Envlron. Sci. Technol. 1992, 26, 1955-1962
A Distributed Reactivity Model for Sorption by Soils and Sediments. 1. Conceptual Basis and Equilibrium Assessments Waiter J. Weber, Jr.," Paul M. McGinley,+ and Lynn E. K a t d
Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125 Contaminant sorption by soils and sediments is characterized as a multiple reaction phenomenon. The approach is predicated on the observation that most natural soils and sediments are intrinsically heterogeneous even at the microscopic scale; that is, variable in composition and structure at both interparticle and intraparticle scales. Heterogeneity is demonstrated for a number of soils which, on the basis of conventional macroscopic properties, would be considered uhomogeneousn. That such heterogeneities are reflected in sorption reactions which differ between soils and between different fractions of soil is also demonstrated. A composite model, the distributed reactivity model (DRM), is introduced to characterize intrinsic heterogeneities in the properties and behaviors of soils and sediments and to capture the resulting nonlinearities of sorption isotherms. Finally, the significance of particlescale heterogeneity and distributed reactivity is illustrated by using measured parameters and DRM calculations to characterize differences in the relative sorption behavior of soils comprising different mass fractions of differently reactive components. Introduction There is virtually universal accord in the scientific community that sorption processes play an important role in the transport and disposition of organic contaminants in subsurface systems. No similar consensus exists regarding mechanistic characterization of these processes. In this paper we argue that sorption processes in environmental systems comprise multiple reaction phenomena involving different reaction mechanisms. Like the environmental solids on which they occur, subsurface sorption processes are intrinsically heterogeneous. Explanations of observed phenomena predicated on single sorption reactions or mechanisms may thus be system and event specific and limited as a basis for characterizing sorption behavior in different systems or under different conditions. The model deriving from the multiple reaction concept is termed the distributed reactivity model (DRM) to acknowledge different distributions of sorption reactions and mechanisms for different solute-solid combinations. Different regions of a soil or sediment matrix may contain different types, amounts, and distributions of surfaces and of soil organic material, even at the particle scale. A variety of different classes of reactions between organic solutes and types of solid surfaces and quasi-solid organic matrices typical of those associated with environmental solids have been identified (I). Given that such interactions differ widely with respect to magnitude and mechanism, it is reasonable to expect distributed reactivities for distributed constituent compositions and surfaces. Background Linear models of the following form are frequently employed to approximate sorption data for soils and sediments: Present address: Department of Civil Engineering, The University of Kentucky, Lexington, KY 40506-0046. *Present address: Department of Civil Engineering, The University of Maine, Orono, ME 04469. 0013-936X/92/0926-1955$03.00/0
where qe is the mass of solute sorbed per unit mass of solid at equilibrium with a solution of solute concentration Ce and K Dis the distribution coefficient. This formulation can be deduced from any of a number of conceptual models, including, as many have suggested, solute partitioning to organic phases ( 2 , 3 ) .Reported observations of correlations among the extent of sorption of organic contaminants, the organic carbon content of sorbents, and the hydrophobicity of solutes are consistent with the behavior expected for partitioning processes. However, strong cautions have been raised about interpreting such observations as confirmation of actual partitioning mechanisms (1,4).
