Environ. Sci. Technol. 2005, 39, 8315-8323
Sorption and Conformational Characteristics of Reconstituted Plant Cuticular Waxes on Montmorillonite B A O L I A N G C H E N †,‡ A N D B A O S H A N X I N G * ,‡ Department of Environmental Science, Zhejiang University, Hangzhou, Zhejiang 310028, China, and Department of Plant, Soil, and Insect Sciences, University of Massachusetts, Amherst, Massachusetts 01003
Plant cuticular waxes are essential barriers that regulate the transport of water and organic molecules to intact cuticular membranes. They also compose a significant fraction of the recalcitrant aliphatic components of soil organic matter (SOM). In this study, we examined the sorption and desorption of three polycyclic aromatic hydrocarbons (PAHs), naphthalene (NAPH), phenanthrene (PHEN), and pyrene (PYR), by cuticular waxes of green pepper (Capsicum annuum) that had been reconstituted by loading them onto montmorillonite (at four different loadings). The reconstituted wax samples, with and without sorbed PAHs, were characterized by solid-state 13C NMR to supply the evidence of melting transition. The sorption isotherms fit well to a Freundlich equation. Sorption isotherms were practically linear except for that of PYR sorption to the lowload wax-montmorillonite sample. The organic-carbonnormalized sorption coefficients (Koc) depended on PAH’s lipophilicity (e.g., octanol-water partition coefficient) and increased with increasing wax-load on clay. Desorption was dependent on PAH’s molecular sizes and sorbed amounts and on the wax load of the clay. Desorption hysteresis was observed only at high loads of NAPH and PHEN, and it decreased with both increasing wax load and molecular size (i.e. NAPH > PHEN >> PYR). Contributing to hysteresis, the melting transition of the reconstituted waxes after sorbing the PAHs was confirmed by solidstate 13C NMR data. Upon adsorption, the intensity of the NMR peak at 29 ppm (attributed to mobile amorphous paraffinic domains) increased, and a peak at 167 ppm (-COOH) appeared, reflecting the transition of solid amorphous to mobile amorphous domains in the reconstituted waxes. The intensity of melting induced by PAH adsorption decreased with increasing PAH molecular size.
Introduction Plant cuticular lipids or waxes, embedded within the cutin matrix of the cuticle and also deposited on cuticle outer surfaces of cuticles (1-3), act as transport barriers of intact cuticular membranes. The reconstituted samples of extracted waxes have been regularly used as a model to study the * Corresponding author phone: (413) 545-5212; fax: (413) 5453958; e-mail:
[email protected]. † Zhejiang University. ‡ University of Massachusetts. 10.1021/es050840j CCC: $30.25 Published on Web 09/14/2005
2005 American Chemical Society
transport properties of plant cuticular waxes (4-6). Affecting soil properties and sorption behavior, lipids are an important class of soil organic substances (SOM), as they are involved in mineral weathering, nutrient mobilization, and humus and soil formation (7). Fatty acids are a major component of lipids in soils, originating from microorganisms (n-C4:0 to n-C26:0) and waxes of plants and insects (n-C26:0 to n-C38:0) (8). Because the short-chained fatty acids are more rapidly decomposed than are the long-chains, the long-chain components of fatty acid are preferentially preserved in soil (7). Plant waxes seem to be the only source of stable longchain fatty acids in soil (2, 7). Moreover, stabilization by organic-mineral bonds or trapping inside aggregates may contribute further to preservation of fatty acids in soils (7, 9). Fatty acids extracted from soils with supercritical carbon dioxide have been interpreted as residuals of waxes (10). Schulten and Schnitzer (9) also reported that the organic matter identified in fine clay fractions was typical of components of natural waxes. Humin, considered being a special form of humic substances tightly associated with the mineral matrix, has been described chemically as having a significant aliphatic nature that was probably attributable to waxlike materials (11, 12). Humin typically represents more than 50% of the organic carbon in a soil and typically more than 70% of total organic carbon in unlithified sediments (11, 13, and refs therein). Therefore, to elucidate the role of organic matter associated with soil minerals, it is of interest to examine the sorption activity of the reconstituted waxes on clay. The structure and molecular dynamics (1, 14-17), phase behavior (3, 18), and organic solute diffusion transport (4-6, 19) of plant cuticular waxes have been extensively investigated by spectroscopic, optical, diffraction, thermal, and mobilityprobe analyses (3). It was argued that cuticular wax consists of a crystalline and an amorphous phase (3, 15, 16). The crystalline domains were dominated by the long aliphatic chains of the wax constituents, which are assembled in a regular lattice; the amorphous zones surrounding the crystallites were made up by chain ends, functional groups, shortchain aliphatics, and nonaliphatic compounds (3, 15, 16). The amorphous zones are further divided into solid amorphous domains and mobile amorphous domains (16). Hydrogen bonding in plant waxes prevents phase separation of short- and long-chain elements (14). Due to the absence of a discrete structural order, the amorphous zone is accessible to permeating molecules, which can easily diffuse in this zone of relatively high fluidity. On the other hand, the crystalline zone excludes molecules that might otherwise diffuse across the barrier. This implies that the physical structure of the wax may be a more important determinant of transport across the cuticle than its chemical composition (3). Similarly, the physical conformation of SOM (e.g. waxes) would play a significant role in sorption of hydrophobic organic contaminants (HOCs) (20, 21). Hysteresis in sorption and desorption reactions is interestingly understood to regulate the fate, transport, and bioavailability of HOCs in soils and sediments. It is welldocumented that SOM predominantly controls sorption and desorption of HOCs in soils and sediments (20-26). Desorption hysteresis of HOCs with SOM has been reported in the literature (23, 25, 27, and refs therein). Irreversible pore deformation has been suggested as a hysteretic mechanism in SOM (23, 24, 26). This process is similar to the “melting away” of nanometer-size holes in the condensed domain (27). The resulting matrix “trapping” effect is thought to lead to different mechanistic pathways for sorption and desorption VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Selected Properties of Naphthalene (NAPH), Phenanthrene (PHEN), and Pyrene (PYR) at Room Temperature (22)a
a M: molecular weight, g/mol. S : water solubility for solid compound, mol/L. S : water solubility as supercooled liquid, mol/L. K : octanolw sl ow water partition coefficient. MP and BP are melting and boiling point temperature (unit is K), respectively. Hfusion: heat of fusion, kcal/mol. 1/2. V 3 δ: solubility parameter, (cal/mL) molar: critical molar volume, cm /mol.
(26). But the direct evidence of phase transition of SOM during sorption process is limited. Aiming to better understand the sorption of PAHs with mineral-associated organic matter (soil and sediment humin), the specific objective of this study was to examine sorption and desorption of three PAHs with reconstituted plant cuticular waxes on montmorillonite and to characterize phase transitions during the sorption process by solid-state13C NMR.
Materials and Methods Preparation of Reconstituted Wax Samples. Cuticular waxes were isolated from the fruits of green pepper (Capsicum annuum). The fruit skins were manually peeled from fresh green pepper fruits. The skins were boiled in water for 1 h, and the pulp was removed manually as much as possible. Then the skins were incubated in a solution of oxalic acid (4 g/L) and ammonium oxalate (16 g/L) at 90 °C for 24 h and washed with deionized distilled water to remove any residual fruit pulp material. This procedure yielded the bulk cuticle. Waxy materials were extracted from the bulk cuticle by Soxhlet extraction with chloroform/methanol (1:1) at 70 °C for 6 h. Boiling the fruit skins for 1 h and subsequently treating them for another 24 h at 90 °C may drastically change wax composition. Polar and shorter-wax components may be lost. In addition, chloroform/methanol Soxhlet extraction of the isolated cuticles may contaminate the extracted waxes with cutin acids release by transesterification. But these isolation processes preserve the long-chain components of waxlike materials in soils, which correspond to the objective of this study to understand sorption of soil and sediment humin or waxlike materials. Therefore, the isolation of cuticles did not use the standard procedures, i.e., enzymatic isolation with subsequent borate buffer treatment and wax extraction with hot chloroform. The extracted waxes, including epicuticular and intracuticular waxes (2), were recovered by rotary evaporation. A given amount (0, 0.075, 0.12, 0.15, and 0.30 g) of the isolated cuticular wax was dissolved in 20 mL of chloroform in a 40-mL screw cap vial with alumina foil liner, to which 1.0 g of Ca2+-montmorillonite was added. The montmorillonite was purchased from Sigma-Aldrich Chemical Co. (Fluka 30 K, a reference clay) and used without