Langmuir 2000, 16, 9819-9824
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Effects of Nonvolatile Organic Contamination on the Surface Areas and Adsorption Energetics of Porous Media Crist S. Khachikian* Department of Civil Engineering, California State University Los Angeles, 5151 State University Drive, Los Angeles, California 90032-8151
Thomas C. Harmon Department of Civil and Environmental Engineering, University of California Los Angeles, Los Angeles, California 90095-1361 Received April 26, 2000. In Final Form: August 14, 2000 Surface areas and pore-size distributions of natural and model porous media were measured before and after contamination by naphthalene, a nonvolatile organic compound (NVOC). The contaminant was added to the solids in a solvent (pentane) which was subsequently volatilized (a scenario meant to mimic field behavior of mixed solvents). The Brunauer-Emmett-Teller (BET) and Frenkel-Halsey-Hill (FHH) models were used to interpret the measured adsorption isotherms. Also, the Barrett-Joyner-Halenda (BJH) model was used to calculate pore-size distributions and pore properties of the samples. The results indicate that contamination of the porous media results in a reduction of surface areas and pore areas and volumes. For the Moffett sand (natural media), the surface area was reduced from 1.57 m2/g to 0.81 m2/g and 0.65 m2/g for the 0.1% and 1.0% contaminated samples, respectively. For the silica gel (model media), the area was reduced from 397 m2/g to 248 and 238 m2/g for the 0.1% and 1.0% contaminated samples, respectively. This reduction is also apparent in the pore-size distributions, where all pore areas and volumes are reduced significantly (for pores between 2.5 and 35 nm). Moreover, adsorption energetics are affected by the presence of the contaminant, although the results are difficult to interpret. It is postulated that the contaminant precipitates into the pores of the solids, reducing the pore and overall areas. Residence in these pores would render the contaminants inaccessible to mobile fluids and difficult to remove during remediation of contaminated soils.
Introduction Environmental porous media are often contaminated with mixtures of liquid and solid compounds. Many aspects of the behavior of single-phase liquid contaminants have been studied (see, e.g., refs 1-7). A common observation in such studies is that the interfacial area of dissolution, an important transport parameter, is difficult to measure.4 A few studies have concentrated on the solid constituents of these contaminant mixtures.8,9 Nonvolatile organic compounds (NVOCs) are usually carried as components in these mixtures and may solidify as the more volatile and soluble components weather away.8,9 The state of the resulting surface-bound NVOC is important to know because it affects contaminant bioavailability or leaching potential. * Corresponding author. Phone, (323) 343-6002; fax, (323) 3436316; e-mail,
[email protected]. (1) Miller, C. T.; Poirier-McNeill, M. M.; Mayer, A. S. Water Resour. Res. 1990, 26 (11), 2783-2796. (2) Miller, C. T.; Christakos, G.; Imhoff, P. T.; McBride, J. F.; Pedit, J. A.; Trangenstein, J. A. Adv. Water Resour. 1998, 21 (2), 77-120. (3) Pennell, K. D.; Pope, G. A.; Abriola, L. M. Environ. Sci. Technol. 1996, 30 (4), 1328-1335. (4) Powers, S. E.; Loureiro, C. O.; Abriola, L. M.; Weber, W. J. Water Resour. Res. 1991, 27 (4), 463-477. (5) Powers, S. E.; Abriola, L. M.; Weber, W. J. Water Resour. Res. 1992, 28 (10), 2691-2705. (6) Seagren, E. A.; Rittmann, B. E.; Valocchi, A. J. Environ. Sci. Technol. 1994, 28 (5), 833-839. (7) Anderson, M. R.; Johnson, R. L.; Pankow, J. F. Groundwater 1992, 30 (2), 250-256. (8) Mukherji, S.; Peters, C. A.; Weber, W. J., Jr. Environ. Sci. Technol. 1997, 31, 416-423. (9) Peters, C. A.; Luthy, R. G. Environ. Sci. Technol. 1994, 28 (7), 1331-1340.
