stability and transportability of water-dispersible soil colloids

8
DIVISION S-9-SOIL MINERALOGY Stability and Transportability of Water-Dispersible Soil Colloids A. K. Seta and A. D. Karathanasis* ABSTRACT Colloid migration in subsurface environments has attracted special attention lately due to its suspected role in facilitating transport of contaminants to groundwater. This study was conducted to evaluate the stability and potential transport of water-dispersible colloids (WDC) through intact soil columns, and the properties of colloids and soil columns facilitating or retarding colloid stability and transportability. Water-dispersible colloids were fractionated from six representative soil samples with diverse mineralogy and physicochenucal characteris- tics. Their stability was evaluated from settling-rate experiments at different pH levels. The results demonstrated that colloid stability was pH dependent. Colloid transportability was assessed by introducing colloid suspensions at a constant flux into intact soil columns represent- ing upper Bt horizons of a Maury (fine, mixed, mesic Typic Paleudalf) and a Loradale (flne-silty, mixed, mesic Typic Argiudoll) soil and evaluating characteristics of the suspensions that were eluted. After five pore volumes of leaching, colloid recovery in the eluents ranged from 35 to 90% depending on type of colloid, initial concentration in the influent, and soil column. The mineralogical composition of the colloid, which was correlated with particle size, appeared to have a profound effect on colloid transportability, following the sequence smectitic > mixed > kaolinitic. Total exchangeable bases (TEB) and pH of WDC also significantly influenced colloid transport. Increasing colloid concentration in the influent slightly increased colloid transport- ability. Soil columns with better macroporosity and less surface charge (Maury) transported more colloids than soil columns with less macro- porosity and higher surface charge (Loradale). E VIDENCE that colloidal particles are mobile in the subsurface environment abounds in the pedological literature. The presence of argillic horizons and the higher ratio of fine clay to total clay in illuvial horizons compared with eluvial horizons are examples of colloid transport resulting from natural pedogenic processes (Birkeland, 1984; Cabrera-Martinez et al., 1989; Net- tleton et al., 1975). Recently, efforts toward understand- ing colloid transport in the subsurface environment have attracted more attention because of the suspected role of colloids in facilitating transport of contaminants to groundwater (McCarthy and Zachara, 1989). However, information on the mechanisms and factors affecting colloid transportability is very limited. The transportability of colloid particles depends on their stability in the aqueous medium. Dispersed colloids will have the tendency to remain in suspension, and therefore be stable, if the repulsive potential (0) between two planar colloid surfaces is maximum (Sposito, 1984). A.K. Seta, Dep. of Soil Science, Univ. of Bengkulu, Indonesia; and A.D. Karathanasis, Dep. of Agronomy, Univ. of Kentucky, Agric. Exp. Stn., Lexington, KY 40506. Contribution from the Dep. of Agronomy, Univ. of Kentucky, Agric. Exp. Stn., Lexington. Journal Article no. 95-06-123. Received7 Aug. 1995. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 61:604-611 (1997). The magnitude of 0 is controlled by ApH (the difference between the pH of the suspension and the pH Z pc of the colloid) and the solution ionic strength (van Olphen, 1977). When ApH increases, </> also increases, thus leading to colloid stability. Increasing the ionic; strength, however, suppresses 4> values and enhances colloid floc- culation (Evangelou, 1990). In addition to the above components, the stability of a colloid is also affected by other factors, such as the presence of organic components and strongly binding agents. The adsorption of organic components on the colloid surface may increase its negative surface charge, thus enhancing the electrostatic repulsive forces and, therefore, the stability of the particle (Jekel, 1986). The presence of binding agents such as Fe and Al, on the other hand, promotes colloid instability (Goldberg et al., 1990). Additionally, colloid stability is also affected by the mineralogical composition of the colloid mixture (Arora and Coleman, 1979), the shape of the particles and their initial concentration in suspension (Oster et al., 1980), the type of cations (Hesterberg and Page, 1990), and the relative proportion of monovalent/divalent cations in the bulk solution (Shainberg and Letey, 1984). Even though a hydrochemical environment may pro- mote their stability, colloids can be mobile only if they are not filtered or adsorbed by the soil matrix. Particle filtration can occur physically through interception, diffusion, and sedimentation (Vinten and Nye, 1985) or by physicochemical interaction with the charged immo- bile porous medium (Fitzpatrick and Spielman, 1972). However, the effect of filtration by porous media can be negligible if colloids move through a preferential flow path with relatively large velocity. Preferential flow paths such as fractures and soil macropores can significantly enhance colloid transport (Smith et al., 1985). The objectives of this study were to: (i) determine the stability and potential transport of WDC fractions with diverse physicochemical and mineralogical composition through intact soil columns, and (ii) evaluate the proper- ties of colloids and soil columns that affect WDC stability and transportability. MATERIALS AND METHODS Colloid Material, Fractionation, and Characterization Water-dispersible colloids were fractionated from upper Bt horizons of six soils representing the series: Beasley silt loam Abbreviations: WDC, water-dispersible colloids; TEB, total exchangeable bases; pHzpc, pH at the zero point of charge; CEC, cation-exchange capacity; OC, organic carbon; EM, electrophoretic mobility; XRD, x-ray diffraction; TG, thermogravimetric; BTG, breakthrough curve; PVC, poly- vinyl chloride; HIV, hydroxy-interlayered vermiculite. 604

