Patterns of isomers of chlorinated dibenzo-p-dioxins as tool for elucidation of thermal formation mechanisms

Chemosphere, Vol. 36, No. 13, pp. 2775-2801, 1998

Pergamon

PII: S0045-6535(97) 10236-3

PATTERNS

OF ISOMERS

FOR E L U C I D A T I O N

OF CHLORINATED OF THERMAL

© 1998ElsevierScienceLtd All rightsreserved.Printedin GreatBritain 0045-6535/9S$19.00+0.00

DIBENZO-P-DIOXINS

FORMATION

AS TOOL

MECHANISMS

A. Wehrmeier', D. Lenoira'', K.-W. Schramm a, R. Zimmermannc, K. Hahn b, B. Henkelmann a, A. KettrupLc

GSF-Forschungszentrum far Umwelt und Gesundheit, Institut far Okologische Chemic', Rechenzentrumb, Ingolst~idter Landstr. 1, D-85764 Neuherberg, Germany Lehrstuhl far Okologische Chemic und Umweltanalytikc, D-85350 Freising, Germany (Receivedin Germany16 September1997;accepted26 November1997)

ABSTRACT

This study gives a statistically derived proof for the existence of typical patterns for isomers of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) formed as trace byproducts of incomplete combustion. A large number and variety of samples related to combustion was analyzed for the concentrations of the PCDD/F congeners. The resulting data set was subjected to Principal Component Analysis to show similarities in isomeric patterns of either homologue group. The first principal component (PRIN1) gave a good (83-91%) description of the tetra- to heptachlorodibenzo-p-dioxins and -furans for most samples. It provided a tool to compare patterns of isomers of PCDD/F formation from laboratory scale combustion of precursors on annealed fly ash, calculated thermodynamic stability and calculated reactivity. The aim of the combined statistical and experimental studies was to find relevant pathways for thermal PCDD/F formation. Therefore, it was necessary to prove which precursors were relevant and if the observed patterns were controlled either by thermodynamic stability of the compounds or kinetic processes. The investigated precursors led to thermal formation of PCDD/F at 350°C and nearly all resulting isomeric patterns corresponded to the statistically derived combustion patterns of PCDD/F. Relative abundance calculated from thermodynamic stability of the PCDD isomers showed similarities, but also distinct differences with the combustion patterns. So did isomer distributions according to calculated HOMO-LUMO energies, which are quantitative measures for reactivity of congeners. A model, however, which was derived from a superposition of thermodynamic stability and reactivity of PCDD isomers, gave a qualitative description of the typical PCDD combustion patterns. ~©1998ElsevierScienceLtd. All rights reserved 2775

2776 1. INTRODUCTION

The results of numerous measurements of concentrations of polychlorinated dibenzo-p-dioxins (PCDD) and -furans (PCDF), either isomer or homologue specific had been reported in the last two decades. As the occurrence of PCDD/F in the environment is mostly related to combustion or other thermal processes, these measurements include a large number of combustion related data. The total number of PCDD/F congeners is 210, therefore the GC-MS measurements led to huge data sets, which are difficult to handle. Hence many data sets were reduced to relevant parameters. For example, as the common interest was focused on the toxicity often toxicity equivalents (I-TEQ) values were compared [1]. Several studies dealt with the formation and fate of tetra- to oetaehlorinated dibenzo-p-dioxins (PCDD) and -furans (PCDF) in the environment [1]. The homologue patterns of polychlorinated dibenzo-p-dioxins apparent in different samples were used for elucidation of possible sources of PCDD or to understand the environmental transformation processes of emitted PCDD [2]. However, if one was interested in the thermal formation mechanism of these compounds analytical information of all PCDD congeners and possible precursors were useful in order to get an insight in relevant reaction pathways. Since only some of the relevant precursors and intermediates for the catalytic thermal PCDD formation are known yet and even therefore a complete analysis is a priori impossible. The previous research was mainly done in either focusing on the analytic of possible precursors, in analyzing the PCDD congener concentrations as products of incomplete combustion or from thermal treatments of precursor under defined laboratory scale conditions. Results of these approaches were often combined to point out the relevant PCDD formation pathway, however more or less different reaction conditions were used in different experiments. For example, the predominant catalytic matrix for PCDD/F formation, the fly ash, was mostly replaced by model fly ash mixtures or annealed fly ashes in laboratory scale experiments. It was suggested that relative concentrations of PCDD isomers found as products of different incomplete combustion tended to be in the same order [3]. In other words, same relative concentrations of isomer pattern were found in each group of homologues. Up to now, statistical proof for these suggestions were only given for the patterns of PCDD/F homologues [2]. However, it was reported that e.g. special model fly ashes show different kinds of PCDD isomer patterns in precursor experiments [4,5] and fly ashes were found containing the so-called characteristic 2,3-, with chlorine in 2,3,7,8-positions, versus 2,6-isomer patterns, where vicinal chlorines weren't favored [6,7,8,9]. The first part of this work filled this gap by comparing patterns of PCDD isomers obtained from residues of various combustion samples by means of a multivariate data analysis, the principal component analysis (PCA). The resulting patterns from this statistical method provided a tool to show the relevance of laboratory scale precursor experiments for PCDD formation in combustion processes and even allowed us

2777 to relate PCDD concentrations in different compartments, e.g. fly ash and flue gas, to possible precursor pathways. In the second part of this work precursor experiments were presented, using fixed bed reactors and typical PCDD/F formation conditions. We concentrated our investigations on compounds like dibenzo-p-dioxin, dibenzofuran, octachlorodibenzo-p-dioxin and active charcoal, which do not form PCDD/F at low temperatures like chlorophenols. It was already proven that condensation of chlorophenols is one relevant pathway in PCDD formation. The condensation of chlorophenols results in similar PCDD isomer patterns as observed in fly ashes [8,10]. However, this pathway is also active at temperatures far below 300°C, where the maximum in the PCDD/F formation is observed [11]. Hence, other PCDD/F pathways should he relevant too. Possible equivalent formation pathways were chlorination and hydrodechlorination reactions of none (e.g. dibenzo-p-dioxin, active charcoal) or perehlorinated (e.g. C18DD) precursors, respectively. Several investigations dealt with these ways, but no reliable picture could be elucidated. Considering the chlorination reactions most authors reported a chlorination in 2,3,7,8-position of the dibenzo-p-dioxin [4,12,13], but patterns resembling the fly ash patterns were found too [4]. It was pointed out that thermal hydrodechlorination of C18DD on different surfaces resulted basically in the same 2,3,7,8-isomer patterns found for chlorination reactions [14], but exceptions, where typical fly ash patterns were found, were reported also [15]. In the third part of this work we tried to explain the chlorination and hydrodechlorination of dioxins by a more theoretical approach discussing the stability and reactivity of different PCDD congeners. It was pointed out that the thermodynamic stability of the PCDD/F congeners may play an important role in the thermal PCDD/F formation [16,17,18]. As far as only one experimental value of the thermodynamic stability of the 2-chlorodibenzo-p-dioxin is available [19], stability of the PCDD were determined using incremental methods [18,20,21] and semiempirical molecular orbital (MO) calculations [16,17,21,22]. Results from these calculations were compared by Huang et al. [21], indicating that two different incremental methods [23,24] predicted enthalpies of formation, which corresponded best to experimental measurements of PCDD related compounds. However, Unsworth et al. [16] showed that the Modified Neglect of Diatomic Differential Overlap (MNDO) hamiltonian in semiempirical Molecular Orbital (MO) calculations gave the best fit to experimental data for chlorinated aromatic compounds. Comparison of typical fly ash isomer patterns and relative Gibbs Free Energy of formation did show some similarities, but also major differences [16,17]. Therefore kinetic control should also play a role in thermal formation mechanism of PCDD/F. One kinetic model for chlorination and hydrodechlorination was evaluated using the same rate constants for each possible substitution reaction [14]. This model itself was not able to explain the PCDD isomer distribution, but a combination of it with thermodynamic stability of PCDD isomers may give a close description of actual PCDD distributions, further indicating that both pathways are important.

