Soil/sediment Organic Matter Research Articles

Chemometric Optimization of a Demineralization Method for Analysing Sediment Organic Matter by Fourier Transform Infrared Spectroscopy

Fourier transform infrared (FTIR) spectroscopy is a widespread and useful technique for characterizing the chemical composition of soil/sediment organic matter (SOM). However, the mineral components of the samples limit spectral analysis since they mask SOM’s distinctive signals. Big efforts have been made in this matter to eliminate the spectral interferences of mineral constituents of the samples, by demineralization with hydrofluoric acid (DHF), or spectral subtraction. This work aimed to upgrade reported DHF methods and optimize a sample preparation methodology for SOM characterization by FTIR spectroscopy combining the potential of spectral and multivariate analysis. Semi-quantitative FTIR ratios were introduced as input variables for principal component analysis to determine the optimal DFH conditions to characterize SOM. One-way analysis of variance (ANOVA, α = 0.05) test revealed that the optimal demineralization was achieved with two cycles of HF 10% v/v at 60 °C, against five HF cycles and room temperature, previously reported. The DHF at the optimal conditions not only improved the demineralization efficiency against that of the reported methods but also did not cause significant undesirable modifications in SOM that could distort the FTIR analysis. These conclusions have statistical significance and are based on the qualitative information of the spectra and also on a comprehensive semi-quantitative study of them.

Assessment of As, Cd, Zn, Cu and Pb Pollution and Toxicity in River Wetland Sediments and Artificial Wetland Soils Affected by Urbanization in a Chinese Delta

The concentrations of As, Cd, Zn, Cu and Pb were analyzed to assess their pollution levels and toxic risks in different wetlands affected by urbanization. The results showed that As and heavy metals (especially Cd) showed higher concentrations in urban and rural river wetland sediments compared with artificial wetland soils. Comparatively, urban river wetland sediments contained higher Cu, Pb and Zn and lower As and Cd levels than rural river wetland sediments and artificial wetland soils. Cu, Pb, Zn, soil/sediment organic matter (SOM) and total phosphorous (TP) were all positively correlated with each other. The highest geoaccumulation indexes and pollution levels were observed in urban river wetlands, followed by rural river wetlands, while artificial wetlands showed the lowest values. In urban and rural river wetlands, the enrichment factor (EF) values for As and heavy metals exceeded 1.5, implying that they might be mostly from anthropogenic sources. In artificial wetlands, the EF values suggested that only Cd was mainly from anthropogenic sources. The sum of toxic units (ΣTUs) values showed that As and heavy metals in both river wetland sediments had moderate toxic risks with ΣTUs values exceeding 4, while a low toxicity risk was observed in artificial wetland soils.

Seasonal and spatial variations in diversity and abundance of bacterial laccase-like genes in sediments of a subtropical mangrove ecosystem

Laccases, involved in degradation of soil/sediment organic matter (SOM), play an important role in C and nutrient cycling in ecosystems. Laccases of fungal origins have been extensively studied, but only few reports are available for bacterial laccase-like genes, especially in coastal mangrove ecosystems. Therefore, we investigated the seasonal diversity and abundance of bacterial laccase-like genes in coastal mangrove sediments, and then compared to other ecosystems including Forest, Grassland, Fen and Bog. Additionally, the correlations between environmental factors and bacterial laccase-like communities were analyzed. The results demonstrated that the diversity and abundance of bacterial laccase-like genes in this mangrove ecosystem largely varied in sediment types and sampling seasons, and the microbial communities inferred by the functional genes included Proteobacteria, Actinobacteria, Bacteroidetes and some Unknown species as the major groups. In comparison, bacterial laccase-like communities in mangrove sediments were significantly different with the referenced ecosystems. The variations of total contents of C and N in soils/sediments are proposed to be the primary reasons for the differences of bacterial laccase-like genes among ecosystems. Moreover, the diversity index was negatively correlated with C:P and N:P, while the abundance showed a positive/negative correlation with water content and redox potential in this mangrove ecosystem, indicating a significant impact of other environmental factors, such as P and redox potential, on the diversity, distribution and the abundance of bacterial laccase-like genes which were underestimated in the previous research. Taking together, hereby P is proposed as a regulatory and limiting element for microbial SOM degradation through the laccases-initiated enzymatic reaction.

