polymer effects on water infiltration and soil aggregation

6
Polymer Effects on Water Infiltration and Soil Aggregation M. Ben-Hur* and R. Keren ABSTRACT The effectiveness of synthetic polymers as soil conditioners was found to depend on polymer properties. We hypothesized that the polymer penetration and movement into the aggregate are important factors in reduction of surface sealing. The effect of nonionic (P-101), cationic (CP-14), and anionic [Complete Green (CG)] commercial polymers on infiltration rate (IR) and aggregate formation was studied in a sandy loam (Typic Rhodoxeralf). Amounts of 25, 50, and 75 kg ha~' of each polymer were spread across the soil surface. After air drying, the surface was subjected to 68 mm of distilled water with impact energy of 18.1 J mm~' m~ 2 using a rainfall simulator. Soil particle association was studied in 10% (w/v) soil suspension with polymer concentration ranging from O to 50 g m" 3 . Viscosity values 3.3 109, and 165 mPa s were determined in 5 g L~' of P-101, CP-14, and CG solutions at shear rate of 21 s' 1 , respectively. The final IR values were 63 to 30 mm h-' at P-101, 18 to 24 mm h^ 1 at CP-14, and 18 to 30 mm h ~' at CG, compared with 8 mm h ~' in untreated soil. The critical time values in suspensions with different concentrations of polymer were 4.3 to 2.7 min for P-101, 1.0 to 3.4 min for CP-14, and 1.0 min for CG. The critical time indicates the size of the aggregate formed in a suspension; the lower the critical time, the larger the aggregates. It was suggested that because the particle surfaces are exposed to polymer molecules in suspension, the large molecule of CG could tie more suspended particles to form aggregates. The greater effectiveness of P-101 in preventing sealing was probably due to its ability to penetrate into aggregates, because of its small molecular size and low viscosity in solution. T ow WATER INFILTRATION, soil erosion, and inefficient I -> water use negatively influence plant growth and survival in arid and semiarid regions (Oster and Singer, 1984). The combined effect of raindrop impact energy and dispersion of clay particles at the soil surface (Agassi et al., 1981) causes seal formation and reduces IR (Morin and Benyamini, 1977). Synthetic polymers effectively increase final IR and reduce runoff and erosion on soils subjected to raindrop impact (Agassi and Ben-Hur, 1992; Ben-Hur et al., 1989; Helalia and Letey, 1988b; Shaviv et al., 1986). The effectiveness of a polymer as a soil conditioner was found to depend on polymer properties such as molecular weight and electrical charge (Ben-Hur and Letey, 1989; Shaviv et al., 1986). Cationic polysaccharide signifi- cantly increased IR when applied at a concentration of 10 g m~ 3 in sprinkled irrigation water. Cationic polymer effectiveness increased with charge density (Ben-Hur and Letey, 1989). Nonionic and anionic polysaccharide had no effect on IR, but a relative low molecular weight 70000-150000 daltons) anionic lignosulfonate at an ap- plication rate of 80 kg ha" 1 was effective (Shaviv et al., 1986). Shainberg et al. (1990) found that surface application of 20 kg ha" 1 of a high molecular weight Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel. Contribution from the Agricultural Research Organi- zation, The Volcani Center, Bet Dagan, Israel. Received 25 Sept. 1995. "Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 61:565-570 (1997). (10-15 million daltons) anionic polyacrylamide was sufficient to maintain a high IR. A soil conditioner's effectiveness is often related to its ability to promote flocculation (Aly and Letey, 1988; Helalia and Letey, 1988a). Polymers induce flocculation (or coagulation) of dispersed clay particles by (i) electro- static absorption of polymer molecules on the clay parti- cles, which helps to compensate the clay surface charge (Black et al., 1966), and (ii) bridging soil particles to- gether (Roberts et al, 1974). These processes are influ- enced by polymer properties and the nature of the poly- mer-particle bonding mechanism. Cationic polymers can compensate the negative elec- trostatic charge on clay particles and therefore can be used as coagulants (Black et al., 1966). Because of their rapid absorption capability and the high affinity to the clay (Ueda and Harada, 1968), only limited interparticle bridging can be achieved. Anionic polymers are effective flocculants, especially in the presence of polyvalent cations (Roberts et al., 1974; Gu and Doner, 1993). For these polymers, only a few segments of the polymer chain are involved in adsorption, while the other segments are present in the form of long loops and tails in solution. Thus, an anionic polymer has a relatively long grappling distance that facilitates the formation of interparticle bridges (Theng, 1982). Compared with charged polymers, nonionic forms are generally less effective as flocculation agents because they exist as randomly coiled units rather than as extended chains (Vincent, 1974). In a dilute colloidal suspension, the particles are sepa- rated at relatively large distances and the most particle surfaces are accessible to polymer molecule absorption. However, accessibility of the soil particles to polymer molecules is limited in soil aggregates, due to proximity of the soil particles. Malik and Letey (1991) observed that relatively large polymer molecules with molecular weights ranging from 0.2 to 15 million daltons did not penetrate into aggregates of three California soils. Ben-Hur et al. (1989) hypothesized that penetration and movement of the polymer molecules into and through intra- or inter-aggregate pores are important factors de- termining the aggregate stabilizing performance of soil conditioners. This hypothesis was confirmed by Malik and Letey (1991) when they studied the effect of soil particles size on adsorption of polyacrylamide and poly- saccharide. Size, configuration of die polymer molecules, and polymer solution viscosity and surface tension should have an effect on polymer penetration into soil aggre- gates. The objective of this study was to test the hypothesis that polymer penetration and movement into aggregates are important in reducing surface seal. We test this hypothesis by comparing the effectiveness of different Abbreviations: IR, infiltration rate; CG, Complete Green; AV, aggrega- tion value; MAV, maximum aggregation value. 565