2778

In addition to the stability of the congeners and possible kinetic formation pathways, the abundance of the congeners is also influenced by their respective reactivity. It was pointed out by Hagenmaler et al. [25] that the formation maximum of PCDD at about 300 °C is a superposition of formation and destruction (Figure 1).

/ Destruction /

/

/'x

\

/ \

/ / ,. //° //

~g

~

,

.... \-~'-/- . . . . . . . . . F'ormation /\ \ ./

'/"' j / / 3oo'e I

\

\ \ Concentration o f , . . dioxins

Temperature Figure 1: The dualistic principle of PCDD/F formation and destruction [25]

Several publications described the potentials of different reactants in destroying PCDD/F (for a summery see [1]). However, the reactivity of the reagents also controls their destructive potential and hence their abundance. Using the Molecular Orbital (MO) approach, the reactivity of the reagent mainly depends on the frontier orbital, namely the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied one (LUMO). In the preliminary works of Fukui et al. it was elaborated that the energy density in the HOMO direct the position of an electrophilic attack, while the energy density distribution in the LUMO controls the position of a nucleophilie attack and a combination of both orbital is responsible for a radical attack [26,27]. Therefore, electron density distributions of the HOMO of the PCDD congeners were calculated in this work to derive pathways of electrophilic reactions involved in PCDD formation. It was possible to extent this theory to compare reactivity not only within molecules but between them. Fukui introduced the term of the superdeloealizibility, which describes the reactivity, depending on a electrophilie, nucleophilic or radical attack, between different molecules at one position within the molecule [28]. Hence, stability of different molecules can be compared using the energies of the HOMO's and LUMO's as measures for reactivity.

2779 2. EXPERIMENTAL

The experimental section contains three parts. First, different samples from combustion processes were analyzed for their concentration of PCDD/F congeners to derive a reliable data set. This data set was analyzed by multivariate data-analysis. Second, results of the data analysis were compared with the PCDD/F formation in the combustion of possible PCDD/F precursors on annealed fly ash in a fixed bed reactor. Third, semiempirieal molecular-orbital-calculations were carded out to get insight in relative thermodynamic stability, position of electrophilie substitution and reactivity between isomers.

2.1.1. PCDD/F Analysis of 121 Combustion Samples

A data set of the concentrations of PCDD/F congeners in combustion samples was derived by analyzing residues from five different municipal waste incinerators, two accidental fires and combustion of plastics under definite technical conditions. Different types of samples-matrices were investigated (Table 1). Generally accepted methods for various sampling techniques have been used [1]. Clean up procedures and analytical measurements used for sample preparation were described elsewhere [29,30].

Table 1: Origin of samples investigated in this study

Source

Number of Samples

Sample-Matrix

N u m b e r Sampfingof Technique Samples

Municipal Waste Incineration

111

Flue Gas

67

Fly Ash

Accidental Fires

6

Combustion of Plastics

4

39

Slag Soot

5 4

Combustion Residue Computer Boxes (Polycarbonates) Computer Devices (Epoxy-Resins) PVC

2 1

!Adsorption on XAD IAdsorption on Zeolith ]Condensate Collected in an Electrostatic Precipitator Collected in Dust Sinks Fibre-Filter Collection Collected by Wiping Collection Collection

Number of Samples 46 12 9 17

18 4 5 4 2 1

1

Collection

1

2

Collection

2

2780

2.1.2. Principal Component Analysis (PCA) Correlation between the patterns of isomers of different samples in each group of tetra- to hepta chlorodibenzo-p-dioxins and -furans were analyzed by means of Principal Component Analysis (PCA) [31 ]. The concentrations of each isomer (cij) were previously normalized to the corresponding homologue (j) concentration according to Equation 1.

Ci d

Nj,j = ~

2

(Equation 1)

C~,J

PCA was carried out for each group of homologues and each sample, in which all isomers are detected. Briefly, the data set for one homologue group consists of Nij-isomers or sums of isomers, which span a ndimensional space. Concentrations of all samples constitute a swarm in this space. If this swarm is highly correlated, one might search for a vector, in whose direction most points of the swarm may be found. Mathematically, a vector with the largest variance of distribution of measured points is calculated. This vector is called 'the first principal component (PRIN1). The second principal component (PRIN2) is calculated orthogonal to the first one and describes the next largest variance. Further principal components can be computed in the same way.

2.1.3. Complete Chromatographic Separation of all PCDD Congeners The data set of PCDD/F congeners of various combustion samples contains data for congeners, which can be resolved by the Rtx2330 (Restek) capillary column. This column is used in routine measurements, because it separates all toxic 2,3,7,8-substituted congeners, is stable against high temperatures and shows low bleeding, which causes a high sensitivity. But this column does not separate all PCDD/F congeners, some are only available as sums [1,11]. The alternatively used column SB-Smectic (Dionex), which contains a liquid crystalline phase, allows separation of all PCDD congeners. However, the disadvantage of this column is its low sensitivity, higher bleeding and lower temperature stability. As a result only relatively high concentrated samples can be measured. Therefore, a higher concentrated, representative combustion samples were measured with this column to get information about congeners, which in routine measurements are only available as sums. Analytical condition and GC/MS hardware are equal to previously reported measurements, identification and quantification was done according published data [1,32].

2781

2.2. T h e r m a l T r e a t m e n t o f P r e c u r s o r s in a Fixed Bed R e a c t o r

Thermal treatment of possible PCDD/F precursors on annealed fly ash in air and at different temperatures were carried out in a fixed bed reactor inside a furnace with a toluene trap downstream (Figure 2). l~qstic tul~

50 all/la~

t

synthetic

. . . . . . . . .

;

ghm t u ~

!

furnance quartz w~_

+ 100 ml toluene

Figure 2: Fixed bed reactor for thermal treatment of PCDD/F precursors on annealed fly ash

Original fly ash of a municipal waste incinerator contains organic compounds and approximately 2% of particulate carbon. Heating original fly ash at elevated temperatures gave significant yields of PCDD/F. Even extracted fly ash forms PCDD/F in de novo synthesis from unextractable particulate carbon. Therefore the original fly ash was heated in synthetic air (140 mL/min) at 1000 °C for five hours to eliminate the organic compounds as well as the particulate carbon. It was assumed that the remaining inorganic matrix showed similar catalytic activity as original fly ash. 2.0 g of this annealed fly ash was taken as the matrix for combustion of precursors. Precursors were dissolved in 50 mL n-hexane, the fly ash is added and the mixture was rotated for 30 min. After evaporation of the solvent the loaded fly ash was thermally treated inside the furnace. Annealed fly ash with active charcoal (Aldrich, Active Charcoal Eponit 114N) was prepared by crushing in a mortar. Combustion experiments were carried out using a flow rate of 50 mL/min of air. Samples and conditions are summarized in Table 2. PCDD/F analysis of these samples was done similar to the analysis accomplished for the other combustion samples [29,30]. The toluene of the impinger was combined with the extract.

2782 Table 2: Precursors and conditions taken for thermal treatment on annealed fly ash Precursor active charcoal

CIsDD dibenzo-p-dioxin dibenzofuran 1,2,3,4-C14DD

Temperature in °C

Concentration

Reaction time in h

250 350 450 350 350 350 350

2.4 % 2.4 % 2.4 % 2 ppm 2 ppm 2 ppm 2 ppm

8 8 8 2 2 2 2

2.3. Thermodynamic Calculations

Thermodynamic stability and reactivity of all PCDD congeners were calculated using the MOPAC 7.0 quantum chemical program package [33]. First a Z-matrix of an approximated structure of each congener was derived using the drawing tools and model builder of the HyperChem program package [34]. This molecular structure was optimized with MOPAC 7.0 using the Austin Model 1 (AM1) hamiltonian [35] by minimizing the calculated Self Consistent Field Energy (EscF) as a function of the atomic position (BFGS-gradient optimizing routine). It should be noted that the PCDD have a slightly non-planar molecular structure in the gas phase (folding angle of about 12° about the oxygen-oxygen axis). In order to achieve correct geometry-optimized structures it was necessary to use a geometry, which already exhibit a folded structure, otherwise the BFGSgradient optimizer converged to a planar structure, which does not represent a minimum, but a saddle point on the potential hyper surface [17]. After optimizing the structure, the energy eigenvalues were calculated, which include the energies for the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). Values for the electronic density at each atom and its charge (valence electron minus electronic density) were available from this calculation. Temperature depending enthalpy (H), entropy (S), heat capacity (%) and the Self Consistent Field Energy (EscF = Heat of Formation) were then calculated from distinct partition functions for vibration, rotation and transition states. These values provided the calculation of temperature dependent Gibbs Free Energy of formation (AGi,598) for each congener (Equation 2).

o ) = H , , , , - H,29, +

- rS,.,,,

(Equation 2)

It should be noted that vibration energies at 0 K were not included. Thus absolute AG values were not available, but the calculation gave reliable data for relative Gibbs Energies of formation (AG~0 between

2783 isomers in one group of homologue. So AG values were related to the most stable isomer in each group of homologue. Assuming a thermodynamic controlled formation mechanism, the abundance of each isomer could be calculated from the relative Gibbs Free Energies of formation of each isomer (Equation 3).

