EQUILIBRIUM AND KINETIC STUDIES OF THE ADSORPTION OF2,4-D AND PICLORAM ON HUMIC ACID

S, U. KHAN'Research Station, Agriculture Canada, Regina, Saskatchewan. Received 22 Mar' 1973,

accepted 19 July 1973.

KneN, S. U. 1,973. Equilibrium and kinetic studies of the adsorption of 2,4-Dand picloram on humic acid. Can. J. Soil Sci. 53: 429-434.

Equilibrium and kinetic studies of the adsorption of 2,4-D (2,4-dichlorophenoxyacetic acid) and picloram (4-amino-3,5,6-trichloropicolinic acid) on a humic acidhave been made. The equilibrium data followed the Freundlich-type isotherm. Rateconstants, activation energies, heats of activation, and entropies of activationwere calculated for the adsorption of the two herbicides on humic acid. The ratedata indicated a physical type of adsorption. In the overall adsorption process therate-limiting step for the initial period was shown to be the difiusion of theherbicide molecules to the surface of humic acid. However, the rate-limitingprocess at longer time intervals was interpreted to be intraparticle diffusion of theherbicide molecules into the interior of the humic acid particies.

On a efiectu6 des 6tudes d'6quilibre et cin6tiques de I'adsorption du 2,4-D (acide2,4-dichloroph6noxy ac6tique) et du picloram (acide 4-amino-3,5,6-trichloropicoli-nique) sur I'acide humique. Les donn6es d'6quilibre suivent I'isotherme du typeFreundlich. On a ca1cu16 les constantes de migration, les 6nergies, les chaleurs, etles entropies d'activation pour l'adsorption des deux herbicides sur I'acide humique.Les donn6es de migration laissent supposer un type physique d'adsorption. Duranttout le processus d'adsorption, la diffusion des mol6cules d'herbicide d la surfacede I'acide humique s'est av6r6e 0tre le point limite de migration pour la phaseinitiale. Cependant, on suppose que le processus limitatif i plus long intervaller6sulte de la diffusion intraparticulaire des mol6cules d'herbicide ir i'int6rieur desparticules d'acide humique.

INTRODUCTION cides by humic substances (Hayes 1970;

The importance of humic substances in in- Khan 1972; Stevenson 1972)' These include

fluencing the activity, behavior, bio-avail- ion exchange' hydrogen bonding, protona-

ability ind OegraOaUitity of herbicides in tion, charge transfer, ligand exchange, co-

soils has been documented in recent reviews ordination through a metal ion, van der

by Hayes (1970),Khan ( 1972), and Steven- Waals forces, and hydrophobic bonding'

son (1972). Numerous reports have ap- Despite the considerable amount of re-

peared in the literature during the past dei- search undertaken, very little attention has

ade indicating that humic substances, which been paid to the- equilibrium and kinetic

represent the most active fraction of organic aspect of the problem. These fundamental

matter in soils, are the most active adsorbent studies are necessary for a better under-

for a wide variety of herbicides (Dunigan standing of the mechanism(s) of adsorption'

and Mclntosn pil Gilmour and Coleman The work described here attempts to obtain

1971;Hance 1965,1969;Hayes et al. 1968; infornation on the equilibrium and kinetics

Li and Felbeck tr972; Sullivan and Felbeck of adsorption of 2,4-D and picloram on a1968; Weber et al. 1969). Several mech- hun.ric acid (HA)' The two herbicides have

anisms or combination of mechanisms have been used extensively for the control ofbeen postulated for the adsorption of herbi- broadleaved weeds in a variety of crops'

'Present address: Chemistry and Biology Re-search Institute, Agriculture Canada, ResearchBranch, Ottawa, Ontario KlA 0C6.

Can. J. Soil Sci,53: 429-434 (Nov. 1973)

MATERIALS AND METHODSMaterialsThe HA originated from a Black Chernozemicsoil of Western Canada. Methods of extraction,

429

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430 CANADIAN JOURNAL OF SOIL SCIENCE

separation, and purification of the HA wereidentical to those described previously (Khan1971). The purified HA contained, on a dryash free basis, 56.4Vo C, 5.57o H, 4.lVo N,1.1% S, and 33.0Vo O. Functional group anal-ysis showed 4.5 meq COOH: 2.1 meq phenolicOH, 2.8 meq alcoholic OH; 4.5 meq C : O;and 0.3 meq OCH., per g of HA. The extractedand purified HA accounted for about 287o ofthe organic matter in the original soil.

