Radiation chemistry of methyl tert-butyl ether in aqueous solution

The chemical kinetics of the free-radical-induced degradation of the gasoline oxygenate methyl tert-butyl ether (MTBE) in water have been investigated. Rate constants for the reaction of MTBE with the hydroxyl radical, hydrated electron, and hydrogen atom were determined in aqueous solution at room temperature, using electron pulse radiolysis and absorption spectroscopy (*OH and e- aq) and EPR free induction decay attenuation (*H) measurements. The rate constant for hydroxyl radical reaction of (1.71 +/- 0.02) x 10(9) M(-1) s(-1) showed that the oxidative process was the dominant pathway, relative to MTBE reaction with hydrogen atoms, (3.49 +/- 0.06) x 10(6) M(-1) s(-1), or hydrated electrons, <8.0 x 10(6) M(-1) s(-1). The hydroxyl radical reaction gives a transient carbon-centered radical which subsequently reacts with dissolved oxygen to form peroxyl radicals, the rate constant for this reaction was (2.17 +/- 0.06) x 10(9) M(-1) s(-1). The second-order decay of the MTBE peroxyl radical was 2k = (6.0 +/- 0.3) x 10(8) M(-1) s(-1). These rate constants, along with preliminary MTBE degradation product distribution measurements, were incorporated into a kinetic model that compared the predicted MTBE removal from water against experimental measurements performed under large-scale electron beam treatment conditions.     

Free radical destruction of beta-blockers in aqueous solution

Many pharmaceutical compounds and metabolites are currently found in surface and ground waters which indicates their ineffective removal by conventional water treatment technologies. Advanced oxidation/reduction processes (AO/ RPs) are alternatives to traditional water treatment, which utilize free radical reactions to directly degrade chemical contaminants. This study reports the absolute rate constants for reaction of three beta-blockers (atenolol, metoprolol, and propranolol) with the two major AO/RP radicals; the hydroxyl radical (*OH) and hydrated electron ((e-)aq). The bimolecular reaction rate constants for *OH are (7.05 +/- 0.27) x 10(9), (8.39 +/- 0.06) x 10(9), and (1.07 +/- 0.02) x 10(10), and for (e-)aq they are (5.91 +/- 0.21) x 10(8), (1.73 +/- 0.03) x 10(8), and (1.26 +/- 0.02) x 10(10), respectively. Transient spectra were observed for the intermediate radicals produced by hydroxyl radical reactions. In addition, preliminary degradation mechanisms and major products were elucidated using 60Co gamma-irradiation and LC-MS. These data are required for both evaluating the potential use of AO/RPs for the destruction of these compounds and for studies of their fate and transport in surface waters where radical chemistry may be important in assessing their lifetime.     

Free radical chemistry of advanced oxidation process removal of nitrosamines in water

Absolute rate constants and degradation efficiencies for hydroxyl radical reactions with seven low-molecular-weight nitrosamines in water have been evaluated using a combination of electron-pulse radiolysis/absorption spectroscopy and steady-state radiolysis/GCMS measurements. The hydroxyl radical oxidation rate constants were found to depend upon nitrosamine size and to have a very good linear correlation with the number of methylene groups in these compounds. This correlation, given by In(k x OH) = (19.72 +/- 0.14) + (0.424 +/- 0.033) (#CH2), suggests that hydroxyl radical oxidation predominantly occurs by hydrogen atom abstraction from constituent methylene groups in each of these nitrosamines. In contrast, the hydrated electron reduction rate constants measured for these compounds were remarkably consistent, with an average value of (1.67 +/- 0.22) x 10(10) M(-1) s(-1). These reduction kinetic data are consistent with this predominantly diffusion-controlled reaction occurring at the N-NO moiety in these carcinogens. From steady-state radiolysis measurements under aerated conditions, specific hydroxyl radical degradation efficiencies for each nitrosamine were evaluated. For larger nitrosamines, the efficiency was constant at 100%; however, for the smaller alkyl substituted species, the efficiency was significantly lower, with a minimum value of only 80% determined for N-nitrosodimethylamine. The reduced efficiency is attributed to radical repair reactions competing with the slow peroxyl radical formation.     

Oxidation of N-nitrosodimethylamine (NDMA) precursors with ozone and chlorine dioxide: kinetics and effect on NDMA formation potential

The oxidation of N-nitrosodimethylamine (NDMA) precursors chlorine dioxide (ClO2). Second-order rate constants for the reactions of model NDMA precursors (dimethylamine (DMA) and 7 tertiary amines) with ozone (kapp at pH 7 = 2.4 x 10(-1) to 2.3 x 10(9) M(-1) s(-1)), ClO2 (kapp at pH 7 = 6.7 x 10(-3) to 3.0 x 10(7) M(-1) s(-1)), and hydroxyl radical (*OH) (kapp at pH 7 = 6.2 x 10(7) to 1.4 x 10(10) M(-1) s(-1)) were determined, which showed that the selected NDMA precursors, with the exception of dimethylformamide (DMFA) can be completely transformed via their direct reaction with ozone. During ozonation, DMFA may be partially transformed through oxidation by the secondary oxidant *OH. ClO2 was also shown to effectively transform most of the precursors, with the exceptions of DMA and DMFA. In the second part of the study, the NDMA formation potentials (NDMA-FP) in synthetic and natural waters were measured with and without pre-oxidation with ozone and ClO2. A significant reduction in the NDMA-FPs was observed after complete transformation of the model NDMA precursors. Ozonation generally led to more effective reduction of the NDMA-FP than ClO2. For most of the precursors, the formation of DMA could account for the NDMA-FPs remaining after complete transformation of the model NDMA precursors. In contrast, dimethylethanolamine and dimethyldithiocarbamate yielded other NDMA precursors (not DMA) as their oxidation products. Pre-oxidation by ozone and ClO2 of several natural waters showed behavior similar to that of the oxidation of model NDMA precursors with a reduction of the NDMA-FP by 32-94% for various natural water sources.     