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DIVISION S-9-SOIL MINERALOGY

Stability and Transportability of Water-Dispersible Soil ColloidsA. K. Seta and A. D. Karathanasis*

ABSTRACTColloid migration in subsurface environments has attracted special

attention lately due to its suspected role in facilitating transport ofcontaminants to groundwater. This study was conducted to evaluate thestability and potential transport of water-dispersible colloids (WDC)through intact soil columns, and the properties of colloids and soilcolumns facilitating or retarding colloid stability and transportability.Water-dispersible colloids were fractionated from six representativesoil samples with diverse mineralogy and physicochenucal characteris-tics. Their stability was evaluated from settling-rate experiments atdifferent pH levels. The results demonstrated that colloid stability waspH dependent. Colloid transportability was assessed by introducingcolloid suspensions at a constant flux into intact soil columns represent-ing upper Bt horizons of a Maury (fine, mixed, mesic Typic Paleudalf)and a Loradale (flne-silty, mixed, mesic Typic Argiudoll) soil andevaluating characteristics of the suspensions that were eluted. Afterfive pore volumes of leaching, colloid recovery in the eluents rangedfrom 35 to 90% depending on type of colloid, initial concentration inthe influent, and soil column. The mineralogical composition of thecolloid, which was correlated with particle size, appeared to have aprofound effect on colloid transportability, following the sequencesmectitic > mixed > kaolinitic. Total exchangeable bases (TEB) andpH of WDC also significantly influenced colloid transport. Increasingcolloid concentration in the influent slightly increased colloid transport-ability. Soil columns with better macroporosity and less surface charge(Maury) transported more colloids than soil columns with less macro-porosity and higher surface charge (Loradale).

EVIDENCE that colloidal particles are mobile in thesubsurface environment abounds in the pedological

literature. The presence of argillic horizons and thehigher ratio of fine clay to total clay in illuvial horizonscompared with eluvial horizons are examples of colloidtransport resulting from natural pedogenic processes(Birkeland, 1984; Cabrera-Martinez et al., 1989; Net-tleton et al., 1975). Recently, efforts toward understand-ing colloid transport in the subsurface environment haveattracted more attention because of the suspected roleof colloids in facilitating transport of contaminants togroundwater (McCarthy and Zachara, 1989). However,information on the mechanisms and factors affectingcolloid transportability is very limited.

The transportability of colloid particles depends ontheir stability in the aqueous medium. Dispersed colloidswill have the tendency to remain in suspension, andtherefore be stable, if the repulsive potential (0) betweentwo planar colloid surfaces is maximum (Sposito, 1984).

A.K. Seta, Dep. of Soil Science, Univ. of Bengkulu, Indonesia; and A.D.Karathanasis, Dep. of Agronomy, Univ. of Kentucky, Agric. Exp. Stn.,Lexington, KY 40506. Contribution from the Dep. of Agronomy, Univ.of Kentucky, Agric. Exp. Stn., Lexington. Journal Article no. 95-06-123.Received7 Aug. 1995. *Corresponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 61:604-611 (1997).

The magnitude of 0 is controlled by ApH (the differencebetween the pH of the suspension and the pHZpc of thecolloid) and the solution ionic strength (van Olphen,1977). When ApH increases, </> also increases, thusleading to colloid stability. Increasing the ionic; strength,however, suppresses 4> values and enhances colloid floc-culation (Evangelou, 1990).

In addition to the above components, the stability ofa colloid is also affected by other factors, such as thepresence of organic components and strongly bindingagents. The adsorption of organic components on thecolloid surface may increase its negative surface charge,thus enhancing the electrostatic repulsive forces and,therefore, the stability of the particle (Jekel, 1986). Thepresence of binding agents such as Fe and Al, on theother hand, promotes colloid instability (Goldberg et al.,1990). Additionally, colloid stability is also affected bythe mineralogical composition of the colloid mixture(Arora and Coleman, 1979), the shape of the particlesand their initial concentration in suspension (Oster etal., 1980), the type of cations (Hesterberg and Page,1990), and the relative proportion of monovalent/divalentcations in the bulk solution (Shainberg and Letey, 1984).

