Atmospheric Chemistry of CF3CH2CH2OH: Kinetics, Mechanisms and Products of Cl Atom and OH Radical Initiated Oxidation in the Presence and Absence of NOX

Global performance and trends of research on per- and polyfluoroalkyl substances (PFASs) between 2001 and 2018 using bibliometric analysis

Per- and polyfluoroalkyl substances (PFASs) are widely used in food packaging, non-stick pots, and surfactants. However, their persistence in the environment, hazardous nature, and potential for bioaccumulation and long-range transport have alarmed an increasing number of scholars and research institutions. Although several literature reviews on PFASs research exist, only a few of them have considered bibliometric indices. In this study, 3,373 PFASs-related articles published between 2001 and 2018 were analyzed using a bibliometric analysis method based on the Science Citation Index (SCI) Expanded. The software tools for mapping knowledge domain (MKD) (VOSviewer and Science of Science (Sci2)) were used to analyze the performance of contributors and PFASs research topics, hotspots, and trends. Our results reveal that the number of PFASs-related articles published annually has increased significantly, with most originating from the United States (followed by those from China). The Chinese Academy of Sciences has published the most articles. A comprehensive analysis of title, keywords, and keywords plus showed that PFASs research hotspots include humans, precursors, and detection methods, with the main focuses being environmental science, toxicology, and environmental engineering. The four main research topics of PFASs were identified, and a literature review was carried out for each one. Overall, this study can supply researchers with a deeper understanding of the development of PFASs studies and provide a comprehensive data reference for researchers to further grasp the research direction in this field.

Atmospheric oxidation of fluoroalcohols initiated by ˙OH radicals in the presence of water and mineral dusts: mechanism, kinetics, and risk assessment

The transport and formation of fluorinated compounds are greatly significant due to their possible environmental risks. In this work, the ˙OH-mediated degradation of CF3CF2CF2CH2OH and CF3CHFCF2CH2OH in the presence of O2/NO/NO2 was studied by using density functional theory and the direct kinetic method. The formation mechanisms of perfluorocarboxylic/hydroperfluorocarboxylic acids (PFCAs/H-PFCAs), which were produced from the reactions of α-hydroxyperoxy radicals with NO/NO2 and the ensuing oxidation of α-hydroxyalkoxy radicals, were clarified and discussed. The roles of water and silica particles in the rate constants and ˙OH reaction mechanism with fluoroalcohols were investigated theoretically. The results showed that water and silica particles do not alter the reaction mechanism but obviously change the kinetic properties. Water could retard fluoroalcohol degradation by decreasing the rate constants by 3-5 orders of magnitude. However, the heterogeneous ˙OH-rate coefficients on the silica particle surfaces, including H4SiO4, H6Si2O7, and H12Si6O18, are larger than that of the naked reaction by 1.20-24.50 times. This finding suggested that these heterogeneous reactions may be responsible for the atmospheric loss of fluoroalcohols and the burden of PFCAs. In addition, fluoroalcohols could be exothermically trapped by H12Si6O18, H6Si2O7, and H4SiO4, in which the chemisorption on H12Si6O18 is stronger than that on H6Si2O7 or H4SiO4. The global warming potentials and radiative forcing of CF3CF2CF2CH2OH/CF3CHFCF2CH2OH were calculated to assess their contributions to the greenhouse effect. The toxicities of individual species were also estimated via the ECOSAR program and experimental measurements. This work enhances the understanding of the environmental formation of PFCAs and the transformation of fluoroalcohols.

Vertical Profiles, Sources, and Transport of PFASs in the Arctic Ocean

The relative importance of atmospheric versus oceanic transport for poly- and perfluorinated alkyl substances (PFASs) reaching the Arctic Ocean is not well understood. Vertical profiles from the Central Arctic Ocean and shelf water, snow and meltwater samples were collected in 2012; 13 PFASs (C6-C12 PFCAs; C6, 8, 10 PFSAs; MeFOSAA and EtFOSAA; and FOSA) were routinely detected (range: <5-343 pg/L). PFASs were only detectable above 150 m depth in the polar mixed layer (PML) and halocline. Enhanced concentrations were observed in snow and meltpond samples, implying atmospheric deposition as an important source of PFASs. Model results suggested atmospheric inputs to account for 34-59% (∼11-19 pg/L) of measured PFOA concentrations in the PML (mean 32 ± 15 pg/L). Modeled surface and halocline measurements for PFOS based on North Atlantic inflow (11-36 pg/L) agreed with measurements (mean, 17, range <5-41 pg/L). Modeled deep water concentrations below 200 m (5-15 pg/L) were slightly higher than measurements (<5 pg/L), suggesting the lower bound of PFAS emissions estimates from wastewater and rivers may provide the best estimate of inputs to the Arctic. Despite low concentrations in deep water, this reservoir is expected to contain most of the PFOS mass in the Arctic (63-180 Mg) and is projected to continue increasing to 2038.

