International Journal of Scientific & Engineering Research Volume 2, Issue 6, June-2011 1

ISSN 2229-5518

Thermodynamic Adsorption of Herbicides on

Eight Agricultural Soils

Rounak M. Shariff, Kafia M. Shareef

Abstract— Thermodynamics of adsorption of three pesticides using a kinetic approach was investigated. The objective of this study is to study the effect of temperature on the sorption behavior of three commonly used pesticides in Kurdistan onto eight natural soil samples collected from different agricultural locations. To elucidate the effect of temperature on the sorption process, the experiments were done at 15, 25, 35 ±1ºC. Values of the standard thermodynamic functions: equilibrium constant Ko, free energy change /1Gº, enthalpy change /1Hº and entropy change /1Sº revealed that adsorption of atrazine, picloram and propanil was spontaneous exothermic and physical in nature to some extent. Data obtained revealed that adsorption coefficient decreases with the incresing temperature. Values of lnKo were in the range 1.238 to 12.75, 1.674 to 14.032 and 29.83 to 173.1

for atrazine, picloram and propanil respectively. The t.Go values were in the range -30.549 to -3.172 KJmol-1, -33.614 to -

4.2897 KJmol-1, and - 414.72 to - 76.412 KJmol-1 for atrazine, picloram, and propanil respectively. Values of t.Ho followed the range -5.010 to -0.738 to KJmol-1, -4.769 to -0.848 KJmol-1 and -75.779 to - 1.628 KJmol-1 for atrazine, picloram, and propanil respectively. t.So followed the range-16.138 to -1.757 Jmol-1 k-1, -14.711 to -2.502 Jmol-1 k-1 and -199.01 to -62.457 Jmol-1k-1for atrazine, picloram, and propanil respectively.

Index Terms — Adsorption isotherms, Adsorption thermodynamic, atrazine, picloram, propanil, HPLC.

—————————— • ——————————

1 INTRODUCTION

HERE is considerable public concem about potential adverse impact of pesticide used on ecosystem and human health. To minimize any such detriments, sound understanding of environmental fate and behavior of pesticides is necessary under local soil and environ- mental conditions [1], [2]. Temperature is an important factor governing the rate of adsorption in soil pore. The existence of a number of solid structures of picloram were suggested and discussed with the application of the val- ues of LHosol as a correction of solubility- temperature effect on the standard enthalpy of the pesticide adsorp- tion processes [3], [4] . The major factors that determine the extent to which herbicides are adsorbed by soil in- clude: i) physical or chemical characteristic of the adsor- bent, ii) physical or chemical properties of the pesticides, and iii) properties of the soil system, such as cleay miner- al composition, pH, kinds and amounts of exchangeable cations, and temperature [5], [6]. Since information on the sorption behavior of pesticides in soil is essential in pre- dicting their leaching potential and contamination of ground water and no data are available in literature for sorption kinetics equilibrium parameters of the three commonly used pesticides (atrazine, picloram and pro- panil) in Kurdistan. Studies were conducted on their sorption and determining the thermodynamic parameters

————————————————

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

Kafia M.Shareef, Departmen of Chemistry, College of Science, University

of Salahaddin- Erbil-Kurdistan region -Iraq. E-mail: skavemy@yahoo.com

associated their sorption onto natural soil samples.

2 METHODLOGY

2.1 Soils

Fresh soil samples were taken from eight main agricul- tural locations, representing awide range of physico- chemical properties. Subsamples of homogenized soils were analyzed for moisture content, organic matter con- tent, particle size distribution, texture, pH, loss on igni- tion and exchangeable basic cations the detail were cha- ractrerized in previous article [7].

2.2 Pesticides

Analytical grad substituted atrazine, picloram and pro- panil were purchased from Riedal-de Haen, Sigma- Aldrich Company. All chemicals used were of analytical grade reagents and used without pre-treatments. Stan- dard stock solutions of the pesticides were prepared in deionised water.

2.3 Adsorption Experiments

The effect of temperature on adsorption of pesticides from aqueous solution was determined at 15, 25, 25±1 Co employing a standard batch equilibrium method [8]. Duplicate air-dried soil samples were equilibrated with different pesticide concentrations (2, 5, 10, and 15 µgml-1) at the soil solution ratios: 4:10, 4:8, and 1:10 for atrazine, picloram and propanil respectively. The samples plus blanks (no pesticide) and control (no soil) were thermos- tated and placed in shaker for 24 h for atrazine and piclo- ram and for 10 h for propanil. The tubes were centrifuged

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for 20 min. at 3500 rpm. Supernatants were analyzed by PerkinElmer series 200 USA family high performance liquid chromatography (HPLC) for each pesticide concen- tration. The detailed information about the soil characte- ristics and their sorption process has been reported in our previous work [8], [9].

