Experimental and Theoretical Studies of the Kinetics of the Reactions of OH Radicals with Acetic Acid, Acetic Acid-d3 and Acetic Acid-d4 at Low Pressure

Alkyl halide formation from degradation of carboxylic acids in the presence of Fe(III) and halides under light irradiation

Advanced oxidation processes (AOPs) have been widely used in water and wastewater treatment and have shown excellent performance in remediating contaminated water. However, their oxidation byproducts, including halogenated organics, have recently attracted increasing attention. Alkyl halides are among the most important environmental pollutants in nature. Here, we report a Fenton-like reaction in which alkyl halides can form during the photodegradation of aliphatic carboxylic acids in the presence of Fe(III) and halides. Chloromethane, chloroethane, and 1-chloropropane were produced from the degradation of acetic acid, propionic acid and n-butyric acid, respectively. CH₃Cl, CH₂Cl₂ and CHCl₃ were all identified as the products of acetic acid with the yields of approximately 5.1%, 0.2% and 0.005%, respectively. It was demonstrated that hydroxyl radicals, halogen radicals and alkyl radicals were involved in the formation of alkyl halides. A possible mechanism of chloromethane formation was proposed based on the results. In real samples of saline water, the addition of carboxylic acid and Fe(III) significantly promoted the generation of CH₃Cl under xenon lamp irradiation. The results indicated that the coexistence of Fe(III), halides and carboxylic acids enhanced the photochemical release of alkyl halides. The reactions described in this paper may contribute to knowledge on the mechanism of halogenated byproduct formation during AOPs.

Comparison of the reactivity of ibuprofen with sulfate and hydroxyl radicals: An experimental and theoretical study

Hydroxyl radical (^•OH) and sulfate radical anion (SO₄^•-) based advanced oxidation technologies (AOTs) are effective methods to treat trace organic contaminants (TrOCs) in engineered waters. Although both technologies result in the same overall removal of TrOCs, the mechanistic differences between these two radicals involved in the oxidation of TrOCs remain unclear. In this study, we experimentally examined the degradation kinetics of neutral ibuprofen (IBU), a representative TrOC, by ^•OH and SO₄^•- at pH3 in UV/H₂O₂ and UV/persulfate systems, respectively. The second-order rate constants (k) of IBU with ^•OH and SO₄^•- were determined to be 3.43±0.06×10⁹ and 1.66±0.12×10⁹M^-1s^-1, respectively. We also theoretically calculated the thermodynamic and kinetic behaviors for reactions of IBU with ^•OH and SO₄^•- using the density functional theory (DFT) M06-2X method with 6-311++G** basis set. The results revealed that H-atom abstraction is the most favorable pathway for both ^•OH and SO₄^•-, but due to the steric hindrance SO₄^•- exhibits significantly higher energy barriers than ^•OH. The theoretical calculations corroborate our experimental observation that SO₄^•- has a smaller k value than ^•OH in reacting with IBU. These comparative results are of fundamental and practical importance in understanding the electrophilic interactions between radicals and IBU molecules, and to help select preferred radical oxidation processes for optimal TrOCs removal in engineered waters.

Atmospheric oxidation of halogenated aromatics: comparative analysis of reaction mechanisms and reaction kinetics

Atmospheric transport is the major route for global distribution of semi-volatile compounds such as halogenated aromatics as well as their major exposure route for humans. Their major atmospheric removal process is oxidation by hydroxyl radicals. There is very little information on the reaction mechanism or reaction-path dynamics of atmospheric degradation of halogenated benzenes. Furthermore, the measured reaction rate constants are missing for the range of environmentally relevant temperatures, i.e. 230-330 K. A series of recent theoretical studies have provided those valuable missing information for fluorobenzene, chlorobenzene, hexafluorobenzene and hexachlorobenzene. Their comparative analysis has provided additional and more general insight into the mechanism of those important tropospheric degradation processes as well as into the mobility, transport and atmospheric fate of halogenated aromatic systems. It was demonstrated for the first time that the addition of hydroxyl radicals to monohalogenated as well as to perhalogenated benzenes proceeds indirectly, via a prereaction complex and its formation and dynamics have been characterized including the respective transition-state. However, in fluorobenzene and chlorobenzene reactions hydroxyl radical hydrogen is pointing approximately to the center of the aromatic ring while in the case of hexafluorobenzene and hexachlorobenzene, unexpectedly, the oxygen is directed towards the center of the aromatic ring. The reliable rate constants are now available for all environmentally relevant temperatures for the tropospheric oxidation of fluorobenzene, chlorobenzene, hexafluorobenzene and hexachlorobenzene while pentachlorophenol, a well-known organic micropollutant, seems to be a major stable product of tropospheric oxidation of hexachlorobenzene. Their calculated tropospheric lifetimes show that fluorobenzene and chlorobenzene are easily removed from the atmosphere and do not have long-range transport potential while hexafluorobenzene seems to be a potential POP chemical and hexachlorobenzene is clearly a typical persistent organic pollutant.

