Microplastics distribution in river side bars: The combined effects of water level and wind intensity.

Rivers are the main pathway for microplastics (MP) transport toward the ocean. However, the understanding of the processes involved in the deposition and mobilization of MP in rivers, specifically in sediment side bars (SB), remains very limited. The objectives of this study were: (i) to examine the effect of hydrometric fluctuations and wind intensity on the distribution of microplastics (MP < 5 mm) in the SB of large river (the Paraná River), (ii) to determine the characteristics of MP to infer their origin and fate, and (iii) to discuss potential similarities or differences between MP suspended in the water column and MP found in sediment. The SB and water column were sampled during the autumn, winter, and spring of 2018, and the summer of 2019 at different river discharges and wind intensities. >90 % of the MP items found were fiber of polyethylene terephthalate (PET; FT-IR analysis), the most common MP color was blue, and most were in the 0.5-2 mm size range. The concentration/composition of MP varied according to the river discharge and wind intensity. During the falling limb of the hydrograph when discharge is decreasing and sediments are exposed for short periods (13-30 days), MP particles transported by the flow were deposited on temporarily exposed SB, accumulating there in high densities (309-373 items/kg). However, during the drought, when sediments remained exposed for a long time (259 days), MP were mobilized and transported by the wind. During this period (no influence of the flow), MP densities significantly decreased on SB (39-47 items/kg). In conclusion, both hydrological fluctuations and wind intensity played a significant role in MP distribution in SB.

Thermal Decomposition of CxF2x+1C(O)OONO2 (x = 2, 3, 4).

The atmospheric degradation of molecules containing the CxF2x+1C(O) moiety, such as perfluoroaldehydes CxF2x+1C(O)H (x = 2-4) formed in the degradation of telomeric alcohols, could lead to the formation of perfluoroacyl peroxynitrates CxF2x+1C(O)OONO2. The thermal decomposition of the CxF2x+1C(O)OONO2 family (x = 2, 3, 4) was investigated by infrared spectroscopy and computational models. Each peroxynitrate synthesis was performed through the photolysis of gas mixtures of the corresponding perfluoroaldehyde, chlorine, nitrogen dioxide, and oxygen. Kinetic analysis for the thermal decomposition of peroxynitrates were performed in the range from 297.0 to 313.7 K at a total pressure of 1000 mbar and the activation energy was experimentally determined. Experimental data were complemented with theoretical data using the Gaussian09 Program Suite. The structures of peroxynitrates were optimized using DFT methods. The activation energies were calculated and investigated taking into account the stereoelectronic effects and using theoretical calculations as well as NBO analysis. The influence of anomeric interaction over the O-N bond was evaluated for all the molecules. Analysis of the results shows that CxF2x+1C(O)OONO2 stability is independent of CxF2x+1 chain length, in contrast to the behavior for perfluoroalkyl peroxynitrates (CxF2x+1OONO2).

Atmospheric Photo-oxidation of Diethyl Carbonate: Kinetics, Products, and Reaction Mechanism.

The rate coefficient for the gas phase of diethyl carbonate with chlorine atoms has been determined at 298 K using a relative method, employing ethyl formate and ethyl acetate as reference compounds. The experimental value, (1.0 ± 0.2) × 10-11 cm3 molecule-1 s-1, is in good correlation with the one estimated by the SAR (Structure-Activity Relationship) method. The photo-oxidation mechanism of diethyl carbonate initiated by chlorine atoms was also studied at 298 K and atmospheric pressure as a function of the oxygen partial pressure. The main products identified by infrared spectroscopy were CH3CH2OC(O)OCHO, CH3CH2OC(O)OCH2CHO, CH3CH2OC(O)OC(O)CH3, CO2, CO, HCOOH, and CH3COOH. The results reveal that the oxidation process occurs by the abstraction of a hydrogen atom from the methyl (43%) and methylene (57%) groups. The relative importance of each reaction path from the primary radicals formed in photo-oxidation and the identity of CH3CH2OC(O)OCHO, CH3CH2OC(O)OC(O)CH3, and CH3CH2OC(O)OCH2CHO were determined using computational methods. The activation energy of reaction paths for the main oxygenated radicals formed during photo-oxidation was determined using Gaussian09 Program.

Photo-oxidation of Dimethyl Malonate Initiated by Chlorine Atoms in Gas Phase: Kinetics and Mechanism.

