Adsorption of different polymers on kaolinite and their effect on flumequine adsorption

Applied Clay Science, 6 (1992) 343-357

343

Elsevier Science Publishers B.V., Amsterdam

Adsorption of different polymers on kaolinite and their effect on flumequine adsorption R.K. Khandal', C. Chenu, I. Lamy and M. Terc6 Station de Science du Sol, INRA, Route de Saint-Cyr, 78026 Versailles, France (Received July 2, 1991; accepted after revision December 19, 1991 )

ABSTRACT Khandal, R.K., Chenu, C., Lamy, I. and Terc6, M., 1992. Adsorption of different polymers on kaolinite and their effect on flumequine adsorption. Appl. Clay Sci., 6: 343-357. The mechanism by which seven different polymers (ionic and non-ionic) influence the adsorption of a weak acid bactericide (flumequine) on kaolinite was investigated, and the amount of flumequine adsorbed in the presence of increasing concentrations of polymers was determined. Adsorption of polymers on clay and flocculation of clay by polymers were studied in order to understand claypolymer interactions. Electrophoretic mobility measurements and potentiometric titrations were performed on selected adsorbates to investigate changes on clay particle interfaces. The effect of increasing concentrations of polymers on flumequine adsorption could be described in three ways: no change, decrease, or decrease followed by increase. The differences were ascribed to competition for adsorption sites by anionic polymers, and blocking of sites by cationic and non-ionic polymers.

INTRODUCTION

The fate of applied toxic agrochemicals (pesticides, herbicides, etc. ) in the soil environment is determined by several factors among which adsorption phenomena are particularly important (Green, 1974). The availability, degradation, transport and residual effects of toxicants are processes directly affected by inter-particle interactions in the soil-water-pesticide system (Weed and Weber, 1974; Khan, 1980). The presence of a material able to modify the surface of an adsorbent may play an important role in controlling the interface interactions responsible for adsorption-desorption. In order to understand the behaviour of pollutants, all aspects pertaining to inter-particle interactions should be thoroughly examined (O'Melia, 1989). Soil organic matter has long been known for its role in altering the behavCorrespondence to: M. Terc6, INRA, Station de Science du Sol, Route de Saint-Cyr, 78026 Versailles, France. 'Present address: 1050, Sector 17 - Near Iffco Area, Office GURGAON 12200 l, India.

0169-1317/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.

344

R.K. KHANI)AL ET AI,

iour of agrochemicals. Generally, organic matter exists in the form of complex macromolecular structures and the following effects have been reported: (i) it renders the mineral surface hydrophobic, thereby facilitating the adsorption of non-polar chemicals (Murphy et al., 1990); (ii) it acts as a major carrier in the transport of highly insoluble pesticides, e.g. DDT (Ballard, 1971 ); (iii) it increases the solubility of certain hydrophobic toxicants (Chiou et al., 1986); and (iv) it reduces fixation of specifically adsorbing anions such as phosphate (Bear and Toth, 1942; Dalton et al., 1952; Parfitt, 1979). Besides the naturally occurring organic macromolecules such as polysaccharides, synthetic polymers and polysaccharides are also found in the soil environment. Whereas some are used as pesticide formulation adjuvants, others are applied as soil conditioners. The use of small quantities of synthetic polymers such as PVA (polyvinyl alcohol ) and guar products has been reported to bring about desirable soil properties such as reduced crust strength, or aggregate stability and improved water infiltration (Page and Quick, 1979; Ben-Hur and Letey, 1989). Polymeric surfactants have been found to improve soil wetting properties and thereby influence the behaviour of herbicides (Bayer, 1967). In brief, as Spurlock and Biggar (1990) have demonstrated, the presence of polymeric materials certainly have an affect on soilpesticide interactions. This paper describes research carried out on the effect of certain polymers (either naturally present or used as soil conditioners ) on flumequine adsorption on kaolinite clay. Flumequine is a bactericide, recently registered in France for agricultural use. Previous studies show that flumequine strongly adsorbs onto kaolinite and goethite (Dur et al., 1991; Khandal et al., 1991 ), and that the presence of polymers influences the adsorption of flumequine on kaolinite (Khandal et al., in press ). The present paper attempts to explain the mechanism by which various polymers influence the adsorption behaviour of this weakly acid molecule. MATERIALS AND METHODS