An appealing aspect of the partitioning analogy is that the information required to predict sorption behavior reduces to two readily obtained parameters, the octanolwater partitioning coefficient, KO,, of the solute and the mass fraction of organic carbon, f, for a particular solid
+
log (KD/f,) = log KO,= a log KO, b
(2)
The coefficients a and b in correlations of this type are obtained experimentally, but they have been related to sorbent properties (5,6). There is abundant evidence that variations in sorption behavior and differences in K , between different soils and sediments can be attributed in part to differences in the origin and types of associated organic matter (7-12). A significant constraint on the conceptual correctness and general utility of eqs 1 and 2 is that they assume and accommodate only linear equilibrium relationships. There is mounting evidence of isotherm nonlinearity in subsurface soils (1, 7, 13-16). Nonlinearity should in fact be expected for surface adsorption phenomena that extend over significant solution concentration ranges and may be exhibited for absorption into organic matrices if interactions between sorbed solute molecules increase or decrease their affinity for those matrices. A variety of conceptual and empirical equilibrium models for representing nonlinear sorption processes exist. The Langmuir model is predicated on an asymptotic approach to some maximum sorption capacity, Q", and a factor, b, relating to the affinity of the surface for the solute: Qe
= QbCe/(1
+ bCe)
(3)
Most environmental sorption processes are less energetically straightforward than those suggested by either the linear free energy partitioning model or the constant-energy limited-surface Langmuir model. Equilibrium data are often best described by models which assume neither homogeneous site energies nor limited levels of sorption, such as the Freundlich model: qe = KFCen (4) The parameter KF in eq 4 is termed the Freundlich unit-capacity coefficient, and n is a joint measure of both the relative magnitude and diversity of energies associated with a particular sorption process. As depicted sche-
0 1992 American Chemical Soclety
Environ. Sci. Technol., Voi. 26, No. 10, 1992
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where f w and Kw are the organic carbon mass fraction and the organic carbon-normalized partition coefficient for the ith sorption region or component. The organic carbon mass fraction, fq, of the bulk soil is then given by Cxim,. As we have noted previously, linear sorption relationships and associated partitioning coefficients are limited in their utility for characterizing adsorptive surface reactions ( 1 , 4 ) . If one or more of the component elements of sorption is governed by a nonlinear relationship between the solution and sorbed phases, then the composite isotherm will deviate from linearity. We suggest that for natural systems, it may be useful to consider composite isotherm behavior as the resultant of a series of near-linear absorption reactions and nonlinear adsorption reactions. The nonlinear reactions may be modeled with a series of any of a number of different models, including the Langmuir and Freundlich models. In that Langmuir behavior is rarely observed even for the most homogeneous of soils, we have elected to use the Freundlich model to characterize the nonlinear reactions. This seems to make particular sense in that, as illustrated in Figure 1, a series of Langmuir reactions, even if individually distinguishable, will sum to yield a composite isotherm of Freundlich character. Because only the linear absorption contributions can be expressed in terms of a summed sorption parameter, KDr,the DRM will take the form m
Be,
= ~ I K D ,+CC~( ~ n l ) i K ~ , C e ~ r c=1
(8)
where x1 is the summed mass fraction of solid phase exhibiting linear sorptions,KD, is the mass-averaged partition coefficient for the summed linear components, and ( x , , ) ~ is the mass fraction of the ith nonlinearly sorbing component. From a practical perspective, the number of operationally distinguishable nonlinear components, m, will typically be only 1 or 2.
where qe, is the total solute mass sorbed per unit mass of bulk solid, x i is the mass fraction of the soil comprising reaction region or component i, and qeiis the sorbed phase concentration expressed per unit mass of that region or component. If each of the individual contributing mechanisms or regions of sorption yields an isotherm which is linear, then the composite isotherm will be linear:
where KD, is the partition coefficient for reaction i expressed on a per mass of component i basis and KDr is the mass-averagedpartition coefficient (6,21). Karickhoff (6) and Curtis et al. (22),among others, have used composite linear isotherms to assess the relative importance of mineral surfaces in soil sorption processes. For true partitioning processes, eq 6 can be rearranged to accommodate the mass fractions of different types of organic matrices: 1956
Environ. Sci. Technol., Vol. 26,
No. 10, 1992
Experimental Evidence of Distributed Reactivity The concentration dependence of the total extent of adsorption, qe,, for each of three solutes of different hydrophobicity was examined for a series of six subsurface soils over concentration ranges relevant to subsurface contamination and remediation. The soils were selected to encompass the general range of organic content, particle size, and surface area typical of aquifer materials. On the basis of common macroscopic characteristics and properties, each of the soils examined would normally be considered “homogeneous”. Methods and Materials. Soils were collected from subsurface environments in southeastern Michigan and northwestern Ohio. The geological origin and association-level classification of the regions are shown in Table I. Five soils were collected at depths greater than 1 m: Augusta, Wagner, Ann Arbor I, Delta, and Ypsilanti soils. Of these, only the Augusta and Ypsilanti soils were saturated at the time of collection. The Ann Arbor I1 soil was collected at a depth less than 1m and was unsaturated. The soils were air-dried and sieved. Organic carbon content was determined by high-temperature oxidation with pure oxygen (Leco WR-112) and by low-temperature persulfate oxidation (Oceanographics International 1-A equipped with a Beckman 915A IR detector). The three organic solutes studied were tetrachloroethylene (TTCE) (Eastman Kodak, Rochester, NY), 1,4dichlorobenzene (DCB) (Eastman Kodak), and 1,2,4-trichlorobenzene (TCB) (Aldrich, Milwaukee, WI). Stock
