocarbon was evolved

as 14CO2 during a 97-day incubation (Fig. 2). Again, after a very brief lag phase,

14CO2 loss was rapid and essentially linear from ca. 8-25 days. Surprisingly, the

degradation rate was significantly faster from treated subsoil samples than from the

top soil, in both experiments. Although soil properties are similar for both soils,

higher total silt + clay content of surface soil (69 % vs 65%), coupled with a very

slightly higher soil microbial population in subsoil (Table 1), accounts for the small

difference in rates between surface and subsoil. Having a higher total clay + silt

content may increase the chlorpyrifos adsorption capacity of surface soil; as a

consequence, there will be relatively less free chlorpyrifos available for microbial

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CHLORPYRIFOS DEGRADATION 83

50

40

T3<D

_ >O>w

OO

30

20

10

0 20 40 60 80 100 120 140

Days of Incubation

FIGURE 1

Mineralization of [14C]chlorpyrifos in Turkish soil during 130-day incubation

(Expt. I).

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84 YÜCEL ET AL.

50

40

•o«

"5UJ

enO

30 -

20

10

0 10 20 30 40 50 60 70 80 90 100

Days of Incubation

FIGURE 2

Mineralization of [:4C]chlorpyrifos in Turkish soil during 97-day incubation

(Expt. II).

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CHLORPYRIFOS DEGRADATION 85

degradation, in that upper-layer soil (and relatively more free chlorpyrifos, in

subsoil).

Subsequent data refer to Expt. II results. In this study, chlorpyrifos

degradation proceeded via three stages for surface and two stages for subsurface

soils, each being first-order reactions with different decay rates. Chlorpyrifos

degradation half-life in the first three days was calculated as 20 days (r2=0.999,

rate constant = -3.4xlO"2 day'1) for surface soils. In the second stage (Days 3-8),

degradation rate increased (1^=0.98, rate constant =10.4xl0'2 day'1) and the

amount of chlorpyrifos decreased to half of its initial value, with a half-life of 7

days. In the third stage (Days 14-28), degradation rate further decreased and the

remaining chlorpyrifos degraded in subsequent 60 days of incubation (Un = 97

days, 1^=0.82, rate constant = -0.7xl0'2 day'1). For subsurface soils, chlorpyrifos

degraded rapidly in the 0-14 day stage (Un = 10 days, r2=0.97, rate constant = -

7.2xlO"2 day"1), then slowly in the succeeding second stage (Un = 74 days, r2 =

0.98, rate constant=-0.9xlO"2 day"1). By comparison Tomlin (1994) reported a

range of ca. 60-120 days for chlorpyrifos' half-life, whereas others have indicated

about 30-60 days (Racke, 1993), or approximately 30-90 days (Hornby et al.,

1996). In spite of the multi-stage degradation processes observed in this study, in

general, it appears that the time required for 50% loss of chlorpyrifos is

approximately 10 days, for both soils (Figs. 3A and 3B). Degradation was then

slower and the remaining chlorpyrifos degrades in 60 days. Decrease in the

probability of hydrolytic and microbial degradation, due to time-dependent binding

of chlorpyrifos in soil, seems to explain the decrease in degradation rates.

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86 YÜCELETAL.

100

go ChlorpyrlfoTCPBoundCO2Extrsctabl«Immobil»

0 10 20 30 40 50 60 70 80 90 100

Days of Incubation

FIGURE 3

Distribution of [14C]chlorpyrifos and its degradation products, following aerobic

incubation under laboratory conditions [Expt. II]: (A) in surface soil (0-15 cm);

(B) in subsurface soil (40-60 cm). The insets show early (2-week) trends in more

detail.

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CHLORPYRIFOS DEGRADATION

100

go

87

ChlorpyrlfosTCPBoundCO2Extraetabl«Immobile

10 -

10 20 30 40 50 60 70 BO 90 100

Days of Incubation

FIGURE 3 Continued

In most studies (Racke et al., 1988, 1990 and 1996; Mahesh and

Nagabovanalli, 1997), it has been stated that chlorpyrifos degradation fit a first-

order equation. On the other hand, Saltzman et al. (1974), observed that an

organophosphorous insecticide, parathion, degrades via two stages, each being

first-order reactions with different decay rates. Getzin (1981) found that

chlorpyrifos hydrolysis on Sultan silt loam neither fit a first-order equation nor

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88 YÜCEL ET AL.

provided two distinct first-order decay curves. Instead, chlorpyrifos decay rates

were curvilinear on soil surfaces. The present study supports Saltzman et al. (1974)

two-stage degradation hypothesis. Such kinetics of pesticide loss in soils are not

unusual, especially in field dissipation observations. It indicates that a pseudo-first-

order assumption of loss is not the best description of chlorpyrifos dissipation in

the Turkish soils. In this laboratory's experiment, chlorpyrifos represented the only

added C-N source. The fact that 14CO2 was rapidly produced indicates that this

chemical was co-metabilized and/or an abiotic degradation product of chlorpyrifos

served as the energy source for soil microbial populations capable of mineralizing

this insecticide. Decrease in the rate of mineralization may have occurred when

most of the substrate(s) was converted into physically or chemically less-accessible

forms such as bound residues.