Materials Adsorbents: Kaolinite clay (Supreme, St. Austell, UK) of fine quality was used without further purification. The clay has a BET nitrogen surface area of 14 m 2 g- ~and a CEC of 12 meq kg- ~at pH 7. Adsorbate: Flumequine (Fig. 1 ) (3M/Riker Laboratory, France), both in 14C-labelled and unlabelled forms, of high purity (99.85%) Polymers: Polyvinyl alcohol (PVA) (Harlow Chemical Co. Ltd., UK) of MW 28,000, containing 12% acetate, and six different polysaccharides: HP-

ADSORPTION OF DIFFERENT POLYMERS ON KAOLINITE ~._

345

5

0

F ~ ~ / ~ II •

COOH Flumequine "~',..i 4c

Molecular pKa

weight

= 261.25

= 6.0

solubility

= 15 ~ g pH4.5

-3 cm

at

Fig. 1. The structure o f f l u m e q u i n e .

8, T-4246, CP-14, Scleroglucan, Xanthan and PGTA. HP-8 (non-ionic), T4246 (anionic) and CP-14 (cationic) are guar derivatives (Celanese Corp., Louisville, USA), i.e., polysaccharides consisting of mannose and galactose units upon which non-ionic cis hydroxyl groups are respectively substituted by hydroxypropyl, carboxyl and quaternary ammonium groups (Ben-Hur and Letey, 1989). Scleroglucan (CECA, France), a non-ionic glucose polymer of MW 1.5 × 106, is the fungal slime of Sclerotium glycanicum. Xanthan (Rhodopol Rh6ne Poulenc, France), an anionic polymer of glucose and mannose as a sodium salt, is a product of Xanthomonas campestris. The polygalacturonic acid (PGTA) (Sigma chemicals, USA), used as a sodium salt, is a product of de-esterified pectin. Scleroglucan, xanthan and the guar derivatives are exudates, or analogs of soil microorganism exudates, whereas PGTA is a major constituent of root mucilages. Xanthan is also used as a formulation adjuvant. The polymer solutions were prepared by dissolution in deionized water and filtered (1.2/~m). Except during titration experiments, sodium azide ( 5 mg dm- 3) was added to prevent microbial degradation.

Methods Adsorption of flumequine in the presence of polymers: A fixed weight (50 mg) of air-dried clay was equilibrated with 20 c m 3 aqueous solution containing 3.8× 10 -2 mmol d m - 3 flumequine and varying amounts of polymers preadjusted to pH 4.5. Each sample was prepared in duplicate. The quantity of flumequine adsorbed was determined by the depletion method as detailed in Khandal et al., 1991. Adsorption ofpolymers: 10 g kaolinite suspension ( 5 g kg- 1) and 10 g polymer solution of defined concentration, separately adjusted to pH 4.5 using 0.1 NHNO3, were mixed for 24 h at 20°C, and centrifuged at 20,000 rpm for 20 min. In order to determine the adsorption isotherm, the supernatant's carbon concentration was measured by dry combustion in a coulometric carbon analyser. Knowing the carbon content of the polymers, the equilibrium concentration in mg polymer kg- 1 solution was calculated. The amount of adsorbed polymer in mg g- l clay was then deduced. All samples were prepared