The rate coefficient for the gas-phase reaction of chlorine atoms with dimethyl malonate (DMM, CH3OC(O)CH2C(O)OCH3) was determined at 298 K using relative methods giving a value of (3.8 ± 0.4) × 10-12, cm3 molecule-1 s-1). The photo-oxidation mechanism of DMM was also investigated. The main products were identified by infrared spectroscopy, and computational calculations were performed in order to support the experimental data. The results reveal that the photo-oxidation occurs mainly by the abstraction of an H atom from the methyl groups. The CH3OC(O)CH2C(O)OCH2O• radical formed subsequently reacts according to three competitive paths: reaction with molecular oxygen to yield CH3OC(O)CH2C(O)OC(O)H, isomerization-unimolecular decomposition to lead finally to CH3OC(O)C(O)H, CO2, and HC(O)OH, and α-ester rearrangement to form monomethyl malonate and carbon monoxide. The yield of products as a function of oxygen pressure was also determined.

Photooxidation of Di-tert-butyl Malonate in the Presence and Absence of Nitrogen Dioxide.

The rate constant for the reaction of di-tert-butyl malonate (DTBM) with chlorine atoms in the gas phase was measured using cyclohexane and pentane as references. The measurements lead to a value of (1.5 ± 0.1) × 10-10 cm3 molecule-1 s-1. The subsequent photo-oxidation mechanism of DTBM in the absence and presence of nitrogen dioxide was investigated. The main carbonated products identified in the first case were acetone, formic acid, carbon monoxide, and carbon dioxide. The addition of nitrogen dioxide lead besides to the formation of (CH3)3CC(O)OONO2 and (CH3)3CONO2. The proposed photo-oxidation mechanism was supported both experimentally and computationally. The results reveal that the (CH3)3COC(O)CH2C(O)OC(CH3)2O• radical formed reacts according to two competitive reactions: decomposition to yield acetone and (CH3)3COC(O)CH2C(O)O• radical 55 ± 2%, and migration of the H atom of the methylene group to the terminal oxygen atom 40 ± 3%.

Photolysis of n-Propyl Formate in the Presence of O2 and NO2: Peroxy Formyl Propyl Nitrate CH3CH2CH2OC(O)OONO2 Synthesis and Characterization.

The photo-oxidation of n-propyl formate (initiated by chlorine atoms) was studied in the presence of NO2, and the products were identified. The Cl atom attack to the molecule occurs in four sites, leading to the formation of formic acid, carbon dioxide, dicarbonylic products, nitrates, peroxy propionyl nitrate (CH3CH2C(O)OONO2, PPN), and a new peroxynitrate, peroxy formyl propyl nitrate (CH3CH2CH2OC(O)OONO2, PFPN). To characterize bulk quantities of the PFPN, its synthesis was carried out by the photolysis of mixtures of CH3CH2CH2OC(O)H, NO2, Cl2, and O2. After purification, its infrared spectrum and thermal stability were determined. The main infrared absorption bands and their corresponding cross sections are 796, 1219, 1302, 1741, and 1831 cm(-1) (1.16, 3.11, 0.88, 2.42, and 1.34 × 10(-18) cm(2) molec(-1), respectively). Thermal decomposition was studied as a function of pressure from 6.0 to 1000 mbar at 298 K, and the activation energy was determined between 293 and 304 K at total pressures of 9.0 and 1000 mbar (Ea = 98 ± 3 and 110 ± 2 kJ/mol, respectively). The atmospheric thermal lifetimes were obtained from kinetic parameters.

Thermal stability of peroxy acyl nitrates formed in the oxidation of C(x)F(2x+1)CH2C(O)H (x = 1,6) in the presence of NO2.

The formation of C(x)F(2x+1)CH2C(O)OONO2 (x = 1,6) from the photooxidation of C(x)F(2x+1)CH2C(O)H (x = 1,6) in the presence of NO2 was investigated. The infrared spectrum of C6F13CH2C(O)OONO2 is reported for the first time, and thermal stability for both peroxynitrates at 295 K and 9.0 mbar is informed. Kinetic parameters (activation energy and pre-exponential factor) for CF3CH2C(O)OONO2 at 9.0 and 1000 mbar are: 108 ± 2 kJ/mol, 1.5 × 10(15) and 114 ± 2 kJ/mol, 2.4 × 10(16), respectively. A comparison is made between fluoro and hydrogenated peroxy acyl nitrates.

