The Heat Capacity of Gaseous and Liquid Polychlorinated Biphenyls, Polychlorinated

The Heat Capacity of Gaseous and Liquid Polychlorinated Biphenyls, Polychlorinated Dibenzo-N-Dioxins and Dibenzofurans

Kulikova T.V., Mayorova A.V., Shunyaev K.Yu.

Institute of Metallurgy, Ural Branch RAS

Ekaterinburg, Russia

E-mail:

Abstract

The study deals with analysis and systematization of the known and calculation of the unknown thermodynamic characteristics (the heat capacity Сро298, temperature dependences of the heat capacity Ср(T) ) of widespread hazardous isomers of gaseous and liquid compounds of polychlorinated biphenyls (PCBs), polychlorinated dibenzo-n-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs).

Introduction

Unique technological and physicochemical properties of polychlorinated biphenyls (PCBs), a huge volume of their production, considerable volatility and solubility, and extreme chemical inertness have led to the world-wide spread of PCB-containing equipment and materials, resulting in the universal contamination with these substances. The most common method used in Russia for destruction of PCBs is their incineration with the formation of polychlorinated dibenzo-n-dioxins (PCDDs) and dibenzofurans(PCDFs), which are among the most hazardous chemical substances known to the mankind.

As often happens, the hazard of PCBs has long been underestimated. With respect to their severe toxicological effect, PCBs are identical to substances that are referred to the high class of hazard. Since these substances are especially toxic, they have been assigned low toxicological standards, which necessitate special requirements on the organization of processes assuming formation of these substances (the so-called dioxinogenic processes) so that industrial emissions meet the norms. Instrumental investigations of these substances are very expensive and, in this connection, interest is attracted to calculation methods for simulation of processes by the data on their thermochemical properties (the standard enthalpy and entropy of formation, the heat capacity, temperature dependences of the heat capacity, the enthalpy increment, etc.).

A quality thermodynamic simulation requires the knowledge of thermodynamic and thermochemical properties of all reliably certified compounds of the system under study in the gaseous or condensed state. Therefore the present study deals with the analysis and systematization of the known and calculation of the unknown the heat capacity of most toxic and hazardous isomers of gaseous PCBs, PCDDs and PCDFs, and liquid PCBs.

CALCULATION OF THE HEAT CAPACITY

It is known that there are 209 individual PCB congeners, 420 polychlorinated dibenzo-n-dioxins and polychlorinated dibenzofurans, which differ by the number and positions of chlorine atoms in a molecule. The most widespread PCB compounds containing up1 to 10 chlorine atoms were chosen for the study. In deciding on isomers, preference was given to ortho-unsubstituted PCBs because they are most toxic and their effect is similar to the effect of PCDDs and PCDFs. Congeners, which do not have chlorine atoms in ortho-positions of molecules (ortho-unsubstituted PCBs), can acquire the planar configuration, which is more favorable in energy terms. Such congeners are isostereoisomeric to PCDDs and PCDFs and present the greatest hazard. As to the PCDD and PCDF isomers, of special hazard to humans and the environment are tri-, tetra-, penta-, and hexa-substituted dioxins and furans containing halogen atoms in lateral positions 2, 3, 7, and 8.

In this study we analyzed the known and calculated the unknown the heat capacity of 17 most widespread and hazardous isomers of PCBs, PCDDs and PCDFs in the gaseous state and 11 compounds of liquid PCBs.

Gaseous PCBs, PCDDs and PCDFs . The literature survey showed that studies dealing with estimation of the heat capacity, the of gaseous PCB, PCDD and PCDF compounds are few. Most of them are based on calculations or are semi-empirical. For example, Saito and Fuwa [1] calculated thermodynamic functions of all PCBs and some PCDDs and PCDFs on the basis of semi-empirical calculations in terms of the PM3 model. O.V. Dorofeeva et al. [2-4] used statistical methods. Table 1 presents the literature data on standard enthalpies and entropies of formation of gaseous and liquid PCBs, PCDDs and PCDFs. The comparison of results obtained in different studies reveals a considerable discrepancy between values reported by highly respected investigators who did very arduous work. In particular, values the heat capacity [1] are 8-70% larger than the corresponding values in [2-4]; the discrepancy grows with the number of chlorine atoms in a molecule. So, we thought it reasonable and topical to attempt an independent result.

Benson's method [5] was used to calculate the heat capacities of the gaseous PCBs, PCDDs and PCDFs.

. We shall dwell briefly on this method.