Amplified solubilization effects of inherent dissolved organic matter releasing from less-humified sediment on phenanthrene sorption.

Soil/sediment organic matter (SOM) releasing with inherent dissolved organic matter (DOM) formed in solution was confirmed both in rhizosphere sediment (S) and uncultivated sediment (P) water systems, and correlations between SOM characteristics subject to sediment's humification degree and its releasing effects on phenanthrene sorption were emphasized. The sequential SOM releasing evidenced by fluorescence and (1)H-NMR profiles coupled with aqueous DOM solubilization was found to make sorption kinetics atypical and sorption capacity reduced, by comparing sorption results among sediments of different pretreatments. More importantly, the tested S was proved less humified with inherent DOM rich in microbial sources than P, and DOM affinity to phenanthrene was thus weakened (K doc values of 2.02-3.63 × 10(4) L kg(-1)), while the inhibitive effects of SOM releasing on sorption were strengthened, ascribing to the enlarged alterations of sediment characters, and particularly the amplified solubilization effects resulted from the larger proportion of soluble SOM and lower critical micelle concentration (5.66 mg L(-1)) of DOM. Moreover, relative contribution of DOM solubilization to the releasing effects enhanced from 0.67 for P to 0.78 for S relative to alterations of sediment characters. Consequently, mobility and exposure risk of polycyclic aromatic hydrocarbons would be enhanced in a plant-soil/sediment-water system.

Sorption of endocrine disrupting chemicals by condensed organic matter in soils and sediments

Sorption of 17α-ethinyl estradiol (EE2) and bisphenol A (BPA) by nonhydrolyzable carbon (NHC), black carbon (BC), and bulk soils and sediments was examined. All sorption isotherms were nonlinear and fitted both Freundlich and Dubinin–Ashtakhov (DA) models. The single-point organic carbon (OC)-normalized distribution coefficient ( K OC) of EE2 for the isolated NHC and BC was 2.7–4.8 times and 5.4–12.9 times greater, respectively, than that of the bulk samples. However, no clear trend in BPA K OC values was observed. Based on the contribution of soil/sediment organic matter (SOM) fractions to the overall sorption of BPA or EE2 by the bulk samples, condensed SOM (NHC and BC) generally played a dominant role to the overall sorption. The BPA adsorption capacity ( Q OC 0 ) from the DA model was higher than that of EE2 on NHC and there was obvious difference in isotherm nonlinearity ( n) between EE2 and BPA. These results suggest that BPA may have more access to the pore sites of NHC samples than EE2. The π– π bonds formed between BPA and NHC or BC may be stronger than that between EE2 and NHC or BC. This would be attributed to the fact that BPA has two benzene rings, and can also be used to explain the difference in hexadecane-water partition coefficient ( K HW )-normalized K OC values ( K OC/ K HW ) of BPA and EE2 after factoring out the hydrophobic effect. These findings could be useful for predicting fate and ecological risks of endocrine disrupting chemicals (EDCs) (e.g., EE2 and BPA) in natural environments especially when soils or sediments become receptors for EDCs.

Part IV—sorption of hydrophobic organic contaminants

Behavior of hydrophobic organic contaminants (HOCs) in the environment has attracted research interest for more than three decades. It has been clearly concluded that humic substances (HSs), which are the main content of soil/sediment organic matter (SOM) and dissolved organic matter (DOM), controls the sorption of HOCs in soils/sediments. In order to predict the movement of HOCs in the environment, many studies have been conducted to relate HOCs sorption characteristics with HS chemical properties. However, no consensus has been reached on precisely what HS chemical properties regulate HOC sorption, indicating that other HS properties (besides chemical properties) may also control HOC-HS interactions. Increasing amounts of research reveal that SOM physical properties can affect the accessibility of HOCs to SOM sorption sites and thus are of great importance for altering HOC-SOM interactions. Therefore, different from the past reviews on HOCs sorption in soils/sediments, this current one emphasizes physical conformation of HSs for both solid and dissolved forms. SOM chemical properties such as aromatic content, aliphatic content, polarity, and molecular weight have all been reported to affect HOC sorption. No general model has been proposed to predict SOM sorption characteristics from any individual chemical properties. Physical conformations of both solid SOM and DOM are of great importance for altering HOC-SOM interactions. The terms of glassy and rubbery domains have been used to describe physical conformations of solid SOM, and efficiency of chemical oxidation and glassy-rubbery transition temperature are indirect methods to describe SOM rigidity. Various techniques and parameters have been employed to study DOM conformation, such as microscopic images, pyrene-probing hydrophobic region, surface tension, and zeta potential. However, DOM nonideal sorption properties are not properly regarded. HOC-DOM interactions are investigated using solubility enhancement, gas-phase partitioning, fluorescence quenching, and dialysis equilibration methods. The limitations of all the methods are discussed in this review. Relatively, a dialysis equilibration experiment is a better design to study the true HOC-DOM interactions. Physical conformation of SOM are of the same importance as, if not more important than, SOM chemical properties for HOC sorption. Although increasing amounts of research focus on SOM physical conformation regarding HOC sorption, proper mathematical description of its physical conformation and the relationship between SOM physical conformation and its sorption properties are still unclear. Quantitative characterization of SOM conformation regarding its sorption properties with HOCs is a topic worth of further research. The HOC-DOM interactions could not be adequately addressed because of the inappropriate research approach; thus, a reevaluation of HOC-DOM interactions is also required.