346

R.K. KHANDAL ET AL

in duplicate and measurements of carbon content were repeated four times. In a previous experiment, it was established that the centrifugation procedure did not cause the polymeric solutions to settle (results not shown). Flocculation of kaolinite by polymers: After mixing, the polymer-clay suspensions (2.5 g kg -~) were allowed to stand for 30 min in test tubes. The supernatant was then pipetted out and its clay concentration determined by measuring the optical density at 650 nm according to Kavanagh et al. ( 1978 ). All suspensions were prepared in duplicate, and measurements of optical density were repeated twice. Electrophoretic mobility measurements: These were carried out using a modified "laser Zee Meter" (Model 501 ) microelectrophoresis apparatus (PEN-KEM, USA) with a rotating prism, a TV monitor and a fiat cell made entirely of quartz, with two platinum electrodes. Measurements were carried out in the presence of 10 -3 M KNO3 with 25 mg dm -3 kaolinite. The effect of different adsorbates and pH on electrophoretic mobility of kaolinite was monitored. Kaolinite suspensions were equilibrated with different polymers ( 10 mg d m - 3) and flumequine ( 15 mg d m - 3) under the same conditions used for adsorption. Measurements of pH were made before and after each electrophoretic mobility measurement. Potentiometric titrations: 50 or 100 cm 3 0.1 M KNO3 solution containing organic material (flumequine: 15 mg dm-3; CP-14:1 g dm-3; or PVA: 1 g dm -3), were mixed with kaolinite (1.25 or 2.5 g). The suspensions were allowed to mix for two hours at 25 °C in an N2 atmosphere, they were titrated potentiometrically with 0.099 M KOH solution by incremental additions. Blank titrations were also performed in a 0.1 M KNO3 medium, without organic material for kaolinite, and without kaolinite for organic molecules. RESULTS

Flumequine adsorption in the presence of polymers The results of flumequine adsorption in the presence of increasing concentrations of different polymers at pH 4.5 are shown in Figs. 2 and 3. The following observations may be made: (i) All the polymers except T-4246 (anionic) and HP-8 (non-ionic) caused a decrease in flumequine adsorption. The presence of PGTA (anionic) resuited in an initial decrease in adsorption but this plateaued off at higher polymer concentrations. Xanthan (anionic) and CP- 14 (cationic) continued to affect adsorption even at higher concentrations. Among the polymers investigated, at the highest polymer concentration, CP-14 had the greatest effect. The presence of T-4246 brought no significant change in adsorption. (ii) Among the non-ionic polymers (Fig. 3 ), PVA showed the maximum effect, i.e., a decrease in adsorption at lower polymer concentrations, fol-

ADSORPTIONOFDI~TF~RENTPOLYMERSON KAOLINITE

l

347

T 4246 (ANIONIC)

1~ I t ' O 'q~

(ANIONIC)

•

XANTHAN

(ANIONIC)

PI 4 •

~

_-"

T

b.

(CATIONIC)

, 80

, 100

, 150

, 200

Polymer Concentration ( mg. dm -s)

Fig. 2. Flumequine adsorption on kaolinite as a function of ionic polymer concentration.

T ~"

pH 4.5

o

E

e

"U

.~CLEROGLUCAN

;7

~ H%

O

0 PVA

11.0

$ E

-'-

"r o

,

,

I

so

lOO

15o

i 200

Polymer Concentration ( mg.dm -3)

Fig. 3. Flumequine adsorption on kaolinite as a function of non-ionic polymer concentration.

lowed by a plateau. In the case of Scleroglucan, an initial decrease in flumequine adsorption was followed by an increase reaching a plateau at higher polymer concentrations.

Adsorption ofpolymers The results for polymer adsorption on clay at pH 4.5 (Figs. 4 and 5 ) show that, among the ionic and non-ionic polymers, respectively, CP-14 (cationic) and scleroglucan adsorbed with the strongest affinity. Scleroglucan and CP14 adsorbed with L-type isotherms (Figs. 4 and 5 ) which indicates absence of competition between polymers and water molecules for adsorption sites on clay whereas the adsorption isotherm of liP-8 may be more usefully described as of the constant partition type (Giles et al., 1960). All the other polymers

;]48

ILK. KHAN I)AL ET A l

80

P E

T f

60

~

-~CP14

o

E

40

•

X/',NmA,N

•

]'4246

1 •

o "o

+

0~ 0

200

400

600

800

pGTA

1000

Equilibrium concentration ( m g . k g l )

Fig. 4. Adsorption isotherms of ionic polymers on kaolinite at pH 4.5.

so 40 30

E g

20

[]

HP8

•

SCLEROGLUCAN

O PVA

10

"o

<

0~ 0

200

400

600

800

1000

Equilibrium concentration (mg.kg-1)

Fig. 5. Adsorption isotherms of non-ionic polymers on kaolinite at pH 4.5.