further treatment. The vials were shaken for 24 h. After centrifugation at 1000g for 30 min, the solid phase was collected as reconstituted wax samples. Five reconstituted-wax samples were prepared with different wax loadings. All samples were freeze-dried to remove completely water and chloroform molecules in the samples and then ground and sieved ( PHEN > NAPH. However, individual PAH KSoc values increased with increasing wax loadings for PHEN and PYR, but KSoc values remained nearly constant for NAPH. The dependence of KSoc on wax loadings may be due to different physical conformation of reconstituted waxes. As the mobile amorphous zone of wax interacts with the mineral surface, the first several molecular layers close to the mineral surface may take a more compact form due to the attractive forces of the mineral surface, while any layers beyond this compact region may be relatively more expanded, as the attractive force becomes much weaker with distance away from the mineral surface (27). Therefore, the proportion of the reconstituted waxes in the expanded phase would be relatively large at high loading, producing high sorption capacity (KSoc). Similar observations of PHEN KSoc dependence
on humic acid loading on minerals have been reported by Wang and Xing (32). But the distinction between the condensed and expanded regions could not be probed by NAPH, due to its small molecular size and thus expressing strong “melting” ability. Desorption data of NAPH, PHEN, and PYR with the selected reconstituted-wax samples are also presented in Figure 3. All of desorption data could be well fitted to the Freundlich equation. The isotherm confidence interval (95%) and all data points for NAPH (A), PHEN (B), and PYR (C) sorption and desorption by wax-mont4 are shown in Figure S1 of the Supporting Information. Upon comparing sorption and desorption data, it was obvious that there was nearly no desorption hysteresis for the lowest concentrations of NAPH, for the low concentrations of PHEN, and for the whole concentration range of PYR. Apparent hysteresis for NAPH and PHEN appeared only at relatively high sorbed amount (S). Due to the different hysteretic behavior of PAHs at low and high concentrations, the different desorption pathways are easily masked at low and high concentrations by using all of desorption data fitting to the Freundlich equation. Furthermore, if all desorption data are included, desorption model parameters should have slightly higher experimental uncertainties, and then desorption hysteresis should be less significant. Therefore, desorption isotherms for PYR and for NAPH and PHEN at the hysteretic regions (high concentration) fit the Freundlich equation; the Freundlich model parameters are listed in Table 4. Again, desorption coefficients (KDd) for NAPH, PHEN, and PYR were calculated from the slopes of the linear fitting desorption isotherms (i.e. high concentration range, Figure 3). Values of KDoc were calculated from normalizing KDd to the carbon level of each reconstituted-wax sample and are listed in Table 4. For NAPH and PHEN at high concentration (hysteretic region), KDoc values decreased with increasing loadings of reconstituted waxes, in contrast with the dependence of sorption data on wax loadings. For PYR, KDoc values increased with loadings of reconstituted waxes, similar to the trend of sorption data with wax loadings. VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Desorption Coefficients and Freundlich Model Parameters of NAPH, PHEN, and PYR with the Reconstituted Cuticular Wax on Montmorillonite PAHs
sorbentsa
ND
log KDfc
NAPH
wax-mont1
1.052 (0.012)b 1.041 (0.010) 1.039 (0.014) 1.033 (0.014) 1.027 (0.026) 1.038 (0.037) 0.931 (0.021) 1.014 (0.019) 0.997 (0.017) 1.001 (0.015)
2.402 (0.013)b 2.426 (0.011) 2.428 (0.015) 2.517 (0.009) 3.369 (0.029) 3.708 (0.045) 3.540 (0.048) 3.926 (0.045) 3.941 (0.040) 4.250 (0.036)
wax-mont2 wax-mont3 wax-mont4 PHEN
wax-mont1 wax-mont4
PYR
wax-mont1 wax-mont2 wax-mont3 wax-mont4
Freundlich r2
KDdd (mL/g)
KDoce (mL/g)
linear r2
KDd/KSdf
ng
Sm rangeh (mg/kg)
0.998
269.1 (4.51)b 276.5 (3.63) 276.4 (2.33) 343.3 (2.50) 2272 (22.83) 4841 (75.69) 4530 (309.9) 7899 (306.6) 8583 (150.9) 17730 (410.6)
6952 (116)b 5741 (75.3) 5490 (46.2) 3705 (27.0) 58680 (590) 52230 (816) 117020 (8008) 164010 (6361) 170460 (2994) 191320 (4429)
0.996
5.19
14
0.5-1.4
0.998
3.70
14
0.6-1.8
0.999
3.60
14
0.7-1.9
0.999
2.35
14
0.9-2.8
0.998
3.01
10
15-60
0.996
1.63
10
54-100
0.938
1.13
16
>150
0.994
1.13
16
>200
0.996
1.12
16
>230
0.993
1.13
16
>330
0.999 0.998 0.999 0.995 0.990 0.993 0.995 0.996 0.994
