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Characterization of Soil Particle Surfaces Using Adsorption Excess Isotherms Marta Berka,*,$,# Sandrine Palau Pla,# and James A. Rice*,# Department of Colloid and EnVironmental Chemistry, UniVersity of Debrecen, Debrecen, H-4010 Debrecen Egyetem te´ r 1, Hungary, and Department of Chemistry and Biochemistry, South Dakota State UniVersity, Brookings, South Dakota 57007 ReceiVed June 24, 2005. In Final Form: October 11, 2005 Measurement of adsorption excess isotherms of methanol-benzene mixtures was applied to the characterization of soil particle surfaces. The sorption capacity and Gibbs energy of sorption of the solid-liquid interface were determined for montmorillonite, three types of soil, and their humin fractions. The soils were found to be less polar or less hydrophilic than the clay, and the humin fraction of soils was found to be less hydrophilic than the whole soils. The soil and humin samples have heterogeneous surfaces which can be divided in two regions on the basis of their relative polarity. The x-axis intersection of the straight section of isotherm assigns the relative proportions of the hydrophilic and hydrophobic regions of the surface.
Introduction Distribution of an organic contaminant between the various organic and inorganic components of a soil influences its persistence, transport, bioavailability, and toxicity. This distribution between the dissolved phase, the colloid-associated, and sorbed soil particles depends on the hydrophobic/hydrophilic character of all components; contaminant, soil, and in particular, the water/soil-particle interface. A surface that is primarily populated by polar groups, such as hydroxyl groups, will have a strong affinity for a polar liquid such as water, and therefore is referred to as hydrophilic. If the surface is populated by nonpolar groups, which is common for surfaces covered by an organic layer, the surface is said to be hydrophobic. Soil particle hydrophobicity is mainly determined by its soil organic matter (SOM) content. It was postulated that the hydrophobic/hydrophilic character is determined not only by the presence of natural organic matter but also its composition. Specifically, the presence of humic substances plays an important role.1-3 Mineral-bound humic substances modify the surfaces of inorganic minerals, changing the nature and number of adsorption sites for contaminants due in part to their complexation, ion buffering, and sorption properties.4 Large hydrophobic molecules have shown the strongest affinity toward the clay surfaces.5 The interpretation of sorption isotherms from organic contaminant binding to soil emphasizes the importance of SOM. The dissolution (partition) model of sorption to SOM has been challenged by evidence that SOM has a nonuniform sorption potential.6 Heterogeneity of the organic matter in soil causes different hydrophobicity of surface and consequently different * To whom correspondence should be addressed. E-mail: mberka@ tigris.unideb.hu (M.B.);
[email protected] (J.A.R.). $ University of Debrecen. # South Dakota State University. (1) Payaperez, A. B.; Cortes, A.; Sala, M. N.; Larsen, B. Chemosphere 1992, 25, 887-898. (2) Allen-King, R.; Grathwohl, P.; Ball, W. AdV. Water Resour. 2002, 25, 985-1016. (3) Lo´pez-Dura´n, J. D. G.; Khaldoun, A.; Kerkeb, M. L.; Ramos-Tejada, M. M.; Gonza´lez-Caballero, F. Clays Clay Miner. 2003, 51, 65-74. (4) Tombacz, E.; Libor, Z.; Illes, E.; Majzik, A.; Klumpp, E. Org. Geochem. 2004, 35, 257-267. (5) Specht, C. H.; Kumke, M. U.; Frimmel, F. H. Water Res. 2000, 34, 40634069. (6) Xing, B.; Pignatello, J. J. EnViron. Sci. Technol. 1997, 31, 792-799.
