Environmental Toxicology and Chemistry

Tomson, Mason B.

doi: 10.1002/etc.5620180801pmid: N/A

Chen, Wei; Kan, Amy T.; Fu, Gongmin; Vignona, Laine C.; Tomson, Mason B.

doi: 10.1002/etc.5620180802pmid: N/A

Neutral organic contaminants commonly reside on the solid portion of soils and sediments. The extent of desorption from these solids determines the fate, reactivity, and toxicity. Numerous researchers have observed that, after an initial exposure of a few days, desorption takes place in two phases, namely, an initial rapid release followed by a long‐term resistant phase. Resistant desorption has been highly unpredictable. In this research, the adsorption and desorption of chlorobenzenes, hexachlorobutadiene, and naphthalene have been studied using historically and freshly contaminated bayou sediments from Lake Charles, Louisiana, USA. After an initial release period, both laboratory‐spiked and historically contaminated sediments exhibit similar desorption profiles. The simulations compare favorably with previous in situ measurements. All desorption results, both laboratory and field, could reasonably be interpreted using a single irreversible isotherm. Consequences of these results in terms of sediment quality criteria (SQC) are also discussed. Finally, an index of fractional irreversible adsorption is proposed and tested.

Leboeuf, Eugene J.; Weber, Walter J.

doi: 10.1002/etc.5620180803pmid: N/A

This evaluation of the polymer‐based Flory–Huggins sorption theory has shown that the model predicts relatively concentration‐independent χ values for rubbery matrices, concentration‐dependent χ values for more glassy matrices, and highly concentration‐dependent, largely negative interaction parameters for sorption of phenanthrene onto Green River kerogen, Ohio Shale II kerogen, Illinois No. 6 and Wyoming coals, and poly(phenyl methacrylate). These greatly negative enthalpic contributions to the interaction parameter are only to be expected where specific sorbate‐sorbent interactions exist, such as in polar sorbate–polar sorbent systems. The failure of the nonlinear partitioning model is explained in terms of its inability to predict these increased interactions of sorbing solutes within microvoids present within glassy matrices. Calculations of sorbent solubility parameters through use of this model do, however, provide good agreement with literature solubility parameter values for specific sorbents; it may thus prove useful for partial characterization of sorbents for which solubility parameters are unknown.

Jepsen, Rich; Lick, Wilbert

doi: 10.1002/etc.5620180804pmid: N/A

Long‐term experiments were done in order to investigate nonlinear isotherms and interactive effects in the sorption of hydrophobic organic chemicals (HOCs) by sediments. In the isotherm experiments, it was demonstrated that the isotherms for all HOCs tested were linear as long as the mass of the sorbed HOC was small by comparison with the mass of organic carbon in the sediments; for larger sorbed HOC concentrations, the isotherms were nonlinear. Sorption experiments also were done with hexachlorobenzene (HCB)–octanol, HCB–ethanol, octanol–ethanol, and HOC–methanol mixtures in water and sediments. Interactive effects were observed and can be described in terms of the partitioning of the primary HOC between the cosolvent, water, and sediments.

Farrell, James; Hauck, Benjamin; Jones, Martin

doi: 10.1002/etc.5620180805pmid: N/A

Adsorption of trichloroethylene (TCE) in adsorbents containing hydrophilic and hydrophobic micropores was investigated in order to determine the mechanisms responsible for TCE adsorption on mineral solids. A high‐pressure liquid chromatography method was used to measure TCE adsorption isotherms on three microporous adsorbents. Silica gel and zeolite type NaX were used as hydrophilic model adsorbents, and hexamethyldisilazane (HMDS)‐treated silica gel was used as a model hydrophobic adsorbent. Batch uptake and desorption isotherms were also measured on the hydrophilic silica gel. Uptake of TCE by all three adsorbents was linear over the concentration range investigated. However, the silica gel desorption isotherm was highly nonlinear, as indicated by its Freundlich isotherm exponent of 0.58. Capillary phase separation into hydrophobic micropores was postulated as being responsible for the isotherm hysteresis. Supporting this hypothesis was the conformance of the TCE adsorption isotherm to Dubinin–Radushkevitch volume filling of micropores theory. The enthalpies for TCE adsorption on all three solids were determined by van't Hoff analysis of distribution coefficients measured over a temperature range from 5 to 90°C. The TCE adsorption enthalpies on the silica gel and HMDS silica gel were exothermic, but on the zeolite adsorption was endothermic. High exothermic adsorption enthalpies on the silica gel adsorbents indicated that TCE adsorption was occurring in hydrophobic micropores, and that adsorption on surfaces with large radii of curvature contributed only minimally to the total uptake. This indicates that the predominant mechanism for TCE adsorption on these mineral solids is not partitioning into the vicinal water layer.

Schulten, Hans‐Rolf

doi: 10.1002/etc.5620180806pmid: N/A

An hypothesis for the structure of dissolved organic matter (DOM) in water is proposed. It is based on previously published humic acid and soil organic matter (SOM) models. Personal computer (PC)‐based molecular modeling and geometry optimization of DOM and humic/xenobiotic complexes in vacuo and water were performed using modern PC software in order to determine low energy conformations and to simulate site‐specific processes such as trapping and binding of biological and anthropogenic substances. Nanochemistry (10−9 m level) allows the evaluation of atomic and molecular space requirements, voids, inter‐ and intramolecular hydrogen bonds, and interactions with water, metal cations, and xenobiotics. The described modeling approach in general allows hydrophilic and hydrophobic reactions to be examined. Structural, molecular, and environmental properties of DOM and its xenobiotic complexes were determined by quantitative structure‐activity relationship software. Focal points were molecular properties, such as solvent accessibility as well as van der Waals surface areas and volumes, partial charges, hydration energy (peptides), hydrophobicity (log P), refractivity, and polarizabilities of humic/xenobiotic complexes were determined. Molecular mechanics calculations show that nonbonded forces (e.g., van der Waals) and hydrogen bonds were the main reasons for temporary immobility of xenobiotic substances retained in DOM. Preliminary experiments to simulate the acidity of water molecules by protonation‐enhanced reactions with polar xenobiotics (e.g., hydroxyatrazine) but left nonpolar substances (e.g., DDT) unchanged.