Table I. Soil Characteristics
soil
series
Augusta Delta Ann Arbor I1 Wagner Ypsilanti Ann Arbor I
Spinks Ottokee Brookston Miami Wasepi Brookston
median grain size (mm)
low T (%)
0.13
0.20 0.16 0.53 0.31 0.21
solutions were prepared in methanol, and working solutions by subsequent dilution in a pH 8.3 aqueous carbonate buffer. The mole fraction of methanol was less than in all experiments. As reported by others, no effects of this carrier were evident at the concentrations used (21). TTCE, DCB, and TCB were extracted into hexane and analyzed by gas chromatography using an electron capture detector (Hewlett-Packard 5880 and 5890). Samples for the isotherm experiments were prepared by mixing working solutions of carbonate buffer and solute with soil in 40-mL glass centrifuge tubes. The tubes were sealed with Teflon-faced silicon septa and tumbled endover-end until apparent equilibrium (Le., no further measurable change in solution concentration) was reached. Fourteen days were required to achieve apparent equilibrium conditions for the Wagner, Ann Arbor I, and Ypsilanti soils, and 7 days for the Augusta, Delta, and Ann Arbor I1 soils. At the completion of each experiment, the samples were centrifuged and the supernatants analyzed for solute concentration. The mass of solute sorbed in each system was calculated by normalizing the difference in aqueous-phase solute mass before and after the test to the soil mass. Solute losses in reactors which did not contain soil averaged 9,4, and 8% of the initial concentrations for TTCE, DCB, and TCB, respectively. Solute recovery efficiencies were evaluated by analyzing the contents of partially decanted vials. Mass balance analyses indicated that for the most volatile solute studied, TTCE, more than half of the loss in the control reactors could be recovered by extraction with hexane. It was therefore assumed that losses occurred primarily from reversible sorption onto reactor components, and it was then possible to correct the data using a method similar to that of Lion et al. (9). Sorption capacities were measured for solution concentrations extending over 3 orders of magnitude in all experiments with the whole soils to provide a sound statistical basis for examining isotherm nonlinearity. Isotherm parameters were determined for the Freundlich isotherm model (eq 4) by fitting the data to the log transform of that equation, using linear regression.
'
Results and Discussion The median grain sizes, nitrogen BET surface areas, and organic carbon contents of the soils are identified in Table I. The surface areas of the soils tested were fairly low (1.2-4.2 m2/g), reasonably typical for aquifer sands. The values for the Ann Arbor I and Ann Arbor I1 soils were higher than those for the other four soils, but for Ann Arbor I, this was not reflected in a smaller grain size distribution. Organic carbon contents obtained using both high- and low-temperature methods are presented in Table I. The f , values for the soils varied from 0.0003 to almost 0.025 (0.03-2.5% by weight). Examination of the data indicates that the alternative methods of organic carbon analysis yielded similar results for the three soils having the lowest organic carbon contents. In contrast, significant differences between the high- and low-temperature results are
total organic carbon high T (%) ratio (L/H)
0.03 0.15 0.55 1.20 0.24 1.10
1.00 0.94 0.95 0.48 0.19 0.85
0.03 0.16 0.58 2.49 1.24 1.29 1000
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observed for the soils having the highest organic carbon contents. The results suggest that the latter soils contain a recalcitrant or "hard" organic fraction which, either because of its composition or its inaccessibility in the soil matrix, is oxidized only by the high-temperature method. Others have observed similar differences in the results obtained for organic carbon by high- and low-temperature techniques (23,24). Single-soluteisotherms for TTCE, DCB, and TCB were obtained for all soils. Typical data and corresponding Freundlich model fits are shown in Figure 2 for TCB. The isotherms for DCB and TTCE on each of the six soils exhibited qualitative behavior and soil-specific patterns similar to those illustrated for TCB in Figure 2. Two features of all of these isotherms are noteworthy in the context of distributed reactivity: (i) the sorption capacities for each solute varied widely among the different soils; (ii) the data and corresponding model fits for the three solutes and six soils exhibited a range of nonlinearities. These features are both characteristic of heterogeneous sorbents. Varied Sorption Capacities. Comparison of Figure 2 and Table I indicates that sorption capacity generally increased with increasing organic content as measured by the high-temperature method, with only the Wagner soil deviating somewhat from a direct rank order. While this trend is consistent with the observations of others for sorption of organic compounds by sediments and soil materials, the capacities themselves are greater than might be expected for even high organic content soils. Comparison of the sorption capacities of different soils for a particular solute can be facilitated by normalizing solidphase solute concentrations, qe, to the respective organic carbon mass fraction, f,, of each soil. As shown in Figure 3a and b, this did not collapse the sorption data to a coincident isotherm, irrespective of whether the high or low organic carbon values were used for normalization. Moreover, comparison with a normalized correlation developed from eq 2 and calibrated with parameters reported by Schwarzenbach and Westall (5) shows that, for all of the soils except the Ann Arbor I1 and to some extent the Environ. Sci. Technol., Vol. 26, No. 10, 1992
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TCB Ce (ugh) Figure 3. Experimental data and Freundlich isotherm models for sorption of TCB by six soils, normalized to the organic carbon fractions of the soils and compared to a prediction using the linear isotherm correlation of Schwarzenbach and Westall (1981): (a, top) low-temperature organic carbon normalization;(b, bottom) high-temperature organic carbon normalization.