This work quantifies geosorbent surface area and pore geometry in the presence of NVOCs. Experiments are conducted to measure the surface and pore areas of solids contaminated with naphthalene, a model NVOC. Specifically, the surface area and pore-size distributions of porous media are measured before and after contamination with naphthalene. These areas can then be incorporated into transport models. Through a study of the adsorption isotherms and the pore areas and volumes, conclusions are drawn regarding the location of the solid contaminants within the solid grains. These conclusions will help practitioners in designing and conducting more efficient remediation projects or in assessing long-term potential for human exposure and groundwater contamination. Theoretical The Brunauer-Emmett-Teller (BET) and the Frenkel-Halsey-Hill (FHH) isotherms are used to analyze the adsorption of nitrogen on solids. The BET isotherm is the most commonly used model for calculating surface areas,10 although its derivation contains assumptions that have previously been criticized.11-13 In particular, the fundamental assumption of a liquidlike structure of adsorbed molecules beyond the first adsorbed layer is suspect. However, a comparative assessment of areas using the BET model, as in this work, is still valid.10 The (10) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982. (11) Halsey, G. J. Chem. Phys. 1948, 16 (10), 931-937. (12) Hill, T. L. Adv. Catal. 1952, 4, 211-258. (13) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66 (8), 1739-1758.
10.1021/la000613p CCC: $19.00 © 2000 American Chemical Society Published on Web 11/16/2000
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FHH isotherm is used to qualitatively assess the effects of contamination on the surface properties of the porous solid. This isotherm has been derived rigorously for ideal, nonporous, spherical particles,12 which is clearly an unrealistic model for environmental solids. Nevertheless, conclusions can be reached on the basis of the parameters in the more general Halsey derivation of the isotherm.11 The more complicated aspect of the analysis is the determination of the pore size distributions (volume or area of pores versus pore size). Many models are based on slit-shaped or cylindrical pores. Again, given the comparative nature of the work, one model, the BarrettJoyner-Halenda (BJH) model, is used exclusively in this study. Details pertaining to each of these models are given below. Adsorption Isotherms. BET Model. The BET isotherm is most commonly given by
c(p/po) n ) nm (1 - p/po)(1 + (c - 1)p/po)
(1)
where n is the moles of adsorbed gas at p (pressure of adsorbate), c is the BET parameter, nm is the monolayer capacity in moles, and po is the saturation pressure of the adsorbate. The slope and intercept of the linearized form of the equation give the essential parameters used in determining the surface area. The monolayer capacity, nm, is a measure of the number of molecules that cover the entire surface of the solid with one complete layer of single adsorbent molecules. Therefore, this value multiplied by the area occupied by each molecule, am, and the number of molecules per mole, LA, yields the surface area of the solid, As. The most widely accepted area value of 16.2 Å2 per nitrogen molecule10 is used here. The BET c parameter is effectively given by exp{(q1-qL)/RT}, where (q1-qL) is the net heat of adsorption.10 Thus, experimental values obtained for c can be used as comparative guides (albeit qualitative ones) to the energetics of the adsorption process. Finally, the BET equation (eq 1) is valid to a relative pressure (p/po) ∼ 0.3, commonly known as point B. From comparisons made between areas calculated using the BET method and those calculated using visual techniques, it has been concluded that the BET equation generally yields results that are within 20% of the true areas.10 FHH Model. The basis for the derivation of the FHH isotherm is that a few molecular layers (two or three) of the adsorbate have adhered to the surface of the solid. At this distance away from the surface, the specific surface interactions are fairly diminished. Therefore, a new adsorbent molecule is attracted to a “liquid” of itself already adhered to the surface. This occurs for relative pressures above approximately 0.35, where the BET equation is no longer valid. Again, whereas the BET model assumes liquidlike behavior after the first molecular layer, the FHH differs by assuming this structure after the third or fourth layer. Hill12 gives a rigorous mathematical derivation of the isotherm for an idealized system (perfect spheres). Halsey11 forwarded the more general expression
ln
po b ) s p θ
(2)
where b is a function of the energy of adsorption in the first layer and θ is the molecular coverage (i.e., moles adsorbed (n)/moles in monolayer (nm)). In the Hill derivation, s is equal to 3, arising from the integration of surface forces. However, this is a fitted parameter in the Halsey
