Envlron. Sci. Technol. 1003, 27, 1587-1594
Sorption of N1 and EGME Vapors on Some Soils, Clays, and Mineral Oxides and Determination of Sample Surface Areas by Use of Sorption Data Cary T. Chlou’ and Davld W. Rutherford
U.S. Geological Survey, P.O.Box 25046, MS 408, Denver Federal Center, Denver, Colorado 80225 Milton Manest
Department of Chemistry, Kent State University, Kent, Ohio 44242
t Present address: Amberson Towers No. 412, 5 Bayard Road, Pittsburgh, PA 16213.
results. The surface area, considered as the solid-gas or solid-vacuum interfacial area, which is external to the material, is assumed to preexist the measurement and to be unchanged by the measurement. The surface area is therefore a property of the solid; Le., within the precision of the method, it should be independent of the choice of suitable adsorbates (1). Although the BET method is widely used in soil science in surface area determination, one also finds reports of surface areas that are determined by the sorptionof vapors that may penetrate into the interior of the bulk solid. For example, the “total surface areas” of expanding clays such as montmorillonite have been measured by the retention of ethylene glycol (EG) (4-7) and ethylene glycol monoethyl ether (EGME) (8-14). The differences between total surface areas by the retention method and external surface areas by the BET (N2) method are considered to be the “internal surface areas”. The areas of such internal surfaces are measured by assuming that the adsorbates exist as monolayers in the interior of the solid. By contrast with the BET model, these internal areas are created by the experiment, and their values may be expected to vary widely with different sorbates. For expanding clay minerals, where the polar sorbate taken up by the interior of the solid is actually by cation solvation (11,15, 16),one may also expect different areas from mineral components that exhibit different extents of such solvation. For soils containing considerable amounts of organic matter, one would similarly expect different areas, depending on the solubility of the sorbate in the organic matter (17-19). As a result, the reported surface areas of soils and minerals by the solvent retention methods may differ widely from the BET areas and from each other. An additional problem with the solvent (EG or EGME) retention method for surface area determination is that the reported surface area is based only on single-retention points of the solvent on the sample under some fixed experimental conditions. The amount of solvent retention thus depends on such conditions as the extent of sample evacuation. For suitable samples, to be illustrated later, one would expect more accurate results from analysis of the solvent-uptake isotherm. For EGME, which is much more volatile than EG, the isotherms on soils and minerals can be readily determined for the intended purpose. In this study, we have determined the N2 isotherms at 77 K and EGME isotherms at room temperature on a wide range of samples: a low-organic-content soil, a highorganic-content peat, sand, aluminum oxide, kaolinite, illite, Ca-montmorillonite, hematite, a synthetic hydrous iron oxide, and a natural hydrous iron oxide. The isotherm data were analyzed by use of the BET equation to obtain N2 monolayer capacities and EGME apparent monolayer capacities, with the expectation that the alternative
0 I993 American Chemical Society
Environ. Scl. Technol., Vol. 27, No. 8, 1993 1587
Vapor sorption isotherms of ethylene glycol monoethyl ether (EGME) at room temperature and isotherms of N2 gas at liquid nitrogen temperature were determined for various soils and minerals. The N2 monolayer capacities [ Q m (Nz)] were calculated from the BET equation and used to determine the surface areas. To examine whether EGME is an appropriate adsorbate for determination of surface areas, the apparent EGME monolayer capacities [ Q m (EGME),] were also obtained by use of the BET equation. For sand, aluminum oxide, kaolinite, hematite, and synthetic hydrous iron oxide, which are relatively free of organic impurity and expanding/solvating minerals, the Qm (EGME),, values are in good conformity with the corresponding Q m (N2) values and would give surface areas consistent with BET (N2) values. For other samples (Woodburn soil, a natural hydrous iron oxide, illite, and montmorillonite), the Qm (EGME),, values overestimate the Q m (N2) values from a moderate to a large extent, depending on the sample. A high-organic-content peat shows a very small BET (N2) surface area; the EGMEI peat isotherm is linear and does not yield a calculation of the surface area. Large discrepancies between results of the two methods for some samples are attributed to the high solubility of polar EGME in soil organic matter and/ or to the cation solvation of EGME with solvating clays. The agreement for other samples is illustrative of the consistency of the BET method when different adsorbates are used, so long as they do not exhibit bulk penetration and/or cation solvation.
