Environ. Sci. Technol. 1997, 31, 240-247
In Situ Spectroscopic Investigations of Adsorption Mechanisms of Nitroaromatic Compounds at Clay Minerals KENNETH W. WEISSMAHR,† STEFAN B. HADERLEIN,* AND R E N EÄ P . S C H W A R Z E N B A C H Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 Du ¨ bendorf, Switzerland ROLAND HANY Swiss Federal Laboratories for Materials Testing and Research (EMPA), CH-8600 Du ¨ bendorf, Switzerland ROLF NU ¨ ESCH Division of Geotechnical Engineering-IGT, ETH, CH-8092 Zu ¨ rich, Switzerland
Nitroaromatic compounds (NACs) including a number of priority pollutants such as explosives (e.g., TNT) and herbicides (e.g., DNOC) have been shown to adsorb strongly and specifically at natural clays. In situ spectroscopic techniques (13C-NMR, ATR-FTIR, UV/VIS, XRD) were applied to investigate the adsorption mechanism(s) of NACs at clays in aqueous systems. Planar NACs with several electronwithdrawing substituents exhibited highest sorption and were used as model compounds. The combined experimental evidence suggests a n-π electron donor-acceptor (EDA) complex between oxygens of the siloxane surface(s) of the clays (e- donors) and NACs (e- acceptors). Other adsorption mechanisms such as H-bonding or direct coordination of NO2 groups to surface sites were not important in aqueous environments, but contributed to the adsorption of NACs from apolar solvents. EDA complex formation took place at both external and, to a lesser extent, interlamellar siloxane surfaces of expandable clays. Adsorbed NACs were oriented coplanar to the siloxane layers and exhibited a high degree of mobility consistent with fast and reversible sorption found in batch experiments. Significant EDA complex formation at clays took place only in the presence of weakly hydrated exchangeable cations (e.g., K+, NH4+). Water coordinated to strongly hydrated exchangeable cations (e.g., Na+, Ca2+) strongly decreased the accessibility of siloxane sites for NACs. EDA complex formation at clays may not only control the transport of NACs in the subsurface but may also affect their reactivity concerning reductive transformation processes.
Introduction Nitroaromatic compounds are widely used as pesticides, explosives, solvents, and intermediates in the synthesis of dyes and other high-volume chemicals (1, 2). Many of these compounds and their transformation products are of significant toxicological concern (3, 4). NACs are ubiquitous * Corresponding author telephone: +41-1-823 55 24; fax: +411-823 54 71; e-mail:
[email protected]. † Present address: University of California at Berkeley, Berkeley, CA.
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environmental pollutants, particularly in subsurface environments (5). One prominent example is the contamination of soils and groundwaters with nitroaromatic munition residues such as 2,4,6-trinitrotoluene (TNT) and other nitro- and aminonitrotoluenes (6, 7). In order to control the mobility and to assess the fate of NACs in the subsurface, the sorption behavior and sorption mechanisms of such compounds must be understood. Previous work (8-11) has demonstrated that NACs, particularly those exhibiting several NO2 groups or other electron-withdrawing substituents, may adsorb strongly and reversibly from aqueous solution to phyllosilicate minerals such as clays. Since other classes of naturally occurring minerals including aluminum and iron (hydr)oxides, carbonates, and quartz were very poor sorbents for NACs, the overall adsorption of such compounds in the subsurface may be dominated by interactions with clay minerals. NACs generally show saturation-type adsorption isotherms at clays and exhibit distinct competition effects when present in mixtures, indicating that NACs adsorb by similar mechanism(s) and to a limited number of distinct adsorption sites at clays. NACs strongly adsorbed to all major groups of clay minerals. However, the affinity of NACs to clays is strongly affected by the type of exchangeable cations present at the clays. Significant adsorption of NACs occurs only in the presence of weakly hydrated cations (i.e., Cs+, Rb+, K+, or NH4+), while strongly hydrated cations such as H+, Na+, Ca2+, Mg2+, or Al3+ prevent NAC adsorption. Various results suggest that NACs adsorb primarily at siloxane surfaces exhibiting a permanent negative charge due to isomorphic substitution. Surface sites of clays exhibiting a variable, pH-dependent charge (i.e., SiOH and AlOH at edge or gibbsite surfaces) are not involved significantly in the adsorption of NACs at clays (8, 10). An electron donor-acceptor (EDA) complex formation between oxygens present at the siloxane surfaces of clays (edonors) and the electron-deficient π-system of NACs (eacceptors) was proposed as adsorption mechanism in aqueous environments. A series of macroscopic experimental evidence supports this hypothesis: Sorption of NACs at clays exhibits high negative