Atmospheric chemistry of ethers, esters, and alcohols on the lifetimes, temperature dependence, and kinetic isotope effect: an example of CF3CX2CX2CX2OX with OX reactions (X = H, D)

Mechanisms and kinetics of the reaction of CF₃CX₂CX₂CX₂OX with OX (X= H, D) radical are investigated on a sound theoretical basis.

Atmospheric chemistry of CF3(CX2)2CH2OH: rate coefficients and temperature dependence of reactions with chlorine atoms and the subsequent pathways of alkyl and alkoxy radicals (X = H, F)

The atmospheric and kinetic properties of CF₃(CX₂)₂CH₂OH (X = H, F) with chlorine atoms were studied by density functional and canonical variational transition state theories in conjunction with the small-curvature tunneling correction.

Temperature effects on the removal of potential HFC replacements, CF3CH2CH2OH and CF3(CH2)2CH2OH, initiated by OH radicals

The gas-phase kinetic coefficients of OH radicals with two primary fluorinated alcohols, CF(3)CH(2)CH(2)OH (k(1)) and CF(3)(CH(2))(2)CH(2)OH (k(2)), potential replacements of hydrofluorocarbons (HFCs), are reported here as a function of temperature (T = 263-358 K) for the first time. k(1) and k(2) (together referred as k(i)) were measured under pseudo-first-order conditions with respect to the initial OH concentration using the pulsed laser photolysis/laser induced fluorescence technique. The observed temperature dependence of k(i) (in cm(3) molecule(-1) s(-1)) is described by the following Arrhenius expressions: k(1)(T) = (2.82 ± 1.28) × 10(-12) exp{-(302 ± 139)/T} cm(3) molecule(-1) s(-1) and k(2)(T) = (1.20 ± 0.73) × 10(-11) exp{-(425 ± 188)/T} cm(3) molecule(-1) s(-1).The uncertainties in the Arrhenius parameters are at a 95% confidence level (± 2σ). Uncertainties in k(i)(T) include both statistical and systematic errors. Activation energies were (2.5 ± 1.2) kJ/mol and (3.6 ± 1.6) kJ/mol for the OH-reaction with CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH, respectively. The global lifetime (τ) at 275 K for CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH due to the OH-reaction was estimated to be ca. 2 weeks and 5 days, respectively. The reported Arrhenius parameters can be used in 3D models that take into account the geographical region and season of emissions for estimating a matrix of instantaneous lifetimes. As a consequence of the substitution of the -CH(3) group by a -CH(2)OH group in HFCs, such as CF(3)CH(2)CH(3) and CF(3)(CH(2))(2)CH(3), the tropospheric lifetime with respect to the OH reaction is significantly shorter and, since their radiative forcing is similar, global warming potentials of CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH are negligible. Therefore, CF(3)CH(2)CH(2)OH and CF(3)(CH(2))(2)CH(2)OH seem to be suitable alternatives to HFCs.

Atmospheric chemistry of n-C6F13CH2CHO: formation from n-C6F13CH2CH2OH, kinetics, and mechanisms of reactions with chlorine atoms and OH radicals

Smog chamber FTIR techniques were used to measure k(Cl + n-C(6)F(13)CH(2)CHO) = (1.84 +/- 0.22) x 10(-11), k(Cl + n-C(6)F(13)CHO) = (1.75 +/- 0.70) x 10(-12), and k(OH + n-C(6)F(13)CH(2)CHO) = (2.15 +/- 0.26) x 10(-12) cm(3) molecule(-1) s(-1) in 700 Torr of N(2) or air diluent at 296 +/- 2K. The chlorine-atom-initiated oxidation of n-C(6)F(13)CH(2)CH(2)OH in air gives n-C(6)F(13)CH(2)CHO in a molar yield of 99 +/- 8%. The atmospheric fate of n-C(6)F(13)CH(2)C(O) radicals is reaction with O(2), while the fate of n-C(6)F(13)C(O) radicals is decomposition to give n-C(6)F(13) radicals and CO. The results are discussed with respect to the atmospheric chemistry of fluorinated alcohols and the formation of perfluorocarboxylic acids.

