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SORPTION OF AMETRYN AND IMAZETHAPYR IN TWENTY-FIVESOILS FROM PAKISTAN AND AUSTRALIARiaz Ahmada; Rai S. Kookanab; Angus M. Alstonc

a AgResearch, Ruakura Research Centre, Hamilton, New Zealand b CSIRO Land and Water, SA,Australia c Department of Soil and Water, The University of Adelaide, SA, Australia

First published on: 31 March 2001

To cite this Article Ahmad, Riaz , Kookana, Rai S. and Alston, Angus M.(2001) 'SORPTION OF AMETRYN ANDIMAZETHAPYR IN TWENTY-FIVE SOILS FROM PAKISTAN AND AUSTRALIA', Journal of Environmental Scienceand Health, Part B, 36: 2, 143 — 160, First published on: 31 March 2001 (iFirst)To link to this Article: DOI: 10.1081/PFC-100103740URL: http://dx.doi.org/10.1081/PFC-100103740

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J. ENVIRON. SCI. HEALTH, B36(2), 143–160 (2001)

SORPTION OF AMETRYN AND IMAZETHAPYRIN TWENTY-FIVE SOILS FROM PAKISTAN

AND AUSTRALIA

Riaz Ahmad,1,* Rai S. Kookana,2 and Angus M. Alston1

1 Department of Soil and Water, The University of Adelaide, WaiteCampus, PMB1, Glen Osmond, SA 5064, Australia

2 CSIRO Land and Water, PMB2, Glen Osmond, SA 5064, Australia

ABSTRACT

Sorption of ametryn and imazethapyr in 25 soils from Pakistan and Australiawas investigated using the batch method. The soils varied widely in their in-trinsic capacities to sorb these herbicides as shown by the sorption coefficients,Kd, which ranged from 0.59 to 47.6 for ametryn and 0.02 to 6.94 for imazeth-apyr. Generally the alkaline soils of Pakistan had much lower Kd values of bothherbicides than the soils of Australia. Both soil pH and soil organic carbon(SOC) were correlated significantly with the sorption of ametryn, whereas onlysoil pH was strongly correlated with imazethapyr sorption. No correlation wasfound between Kd values of the herbicides and the clay contents of the soils.Multiple regression analysis showed that Kd values were better correlated(r2�0.94 and 0.89 for ametryn and imazethapyr, respectively) if SOC and pHwere simultaneously taken into account. The study indicated that sorption ofthese herbicides in the alkaline soils of Pakistan was low and consequentlythere is considerable risk of groundwater contamination.

Key Words: Sorption; Ametryn; Imazethapyr; Kd; Koc; Soil organic carbon;pH; Clay.

143

Copyright � 2001 by Marcel Dekker, Inc. www.dekker.com

* Corresponding author. Current address: AgResearch, Ruakura Research Centre, Private Bag 3123,Hamilton, New Zealand. E-mail: ahmadr�agresearch.cri.nz

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INTRODUCTION

Herbicides have become an indispensable component of modern agriculturalsystems and are being increasingly used to ensure the production of an adequatesupply of food and enhance the quality of produce. However, the occurrence ofherbicides in groundwaters has become an important environmental concern inmany countries (1–5). Among various pesticides detected in groundwaters, herbi-cides predominate (4).

Sorption to soil and sediments is central to the fate and behaviour of herbi-cides in the environment. The leaching and movement of herbicides through theroot zone, the vadose zone and into groundwater is greatly dependent on the extentto which they interact with and become sorbed by soil constituents. The sorptionbehaviour of herbicides in soil may be influenced by a number of soil propertiessuch as clay content and soil organic carbon (SOC) content (6 –10), soil pH (11–15) and amount and type of clay (16). However, all these soil properties varywidely with soils from different countries or even regions.

Most soil-herbicide interactions have been studied in acidic soils of Europeand North America. The sorption behaviour of herbicides in Australian soils isrelatively poorly understood (17), and data on sorption behaviour of herbicides inthe alkaline soils of Pakistan are lacking. The intrinsic properties of the soils ofboth these countries differ from those in Europe and North America (18). There-fore, it is imperative that studies on herbicides be conducted on soils of Pakistanand Australia to understand better the sorption behaviour of pesticides in thesesoils.