Wagner soil for a high-carbon normalization only, the observed sorption capacities are significantly higher than predicted by a simple KO,- KO,correlation. Previous observations of sorption capacities in excess of those predicted by correlations between KO,and KO, have been attributed to a variety of phenomena, including the following: (i) sorption to reactive mineral components, (ii) sorption mechanisms other than simple organic phase partitioning, and (iii) differences in the compositional character of soil organic matter. For the first two conditions, it would be reasonable to expect some dependence of sorption capacity on soil surface area. To evaluate this for the soils tested, the TCB isotherms H Figure 3 were normalized to the respective BET surface areas of the soils. As shown in Figure 4, the normalized capacities exceed by at least 1 order of magnitude those reported by Schwarzenbach and Westall (5) for mineral phases such as silica or kaolonite. It may be noted in Figure 4 that surface area normalization of the Ypsilanti, Ann Arbor I, and Wagner sorption data results in bringing these isotherms closer together, particularly at higher solution concentrations. The high-temperature measurements of organic carbon for the first two of these soils, 1.24 and 1.29%,respectively, are very similar, but, as illustrated in Figure 2, TCB sorption is significantly higher on a total weight basis for the Ann Arbor I soil than for the Ypsilanti soil. This apparently relates in major part to the larger surface area of the Ann Arbor I soil. However, the fact that the surface area normalization does not completely collapse these isotherms on each other, coupled with the somewhat 1958
10
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1000
10 100
Flgure 4. Experimental data and Freundlich isotherm models for sorption of TCB by SIX soils, normalized to the surface areas of the soils.
1 I I 1
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Environ. Sci. Technol., Vol. 26, No. IO, 1992
greater degree of nonlinearity exhibited by the Ann Arbor I isotherm, suggests that the Ann Arbor I soil may experience surface reactions not operative to the same extent for the Ypsilanti soil. The Wagner soil, on the other hand, has a surface area very close to that of the Ypsilanti soil, but an organic carbon content that is twice as high. The fact that its surface-area normalized isotherm lies somewhat below that of the Ypsilanti soil appears to relate to a much higher percentage of its total organic content being comprised of “low temperature” or “soft carbon”. It can be noted in Figure 3 that normalization 0f sorption for the Ypsilanti soil to its total organic carbon value (1.24%), much of which was “hard” carbon (1.24-0.24% = LO%), yields a somewhat higher isotherm (Figure 3b) than does normalization of the Wagner soil to its similar level (1.2%) of soft organic carbon (Figure 3a). Normalizing the three highest organic content soils to either organic carbon or to surface area brings the isotherms for the Ann Arbor I and Ypsilanti soils closer together. The normalized isotherms for Wagner soil differ from the other two, however, suggesting differences in the organic matter associated with this soil. This was reinforced by subsequent analyses of the oxygen/carbon ratios of the reactive components of the Wagner and Ypsilanti soils, which were found to be 0.47 and 0.60, respectively. Differences in the character of soil organic matter also affect the sorption behavior of the three lowest carbon soils. As indicated in Figure 4, organic carbon and surface area normalization of TCB sorption data for these soils yields isotherms which are remonably coincident but significantly lower than those for the three higher organic content soils. The foregoing analyses of sorption data for ostensibly similar aquifer soils demonstrate that variations in the character of the organic matter associated with the soils or in the reactivities of available surfaces yield rather remarkable differences