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Page 1: Polymer Effects on Water Infiltration and Soil Aggregation

Polymer Effects on Water Infiltration and Soil AggregationM. Ben-Hur* and R. Keren

ABSTRACTThe effectiveness of synthetic polymers as soil conditioners was

found to depend on polymer properties. We hypothesized that thepolymer penetration and movement into the aggregate are importantfactors in reduction of surface sealing. The effect of nonionic (P-101),cationic (CP-14), and anionic [Complete Green (CG)] commercialpolymers on infiltration rate (IR) and aggregate formation was studiedin a sandy loam (Typic Rhodoxeralf). Amounts of 25, 50, and 75 kgha~' of each polymer were spread across the soil surface. After airdrying, the surface was subjected to 68 mm of distilled water withimpact energy of 18.1 J mm~' m~ 2 using a rainfall simulator. Soilparticle association was studied in 10% (w/v) soil suspension withpolymer concentration ranging from O to 50 g m"3. Viscosity values3.3 109, and 165 mPa s were determined in 5 g L~' of P-101, CP-14,and CG solutions at shear rate of 21 s'1, respectively. The final IRvalues were 63 to 30 mm h-' at P-101, 18 to 24 mm h^1 at CP-14,and 18 to 30 mm h ~' at CG, compared with 8 mm h ~ ' in untreated soil.The critical time values in suspensions with different concentrations ofpolymer were 4.3 to 2.7 min for P-101, 1.0 to 3.4 min for CP-14, and1.0 min for CG. The critical time indicates the size of the aggregateformed in a suspension; the lower the critical time, the larger theaggregates. It was suggested that because the particle surfaces areexposed to polymer molecules in suspension, the large molecule of CGcould tie more suspended particles to form aggregates. The greatereffectiveness of P-101 in preventing sealing was probably due to itsability to penetrate into aggregates, because of its small molecularsize and low viscosity in solution.

T ow WATER INFILTRATION, soil erosion, and inefficientI -> water use negatively influence plant growth andsurvival in arid and semiarid regions (Oster and Singer,1984). The combined effect of raindrop impact energyand dispersion of clay particles at the soil surface (Agassiet al., 1981) causes seal formation and reduces IR (Morinand Benyamini, 1977).

Synthetic polymers effectively increase final IR andreduce runoff and erosion on soils subjected to raindropimpact (Agassi and Ben-Hur, 1992; Ben-Hur et al., 1989;Helalia and Letey, 1988b; Shaviv et al., 1986). Theeffectiveness of a polymer as a soil conditioner wasfound to depend on polymer properties such as molecularweight and electrical charge (Ben-Hur and Letey, 1989;Shaviv et al., 1986). Cationic polysaccharide signifi-cantly increased IR when applied at a concentration of10 g m~3 in sprinkled irrigation water. Cationic polymereffectiveness increased with charge density (Ben-Hur andLetey, 1989). Nonionic and anionic polysaccharide hadno effect on IR, but a relative low molecular weight70000-150000 daltons) anionic lignosulfonate at an ap-plication rate of 80 kg ha"1 was effective (Shaviv etal., 1986). Shainberg et al. (1990) found that surfaceapplication of 20 kg ha"1 of a high molecular weight

Agricultural Research Organization, The Volcani Center, P.O. Box 6, BetDagan 50250, Israel. Contribution from the Agricultural Research Organi-zation, The Volcani Center, Bet Dagan, Israel. Received 25 Sept. 1995."Corresponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 61:565-570 (1997).