Since information on the sorption behaviour of pesticides in soil is essentialin predicting their leaching potential and contamination of groundwater, studieswere conducted on the sorption of two ionizable herbicides, ametryn (a triazine)and imazethapyr (an imidazolinone) in a range of soils collected from Pakistan andAustralia, covering a wide range of physical and chemical properties. Extensiveresearch on sorption of triazine herbicides has been done e.g. for atrazine (10, 19–21), and for imazapyr (22–24) and imazaquin (25–26) among the imidazolinonegroup of herbicides. However, little is known about sorption behaviour of ametrynand imazethapyr in soil. Therefore, our experimental objectives were to evaluatethe sorption characteristics of ametryn and imazethapyr in 25 soils from Pakistanand Australia and to evaluate the soil characteristics that influence sorption of thesepesticides.

MATERIALS AND METHODS

Soils

Thirteen soils from Pakistan and 12 from Australia, with no recent history ofapplication of the herbicides, were collected. The soils used were chosen to repre-sent a wide range of characteristics of soil properties (SOC, 2.8–138 g kg�1; pH,

144 AHMAD, KOOKANA, AND ALSTON

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4.7–8.6; clay, 130 –755 g kg�1). Illite was the dominant clay mineral in Pakistanisoils and kaolinite dominated in the Australian soils. The description and charac-teristics of the soils have been given elsewhere (18). The key properties affectingpesticide sorption e.g. SOC, pH and clay contents for these soils have been givenin Table 2 in Results and Discussion section.

Herbicides

The ametryn (N2-ethyl-N4-isopropyl-6-methylthio-1,3,5-triazine-2,4-di-amine), and imazethapyr [(RS)-5-ethyl-2-(4-isopropyl-4-methyl-5-oxo-2-imida-zolin-2-yl)nicotinic acid] used in the study had �99% purity. Some properties ofthe herbicides are given in Table 1 and their structures are shown in Figure 1.

Herbicide Sorption Equilibria

To establish when herbicide sorption reached equilibrium in the soil systems,10 mL of ametryn solution in 0.01M CaCl2 was added to 2 g of air dried soils (1 :5 soil solution ratio) in 14 mL glass tubes fitted with teflon-lined screw caps. Forimazethapyr, a soil solution ratio of 1 :2 was employed due to its very low sorptionand to avoid errors due to small changes in the solution concentrations after equili-bration. Duplicate samples plus blanks (no soil) were shaken for 0.5, 1, 2, 4, 8, 16,20 and 24 h. The tubes were centrifuged for 20 min at 1800 rpm. About one mLof the supernatant was filtered through 0.45 mm nylon membrane syringe filtersand concentrations of the herbicides were determined by HPLC. Shaking times atwhich sorption equilibria reached were assessed.

Herbicide Sorption Measurements

Sorption of ametryn and imazethapyr from aqueous solution was determinedat ambient laboratory temperature (22 � 2�C) employing the standard batch equili-bration method, which has become the most widely used procedure for character-izing the sorptive properties of chemicals and sorbents (30). Sorption was mea-sured using 0.01M CaCl2 as a background electrolyte to minimize changes in ionicstrength and promote flocculation. For determination of sorption isotherms, foursolution concentrations (0.5, 1.0, 1.5, and 2.0 mg L�1) for ametryn and three con-centrations (0.5, 1.0, and 1.5 mg L�1) for imazethapyr were used. This concentra-tion range represents values that are typical of concentrations in soil resulting fromconventional application rates of herbicides. Duplicate soil samples were equili-brated with pesticide solutions at the soil solution ratios of 1 :5 for ametryn and 1:2 for imazethapyr by shaking for 16 h in glass tubes sealed with teflon-lined caps.At the end of the shaking time, the suspension was centrifuged at 1800 rpm for20 minutes to obtain a clear solution and �1 mL of the clear supernatant wasfiltered directly into the HPLC autosampler vial through a nylon syringe filter. The

SORPTION OF AMETRYN AND IMAZETHAPYR 145

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146 AHMAD, KOOKANA, AND ALSTON

Tabl

e1.

