Sorption and conformational characteristics of reconstituted plant cuticular waxes on montmorillonite

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 ofthe 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 low-load wax-montmorillonite sample. The organic-carbon-normalized 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 solid-state 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.     

Black carbon and kerogen in soils and sediments. 2. Their roles in equilibrium sorption of less-polar organic pollutants

The first paper of this series reported that soil/sediment organic matter (SOM) can be fractionated into four fractions with a combined wet chemical procedure and that kerogen and black carbon (BC) are major SOM components in soil/sediment samples collected from the industrialized suburban areas of Guangzhou, China. The goal of this study was to determine the sorptive properties forthe four SOM fractions for organic contaminants. Sorption isotherms were measured with a batch technique using phenanthrene and naphthalene as the sorbates and four original and four Soxhlet-extracted soil/sediment samples, 15 isolated SOM fractions, and a char as the sorbents. The results showed that the sorption isotherms measured for all the sorbents were variously nonlinear. The isolated humic acid (HA) exhibited significantly nonlinear sorption, but its contribution to the overall isotherm nonlinearity and sorption capacity of the original soil was insignificant because of its low content in the tested soils and sediments. The particulate kerogen and black carbon (KB) fractions exhibited more nonlinear sorption with much higher organic carbon-normalized capacities for both sorbates. They dominate the observed overall sorption by the tested soils and sediments and are expected to be the most important soil components affecting bioavailability and ultimate fate of hydrophobic organic contaminants (HOCs). The fact that the isolated KB fractions exhibited much higher sorption capacities than when they were associated with soil/sediment matrixes suggested that a large fraction of the particulate kerogen and BC was not accessible to sorbing HOCs. Encapsulation within soil aggregates and surface coverage by inorganic and organic coatings may have caused large variations in the accessibility of fine kerogen and BC particles to HOCs and hence lowered the sorption capacity of the soil. This variability posts an ultimate challenge for precisely predicting HOC sorption by soils from the contents of different types of SOM.     

Sorption of phenanthrene by nonhydrolyzable organic matter from different size sediments

Nonhydrolyzable organic carbon (NHC) and sorption isotherms of phenanthrene (Phen) on six size-fractionated NHC fractions in two sediments from the Pearl River and Estuary, South China, were investigated. It was found that NHC including ancient organic carbon, black carbon, resistant aquatic organic carbon, and aged soil organic carbon consists mainly of aliphatic and aromatic carbon using 13C nuclear magnetic resonance spectroscopy. The sorption isotherms of Phen by the size-fractionated NHC fractions are nonlinear and are well-fitted to the Freundlich model. For the estuary sediment, the NHC contents and the organic carbon-normalized distribution coefficients (Koc) in the size fractions increase with decreasing particle size. The clay NHC fraction contributes to 70% of the Phen sorption by the bulk NHC isolate. However, for the contaminated river sediment, the NHC contents and the Koc values exhibit no regular variations among the size fractions. The Phen sorption capacities on the size-fractionated NHC fractions of the two sediments are significantly related to their H/C ratios and aliphatic carbon, but negatively to aromatic carbon. The fine-particle NHC fractions with high aliphatic carbon and H/C ratio play a very important role in the sorption, transport, and fate of Phen by the investigated sediments.     

Sorption of polar and nonpolar aromatic organic contaminants by plant cuticular materials: role of polarity and accessibility

In both forest and agricultural soils, plant derived cuticular materials can constitute a significant part of soil organic matter. In this study, the sorption of nonpolar (naphthalene and phenanthrene) and polar (phenol and 1-naphthol) aromatic organic pollutants to aliphatic-rich cuticularfractions of green pepper (Capsicum annuum) (i.e., bulk (PC1), dewaxed (PC2), nonsaponifiable (PC3), nonsaponifiable-nonhydrolyzable (PC4), and dewaxed-hydrolyzed residue (PC5)) were examined to better understand the influence of polarity and accessibility on their sorption behavior. The polarity and structures of cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy, and solid-state 13C NMR. The sorption isotherms fit well to the Freundlich equation. Sorption of the tested organic compounds to PC4, which had more condensed domains, was nonlinear (Freundlich N(s) values of 0.766-0.966). For naphthalene and phenanthrene, the largest sorption capacity (K(oc)) occurred in PC5, which contained the highest paraffinic carbons (63%) and the lowest polarity: approximately 2 and aproximately 3 times higher than the respective carbon-normalized octanol-water partition coefficient (K(owc)), indicating that PC5 was a powerful sorption medium. For phenol and 1-naphthol, the largest K(oc) values occurred in PC4 with polar aromatic cores: approximattely 17 and approximately 7 times higher than the respective K(owc), suggesting that PC4 was much more accessible and compatible to polar aromatic pollutants than nonpolar aromatic pollutants. There was little or no correlation of K(oc) with either aliphatic or aromatic components of the tested aliphatic-rich sorbents because the polarity and accessibility apparently played a regulating role in the sorption of organic contaminants.     