Atmospheric perfluorinated acid precursors: chemistry, occurrence, and impacts

Perfluorocarboxylic acids (PFCAs) can be found from the hydrolysis of perfluoroacyl fluorides and chlorides, which can be produced in three separate ways in the atmosphere. Alternatively, PFCAs can be formed directly in the gas phase through reaction of perfluoroacyl peroxy radicals or perfluorinated aldehyde hydrates. All five mechanisms have been elucidated using smog chamber techniques. Yields of the PFCAs from this process vary from less than 10% to greater than 100%, depending on the mechanism. The formation of perfluorosulfonic acids in the atmosphere can also occur, though the mechanism has not been entirely elucidated. A large number of compounds have been confirmed as perfluorinated acid precursors, including CFC-replacement compounds, anesthetics, fluorotelomer compounds, and perfluorosulfonamides. Levels of some of these compounds have been measured in the atmosphere, but concentration for the majority have yet to be detected. It is clear that atmospheric oxidation of volatile precursors contributes to the overall burden of PFAs, though the extent to which this occurs is compound and environment dependent and is difficult to assess accurately.

Extraction and clean-up strategies for the analysis of poly- and perfluoroalkyl substances in environmental and human matrices

The rapidly expanding field of per- and polyfluorinated alkyl substances (PFASs) research has resulted in a wide range of analytical methodologies to determine the human and environmental exposure to PFASs. This paper reviews the currently applied techniques for sample pre-treatment, extraction and clean-up for the analysis of ionic and non-ionic PFASs in human and environmental matrices. Solid phase extraction (SPE) is the method of choice for liquid samples (e.g. water, blood, serum, plasma), and may be automated in an on-line set-up for (large volume) sample enrichment and sample clean-up. Prior to SPE, sample pre-treatment (filtration or centrifugation for water or protein precipitation for blood) may be required. Liquid-liquid extraction can also be used for liquid samples (and does not require above mentioned sample pretreatment). Solid-liquid extraction is the commonly applied method for solid matrices (biota, sludge, soil, sediment), but automation options are limited due to contamination from polytetrafluorethylene tubings and parts applied in extraction equipment. Air is generally preconcentrated on XAD-resins sandwiched between polyurethane foam plugs. Clean-up of crude extracts is essential for destruction and removal of lipids and other co-extractives that may interfere in the instrumental determination. SPE, (fluorous) silica column chromatography, dispersive graphitized carbon and destructive methods such as sulphuric acid or KOH treatment can be applied for clean-up of extracts. Care should be taken to avoid contamination (e.g. from sample bottles, filters, equipment) and losses of PFASs (e.g. adsorption, volatilization) during sampling, extraction and clean-up. Storage at -20 degrees C is generally appropriate for conservation of samples.

The gas phase tropospheric removal of fluoroaldehydes (CxF2x+1CHO, x = 3, 4, 6)

The rate coefficient of the OH reaction with the perfluoroaldehydes C(3)F(7)CHO and C(4)F(9)CHO have been determined in the temperature range 252-373 K using the pulsed laser photolysis-laser induced fluorescence (PLP-LIF) method: k(C(3)F(7)CHO+OH) = (2.0 +/- 0.6) x 10(-12) exp[-(369 +/- 90)/T] and k(C(4)F(9)CHO+OH) = (2.0 +/- 0.5) x 10(-12) exp[-(356 +/- 70)/T] cm(3) molecule(-1) s(-1), corresponding to (5.8 +/- 0.6) x 10(-13) and (6.1 +/- 0.5) x 10(-13) cm(3) molecule(-1) s(-1), respectively, at 298 K. The UV absorption cross sections of these two aldehydes and CF(3)(CF(2))(5)CH(2)CHO have been measured over the range 230-390 nm at 298 K and also at 328 K for CF(3)(CF(2))(5)CH(2)CHO. The obtained results for C(3)F(7)CHO and C(4)F(9)CHO are in good agreement with two recent determinations but the maximum value of the absorption cross section for CF(3)(CF(2))(5)CH(2)CHO is over a factor of two lower than the single one recently published. The photolysis rates of C(3)F(7)CHO, C(4)F(9)CHO and CF(3)(CF(2))(5)CHO have been measured under sunlight conditions in the EUPHORE simulation chamber in Valencia (Spain) at the beginning of June. The photolysis rates were, respectively, J(obs) = (1.3 +/- 0.6) x 10(-5), (1.9 +/- 0.8) x 10(-5) and (0.6 +/- 0.3) x 10(-5) s(-1). From the J(obs) measurements and calculated photolysis rate J(calc), assuming a quantum yield of unity across the atmospheric range of absorption of the aldehydes, quantum yields J(obs)/J(calc) = (0.023 +/- 0.012), (0.029 +/- 0.015) and (0.046 +/- 0.028) were derived for the photodissociation of C(3)F(7)CHO, C(4)F(9)CHO and CF(3)(CF(2))(5)CHO, respectively. The atmospheric implication of the data obtained in this work is discussed. The main conclusion is that the major atmospheric removal pathway for fluoroaldehydes will be photolysis, which under low NO(x) conditions, may be a source of fluorinated carboxylic acids in the troposphere.