Enhanced Mobilization of Field Contaminated Soil-bound PAHs to the Aqueous Phase under Anaerobic Conditions

Although microbially-mediated redox environments can alter the characteristics of soil/sediment organic matter (SOM) and its interactions with persistent hydrophobic organic contaminants (HOCs) bound to soils and sediments, the nature of their effects has not been adequately addressed. In this study, a field soil collected from a manufacturer gas plant site and contaminated historically with creosotes was incubated under aerobic and anoxic/anaerobic conditions along with various amendments (extra carbon and enrichment minerals) for stimulating microbial activities. Anaerobic conditions stimulated significant fractions of bound polycyclic aromatic hydrocarbons (PAHs) encompassing naphthalene through benzo[g,h,i]perylene to be mobilized to the aqueous phase, leaving their aqueous phase concentrations far in excess of solubility (increases in their apparent aqueous phase concentrations by factors as high as 62.8 relative to their initial aqueous phase concentrations). Such effects became more evident for high molecular weight PAHs. Dissolved organic matter exhibiting a high affinity for PAHs was liberated from soils during the anaerobic soil incubations. Feasibility of this concept for field applications was evaluated with a lab-scale continuous flow system composed of an anaerobic soil column followed by an aerobic bioreactor inoculated with PAH-degrading microbes. High quantities of PAHs exceeding their aqueous solubilities were eluted from the anaerobic soil column and those mobilized PAHs were readily bioavailable in the secondary aerobic bioreactor. This study may offer a potential method for cost-effective and performance-efficient ex situ remediation technologies (or in situ if appropriate hydrological control available in the contaminated field site) and risk assessment for the HOC-contaminated soils/sediments.

Impacts of microbial redox conditions on the phase distribution of pyrene in soil–water systems

Variations in the soil/sediment organic matter (SOM)-hydrophobic organic contaminant (HOC) bindings upon microbially mediated redox conditions were examined. While the extractability of pyrene associated with soil declined after its biodegradation began during aerobic incubation, its variations were almost constant (±3.0–4.4%) during anoxic/anaerobic incubations. The dissolved organic matter released from the soil incubated under highly reduced conditions became more humified and aromatic, had a higher average molecular weight, and was more polydispersed compared to that obtained from oxic incubation, similar to the SOM alterations in the early stage of diagenesis (humification). The concentrations of pyrene in the aqueous phase increased significantly during the soil incubations under highly reduced conditions due to its favorable interaction with the altered DOM. Our results suggest that the microbially mediated redox conditions have significant impacts on SOM and should be considered for the transport, fate, bioavailability, and exposure risk of HOCs in the geo-environments.

Sorption of phenanthrene, naphthalene and o-xylene by soil organic matter fractions

Sorption of hydrophobic organic compounds (HOCs) is an important process for determining their fate and transport in soils and sediments. Soil/Sediment organic matter (SOM) is believed to be the dominant sorbent but the mechanisms are not thoroughly understood. In this study, we examined the sorption of phenanthrene, naphthalene and o-xylene by a mineral soil, its base-extracted soil (BE), humic acid (HA), and deashed humin (DH) using a batch equilibration technique. All isotherms were nonlinear and nonlinearity increases in the order: soil HA < soil deashed humin < soil < BE. Aliphatic-rich deashed humin (DH) exhibited the highest sorption capacity for HOCs while mineral associated humin (BE) had the strongest nonlinearity. The results indicate that aliphatic components of SOM contribute greatly to sorption of HOCs in soils and sediments, which needs to be accounted for in determining the fate and availability of HOCs in the environment.