types of contaminant sorption.7-10 In previous work, sorption of organic compounds to soils from different sources and on humic substances has resulted in the postulation of the existence of different sorption domains2,11 and that the hydrophobic interactions and conformational flexibility or rigidity in the aliphatic portions of humic components can control contaminant adsorption/desorption.6,12,13 Despite these efforts, a molecular-level understanding of the interactions between organic contaminants and soil14-21 is far from complete. The hydrophilicity of soil and clays can be described by its wettability or by water-vapor sorption.22,23 Measurement of adsorption excess isotherms of the binary solvent with different polarity makes the determination of the polar character of soil surface possible, which correlates with its hydrophilicity. The hydrophilic/hydrophobic balance of adsorbent can be determined by adsorption studies from binary liquid mixtures such as alcohol-water, alcohol-hydrocarbon, or aromatic-aliphatic hydrocarbons.24 The polarity of the surface can be characterized through the shape of the excess isotherms and the azeotropic composition, x1,a, of the solvent mixture. Though the method is widely used for characterization of clays, (7) Gunasekara, A. S.; Xing, B. J. EnViron. Qual. 2003, 32, 240-246. (8) Ran, Y.; Huang, W.; Rao, P. S. C.; Liu, D.; Sheng, G.; Fu, J. J. EnViron. Qual. 2002, 31, 1953-1962. (9) Wang, Q.; Liu, W.; Wang, Q. Q.; Liu, W. P. Soil Science 1999, 164, 411-416. (10) Chiou, C. T.; Kile, D. E.; Rutherford, D. W.; Sheng, G.; Boyd, S. A.; Sheng, G. Y. EnViron. Sci. Technol. 2000, 34, 1254-1258. (11) Gunasekara, A. S.; Simpson, M., J.; Xing, B. EnViron. Sci. Technol. 2003, 37, 852-858. (12) Piccolo, A.; Conte, P.; Scheunert, I.; Paci, M. J. EnViron. Qual. 1998, 27, 1324-1333. (13) Kang, S.; Xing, B. EnViron. Sci. Technol. 2005, 39, 134-140. (14) Kohl, S. D.; Toscano, P. J.; Hou, W.; Rice, J. A. EnViron. Sci. Technol. 2000, 34, 204-210. (15) Kohl, S. D.; Rice, J. A. Org. Geochem. 1999, 30, 929-936. (16) Luo, J.; Farrell, J. EnViron. Sci. Technol. 2003, 37, 1775-1782. (17) Zhu, D. Q.; Hyun, S. H.; Pignatello, J. J.; Lee, L. S. EnViron. Sci. Technol. 2004, 38, 4361-4368. (18) Hyun, S.; Lee, L. EnViron. Sci. Technol. 2004, 38, 5413-5419. (19) Borisover, M.; Graber, E. R. EnViron. Sci. Technol. 2002, 36, 45704577. (20) Borisover, M.; Graber, E. R. Langmuir 2002, 18, 4775-4782. (21) Borisover, M.; Graber, E. R. EnViron. Sci. Technol. 2004, 38, 41204129. (22) Likos, W. J.; Lu, N. Clays Clay Miner. 2002, 50, 553-561. (23) Cancela, G. D.; Huertas, F. J.; Taboada, E. R.; SanchezRasero, F.; Laguna, A. H. J. Colloid Interface Sci. 1997, 185, 343-354. (24) Lagaly, G.; De´kany, I. AdV. Colloid Interface Sci. 2005, 114, 189-204
10.1021/la051705n CCC: $33.50 © 2006 American Chemical Society Published on Web 12/16/2005
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surface-modified organophillic clays,25-27 or carbon surface heterogeneity28 and nanoparticles,29 it has not been applied to the characterization of soil particles. The method is simple. The solvent mixture has to meet the following requirements: the two liquids in the mixture are miscible, their polarities differ substantially from each other, they possess different refractive indexes, and they do not dissolve anything from the surface of the solid. Adsorption of binary liquid mixtures on solid adsorbents allows us to determine the sorption capacity and Gibbs energy of sorption of the solid-liquid interface. The binary mixture of methanol (as polar or hydrophilic component) and benzene (as nonpolar or hydrophobic component) is very often used to describe the overall surface character because these two solvents are miscible over the entire concentration range despite their significantly different polarities. In the case of soils, a preextraction must be done with the solvent mixture before the adsorption isotherms are determined in order to remove organic compounds soluble in the solvent mixture such as fatty acids, waxes, etc. Though this extraction may change the soil characteristics, this effect was disregarded because these solvent extractable compounds represent only a small portion of the soil’s or humin’s total organic carbon. The sorption of contaminant is mainly controlled by the insoluble fraction of SOM such as humin,30 which is associated with mineral components. Humin is described chemically as having a significant aliphatic nature that is probably attributed to the lipids that comprise a significant portion of its organic components. The literature of humin is reviewed by Rice.31 The purpose of this paper is to characterize and compare the polarity or hydrophilicity of soils and their humin fractions by adsorption excess isotherms of binary liquid mixtures and to detect the two regions of the soil surface, if they exist.