adsorption enthalpies (∆Hads of about -40 kJ mol-1) and is governed by electronic and steric substituent effects rather than by hydrophobic interactions. Generally, good electron acceptors, i.e., NACs exhibiting an electrondeficient π-system due to several electron-withdrawing and electron-delocalizing substituents, strongly adsorb to clays [e.g., 1,3-dinitrobenzene (1,3-DNB); 1,3,5 trinitrobenzene (TNB)]. Poor electron acceptors, i.e., NACs with several electron-donating substituents (e.g., CH3, NH2) as well as nonaromatic nitro compounds exhibit very low affinities. Factors that prevent coplanarity and/or optimal resonance of the aromatic NO2 groups(s) with the aromatic ring strongly hinder the adsorption of NACs (e.g., ortho substitution or bulky substituents), suggesting that a coplanar arrangement of NACs and the donor sites at clay minerals is necessary for significant EDA complex formation. Note that very similar substituent effects were found for EDA complex formation of NACs in solution (12). Adsorption of NACs to clay minerals in organic solvent systems has been addressed previously using various spectroscopic techniques. Surface interactions of NACs in such systems were interpreted in terms of H-bonding and/or direct coordination of NO2 group(s) to exchangeable cations or hydration water of exchangeable cations (13-16). These studies did not consider EDA complex formation as a potential adsorption mechanism. In the present study, we report spectroscopic evidence for EDA interactions of NACs with clay minerals in aqueous systems. The adsorption of two strongly adsorbing NACs (1,3-DNB and TNB) was investigated
S0013-936X(96)00381-1 CCC: $14.00
1996 American Chemical Society
TABLE 1. Names, Abbreviations, Cation Exchange Capacity (CEC), Surface Area, and Layer Charge of Clay Minerals Investigated
clay mineral hectorite, SHCa-1 beidellite, SBCa-1 montmorillonite, Camp Berteau montmorillonite, SAz-1 illite, Sarospatak
origin
CEC surface area layer (mol (m2 g-1) kg-1) totala/externalb chargec
Hector, CA 0.67 California 0.66 Morocco 0.85
520/120 330/23 570/70
0.33d 0.35e 0.42f
Arizona
0.95
530/100
0.48f
Hungary
0.18
70/40
0.60g
a
b
Surface area accessible for H2O. Surface area accessible for N2 (BET method). c Per half unit cell; calculated from elemental analyses. d Data from ref 43. e Data from Dr. Post of the California State University at Sacramento. f Data from ref 44. g Data from ref 45.
using various in situ techniques, namely, 13C-nuclear magnetic resonance (NMR), infrared (FTIR), and ultraviolet (UV) spectroscopy, as well as X-ray diffraction (XRD). The objectives of this study were to identify the adsorption mechanism(s) of NACs at clay minerals and to evaluate the possibilities and limitations of current in situ spectroscopic methods to study interactions of sparingly soluble organic pollutants with natural surfaces in aqueous systems.
Experimental Section Chemicals and Clay Minerals. 1,3,5-Trinitrobenzene (TNB; obtained from EMS Dottikon, Switzerland) was recrystallized from ethanol before use. Fully 13C-labeled 1,3-dinitrobenzene (1,3-DNB) was purchased from Dr. Glaser AG, Basel. Table 1 compiles the names and some important characteristics of the clay minerals used. SAz-1, HCa-1, and SBCa-1 were purchased from the Source Clay Minerals Repository (University of Missouri, Columbia, MO). Dr. G. Kahr, Laboratory of Clay Mineralogy, ETH Zurich, Switzerland, provided illite (Sarospatak) and Camp Berteau montmorillonite. The clays were chosen to cover a range of pertinent mineralogical properties such as type and location of isomorphic substitution within the clay lattices. Hectorite is particularly suited for spectroscopic investigations. Its very low content of paramagnetic iron is essential for NMR measurements because paramagnetic impurities broaden the signals significantly (17). In addition, the interpretation of IR spectra is facilitated at hectorite because the region of the structural OH vibrations of the clay does not overlap with the stretching vibrations of water (18). Finally, hectorite is suited for preparing oriented self-supporting clay films that can be used to study the orientation of adsorbed NACs. Calcite impurities present in the raw hectorite were removed by sedimentation and acidification to pH 2. The suspension was neutralized by the addition of 0.1 M KCl (pH 9). Homoionic clays were prepared by repeated washing with 0.1 M KCl or CaCl2 followed by dialysis and lyophilization. The clays used exhibited some minor impurities such as quartz (all samples), kaolinite (all samples except illite Sarospatak), and feldspars (SAz-1 and Camp Berteau montmorillonite). Adsorption Experiments. The Kd values of NACs were determined within the quasi-linear initial part of the NAC adsorption isotherms by batch experiments (8) in aqueous and cyclohexane suspensions of homoionic K+- and Ca2+illites. Dissolved concentrations of NACs were measured by UV spectroscopy. Concentrations of adsorbed NACs were calculated by difference. XRD Measurements. A stock suspension of homoionic K+-clays (1 g of clay / 10 mL of 0.1 M KCl) was prepared, and aliquots of this suspension were diluted in 0.1 M KCl with and without TNB. The suspensions were equilibrated for 1-2 h at 22 °C and then centrifuged. Until otherwise stated,