Introduction Studies of the surface chemistry of solids are frequently concerned with the extent of their subdivision, which in turn is frequently estimated by determinations of their surface areas. The standardmethod for the determination of surface areas is the Brunauer-Emmett-Teller (BET) method (1-3), in which one determines the adsorption isotherm of any of a number of vapors or gases. Suitable adsorbates must be chemically inert, not subject to molecular sieving, and confined to the exterior of the solid, i.e., there must be no significant bulk penetration or sitespecific interaction of the adsorbate with the solid. The BET model provides a method for calculating the monolayer capacity from the adsorption isotherm; the surface area per molecule of adsorbate is estimated from its liquid density. Although nitrogen at liquid nitrogen temperature is the most frequently used adsorbate, the method is by no means limited to nitrogen, and a wide variety of suitable adsorbates (e.g., krypton) on the same solid yield similar
0013-936X/93/0927-1587$04.00/0
methods would yield similar surface areas on samples that do not exhibit bulk penetration by EGME and discrepant results to the extent that such penetration occurs. The former expectation follows both from the BET model and from the semiquantitative correlation of Tiller and Smith (11)between BET (Nz)surface areas and EGME retentions on a variety of smectite-free minerals. The latter expectation follows from extensive earlier studies, e.g., the work of Dyal and Hendricks ( 4 ) on the EG retention and BET(Nz)areas of smectites, the work of Bower and Gschwend (5) on the EG retention by soils and soil organic matter (SOM),and the work of Chiou et al. (17)on the BET-(Nz) surface area of SOM. These expectations have been largely realized. The results illustrate the extent to which discrepancies between the alternative methods depend on the amount and solvency of the organic content as well as on the expanding/solvating clay content of the sample. In addition, they illustrate the close correspondence between surface areas determined by Nz and by EGME adsorption on samples that conform to the postulates of the BET model.
Experimental Section Materials. Two surface-area reference standards of aluminum oxide samples, with surface areas of 109 m2/g (referencestandard A) and of 29.9 m2/g(referencestandard B), were provided by the Quanta Chrome Corp. An additional aluminum oxide sample, a nonactivated reagent grade, was obtained from Aldrich with a purity of 99.99 % . The Ottawa sand is a standard sand obtained from Fisher Scientific with a 20-30-mesh particle size. The sample was washed with 10% HC1 and treated with HzOz to remove any residual iron oxides and organic matter. The kaolinite and montmorillonite samples were obtained from the Source Clays Repository, the Clay Minerals Society, Department of Geology, University of Missouri, Columbia, MO. The kaolinite sample (KGa-2) is from Warren County, GA; it is a poorly crystallized material with a cation-exchange capacity of 3.3 mequiv/100 g and an organic carbon content less than 0.02 % by weight. The montmorillonite sample is from Apache County, AZ, originated from Bidahochi formation (Pliocene); it has a cation-exchange capacity (CEC) of 120mequiv/100 g with Ca as the major cation. The illite sample, originated from Fifthian, IL, was obtained from Ward's Natural Science Establishment, Inc., Rochester, NY. The sample has a greater than 90% illite content (with minor amounts of quartz and muscovite), a CEC of 25 mequiv/100 g (with K as the major cation), and an organic carbon content of 0.8%. The illite sample was treated with HzOz to reduce the organic content before its use in sorption experiments. The treated sample has a CEC of 15.5 mequiv/100 g and contains 0.4% organic carbon. Hematite was obtained from Ward's Natural Science Establishment, Inc. It is a highly pure sample with a total Fe content of 70.28% on a dry weight basis; X-ray diffraction datashowed the sample to be a well-crystallized material (20). The synthetic hydrous iron oxide was prepared in the laboratory by hydrolyzing 0.5 M aqueous ferric nitrate with 10% NaOH at pH 8.0-8.5. The sample has a total Fe content of 62.4%. X-ray diffraction data showed it to be an essentially amorphous iron oxide with low goethite content (20). The natural hydrous iron oxide was collected from a drain pond of the Iron Mountain mine in the West Shasta sulfide district near Redding, 1588 Envlron. Scl. Technol., Vol. 27, No. 8, 1993
CA. It is a relatively impure iron oxide sample with the total Fe being 9.1% . X-ray diffraction showed it to be an amorphous iron oxide with sharp quartz and pyrite peaks (20). The peat sample is a reference peat of the International Humic Substances Society collected from Everglades, FL. The dry-weight chemical composition of the peat is 50.8 % carbon, 4.0% hydrogen, 30.1 % oxygen, 3.2% nitrogen, 0.58% sulfur, 0.26% phosphorus, and 13.6% ash. The Woodburn soil is a slit loam from Oregon, and its dryweight composition gives 1.9% organic matter, 68% slit, 21 % clay, and 9 % sand. X-ray diffraction showed that fine-grained mica (illite) and kaolinite are the dominant clays. EGME Sorption Experiments. The apparatus and procedure used for the determination of vapor uptake by soils and minerals has been described previously (18). Briefly, the experiment consists of equilibrating the vapor in a sorption chamber containing a Cahn Model 2000 electrical microbalance from which a test solid sample (sorbent) hangs in a small glass cup. The sorbent sample was heated at 100 "C for 8-10 h inside the chamber and then cooled to room temperature for 6-8 h under a vacuum of lo4 Torr to remove moisture and to determine the dry sample weight. Typically, 100-500 mg of samples was used in sorption experiments. The test liquid (EGME) was purified by vacuum distillation to remove residual air and then introduced into the sorption chamber for establishment of equilibrium. A change in sample weight resulting from