International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 1

ISSN 2229-5518

Adsorption - Desorption of Metolachlorand 2, 4-D

on Agricultural Soils

Rounak M. Shariff

(Dept. of Chemistry, College of Scince, University of Salahaddin - Erbil-Kurdistan Region- Iraq)

AbstractAdsorption - desorption behavior of metolachlor [ 2-chloro-N- (2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide)] which is nonionic herbicide, and 2,4-D (2,4- dichlorophenoxyacetic acid) as anionic herbicide have been studied, by performing batch equilibrium experiments on six agricultural soil samples which has different texture. The adsorption processes of metolachlor and 2, 4-D on the soil solid matrix exhibited moderate rate of accumulation with 20.30% and, 24.71 % respectively after 0.5h. Data revealed that the adsorption- desorption of metolachlor and 2, 4-D on the selected soil samples followed the first order rate law. Linear and Freundlich models were used to describe the adsorption- desorption of the two pesticides. Variation in adsorption affinities of

the soils to the pesticides was observed, distribution coefficient Kd values for adsorption process varied between 1.882 - 3.025 mlg-1 and

2.123- 3.989 mlg-1 for metolachlor and 2,4-D respectively , and for desorption process varied between 4.222- 10.986 and 4.755- 14.54 mlg-

1 for metolachlor and 2,4-D respectively. Freundlich coefficient KF for metolachlor and 2,4-D ranged between 0.105-0.312 and 0.119-0.355 mlg-1 for adsorption processes. The value Freundlich coefficient for desorption process KFdes ranged from 0.479 to 1.130 mlg-1 and 0.284 to

1.012 mlg-1 for metolachlor and 2,4-D respectively. Values of equilibrium constant Ko conducted by the ratio between the constant rate of adsorption to the constant rate of desorption, equilibrium constant for metolachlor and 2,4-D on selected soil samples were in the following

from 1.345 to 1.572 and from 1.157 to 1.706 respectively. All desorption isotherms exhibited hysteresis. Higher desorption hystersis (for metolachlor and 2,4-D was less readily desorbed).

Index TermsAdsorption desorption kinetics and isotherms, HPLC, Metolachlor, 2, 4-D.

—————————— ——————————

1 INTRODUCTION

ESTICIDES are the most extensively used agrochemicals in the agricultural fields throughout the world. It has been estimated that pesticides application however, most of the pesticides enter the environment, and contaminate soils, water and air, throughout the world, the accumulation of pesticides in food and drinking water has been generally recognized as dangerous[1]. The adsorption desorption behavior of pesti- cides in soil are primarily regulated by soil texture. As soil have a great role in the environment by controlling the organic compounds because of their sorption capability, it is also an important factor governing the migratory behavior of the pes- ticide in soil and ground water and may also influence the uptake and metabolism by plants or microorganisms and the other organisms present in soil [2], [3]. The Equilibrium for adsorption-desorption processes was attained for most of the systems in 4-24h. Desorption was slower than sorption, it has generally been characterized by an initial rapid rate followed by a much slower approach to an apparent equilibrium. The initial reaction has been associated with diffusion of the pesti- cides to and from the surface of the sorbent[4]. The purpose of the desorption kinetics study is to investigate whether a chem- ical is reversibly or irreversibly adsorbed on a soil, which plays an important role in the behavior of a chemical in field soil [5], [6]. Hysteresis which is commonly observed in pesti- cide adsorption-desorption studies with soils. Once adsorbed, some adsorbats may react further to become covalently and irreversibly bound, while others may become physically

————————————————

Rounak M.Shariff, Departmen of Chemistry, College of Science, University of Salahaddin- Erbil-Kurdistan Region-Iraq. E-mail: rou- nakm2000@yahoo.com

trapped in the soil matrix. Moreover, hysteresis may increase with adsorbent- adsorbate contact time [7], [8]. The selected herbicides were used in corn crops and cotton as preemer- gence and post-emergence weed control. Metolachlor belongs to chloroacetanilide herbicides its persistence in the soil, its soluble in water, and poorly bound to most soils so it leaching down towards the ground water [9], [10]. The acidic model: 2,
4-D is anionic herbicide (auxin mimic family) which used to kill or suppresses unwanted plants by its chemical structure through the herbicide’s mode of action mechanism (biochemi- cal or physical) [11], [12]. Its adsorption involved ionic inte- raction with positive charges in soil and also the less energetic Van der Waals forces and charge transfer[13], [14].