in sorption capacities. These findings suggest intrinsic heterogeneities that relate to certain features of the soils which, though not obvious in measurements of bulk properties, apparently play significant roles in determination of relative sorption reactivity. Isotherm Nonlinearity. The Freundlich isotherm model was found to provide a reasonable fit to the sorption equilibrium data for all six soils and three solutes over broad concentration ranges, as evidenced for TCB in Figure 2. The slopes of the model fits, corresponding to the Freundlich exponent parameter, n, are measures of isotherm linearity. The closer n is to 1, the more linear is the isotherm. It is evident from Figure 2 that the degree of nonlinearity increases with increasing sorption capacity
Table 11. Freundlich Sorption Isotherm Model Parameters and 95% Confidence Limits (in Parentheses)
soil
data
Augusta Delta Ann Arbor I1 Wagner
3 58 27 64 6 75
Ypsilanti
Ann Arbor I
TTCE log KF (rg/g)(L/rg)" -3.94 (5.90) -2.96 (0.26) -2.98 (0.27) -1.22 (0.10) -1.28 (0.25)
-0.75 (0.09)
n
data
1.20 (2.18) 0.93 (0.08) 1.06 (0.08) 0.78 (0.04) 0.79 (0.11) 0.78 (0.03)
10 35 15 7 8 50
DCB 1% KF (rg/g)(L/rg)" -2.40 -2.68 -2.34 -1.05
(0.32) (0.16) (0.60) (0.63) -1.11 (0.11) -0.29 (0.07)
n
data
0.76 (0.12) 0.89 (0.05)
8 53 31 28 6 46
0.88 (0.18)
0.77 (0.23) 0.78 (0.10) 0.69 (0.03)
TCB log KF (rg/g)(L/rg)" -2.11 (0.14) -1.86 (0.09) -1.69 (0.25) -0.66 (0.06) -0.53 (0.51)
0.00 (0.04)
n 0.84 (0.06) 0.85 (0.03) 0.89 (0.08) 0.77 (0.02) 0.75 (0.25) 0.68 (0.02)
'KF = l.Ol(pg/g)(L/fig) for TCB on Ann Arbor I soil. 1000
100 TT E:
'
0.01 10
/
I
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100
1000
J loo00
Ce (ugll)
0.1
1
10
100 Ce (ugll)
1000
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Flgure 5. Comparison of linear and Freundlich isotherm models to experimental data for sorption of TCB and TTCE by Delta soil.
Figure 6. Comparison of llnear and Freundllch isotherm models to experimental data for sorption of TCB and TTCE by Wagner soil.
for the soils tested. To expand on this point, data for sorption of both TTCE and TCB by the lower capacity Delta and higher capacity Wagner soils are shown in Figures 5 and 6, respectively. The best possible linear isotherm model fit of the data is also shown for each case, along with the 95% confidence limits for the corresponding best-fit Freundlich model isotherm. Comparison of the best linear isotherm model fits to the Freundlich model confidence limits indicates clearly the extent to which the measured isotherms are nonlinear. The summary of Freundlich parameters for the three solutes and six soils given in Table I1 further indicates that the measure of isotherm nonlinearity, n, differs only slightly among solutes for a particular soil but varies significantly among the soils for a given solute. It is evident from the information given in Table I1 that both the sorption capacity and the heterogeneity of the process increase markedly with increased levels of hard carbon (i.e., the difference in carbon between values measured by the low- and high-temperature oxidation techniques). Isotherm nonlinearity was most notable (0.68 < n < 0.79) for the high-capacity Ann Arbor I, Wagner, and Ypsilanti soils, each of which contained a significant fraction of organic carbon which was not measured by mild oxidation; i.e., hard carbon. In contrast, for the low-carbon soft carbon soils, the corresponding isotherms more closely approached a linear relationship (0.88 < n < 1.06). In the case of the Ann Arbor I1 soil, only the TCB isotherm yielded a confidence interval which excluded an n value of 1,while sorption isotherms on Delta and Augusta soils were intermediate in linearity. The extent of isotherm nonlinearity remained constant over the entire concentration range for all of the soils examined and, in general, increased with solute hydrophobicity. The trend is consistent with observations of sorption on other hydrophobic surfaces, such as synthetic resins and activated carbons (29,20).
