Polymer Type and Water Quality Effects on Soil DispersionAWAD M. HELALIA AND J. LETEY*

ABSTRACTTen polyacrylamide (PAM) and derivatized guar compounds, with

different charges were tested at 0,10, and 50 mg L~' concentrationwith low electrolyte solution (1 molc m-1) at 0, 5, and 20 sodiumabsorption ratio values for their ability to reduce dispersion of threesoils. Dispersion was determined by measuring the soil settling ratein solution. All compounds were effective in promoting clay floccu-lation at a concentration of 10 mg L '. When compared at compa-rable charge, the PAM compounds were more effective than the guarcompounds. This result is consistent with their higher molecularweight. The order of effectiveness of the compounds was cationic >nonionic > anionic. No cationic PAM compounds were availableand the cationic guar compound (CP-14) was comparable to the non-ionic PAM. Flocculation decreased very slightly as the SAR in-creased, suggesting that the effect of low electrolyte concentrationwas the prominent factor in dispersion. The cationic guar "CP-14"was tested separately with low (0.05 dS nr') and moderate (0.7 dSm ') electrolyte concentrations simulating canal and well waters fromCalifornia. Its effect on flocculation was very high at 5 mg L ' con-centration in both waters after only 10-min settling. Increasing theconcentration > 10 mg L ' did not improve the flocculation results.

Additional Index Words: flocculation, soil structure, aggregation,polyacrylamide, guar, soil conditioners.

CLAY DISPERSION , aggregate breakdown, and soilcrusting affect infiltration rate, mechanical

strength, credibility, and air-water relations, which canbe major problems in some irrigated arid and semiaridsoils. Considerable research was conducted in the1950s and 1960s on the utility of synthetic polymersto stabilize soils. Gardner (1972) reported that syn-

Department of Soil and Environmental Sciences, Univ. of Califor-nia, Riverside, CA 92521. Research was supported by the Univ. ofCalifornia Kearney Foundation of Soil Science. Received 22 May1987. Corresponding author.

Published in Soil Sci. Soc. Am. J. 52:243-246 (1988).

thetic materials have remarkable capabilities for sta-bilizing soil aggregates formed previously or duringthe mixing process. When properly applied, polymersimpart desirable stability characteristics to many soilmaterials that are structurally unstable in their ordi-nary condition. Despite these positive results, syn-thetic polymers have not been used commercially forsoil aggregate stabilization. Factors contributing totheir commercial failure include cost of material anddifficulty in proper treatment. Typically, the polymerwas applied as a dry powder, mixed, moistened withthe soil, and the soil tilled to form desirable-sized ag-gregates. Upon drying, the formed aggregates were verystable.

Polymers are currently available that have a rangeof chemical and physical properties, which might en-hance their utility for soil stabilization. Polymers canbe synthesized that are either nonionic, cationic, oranionic. Some polymers are soluble in water and maytherefore be applied with the irrigation water. Underthese conditions, the goal is to stabilize soil aggrega-tion in its initial state rather than to use a treatmentprocess that includes formation of soil aggregates. Po-tentially, the chemicals could be applied at lower con-centrations, enhancing their economic feasibility.

Clay is the reactive soil component expected togreatly affect soil structure. Clay flocculation is a pro-cess whereby particles associate into larger units,known as floes (Hiemenz, 1986). LaMer (1966) de-fined flocculation as the process of forming aggregatesby the bridging of several particles by a polymer chain,whereas coagulation is the process of forming aggre-gates by electrolyte addition. Cationic polymers arepostulated to act as both a coagulant in compressingthe double layers of the clay particles, and as a floc-culant by bridging the particles. The latter process iscontrolled by the initial clay concentration of the sus-pension (Black, 1966).

