Mechanisms of Dissolved Organic Carbon Adsorption On SoilP. M. Jardine,* N. L. Weber, and J. F. McCarthy

ABSTRACTThe subsurface transport of inorganic and organic contaminants

may be strongly related to the movement of dissolved organic carbon(DOC) through a soil profile. A variety of soil chemical and hy-drologic factors control the mobility of the DOC, which may enhanceor impede the transport of the associated contaminants. In thisstudy, the sources of DOC adsorption on two proposed waste-sitesoils are defined, and the chemical mechanisms operative during theadsorption process are specified. Adsorption isotherms for the twosoils determined at constant pH, ionic strength (/), and temperatureindicated that DOC adsorption increased with increasing soil profile

P.M. Jardine and J.F. McCarthy, Environmental Sciences Div., OakRidge National Lab., P.O. Box 2008, Oak Ridge, TN 37831-6038.N.L. Weber. Chemistry Dep., Blackburn College, Carlinville, IL62626. Joint contribution from Oak Ridge National Lab., Univ. ofTennessee, Knbxville, and Blackburn College. This research wasfunded by the Subsurface Science Program of the Ecological Re-search Div., Office of Health and Environmental Research, U.S.Dep. of Energy under contract DE-AC05-84OR21400 with MartinMarietta Energy Systems, Inc. Publication no. 3354. Received 6Feb. 1989. "Corresponding author.Published in Soil Sci. Soc. Am. J. 53:1378-1385 (1989).

depth. Different adsorption capacities were exhibited by the twosoils, however, which was related to their contrasting indigenousorganic matter contents and mineralogies. The adsorption of DOCby the soils was not a function of solution / (/ = 0.001 to 0.1 molL ' using NaCl); however, DOC adsorption was dependent on so-lution pH, with maximum adsorption occurring at ^4.5. Competi-tive ion-exchange studies using Na2SO4 as an ionic-strength adjustersuggested that a portion of the DOC was electrostatically bound tothe soil via anion exchange. By using thermodynamic principles, thepredominant mechanism of DOC retention by the soil was found tobe physical adsorption driven by favorable entropy changes. This issupported by preferential adsorption of the hydrophobic organic sol-utes to the soil relative to the hydrophilic organic solutes.

DISSOLUTION of soil organic matter during stormevents is a common phenomena. Dissolved or-

ganic C is thus mobilized with the aqueous phase tolower profile depths. Since this natural organic ma-terial acts as a sink for numerous organic and inor-

JARDINE ET AL.: ORGANIC CARBON ADSORPTION MECHANISMS 1379

ganic contaminants (Bertha and Choppin, 1978; Lan-drum et al., 1984; McCarthy and Jimenez, 1985),mobile DOC is believed to enhance the transport ofthe associated contaminants through the porous me-dia (Champ et al., 1984; Nelson et al., 1985;Gschwend and Wu, 1985; McCarthy and Zachara,1989). If, however, the DOC is immobilized duringtransport, the mobility of the associated contaminantswill be impeded.

Research has shown that DOC may be immobilizedthrough complex interactions with mineral surfaces(Greenland, 1971; Davis and Glour, 1981; Sibandaand Young, 1986). Davis (1980) demonstrated thatextracts of fulvic and humic acids from lake sedimentswere significantly adsorbed on gibbsite, with maxi-mum adsorption occurring near pH 5. He suggestedthat the adsorbed DOC may act as a template formetal complexation and binding. A variety of syn-thetic Fe oxides and hydroxides also adsorb apprecia-ble quantities of DOC (Schwertmann, 1966; Sibandaand Young, 1986). Tipping (1981a,b) has shown thathumic substances are bound to lab-synthesized goe-thite, hematite, and amorphous Fe gels, with largermolecular-weight humics exhibiting a greater adsorp-tion potential relative to smaller humic molecules.Phyllosilicates have also been noted to adsorb signif-icant amounts of DOC (Kodama and Schnitzer, 1974;Wershaw and Pinckney, 1980; Davis, 1982). Davis(1982) suggested that fulvic and humic extracts fromlake sediments were adsorbed on the edge sites of ka-olinite, while Inoue and Wada (1968) found that DOCwas extensively adsorbed by halloysite, montmoril-lonite, and allophane.