Importance of black carbon to sorption of native PAHs, PCBs, and PCDDs in Boston and New York harbor sediments

The solid-water distribution ratios (Kd values) of "native" PAHs, PCBs, and PCDDs in Boston and New York Harbor sediments were determined using small passive polyethylene samplers incubated for extended times in sediment-water suspensions. Observed solid-water distribution coefficients exceeded the corresponding f(oc)Koc products by 1-2 orders of magnitude. It was hypothesized that black carbon (fBC), measured in the Boston harbor sediment at about 0.6% and in the New York harbor sediment at about 0.3%, was responsible for the additional sorption. The overall partitioning was then attributed to absorption into the organic carbon and to adsorption onto the black carbon via Kd = f(oc)Koc + f(BC)K(BC)C(w)n-1 with Cw in microg/L. Predictions based on published Koc, K(BC), and n values for phenanthrene and pyrene showed good agreement with observed Kd,obs values. Thus, assuming this dual sorption model applied to the other native PAHs, PCBs, and PCDDs, black carbon-normalized adsorption coefficients, K(BC)S, were deduced forthese contaminants. Log K(BC) values correlated with sorbate hydrophobicity for PAHs in Boston harbor (log K(BC) approximately 0.83 log gamma w(sat) - 1.6; R2 = 0.99, N= 8). The inferred sorption to the sedimentary BC phase dominated the solid-water partitioning of these compound classes, and its inclusion in these sediments is necessary to make accurate estimates of the mobility and bioavailability of PAHs, PCBs, and PCDDs.     

Chemical extractions affect the structure and phenanthrene sorption of soil humin

Humin is a major fraction of soil organic matter and strongly affects the sorption behavior and fate of organic contaminants in soils and sediments. This study evaluated four different extraction methods for soil humins in terms of their organic carbon structural changes and the consequent effects on phenanthrene sorption. Solid-state 13C NMR demonstrated that 0.1 M NaOH exhaustively extracted humin and humin extracted with 6 M HF/HCl at 60 degrees C had a relatively high amount of aliphatic components as compared with 1 M HF-extracted humin. The treatment of 6 M HF/HCl at 60 degrees C greatly reduced carbohydrate components (50-108 ppm) from humin samples, i.e., more than 50% reduction. In addition, the humin from this 6 M HF/HCl treatment contained relatively more amorphous poly(methylene) domains than the humins extracted by other methods. With the respect to phenanthrene sorption, the linearity of sorption isotherm (N) and sorption affinity (Koc) varied markedly among the humin samples extracted by different methods. The NaOH exhaustively extracted humin had the most nonlinear sorption isotherm and the HF-extracted humin had the lowest Koc. It is concluded that humin samples from different extraction procedures exhibited substantial differences in their organic carbon structure and sorption characteristics, even though they were from the same soil. Therefore, one needs to be cautious when comparing the structural and sorption features of soil humins, especially when they are extracted differently. The 6 M HCl/HF extraction at elevated temperature is not encouraged, due to the modifications of chemical structure and physical conformation of organic matter.     