(10-15 million daltons) anionic polyacrylamide wassufficient to maintain a high IR.

A soil conditioner's effectiveness is often related toits ability to promote flocculation (Aly and Letey, 1988;Helalia and Letey, 1988a). Polymers induce flocculation(or coagulation) of dispersed clay particles by (i) electro-static absorption of polymer molecules on the clay parti-cles, which helps to compensate the clay surface charge(Black et al., 1966), and (ii) bridging soil particles to-gether (Roberts et al, 1974). These processes are influ-enced by polymer properties and the nature of the poly-mer-particle bonding mechanism.

Cationic polymers can compensate the negative elec-trostatic charge on clay particles and therefore can beused as coagulants (Black et al., 1966). Because of theirrapid absorption capability and the high affinity to theclay (Ueda and Harada, 1968), only limited interparticlebridging can be achieved.

Anionic polymers are effective flocculants, especiallyin the presence of polyvalent cations (Roberts et al.,1974; Gu and Doner, 1993). For these polymers, onlya few segments of the polymer chain are involved inadsorption, while the other segments are present in theform of long loops and tails in solution. Thus, an anionicpolymer has a relatively long grappling distance thatfacilitates the formation of interparticle bridges (Theng,1982). Compared with charged polymers, nonionic formsare generally less effective as flocculation agents becausethey exist as randomly coiled units rather than as extendedchains (Vincent, 1974).

In a dilute colloidal suspension, the particles are sepa-rated at relatively large distances and the most particlesurfaces are accessible to polymer molecule absorption.However, accessibility of the soil particles to polymermolecules is limited in soil aggregates, due to proximityof the soil particles. Malik and Letey (1991) observedthat relatively large polymer molecules with molecularweights ranging from 0.2 to 15 million daltons did notpenetrate into aggregates of three California soils.

Ben-Hur et al. (1989) hypothesized that penetrationand movement of the polymer molecules into and throughintra- or inter-aggregate pores are important factors de-termining the aggregate stabilizing performance of soilconditioners. This hypothesis was confirmed by Malikand Letey (1991) when they studied the effect of soilparticles size on adsorption of polyacrylamide and poly-saccharide. Size, configuration of die polymer molecules,and polymer solution viscosity and surface tension shouldhave an effect on polymer penetration into soil aggre-gates.

The objective of this study was to test the hypothesisthat polymer penetration and movement into aggregatesare important in reducing surface seal. We test thishypothesis by comparing the effectiveness of different

Abbreviations: IR, infiltration rate; CG, Complete Green; AV, aggrega-tion value; MAV, maximum aggregation value.

565

Page 2: Polymer Effects on Water Infiltration and Soil Aggregation

566 SOIL SCI. SOC. AM. J., VOL. 61, MARCH-APRIL 1997

commercial polymers on IR of soil exposed to rainfalland on soil aggregation.

MATERIALS AND METHODSThree different commercial, synthetic polymers, designated

P-101, CP-14, and CG were tested. Some properties of thesepolymers, according the manufacturers' information, and pre-sented in Table 1. The charge density of the charged polymerswas determined by the percentage of the substitutional groups.For CG substitution of NH2 by OH was 20% and for CP-14substitution of H by NH4 was 10%. Solutions with variousconcentrations of each polymer were prepared, 1 d before theuse, by mixing different amounts of polymer in tap water(electrical conductivity = 1 dS irr1 and SAR 2) for 18 h withcontinuous stirring.

A sandy loam (Typic Rhodoxeralf) from the coastal plainof Israel was used in this study. The soil was collected fromthe 0.3- to 0.6-m layer in a field, air dried, ground, and passedthrough a 2-mm sieve. The soil texture was 11.4% clay, 3 % silt,and 85.6 % sand, where the dominant clay was montmorillonite.The soil contained <1 g kg"1 CaCOs and organic matter andhad a cation-exchange capacity of 6.1 cmolc kg"1 and exchange-able Na percentage of 1.9%. The saturated paste pH was 7.8.Four studies were conducted.

