Marina Lasagni

A hypothesis on the mechanism of PCDD biological activity based on molecular electrostatic potential modeling. Part 2

We have focused our attention on the electrostatic properties of a set of PCDDs, polychlorinated dibenzo-p-dioxins, of known binding affinities for the Ah receptor. In this paper we analyze the molecular electrostatic potential (MEP) of the six selected isomers, 2,3,6-TrCDD, 1,2,4,7,8-PeCDD, 1,2,3,4-TCDD, 1,2,3,4,7-PeCDD, 1,2,4-TrCDD and 1-MCDD, in order to validate the results previously obtained on a test set of eight PCDDs. A visual analysis of the MEP distributions shows that the stepwise differentiation from the electrostatic characteristics typical of the most active isomers is associated with a decrease in binding affinity; however, the simultaneous occurrence of different trends for different electrostatic requirements for activity does not make for a simple definition of the relationships between MEP patterns and affinities for the whole set of isomers. The information retained in the overall MEP distributions is summed up in a few carefully chosen descriptors, i.e. the MEP values in particular points located around the molecules. A statistical analysis performed by linear and non-linear methods elucidates the electrostatic requirements effective in the recognition process with the Ah receptor; from this there also emerges a physical interpretation of the process. In particular, two different types of electrostatic feature associated with two subsets of PCDD isomers appear to be recognized by the Ah receptor.

The combustion of municipal solid wastes and PCDD and PCDF emissions. Part 2. PCDD and PCDF in stack gases

Abstract Literature data on the presence of PCDDs and PCDFs in the feeding as well as in the emissions of municipal solid waste incinerators have been collected and incorporated in a data base. The presence of PCDDs and PCDFs in MSW and their possible sources are discussed. The congener profiles of PCDD and PCDF have been identified and used as fingerprints for the comparison of the various samples. The possibility that PCDDs and PCDFs present in MSW and RDF come from commercial 5CP, as well as from products which use 5CP or its sodium salt in the manufacturing process, is shown here.

Degradation of Octachlorodibenzofuran and Octachlorodibenzo-p-dioxin Spiked on Fly Ash: Kinetics and Mechanism.

Octachlorodibenzofuran (OCDF) degradation kinetics in a moist nitrogen stream and those of octachlorodibenzo-p-dioxin (OCDD) in a dry or moist airstream on pretreated fly ash were studied in a laboratory pilot plant with temperature ranging from 200 to 350 °C and reaction times varying from 15 to 240 min. The mathematical treatment of experimental data indicated that two reaction pathways, decomposition and dechlorination, operated independently because of a partition of the reagents in the fly ash according to a physical and a chemical adsorption. The decomposition reaction was of the first order with respect to the reactant adsorbed physically. The dechlorination reaction involved first-order series reactions. Here, the kinetic scheme for the first two dechlorination steps is discussed. Kinetic constants and partition constants at different temperatures as well as Arrhenius and Eyring parameters and the isosteric enthalpy of adsorption are calculated.

A selected bibliography on PCDD and PCDF formation

Abstract A list of papers appeared in the worldwide literature in the period 1970-1987 is reported. The papers are classified according to a list of subjects which outlines the most significant aspects in the study of PCDD and PCDF formation.

Toward a mechanistic understanding of PCDD biological activity based on molecular electrostatic potential modeling

Abstract The mechanism of action of polychlorinated dibenzo-p-dioxins (PCDDs) and related compounds has received extensive study but has not yet been completely elucidated. Most of the toxic responses and biological effects appear to be mediated by the binding to the Ah receptor. Our study is focused on the ligand-receptor complex which has a key role in the dioxin mechanism of action. In this paper we seek patterns in the molecular electrostatic potential (MEP) of PCDDs that can be related to their binding affinity. To this aim we analyse the three dimensional distributions of the MEP, obtained in the framework of the HF--SCF theory by ab initio calculations with the 3-21G basis set. A series of isomers showing a significant range of binding affinity values are studied: the choice of isomers belonging to the tetrachlorinated dibenzo-p-dioxin (TCDD) congener group allows us to study the effects of substitution patterns; the comparison with other PCDDs leads us to study the effects of different degrees of chlorination. On the basis of the significant electrostatic features we propose a structural hypothesis for the activity. Relevant information retained in the overall MEP distribution is summarized in a few descriptors, i.e. the MEP values in points properly located around the molecules. A good quantitative model is obtained which relates these descriptors with the experimental pEC50 values.

Biodegradation of variable-chain-length n -alkanes in Rhodococcus opacus R7 and the involvement of an alkane hydroxylase system in the metabolism

Rhodococcus opacus R7 is a Gram-positive bacterium isolated from a polycyclic aromatic hydrocarbon contaminated soil for its versatile metabolism; indeed the strain is able to grow on naphthalene, o-xylene, and several long- and medium-chain n-alkanes. In this work we determined the degradation of n-alkanes in Rhodococcus opacus R7 in presence of n- dodecane (C12), n- hexadecane (C16), n- eicosane (C20), n- tetracosane (C24) and the metabolic pathway in presence of C12. The consumption rate of C12 was 88%, of C16 was 69%, of C20 was 51% and of C24 it was 78%. The decrement of the degradation rate seems to be correlated to the length of the aliphatic chain of these hydrocarbons. On the basis of the metabolic intermediates determined by the R7 growth on C12, our data indicated that R. opacus R7 metabolizes medium-chain n-alkanes by the primary alcohol formation. This represents a difference in comparison with other Rhodococcus strains, in which a mixture of the two alcohols was observed. By GC-MSD analysis we also identified the monocarboxylic acid, confirming the terminal oxidation.Moreover, the alkB gene cluster from R. opacus R7 was isolated and its involvement in the n-alkane degradation system was investigated by the cloning of this genomic region into a shuttle-vector E. coli-Rhodococcus to evaluate the alkane hydroxylase activity. Our results showed an increased biodegradation of C12 in the recombinant strain R. erythropolis AP (pTipQT1-alk R7) in comparison with the wild type strain R. erythropolis AP. These data supported the involvement of the alkB gene cluster in the n-alkane degradation in the R7 strain.

