토양에서의 pesticides의 동역학적인 고찰

임 광희, 이 영득．

대구대학교 화학공학과, 대구대학교 농화학과．

The speculation on kinetic behaviour of pesticides in soil

Kwang-Hee Lim, Young-Deuk Lee．

Dept. of Chemical Engineering, and Dept. of Agricultural Chemistry*       Taegu University

1. INTRODUCTION

The soil bound residue has been defined as unextractable and chemically unidentifiable pesticide residue remaining in fulvic acid, humic acid and humin fractions after exhaustive sequential extraction with nonpolar organic and polar solvents.1  As long as one is interested in the toxicity of the remaining parent pesticide, the experimental data and the kinetic modeling of the persistence of pesticides should distinguish intact pesticide from the degraded one.  Moreover when one cultivates the pesticide-sensitive plants, one needs not only to measure pesticide residues in soil but also to be able to predict the residue concentration, particularly behaviour of intact pesticide molecule.  Thus it is the motivation to carry out experiments and kinetically model the intact molecule behaviour to predict the level of toxicity of a designated pesticide, which may be above those that might be toxic to soil-environment.  This paper focuses on kinetically modeling the persistence of pesticides under laboratory condition which leads one to be able to assume that each soil sample is prepared in homogeneous condition and to ignore transport (to outside of the soil sample) terms, such as leaching and runoff, and photodecomposition.  Throughout this paper the term of extractable residue or pesticide is used as the extractable one in solvent extraction, which has opposite meanings to the bound residue.  In near future, this kinetic model shall be combined with its overall transport equation as a sink term. Then it will contribute to understanding the fate of behaviour of pesticide under field condition, cultivating the sensitive plant, and pesticide-related soil remediation.

2. MATERIALS AND EXPERIMENTAL METHODS

2.1. Chemicals

Analytical standards of pesticides were kindly supplied by corresponding basic producers.  Stock standard solutions of 500mg/L or 1000mg/L were prepared in acetone, methanol or acetonitrile. Working standard solutions for soil treatment and calibration were prepared in appropriate organic solvents according to pesticide properties. All solvents were pesticide residue (PR) grade or reagent grade freshly redistilled in glass. Florisil (60∼100mesh, PR grade) and silica gel (70∼230mesh, for column chromatography) were purchased from Aldrich Chemical (USA) and Merck (Germany), respectively and activated at 130℃ for more than 5 hrs prior to use. Other reagents were also PR grade unless otherwise specified.

2.2. Soil Samples

Agricultural fields for soil sampling were selected in Kyungbuk Province, Korea, among the major cultivating areas of paddy rice, fruits and upland crops. Two sites of different soil texture, at least, were chosen for each pesticide to investigate the effect of soil properties on the persistence of pesticides. Bulk soil samples were collected from paddy or upland fields to the depth of 10 cm during the cultivating season, air-dried, and passed through 2-mm sieve before use.

2.3. Preparation of Soil Extracts

Soil extracts are prepared appropriately according to each pesticides treated in this experiments.

3. THE HIERARCHY OF KINETIC MODEL

3.1. The hierarchy of kinetic model

Hamaker et al.2 assumed that the bound material can be adequately represented for mathematical purposes as a single compartment with average behaviour for the unavailable material.  The state of the chemical in the unavailable condition is not characterized beyond being immobile, resistant to chemical attack and slowly exchanged with the chemical in the labile condition.

Hamaker et al.'s model:

[eqn: & C_2 sup' (unavailable) rel exarrow { }{ } C_1 sup ' (labile) rarrow O]                       (1)

The unavailable pool includes all kinds of bound residue in their kinetic model.  They made use of the values of the sum of the size of labile pool and unavailable pool as the set of experimental data, to determine each rate constant of their model.  Thus they distinguished labile pool from unavailable pool, i.e., bound residue, that includes the degraded pesticide.  It may be substantiated by the experimental fact that microbially degraded pesticide is in some pesticides more tightly bound to soil than parent one.3,4  Khan et al. also indicated that the organic fraction of soil appears to have the potential for forming bound residues with pesticides or products arising from their degradation.5

Kinetically modeling the persistence of the pesticide, in the light of the toxicity of intact pesticides, one needs to distinguish the intact pesticide from the degraded to predict and extrapolate the active toxicity of the pesticide.  To investigate the behaviour of intact pesticide molecules, the following hierarchy of kinetic model is proposed, adopting and manipulating the intact pesticide (C1) that is reversibly bound or solubilized in soil solution ( For the pesticide of very large n-octanol/water distribution coefficient, the water-solubilized fraction of intact pesticide may be ignored) and the irreversibly bound intact pesticide (C2) that is assumed to be resistant to chemical and microbial attack, as variables.