2 METHODLOGY

2.1 Soils

Fresh soil samples were taken from six soil samples were col- lected from six main agricultural, representing a range of phy- sico-chemical properties. Subsamples of homogenized soils were analyzed for moisture content, organic matter content, particle size distribution, texture, pH, loss on ignition and ex- changeable basic cations the detail were characterized in pre- vious article[15].

2.2 Pesticide

Analytical grad substituted Metolachlor (purity 97.8%), and 2,
4-D (purity 98%) were both purchased from Riedal-de Haen, Sigma-Aldrich company ltd. All chemicals used were of ana-
lytical grade reagents and used without pre-treatments. Stan- dard stock solutions of the pesticides were prepared in deio- nised water.

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2.3 Adsorption Experiments

Adsorption of two pesticides from aqueous solution was de- termined at temperature (25±1 C˚) employing a standard batch equilibrium method [16], [17]. Duplicate air-dried soil samples were equilibrated with different pesticide concentrations (3, 5,
10, and 15 µg ml-1 ) were for both pesticides at the soil solution ratios 4:8, and 5:10 for metolachlor and 2,4-D respectively in 16 ml glass tube fitted with Teflon-lined screw caps. The samples plus blanks (no pesticide) and control (no soil) were thermos- tated and placed in shaker for 0.5, 1, 3, 6, 9, 12 and 24h for me- tolachlor and 0.5, 1, 1.5,2, 2.5, 3, 3.5, 4, 6 and 24h for 2,4-D. The tubes were centrifuged for 20 min. at 3000 rpm. One ml of the clear supernatant was removed and analyzed for the pesticide concentration [18], [19]. Pesticide identification was done by PerkinElmer series 200 USA family high performance liquid
chromatography (HPLC) equipped with a changed loop
of desorption in all experiments and on all soil samples [22]. Where k is kdes is the desorption rate constant (h-1 ), Ct is the amount of released pesticides at time t and C is Ce is the amount of released pesticides at equilibrium and kdes is the slope of straight line which is equal to coefficient release rate of kdes. A plot of log (Ce – Ct) versus t should give a straight line with slope –kdes/2.303 and intercept of log Ce of both pes- ticides demonstrated in (Table 1&2).

3.1.2 Equilibrium Constant

Considering the experimental equilibrium constant ko can be conducted by the ratio of adsorption rate constant ka to the desorption rate constant kdes, Ko were calculated using the fol- lowing expression[23].
(20µl), C18 reversed phase column, flow rate 1.0 ml min-1, and a variable wave length UV detector at wavelength 220 nm 200 nm for metolachlor and 2,4-D respectively. Separation of me-

Ko

ka

kd es

(2)
tolachlor in aqueous phase was achieved with a mobile phase
of ratio 80:20 acetonitrile to water, while for 2, 4-D 60:40 (acidi-
fied with 1gL-1 phosphoric acid). Under these conditions the
retention time of metolachlor, and 2, 4-D was 5.37, 4.69 min
respectively. Each sample was injected twice to determine the
pesticide content by integrating the obtained peak with the respective standard pesticides. The pesticide content was av-
erage of two measurements, with no more than 5% deviation between the measurements.

2.4 Dedsorption Experiments

Desorption processes were done as each test tube was placed in a thermostated shaker at (25±1 C˚) after equilibration for 24 h with different pesticide concentrations (3, 5, 10 and 15
µg ml-1) the samples were centrifuged, 5ml of supernatant was removed from the adsorption equilibrium solution and imme-
It indicates that adsorption in the systems studied may be
viewed as a reaction in which a solute molecule collides with
an adsorption site to form the adsorption complex. Desorption
may be viewed as "unimolecular" process by which the ad-
sorption complex dissociates to a free site and solute molecule.
The large difference in the equilibrium adsorption arise main- ly from the difference in the rate of desorption.

3.2 Adsorption-Desorption Isotherms

3.2.1 Distribution Coefficient

The distribution coefficient (Kd) was calculated by the using the following expression[20], [22].
(3)

Cs

diately replaced by 5ml of water and this repeated for four

K d Ce

times [20]. The resuspended samples were shaken for men- tioned time previously for the kinetic study for each pesticide.