Even though a hydrochemical environment may pro-mote their stability, colloids can be mobile only if theyare not filtered or adsorbed by the soil matrix. Particlefiltration can occur physically through interception,diffusion, and sedimentation (Vinten and Nye, 1985) orby physicochemical interaction with the charged immo-bile porous medium (Fitzpatrick and Spielman, 1972).However, the effect of filtration by porous media canbe negligible if colloids move through a preferential flowpath with relatively large velocity. Preferential flow pathssuch as fractures and soil macropores can significantlyenhance colloid transport (Smith et al., 1985).

The objectives of this study were to: (i) determine thestability and potential transport of WDC fractions withdiverse physicochemical and mineralogical compositionthrough intact soil columns, and (ii) evaluate the proper-ties of colloids and soil columns that affect WDC stabilityand transportability.

MATERIALS AND METHODSColloid Material, Fractionation,

and CharacterizationWater-dispersible colloids were fractionated from upper Bt

horizons of six soils representing the series: Beasley silt loam

Abbreviations: WDC, water-dispersible colloids; TEB, total exchangeablebases; pHzpc, pH at the zero point of charge; CEC, cation-exchangecapacity; OC, organic carbon; EM, electrophoretic mobility; XRD, x-raydiffraction; TG, thermogravimetric; BTG, breakthrough curve; PVC, poly-vinyl chloride; HIV, hydroxy-interlayered vermiculite.

604

SETA & KARATHANASIS: STABILITY AND TRANSPORTABILITY OF SOIL COLLOIDS 605

Table 1. Physicochemical propertiest of the water-dispersible colloids (WDC) used in the leaching experiments.t

WDC

BeasleyLoradaleMauryShroutsSmithdaleWaynesboro

PH

6.26.76.35.85.95.2

OC

gkg-1

8348854

CEC

—— cmol63.481.847.546.437.229.0

TEB. _,

v *^b26.529.220.617.318.48.1

FeH

15.915.916.216.467.975.7

Al.„-!L8

6.15.2

10.99.2

57.161.3

EM

um cm V"1 s"'-1.8-1.9-1.4-1.6-0.7-0.8

Mean colloiddiameter

nm220300700270400

1050

t OC = organic carbon; CEC = cation-exchange capacity; TEB = total exchangeable bases; Fe and Al (amorphous and crystalline); EMmobility,

t Analytical precision for pH = +0.03 pH units; OC = ±7%; CEC, TEB, Fe, Al, EM, and mean collloid diameter = ±10%.

: electrophoretic

(fine, montmorillonitic, mesic Typic Hapludalf), Loradale siltloam (fine-silty, mixed, mesic Typic Argiudoll), Maury siltloam (fine, mixed, mesic Typic Paleudalf), Shrouts silty clayloam (fine, mixed, mesic Typic Hapludalf), Smithdale loam(fine-loamy, siliceous, thermic Typic Hapludult), and Waynes-boro silt loam (clayey, kaolinitic, thermic Typic Paleudult).The extraction of the WDC fractions was accomplished bymixing about 10 g of soil with 200 mL of deionized H2O(without addition of dispersing agent) in plastic bottles, shakingovernight, centrifuging at 750 rpm for 3.5 min, and decanting.The concentration of the colloid fraction was determined gravi-metrically, and before it was stored as a stock suspension,0.002% (by weight) of NaN3 was added to suppress microbialactivity. A subsample of stock colloid suspension was airdried, gently crushed, and passed through a 0.23-mm openingdiameter sieve for physicochemical characterization.

The physicochemical properties of the colloid particles usedin the experiments (Table 1) were determined according toprocedures outlined by the Soil Survey Laboratory Staff (1992).Basic cations and CEC were determined by the 1 M ammoniumacetate (pH 7.0) method. Organic C was measured using aLeco Carbon Analyzer, Model CR12 (Leco Corp., St. Joseph,MI). Triplicate determinations of colloid EM were made witha Delsa 440 Doppler Electrophoretic Light Scattering Analyzer(Coulter Electronics Co., Hialeah, FL), using a 30-s counttime, 100-mV applied voltage, and 3-mA current. Colloidparticle-size distributions were determined in duplicate with amicroscan particle-size analyzer (Quantachrome Corp., Boyn-ton Beach, FL).