Major products of degradation detected in soil from both depths, after 97

days of incubation, included (in the order of decreasing concentration): 14CO2,

extractable-immobile residues, chlorpyrifos, soil-bound residues, and TCP. The

respective amounts were 40.6, 21.1, 13.1, 11.7, 3.8% for surface soils and 42.6,

21.4, 12.4, 11.4, 4.8% for subsurface soils, respectively (Figs. 3A and 3B). The

insets of each figure show early (2-week) trends in more detail. The percentages of

chlorpyrifos and TCP are given as the sum of the amounts recovered from solvent

extraction and SFE. Most of the chlorpyrifos (85-98%) was removed by

conventional solvent extraction. The reverse behaviour was exhibited for TCP :

most (82-99%) was recovered by SFE.

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CHLORPYRIFOS DEGRADATION 89

Thin-layer Chromatographie studies showed that for SFE extracts, 40-50%

of applied radioactivity on TLC plates remained at the origin. When these amounts

were calculated with respect to initial activity, the relative amount remaining at the

origin was in the range of 0.8-21.4% and increased with incubation time. This

radioactivity may consist of highly polar metabolites, intermediate between TCP

and CO2 formation after ring cleavage. Alternatively, it may be explained as

radioactivity bound to the soil structure. SFE did remove some of the bound

residues, but it was impossible to identify them with the TLC solvent system used

successfully for chlorpyrifos and TCP. In a somewhat analogous study, Printz et al.

(1995) investigated the bound residues of methabenthiazuron in soil fulvic and

humic acids by further fractionating solvent-extracted soils with suitable solvents.

In the present study, the portion of radioactivity extracted by SFE, but remaining

at the origin of the TLC plate, might also be residue bound to fulvic and humic

acids. If this is the case, the values given in Figs. 3A and 3B for the bound residues

should increase at least two-fold.

Overall recoveries of the radioactivity (calculated as the sum of the

percentages of evolved CO2, chlorpyrifos, TCP, bound and extractable-immobile

residues) ranged from 85-99.5%. Some activity might be lost during sampling of

KOH traps and/or removal of soil samples, but this is thought to be relatively

minor. Because the experiment was designed to study chlorpyrifos fate in

biologically active soils only, no sterilized controls were included. However, this

led to initial concern that volatilized chlorpyrifos might contribute to some of the

observed radioactivity within the KOH traps. Evaluation of the observed data as

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90 YÜCEL ET AL.

well as theoretical calculations seem to dispell this concern. First, such loss should

have been greatest at earliest sampling times, yet apparent 14CO2 accumulation in

the KOH traps exhibited a short lag phase (Fig. 1), and this actually lagged behind

the production of major metabolite TCP during the first week (Fig. 3A, 3B), as

expected. Second, when the amount of chlorpyrifos expected in the free air phase

(ca. 240 ml) was calculated, only 0.01-0.03% of applied insecticide was predicted

at very early times after treatment. Among the assumption used were : soil water

volume = 25 ml; dimensionless Henry's Law constant for chlorpyrifos (at 25°C) =

1.70 x 10"4; chlorpyrifos water solubility within the range of 0.4-1.4 ppm; and

no adsorption by soil. After incorporating soil sorption (which is expected to be

rapid), and testing a wide range of soihwater distribution coefficients (Kj = 10-60)

based on values from the literature, applied to the known organic C content of

these soils, the predicted chlorpyrifos steady-state redistribution into the air ranged

from 0.7*10* to 4x10° % of applied 14C.