3 DATA ANALYSIS

3.1 Adsorption-Desorption Kinetics

The rate constants for adsorption of each pesticide on soils were calculated us-
The distribution coefficient (Kd) was calculated by taking the ratio of adsorption concentration in soil (Cs) and equilibrium concentration in solution (Ce), and averaged across all equili- brium concentration to obtain a single estimate of Kd of both pesticides demonstrated in (Table 3&4).

logC Ct  log C

(1)

k t

2.303

ing the first order rate expression[21].

3.2.2 Freundlich Coefficient

Adsorption isotherm parameters were calculated using the linearized form of Freundlich equation[22], [23].

1

Where k : ka is the rate constant for adsorption (h-1), t the time

log Cs  log K F  log Ce

n

(4)
(h), C is Co the concentration of pesticide added (µg ml-1) and Ct the amount adsorbed (µg ml-1) at time t. In all cases, first order equation provided satisfactory fit for the data as linear plots of log (Co – Ct) against t of both pesticides demonstrated in (Table 1&2). The same equation used to describe the process
Cs and Ce were defined previously, KF is Freundlich adsorp- tion coefficients, and n is a linearity factor, it is also known as adsorption intensity, 1/n is the slope and logKF is the intercept of the straight line resulting from the plot of logCs versus logCe as shown in fig 1a&b. The values of KF and 1/n calcu-

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lated from this regression equation showed that Freundlich adsorption model effectively describes isotherms for both pes- ticides in all cases. Desorption isotherms of both pesticides were fitted to the linearzed form of the Freundlich equation [24].

1

3.3 Hysteresis Coefficient

A study of both pesticides desorption isotherms show positive

log Cs  log K Fdes

ndes

log Ce

(5)
hysteresis coefficients H1 on the six selected soil samples. Hys- teresis coefficients (H1) can be determined by using the follow- ing equation [25].

na

Where Cs is the amount of pesticides still adsorbed (μg g-1), Ce is the equilibrium concentration of pesticides in solution after desorption (μg mL-1), and KFdes (μg g1-nfdes /mlnfdes g-1) and nfdes are two characteristic constants of both pesticides desorption

H 1

nd es

(6)
[25]. The value of the KFdes and nfdes constants of both pesti- cides demonstrated in (Table3&4).
Where na and ndes are Ferundlich adsorption and desorption constants, respectively, indicating the greater or lesser irrever- sibility of adsorption in all samples, the highest values corres- ponding for which the highest adsorption constant was ob- tained. The coefficient H1 is a simple one and easy to use, Data in table 5 demonstrated H1 values for metolachlor, and 2, 4-D respectively.
The extent of hysteresis was quantified by using hysteresis coefficient (ω), it was defined on the discrepancy between the sorption and desorption isotherms, and calculated by using Freundlich parameters estimated from sorption and desorp- tion isotherms separately, (ω) expressed as[26].

 (

na 1) x100

ndes

(7)

Fig. 1.a Fitted adsorption isotherm Ferundlich model for metolachlor on the selected soil samples (♦ S1, ■ S2, ▲ S3, x S4, * S5, ●S6).

Recently Zhu et. al [27] proposed an alternative hysteresis coefficient )λ( based on the difference in the areas between adsorption and desorption isotherms, they derived the follow- ing expression for the parameter λ for the traditional iso- therms

 (

na 1

ndes  1

1) x100

(8)

3.4 Organic Matter Normaliz Adsorption Coefficient

The linear or distribution coefficient (Kd) is related to soil or- ganic carbon (OC) and soil organic matter (OM) by the follow- ing equations [28].

Fig. 1. b Fitted adsorption isotherm Ferundlich model for

2,4-D on the selected soil samples (♦ S1, ■ S2, ▲ S3, x S4, * S5, ●S6).

%OC

%OM

1.724

(9)

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K  100K d

OM %OM

International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 4

ISSN 2229-5518

K OC

100K

d

(10)
In present study, the value of n were less than unity for S3 in the adsorption of metolachlor indicating the non-linear rela- tionship between concentration of metolachlor and the soil, while a linear relationship between the other soils and meto- lachlor. The adsorption processes for 2, 4-D on soil samples, the values of n>1 indicating a linear relationship between as

%OC

(11)
shown in fig 1 a & b. The variable slopes of the adsorption isotherm obtained for different soil systems studied reveal that the both pesticides adsorption on soil complex phenomena