AG I ,tel

e XI( AGI'rel)

RT

(Equation 3)

En e AG~,tel i=l

3. RESULTS

Results for the PCA are shown for the tetrachlorinated dioxins (C14DD) in Figure 3, because many results of this group are transferable to other groups. Tetrachlorodibenzo-p-dioxins are also the basis for the comparison of combustion patterns with model experiments and calculations.

3.1.1. PCDD/F Analysis of Combustion Samples

Consistent with the different types of analyzed samples a great variety of concentrations of PCDD/F were found (e.g. 2 ng/kg < CI4DD < 20387 ng/kg). In Figure 3 the isomer pattern for CI4DD of all of these samples is plotted against the isomer concentration. A general trend to build up two classes of parallel (proportional) patterns can be observed from the lines of the isomer-concentration plot. 10~0.00

IIOQ.00

100.00

t.O0

0.10

0.01

q

,

ii -

~-

i

_~

i

._

i

-

...-

i

,

J

i

i

iiii ii

i

-

~

~

Figure 3: Cl4DD-isomers and their concentrations in different samples (lines in this plot) of combustion processes

2784

3.1.2. Principal Component Analysis The normalized data set was analyzed according to its principal components. Figure 4 shows a plot of the first and second principal component of the CI4DD data set. The points in this plot represent different types of samples. It can be seen, that for most of these samples 'PRIN 1' is close to one; 91% of the C14DD data set information (variance, not corrected for the mean) can be described by the first principal component. A nearly complete description is achieved by adding the second principal component, which contains 7% of the information.

tt

,e

"1,P

oo

PRIN

1

Figure 4: Principal component one ('PRIN 1') and two ('PRIN 2') of the CI4DD data set

This procedure described for the CI4DD isomers was performed for the other tetra- to heptachloro-dibenzop-dioxins and -furans too. Similar results were obtained. For each of these eight groups of homologues the isomer distribution could nearly be described by two principal components, at which the first principal component alone gave already a very good description (Table 3).

Table 3: Percentage of the first and second principal component on the description of the isomer data set of each group of homologues

Group of Homologues CI4DD CIsDD CI6DD CbDD CI4DF CIsDF CI6DF CITDF

Number of Samples 1. Principal Component 2. Principal Component 42 63 85 121 47 70 66 111

91% 92 % 92 % 95 % 86 % 83 % 85 % 98 %

7% 5% 4% 5% 6% 13% 7% 2%

2785 Note that the number of samples, which span the n-dimensional space, is different for each group of homologues, because only complete data set were included.

3.1.3. Complete Separation and Identification of PCDD Congeners

It was possible to separate all tetra- to octachlorodibenzo-p-dioxins by means of a SB-Smectic capillary column. The normalized concentrations of all C14DD of one representative fly ash, which shows similar relative ratios in the patterns of PCDD/F congeners as the first principal component (PRIN 1), are shown in Figure 5. The CI4DD isomer pattern is dominated by the 1,3,6,8- and 1,3,7,9-C14DD (Figure 5). The most abundant CIsDD isomers (in decreasing order) are 1,2,3,6,8-, 1,2,4,7,9-, 1,2,3,7,9- and 1,2,4,6,8-ClsDD (Figure 6). 1,2,3,4,6,8-C16DD is the isomer with highest concentrations in combustion residues, only small amounts of other CInDD isomers were found (Figure 7). The 1,2,3,4,6,7,8-C17DD is a little bit more abundant in combustion samples than the 1,2,3,4,6,7,9-C17DD (not shown in figures).

3.2. Precursor Experiments

3.2.1. Thermal Treatment of Active Charcoal Carbon-like structures were considered to be the predominant compounds in PCDD/F formation in combustion processes (for a review see [37,38]). In this study we examined the thermal treatment of active charcoal on annealed fly ash. PCDD/F isomer patterns for all groups of PCDD/F homologues produced by this de-novo reaction at 350°C are in good agreement with the combustion patterns found in PCA. For an example of the CI4DD isomers see Figure 5.

3.2.2. Thermal Treatment of CIsDD Hydrodechlorination of octaehlomdibenzo-p-dioxin did occur on annealed fly ash at 350°C. Patterns of PCDD isomers of all groups of PCDD/F homologues were nearly the same as in other combustion samples. For an example of the C14DD isomers see Figure 5. It should be noted that chlorinated dibenzo-p-furans were also formed. Pattems of isomers for these homologues were comparable to other combustion patterns tOO.

3.2.3. Thermal Treatment of Dibenzo-p-dioxin Annealed fly ash is a catalyst and reagent for the chlorination of dibenzo-p-dioxin at 350°C. Predominant congeners, which were formed in various combustion processes were formed in the thermal chlorination in equal relations of dibenzo-p-dioxin, too. Compare Figure 5 for an example. Like CIsDD also dibenzo-p-

2786 dioxin decomposed in the presence of chlorine-containing annealed fly ash to chlorinated dibenzofurans with characteristic pattems of isomers, 1.00

iRepresentativei Fly Ash

o.so ss

f

I I

..........

! ooO.oo

.

.

0.00 ~

i.I

1.00

_ ~

~

'o,o 1

.-~

....

!

|

i 0.60

~o~o},

..

I.II

. _ .

0.00

1,00

LClaCDDi

e o.9o

0.80 -

I

0.70

0.60 ~ 0.50 ~ 0.40 -- 0.30 [g 0.20 0.10 0.00

I

l

I

ii

1,00. ibenzo-p-daoxm

0.8O

0,O0

I i

0,40

0,20 0,00

: -- :

.

.

.

.

.

.

.

]0..!

'"1

I

o.zo

"CI~D .

.

.

I .

Figure 5: Normalized concentration of CI4DD isomers of a representative fly ash according to 'PRIN 1' and combustion residues of thermal treatment of active charcoal, ClsDD, dibenzo-pdioxin and dibenzofuran on annealed fly ash at 350°C.

2787 3.2.4. Thermal Treatment of Dibenzofuran Dibenzofuran was chlorinated on annealed fly ash at 350°C and was partly converted to chlorinated dibenzo-p-dioxins. Patterns of PCDF isomers formed in this reaction did fit to various combustion patterns for the PCDF. However, patterns of PCDD isomers were different to typical combustion pattern. The difference for the C14DD isomers can be seen in Figure 5.3.2.5. Thermal Treatment of 1,2,3,4-C14DD

Thermal treatment of 1,2,3,4-C14DD on annealed fly ash at 350°C showed hydrodechlorination and chlorination products in combustion residues. So it is not quite clear if an intramolecular isomerization did occur, although one different CI4DD isomer, the 1,3,6,8-C14DD, was found. Intermolecular chlorination occurred with fly ash as reagent. In comparison with the typical combustion pattern of the C15DD isomers it could be worked out that chlorination preferably took place in 7-position. The yield of 1,2,3,4,7-C15DD was ten times higher than that of 1,2,3,4,6-C15DD. For typical combustion samples the ratio is 1:4.5 (Figure 6). i.00 0.80

I

1.00 ] 0.80 1

Chl~nafion of I~,~CI4DD

e "0.40 m 0.20

i,

0.00

!

