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Published online 1 May 2008
Published in Soil Sci Soc Am J 72:586-594 (2008)
DOI: 10.2136/sssaj2007.0030
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

Fractional Availability of Smectite Surfaces in Soils for Adsorption of Nitroaromatic Compounds in Relation to Soil and Solute Properties

Simone M. Charles, Brian J. Teppen, Hui Li and Stephen A. Boyd*

Department of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824

* Corresponding author (boyds{at}msu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Efficacy of soil clay minerals for the sorption of nonionic organic compounds may be reduced by soil organic matter (SOM) association with clays. Previously, we quantified the fractional availability (fa) of soil clay surfaces for sorption of para-nitrocyanobenzene (p-NCB). Removal of SOM from a soil increased p-NCB sorption, demonstrating that SOM reduced the availability of clay surfaces for sorption of p-NCB. In this study, we extended our investigation of SOM–smectite interactions by measuring the availability of smectite surfaces in four soils to four nitroaromatic compounds (NACs) including p-NCB, 2,4-dinitrotoluene (2,4-DNT), 1,4-dinitrobenzene (1,4-DNB), and 1,3,5-trinitrobenzene (1,3,5-TNB). Increased NAC sorption was consistently observed after SOM was removed from whole soils, indicating that SOM blocks the availability of soil smectite surfaces to NACs. Estimates for the fa of smectite surfaces in the four soils ranged from 1 to 0.3. These fa values were strongly negatively correlated with the ratio of SOM/smectite contents. Also, fa values differed systematically with NAC probe molecules, with SOM blocking fewer clay mineral sites from more strongly sorbing NACs. This suggests that smectite surface availability in soils may be an operationally defined quantity whose value depends on the probe molecule. Values of fa approaching 1 for strongly sorbing NACs suggest displacement of SOM from smectite surfaces by the sorbed compound.

Abbreviations: 1,3,5-TNB, 1,3,5-trinitrobenzene • 1,4-DNB, 1,4-dinitrobenzene • 2,4-DNT, 2,4-dinitrotoluene • NAC, nitroaromatic compound • NOC, nonionic organic compound • p-NCB, para-nitrocyanobenzene • SOM, soil organic matter


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The ever-increasing manufacture and use of synthetic organic chemicals has amplified the need for development and improvement of models to quantitatively predict chemical fates in the environment. Two significant fate processes for organic contaminants and pesticides in soils are their sorption by clay minerals and SOM, generally considered the most important sorptive compartments in soil. Due to existing dogma that nonionic organic compound (NOC) sorption by SOM greatly exceeds that by clay minerals, fate and transport models typically include terms representing sorption of organic contaminants and pesticides to SOM (e.g., Kom or Koc), but do not contain terms to account for the potential contribution of soil clays to sorption.

Previous studies have shown that, when considered as isolated components, soil clays can play an equal or dominant role, compared with SOM, in the immobilization of certain important classes of organic contaminants and pesticides including triazines and NACs (Laird et al., 1994; Sheng et al., 2001). For example, smectites can adsorb up to 10% NACs by weight (Johnston et al., 2001), and there is unequivocal evidence that such sorption occurs in the smectite interlayers (Boyd et al., 2001; Sheng et al., 2001, 2002; Li et al., 2004). The sorptive contributions of SOM and clay components should be interrelated, however, because they are known to be associated as clay–SOM complexes or assemblages in soils (Stevenson, 1994). Describing sorption of organic and inorganic species by soils as the additive product of the isolated SOM and mineral components is inadequate because it is based on unrealistic assumptions, i.e., that the components act independently when it is known they are intimately associated in soils (Vermeer et al., 1991; Celis et al., 1998). In fact, several studies investigating the sorption of organic contaminants and pesticides by synthetic humic acid (HA)–clay complexes have shown that the extent of sorption to the complex does not equate to the sum of sorption to the individual isolated components (Rebhun et al., 1992; Pusino et al., 1992, 1994; Celis et al., 1997).

