Gibbs Free Energies of Formation of Pcdds: Evaluation of Estimation Methods and Application for Predicting Dehalogenation Pathways

Abstract

Abstract-Gibbs free energies of formation for the ideal gas, liquid phase, and species in solution were estimated for 75 polychlorinated dibenzo-p-dioxins (PCDDs) at 298.15 K and 100,000 Pa. These values were used to calculate standard redox potentials, with H 2 as the electron donor. Owing to the lack of experimental thermodynamic data for the involved chemical species, the standard-state ideal gas enthalpies of formation were evaluated by several semiempirical methods including the Modified Neglect of Diatomic Differential Overlap (MNDO), Austin Model 1 (AM1), and MNDO-Parametric Method 3 (PM3). Three different implementations of Benson's group additivity method for estimation of ideal gas enthalpies of formation were also evaluated. As a result of the evaluation, the CHETAH program based on Benson's group additive method was chosen for ideal gas enthalpy of formation calculations. Entropies, vapor pressure, and aqueous solubility were calculated by existing methods in order to complete the thermodynamic cycle. Calculated redox potentials indicate that chlorodioxins, including 2,3,7,8-tetrachlorodioxin, can serve as electron acceptors in anaerobic environments, at least from a thermodynamic perspective.

Biodiversity of Dehalorespiring Bacteria with Special Emphasis on Polychlorinated Biphenyl/Dioxin Dechlorinators

“…As described above and previously 37,71) , it is clear that the hydrogenotrophic redox process with PBDs as terminal electron acceptors can provide energy sufficient for the growth and survival of dehalogenating microorganisms.…”

Biodiversity of Dioxin-Degrading Microorganisms and Potential Utilization in Bioremediation.

“…Bacterial dehalorespiration with tri-or tetrachloroinated benzene as a terminal electron acceptor has also been reported 4) . Gibbs free energy of formation of various PCDD/Fs and redox potentials for PCDD/F substrate/product couples indicate that the reductive dehalogenation of PCDD/Fs is an exergonic reaction 91) and imply that microorganisms acquire energy via anaerobic electron transport with PCDD/Fs as terminal electron acceptors.…”

“…Earlier work related to this subject showed changes in PCDD/F congener distribution patterns and the resultant accumulation of less chlorinated forms in sediments 183) . 91) assumed that 2,3,7,8-TCDD is not a predicted intermediate in any theoretically preferred pathway of PCDD dechlorination. If this toxic congener is to be produced, it is from 1,2,3,7,8-PeCDD.…”

Molecular orbital calculations to describe microbial reductive dechlorination of polychlorinated dioxins

“…This method involves least‐squares fitting of the atomic charges in order to reproduce the charge density, which has been calculated quantum mechanically (DFT method) as a set of points in the space surrounding the molecule. The Gibbs free energy for reductive dehalogenation of chlorinated dioxins used in this study was calculated by Huang et al [9] with H 2 as the electron donor.…”

“…Though not conclusive, these results are supportive of the laboratory data using both historical and freshly spiked dioxin congeners. Furthermore, based on the Gibbs free energy of reductive dechlorination and standard redox potentials for this process, chlorinated dioxins can from a thermodynamic perspective serve as electron acceptors in anaerobic environments [9].…”