Emissions of tar-containing binders: field studies.

This study describes the measurement of emissions during field construction of asphalt pavements using tar-containing recycled asphalt pavement (RAP), which is known to release harmful substances, such as polycyclic aromatic hydrocarbons (PAH). At three different test sites, the main emission sources were identified and the total emission rates of fumes and PAHs of the paving process were determined. For this purpose, the paver was temporarily enclosed. While the screed area was the main emission source, the hopper area and freshly compacted pavement were also significant. In comparison with previous laboratory tests, the binder composition and the resulting emissions were comparable, except for Naphthalene. Benzo(a)pyrene (BaP) as a representative for carcinogenic PAHs was identified as a good leading compound, correlating well with the toxicity weighted sum of PAHs. In contrast, the unweighted, mass related sum of all EPA PAHs does not seem to be a good parameter to assess workplace concentrations because emissions by mass are dominated by the less hazardous 2-, 3- and 4-ring PAHs. Workplace concentrations for bitumen fumes and PAHs were below limit values in all three field studies. However, the margin was not large and the field tests were done under favourable meteorological conditions. Therefore, we suggest maintaining the current Swiss limit of 5000 mg EPA-PAH per kg binder in the RAP-containing hot mix.

Regio- and stereoselective isomerization of hexabromocyclododecanes (HBCDs): kinetics and mechanism of gamma- to alpha-HBCD isomerization.

Hexabromocyclododecanes (HBCDs) are high production volume chemicals (>20000 ty(-1)) used as flame retardants for plastics and textiles. Lately, we reported on the stereoselective isomerization of beta-HBCDs. Herein we present insights into the mechanism and kinetics of (+)gamma- to (+)alpha- and of (-)gamma- to (-)alpha-HBCD isomerization. Only two of the six bromine atoms migrated, indicating that rearrangements of gamma- to alpha-HBCDs are regio- and stereoselective as well. The apparent first-order isomerization rate constants increased from 0.0013 to 0.0031 to 0.0070 min(-1) at 120, 130, and 140 degrees C, respectively, corresponding to half-lives of 540, 230, and 99 min. Thus, a thermal treatment of materials containing gamma-HBCDs at temperatures >100 degrees C may induce the formation of alpha-HBCDs and, hence, may alter the diastereomeric ratio of a HBCD mixture. The inversion of vicinal dibromides in like-configurations (RR/SS) prevailed, whereas unlike-configurations (RS/SR) were not affected. An intramolecular, stereoselective migration of neighboring bromine atoms via a four-center transition state would explain the observed stereoisomer pattern and first-order kinetics. Despite the fact that vicinal dibromides in HBCDs prefer synclinal (gauche) conformations, antiperiplanar (staggered) conformations are assumed to facilitate concerted 1.2-shifts of both bromine atoms. A conformation analysis revealed that under kinetic control, only those bromine atoms in the more flexible part of the molecules are migrating, whereas those in the conserved triple-turn motive were not affected. Thus, this structural motive, common to all alpha-, beta-, and gamma-HBCDs, is more rigid and less reactive than the flexible part, containing the reacting dibromides in like-configurations.

Regio- and stereoselective isomerization of hexabromocyclododecanes (HBCDs): kinetics and mechanism of beta-HBCD racemization.

Hexabromocyclododecanes (HBCDs) are high production volume chemicals currently produced in quantities exceeding 20000ty(-1). They are used as flame retardants for plastics and textiles. HBCDs are thermally labile compounds, rapidly decomposing at temperatures above 250 degrees C to form bromine radicals, which scavenge other radicals formed during pyrolysis. But certain HBCD stereoisomers must reach the environment without decomposition, because their levels in soils, sediments, and biota are increasing worldwide. The fate of individual HBCD stereoisomers during production, product use, disposal, and transformation in the environment remains unclear. Herein we report on the thermally induced, highly selective isomerization of (+) and (-)beta-HBCD. Regio- and stereoselective migration of only two of the six bromine atoms resulted in the racemization of both beta-HBCDs. First order rate constants (k(rac)) increased from 0.005, 0.011, 0.021, to 0.055min(-1) at 130, 140, 150, and 160 degrees C, corresponding to half life times tau(1/2) of 143, 63, 29, and 14min, respectively. From the deduced kinetic model, we conclude that any thermal treatment of enantiomerically enriched beta-HBCDs in the range of 100-160 degrees C will result in a loss of most optical activity within few hours. The simultaneous inversion of two asymmetric centers occurred with perfect stereocontrol. Selectively, vicinal dibromides with the RR- and the SS-configurations migrated at these temperatures. An intramolecular reaction mechanism with a four-center transition state is postulated, based on the obtained stereoisomer pattern and the observed reaction kinetics. Crystal structure analysis revealed that all vicinal dibromides in beta-HBCDs prefer synclinal (gauche) conformations. However, an antiperiplanar (staggered) conformation is assumed to facilitate the concerted 1.2-shifts of both bromine atoms, resulting in an inversion of both neighboring carbon atoms. First experiments with other HBCD stereoisomers suggest that the presented isomerization mechanism is of relevance for those stereoisomers as well.