Analytically pure 2,4-dichlorophenoxyaceticacid (2,4-D) and 4-amino-3,5,6-trichloropico-linic acid (picloram) were used in this study.

Equilibrium StudyA known amount of finely ground HA (20 mg)was weigheci into several glass-stoppered centri-fuge tubes and shaken for 48 h with a knownvolume of aqueous herbicide sohltion (20 ml)of varied concentration. The experiments wereconducted at 5 and 25 C. The pH of the sus-p:nsions was in the range of 3.3-3.6. HA wasremoved frorn the suspension by centrifugationat the appropriate temperature. The supernatantsolution was analyzed for the herbicide con-centration. 'fhe amount of the herbicide ad-sorbed by HA was determined by subtractingthe concentration remaining in solution afterequilibrium from the initial concentration.

Kinetic StudyFinely ground HA was weighed into glass-stoppered centrifuge tubes and a known quan-tity of the herbicide solution was added to eachtube. The samples were shaken continuouslyat 5 and 25 C. At appropriate time intervalsa tube was removed, centrifuged immediatelyfor a very short period of time, and the super-natant liquid analyzed for herbicide concentra-tion. During the short centrifugation period thetemperature variation was minimal.

AnalysisThe concentration of 2,4-D or picloram in theabove experiments was determined as follows.An afiquot of the solution was acidified to pH 1

with 6 N H,SO, and extracted several timeswith ether. After drying the ether extract overanhydrous Na,SO,, the volume of the solventin the flask was reduced to about 0.5 ml usinga rotary evaporator. The residue was taken upin a small volume of methanol and methylatedwith an ether solution of diazomethane, gen-erated from Diazald. A few drops of hexanewere added in the flask and the excess of diazo-methane removed by allowing it to evaporatejust to dryness in the fumehood. The resultingmaterial was taken up in hexane and an appro-priate aliquot of the solution injected into thegas chromatograph for quantitative analysis of

the herbicide. Esters of 2,4-D and piclorampresent in the samples were calcuiated by com-paring the sample peak heights (in the linearresponse region) with those of the appropriatestandards. Analysis of untreated blanks con-firmed the absence of interfering substances.

A Hewlett-Packard model 7610 A gas chro-matograph equipped with a Ni* electron cap-ture detector was used in this study. The glasscolumn (1,200 X 4 mm) was packed with 5o/o

ethyl acetate fractionated Dow Corning highvacLrum grease on Chromosorb W DCMS, 80-100 mesh. The carrier gas was 95/o argon and5ok methane mixture, which was also used forpurging the detector.

RESULTS AND DISCUSSIONEquilibrium StudyAn analysis of the data established the factthat they can be best represented in terms ofthe empirical Freundlich adsorption iso-therm. The Freundlich equation can be ex-pressed as (Glasstone and Lewis 1960):

X : KC" (1)

where X is the amount of adsorbate takenup per unit mass of the adsorbent; C is theequilibrium concentration in solution; n andK are constants representing the slope andthe intercept of the isotherm, respectively.In the present study, the data obtained gavereasonably good straight lines by plottinglog X against log C. A typical Freundlichplot for adsorption of 2,4-D on humic acidat 5 C is shown in Fig. 1. The values of nand log K (C - 1 ppm) were estimatedfrom the Freundlich plots by using themethod of least-square fit (T'able 1).

The constants ,K and n may provide roughestimates of the adsorbent capacity and theintensity of adsorption, respectively (Adam-son 1967). For both 2,4-D and picloram,the value of slope n decreased with decreasein temperature (Table 1). This is in accord-ance with the Freundlich-tvpe isotherm

'Iable 1. Freundlich isotherm constants for theadsorption of 2,4-D and picloram on hun.ric acid

Slope z Intercept log K

Herbicide 5C 25C 5C 25C

2,4-D 0.748 0.789Picloram 0.877 0.912

1.034 0. 8691.132 1.046

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KHAN-ADSORPTION STUDIES OF 2.4-D AND PICLORAM 43r

oooE

_l

ol

-t 0 I

log C (ppm)

Fig. 1. Freundlich plot for the adsorption of2,4-D on humic acid at 5 C.