Theoretical Background Adsorption Excess Isotherm. The theory of adsorption from liquid mixtures onto solid surfaces has been described32-35 and is cited therein; several additional examples were reviewed by Dabrowski et al.36 When solid adsorbents are immersed in a liquid medium, solid-liquid interfacial interaction will cause formation of an adsorption layer on the adsorbent surface formed by solvent molecules. In the case of a purely physical adsorption of binary mixtures, the adsorption layer is determined solely by the range of the forces and the material composition of this layer. The mass balance for component 1 is described by the following equation:
nσ(n) ) n0(x1,0 - x1) ) nsxs1 - nsx1 1 where
nσ(n) 1
(1)
is the specific surface excess amount (mmol/g) of
(25) Dekany, I.; Farkas, A.; Regdon, I.; Klumpp, E.; Narres, H. D.; Schwuger, M. J. Colloid Polym. Sci. 1996, 274, 981-988. (26) Farkas, A.; Dekany, I. Colloid Polym. Sci. 2001, 279, 459-467. (27) Regdon, I.; Dekany, I.; Lagaly, G. Colloid Polym. Sci. 1998, 276, 511517. (28) Laszlo, K.; Bota, A.; Nagy, L. G. Carbon 2000, 38, 1965-1976. (29) Kanyo, T.; Konya, Z.; Kukovecz, A.; Berger, F.; Dekany, I.; Kiricsi, I. Langmuir 2004, 20, 1656-1661. (30) Kohl, S. D.; Rice, J. A. Chemosphere 1998, 36, 251-261. (31) Rice, J. A. Soil Science 2001, 166, 848-857. (32) Schay, G. Adsorption from solutions of nonelectrolytes; Matijevic, E., Ed.; Wiley: London, 1969; Vol. 2, p 155. (33) Everett, D. H. Adsorption at the Solid-Liquid Interface; The Chemical Society: London, 1973; Vol. 1. (34) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: New York, 1995; Vol. II. (35) Dekany, I.; Berger, F. Adsorption from liquid mixtures on solid surfaces; Toth, J., Ed.; Dekker: New York, 2002; Vol. 107, pp 573-629. (36) Dabrowski, A.; Jaroniec, M. AdV. Colloid Interface Sci. 1990, 31, 155223.
component 1 in the interfacial layer, n0 is the specific amount of the initial bulk liquid phase (mmol/g), x1,0 and x1 are the initial and equilibrium molar fractions of component 1, respectively, in the bulk phase, ns is the specific amount of the adsorbed liquid layer in the interfacial layer, and xs1 is its composition in the equilibrium. Excess thermodynamic quantities referred to the Gibbs surface are denoted by superscript σ to distinguish them from quantities relating to the interfacial layer, for which superscript s is employed. The surface excess amount or Gibbs adsorption of component i, nσ(n) i , which may be positive or negative, is defined as the excess of the amount of this component actually present in the system over that present in a reference system of the same volume as the real system and in which the bulk concentrations in the two phases remain uniform up to the Gibbs dividing surface. Due to the adsorption, x1,0 changes to the equilibrium concentration, x1. This change ∆x1 ) (x1,0 - x1), can be determined by simple analytical methods, for example from the change in refractive index of liquid phase after its interaction with the adsorbent. Five types of isotherms have been proposed.32,34 Types I and II have only hydrophilic moieties of similar polarity, while the isotherms of types III, IV, and V characterize surface contains two regions of differing polarity. One part interacts strongly and the other part weakly with component 1 from the binary liquid mixture. Solvent components adsorb better to a moiety with a similar polarity. The adsorption azeotropic composition, x1,a (the intersection of the linear section with the x axis), is characterized by identical of adsorbate and solution compositions. x1,a is used a measure of hydrophobic/ hydrophilic character of surface. The amount of material actually present on the surface is the adsorption capacity of the solid adsorbent, which may be determined in binary liquid mixtures if the adsorption isotherm is known.37-39 The Schay-Nagy extrapolation or reciprocal isotherm representation method can be used for the determination of adsorption capacity on the basis of adsorption isotherms.32,37-39 Instead of a single value of adsorption capacity, the system can also be characterized by the ‘equivalent layer thickness’, tequ, given as a function of the composition of the bulk phase.40 This means the thickness of a homogeneous surface layer which has the same composition as the first monolayer, xs1,1 (i.e. xs1 ) xs1,1), and contains the equivalent excess amount of components. The calculation consists of the following principal theoretical steps. After the adsorption ) f(x1), and the activity coefficient, γ1, excess isotherms, nσ(n) 1 of the solute in the bulk liquid phase are determined, the function of the Gibbs energy of adsorption of solid-liquid interfaces, ∆21G ) f(x1) is obtained by integrating the Gibbs equation.35