We recently developed a novel approach for quantifying the availability of clay surfaces in soils for adsorption of NOCs (Charles et al., 2006a). We reported that the association of SOM with smectite clays in soil reduced the efficacy of soil mineral fractions for NOC sorption. This previous study revealed that p-NCB sorption by the A horizon of a K-saturated, smectitic Webster soil with SOM removed was greater than that by the organic-rich parent soil, indicating that the association of SOM with clay minerals reduced the availability of clay sorptive sites. In addition, SOM suppressed sorption of the pesticide diuron [N-(3,4-dichlorophenyl)-N,N-dimethyl urea] to soil clay surfaces by relatively more than SOM suppressed p-NCB sorption, suggesting that the apparent blockage of clay mineral surfaces by SOM was solute dependent (Charles et al., 2006a). Liu et al. (2002) reported that the presence of HA on HA–clay complexes decreased sorption to a different extent dependent on the pesticides used; however, their study used synthetic HA–clay complexes so that extrapolation to natural soils is uncertain.

The objectives of this study were (i) to determine the broader applicability of our novel method for quantifying fractional availability by using additional soils and solutes, (ii) to determine whether our earlier conclusion that SOM reduces mineral surface availability drawn from one soil horizon might be generalized by quantifying the fractional availability of mineral sorption sites in two horizons from each of two soils, (iii) to determine whether trends exist in the apparent dependence of fractional availability on solute properties by using four NACs to probe the availability of mineral surfaces in four soil samples, and (iv) to determine how SOM and smectite contents in soils influence the availability of clay mineral surfaces. The NAC molecules used in the present study are excellent probes of mineral surface chemistry because, although NOCs, the NACs interact more strongly with the clay minerals than with SOM (Charles et al., 2006a).


   EXPERIMENTAL METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Two mineral soils, Webster (a fine loamy, mixed, mesic Typic Endoaquoll) and Clarion (a fine loamy, mixed, mesic Typic Hapludoll), A and B horizons (Soil Survey Staff, 2007), and an organic soil (Pahokee peat, a euic, hyperthermic Lithic Medisaprist) were air dried and sieved to remove coarse fragments (>2 mm). The mineral soils were fractionated according to the procedures detailed in Charles et al. (2006b). Soil organic matter was removed using 30% H2O2 at 80°C (Kunze and Dixon, 1986). Homoionic soils and soil fractions were prepared by washing the soils with approximately 200 mL of 0.1 mol L–1 solution of KCl or MgCl2 overnight, thereafter centrifuging at 4068 x g for 30 min and discarding the supernatant. This procedure was repeated four times. The soils were then washed with 200 mL of Millipore Milli-Q (Billerica, MA) deionized H2O, and thereafter centrifuged and the supernatant tested with AgNO3 until a negative Cl– test was obtained. The soils were then freeze-dried and stored at room temperature (23 ± 1°C) until used. A K+–saturated beidellite (Ward's Natural Science, Rochester, NY) was prepared as described above, freeze-dried, and stored at room temperature until used. Clay minerals present in this Webster soil were previously identified as mica or illite, smectite, and kaolinite (Charles et al., 2006b). The Michigan State University Soil and Plant Nutrient Laboratory (East Lansing, MI) determined soil texture by the hydrometer method (Gee and Bauder, 1986) and SOM content using dry combustion (LECO C analyzer, St. Joseph, MI) (Nelson and Sommers, 1996).

The 1,4-DNB was purchased from Sigma Chemical Co. (St. Louis, MO) with a purity of 98%, while p-NCB and 2,4-DNT were purchased from Aldrich Chemical Co. (Milwaukee, WI) with purities of 97%, and 1,3,5-TNB was purchased from ChemService (West Chestnut, PA) with a purity of 99%. All compounds were used as received.