(Hayward and Trapnell 1964) and indi-cates that the intensity of the herbicide ad-sorption was lower at 5 C than that at 25C. At both temperatures the value of nwas less than unity, indicating a convex orZ-type isotherm. This kind of isotherm mayarise due to minimum competition of solventfor sites on the adsorbing surface (Hayw,ardand Trapnell 1964). However, it is possiblethat in the adsorption process a few layersof water molecules may be always presentbetween the herbicide and humic acid sur-face. Examination of I( values in Table 1

shows that the adsorptive capacity of humicacid for picloram was slightly greater thanfor 2,4-D at both temp,eratures. An increasein temperature resulted in a decrease of Kvalues for both herbicides, thereby indicat-ing the exothermic nature of adsorption(Kipling 1965). Ya,mane and Green (1972)obtained similar results in a study of s-tri-azine herbicides adsorption on soil material.However, they attributed the apparent exo-thermic nature of adsorption of triazines tothe temperature dependence of the herbicide-water interaction.

Kinetic StudyAn examination of the data revealed thatthere was an initial rapid rate of adsorptionof 2,4-D and picloram on humic acid at bothtemperatures. This was followed by slower

rates at longer times. Weber and Gould( 1966) studied adsorption of 2,4-D and sev-eral other organic pesticides from diluteaqueous solution by porous activated char-coal and have suggested a mechanism in-volving intraparticle transport of the solutein the pores and capillaries of the adsorbent.For such systems, the amount of solute ad-sorbed from solution is directly proportionalto the square root of the time elapsed (Crank1965). In accordance with this the amountof herbicide adsorbed X was plotted as a

function of the square root of time t. A typicalcurve for 2,4-D at 5 C is shown in Fig. 2.In each instance the linearity in the plotswas usually attained after about 1 or 1.5 h.Thus it appears that, at longer times, intra-particle transport was the dominant rate-limiting process in the adsorption of 2,4-Dand picloram on humic acid. These findingsfit in well with the surface geometry andstructural concepts of humic substances. Inthe solid state the humic acid is consideredto have a laminated, textured makeup (Orlovand Glebova 1972). It has been postulatedthat the structure of humic substances isloose or open (Kodama and Schnitzer 1.967)and contains voids or holes of different mo-lecular dimensions (Schnitzer and Khan1972). lt follows, therefore, that at longertimes, adsorption is governed by the diffusionof the herbicide molecules from the exteriorsurface to the interior of the pores of humicac!d structure.

As linearity of the data was not observedfor times less than about 1.5 h, the rate-limiting adsorption mechanism, during thisinitial period, was considered to be differentthan predicted by intraparticle transport.Therefore, the kinetic data for this initialperiod was examined in the light of the gen-eralized equilibrium theory proposed by Favaand Eyring ( 1956). Accordingly, the follow-ing equation takes into account both adsorp-tion and desorption in obtaining a rate equa-tion for adsorption of a solute from solution(Fava and Eyring 1956; Haque and Sexton1 968 ).

# : ,"'(1 - d) sinh {b (1 - d)\ : y, (2)

where { is the fraction adsorbed (amountadsorbed at time r divided by the amount

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432 CANADIAN JOURNAL OF SOIL SCIENCE

2468to'5 (t',our.)o'5

Fig. 2, Adsorption rate curve of 2,4-D on humic acid at 5 C as a function of the square root oftime.

o)E6)

tr

x

adsorbed at equilibrium); K/ is the rate con-stant for adsorption; and D is a constant thatyields a measure of the surface stressingenergy due to loading with molecules. Plotsof 4 vs. / were obtained. An example for theadsorption of 2,4-D on humic acid at 5 Cis shown in Fig. 3. From these plots thedifferential dq/ dt for various values of 4were obtained. A computer program wasused to estimate the rate constant I( for aseries of substituted 6 values so that f (K',b)is minimized, where

f (x"tt) :

,\rv, - zK' 0 - d) Sinh [a (r - d) ]1,. (3)

The rate constants K' thus estimated for the

adsorption of 2,4-D and picloram on humicacid at two different temperatures are shownin Table 2.