3.1 Equilibrium constant

The equilibrium constant Ko was enabled us to calculate the thermodynamic parameters for physico-chemical equilibrium between pesticides and soils [10].
(3)
The values of Go at 288.15, 298.15 and 309.15K were summarized in Tables 1, 2, and 3 for adsorption of atra- zine, picloram and propanil respectively. The values of
Go for adsorption of studied pesticides were negative
and decreased with temperature rise indicating that ad-
sorption of pesticides on the soils were spontaneous with
a high preference of the soil surface. The data revealed
that adsorption of pesticides were in the following order: propanil > atrazine > picloram. The magnitude of Go also showed that the interactions of pesticides with the soil were thermodynamically spontaneous process and

Ko = a ae

= ys Cs

ye Ce

(1)
adsorption occurred through a bonding mechanism. The
Go values were in the range -30.549 to -3.172 KJmol-1, -
33.614 to - 4.2897 KJmol-1, and - 14.72 to - 76.412 KJmol-1
Where as: activity of the adsorbed solute, ae: activity of the solute in the equilibrium solution, Cs: }lg of solute ad- sorbed per milliliter of solvent in contact with the adsor-
for atrazine, picloram and propanil respectively. The re- sults obtained in the present study are similar to those of Gupta et al [14] who reported an increase in the values of
bent surface, Ce: }lg of solute per milliliter of solvent in

Go

with temperature. Variation of Go
with temperature
the equilibrium solution, ys: activity coefficient of the ad- sorbed solute. ye: activity coefficient of the adsorbed so- lute in the equilibrium solution. As the concentration of the solute in the solution approaches zero, the activity coefficient, y, approaches unity. “Equation (1),“may then be written as:
may be due to the increase in the degree of freedom of
adsorbed molecules, which enhances desorption rather
than adsorption at higher temperatures [15], [16].

3.3 Standard Enthalpy Change

The standard enthalpy change of adsorption Ho represents the difference in binding energies between the solvent and the soil with the pesticides. Values of Ho

lim

s - - �

C s =

C e

a s = K

a e

(2)
were determined graphically from the slope of the plot of lnKo vs. 1/ T Fig. 1 a, b, and c using the following “equa- tion” [11].
Values of lnKo were obtained from the plot of ln (Cs/Ce) vs. Cs, lnKo was obtained at Cs = 0, as described by Biggar and Chenung [11]. The results were summarized in Table
1, 2, and 3 for atrazine, picloram and propanil respective- ly. Values of lnKo were in the range 1.238 to 12.75, 1.674 to

[ dLnKo ] =

d ( 1 )

T

H o

R

(4)
14.032 and 29.83 to 173.1 for atrazine, picloram and pro- panil respectively. It is well known that Ko is constant at constant temperature and the position of equilibrium de- pends only on thermodynamic quantities and is indepen- dent of any consideration of kinetics or mechanism. Val- ues of Ko obtained can vary among soils due to the quan- tities and composition of soil components. The Ko values were decreased with rise in temperature, confirming that the pesticides had a high preference for adsorption at low temperature.

3.2 Standard Free Ernergy Change

Adsorptions equilibrium constant Ko can be expressed in terms of the standard Gibbs free energy Go for adsorp- tion [12], [13].

o

Values of Ho were summarized in Table 4, followed the range -5.010 to -0.738, -4.769 to -0.848 and -75.779 to -
1.628 KJmol-1 for atrazine, picloram and propanil respec- tively. The negative values of Ho indicated the exother- mic behaviors of the reaction. The linear nature of the plot indicates that the mechanism of adsorption is not changed as temperature is changed. The values of R2 were in the range 0.9999 to 0.902, 0.989 to 0.765, and 0.902 to
0.999 for atrazine, picloram and propanil respectively. The negative enthalpy of adsorption indicates an exo- thermic binding [17], [18]. Showing that the interaction of pesticides with the soil is an energetically stable exother- mic process and the adsorption occurred through a bond- ing mechanism. The Ho values explain the binding strength of pesticides to the soil; the lower negative value of Ho indicates stronger binding. Thus low values of Ho pointed towered chemisorptions; hence the herbicides adsorption may be due to coordination and /or protona-

G = RTLnKo

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tion, hydrogen bonding and dipole association and van der Waal’s forces. This indicates that the interactions be- tween the pesticides and the studied soil samples were stronger at lower temperature.