Som

eSe

lect

edP

rope

rtie

sof

the

Pest

icid

esU

sed

Com

mon

nam

eC

hem

ical

nam

eC

hem

ical

form

ula

Mol

ecul

arw

eigh

tA

queo

usso

lubi

lity

Ioni

zabi

lity

(pK

a)

Am

etry

n*N

2 -et

hyl-

N4 -

isop

ropy

l-6-

met

hylth

io-1

,3,5

-tri

azin

e-2,

4-di

amin

eC

9H17

N5S

227.

320

0m

gL

�1

(20�

C)

4.1

Imaz

etha

pyr*

*(R

S)-5

-eth

yl-2

-(4-

isop

ropy

l-4-

met

hyl-

5-ox

o-2-

imid

azol

in-2

-yl)

nico

tini

cac

idC

15H

19N

3O3

289.

31.

4g

L�

1

(25�

C)

2.1,

3.9*

**

*To

mlin

(27)

;**

Har

tley

(28)

;***

Hor

nsby

etal

.(29

).

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herbicide concentrations in solution were measured on the HPLC (Novapack C18,10 cm 5 mm ID cartridge and Waters appliance with UV detector). The mobilephase was acetonitrile :water (35:65) for ametryn; methanol :water :acetic acid(40:60 :0.5%) for imazethapyr; and the assay was performed at 220 and 254 nmfor ametryn and imazethapyr, respectively. Differences between initial and equilib-rium concentrations were assumed to be sorbed and the values were used to cal-culate sorption. Blanks which included each of the soils with only 0.01M CaCl2solution (no test chemical) and a single control of the test chemical solution withno soil were also performed to check for analytical interference due to soil extractsand for interaction between the chemicals and the tubes.

Determination of Sorption Coefficient (Kd)

For linear sorption isotherms, the sorption coefficient, Kd, is defined in equa-tion (1).

pesticide sorbed (mg / kg of soil)K � (1)d pesticide in solution (mg / L)

The values of Kd were determined either from the slope of the linear plots of sorbedv aqueous herbicide concentrations or from the single solution concentration ofeach herbicide using equation (1).

RESULTS AND DISCUSSION

Sorption Kinetics

Changes in solution concentration with time for ametryn and imazethapyrare shown for selected soils in Figure 2. The kinetic study results showed that

SORPTION OF AMETRYN AND IMAZETHAPYR 147

Figure 1. Structures of ametryn and imazethapyr.

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�90% of the sorption occurred within the first 2 h. On the basis of these experi-ments, an apparent equilibrium was reached in 8 h for ametryn and 12 h for ima-zethapyr, as no changes were observed in concentrations of the herbicides in so-lution after these time periods. Earlier determinations of sorption coefficients werebased on 4 h equilibrations for ametryn (31) and 10 h (11) or 15 h (13) for ima-

148 AHMAD, KOOKANA, AND ALSTON

Figure 2. The effect of time on sorption of ametryn and imazethapyr in selected soils.

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zethapyr with no significant changes in solution concentrations beyond 4 h. Anequilibrium time of 16 h for both herbicides was used in our sorption studies.

Sorption of Ametryn and Imazethayr

The sorption isotherms for ametryn and imazethapyr (Figures 3 & 4) werelinear over the concentrations used and showed good fit to the measured data(r2 � �0.96) which suggested that the Kd is appropriate to use for these chemicals.The Kd values for ametryn and imazethapyr determined for 25 soils are reportedin Table 2. The magnitude of the Kd values is indicative of moderate to strongsorption for ametryn (mean Kd � 9.45 L kg�1) and weak to moderate sorption forimazethapyr (mean Kd � 1.47 L kg�1). These findings are in agreement with thehydrophobicity of the herbicides as represented by the Kow values, and they sup-port the comment by Calvet (32) about the use of the octanol-water partition co-efficients to predict the sorption of organic compounds in soils. Their use is validonly if the mineral constituents of the soil do not play a role, and interactions ofthe pesticides with the organic matter do not cause deviations from the predictedbehaviour.