the concentration of adsorbed TNB was close to the sorption capacity ([TNBsorb]max) of the respective clays (see Table 2). The wet clay paste was spread on glass disks or silicon single crystal supports, dried at 60 °C for 12 h, and equilibrated at 13% or 50% RH. K+-clays with adsorbed TNB showed less plasticity and swelling than untreated reference samples and required longer supersonic treatment to disperse completely during sample preparation. The samples were analyzed on a Phillips PW 1729 or a Siemens D500 diffractometer using So¨ller slits and Cu KR radiation. A chamber saturated with ethylene glycol vapor was used for measuring glycolated clay samples. The XRD spectra were highly reproducible. The precision of d-spacing measurements was (0.01 × 2Θ (Å). 13C-NMR Measurements. NMR samples of K+-hectorite were prepared similarly to XRD samples, but fully enriched [13C]1,3-DNB dissolved in 0.1 M KCl was used instead of TNB. Samples with two different concentrations of adsorbed 1,3DNB (5 and 40 mmol kg-1 corresponding to 1.6% and 13% of maximum adsorption capacity, respectively) were used. Measurements were performed using wet clay slurries, airdry samples (kept for 12 h in a desiccator over silica gel and then equilibrated at 45-50% RH), and dehydrated samples (treated at 373 K for 12 h). 13 C-NMR spectra were acquired at 100.6 MHz and 295 ( 2 K on a Bruker ASX-400 NMR spectrometer using a 4-mm magic angle spinning (MAS) probe head. The digital resolution was 0.18 ppm. The carbon polarization was generated either by 90° single-pulse (SP) excitation without 1H decoupling during data acquisition of the 13C free induction decay or by using cross-polarization (CP) with a contact time of 2 ms. Proton decoupling during data acquisition induced a severe line broadening of the 13C signals and was not applied routinely in our study. Such line broadening effects in solidstate NMR have been attributed to an interplay between molecular motion, MAS frequency, and decoupling power (19-21). The 13C spin-lattice relaxation times, T1, were determined at a spinning rate of 5 kHz by using the inversion-recovery technique. Typical conditions for recording spectra in MAS experiments were as follows: recycle delay 3 s; acquisition time 28 ms; sweep width 36 kHz; no line broadening applied to any spectra before Fourier transformation. UV/VIS Measurements. Absorption spectra were recorded against water on an Uvikon 860 instrument equipped with a head-on detector. This detector design allowed us to place cuvettes close to the photomultiplier tube, thus minimizing intensity losses due to light scattering. Both stable clay suspensions (22) in 1-cm quartz cuvettes and thin clay films coating the inside of a 1-cm standard Suprasil fluorescence cuvette were used. The spectra obtained were highly reproducible within the spectral resolution of 1 nm. Homoionic clays (Na+-, K+-, and Cs+-exchanged hectorite and SAz-1 montmorillonite) were size fractionated by sedimentation. The suspended fine fractions (95%), and the spectra c and d showed a red shift of the π-π* transition band of TNB of about 10 nm. Identical spectra of adsorbed TNB were obtained using homoionic suspensions of montmorillonite. TNB adsorbed to films of K+-illite and K+-beidellite also showed an absorption maximum at 238 nm, as was found for K+-hectorite (data not shown). The type and location of
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TABLE 3. Wave Numbers of Symmetric (νsym) and Asymmetric (νasym) Stretching Vibrations of NO2 Groups of 1,3,5-Trinitrobenzene (TNB) as Pure Solid, in CCl4 and Aqueous Solution, and Adsorbed to Homoionic Clay Minerals 1,3,5-trinitrobenzene (TNB) as pure solid dissolved in:
CCl4 (10 mM) water (1 mM)
adsorbed to: K+-hectorite; aqueous supernatant air-dried (50% RH) N2 dried (0% RH) Cs+-hectorite; aqueous supernatant air dried (50% RH) N2-dried (0% RH) + K -beidellite; aqueous supernatant air-dried (50% RH) N2-dried (0% RH) K+-illite; aqueous supernatant air-dried (50% RH) N2-dried (0% RH) K+-montmorillonite; aqueous supernatant
νsyma νasyma As/Aasb (cm-1) (cm-1) (-) 1342
1539
1.1
1343 1348
1552 1550
0.7 0.8
1353 1353 1353 1350 1350 1350 1352 1352 1354 1352 1352 1352
1547 1547 1547 1547 1547 1547 1549 1547 1549 1545 1545 1545
1.4 1.4 1.4 1.2 1.2 1.2 1.5 1.5 1.5 1.2 1.2 1.2
1353
1548
1.6
a Wave numbers were highly reproducible within the spectral resolution. b Ratio of absorption intensities of symmetric (As) and asymmetric (Aas) stretching vibrations of the NO2 groups.