14

3.4 Isosteric Enthalpy Of Adsorption

The isosteric enthalpy of adsorption H is the standard enthalpy of adsorption at a fixed surface coverage. Values of H were calculated by the expression [17].

12

10

H = R[

6

dLnCe ]x

1

(5)

4 d ( )

T

2

0

0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035

1/T (K-1)

Fig 1 a Variation of ln Ko with 1/T for adsorption of atrazine on the eight soil samples (+ S1, • S2, A S3, x S4, * S5, •S6,

+S7, -S8).

18

16

14

12

10

8

6

4

2

0

0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035

1/T(K-1)

Where x is amount of pesticide adsorbed, and the average was calculated for each concentration. The values of LH Table 5 were in the following range -0.05 to -0.0084 kJmol-

1, -0.0057 to - 0.014 kJmol-1, and -0.0141 to -0.0034 kJmol-1 for atrazine, picloram and propanil respectively. The val- ues of LH of adsorption as a function of amount of pesti- cide adsorbed was almost the same for all the three pesti- cides on eight soils that support our inference regarding the mechanism of adsorption. These values were relative- ly small and were of the order which was consistent with a physical type of adsorption [18].

3.5 Standard Entropy Change

The values of standard entropy changeSo of adsorption were determined by using the “equation bellow” [18].

Fig.1. b Variation of ln Ko with 1/T for adsorption of picloram

LnKo =


H + S

(6)

on the eight soil samples (+ S1, • S2, A S3, x S4, * S5, •S6,

+S7, -S8).

RT R

250

200

150

100

50

0

0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035

1/T(K-1)

Fig. 1 c Variation of ln Ko with 1/T for adsorption of propanil, on the eight soil samples (+ S1, • S2, A S3, x S4, * S5, •S6,

+S7, -S8).

The values of So were determined from the plot of -lnKo
against 1/ T and the results were summarized in Table 4. The values of So followed the range -16.138 to -1.757
Jmol-1 k-1, -14.711 to -2.502 Jmol-1 k-1 and -199.01 to - 62.457
Jmol-1k-1 for atrazine, picloram, and propanil respectively. The negative values of So indicate that the adsorption of the herbicide formed on all eight soil samples were stabi- lization, association, fixation or immobilization of the pes- ticides molecules as a result of adsorption decreased the degree of freedom, causing negative entropy effect[19], [20]. The values of So pointing to the formation of the complexity by coordination or association of the herbi- cides and an exchangeable cation with the resultant of the loss in the degree of freedom of the pesticide.

3.6 Organic Matter Normalized Free Energy Change

Of Adsorption

Organic matter was the most important factor that go- verned the adsorption of pesticides on soils. The organic

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matter normalized free energy changes LGo
of adsorp-
sorption associated to the temperature. Adsorption effi-
tion of atrazine, picloram and propanil were calculated by using the following equation [21].

o

ciency for the studied pesticides was found to depend on the nature of adsorbent and adsorbate and the nature of the interactions between them.

G OM =

RTLnKOM


(7)
The values of LGo
in Table 6 were in the range -8.0106

1.4

to -10.188, -10.629 to -7.7422, and - 7.7984 to - 2.0458
KJmol-1 for atrazine, picloram and propanil respectively.

1.2

1

The negative values of LGo
revealed that the adsorption

0.8

of the three pesticides on the eight soil samples are spon-
taneous process and the adsorption has physical nature
[22]. This signifies that there is a constant partitioning of
the three pesticides. Between soil and solution, and water
molecules do not pose strong competition for the adsorp-
tion sites, and also more affinity of the studied pesticides
towards soil particulate matter than soil solution. Because

0.6

0.4

0.2

0

285 290 295 300 305 310

T (K)


most of the available sites in these soils are probably present at the surface of SOM and are therefore readily available for adsorption. The values of GoOM for adsorp- tion under the effect of temperatures were in the order T1> T2 >T3 for the studied pesticides (with some excep- tions). This indicates that as the temperature increases the adsorption coefficients decrease for atrazine, picloram and propanil.

Fig. 2.a Variation of adsorption coefficient with Temperature for Atrazine on the eight soil samples(+ S1, • S2, A S3, x S4,

* S5, •S6, +S7 , -S8).