Sorption and Displacement of Pyrene in Soils and Sediments

Sorption isotherms of pyrene on soils and sediments were examined to understand its sorption behavior. All systems examined exhibited nonlinear sorption. Sorption nonlinearity was found to be a function of the polarity index of soil/sediment organic matter (SOM), suggesting that the degree of condensation of SOM, characterized by its polarity index, was correlated with the sorption behavior of pyrene. The polarity index of SOM could be a new factor for explaining the sorption nonlinearity. The sorption affinity of two soils and two sediments for pyrene increased with decreasing SOM polarity. A higher sorption affinity in the two soils was associated with a higher degree of condensation of SOM compared to that of the two sediments. A displacement test was performed after pyrene sorption using phenanthrene as a displacer. Pyrene was displaced in all systems examined, and nonlinearity became less pronounced after displacement. Such an increase in isotherm linearity implied that sorption site energies became more homogeneous after displacement. Furthermore, the site energy distribution IE*) derived from the Freundlich model parameters showed that energy reduction of high-energy sites was more significant than that of low-energy sites after displacement. In addition, a decrease in sorption capacity after displacement could be ascribed to the partial depletion of sorption sites by the displacer. The displacement data indicated that the cocontaminant can have potential effects on the fate and bioavailability of anthropogenic organic pollutants sorbed in soils and sediments, thus affecting their exposure risks.

Polyparameter Linear Free Energy Relationships for Estimating the Equilibrium Partition of Organic Compounds between Water and the Natural Organic Matter in Soils and Sediments

Values of the organic-carbon-based partition coefficient (Koc) have often been estimated using one-parameter linear free energy relationships (op-LFERs), which include both correlations between log Koc and log Kow, where Kow is the octanol-water partition coefficient, and op-LFERs that are based on first-order molecular connectivity indices. For chemicals with tendencies toward strong hydrogen-bonding or other specific interactions with the organic phase, however,these methods are notsufficientlyaccurate. Polyparameter LFERs (pp-LFERs) address these shortcomings by explicitly considering contributions toward free energy change from multiple kinds of molecular interactions with both water and bulk organic phases. This paper reviews pp-LFER theory and presents the development of a new pp-LFER for organic chemical partitioning with soil/sediment organic matter (SOM) using a data set of 356 carefully selected experimental values of log Koc for 75 chemicals, including apolar, monopolar, and bipolar compounds. The paradigm of representing SOM by a single pp-LFER is qualitatively supported by our results, and the set of coefficients for the regression log Koc= (1.10+/-0.10)E - (0.72+/-0.14)S + (0.15+/-0.15)A - (1.98+/-0.14)B + (2.28+/-0.14)V + (0.14+/-0.10) represents a proposed set of water-SOM-specific properties for estimating log Koc. The developed correlation outperformed other currently recommended approaches with the given Koc data set and also compared favorably against the use of new multiple class-specific op-LFER regressions. Overall, the pp-LFER approach is recommended over other current methods for the purpose of Koc estimation and especially for polar chemicals.

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.

Black carbon and kerogen in soils and sediments. 1. Quantification and characterization.

A comprehensive wet chemical procedure was developed by combining acid demineralization, base extraction, and dichromate oxidation for fractionation and quantitative isolation of soil/sediment organic matter (SOM) into four fractions: (1) humic acids + kerogen + BC (HKB); (2) kerogen + BC (KB); (3) humic acid (HA); and (4) BC. The soil/sediment samples tested were collected from the suburban areas of Guangzhou, a rapidly developing city of China. The results show that BC and kerogen constitute 57.8-80.6% of the total organic carbon (TOC) and that the relative content of BC ranges from 18.3% to 41.0% of the TOC, indicating that both BC and kerogen are major organic components in soils and sediments from this industrialized region. Systematic characterization of the isolated SOMs shows that both BC and kerogen have sizes ranging from a few microns to above 100 microm, relatively low O/C and H/C atomic ratios, and low contents of oxygen-containing functional groups. The isolated BC has unique fusinite and semifusinite macerals, highly porous nature, and structures indicative of its possible origins. The study indicates that SOM is highly heterogeneous and that humin, the nonextractable humus fraction, consists mainly of kerogen and BC materials in the tested soil/sediment samples. The presence of these materials in soils and sediments may have significant impacts on pollutant mass transfer and transformation processes such as desorption and bioavailability of less polar organic chemicals in surface aquatic and groundwater environments.