This experiment was designed to ascertain chlorpyrifos loss from soil,

under controlled laboratory conditions. Because nonsterile soil only was used, one

could speculate on the mechanism of this loss. Chemical hydrolysis seems likely to

account for most of the initial conversion of chlorpyrifos to TCP, especially at high

pH and low organic content (see Table 1) of surface and subsurface soils has

increased hydrolysis and decreased adsorption of chlorpyrifos (Racke, 1993; Racke

et al, 1988, 1990 and 1996; Felsot and Dahm, 1979; Getzin, 1981). Hydrolysis of

chlorpyrifos yields ethanol and 0-(3,5,6-pyridyl)-0-ethyl phosphorothioic acid at

the pH range of 1-7.5, while alkaline hydrolysis yields TCP and phosphorothioic

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CHLORPYRIFOS DEGRADATION 91

acid (pH 8-13) (Macalady and Wolfe, 1983). pH is not the only factor affecting the

hydrolysis. In addition, soil texture (silt and clay content), type of clay, soil

moisture, organic matter content, application rate of insecticide and temperature

also influence the rate of hydrolysis (Racke et al., 1996). The capacity of soil for

catalyzing the degradation of chlorpyrifos decreases with increasing moisture and

+ H,O

S = P

C2H5O O C2H5

Chlorpyrifos

C I . C I

N +C2H5OH

o= P-OC2H5

HO

Acidic

+ OH

_ > ' C 2 H 5 O X / OC 2 H 5

S = PIOH

Basic

S = P

Chlorpyrifos TCP

organic matter content, but increases with increasing clay content. Having a high

clay content increases the probability of clay-catalyzed hydrolysis reaction for the

soil used in this study. Chlorpyrifos-degrading activities of some clays were

reported as illite = vermiculite > kaolinite > montmorillinite (Racke et al., 1996).

Although montmorillinite has the lowest activity, illite and kaolinite (also present in

our soils) may cause an increase in clay-catalyzed hydrolysis rate. But, in moist

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92 YÜCELETAL.

soils, clay-catalyzed hydrolysis rate decreases since active sites of clay surfaces are

covered by water molecules. In surface and subsurface soils, while high moisture

decreases the clay-catalyzed reaction rate, high pH increases the basic hydrolysis

rate. On the other hand, silt content of soil plays an important role in hydrolysis; as

silt content and pH increase, hydrolysis reaction rate also increases (Racke et al.,

1996).

In the surface and subsurface soils used in this study, based on (a) their

existing properties, (b) known postulated causes for chlorpyrifos dissipation in

soils, and (c) the observed chlorpyrifos degradation rates in this study, it can be

concluded that clay and silt content, high moisture and high pH of the soils are

effective in hydrolytic degradation of chlorpyrifos.

However, based on the appearance of a short lag phase as well as the fact

that 14C labeling was in a relatively recalcitrant position (the pyridinyl ring), it

could be surmised that microbial degradation was most important thereafter.

Ample soil moisture, as in this experiment, also tends to allow for optimal

microbial processes. Although the pathway of chlorpyrifos degradation in soil has

been reported as initial formation of TCP by several mechanisms, including

hydrolysis, followed by microbial transformation of this primary degradation

product to yield mineralized and soil-organic matter incorporated carbon (Racke et

al., 1996) apparently no one has successfully identified chemical intermediates

following ring cleavage of TCP and before formation of CO2 (Bollag, J.-M.

Pennsylvania State Univ., 1997, pers. comm.). However, Feng et al. (1997) did

develop an immobilized bacteria system that effectively degrades TCP in

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CHLORPYRIFOS DEGRADATION 93

chlorpyrifos-production wastewater. Within soil, complicating the effects of

transformations (and metabolite identification) is the redistribution, of chlorpyrifos,

TCP, and other degradation products. Sorption onto or within soil particles

reduces the solution concentration of substrates and, therefore, their availability for

microbial degradation (Smith et al., 1967). The fact that most TCP was

recoverable only after the rigorous SFE treatment seems to support the importance

of competing physicochemical processes, just as does the increasing proportion of

I4C being recovered in an extractable (by SFE), but immobile (on TLC) form.

Among several potential environmental consequences of pesticide residues

in soils are transport of contaminates into surface or groundwater, and uptake of

residues into crops growing on the site. Apart from other research which has

suggested both little leaching (Racke, 1993; Somasundaram et al., 1991) and

minimal root uptake (Racke, 1993; Smith et al., 1967) the relatively short half-life

and high mineralization rates of chlorpyrifos, as found in this study, further reduces

the likelihood of substantial accumulation of chlorpyrifos in potato, tomato, maize

or other plants grown in chlorpyrifos-containing soils.

ACKNOWLEDGEMENT

This research was supported in part by the International Atomic Energy Agency

(IAEA), and was presented at the "International Symposium on the Use of Nuclear

and Related Techniques for Studying Environmental Behaviour of Crop Protection

Chemicals", Vienna, Austria, 1-5 July 1996.

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94 YÜCEL ET AL.

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Received: May 28, 1998

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