vapor uptake was recorded as an electrical signal from the balance. The partial pressure of EGME at the point of vapor-sample equilibrium was recorded by the Baratron pressure gauge. The equilibrium of EGME with the samples was generally quite rapid for all samples except peat, taking about 1 day or less. For the latter, the normal time for equilibrium was between 3 and 4 weeks. To accelerate the equilibrium of EGME on peat, a relatively high partial pressure of EGME was initially introduced to the peat sample to enhance the EGME uptake during the first 1-2 weeks. The partial pressure of EGME in the sorption chamber was then reduced gradually in a manner such that no desorption of EGME from the sample occurred until the point that a further reduction in EGME partial pressure would cause a desorption. This procedure shortened the equilibrium time of EGME to about 2-3 weeks. The EGME isotherm with a given sample is plotted as the weight of uptake per unit of dry weight of the sample against the relative pressure of EGME. The saturation vapor pressure of EGME at room temperature (about 5 mmHg at 24 "C) was determined by monitoring the pressure of saturated EGME vapor prior to the sorption experiment, and it was used for the isotherm plot. (EG was not used in this study because of its low vapor pressure). Nitrogen Adsorption Experiments. The vapor uptake of N2gas by the sample was determined at liquid NZ temperature. A Quantasorb surface analyzer (Quanta Chrome Corp.) was used to measure the amount of Nz gas uptake at given relative pressure of Nz from a stream of the Nz-He gas mixture. A flow controller was used to fix the NZ-He mixture composition for establishing the isotherm. The test samples were prepared by heating at 100 "C under a flow of pure He gas for 24 h. This outgassing condition is relatively comparable with that used in the
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Relative Pressure, P/Po Figure 1. Isotherms of EGME vapor at room temperature and N2 vapor at 77 K on Ottawa sand and hematite. 10
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Figure 3. Isotherms of EGME vapor at room temperature and N2 vapor at 77 K on natural hydrous iron oxide ("10) and synthetic hydrous Iron oxide (SHIO). 350
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EGME uptake experiments. The surface areas of samples were calculated by use of the linear BET plot of the N2 isotherms at relative pressures of 0.05-0.30 for obtaining the monolayer adsorption capacities, together with the molecular area of N2. The instrument has been calibrated against various surface area standardswith values ranging from 0.51 to 239 m2/g.
Results and Discussion Adsorption isotherms of N2 vapor at liquid nitrogen temperature and sorption isotherms of EGME vapor at room temperature on Ottawa sand, hematite, aluminum oxide, kaolinite, natural hydrous iron oxide, synthetic hydrous iron oxide, illite, Ca-montmorillonite, Woodburn soil, and peat are shown in Figures 1-5. The isotherms were obtained over a considerable range of relative pressures (PIP")for both vapors to illustrate the shape of the isotherms that is characteristic of the transition from submonolayer to multilayer adsorption. Because of the very long equilibrium time for EGME with peat, the EGMEIpeat isotherm was extended only to PIP" = 0.4; this range is, however,sufficient for illustrating the sorption effect. According to the BET theory (1-3)) the adsorption capacity at the beginning of the linear range following the
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initial sharp rise (called "the point B") corresponds roughly to the adsorption monolayer (Qm);multilayer adsorption is signified by an upward concavity as PIP" approaches 1. The Qm value of the adsorbate and the properly assigned molecular area of the adsorbate are then used to calculate the surface are of the solid (adsorbent). The linear BET plot allows the value of Qm of an adsorbate on a given solid to be calculated according to
where Q is the amount of vapor adsorbed at given PIP", P i s the equilibrium partial pressure of the adsorbate, PO is the saturation vapor pressure of the adsorbate, and C is a constant related to the net heat of monolayer adsorption, which affects the sharpness of the curvature. A plot of lIQ(PoIP- 1)against PIPoyields a straight line with a slope of (C - l)lCQmand an intercept of l/CQm, from which Qm and C are calculated. The linearity of the BET plot is usually observed at PIP" = 0.05-0.30 in systems involving physical adsorption (1-3). The required molecular area of the adsorbate (am) may be estimated on the assumption that the arrangement of the adsorbate molecules on the solid surface is the same as it would be on a close-packed plane surface within the Environ. Sci. Technol., Voi. 27, No. 8, 1993 1589
Table I. Qm (Nz) Monolayer Capacities, BET (Nz)Surface Areas, Qm (EGME),, Equivalent Monolayer Capacities, and (EGME),, Apparent Monolayer Capacities of Selected Soils and Minerals8
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Qm
sample Ottawa sand hematite peat aluminum oxide Woodburn soil natural hydrous iron oxide kaolinite illite Ca-montmorillonite synthetic hydrous iron oxide
Qm
(Nz)(mg/g) 0.032 0.14 0.36 0.87 3.22 3.31 6.03 19.3 21.8 53.4
BET (Nz)area (In2/@
Qm
QDl
(EGME),, (mg/g)
(EGME), (mg/g)
0.11 0.50 1.26 3.03 11.2 11.5 21.0 67.2 75.9 186
0.042 0.18 0.47 1.13 4.19 4.30 7.84 25.1 28.3 69.4
0.035 0.18 1.02 13.4 7.98 7.90 38.0 215 61.2
(EGME)ap/ Qm (EGME), 0.83 1.00 0.90 3.20 1.86 1.01 1.51 7.60 0.88
a The Qm (Nz)values are from Nz adsorption isotherms and the BET equation; Qm (Nz) values are used to determine the surface areas of samples; Qm(EGME), are values equivalent to Qm (Nz)for the same surface areas of samples; Q (EGME)., values are from the EGME sorption isotherms and the BET equation, assuming that no penetration or specific interaction occurs. The ratio of Q, (EGME)., to Qm (EGME),, when significantly greater than 1, expresses the extent of EGME penetration or specific interaction.
