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. Desorption rate constants appear to decrease with increasing contaminant n-octanol-water partition coefficient. The ratio of the fraction of contaminant desorbed from sediment to Tenax in 6h and the rapidly desorbing fraction appeared to slightly decrease on increasing contaminant hydrophobicity. This was not the case for the extraction for 24h. Rapidly desorbing fractions or bioavailable fractions can be estimated, within a factor of 1.4, by multiplying the fraction desorbed in 24h by a factor of 0.7.

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. For air-organic solvent partition, the Abraham solvation parameter V can be omitted from the Goss-modified Abraham solvation equation without any loss of statistical quality. For air-water partitioning, organic biphasic system partitioning, as well as water-organic solvent partitioning, omitting the V parameter from the Goss-modified Abraham solvation equation leads to only a small deterioration of statistic quality.

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. To that end, a modification of the description of the Ben-Naim standard state into the van der Waals volume is proposed. Differences between predicted and fitted values of the Abraham solvation equation intercept for the enthalpy of solvation, the entropy of solvation, solvent-water partitioning, air-solvent partitioning, partitioning into micelles, partitioning into lipid membranes and lipids, and chromatographic retention indices are comparable to experimental uncertainties in these values.

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). For branched alkane vapor pressures, the McGowan specific volume is a poor predictor. Therefore, in this study a correction on the McGowan specific volume is derived in terms of the number of branches and the number of pairs of vicinal branches to improve the prediction of branched alkane vapor pressures. This branching correction also brought branched/alkane solvent accessible volumes, octanol/water partition coefficients, air/hexadecane partition coefficients, and aqueous solubilities as well as alkyl-branched substituted aliphatic hydrocarbon air/hexadecane partition coefficients more in line with corresponding linear hydrocarbon properties when compared on a McGowan specific volume basis. Even for air-hexadecane partition coefficients of substituted aliphatic hydrocarbons with substituents at non-terminal carbons, application of the branching correction to the carbon bearing the substituent caused these partition coefficients to be more in line with those for linear compounds. Values for the Abraham A and B solvation parameters for nonlinear aliphatic ethers, amines, and alcohols, recalculated using branching corrected McGowan specific volumes, turned out to be closer to chemical expectations based on linear aliphatic ether, amine and alcohol values compared to previously reported experimental values obtained using uncorrected McGowan specific volumes. A comparison of alkylbenzene and alkene partition coefficient estimates from two different linear solvation energy relations, one containing a McGowan specific volume term and one without such a term, suggests that no branching correction is needed for alkyl groups at sp2 carbons. The main advantage of using branching corrected McGowan specific volumes is that the values of other solvation parameters become chemically more consistent.

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. The updated PCB solvation parameters were tested on partitioning between five other phase combinations. Compared to the original PCB solvation parameter set, the updated PCB solvation parameters resulted in substantially improved estimates from Abraham solvation equations for (subcooled) liquid vapor pressures, aqueous solubilities, HPLC capacity factors, and for coefficients of air-n-hexadecane, air-water, organic carbon-water, and n-octanol-water partitioning. For water to polydimethyl siloxane and sodium dodecylsulphate (SDS) partitioning, the updated PCB solvation parameters yielded no improvement compared to the original data set. The main difference between the updated and the original parameter set is that updated PCB McGowan specific volumes depend on the degree of ortho-chlorination, which is qualitatively confirmed by trends in the PCB solvent accessible volumes and surface areas. The use of the updated PCB solvation parameters instead of the original values is therefore recommended.

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. The analysis also identified which type of transport resistance governed sediment water exchange. For seven inland locations, exchange was limited by benthic boundary layer transport, showing no dependencies on sediment or chemical properties other than concentration. For one river mouth location, exchange was limited by slow in-bed intraparticle diffusion. A biphasic dual compartment radial diffusion model adequately described the data for this location. Fast desorption was interpreted as molecular diffusion retarded by microscale dual domain sorption to amorphous as well as black carbon (BC). Slow desorption was invariant with LogK(ow) and consistent with intraorganic matter diffusion through BC particles. Finally, it is discussed how these findings can be translated into a general framework for flux based exposure assessment.

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. The estimation procedure uses a linear combination of simple molecular descriptors for the molecular structure variation within the compound class and a polynomial for the temperature of melting. This procedure is applied to the vapour pressure of polycyclic aromatic hydrocarbons (PAHs), alkylated PAHs, polychlorinated dibenzo-p-dioxins and biphenyls and to the aqueous solubility of PAHs, methylated PAHs, chlorinated benzenes, polychlorinated and polybrominated biphenyls, chlorinated phenols, cresols, and chlorinated 2-methoxyphenols. The standard error of the solid vapour pressure or aqueous solubility estimates from the various compound-class specific regression equations was about 0.2 log units. For PAHs, chlorobenzenes, and PCBs used in the present study, aqueous solubility estimated from the regression equations taking the temperature of melting equal to 298 K, i.e. assuming that the compounds are in a hypothetical liquid state, was equal, within 0.1-0.3 log units to the subcooled liquid solubility estimated from literature regression equations.

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, 0.15 and 0.12-0.3 log units, respectively.