The activation energy AE was calculatedfrom the Arrhenius equation (Castellan1964):

Ly' : sp- nr, (4)

where K' is the rate constant, 7 the absolutetemperature, R the gas constant and .E theactivation energy for the process. ,4 is theconstant related to the frequency of col-lision. Converting equation (3) to logarith-mic form, we have

' -,|ln l! : lll -11 - (s)E

l{I

Table 2. Kinetic parameters for the adsorption of 2,4-D and piclorarn on hurnic acid

HerbicideTemperature,

C

RateconstantK, sec-l

Energy ofactivation

AEK cal mole I

Heat of Entropyactivation activation

AHf ASfK cal mole-l e.u.

2,4-D

Picloram

15 x 10-553 x 10-513 x 10-513 x 10-5

I. YJ

1.81

1. 36

1.22

-118.9

- 118.5

5

68

10

25

25

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For two rates, K/, and K'r, correspondingto absolute temperatures 7' and 7", theequation for the activation energy based onthe rate constants may be written as:

433

of adsorption. The transference rate wouldbe dependent upon diffusion of the herbicidemolecules across the water film surroundingthe humic acid particles and on the shakingrate of the suspension. The attractive forcesin physical adsorp,tion process involve ac-tivation energies that are less than a fewKcal,/mol (Adamson 1967). The values ofthe activation energy AE reported in Table 2are small, which further suggests physicaladsorption, and characterize diffusion-con-trolled processes. The low magnitude of heat

of activation AIlt ruled out the possibilityof chemisorption (Hayward and Trapnell1964) and suggests that the most probablenature of the adsorption is physical. Thelarge negative values for entropy of activa-

tion ASI (Table 2) indicate a lesser degreeof freedom for 2,4-D and picloram in thetransition state.

CONCLUSIONSThe adsorption of 2,4-D and picloram onhumic acid followed the Freundlich-type iso-therm. The adsorption at the initial times ap-pears to be a diffusion of the herbicide mole-cules to the surface of humic acid. How-ever, at longer times the adsorption rate be-comes slow as it is controlled by the intra-particle diffusion of the herbicide moleculesinto the interior of the humic acid particles.

sruDrEs on 2,4-o AND PTcLoRAM

t (minutesl

Fig. 3. Rate of approach to equilibrium of 2,4-D on humic acid at 5 C.

6030

, A2

K't -*(; ;) (6,

Equation (6) was used to estimate he ac-tivation energy AE for 2,4-D and picloramadsorption on humic acid.

The heat of activation A11* is related tothe activation energy AE as:

6p: 6p+ 4 nr. (7)

Applying the absolute reaction rate theorywe can calculate the entropy of activation

ASI from the following relationship (Cas-tellan 1964):

.., /er\ / tr+, /rrr\^

: l, texpl -- l"*p(:}), fs.l\h/ \ 1(r/ \K/

where ft is the Boltzman constant and ft thePlank's constant.

A summary of the rate parameters calcu-lated for the adsorption of 2,4-D and pi-cloram on humic acid is given in Table 2.The values of the rate constant K' are of theorder that indicates that the initial rate wascontrolled by the herbicide movement to thehumic acid surface involving a physical type

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434 CANADIAN JOURNAL OF SOIL SCIENCE

The relatively fast rate of adsorption, lowvalues of activation energy, and heat of ac-tivation suggest the physical type of adsorp-tion possibly involving van der Waals forcesand hydrophobic bonding between the herbi-cide molecules and humic acid surface in anaqueous system.

