Analysis of organic contaminant desorption kinetic data for sediments and soils: implications for the Tenax extraction time for the determination of bioavailable concentrations.

Solid-phase extractions with adsorbents like Tenax have been widely used to assess bioaccessible or bioavailable concentrations and non-extractable residues (NER) of organic contaminants in soils or sediments. This paper presents an analysis of literature rate constants and fractions for rapid, slow and very slow contaminant desorption from soils and sediments. Contaminant fractions desorbed from sediment to Tenax in 6 or 24h were evaluated as to their adequacy as a proxy for rapidly desorbing fractions, which have been shown to correlate with bioavailable concentrations.

A possible simplification of the Goss-modified Abraham solvation equation.

Abraham solvation equations find widespread use in environmental chemistry and pharmaco-chemistry. Recently Goss proposed a modified Abraham solvation equation. For various partitioning processes, the present study investigates the consequences for the fit when the Abraham solvation parameter V is left out of this modified solvation equation.

Solvation thermodynamics and the physical-chemical meaning of the constant in Abraham solvation equations.

Abraham solvation equations find widespread use in environmental chemistry. Until now, the intercept in these equations was determined by fitting experimental data. To simplify the determination of the coefficients in Abraham solvation equations, this study derives theoretical expressions for the value of the intercept for various partition processes.

A simple McGowan specific volume correction for branching in hydrocarbons and its consequences for some other solvation parameter values.

Differences in molecular properties between linear and branched alkanes as well as between compounds with branched alkyl groups is of relevance due to the large number of branched isomers of environmentally relevant compounds (e.g. fuels, fuel additives, surfactants).

Updated Abraham solvation parameters for polychlorinated biphenyls.

This study shows that the recently published polychlorinated biphenyl (PCB) Abraham solvation parameters predict PCB air-n-hexadecane and n-octanol-water partition coefficients very poorly, especially for highly ortho-chlorinated congeners. Therefore, an updated set of PCB solvation parameters was derived from four PCB properties and associated Abraham solvation equations. Additionally, the influence of ortho-chlorination on PCB solvent accessible volume and surface area was investigated.

Estimation of in situ sediment-to-water fluxes of polycyclic aromatic hydrocarbons, polychlorobiphenyls and polybrominated diphenylethers.

Sediment-water fluxes of hydrophobic organic chemicals (HOC) may affect the quality of surface waters. Here, we present an approach to derive such fluxes from (a) in situ HOC concentration gradients measured with passive samplers and (b) mass transfer coefficients measured with a novel flux method using Empore disks. For eight undisturbed sediments, this method identified whether the sediment acted as a source or as a sink for HOCs.

QSPRs for the estimation of subcooled liquid vapor pressures of polycyclic aromatic hydrocarbons, and of polychlorinated benzenes, biphenyls, dibenzo-p-dioxins, and dibenzofurans at environmentally relevant temperatures.

This study aims to develop estimation procedures for subcooled liquid vapor pressures of polycyclic aromatic hydrocarbons (PAHs), and of polychlorinated benzenes, biphenyls (PCBs), dibenzo-p-dioxines (PCDDs), and dibenzofurans (PCDFs) based on quantitative structure-property relationships (QSPRs) for the subcooled liquid vaporization enthalpy and entropy in terms of simple molecular structure descriptors and the system temperature. It turned out that subcooled liquid vaporization enthalpies and entropies for these compound classes can be estimated from the number of carbon atoms, the number of chlorine atoms, the number of PCB ortho-chlorine atoms and the system temperature. Subcooled liquid vapor pressures at 298 K calculated from the estimated vaporization enthalpies and entropies were equal to directly measured experimental values as well as to experimental values determined by gas chromatographic methods within, on average, 0.

Compound-class specific estimation of solid organic compound vapour pressure and aqueous solubility from simple molecular structure descriptors and the temperature of melting.

For many solid organic compounds, experimental data for their aqueous solubility and vapour pressure are lacking. Therefore, estimation procedures for these compound properties are needed. On theoretical grounds, this study derives a general compound-class specific estimation procedure for solid organic compound aqueous solubility and vapour pressure.