4 TEMPERATURE DEPENDENCY OF ADSOPTION

COEFFICIENT

Variation of adsorption coefficient with temperature Fig.
2 a, b and c indicted that as the temperature increased the
values of adsorption coefficient for these herbicides de-
creased. This can be explained by the types of the interac-
tions between the pesticides and soil through adsorption
that may occur as a result of two types of forces: enthal-
py-related and entropy-related forces [23]. Hydrophobic
bonding is anexample of an entropy-driven process; it is

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

285 290 295 300 305 310

T (K)

due to a combination of London dispersion forces (instan- taneous dipole-induced dipole) associated with large en- tropy changes resulting from the removal of the sorbate from the solution. For polar chemicals, the enthalpy- related forces are greater, due to the additional contribu- tion of electrostatic interactions. A small temperature ef- fect was detected with the adsorption of studied pesti- cides by the soil samples; this behavior has been inter- preted as due to physical adsorption. Results obtained in the present study are similar to those reported by Biggar et al [11].

4 CONCLUSION

Adsorption experiments were conducted at 15, 25, and
35oC to study the thermodynamic (equilibrium) parame-
ter, associated the adsorption of the studied pesticides on
the selected soil samples. The net effect of pesticide ad-

Fig.1. b Variation of ln Ko with 1/T for adsorption of picloram on the eight soil samples (+ S1, • S2, A S3, x S4, * S5, •S6,

+S7, -S8).

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International Journal of Scientific & Engineering Research Volume 2, Issue 6, June-2011 5

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0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

285 290 295 300 305 310

T (K)

and its Cation Interaction Behavior."Journal of Col- loidal and Interface Science. 2 269(2004):259-264

[10] OP. Bansal. "Kinetics of Interaction of three Carba- mate Pesticides with Indian soils: Aligarh district. Pest Manag Sci. 60(2004):1149-1155

[11] Biggar, J. W. and M. W. Chenung "Adsorption of pic- loram (4-Amino 3, 5, 6-Trichloropineolinic Acidon Panoche. Ephrata and Palouse Soils a Thermodynam- ic Approach to Adsorption Mechanism". Soil Sci. Soc. Am. Proc. 37(1973): 863-868

[12] K. Chaudhary and B. Prasad. "Thermodynamics of Potassiume Exchange Reaction in Entisol and vertisol using a Kinetic Approach by Miscible Displacement

Fig 2 c Variation of ln Ko with 1/T for adsorption of propa- nil, on the eight soil samples (+ S1, • S2, A S3, x S4, * S5,

•S6, +S7, -S8).

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[5] J.B. Alam, A. K. Dikshit and M. Bandy Opadhayay.

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[8] G. Suman, and V. T. Gajbhiye, J. of Environ. Sci. and health, 6 B37 (2002):573-586.

[9] Alexandre G. S. Prado, Andreia H. Tosta and Glaudio Airoldi. "Asorption, Separation, and Thermodynamic Data on Herbicide Picloram Anshored on Silica Gel

Technique ". Journal of the Indian Society of Soil
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[13] T. Sismanoglu, A. Ercage, S. Pura and E. Ercage. "Ki-

netics and Isotherms of Dazomet Adsorption on Nat-
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[14] H. K. Gupta, Devprakash, P. K. M. Ishra and vishal
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[15] Michael Sander, Yuefeng Lu and Josseph J. Pingnatel- lo. "A Thermodynamically Based method to Quantify Trine Sorption Hystersis. J. Environ. Qual.

34(2005):1063-1072
[16] Young –HakKim, Thomas M. Heinze, Seong-Jae Kim,
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[18] Elsayed A. Elkhatib, A. M. Mahdy and N. H. Bbrakat
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as Affected by Citrate and Succinate" Soil & Water
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[21] Sarah Taylor –lovel, Gerald k. Sims, and Loyd M.
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TABLE 1

EQUILIBRIUM CONSTANTS AND STANDARD FREE ENERGY CHANG AT THREE TEMPERATURES FOR ADSORP- TION OF ATRAZINE ON THE SELECTED SOIL SAMPLES

Soil

T1 K

T2 K

T3K

Soil

lnK0 Go (KJ/mol)

lnK0 Go(KJ/mol)

lnK0 Go(KJ/mol)

S1

S2

S3

S4

S5

S6

S7

S8

8.154 -19.534

10.64 -25.482

8.365 -20.039

6.820 -16.339

12.75 -30.549

6.918 -16.573

10.24 -24.526

12.20 -29.218

7.271 -18.023

5.783 -14.334

5.106 -12.657

5.713 -14.162

4.467 -11.073

5.703 -14.136

5.557 -13.775

8.26 -20.465

4.365 -11.185

1.238 -3.172

2.973 -7.618

5.446 -13.953

1.754 -4.493

3.273 -8.386

2.711 -6.944

4.24 -10.856

TABLE 2

EQUILIBRIUM CONSTANTS AND STANDARD FREE ENERGY CHANG AT THREE TEMPERATURES

FOR ADSORPTION OF PICLORAM ON THE SELECTED SOIL SAMPLES

l)