Solid-state conformations and absolute configurations of (+) and (-) alpha-, beta-, and gamma-hexabromocyclododecanes (HBCDs).

Hexabromocyclododecanes (HBCDs) are high production volume chemicals used as flame retardants for plastics and textiles. They are currently produced in quantities exceeding 20,000 t/y. Despite this fact, the correct stereochemistry of most HBCDs is still not known. Six stereocenters are formed during bromination of cyclododecatrienes, resulting in mixtures of different stereoisomers. Considering all elements of symmetry, 16 different stereoisomers including six pairs of enantiomers as well as 4 meso forms are possible theoretically. Recently, we isolated 8 of the 16 possible stereoisomers from a technical HBCD mixture and assigned their relative configurations. Herein, we report on the isolation of 6 enantiomerically pure alpha-, beta-, and gamma-HBCDs, obtained from preparative chiral-phase liquid chromatography, and we present their absolute configurations, which were determined from X-ray diffraction analysis. The absolute configuration of (-) alpha-HBCD was found to be (1R,2R,5S,6R,9R,10S), while the one of (+) beta-HBCD is assigned to (1S,2S,5S,6R,9S,10R), whereas the one of (-) gamma-HBCD corresponds to (1S,2S,5S,6R,9R,10S). The given structural information allows the unambiguous identification of the six most important HBCD stereoisomers, which typically account for more than 95% of technical HBCDs. In addition, we compared the solid-state conformations of racemic and enantiomerically pure alpha-, beta-, and gamma-HBCDs. In all cases, vicinal dibromides adopted a synclinal (sc) conformation with torsion angles of 69+/-6 degrees. A unique structural motive was common to all examined HBCD solid-state conformations. This conserved structure was described as an extended triple turn consisting of an arrangement of three pairs of synclinal and two antiperiplanar torsion angles.

Emissions of tar-containing binders: a laboratory study.

In Switzerland, hot recycling of tar-containing pavements is a subject of much dispute between environmentalists, road authorities and constructors. The main reason for this controversy comes from a lack of knowledge about the amount of hazardous compounds emitted, especially polycyclic aromatic hydrocarbons (PAHs), and the resulting health risk for road workers. On this background we decided to initiate a research project to study the emission behaviour of tar-containing materials. Mixtures of tar and bitumen with variable PAH content were heated in an open reactor flask at different temperatures to identify and quantify the key parameters of PAH emissions. The expected linear correlation between PAH concentration in the fumes and in the binder was found only for binder mixtures with PAH concentrations above 5000 ppm. This was traced back to the problem that a change of the PAH content in the binder was always accompanied by a change of other parameters, like viscosity. In the experiments with temperature variation, emissions of individual PAHs correlated well with vapor pressure. However, for Naphthalene and, in a lesser degree, for 3-ring PAHs too, a partial depletion of these compounds in the vapour was observed in some experiments. The effect is a slower increase with temperature for these compounds compared to 4-6-ring PAHs. This is one reason why the commonly used set of EPA-PAHs, which includes naphthalene and 3-ring PAHs, is considered inappropriate for the assessment of the health hazard in the case of tar-containing materials in hot recycling.

Crystal structure analysis of enantiomerically pure (+) and (-) beta-hexabromocyclododecanes.

The molecular structures of individual HBCD stereoisomers are not elucidated yet. Recently, we isolated 8 of the 16 possible stereoisomers from a technical HBCD mixture and tentatively assigned their relative configurations. Herein we report on the isolation of enantiomerically pure (+) and (-) beta-HBCDs, both obtained from preparative chiral-phase liquid chromatography, and we present their absolute configurations determined from X-ray diffraction analysis. The absolute configuration of (+) beta-HBCD was found to be (1S,2S,5S,6R,9S,10R), while the one of (-) beta-HBCD was assigned to (1R,2R,5R,6S,9R,10S). The given structural information allows, for the first time, the unambiguous identification of these two important HBCD stereoisomers, which are typically found in technical products at proportions of about 3-5% for each enantiomer.