Distributed Sorption Reactivity. To further explore the hypothesis of distributed reactivity, and to relate this to the distribution of properties among the individual components of a bulk soil, several of the most evidently different fractions of the soils tested were isolated and analyzed. These analyses were then used to identify the particular soil components principally responsible for sorption reactivity and nonlinearity. The methodology of separation involved visual observation under a magnifying lens and manual sorting. This limited the separation to larger and more evident soil grains, but it is possible to extrapolate the results to bulk soils for qualitative comparisons. Separable fractions included quartz, carbonates, and shales. The results of sorption tests with TCB, the most hydrophobic solute of those studied, indicated that the shale fraction was by far the most significantly reactive. Shale was most evident in the three soils having the highest organic carbon content and exhibiting more nonlinear sorption, Ann Arbor I, Wagner, and Ypsilanti soils. Its presence in these soils likely reflects entrainment by glacial activity in southeastern Michigan. The Wagner soil, for example, was collected in an area with a shale bedrock overlain by approximately 200 f t of glacially deposited materials. There were some dark fragments in the other soil mixtures as well, but these had materials distinctly different in appearance upon microscopic examination. Figure 7 shows TCB isotherms on pulverized samples of shale-like materials isolated from Ypsilanti, Wagner, and Augusta soils, along with the corresponding isotherms for the bulk soils. It is evident that these isolated materials exhibit higher sorption capacities per unit weight than do the bulk soils. Although we did not specifically measure the mass fractions of reactive components, it is evident from Figure 7 that these fractions were relatively small on a mass basis. This is particularly the case for Wagner and Ypsilanti soils, for which the reactive fractions have capacities per unit weight more than 2 orders of magnitude Environ. Sci. Technol., Vol. 26, No. 10, 1992
1959
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Flgure 7. Sorption of TCB by three isolated reactive componentsand by the corresponding parent soils: (a) Wagner; (b) Ypsilanti; (c) Augusta.
greater than do the bulk soils. This would place the mass fractions of the reactive components somewhere in the neighborhood of 0.01, or about 1% by weight. Surface area measurements were conducted for both unpulverized and pulverized (mortar and pestle) shale from the Wagner soil to assess how much of this increase might be attributable to smaller particle sizes. The results, 11.0 and 13.0 m2/g, respectively, indicate that, while pulverizing made little difference, the shale itself has a much higher surface area than the bulk soil. The results are clearly consistent with the hypothesis of distributed reactivity and its relationship to implicit heterogeneity. It is noteworthy that the reactive fractions separated from the bulk soils seem to exhibit different trends with respect to their relative degrees of homogeneity. The reactive component from the Wagner shale had a Freundlich n value of 0.9 for the reactive component and 0.8 for the bulk soil), suggesting that the reactive component was more homogeneous with respect to sorption. The n values for the Ypsilanti reactive fraction and its parent soil were virtually the same. For the Wagner and Ypsilanti soils, isotherm nonlinearity can likely be linked to properties of the reactive shale fractions which lead to sorption heterogeneity. These fractions contain a relatively high organic carbon content ( 5 4 % ) with little inorganic carbon and are mineralogically similar, containing feldspar, quartz, mica, and chlorite. The organic matter is expected to differ from that found in most soils in that it has been diagenetically altered over a long time period. The bedrock shales in Michigan, for instance, are of Devonian or Mississippian age. Elemental 1960
Envlron. Sci. Technol., Vol. 26, No. 10, 1992
analysis of the reactive fractions confirmed potential differences between these fractions and typical soil organic matter. The atomic oxygen/carbon (O/C) ratios for the Wagner and Ypsilanti shales, 0.47 and 0.60, respectively, were intermediate with respect to those reported for cellulose and soil humic acids, 0.84 and 0.37, respectively (10, 25). Similarly, the elemental hydrogen/carbon (H/C) ratios for the Wagner and Ypsilanti shales, 1.2 and 1.4, respectively, are in the midrange of values (0.7-2.0) reported for cellulose and humic acids. Reductions in the quantities of oxygen and hydrogen relative to the quantity of carbon are anticipated during diagenesis, and these changes are expected to lead to more aromatic, and likely more hydrophobic organic fractions. The O/C and H/C ratios for the Wagner and Ypsilanti fragmented and entrained materials comprising the reactive fractions are not as low as have been reported for undisturbed shales (shale “rock”),which are typically O/C C 0.3, and H/C C 1.0 (12, 26, 27). This may reflect a lower degree of diagenesis, different source materials, and possible oxidation after deposition within the subsoil (28, 29). Characterizationsof organic material in aged shales have identified an abundance of aromatic functionalities, with most of the organic matter having only limited solubility in aqueous solution (27,28). It is likely that the organic substances within the Wagner and Ypsilanti reactive fractions reflect some of these trends, and while the extent to which their origin, processing, and postdeposition oxidation have affected the composition is unknown, they both likely contain a variety of organic matter which is insoluble and exhibits a range of sorption energies. It is likely that the reactive fractions of both of these soils are mixtures of hard and soft carbon materials, which would account for both their higher capacities for adsorption and their greater degree of implicit heterogeneity and isotherm nonlinearity. In the same context, a reactive component of the Augusta soil, which had only a soft carbon fraction, showed a slightly more linear sorption of TCB than did the parent soil, consistent with a larger amount of soft carbon matrix and correspondinglygreater linear sorption behavior than the parent soil. To further assess the character of organic matter associated with the soils which contained the most evident shale components, TCB sorption by Wagner soil was measured before and after the following: (i) extraction of the bulk soil with 0.5 M sodium hydroxide to remove any base-soluble (e.g., humic-type) organic material and (ii) heat treatment at 430 “C for 12 h to more completely oxidize all associated organic material. As noted earlier in Table I, the Wagner soil had both a soft carbon component (1.2% OC) and a hard carbon component (1.29% 06). Equilibrium sorption data for the soils after each treatment are compared in Figure 8 to the sorption data and fitted isotherm for the untreated Wagner soil. It is evident from these data that base extraction had only a modest affect on the sorption capacity of this soil and virtually no effect on isotherm nonlinearity. Conversely, heat treatment significantly reduced the sorption capacity of the soil. The limited number of data points for the heat-treated soil and their relatively high degree of scatter make it difficult to quantitatively assess any effects that such treatment might have had on isotherm linearity. The conclusion to be drawn from this analysis is that the most reactive organic fractions of the high-capacity soils tested are apparently not composed of humic-type or other base-extractable materials. The increased hydrophobicity of the surfaces associated with these hard carbon soils is most likely the underlying cause of surface
1000 Untreated
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Figure 8. Effects of base extraction and heat treatment of Wagner soil on its sorption of TCB. Confidence limits shown for an individual value at any solution-phase concentration.
reactions which are typically nonlinear and mechanistically different from partitioning processes ( I ) . On the other hand, the more linearly sorbing and apparently less heterogeneous soils tested appear to have reactivity associated with soft carbon, as suggested by the ease with which most of the organic matter of these soils was oxidized under relatively mild conditions.
Significance of Distributed Reactivity To place the findings of this work and the concept of distributed reactivity in practical perspective, it is useful to examine the significance of different combinations of sorptive components on the nature of composite isotherms for soils and sediments. For this purpose, consider the sorption of W C E by a series of cornposited soils made up by different combinations of components which behave in a manner similar to the two different groups of six soils studied here, that is, the more linearly sorbing soft carbon soils and the more nonlinearly sorbing hard carbon soils, as exemplified by the Wagner soil. As noted earlier, on a reactivity-normalized basis, the nonlinearly sorbing component of this soil would apparently comprise about 1% of its mass, or xnI-0.01. The composited soils will thus have a single nonlinear component of mass fraction x,,, and adsorption characteristics identical to the Wagner soil and multiple linearly sorbing components of cumulative mass fraction, xI,average organic content, foc, and mass-averaged organic carbon normalized partitioning coefficient, K,, corresponding closely to the behavior of our soft carbon soils. For purposes of this illustration, we can similarly normalize the capacity term, KF, of the nonlinear component to the organic carbon content of that component to yield a KFW The composite sorption isotherm for this case can then be formulated from eqs 7 and 8 as qe, = ( x f o c ) l K w c e
+ (xfoc)nlKFocCen
(9)
Equation 9 may neglect portions of the subsoil, such as mineral components, which can be considered as unreactive in comparison to the components characterized. The KO,for TTCE can be estimated from its octanolwater partition coefficient (Kow= 500) and a correlation such as that shown in eq 2. It can be observed in Figure 3 that the Schwarzenbach and Westall (5) correlation provides a reasonable estimate of the soft carbon soils studied in this work, and thus a reasonable overall representation of the more linearly behaving components. Application of their calibration coefficients (a = 0.72, b = 0.49) to eq 2 gives a KO,of 271 cm3/g. The values of the
0.05
2
0.025
li
.-t
L
o -0.025 -0.05
1
10
100 Ce (ug/l)
1000
10000
Figure 9. Composite DRM models for sorptlon of TTCE: (a) single Freundllch model fits to DRM generated points (n values indicated); (b) relative error associated with single Freundlich fits.