bulk liquid (3). For two dimensional close packing, this gives a, = 1.09(M/d,N)2/3 (2) where M is the molecular weight of the adsorbate, d l is the adsorbate liquid density, and N is the Avogadro number. Insertion of dl = 0.81 g/cm3 for nitrogen at 77 K gives a, (Nz) = 16.2 X m2. For other gases of small molecular size (e.g., inert gases), the a, values thus calculated give consistent surface areas on the same sample (1-3). This calculation method, however, is not necessarily as accurate for larger molecules, where the packing on solid surfaces may exhibit variations. Thus, for EGME, with dl = 0.930 g/cm3, the calculated molecular area a, (EGME) is 32.3 X m2. A better value can be determined by calculating a, from the monolayer capacities on samples of known surface areas (by Nz adsorption). We have determined the EGME monolayer capacities on two alumina reference samples (29.9 and 109 m2/g,respectively) as 11.3 and 40.4 mg/g from the BET plot (eq 1)of the respective EGME isotherms (not shown);the corresondingam (EGME) values and 40.4 X m2, giving an average of are 39.6 X 40.0 X 10-20m2. The accuracy of the surface areas of the two reference standards was confirmed (32.2 and 107 m2/ g, respectively) by BET plots of the experimental Nz (77 K) isotherms. We have used a, (EGME) = 40.0 X m2 in calculating the surface areas of samples. As seen, except for the EGME/peat systems, Nz and EGME show nonlinear isotherms with all samples and exhibit a concave-down curvature at PIP" = 0.03-0.20. With the exception of the data for peat, the shape of the EGME isotherms with the samples is largely comparable with that of the respective Nz isotherms, except that the former exhibits a sharper curvature for many systems in the low PIP" range. This difference may be attributed to the enhanced polar interaction of EGME with the minerals. Type I1 isotherms of Brunauer-Emmett-Teller ( I ) are observed for Nz and EGME with aluminum oxide, sand, hematite, kaolinite, illite, Ca-montmorillonite, natural hydrous iron oxide, and Woodburn soil; type IV isotherms are found for both vapors with the synthetic hydrous iron oxide. For the peat sample, however, the Nz isotherm is type I1 whereas the EGME isotherm is linear. The Nz and EGME isotherms with the samples can also be categorized by the differences in their uptake capacities. Sand, hematite, and nonactivated aluminum oxide all show 1590 Envlron. Scl. Technol., Vol. 27, No. 8, 1993
very small but comparable uptakes of Nz and EGME (Figures 1 and 2); peat shows a very small uptake of NZ but a large and linear uptake of EGME (Figure 5);kaolinite shows moderately increased and comparable uptakes of N2 and EGME (Figure 2); natural hydrous iron oxide and Woodburn soil show a moderate uptake of EGME but a relatively small uptake of NP(Figures 3 and 5); illite shows a relatively high uptake of Nz and a somewhat greater uptake of EGME (Figure 4); Ca-montmorillonite shows an uptake of Nz comparable to that by illite but shows a greatly enhanced uptake of EGME (Figure 4); and synthetic hydrous iron oxide shows large and comparable uptakes of Nz and EGME (Figure 3). Although all the EGME isotherms, except the EGMEI peat system, exhibit curvatures at low PIP" that resemble type I1 isotherms, the Q, (EGME) values obtained from these isotherms are not necessarily proper measures of surface monolayer capacities if there is bulk penetration or site-specific interaction in the solid. Therefore, the Q, (EGME) values derived from the EGME isotherms according to eq 1are denoted as the apparent monolayer capacities, pending the validation of their consistency with the monolayer capacities derived from inert gases such as Nz. From the experimental Q, (Nz)values and calculated surface areas, one may then calculate the values of Q, (EGME),, equivalent to Q, (Nz) for the surface areas of the samples by using the previously determined a, (EGME) value. This gives Q m (EGME)eq = 1.3OQm (Nz). A comparison of the Q, (EGME)eq values with the apparent Qm (EGME), values from the isotherms defines the extent of EGME uptake by penetration (or site-specific interaction) for each of the solid samples. The Q m (Nz) values, BET (Nz) surface areas, Q, (EGME),, values, and Q, (EGME),, values are given in Table I. Consider first the results with sand, hematite, aluminum oxide, kaolinite, and synthetic hydrous iron oxide, which are of relatively pure composition and are not expected to exhibit bulk penetration. The purity of the samples and the absence of organic impurity and smectite components should eliminate potential complications in the surface area measurement. In these systems, a reasonable agreement is found between the surface areas obtained by Q, (Nz) and those obtained by Qm (EGME),,, since the ratios of Q, (EGME),, to respective Q, (EGME)eq are close to 1,with the BET (N2) surface areas ranging from about 0.1 mZ/g for sand to 186 m2/g for the synthetic hydrous iron