Limited research exists that expresses the potentialimmobilization of DOC by soil. Adsorption of humicsubstances on Fe-rich tropical soils was shown by Si-banda and Young (1986), while Leenheer (1980)found that the hydrophpbic components of DOC hada higher affinity for soils relative to the hydrophiliccomponents. Investigating the spatial heterogeneityand rate of nutrient transport in a forest soil, Jardineet al. (1989) found that natural DOC was highly re-active with the soil even under conditions of extremepreferential flow.

The mechanisms by which DOC is adsorbed onmineral surfaces is largely unknown. Parfitt et al.(1977) suggested that the carboxyl groups of fulvic andhumic acid replaced surface OH" from gibbsite, goe-thite, and imogolite. Ligand exchange of surface co-ordinated OH- and H2O from Fe oxides by humicsubstances has also been noted by Tipping (1981a,b).

Inoue and Wada (1968) speculated that DOC was ad-sorbed on allophane through an anion-exchangemechanism. Kodama and Schnitzer (1974) believedthat fulvic acid displaced surface water in sepolitethrough a nonionic hydrophobic adsorption mecha-nism. Many mechanisms postulated for DOC adsorp-tion on mineral surfaces are hypothetical. Few studieshave actually sought to decipher the various modes ofDOC adsorption in soil systems and how this im-mobilization process affects the transport of inorganicand organic contaminants.

In order to predict the fate of colloid-mediated con-taminant transport through the subsurface, the mech-anisms by which DOC is attenuated to soil must beclearly understood. In this study, we define the sourcesof DOC adsorption in complex soil systems and spec-ify the chemical mechanisms operative during the ad-sorption process.

MATERIALS AND METHODSSite Description and Sample Preparation

Bulk soil samples were obtained from two contrasting for-ested watersheds on the Oak Ridge Reservation in east Ten-nessee. The samples were air dried and passively ground topass a 2-mm sieve. One site was located in the WalkerBranch Watershed and samples were obtained from the A,E, Btl, and Bt2 horizons. This soil (a fine loamy, siliceous,thermic Typic Paleudult) has a deep profile of 30 m withpH values of 4 to 6 and CEC values of 4 to 6 cmolc kg-'.The second site was located in the Melton Branch Wa-tershed and samples were obtained from the A and B ho-rizons. This soil (a loamy skeletal, mixed, thermic, shallowTypic Dystrocrept) has a shallow profile of 0.5 to 3.0 m withpH and CEC values of 4 to 6 and 10 to 20 cmolc kg-', re-spectively. Select chemical, physical, and mineralogicalproperties of the two soils are provided in Table 1. Particle-size analysis was performed using the hydrometer method(Day, 1965); the organic matter content was determined us-ing a Leco total carbon analyzer (Leco Corp., St. Joseph,MI); and dithionite-citrate-bicarbonate (DCB) extractableFe was determined by standard methods (Mehra and Jack-son, 1960). Mineralogy of the <2-fim clay fraction was es-timated using x-ray diffraction with kaolinite and gibbsitequantified through differential scanning calorimetry (DSC).

Solution PreparationThe DOC solution used in this study was obtained from

a surface stream near a peat deposit in Hyde County, NorthCarolina. The total organic C concentration was r~53 ng,mL-' with 69% of the total C as hydrophobic solutes (34%hydrophobic acids and 36% hydrophobic neutrals) and 31%

Table 1. Selected chemical, mineralogical, and physical properties of the Walker and Melton Branch soils.

Horizon

Particle size analysis

Sand Silt ClayOrganic

matter contentDCB extractable

FeMineralogy of

<2 jim clay fraction!

Walkergkg-

AEBtlBt2

MeltonB

34.938.232.234.2

30.8

58.952.944.241.5

50.4

6.28.9

23.624.3

18.8

3.50.80.20.1

0.6

9.5813.0616.8221.28

28.71

K27 V27 VC14 Q13 110 IS5 G3 F,K« VCM I, Q, IS6 V5 M5 G2 F,K^ VC22 Mn V, IS6 15 Q4 F, G,

I45 IS20 V10 K, VC6 M5 Q3 F,t K = kaolinite; V = vermiculite; VC •= chloritized vermiculite; I = illite (soil mica); IS = interstratifled 2:1; Q = quartz; G = gibbsite; M = montmorillonite;

F = feldspar. Subscripts refer to the percent by weight of each mineral, where t = trace.

