Sorption of comparatively nonpolar organic chemicals by natural solids not only can be predominated by partitioning with organic matter but also can reflect a substantial contribution from adsorption at low relative concentration. Sorption of nine polycyclic aromatic hydrocarbons (PAHs) and chlorinated benzenes (CBs) was investigated on a subsurface aquitard through batch study, with results interpreted by a composite adsorption-partitioning model. For both PAHs and CBs, the low-concentration adsorption slope and the coefficient for partitioning each correlated well with K ow ; however, PAHs consistently sorbed more strongly than CBs at given K ow . For all chemicals, adsorption contributions were only important at low relative concentration and could be successfully modeled by assuming either Langmuir-type or Polanyi-type isotherms. Isotherms for all liquid chemicals fell on a single isotherm when plotted on a Polanyi basis (adsorbed volume per mass of sorbent versus adsorption potential density), providing evidence that a pore-filling phenomenon is involved. Adsorbed volumes of solid chemicals were observed to be less than those of liquids at the same adsorption potential density, consistent with a reduction in packing efficiency and as previously reported for activated carbons. These results suggest that the adsorption contribution is from pore filling within microporous solids.

Sorption isotherms of 13 apolar liquids and solids and polar solids-six in unprecedented detail-are used to evaluate a polymer-based model for natural organic matter. While all isotherms are nonlinear, the "running" Freundlich exponent n varies markedly with concentration. The isotherms show linear-scale inflection consistent with the presence of flexible (deformable) porosity as predicted by the glassy polymer-based Extended Dual-Mode Model (EDMM). The EDMM assumes dissolution and hole-filling domains in the organic solid, with provision for sorbate-caused plasticization of the solid and "melting" of the holes. Features of the EDMM are illustrated for chlorinated benzenes in poly(vinyl chloride). The solutes fall into categories of "hard" (aliphatics and 2,4-dichlorophenol) and "soft" (chlorinated benzenes, 2-chloronitrobenzene) according to their ability to plasticize organic matter. Comparison of domain coefficients at infinite dilution reveals that organic solutes have a modestly greater affinity for holes than dissolution sites (by 0.1-0.6 log unit), as expected by the polymer model. Sorption of CHCl3 shows time-dependent hysteresis diminished at high concentrations by the plasticizing effect. Sorption of CHCl3 also shows a type of hysteresis for glassy solids known as the "conditioning effect" in which high loading of sorbate increases hole population upon its removal and thus leads to enhanced uptake and nonlinearity when a second sorption is performed. A Polanyi-based, fixed-pore filling model applied to the adsorption component of the isotherms gave widely variant volumetric pore capacity, contrary to its own stipulations, and could not explain the hysteresis.

Page 89. The last paragraph cites a personal communication with Ö rjan Gustafsson, Stockholm University, that the chemothermal oxidation method for determining soot C in natural samples "is suspected of overestimating soot C due to charring or possibly underestimating soot C if minerals are present". The statement incorrectly (and inadvertently) implies that the method is suspected of being generally unsuitable, even for the marine sediments investigated in ref 21. A more accurate statement would be that the method has unknown applicability for high OC soils where artifactual charring or possible losses of soot are possible.Page 90. In first full paragraph of the second column, we stated "Note the strong curvature on the linear scale of the DBP and DCCH isotherms, which contrasts with the "linear" isotherms reported for 1,2-dibromoethane and other apolar compounds in the same soil by others (25, 26).". The isotherm of DBP (1,2-dibromopropane) is nonlinear up to 10% or more of its solubility (1510 mg/L). The isotherm of DCCH (trans-1,2-dichlorocyclohexane) is nonlinear up to at least 50% of its solubility (240 mg/L) and shows reverse sigmoid shape. The isotherms in refs 25 and 26 of similarly apolar compounds (1,2-dibromoethane, trichloroethene, and lindane), in fact, are reported to be linear above ∼1-1.5% of their solubility and nonlinear below.

Abstract. Analysis of subsurface soil cores from the site of a field-scale groundwater remediation experiment at Dover Air Force Base, Delaware, has revealed that tetrachloroethene (PCE) and trichloroethene (TCE) contamination extends into an aquitard underlying a groundwater aquifer. The site location is well doWngradient of the locations of contaminant release, and the aquitard contamination is believed to have begun when contaminated groundwater first arrived in the overlying aquifer. Using independent estimates of sorption and diffusion properties in the aquitard layers, mathematical modeling based on diffusion in laminate slabs has been used to make inferences regarding the historical concentration conditions in the overlying aquifer. The results suggest that plume arrival occurred within the last two decades, with some important differences in the inferred TCE and PCE plume histories. The diffusion model was also applied toward predicting future aquitard concentrations and fluxes under scenarios based on the current condition as a starting point and hypothesized conditions of future groundwater cleanup. The results demonstrate how aquitard sampling and diffusion modeling can provide essential information relevant to forensic analysis, risk assessment, and subsurface cleanup. IntroductionClayey subsurface deposits are often of sufficiently low hydraulic conductivity that molecular diffusion is the dominant process of contaminant transport under many typically encountered conditions of hydraulic gradient [Barone el al., 1992; Johnson el al., 1989; Shackelford and Daniel, 1991]. As a consequence, the contamination of clay and silt aquitards underlying polluted aquifers may proceed primarily by this process, and subsequent remediation will be correspondingly slow. Investigations of contaminant diffusion in aquitards can help us to understand the effect of this rate-limiting process on remediation time and can also provide important clues regarding the contamination history of the overlying groundwater.Organic contaminant diffusion i n low-permeability materials has been studied previously iri the context of clay landfill liners or cutoff barriers, using both laboratory experiments [Barone el al., 1992; Molt and Weber, 1991] and field diffusion profiles [Johnson el al., 1989]. Johnson el al. [1989] have reviewed some of the literature related to solute diffusion through clay liners and other low-permeability materials underlying landfill sites and provide compelling evidence that simple Fickian diffusion can be an important mechanism of vertical contaminant transport in many field situations. These authors were among the first to investigate in situ diffusive transport through field material, which they accomplished by means of pore-water analyses from squeezed sections of core taken from directly below a 5-year-old landfill. Results were successfully modeled on the basis of the known exposure time, using the diffusion coefficient as a fitting parameter. Diffusion coefficients of organic Copyright 1997 by the ...

The development of appropriate equilibrium sorption relationships for anthropogenic organic contaminants with soils and sediments is essential to predicting the extents and rates of solid-water interactions in the environment. In this context, we previously reported results that showed how nonlinear sorption isotherms with nine lowpolarity organic chemicals on a silty aquitard material could be modeled as a combined adsorption-partitioning process. The adsorption component was ascribed to presumed traces of a carbonaceous adsorbent, for which adsorption was well described by a Polanyi-based isotherm. In this work, we report competitive sorption behavior on the same material, as observed from batch experiments in binary solute systems that contain combinations of the previously studied sorbates (chorinated benzenes and polyaromatic hydrocarbons) over wide ranges of concentration. The results confirmed our expectation that partitioning is an increasingly dominating contribution to overall sorption when cosorbates are present. Moreover, the competitive effects could be very well predicted on the basis of Polanyi-Manes adsorption theory and the prior single solute results. Overall, the results further validate the applicability of a Polanyi-based modeling approach and lend additional support to the hypothesis that this sorbent contains trace quantities of high surface area carbonaceous material.