244 SOIL SCI. SOC. AM. J., VOL. 52, 1988

The stability of dispersed suspensions against co-agulation is governed by interparticle forces. The mainforces are van der Waals attraction, electrostatic re-pulsion, and repulsion due to interaction of adsorbedlarge molecules. The latter force may lead to attractionrather than repulsion if the particles contain two ormore anchor groups adsorbed on different particles(Overbeek, 1977). The stability of a clay colloid sus-pension is influenced by factors such as the pH andthe ionic strength of the medium and the molecularweight of the polymer. Ueda and Harada (1968) stud-ied the behavior of different cationic polymers withkaolinite suspensions as a function of polymer mo-lecular weight and pH of the solution. The extent ofaggregation was assessed by measuring the change intransmittance of supernatant liquid. The amount ofpolymer required to increase the transmittance of ka-olinite suspensions by 50% was denned as the aggre-gation value. Udea and Harada (1968) also indicatedthat less of the higher molecular weight polymers wasneeded to achieve aggregation when compared to thelower molecular weight polymers.

Adsorption of a polymer depends on its charge.Theng (1982) reported that positively charged poly-mers are adsorbed largely through electrostatic inter-actions between cationic groups of the polymer andnegatively charged sites of the clay surface. Little ad-sorption occurs with negatively charged polymers dueto charge repulsion between the polymer and the claysurface. Low pH, high ionic strength, and the presenceof polyvalent cations in the system enhance polyanionadsorption. On the other hand, Greenland (1963) re-ported that uncharged polymers exist as randomlycoiled molecules in solution and therefore are consid-ered to be relatively poor flocculants compared to thepolyanions. However, the molecules can improve ag-gregate stability because they tend to uncoil in an ex-tended form on contact with the soil and spread overadjacent surfaces, linking them together like a "coatof paint" lining the pores of aggregates.

Soil structural stability is dependent on the irriga-tion water quality. Very low electrolyte concentrationsand/or high sodium adsorption ratios (SAR) contrib-ute to soil dispersion (Shainberg and Letey, 1984).Frenkel et al. (1978) demonstrated that plugging ofpores by dispersed clay particles is the major cause ofreduced hydraulic conductivity in some soils. Osterand Schroer (1979) showed that soil dispersion de-creased with increasing electrolyte concentration.

The purpose of this study was to investigate theeffectiveness of 10 polymers of differing chemicalproperties in reducing dispersion of three soil mate-rials. The initial investigations were conducted usingwaters conducive to soil dispersion. An extended study

was conducted using two synthesized waters typical ofthe irrigation water qualities of the San Joaquin Valleyof California.

MATERIALS AND METHODSThe three soils used were Arlington (coarse-loamy, mixed,

thermic Haplic Durixeralfs); Pachappa (fine-loamy, mixed,thermic Mollic Haploxeralfs); and Fallbrook (fine-loamy,mixed, thermic Typic Haploxeralfs). The soils had low or-ganic C content and similar clay mineralogy (Table 1). Thesoils were air-dried, ground, and passed through a 2-mmsieve.

The compounds tested were either polyacrylamides (PAM)or derivatized guar. The compounds could be further cate-gorized as being either nonionic, anionic, or cationic withdiffering charge density. Nine of the compounds were pro-vided by Celanese Corp. (Louisville, KY), and are identifiedin Table 2. Another anionic polyacrylamide marketed underthe trade name Complete Green® was provided by Com-plete Green (Los Angeles).

The effectiveness of the compounds in promoting floc-culation was checked by determining the rate at which soilsettled after being shaken in a polymer solution. Waters con-taining 1 mol. m-3 of CaCl2 and/or NaCl to produce SARvalues of 0, 5, and 20 were prepared. The chemical com-position of these waters was expected to promote dispersion.The polymers were each tested in each water at concentra-tions of 0, 10, and 50 mg L~ ' .

One-half gram of air-dried soil sample was placed in a 30-mL pyrex test tube and 25 mL of the test solution was added.The tubes were shaken on a reciprocating shaker at 3 cycless"' for 15 min. The test tubes were then gently turned upsidedown three times and placed vertically in a fixed position.The time required for particles or aggregates >2 nm to passthrough a plane below the top 5 mL of solution was calcu-lated using Stoke's law. At that time, the top 5 mL wassiphoned off using an automatic pipette fixed to the labo-ratory bench to eliminate disturbance of the sample duringsiphoning. The optical transmittance (T %) of the removedsolution was measured at the 410-nm wavelength using acolorimeter. Each treatment was replicated six times.