∆21G ) -RT
∫0
x1
(
nσ(n) 1 (1 - x1)x1
1+
)
d ln γ dx1 d ln x1
(2)
To calculate the volume fraction, φs2, of component 2 in the first monolayer, the following equation is used s -∆G am,1 1 φ2 r - 1 s + + ln (φ2 - φ2) ) 0 RT as r φ2 r
(3)
where am,1 is the molar cross-sectional area of component 1, as is the specific surface area of the adsorbent, φ2 is the volume fraction of component 2 in the bulk phase, and φ2 ) r(1 - x1)/(x1 (37) Dekany, I.; Szanto, F.; Nagy, L. G.; Schay, G. J. Colloid Interface Sci. 1983, 93, 151-161. (38) Everett, D. H. Trans. Faraday Soc. 1965, 61, 2478. (39) Schay, G.; Nagy, L. G. J. Chim Phys 1961, 149. (40) Berger, F.; Dekany, I. Colloids Surf., A 1998, 141, 305-317.
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+ r(1 - x1)), r being the value of the ratio of the molar volumes, r ) Vm,2/Vm,1. as can be determined from BET adsorption or from monomolecular adsorption capacity of solvents by application of the graphical extrapolation method for the linear sections of the excess isotherm.35 The molar volumes of methanol and benzene are 4.05 × 10-5 and 8.89 × 10-5 m3/mol, respectively.41 The cross-sectional areas of components calculated from the molar volumes of 99 nm2/mmol for methanol and 168 nm2/mmol for benzene were applied. After the determination of φs2, the xs1,1 composition in the first monolayer and the equivalent layer thickness, tequ, can be calculated as follows
xs1,1 )
tequ )
r(1 - φs2) φs2 + r(1 - φs2)
s s nσ(n) 1 Vm,1 x1,1 + r(1 - x1,1)
as
xs1,1 - x1
(4)
(5)
Figure 1. Refractive index as a function of the molar concentration of amethanol (1)/benzene (2) mixture before and after adsorption on montmorillonite.
Experimental Section Material and Methods. Methanol and benzene of analyticalgrade purity and dried over molecular sieves were used for the experiments. Sodium montmorillonite clay (SWy-2, Source Clay Repository) was used as a well-studied hydrophilic material. The montmorillonite was extracted for 24 h using a benzene/methanol mixture (1:1, v/v) in a Soxhlet apparatus. Three different types of soils collected from South Dakota and their humin fractions were used as adsorbents. The humin fraction of soil is that fraction of the SOM which is insoluble in aqueous solvent systems at any pH value and that remains after fulvic and humic acids have been extracted from the soil. Humin fractions of different soil types were isolated by fractionation with a modified MIBK method described by Kohl et al.30 Properties of the soils have been described earlier;42 the Hetland soil has a clayey texture (asBET ) 35 m2/g), the Poinsett soil a fine-silty texture (asBET ) 10.7 m2/g), and the Allivar soil possesses a sandy texture (asBET ) 9.4 m2/g). To produce adsorbents without solvent-soluble solutes, solvent pre-extraction was performed42 by extracting the soil and humin samples with a benzene/methanol azeotrope (3:1, v/v) for 3 days and then with a benzene/methanol (1:1, v/v) mixture for 2-3 days using a Soxhlet apparatus until the solvent was colorless. After extraction, the samples were allowed to air-dry and then were heated at 100 °C and evacuated at 20 µm Hg for 1 day in a vacuum oven to eliminate the potential for sorbed extraction solvent remaining on the surface. Adsorption excess isotherms of methanol/benzene mixtures were studied in a static solvent adsorption system at 25 ( 0.1 °C. Equilibrium was reached within 6 h of shaking time. The adsorbent/solvent ratio was always 1:2; so 4.000 g soil sample (or 3.000 g humin) were shaken for 24 h in well-sealed vials with 8.00 cm3 (or 6.00 cm3 in the case of humin) of solvent mixtures containing different methanol/benzene ratios. After one-half day of sedimentation, the supernatant was filtered through a 0.45 µm membrane filter. The change of the solvent refractive index was (∆RIU ≈ 1 × 10-2-10-3) measured by a BI-DNDC differential refractometer (Brookhaven Instruments Corporation) at 30 ( 0.01 °C. The refractive index sensitivity of the instrument was 2.5 × 10-9 ∆RIU. All measurements were replicated three times with ∆RIU ≈ 1 × 10-4-10-5 reproducibility. The calibration curve was constructed by the measurements of the refractive index of methanol/benzene mixtures using an ABBEMATPR automatic process refractometer. Activity coefficients for the methanol (1)/benzene (2) systems were calculated from the RedlichKister equation43 from the vapor-liquid equilibrium data.41 The quantity d ln γ/d lnx1 in eq 2 was fitted as y ) -0.2018953456 + (41) Perry, R. H.; Green, D. W. Chemical Engineers' Handbook, 7th ed.; McGraw-Hill: New York, 1997. (42) Malekani, K.; Rice, J. A.; Lin, J. S. Soil Science 1997, 162, 333-342. (43) Redlich, O.; Kister, A. T. Ind. Eng. Chem. 1948, 40, 341-345.