The value of Kd for ametryn in the 13 Pakistani soils varied from 0.59 L kg�1

(Pk4) to 4.30 L kg�1 (Pk16) whereas in the Australian soils, it varied from 0.97L kg�1 (Avon2) to 47.6 L kg�1 (SS7). For imazethapyr the Kd varied from �0(Pk18) to 0.55 L kg�1 (Pk16) in Pakistani soils and from �0 (Avon2) to 6.94L kg�1 (ACU1) in the Australian soils. For both herbicides, the soils from Pakistanshowed relatively lower Kd than the Australian soils, which may be due in part tothe higher alkalinity of the Pakistani soils.

Influence of Soil pH

The Kd values of ametryn and imazethapyr are plotted against pH in Fig-ure 5. As expected for the interaction of ionizable pesticides with SOC (33),sorption of both herbicides to soil increased with the decrease of soil pH from8.6 to 4.7. There was a good negative exponential correlation between Kd and pHfor both ametryn (r2�0.85) and imazethapyr (r2�0.76). Ametryn showed muchhigher sorption than imazethapyr across all pH levels (Table 2). One Pakistani soil(Pk18) and one Australian soil (Avon2) sorbed very little imazethapyr (Kd, 0.02).In case of ametryn, soils Pk4 (Kd, 0.59) and Avon2 (Kd, 0.97) had lowest sorptionin the Pakistani and Australian groups of soils, respectively. All these soils withlow Kd values had pH �8.

These results are consistent with the reported decrease in imazethapyr ac-tivity at lower pH, attributed to increased sorption (13, 23, 34). In the case ofametryn, a basic compound, the decrease in pH causes protonation and formationof cations at pH � pKa. However, Gan et al. (35) did not observe any significant

SORPTION OF AMETRYN AND IMAZETHAPYR 149

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150 AHMAD, KOOKANA, AND ALSTON

Figure 3. Sorption isotherms of ametryn in some Pakistani and Australian soils.

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SORPTION OF AMETRYN AND IMAZETHAPYR 151

Figure 4. Sorption isotherms of imazethapyr in some Pakistani and Australian soils.

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difference in the behaviour of imazethapyr in four Minnesota soils in the pH rangeof 4.8 to 7.1, which is contrary to the reports that greatest change in sorption occurswhen pH decreases below 6, with little or no response occurring above 6 (11, 13).In another study by Loux et al. (36), sorption of imazethapyr showed that sorptionwas greater at pH below 6 than at higher pH, which is in accord with the studiesreported here.

Sorption is generally characterized by an envelope showing a maximumsorption at a pH in the vicinity of pKa of weakly acidic (e.g. imazethapyr) andweakly basic (e.g. ametryn) compounds (37–39). The soil pH influences sorptionby influencing the degree of ionization of herbicides. Imazethapyr is an amphotericcompound because of the presence of acidic and basic functional groups (26). Ion-izable carboxyl group for imazethapyr has acidic and basic pKa values of 2.1 and3.9, respectively (26). Therefore, it should be predominantly in anionic form at pH

152 AHMAD, KOOKANA, AND ALSTON

Table 2. Soil Sorption Coefficients (Kd) for Ametryn and Imazethapyr in the Soils Studied

No Soil

SOC(Leco)

(g kg�1)pH

(1:5 CaCl2)Clay

(g kg�1)

Kd

(Ametryn)(L kg�1)

Kd

(Imazethapyr)(L kg�1)

Pakistani Soils

1 Pk 2 3.88 8.16 252 1.10 0.172 Pk 4 6.66 8.07 156 0.59 0.253 Pk 6 13.82 8.56 268 3.25 0.364 Pk 7 6.84 8.08 152 1.75 0.365 Pk 8 3.88 8.35 140 0.85 0.166 Pk 9 6.54 7.80 156 1.51 0.437 Pk 10 2.79 8.07 180 0.63 0.138 Pk 11 6.92 8.30 292 2.16 0.229 Pk 12 6.18 8.51 172 2.07 0.13