(non-ionic and ionic) adsorb according to an S-type isotherm (Giles et al., 1974). The stronger adsorption of CP- 14 (Fig. 4) was attributed to interactions between positively charged polymer molecules and negatively charged clay faces. Since CP-14 is a quaternary ammonium derivative of guar, adsorption is probably due to the quaternary ammonium group; similar results are reported for quaternary ammonium surfactant molecules (Smith and Bayer, 1967). The charge neutralisation mechanism is also reported for cationic guar derivative adsorption on montmorillonite clay (Aly and Letey, 1988 ). In all the other polymers, adsorption would seem to be due to forces such as Van der Waals and H-bonding.

ADSORPTION OF DI~'FERENT POLYMERS ON KAOLINITE

349

Electrophoretic mobility The results of electrophoretic mobility of clay alone at different pH values and of clay in the presence of three polymers and flumequine (Fig. 6) give rise to the following comments: (i) The negative values of electrophoretic mobility suggest that, in the absence of adsorbates, the clay particles were negatively charged. As the pH increased, electrophoretic mobility became more negative. (ii) The increase in electrophoretic mobility towards the negative side suggest that, in the presence of flumequine, xanthan and PGTA, the negative charge of clay particles increased, and hence: (iii) Since the electrophoretic mobility values of clay were on the positive side in the presence of CP-14, this indicates that the charge of clay particles had switched from negative to positive. This charge-reversal phenomenon was found to occur even in the presence of less than 5 mg dm-3 polymer (results not shown). The results of kaolinite electrophoretic mobility are in accordance with the concept that the particles have negatively-charged faces and positively charges edges. The increase in pH results in the decrease of the edges' positive charge

~CP 0 mg.dm'al 14 PGTA

l"

"

+

Kaolinite

+

L = Xanthan + -3 5mg.dm--~Flumecluine + ~Kaolinite alone

+5

~> +4 o ,

o

+3

~

+2

.0 O

E .~ h. 0

4

5

6

7

8

pH

-2

~"

o. o

-3

o@

-4

U.I

-5

F i g . 6. Electrophoretic mobility o f kaolinite in the presence o f polymers.

350

R,K. KI-IANDAL ET A I

and consequent increase in overall negative charge, as reported by Williams and Williams (1978). In the presence of flumequine, xanthan or PGTA, the increased negative charge supports the assumption that these adsorbates interact with the positive edges, making the system more negative. The phenomenon of charge reversal in the presence of CP-14 was attributed to adsorption of the cationic polymer. Since this polymer molecule is large, one molecule can interact with several negative surface sites. In addition to surface charge neutralisation, the clay particles may themselves become positive due to the charge of the polymer. Considering the chemical formula given for CP-14 in Ben-Hur and Letey (1989), the charge of the polymer was calculated to be 0.386 meq positive charges g-~ guar. Assuming the maximum adsorption of CP- 14 on kaolinite to be about 70 mg CP- 14 gclay (Fig. 4 ), the result is 0.27 meq positive charges g- ~kaolinite. This by far compensates and exceeds the negative charge of the kaolinite which only amounts to 0.12 meq g- l clay. Flocculation