a The reconstituted cuticular wax of green pepper on montmorillonite. b The values in the parentheses are the standard deviation. c Desorption isotherms fit Freundlich equation at hystersis ranges for PHEN and PYR due to different desorption behavior at low and high concentrations and fit the whole tested concentrations for PYR. KDf is desorption capacity coefficient [(mg/kg)/(mg/L)ND]. d KDd is desorption coefficient, calculated from the slope of linear desorption isotherms at hysteretic ranges. e KDoc ) KDd/foc, where foc is the organic carbon content of sorbent, %. f KDd/KSd is the ratio of desorption and sorption coefficients at hysteresis ranges. g n is the number of desorption data. h Sm range is the melting-transition concentration ranges of sorbed PAH, mg/kg. The Sm range was estimated from transition region of no-hysteresis to hysteresis of sorbate in sorption-desorption isotherms in Figure 3.
Hysteresis or isotherm nonsingularity is a confounding issue in sorption research that undermines the commonplace assumption of reversibility in environmental fate and risk assessment models for organic compounds in soil media (24). Many examples of hysteresis in the literature appear to be true hysteresis (23-27). The apparent sorption-desorption hysteresis was quantified for each sorbent-solutesolution system using the hysteresis index (HI):
HI )
SD SS
|
|
KD d
)
KSd
T,Ce
T,Ce
)
|
KD oc KSoc
(2) T,Ce
According to eq 1, sorption and desorption equations are as follows
sorption: desorption:
log SS ) log KSf + NS log Ce log SD ) log KD f + ND log Ce
(3) (4)
At a given concentration, eq 4 minus eq 3 results in
log
|
SD SS
) log
T,Ce
|
KD f KSf
+ (ND - NS) log Ce
(5)
Thus,
log HI ) log
KD f KSf
+ (ND - NS) log Ce
(6)
where SS and SD are solid-phase solute concentrations for the single-cycle sorption and desorption experiment, respectively, and T and Ce specify conditions of constant temperature and residual solution phase concentration. As shown in Figure 3, at Ce < Ca, HI ) 1, indicating that there was no hysteresis for NAPH and PHEN at relatively low concentrations. At Ce > Cb (Figure 3), the HI values are equal to KDd/KSd ratios because of the practically linear isotherms 8320
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of both sorption and desorption, and their values jumped up to larger than 1 (Table 4). The HI (or KDd/KSd) values increased with decreasing reconstituted-waxes loadings at a given solute concentration. These observations were consistent with results of previous investigations. Entrapment of sorbing molecules within condensed SOM matrixes contributes significantly to sorption-desorption hysteresis (26). Higher hysteresis of NAPH with humin (condensed domain) than with humic acid (expanded domain) was observed earlier (27). In comparison to the single-solute system, hysteresis of PHEN with SOM increased in the presence of high concentrations of cosolute, dichlorobenzene (DCB) and trichloroethylene (TCE) (25). This effect increased with the increases of condensed degree of SOM and cosolute concentration (25). In the current study, the dependence of the HI on PAHs molecular size was observed. Sorption hysteresis decreased with increasing PAH molecular size (molar volume in Table 1): NAPH > PHEN >> PYR. This is consistent with the observation that desorption hysteresis is not controlled by diffusion of the sorbate during the desorption process (24). For NAPH and PHEN, the reconstituted-wax medium was induced to be a more powerful sorption domain by high PAH-loading (i.e. KDd/KSd > 1). The transition of reconstitutedwax medium was hardly induced by sorbed-PYR (i.e., KDd/ KSd ≈ 1). Hysteresis of PHEN sorption in the presence of TCE (Sw ) 1100 mg/L) was reported to be larger than in the presence of DCB (Sw ) 83.1 mg/L) (25), consistent with the effect of solute molecular size on hysteresis in our present study. Schreiber and Schonherr (5) also reported that diffusion coefficients decreased rapidly with increasing molar volumes of solutes in reconstituted waxes. An increase in molar volume by a factor of 2 caused a decrease in the diffusion coefficients of aromatic molecules by a factor of 7.6 (5). It is possible that desorption in such cases is ratelimited by physical changes (matrix rearrangement) in the sorbent (24), because the intensity of matrix rearrangement depended on molecular size (negative correlation) and aqueous solubility (positive correlation). The reported results
FIGURE 4. Diagram of the phase transition model in the mineral-waxes-PAHs interaction systems. MA: mobile-amorphous domains; SA: solid-amorphous domains; CY: crystalline domains; OMA: original MA; TMA: transformed MA by melting as a result of sorbed PAH. Sm1 (for NAPH) and Sm2 (for PHEN) in panel B refer to phase transition concentration of sorbed PAH.