10 Pk 13 9.70 8.20 292 2.36 0.3211 Pk 15 6.50 8.03 320 1.05 0.3112 Pk 16 22.20 7.56 292 4.30 0.5513 Pk 18 5.72 8.47 172 0.90 0.02

Australian Soils

14 SS 6 70.10 5.40 479 40.55 2.8415 SS 7 138.00 4.69 160 47.62 4.9816 SS 8 23.20 7.21 679 7.68 0.9717 Buckelboo 12.94 7.29 260 2.65 0.1918 Avon 1 12.90 7.38 130 1.53 0.1819 Avon 2 3.84 8.02 180 0.97 0.0220 Mt Mary 10.20 7.63 210 2.21 0.1621 Pinnaroo 12.00 7.54 182 1.78 0.1822 ACU1 25.60 4.72 212 31.02 6.9423 B211 34.30 5.94 755 20.45 4.7224 Qld 38.40 5.20 515 26.23 5.6425 URB-P 26.70 4.90 172 31.06 6.44

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SORPTION OF AMETRYN AND IMAZETHAPYR 153

Figure 5. Relationship of Kd values of ametryn and imazethapyr with soil pH.

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values greater than the pKa value. Because of its anionic form, the molecule isrepulsed by negatively charged clay and organic surfaces, and little or no sorptionwas observed in soils having high pH. Protonation of imidazolinone nitrogens mayoccur at low pH, resulting in an undissociated or even a positively charged mole-cule with a potential for the binding to the soil (36, 13). Moreover, the water solu-bility of acidic compounds generally decreases with decreasing pH and results inan increase in their Kow. This increase in Kow of imazethapyr at low pH could haveresulted in its higher sorption.

The dependence of imazethapyr sorption on soil pH is similar to the sorptionof acidic herbicides. The characteristics of imazethapyr sorption are somewhatsimilar to those of chlorsulfuron, an acidic herbicide with pKa of 3.58 (40). Chlor-sulfuron sorption decreases as the soil pH increases and pH changes below 6 havethe greatest influence on its sorption (41– 43).

Basic herbicides like ametryn [pKa 4.1, Hornsby et al.(29)] also protonate asthe soil pH decreases and the resulting cations are sorbed to the negatively chargedsoil colloids (44). Reduced protonation of ametryn in soils with high pH is likelyto have been responsible for its low sorption. Relationships between increasedsorption with decreasing soil pH have been reported for other weak base herbicidessuch as terbutryn (39) and atrazine (45).

Influence of Soil Organic Carbon

Sorption distribution coefficients of ametryn and imazethapyr would alsovary between soils because of the varying quantities (and qualities) of SOC pres-ent. This would be particularly true for Pakistani soils, which covered a narrow pHrange. The high SOC contents of Pk16, SS6, SS7, ACU1, B211, Qld and URB-Pare most likely responsible for the high sorption of both chemicals by these soils.In soils with low OC e.g. Pk18 (OC, 5.7 g kg�1) and Avon2 (OC, 3.8 g kg�1), thesorption of ametryn and imazethapyr was very low (�1 and 0.02 L kg�1, respec-tively). In soils with low OC contents, contributions of other properties (e.g. pH)to sorption become more important. Soils Pk18 and Avon2, which had � 6 g kg�1

SOC and pH �8, showed almost zero sorption of imazethapyr. Sorption of ame-tryn was highest in soils SS6 and SS7 with highest OC (70 and 138 g kg�1, re-spectively) and acidic pH (5.4 and 4.7) compared with other acidic soils (ACU1,B211, Qld, and URB-P) with medium OC. Figure 6 shows a better correlation (r2

� 0.74) of SOC with Kd of ametryn than with imazethapyr (r2 � 0.35). It seemsthat SOC may play a greater role in sorption of ametryn than of imazethapyr.

It is evident from Table 2 that soil SS7 (OC, 138 g kg�1), did not sorb asmuch of the herbicides on the basis of per unit mass of SOC as other soils (ACU1,B211, Qld, URB-P) having similar pH but much lower OC. It appears that differ-ences in the physical and/or chemical composition of SOC between soils may beplaying an important role in the sorption of the herbicides. Pakistani soil Pk6,sorbed a high amount of ametryn (3.2 L kg�1) in spite of having high pH (8.6) and

154 AHMAD, KOOKANA, AND ALSTON

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SORPTION OF AMETRYN AND IMAZETHAPYR 155

Figure 6. Relationship of Kd values of ametryn and imazethapyr with SOC.