equation and, for porous solids, should be less than 3.10 The BET and FHH isotherms are independent of each other because these models use separate portions of the experimental data and are based on different physical assumptions. Therefore, similar conclusions drawn from the analysis of the data with each isotherm aid in strengthening the hypotheses proposed in this paper. Pore-Size Distribution. The calculation of pore-size distributions is based on the premise that the sample contains mesopores (between 2 and 50 nm10). These pores usually manifest themselves in Type IV adsorption isotherms. Hysteresis loops obtained in this study are of Type A (silica gel) and Type B (natural solids). According to Gregg & Sing,10 pore size calculations for these types of loops are valid. Both the adsorption and the desorption legs of the isotherms can be used to determine pore properties. Gregg and Sing10 recommend using the adsorption branch if network effects (interconnection of pore networks) cannot be discarded. These effects may be present for natural solid samples. Also, Type B hysteresis loops are unlikely to yield correct pore-size distribution results if the desorption branch is used.10 Therefore, only the adsorption branch is used in this study. The BJH model assumes that the pores of the solid are cylindrical in shape (details given in refs 10 and 14). Other models exist in which pores are taken as slit-shaped. However, given the comparative nature of this study and the ease of application of the BJH method, the BJH model is used exclusively. The solid samples are stepped through a series of relative pressures ranging from 0.1 to 0.99. At each pressure step, the pore radius, rp, is calculated as the sum of the core radius, rk, and the thickness of an adsorbed film, t. The core radius is calculated using the Kelvin equation
ln
2γVL 1 p )po RT rk
(3)
where γ is the surface tension of the adsorbent and VL is the molar volume of the adsorbent. The Halsey equation
t ) 3.540
[
-5.000 ln(p/po)
]
0.333
(4)
is used to calculate the thickness, t, of the adsorbed film.11 The pore radius at each pressure point is combined with adsorption data (volume adsorbed per pressure step) to calculate geometric mean radius, area, and volume for each pore size. Materials and Methods Porous Media. Two natural sands, Moffett sand (MS) and Borden sand (BS), as well as a model porous media, Silica gel (SG), are used in this study. The properties of the porous materials are given in Table 1. The solids were chosen to represent a range of surface areas from very small (approximately 1 m2/g) to fairly large (approximately 500 m2/g). Silica gel of a size fraction similar to that of the natural sands was not used because of its very large surface area (approximately 800 m2/g). The “clean” surface areas were obtained by riffle-splitting samples into 3-g portions. A large sample would yield more accurate results because of the increased overall adsorption of the sample. However, the experimental setup is limited by the free-space correction (proportional to sample size). Preliminary results indicated that 3-g samples produced negligible free-space corrections. Solvents were used extensively in this study to facilitate the contamination and remediation of the solids. Therefore, the solid (14) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380.
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Table 1. Average Grain Diameter and Previously Obtained Specific Surface Areas of the Solids Used in This Study solid
nominal grain size (mm)
published area (m2/g)
Borden sand (BS) Moffett sand (MS) Silica gel 1 (SG1)
0.33 0.33 0.097
0.40 NDb 500a
ref 15 16
a Provided by manufacturer, Fisher Scientific. b ND ) not determined. Data reported in Harmon16 includes surface area for -60+80 size fraction of Moffett sand (area ) 5.2 m2/g).
samples were washed to determine the effects of solvents on the surface areas. Solids were washed by weighing a sample in a beaker, adding the solvent, and mixing thoroughly. The sample was allowed to evaporate until the weight of the residual remained steady, at which time the surface area was measured. The weight was used as an indicator of the disappearance of the solvent. The solvents used included acetonitrile, benzene, methanol, and pentane, and preliminary results indicated that the solvent type did not affect the experimental results. These solvents were chosen because they represent chemicals with a range of properties (polarity, boiling point, and viscosity) that are fairly relevant to environmental work and are commonly used in industry. Pentane, the solvent with the lowest boiling point and the highest vapor pressure (i.e., the fastest evaporation rate), was selected for use in this study. Contamination Procedure. The solids were contaminated