Sorption Of Hydrophobic Organic Compounds Research Articles

New modeling paradigms for the sorption of hydrophobic organic chemicals to heterogeneous carbonaceous matter in soils, sediments, and rocks

Heterogeneity in naturally occurring carbonaceous materials (CMs) causes sorbed hydrophobic organic compound (HOC) concentrations in soils, sediments, and rocks to occur as a combination of surface adsorption and phase partitioning, with the latter typically more linearly dependent on aqueous concentration. In this manuscript, we describe a model to simulate HOC sorption as the combined effect of adsorption to thermally altered CM and a more linear solvation-driven absorption into gel-like CM (organic matter). We describe different forms of thermally altered CM (such as soots, chars, coals, and kerogen), the manner in which these materials can serve as especially strong adsorbents, and the conditions under which they can control solid–aqueous distribution. Specific examples of model fits to soil, sediment and rock samples with identified thermally altered CM components provide a linkage between sorption components and sorbent material properties. Because both the adsorption and partition components are scalable by compound solubility, it may often be possible to estimate nonlinear isotherms for a wide range of chemicals based on comparatively few experimental measurements. Thermally altered CM is widespread in the environment and can serve as an important sorbent even when present in small quantities (especially at low concentrations of adsorbates). In this context, the sorption modeling refinements described in this work are expected to have wide applicability. Given that solid/water distribution is a central process affecting contaminant fate, such refined models are an essential element for better estimates of risk and improved remediation design.

Sorption of Hydrophobic Organic Compounds by Soil Materials: Application of Unit Equivalent Freundlich Coefficients

Prediction of the fate of hydrophobic organic compounds (HOCs) in the soil environment is difficult due to the highly complex interaction between the organic molecules and the soil components. The Freundlich isotherm is the most commonly used model to describe retention of HOCs by heterogeneous sorbents. Because of inconsistent units of constants calculated from the Freundlich model, it is difficult to strictly compare the sorption of organic molecules by different sorbents. In this study, we modified the Freundlich equation to better assess the effects of organic carbon content and physical heterogeneity on the sorption of HOCs by soils and microaggregate fractions, using naphthalene and phenanthrene as probes. Isotherms for naphthalene and phenanthrene sorption were nonlinear and well described by the Freundlich equation. Unit-equivalent Freundlich coefficients (K‘f) were calculated by normalizing the liquid-phase equilibrium concentrations (Ce) to the supercooled liquid-state solubility (Sscl) of napht...

Variability of chlorinated-solvent sorption associated with oxidative weathering of kerogen

The perchloroethylene (PCE) sorption distribution coefficient ( K d, measured at 40–100 μg/l solution concentration in batch experiments) ranged from 4.0 to 134 l/kg over the sampled thickness (1.0–6.0 m depth) of a moderately-low organic carbon (0.002–0.005), fractured and weathered diamict (poorly sorted, glacially deposited) aquitard. The organic carbon-normalized K d ( K oc) for deeper, unoxidized (reduced) samples from this deposit (log K oc=4.4–4.59) were generally consistent with the kerogen in the underlying Devonian-age shale (log K oc=4.57), a likely source rock for these sediments. The similarities between these samples suggest that the sorption capacity of the kerogen was minimally altered during transport and deposition, and that humic substances were not co-deposited with the kerogen in the glacially-deposited sediments. The K oc within the diamict was correlated to the oxidation state of the organic matter, characterized by the H/O ratio through elemental analysis. The K oc for more oxidized samples from shallower depth samples (log K oc=3.35–3.65) were approximately an order of magnitude less than those determined for the reduced samples. The K oc for 1.5- to 2.5-cm thick oxidation haloes surrounding fractures and characterized by visible Fe-oxidation, were approximately 20–30% lower than the adjacent unaltered matrix samples. The linear trend observed between log K oc and log H/O is similar to, but steeper than, the relation reported previously, which represents increasing sorption magnitude with increasing diagenesis in coal and shale samples. The spatial distribution of the K oc within this aquitard, in concert with qualitative geologic observations, suggests that oxidative weathering of the native kerogen exerts an important control on hydrophobic organic compound sorption.