Distributed reactivity model for sorption by soils and sediments. 15. High-concentration co-contaminant effects on phenanthrene sorption and desorption.

Soil and sediment materials having organic matter matrixes of different geochemical character were examined with respect to their sorption and desorption of phenanthrene in the presence of order-of-magnitude larger concentrations of trichloroethylene (TCE) and dichlorobenzene (DCB). These co-contaminants depressed phenanthrene sorption in the lowest residual solution phase concentration ranges of that target solute investigated, whereas in its highest residual concentration regions phenanthrene sorption was either not affected or was actually enhanced. In both concentration ranges, the effects observed varied with the hydrophobicity and relative concentration of the co-contaminant and with the geological maturity and associated degree of condensation and aromatization of the soil/sediment organic matter (SOM). Desorption isotherms for phenanthrene indicate the occurrence of increased hysteresis in the presence of high concentrations of DCB and TCE, the effect increasing with increased degree of associated organic condensation. Tests in which high concentrations of DCB and TCE were added after completion of the phenanthrene desorption experiments show clear evidence of partial displacement of sorbed phenanthrene to the solution phase. The results of the work support the concept of SOM glass-transition concentrations, above which matrix deformation occurs and so-called "conditioning effects" are observed.

Amorphous and condensed organic matter domains: the effect of persulfate oxidation on the composition of soil/sediment organic matter.

The composition of amorphous and condensed soil/sediment organic matter (SOM) domains was investigated for one soil sample and four sediment samples. These samples were oxidized with persulfate to remove amorphous SOM, before and after which the composition of SOM was studied by thermogravimetric analysis, pyrolysis-GC/MS, and cross polarization magic angle spinning 13C-NMR. Comparison of the SOM composition before and after oxidation showed that condensed SOM was more thermostable and less polar than amorphous SOM. Condensed SOM was relatively low in O-alkyl C and carboxyl C and it was likely to contain only small amounts of labile organic components (carbohydrates, peptides, fatty acids). Apart from these general characteristics, the composition of the condensed and amorphous domains appeared to be highly dependent on the origin and nature of the SOM investigated. Condensed domains in relatively undecomposed SOM were enriched in aliphatic C, whereas condensed domains in relatively weathered SOM were enriched in aromatic C. Altogether, the compositional changes upon persulfate oxidation were similar to the compositional changes upon humification, which supports the idea that weathered SOM is more condensed than the original material.

Peroxidase-catalyzed oxidative coupling of phenols in the presence of geosorbents: rates of non-extractable product formation.

Oxidative coupling processes in subsurface systems comprise a form of natural contaminant attenuation in which hydroxylated aromatic compounds (HACs) are incorporated into soil/sediment organic matter matrices. Here we describe the oxidative coupling of phenol catalyzed by horseradish peroxidase (HRP) in systems containing two geosorbents having organic matter of different composition; specifically Chelsea soil, a near-surface geologically young soil having a predominantly humic-type soil/sediment organic matter (SOM) matrix, and Lachine shale, a diagenetically older natural material having a predominantly kerogen-type SOM matrix. It was found that each of these two different types of natural geosorbents increased the formation of non-extractable coupling products (NEPs) over that which occurred in solids-free systems. The extent of coupling was higher in the systems containing humic-type Chelsea SOM than in those containing kerogen-type Lachine SOM. It was observed that HRP inactivation by free radical attack was significantly reduced in the presence of each geosorbent. A rate model was developed to facilitate quantitative evaluation and mechanistic interpretation of such coupling processes. Experimental rate measurements revealed thatthe greater extent of reaction observed in the presence of Chelsea soil than in the presence of Lachine shale can be attributed to two factors: (i) more effective protection of HRP from inactivation by the Chelsea SOM and (ii) the greater reactivity of Chelsea SOM with respect to cross-coupling. Interrelationships among enzyme protection, cross-coupling reactivity, and SOM chemistry are discussed.