with naphthalene. Pentane was used as the solvent. Naphthalene is used for two reasons: it is fairly nontoxic as compared to the heavier polycyclic aromatics, and its vapor pressure and solubility are relatively low. The latter properties are useful in subsequent experiments involving the volatilization and solubilization of naphthalene. The solid/naphthalene/pentane mixture was stirred in a beaker until the pentane volatilized. The remaining solid mixture was used for all contaminated sample runs. Approximately 0.1% (1000 ppm) and 1.0% (10,000 ppm) of naphthalene (by weight) were added to the solids. Two potential experimental artifacts were tested and discounted. First, the pentane may not volatilize completely and may affect the surface areas. A series of experiments were conducted (data not shown) to test this effect, and it was concluded that washing with solvents did not affect the surface areas significantly. Also, repeated surface-area runs on a single sample, wherein a vacuum is imparted and maintained on the sample, yielded constant results. A related issue is that solvent washing may have affected the morphology of the organic matter associated with the solid grains, which, in turn, may have had an impact on naphthalene deposition. However, these effects are assumed to be negligible in comparing surface areas of clean and contaminated solids. Also, solids used in this study have no organic carbon (SG) or have very small amounts of organic carbon (BS and MS; see ref 15). Therefore, the overall contribution of this effect on contaminant transport is not expected to be significant. The second potential artifact is that the naphthalene may volatilize as the pentane evaporates. The final naphthalene concentrations were checked in samples of the porous media by extraction using a variety of solvents (pentane, hexane, and acetonitrile). The extract was analyzed using an HPLC (Waters Corporation) equipped with a photodiode array detector (PDA 990, 80:20 methanol:water mixture). The actual contaminant loading for the sample initially contaminated at 0.1% was 0.084% ((0.003%), and that for the 1.0% samples was 0.757% ((0.030%). Therefore, the goal of producing an order-of-magnitude difference in contamination levels was realized. The results here, quoted as 0.1% and 1.0% contamination, are meant to represent “intermediate” and “heavy” field contamination scenarios, respectively. (15) Ball, W. P.; Buehler, C. H.; Harmon, T. C.; Mackay, D. M.; Roberts, P. V. J. Contam. Hydrol. 1990, 5, 253-295. (16) Harmon, T. C. Determining and Modeling Diffusion-Limited Desorption Rates in Heterogeneous Aquifer Solids. Ph.D. Dissertation, Stanford University, 1992.
Surface-Area Analysis. Surface areas were measured using a Gemini III 2375 surface-area snalyzer (Micromeritics Corp., Norcross, GA). Nitrogen and helium of high purity (99.999% and 99.995%, respectively) were used in all experiments. The detection limit of the Gemini, given the current configuration, is approximately 0.1 m2/g. The Gemini consists of a sample vial that contains the porous media sample and a balance vial that is either left blank or filled with inert (nonadsorbing) glass beads. These glass beads are used to counterbalance the volume occupied by the sample, because calculations are most accurate when the difference in occupied volumes between the two vials is at a minimum. This difference in volumes, or the free space, is measured using He as an inert gas, and the entire analysis is conducted with the vials immersed in a dewar of liquid nitrogen. The adsorption isotherms are measured using the same experimental setup. The analysis is stepped through a set of preset relative pressures in which each step is maintained until the adsorbed volume remains steady. Then, more nitrogen is added to reach the next pressure step, and in this way the complete adsorption isotherm is obtained. A few desorption isotherms were also measured in which after the highest relative pressure is reached, a vacuum pump is used to step back down the isotherm. To determine true areas using the BET method, it is imperative that care be taken to remove the adsorbed water (condensed from ambient humidity) from the surface of the solid. The removal of this water requires one to heat the sample while imparting a vacuum on it. The contamination procedure used in this study precludes the use of these conditioning steps. Moreover, the conditions of the experiments are meant to mimic field conditions, which include ambient humidity. It should be clearly stated that the data presented here are incorrect in absolute value for the clean or contaminated solids. They are, however, representative of the properties of field-conditioned solids; that is, for solids exposed to environmental conditions such as humidity. Also, for comparative studies similar to this, the true values of the surface areas are unnecessary.