Sorption of the solutes by soils, soil fractions, and K+–saturated beidellite was measured in triplicate using a batch equilibrium technique. This involved adding solute solutions with a range of initial aqueous concentrations (Ci) of p-NCB (1–42 mg L–1), 1,4-DNB (1–42 mg L–1), and 2,4-DNT (1–34 mg L–1) to known masses of soil. Solutions of the compounds were prepared in 0.05 mol L–1 KCl or MgCl2, corresponding to the exchangeable cation saturating the sorbent, and 5 mL of each solution was pipetted into 7.5-mL borosilicate glass vials containing between 0.1 and 0.4 g of sorbent but fixed for a given combination of solute and sorbent. The vials were rotated continuously for 24 h in the dark at room temperature to ensure that sorption equilibrium was attained; previous evaluation showed that sorption equilibrium for each solute was reached within 24 h (Charles et al., 2006b). The liquid and solid phases were separated by centrifugation at 4068 x g for 30 min. The solute concentration in the supernatant (Ce) was determined using a high-performance liquid chromatograph (HPLC, Perkin Elmer, Norwalk, CT) consisting of a Model 250 binary LC pump with a Series 200 autosampler and an ultraviolet-visible detector set at the following wavelengths: 254 nm for p-NCB, and 265 nm for 1,4-DNB and 2,4-DNT. A Platinum EPS C18 column (Alltech Assoc., Deerfield, IL) was eluted with an isocratic mixture of methanol (HPLC grade) and Milli-Q deionized water (55:45 methanol/water for p-NCB, 65:35 for 1,4-DNB and 2,4-DNT) with a flow rate of 1.0 mL min–1. The amount of solute sorbed was calculated as the difference between the initial (Ci) and final (Ce) aqueous-phase solute concentrations. Compound recoveries in control vials without sorbent were >95% and not used to adjust the data. Sorption data were described using the Freundlich equation, Q = KfCen, where Q is the solute mass sorbed per unit mass of sorbent, Kf is the Freundlich sorption coefficient, and n is an exponent describing the shape of the sorption isotherm (Chiou, 2002).

Removal of organic matter with H2O2 could result in the liberation and oxidation of heavy metals. The potential contribution of sesquioxide presence or formation after H2O2 treatment to solute sorption was evaluated using a K+–saturated ferrihydrite, prepared using FeCl3 and KOH (Charles et al., 2006b). Sorption of p-NCB, 1,4-DNB, and 2,4-DNT by this ferrihydrite was measured in duplicate using the batch equilibrium technique described above.

Using the Freundlich parameters Kf and n, Q values were calculated at four selected Ce values, i.e., 5, 10, 15, and 20 mg L–1. Then values for fa were calculated utilizing sorption data from the K+– and Mg2+–saturated sorbents and our method of data analysis (Charles et al., 2006a). Fractional availability of clay mineral surfaces can range from 0 (unavailable) to 1 (100% available). Standard deviations for fa values were calculated using error propagation procedures (Skoog and West, 1986). Fractional availability was also evaluated using the approach proposed by Karickhoff (1984). For this method, sorption data for the K+–saturated sorbents was used. Since the Karickhoff method of data analysis includes a solute Kom value that cannot be determined directly for the soils under study, the Pahokee peat was used as a surrogate to estimate Kom values for p-NCB, 1,4-DNB, and 2,4-DNT. This peat does not contain minerals implicated in the sorption of the test solutes, so SOM is the only sorptive compartment present (Charles et al., 2006b).