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. Heat capacity data are even scarcer. In the present study, semi-empirical compound class specific equations were derived to estimate fugacity ratios from molecular weight and melting temperature for polycyclic aromatic hydrocarbons and polychlorinated benzenes, biphenyls, dibenzo[p]dioxins and dibenzofurans. These equations estimate fugacity ratios with an average standard error of about 0.05 log units. In addition, for compounds with known fusion entropy values, a general semi-empirical correction equation based on molecular weight and melting temperature was derived for estimation of the contribution of heat capacity differences to the fugacity ratio. This equation estimates the heat capacity contribution correction factor with an average standard error of 0.02 log units for polycyclic aromatic hydrocarbons, polychlorinated benzenes, biphenyls, dibenzo[p]dioxins and dibenzofurans.

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. Such was also the case for logS(l). Slopes for the linear relation of chlorobenzenes and PCB logK(oc) values with the number of chlorines were similar for the various soils and sediments, though intercepts were different. The ortho-chlorine correction factor for PCB logK(oc) was equal to the ortho-chlorine correction factor for PCB logK(ow) and logS(l). For PCDDs and PCDFs, a relation between logK(oc) and the number of chlorine atoms was derived by combining the chlorobenzenes/PCB logK(oc)-logK(ow) and logK(oc)-logS(l) relationships with logK(ow) (or S(l))-chlorine number relations for PCDDs and PCDFs.

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. From the linear regressions for linear partitioning into the various amorphous organic carbons, an average intercept for the linear partitioning regression equation was derived to represent average organic carbon in soils and sediments.

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. Rate constants for desorption to water from these two types of sites in the three activated carbons did not reveal a relation with activated carbon grain size. These rate constants were comparable to those for desorption of various organic compounds from hard carbon in various sediments.

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. The values for the maximum amounts adsorbed onto high-energy sites revealed that, after equilibration of the first solute, still some high-energy sites could be occupied by the second solute. Phenanthrene, chrysene, and 2,5-dichlorobiphenyl seem to share about 30% of the accessible high-energy adsorption sites in the sediment employed.

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. Fugacity ratios are usually calculated from fusion thermodynamic data. For 41 polychlorinated biphenyls (PCBs), fusion enthalpy was found to be correlated with fusion entropy. Highly linear correlations were observed for non-ortho-PCBs, mono-ortho-PCBs, and diortho-PCBs. Fugacity ratios estimated from the fusion enthalpy-entropy linear regression parameters were equal, within 10% on average, to fugacity ratios calculated from fusion enthalpy for ortho chlorinated PCBs with melting points below 380 K and for non-ortho-PCBs. For ortho chlorinated PCBs with melting points above 380 K, fugacity ratios were better estimated from a nonlinear regression of fugacity ratios against the melting point and the system temperature. For all 209 PCB congeners, fugacity ratios at 298 K are listed on the basis of experimental fusion data or estimates from the regressions.

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). A statistical analysis, accounting for differences in the number of fitting parameters, demonstrates that the dual Langmuir equation is in general superior and/or preferable to the single and triple Langmuir equation, the Freundlich equation, a Polanyi-Dubinin-Manes equation, and the Toth equation. Consequently, the analysis suggests the presence of two types of adsorption sites: a high-energy (HE) type of site and a low-energy (LE) type of site. Maximum adsorption capacities for the HE domain decreased with temperature while those for the LE domain increased. Average Gibbs free energies for adsorption from the hypothetical pure liquid state at 298 K were fairly constant at -15+/-4 and -5+/-4 kJ mol(-1) for the HE and LE domain, respectively.

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. For the sediments, 41-84% of the adsorption sites appeared to be occupied by native compounds.

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. The time needed to reach apparent equilibrium in the experimental setup, with 10 g sediment/L water, ranged from 13 to 166 h, depending on the sorbate and the adsorption process. These short times to equilibrium suggest no need to assume rate-limiting diffusion from and to adsorption sites in this sediment. Average Gibbs free energies for adsorption of the four organochlorine compounds from the pure solid state were -10 +/- 3 and -20 +/- 3 kJ/mol for low-energy and high-energy sites, respectively, pointing to two different adsorption mechanisms.

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. We review evidence that black carbon may strongly reduce the risk posed by organic contaminants in sediments and soils. Extremely efficient sorption to black carbon pulls highly toxic polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins, polybrominated diphenylethers and pesticides into sediments and soils. This increased sorption is general, but strongest for planar (most toxic) compounds at environmentally relevant, low aqueous concentrations. Black carbon generally comprises about 9% of total organic carbon in aquatic sediments (median value of 300 sediments), and then may reduce uptake in organisms by up to two orders of magnitude. This implies that current environmental risk assessment systems for these contaminants may be unnecessarily safe.

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. Generally, for a broad range of nonpolar organic compounds, from substituted benzenes to five-ring polycyclic aromatic hydrocarbons and hexachlorinated biphenyls, sorbate molecular symmetry-corrected Gibbs free energies for high-energy and low-energy adsorption relative to the pure solid state were within a narrow range or approximately -23 and -11 kJ/mol, respectively. These two average values for the geosorbents were comparable to corresponding values for adsorption onto activated carbon.