ACKNOU/LEDGITENTSThe technical assistance of R. Mazurkewich ismuch appreciated. I express my thanks to L. P.Lefkovitch of Statistical Research Service, Ot-tawa, for his valuable help in computing work"

LITERATURE CITEDADAMSON, A. W. 1967. Physical chemistryof surfaces. Academic Press, Inc., New York,N.Y. 402 pp.CASTELLAN, G. W. 1964. Physical chemistry.Addison-Wesley Publishing Co. Inc., London.pp. 607-643.CRANK, J. 1965. The mathematics of dif-fusion. Clarendon Press, London. 147 pp.DUNIGAN, E. P. and McINTOSH, T. H.7971. Atrazine soil organic matter interaction.Weed Sci. 19:279-282.FAVA, A. and EYRING, H. 1956. Equilibriumand kinetic of detergent adsorption - a gen-eralized equilibrium theory. J. Phys. Chem. 60:890-898.GILMOUR, J. T. and COLEMAN, N. T. 1971.s-Triazine adsorption studies: CA-H humic acid.Soil Sci. Soc. Amer. Proc. 35: 256-259.GLASSTONE, S. and LEWIS, D. 1960. Ele-ments of physical chemistry. D. Van NostrandCompany, Inc., New York, N.y. pp. 56j.HANCE, R. J. 1965. Observation on the rela-tionship between the adsorption of diuron andthe nature of the adsorbent. Weed Res. 5: 108-114.HANCE, R. I. 1969. The adsorption of linuron,alrazine and EPTC by model aliphatic adsorb-ents and soil organic preparations. Weed Res.9: 108-1 13.HAQUE, R. and SEXTON, R. 1968. Kineticand equilibrium study of the adsorption of 2,4-dichlorophenoxyacetic acid on some surfaces. J.Colloid Interface Sci. 27: 818-827.HAYES, M. H. B. 1970. Adsorption of triazineherbicides on soil organic matter, including ashort review on soil organic matter chemistry.Residue Rev. 32: 131-174.

HAYES, M. H. 8., STACEY, M. and THOMP-SON, J. M. 1968. Adsorption of atrazine herbi-cides by soil organic matter preparations. Pages75-90 in Isotopes and radiation in soil organicmatter studies. Int. Atomic Energy Agency,Vienna. Austria.HAYWARD, D. O. and TRAPNELL, B. M.W. 1964. Chemisorption. Butterworths, London.91 pp.KHAN, S. U. 1971. Distribution and charac-teristics of organic matter extracted from theblack solonetzic and black chernozemic soils ofAlberta. The humic acid fraction. Soil Sci. 112:401-409.KHAN, S. U. 1972. Adsorption of pesticide byhumic substances: a review. Environ. Lett. 3:1 1a

KIPLING, J. J. 1965. Adsorption from solu-tions of non-electrolytes. Academic Press, Inc.,New York, N.Y. 129 pp.KODAMA, H. and SCHNITZER, M. 1967. X-ray studies of fulvic acid, a soil humic com-pound. Fuel 47:87-94.LI, G. and FELBECK, G. T. Ir. 1972. A studyof the mechanism of atrazine adsorption byhumic acid from muck soil. Soil Sci. 113: 14G-148.ORLOV, D. S. and GLEBOVA, G. I. 1972.Electron-microscopic investigation of humicacids. Sov. Soil Sci. (Engl. Transl. Pochvoved-enie) 4: 445-452.SCHNITZER, M. and KHAN, S. U. 1972. Hu-mic substances in the environment. MarcelDekker, Inc., New York. pp. 137-2A1.STEVENSON, F. I. 1972. Organic matter re-actions involving herbicides in soil. J. Environ.Qual. 1: 333-343.SULLIVAN, J. D. and FELBECK, G. T. Jr.1968. A study of the interaction of atrazineherbicides with humic acids from three differentsoils. Soil Sc|. 106: 42-52.WEBER, W. J. Jr. and COULD, J. P. 1966.Sorption of organic pesticides from aqueoussolutions. In R. F. Gould, ed. Organic pesticidesin the environment. Advan. Chem. Ser. 60:280-304.WEBER, J. 8., WEED, S. B. and WARD, T.M. 1969. Adsorption of atrazine by soil organicmatter. Weed Sci. l7: 417-421.YAMANE, V. K. and GREEN, R. E. 1972.Adsorption of ametryne and atrazine on anoxisol, montmorillonite, and charcoal in rela-tion to pH and solubility effects. Soil Sci. Soc.Amer. Proc. 36: 58-64.

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