Formation of Products of Incomplete Destruction (PID) from the Thermal Oxidative Decomposition of Perfluorooctanoic Acid (PFOA): Measurement, Modeling, and Reaction Pathways.

The thermal decomposition of perfluorooctanoic acid (PFOA) under oxidative conditions was investigated using air (O2) and N2O as oxidants over temperatures ranging from 400 to 1000 °C in an α-alumina reactor. In the presence of air, PFOA was found to decompose into perfluorohept-1-ene (C7F14) and perfluoroheptanoyl fluoride (C7F14O) in addition to HF, CO, and CO2. At temperatures above 800 °C, both C7F14 and C7F14O were no longer detected. A comprehensive analysis of the reaction mechanisms through quantum chemical analysis and kinetic modeling in combination with experimental observations was utilized to identify key reaction pathways. Quantum chemical analysis led to the conclusion that oxygen atoms are crucial in decomposing perfluoroalk-1-enes, especially the stable perfluorohept-1-ene (C7F14). Under oxidative conditions, it was found that significant quantities of C2F6 and CF4 were formed. Further quantum chemical analysis suggests that the O atoms facilitate the formation of volatile fluorinated compounds (VFCs) such as tetrafluoromethane (CF4) and hexafluoroethane (C2F6), particularly at higher temperatures. By elucidating these key reactions, an improved understanding of the potential formation products of incomplete combustion (PICs) or products of incomplete destruction (PIDs) is made.

Products and mechanism of thermal decomposition of chlorpyrifos under inert and oxidative conditions.

Chlorpyrifos (CPF) is a widely used pesticide; however, limited experimental work has been completed on its thermal decomposition. CPF is known to decompose into 3,5,6-trichloro-2-pyridinol (TCpyol) together with ethylene and HOPOS. Under oxidative conditions TCpyol can decompose into the dioxin-like 2,3,7,8-tetrachloro-[1,4]-dioxinodipyridine (TCDDPy). With CPF on the cusp of being banned in several jurisdictions worldwide, the question might arise as to how to safely eliminate large stockpiles of this pesticide. Thermal methods such as incineration or thermal desorption of pesticide-contaminated soils are often employed. To assess the safety of thermal methods, information about the toxicants arising from thermal treatment is essential. The present flow reactor study reports the products detected under inert and oxidative conditions from the decomposition of CPF representative of thermal treatments and of wildfires in CPF-contaminated vegetation. Ethylene and TCpyol are the initial products formed at temperatures between 550 and 650 °C, although the detection of HOPOS as a reaction product has proven to be elusive. During pyrolysis of CPF in an inert gas, the dominant sulfur-containing product detected from CPF is carbon disulfide. Quantum chemical analysis reveals that ethylene and HOPOS undergo a facile reaction to form thiirane (c-C2H4S) which subsequently undergoes ring opening reactions to form precursors of CS2. At elevated temperatures (>650 °C), TCpyol undergoes both decarbonylation and dehydroxylation reactions together with decomposition of its primary product, TCpyol. A substantial number of toxicants is observed, including HCN and several nitriles, including cyanogen. No CS2 is observed under oxidative conditions - sulfur dioxide is the fate of S in oxidation of CPF, and quantum chemical studies show that SO2 formation is initiated by the reaction between HOPOS and O2. The range of toxicants produced in thermal decomposition of CPF is summarised.

Pyrolysis of Glyphosate and Its Toxic Products.

With health concerns developing about the use of glyphosate (phosphonomethylglycine, PMG), the world's most used herbicide, the possibility of destruction of stockpiles via incineration arises. Little is known, however, about the possible toxic products of decomposition. We have performed a quantum chemical computation of the mechanism of thermal decomposition of PMG. Two initiation channels, one producing sarcosine and the other producing N-methylaminomethylphosphonic acid, have been located. Both the initial products decompose to dimethylamine (DMA), and the mechanism of further decomposition and toxic products is explored. Global potential energy surfaces for the initial decomposition of DMA are presented together with chemical kinetic modeling wherein the rate constants employed have been calculated from the quantum chemical data. Time and temperature evolution of the expected toxic products are presented and discussed.