CIsDD

~o.~

Representative Fly Ash

I

~ 020 0.00

CIsDD

Figure 6: Normalized concentration of ClsDD isomers in thermal reaction of 1,2,3,4-C14DD o n annealed fly ash in comparison to a representative fly a s h

Considering the formation of sixfold chlorinated dibenzo-p-dioxins, the abundance of 1,2,3,4,7,8-C16DD was twice as much as for the 1,2,3,4,6,7- and the 1,2,3,4,6,8-isomer. This suggested that the primary chlorination product of 1,2,3,4-C14DD, the 1,2,3,4,7-CI5DD, preferably is chlorinated in 8-position. Abundance of CI6DD isomers in typical combustion samples were different, major 1,2,3,4-C14DD chlorination products were of low abundance (Figure 7). It should further be mentioned that the abundance of C16DD isomers like 1,2,3,6,8,9,-C16DD, which aren't perchlorinated in one ring like 1,2,3,4-C14DD, shows the importance of hydrodechlorination reactions at these reaction conditions.

2788 1.00 0.8O

!,-

1O0 --

Chlorlultoa of 1,2,3,4-C1~DD

I ~

Represeutat/ve Fly Ash

[ ,

0 so

I

I i°'l ' ~

~

~

:-

~

~

o.,lm

~

.~ .~ ~: .~

C~DD

Figure 7: Normalized concentration of CI6DD isomers in thermal reaction of 1,2,3,4-C14DD on annealed fly ash in comparison to a representative fly ash

Further, not shown in diagrams is that thermal reaction of 1,2,3,4-C14DD on annealed fly ash is not timedependent in the time range between 0.5 and 5 h and in coincidence with other combustion experiments with dibenzo-p-dioxin precursors, PCDF are formed besides PCDD. For 1,2,3,4-C14DD as a precursor these PCDF isomer patterns are in good agreement with typical combustion patterns.

3.3. Semiempirical Molecular Orbital Calculations

Semiempirical molecular orbital calculations pointed out that the thermodynamic stability of PCDD congeners at 598 K are different. A general trend to stable congeners, which are chlorinated in 2,3,7,8position, can be observed. Furthermore chlorination in different rings were thermodynamically favored (Table 4). The thermodynamic stability of a PCDD isomer in each homologue group followed the subsequent rules in decreasing order:

1. The more chlorine is present in 2,3,7 or 8-position, the more stable is the isomer in one homologue group. 2. Different rings are favored for chlorine atoms in PCDD in equivalent positions. 3. Vicinal chlorine atoms are less stable.

These findings were consistent to other studies [16,18,20,22]. Relevant trends for the stability of PCDD isomers could be concluded from the stability of the CI4DD isomers (Figure 8). It should further be noted that relative stability for PCDD isomers were not significantly influenced by temperature only overall stability rose in general with decreasing temperature.

2789 Table 4: Calculated Gibbs Free Energy of for formation (AGi.598 - TSi.0), relative abundance according to

AGi,rc I (Xi(DGi,r¢I)), normalized relative abundance (n(xi)) and HOMO-LUMO energies for the C14DD to C17DD isomers

Gibbs relative Gibbs Energy AGi (-TSo) Energy AGi,r¢|. '[kcai/mol] [kcal/mol]

normalized HOMO-LUMO relative Energy abundance (n(xi)) [eV]

CIIDD -51.634 -53.820

2.186 0.000

0.5416 0.8407

-8.44614 -8.35611

-59.613 -60.759 -58.399 -57.934 -61.139 -61.401 -58.420 -61.929 -63.411 -63.809

4.196 3.050 5.410 5.875 2.670 2.408 5.389 1.880 0.398 0.000

0.2193 0.2762 0.1718 0.1565 0.2981 0.3143 0.'1726 0.3495 0.4709 0.5101

-8.33367 -8.12719 -8.44071 -8.47681 -8.37015 -8.07836 -8.48085 -8.24453 -8.30152 -8.25933

-67.488 -66.299 -66.922 -69.225 -69.326 -66.788 -67.900 -70.472 -70.476 -65.619 -68.076 -68.121 -69.172 -72.422

4.934 6.123 5.501 3.198 3.097 5.635 4.522 1.950 1.946 6.803 4.347 4.302 3.250 0.000

0.1911 0.1505 0.1705 0.2710 0.2766 0.1660 0.2076 0.3483 0.3486 0.1312 0.2151 0.2170 0.2682 0.5156

-8.20673 -8.27614 -8.36889 -8.28702 -8.27383 -8.36696 -8.37135 -8.26512 -8.27499 -8.48248 -8.37509 -8.37180 -8.27290 -8.16130

-72.263 -74.522 -76.964 -77.073 -74.442 -73.378 -75.873 -75.887 -73.327 -74.209 -76.045 -73.748 -77.358

6.556 4.298 1.856 1.747 4.377 5.442 2.946 2.932 5.492 4.610 2.774 5.071 1.461

0.0993 0.1564 0.2556 0.2612 0.1539 0.1242 0.2052 0.2058 0.1230 0.1469 0.2125 0.1339 0.2767

-8.18973 -8.25142 -8.16893 -8.16857 -8.24936 -8.33152 -8.25529 -8124255 -8.34110 -8.29174 -8.28099 -8.38155 -8.20161

CI2DD 1,2 1,3 1,4 1,6 1,7 1,8 1,9 2,3 2,7 2,8

ChDD 1,2,3 !1,2,4

1,2,6 1,2,7 1,2,8 1,2,9 1,3,6

1,3,7 1,3,8 1,4,6 1,4,7 1,4,8 1,7,8 2,3,7 CI4DD 1,2,3,4 1,2,3,6 1,2,3,7 1,2,3,8 1,2,3,9 1,2,4,6 1,2,4,7

1,2,4,8 1,2,4,9 1,2,6,7 1,2,6,8 1,2,6,9 1,2,7,8

2790

Table 4: Calculated Gibbs Free Energy of for formation (AGi.598 - TSi.0), relative abundance according to AGi,reI (Xi(DGi,rel)),normalized relative abundance (ntxi)) and HOMO-LUMO energies for the C14DDto C17DD isomers; continued

Gibbs relative Gibbs Energy AGI (-TS0) Energy AGi,reL [kcal/mol] [kcai/mol] 1,2,7,9 1,2,8,9 1,3,6,8 1,3,6,9 1,3,7,8 1,3,7,9 1,4,6,9 1,4,7,8 2,3,7,8

normalized relative abundance (nlxtl)

HOMO-LUMO Energy

leVI

-76.031 -74.692 -76.371 -74.886 -78.309 -77.149 -71.757 -76.446 -78.819

2.788 4.127 2.448 3.934 0.511 1.670 7.062 2.373 0.000

0.2119 0.1618 0.2268 0.1683 0.3350 0.2653 0.0897 0.2303 0.3712

-8.28686 -8.29097 -8.29298 -8.37619 -8.20325 -8.26509 -8.49904 -8.26463 -8,13202

-79.288 -81.862 -82.513 -83.743 -81.354 -84.862 -83.574 -82.493 -81.368 -82.490 -80.311 -84.065 -82.551 -81.505

5.575 3.000 2.350 1.119 3.508 0.000 1.288 2.369 3.494 2.372 4.552 0.797 2.311 3.357

0.1350 0.2266 0.2583 0.3308 0.2046 0.4143 0.3197 0.2572 0.2051 0.2571 0.1658 0.3529 0.2603 0.2109

-8.23929 -8.14847 -8.19365 -8.18690 -8.27226 -8.11659 -8.18433 -8.18711 -8.28601 -8.26837 -8.36922 -8.17474 -8.28728 -8.26918

-87.386 -88.475 -86.252 -89.793 -89.206 -88.869 -89.033 -90.009 -86.986 -87. 890

2.623 1.534 3.757 0.216 0.803 1.140 0.977 0.000 3.023 2.119

0.2454 0.3055 0.1954 0.3983 0.3539 0.3308 0.3418 0.4160 0.2265 0.2716

-8.17928 -8.18249 -8.26360 -8.09608 -8.11873 -8.19572 -8.19610 -8.11318 -8.30981 -8.27749