oxide. This consistency is evidence that EGME exhibits no significant molecular sievingon these samples; the small differences, especially for small-surface-area solids, are due largely to experimental errors and, perhaps, to minor differencesin sample outgassing conditions. In the absence of such molecular sieving effects, EGME satisfies the postulates of the BET model as well as does N2 on solids that do not exhibit bulk penetration and site-specific interaction; on such solids EGME is practically the equivalent of N2. The consistency of the results using these alternative methods on appropriate samples is such that the Q m (EGME), values that are significantly higher than the corresponding Qm (EGME), values may be taken as evidence of bulk penetration and/or specific interaction. We now consider the other samples in Table I, except for peat which will be considered later, where the ratios of Q m (EGME)apto Q m (EGME), values vary from 1.85 for natural hydrous iron oxide to 7.6 for Ca-montmorillonite. The data of Figure 4 showing the EGME/Camontmorillonite isotherm extend the earlier observations, e.g., by Carter et al. (€9,of the single-point EGME retention by Ca-montmorillonite. These authors assumed that the EGME forms internal monolayers that are intercalated between silicate layers of the clay. Taking the total surface area (i.e., the external surface and interlayer plane) as 810 m2/g for montmorillonite from Dyal and Hendricks (41, they estimated the molecular area of EGME as 52 X m2, Since this value is only 30% higher than our estimate of 40 X 10-20 m2, it may be taken as a reasonable confirmation of their model for Ca-montmorillonite. However, since the uptake of polar solvents by expanding clays is mainly by cation solvation (15,16),the method would not give similar internal surface areas for clay minerals of similar structure but containing cations of weaker solvating power than Ca. Therefore, although EGME may be a fortuitous choice of a solvent that forms approximate intercalated monolayers in Ca-montmorillonite, the sorption of volatile polar solvents cannot be a general method for internal surfaces in the same sense of the BET method for external surfaces. Moreover, since the solvation of EGME (or EG) with expanding clay minerals is more of a stoichiometric than a surface phenomenon (15, 16), it gives little information on the state of subdivision of the sample. On the other hand, if one wishes to detect the solvation effect of the liquid with clay minerals, the isotherm method for EGME should give a more complete account over the single-point EGME (or EG) retention method. The large discrepancy between the Q m (EGME),, and Q m (EGME),, values for Ca-montmorillonite suggests that the Q m (EGME), value would give a highly inaccurate estimate of the surface area of this sample, due to the extensive penetration of EGME into the clay structure (by virtue of cation solvation). Such penetration violates the basic assumptions of the BET model in surface area determination and hence leads to the discrepancy between Q m (EGME)apand Q m (EGME),,. Accordingly, the use of Qm (EGME)ap would overestimate the surface area of a mineral sample containing 1 % Ca-montmorillonite by about 5 m2/g. A greater overestimate (by about 7 m2/g) would result if the EGME retention at near vapor saturation is used. Continuing our examination of the data in Table I, we note that for illite the ratio of Q m (EGME),, to Qm (EGME)eq is a much more moderate 1.5. Although the
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Flgure 5. Isotherms of EGME vapor at room temperature and N2 vapor at 77 K on Woodburn soli anb Florida peat.
"internal surface area" of illite by crystallographic calculations should be comparable to that of montmorillonite, it is clear from this and earlier work ( 4 , B ) that it is not measured by either EG or EGME. The corresponding ratio of 1.85 for the natural hydrous iron oxide may be attributed to expanding/solvating clay impurities. Since the ratio of Q m (EGME),, to Q m (EGME),, is moderate for illite, a small amount of illite in minerals should have a relatively small effect on surface area determination by use of EGME or other vapor isotherms. We will consider Woodburn soil following our account of peat. The EGME on peat (Figure 5) shows a large uptake and linear isotherm that is characteristic of bulk solubility (17-19) and far exceeds the small surface adsorption exhibited by N2. The limiting capacity (solubility) of EGME at P / P = 1, normalized for the organic content of peat, is 190 mg/g. This is comparable with the finding of Bower and Gschwend (5) that 170-250 mg of EG is retained by 1 g of soil organic matter (SOM), to which they assigned 560-800 m2/g as the apparent surface area on the assumptions that EG forms monolayers on SOM m2. Since the EGME and that a m (EG) = 33.2 X isotherm on peat is linear (rather than type 11),the BET calculation does not apply, and there is no theoretical basis for calculating Q m (EGME)ap The N2 data give a surface area of about 1.3 m2/gfor peat. The high apparent surface area for SOM as reported by Bower and Gschwend results from calculations in which the observed sorption of EG on SOM is attributed to surface adsorption rather than to bulk solubility. Following the procedure of Bower and Gschwend (5) and using our EGME/peat isotherm, one can estimate the extent to which the organic matter content in samples introduces errors into the determination of surface areas by EGME retention. If one assumes the EGME uptake capacity of SOM to be the same as the organic content of peat, then the overestimated surface area for a sample containing 1% SOM would be 5 m2/g if the EGME retention is measured near saturation and about 0.3-0.5 m2/g if the retention is measured at a relative pressure ( P / W of 0.05-0.1. The error caused by SOM content is much more sensitive to retention conditions than the error caused by montmorillonite content because of the different shapes of the EGME isotherms (Figures 4 and 5). As shown in this study, since the monolayer capacity (or apparent Envlron. Scl. Technol., Vol. 27, No. 8, 1993 l S # l