Thermal oxidation of dieldrin and concomitant formation of toxic products including polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F).

This paper examines the gas phase thermal decomposition of dieldrin and associated formation of toxic combustion products including polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F). Volatile Organic Carbon (VOC) analysis revealed the formation of pentachlorostyrene (PCS), hexachlorostyrene (HCS) and polychlorinated naphthalene as toxic combustion products generated during the combustion of dieldrin. The thermal pyrolysis of dieldrin resulted in the formation of chlorinated benzenes and chlorinated phenols, which are known PCDD/F precursors. The formation of PCDD/F commenced around 823 K @ 5s residence time and results indicate a preference for the formation of PCDF over PCDD under all experimental conditions studied. Subsequent experiments, to examine the yield of PCDD/F as a function of temperature, reveal the progressive chlorination of PCDD/F with temperatures up to 923 K. Octachlorodibenzofuran (OCDF) was the major dioxin congener detected in the oxidation of dieldrin. The highest toxicity factor for dioxin formation was recorded at 923 K with a 6% O2 content in the feed gas and corresponds to 6.24 ng TEQ WHO 2005/mg of dieldrin and total PCDD/F concentration of 96.8 ng/mg of dieldrin.

Mechanism of the Thermal Decomposition of Chlorpyrifos and Formation of the Dioxin Analog, 2,3,7,8-Tetrachloro-1,4-dioxino-dipyridine (TCDDpy).

Thermal decomposition of the pesticide, chlorpyrifos (CPf) and its major degradation product, 3,5,6-trichloro-2-pyridinol (TCpyol), has been studied by quantum chemical calculation using density functional methods at the M06-2X/GTLarge//M06-2 X/6-31+G(d,p) level of theory. Chlopyrifos was found to undergo a series of unimolecular stepwise elimination reactions releasing two molecules of ethylene and finally HOPOS to form TCpyol. TCpyol underwent oxidative decomposition initiated by abstraction of its phenolic H atom by O2. Two phenoxy radicals so produced underwent combination leading to the formation of 2,3,7,8-tetrachloro-[1,4]dioxinodipyridine (TCDDpy). Via Smiles rearrangement both cis and trans TCDDpy are formed. Kinetic models have been constructed to model the decomposition of CPf into TCpyol and of the latter into cis and trans TCDDpy. Modeled results are compared with the experiments of Sakiyama et al. ( Organohalogen Compounds, 2012, 74, 1441-1444).

Mechanism and Rate of Thermal Decomposition of Hexachlorocyclopentadiene and Its Importance in PCDD/F Formation from the Combustion of Cyclodiene Pesticides.

Thermal decomposition of hexachlorocyclopentadiene (HCCP) has been studied in inert gas and under oxidative conditions in a silica flow reactor at a residence time of 5.0 s between 690 and 923 K and 1 atm pressure. Pyrolysis was initiated by Cl bond fission to form pentachlorocyclopentadienyl radical; two such radicals then combined to undergo a series of intramolecular rearrangements and Cl fissions, producing principally octachloronaphthalene (8ClNP) and Cl2. This process has been studied by quantum chemical calculation, and a reaction potential energy surface has been developed. The rate constant of initial Cl atom fission has been calculated by canonical variational transition state theory as k = 1.45 × 1015 exp(-222 ± 9 kJ mol-1/RT) s-1 between 500 and 2000 K. A minimal kinetic model was developed to model the decomposition and major products. Oxidative decomposition was studied in nitrogen with O2 contents of 1, 6, 12, and 20 mol %. Increasing O2 to 6-8% increased the rate of decomposition of HCCP and decreased the yield of 8ClNP. Above 823 K, hexachlorobenzene (HCB) and CO became major products. The oxidative reaction has also been studied quantum chemically. At high O2 content (>∼10%), the rate of decomposition of HCCP declined as did yields of 8ClNP and HCB, but CO yields increased.