Viscosity StudyShear stress and shear rate values of the various polymer

solutions at various concentrations were determined at a tem-perature of 25 °C using a Rotoviscometer (Gebruden, Haake,Germany). The unit was a couvette-type with a rotating outercylinder and a stationary inner cylinder. Each treatment inthis study was replicated twice.

Surface Tension StudyThe measurements were performed with a Landa tensiometer

equipped with a platinum-iridium ring. Polymer solutions werefreshly prepared at different concentrations at a temperatureof 25°C. Each treatment was replicated twice.

Flocculation StudyThe effect of polymer concentration (O, 10, 20, and 50 g

m~3) on flocculation was tested in three replications. One-hundred grams of air-dried soil was placed in a 1-L plasticbottle, and 0.5 L of distilled water was added. The bottle wasshaken on a reciprocating shaker at 3 cycles s"1 for 4 h. Aftershaking, the suspension was poured into a 45-cm-long plasticcylinder with an inside diameter of 5 cm, and 0.5 L of thedesired polymer solution in tap water was added. The cylinderwas then gently inverted three times and placed vertically ina fixed position, and a hydrometer was put into the suspension.The hydrometer level was recorded every minute. The suspen-sion's electrical conductivity was 0.6 dS m~', and its tempera-ture was 27 °C.

Table 1. Some general properties of the studied polymers.

Rainfall Simulator StudyPolymer effects on IR were studied using a rainfall simulator

with rotating disk (Morin et al., 1967). The typical mechanicalparameters of the simulated rainfall were rainfall intensity, 68mm h~'; raindrop median diameter, 1.9 mm; median dropvelocity, 6.2 m s~'; and kinetic energy, 18.1 J mm"1 m~2.

Air-dried soil was packed 2 cm deep over a layer of coarsesand in a perforated tray measuring 30 by 50 cm. The packedsoil tray was placed at a slope of 25 % under a rainfall simulator.Solutions of CG, CP-14, and P-101 with concentrations of0.5, 1, and 10 g L"1, respectively, were spread over the soilsurface with a hand sprayer at rates of O, 25, 50, and 75kg polymer ha"1. These solution concentrations enabled thepolymers to be uniformly distributed on the soil surface. Afterthe spraying, the soil samples were left to dry overnight. Eachtreatment was replicated four times. The treated soils in therainfall simulator trays were saturated slowly from the bottomwith tap water and were then exposed to a simulated rainstormof 68 mm of distilled water. During the rainstorm, waterpercolating through the soil was collected and measured.

The differences between the replicates were <5% in theviscosity and surface tension studies. Therefore, only tworeplicates were measured in each treatment. For the final IRand flocculation parameters, mean separations (P < 0.05) weredetermined using Tukey's multiple comparison procedure (Steeland Torrie, 1960).

RESULTS AND DISCUSSIONThe effects of the polymer type and concentration on

IR as a function of cumulative rainfall are presented inFig. 1. In the absence of polymer (control), the IRdecreased sharply with increasing rainfall depth, and afinal IR (8 mm h"1) was obtained after =30 mm ofrainfall. The IR reduction was caused mainly by a sealthat formed at the soil surface. Whereas a sharp dropin soil IR was observed in the control, it remainedrelatively high and stable when polymers were applied,in spite of the high rain intensity (68 mm h"1) and thesteep slope of 25% (Fig. 1).

The polymer P-101 was the most effective. At applica-tion rates of 50 and 75 kg ha"1, no IR reduction wasobserved throughout the rainstorm. At the lowe:st applica-tion rate (25 kg ha"1) the IR remained high during thefirst 55 mm of the rainfall and then a reduction in IRwas observed.

The IR of the soil in the presence of CP-14 remainedstable during the first 30 mm of rainfall, and men a dropin IR was observed. A final IR of 18 mm h"1 wasobtained at 60 mm of rainfall, which was about twicethe final IR of the soil in the control. The effectivenessof this polymer on IR was independent of the polymerapplication rate, within the range studied.