Thermal analytical investigation of biopolymers and humic- and carbonaceous-based soil and sediment organic matter

Improved understanding of the physical, chemical, and thermodynamic properties of soil and sediment organic matter (SOM) is crucial in elucidating sorption mechanisms of hydrophobic organic compounds (HOCs) in soils and sediments. In this study, several thermoanalytical techniques, including thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), temperature-modulated differential scanning calorimetry (TMDSC), and thermal mechanical analysis (TMA) were applied to 13 different organic materials (three woods, two humic acids, three kerogens, and five black carbons) representing a spectrum of diagenetic and/or thermal histories. Second-order thermal transition temperatures (T(t)) were identified in most materials, where the highest observed T(t) values (typically characterized as glass transition temperatures (T(g were shown to closely relate to chemical characteristics of the organic samples as influenced by diagenetic or thermal alteration. Results further suggest a positive correlation between glass transition temperature and a defined diagenetic/thermal index, where humic-based SOM (e.g., humic and fulvic acids) possess lowertransition temperatures than more "mature" carbonaceous-based SOM (i.e., kerogens and black carbons). The observed higher thermal transition temperature of aliphatic-rich Green River shale kerogen (approximately 120 degrees C) relative to that of aromatic-rich humic acids suggests that a sole correlation of aromaticity to thermal transition temperature may be inappropriate.     

Investigation of the role of structural domains identified in sedimentary organic matter in the sorption of hydrophobic organic compounds

The role of composition and structure of sedimentary organic matter (SOM) in the sorption of hydrophobic organic compounds (HOCs) was investigated by spiking 13C-labeled phenanthrene onto six estuarine sediments known to vary in SOM content and character. After equilibration and HF treatment, 13C NMR cross polarization and stable carbon isotope analyses indicated that the amount of desorption-resistant phenanthrene was related to aromatic carbon content. Application of the 13C NMR spectral editing technique proton spin relaxation editing (PSRE) demonstrated that all samples consisted of a rapidly relaxing and a slowly relaxing component, further evidence that SOM can be described as a structurally heterogeneous sorbent. Further, comparison of corresponding control and spiked PSRE subspectra revealed that, for each of the six sediments, desorption-resistant phenanthrene had become associated almost exclusively with the rapidly relaxing component. In only two of the sediments were there even small amounts of phenanthrene discernible in the slowly relaxing component, which is signficant as it was not always true that aromatic carbon was concentrated exclusively in the rapidly relaxing phase. The implication of these findings is that not all aromatic fractions have the same affinity for phenanthrene and that some fractions may indeed have little affinity at all. These results were interpreted as indicative that rapidly relaxing aromatic carbon associated with either sediment-associated charcoal or diagenetic organic matter plays a controlling role in the sorption of HOCs. However, the exact manner in which this rapidly relaxing aromatic phase relates to models presented elsewhere remains unclear.     

The nature of soil organic matter affects sorption of pesticides. 1. Relationships with carbon chemistry as determined by 13C CPMAS NMR spectroscopy

The structural composition of soil organic matter (SOM) was determined in twenty-seven soils with different vegetation from several ecological zones of Australia and Pakistan using solid-state CPMAS 13C NMR. The SOM was characterized using carbon types derived from the NMR spectra. Relationships were determined between Koc (sorption per unit organic C) of carbaryl(1-naphthylmethylcarbamate) and phosalone (S-6-chloro-2,3-dihydro-2-oxobenzoxazol-3-ylmethyl O,O-diethyl phosphorodithioate) and the nature of organic matter in the soils. Substantial variations were revealed in the structural composition of organic matter in the soils studied. The variations in Koc values of the pesticides observed for the soils could be explained only when variations in the aromatic components of SOM were taken into consideration. The highly significant positive correlations of aromaticity of SOM and Koc values of carbaryl and phosalone revealed that the aromatic component of SOM is a good predictor of a soil's ability to bind such nonionic pesticides.     

Sorption of PAHs by aspen wood fibers as affected by chemical alterations

Sorption and desorption experiments for phenanthrene and pyrene, using untreated (UTR) and treated (bleaching and hydrolysis) aspen wood fibers, were examined to understand their sorption mechanisms. The wood was characterized by elemental and porosity analysis, solid-state 13C NMR, and diffuse reflectance infrared Fourier transform spectroscopy. Bleaching removed aromatic components, yielding the highest polarity and increased porosity, whereas hydrolysis removed a large percentage of hemicellulose and parts of amorphous cellulose, producing a matrix with more aromatic moieties, lower polarity, and higher porosity than that of the UTR wood fibers. All isotherms fitted well to the Freundlich equation and the N values had a decreasing trend from bleached (BL), UTR, low-temperature hydrolyzed to high-temperature hydrolyzed (HHY) wood fibers. BL fibers had the lowest sorption capacity (Koc) for both phenanthrene and pyrene. HHY had the highest Koc because of its high aromatic carbon content and low polarity. The results suggest that aromatic moieties and polarity of wood fibers play significant roles in polycyclic aromatic hydrocarbon (PAHs) sorption and desorption. Thus, both aromatic components and polarity should be considered when predicting the PAHs sorption/desorption by aspen wood fibers. This study demonstrated that aspen wood fibers are a potential sorbentfor PAHs and that chemical modifications of the wood matrix can effectively increase its sorption efficiency. These results may have implications for the treatment of stormwater runoff and other PAH-contaminated liquids.     