Figure 2. Adsorption excess, nσ1 (∆) and the equivalent layer thickness, t (b), in a methanol(1)/benzene(2) mixture on montmorillonite. The solid line is a fitted curve; the horizontal line indicates the calculated monolayer thickness of solvent. From the linear part of the isotherms, ns1 ) 2.92 mmol/g (see text).
Figure 3. The Everett-Schay representation, x1x2/nσ(n) as a 1 function x1 of the calculated excess isotherms in methanol (1)/benzene (2) mixtures on montmorillonite. (A) Calculated from noncorrected data; (B) calculated from the fitted isotherm in Figure 2. 1.845852043 ln[x1] + 1.8729835722 ln[x1]2 + 0.9081259768 ln[x1]3 + 0.2394681475 ln[x1]4 + 0.0257813028 ln[x1]5. The measured adsorption isotherms were approximated by the nσ(n) ) bxa1(1 - x1) 1 σ(n) function for the isotherm of type II and by the n1 ) bxa1(1 - x1)c function for type III isotherms. These fitted isotherms were applied for eq 2. Calculations were performed with Mathematica 5.0 (Wolfram Research). Organic carbon contents were determined on a Shimadzu TOC 500 total organic carbon analyzer.
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Figure 4. Adsorption excess isotherm in a methanol (1)/benzene (2) mixture on (O) Poinsett soil and on its (4) humin fraction. From the linear part of the isotherms, ns1 ) 1.80 mmol/g, ns2 ) 0.58 mmol/g, x1,a ≈ 0.76 for soil and ns1 ) 1.35 mmol/g, ns2 ) 0.55 mmol/g, x1,a ≈ 0.71 for its humin.
Results and Discussion Figure 1 shows the experimental data, i.e., the refractive index as function of the composition of methanol/benzene mixture before and after adsorption on montmorillonite. The adsorption excess calculated from the change of refractive index using eq 1 and the equivalent layer thickness calculated using eqs 2-5 in the methanol (1)/benzene (2) mixture on clay are shown in Figure 2. Montmorillonite is a hydrophilic clay mineral, and therefore, it is expected, as well as observed, that methanol is preferentially adsorbed (type II). A maximum occurs at relatively low concentration, and the final decrease of the isotherm (x1 ) 0.5-1) is practically linear (RQS > 0.96). During adsorption experiments, an increasing peptization of the clay was observed at higher methanol concentrations, so it is possible that the small ‘hump’ on the isotherm (Figure 2) arises from an increase in specific surface area resulting from the disaggregation. The calculated equivalent layer thickness is about 0.6 nm, approximating a 1.5 monomolecular layer solvent thickness. The widely used Everett-Schay representation39 is not linear, showing a deviation on the curve of x1x2/nσ(n) ) f(x1) at higher 1 mole fractions of methanol (see curve A in Figure 3). This deviation is caused by the dissaggregation and almost disappears after correction of the isotherm (curve B in Figure 3). The residual nonlinearity in curve B in Figure 3 is caused by the higherthan-unity value of r.37 From the linear portion of the isotherms, the adsorption capacity of methanol was calculated to be ns1 ) 2.92 mmol/g. Similar experiments and calculations were performed on soil and humin samples. Soil samples have lower methanol adsorption capacity than montmorillonite. In the case of the Poinsett soil, a type III isotherm shows a transition between the type II and type IV (or S type) as a result of decreasing polarity of the samples (see Figure 4). The adsorption excess amount is always positive; hence, the whole surface is polar or hydrophilic. The isotherm is characterized by an inflection after a substantially straight part that asymptotically decreases to zero at x1 ) 1. There is an intersection of the linear portion of the curve with the x axis, suggesting that the surface contains two regions with differing degrees of polarity or hydrophilicity/hydrophobicity. One part interacts strongly and the other weakly with polar methanol from the methanol/benzene solvent mixture. As expected, it was found that the humin fraction of the soil is less polar, or more hydrophobic, than the whole soil. The less-polar sample has a lower methanol adsorption capacity, and its isotherm intersects the x-axis intersection at a lower value (see Figure 4). as a function x1, is The Everett-Schay representation, x1x2/nσ(n) 1 nonlinear and shows a strong deviation from linearity for the Poinsett soil (see Figure 5) and its humin fraction. Thus,
Figure 5. The Everett-Schay representation, x1x2/nσ(n) as a 1 function x1 of the excess isotherms in Figure 4. (S), the Poinsett soil; (H), the Poinsett soil humin.