SPECTROSCOPIC EVIDENCE FOR CONDENSED DOMAINS IN SOIL ORGANIC MATTER

Soil organic matter (SOM) has previously been proposed as a dual-mode sorbent for hydrophobic organic compounds (HOC), i.e., partitioning takes place in both expanded (rubbery) and condensed (glassy) domains, whereas competitive sorption takes place only in condensed domains. The objective of this paper was to provide some spectroscopic evidence of condensed domains in SOM. Six humic acids (HA) extracted from different depths of a single soil profile were analyzed using X-ray diffraction (XRD) and solid-state [sup 13]C nuclear magnetic resonance (NMR) with cross-polarization (CP) and magic angle spinning (MAS) techniques. The XRD results showed a strong peak at 0.35 nm for the condensed aromatic structure of the HA from mineral horizons and a dominant peak at 0.43 nm for less tightly packed side chains of the HA from surface organic horizons. The [sup 13]C CP/MAS NMR results indicated further that HA becomes more aromatic from surface organic to mineral horizons. This increase of aromatic structure in HA was confirmed by the increase of nonprotonated carbon signals at 130 ppm using a dipolar-dephasing technique and by the increase of atomic C/H and C/O ratios. These spectroscopic and elemental data corroborate further the dual-mode model for the sorption of HOC in SOM.

Rate-Limited Sorption of Hydrophobic Organic Compounds by Soils with Well-Characterized Organic Matter

This study is focused on the rate-limited sorption of homologous series of polycyclic aromatic hydrocarbons, alkylated benzenes, chlorinated benzenes, and chlorinated alkenes by two soils, one in w...

Estimating Sorption Rates of Hydrophobic Organic Compounds in Iron Oxide- and Aluminosilicate Clay-Coated Aquifer Sands

We hypothesized that retarded diffusion in coatings controls the rate of sorption of hydrophobic organic compounds (HOCs) on quartzitic aquifer sands. Microscopic examination of three sands was used to quantify the coating thicknesses. With measures of the coatings' organic contents and porosities, we predicted the relative sorption rates for (a) multiple HOCs on one sand and (b) one HOC with three sands. The predicted relative rates and equilibrium coef ficients were assessed using observations of HOC transport through short sand columns operated at varying flow rates. We found that the column Kd values were always much less than predicted from Kocfoc or observed in batch tests. This suggests that diagenetically produced sorbents may include organic matter that is completely inaccessible for HOC sorption; procedures that disaggregate these sands could expose organic matter that does not sorb HOCs in the environment. Second, by modifying our retarded diffusion expectations with the inferred fraction of available organic carbon, favail, all observed sorption rates were consistent (within a factor of 4) with kr ≈ 0.001Daq/δ2(1 + Koc favail), where kr is the desorption rate constant, Daq is the HOC's aqueous diffusivity, δ is the coating thickness, is the ratio of solids-to-water in the coatings, is the organic carbon content of the coating solids, and all other factors affecting sorption rates (e.g., tortuosity) were set equal to 0.001. Since oxide coatings are ubiquitous in aquifer sands, the model described here should have wide applicability.

The role of sorbed humic substances on the distribution of organic and inorganic contaminants in groundwater

Mineral-bound humic substances modify inorganic surfaces in subsurface sediments, changing the nature and number of complexation sites for contaminants. Because of adsorptive enrichment, the reactive surface area or site concentration contributed by mineral-bound humic substances can exceed that of dissolved or colloidal humic substances by two orders of magnitude. Mineral-bound humic materials may, therefore, provide a major sink for the removal of contaminants in groundwater. The reactivity of the humic substance is primarily determined by the structural and bulk chemical properties of the humic substance and the aqueous solution chemistry. Organic and inorganic contaminants sorb readily to mineral-bound humic substances. The sorption of hydrophobic organic compounds increases as ionic strength decreases, is enhanced by divalent cations, and displays non-linear isotherms and competitive adsorption behavior. Collectively, these results suggest that hydrophobic adsorption, rather than phase partitioning, is the primary sorption mechanism for neutral organic molecules on these particle coatings. Mineral-bound humic substances augment, rather than change, the intrinsic complexation properties of mineral surfaces for metal cations. The degree of sorption enhancement promoted by mineral-bound organic material varies strongly with pH and depends on the magnitude of the stability constants between the metal cation and the humic substance, the strength and magnitude of adsorption of the humic substance by the mineral surface, and the extent of aqueous complex formation between the non-sorbed humic substance and metal. The simplest sorption model for humate-modified surfaces is the linear additivity model (LAM). Sorption data for certain hydrophobic organic compounds and metal cations appear to conform to this model.