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only a medium amount of OC (13.8 g kg�1). Further investigations of the qualityand properties of OC in soils of different origins are necessary to elucidate themechanisms responsible for the observed differences in sorption.

Influence of Clay

Sorption of ametryn and imazethapyr appeared to be independent of the claycontents of soils. No correlation was found between Kd values of either herbicideand clay content (Figure 7). Indeed, two soils with low clay contents, SS7 (clay,160 g kg�1) and URB-P (clay, 172 g kg�1), showed very high sorption for ametryn(47.6 and 31.1 L kg�1, respectively) and imazethapyr (5.0 and 6.4 L kg�1, respec-tively). Valverde-Garcia et al. (46) also found no correlation between the Kd ofatrazine (same chemical group as ametryn) and total clay content, although theydid find a strong correlation between the illite content and Kd. However, studiesconducted by Seybold et al. (10) on influence of the type of clay on sorption ofatrazine showed no significant correlation between any of the individual clay min-erals including illite. However, a study by Loux et al. (36) showed that imazethapyrsorption on soils was positively correlated with soil clay content. Che et al. (34)observed greater sorption of imazethapyr on illite and kaolinite at pH 3.0, com-pared with that on hectorite (OC, 0.1%), which was consistent with the higher Ccontents of illite (OC, 0.17%) and kaolinite (OC, 0.16%). In the present study,Pakistani soils contained illite as the most dominant clay type, followed by kaolin-ite. However, they were generally lower in sorption of both herbicides than theAustralian soils, which might be due to the lower SOC contents and higher pH ofthe Pakistani soils. Dolling (47) concluded that the availability of imidazolinoneherbicides was unlikely to be affected by the soil clay content because of theiroverall weak affinity for clays.

Multiple Linear Regression

The two soil parameters, SOC and soil pH, accounted for 94% of the totalvariance in ametryn and 89% in imazethapyr sorption by 25 soils.

2Ametryn (K ) � 58.0 � 0.189 SOC � 7.195 pH r � 0.94, p �0.001d2Imazethapyr (K ) � 15.63 � 0.016 SOC � 1.901 pH r � 0.89, p �0.05d

The correlation suggests that the combination of a high SOC content and a low pHvalue could have been responsible for the Kd values of SS6, SS7, ACU1, B211,Qld, and URB-P soils from Australia being significantly higher than the others. Itshows that the sorption of ametryn is dependent on both SOC and pH, and thatboth parameters need to be considered to explain variations in Kd of ametryn invarious soils. By contrast, the Kd of imazethapyr was not significantly (p�0.05)related to SOC and pH.

Pakistani soils, after years of intensive cultivation resulting in low SOC, ex-

156 AHMAD, KOOKANA, AND ALSTON

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hibit low sorption of weakly basic herbicides such as ametryn. Sorption of acidicchemicals such as imazethapyr depends greatly on pH of the soil. As most of thePakistani soils are alkaline in nature (pH �7.5), they have low capacities for re-taining acidic herbicides like imazethapyr and eventually their leaching is likely tocause contamination of the groundwater.

SORPTION OF AMETRYN AND IMAZETHAPYR 157

Figure 7. Relationship of Kd values of ametryn and imazethapyr with clay contents.

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In conclusion, the results of the study showed that the sorption of ametrynand imazethapyr was affected by the soil pH and the amount of SOC. Therefore,care is needed while extrapolating results generated on acidic soils to alkalinesoils. The widespread use of herbicides warrants further studies of herbicide sorp-tion on alkaline soils of Pakistan and Australia to generate local data rather thanfollowing the data from overseas studies.

ACKNOWLEDGMENTS

We wish to thank Trevor James for his review of an earlier draft of the manu-script. The senior author greatly acknowledges the World Bank and the Govern-ment of Pakistan for financing his doctoral study.

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Received July 13, 2000

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