Amorphous oxides of Fe and Al in the WDC were extractedusing ammonium oxalate and the crystalline Fe oxides wereextracted using sodium citrate-bicarbonate-dithionite, ac-cording to procedures outlined by Shuman (1985) and McDan-iel and Buol (1991). The mineralogical composition of WDC(Table 2) was determined by XRD and TG analysis. Quantita-tive mineral estimations were based on procedures describedby Karathanasis and Hajek (1982a). Thermogravimetric analy-sis was also used to calculate the surface area of the colloidsas described by Karathanasis and Hajek (1982b).

Colloid Stability ExperimentsDuplicate colloid suspensions with a concentration of 300

mg L~' were placed in a series of 50-mL test tubes of 20-cmheight. The suspensions were adjusted to pH levels rangingfrom 2.5 to 10.0 with HC1 or KOH. A 200-nL colloid-suspension sample was pipetted at the 5-cm depth from eachtube after 4,8, 16, and 24 h of settling time. The concentrationof the pipetted colloid suspension was scanned with a Bio-TexInstruments microplate reader (Bio-Tex Instruments, Winoo-ski, VT) at 540 nm. Colloid stability at each pH level wasexpressed in terms of the percentage of colloid concentrationremaining in suspension vs. pH. The higher the percentageof the colloid remaining in suspension, the higher its colloidstability.

Preparation of Intact Soil Columns

Intact soil columns were taken from upper Bt horizons of thesame Maury and Loradale soils used for colloid fractionation.These two soils were selected because they have considerablydifferent hydraulic conductivities and OC contents. The upperBt horizon depth was sampled to represent a rooting-depthsubsurface soil layer. Each column was prepared by carvingthe soil into a cylindrically shaped pedestal of 13-cm diameterand 20-cm length and encasing with an equal length of PVCpipe of 16-cm diameter. The size of the columns was selectedto compensate for spatial variability, especially in soil hydraulicconductivity. The space between the intact soil column andthe PVC pipe was sealed with expansible polyurethane foam.The columns were left in the field overnight to allow thefoam to dry before they were separated from their base andtransported to the laboratory. A total of 26 columns for eachsoil was used in the leaching experiments (6 WDC x 2 colloidconcentrations x 2 replications -I- 2 for the Cl" tracer). Physi-cochemical properties of the undisturbed soil columns arepresented in Table 3.

Table 2. Mineralogical compositiont and surface area (SA) of water-dispersible colloid fraction.^Colloid

BeasleyLoradaleMauryShroutsSmithdaleWaynesboro

Sm + HIV

_—-—

290-

Sm + V

600—-

170—-

Int.

_—-

300—-

HIS + HIV„ !,_-!

440460

——-

HIV

_—-——

210

Mica

20015010030070

110

Kaolinite

160350360200580

56

Quartz

4060803060

120

SAm2g-'

386186190123121114

t Sm = smectite; HIV = hydroxy-interlayered vermiculite; V = vermiculite; HIS = hydroxy-interlayered smectite; Int. = interstratified mica-venniculite-smectite.

t Analytical precision for mineralogical estimations = ±10%; SA = ±7%.

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

Table 3. Physicochemical propertiest of the soil columns used in the leaching experiments.^Soil

LoradaleMaury

Sand

12090

SiltB

670560

Clay.~-ikb

210350

OC

215

BS

%59.446.0

PH

6.35.8

D,

gem"3

1.41.6

HC

cm min"1

0.3 ± 0.22.6 ± 1.7

CEC

cmolc kg"'25.221.9

Fe

— S*6.58.3

Al_i

•84.42.8

t OC = organic carbon; BS = base saturation; DI, = bulk density; CEC = cation-exchange capacity; HC = hydraulic conductivity (average of triplicatesamples); Fe and Al (amorphous and crystalline).

t Analytical precision for sand, silt, and clay = +3%; OC = ±7%; BS, CEC, Fe, and Al = ±10%; pH = ±0.03 pH units; Db = ±9%.

Leaching ExperimentsPrior to setting up the leaching experiments, the soil columns

were saturated from the bottom upward with deionized H2Oto remove air pockets. Then, the columns were oriented verti-cally and about five pore volumes of deionized H2O containing0.002% of NaN3 were introduced at the top of each column(downward flow) using a peristaltic pump at a constant flux(Darcian velocity of 2.21 cm h"1) to remove loose materialfrom the pores of the soil column and to suppress biologicalactivity. Following the washing with deionized F^O, solutedispersion was evaluated in each column by applying a stepinput of CaCl2 and monitoring the eluent Cl" concentrationcontinuously. Breakthrough curves were constructed based onreduced Cl" concentration (ratio of effluent concentration toinfluent concentration = C7C0) and pore volume (flux-averagedvolume of solution pumped per column pore volume).