It is evident from Fig. 1, that the effectiveness of CG

Polymer

P-101CP-14CG

Composition

INRtPolysaccharidePolyacrylamide

Molecular weight

daltons1 x lff-2 x 10s

2 x W-2 x 106

1.0 x 10'-1.5 x 10'

Charge

NonionicCationicAnionic

Charge density

%<21020

Manufacturer

Hydropolymer, .IsraelCelanese Corp., USAComplete Green Corp., USA

t IMR = information not released.

Page 3: Polymer Effects on Water Infiltration and Soil Aggregation

BEN-HUR & KEREN: POLYMER EFFECTS ON WATER AND SOIL 567

P-IOI60

40

20

L O

E 60u< 40(T

O 20

£ O^u.? 60

40

2O

O

CP-14

20 40 60CUMULATIVE RAINFALL, mm

80

Fig. 1. Average values of infiltration rate as a function of waterapplication for different polymer types and concentrations. Differentletters at the end of the lines indicate significant difference (P <0.05) among the final IR values for each polymer

was significantly less than P-IOI and somewhat less thanCP-14. At a CG application rate of 25 kg ha"1, the IRdecreased moderately to a final value of 18 mm h~' ata cumulative rainfall of 40 mm. At 50 and 75 kg ha"1

CG, the IR reduction with cumulative rainfall was less,and a final IR value of 28 mm h~' was obtained (Fig. 1).

The polymer molecules were absorbed on the soilparticle surfaces and acted as a cementing material,holding particles together against the destructive forcesof water drops. Thus, the destruction of the aggregatesat the soil surface was diminished and a high IR wasmaintained (Ben-Hur and Letey, 1989; Helalia and Letey,1988b; Shaviv et al., 1986).

The lower efficiency of CG in relation to the otherpolymers could be explained by its lower adsorptionon soil constituents, which leads to a decrease in thestabilizing effect of the polymer (Ben-Hur and Letey,1989). However, Malik and Letey (1991) found thatthe adsorption of anionic polyacrylamide, with similarmolecular weight and charge density to CG, on threeCalifornia soils was significantly higher than CP-14.

Another possible explanation to the low efficiency ofCG is the limiting movement of the CG molecules intosoil aggregate. In this case, most of the polymer moleculeadsorption would take place on the external surface ofthe aggregate. Once the aggregate broke, internal soilsurfaces without polymers would be exposed, and IRwould decrease due to seal formation (Ben-Hur et al.,1989). This factor can be evaluated from the effect ofthe polymer on aggregate formation and the viscosityand surface tension of polymer solutions.

The degree of soil aggregation, at a given time, can

• 50mg/Lo 20mg/Lo lOmg/L• CONTROL

I___IO 10 124 6 8

TIME, minFig. 2. Average aggregation values of soil suspension for three polymer

solutions at various concentrations (CG = Complete Green).

be defined by the AV, which was computed using Eq.[1].

Ro[1]

where Rt and RO are the hydrometer readings in thesuspension with and without polymer, respectively. Thehydrometer reading value of the control suspension atthe first minute, before aggregation took place, wasselected as a reference value because it was observedthat the change in concentration of the suspended particlesin the absence of polymer was negligible during the firstminute.

The aggregation-flocculation process in the presenceof polymer can be evaluated from the changes in theAV with time (Fig. 2). The AV of the control suspensionincreased during the first 10 min of settling, to a MAVof 0.85 (Table 2). This increase was a result of thesettling of sand, silt, and microaggregate particles X).02mm. Only particles <0.02 mm remained in suspensionafter 10 min of settling (Van Olphen, 1977).

Whereas a gradual increase in AV was observed in theabsence of the polymers, a sharp increase was obtained inthe suspensions with the polymers (Fig. 2), and theirMAV were reached after a shorter time. Moreover, theMAV of the suspensions with the polymers were greaterthan those of the control suspensions (Table 2). Thegreater the MAV, the more particles are associated withthe aggregates that the polymer forms. Likewise, thetime at which the AV achieved its maximum value (the

Page 4: Polymer Effects on Water Infiltration and Soil Aggregation

568 SOIL SCI. SOC. AM. }., VOL. 61, MARCH-APRIL 1997

Table 2. Maximum aggregation values (MAY) and critical time values of the different suspensions.Polymer concentration

Parameter

MAVCritical time, min

Control

0.85 bt9.7 a

lOgm"3

0.91ab4.3b

P-101

20 g m-J

0.91ab3.7bc

50gm-3

0.87b2.7cd

10 gm- 1

0.91ab3.7bc

CP-14

20 g m-'

0.91ab2.0de

50 g m'1

0.95al.Oe

10 gm- 1

0.94al.Oe

CG

20 g m"1

0.94al.Oe

50 g m-'0.94al.Oe

t Different letters within rows indicate significant difference among the suspensions in the value of the parameter (P < 0.05).

critical time) can be related to the size of the aggregateformed; the lower the critical time, the larger the aggre-gates.

