Rates of phenanthrene sorption by four different types of soils and sediments were characterized by examining the time dependence of solute phase distribution relationships (PDRs) in completely-mixed batch reactors. Unlike conventional single-level concentration methods, the experiments were conducted using a range of concentrations to obtain a time series of nonequilibrium PDRs for each sorbent−sorbate system over reaction periods ranging from 1 min to 14 days. In all cases tested, the nonequilibrium PDRs changed from approximately linear form to increasingly nonlinear form as the time of reaction increased. A Freundlich-type relationship, q(t) = K F(t)C(t) n ( t ), was used to relate values of measured temporal solid-phase solute concentrations, q(t), to corresponding solution-phase solute concentrations C(t). After a short “initiation” stage, the parameters n(t) and K F(t) were observed to vary functionally with logarithmic time. A three-domain particle-scale model predicated on the existence of discrete soil components (exposed inorganic surfaces and amorphous and condensed soil organic matter) is invoked to explain the observed sorption behavior and functional relationships underlying the time dependence of the PDRs.

Phenanthrene sorption and desorption equilibria were measured for 10 natural sorbents having significantly different geological ages and organic matter compositions. Three geologically young peats, one humic acid, three geologically old shales, and samples of kerogen isolated from each of the shales were examined. Elemental analyses and solid-state 13C-NMR spectra reveal that the oxygen/carbon (O/C) atomic ratios of the soil organic matter (SOM) as sociated with the samples decrease with increased age and, thus, apparently with diagenetic alteration. The sorption affinities of these materials for phenanthrene as well as their respective isotherm nonlinearities and hysteretic behaviors were found to correlate inversely with the O/C atomic ratio; samples containing more physically condensed and chemically reduced SOM matrices exhibited greater solute affinity, more nonlinear sorption equilibria, and more pronounced hysteresis. Observed relationships between the chemical and structural characteristics of associated organic matter and the sorption and desorption behaviors of the samples are captured effectively by the concepts underlying the Dual Reactive Domain Model advanced earlier in this series. This study thus extends that model to include the desorption process, supporting its general ap plicability for characterizing the overall behavior of soils and sediments with respect to solute uptake and release.

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Sorption isotherms for a hydrophobic solute probe, phenanthrene, were measured experimentally for 27 different soils and sediments. The linear and Freundlich isotherm models and the Dual Reactive Domain Model (DRDM) were used to fit the resulting data. The results reveal for all soils and sediments studied that (i) the Freundlich model and the DRDM fit the data well, whereas a linear model fails to do so; (ii) values of the organic carbon-normalized distribution coefficient, K OC, calculated from individual isotherm points for a specific sorbent−solute system vary significantly with the aqueous-phase solute concentration, C e; and (iii) all commonly used correlations of K OC with octanol−water partitioning coefficients and solute solubility limits significantly underestimate K OC for C e values smaller than approximately one-tenth of aqueous-phase solute solubility, C S. The sorption behaviors of all of the soils and sediments studied are thus inconsistent with the simple concept of linear phase partitioning. The general applicability of the DRDM, a polymer-based limiting case form of the Distributed Reactivity Model, for all systems studied supports mechanistic arguments based on polymer sorption theory.

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.

Long-term temporal phase distribution relationships (PDRs) were measured for sorption of a hydrophobic organic contaminant probe by seven EPA reference soils and sediments and six shale and kerogen samples. The times required for attainment of apparent sorption equilibrium by the phenanthrene probe were found to be highly dependent upon the aqueous phase-solute concentration, C(t), for a given sorbent, and the type of soil organic matter (SOM) associated with a particular sorbent. Organic-carbonnormalized single-point temporal distribution ratios corresponding to low residual solution phase concentrations were found to approach their respective apparent equilibrium values after times ranging from several days to 90 days for the EPA soils and sediments and from 90 days to g 368 days for the shales and kerogens. Conversely, at residual solution phase concentrations 2 orders of magnitude larger, apparent equilibrium conditions were attained within a few hours for the EPA soils and sediments and within a year for the shale and kerogen samples. The observed dependencies of sorption rate on C(t) and on the type of SOM appear to result from differences in solute diffusion behavior within chemically reduced and structurally condensed SOM domains and that in highly amorphous SOM domains. In the former case the very slow and concentration-dependent non-Fickian behavior observed is likely attributable to the slow and energetically driven reconfiguration of local condensed SOM structures to accommodate solute migration into the matrixes. The results of the study extend the applicability of the Dual Reactive Domain Model introduced earlier in this series of papers to the interpretation and description of sorption rate behavior.

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Steroid estrogens at sub-micrograms per liter levels are frequently detected in surface water, and increasingly cause public concern of their potential impacts on ecosystems and human health. Assessing the environmental fate and risks of steroid estrogens requires accurate characterization of various physicochemical and biological processes involving these chemicals in aquatic systems. This paper reports sorption of three estrogens, 17beta-estradiol (estradiol), estrone, and 17alpha-ethinyl estradiol (EE2), by seven soil and sediment samples at both equilibrium and rate-limiting conditions. The results indicated that attainment of sorption equilibrium needs about 2 d when aqueous estrogen concentrations (C(t)s) are 25 to 50% of their solubility limits (S(W)S), but equilibrium requires 10 to 14 d when the Ct is 20 times lower than the S(W). The measured sorption isotherms are all nonlinear, with the Freundlich model parameter n ranging from 0.475 to 0.893. The observed isotherm nonlinearity correlates to a gradual increase of single-point organic carbon-normalized sorption distribution coefficient (capacity) (K(OC)) as the equilibrium estrogen concentration (Ce) decreases. At Ce = 0.5S(W), all three estrogens have log K(OC) values of 3.14 to 3.49, whereas at Ce = 0.02S(W), the log K(OC) values for estrone, EE2, and estradiol are within ranges of 3.40 to 3.81, 3.45 to 3.85, and 3.71 to 4.12, respectively. This study suggests that, when at sub-micrograms per liter levels, these estrogenic chemicals may exhibit even slower rates and greater capacities of sorption by soils and sediments.