Following the washing with deionized H2O, each columnwas leached with a colloid suspension, which was applied tothe top of each column at a flow rate of 2.21 cm h~'. Thisapplication rate did not allow any ponding on the top of thecolumn. All leaching experiments were run in duplicate. Anew set of soil columns was used for each leaching experiment.Two levels of influent colloid concentrations were used: 30 and300 mg L~ ' . The leaching outputs were monitored continuouslywith respect to volume of eluent and colloid concentration andwere used in constructing the BTCs. After five pore volumesof leaching, the input solution was switched to deionized H2Oto evaluate colloid desorption and detachment from the soilcolumns. Colloid concentrations in the eluent were determinedturbidimetrically by measuring the optical density of the suspen-sion at 540 nm with a Bio-Tex Instruments microplate readerand converting it to milligrams per liter of colloid accordingto a standard colloid curve. The mineralogical composition ofthe eluted colloids was determined by XRD and TG analysis,and checked against the composition of the colloids in thestock suspension. This comparison was done for the purposeof assessing colloid contamination by the column matrix andpreferential filtration of specific minerals.

RESULTS AND DISCUSSIONColloid Stability

Settling-rate characteristics of WDC fractions asaffected by pH are shown in Fig. 1. The influence ofpH on colloid stability is evident, but varies among thesamples. Regardless of the colloid type, Fig. 1 showsthat decreasing the pH below 4 results in a sharp decreasein the amount of colloids remaining in suspension. Thisinstability indicates that, at that pH range, the colloid isapproaching its pHZpc, at which the net surface potentialdecreases to the point that coagulation occurs. As thepH of the colloid approaches its pHzpc, edge-to-facebonding takes place, as well as bonding of positive Feand Al oxides to negative clay surfaces (van Olphen,

1977). At pH levels above 4, edge-to-face bonding andFe-Al bonding to particle surfaces are limited, thusincreasing colloid dispersion and stability, which reachesan optimum level at about pH 7. Table 1 shows that theoriginal pH of the Waynesboro colloid (5.2) is the lowestand that of the Loradale colloid (6.7) the highest amongthe colloids. Therefore, the Waynesboro colloid, beingcloser to its pHZPc, is expected to show a lower stabilitythan the Loradale colloid (Fig. 1).

Furthermore, high amounts of Fe and Al hydroxyox-ides present in the Waynesboro colloid (Table 1) contribrute significantly to its instability. In addition to the netflocculative power of the free Fe and Al hydroxides,additional hydroxy-interlayered Al released from HIVat low pH may have promoted the flocculation process.The abundance of kaolinite in the Waynesboro, Smith-dale, and Maury colloids may have also contributed totheir accelerated decrease in stability with decreasingpH. Kaolinite particles have a tendency to remain floccu-lated at pH <7.5 due to strong attraction of oppositelycharged crystal faces (Tama and El-Swaify, 1978). Ionic-strength changes with pH in the absence of an ionic-strength buffer would also tend to cause flocculation,especially in low-charge colloids. In contrast, the stabilityof the Loradale colloid was probably enhanced by itsOC content (Table 1). Organic coatings have been foundto promote colloid dispersion by stearic hindrance(Kretzschmar et al., 1995). The Beasley and ShroutsWDCs (Table 1) exhibited moderate but similar stabilitytrends, in spite of the abundance of smectite in theBeasley colloid. The comparable stability behavior ofillite and smectite at low pH is apparently due to theloose, smaller size floes formed by illitic particles com-pared with the denser, larger size floes formed by smectite(Greene et al., 1978).

Most colloids showed a decrease in stability with timebut to a different extent. Morrison and Boyd (1973)suggested that the rate of flocculation is a function ofcollision frequency, and is directly related to (i) thefraction of collisions that have sufficient energy, and (ii)the fraction of collisions that have proper orientation.Therefore, increasing the elapsed time will allow morecollisions and interaction among colloids to occur and,therefore, improve the chances for flocculation. In addi-tion, particle size might also play a role in colloid-settlingrates. This effect, however, is only apparent in thelow-pH range for the Waynesboro, Smithdale, andMaury colloids, which had some of the largest meandiameters (Table 1). At higher pH levels, surface-chargephenomena, such as increases in pH-dependerit chargesdue to Fe and Al hydroxides and kaolinite (Waynesboro

ICAe

^e'3I

100

75

50

25

100

75

50

25

SETA & KARATHANASIS: STABILITY AND TRANSPORTABILITY OF SOIL COLLOIDS

100

607

Loradale

10 12

= oMaury

10 12

75

50

25

g too

g 75C00C 50

§ 25b*

73

"JS O

100

10 12

Shrouts

10 12

Fig. 1. Mean settling rates of water-dispersible colloids (VVDC) fractionated from (a) Loradale, (b) Maury, (c) Waynesboro, (d) Beasley,(e) Shrouts, and (f) Smithdale soils at various pH values.

and Smithdale colloids) or organic coatings (Loradalecolloid) and coagulation of particles due to the increase inionic strength have probably overshadowed colloid-sizeeffects. Finally, the heterogeneous mineralogical compo-sition of the colloids may contribute to this phenomenon(Arora and Coleman, 1979).