A follow-up study was conducted on the cationic guarcompound identified as CP-14. In this case, waters were syn-thesized to represent typical irrigation waters in the San Joa-quin Valley of California. Canal water, which has its originfrom Sierra Nevada Mountains snowmelt and has a verylow electrolyte concentration, and a water typical of wellwater (El-Tayib et al., 1979), were synthesized. The chemicalcomposition of these waters is given in Table 3. Note thatthe main difference between these waters is the electrolyteconcentration, reported in terms of the electrical conductiv-ity (EC), and that both waters have a very low SAR value.The polymer was added at concentrations of 0, 5, 10, 20,and 50 mg L '. The same procedures were followed as pre-viously, except that samples were siphoned off at three dif-ferent times, representing the time at which particles and/or aggregates >2, 3, or 6 fim would have passed the plane.It should be noted that in using Stoke's law, it is assumedthat aggregates have the same density as soil particles. This

Table 1. Characteristics of the soils.

Soil

ArlingtonPachappaFallbrook

Clay

21.216.219.6

Silt

—— % ——37.132.618.6

Sand

41.751.261.8

CECt

mmolc kg"1

150111116

OCT

gkg-6.62.42.1

Texture

LoamLoamSandy loam

Dominantclay

minerals}

M.V&KM,V &KM & V,K

Saturation extract

ECdS m-'

0.831.310.51

pH

7.17.17.3

OC

mg L '26411660

SP§

35.530.927.4

t CEC = cation exchange capacity; OC = organic carbon. t M = mica, V = vermiculite, K = kaolinite. § SP = saturation percentage.

HELALIA & LETEY: POLYMER TYPE AND WATER QUALITY EFFECTS ON SOIL DISPERSION 245

100

jao

leo

!40

20

SflR=0 D SflR=5 S SRR=20

10 mg L"

0.0 PN Pfl Pfl PR GC GC GN GN Gfl Gfl

100

.80

60

40

50 mg L"

0.0 PN PR PR Pfl GC GC GN GN Gfl Gfl1 2 3 4 5 6 7 8 9 1 0

Fig. 1. Flocculation of Arlington soil with 10 different polymers. Thefirst letter identifies polyacrylamide (P) or guar (G); and the sec-ond letter identifies charge: nonionic (N), cationic (C), or anionic(A). The specific compounds are: (1) Polyhall-2J, (2) Polyhall-21J, (3) Polyhall-40J, (4) Complete Green, (5) T4141, (6) CP-14,(7) 6003VT, (8) HP-8, (9) R-600, and (10) T4246.

assumption is, of course, not absolutely true. The size of theaggregates passing the plane are actually somewhat largerthan the specified diameter. Viscosity of the solutions wasmeasured using an Ostwald viscometer and was not alteredby the polymer additions.

RESULTS AND DISCUSSIONThe results for the Arlington soil are summarized

in Fig. 1. The T % was 20.5 at zero polymer additionand SAR = 0. The T % decreased slightly as the SARincreased. The SAR had a relatively small effect ondispersion in this solution of extremely low electrolyteconcentration. The relationship between T % and clayconcentration in solution is presented in Fig. 2. These

Table 2. Identification of compounds tested.__________Type of compound PAM GuarNonionicAnionic (lower charge)Anionic (higher charge)Cationic (lower charge)Cationic (higher charge)

Polyhall 2JPolyhaU 21JPolyhall 40J

HP-8 and 6003VTT-4246R-600CP-14T-4141

I00<>

80

60

40

20

CW T% = 101 -3.2X"J(r2 = 1.0)W W T % = !04-3.2X~5(r2=I.O)

o CW * W

800 10000 200 400 600CLHY CONCENTRRTION mg.L'1

Fig. 2. Relation between transmittance percent and clay concentra-tion for two waters.

data were obtained using Fallbrook soil and the twosynthesized irrigation waters.

All 10 products were effective in promoting floc-culation. The effect of charge on the PAM compoundswas quite small, although the nonionic (Polyhall-2J)appeared to be somewhat superior to the anionic com-pounds (Polyhall-21J, Polyhall-40J, and CompleteGreen), particularly at the higher SAR values.