Strong sorption of native PAHs to pyrogenic and unburned carbonaceous geosorbents in sediments

It has recently been shown that the presence of carbonaceous geosorbents (CG, including black carbon (BC), unburned coal, and kerogen) can cause strong sorption of polycyclic aromatic hydrocarbons (PAHs) in sediments. We studied sorption of native PAHs in four Norwegian harbor sediments of which high fractions (21-56%) of the total organic carbon (TOC) consisted of CG carbon (CGC), as shown by organic petrography. PAH sorption coefficients were 1-2 orders of magnitude above predictions based on amorphous organic carbon partitioning alone. In recent studies, such strong sorption was attributed solely to BC sorption under the implicit assumption that sorption is linear for coal and kerogen. The most important result of the present study is that total sorption is better explained by considering all three nonlinearly sorbing CGC materials than by only considering BC. In addition, it was evaluated whether activated carbon (AC) amendments could be effective in reducing the freely dissolved pore-water concentrations (CW) and thus the environmental risks of the PAHs in such strongly sorbing sediments. The results indicated that an addition of 2 weight % AC reduced the Cw by factors of 21-153 for the four sediments (average values for all PAHs). Itwas shown that phenanthrene sorption to AC was, on average, reduced by a factor of 6 in sediment-AC mixtures compared to pure AC.     

Strong sorption of phenanthrene by condensed organic matter in soils and sediments

The nonhydrolyzable carbon (NHC) and black carbon (BC) in three contaminated soils and seven sediments from the Pearl River Delta and Estuary, China, were isolated upon treatments with an acid hydrolysis method and with a combustion method at 375 degrees C, respectively, and their sorption isotherms for phenanthrene (Phen) were established. It was found that NHC is chemically and structurally different from the biopolymer and humic substances and consists mainly of aliphatic and aromatic carbon using elemental analysis, 13C nuclear magnetic resonance spectroscopy (13C NMR), and Fourier transformed infrared spectroscopy (FTIR). All the sorption isotherms are nonlinear and are well fitted by the Freundlich model. The single-point organic carbon-normalized distribution coefficient (K(oc)) measured for the isolated NHC is 1.3-7.7 times higher than that for the bulk samples at the same aqueous concentration of Phen. The NHC fractions play a dominant role to the overall sorption in the bulk samples. The bulk soils and their NHC fractions have lower sorption capacity than the bulk sediments and their NHC fractions, relating to the different source of organic matter between soils and sediments. The Phen sorption capacity in the NHC samples is related significantlyto H/C ratios and aliphatic carbon, but negatively to aromatic carbon, demonstrating the important role of aliphatic carbon to the Phen sorption and the fate in the investigated soils and sediments.     

Variation in phenanthrene sorption coefficients with soil organic matter fractionation: the result of structure or conformation?

Sorption of phenanthrene to varying soil types was investigated to better understand sorption processes. Humic acid and humin fractions were isolated from each soil sample, and sorption coefficients were measured by batch equilibration. Samples were characterized by carbon analysis and 13C cross polarization magic angle spinning (CP/ MAS) nuclear magnetic resonance (NMR) spectroscopy. Measured organic carbon-normalized sorption coefficients (Koc) of the fractions were greater in all cases when compared to the soils. The humin fractions exhibited greater Koc values than did source samples, suggesting that fractionation may reorganize organic matter in humin resulting in an increased availability of and/or more favorable sorption domains. Mass balance calculations revealed that the sum of sorption to the fractions is greater than sorption to the whole sample. The greatest difference between sorption values was found to occur with the mineral soils, suggesting that clay minerals influence the physical conformation of soil organic matter (SOM) and availability of sorption domains. The mass balance, sorption data, and a lack of consistent trends between observed Kco values and solid-state 13C NMR data suggest that the physical conformation of SOM and chemical characteristics both play important roles in sorption processes.     