The results of flocculation measurements in the presence of polymers are shown in Fig. 7. These measurements were carried out at pH 4.5 and the clay particles showed a tendency to flocculate even without a polymer being added. Slight stabilization was observed in the presence of all polymers except CP14. Stabilization may result from an increased overall charge of the clay particles (inter-particle repulsion ) in the case of anionic polymers, or from steric stabilization in non-ionic polymers. An increase in the medium's viscosity due to addition of polymers could also explain the settling of clay particles being hindered. In the case of CP- 14, transmittance decreased sharply, indicating enhanced aggregation of clay particles. CP- 14 is reported to cause flocculation of montmorillonite clay (Aly and Letey, 1988). Flocculation enhanced by cationic guar derivatives is also reported by Helalia and Letey (1988) and Aly and Letey ( 1990 ) for various soil constituents. Potentiometric titration

Potentiometric titration curves of clay, of clay in the presence of flumequine, CP-14 or PVA, and of these three polymers individually are shown in Fig. 8. The titration curve of kaolinite alone shows the day's weak acid behaviour. The source of kaolinite acidity is attributed to reactions with edge surfaces, due to either the silanol or aluminol groups (Hanna and Somasundaran, 1979; Nabzar et al., 1984). When kaolinite is in an acid medium, the positive charge

ADSORPTIONOFDIF~'~:RBNTPOLYMERSONKAOLINITE

351

pH 4.5

150

125

100~ "1o

o PVA • Xanthan • PGTA ~'HP

m o

o

•

8

75

Scleroglucan ~T 4246 •

II

A Cp 14

50

35 I

I

0 25 50

!

I

100

250 Polymer c o n c e n t r a t i o n ( mg.Kg " 1 )

I

500

Fig. 7. Flocculation of kaolinite as influenced by polymers. Turbidity of the supernatants is expressed as optical density at 650 nm, relative to that of the pure kaolinite.

is located on the edge faces; this results from protonation of the edge aluminol groups according to the generally accepted formula: A1-OH+ H30 + ~A1-OH~- + H 2 0 Consequently, as the pH increases, the dissociation of kaolinite edge functional groups decreases the number of positive sites, and, as demonstrated by microelectrophoresis, the overall negative charge increases. In order to evaluate the acidity due to adsorbates, in this case flumequine since it exhibits weak acid behaviour, experimental curves of kaolinite+adsorbate may be compared with the synthesized curves drawn from the algebraic sum of the corresponding blanks.

~]52

iLK. KHANDAI,

lo/./ ~

/ -/

ET AL

/~

I01

i/ i'

81

//

// //

i;~

®

0.5

1

0.5

1

ml KOH

Fig. 8. Potentiometric titration curves of kaolinite alone (a), adsorbate alone (b), and their complexes (c). Synthesized curve (d) is the algebraic sum of curves (a) and (b). The adsorbates are (1) CP-14; (2) PVA; (3) flumequine.

(i) The titration curve of kaolinite + CP- 14 was always above that of kaolinite alone, until pH 10 was reached, where they overlapped. This indicates that H + ions disappeared from the solution between pH 4 and 10. Here, the effect of CP-14 adsorption on kaolinite was seen as a blocking of the clay's acid sites. (ii) For PVA, an initial decrease in H + concentration between pH 4 and 5 was followed by further release of H + at pH > 5. This effect may be considered as similar to that reported by Pefferkom et al. ( 1985, 1987) for the adsorption of polyacrylamide on kaolinite: in an acid medium, PVA (neutral) interacts with silanol groups and affects the dissociation of aluminol groups, whereas in a basic medium, it is the aluminol groups which are involved directly in PVA adsorption.

ADSORPTIONOF DIFFERENT POLYMERSON KAOLINITE

353

(iii) The titration curve for kaolinite + flumequine was always below that of kaolinite alone. However, the experimental curve was not consistent with the synthesized curve. Below pH 4.8, further liberation of H ÷ was observed. This behaviour is in accordance with the proposed mechanism of flumequine fixation on the edges of kaolinite (Khandal et al., 1991 ) which suggests that the anionic form of flumequine is formed prior to its adsorption, and H ÷ is liberated earlier than with flumequine alone. For pH > 4.8, following the addition of the same amount of O H - as in the synthetic curve, the experimental curve of kaolinite + flumequine gives a higher pH value. This indicates the blocking of the acid sites of clay due to interaction with flumequine. DISCUSSION

In this study, the adsorption of flumequine on the edges of clay particles was supported by the results of electrophoretic mobility and potentiometric titration, as previously suggested by Khandal et al. ( 1991 ). Thus, any process which affects the edges of clay may also be expected to show an effect on the adsorption behaviour of flumequine.