The results in Fig. 2 and Table 2 indicate that thepolymers significantly increased aggregate formation andthe size of the aggregates formed. However, the capabili-ties of the three polymers to form aggregates were notthe same. Although the MAV of the soil for all threepolymers was independent of the polymer concentration,the time to reach this value depended on concentration(except for the CG polymer); the lower the concentration,the longer the time required. The largest aggregates wereobtained in the presence of CG at all concentrations. Inaccordance with the results of Black et al. (1966), themost effective polymer in the aggregation-flocculationprocess was CG, with the largest molecular weight. Thisindicates that the adsorption of CG on soil particles in

200 400 600 800 1000SHEAR RATE, s"1

Fig. 3. Shear stress as a function of shear rate for three polymersolutions at various concentrations (CG = Complete Green).

suspension and its capability to hold them together arehigh.

The effectiveness of the polymers in association ofsoil particles in suspension was in the order, CG > CP-14> P-101 (Fig. 2 and Table 2). This order is opposite tothat obtained from the IR experiment P-101 > CP-14 >CG) (Fig. 1). This difference is probably a result ofdifferences in the accessibility of the soil particle surfacesto polymer molecules. In a soil suspension, the soilparticles are initially separated, so that all particle sur-faces are exposed to the polymer molecules. In this case,the larger the polymer molecule, the more suspendedparticles can be bounded to form aggregates.

The size (conformation) of the polymer molecule ina solution is controlled mainly by (i) the molecular weight(the higher the molecular weight, the larger the molecularsize), and (ii) the charge density of the molecule. Chargesalong the molecule would be expected to cause the mole-cules chain to stretch out (Malik and Letey, 1991). Themolecular weight of CG is one to two orders of magnitudegreater than CP-14 and two orders of magnitude greaterthan P-101 (Table 1). Likewise, the charge density ofthe polymers was in the order, CG > CP-14 > P-101.Therefore, it can be concluded that the molecule size ofthe polymers in a solution was in the order, CG >CP-14 > P-101. This order is similar to that obtainedin aggregate formation in a soil suspension (Fig. 2 andTable 2).

Conversely, in the rainfall simulator study, the poly-mer solutions were sprayed over a surface of dry soilaggregates, so that the accessibility of the internal aggre-gate surface to the polymer molecules was limited (Ben-Hur et al., 1992; Malik and Letey, 1991). Hence, theeffectiveness of the polymer in stabilizing soil aggregatescould be related to the capability of the polymer to moveinto aggregates. The movement of polymer solution indry soil depends mainly on the polymer-soil interactionand the viscosity and surface tension of the appliedpolymer solutions.

Shear stress (T) as a function of shear rate (D) forthe three polymer solutions at various concentrations ispresented in Fig. 3. The dynamic viscosity (r|) of thepolymer solution is defined as

[2]

A linear relationship between shear stress and shearrate values was obtained for the P-101 solutions at con-centrations below 50 g L~' (Fig. 3). The linear relation-ship indicates that the polymer solution behaves as anewtonian liquid in this concentration range. Conversely,

Page 5: Polymer Effects on Water Infiltration and Soil Aggregation

800

600

400

200

P-IOI

LOW SHEAR

HIGH SHEAR

BEN-HUR & KEREN: POLYMER EFFECTS ON WATER AND SOIL

80

70

60

50

569

20 40 60 80 100

CP-14

10 12

O 1 2 3 4 5 6 7POLYMER CONCENTRATION, g LT1

Fig. 4. Viscosity value of different polymer solutions as a function ofpolymer concentration for two shear rates (different concentrationswere used for each polymer; CG = Complete Green).

a psuedoplastic behavior was observed for the P-IOIsolution at concentrations >50 g LT1 and for the othertwo polymers at all concentrations. The viscosity changewith the increase of shear rate is a result of the changeof the stearic structure of the polymer molecules in thesolution. At rest or at very low shear rate, the polymermolecules maintain an irregular internal order (Theng,1982), which increases the internal resistance againstflow (i.e., high viscosity). With an increase of shearrates, the polymer molecules disentangle, stretch, andorientate parallel to the driving force. As a result, the flowbehavior becomes newtonian and the viscosity becomesconstant at a high shear rate.