Gas Phase Thermal Oxidation of Endosulfan and Formation of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans.

This paper investigates the thermal decomposition of technical endosulfan under oxidative conditions and the subsequent formation of polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans (PCDD/F, dioxins), and their precursors. Both quantum chemical calculations and laboratory experiments were employed to investigate the pathways of oxidation of endosulfan. The laboratory scale apparatus used consists of a tubular reactor and product collection system and analysis train. The results report the effect of temperature (523-923 K) and O2 concentration on PCDD/F formation in a N2 bath gas at a residence time of 5 s. The decomposition of endosulfan produces two types of PCDD/F precursors, involving all chlorinated benzenes (CBz) and chlorinated phenols (CPh). Oxidation of endosulfan favors the formation of PCDF over PCDD. Octachlorodibenzofuran is the most abundant homologue group detected in all experiments. The maximum emission factor for PCDD/F was observed at 923 K and O2 content of 6% and corresponds to 64 ng TEQ-WHO2005 per mg of endosulfan and a total dioxin concentration of 1131 ng/mg of endosulfan.

S-Nitrosation of Aminothiones.

Nitrosation reactions span a diverse range of applications, from biochemistry to industrially important processes. This study examines nitrosation of aminothiones in acidic solutions and re-evaluates currently accepted diffusion limits and the true nature of the nitrosating agent for nitrous acid initiated reactions. Experimental measurements from stopped-flow UV/vis spectrophotometry afforded derivation of equilibrium constants and reaction enthalpies. Apparent Keq corresponds to 559-382 M(-2) for thioacetamide (TA, 15-25 °C) and 12600-5590 M(-2) for thiourea (TU, 15-35 °C), whereas the reaction enthalpies amount to -27.10 ± 0.05 kJ for TA and -29.30 ± 0.05 kJ for TU. Theoretical calculations via a thermochemical cycle agree well with reaction free energies from experiments, with errors of -2-4 kJ using solvation method SMD in conjunction with hybrid meta exchange-correlation functional M05-2X and high-accuracy multistep method CBS-QB3 for gas-phase calculations. The kinetic rates increase with acidity at activation energies of 54.9 (TA) and 66.1 kJ·mol(-1) (TU) for the same temperature range, confirming activation-controlled reactions. At pH 1 and below, the main decomposition pathway for the S-nitroso species leads to formation of nitric oxide.

Thermodynamic stability and structure of cuprous chloride surfaces: a DFT investigation.

Density functional theory together with ab initio atomistic thermodynamics has been utilized to study the structures and stabilities of the low index CuCl surfaces. It is shown that the Cl-terminated structures are more stable than the Cu-terminated configurations, and that the defective CuCl(110)-Cu structure is more stable than the stoichiometric CuCl(110) surface. The equilibrium shape of a cuprous chloride nanostructure terminated by low-index CuCl surfaces has also been predicted using a Wulff construction. It was found that the (110) facets dominate at low chlorine concentration. As the chlorine concentration is increased, however, the contributions of the (100) and (111) facets to the Wulff construction also increase giving the crystal a semi-prism shape. At high chlorine concentration, and close to the rich limit, the (111) facets were found to be the only contributors to the Wulff construction, resulting in prismatic nanocrystals.

Accurate rate constants for decomposition of aqueous nitrous acid.

Decomposition of nitrous acid in aqueous solution has been studied by stopped flow spectrophotometry to resolve discrepancies in literature values for the rate constants of the decomposition reactions. Under the conditions employed, the rate-limiting reaction step comprises the hydrolysis of NO(2). A simplified rate law based on the known elementary reaction mechanism provides an excellent fit to the experimental data. The rate constant, 1.34 × 10(-6) M(-1) s(-1), is thought to be of higher accuracy than those in the literature as it does not depend on the rate of parallel reaction pathways or on the rate of interphase mass transfer of gaseous reaction products. The activation energy for the simplified rate law was established to be 107 kJ mol(-1). Quantum chemistry calculations indicate that the majority of the large activation energy results from the endothermic nature of the equilibrium 2HNO(2) ⇆ NO + NO(2) + H(2)O. The rate constant for the reaction between nitrate ions and nitrous acid, which inhibits HNO(2) decomposition, was also determined.