Interactions with anionic polymers The results of electrophoretic mobility indicated that anionic polymers such as PGTA and xanthan also interact with the clay edges. These interactions would result in competition with flumequine for adsorption sites. Absence of flocculation also backs up the idea that the anionic form of polymers are adsorbed at clay edges. All three anionic polymers caused stabilisation of the clay suspension by occupying the same adsorption sites as flumequine. Competitive effects such as this are also recognised for other naturally occuring acids (Sibanda and Young, 1986 ). The competition mechanism proposed is that affinity to the sorbent is greater in the polymer than the pesticide. Thus, that T-4246 had no effect on flumequine adsorption may be due to its relatively weaker interaction with clay edges. The weakness of this polymer's interactions with montmoriUonite is also reported by Aly and Letey (1988).

Interactions with cationic polymers The mechanism for the changes observed in the case of CP- 14 may be due to two phenomena, the first being a blocking of sites. The charge-reversal phenomenon observed during electrophoretic mobility, the disappearance of H + from the solution during titration, and charge neutralisation during adsorption and flocculation, suggest that the quaternary ammonium group of CP-14 interacts with negatively charged clay faces. Flocculation would render the edges of kaolinite physically inaccessible to flumequine. Since flumequine ad-

354

R.K. KHANDAL ET AI,

sorption decreased continuously as the concentration of CP- 14 increased, it may be hypothesized that little flumequine adsorption occurs in the clay-CP14 complex. A second possible mechanism is the direct interaction of CP-14 and flumequine in solution.

Interactions with non-ionic polymers For a system like flumequine-kaolinitewhere adsorption is due to the strong interactions between two differently charged species (Khandal et al., 1991 ), few changes would be expected with non-ionic polymers. In this respect, the absence of significant effect with HP-8 seems logical since there would be no competition between polymer and flumequine for adsorption sites. However, the other non-ionic polymers, i.e., PVA and scleroglucan, were found to influence the adsorption behaviour of flumequine. The effect exhibited by PVA may be attributed to its long tails trailing in the suspending medium while the trains of different segments remain in contact with the clay surface. These tails modify the interface between clay surface and flumequine; the role of the tails and loops in the adsorption properties of PVA has been well demonstrated by Burchill and Hayes (1980) and Tadros ( 1982 ). At higher concentrations of PVA, flumequine adsorption attains a plateau. In the case of scleroglucan, the decrease in flumequine adsorption at low scleroglucan concentrations may be due to a blocking of sites through flocculation, as hypothesized for CP-14 (Fig. 7). Enhanced flumequine adsorption at higher scleroglucan concentrations may be interpreted as flumequine adsorption onto the polymer located at the clay surface. Based on the results of electrophoretic mobility and titration, it was assumed that the behaviour of clay according to pH may be described as the progressive dissociation of edge surface groups (cf. Pefferkorn et al., 1985; 1987). Since the measurements in the present study were carried out in an acid medium, the interactions between clay and the anionic species such as flumequine, xanthan, PGTA and T-4246 may occur as follows: OH Si OH

f ~

OH~ --

A_

ADSORPTION OF D~'FERENT POLYMERS ON KAOLINITE

355

As concerns PVA in acid medium, it was assumed that it interacts with silanol OH and affects flumequine adsorption as a result of its long trailing tails: OH - - -PVA

siJ OH

/

~OHf

This proposal would seem to be borne up by the changes observed during pH metric titration of PVA. A similar situation may also occur in the other two non-ionic polymers. CONCLUSION