Semi-empirical estimation of organic compound fugacity ratios at environmentally relevant system temperatures.

Fugacity ratios of organic compounds are used to calculate (subcooled) liquid properties, such as solubility or vapour pressure, from solid properties and vice versa. They can be calculated from the entropy of fusion, the melting temperature, and heat capacity data for the solid and the liquid. For many organic compounds, values for the fusion entropy are lacking.

Estimation of amorphous organic carbon/water partition coefficients, subcooled aqueous solubilities, and n-octanol/water distribution coefficients of alkylbenzenes and polycyclic aromatic hydrocarbons.

The aim of this work was to derive a relation between the number of specific carbon atoms in alkylbenzenes and PAHs and the average logK(oc) for linear partitioning between amorphous organic carbon in soils and sediments and water. The relation between the number of specific carbon atoms and n-octanol/water partitioning and subcooled aqueous solubility was sought first, as the number of data for partitioning into amorphous organic carbon was relatively sparse. It turned out that linear partitioning into amorphous organic carbon could be described by a linear relation based on the number of aromatic carbons, the number of alkyl carbons and the number of alicyclic carbons in the same way as for n-octanol/water partitioning and subcooled liquid aqueous solubility.

Estimation of amorphous organic carbon/water partition coefficients, subcooled liquid aqueous solubilities, and n-octanol/water partition coefficients of nonpolar chlorinated aromatic compounds from chlorine fragment constants.

This study aims to derive the relation between the number of chlorine atoms in chlorobenzenes, polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs) and logK(oc) for linear partitioning between water and average amorphous organic carbon in soils or sediments. Because reliable determinations of logK(oc) are relatively sparse for chlorobenzenes and PCBs, and are even absent for PCDDs and PCDFs, a work-around solution was developed: First, the relation of n-octanol/water partitioning (K(ow)) and (subcooled) liquid solubility (S(l)) to the number of chlorines was investigated. Slopes for the linear correlation of logK(ow) with the number of chlorines (corrected for the number of ortho-chlorines in the case of PCBs) appeared identical for chlorobenzenes, PCBs, PCDDs, and PCDFs.

Kinetics of phenanthrene desorption from activated carbons to water.

We determined the kinetics of phenanthrene desorption from three activated carbons to water using Tenax beads as an infinite sink for organic compounds in water. Desorption kinetic data very well fitted a biphasic kinetic model based on the presence of two different adsorption sites, viz. low-energy sites and high-energy sites.

Competition between phenanthrene, chrysene, and 2,5-dichlorobiphenyl for high-energy adsorption sites in a sediment.

We determined the maximum amounts of added phenanthrene, chrysene, and 2,5-dichlorobiphenyl sorbed onto high-energy adsorption sites in a sediment on bi-solute experiments. The bi-solute pairs were phenanthrene/chrysene and phenanthrene/2,5-dichlorobiphenyl. On the bi-solute sorption experiments, one solute was introduced and equilibrated with sediment prior to addition of the second solute.

Gibbs free energies for dual Langmuir-like adsorption onto hard carbon materials in sediment and soils.

Adsorption of organic compounds onto hard carbon constituents of soils and sediments may be described by a dual Langmuir-like equation for adsorption onto high-energy sites and low-energy sites. To describe quantitatively the sorbate-sorbent interactions on these high-energy sites and low-energy sites, Gibbs free energies for adsorption onto several soils and sediments were calculated using suitable experimental sorption data from the literature. A large part of the variation in these Gibbs free energies relative to the pure solid state appeared to be related to differences in sorbate molecular symmetry.

Estimation of polychlorinated biphenyl fugacity ratios.

On the quantitative comparison of solubilities or vapor pressures of homologous series, the variation in the effect of crystal structure on solid properties may substantially influence the outcome of the comparison. Usually, the effect of this variation is eliminated by comparing values of the liquid state. The ratio of solid to liquid properties is called the fugacity ratio.