nonlinear isotherm coefficients for TTCE on Wagner soil are, from Table 11,n = 0.78, KF= 13.2 (pg/g)(cm3/g)". On a per unit organic carbon basis, and making no differentiation between the carbon for the whole soil,the KFQ for the nonlinear component is 530 (pg/g)(cm3/g)". To evaluate the effect of differently distributed reactivities, the sorption patterns for several composite soils were computed. An (xf0Jl of 0.001 was assumed for each of four soils having different (xf,Jd values: 0,0.002,0.005, and 0.001.The composite sorption isotherms for these four soils, shown in Figure 9, reveal how differences in sorption capacities for composite soils will vary with concentration. In environmentally critical low-concentration regions, the differences represent greater percentages of total sorption than they do at higher concentrations. Using values for qe,at one-decade concentration intervals from 1 pgjL to 10 mg/L, log-linear fits to single Freundlich isotherms were obtained. The results are shown as the linear lines in the logarithmic plots. The resulting Freundlich n values range from 0.83 to 1.0. It is thus evident from Figure 9a that the composite isotherm points calculated from the DRM in eq 9 are fit well by simple Freundlich models of the type given in eq 4. However, if the nonlinearity is indeed attributable to intrinsic heterogeneity and distributed reactivity, then single Freundlich isotherm models which lump all heterogeneity characteristics in a single empirical coefficient, n,while capable of fitting the data, do not adequately characterize or represent the sorption processes. The extent to which the single Freundlich model deviates from the DRM can be deduced from a relative error computed as
The results of such relative error determinations are presented in Figure 9b. Environ. Sci. Technoi., Vol. 26, No. 10, 1992
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This example indicates the extent to which small quantities of nonlinearly sorbing components can significantly impact sorption isotherm nonlinearity. The extent to which the components of distributed reactivity leading to such nonlinearity must be individually characterized depends upon the context in which the resulting isotherm models are applied. For the present, suffice it to note that, as we have demonstrated in an earlier paper ( I ) , differences in sorbed-phase quantities at low solution concentrations can be highly significant, given that the goals of transport prediction and remediation generally necessitate accurate quantification of contaminant concentrations at such low levels. Summary
The experimental evidence is convincing. Soils which appear homogeneous at a macroscopic level can be implicitly heterogeneous, comprising a variety of matrix components and interfaces exhibiting a variety of reactivities with respect to organic solutes. These reactivities appear to be variously distributed in different soil fractions, supporting the hypothesis that reactivity itself is distributed, and supporting the underlying concept of the distributed reactivity model for soil and sediment sorption. Calculations using measured parameters to represent differences in the relative sorption behavior of soils comprising different mass fractions of differently reactive components suggest that particle-scale heterogeneity and distributed reactivity can significantly affect contaminant transport processes at field scale. This is the first paper in a series on the DRM. Its purpose is to configure the model, explain its basis, and illustrate its application to characterization of a set of observations regarding the sorption of typical hydrophobic organic contaminants by typical subsurface soils. Papers to follow in the series will address applications of the model to multicontaminant systems and to nonequilibrium sorption behavior (30, 31). Acknowledgments
We appreciate the assistance of Patti Hayes, Maureen McGraw, Steve Westenbroek, Mary Lynam, and Guisepe Ricco in the sorption measurements and that of James Surhigh for his work on soil fractionation. High-temperature organic carbon determinations were performed by Robert Powell of the U.S.EPA Kerr Laboratory in Ada, OK. Joseph Pedit, a former undergraduate student at the University of Michigan and currently a Ph.D. candidate at the University of North Carolina at Chapel Hill, and Jeff Kremarik, Washtenaw County Environmental Health Department, assisted in soil collection. The Center for Applied Energy Research at the University of Kentucky carried out the elemental analyses of the soils reported in this paper. Registry NO. TTCE, 127-18-4;DCB, 106-46-7;TCB, 120-82-1.
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Received for review December 18, 1991. Revised manuscript received June 1,1992. Accepted June 19,1992. This research was supported in part by the National Science Foundation, Research Grant ECE8503903, and in part by Research Grant l-P42-ES04911-0-01 from the National Institutes for Enuironmental and Health Sciences. Elements of the work were presented at the American Chemical Society’s 65th Colloid and Surface Science Symposium in Norman, OK, 17-19 June, 1991.