For the guar products, the cationic (CP-14 andT4141) were somewhat more effective than the non-ionic (6003VT and HP-8) products, which were con-siderably superior to the anionic (R-600 and T4246)products.

A comparison of the PAM to the guar products withthe same type of charge reveals that PAM productswere superior in promoting flocculation. This resultmight be related to the molecular weight. The molec-ular weights of the PAM products range from 10 to15 million, whereas those of the guar products rangefrom 200 000 to 2 million, (Steve Hoye, Hi-Tek Poly-mers, Inc., Louisville, KY, 1987, personal commu-nication). Overbeek (1977) and Theng (1979) reportedthat polymer effects on aggregate formation increasedas their molecular weight increased. Our results are,therefore, consistent with Overbeek (1977) and Theng(1979). Very little difference was observed between the10 and the 50 mg L~' additions, except for the anionicguar compounds, particularly R-600.

Detailed results for the Pachappa and Fallbrook soilsare not presented because they are quite similar tothose for the Arlington soil. Minor differences includelower T % for the anionic PAM products in the Pa-chappa and Fallbrook soils. Likewise, the nonionicguar products resulted in slightly lower T % with theFallbrook than the Arlington soil.

From an economic point of view, all PAM products,HP-8, T4246, and CP-14, have about the same price,whereas 6003VT is somewhat cheaper and R-600 andT4141 are somewhat more expensive. The more ex-pensive products did not have sufficient improvement

Table 3. Chemical composition of the two waters used.Water type EC PH SAR NO.-N Ca Na Cl SO. HCO,

dS m"' mg L'1 ———— molc m~' —————Canal water 0.05 7.2 0.43 0 0.28 0.16 0.04 0.04 0.36Well water 0.70 8.0 1.75 1 4.4 2.6 1.1 1.5 4.4

246 SOIL SCI. SOC. AM. J., VOL. 52, 1988

I- 100ZLL)£ 80LUCLLJ 60O

£* 40

20

CCARLINGTON

——— CW 85——— WW 10........... ww 85

PACHAPPA FALLBROOKi__i i

10 1020 0 10 20 0CP-14 CONCENTRATION, mg L~'

Fig. 3. Flocculation of soils at different CP-14 concentrations and times.

20

in performance to justify their increased cost. Thecheapest product (6003VT) did not produce the high-est T %, even at the higher concentration, so that itscomparative economic advantage is questionable.

The results of various additions of CP-14 to the twosynthesized irrigation waters are illustrated in Fig. 3.Although data were taken from the T % of the top 5mL after three settling times, only the extremes (10and 85 min) are illustrated. The CP-14 greatly in-creased the flocculation for all three soils in both ir-rigation waters. The polymer was most effective onthe Pachappa soil, where 5 mg L ' was sufficient toprovide a solution with T % approximately 90% afteronly 10 min of settling. Additional concentrations ofCP-14 over time did not improve the results.

Dispersion without polymer treatment was en-hanced more by the canal than by the well water forall three soils. Likewise, the T % of the Arlington soilfor a given time of settling was higher for the wellwater than the canal water at CP-14 concentrations of5 and 10 mg L '. There was no effect of either timeof settling or type of irrigation water at concentrationsof CP-14 >10 mg L~'. A concentration of 5 mg L~'of CP-14 in well water was adequate to provide max-imum T % in the Fallbrook soil. On the other hand,there is a gradual increase in T % with increased poly-mer concentration when canal water was used withFallbrook soil.

Overall, a relatively low solution concentration of5 mg L~' of CP-14 was effective in promoting floc-culation of the three soils investigated. It must be rec-ognized, however, that a relatively small amount ofsoil was exposed to a relatively large volume of so-lution. Since the soil and solution were intimatelymixed, the entire soil mass was exposed to the totalamount of polymer in solution. A 5 mg Lr' solutionconcentration is equivalent to a 250 mg kg~' ratio of

polymer to soil. Effectiveness of these very low solu-tion polymer concentrations in promoting coagulationwhen applied with the irrigation water in the field isstill somewhat in question because a much lower so-lution-to-soil ratio would occur. Nevertheless, the re-sults of this study suggest that certain polymers canbe quite effective in promoting soil flocculation.