Colloid TransportData from the leaching experiment were transformed

into BTCs based on reduced concentration (ratio of effluentconcentration to influent concentration = C/C0) vs. porevolume of suspension passed through the columns. Boththe Cl" (conservative tracer) and most of the colloids(except for Waynesboro) showed an initial rapid break-through (C/C0 = 0.75), suggesting that preferential flowis controlling colloid transport through the columns (Fig.2). Size exclusion appears to be the dominant mechanism

in the transport of these WDCs, as evidenced by theinitial rapid breakthroughs that occur even prior to thatof the conservative tracer. Enfield et al. (1989) havedemonstrated that large colloid particles can be physicallyexcluded from passage through smaller pore spaces inthe porous medium due to their size, thus resulting ina reduced path length and faster transport relative to thedissolved solutes. In contrast, the breakthrough of theWaynesboro colloid, which had the largest diameterparticles, occurred after that of the conservative tracer,suggesting that its initial transport through the column isprobably controlled by adsorption or screening processesrather than size-exclusion processes. After the initialbreakthrough at approximately 0.4 pore volumes, thecolloid BTCs fell below the BTG of the conservativetracer and showed a very slow approach toward a steadyC/C0. This suggests that hydrodynamic dispersion and

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

Loradale Column Maury Column

1.00

OOU 0.30

OOO 0.30

0.23

0.00

I.OO

0.73

oUD' 0.50

0.25

0.00

I.OO

0.75

OUU 0.50

0.25

0.00

- Cl- (Tracer)

—*— Loradale

—*— Shrouts

—T— Smithdale

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Pore Volume Pore Volume

Fig. 2. Mean breakthrough curves (BTCs) for water-dispersible colloids of 300 mg L"1 concentration eluted from intact Loradale and Maurysoil columns.

screening processes may control this phase of colloidtransport. The extended tailing of the BTCs may alsobe indicative of slow adsorption (Dunnivant et al., 1992;Jardine et al., 1992) of WDC by the column matrix.No indication of pore clogging by the colloids passingthrough the columns was observed, however, after fivepore volumes of leaching.

As the input solution was switched to deionized H2Oafter five pore volumes of leaching, a steep drop in theconcentration of the WDC eluents was observed (Fig.2). This suggests that desorption of WDC from thecolumn matrix was minimal. Similar results were re-ported by Ryan and Gschwend (1990) and Kretzschmaret al. (1995), who observed irreversible adsorption oforganic and mineral colloids on aquifer and saprolitematerials.

Table 4. Correlation coefficients between colloid properties andcolloid transport.

Colloid properties Correlation coefficientPHOrganic C, g kg-1

Cation-exchange capacity, cmol kg~Total exchangeable bases, cmol kg~Iron, g kg-.1Aluminum, g kg"1

Kaolinite, %Surface area, m2 g"1

Electrophoretic mobility, urn cm V"

0.884.90.760.94**

-0.67-0.67-0.59

0.580.67

** Significant at <0.05 and 0.01 probability levels, respectively.

Effect of Colloid PropertiesAfter five pore volumes of leaching, colloid recovery

in the eluent ranged from 35 to 90%, depending on thetype and concentration of the colloid and the type ofsoil column. Colloids fractionated from the Beasley andLoradale soils had the highest transportability, whilethose fractionated from the Waynesboro soil had thelowest. Among the colloid properties, pH and TEB weresignificantly correlated with colloid transport (Table 4).The effect of colloid pH on colloid migration can beexplained from its effect on colloid stability. Colloidswith pH near their pHZpc will tend to flocculate (Fig.1). Since all the colloids used in this study had pHZpcvalues below 4, the higher their pH above 4 the greatertheir stability. Increased colloid stability tends to reducethe filtering effect of the column matrix and, thus, facili-tate colloid transport. The strong correlation betweencolloid transport and TEB was anticipated because ofits high correlation with pH (Table 5). However, anadditional contribution of TEB to colloid transport mayderive from cation-exchange reactions between the col-loid-saturating cations and the column matrix, whichreduces colloid interaction with matrix surfaces and thusenhances colloid transport. The high pH and TEB ofthe Beasley and Loradale colloids explain their hightransportability among the colloids.