Formation of toxic species and precursors of PCDD/F in thermal decomposition of alpha-cypermethrin.

This article examines the thermal decomposition of alpha-cypermethrin, one of the most common pyrethroid pesticides. The objective was to identify its decomposition pathways and to gain an understanding into the formation of toxic species in the environment, including those that may behave in combustion systems, especially in fires in the environment, as precursors for PCDD/F (polychlorinated dibenzo-p-dioxins and dibenzofurans). The experiments were conducted under non-oxidative conditions using a tubular reactor housed in a three-zone heating furnace and operated with a dilute stream of alpha-cypermethrin in 99.999% nitrogen. The condensable products were identified and quantitated, after being collected in a cold solvent trap and in an activated charcoal cartridge. The study revealed the formation of pollutants including precursors of toxic PCDD/F such as diphenyl ether and phenol. It was also found that the decomposition of alpha-cypermethrin involved parallel pathways of an unusual vinylcyclopropane rearrangement-cum-aromatisation reaction transforming alpha-cypermethrin and a rupture of the C(=O)O-C(-C≡N) linkage. The former is similar to that occurring in the decomposition of permethrin pesticide, whereas the latter constitutes a newly discovered channel for the formation of pollutants. Density functional theory (DFT) calculations allowed us to attribute the occurrence of the second pathway to the effect of the cyanide group in significantly weakening the O-C bond.

Experimental study of decomposition of aqueous nitrosyl thiocyanate.

This study has examined the kinetics of the decomposition of nitrosyl thiocyanate (ONSCN) by stopped flow UV-vis spectrophotometry, with the reaction products identified and quantified by infrared spectroscopy, membrane inlet mass spectrometry, ion chromatography, and CN(-) ion selective electrode. The reaction results in the formation of nitric oxide and thiocyanogen, the latter decomposing to sulfate and hydrogen cyanide in aqueous solution. The rate of consumption of ONSCN depends strongly on the concentration of SCN(-) ions and is inhibited by nitric oxide. We have developed a reaction mechanism that comprises three parallel pathways for the decomposition of ONSCN. At high thiocyanate concentrations, two reaction pathways operate including a second order reaction to generate NO and (SCN)(2) and a reversible reaction between ONSCN and SCN(-) producing NO and (SCN)(2)(-), with the rate limiting step corresponding to the consumption of (SCN)(2)(-) by reaction with ONSCN. The third reaction pathway, which becomes significant at low thiocyanate concentrations, involves formation of a previously unreported species, ONOSCN, via a reaction between ONSCN and HOSCN, the latter constituting an intermediate in the hydrolysis of (SCN)(2). ONOSCN contributes to the formation of NO via homolysis of the O-NO bond and subsequent dimerization and hydrolysis of OSCN. Fitting the chemical reactions of the model to the experimental measurements, which covered a wide range of reactant concentrations, afforded estimation of all relevant kinetic parameters and provided an excellent match. The reaction mechanism developed in this contribution may be applied to predict the rates of NO formation from ONSCN during the synthesis of azo dyes, the gassing of explosive emulsions, or nitrosation reactions occurring in the human body.

An equilibrium ab initio atomistic thermodynamics study of chlorine adsorption on the Cu(001) surface.