A modified form of the equation proposed by Karickhoff (1984) was used to derive a linear equation relating fa to the SOM and smectite contents of the test soils:

Formula 1[1]
where Kwhole soil is the sorption coefficient of the solute by the whole (unfractionated) soil, Kom is the solute sorption coefficient normalized for fractional SOM content (obtained in our study using the Pahokee peat), Kmin is the fractional mineral solute sorption coefficient (obtained in our study using the K-beidellite), and fom and fmin are the fractional SOM and mineral contents of the test soil, respectively. Plotting fa vs. fom/fmin for each soil should give a straight line with an intercept of 1 when the SOM content is zero (100% availability of clay surfaces, i.e., Kwhole soil = Kmin). To estimate the Kom and Kmin terms in Eq. [1], sorption data for each solute by the Pahokee peat soil and K-beidellite were fitted to the Freundlich equation, and the resultant n and Kf sorption parameters were used to calculate Qom and Qmin (sorption per unit mass of Pahokee peat and K-beidellite) at an equilibrium concentration of 10 mg L–1. The Qom and Qmin values were then substituted for Kom and Kmin values in the generalized Eq. [1]:

Formula 2[2]
The use of Q values (Eq. [2]) rather than K values (Eq. [1]) for the correlation analysis was necessary because the sorption isotherms were nonlinear (n != 1). The use of the Pahokee peat soil and K-beidellite to estimate Kom and Kmin values is justified based on our previous studies (Charles et al., 2006a,b) because Kom values have been shown to be relatively constant among soils (Kile et al., 1995) and because Laird et al. (1994) reported the dominant smectite in another Webster soil to be beidellite. Correlation coefficients were obtained describing the relationship of measured fa values to SOM content, smectite content, and the ratio of SOM to smectite content (fom/fsmectite) in the whole soils.

Statistical analyses of differences among sorption isotherms were made by comparing the Freundlich Kf and n values for each soil–solute system using nonlinear regression (SAS Institute, 1999). A significant difference between isotherms was assigned when there was a significant difference ({alpha} = 0.05) between Kf values of compared isotherms (Laboski and Lamb, 2004).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
The Webster and Clarion soils were chosen for this study because they are organic rich and contain significant amounts of smectite clays (13–24% smectite, Table 1 ), which are implicated in the strong adsorption of NACs (Haderlein and Schwarzenbach, 1993; Boyd et al., 2001). The amount of free Fe oxides associated with the Webster and Clarion soils was <1%. Sorption of p-NCB, 1,4-DNB, and 2,4-DNT by ferrihydrite synthesized in the laboratory was negligible, consistent with studies by Haderlein and Schwarzenbach (1993). We therefore conclude that FeOx did not contribute to sorption of the test solutes in the soils studied.


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  		Table 1. Characteristics of Webster and Clarion A and B horizons and corresponding soils from which soil organic matter (SOM) was removed.

 
Isotherms representing sorption of p-NCB, 1,4-DNB, and 2,4-DNT by each soil were L shaped (Fig. 1–4GoGoGo ). The amount of NAC sorbed did not plateau in our sorption experiments, hence the Freundlich equation was used to describe the isotherms (solid lines in Fig. 1–4GoGoGo; Chiou, 2002). Freundlich sorption coefficients are presented in Table 2 . Sorption of p-NCB, 1,4-DNB, and 2,4-DNT by the Mg2+–saturated soil was equal to or greater than that by the Mg2+–saturated SOM-removed soil (Fig. 1–4GoGoGo). Removal of SOM probably liberates potential NAC adsorption sites on smectite surfaces but sorption of NACs by such Mg2+–exchanged smectite surfaces is minimal due to the large hydrated radius of the Mg2+ cation (Boyd et al., 2001). It is now well established that smectites exchanged with cations having comparatively larger hydrated radii (e.g., Mg2+) have much lower affinities for NACs than smectites exchanged with more weakly hydrated cations (e.g., K+) (Haderlein et al., 1996; Weissmahr et al., 1997; Boyd et al., 2001; Sheng et al., 2002; Johnston et al., 2001, 2002; Li et al., 2003, 2004). Adsorption of NACs occurs primarily in the clay interlayers (Boyd et al., 2001; Sheng et al., 2002; Li et al., 2004), and lower cation hydration manifests larger siloxane surface adsorption domains between exchange sites, promotes formation of complexes between NO2 groups and exchangeable cations, and optimizes interlayer distances so that NACs can be substantially dehydrated by aligning parallel to, and interacting directly with, the opposing siloxane sheets. Hence, the smectites in the Mg2+–saturated SOM-removed soils are ineffective as sorbents for sorption of the NACs, and SOM is the principal sorbent phase. As such, SOM removal diminished p-NCB, 1,4-DNB, and 2,4-DNT sorption in the Mg2+–saturated soils (Fig. 1–4GoGoGo).


