Polymer Effects on Water Infiltration and Soil Aggregation
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Polymer Effects on Water Infiltration and Soil AggregationM. Ben-Hur* and R. KerenABSTRACTThe effectiveness of synthetic polymers as soil conditioners wasfound 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 weightAgricultural 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@example.com).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 differentAbbreviations: IR, infiltration rate; CG, Complete Green; AV, aggrega-tion value; MAV, maximum aggregation value.565566 SOIL SCI. SOC. AM. J., VOL. 61, MARCH-APRIL 1997commercial polymers on IR of soil exposed to rainfalland on soil aggregation.MATERIALS AND METHODSThree different commercial, synthetic polymers, designatedP-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 BEN-HUR & KEREN: POLYMER EFFECTS ON WATER AND SOIL 567P-IOI604020L OE 60u< 40(TO 20 O^u.? 60402OOCP-1420 40 60CUMULATIVE RAINFALL, mm80Fig. 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 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. .Pgrwhere 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 to1 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 tension570 SOIL SCI. SOC. AM. J., VOL. 61, MARCH-APRIL 1997should 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.
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