The effect of chlorine (Cl) chemisorption on the energetics and atomic structure of the Cu(001) surface over a wide range of chlorine pressures and temperatures has been studied using equilibrium ab initio atomistic thermodynamics to elucidate the formation of cuprous chloride (CuCl) as part of the Deacon reaction on copper metal. The calculated surface free energies show that the 1/2 monolayer (ML) c(2 × 2)-Cl phase with chlorine atoms adsorbed at the hollow sites is the most stable structure for a wide range of Cl chemical potential, in agreement with experimental observations. It is also found that at very low pressure and exposure, but elevated temperature, the 1/9 ML and 1/4 ML phases become the most stable. By contrast, a high coverage of Cl does not lead to thermodynamically stable geometries. The subsurface adsorption of Cl atoms, however, dramatically increases the stability of the 1 ML and 2 ML adsorption configurations providing a possible pathway for the formation of the bulk-chloride surface phases in the kinetic regime.

Rate constants for hydrogen abstraction reactions by the hydroperoxyl radical from methanol, ethenol, acetaldehyde, toluene, and phenol.

An important step in the initial oxidation of hydrocarbons at low to intermediate temperatures is the abstraction of H by hydroperoxyl radical (HO(2)). In this study, we calculate energy profiles for the sequence: reactant + HO(2) → [complex of reactants] → transition state → [complex of products] → product + H(2)O(2) for methanol, ethenol (i.e., C(2)H(3)OH), acetaldehyde, toluene, and phenol. Rate constants are provided in the simple Arrhenius form. Reasonable agreement was obtained with the limited literature data available for acetaldehyde and toluene. Addition of HO(2) to the various distinct sites in phenol is investigated. Direct abstraction of the hydroxyl H was found to dominate over HO(2) addition to the ring. The results presented herein should be useful in modeling the lower temperature oxidation of the five compounds considered, especially at low temperature where the HO(2) is expected to exist at reactive levels.

Toxic pollutants emitted from thermal decomposition of phthalimide compounds.

Phthalimide (PI) and tetrahydrophthalimide (THPI) are two structurally similar compounds extensively used as intermediates for the synthesis of variety of industrial chemicals. This paper investigates the thermal decomposition of PI and THPI under oxygen rich to oxygen lean conditions, quantifying the production of toxicants and explaining their formation pathways. The experiments involved a plug flow reactor followed by silica cartridges, activated charcoal trap and a condenser, with the decomposition products identified and quantified by Fourier transform infrared spectroscopy (FTIR), gas chromatography-mass spectrometry (GC-MS) and micro gas chromatography (µGC). The density functional theory (DFT) calculations served to obtain dissociation energies and reaction pathways, to elucidate the reaction mechanism. The oxidation of PI and THPI produced several toxic nitrogen-containing gases and volatile organic compounds, including hydrogen cyanide, isocyanic acid, nitrogen oxides, benzonitrile, maleimide and tentatively identified benzenemethanimine. The detection of dibenzo-p-dioxin (DD) and dibenzofuran (DF) suggests potential formation of the toxic persistent organic pollutants (POPs) in fires involving PI and THPI, in presence of a chlorine source. The oxidation of THPI produced 2-cyclohexen-1-one, a toxic unsaturated ketone. The results of the present study provide the data for quantitative risk assessments of emissions of toxicants in combustion processes involving PI and THPI.

Air pollutants formed in thermal decomposition of folpet fungicide under oxidative conditions.

This contribution studies the decomposition of folpet fungicide under oxidative conditions and compares the product species with those of captan fungicide, which is structurally related to folpet. Toxic products arising from folpet comprised carbon disulfide (highest emission factor of 4.9 mg g(-1) folpet), thiophosgene (14.4), phosgene (34.1), hydrogen cyanide (2.6), tetrachloroethylene (111), hexachloroethane (167), and benzonitrile (4.5). Owing to their related molecular structures, folpet emitted similar products to captan but at different yields, under the same experimental conditions. It appears that the availability of easily abstractable H atoms, in the structure of captan but not in that of folpet, defines the product distribution. In conjunction with the quantum chemical calculations, these experimental measurements afford an enhanced explanation of the formation pathways of hazardous decomposition products of these two structurally related fungicides.

Theoretical study on the thermodynamic properties and self-decomposition of methylbenzenediol isomers.