determination of adsorption capacity from this representation is not possible. The shapes of the isotherms of the Hetland soil and its humin fraction are similar to that of the Poinsett soil, but they have more hydrophilic surfaces with a higher sorption capacity (Figure 6). The x-axis intersection of the linear part of isotherm of the Hetland soil is higher than that of its humin, indicating that the humin is less hydrophilic than the whole soil. The Allivar soil was found to be the most hydrophobic of the soils on the basis of the isotherms shown in Figure 7. By comparison, the Allivar soil humin has a higher adsorption capacity both for methanol and for benzene than the Allivar soil, but it has a lower intersection x1,a value. This anomaly means that the Allivar humin has a stronger hydrophobic character but larger specific surface area than the ‘whole’ Allivar soil. On the basis of their isotherms, the hydrophilicity of the materials decreases in the order: montmorillonite > Hetland soil > Poinsett soil > Allivar soil (Figure 8a) and Hetland humin > Poinsett humin > Allivar humin (Figure 8b). The adsorption excess isotherms of all soils (Figures 6-8) show type III behavior. There is an intersection of the linear section of the curves with the x axis, indicating that the surface of all the soils is hydrophilic, but not uniformly so. The shift of the solvent’s composition (x1,a) indicates that the methanol affinity approximately decreases with increasing humin content per surface area of samples (humin in mg/m2), as presented in Tables 1 and 2. The order of the calculated Gibbs energy of sorption (column 4 in Tables 1 and 2) is nearly the same as the intersection values (column 1 in each table). The Allivar humin is an exception and requires additional examination. The Allivar soil with its
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Figure 6. Adsorption excess isotherm in a methanol (1)/benzene (2) mixture on (O) the Hetland soil. From the linear part of the isotherms, ns1 ) 2.11 mmol/g, ns2 ) 0.46 mmol/g, x1,a ≈ 0.82 for soil and ns1 ) 1.69 mmol/g, ns2 ) 0.50 mmol/g, x1,a ≈ 0.77 for its humin.
Figure 7. Adsorption excess isotherm in a methanol (1)/benzene (2) mixture on the Allivar soil and its humin. From the linear part of the isotherms, ns1 ) 1.4 mmol/g, ns2 ) 0.40 mmol/g x1,a ≈ 0.76; ns1 ) 1.85 mmol/g, ns2 ) 1.2 mmol/g x1,a ≈ 0.61.