The viscosity of the polymer solutions as a functionof polymer concentration for two shear rates, 27 s"1

(low shear rate) and 1000 s~l (high shear rate), arepresented in Fig. 4. The viscosity of P-101 solutionincreased linearly with increasing polymer concentrationup to 50 g LT1, and no differences between the viscosityvalues at high and low shear rates were observed at thislow concentration range. At concentrations above 50 gLT1, the viscosity of P-101 solutions increased sharplyat low shear rate and moderately at high shear rate.

In contrast to the case of P-101, the viscosities ofCP-14 and CG solutions were greater at the low shearrate than at the higher shear rate, at any given polymerconcentration (Fig. 4). This difference was small at lowpolymer concentration and increased sharply as the poly-mer concentration increased.

P-IOI

70

60

50

40) 0.2 0.4 0.6 0.8 1.0

POLYMER CONCENTRATION, g IT1

Fig. 5. Surface tension of different polymer solution as a function ofpolymer concentration (different concentrations were used for eachpolymer; CG = Complete Green).

The polymer solution viscosities at a given polymerconcentration were in the order, CG > CP-14 > P-101for both selected shear rates. This progression in viscosi-ties was probably due to the differences in the molecularsizes and the charge density of the polymers; in general,the larger the molecular size and the charge density,the greater the viscosity (Yariv and Cross, 1979). Theviscosity order was opposite to that obtained for IR.Higher viscosity decreases the solution flow rate in theconducting pores and, therefore, may increase the num-ber of polymer molecules that interact with the soilparticles, thus in turn, decreasing the penetration ofpolymer molecules into the aggregate.

The relationship between the matrix suction (5) andthe surface tension (y) of solution in capillary tubes ispresented in Eq. [3].

[3]Pgr

where p is the density of the solution, g is the gravityacceleration, r is the radius of the pore, and a is thecontact angle. It was assumed that COS a is equal to«1 due to the low concentration of the polymer solutions.This equation indicates that an increase of the surfacetension of the solution increases the matrix suction.Hence, a polymer solution with a higher surface tension

Page 6: Polymer Effects on Water Infiltration and Soil Aggregation

570 SOIL SCI. SOC. AM. J., VOL. 61, MARCH-APRIL 1997

should have a higher capability to penetrate into soilaggregate when it is sprayed on dry soil.

Surface tensions of the different polymer solutions asa function of their concentration in water are presentedin Fig. 5. Addition of CG to water had no effect on thesurface tension, whereas addition of P-101 and CP-14to the water decreased the water surface tension. Thisdecrease of the surface tension may be due to the occur-rence of hydrophobic groups in the CP-14 and P-101molecules.

The surface tensions of the applied polymer solutionsin the rainfall simulator study were in the order, CG >CP-14 > P-101. This order is opposite to that of theireffectiveness in the IR experiment (Fig. 1). These resultssuggest that the surface tension is not the dominant factorthat affects polymer penetration into the soil aggregate.Probably, the large size of the CG molecules limitedtheir penetration into small pores, despite the high matrixsuction.

Penetration of large polymer molecules into narrowpores can also be limited because of a stearic interference.For an anionic polymer, a few segments of the polymermolecule are adsorbed on soil particles and other seg-ments extend away from the surface into the pore asloops and tails (Greenland, 1972). These loops and tailsmay limit the penetration of other polymer molecules.Hence, the penetration of the large CG and CP-14 mole-cules, particularly the anionic CG molecules that havea stretched-out chain structure, into intra-aggregate porescould be limited in comparison with that of the smallerP-101 molecule.

The above results support the hypothesis that the highereffectiveness of P-101 in preventing surface seal forma-tion is due to its capability to penetrate into aggregatesand to stabilize them, because of its small molecular sizeand the low viscosity of its solution.

ACKNOWLEDGMENTSThe authors thank Ms Eva Klein and Messrs. N. Shapir,

A. Greenberg, and Y. Zarchia for their help in the laboratoryand rainfall simulator work.