The statistical analysis (Table 4) also showed a negative

SETA & KARATHANASIS: STABILITY AND TRANSPORTABILITY OF SOIL COLLOIDS 609

Table 5. Linear correlation coefficients between colloid proper-ties, t

Beasley colloid]

ocCECTEBFeAlKao.SAEM

pH0.740.87*0.95**

-0.76-0.76-0.53

0.440.10

OC

0.87*0.70

-0.47-0.51-0.25

0.09-0.12

CEC

0.93**-0.74-0.78-0.64

0.530.15

TEB

-0.74-0.76-0.64

0.630.13

Fe

0.99**0.90*

-0.52-0.57

Al

0.91*0.55

-0.57

Kao.

-0.71-0.76

SA

0.25

*,** Significant at <0.05 and 0.01 probability levels, respectively.t OC = organic C; CEC = cation-exchange capacity; TEB = total ex-

changeable bases; Fe and Al (amorphous and crystalline); Kao. = kaolin-ite; SA = surface area; EM = electrophoretic mobility.

influence of Fe, Al, and kaolinite on colloid transport.For example, the Waynesboro colloid, which had a veryhigh kaolinite content and the highest amounts of Fe andAl, showed the lowest recovery percentage (average of41%), while the Beasley colloid, which had very lowkaolinite, Fe, and Al, had the highest recovery percentage(average of 82%). Iron and Al are known to be strongflocculants (Goldberg et al., 1990) and their abundancein the Waynesboro colloid should considerably decreaseits stability and transportability. Moreover, kaoliniticcolloids, due to their larger particle size (as indicatedby the negative correlation between kaolinite and surfacearea shown in Table 5), could be more easily filteredby the soil columns. These factors explain the low trans-portability of the Waynesboro colloid, which had thelargest mean diameter and one of the lowest EMs (Table1). Electrophoretic mobility is the response of a chargedparticle to an applied constant electric field. The closerthe EM value is to zero, the lower the charge at thesurface of the colloid (Sposito, 1984) and, consequently,the lower its stability.

Although the Smithdale colloid fraction had amountsof kaolinite, Fe, and Al comparable to the Waynesborocolloids (Tables 1 and 2), it showed nearly twice as highcolloid recovery (Fig. 2). This may be attributed to itshigher CEC and smectite-HIV content, and its smallermean diameter. The effects of OC and CEC on colloidmigration can be seen more clearly by comparing theMaury and Loradale colloids. Tables 1 and 2 show thatcolloid fractions from the above two soils have similarphysicochemical and mineralogical properties, but theLoradale colloids are smaller, have four times more OC,and nearly twice the CEC of the Maury colloids. Thesedifferences are apparently responsible for the highertransportability (Fig. 2) of the Loradale colloid. Thehigher OC and CEC reflect a higher negative charge onparticle surfaces, and more repulsive forces driving theparticles apart, thus inducing greater stability and trans-portability (Goldberg et al., 1990). In addition, the likelypresence of organic coatings on the Loradale colloidswould tend to inhibit attachment on the column matrixand enhance colloid mobility due to steric stabilizationby the adsorbed organic components (Kretzschmar etal., 1995).

In terms of colloid mineralogy, the magnitude of col-loid transport decreased according to the sequence smec-titic > mixed > kaolinitic. However, it is very difficult

40 34 26 18 1026

Smithdale colloid

40 34 26 18 10

Fig. 3. X-ray diffractograms of Beasley and Smithdale colloids before(A) and after elution through Loradale (B) and Maury (C) soilcolumns.

to isolate the effect of colloid mineralogical compositionor of specific minerals on colloid transportability. Forexample, in addition to the high negative charge, thetransportability of the smectitic colloids (Beasley) mayhave been induced by their smaller particle size (Table1), which minimizes filtration by the column matrix.This can be deduced from the positive correlation be-tween specific surface area and colloid transportability(Table 4). A similar argument can be made for thereduced transportability of the kaolinitic colloids, whichhad some of the largest particles. A surprising resultwas the high transportability shown by the Smithdalecolloid in spite of the high kaolinite content, the relativelylarge particle size, and the low EM. Apparently, thepresence of considerable amounts of smectite in thecolloid can overcome the inhibiting role of these factorsin the transport process.

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

Table 6. Multiple linear regression relationships between colloidtransportability (Y) and total exchangeable bases (TEB), pH,and cation-exchange capacity (CEC).