Sorption of organic compounds to activated carbons. Evaluation of isotherm models.

Sorption to "hard carbon" (black carbon, coal, kerogen) in soils and sediments is of major importance for risk assessment of organic pollutants. We argue that activated carbon (AC) may be considered a model sorbent for hard carbon. Here, we evaluate six sorption models on a literature dataset for sorption of 12 compounds onto 12 ACs and one charcoal, at different temperatures (79 isotherms in total).

Kinetics of adsorption and desorption of some organochlorine compounds on black carbon in a sediment.

Rate constants for adsorption and desorption of four organochlorine compounds on black carbon in a sediment were determined from measurements of the rate of removal, by gas purge, of the organochlorine compounds as single solutes from a water-sediment mixture immediately after addition of the solute to the system. The rates of removal fitted to a kinetic scheme based on Langmuir adsorption onto two types of sites in black carbon. The first-order rate constants for desorption from these sites were comparable to those for slow and very slow desorption from sediment.

Quantification of total native compounds on phenanthrene-specific adsorption sites in the very slow desorption domain of 16 sediments and soils.

We determined the maximum amount of added phenanthrene that could be adsorbed in the very slow desorption domain of 16 sediments and soils with and without native compounds present. The differences in the amount of phenanthrene taken up in this domain with and without native compounds present indicates to what extent native compounds occupy those adsorption sites in the very slow desorption domain which may accommodate phenanthrene. For the two aquifer materials, presence of native compounds was less than the uncertainty associated with the methodology.

Black carbon: the reverse of its dark side.

The emission of black carbon is known to cause major environmental problems. Black carbon particles contribute to global warming, carry carcinogenic compounds and cause serious health risks. Here, we show another side of the coin.

Extensive sorption of organic compounds to black carbon, coal, and kerogen in sediments and soils: mechanisms and consequences for distribution, bioaccumulation, and biodegradation.

Evidence is accumulating that sorption of organic chemicals to soils and sediments can be described by "dual-mode sorption": absorption in amorphous organic matter (AOM) and adsorption to carbonaceous materials such as black carbon (BC), coal, and kerogen, collectively termed "carbonaceous geosorbents" (CG). Median BC contents as a fraction of total organic carbon are 9% for sediments (number of sediments, n approximately 300) and 4% for soils (n = 90). Adsorption of organic compounds to CG is nonlinear and generally exceeds absorption in AOM by a factor of 10-100.

Influence of long contact times on sediment sorption kinetics of spiked chlorinated compounds.

The desorption kinetics of hexachlorobenzene (HCB) and 2,4,4'-trichlororbiphenyl (PCB 28) spiked to a field sediment were studied using a gas-purge technique. A contact time of up to 1,461 d was used to assess long-term changes in desorption kinetics. Purge-induced desorption experiments lasted from 300 to more than 4,000 h.

Modeling maximum adsorption capacities of soot and soot-like materials for PAHs and PCBs.

Recent studies have shown that not partitioning but adsorption is the main mechanism for sorption of hydrophobic organic compounds to soot and soot-like materials. For compounds that adsorb by van derWaals forces only, variation in soot-water distribution coefficients will result from differences in these forces for adsorption, as well as the maximum number of accessible sites. This maximum number of accessible sites may a priori be expected to vary due to differences in both sorbent characteristics and sorbate dimensions.

Temperature effects on very slow desorption of native chlorobenzenes from sediment to water.

The temperature dependence of the kinetics of very slow desorption of eight chlorobenzenes was studied in laboratory batch experiments on a field-contaminated sediment from Lake Ketelmeer, The Netherlands. The observed rate constants for very slow desorption averaged (1.5 +/- 0.

Fugacity ratio estimations for high-melting rigid aromatic compounds.

Prediction of the environmental fate of organic compounds requires knowledge of their tendency to stay in the gas and water phase. Vapor pressure and aqueous solubility are commonly used descriptors for these processes. Depending on the type of distribution process, values for either the pure solid state or the (subcooled) liquid state have to be used.