It is believed that there is considerable suppression of strong mineral adsorption of organics including organic pesticides, due to strong dipole interaction between soil minerals and water.6,7  Even though mineral are not completely shielded by soil organic matter8, inorganic soil constituents' role is apparently masked by organic matter at levels above 2%.9  Moreover adsorption by the soil mineral fraction is relatively unimportant in wet soils.10,11,12  In these persistence experiments, soils are water-saturated or wet to the degree of at least 60% of field moisture capacity.  Thus one may assume that microbial degradation dominates soil surface (i.e., clay and minerals) catalytic degradation where other degradation processes such as photodegradation are blocked.  Further, for the simplicity of modeling microbial degradation, Michaelis-Menten kinetics is adopted where specific enzyme concentration is expressed by means of specific microbes concentration in soil.

Hui et. al.13 found that using 14C-labeled pesticides, most of the applied radioactivity is bound irreversibly to the humin fractions of soil organic matter.  Spillner et al.14 observed that bound radiocarbon [(ring-14C) fenitrothion] was associated mainly with HA and FA fractions.  The bulk of the immobilized aromatic propanil moiety is chemically bound to HA to form a humus-3,4-dichloroaniline complex in the formation of bound residues with the herbicide propanil.15  The bound residues are protected from chemical degradation and are attached by a chemical bond with soil organic matter.2  Moreover the forementioned experimental facts and arguments in last paragraph may also support the assumption that irreversible binding site is mostly located on the interior or exterior binding surface of soil organic matter for such wet soil as in these persistence experiments.

The hierarchy of the proposed model:

C2 [eqn: rel exarrow k _1' k_2']  C1 + Edv

Ed = Edv + C2 + [O·Ed]

C1 + E [eqn: rel exarrow { } { }] [C1·E] [eqn: ->] O (microbial degradation)                         (2)

where	C1 :"intact pesticide" that is reversibly bound or solubilized in soil                     solution

C2 : irreversibly bound or trapped "intact pesticide".

Ed : total irreversibly binding (or trapped) site concentration in OM.

EdV : vacant irreversibly binding (or trapped) site concentration in OM.

[O·Ed] : irreversibly bound "degraded pesticide"

E : specific microbes concentration

The diffusion of reversibly bound C1 to EdV and irreversible adsorption of intact pesticide from soil solution are lumped in the rate constant of k2'.  Kloskowski et. al.16 observed that the biological availability of the non-extractable residues was reduced by a factor of 4-8 (anilazine, methabenzthiazuron) up to 675 (smizine) in comparison to freshly applied active substance.  F□hr17 also observed that the biological availability of the bound residues decreased by a factor between 5 and 500.  Since microbially degraded pesticide is in some pesticides, however, more tightly bound to soil than intact pesticide3, biological availability of degraded bound residue may be much less than that of parent(intact) bound residue.  Thus the biological availability of parent(intact) bound residue is, one might say, reduced or decreased by the less factor than the above factors.  The biological availability of C2 may be  considered in the rate constant of k1' and the rate of microbial degradation of C1 as serial processes.  .

For low concentration of pulse input of C1, the effect of the model-induced nonlinearity diminishes and the model becomes linear in intact pesticide concentration.

[eqn: & C_2 rel exarrow k_1 k_2 C_1 buildrel rarrow k sub 3 O]                                             (3)

Thus model (2) may be reduced to linear model (3) which has same form as model (1) does.  However the variables of model (3) are different from those of model (1).  According to model (3), C1 decays in two time scales with a pulse input of C1.  Based upon model (3), one can estimate the rate constants, utilizing the experimental time dependent data of the active concentration of each extractable intact pesticide, as determined by the methods of least square of nonlinear regression using Kirkpatrick's simulated annealing procedure or as approximated in such limiting cases as k3 << k1, and k3<<k2 or k1 << k2 and k1 << k3.

4. DISCUSSION AND CONCLUSION

Based on the appropriate assumptions, the hierarchy of kinetic model is established and  gives the simplified-linear kinetic model where the rate constant of k2 is proportional to the fraction of soil organic matter.  In fact, for Cyhexatin and Tricyclazole, factor of two difference of soil organic matter between soil A and B (for Cyhexatin), and soil H and C (for Tricyclazole), gives about factor of two difference of k2 respectively, which is consistent with a priori assumption of this kinetic model.

In most pesticides of each experiment, not a small amount of intact bound residue is accumulated in soil and behaves as a storage of intact pesticide for its slow release.

5. ACKNOWLEDGEMENT

This research was supported (in part) by the Taegu University Research Grant, 1997.

6. REFERENCE

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