-94.874 -93.691

0.000 1.182

0.7853 0.6191

-8. I 1857 -8.21105

C~DD 1,2,3,4,6 1,2,3,4,7 1,2,3,6,7 1,2,3,6,8 1,2,3,6,9 1,2,3,7,8 1,2,3,7,9 1,2,3,8,9 1,2,4,6,7 1,2,4,6,8 1,2,4,6,9 1,2,4,7,8 1,2,4,7,9 1,2,4,8,9

Ci6DD 1,2,3,4,6,7 1,2,3,4,6,8 1,2,3,4,6,9 1,2,3,4,7,8 1,2,3,6,7,8 1,2,3,6,7,9 1,2,3,6,8,9 1,2,3,7,8,9 1,2,4,6,7,9 1,2,4,6,8,9

CITDD 1,2,3,4,6,7,8 1,2,3,4,6,7,9

2791 Vice versa the reactivity of an isomer calculated from the HOMO-LUMO energy rose with chlorine in its 2,3,7 or g-positions. The reactivity is characterized by high HOMO, which led to an easy oxidative decomposition, and a low LUMO, which favored the reductive reactions like hydrodeehlorination (Table 4). Figure 8 gives an example for the relative gas phase stability and reactivity of the CI4DD isomers. f

0 40 ~

q

!Calculated Relative Gibbs Freei

|

o.. O.lO. 1mII,,ll,,I,II,i,II •

0.1~

ii)

CI4DD 8.6000

iCalc~at~ ~HOMO-LUMO~Ene~I

8.5000

1

8.4000 8.3000

8.2000 8.1000 8.0000

n

~

~44n

4 5

4~45,

5~

CI4DD

Figure 8: Normalized relative Gibbs Free Energies of Formation and HOMO-LUMO energies of C14DD isomers

The relative reactivity according to electrophilic substitution can be estimated for each single congener according to the frontier orbital theory. The distribution of electron density is determined by the amplitude of the wavefunction of the congener. The HOMO represents the wavefunction that is responsible for the directive interaction of an eleetrophilie attack. Therefore the eigenvalue and the distribution of the wavefimction~s amplitude determine the relative reactivity at the carbon position of the congener. The lowest charge/electronic density at a carbon atom in an individual congeners should than describe the position of an eleetrophilic attack. These densities and the equivalent distribution of charges were calculated for all atoms in each PCDD congener and are listed for the carbon atoms in 1,2,3,4,6,7,8 and 9-position, respectively (Table 5).

2792 Table 5: Calculated atomic charge distributions of the carbon atoms in 1,2,3,4,6,7,8 and 9-position of selected dibenzo-p-dioxins (lowest charges for each congener are highlighted)

Congener

C-I

DO 2-CllDD 2,8-CI2DD 2,3,7-C13DD 2,3,7,8-C14DD 1,2,3,7,8-CIsDD 1,2,3,4,7,8-C16DD 1,2,3,4,6,7,8-CbDD CIsDD

-0.1159 ~l!~!!i -0.1131 -0.0613 -0.1117 -0.0603 -0.1072 -0.0593 :~i~:~:~:~i~ii~i-0.0591 -0.0453 -0.0636 -0.0482 -0.0570 -0.0478 -0.0560 -0.0452 -0.0588

C-2

C-7 C-8 C-9 C-3 C-4 U-6 ~.~;~!i!/!i!iiiiiii.0.1167 -0.1165 ~ii~iiii!!i!i~ ~ - 0 . 1 1 6 6 -0.1217 -0.1101 -0.1153 -0.1257 :;:~.~.~iiiiiii::i::i::i::i -0.1144 ~ ~iii~i~i~i!~ii~i -0.1080 i-0.1082 ~iiiiii~iiii!~i.0.0602 -0.1116 -0.0594 -0.1065 -0.1164 -0.0593 ~iiii~i!;i~i~!il .0.1059 .00589 ii iiii .0.0586 ~.......... : ~ .!..:i~!iiiiiiiiiiii ~:~i~i~i~ii .0.0588 -0.0519 ~ ! ~ i i .0.1034 -0.0584 -0.0586 -0.1014 iiii -0.0569 i-0.0486 ~ii~iiiiiiiii!ii-0.0579 -0.0582 ~ i ~ i i-0.0572 -0.0456 .0.0419 -0.0632 -0.0508 ~ i l i ~ i ~ i -0.0566 -0.0488 .0.0448 -0.0563 -0.0560 .0.0453

The preferred position of the electrophilic substitution of dibenzo-p-dioxin can be concluded form this table. The reactivity falls in the sequence 2 ---) 8 ~ 3 --~ 7 ~ 1 --) 4 ~ 6 --~ 9. Because of the large -I-effect of the chlorine atom bonded to a carbon atom and a lower +M-effect of a chlorine bonded to an aromatic system, electronic density on a chlorine bonded carbon atom is always the lowest. Therefore electrophilic substitution should preferably take place at C-H bonds, hydrodechlorination should not be favored as an electrophilic reaction. However, in cases of a large excess of H+-ions a reverse reaction may occur. Considering the electronic densities in the HOMO of the CIsDD an electrophilic substitution of chlorine in PCDD by H+-ions is not a reverse chlorination reaction. Hydrodechlorination of ClsDD will lead to substitution in the 2,3,7 or 8-positions, respectively. So an electrophilic hydrodechlorination should take place in the same sequence as for the chlorination, but results in most stable products 1-CIIDD, 1,9-C12DD, 1,4,6-C13DD, 1,4,6,9-C14DD, 1,2,4,6,9-C15DD, 1,2,4,6,7,9-C16DD and 1,2,3,4,6,7,9-C17DD (Table 6).

Table 6: Calculated atomic charge distributions of the carbon atoms in 1,2,3,4,6,7,8 and 9-position of selected dibenzo-p-dioxins, continued (lowest charges are highlighted except charges at carbons, which are bonded to hydrogen, they are printed italic)

Congener CIsDD

E-I

C-2

-0.0452

~i~!~

1,2,3,4,6,7,9-C!7DD .0.0462

-0.0562

C-3 ~

C-4

C-6

.0.0488

~ $ : : ~ i ~ i -0.0454 -0.0431

-0.0448 ~ili~i! ~i~!]-0.0453 -0.0502 -0.0544 .o.1132 -0.0428 -0.0513 ~iiiiiiiiiii!iiiiiii.0.1137 -0.0435