monolayer capacity) of a sample as determined from the isotherm is usually located at relatively low PIP", the isotherm method tends to minimize the effect of SOM on surface area determination. Completing our analysis of data in Table I, we note that the Qm (EGME),, is more than 3 times as large as Qm (EGME),, Le., the equivalent Q, (N2)value, for Woodburn soil. This difference cannot be accounted for by the organic content of 1.9% in the sample, but can be accounted for by small amounts of expanding and/or solvating clay minerals. A more detailed account of the excess EGME uptake over its surface adsorption on Woodburn soil will be provided in the following discussion. We now consider the solvation of EGME with solvating clays in soils and minerals. Although the excess EGME (or EG) sorption does reflect a sample's content of organic matter and solvating clay, the foregoing analysis suggests that it is improper to report EGME retention results in terms of surface areas when bulk penetration or sitespecific interaction occurs. On the other hand, the sorption isotherms of polar, solvating sorbates (like EGME) and adsorption isotherms of inert sorbates (such as Nz) may be used to account for the solvating effect of polar solvents with soils and minerals. In all likelihood, the solubility of EGME in SOM (i.e,, the slope of the corresponding linear isotherm) would be quite similar on a wide variety of soils. For a sample containing solvating clays, other inorganic matter, and organic matter, a useful analytical procedure would involve (i) determination of vapor isotherms of EGME and of Nz on the sample; (ii) determination of the sample's organic and ash contents; (iii) subtraction from the total EGME isotherm of the portion of the EGME surface isotherm, the latter being obtained by multiplying Q (Nz) by 1.30 to account for differences in molecular weight and molecular area of EGME and Nz on the constraint of equal surface coverage; and (iv) subtraction from the resulting curve of the linear isotherm calculated for the organic content of the sample (based on EGME solubility in peat), if significant. The result would be an isotherm that reflects the effect of cation solvation of EGME with the sample. These calculations have been carried out for Ca-montmorillonite, Fifthian illite, and Woodburn soil. The EGME solvation capacities with Ca-montmorillonite and Fifthian illite at different relative vapor pressures (P/Po)of EGME are shown in Figure 6. As noticed, the solvation isotherm of EGME on Ca-montmorillonite is very flat beyond P/P" > 0.1, showing the high constancy of the amount of EGME associated with montmorillonite clay. The isotherm with Fifthian illite shows a slight increase in uptake with P/P" from 0.1 to 0.4 and stays relatively unchanged beyond that point. On Ca-montmorillonite, the data show a close stoichiometric relation of about 4 EGME molecules for each exchangeable Ca ion at P/P" > 0.1, based on the clay's cation-exchange capacity (CEC) (120 mequiv/100 g). In the case of Fifthian illite, it gives 0.9-1.4 EGME molecules for each exchangeable K ion over the range of P/P" = 0.1-0.9, according to the CEC of the clay (15.5 mequiv/100 g) and assuming that all exchangeable cations are K ions. The lower coordination number for illite (than for Ca-montmorillonite) is expected partially from the greater electrostatic charge per unit formula weight for illite (about 0.8 charge) than for montmorillonite (about 0.4 charge) (21);the high charge in illite strongly holds the silicate interlayers against 1592 Envlron. Scl. Technol., Vol. 27. No. 8, 1993
-
0
1
300 I
250
I 50
e Ca-Montmoriiionile 0 illite (RlghlScale)
~
25 20
-
15
- 10 5
01 0
" '
'
0.1
,
3,
'
0.3
0.2
0.4
0.5
0.6
0.7
0.8
0.9
1
Relative Pressure, PiPo Flgure 6. Isotherms of EGME vapor on Ca-montmorlllonlteand Flfthlan llllte by cation solvation.
0 Cation Solvation A Surface Adrorplion
0 Partillon In SOM
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
i
Relative Pressure, P/Po Flgure 7. Isotherms for uptake of EGME vapor on Woodburn soil by cation solvation, surface adsorption, and partition in SOM.
expansion by EGME. Since illite is known not to swell significantly, the EGME solvation with exchangeable K ions likely takes place principally at the edges of the clay, which may be considered a site-specific chemisorption rather than a bulk penetration as for montmorillonite. The coordination number for illite should represent the average of EGME solvation capacities with exchangeable K ions from all different locations. A similar analysis of EGME sorption by Woodburn soil is given in Figure 7. Here the total EGME uptake is broken down into surface adsorption, partition (solubility) in SOM, and cation solvation. The cation solvation effect is again relatively flat with P / P = 0.1-0.7. However,without the knowledge of individual clay mineral contents and of exchangeable cation(s),it is impossible to relate the results to specific clay type and exchangeable cation(s). The relatively small EGME solvation capacity in Woodburn soil can be accounted for by either a small amount of montmorillonite (about 5 76 ) or any combinations of small quantities of montmorillonite and other solvating clay minerals. Since mixed layer minerals in soils and clays having the swelling and base-exchange properties cannot be readily detected by X-ray diffraction methods (41, the lack of identification of montmorillonite in Woodburn soil by X-ray does not exclude its presence in small quantities.