1380 SOIL SCI. SOC. AM. J., VOL. 53, SEPTEMBER-OCTOBER 1989

as hydrophilic solutes using the DOC-fractionation methoddescribed by Leenheer and Huffman (1979). The propor-tions of hydrophobic and hydrophilic DOC in this surfacestream are very similar to those found for soil-solution DOCobtained from the surface horizon of the Walker Branch soilduring storm events (Jardine, 1989, unpublished data). Buf-fle et al. (1978) similarly found that distilled-water extractsof soil yielded DOC solutions with molecular-weight distri-butions similar to natural freshwater DOC. They noted thatalkaline extracts of soils and peat contained organic com-pounds of much higher molecular weight than was typicallyfound in surface streams. In order to initiate the adsorptionstudies, the stock DOC solution was diluted 2X, 3X, 4X,and 5X with deionized water to obtain five solutions ofvarying DOC concentration. The pH of the solutions was6.0.Adsorption Studies

Whole Soils. Batch equilibrium adsorption of DOC onall soils was performed. Subsamples of Walker A, E, Btl,and Bt2 horizon soils (1.5, 1.0, 0.5, and 0.25 g, respectively)and Melton A and B horizon soils (1.5 and 0.5 g, respec-tively) were weighed into 50-mL glass centrifuge tubes. Thediffering quantities of soil used were related to the DOCadsorption capacity of each soil with the quantities of soilchosen to provide isotherms with an equilibrium solution-DOC concentration range of 0 to =;20 mg/L. This concen-tration range represents typical soil-solution DOC levels en-countered at these sites throughout the year (Jardine et al.,1989, unpublished data). Known volumes of 0.03 mol Lr1

NaCl, adjusted to pH 6.0, and the various DOC solutions(10 and 20 mL, respectively) were added to the tubes. Ablank sample containing no DOC was prepared by addingdeionized water to the NaCl solution and soil. Ionicstrength, pH, and temperature were kept constant for eachparticular adsorption isotherm. To investigate the effect ofsolution I on DOC adsorption by soil, three concentrationsof NaCl (i.e., 0.3, 0.03, and 0.003 mol L-', pH = 6) wereused, resulting in final solution 7s of 0.1, 0.01, and 0.001mol Lr1. The effect of solution pH on DOC adsorption wasinvestigated by premixing the NaCl and DOC solutions andadjusting the pH with NaOH or HC1 prior to equilibrationwith the soil. The temperature was maintained at 296 K forall adsorption isotherms except those involved in the tem-perature study.

The soil solutions were gently agitated on a reciprocalshaker for 2 d at 120 rpm until equilibrium was reached, asdetermined from kinetic analysis. The mixtures were cen-trifuged at 3000 rpm for 20 min and then filtered througheither 0.4- or 0.8-/tm polycarbonate nuclepore filters. Super-natant was analyzed for pH and DOC, the latter analyzedwith a total organic C analyzer (Model 700 Total CarbonAnalyzer, OI Corp., College Station, TX). All experimentswere done in duplicate and adsorption isotherms were plot-ted as adsorbed DOC (mg DOC kg-1 solid) vs. equilibriumsolution concentration (mg DOC L"1).

Soils with Organic Matter Removed. It was of interest todetermine the effect of indigenous organic C on the subse-quent adsorption of added DOC. Therefore, organic C wasremoved from select soils using hot 30% H2O2 in a NaOAcbuffer. The samples were then washed with 0.01 mol L-'NaCl to remove the entrained salts, air dried, and DOC-adsorption studies performed in the same manner as.pre-vious described.

Soils with Iron Oxides Removed. It was of interest to de-termine which soil mineral constituents were responsible forDOC adsorption. Therefore, Fe oxides were removed fromsubsamples of Walker E, Btl, and Bt2 and Melton B hori-zons. Initially, the organic matter was removed from the

samples with two washings of hot 10% NaOCl solution (pH= 9.5), followed by two washings with 2% Na2CO3 (pH =9.5) and deionized water. The samples were then treated at353 K three times with the DCB treatment, using appro-priate quantities of 0.3 mol L-1 Na citrate, 1 mol L-1

NaHCO3, and crystalline Na2S2O4 to remove Fe oxides(Mehra and Jackson, 1960). The samples were washed ex-tensively with 0.01 mol L-1 NaCl to remove entrained salts.Samples were air dried and adsorption studies were thenperformed in the same manner as previously described.