Insights into the sorption properties of cutin and cutan biopolymers

Plant cuticles have been reported as highly efficient sorbents for organic compounds. The objective of this study was to elucidate the sorption and desorption behavior of polar and nonpolar organic compounds with the major structural components of the plant cuticle: the biopolymers cutin and cutan. The sorption affinity values of the studied compounds followed the order: phenanthrene > atrazine > chlorotoluron > carbamazepine. A higher sorption affinity of phenanthrene and atrazine to cutin was probably due to the higher level of amorphous paraffinic carbon in this biopolymer. Phenanthrene exhibited reversible sorption behavior and a high ratio of organic-carbon-normalized distribution coefficient (Koc) to carbon-normalized octanol-water partitioning coefficients (Kowc) with both biopolymers. This suggests that both biopolymers provide phenanthrene with a partition medium for hydrophobic interactions with the flexible long alkyl-chain moieties of the biopolymers. The low Koc/Kowc ratios obtained for the polar sorbates suggest that the polar sites in the biopolymers are not accessible for sorption interactions. Atrazine and carbamazepine exhibited sorption-desorption hysteresis with both sorbents, indicating that both sorbates interact with cutin and cutan via both hydrophobic and specific interactions. In general, the sorptive properties of the studied biopolymers were similar, signifying that the active sorption sites are similar even though the biopolymers exhibit different properties.     

Importance of structural makeup of biopolymers for organic contaminant sorption

Sorption of pyrene, phenanthrene, naphthalene, and 1-naphthol by original (lignin, chitin, and cellulose) and coated biopolymers was examined. Organic carbon normalized distribution coefficients (Koc) of all compounds by the original biopolymers followed the order lignin > chitin > cellulose, in line with the order of their hydrophobicity. Hydrophobicity of structurally similar organic compounds is the main factor determining their ability to occupy sorption sites in biopolymers. Specific interactions (e.g., H-bonding) between 1-naphthol and chitin or cellulose increased its ability to occupy sorption sites. Lignin coating resulted in an increased Koc for phenanthrene (13.6 times for chitin and 6.9 times for cellulose) and 1-naphthol (6.0 times for chitin and 3.7 times for cellulose) relative to the acetone-treated chitin and cellulose. Also, these coated biopolymers had increased isotherm nonlinearity, due to the newly formed condensed domains. An increase in phenanthrene and 1-naphthol sorption by lignin-coated biopolymers as compared to chitin and cellulose was contributed by the newly created high-energy sites in condensed domains and coated lignin. Results of this study highlight the importance of the structural makeup of biopolymers in controlling the sorption of hydrophobic organic compounds.     

Phenanthrene sorption to soil humic acid and different humin fractions

This study was undertaken to provide an insight into the effect of heterogeneous soil organic matter (SOM) on the sorption of phenanthrene. Humic acid (HA) and humin were extracted from a peat soil. Humin was further fractionated into bound-humic acid (BHA), lipid, and insoluble residue (IR) fractions. Heterogeneous natures of these fractions were characterized by elemental analysis, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, and solid-state 13C NMR. Aliphaticity of the fractions followed the order lipid >BHA > HA > IR, while the polarity order was IR > BHA> HA > lipid. Sorption of phenanthrene on these fractions fitted the Freundlich equation, suggesting that phenanthrene sorption isotherms of lipid were almost linear (N = 0.993), while those of HA, BHA, and IR were nonlinear, with N values ranging from 0.723 to 0.910. The N values followed the order lipid > HA > BHA > IR and were significantly correlated inversely with their polarities (p < 0.05). Organic carbon-normalized sorption coefficients (K(FOC)) were independent of aliphatic or aromatic contents of the SOM fractions. The results suggested that SOM, especially for the humin fractions, was highly heterogeneous in terms of elemental composition, structure, and polarity. Such heterogeneity was considered to be responsible forthe nonlinear sorption of phenanthrene.     