FIGURE 5. FTIR spectra of 1,3,5-trinitrobenzene adsorbed at a selfsupporting, oriented K+-hectorite film recorded at various angles of incidence of the light beam. The intensities of the in-plane NO2stretching vibrations (νsym at 1353 cm-1 and νasym at 1545 cm-1) were not affected by variations of the angle of the incident light. Outof-plane stretching vibrations (at 710 and 924 cm-1) increased with increasing angle of the incident light (dichroism). adsorbed to clay minerals were quite similar. Compared to CCl4 solution, wave numbers for νasym of TNB adsorbed to clay minerals were red-shifted by 5-10 cm-1, whereas values for νsym were blue-shifted by about 10 cm-1. Moreover, the ratio of the band intensities was inverted. The spectra of TNB adsorbed to a given clay were not affected by varying moisture contents nor by the type of exchangeable K+ or Cs+ cations or the concentration of TNB. No evidence for disturbed symmetry, such as line splitting or line broadening, was found for adsorbed TNB as compared to dissolved TNB. In order to determine the orientation of TNB adsorbed to clays, the dichroic properties of the absorption bands, i.e., the impact of the angle of the incident light on the absorption intensities, were studied at oriented, self-supporting K+-hectorite films (Figure 5). The angle between the principal plane of the NACs and the siloxane plane of the clay mineral was estimated by comparing the ratios of the band intensities of transmission spectra recorded under several angles of incidence (25). The stretching vibrations of the NO2 groups hardly changed their intensity between 0° and 30°, whereas bands at 705 and 924 cm-1 (tentatively assigned to N-O and C-H out-of-plane wagging deformations) increased significantly.
Discussion
FIGURE 4. ATR-FTIR spectra of 1,3,5-trinitrobenzene as solid, dissolved and adsorbed to various homoionic clay minerals: (a) Dissolved in CCl4 (10 mM); (b) pure (solid) TNB; (c) adsorbed to K+-hectorite, 100 mmol kg-1 (33% of adsorption capacity) with aqueous supernatant (0.1 M KCl); (d) adsorbed to dry K+-hectorite (100 mmol kg-1), 50% RH; (e) adsorbed to dry K+-hectorite (100 mmol kg-1), 0% RH; (f) adsorbed to Cs+-hectorite, 130 mmol kg-1 (43% of adsorption capacity) with aqueous supernatant (0.1 M CsCl). isomorphic substitution of the various clays had no detectable effect on the spectra of adsorbed TNB. Drying of the clay film under N2 had no measurable effect on the spectrum of adsorbed TNB. Additional absorption bands of adsorbed TNB in the range of 200-700 nm were not detected in any of the experiments. FTIR. The wave numbers of the symmetric and asymmetric NO2-stretching vibrations of TNB as pure solid, dissolved in both CCl4 and water, and adsorbed to several clay minerals are summarized in Table 3. Figure 4 shows some representative spectra. The absorption maxima of TNB dissolved in apolar CCl4, dissolved in polar water, and
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Role of External and Interlayer Siloxane Surfaces for NAC Adsorption. NAC adsorption is known to take place at external siloxane surfaces of two-layer clays such as kaolinites (8). In this work, the role of interlayer surfaces of three-layer clays with respect to NAC adsorption was examined using primarily XRD techniques. The interlayer spacing of all dehydrated smectites (equilibrated at 13% RH) increased upon adsorption of TNB. Assuming coplanar orientation of adsorbed TNB (see below) and using the van der Waals diameter of the aromatic π-system, a value of 3.5 Å can be considered as a lower estimate of smectite layer expansion due to complete intercalation of TNB. Since measured layer expansions of the smectites even at saturation levels of TNB only ranged from 0.8 to 2.14 Å, complete intercalation of NACs can be excluded. However, according to Merings rule (26), the modest layer expansions found in the presence of adsorbed TNB as well as the position, the decreased intensity, and the broadening of the 002 and 005 signals strongly indicate partial intercalation of TNB. Partial intercalation of NACs is also suggested by the systematic increase of interlayer spacings of K+hectorite with increasing amounts of adsorbed TNB (Figure 1). Note that measurable d(001) layer expansions occurred only at relatively high concentrations of adsorbed TNB (about 10% of the adsorption capacity of K+-hectorite). The results