Experimental and Theoretical Study of the Kinetics of the OH + Propionaldehyde Reaction between 277 and 375 K at Low Pressure

Measurements of the rate constant for the reaction of OH radicals with propionaldehyde as a function of temperature were performed using low-pressure discharge-flow tube techniques coupled with laser-induced fluorescence detection of OH radicals. The measured room-temperature rate constant of (1.51 ± 0.22) × 10(-11) cm(3) molecules(-1) s(-1) at 4 Torr was generally lower but in reasonable agreement with previous absolute and relative rate studies at higher pressures. Measurements as a function of temperature resulted in an Arrhenius expression of (2.3 ± 0.4) × 10(-11) exp[(-110 ± 50)/T] cm(3) molecules(-1) s(-1) between 277 and 375 K at 4 Torr. The observed temperature dependence at low pressure is in contrast to previous measurements of a negative temperature dependence at higher pressures. Ab initio calculations of the potential energy surface for this reaction suggest that the primary reaction pathway involves the formation of a hydrogen-bonded prereactive complex, which could account for the difference in the observed temperature dependence at lower and higher pressures.

Mechanisms and reaction-path dynamics of hydroxyl radical reactions with aromatic hydrocarbons: the case of chlorobenzene

All geometries and energies significant for the first step of tropospheric degradation of chlorobenzene were characterized using the MP2/6-31+G(d,p) and G3 methods. A pre-reaction complex for the addition of OH radical to chlorobenzene was found and the associated transition state was determined for the first time. The reaction path for the association of OH radical and chlorobenzene into the pre-reaction complex was extrapolated from the selected low frequency normal mode of pre-reaction complex. The reaction rate constant for addition of OH radical to chlorobenzene was determined for the temperature range 230-330K, using RRKM theory and G3 energies. The calculated rate constants are in agreement with the experimental results. Regioselectivity was also determined for the title reaction from the ratio of respective reaction rates and our results are in very good agreement with the experimental results, which show the dominance of the ortho and para channels as well as a negligible contribution by the ipso channel.

Experimental evidence for the pressure dependence of the reaction rate constant between acetic acid and hydroxyl radicals

The reaction rate constant of acetic acid with the hydroxyl radical is measured at 93 Torr with our high-pressure flow system (HPFS) and found to display a negative temperature dependence that can be described by the Arrhenius expression, k(T) = (2.44 ± 0.22) × 10(-14) exp ((1027 ± 24)/T)) cm(3) molecule(-1) s(-1). Compared with our previously reported 7 Torr data, we find a noticeable pressure dependence. This dependence is observed to increase with decreasing temperature. This finding is consistent with a termolecular reaction mechanism. It is the first experimental evidence of the pressure dependence for this rate constant. A kinetics model is constructed, and the model results agree qualitatively with our experimental data. The extrapolated rate constant of the title reaction would be faster than previously believed at conditions of the upper troposphere/lower stratosphere, suggesting that the importance of acetic acid in its impact on HO(x) chemistry is currently underestimated.

Experimental study of the kinetics of the reaction of acetic acid with hydroxyl radicals from 255 to 355 K

The rate constant of the reaction of OH with acetic acid over the temperature range of 255-355 K was determined using our High-Pressure Flow System with laser-induced fluorescence detection of the OH radicals and FTIR spectrometry for acetic acid quantification. The rate constant displays a negative temperature dependence and can be described by the Arrhenius expression: k(1)(T) = (5.38 +/- 0.28) x 10(-14) exp(740 +/- 51/T) cm(3) molecule (-1) s(-1), with k(1) = (6.77 +/- 0.14) x 10(-13) cm(3) molecule (-1) s(-1) at 295 K. The negative temperature dependence suggests a pre-reactive complex formation between the OH radicals and the acetic acid monomer, and this result is consistent with previous reports. The use of FTIR spectrometry allows for separation of the acetic acid monomer and dimer in the spectrum and gives a measurement of the acetic acid monomer that is independent of the temperature measurement and free of reliance on an equilibrium constant expression that can introduce high uncertainty. The highly sensitive laser-induced fluorescence for OH detection coupled with the FTIR spectrometry result in a rate constant measurement with low uncertainty, and the data set presented here in the temperature range of 255-355K serves to bridge existing data sets that are obtained either above or below room temperature.

Experimental and ab initio dynamical investigations of the kinetics and intramolecular energy transfer mechanisms for the OH + 1,3-butadiene reaction between 263 and 423 K at low pressure

The rate constants for the reaction of the OH radical with 1,3-butadiene and its deuterated isotopomer has been measured at 1-6 Torr total pressure over the temperature range of 263-423 K using the discharge flow system coupled with resonance fluorescence/laser-induced fluorescence detection of OH. The measured rate constants for the OH + 1,3-butadiene and OH + 1,3-butadiene- d 6 reactions at room temperature were found to be (6.98 +/- 0.28) x 10 (-11) and (6.94 +/- 0.38) x 10 (-11) cm (3) molecule (-1) s (-1), respectively, in good agreement with previous measurements at higher pressures. An Arrhenius expression for this reaction was determined to be k 1 (II)( T) = (7.23 +/- 1.2) x10 (-11)exp[(664 +/- 49)/ T] cm (3) molecule (-1) s (-1) at 263-423 K. The reaction was found to be independent of pressure between 1 and 6 Torr and over the temperature range of 262- 423 K, in contrast to previous results for the OH + isoprene reaction under similar conditions. To help interpret these results, ab initio molecular dynamics results are presented where the intramolecular energy redistribution is analyzed for the product adducts formed in the OH + isoprene and OH + butadiene reactions.