Results and Discussion This section begins by presenting the adsorption isotherms for clean and contaminated samples. The areas of the clean and contaminated samples are compared and the effects of the contaminant are discussed. Next, the energetics of adsorption suggested by the data are discussed. Both the BET model and the FHH model are used for this purpose. Finally, the BJH model is used to calculate the areas and volumes of the sample pores. Adsorption Isotherms. In general, isotherms are represented using straight scales. However, log scales are frequently used in this study because they help distinguish between the small differences in adsorbed volume of clean and contaminated samples. The adsorption of nitrogen on SG (Figure 1) follows a Type IV isotherm typical of mesoporous solids.10 The clean areas obtained here are approximately 20% less than the true value (Tables 1 and 2), a deficit attributed to the lack of sample pretreatment. It is evident in Figure 1 that the contaminant has reduced the nitrogen adsorption capacity of the silica gel somewhat uniformly across the entire pressure range. The adsorption behavior at the high-pressure end of the isotherm is suspect because the overall adsorption for silica gel is very high and at these pressures, small variations in temperature can cause large discrepancies in the data. The analysis for each silica gel sample takes from 3 to 4 h, during which time the liquid nitrogen level in the dewar decreases and cannot be accessed. Therefore, a portion of the vial is not in contact with liquid nitrogen, resulting in an uneven temperature distribution in the vial. This effect may account for the scatter observed at the higher pressures. The isotherms for the natural BS and MS samples are of Type IV (see Figure 2 for example). The data for BS are less amenable to analysis than those for MS because the
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Figure 1. Adsorption isotherms for clean (O) and 1.0% contaminated (0) Silica gel (SG). Contamination reduces the affinity of the SG surface for the adsorbate (nitrogen). This reduction is fairly uniform across the entire pressure range. Table 2. Measured Surfaces Areas for Clean and Contaminated Moffett Sand, Borden Sand, and Silica Gel solid
contam level
area (m2/g)
Moffett sand Moffett sand Moffett sand
clean 0.1% 1.0%
1.57 ((0.09) 0.81 ((0.03) 0.65 ((0.02)
Borden sand Borden sand
clean 1.0%
0.27 ((0.02) 0.15 ((0.01)
Silica gel Silica gel Silica gel
clean 0.1% 1.0%
396.5 ((5.8) 248.1 ((1.0) 238.3 ((6.0)
Figure 2. Adsorption and desorption isotherms for clean Moffett sand (MS). The hysteresis loop clearly indicates that the MS samples follow a Type IV isotherm with a Type B hysteresis loop.
overall surface area of BS is much lower than that of MS (Table 2). Although the surface-area data of both sands are presented in Table 2, the remaining results (tabular and graphical) are for MS samples only. However, the conclusions discussed below are valid for both natural sands. A desorption isotherm is shown in Figure 2 which clearly indicates that the Moffett sand follows a Type IV isotherm with a hysteresis loop of Type B. From Figure 3, it is evident that the adsorption capacity of MS is reduced by the presence of the contaminant (open and solid
Figure 3. Adsorption isotherms for clean (O) and contaminated (9 for 0.1% and 0 for 1.0%) Moffett sand (MS). The presence of contamination reduces the affinity of the surface for the adsorbate molecules. As contaminant load increases, the volume of adsorbate adsorbed decreases.
squares). Also, an increase in contaminant loading further reduces the adsorption capacity. The surface areas were calculated using BET fits to data gathered at p/po < 0.31, and the data are summarized in Table 2. As the contaminant loading is increased, the surface area is gradually decreased for the BS, MS, and SG samples. A number of possibly concurrent reasons may explain the reduction of areas due to contamination: (1) the contaminant reduces surface roughness, thus reducing adsorption capacity; (2) the contaminant covers the mineral surface and presents a less-favorable adsorption site to the nitrogen molecules; and (3) the contaminant fills or “blocks” deeper pores, rendering their surface area inaccessible. A closer look at the data presented in Table 2 reveals that the surface areas of the solids are not similarly affected. Whereas coating at 0.1% reduces the areas of the MS and SG samples by 48% and 37%, respectively, coating at 1.0% reduces the areas of the MS and SG areas by 59% and 40%, respectively. These results suggest that, although higher contamination further reduces MS areas, the reduction of SG areas seems to be independent of the contaminant loading. The discrepancy is probably due to the SG having large pore areas which are coated with naphthalene at the 0.1% loading. This coating significantly reduces the affinity of the SG for nitrogen molecules, and if further contamination (1.0%) thickens this coating, then the surface area at the higher loading should not change substantially. In contrast, the MS samples seem to be affected more by the “pore-blocking” mechanism, in which higher mass loading results in more pores blocked. This pore-blocking manifests itself as lower surface-area values. More experiments, including development of techniques for direct observation of the contaminated samples, are necessary to discover the location of the contaminant on the solid surface and its effects on gas adsorption. Also, it is expected that much higher loading for the SG (say 10%) will produce a more pronounced effect on surface areas as pores begin to be filled. It is evident that contamination of porous media by solid NVOCs reduces the capacity of solid media to adsorb nitrogen. The decrease in surface area is related to the contamination loading; that is, at higher loading, the surface area is reduced further. It is concluded that the NVOC is precipitating on the porous media grains and
Nonvolatile Organic Contamination in Porous Media Table 3. Adsorption Energetics Estimated from the FHH and BET Isothermsa solid
contam level
Moffett sand Moffett sand Moffett sand
clean 0.1% 1.0%
95.6 ((12.1) 2.25 ((0.07) 2.58 ((0.08) 55.6 ((10.2) 1.80 ((0.08) 2.11 ((0.12) 56.8 ((4.9) 2.01 ((0.03) 2.21 ((0.08)
Silica gel Silica gel Silica gel
clean 0.1% 1.0%
76.6 ((1.5) 712 ((215) 237 ((174)
c
s
b
1.54 ((0.02) 1.88 ((0.02) 1.92 ((0.02) 2.25 ((0.09) 1.79 ((0.02) 1.89 ((0.03)
a The energetics is described by the BET c parameter and the FHH s and b parameters.