and our present work support the concept of critical concentration of solute to induce the phase-transition of the SOM, above which matrix deformation (rearrangement) occurs (24, 25). The different phase transitions during PAHs sorption process will be confirmed later by solid-state 13C NMR data. The Evidence of Melting Transition. Cuticular waxes are multiphase systems consisting of crystalline and amorphous zones. The amorphous zones are subdivided into a solid zone and a mobile zone (3, 15, 16). The physical conformation of wax-mont4 in the presence and absence of sorbed PAHs was characterized by solid-state 13C NMR (Figure 2). In DRIFT spectra of waxes (Figure 1), a peak at 1712 cm-1 (-COOH) was observed, but the -COOH peak in the 13C NMR spectra of 167-170 ppm was very weak. Mao et al. (34) reported that the sp2 carbons (108-220 ppm) or -COOH tend to be underrepresented by CP MAS 13C NMR due to their longer relaxation times (T1, spin-lattice relaxation time). According to this observation, -COOH of the reconstituted waxes may be in rigid domains, corresponding to longer T1 (31, 33, 34). Interestingly, a pronounced peak at 167 ppm (-COOH)
appeared for reconstituted waxes with sorbed NAPH and PHEN, indicating that the rigid domains in reconstituted waxes changed (or melted) to more mobile domains, corresponding to the reduced T1 of 13COOH. The carboxyl carbon contents of reconstituted waxes, “without and with” sorbed PAHs, were 3.7% (wax-mont4), 6.2% (wax-mont4PHEN), 8.0% (wax-mont4-low NAPH), and 8.6% (waxmont4-high NAPH); the increasing sequence was consistent with the trend of melting capability of PAHs, i.e., NAPH > PHEN, and high PAH loading > low PAH loading. The two NMR peaks of 32 ppm (rigid domain: crystalline and solid amorphous) and 29 ppm (mobile amorphous domain) in paraffinic carbons (0-50 ppm) have been used to characterize the physical conformation of organic matter (16, 31, 34). In this study, the percentages of mobile amorphous paraffinic C (28-30.5 ppm) in total paraffinic components (0-50 ppm) (35), calculated to further describe the phase transition of reconstituted waxes during the sorption process, were 13.8% (wax-mont4), 13.5% (waxmont4-PYR), 14.6% (wax-mont4-PHEN), 14.4% (waxmont4-low NAPH), and 15.2% (wax-mont4-high NAPH). VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The NMR evidence (at 167 and 29 ppm) confirmed the physical phase transition (melting) of reconstituted waxes induced by sorbed PAH during sorption process. A new phase transition model for PAHs-waxes-mineral systems is proposed (Figure 4). In this model, reconstituted waxes consist of three domains: crystalline domains (CY), solid-amorphous domains (SA), and mobile-amorphous domains (MA) (16). Painter and Coleman (36) reported that semicrystalline polymer (solid-amorphous domains) chains exist in a crystalline-amorphous domain as folded polymer crystals. Because of interaction of the crystalline-amorphous complex of waxes with mineral surfaces (Figure 4A), the polar portions of the wax molecule are likely to displace water molecules from exchangeable Ca2+ and allow the aliphatic portions to be retained near the uncharged region on the clay surfaces by van der Waals interactions. At low wax loads, a large volume fraction of the wax molecules is likely to form an ordered structure near the clay surface. But as loading increases and the availability of an ordered template (the clay surface) diminishes, more and more of the wax molecules assume the “amorphous” conformation. Thus, the first several molecular layers of the mobile amorphous region may rearrange to take a more condensed form (27). This condensed region can cause enhanced nonlinear sorption, but both condensed and expanded (mobile) amorphous aliphatic regions will contribute to PAH sorption (27, 32). Studies using synthetic polymer membranes, composed of amorphous and crystalline regions, have shown that crystalline domains are not accessible to solutes, and solute diffusion (sorption) is limited to amorphous regions (5, 30). For cuticular waxes, solute diffusion occurred in the amorphous wax domain (6). Therefore, the phase transitions during the sorption process of PAHs were thought to be limited to the solid (rigid) amorphous domains of