Figure 1
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  		Fig. 1. Sorption isotherms of p-nitrocyanobenzene (p-NCB), 2,4-dinitrotoluene (DNT), and 1,4-dinitrobenzene (DNB) by K+– and Mg2+–saturated Webster A horizon soil (WA) and Webster A horizon soil from which soil organic matter was removed (WAO).

 

Figure 2
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  		Fig. 2. Sorption isotherms of p-nitrocyanobenzene (p-NCB), 2,4-dinitrotoluene (DNT), and 1,4-dinitrobenzene (DNB) by K+– and Mg2+–saturated Webster B horizon soil (WB) and Webster B horizon soil from which soil organic matter was removed (WBO).

 

Figure 3
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  		Fig. 3. Sorption isotherms of p-nitrocyanobenzene (p-NCB), 2,4-dinitrotoluene (DNT), and 1,4-dinitrobenzene (DNB) by K+– and Mg2+–saturated Clarion A horizon soil (CLA) and Clarion A horizon soil from which soil organic matter was removed (CAO).

 

Figure 4
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  		Fig. 4. Sorption isotherms of p-nitrocyanobenzene (NCB), 2,4-dinitrotoluene (DNT), and 1,4-dinitrobenzene (DNB) by K+– and Mg2+–saturated Clarion B horizon soil (CLB) and Clarion B horizon soil from which soil organic matter was removed (CBO).

 

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  		Table 2. Freundlich equation sorption coeffiecient Kf and isotherm curvature coefficient n values for p-nitrocyanobenzene, 1,4-dinitrobenzene, and 2,4-dinitrobenzene sorption by K+– and Mg2+–exchanged Webster and Clarion (A and B horizons) whole soils and the corresponding soils from which soil organic matter (SOM) was removed.

 
The K+-saturated soils (unfractionated and SOM removed) sorbed p-NCB, 1,4-DNB, and 2,4-DNT to a greater extent than Mg2+–saturated soils (Fig. 1–4GoGoGo). These results are consistent with several studies (Haderlein and Schwarzenbach, 1993; Haderlein et al., 1996; Boyd et al., 2001; Sheng et al., 2002) that reported stronger sorption of NACs by smectite clays saturated with cations of relatively low hydration energies (e.g., K+, Cs+) compared with that by systems saturated with cations of relatively higher hydration energies (e.g., Ca2+, Mg2+), and with the fact that smectite is the most abundant clay mineral in the Webster and Clarion soils (Table 1). The K+–saturated SOM-removed soils sorbed more p-NCB, 1,4-DNB, and 2,4-DNT than the unfractionated K+–saturated soils (Fig. 1–4GoGoGo), even though SOM undoubtedly acts as a sorptive compartment for these solutes (Sheng et al., 2001). Increased sorption of NACs by each soil after SOM was removed implies that SOM obscures adsorptive surfaces in the Webster and Clarion soils. This effect is probably due to obscuration of smectite surfaces by SOM since smectites are abundant in the soils studied and they are the mineral phases with the greatest affinities for NACs. There are several previous reports of decreased sorption of organic solutes to smectite clays when associated with SOM (Pusino et al., 1992, 1994; Celis et al., 1997; Liu et al., 2002), although these studies utilized synthetic clay–organic matter complexes. Our data (Fig. 1–4GoGoGo) indicate that in natural soils, removal of SOM from soil also increases sorption of NACs, although the degree to which SOM renders clays surfaces unavailable has only recently been quantified for one soil and two solutes (Charles et al., 2006a).