Kubicki, James D.; Blake, Geoffrey A.; Apitz, Sabine E.

doi: 10.1002/etc.5620180807pmid: N/A

Energy gaps between the highest‐occupied molecular orbital and lowest‐unoccupied molecular orbital (ΔEHOMO‐LUMO) for a suite of common polycyclic aromatic hydrocarbons (PAHs) in the gas‐phase were calculated with three different molecular modeling methods: semiempirical, ab initio Hartree‐Fock, and density functional calculations. Results indicate that semiempirical, Hartree‐Fock, and density functional calculations may provide useful relative HOMO‐LUMO gap information, but these methods overestimate the actual ΔEHOMO‐LUMO. Based on vibrational frequency analyses, density functional calculations reliably produce dynamically stable structures that can be used to predict model ΔEHOMO‐LUMO values. Both the semiempirical and ab initio Hartree‐Fock methods were unreliable in predicting dynamically stable structures; hence prediction of ΔEHOMO‐LUMO values was not possible for several PAHs. Changes in the HOMO‐LUMO gap of benzene and selected PAHs due to solvation effects were calculated using self‐consistent reaction field methods and explicit solvation. Self‐consistent isodensity polarized continuum model calculations modeling water and octanol solvation do not change calculated ΔEHOMO‐LUMO values enough to affect predicted phototoxicities; thus, gas‐phase values may be used for PAHs in solution and in vivo. Energetics of PAH bonding to mineral surface groups were also modeled. In some cases, interaction of PAHs with model aluminate surface defects suggests that ΔEHOMO‐LUMO values may be lowered significantly by adsorption that would lower chemical stabilities. Significant increases in calculated ΔEHOMO‐LUMO that would increase chemical stability of the compounds were not predicted.

Novak, Jeffrey M.

doi: 10.1002/etc.5620180808pmid: N/A

The effects of two soil properties—soil organic carbon (SOC) content and landscape position—and a management factor (tillage) from Iowa and South Carolina soils on the sorption of the herbicide atrazine (6‐chloro‐N‐ethyl‐Nǐ‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine) were evaluated to assess their influence on atrazine fate in the soil environment. In both Iowa and South Carolina, the magnitude of atrazine sorption was strongly and positively correlated with SOC, landscape position, and tillage. Landscape position was especially important for Iowa soils because SOC‐enriched depressional areas (potholes) on the Wisconsin glacial surface could sorb more atrazine than soils in sloping positions. Our data suggest that herbicide management strategies such as application rates or herbicide selection will require some adjustments to account for the effects of SOC, landscape position, and tillage management on herbicide sorption in to augment effective weed control and prevent herbicide movement to groundwater.

Laird, David A.; Fleming, Pierce D.

doi: 10.1002/etc.5620180809pmid: N/A

Mechanisms by which organic bases are adsorbed on hydrated smectite surfaces were investigated. Three Ca‐saturated reference smectites (Otay, SPV, and Panther Creek) were dispersed in distilled water containing 5 μmol of pyridine or 3‐butylpyridine. The pH was adjusted to between 7.5 and 3 using 0.01 M HCl. After a 2‐h equilibration, the amount of pyridine or 3‐butylpyridine adsorbed on the clay and the amount of Ca desorbed from the clay were determined. Negligible amounts of pyridine were adsorbed by the Ca‐smectites in the neutral systems (pH > 7); however, most of the added pyridine was adsorbed on the smectites in the acidified systems (pH < 5). Equivalent amounts of Ca2+ were desorbed from the clays, indicating that pyridine was adsorbed as a protonated species by cation exchange. By contrast, 40 to 90% of added 3‐butylpyridine was adsorbed on the smectites at neutral pHs, whereas only small amounts of Ca2+ were desorbed. The results suggest that 3‐butylpyridine is initially retained by hydrophobic bonding between the alkyl side chain of the molecule and hydrophobic nanosites located between the charge sites on smectite surfaces. Surface acidity catalyzed protonation 1 to 1.5 pH units above the pKa of the bases.

Kleineidam, Sybille; Rügner, Hermann; Grathwohl, Peter

doi: 10.1002/etc.5620180810pmid: N/A

Groundwater contamination by dissolved organic compounds frequently occurs in valley aquifers that consist of highly heterogeneous sand and gravel sediments. Remediation and risk assessment (e.g. reactive transport modeling) requires detailed information on the sorption/desorption kinetics in such aquifer materials. In this paper we present data on slow sorption kinetics of phenanthrene and the composition of several aquifer materials that are typical for southern Germany and Switzerland. The heterogeneity of the aquifer material is described in terms of the physical and chemical properties (e.g., grain size, organic carbon content, intraparticle porosity, sorption parameters, and rate constants for intraparticle diffusion) of the sediment constituents (lithocomponents). Phenanthrene sorptive uptake in a heterogeneous bulk sample can be predicted using a numerical model only if the composition and geochemical heterogeneity (different sorptivities and porosities of the lithocomponents) are considered. It could be shown that even within a narrow grain size fraction, the geochemical heterogeneity has to be incorporated for the prediction of long‐term sorptive uptake or release of organic contaminants.