Equation R2 valuey = -0.23 + 0.73 logTEB 0.97y = 0.52 - 1.39 log pH + 0.99 logTEB 0.98y = 0.62 - 1.14 log pH - 0.31 logCEC + 1.17 logTEB 0.99

The nearly identical mineralogical composition of thecolloids in the eluent suspensions compared with that inthe input suspensions (Fig. 3) suggests that no preferentialfiltration or adsorption of specific types of mineral parti-cles took place within the soil columns. These observa-tions may appear to contradict the findings of Kaplan etal. (1993), who observed differential mobility of mineralsleached through soil columns, when comparing gibbsitewith kaolinite, quartz, or HIV. None of the colloidsamples in this study contained any gibbsite, however.Therefore, although the mineralogical composition ofthe colloids studied was a significant factor in theirpropensity for migration, it was confounded by otherfactors such as particle size (i.e., physical filtration) andwas not the dominant factor controlling colloid migrationto subsurface environments.

Multiple linear regression analyses were also carriedout employing the variables exhibiting high correlationswith colloid transportability. Among various combina-tioi s of variables and functions, it was found that thebes single independent variable predicting colloid trans-poilability was the log of TEB, the best double variablewas the log of pH-TEB, and the best triple variable wasthe log of pH-CEC-TEB (Table 6).

Effect of Colloid ConcentrationAn average of about 5% (range 1-14%) more colloids

were recovered from the 300 mg L~' suspensions thanthe 30 mg LT1 suspensions, suggesting that colloid con-centration may influence colloid migration. An exampleis provided in Fig. 4 showing the effect of colloid concen-tration on the BTG of the Maury colloid eluted throughthe Maury soil column. Increasing colloid concentrationstends to decrease the stability (i.e., enhance coagulation)of some colloid suspensions by increasing the frequency

of collisions between colloid particles in suspension.Formation of larger colloids may result in their physicalexclusion from passage through the smaller pores of thecolumn, thus shortening their path length and facilitatingtransport (Enfield and Bengtsson, 1988). In addition,increasing the colloid concentration in the input solution(and therefore, the cation load) may result in a greatersaturation of binding sites in the soil-matrix columns(Dunnivant et al., 1992), thus rendering them inactive.This inactivation phenomenon could increase tide colloidrecovery in the eluent of the 300 mg LT1 colloid suspen-sions. Kretzschmar et al. (1995) also suggested that onlymonolayers of natural colloids are deposited on pore-wallsurfaces, which may block further colloid-matrix colli-sions. Subsequent collisions between mobile and attachedcolloids are not as limiting to colloid transport due to theirsimilar charge characteristics. Therefore, a suspensioncontaining a high concentration of colloids will form themonolayer filter faster and consequently enhance colloidtransport.

Effect of Soil ColumnRegardless of colloid type, colloid transport was also

influenced by the type of the soil column. Maury columnsproduced about 26% (range 2-32%) higher recoverythan the Loradale columns. An example is provided inFig. 5 showing the effect of soil-column type (Mauryvs. Loradale) on the BTG of the Waynesboro colloid at300 mg LT1 concentration. The increased colloid recov-ery observed in the Maury columns may be due to thelower OC content and CEC of the soil matrix, whichallows less interaction with the eluting colloids.

The steeper slope in the ETC of the conservative tracerin the Maury soil column than in Loradale (Fig. 2) alsosuggests a better macropore system in the former soilthat facilitates colloid migration. A better macroporesystem in the Maury soil column is also supported byits higher saturated hydraulic conductivity compared withthat of the Loradale (Table 3). Macropores can be formedby soil fauna, soil cracks, plant roots, or natural soilpipes and can significantly increase preferential flow andcolloid transport through the soil matrix (Smith et al.,1985). Earlier investigations with the Maury soil have

U

1.00

0.75

0.50

0.25

o.oo*-0

-•- 300 mg/L

-Q- 30 mg/L

Pore VolumeFig. 4. Effect of colloid concentration on the breakthrough curve of

the Maury colloid eluted through Maury soil columns.

oy.U

1.00

0.75

0.25

—•— Maury

—D— Loradals

0.00 •—————'——————'——————'—————0 1 2 3 4

Pore VolumeFig. 5. Effect of soil column type (Maury vs. Loradale) on the break-

through curve of the Waynesboro colloid at 300 mg L"1 concen-tration.

SETA & KARATHANASIS: STABILITY AND TRANSPORTABILITY OF SOIL COLLOIDS 611

shown that its extensive macropore system plays animportant role in transport processes, especially if thethe flow rate of water percolating through the soil is1 cm h"1 or higher (Quisenberry et al., 1994). Highermacroporosity also results in a smaller pore-wall surfacearea in contact with the flowing suspension, which allowsfewer colloid-matrix interactions and more rapid devel-opment of the colloid monolayer filter. As a consequence,colloid transport is facilitated through the column(Kretzschmar et al., 1995).