of the NMR study, however, suggest that partial intercalation of NACs occurred already at much lower concentrations (see below). At 50% RH, the XRD patterns of clays with and without adsorbed TNB were quite similar and were dominated by the 12.5 Å d(001) spacings of hydrated smectite layers. However, partial intercalation of TNB may also have occurred in the presence of hydrated interlayers without noticeable changes in d(001) spacings if adsorbed TNB had replaced interlayer water. Glycol treatment significantly expands the interlamellar regions of smectites and is frequently used to investigate intercalation processes (27). Some of the glycolated smectites showed slightly smaller d(001) spacings in the presence of adsorbed TNB than the glycolated reference samples, suggesting that part of the adsorbed TNB was intercalated and not displaced by glycol. The disappearance of the 10.1 Å spacing of hectorite upon TNB adsorption indicates that TNB may have entered parts of the collapsed interlayers that were not accessible for water nor for glycol. The results demonstrate that certain regions of siloxane interlayers of expandable clays are accessible for NAC adsorption in aqueous environments. However, complete intercalation of NACs did not occur even in the saturation range of their adsorption isotherms. Partial intercalation of TNB lead to interstratification of 13.5 Å TNB intercalated layers with other components of smectites in aqueous environments such as 10.1 Å dehydrated layers and 12.5 Å hydrated layers. The degree of NAC intercalation was inversely related to the layer charge of the clays (Table 1) and was highest for hectorite and lowest for SAz-1 montmorillonite. Orientation of Adsorbed NACs. The intensity of the IR absorption bands of spatially oriented compounds (e.g., in crystals or adsorbed to oriented surfaces) is a function of the angle between the direction of the oscillating dipoles of a vibrational mode of the compounds and the direction of the electric vector of the incident light. Therefore, the (average) orientation of the compound can be inferred from the dichroism of their absorption bands, if the type of symmetry of the respective vibrational modes is known (25). The high symmetry of the planar TNB (point group D3h) greatly simplifies its IR spectrum. While the dipole moments of all (in-plane) stretching vibrations of TNB are parallel to its principal symmetry plane, the dipole moments of the outof-plane wagging vibrations are normal to it. The selfsupporting oriented K+-hectorite films used exhibit siloxane surfaces parallel to the plane of the films. The absorption intensity of vibrations parallel to the siloxane planes remains almost constant for incident angles of light between 0° and about 30°. Absorption intensities of vibrations perpendicular to the siloxane planes are very sensitive to changes of incident angles in this range. A quantitative treatment of dichroism was given by Prost (28). The dichroic properties of the NO2stretching and N-O and C-H wagging vibrations of TNB adsorbed to K+-hectorite clearly demonstrate a coplanar orientation of TNB and the siloxane surface of the clays. A coplanar orientation is essential for an EDA adsorption mechanism, because only coplanarity allows significant overlap of the orbitals of the non-bonding electrons of the siloxane surface and the electron-deficient π-system of NACs. Mobility of Adsorbed NACs. The NMR results of air-dried samples reveal that two distinct fractions of adsorbed NACs with distinguishable degrees of mobility were present at hectorite. Short 13C relaxation times of only a few hundred milliseconds (17), attenuated cross-polarization transfer efficiencies, and the fact that low MAS rates without 1H decoupling were sufficient to produce narrow resonance lines suggest that the mobility of both 1,3-DNB species was high and in the same order of magnitude. The 13C line broadening induced by proton decoupling also points to vigorous molecular dynamics, with motions on the time scales of the