4 RESULT AND DISCUTION

4.1 Adsorption-Desorption Rate

Data in Table 1&2 showed that adsorption –desorption of the both pesticides in all cases followed first order rate law [29], [30], (values of rate constants for adsorption and desorption of metolachlor on selected soil samples were in the range from
1.001 to 1.886 h-1 and 0.132 to 2.153h-1 respectively, and the
value of R2 for adsorption-desorption of metolachlor on se-
lected soil samples ranged from 0.762 to 0.976 and from 0.735
to 0.995 respectively. The value of standard error (S.E.) for
adsorption-desorption of metolachlor on selected soil samples
ranged from 0.100 to 0.204 and from 0.084 to 0.244 respective- ly. Values of rate constants for adsorption-desorption of 2, 4-D
on selected soil samples were in the range from 1.073 to 2.683 h-1 and 0.731 to 1.277h-1 respectively, and the value of R2 for adsorption-desorption of 2,4-D on selected soil samples ranged from 0.729 to 0.988 and from 0.739 to 0.987 respective- ly. The value of standard error (S.E.) for adsorption-desorption of 2,4-D on selected soil samples ranged from 0.195 to 0.269 and from 0.192 to 0.202 respectively. The values of rate con- stant for adsorption ka for the both pesticides are greater than desorption rate constant kdes. Thus desorption during the first few hours is likelyto come from the or more accessible sites and /or from the low-energy –release sorption mechanisms, whereas metolachlor sorbed on less accessible sites and/or more strongly adsorbed sites is not susceptible to desorption initially and is subsequently subject to slow release over time [31], [32]. Data in table 1&2 demonstrated the values of equili- brium constant Ko equilibrium constant for metolachlor, and 2,
4-D on selected soil samples were in the following from1.345 to 1.572 and from 1.157 to 1.706 respectively.

4.2 Adsorption-Desorption Isotherms

Preliminary studies have shown that more than 90% of the
2,4-D sorption occurs within 2h. Orgam et al .1985 [33] sug-
gested that microbial degradation occurred only in solution
phase and not when the pesticide was sorbed. Kd values for adsorption process varied between 1.882 - 3.025 mlg-1 and
2.123- 3.989 mlg-1 for metolachlor and 2,4-D respectively, and for desorption process varied between 4.222- 10.986 and 4.755-
14.54 mlg-1 for metolachlor and 2,4-D respectively. Freundlich coefficient KF for metolachlor and 2,4-D ranged between 0.105-
0.312 and 0.119-0.355 mlg-1 for adsorption processes. The value Freundlich coefficient for desorption process KFdes ranged from 0.479 to 1.130 mlg-1 and 0.284 to 1.012 mlg-1 for metolach- lor and 2,4-D respectively. Ferundlich desorption isotherms were determined on the soils used in the adsorption isotherms experiment, the KFdes values indicate that a small proportion of the chemical has desorbed into solution [34], [35].
involving different types of adsorption sites with different surface energies .The value of nfdes describes nonlinearity cur- vature in the desorption isotherm and is often used as an in- dex of hysteresis. Results obtained in the present work showed that the values of ndes values were smaller than the values for the other works [36], [37]. Data in table 5 demon- strated H1 values for metolachlor and 2,4-D from the selected soil samples in the range from 1.450-2.352 and 0.568-2.595 re- spectively, indicating an increase in the irreversibility of the adsorption of herbicide as the clay content increases, and indi- cate the increased difficulty of the sorbed analyte to desorb from the matrix. The calculated values of hysteresis coefficient (ω) for adsorption-desorption of for metolachlor and 2, 4-D on the selected soil samples were summarized in table 5 ranged from 90 to 135 and from -43 to 159 respectively. Whereas hys- teresis coefficient (ω ) is only applicable for the traditional type isotherms of the successive desorption [38], [39]. The data in table 5 demonstrated hysteresis coefficient (λ) according to equation 8 for adsorption-desorption of for metolachlor and
2,4-D from the selected soil samples were ranged from 132 to
678 and from 72 to 750 respectively.

5 CONCLUSION

The batch kinetics experiments were used to differentiate the behavior of two pesticides on six agricultural soil samples. The experimental data were evaluated by employing first-order rate law, the regression equations relating that the highest values are in first-order which is the most suitable to be used. Accessibility of soil organic OC and clay content and the chemical nature of the constituents determined the adsorption affinity of the soil. All desorption isotherms exhibited hystere- sis. Higher desorption hystersis (both was less readily de- sorbed), the increasingly difficult desorption with decreasing solute concentration which can be explained by the limited number of the available sites for the high-energy. Most of these sites were occupied at low solute concentrations, whe- reas at high solute concentrations, more molecules are taken up by low-energy binding sites and therefore they can more readily desorbed. This could also be explained by the possible hysteresis effect taking place during desorption involving var- ious forces that caused the amount of compounds retained to be higher after desorption than after adsorption at the unit of equilibrium concentration.