reducing the specific interactions between the surface and the adsorbate molecule (or reducing the energy of adsorption, as discussed below). Surface Energetics. Each of the energy parameters to be discussed (BET c, and FHH s and b) has a rigorous theoretical definition, but one that either is for highly idealized systems or depends on other theoretical values that cannot be measured experimentally. Therefore, the following discussion is useful in comparing the nature of surface forces giving rise to gas adsorption, but it does not yield quantitative conclusions. The BET c parameter is related to the net heat of adsorption in the first layer. The solid surfaces present sites that have potential specific interactions with the nitrogen molecules. A cover of naphthalene on this surface attenuates these specific interactions and presents a potentially weaker, electrostatic surface. Therefore, the c values of the contaminated samples should be lower than the c values of the clean samples. The reduction is approximately 40% for the MS samples (Table 3). For the SG samples, c varies considerably and in conflict with expected trends. The average c values for both the contaminated SG samples (especially at 0.1% contamination) increase considerably from their clean values (Table 3). The results for the individual samples exhibited a high degree of variability and were very sensitive to the fitting criteria. More studies are under way to explain this behavior. The FHH model is fit to the midrange of the adsorption isotherms. The range of validity for this isotherm is ambiguous in the literature, but a limiting condition is that the lower limit should be above the relative pressure where multiple layers form. Presumably this is beyond point B, the upper limit of validity for the BET isotherm (where monolayer coverage is complete at p/po ∼ 0.3). The upper bound on the applicability of the FHH model is less clear. Gregg and Sing10 suggest testing the validity by linearizing the data and trying different subsets until a good fit is obtained. Here, the FHH model fits the data best for a relatively large range of relative pressures (i.e., 0.35-0.85). From this fit, the FHH parameters s and b are calculated. Many experiments on nonporous solids have yielded s values of approximately 2.7.10 For mesoporous solids, this value is generally between 1.4 and 1.7. For clean MS and SG, s was estimated to be 2.25 and 1.54, respectively. These numbers affirm that mesopores exist in these samples and that SG is more mesoporous than MS (because s is smaller), an expected result. Upon contamination at 0.1%, the s value of MS drops to 1.80. At 1.0%, the s value becomes 2.01. This result is counter intuitive; the “blocking” of pores should increase this number to something approaching 3, but in both cases the s values are lower. Also, the decrease of s is not consistent in that higher contamination does not result in a further decrease. A possible explanation is that at 0.1%, naphthalene is blocking some pores while covering most of the solid
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surface. This would change a two-region adsorption solid (pores and external surface) to a three-region solid (pores, external surface, and naphthalene). So, if the naphthalene is covering most of the surface and blocking some pores, then the adsorbing nitrogen molecules have two choices of sites: the naphthalene surfaces and the mineral mesopores. This type of distribution would increase the mesoporous adsorption site proportion but not the overall quantity of adsorption. At higher loadings (as in the 1.0% samples), more of the exposed pores would be filled or blocked, resulting in a reversal of the trend (i.e., increasing s values). It is expected that higher loadings will further increase s values. The SG s values increase for both the 0.1% and 1.0% samples in accord with the expected trend. However, the increases are inconsistent, because the s values are smaller for the higher contaminant loading (1.0%). At this point, the reasons for the discrepancy in the observed trends in the data for the MS and the SG are unclear. More studies with well-characterized solids must be conducted before this phenomenon will be understood. The FHH b value is related to the energy of adsorption in the first layer.10 The procedure for obtaining b is more reliable than that used to obtain c because it uses data in the midrange of the isotherm (p/po between 0.35 and 0.85), which supports a more stable parameter estimate. Thus, the parameter values of b present a clearer picture of the adsorption process. It is expected and observed that the adsorption on clean surfaces should yield higher values of b than adsorption on the surface of naphthalene. Experimentally, we verified this