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TABLE 3

EQUILIBRIUM CONSTANTS AND STANDARD FREE ENERGY CHANG AT THREE TEMPERATURES FOR AD-

SORPTION OF PROANIL ON THE SELECTED SOIL SAMPLES

Soil

T1 K

T2 K

T3 K

lnK0 Go(KJ/mol)

lnK0 Go(KJ/mol)

lnK0 Go(KJ/mol)

S1

S2

S3

S4

S5

S6

S7

S8

83.00 -198.85

146.2 -350.18

95.06 -227.74

170.6 -408.79

110.4 -264.51

173.1 -414.72

155.7 -372.98

119.3 -285.73

79.87 -197.99

111.3 -275.97

66.88 -165.79

122.3 -303.13

97.73 -242.28

160.6 -397.98

135.6 -336.15

107.3 -265.95

41.35 -105.93

40.54 -103.87

38.87 -99.593

53.42 -136.87

41.35 -105.93

29.83 -76.412

36.40 -93.257

33.09 -84.764

TABLE4

STANDARD ENTHALPY CHANGE AND STANDARD ENTROPY CHANGE (DETERMINED GRAPHICALLY) FOR ADSORPTION

OF ATRAZINE, PICLORAM, AND PROPANIL ON THE SELECTED SOIL SAMPLES

Soil

atrazine

picloram

propanil

Ho R2 So

(KJ/mol) J/mol.k

Ho R2 So

(KJ/mol) (J/mol.k)

Ho R2 So

(KJ/mol) (J/mol.k)

S1

S2

S3

S4

S5

S6

S7

S8

-2.010 0.902 -5.953

-5.019 0.999 -16.14

-2.885 0.989 -9.023

-0.738 0.901 -1.757

-5.904 0.931 -19.057

-1.938 0.957 -5.869

-4.029 0.985 -12.781

-4.247 0.999 -13.266

-4.396 0.989 -13.597

-1.757 0.988 -5.442

-4.769 0.797 -14.711

-2.003 0.823 -6.235

-2.009 0.972 -6.015

-0.848 0.838 -2.502

-1.973 0.995 -5.679

-2.208 0.841 -6.318

-22.025 0.791 -65.57

-56.168 0.955 -176.64

-29.995 0.999 -92.628

-1.628 0.986 -62.457

-36.605 0.869 -112.86

-75.779 0.799 -239.79

-63.204 0.859 -199.01

-45.634 0.838 -142.76

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TABLE 5

ISOSTERIC HEAT CHEAHG OF ADSORPTION OF ATRAZINE, PICLORAM, AND PROPANIL ON THE SELECTED

SOIL SAMPLES.

T3

TABLE 6

ORGANIC MATTER NORMALIZED FREE ENERGY CHANGE OF ADSORPTIO OF ATRAZIN, PICLORAM, AND PROPANIL ON THE

SELECTED SOIL SAMPLES

Soil

atrazine

picloram

propanil

Soil

AGoOM (KJ/mol)

AGoOM (KJ/mol)

AGoOM (KJ/mol)

Soil

T1 T2 T3

T1 T2 T3

T1 T2 T3

S1

S2

S3

S4

S5

S6

S7

S8

-9.001 -8.935 -8.746

-9.967 -9.546 -9.861

-9.905 -9.662 -9.297

-10.19 -9.707 -9.100

-9.029 -8.545 -8.011

-9.720 -9.204 -9.517

-9.795 -9.137 -9.354

-9.319 -8.969 -9.236

-8.757 -8.854 -8.8203

-10.22 -10.311 -8.5742

-10.01 -10.325 -9.8902

-10.63 -10.942 -10.011

-8.247 -8.427 -8.5593

-10.26 -10.599 -7.7422

-9.863 -10.009 -9.7379

-8.928 -9.051 -8.7286

-7.449 -6.890 -6.792

-6.566 -6.564 -6.564

-7.798 -7.297 -6.116

-5.459 -5.181 -3.908

-6.597 -6.719 -4.978

-7.031 -6.669 -8.424

-5.257 -4.376 -2.046

-5.829 -5.732 -5.759

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