Pure Minerals. To determine more specifically what min-erals were contributing to DOC adsorption in the Walkerand Melton soils, pure minerals representing those foundnaturally in these soils (Table 1) were obtained for study.The minerals kaolinite, illite, gibbsite [A1(OH)3], and he-matite (a-Fe2O3) were used. Poorly crystalline Georgia ka-olinite was obtained from the clay minerals repository, Dep.of Geology, Columbia, MO. Illite was obtained from Bea-vers Bend in McCurtain Co., OK. Gibbsite was obtainedfrom Reynolds Metal Co. with excellent purity as deter-mined by DSC, and hematite was obtained from Ironwood,MI via Wards Natural Science Establishment, Inc., Roch-ester, NY. Hematite was ground in a ball mill for 5 minprior to use, illite was hand ground with a mortar and pestle,and the remaining minerals were used directly without al-teration. The pure minerals all had <0.08% total C as de-termined by the Leco total carbon analyzer. Adsorptionstudies were performed on all minerals in the same manneras previously described under conditions of constant 7, pH,and temperature.

Fractionation of Equilibrium DOC SolutionsTo determine which components of the DOC were ad-

sorbed by the Walker and Melton soils, fractionation anal-ysis was performed on various equilibrium DOC solutions(Leenheer and Huffman, 1979). Subsamples of each soil ho-rizon were equilibrated with stock DOC solution, using pre-viously described methods. The equilibrium supernatantwas fractionated by an Amberlite XAD-8 resin (Rohm HaasCo., Philadelphia, PA) into acid and neutral hydrophobicorganic solutes and hydrophilic organic solutes. The hydro-phobic solutes are defined as those sorbed onto the nonionicresin, and are differentiated into acid and neutral fractionsby sorption/desorption controlled by pH adjustment. Thehydrophilic solutes pass unaltered through the resin.

RESULTS AND DISCUSSIONAdsorption of Dissolved Organic Carbon

by Whole SoilsAdsorption isotherms for DOC interaction with

Walker and Melton Branch soils indicated that a sig-nificant quantity of DOC was attenuated by the soils(Fig. 1-3). The isotherms were maintained at constant/(i.e., 0.1, 0.01, and 0.001 mol L-1), temperature (296K), and pH. The initial pH of each system was 6.0, tomimic the natural pH of the DOC stock solution, butdecreased relative to the characteristic equilibrium pHof the soil (Fig. 1-3). The decreasing equilibrium pHvalues were a function only of the soil, since blankscontaining no added DOC had the same equilibriumpH as samples with added DOC. Thus, each pointalong a particular curve had the same equilibrium pH,which is an essential feature of an isotherm (Fig. 1-3). Variations in solution 7 from 0.001 to 0.1 mol Lr1

using NaCl had a negligible effect on DOC adsorptionfor the soils studies. This contrasts with the results of

JARDINE ET AL: ORGANIC CARBON ADSORPTION MECHANISMS 1381

Da vis (1982), who showed a decrease in DOC ad-sorption on Al oxides with increasing 7 from 0.01 to0.1 molL-1 NaCl.

Significantly less DOC was adsorbed by the WalkerE horizon soil relative to the Walker Btl and Bt2 ho-rizon soils, with the latter soils adsorbing similarquantities of DOC (Fig. 1 and 2). Later in the man-uscript, it will become apparent that the difference inDOC adsorption between the E and B horizon soils isthe result of differing quantities of minerals and in-digenous organic C characteristic to each horizon (Ta-ble 1). The Melton B horizon soil had a significantlylower adsorption capacity for DOC relative to theWalker B horizon soils but a similar adsorption ca-pacity to the Walker E horizon soil (Fig. l-3).This wasprimarily due to the large quantity of indigenous or-ganic C on the Melton B horizon soil (Table 1). Re-moval of the initial organic C from the soil resultedin an approximate four-fold increase in DOC adsorp-tion (Fig. 4), suggesting that the indigenous organicmatter impeded further adsorption of added DOC.This effect is not as pronounced on the Walker Btland Bt2 soils, since they have very low amounts of

indigenous organic C (Table 1). These results indicatethat the Walker and Melton soils have a limited ad-sorption capacity for DOC, suggesting monolayer cov-erage. Impeded adsorption of added DOC to soils con-taining appreciable quantities of indigenous organic Cwas also evident when DOC was added to Walker andMelton A horizon soils with an overall desorption oforganic C observed rather than adsorption. Later inthe text (i.e., Table 2), it will be shown that a slightredistribution of the added DOC with the indigenousorganic C occurs during the equilibration process.