Atmospheric Chemistry of n-CxF2x+1CHO (x = 1, 2, 3, 4): Fate of n-CxF2x+1C(O) Radicals

Smog chamber/FTIR techniques were used to study the atmospheric fate of n-C(x)F(2)(x)(+1)C(O) (x = 1, 2, 3, 4) radicals in 700 Torr O(2)/N(2) diluent at 298 +/- 3 K. A competition is observed between reaction with O(2) to form n-C(x)()F(2)(x)()(+1)C(O)O(2) radicals and decomposition to form n-C(x)F(2)(x)(+1) radicals and CO. In 700 Torr O(2)/N(2) diluent at 298 +/- 3 K, the rate constant ratio, k(n-C(x)F(2)(x)(+1)C(O) + O(2) --> n-C(x)F(2)(x)(+1)C(O)O(2))/k(n-C(x)F(2)(x)(+1)C(O) --> n-C(x)F(2)(x)(+1) + CO) = (1.30 +/- 0.05) x 10(-17), (1.90 +/- 0.17) x 10(-19), (5.04 +/- 0.40) x 10(-20), and (2.67 +/- 0.42) x 10(-20) cm(3) molecule(-1) for x = 1, 2, 3, 4, respectively. In one atmosphere of air at 298 K, reaction with O(2) accounts for 99%, 50%, 21%, and 12% of the loss of n-C(x)F(2)(x)(+1)C(O) radicals for x = 1, 2, 3, 4, respectively. Results are discussed with respect to the atmospheric chemistry of n-C(x)F(2)(x)(+1)C(O) radicals and their possible role in contributing to the formation of perfluorocarboxylic acids in the environment.

Reaction of Cl Atoms with C6F13CH2OH, C6F13CHO, and C3F7CHO

The Cl atom initiated oxidation of C(6)F(13)CH(2)OH, C(6)F(13)CHO, and C(3)F(7)CHO was investigated at 298 K and 1000 mbar pressure of air in a photoreactor using in situ Fourier transform infrared (FTIR) analysis. The rate coefficient for the reaction Cl + C(6)F(13)CH(2)OH (reaction 2) was measured using a relative method: k(2) = (6.5 +/- 0.8) x 10(-13) cm(3) molecule(-1) s(-1). C(6)F(13)CHO was detected as the major primary product, while CO and CF(2)O were found to be the major secondary products. A fitting procedure applied to the concentration-time profiles of C(6)F(13)CHO provided a production yield of (1.0 +/- 0.2) for this aldehyde in reaction 2, and the rate coefficient for the reaction Cl + C(6)F(13)CHO (reaction 4) was k(4) = (2.8 +/- 0.7) x 10(-12) cm(3) molecule(-1) s(-1). A high CO yield observed in the oxidation of C(6)F(13)CH(2)OH, (52 +/- 1)%, is attributed to the Cl atom initiated oxidation of C(6)F(13)CHO. High CO yields, (61 +/- 2)% and (85 +/- 5)%, were also measured in the Cl atom initiated oxidation of C(3)F(7)CHO in air and nitrogen, respectively. These high CO yields suggest the occurrence of a decomposition reaction of the perfluoroacyl, C(6)F(13)CO, and C(3)F(7)CO radicals to form CO which will compete with the combination reaction of these radicals with oxygen to form perfluoroacyl peroxy radicals in the presence of air. The latter radicals C(n)F(2)(n)(+1)CO(O)(2) (n = 6-12), through their reaction with HO(2) radicals, are currently considered as a possible source of persistent perfluorocarboxylic acids which have been detected in the environment. The consequences of the present results would be a reduction of the strength of this potential source of carboxylic acids in the atmosphere.