New Understanding of the Formation of PCDD/Fs from Chlorophenol Precursors: A Mechanistic and Kinetic Study

Abstract

Although chlorophenols (CPs) are considered to be the most important and direct precursors of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), our understanding of the formation mechanism of PCDD/Fs is exclusively limited to an invariable idea that chlorophenoxy radicals (CPRs) are only necessary intermediates. The present work presents a systematic theoretical study that aims at providing new insight into the homogeneous formation of PCDD/Fs from CPs. Two different types of radicals from CPRs, i.e., substituted phenyl radicals and phenoxyl diradicals, are proposed to serve as potential sources contributing to the formation of PCDD/Fs. The thermodynamic and kinetic properties of reactions of 2-chlorophenol (2-CP), as a representative of CP congeners, with atomic H to produce various potential radicals forming PCDD/Fs are studied by performing density functional theory calculations and direct kinetics studies. The newly proposed radicals, especially substituted phenoxyl diradicals (the most direct intermediates of PCDD/Fs), can be formed via reactions of 2-CP with atomic H with small barriers and large reaction energies. They should be expected to be responsible for the homogeneous formation of PCDD/Fs under high temperature. Several typical PCDD/F products are predicated through direct self- and cross-couplings of the newly proposed radicals. The radical coupling patterns proposed in the present work expand our understanding of the formation mechanism of PCDD/Fs from CP precursors.

An Overview of PCDD/F Inventories and Emission Factors from Stationary and Mobile Sources: What We Know and What is Missing

“…Detailed mechanisms have been addressed in previous studies by (Zhang et al, 2008;Qu et al, 2009;Zhang et al, 2014), therefore here we only briefly describe the important mechanisms that are most related to those in combustion sources, which include De novo synthesis and precursor pathway. Through research work, the pathways for PCDD/F formation have been determined to be either through heterogeneous or homogenous mechanisms (Stanmore, 2004;Altarawneh et al, 2009).…”

Modeling study of polychlorinated dibenzo-p-dioxins and dibenzofurans behavior in flue gases under electron beam irradiation

“…The homogeneous gas-phase precursor's mechanism of PCDD/Fs formation under oxidation conditions has been extensively studied both experimentally (Evans and Dellinger, 2004;Briois et al, 2006) and theoretically (Altarawneh et al, 2007;Xu et al, 2011;Zhang et al, 2014). The available kinetic models of this mechanism proposed by Babushok and Tsang (2003) and Khachatryan et al (2003) are originated from Shaub and Tsang (1983) model, and may be used in the low temperature range for estimation of radiationinduced PCDD/Fs formation in the EBDS process.…”

Assessment of PCDD/Fs formation in the Fenton oxidation of 2-chlorophenol: Influence of the iron dose applied

“…CPs are ubiquitously in the environment due to their extensive and long-term use in industrial and daily life (Zhang et al, 2014), having been detected in industrial wastewaters, surface and groundwater and soils (Detomaso et al, 2003;Oturan et al, 2009). Together, reduced input of these compounds into the environment and the development of effective methods for their removal are important objectives for mitigating these contaminants.…”

“…The oxidative coupling of CPRs/CPs leads to bi-aromatic intermediates whose cyclization produce PCDD/Fs (Zhang et al, 2008;Altarawneh et al, 2009). Other intermediates, such as phenyl radicals and phenoxyl diradicals, can also be formed from 2-CP and contribute to the formation of PCDD/Fs (Pan et al, 2013;Zhang et al, 2014). In addition, as a result of the chlorine content of 2-CP as well as the ubiquitous chloride in the reaction medium, the effectiveness of the Fenton process may be diminished owing to the formation of iron(III)-chlorocomplexes and the scavenging capacity of the chloride ion, although the inhibition caused by ferric complexation may be avoided at chloride concentrations lower than 200 mM as it was observed by Lu et al (2005) and Vallejo et al (2014).…”