MAS frequency and 1H decoupling strength (several kHz). The degree of mobility of adsorbed 1,3-DNB strongly depended on the water content of the samples. This is evident from the highly attenuated CP signal intensities of wet clay samples (due to motional averaging of the static dipolar 1H13 C interaction). At dehydrated samples, the reduced mobility of adsorbed 1,3-DNB is reflected by broadened NMR signals due to incomplete averaging of the anisotropic interactions by MAS alone and/or exchange broadening between adsorbed NACs exhibiting slightly different chemical shifts as a result of siloxane site heterogeneity. Fast initial ad- and desorption kinetics of NACs at smectites observed in batch sorption experiments (10) are consistent with the picture of highly dynamic and mobile sorbed NAC species. The chemical shift difference between the two fractions of adsorbed NACs is about 100 Hz. The coalescence of two lines separated by these 100 Hz requires a slow exchange rate of only about π‚∆ν‚2-(0.5) ) 222 Hz (29). The molecular dynamics of adsorbed NACs, however, was much higher (see above). Thus, it is conceivable that the two fractions of 1,3DNB adsorb at spatially separated sites, where fast exchange among each other is not possible. This interpretation is consistent with two other findings: The XRD results indicate that NACs not only adsorb at external surfaces but also may penetrate the interlamellar regions of smectites. Furthermore, the slow desorption kinetics observed for a minor fraction of NACs at smectites (10) supports the idea of two spatially separated sorption sites of NACs at expandable clay minerals. Role of Aromatic NO2 Groups: Disproof for Direct Coordination to Surface Sites. Sorption of NACs at clay minerals in non-aqueous systems has already been studied (13-16). H-bonding to water ligands of exchangeable cations or direct coordination of NO2 groups to such cations were identified as adsorption mechanisms of NACs in such systems. However, the results of our FTIR and UV measurements suggest that the role of NO2 group(s) in the adsorption of NACs is fundamentally different in aqueous environments. Both, UV and IR spectra of TNB adsorbed to homoionic clays were very similar regardless of the amount of water and the type of cations (K+ or Cs+) present at the clays. This suggests that neither exchangeable cations nor water ligands of the cations or the siloxane surfaces were directly involved in the adsorption of NACs at aqueous clays. The (small) shifts observed in the IR spectra (Figure 4) of adsorbed TNB compared to aqueous or solid TNB were caused by other factors: The minor effect of the type of solvent (CCl4 versus H2O) on the IR spectra of dissolved TNB reflects the low Lewis basicity (H-accepting capability) of the NO2 groups of TNB. Apolar and aprotic CCl4 was chosen as an inert solvent where solute-solvent interactions are minimal as compared to water where significant H-bonding to NO2 groups may be expected. Since the Lewis basicity of aromatic NO2 groups is decreased by electron-withdrawing substituents (e.g., additional NO2 groups) but increased by electron donating substituents (30, 31), TNB can be considered as a rather poor H-acceptor. However, compared to p-nitroaniline, which is a much better H-acceptor, TNB exhibits more than 3 orders of magnitude higher affinity for aqueous clay minerals (10). Therefore, it can be concluded that H-bonding or direct coordination of the aromatic NO2 groups is negligible for the adsorption of NACs from aqueous solution to clay minerals. This conclusion is further supported by a comparison of the Kd values of 1,3-DNB and 1,2-DNB at homoionic K+- and Ca2+-clays in aqueous and in cyclohexane suspensions (Table 4). In aqueous suspensions, significant adsorption of 1,3DNB occurred only at K+-illite (low sorption of 1,2-DNB due to the ortho substituent effect; low sorption of both NACs at Ca2+-clay due to the presence of strongly hydrated exchangeable cations). In cyclohexane suspensions, significant adsorption of 1,3-DNB and 1,2-DNB occurred at K+- and Ca2+illite. Kd values for the Ca2+-clay were very similar for 1,3-
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TABLE 4. Affinity (Kd Values; L kg-1) of o- and m-Dinitrobenzene to Homoionic K+- and Ca2+-Illites in Aqueous and Cyclohexane Suspensions 1,3-dinitrobenzene 1,2-dinitrobenzene
K+-illite (H2O)
Ca2+-illite (H2O)
K+-illite (cyclohexane)
Ca2+-illite (cyclohexane)
2100 2.5