Kinetic modeling of the formation and destruction of polychlorinated dibenzo-p-dioxin and dibenzofuran from fly ash native carbon at 300 °C.

The kinetics for the oxidative breakdown of native carbon in raw fly ash samples (RFA) and for the formation and destruction of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF), abbreviated PCDD/F, were investigated using a flow-over solid system in which the RFA samples were thermally treated at 300 °C under synthetic air. This study investigated the correlation between the disappearance of the reagent and the formation of the products to gain insight into the underlying mechanisms that govern these reactions at congener groups level. The detailed analyses of the experimental concentration-time data revealed significant differences in the behavior between the 2,3,7,8-substituted PCDD and the 2,3,7,8-substituted PCDF, non-2,3,7,8-substituted PCDD and PCDF. The chlorine balance for the former was always negative, that is, chlorine was released regardless of reaction time and primarily resulted from the dechlorination of the hepta- and octa-homologues. However, for the others, the balance was substantially positive up to approximately 240 min and became negative at longer intervals when the dechlorination reactions took over. The processes involving PCDD and PCDF in which the thermal destruction was only partial were found to increase the total equivalent toxicity (TEQ) levels rather than reduce them.

Characterization of fly ash from municipal solid waste incinerators using differential scanning calorimetry

Abstract Endothermic (in the 30–300°C range) and exothermic (in the 300–500°C range) processes were identified through differential scanning calorimetry experiments on fly ash in an air atmosphere. The endothermic processes are thermodynamically controlled and include the desorption of organic compounds along with a phase transition. Instead, exothermic processes are kinetically controlled and include two distinct combustion reactions. Their nature was confirmed by the flatness in DSC scans performed in nitrogen. The apparent activation energies of the two reactions are in very good agreement with those previously obtained from kinetic studies. Based on these laboratory experiments, the energy balance of the thermal process on the fly ash was performed.

The Toxicity Equivalency Factor Scheme Applied To Municipal Incinerator Pcdd/Pcdf Emissions When Specific Congener Information Is Lacking

Abstract The 2,3,7,8-TODD Toxicity EQuivalents (TEQ) approach provides a useful method for estimating the environmental risk of polychlorodibenzodioxin and polychlorodibenzofuran complex mixtures, thus making it possible to express their toxicological significance with only one number. Generally the conversion of the concentration units into those of TEQ requires an isomer-specific analysis and the appropriate Toxicity Equivalency Factors (TEF). In this paper we propose a “theoretical” model for estimating TEQ values of PCDD + PCDF mixtures which, starting from the experimental value of the PCDD + PCDF concentration (μg Nm −3 ) and the theoretical value of PCDD and PCDF fingerprints in the emissions from municipal incinerators, to make an estimate of the global toxicity in terms of ng TEQ Nm −3 . The TEQ values estimated for a number of samples are in good agreement with those calculated from the isomer and congener experimental data. Given the investment and running costs plus the time and the analytical complexity in determining the congeners and single isomer concentrations, it seems that the proposed “theoretical” model is a reasonable tool for estimating the exposure hazard related to a complex mixture of PCDD/PCDFs emitted from municipal solid waste (MSW) incinerators. Another objective of this paper is to explore the consistency between scientific knowledge and the authority regulatory guidelines. This work shows that the Italian national emission standard (4 μg Nm −3 ) for the total PCDD+PCDF concentration is higher than the international target of 0.1 ng TEQ Nm −3 . We suggest a value of 1 μg Nm −3 for TCDD + TCDF concentration if Italian regulators want to add a further constraint.

Lab-scale pyrolysis of the Automotive Shredder Residue light fraction and characterization of tar and solid products

The general aim of this study is the recovery of Automotive Shredder Residue (ASR). The ASR light fraction, or car fluff, that was collected at an Italian shredding plant was pyrolysed at various temperatures (500-800°C) in a lab-scale reactor. The condensable gases (tar) and solid residue yields increased with decreasing temperature, and these products were characterized to suggest a potential use to reclaim them. The higher heating value (HHV) of tar was 34-37MJ/kg, which is comparable with those of fossil fuels. Furthermore, the ash content was low (0.06-4.98%). Thus, tar can be used as an alternative fuel. With this prospect, the concentrations of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in tar were determined. The toxicity of tar changes with temperature (1-5ng I-TEQ/g), and the PCDFs significantly contribute to tar toxicity, which was 75-100% with a maximum of 99.6% at 700°C. Regarding the characterization of the solid residue, the low HHV (2.4-3.3MJ/kg) does not make it suitable for energy recovery. Regarding material recovery, we considered its use as a filler in construction materials or a secondary source for metals. It shows a high metal concentration (280,000-395,000mg/kg), which is similar at different pyrolysis temperatures. At 500°C, polycyclic aromatic hydrocarbons (PAHs) were not detected in the solid residue, whereas the maximum total PAH concentration (19.41ng/g, 700°C) was lower than that in fly ash from MSWI. In conclusion, 500°C is a suitable pyrolysis temperature to obtain valuable tar and solid residue.