Adsorption of Dissolved Organic Carbonby DCB-treated Soils and Pure Minerals

It was of interest to determine what indigenous soilminerals were the primary sinks for DOC adsorption.To determine the extent of DOC adsorption by Feoxides and hydroxides in the various soils, subsam-ples with indigenous organic C removed were treatedwith a DCB solution that selectively removed Fe min-erals from the soil. Adsorption isotherms for DOCinteraction with these treated soils were determined.

600

!1o

300-

200

100-

WALKER E HORIZON

1=0.1, pH=4.9 *

1=0.01, pH=5.1 o

1=0.001, pH=5.2 a

4 6 6 10 12EQUIL SOLN CONC (mg DOC/L)

16

Fig. 1. Isotherms for dissolved organic C adsorption on Walker Esoil horizon at constant ionic strength (I), pH, and temperatureof 296 K.1800

1500-

1200-

oQQEdm§C/}

900-

eoo

300-

WALKES SOIL

1=0.1 Btl, pH=4.7 •B12, pfes.7 •

1=0.01 Btl, pH=5.0 oBt2, pH=5.7 D

1=0.001 Btl. pH=5.0 •pH=5.6 •

5 10 15EQUIL SOLN CONC (mg DOCA)

20

Fig. 2. Isotherms for dissolved organic C adsorption on Walker Btland Bt2 soil horizons at constant ionic strength (/), pH, and tem-perature of 296 K.

500

400-

300-_OQQ

200 HoC/3

100-

MELTON B HORIZON1=0.1, pH=4.5 *1=0.01, pH=4.6 o1=0.001, pH=4.3 °

0 5 10 15 20EQUIL SOLN CONC (mg DOC/L)

Fig. 3. Isotherms for dissolved organic C adsorption on Melton Bsoil horizon at constant ionic strength (/), pH, and temperatureof 296 K.1800

OoQ 900QWCO

MELTON B HORIZOK

—without initial O.C.

with initial O.C.

0 5 10 IS 20EQUIL SOLN CONC (mg VOC/L)

Fig. 4. Isotherms for dissolved organic C adsorption on Melton Bsoil with indigenous organic C (O.C.) present and removed at /= 0.01, pH = 4.6, and temperature of 296 K.

1382 SOIL SCI. SOC. AM. J., VOL. 53, SEPTEMBER-OCTOBER 1989

Table 2. Total quantities and percentages of total hydrpphobic andhydrophilic dissolved organic C adsorbed by the various Walkerand Melton Branch soil horizons.

HorizonTHbf

adsorbedTHlf

adsorbed% THb % TH1

adsorbed adsorbed-mg/kg-

WalkerAEBtlBt2MeltonB

12.6430.2973.8

1122.0

396.0

-140.488.2

277.2207.6

76.8

_

83.078.084.0

83.8

_

17.022.016.0

16.2t THb = Total hydrophobic organic C; TH1 = Total hydrophilic organic C.

A significant decrease in adsorption occurred on theWalker Btl soil after removal of Fe oxides and hy-droxides (Fig. 5), suggesting that Fe minerals play asignificant role in DOC adsorption. Although notshown, similar results were observed for the WalkerE and Bt2 and Melton B soils. The Fe minerals foundin the Walker soil are primarily hematite (a-Fe2O3)and maghemite (7-Fe2O3), which exist mainly as dis-crete particles (S.Y. Lee, 1989, personal communica-tion). Their abundance increases with depth (Table 1)in direct correspondence with increasing DOC ad-sorption with depth. Tipping (1981b) and Ho andMiller (1985) have shown dramatic adsorption ofhumic acid by hematite. At an arbitrary equilibriumDOC solution concentration of 28 mg Lr1 and pH 7,=:750 mg DOC kg-1 solid was adsorbed by the Feminerals in the Walker Btl soil (Fig. 5). This com-pares surprisingly well with 695 mg kg-1 adsorbedDOC on the pure mineral hematite at a similar pH,even though surface area differences between the sam-ples were not considered.