assumption by measuring the surface area of solid naphthalene (data not shown), which yielded a negligible area, indicating that the nitrogen molecules do not have a strong affinity for pure naphthalene. The b values are reduced because the contamination is causing the nitrogen molecules to adsorb to less favorable sites and to sites farther from the surface, thus reducing the interaction with the solid surface. The b values for the MS samples decrease with contamination (Table 3) as expected. For the SG samples, the situation is slightly different. Upon contamination at 0.1%, the b values for the SG samples increase. Further contamination at 1.0% causes a decrease in the average of the b value from the 0.1% value. Although inconsistent, this trend is similar to that observed for the SG s values. It is clear that additional studies are needed to elucidate the relationship between contamination and these energyparameter values. It is evident that the adsorption energy, as quantified by the BET c and the FHH s and b parameters, is changing in the presence of contamination. Some of the results are consistent with the decreasing surface areas, whereas others deviate from this trend. More careful examination using ideal solids (e.g., zeolites) is needed to understand this behavior. Pore-Size Distributions. The contaminant reduces the adsorption capacity of the pores in a fairly uniform manner (Figures 4-6). This uniformity can be explained in terms of the following scenario. First, the pentane naphthalene mixture penetrates the pores of the solid media. Then, the pentane evaporates and the naphthalene precipitates, lining, filling, or blocking pores. This “poreblocking” mechanism is speculative at this point. Also, data presented are for pore sizes > 2.5 nm, because it is generally accepted that this methodology (i.e., the Kelvin equation) is valid only for pores of this size. The data for the SG samples (Figure 4) show a discernible trend: for pores less than 12 nm in diameter, the presence of naphthalene reduces the pore volumes. This observation is in accord with the surface-energetics
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Figure 4. Pore-size distribution (dvD/dD) for clean (O) and contaminated (9 for 0.1% and 0 for 1.0%) silica gel (SG). The contaminants reduce the capacity of the pores to adsorb nitrogen, thus reducing their contribution to the total area of the solid.
Figure 5. Pore-size distribution (dvD/dD) for clean (O) and contaminated (9 for 0.1% and 0 for 1.0%) Moffett sand (MS). Again, it is evident that the contamination reduces the capacity of pores to adsorb nitrogen. Scatter at small pore sizes is due to the small measured volumes.
arguments given in the previous sections. The data for pores greater than 12 nm in diameter are suspect, as previously noted for the SG adsorption isotherms. Contamination at 0.1% reduces the pore volumes substantially (∼40% on average). Further contamination (loading of 1.0%) has a less pronounced effect on pore volumes (∼25%
Khachikian and Harmon
Figure 6. Cumulative pore-size distribution (Σ(dvD/dD)) areas for clean (O) and contaminated (9 for 0.1% and 0 for 1.0%) Moffett sand (MS). The contribution of each pore size to the total pore-size distribution of the solid is apparent. Moreover, the effects of contaminant loading on the pores are accentuated in this figure.
on average). This observation is in accord with the previously discussed trends observed for the SG areas. For the MS samples, a relatively equal reduction of pore volumes seems to occur across the entire pore-size distribution (Figures 5 and 6). This observation suggests that the contaminant-solvent mixture is filling the pores fairly indiscriminately. The larger scatter at the lower pore sizes can be attributed to the method’s inability to resolve the small areas associated with these pore sizes for MS. The same data can be viewed as a cumulative distribution (Figure 6) where the contribution of each pore range to the total surface area is more apparent. Again, it appears that all pores are affected equally by the contamination and contribute equally to the observed total pore volume reduction. The data here present a picture in which the contaminants are deposited in the pores of the solid grains, reducing their surface areas. The contaminant may also block pores from access to nitrogen. Currently, the actual mechanism is not known; more advanced analyses are needed to fully characterize the solids and the fate of NVOCs in these solids. Acknowledgment. This work was sponsored by the National Science Foundation (BES-9502170). The results have not been subjected to NSF review, and an endorsement should not be inferred. We thank the two anonymous reviewers for their insightful and helpful comments. LA000613P