Figure 8. Measured adsorption excess isotherms in a methanol (1)/benzene (2) mixture on (A) soils and (B) their humin organic matter fractions. Table 1. Characteristic Properties of the Different Soil Samplesa sample
x1,a
ns1 mmol/g
ns2 mmol/g
-∆G J/g
TOCb %
TOC mg/m2
huminb %
humin mg/m2
s aBET m2/g
asequ m2/g
Vs cm3/g
Montmorillonite Hetland Soil Poinsett Soil Allivar Soil
1.0 0.82 0.76 0.78
2.92 2.11 1.80 1.40
0 0.46 0.58 0.40
6.4 4.64 3.63 2.45
0 2.41 2.72 1.95
0 0.69 2.54 2.07
0 1.64 1.63 1.35
0 0.47 1.52 1.43
750 35 10.7 9.4
581 287 278 204
0.118 0.126 0.125 0.092
a x , azeotropic composition; ns and ns , adsorption capacities from the intercepts of linear section; ∆G, calculated Gibbs energy of adsorption; 1,a 1 2 TOC, total organic carbon; humin, insoluble organic component, see text; as, specific surface area; Vs, volume of the interfacial phase. b Determined by Kohl et al.29
lower humin content shows a greater degree of hydrophobicity than the Poinsett soil, indicating that its hydrophobic/hydrophilic character is determined not only by the amount of humin but also by its composition. The Allivar soil, and hence its humin, contains the greatest amount of bound-humic acid and bound-lipid fraction which seem to play important roles in the surface hydrophobicity (Table 2). Comparing the humin fractions with their respective whole soils (Tables 1 and 2), we can conclude that the same humin content and composition causes a greater degree of
hydrophobicity because in the absence of fulvic and humic acids, humin’s contributions to the overall soil material dominate its surface characteristics. It was found that eq 5 was not applicable for calculation of the equivalent layer thickness vs x1 function in the case of soils because the tequ decreased below one molecular size of solvent and approached zero, which is not a realistic value for a monolayer. s s The shape of the isotherms (nσ(n) 1max, n1, n2, x1,a) and the calculated
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Table 2. Characteristic Properties of Humins from Different Soilsa sample
x1,a
ns1 mmol/g
ns2 mmol/g
-∆G J/g
TOC %
humin mg/m2
BHAb %
I.R.b %
BLb %
asequ m2/g
vs cm3/g
Hetland Humin Poinsett Humin Allivar Humin
0.77 0.71 0.61
1.69 1.35 1.85
0.50 0.55 1.20
3.64 2.0 2.98
1.68 1.62 1.58
0.48 1.51 1.68
61.3 61.4 67.3
30.1 31.3 23.7
8.6 7.3 8.9
251 227 393
0.113 0.104 0.182
a Symbols as in Table 1; BHA, bound humic acid; I.R., insoluble residue; BL, bound lipid. b Mean percent organic carbon in fractions of humin determined by Kohl et al.29
Gibbs energy of sorption by themselves can be considered as measures of a material’s hydrophilicity. The specific surface areas calculated from isotherms were found to be much higher than the BET surface areas. This indicates that adsorption proceeds not only on the external surfaces of soils but on internal surfaces as well.
Conclusions We report the first characterization of surface polarity of soils and SOM fractions by analyzing the binary solvent adsorption excess isotherms of whole soils and their humin fractions. Polarity was characterized by the Gibbs energy of sorption of the solidliquid interface. The ratio of the hydrophilic and hydrophobic (or less-hydrophilic) surface regions was determined from the molar ratio of a methanol/benzene solvent mixture. The order of the sample material’s surface hydrophilicity was found to be montmorillonite > Hetland soil > Poinsett soil > Allivar soil > Hetland humin > Poinsett humin > Allivar humin. There was a direct correlation between the polarity and the specific BET surface area. A correlation was also found between the hydrophobicity of the soil samples and their humin contents. Results showed that the hydrophobic character of a soil was determined not only by its humin content but also its composition.
From this small number of samples, it seems to be that the boundhumic acid and the bound-lipid components of humin may make the main contributions to the particle surface hydrophobicity. The adsorption excess isotherms show that both the soil and their humin samples have heterogeneous surfaces that display both hydrophobic and hydrophilic character. The difference in polarity between the two regions is smaller than between the polarity of methanol and benzene. By changing the components of the binary solvent mixture (as alcohol-water, alcoholhydrocarbon, or aromatic-aliphatic hydrocarbons) it should be possible to find a solvent mixture in which one component has a polarity similar to the average polarity of each surface region. This method is a simple and useful technique for comparing of hydrophilic/hydrophobic properties of soil or soil fraction particle surfaces. The benzene and methanol mixture appears to be wellsuited to describe the overall character of the particle surfaces. Acknowledgment. This work was supported by a grant from the South Dakota Water Resources Research Institute, the National Science Foundation/EPSCoR Grant No. EPS-0091948 and by the State of South Dakota. Marta Berka acknowledges the support of the Be´ke´si Gyo¨rgy postdoctoral fellowship and OTKA T049044. LA051705N