The fractional availabilities (fa) of mineral surfaces in the soils under study were quantified from sorption data using two methods of data analysis. The first method was given by Karickhoff (1984) and has only recently been tested experimentally (Charles et al., 2006a):

Formula 3[3]
Although straightforward, this equation requires determination of the isolated SOM contribution to sorption (Qom or Kom). This can present difficulty since SOM cannot be isolated in total or without alteration from the test soil. The experimental determination of fom may also introduce error in some instances, for example when fom < 1%. These potential sources of error become increasingly problematic as SOM becomes an increasingly important contributor, compared with the mineral fraction, to overall sorption (Charles et al., 2006a).

We recently presented a new equation for determination of fa, which is predicated on the observations that NAC sorption by SOM was nearly identical whether the cation exchange sites were saturated with K+ or Mg2+ (Charles et al., 2006b), whereas NAC sorption by smectite clay was strongly dependent on the cation saturating the clay cation exchange sites; K-smectites have high affinities for NACs while Mg-smectites do not (Charles et al., 2006a). Thus, if two experiments are done in which NAC sorption is measured for a soil saturated with K+ vs. Mg2+, then

Formula 4[4]
where the subscripts K-sat and Mg-sat are added to differentiate the K+– and Mg2+–saturated soils. Since Qom, K-sat {cong} Qom, Mg-sat, then

Formula 5[5]
Equation [5] eliminates the Qom and fom terms needed in Eq. [3]. In instances where sorption by SOM equals or exceeds that by clays, Eq. [5] is clearly the preferred method of data analysis (Charles et al., 2006a).

The fa values derived using Eq. [3] and [5] are summarized in Table 3 . The fa values derived from Eq. [3] using K+–saturated Pahokee peat to estimate Qom were similar to those derived using Mg2+–saturated peat. The fa values calculated for each soil using Eq. [3] were similar to those calculated using Eq. [5]. For p-NCB and 1,4-DNB, fa values decreased in the order Clarion A < Webster B < Webster A < Clarion B (P < 0.05). For 2,4-DNT, fa values decreased in the order Clarion A < Webster A = Clarion B < Webster B (P < 0.05). For Clarion A, Webster A, and Webster B soils, at each concentration, the fa values decreased in the order p-NCB < 1,4-DNB < 2,4-DNT. For Clarion B, at each concentration, the fa values decreased in the order p-NCB < 1,4-DNB = 2,4-DNT. Both equations yielded fa values for the Clarion B horizon soil that were greater than for the Clarion A horizon soil; fa values for Webster A and B horizon soils were similar.


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  		Table 3. Fractional availability (fa) values of mineral surfaces in Webster and Clarion (A and B horizons) soils, as quantified from sorption of nitroaromatic compounds (NACs) and Eq. [3] (using K+– and Mg2+–saturated Pahokee peat to determine the solute sorption coefficient normalized by the fractional soil organic matter content [Kom]) or Eq. [5]. Standard deviations in parentheses.

 
Soil organic matter is associated with soil clays in a variety of ways that may influence the availability of mineral surfaces for adsorption of organic contaminants and pesticides. Soil organic matter may coat clay surfaces, bridge clay packets resulting in aggregate formation, inhibit shrinking and swelling of the smectite interlayers, and partially block the entrance of sorbates into the interlayer regions (Walker and Crawford, 1968; Pusino et al., 1992, 1994; Onken and Traina, 1997; Celis et al., 1999). It is unlikely that the reduced surface availability reported here was related to carbonate minerals, which are known to play a role in soil particle aggregation; in a previous study (Laird et al., 2001), no carbonates were detected in the Webster soil.