1,2,4,6,7,9-C16DD -0.0514 ~ i 1,2,4,6,9-CIsDD .0.0521

.0.1145

1,4,6'9-CI4DD 1,4,6"C13DD I'9"CI2DD I-CIIDD DD

C-7

C-8

C-9

o.,,,

.0.0445

.0.0505

.o11,,

.o11,8

.0.0503

~ i ~

•o.,6~

~.116~

.o.,s,

~ i i

~/il

.0.1167

.0.1173

~ii!i~ii -0.0485

.o.11,0

~

~ i ~ -0.0519

.0.1245

.0.1164

.0.1124

~ ~

~I ~!iii

.0,2~7

.0.,7~

.0.,63

.0,6,

.0.117~

.0 , ~

~!~iii!i!i!~

.0.1262

.0.I 192

.0.I 179

.0.I 150

.0.1262

.0.1264

-0.1127

-01279

.0.1279

.0.1167

.0.1165

.0.1277

.0.1279

.0.1166

.0.1159

2793 4. DISCUSSION AND CONCLUSION

The principal component analysis of various combustion samples shows the existence of one common combustion pattern for PCDD/F isomers. It is described by the first principal component, which resembles the 2,6-patterns of fly ash samples published by Ballschmiter et al. [6]. Regarding these patterns, it is slriking that the congeners with vicinal chlorine are not favored. Ballschmiter et al. pointed out that other fly ash patterns exist, which he calls the 2,3-patterns [6,7,8,9]. The characteristic of these patterns is a predominant substitution in 2,3,7 and 8-positions. These patterns are somehow related to the second principal .component evaluated in this study, which describes an orthogonal (independent) patterns to the first principal component. It is shown by our statistical analysis, that only two to thirteen percent of the combustion samples are related to this class. Concluding the discussion of the principal component analysis it is stressed that two different combustion patterns together describe nearly all PCDD isomer patterns found in combustion processes and especially one, the first principal component itself, gives a very good description for the majority of samples. Model combustion of different precursors on annealed fly ash very often give equivalent isomer patterns (see Figure 5), closely related to the first principal component. It can be concluded from these results that characteristic formation mechanisms should be operative in all thermal formations of PCDD/F. However, our statistical approach points out the existence of one dominant pathway, represented by the first principal component or the 2,6-isomer patterns, respectively and a less dominant second pathway leading to what we basically call the second principal component or the 2,3-isomer patterns. Another indication for a different, less dominant pathway is given by discrepancies in experimental results for similar experiments. The thermal chlorination of dibenzo-p-dioxin on annealed fly ash in our and other [4] published experiments resulted in a PCDD isomer distributions, which contributes to the first principal component, but other investigators do find distributions, which resemble the 2,3-patterns [4,12,13]. The same is observed for the hydrodechlorination of C18DD [14,15]. Although experimental conditions in the investigations are only slightly different, e.g. different catalytic matrices are used, no common trend could be figured out. An explanation of the two classes of PCDD isomer patterns found in combustion residues of thermal processes may be based on the commonly discussed dualistic principle of the temperature dependent PCDD/F formation and destruction (Figure 1). Hence, the thermal formation maximum of PCDD/F at about 300 °C is a superposition of a formation and a destruction rate. Therefore, the resulting PCDD/F isomer patterns are effected by both processes. It is suggested that the thermodynamic stability of the different congeners should be the predominant factor in the PCDD/F formation [16,32]. Thermal formations of PCDD/F takes place at elevated temperatures between 250 and 450°C, as a result PCDD/F congeners with the lowest Gibbs Free Energy of formation should be formed predominantly. However, the patterns of isomers according to their calculated gas phase

2794 stability do not correspond exactly to abundance of PCDD isomers found in combustion processes [17]. We repeated these calculation with the AM1 hamiltonian in contrast to the MNDO hamiltonian used in the previous publication [ 16], because in many cases results of calculations with AM 1 give better description of experimental values for PCDD than MNDO calculation (e.g. the ionization potentials of PCDD and polychiorinated benzenes are predicted best by AM1 calculations [39,40]). However, the abundance determined from the calculated relative Gibbs Free Energies of formation of PCDD isomers (AM1) do not match exactly to typical combustion patterns (compare Figure 5 and Figure 8). In particular congeners with chlorine in 2,3,7,8-position are not as abundant in combustion samples, which correspond to the first principal component, as predicted by their thermodynamic stability. However, formation of PCDD/F ma~, also be a partly kinetic controlled process. If kinetic control plays a role, the energy of the transition state of the formation reaction should determine the formation rate of products. According to the Hammond postulate the stability of the transition state is reagent-like for exothermic reactions [41]. One possible reaction pathway in PCDD formation may be a chlorination of unehlorinated precursors, which already exhibit a dioxin like structure. This is discussed by the so called de novo theory, where particulate carbon is partially chlorinated [37,38]. A model for this pathway is the chlorination of dibenzo-p-dioxin. It is generally believed that this should be an electrophilic substitution of hydrogen in the aromatic rings [42,43,44]. As far as halogen substitution of a C-H bond in an aromatic system lowers the heat of formation in the products, the properties of the reagent should determine the product. In case of an electrophilic aromatic substitution the electronic density at the carbon atoms is a good measure for an electrophilic attack [26]. Calculation of the electronic density for the PCDD congeners suggest that electrophilic substitution of dibenzo-p-dioxin results predominately in 2-CllDD, 2,8-C12DD, 2,3,7-C13DD, 2,3,7,8-C14DD, 1,2,3,7,8-C15DD, 1,2,3,4,7,8-C16DD and 1,2,3,4,6,7,8-C17DD as most favored products for each group of homologues. These theoretical predictions match well with experimental findings of others [14,45] and our results for the chlorination of 1,2,3,4-C14DD on annealed fly ash (direction to the 7,8-positions). Therefore, chlorination of dibenzo-p-dioxin according to the theory of an electrophile substitution and many experimental measurements lead to relative abundance of PCDD isomers with the highest thermodynamic stability (except 1,2,3,6,7,8-C16DD, which is more stable than 1,2,3,4,7,8C16DD). Both approaches tend to describe the PCDD combustion patterns according to the second principal component or the 2,3-patterns, which is only found in a few combustion samples, but they cannot explain the typical PCDD isomer patterns found in most combustion residues and also do not match to our results of dibenzo-p-dioxin chlorination on annealed fly ash, which are similar to findings of an other investigation

[4]. As it is pointed out above, a description of the patterns of PCDD isomers in residues of combustion processes should include both formation and destruction mechanisms. We ha~,e discussed the PCDD formation mechanisms from the molecular point of view of the reagent, even though the reactant can effect

2795 the isomer patterns too. Conclusively, a discussion of the PCDD destruction mechanisms from relevant properties of the reagent is made, bearing in mind that reactants can effect the PCDD isomer patterns too. The molecular property responsible for the destruction of a molecule is its reactivity. The preliminary work of Fukui [26] pointed out that the frontier orbitals (HOMO and LUMO) are a good measure for the reactivity of a molecule. A molecule, whose HOMO exhibits a low energy is relatively stable against an eleetrophilic/oxidative attack. If the LUMO exhibits a high energy, a low reactivity against a nucleophilic/reductive attack is expected. Hence, the difference of the HOMO-LUMO energies (eigenvalues) may be a measure for the overall reactivity of a molecule. This relative reactivity is calculated for all PCDD congeners (Table 4) and it is shown that PCDD congeners with chlorine in 2,3,7,8-positions are more reactive than others. Therefore a destruction of PCDD, whether it is oxidative or reductive in nature, should result in PCDD isomer patterns, where congeners with chlorine in 1,4,6,9-position are predominant (for an example see Figure 8). A possible destructive reaction of PCDD is their hydrodechlorination. This may be electrophilic in nature and should then be effected by the charge distribution at the carbon atoms of the PCDD congeners. The electrophilic agent could be an excess of H÷-ions (like it is assumed in Chapter 3.3) or more likely a single electron process catalyzed by metals, which easily change their oxidation state (e.g. Fe2÷ / Fe 3÷ or Cu + / 2+

Cu ), like it is proposed by other investigators [44,47,48]. Both, the electrophilic hydrodechlorination according to charge distribution at the carbon atoms and the reactivity approach based on HOMO-LUMO energies of PCDD congeners then predict that PCDD congeners with chlorine in 1,4,6,9-position are formed predominately in destructive reactions. These findings are consistent with a previously reported approach, where reaction enthalpies for possible hydrodechlorination reactions are calculated from calculated Gibbs Free energies of formation for PCDD congeners [21 ]. It was pointed out that hydrodechlorination pathways leading to PCDD chlorinated in 2,3,7,8-position are less exothermic. Hence, PCDD congeners, which do not

exhibit

two

adjacent

chlorine

are

formed

predominately

in

thermodynamic

controlled

hydrodechlorination reactions. The experimental results do not fit to these predictions. Some [15] like our results of the hydrodechlorination of C18DD lead to the PCDD isomer patterns, which resembles 'PRINI' and others [14] match with 'PRIN2'. Destruction of PCDD with OH-radicals also does not lead to predicted 1,4,6,9-patterns, but resulted in a PCDD isomer patterns, which are close to 'PRINI' [46].