Benson's method is the group additivity method involving analysis of the molecule structure. Atomic or molecular groups are separated and the nearest neighbors of the atom or the group are considered. Table 2 gives the number of groups necessary for determination of group increments in structural formulas of PCBs, PCDFs and PCDDs. Values of the thermodynamic characteristics of group increments were determined from available reference and literature data [5-6]. Information about the energy contribution of each group and the number of groups (see Table 3) was used to calculate the heat capacities of the PCBs, PCDDs and PCDFs.

Table 2. Number of groups for determination of group increments in structural formulas of PCBs, PCDFs and PCDDs

Св* is the carbon atom in an aromatic ring.

Table 3. Values of the thermodynamic characteristics determined by the method of group increments[5],[9].

Св* is the carbon atom in an aromatic ring

Values presented in Table 1 show the heat capacities of PCBs, PCDDs and PCDFs calculated in this study and by other investigators. It follows from the comparison of these results that the values of the heat capacity at T = 298 K obtained by using statistical methods (O.V. Dorofeeva et al. [2-4, 7,10]) for 17 isomers of PCBs, PCDDs and PCDFs are in good agreement with the values calculated by other investigators [9, 11-13] and with the our values.

Benson's method was used to calculate temperature dependences of the heat capacity.

Values of the thermodynamic characteristics of group increments were determined from available reference and literature data [5-6]. Information about the energy contribution of each group and the number of groups (see Table 2 and 4) was used to calculate the heat capacity dependence of molecules vs temperature. The obtained results were processed by a polynomial:

Ср = а + bx + cx2 + dx3 + e·105T-2, (1)

where х = T·10-3.

Values of the coefficients a, b, c, d and e are given in Table 5.

Liquid PCBs . It should be noted that ample literature data on the heat capacity of liquid ecotoxicants is only available for biphenyl (C12H10) [9, 14], dibenzo-n-dioxin (C12H8O2) [7, 15] and dibenzofuran (C12H8O) [5, 15]. The only study dealing with calculation of the heat capacity for the whole series of liquid PCDD and PCDF homologues was published by V.S. Iorish et al. [7]. As to liquid PCB compounds, the literature data on their the heat capacity are scarce [9, 14].

The heat capacity of liquid PCBs were calculated was calculated by two options: using the group additivity method due to Domalski [9] and from the equation 2. Information about the energy contribution of each group and the number of groups (see Table 1 and 2) was used to calculate the heat capacity .

Table 4. Heat capacity of group increments according to Benson [5, 6]

Св* is the carbon atom in an aromatic ring.

Data on the heat capacity of liquid PCBs can be found in [7, 9, 14]. Using the literature data on the heat capacity (Ср0298) of biphenyl and values of the thermodynamic characteristics of the Св-H and Св-Cl groups in Table 2, we calculated Ср0298 for the series of PCBs compounds:

Ср0298 (PCB) = Ср0298 (BP) - (10 - n) Ср0298 (Св- H) + n · Ср0298 (Св-Cl) +(m*orto corr Cl- Cl ) (2)

Values presented in Table 4 show the heat capacities of PCBs, PCDDs and PCDFs calculated in this study and by other investigators.

Missenar's group additivity method [12] was used to calculate temperature dependences of the heat capacity of liquid PCBs (heat capacities of group contributions at different temperatures are given in Table 6). The calculated temperature dependences were processed by the polynomial (1). Coefficients of the polynomial (1) are tabulated in Table 5. Data on the heat capacity of liquid PCBs at T = 298 K are given in Table 5. It is seen that the values of the heat capacity at 298 K, which were calculated in this study by Missenar's method [12] and the equation (6), agree well with each other and with those obtained in [9].

Table 6. Heat capacity of group increments according to Missenar [6]

Conclusions

1. The literature data on the heat capacity of 17 most widespread and hazardous isomers of PCBs, PCDDs and PCDFs in the gaseous state and 11 compounds of liquid PCBs have been analyzed and systematized for the first time.
2. Methods have been developed for calculating of the heat capacity of organic compounds. Values of the heat capacity of gaseous and liquid PCBs, PCDDs and PCDFs have been calculated for the first time.
3. The comparison of the calculated values of the heat capacity with the known literature data demonstrated their good mutual correlation.
4. The obtained data were added to the TERRA database and were used for thermodynamic simulation of the thermal stability of PCBs, PCDDs and PCDFs.
5. The obtained data can be used for simulating of the behavior of complex heterogeneous systems, including ecotoxicants, over a wide interval of temperatures and initial compositions.

This study were supported by RFBR (project No. 08-03-00362-a)

and program for young scientists and graduate students of

Ural Branch RAS.

References

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Table 1. Heat capacities of gaseous and liquid PCBs, PCDDs and PCDFs at T = 298 K

Table 5, cont'd

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Table 5. Temperature dependences of the heat capacity PCBs, PCDDs and PCDFs compounds

(*) gas

(**) liquid

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