The Fe minerals found in the Melton soil are pri-marily amorphous oxide coatings on phyllosilicateswith no goethite detected using powder-mount X-raydiffraction (R. Arnseth, 1989, personal communica-tion). The amorphous Fe accounts for as much as 70%of the DOC adsorbed by the Melton soil at pH 7 (datanot shown). In comparison, the contribution of Feminerals to DOC adsorption in the Walker soils at pH

1200-

IfALKER Btl HORIZON

WHOLE SOIL, pH=7.0 °DCB TREATED SOIL, pH=7.0

10 15 20EQUIL SOLN CONC (mg DOC/L)

25

Fig. S. Isotherms for dissolved organic C adsorption on the WalkerBtl soil with Fe oxides removed and without Fe oxides removed(whole soil) at / = 0.01, pH = 7.0, and temperature of 296 K.

7 is in the vicinity of 50% (Fig. 5). This agrees withthe larger quantity of DCB-extractable Fe in the Mel-ton soil relative to the Walker soil (Table 1). Also, thenoncrystalline Fe oxides and hydroxides in the Meltonsoil may have a more extensive and active surfacethan the more crystalline Fe minerals in the Walkersoil, with the former minerals able to adsorb largeramounts of DOC (Greenland, 1971; Tipping, 1981b).

Significant quantities of DOC were still adsorbed tothe Walker and Melton soils even after Fe oxide re-moval (Fig. 5), suggesting that the remaining clay min-erals in the soil played an important role in DOC ad-sorption. The pure minerals kaolinite and gibbsitewere used to mimic soil minerals common to theWalker soil, with kaolinite being the predominant claymineral in the <2-jum clay fraction (Table 1). Theseminerals adsorbed appreciable quantities of DOCsimilar in magnitude to the Walker B horizon wholesoils (Fig. 2 and 6). Parfitt et al. (1977) and Davis(1982) also have shown significant adsorption of DOCon gibbsite and kaolinite, respectively. The isothermfor DOC adsorption on kaolinite had a constant pHof 4.5; however, the gibbsite "isotherm" had a vari-able pH which increased with increasing concentra-tion of adsorbed DOC (Fig. 6). The mechanism ofDOC adsorption of the latter mineral is suggestive ofa ligand exchange reaction in which DOC displacessurface hydroxyls from the gibbsite. Adsorption ofDOC onto kaolinite may proceed by alternate mech-anisms, since its equilibrium solution pH is not afunction of the quantity of DOC adsorbed. The mech-anism^) may be similar to DOC adsorption on theWalker soils since their equilibrium pH values are alsoconstant, suggesting kaolinite plays an important rolein DOC adsorption on this soil relative to gibbsite.This is consistent with the fact that the quantity ofkaolinite increases with depth, as does the DOC ad-sorption capacity. The reverse is true for gibbsite (Ta-ble 1).

One cannot neglect the role of other potential ad-sorbents found in the Walker clay fraction. Thesmaller quantities of higher charged 2:1 minerals mayalso have a substantial contribution toward DOC ad-

Q 1200-

§DaK

1 600-

KAOLINITE pH=4.5 o

GIBBSITE pH=4.2-5.3 o

0 5 10 15 20 25EQUIL SOLN CONC (mg DOC/L)

Fig. 6. Isotherms for dissolved organic C adsorption on kaoliniteand gibbsite at / = 0.01 and temperature of 298 K. The kaoliniteisotherm maintained a constant pH of 4.5 while the gibbsite "iso-therm" had a variable pH ranging from 4.4 to 5.3 at low and highquantities of adsorbed dissolved organic C, respectively.

JARDINE ET AL.: ORGANIC CARBON ADSORPTION MECHANISMS 1383

sorption (Inoue and Wada, 1968). However, the 2:1phyllosilicate illite, used to mimic soil minerals com-mon to the Melton soil (Table 1), was found to adsorb~85% less DOC relative to kaolinite. It is possiblethat the large DOC macromolecules are sterically hin-dered from entering the interlayer region of the 2:1nonexpanding clays.