ACKNOWLEDGMENT

The author wish to thank The authors wish to thank all the chemistry staff in Salahaddin University. I express my grati-

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tude to Assit proff Dr. Kasim and Dr. Kaffia.

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TABLE 1

ADSORPTION -DESORPTION RATE CONSTANTS AND EQUILIBRIUM CONSTANTS FOR METOLACH- LOR ON THE SELECTED SOIL SAMPLES

Soil

Conc

.ppm

Adsorption

Desorption

Ko

Soil

Conc

.ppm

Kads S.E R2

calc

(h-1)

Kdes S.E R2

calc

(h-1)

Ko

S1

S2

S3

S4

S5

S6

3

5

10

15

3

5

10

15

3

5

10

15

3

5

10

15

3

5

10

15

3

5

10

15

1.285 0.185 0.828

1.515 0.100 0.942

1.582 0.104 0.904

1.001 0.158 0.808

1.886 0.175 0.801

1.414 0.193 0.819

1.382 0.168 0.955

1.327 0.165 0.849

1.765 0.171 0.834

1.609 0.124 0.927

1.639 0.113 0.843

1.115 0.154 0.884

1.422 0.199 0.852

1.386 0.202 0.848

1.369 0.204 0.873

1.261 0.197 0.983

1.749 0.164 0.973

1.161 0.971 0.944

1.281 0.165 0.762

1.326 0.153 0.976

1.669 0.197 0.954

1.744 0.195 0.962

1.695 0.163 0.933

1.698 0.159 0.955

1.187 0.119 0.879

0.913 0.170 0.776

1.120 0.128 0.809

0.796 0.199 0.918

1.127 0.244 0.899

1.236 0.176 0.874

1.117 0.186 0.922

0.989 0.121 0.802

1.150 0.158 0.914

1.111 0.188 0.930

1.161 0.192 0.963

1.070 0.193 0.969

0.981 0.089 0.793

0.987 0.093 0.892

0.984 0.094 0.936

0.983 0.084 0.995

1.025 0.190 0.979

0.847 0.185 0.735

0.985 0.188 0.946

0.802 0.186 0.967

1.627 0.194 0.974

0.132 0.122 0.823

2.153 0.189 0.902

0.406 0.168 0.863

1.360

1.345

1.364

1.370

1.449

1.572

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ISSN 2229-5518

TABLE 2

ADSORPTION -DESORPTION RATE CONSTANTS AND EQUILIBRIUM CONSTANTS FOR 2,4-D ON THE

SELECTED SOIL SAMPLES

Soil

Conc

.ppm

Adsorption

Desorption

Ko

Soil

Conc

.ppm

Kads S.E R2

calc

(h-1)

Kdes S.E R2

calc

(h-1)

Ko

S1

S2

S3

S4

S5

S6

3

5

10

15

3

5

10

15

3

5

10

15

3

5

10

15

3

5

10

15

3

5

10

15

1.285 0.185 0.828

1.515 0.100 0.942

1.582 0.104 0.904

1.001 0.158 0.808

1.886 0.175 0.801

1.414 0.193 0.819

1.382 0.168 0.955

1.327 0.165 0.849

1.765 0.171 0.834

1.609 0.124 0.927

1.639 0.113 0.843

1.115 0.154 0.884

1.422 0.199 0.852

1.386 0.202 0.848

1.369 0.204 0.873

1.261 0.197 0.983

1.749 0.164 0.973

1.161 0.971 0.944

1.281 0.165 0.762

1.326 0.153 0.976

1.669 0.197 0.954

1.744 0.195 0.962

1.695 0.163 0.933

1.698 0.159 0.955

1.187 0.119 0.879

0.913 0.170 0.776

1.120 0.128 0.809

0.796 0.199 0.918

1.127 0.244 0.899

1.236 0.176 0.874

1.117 0.186 0.922

0.989 0.121 0.802

1.150 0.158 0.914

1.111 0.188 0.930

1.161 0.192 0.963

1.070 0.193 0.969

0.981 0.089 0.793

0.987 0.093 0.892

0.984 0.094 0.936

0.983 0.084 0.995

1.025 0.190 0.979

0.847 0.185 0.735

0.985 0.188 0.946

0.802 0.186 0.967

1.627 0.194 0.974

0.132 0.122 0.823

2.153 0.189 0.902

0.406 0.168 0.863

1.360

1.345

1.364

1.370

1.449

1.572

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International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 8