Mechanism of photo-oxidation of heptafluorobutyric anhydride in the presence of NO2. Synthesis and characterization of heptafluoropropyl peroxynitrate, CF3CF2CF2OONO2.

The photolysis of heptafluorobutyric anhydride at 254 nm in the presence of NO(2) and O(2) has been studied. It leads to the formation of CF(3)CF(2)CF(2)OONO(2), CF(3)CF(2)OONO(2), and CF(2)O as the only fluorine-containing carbonaceous products. The formation of the new heptafluoropropyl peroxynitrate (HFPN, CF(3)CF(2)CF(2)OONO(2)), as one of the main products, is a consequence of the formation of CF(3)CF(2)CF(2)OO(•) radicals followed by the reaction with NO(2). To characterize HFPN, the UV absorption cross sections and their temperature dependence between 245 and 300 K have been measured over the wavelength range 200-300 nm as well as the infrared absorption cross sections. Kinetic parameters for its thermal decomposition are also presented in the temperature range between 281 and 300 K. The Rice-Ramsperger-Kassel-Marcus calculation reveals that the rate coefficient for the thermal decomposition at 285 K is almost independent of total pressure. The mechanism for the decomposition of CF(3)CF(2)CF(2)OONO(2) in the presence of NO was adjusted by a kinetic model, which enabled the calculation of important rate coefficients.

Atmospheric chemistry of perfluoroaldehydes (CxF2x+1CHO) and fluorotelomer aldehydes (CxF2x+1CH2CHO): quantification of the important role of photolysis.

The UV absorption spectra of CF(3)CHO, C(2)F(5)CHO, C(3)F(7)CHO, C(4)F(9)CHO, CF(3)CH(2)CHO, and C(6)F(13)CH(2)CHO were recorded over the range 225-400 nm at 249-297 K. C(x)F(2)(x)(+1)CHO and C(x)F(2)(x)(+1)CH(2)CHO have broad absorption features centered at 300-310 and 290-300 nm, respectively. The strength of the absorption increases with the size of the C(x)F(2)(x)(+1) group. There was no discernible (<5%) effect of temperature on the UV spectra. Quantum yields for photolysis at 254 and 308 nm were measured. Quantum yields at 254 nm were 0.79 +/- 0.09 (CF(3)CHO), 0.81 +/- 0.09 (C(2)F(5)CHO), 0.63 +/- 0.09 (C(3)F(7)CHO), 0.60 +/- 0.09 (C(4)F(9)CHO), 0.74 +/- 0.08 (CF(3)CH(2)CHO), and 0.55 +/- 0.09 (C(6)F(13)CH(2)CHO). Quantum yields at 308 nm were 0.17 +/- 0.03 (CF(3)CHO), 0.08 +/- 0.02 (C(4)F(9)CHO), and 0.04 +/- 0.01 (CF(3)CH(2)CHO). The quantum yields decrease with increasing size of the C(x)F(2)(x)(+1) group and with increasing wavelength of the photolysis light. The photolysis quantum yield at 308 nm for CF(3)CHO measured here is a factor of at least 8 greater than that reported previously. Photolysis is probably the dominant atmospheric fate of C(x)F(2)(x)(+1)CHO (x = 1-4) and is an important fate of C(x)F(2)(x)(+1)CH(2)CHO (x = 1 and 6). These results have important ramifications concerning the yield of perfluorocarboxylic acids in the atmospheric oxidation of fluorotelomer alcohols.

Synthesis and characterization of trifluoromethyl fluoroformyl peroxycarbonate, CF3OC(O)OOC(O)F.

The synthesis of CF(3)OC(O)OOC(O)F is accomplished by the photolysis of a mixture of (CF(3)CO)(2)O, FC(O)C(O)F, CO, and O(2) at -15 degrees C using a low-pressure mercury lamp. The new peroxide is obtained in pure form in low yield after repeated trap-to-trap condensation and is characterized by NMR, IR, Raman, and UV spectroscopy. Geometrical parameters were studied by ab initio methods [B3LYP/6-311+G(d)]. At room temperature, CF(3)OC(O)OOC(O)F is stable for many days in the liquid or gaseous state. The melting point is -87 degrees C, and the boiling point is extrapolated to 45 degrees C from the vapor pressure curve log p = 8.384 - 1715/T (p/mbar, T/K). A possible mechanism for the formation of CF(3)OC(O)OOC(O)F is discussed, and its properties are compared with those of related compounds.