Mechanisms of Dissolved Organic Carbon Retentionby Soil

Once the soil constituents contributing to DOC ad-sorption were identified, the underlying chemicalmechanisms operative during this adsorption processwere determined. A competitive exchange study wasinitiated on the Walker Btl soil to determine the con-tribution of anion exchange during DOC adsorption.The typical ionic strength adjuster NaCl was replacedwith Na?SO4 (/ = 0.1, 0.01, 0.001 mol L-1) since the804" anions exhibits strong electrostatic interactionswith soil positive charges. The SO^ anion should bindonly on the positive side of the soil zero point ofcharge (ZPC) and should not induce ligand exchangereactions where surface OH- groups are replaced. Thisis supported by similar equilibrium pH values for thesulfate and chloride systems (Fig. 2 and 7). Wildenseeand Baham (1988) have also shown that ligand ex-change is not a suitable mechanism describing sulfateadsorption on kaolinite. In contrast to the Cl~ system,DOC adsorption on the Walker soil decreased as theconcentration of SO£- increased; suggesting that a por-tion of the DOC was bound to the soil via anion ex-change (Fig. 2 and 7). The highest concentration of804" used far exceeds the sulfate adsorption capacityof the soil (Johnson et al., 1986). Therefore, it is rea-sonable to suggest that this concentration of SO^ suf-ficiently eliminated all anion-exchange reactions ofthe DOC, which account for =^25% of the total DOCadsorbed by the soil (Fig. 7).

Although anion exchange is a significant mecha-nism of DOC adsorption on these soils, it cannot ac-count for a large portion of the adsorption process(Fig. 7). This is substantiated by investigations of ad-sorbed DOC in a Cl~ matrix as a function of the equi-librium solution pH (Fig. 8). The characteristic ad-

1500-

1200-

O 900QO

O<

600

300-

SULFATE MATRIXWALKER Btl HORIZON

1=0.1, pH=5.11=0.01, pH=5.21=0.001, pH=5.2

10 15 20EQUIL SOLN CONC (mg DOC/L)

25

Fig. 7. Isotherms for dissolved organic C adsorption on the WalkerBtl soil at constant ionic strength (I), pH, and temperature of296 K, using Na2SO4 as the / adjuster.

sorption curves with pH at a variety of equilibriumsolution DOC concentrations are similar to thosefound for pure minerals (Davis and Glour, 1981). Thecurves show maximum DOC adsorption on theWalker soil at pH 4.5, which monotonically decreasesat higher and lower pH values (Fig. 8). The fact thatsignificant DOC adsorption exists at pH values >7suggests that mechanisms other than anion exchangeare also operative in this system. In this region ofhigher pH, the DOC macromolecules will be ionizedand have predominately negative charges, as will thesurfaces of most soil minerals, since the solution pHwill exceed their ZPC values. The DOC macromole-cules must overcome large repulsive forces during ad-sorption to the soil in this region of high pH.

To decipher what additional mechanisms were con-tributing to DOC adsorption by the Walker soil, iso-therms were prepared at two temperatures in aNa2SO4 (/ = 0.1 mol Lr1) matrix (Fig. 9). The con-centration of the SOI" used far exceeds the sulfate ad-sorption capacity of the soil (Johnson et al., 1986);thus, anion exchange mechanisms by the DOC areeliminated. The alternate mechanism(s) of DOC ad-

2000-

g 1500-

8«9 1000

WALKER Btl HORIZON

20 mg DOC/L

15 mg DOC/L

10 mg DOC/L

5 mg DOC/L

EQUIL SOLN pHFig. 8. Adsorbed dissolved organic C on the Walker Btl soil as a

function of equilibrium solution pH at a variety of equilibriumsolution-dissolved organic C concentrations, temperature of 296K, and / = 0.01 using NaCl.

1o8Q

1Q

WALKER Btl HORIZON

296K2B3K

0 5 10 15 20 25EQUIL SOLN CONC (mg DOC/L)

Fig. 9. Isotherms for dissolved organic C adsorption on Walker Btlsoil at temperatures of 283 and 296 K with pH = 5.0 and / =0.01 using Na2SO4.

1384 SOIL SCI. SOC. AM. J., VOL. 53, SEPTEMBER-OCTOBER 1989

Table 3. Total quantity of hydrophobic dissolved organic C and thepercentage contribution of hydrophobic acid and neutral organicsolutes to the total hydrophobic dissolved organic C adsorbed bythe various Walker and Melton Branch soil horizons.