Alkylated hydroxylated aromatics are major constituents of various types of fuels, including biomass and low-rank coal. In this study, thermochemical parameters are obtained for the various isomeric forms of methylbenzenediol isomers in terms of their enthalpies of formation, entropies, and heat capacities. Isodesmic work reactions are used in quantum chemical computations of the reaction enthalpies for O-H and H2C-H bond fissions and the formation of phenoxy- and benzyl-type radicals. A reaction potential energy on the singlet-state surface surface is mapped out for the unimolecular decomposition of the 3-methylbenzene-1,2-diol isomer. According to the calculated high pressure-limit reaction rate constants, concerted hydrogen molecule elimination from the methyl group and the hydroxyl group, in addition to intermolecular H migration from the hydroxyl group, dominates the unimolecular decomposition at low to intermediate temperatures (T ≤ 1200 K). At higher temperatures, O-H bond fission and concerted water elimination are expected to become the sole decomposition pathways.

Conversion of a CFCs, HFCs and HCFCs waste mixture via reaction with methane.

The gas-phase reaction of a mixture of waste refrigerant gases, namely R22 (CHClF(2)), R12 (CCl(2)F(2)) and R134a (CH(2)FCF(3)) with CH(4) has been investigated over the temperature range of 873-1133K. The investigation was undertaken as an initial assessment of the viability of this process as a treatment option for waste mixtures of hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFC), chlorofluorocarbons (CFCs) and as a potential route for the synthesis of CH(2)=CF(2) (VDF). During the reaction, CH(2)=CF(2) is observed as the major product formed and a 43% selectivity to CH(2)=CF(2) is obtained at 1073K. A detailed mechanism is developed based on the mechanistic analysis from kinetic modeling, with the initiation reaction involving the formation of Cl radicals from CCl(2)F(2). Good agreement is achieved between the predictions and experimental results. Based on a mechanistic analysis, a summary of the major reaction pathways is proposed, which is consistent with the experimental observations.

Conversion of CHF(3) to CH(2)CF(2) via reaction with CH(4) in the presence of CBrF(3): An experimental and kinetic modelling study.

Gas-phase reaction of CHF(3) (HFC 23) with CH(4) in the presence of CBrF(3) (halon 1301) to produce CH(2)CF(2) (VDF) is presented. Experiments were carried out in a plug-flow reactor at temperatures between 873 and 1173 K. Under these conditions, CH(2)CF(2) was a dominant product observed, with CH(2)F(2), C(2)H(2), CH(2)CHF, C(2)F(4), C(2)H(6) and CHFCF(2) also detected. In the presence of less than 6000 ppm of CBrF(3), the rate of formation of CH(2)CF(2) is significantly enhanced, and a much lower rate of formation of the major by-product, C(2)F(4) is observed. Further increasing the proportion of CBrF(3) in the feed resulted in an even higher rate of formation of CH(2)CF(2). Experimental results are fitted very well by a model derived from the NIST and GRI-Mech 3.0 mechanisms, modified to include additional published kinetic data.

Thermal decomposition of captan and formation pathways of toxic air pollutants.

This study investigates the thermal decomposition of a widely used fungicide, captan, under gas phase conditions, similar to those occurring in fires, cigarette burning, and combustion of biomass treated or contaminated with pesticides. The laboratory-scale apparatus consisted of a plug flow reactor equipped with sampling trains for gaseous, volatile organic compounds (VOC) and condensed products, with analysis performed by Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS), respectively. Under oxidative conditions, the thermal decomposition of captan generated gaseous pollutants including carbon disulfide, thiophosgene, phosgene, and hydrogen cyanide. The VOC analysis revealed the formation of tetrachloroethylene, hexachloroethane, and benzonitrile. Quantum chemical calculations indicated that captan decomposes unimolecularly, via fission of the C-S bond, with the ensuing radicals reacting with O(2). The results of the present study provide an improved understanding of the formation pathways of toxic air pollutants in the accidental or deliberate combustion of captan.