Formation of persistent organic pollutants from 2,4,5-trichlorothiophenol combustion: a density functional theory investigation

Abstract

Polychlorinated dibenzothiophene (PCDT) and polychlorinated thianthrene (PCTA) are sulfur analogues of dioxins, such as polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). In this work, we present a detailed mechanistic and kinetic analysis of PCDT and PCTA formation from the combustion of 2,4,5-trichlorothiophenol. It is shown that the formation of these persistent organic pollutants is more favourable, both kinetically and thermodynamically, than their analogous dioxin counterparts. This is rationalised in terms of the different influences of the S-H and O-H moieties in the 2,4,5-trichlorothiophenol and 2,4,5-trichlorophenol precursors. Kinetic parameters also indicate that the yield of PCDT should exceed that of PCDD. Finally, we demonstrate here that the degree and pattern of chlorination on the 2,4,5-trichlorothiophenol precursor leads to subtle thermodynamic and kinetic changes to the PCDT/PCTA formation mechanisms. Graphical abstract Formation mechanisms of persistant organic pollutants, PCDT and PCTA, from 2,4,5-trichlorothiophenol combustion, has been investigated using density functional theory.

Quantum Chemical and Kinetic Study on Radical/Molecule Formation Mechanism of Pre-Intermediates for PCTA/PT/DT/DFs from 2-Chlorothiophenol and 2-Chlorophenol Precursors

“…Careful examination of the literature shows that two heterogeneous PCTA/DT formation pathways were proposed: Reactions from elemental carbon, sulfur, and chlorine- and sulfur-containing pesticides [22], and reactions from chemical precursors [23,24,25,26,27], which is the most direct route to the formation of PCTA/DTs. Among different precursors of PCTA/DTs, chlorothiophenols (CTPs) are demonstrated to be the most important precursors of PCTA/DTs, which is consistent with the widely recognized fact that chlorophenols (CPs) are identified to be predominant precursors of PCDD/DFs [23,24,28].…”

“…Similar to the formation of PCDD/DFs from CPs precursor, the homogeneous gas-phase formation of PCTA/DTs involves radical/radical condensation of CTPRs and radical/molecule recombination of CTPR and CTP [23,24,25,37,38,39,40]. Dar et al presented that radical/radical couplings are thermodynamically comparable to radical/molecule recombinations for the PCTA/DT formation [23,24].…”

“…Dar et al presented that radical/radical couplings are thermodynamically comparable to radical/molecule recombinations for the PCTA/DT formation [23,24]. Therefore, following the radical/radical routes, a series of theoretical studies on PCTA/DT formation mechanisms from the coupling of 2-CTPRs, 2,4,5-TCTPRs, 2,4-DCTPRs, and 2,4,6-TCPRs were carried out by Dar et al and our group [23,24,25,26]. These studies showed that the formation of PCTAs was easier than that of PCDT due to the fact that the PCTA can form via one elementary step less than PCDT and the potential barrier of the rate-determining step of PCTA is about 10 kcal/mol lower than that of PCDT [23,24,25,26].…”

“…Therefore, following the radical/radical routes, a series of theoretical studies on PCTA/DT formation mechanisms from the coupling of 2-CTPRs, 2,4,5-TCTPRs, 2,4-DCTPRs, and 2,4,6-TCPRs were carried out by Dar et al and our group [23,24,25,26]. These studies showed that the formation of PCTAs was easier than that of PCDT due to the fact that the PCTA can form via one elementary step less than PCDT and the potential barrier of the rate-determining step of PCTA is about 10 kcal/mol lower than that of PCDT [23,24,25,26]. However, this conclusion failed to give reasonable explanation for the much higher concentration of PCDTs than that of PCTAs under the pyrolysis or combustion conditions [16,17].…”

Mechanistic and Kinetic Study on Self-/Cross- Condensation of PCTA/DT Formation Mechanisms from Three Types of Radicals of 2,4-Dichlorothiophenol

“…Among different precursors, chlorothiophenols (CTPs) are the most direct precursors of PCTA/DTs [20,21,22], which have been widely used in large quantities in various chemical industries, such as in manufacturing of dyes, insecticides, printing inks, pharmaceuticals, and polyvinyl chloride [23]. Similar to the formation of PCDD/DFs from chlorophenol (CP) precursors, the gas-phase formation of PCTA/DTs from CTP precursors was also proposed involving radical-radical coupling of two CTPRs and radical-molecule recombination of CTPR and CTP [21,22,24,25,26,27,28]. Dar et al have shown that radical-radical coupling is more competitive thermodynamically than radical-molecule recombination for PCTA/DT formation [21,22,24].…”