However, the typical PCDD isomer patterns can be qualitatively described by a superposition of the stability and reactivity from the results of our calculations (Table 4). We propose that the rates of formation of different PCDD congeners are controlled by their thermodynamic stability, while their destruction is related to their reactivity (qualitatively described by the HOMO-LUMO energies). In other words, the amount of PCDD congeners, which are formed according to their thermodynamic stability is reduced according to their reactivity. Figure 9 gives an example for three

2796 different CI4DD isomers, 1,4,6,9-, 1,3,6,8- and 2,3,7,8-C14DD. The 2,3,7,8-C14DD is formed predominantly according to its highest thermodynamic stability. Due to its low relative Gibbs Energy of formation, the 1,4,6,9-C14DD is only formed in small amounts. However, 1,4,6,9-C14DD is destroyed very slowly after being formed, whereas 2,3,7,8-C14DD is destroyed faster due to its higher reactivity. So the abundance of 2,3,7,8-C14DD is smaller than expected from its thermodynamic stability. The properties of 1,3,6,8-C14DD are somewhere in between. Its thermodynamic stability is higher than the one of 1,4,6,9-C14DD, but lower than the one of 2,3,7,8-C14DD. The same is true for its reactivity. So 1,3,6,8-C14DD is formed to some extent and not destroyed in large amounts. The sum of both is a higher abundance than the more stable 2,3,7,8-C14DD (compare this model with actual concentrations in Figure 5).

0,40

O,35

[• reactivity index i • relative concentration

O,3O e~

N 0,25

.o al

0,20

j

0,15 0,10

0,05 0,00

1,3,6,8

1,4,6,9

2,3,7,8

CI4DD Figure 9: Qualitative picture of thermodynamic stability (whole bars), reactivity and the resulting relative concentration in combustion samples of 1,4,6,9-, 1,3,6,8- and 2,3,7,8-C14DD (the thermodynamic stability is the relative Gibbs Free Energy of formation; the reactivity index was calculated from the HOMO-LUMO energies (see Table 4))

We investigated PCDD formations from active charcoal at different temperatures to support this theory. According to the dualistic principal of PCDD formation and destruction, the patterns of PCDD isomers at low temperatures (e.g. 250 °C) should then resemble the patterns derived from the thermodynamic stability, while the observed patterns at high temperature processes (e.g. 450 °C) should be dominated by the reactivity of the congeners. Figure 10 gives an example for the formation of both CIIDD isomers at three different temperatures from thermal treatment of active charcoal on annealed fly ash. Normalized concentrations at these temperatures are compared with abundance according to their relative Gibbs Free Energy of formation and their relative HOMO-LUMO energies.

2797 1,00

0,90

1 ! 0.70

0,70 I 0,10

fReltlive Gibl~ IEnergy of Formlldolal ~

0,$0 0,90

0,60

jo..

0,60 0,50 0,40 0.30 0~0 0,10 0,00

i

L

o,so

0,30 O.2O

!1

Superposition of thermodynamic stability (Gibbs Energy of formation) and reactivity (HOMO-LUMO Energy)

8,5

8,45 8,4

i-(HOMO-LUMO)Energyl

.I 835 t 8,3 $,25

I-CllDD

2-CIIDD

1-CIIDD

2-CIIDD

Figure 10: Normalized concentrations of I-CIIDD and 2-CIIDD according to their formation at 250°C, 350°C, 450°C, relative Gibbs Free Energy of formation and HOMO-LUMO-energy

This picture is in agreement with the predictions drawn from the model. The 2-CIIDD is formed predominantly at low temperatures (250 °C) according to its higher thermodynamic stability. On the other hand, 2-CIIDD is destroyed faster due to its higher reactivity. Hence, no 2-CljDD is detected at high temperatures (450 °C), only 1-CIIDD is found in small amounts due to its lower reactivity. The relative abundance at 350°C are dominated by both processes like it is already discussed for the three CI4DD isomers. The favored degradation of the 2-CIIDD is also found by others [49]. Unfortunately the situation is not as clear for the higher homologues. Here the picture is less clear due to the multitude of possible degradation and formation pathways, pointing toward each single congener. Further on sterieal effects are not included in the picture yet. For example, the sterical shielding of huge chlorine atoms may discriminate reaction pathways for different congeners. Some of these kinetic arguments have been discussed in a previous paper [50]. However, the pattern of the CI2DD isomers is somehow influenced by temperature, but for higher chlorinated PCDD homologues no distinct temperature trend could be observed. The temperature independence of higher chlorinated PCDD isomer patterns is found by others too [4,36].

2798 In conclusion the evaluated combustion patterns of PCDD/F isomers and the laboratory experiments show that many different compounds are precursors in PCDD/F formation reactions and the statistical approach provides a powerful tool to show similarities and differences between isomer patterns. It could be traced out that the abundance of PCDD in combustion samples can be a superposition of formation and a destruction mechanisms. The formation of PCDD is believed to be controlled by the thermodynamic stability of the congeners. One possible destructive mechanism is the hydrodechiorination of higher chlorinated PCDD.

5. Acknowledgment

The authors like to thank Harald Bartl for carrying out the semiempirical calculations and a lot of additional computer work and Torsten Wottgen for his help in the PCDD/F clean up of several combustion samples.

6. Literature

1) K. Ballschmiter, R. Bacher: Dioxine: Chemie, Analytik, Vorkommen, Umweltverhalten und Toxikologie der Halogenierten Dibenzo-p-dioxine und Dibenzofurane. VCH, Weinheim 1996 2) H. Hagenmaier, C. Lindig, J. She: Correlation of Environmental Occurrence of Polychlorinated Dibenzop-dioxins and Dibenzofurans with Possible Sources. Chemosphere, 29, 2163-2174 (1994) 3) K.-W. Schramm, A. Wehrmeier, D. Lenoir, B. Henkelmann, K. Hahn, R. Zimmermann, A. Kettrup: Deduction of a Typical Pattern for the Isomers of Tetra- to Heptachloro-Dibenzo-p-dioxins and -furans Formed in Combustion Processes. Organohalogen Compd., 27, 196-200 (1996) 4) J. Blaha: Untersuchungen zur Thermisehen Bildung yon Polychlorierten Dibenzo-p-dioxinen und Dibenzofuranen. Dissertation Eberhard-Karls Universit~t Ttibingen, 1995 5) L. Stieglitz: The De-Novo-Synthesis of PCDD/PCDF on Fly Ash. 5th International Congress on Toxic Combustion Byproducts, Dayton 1997 6) K. Ballschmiter, W. Zoller, C. Scholz, A. Nottrodt: Occurrence and Absence of Polychlorodibenzofurans and Polychlorodibenzodioxins in Fly Ash from Municipal Incinerators. Chemosphere, 12, 585-594 (1983) 7) K. Ballschmiter, M. Swerev: Reaction Pathways for the Formation of Polychlorodibenzodioxins (PCDD) and Polychlorodibenzofurans in Combustion Processes I. Fresenius Z. Anal. Chem., 328, 125-127 (1987) 8) K. Ballschmiter, I. Braunmiller, R. Niemczyk, M. Swerev: Reaction Pathways for the Formation of Polychloro-dibenzodioxins (PCDD) and -dibenzofurans (PCDF) in Combustion Processes. Chemosphere, 17, 995-1005 (1988)

2799 9) M. Swerev, K. Ballschmiter: Pattern Analysis of PCDDs and PCDFs as an Approach to an Occurrence / Source Correlation. Chemosphere, 18, 609-616 (1989) 10) W. Zoller, K. Ballschmiter: Formation of Polychlorinated Dibenzodioxins and Dibenzofurans by Heating Chlorophenols and Chlorophenates at Various Temperatures. Fresenius Z. Anal. Chem., 323, 19-23 (1986) 11) H.R. Buser, C. Rappe: Isomer-Specific Separation of 2,3,7,8-Substituted Polychlorinated Dibenzo-pdioxins by High-Resolution Gas Chromatography/Mass Spectrometry. Anal. Chem., 56, 442-454 (1984) 12) R. Addink, B. Van Bavel, R. Visser, H. Wever, P. Slot, K. Olie: Surface Catalyzed Formation of Polychlorinated Dibenzo-p-dioxins/Dibenzofurans During Municipal Waste Incineration. Chemosphere, 20, t929-1934 (1990) 13) R. Lujik, K. Dorland, P. Smit, H.A.J. Govers: The Halogenation of Dibenzo-p-dioxin and Dibenzofuran in a Model Fly Ash System. Organohalogen Compd, 8, 273-276 (1992) 14) T. Wiesmtlller: Untersuchungen zur Katalytischen Dechlorierung von Octachlordibenzo-p-dioxin und Octachlordibenzofuran

und Anwendungen der Erhaltenen Gemische in Toxikologischen

Studien.