The effect of sorption on the oxidation of polychlorinated biphenyls (PCBs) by hydroxyl radical

Sorption of hydrophobic organic compounds, such as polychlorinated biphenyls (PCBs), to particulate matter (diatomaceous earth) affects transformation rates by aqueous-phase hydroxyl radical. The reaction of three PCB congeners with hydroxyl radical (OH .), generated in a photo-Fenton system (i.e. iron, hydrogen peroxide and light at pH 3), was followed to evaluate predictive models of transformation kinetics in the presence of particulates. The OH . generated by this system only reacted with dissolved PCBs, and the overall rate of removal for each congener could be modeled using congener-specific data on desorption kinetics and solution phase reaction rates. Observed kinetics for the least hydrophobic congener evaluated, 2-monochlorobiphenyl, agreed with predictions based upon measured steady-state hydroxyl radical concentrations and second-order rate constants. 2,2′,5-Trichlorobiphenyl was also oxidized by OH ., but sorption to particulate matter resulted in slower transformation rates. A predictive model which utilized measured adsorption/desorption rate constants, steady-state hydroxyl radical concentrations and second-order rate constants adequately predicted 2,2′,5-trichlorobiphenyl transformation kinetics. Concentrations of 2,2′,4,5,5′-pentachlorobiphenyl, the most hydrophobic compound analyzed, did not change over the time scale of the experiments because dissolved concentrations were extremely low and desorption kinetics were very slow.

Effect of clay minerals present in aquifer soils on the adsorption and desorption of hydrophobic organic compounds

Adsorption of hydrophobic organic compounds (HOCs) onto clay minerals and organic matter present in soils results in retarding their mobility. To study the impact of clay minerals on HOC sorption, kinetic and equilibrium studies were performed using naphthalene as a test surrogate contaminant. The results of these studies indicated that expandable clay minerals (clays that expand and expose large internal surface area on wetting), such as montmorillonite and vermiculite, had a significant impact on naphthalene partitioning. A mathematical model was developed from the equilibrium data which related clay mineral concentrations with the naphthalene partition coefficient. Equilibrium desorption studies were also performed by adding a micellar solution of a surfactant mixture (50:50) of Tween 20 and Aerosol AY-65 to mobilize the adsorbed naphthalene. The surfactant mixture was generally unable to mobilize the sorbed contaminant due to sorption irreversibility and adsorption hysteresis.

Surfactant Solubilization of Organic Compounds in Soil/Aqueous Systems

Hydrophobic organic compounds (HOCs) are modeled as being distributed between aqueous, micellar, and sorbent compartments in a soil/aqueous system containing nonionic surfactant micelles. The partitioning of HOC between the hydrophobic interiors of the nonionic surfactant micelles and the surrounding solution can be characterized with a mole fraction partition coefficient. Nonionic surfactant sorption onto soil may be described by a maximum sorption parameter. Sorbed surfactant molecules tend to increase HOC sorption, and free surfactant monomers in solution tend to decrease HOC sorption by increasing the HOC apparent aqueous solubility; these effects can be represented by a modified HOC soil/water partition coefficient. The distribution of HOC between the three compartments can be estimated using a physicochemical model, for which model parameter values conceptually may be obtained from independent experiments.

A distributed reactivity model for sorption by soils and sediments. 2. Multicomponent systems and competitive effects

The uptake of individual hydrophobic organic compounds (HOCs) from aqueous solution by soils which exhibit heterogeneous reactivity is shown to be reduced in the presence of other HOCs. The observed behavior is consistent with expectations for competitive surface adsorption phenomena, being most marked at low solution-phase concentrations and coinciding with single-solute isothermal nonlinearity. Nonlinear sorption and competitive effects appear to relate to components or fractions of soil having different reactivity than that commonly attributed to soil organic matter. In particular, HOC uptake by the organic components of soils deriving from certain sedimentary rocks appears to occur by surface reactions rather than by partitioning to organic matrices. Ideal adsorbed solution theory (IAST) is used to predict mixed-solute competitive effects from single-solute sorption isotherms. The results are examined within the context of the distributed reactivity model (DRM), wherein overall sorption behavior by heterogeneous solids is explicitly attributed to different mechanisms. Competitive reductions in the sorption of HOCs in systems dominated by nonlinearly sorbing soil components and characterized by a two-compartment DRM are found to be modeled reasonably well with IAST. 22 refs., 12 figs., 3 tabs.