Determining PCB Sorption/Desorption Behavior on Sediments Using Selective Supercritical Fluid Extraction. 2. Describing PCB Extraction with Simple Diffusion Models

A simple diffusion model is fitted to selective SFE profiles obtained in Part 1 for native (not spiked) PCBs in historically contaminated sediments and soils. The model takes two types of geometry into consideration, a spherical one (Hot Ball, HB) and a planar one (Finite Slab, FS) and is extended to describe the influence of increasing temperature during the extraction. In this work, soil/sediment organic matter (SOM) is treated as a polymer-like organic material (POM). Consequently, diffusion is considered to be an activated process where the diffusion coefficients depend on the temperature according to the Arrhenius equation. When using one type of POM in the model, the characteristic “temperature humps” obtained in the extraction profiles are simulated relatively well, but at least three types of POM are needed for excellent fits to experimental data. The final model, consisting of three different POMs with spherical geometry, was fitted to all matrixes to estimate diffusion coefficients and activation energies. These data correlated with relevant literature values obtained from long-term water desorption experiments, indicating that it might be possible to replace these experiments with rapid SFE to characterize soils and sediments and to predict the rate of release of pollutants under field conditions.

Correlation of Soil and Sediment Organic Matter Polarity to Aqueous Sorption of Nonionic Compounds

Polarities of the soil/sediment organic matter (SOM) in 19 soil and 9 freshwater sediment samples were determined from solid-state 13C-CP/MAS NMR spectra and compared with published partition coefficients (Koc) of carbon tetrachloride (CT) from aqueous solution. Nondestructive analysis of whole samples by solid-state NMR permits a direct assessment of the polarity of SOM that is not possible by elemental analysis. The percent of organic carbon associated with polar functional groups was estimated from the combined fraction of carbohydrate and carboxyl−amide−ester carbons. A plot of the measured partition coefficients (Koc) of carbon tetrachloride (CT) vs. percent polar organic carbon (POC) shows distinctly different populations of soils and sediments as well as a roughly inverse trend among the soil/sediment populations. Plots of Koc values for CT against other structural group carbon fractions did not yield distinct populations. The results indicate that the polarity of SOM is a significant factor in accounting for differences in Koc between the organic matter in soils and sediments. The alternate direct correlation of the sum of aliphatic and aromatic structural carbons with Koc illustrates the influence of nonpolar hydrocarbon on solute partition interaction. Additional elemental analysis data of selected samples further substantiate the effect of the organic matter polarity on the partition efficiency of nonpolar solutes. The separation between soil and sediment samples based on percent POC reflects definite differences of the properties of soil and sediment organic matters that are attributable to diagenesis.

Partition Characteristics of Polycyclic Aromatic Hydrocarbons on Soils and Sediments

The partition behavior was determined for three polycyclic aromatic hydrocarbons (PAHs), i.e., naphthalene, phenanthrene, and pyrene, from water to a range of soil and sediment samples. The measured partition coef ficients of the individual PAHs between soil/sediment organic matter (SOM) and water (i.e., Koc values) are relatively invariant either for the “clean” (uncontaminated) soils or for the clean sediments; however, the mean Koc values on the sediments are about twice the values on the soils. This disparity is similar to the earlier observation for other nonpolar solutes and reflects the compositional differences between soil and sediment organic matters. No significant differences in Koc are observed between a clean coastal marine sediment and freshwater sediments. The coastal sediments that are significantly impacted by organic contaminants exhibit higher Koc values. At given Kow values (octanol−water), the PAHs exhibit much higher Koc values than other relatively nonpolar solutes (e.g., chlorinated hydrocarbons). This effect is shown to result from the enhanced partition of PAHs to SOM rather than from lower Kow values of PAHs at given supercooled liquid solute solubilities in water. The enhanced partition of PAHs over other nonpolar solutes in SOM provides an account of the markedly different correlations between log Koc and log Kow for PAHs and for other nonpolar solutes. The improved partition of PAHs in SOM stems apparently from the enhanced compatibility of their cohesive energy densities with those of the aromatic components in SOM. The approximate aromatic fraction in soil/sediment organic matter has been assessed by solid-state 13C-NMR spectroscopy.