ISSN 2229-5518

TABLE3

ADSORPTION -DESORPTION ISOTHERM PARAMETERSTHE LINEAR AND FREUNDLICH

MODELS FOR METOLACHLOR ON THE SELECTED SOIL SAMPLES

Adsorption Desorption Models

Parameter

Soils

Adsorption Desorption Models

Parameter

S1 S1 S1 S1 S1 S1

Adsorption Freundlich Dedsorp- Freundlich

Distr.Coffi Adsorption tion Desorption

Distr.Coffi

Kd (calc)

S.E R2

KOC(ml/g)

KOM(ml/g)

KF(ml/g)

S.E nF R2

Kd (calc)

S.E

R2

KFdes(ml/g)

S.E

nFdes

R2

2.429 2.628 3.025 2.206 1.882 2.216

0.259 0.181 0.149 0.279 0.263 0.277

0.785 0.985 0.965 0.879 0.943 0.847

87 253 95 94 98 147

1.496 4.361 1.632 1.614 1.695 2.532

0.105 0.202 0.214 0.191 0.312 0.169

0.286 0.282 0.310 0.322 0.283 0.281

1.336 1.668 0.943 1.483 1.684 1.430

0.826 0.936 0.921 0.889 0.833 0.923

8.915 6.773 4.222 5.889 9.203 10.986

0.099 0.079 0.072 0.068 0.100 0.039

0.757 0.765 0.986 0.846 0.968 0.865

0.817 0.479 1.066 1.130 0.724 0.742

0.290 0.234 0.358 0.243 0.133 0.121

0.767 1.051 0.401 0.692 0.885 0.986

0.908 0.857 0.992 0.886 0.998 0.937

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International Journal of Scientific & Engineering Research, Volume 2, Issue 11, November-2011 9

ISSN 2229-5518

TABLE4

ADSORPTION -DESORPTION ISOTHERM PARAMETERSTHE LINEAR AND FREUNDLICH

MODELS FOR 2,4-D ON THE SELECTED SOIL SAMPLES

Adsorption Desorption Models

Parameter

Soils

Adsorption Desorption Models

Parameter

S1 S1 S1 S1 S1 S1

Adsorption Freundlich Dedsorp- Freundlich

Distr.Coffi Adsorption tion Desorption

Distr.Coffi

Kd (calc)

S.E R2

KOC(ml/g)

KOM(ml/g)

KF(ml/g)

S.E

nF

R2

Kd (calc)

S.E R2

KFdes(ml/g)

S.E

nFdes

R2

2.123 2.949 3.857 3.112 2.124 3.989

0.279 0.283 0.179 0.207 0.206 0.209

0.898 0.797 0.977 0.846 0.968 0.752

105 284 121 173 758 298

1.816 4.893 2.081 2.974 13.52 5.144

0.178 0.119 0.185 0.175 0.355 0.154

0.256 0.280 0.325 0.350 0.254 0.306

1.409 1.608 1.128 1.021 1.111 1.516

0.944 0.823 0.803 0.979 0.964 0.754

4.755 14.54 10.21 8.258 4.762 7.039

0.113 0.160 0.272 0.093 0.146 0.142

0.915 0.765 0.977 0.955 0.853 0.953

0.720 1.012 0.419 0.303 0.613 0.284

0.530 0.578 0.212 0.349 0.380 0.329

0.543 0.808 1.147 1.799 0.750 1.022

0.800 0.978 0.773 0.981 0.959 0.850

TABLE 5

HYSTERSIS COEFFICENT FOR ADSORPTION- DESORPTION FOR METOLACH-

LOR AND 2,4-D ON THE SELECTED D SOIL SAMPLES

Soi

Metolachlor

2,4-D

Soi

H1 ω λ

H1 ω λ

S1

S2

S3

S4

S5

S6

1.742 74 678

1.587 59 173

2.352 135 398

2.143 114 492

1.903 90 132

1.450 45 339

2.595 159 304

1.990 99 750

0.983 -1.656 126

0.568 -43.25 73

1.481 48.13 72

0.749 -25.02 16

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