The sorption of humic acids to mineral surfaces and their role in contaminant binding

Humic substances dissolved in groundwater may adsorb to certain mineral surfaces, rendering hydrophilic surfaces hydrophobic and making them sorbents for hydrophobic organic compounds (HOC). The sorption of humic and fulvic acids (International Humic Substances Society, IHSS, reference samples) on hematite and kaolinite was investigated to determine how natural organic coatings influence HOC sorption. The sorption behavior of the humic substances was consistent with a ligand-exchange mechanism, and the amount of sorption depended on the concentration of hydroxylated surface sites on the mineral and the properties of the humic substance. The sorption of the humic substances to two solids was proportional to their aromatic carbon content and inversely proportional to the O/C ratio. Increasing quantities of sorbed humic substances ( f oc, 0.01–0.5%) increased the sorption of carbazole, dibenzothiophene and anthracene. Peat humic acid, the most aromatic coating, showed the greatest sorption enhancement of HOC when sorbed to hematite. In addition, HOC sorption was greater on organic coatings formed at low ionic strength ( I = 0.005) as compared to higher ionic strength ( I = 0.1). We suggest that both the mineral surface and the ionic strength of the electrolyte affect the interfacial configuration of the sorbed humic substance, altering the size or accessibility of hydrophobic domains on the humic molecule to HOC.

Influence of mineral-bound humic substances on the sorption of hydrophobic organic compounds

The sorption of three hydrophobic organic compounds (HOC) was investigated on hematite and kaolinite that had been coated with natural humic substances over a mass percent carbon range of 0.01-0.5%. Increasing quantities of sorbed humic substances increased the sorption of HOC. Anthracene, the most hydrophobic HOC, showed the greatest sorption enhancement, while the most aromatic coating, peat humic acid, was the strongest sorbent. Depending on the type of humic acid coating and the mineral substrate, the experimental K{sub oc} values were either higher or lower than those predicted by the K{sub ow}. The sorptivity of a given humic acid for HOC was not the same on kaolinite and hematite, suggesting that the orientation and structure of the humic substance on the mineral may affect the surface area of the organic phase and the accessibility of hydrophobic domains that control HOC sorptivity. Sorption isotherms for HOC on the humic-coated mineral substrates were nonlinear, implying that the sorption phenomenon was adsorption onto rather than partitioning into the surface organic phase.

Estimating the effects of dispersed organic polymers on the sorption of contaminants by natural solids. 2. Sorption in the presence of humic and other natural macromolecules

A triphase distribution model was used in conjunction with experimental observations to characterize the effects of humic polymers dispersed in an aqueous phase on the sorption of hydrophobic organic compounds from that phase by natural solids

Numerical modeling of sorption kinetics of organic compounds to soil and sediment particles

A numerical model is developed to simulate hydrophobic organic compound sorption kinetics, based on a retarded intraaggregate diffusion conceptualization of this solid‐water exchange process. This model was used to ascertain the sensitivity of the sorption process for various sorbates to nonsteady solution concentrations and to polydisperse soil or sediment aggregate particle size distributions. Common approaches to modeling sorption kinetics amount to simplifications of our model and appear justified only when (1) the concentration fluctuations occur on a time scale which matches the sorption timescale of interest and (2) the particle size distribution is relatively narrow. Finally, a means is provided to estimate the extent of approach of a sorbing system to equilibrium as a function of aggregate size, chemical diffusivity and hydrophobicity, and system solids concentration.

Solvophobic approach for predicting sorption of hydrophobic organic chemicals on synthetic sorbents and soils

The application of a solvophobic approach for predicting the sorption of hydrophobic organic compounds (HOC) was evaluated with data collected using synthetic sorbents and soils. The experimental data consisted of batch equilibrium sorption coefficients ( K D), as well as soil-TLC and reversed-phase liquid chromatographic (RPLC) retention factors (κ′). All data were collected using aqueous solutions and binary or ternary solvent mixtures of water, methanol, acetone, and acetonitrile. As predicted by the theory, the chromatographic retention factors and sorption coefficients for HOC decreased log-linearly with increasing fraction of organic cosolvent in binary solvents. Model parameters estimated from the binary solvent data could be used to predict sorption (or retention) from ternary solvents. Reasonable agreement was found between model parameters reported in the literature and those estimated using the data from batch sorption, soil-TLC, and RPLC studies.