Sorption mechanisms of phenanthrene, lindane, and atrazine with various humic acid fractions from a single soil sample

The sorption behavior of organic compounds (phenanthrene, lindane, and atrazine) to sequentially extracted humic acids and humin from a peat soil was examined. The elemental composition, XPS and (13)C NMR data of sorbents combined with sorption isotherm data of the tested compounds show that nonspecific interactions govern sorption of phenanthrene and lindane by humic substances. Their sorption is dependent on surface and bulk alkyl carbon contents of the sorbents, rather than aromatic carbon. Sorption of atrazine by these sorbents, however, is regulated by polar interactions (e.g., hydrogen bonding). Carboxylic and phenolic moieties are key components for H-bonding formation. Thermal analysis reveals that sorption of apolar (i.e., phenanthrene and lindane) and polar (i.e., atrazine) compounds by humic substances exhibit dissimilar relationships with condensation and thermal stability of sorption domains, emphasizing the major influence of domain spatial arrangement on sorption of organic compounds with distinct polarity. Results of pH-dependent sorption indicate that reduction in sorption of atrazine by the tested sorbents is more evident than phenanthrene with increasing pH, supporting the dependence of organic compound sorption on its polarity and structure. This study highlights the different interaction mechanisms of apolar and polar organic compounds with humic substances.     

Sorption of polycyclic aromatic hydrocarbons to oil contaminated sediment: unresolved complex?

Oil is ubiquitous in aquatic sediments and may affect partitioning and bioavailability of hydrophobic organic chemicals (HOCs). In contrast to other sedimentary hydrophobic carbon phases (natural organic matter, soot-like materials), oil residues have hardly received any attention as far as it concerns effects on HOC sorption. This paper describes experimental work dealing with such effects of oil on polycyclic aromatic hydrocarbon (PAH) sorption to sediments. Three different oils were spiked to a marine sediment in concentrations between 0 and 100 g/kg. Sediment-water distribution coefficients (Kd) for six deuterated PAHs were then determined either directly after spiking the oil or after a semi-natural weathering process in the lab (lasting for more than 2 yr). Resulting Kd values demonstrated sorption-reducing (competitive) effects at relatively low oil concentrations and sorption-enhancing effects at high oil concentrations. The latter effects only occurred above a certain threshold [i.e., ca. 15% (w/w) of oil on a sedimentary organic carbon basis] marking the oil concentration at which the hydrocarbon mixture presumably starts forming separate phases. Assuming a two-domain (organic carbon + oil) distribution model, oil-water distribution coefficients (K(oil)) for PAHs were estimated. For fresh oils, log K(oil) values appeared to be very similar for different types of oils, proportional to log K(OW) values and indistinguishable from log K(OC) values. For weathered oils, K(oil) values were also rather independent of the type of oil, but the affinity of low molecular weight PAHs for weathered oil residues appeared to be extremely high, even higher than values reported for most types of soot. Because affinities of high molecular weight PAHs for oils had not changed upon weathering, sorption of all PAHs studied (comprising a log K(OW) range of 4.6-6.9) to the weathered oil residues appeared to be more or less constant (averaged log K(oil) = 7.0 +/- 0.24). These results demonstrate that it is crucial to take the presence of oil and its weathering state into account when assessing the actual fate of PAHs in aquatic environments.     

Investigating the role of mineral-bound humic acid in phenanthrene sorption

Contaminant-soil interaction studies have indicated that physical conformation of organic matter atthe solid-aqueous interface is important in governing hydrophobic organic compound (HOC) sorption. To testthis, organo-clay complexes were constructed by coating montmorillonite and kaolinite with peat humic acid (PHA) in Na+ or Ca2+ dominated solutions with varying pH and ionic strength values. The solution conditions encouraged the dissolved PHA to adopt a "coiled" or "stretched" conformation prior to interacting with the clay mineral surface. Both kaolinite and montmorillonite organo-clay complexes exhibited higher phenanthrene sorption (Koc values) with decreasing pH, indicating that the coiled configuration provided more favorable sorption conditions. Evidence from 1H high-resolution magic angle spinning (HR-MAS) nuclear magnetic resonance (NMR) indicated that polymethylene groups were prevalent at the surface of the organo-clay complexes and may enhance sorptive interactions. Preferential sorption of polymethylene groups on kaolinite and aromatic compounds on montmorillonite may also contribute to the difference in phenanthrene sorption by PHA associated with these two types of clay. This study demonstrates the importance of solution conditions in the sorption of nonionic, hydrophobic organic contaminants and also provides evidence for the indirect role of clay minerals in sorption of contaminants at the soil-water interface.