Horizon

WalkerEBtlBt2MeltonB

THbfadsorbedmg kg-'

430.2973.8

1122.0

396.0

% HbAf adsorbedofTHb

48.846.952.2

44.4

% HbNf adsorbedofTHb

51.253.147.8

55.6

t THb = total hydrophobic; HbA = hydrophobic acid; HbN = hydrophobicneutral.

sorption had a very low temperature dependency thatwas slightly endothermic, since more DOC was ad-sorbed at the higher temperature. Using the Clausius-Clapeyron equation (Atkins, 1982), isosteric heats ofadsorption, AH, were calculated that ranged from 4 to8 kJ mol"1. Since the adsorptive process is an endo-thermic reaction with a low temperature dependency,ligand exchange cannot be a plausible mechanism. Li-gand-exchange reactions are very temperature depen-dent, exhibiting large exothermic AH values (Ha-maker and Thompson, 1972; Hassett et al., 1981). Anearlier observation, that the equilibrium solution pHwas not a function of the quantity of DOC adsorbedby the soils (Fig. 1-3), further supports the contentionthat ligand exchange is insignificant during the DOCadsorptive process. The virtual temperature indepen-dency of this reaction (Fig. 9) suggests that the DOCadsorptive process is entropy driven, and a physicaladsorptive mechanism is dominant. Evans and Rus-sell (1959) also noted that fulvic and humic acid ad-sorption on montmorillonite was temperature inde-pendent, while Vansant and Uytterhoeven (1972)have shown the adsorption of alkyammonium ions onmontmorillonite to be temperature independent andcontrolled by entropy effects.

To strengthen the argument that physical adsorp-tion mechanisms significantly contribute to DOC ad-sorption by soil, select equilibrium DOC solutions foreach soil were fractionated into the hydrophobic andhydrophilic organic portions (Leenheer and Huffman,1979). Fractionation analysis revealed that the hydro-phobic organic solutes were preferentially adsorbed bythe soil compared to the hydrophilic organic solutes(Table 2). Of the total DOC adsorbed by the soils, over80% was hydrophobic organic solutes and less than20% was hydrophilic organic solutes. The Walker Ahorizon, which had a net desorption of DOC, releasedmore hydrophilic solutes into solution while remov-ing some hydrophobic solutes (Table 2). The hydro-phobic organic solutes were further separated andcharacterized as acidic and neutral hydrophobic sol-utes, both of which showed equivalent adsorption forthe soils (Table 3). Of the total hydrophobic organicsolutes adsorbed, 50% were hydrophobic acids and50% were hydrophobic neutrals. The preferential ad-sorption of hydrophobic organic solutes on soil rela-tive to hydrophilic organic solutes is most likely be-cause of the additive effect of physical sorption(Leenheer, 1980). Because weak positive sorbate-sor-bent interactions overcome extremely weak solute-sol-

vent interactions, the hydrophobic organic solutes areadsorbed by the soil through an entropy driven pro-cess related to the destruction of highly structuredwater surrounding the solvated DOC macromolecules(Hassett et al., 1981).

CONCLUSIONAdsorption isotherms for DOC interaction with two

proposed waste-site soils have been determined atconstant pH, /, and temperature. The adsorption ofDOC increased with increasing profile depth; how-ever, different adsorption capacities were exhibited bythe two soils, which was related to their contrastingindigenous organic matter contents and mineralogies.Large amounts of indigenous soil organic mattertended to impede the adsorption of added DOC. Bothcrystalline and noncrystalline Fe oxides and hydrox-ides from the soils retained ~50 to 70% of the totaladsorbed DOC. Phyllosilicates in the <2 pm clay frac-tion adsorbed the remaining DOC, with kaolinite ex-hibiting a larger adsorption capacity than illite.

Hydrophobic organic solutes were preferentially ad-sorbed by the soils relative to hydrophilic organic sol-utes. The primary adsorption mechanism of these or-ganic solutes is believed to be physical adsorptiondriven by favorable entropy changes. An anion-ex-change mechanism accounts for the remaining ad-sorbed DOC, which is ^25% of the total attenuatedDOC. Cation-bridging mechanisms during the DOCadsorption process cannot be entirely ruled out; how-ever, their contribution is unlikely since we utilizedNa+-saturated conditions. Ho and Miller (1985) pre-sented spectroscopic evidence suggesting metal-humicacid bridging mechanisms to oxide surfaces were un-likely.

Knowledge of the DOC adsorption sources in com-plex soil systems, and identification of the chemicalmechanisms operative during the adsorption process,allow more accurate prediction of the fate of colloid-mediated contaminant transport through the subsur-face.

ACKNOWLEDGMENTSWe appreciate the efforts of Dr. L.W. Zelazny, Steve Feld-

man, and Paul Gassman for assistance in X-ray diffractionanalyses and Dr. Frank Wobber, the contract officer forDOE.

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