reconstituted waxes, i.e., transition from solid-amorphous domains to mobile-amorphous domains (Figure 4B). This phase transition did not happen to PYR, with the largest molecular size among the tested PAHs, but appeared at high loading of PHEN and NAPH on reconstituted waxes. The phase-transition intensity increased with decreasing of molecular size, i.e., PYR PHEN >> NAPH) and depended on wax loads on the montmorillonite. The Sm parameter was negatively related to the melting ability of sorbates. For the three tested PAHs, naphthalene with the smallest molecule volume expressed the strongest melting ability and then led to the lowest Sm value. It was reported that the Sm value of PHEN to highly condensed kerogen was in the range of 20-100 mg/g (26), which was 3 orders of magnitude higher than the Sm range (15-100 mg/kg) of PHEN to reconstituted waxes in this study. The hysteresis of PAHs with the reconstituted waxes could be well explained by the proposed phase transition model. In our current study, for PYR and for NAPH and PHEN at low loading, matrix rearrangement of reconstituted waxes did not occur (Figure 4B). Sorbing molecules can diffuse freely 8322
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into and out of highly mobile amorphous regions (MA), which act as liquidlike partitioning phases for PAHs, exhibiting correspondingly linear isotherms and no hysteretic sorption (22). At high PAH loading, sorption of NAPH and PHEN occurred in both original and transformed mobile-amorphous domains (OMA and TMA, Figure 4C). The transformed mobile-amorphous domains (TMA) served as liquidlike partitioning phases similar to original mobile-amorphous domains (OMA), suggested by the almost linear sorption isotherms in the whole concentration range, including low and high concentrations. This explanation is consistent with reported observations (25, 37) that partitioning becomes a more significant contributor to the overall sorption behavior of a specific target sorbate when cosorbates are present to swell SOM matrixes. When the aqueous solute concentration was decreased sharply by dilution, sorbed NAPH and PHEN in the transformed-mobile-amorphous domains were “trapped” by collapsed matrix, particularly in solidamorphous domains (Figure 4D), resulting in desorption hysteresis. The sorption and desorption follow different mechanistic pathways due to sorbent matrix deformation (rearrangement) (24). In summary, this study examined sorption-desorption of PAHs and conformation of reconstituted plant cuticular waxes, which greatly contribute to recalcitrant aliphatic fractions of SOM. Sorption capacity (Koc) referred to organic C of the waxes and depended on the lipophilicity of PAHs due to partition-like mechanisms of original and/or transformed mobile amorphous domains. Desorption hysteresis was due to the “trapping” effect of transformed mobile amorphous domains for sorbed PAH, thus regulated by PAH molecular sizes rather than diffusion of solute in desorption process. The phase transition from solid-amorphous to mobile-amorphous domains was identified by solid-state 13C NMR of reconstituted waxes in the presence and absence of sorbed PAH. A new phase transition model for mineralwaxes-PAHs interaction systems was proposed to explain the sorption-desorption behavior of PAHs with reconstituted waxes on montmorillonite. It may be useful to advance the understanding of soil humin, which is highly aliphatic in nature (waxlike materials).
Acknowledgments This project was supported in part by Research Grant No. IS-3385-03R from BARD, the United States-Israel Binational Agricultural Research and Development Fund, the CSREES, USDA National Research Initiative Competitive Grants Program (2005-35107-15278), and the Federal Hatch Program (Project No. MAS00860). B.C. also thanks the National Natural Science Foundation of China (NSFC) (20207007) for support.
Supporting Information Available Isotherm confidence interval (95%) and all of data points for NAPH (A), PHEN (B), and PYR (C) sorption and desorption by wax-mont4. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review May 2, 2005. Revised manuscript received July 26, 2005. Accepted August 10, 2005. ES050840J
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