Dissertation, Universitat Ttibingen 15) H. Hagenmaier, H. Brunner, R. Haag, M. Kraft: Copper Catalyzed Dechlorination/Hydrogenation of Polychlorinated Dibenzo-p-dioxins and Polychlorinated Dibenzofurans, and Other Chlorinated Aromatic Compounds. Environ. Sci. Technol., 21, 1085-1088 (1987) 16) J.F. Unsworth, H. Dorans: Thermodynamic Data for Dioxins from Molecular Modeling Computations: Prediction of Equilibrium Isomer Composition, Chemosphere, 27, 351-358 (1993) 17) R. Zimmermann, A. Wehrmeier, D. Lenoir, K.-W. Schramm, A. Kettrup: A Thermodynamic Study on the Isomer Composition of Tetrachlorinated Dibenzo-p-dioxins Formed in Combustion Processes. Organohalogen Compd., 27, 237-242 (1996) 18) D. Thompson: An Evaluation of the Heat of Formation of Chlorinated Dioxins and Its Application to Isomer Abundance Prediction. Chemosphere, 29, 2545-2554 (1994) 19) T.S. Papina, V.P. Kolesov, V.P. Vorobieva, V.F. Golovkov: The Standard Molar Enthalpy of Formation of 2-Chlorodibenzo-p-dioxin. J. Chem. Thermodynamics, 28, 307-311 (1996) 20) M.W. Shaub: Procedure for Estimating the Heats of Formation of Aromatic Compounds: Chlorinated Benzenes, Phenols and Dioxins. Thermochem. Acta, 55, 59 73 (1982) 21) C.-L. Huang, B.K. Harrison, J. Madura, J. Dolfing: Gibbs Free Energies of Formation of PCDDs: Evaluation of Estimation Methods and Application for Predicting Dehalogenation Pathways. Environ. Toxicol. and Chem., 15, 824-836 (1996) 22) C.J. Koester, R.A. Hites: Calculated Physical Properties of Chlorinated Dibenzo-p-dioxins and Dibenzofurans. Chemosphere, 17, 2355-2362 (1988)

2800 23) D.J. Frurip, E. Freedman, G.R. Hertel: A New Release of the ASTM CHETAH Program for Hazard Evaluation: Version for Mainframe and Personal Computer. Plant/Operation Progress, 8, 100-104 (1989) 24) National Institute of Standards and Technology: NIST Standard Reference Database 25, Version 1.1 (1991) Gaithersburg, MD, USA 25) H. Hagenmaier, H. Brunner, R. Haag, M. Kraft: Die Bedeutung Katalytischer Effekte bei der Bildung und Zerstfrung von Polychlorierten Dibenzodioxinen und Dibenzofuranen. VDI-Berichte, 634, 557-584 (1987) 26) K. Fukui, T. Yonezawa, H. Shingu: A Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons. J. Chem. Phys., 20, 722-725 (1952) 27) K. Fukui, T. Yonezawa, C. Nagata, H. Shingu: Molecular Orbital Theory of Orientation in Aromatic, Heteroaromatic, and Other Conjugated Molecules. J. Chem. Phys., 22, 1433-1442 (1954) 28) K. Fukui, T. Yonezawa, C. Nagata: MO-Theoretical Approach to the Mechanism of Charge Transfer in the Process of Aromatic Substitution. J. Chem. Phys., 27, 1247-1259 (1957) 29) K.-W. Schramm, M. Merk, B. Henkelmann, A. Kettrup: Leaching of PCDD/F from Fly Ash and Soil with Fire-Extinguishing Water. Chemosphere, 30, 2249-2257 (1995) 30) B. Henkelmann, K.-W. Schramm, C. Klimm, A. K e m p : Quality Criteria for the Isotope Dilution Method with HRGC/MS. Fresenius Z. Anal. Chem., 354, 818-822 (1996) 31) SAS / STAT User's Guide, Vers,6 / Vol.2, SAS Institute Inc., 1241, (1989) 32) K. Ballschmiter, R. Bacher, A. Mennel, R. Fischer, U. Riehle, M. Swerev: The Determination of Chlorinated Biphenyls, Chlorinated Dibenzodioxins, and Chlorinated Dibenzofurans by GC-MS. J. High Res. Chrom., 15, 260-270 (1992) 33) J.J.P. Stewart: MOPAC 7.0, F.J, Seiler Res. Lab., USAF Academy 34) HyperChem 2.0, Autodesk Inc. 1992 35) M.J.S. Dewar, E.G. Zoebisch, E.F. Healy, J.J.P. Stewart: AMI: A New General Purpose Quantum Mechanical Molecular Model. J. Am. Chem. Soc., 107, 3902 (1985) 36) R. Addink: Mechanisms of Formation and Destruction of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans in Heterogeneous Systems. Ph.D. Thesis University of Amsterdam Feb. 1995 37) H. Huang, A. Buekens: On the Mechanisms of Dioxin Formation in Combustion Processes. Chemosphere, 31, 4099-4117 (1995) 38) H. Huang, A. Buekens: De Novo Synthesis of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans: Proposal of a Mechanistic Scheme. Sci. Total Environ., 193, 121-139 (1996) 39) R. Zimmermann, U. Boesel, D. Lenoir, A. Kettrup, T.L. Gebner, H.J. Neusser: The Ionisation Energy of Polychlorinated Dibenzo-p-dioxins: New Experimental Results and Theoretical Studies. Int. J. Mass. Spec. Ion Processes, 145, 97-108 (1995)

2801 40) R. Zimmermann: Quantum Chemical Calculations on Chlorinated Aromatics for Determination of Molecular Constants Relevant for New Laser Based Analytical Methods: Comparison of Semiempirical Methods. Organohalogen Compd., 19, 161-166 (1994) 41) J. March: Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. (3th Edition), p.188, Wiley, New York (1985) 42) B.K. Gullet, K.R. Bruce, L.O. Beach: The Effect of Metal Catalyst on the Formation of Polychlorinated Dibenzo-p-dioxin and Polychlorinated Dibenzofuran Precursors. Chemosphere, 20, 1945-1952 (1990) 43) F.W. Karasek, A. Beard, P. Krislmat: Formation of Polychlorinated Dibenzofurans by Chlorination and de Novo Reactions with FeC13 in Petroleum Refining Processes. Environ. Sci. Technol., 27, 1505-1511 (1993) 44) R.V. Hoffman, G.A. Eiceman, Y.-T. Long, M.C. Collins, M.-Q. Lu: Mechanism of Chlorination of Aromatic Compounds Adsorbed on the Surface of Fly Ash from Municipal Incinerators. Environ. Sci. Technol., 24, 1635-1641 (1990) 45) O.M. VanBerkel, K. Olie, M. Van Den Berg: Thermal Degradation of Polychlorinated Dibenzo-pdioxins and Dibenzofurans on Fly Ash from Municipal Incinerators. Inter. J. Environ. Anal. Chem., 34, 5167 (1988) 46) S. Leichsenring, D. Lenoir, G. May, A. K e m p : Catalytic Oxidation of Chloroaromatic Trace Contaminants Adsorpted on Wessalith Day by Ozone. Chemosphere, 33, 343-352 (1996) 47) K. Jay, L. Stieglitz: On the Mechanism of Formation of Polychlorinated Aromatic Compounds with Copper(II) Chloride. Chemosphere, 22, 987-993 (1991) 48) L. Stieglitz, H. Vogg, G. Zwick, J. Beck, H. Bautz: On Formation Conditions of Organohalogen Compounds from Particulate Carbon on Fly Ash. Chemosphere, 23, 1255-1264 (1991) 49) L. Stieglitz: Remark on '5th International Congress on Toxic Combustion Byproducts 1996, Dayton, OH, USA' . 50) D. Lenoir, A. Wehrmeier, R. Zimmermann, A. Kaune, R. Zimmermann: Thermal Formation of Polychlorinated Dibenzo-p-dioxins and -furans: Investigation on Relevant Pathways. Proceedings '5th International Congress on Toxic Combustion Byproducts 1996, Dayton, OH, USA'