The physical meaning of fa in terms of the mechanisms by which SOM obscures soil clay surfaces remains unclear. If all C in the SOM were assumed to cover the basal surfaces of the soil smectites, then the formula for surface coverage used in the alkylammonium method (Lagaly, 1981) and detailed in Charles et al. (2006a) predicts that this C would cover approximately 49, 38, 31, and 10% of the smectite surface area in the Clarion A, Webster A, Webster B, and Clarion B horizons, respectively (Table 1). Note that these values are in modest agreement with the complements of the average fa values for each horizon (Table 3), which are 0.64, 0.36, 0.53, and 0.17, respectively. This agreement may be strictly fortuitous, but would provide a good explanation for the strong correlation between fa and the ratio of SOM to smectite (discussed below). Thus, one conclusion from this analysis is that the basal surfaces of smectite clays are partially covered by SOM, as suggested by Stevenson (1994). Adsorbed organic matter in the form of basic amino acids such as arginine in smectite interlayers could plausibly be responsible for coverage of interlayer sites that would otherwise be available for sorption of NACs (Laird et al., 2001). Evidence for the intercalation of larger humus components, i.e., humic materials, is lacking, however, so it seems an unlikely cause of reduced surface availability (Celis et al., 1997, 1999; Ahlrich, 1972).

The fa values for p-NCB, 1,4-DNB, and 2,4-DNT were strongly negatively correlated (61–88%) with the ratio of SOM/smectite contents in soils (Table 4 ). The fa values for p-NCB, 2,4-DNT, and 1,4-DNB were moderately negatively correlated with the SOM contents of the soil horizons, with 45 to 51, 24 to 27, and 21 to 33%, respectively, of the difference in fa values among soil horizons attributed to differences in SOM contents (Table 4). The fa values for p-NCB, 2,4-DNT, and 1,4-DNB were less correlated with the smectite contents of the soil horizons, with 17 to 21, 31 to 33, and 33 to 43%, respectively, of differences in fa values among soil horizons attributed to differences in smectite contents (Table 4). Linear regression analysis of the fa values to the ratios of SOM/smectite indicated that the fa values calculated were reasonable since, for each regression, the intercepts were all approximately unity at 0% SOM (Table 4). This observation further validates our recently presented method of data analysis for quantitative determination of fa (Charles et al., 2006a).


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  		Table 4. Correlations between fractional availability (fa) values for sorption of the nitroaromatic compounds (NACs) p-nitrocyanobenzene (p-NCB), 1,4-dinitrobenzene (1,4-DNB), and 2,4-dinitrotoluene (2,4-DNT), estimated using Eq. [3] (Karickhoff, 1984) (using solute sorption by K+–saturated Pahokee peat to determine the solute sorption coefficient normalized by the fractional soil organic matter content [Kom]) and our alternative Eq. [5], and percentage of soil organic matter (SOM) and smectite and the SOM/smectite ratio. Correlations were performed across the Webster and Clarion (A- and B-horizon) soils ({alpha} = 0.5). Standard deviations in parentheses.

 
Among the NACs studied, the overall fa values decreased in the order 2,4-DNT > 1,4-DNB > p-NCB for each soil horizon, suggesting that the availability of clay surfaces for NAC adsorption is influenced, in part, by the solute. Specifically, higher fa values were associated with solutes that were adsorbed more strongly by smectites.

To further explore the dependence of fa on solute and soil properties, we correlated our fa values with sorption parameters of the three probe molecules used to determine fa. The affinities of the NACs for SOM and pure soil smectites were modeled by our sorption data for Pahokee peat and beidellite clay (Fig. 5 ), respectively. Correlation analysis revealed that the fa values for the Webster A, Webster B, and Clarion A horizon soils across solutes were strongly negatively correlated with Qom/Qbeidellite (i.e., sorption distribution coefficient for each NAC to Pahokee peat and K+-beidellite at an equilibrium concentration of 10 mg L–1) with 68 to 89% of the differences in fa values among solutes attributed to differences in the relative affinities of the solute for SOM and beidellite in these soils (Table 5 ). For each of these soils, the fa and Qbeidellite values were even more strongly correlated, with >90% of the difference in fa values for each soil across NACs attributed to the difference in affinity of each NAC for the representative smectite clay (Fig. 5, Table 4). These results suggest that the greater the extent of sorption of the NAC to swelling clays, the less impact SOM may have on the availability of sorptive surfaces in soils. For the Clarion B horizon, fa values were less correlated with Qom/Qbeidellite (9–11%) or Qbeidellite (47–49%). This may be due to the low SOM content (?0.6%) of this soil, which can theoretically cover a maximum of only ~10% of the clay surface in whole soil. Thus the efficacy of SOM to obscure sorptive clay surfaces in this case may be minimal.