The presence of polymers in the system modified the bactericide adsorption behaviour on kaolinite due to polymer adsorption on the clay changing the adsorbent surface properties. Several mechanisms occurred according to the respective charge of polymers and pesticide. With an anionic pesticide such as flumequine, competition for adsorption sites occurred with anionic polymers, and a physical blocking of sites seemed to occur with cationic and non-ionic polymers. Previous studies on the effect of organic matter on pesticide adsorption have dealt with very complex and highly aromatic organic, i.e. humic, substances. An increase in pesticide adsorption was generally found and this was related to the hydrophobic nature of humic substances which may facilitate retention of poorly water-soluble pesticides. However, a significant proportion of soil organic matter consists of transient hydrophilic macromolecules such as polysaccharides. In the present study, such polymers were also demonstrated to affect pesticide adsorption but the general effect observed here was to decrease adsorption. Hence, natural organic constituents may increase, as well as decrease, the retention of applied organic pollutants in soil, and the specific role of these various soil organic polymers on the behaviour of agrochemicals must be assessed thoroughly in order to gain a better understanding of the fate of applied agrochemicals in soils.

7156

R.K KHANI)AI, ET AL

,ACKNOWLEDGEMENTS T h e a u t h o r s wish to t h a n k A.M. T a b a r e a u , M. P e r r i e r a n d M. P h a n for t e c h n i c a l assistance. R . K . K h a n d a l w o u l d like to t h a n k I . N . R . A . for financial s u p p o r t d u r i n g his stay in Versailles.

REFERENCES Aly, S.M. and Letey, J., 1988. Polymer and water quality effects on flocculation of montmoriilonite. Soil Sci. Soc. Am. J., 52: 1453-1458. Aly, S.M. and Letey, J., 1990. Physical properties of sodium-treated soil as affected by two polymers. Soil Sci. Soc. Am. J., 54: 501-504. Bailard, T.M., 1971. Role of humic carrier substances in DDT movement through forest soil. Soil Sci. Soc. Am. Proc., 35: 145-147. Bayer, D.E., 1967. Effect of surfactants on leaching of substituted urea herbicides in soil. Weeds, 15: 249-252. Bear, F.E. and Toth, S.J., 1942. Phosphate fixation in soil and its practical control. Ind. Eng. Chem., 34: 49-52. Ben-Hur, M. and Letey, J., 1989. Effect of polysaccharides, clay dispersion and impact energy on water infiltration. Soil Sci. Soc. Am. J., 53: 233-238. Burchill, S. and Hayes, M.H.B., 1980. Adsorption of poly (vinyl alcohol) by clay minerals. In: A. Benin and U. Kafkafi (Editors), Agrochemicals in Soils. Israel Soil Science Society, pp. 109-121. Chiou, C.T., Malcolm, R.L., Briton, T.1. and Kile, D.E., 1986. Water solubility enhancement of some organic pollutants and pesticides by dissolved humic and fulvic acids. Environ. Sci. Technol., 20: 502-508. Dalton, J.D., Russell, G.C. and Sieling, D.H., 1952. Effect of organic matter on phosphate availability. Soil Sci., 73:173-181. Dur, J.C., Khandal, R.K. and Terc6, M., 1991. Adsorption of two weak acids on goethite. In: M. Mansour (Editor), Proc. 3rd workshop on Study and Prediction of Pesticides Behaviour in Soils, Plants and Aquatic Systems, Munich, May 30-June 1, 1990. GSF, Munich. Giles, C.H., MacEwan, T.H., Nakhwa, S.N. and Smith, D., 1960. A system of classification of solution adsorption isotherms and its use in diagnosis of adsorption mechanisms and in measurement of specific areas of solids. J. Chem. Soc., III: 3973-3993. Giles, C.H., D'Silva, A.P. and Easton, 1.A., 1974. A general treatment and classification of the solute adsorption isotherm, Part II. Experimental interpretation. J. Colloid Interface Sci., 47: 766-778. Green, R.E., 1974. Pesticide-clay-water interactions. In: W.D. Guenzi (Editor), Pesticides in Soil and Water. Soil Sci. Soc. Am. Inc. Publ., New York, N.Y., pp. 3-37. Hanna, H.S. and Somasundaran, P., 1979. Equilibration of kaolinite in aqueous inorganic and surfactant solutions. J. Colloid Interface Sci., 70:181-190. Helalia, A. and Letey, J., 1988. Effects of different polymers on seedling emergence, aggregate stability and crust hardness. Soil Sci., 148:199-203. Kavanagh, B.V., Posner, A.M. and Quirk, J.P., 1978. Adsorption of PVA on soil colloids. In: W.W. Emerson, R.D. Bond and A.R. Dexter (Editors), Modification of Soil Structure. Wiley, Chichester, pp. 165-174. Khan, S.U., 1980. Pesticides in Soil Environment. Elsevier, New York, N.Y., pp. 29-118. Khandal, R.K., Dur, J.C. and Terc6, M., 1991. Effect of polymers on adsorption of flumequine