Table 11. Comparison of Surface Areas of Low-Organic-Content Soils and Clay Minerals Determined by BET Equation Using Na and Low-Polarity Organic Vapors as Adsorbates.
soil/clay mineral
organic carbon (% )
BET surface area (m2/g) low-polarity N2 organic vapor
Woodburn soil
1.10
11.2
Ashurst field soil Ashurst garden soil Whittlesey Black Fen soil Boston silt bentonite (Wyoming) Webster soil kaolinite bentonite
2.41 4.55 0.29 2.66 NDB 3.02 0.07 0.48
1.9 6.3 71.0 28.6 65.0 4.2 13.6 14.4
13.2: 13.8: 10.9,d 12.1e 3 9 4.6f 50.5’ 239 61.u 5.0“ 9.0,h 10,’ 16) 17’
The data for Woodburn soil are from Chiou and Shoup (23). The data for Ashurst field soil, Ashurst garden soil, Whittlesey Black Fen soil, Boston silt, and bentonite (Wyoming) are from Call (24). The data for Webster soil, kaolinite, and bentonite are from Rhue et al. (25). From benzene data. c From chlorobenzene data. d From rn-dichlorobenzenedata. e From 1,2,4-trichlorobenzenedata. f From ethylene dibromide data. 8 ND = not determined. From toluene data. i From p-xylene data.
We have seen that in the absence of bulk penetration or specific interaction EGME is essentially equivalent to Nz in determining the surface areas of solids. One would expect that any vapor that does not penetrate into or react specifically with clays nor significantly dissolve in organic matter should exhibit isotherms that would give essentially the same surface areas. In this respect, relatively nonpolar vapors have been found to exhibit low solubility in SOM (18, 19, 22), and these vapors would not be expected to exhibit significant penetration into expanding clays. Table I1 gives surface areas of a number of soils and minerals (including smectite) from the literature, as determined by BET plots derived from isotherms of Nz and a number of low-polarity vapors. A relatively good agreement exists between the surface areas obtained from the isotherm data of Nz and low-polarityvapors. Part of the small differences may be attributed to different outgassing conditions of the samples in the Nz and vapor uptake experiments and part to the fact that the various vapors and NZwere not standardized on the same standard sample. However, one would not expect similarly good agreement for highorganic-content peat and muck (22),where the high organic content, even with relatively low solubility of low-polarity organic liquids, would lead to significant partition (solubility). It appears that the concept of surface area based on solvent retention for soils and minerals has become increasingly imprecise. In 1950, Dyal and Hendricks (4) introduced the term “internal surface” to refer to the areas of crystallographic silicate planes within swelling clays. By this designation and the amount of EG retained by montmorillonite, they differentiated between external surfaces that preexist the measurement and ”internal surfaces” for the EG-silicate interfaces that result from EG penetration. The concept of internal surfaces was then extended by others to minerals and soils to denote the difference of the total surface area by the solvent retention method and the external surface area by standard BET method. In such cases, the correspondence between internal surfaces and crystallographic plane areas is often nonexistent because the areas of crystallographic planes
of all expanding clays that one can draw within their structures are of the same magnitude, whereas the EG or EGME retention depends not only on the specific expanding clay but also on the type of cation associated with the clay. Bower and Gschwend (5) used the DyalHendricks value of 3.1 X lo4 g of EG per m2 of surface area from montmorillonite along with the EG retention by SOM to calculate what they called “apparent surface areas” of SOM. Subsequently, the large excess of the SOMs apparent surface area (5) over its BET (Nz) surface area (17)was assumed by others (14)as the internal surface area of SOM, in spite of the fact that SOM and silicate clays have entirely different structures. From this and earlier studies ( I n , the high uptake of EG by SOM may be better described in terms of bulk solubility, similar to that of EGME in peat. In 1965Carter et al. (B), in the course of introducing the more convenient EGME as a substitute for EG in measuring the retention by clays, stated that the total surface area is a fundamental property of layer silicates. Using EGME retention, they reported the total surface areas of montmorillonite, illite, and kaolinite as 810,192.6, and 48.3 m2/g,respectively. If one takes the view of Dyal and Hendricks ( 4 ) for montmorillonite that the total surface area is the sum of preexisting surfaces and internal crystallographic planes, the lower value for illite would only account for part of the crystallographic planes, and the small value for kaolinite presumably only accounts for intrinsic (external) surfaces. These total surface areas may represent the sum of preexisting surfaces and