Figure 5
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  		Fig. 5. Sorption isotherms of p-nitrocyanobenzene (p-NCB), 2,4-dinitrotoluene (2,4-DNT), and 1,4-dinitrobenzene (1,4-DNB) by K+–beidellite. Isotherms with different letters are statistically different at {alpha} = 0.05.

 

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  		Table 5. Amount of solute sorbed to K+–beidellite (Qbeidellite) and peat (Qom) soil at an equilibrium concentration of 10 mg L–1 and their ratio for the nitroaromatic compounds (NACs) p-nitrocyanobenzene (p-NCB), 1,4-dinitrobenzene (1,4-DNB), and 2,4-dinitrotoluene (2,4-DNT). Regressions were performed between Qbeidellite or Qom/Qbeidellite and fa values estimated using Eq. [3] (Karickhoff, 1984) (using solute sorption by K+–saturated peat to determine the solute sorption coefficient normalized by the fractional soil organic matter content [Kom]) or our alternative Eq. [5]. These regression coefficients are reported for four soil samples: the Webster A horizon (WA) and B horizon (WB) and the Clarion A horizon (CLA) and B horizon (CLB).

 
Our data suggest that fa values for NACs in whole soils approach 1 in cases where the NAC is strongly sorbed and the SOM/smectite ratio low. For example, fa values for 2,4-DNT sorbed by the Webster B horizon soil approach 1 (Table 3). Since in theory fa cannot be >1, it was of interest to evaluate fa using a compound that sorbs more strongly to smectite clays than DNT, i.e., 1,3,5-TNB. We measured sorption of 1,3,5-TNB to K+– and Mg2+–exchanged Webster B horizon soil and the corresponding SOM-removed soil (Fig. 6 ) and calculated fa values at Ce of 5, 10, 15, and 20 mg L–1, as described above. The determined fa values were close to unity: 1.05 ± 0.04 to 1.09 ± 0.09 and 1.06 ± 0.04 to 1.10 ± 0.09 using both Eq. [3] and [5]. This observation provides further validation of our approach for the experimental evaluation of clay mineral surface availability (Charles et al., 2006a).


Figure 6
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  		Fig. 6. Sorption isotherms of 1,3,5-trinitrobenzene (TNB) by K+– and Mg2+–saturated Webster B horizon soil (WB) and Webster B horizon soil from which soil organic matter was removed (WBO).

 
These results suggest that the relative SOM and smectite contents of the soil horizons, as well as the sorption affinity of the solute to smectite clays, influence the efficacy of SOM to obscure sorptive surfaces of soil clays and hence the availability of these sites in soil for sorption of NACs. Apparently fa is not a fixed intrinsic value for a given soil. Rather, fa is more properly viewed as being operationally defined, similar to SOM surface area measurement (Pennell et al., 1995) because its value depends on the probe molecule used. That is, solutes with higher affinities for smectite surface adsorption sites are more effective in accessing these sites, manifesting higher fa values. Conversely, this study shows that SOM in undisturbed soils obscures many clay mineral surfaces and inhibits the sorption of small organic solutes by smectites, with a stronger inhibition of more weakly sorbing solutes. Thus, SOM acts like an organic solute of moderate adsorption strength, competing with NOCs for adsorption sites on smectites.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 EXPERIMENTAL METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
This project was supported by National Research Initiative Competitive Grant no. 2005-35107-15237 from the USDA Cooperative State Research, Education, and Extension Service.

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Received for publication January 19, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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