ADSORPTION OF DIFFI~RENT P O L Y M E R S O N KAOLINITE

357

on kaolinite. In: M. Mansour (Editor), Proc. 3rd workshop on Study and Prediction of Pesticide Behaviour in Soils, Plants and Aquatic Systems, Munich, May 30-June l, 1990. GSF, Munich, in press. Khandal, R.K., Dur, J.C. and Tercd, M., 1991. Adsorption characteristics of flumequine on kaolinitic clay. Geoderma, 50: 95-107. Murphy, E.M., Zachara, J.M. and Smith, S.C., 1990. Influence of mineral-bound humic substances on the sorption of hydrophobic organic compounds. Environ. Sci. Technol., 24: 15071516. Nabzar, L., Pefferkorn, E. and Varoqui, R., 1984. Polyacryl amide sodium kaolinite interactions: flocculation behaviour of polymer clay suspensions. J. Colloid Interface Sci., 102: 380388. O'Melia, C.R., 1989. Particle-particle interactions in aquatic systems. Colloids Surf., 39: 255271. Page, E.R. and Quick, M.J., 1979. A comparison of the effectiveness of organic polymers as soil anti-crusting agents. J. Sci. Food Agric., 30:112-118. Parfitt, R.L., 1979. The availability of P from phosphate-goethite bridging complexes. Desorption and uptake by Ryegrass. Plant Soil, 73: 55-65. Pefferkorn, E., Nabzar, L. and Carroy, A., 1985. Adsorption ofpolyacrylamide to Na Kaolinite: correlation between clay structure and surface properties. J. Colloid Interface Sci., 106: 94103. Pefferkorn, E., Nabzar, L. and Varoqui, R., 1987. Polyacrylamide XXX (See above: Na Kaolinite) Na-kaolinite interactions; Effect of electrolyte concentration on polymer adsorption. Colloid Polym. Sci., 265: 889-896. Sibanda, H.M. and Young, S.D., 1986. Competitive adsorption of humus acids and phosphate on goethite, gibbsite and two tropical soils. J. Soil. Sci., 37: 197-204. Smith, L.W. and Bayer, D.E., 1967. Soil adsorption of diuron as influenced by surfactants. Soil Sci., 103: 328-330. Spurlock, F.C. and Biggar, J.W., 1990. Effect of naturally occuring soluble organic matter on the adsorption and movement of simazine [2-chloro-4,6-bis (ethylamino)-s-triazine] in Hanford sandy loam. Environ. Sci. Technol., 24:736-741. Tadros, Th. F., 1982. Polymer adsorption and dispersion stability. In: Th.F. Tadros (Editor), The Effect of Polymers on Dispersion Properties. Academic Press, New York, N.Y., pp. 138. Weed, S.B. and Weber, J.B., 1974. Pesticide-organic matter interactions. In: W.D. Guenzi (Editor), Pesticides in Soil and Water. Soil Sci. Soc. Am. Inc. Publ., New York, N.Y., pp. 3966. Williams, D.J.A. and Williams, K.P., 1978. Electrophoresis and zeta potential of kaolinite. J. Colloid Interface Sci., 65: 79-87.