created surfaces between sorbed EGME and clay, if they all exist as monolayers. In a 1986 paper, Carter et al. (26) recognized that the assumption of EGME or EG forming monolayers on layer silicates is “difficult to prove” and cited reasons why it may be invalid. It is evident that the excess EGME sorption by SOM is caused by bulk solubility and that the excess EGME sorption by clay is related to cation solvation and thus affected by the type of cation and solvent (15,16). As a result, the concept of “internal surfaces” would require a series of ad hoc explanations to fit each new set of data when such bulk penetration and/or cation solvation occurs. It would therefore be best to restrict the term “surface area” to the solid-gas interfacial area accessible to inert gases, as is done in allied fields. While the polar solventretention method is useful for characterization of clay minerals, the results can be better described by the term “sorption capacity” rather than total surface area or internal surface area to account for the combined effect of surface adsorption and cation solvation involved. In summary, the generality and relative simplicity of the BET method (using nonpenetrating and nonreactive adsorbates) have made it the accepted standard method for determination of the surface areas of divided solids without prior knowledge of their compositions. The solvent-retention method with EG or EGME has useful environmental applications, but the results thereof are not well rationalized in terms of internal surface areas. With the use of the BET equation, EGME is shown to be essentially equivalent to Nz in surface area determination for minerals that are nearly free of organic and solvating clay content. For soils and minerals containing significant amounts of solvating clay and organic matter, the solvation of EGME with clay cations and its solution in the organic matter complicatesthe use of EGME data in determination Environ. Sci. Technoi., Vol. 27, No. 8, 1993
15DS
of their surface areas. The use of low-polarity vapor isotherms and the BET equation should provide a reasonable estimation of the surface areas for most soils and minerals, except for high-organic-contentsoils. This result is attributed to the reduction of the vapor solubility in SOM and the elimination of vapor-cation solvation in expanding/solvating clays. The use of both EGME and Nz isotherms may, however, lead to a useful analysis of the cation solvation in clay minerals and soils.
Acknowledgments We thank Dr. T. T. Chao of the US. Geological Survey (Denver, CO) for the gift of the synthetic hydrous iron oxide and natural hydrous iron oxide samples used in this work. The use of trade and product names in this article is for identification purposes only and does not constitute endorsement by the US.Geological Survey.
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1938,60,309. Adamson, A. W. Physical Chemistry of Surfaces, 2nd ed.; Interscience Publishers: New York, 1967;pp 565-648. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982;pp 61-
84. Dyal, R. S.;Hendricks, S. B. Soil Sci. 1950,69,421. Bower, C. A,; Gschwend, F. B. Soil Sci. SOC.Am. Proc. 1952,
17,342. Martin, R. T. Soil Sci. SOC.Am. Proc. 1965,19,160. Sor, K.; Kemper, W. D. Soil Sci. SOC.Am. Proc. 1959,23,
(9) Heilman, M. D.;Carter, D. L.; Gonzalez, C. L. Soil Sci. 1965,100,409. (10) Cihacek, L. J.; Bremner, J. M. Soil Sci. SOC.Am. J . 1979, 43,821. (11) Tiller, K. G.; Smith, L. H. Aust. J. Soil Res. 1990,28, 1. (12)Eltantawy, I. M.; Arnold, P. W. J . Soil Sci. 1973,24,232. (13) Eltantawy, I. M.; Arnold, P. W. J. Soil Sci. 1974,25,99. (14)Pennel, K.D.;Rao, P. S. C. Enuiron. Sci. Technol. 1992,26, 402. (15) McNeal, B. L. Soil Sci. 1964,97,96. (16)Dowdy, R. H.; Mortland, M. M. Clays Clay Miner. 1967,15, 259. (17) Chiou, C. T.; Lee, J.-F.; Boyd, S. A. Enuiron. Sci. Technol. 1990,24,1164. (18)Chiou, C. T.; Kile, D. E.; Malcolm, R. L. Enuiron. Sci. Technol. 1988,22, 298. (19)Chiou, C. T.; Lee, J.-F.; Boyd, S. A. Environ. Sci. Technol. 1992,26,404. (20)Chao, T. T.; Zhou, L. Soil Sci. SOC.Am. J . 1983,47, 225. (21)Brady,N. C. TheNatureandPropertiesojSoils;MacMillan Publishing Co.: New York, 1984;Chapter 5,pp 160-164. (22) Rutherford, D. W.; Chiou, C. T. Environ. Sci. Technol. 1992, 26,965. (23) Chiou, C. T.; Shoup, T. D. Enuiron. Sci. Technol. 1985,19, 1196. (24) Call, F.J . Sci. Food Agric. 1957,8, 630. (25) Rhue, R. D.;Rao, P. S. C.; Smith, R. E. Chemosphere 1988, 17,727. (26) Carter, D. L.; Mortland, M. M.; Kemper, W. D. In Method of Soil Analysis, Part I . Physical and Mineralogical Methods; A. Klute, Ed.; Agromony Monograph No. 9,2nd ed.; American Society of Agronomy and Soil ScienceSociety of America: Madison, WI, 1986;Chapter 16,pp 412-423.
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$684 Envlron. Sci. Technol., Vol. 27, No. 8, 1993
Received for review April 20, 1992.Revised manuscript received March 30, 1993.Accepted April 26, 1993.