Free radical chemistry of disinfection byproducts. 2. Rate constants and degradation mechanisms of trichloronitromethane (chloropicrin)

Absolute rate constants for the free-radical-induced degradation of trichloronitromethane (TCNM, chloropicrin) were determined using electron pulse radiolysis and transient absorption spectroscopy. Rate constants for hydroxyl radical, *OH, and hydrated electron, e(aq)-, reactions were (4.97 +/- 0.28) x 10(7) M(-1) s(-1) and (2.13 +/- 0.03) x 10(10) M(-1) s(-1), respectively. It appears that the *OH adds to the nitro-group, while the e(aq)- reacts via dissociative electron attachment to give two carbon centered radicals. The mechanisms of these free radical reactions with TCNM were investigated, using 60Co gamma irradiation at various absorbed doses, measuring the disappearance of TCNM and the appearance of the product nitrate and chloride ions. The rate constants and mechanistic data were combined in a kinetic computer model that was used to describe the major free radical pathways for the destruction of TCNM in solution. These data are applicable to other advanced oxidation/reduction processes.     

Photochemical fate of pharmaceuticals in the environment: cimetidine and ranitidine

The photochemical fates of the histamine H2-receptor antagonists cimetidine and ranitidine were studied. Each of the two environmentally relevant pharmaceuticals displayed high rates of reaction with both singlet oxygen (1O2, O2(1delta(g))) and hydroxyl radical (*OH), two transient oxidants formed in sunlit natural waters. For cimetidine, the bimolecular rate constant for reaction with *OH in water is 6.5 +/- 0.5 x 10(9) M(-1) s(-1). Over the pH range 4-10, cimetidine reacts with 1O2 with bimolecular rate constants ranging from 3.3 +/- 0.3 x 10(6) M(-1) s(-1) at low pH to 2.5 +/- 0.2 x 10(8) M(-1) s(-1) in alkaline solutions. The bimolecular rate constants for ranitidine reacting with 1O2 in water ranges from 1.6 +/- 0.2 x 10(7) M(-1) s(-1) at pH 6-6.4 +/- 0.2 x 10(7) M(-1) s(-1) at pH 10. Reaction of ranitidine hydrochloride with *OH proceeds with a rate constant of 1.5 +/- 0.2 x 10(10) M(-1) s(-1). Ranitidine was also degraded in direct photolysis experiments with a half-life of 35 min under noon summertime sunlight at 45 degrees latitude, while cimetidine was shown to be resistant to direct photolysis. The results of these experiments, combined with the expected steady-state near surface concentrations of 1O2 and *OH, indicate that photooxidation mediated by 1O2 is the likely degradation pathway for cimetidine in most natural waters, and photodegradation by direct photolysis is expected to be the major pathway for ranitidine, with some degradation caused by 1O2. These predictions were verified in studies using Mississippi River water. Model compounds were analyzed by laser flash photolysis experiments to assess which functionalities within ranitidine and cimetidine are most susceptible to singlet-oxygenation and direct photolysis. The heterocyclic moieties of the pharmaceuticals were clearly implicated as the sites of reaction with 1O2, as evidenced by the high relative rate constants of the furan and imidazole models. The nitroacetamidine portion of ranitidine has been shown to be the moiety active in direct photolysis.     

Selective oxidation of key functional groups in cyanotoxins during drinking water ozonation

Chemical kinetics were determined for the reactions of ozone and hydroxyl radicals with the three cyanotoxins microcystin-LR (MC-LR), cylindrospermopsin (CYN) and anatoxin-a (ANTX). The second-order rate constants (k(O3)) at pH 8 were 4.1 +/- 0.1 x 10(5) M(-1) s(-1) for MC-LR, approximately 3.4 x 10(5) M(-1) s(-1) for CYN, and approximately 6.4 x 10(4) M(-1) s(-1) for ANTX. The reaction of ozone with MC-LR exhibits a k(O3) similar to that of the conjugated diene in sorbic acid (9.6 +/- 0.3 x 10(5) M(-1) s(-1)) at pH 8. The pH dependence and value of k(O3) for CYN at pH > 8 (approximately 2.5 +/- 0.1 x 10(6) M(-1) s(-1)) are similar to deprotonated amines of 6-methyluracil. The k(O3) of ANTX at pH > 9 (approximately 8.7 +/- 2.2 x 10(5) M(-1) s(-1)) agrees with that of neutral diethylamine, and the value at pH < 8 (2.8 +/- 0.2 x 10(4) M(-1) s(-1)) corresponds to an olefin. Second-order rate constants for reaction with OH radicals (*OH), k(OH) for cyanotoxins were measured at pH 7 to be 1.1 +/- 0.01 x 10(10) M(-1) s(-1) for MC-LR, 5.5 +/- 0.01 x 10(9) M(-1) s(-1) for CYN, and 3.0 +/- 0.02 x 10(9) M(-1) s(-1) for ANTX. Natural waters from Switzerland and Finland were examined for the influence of variations of dissolved organic matter, SUVA254, and alkalinity on cyanotoxin oxidation. For a Swiss water (1.6 mg/L DOC), 0.2, 0.4, and 0.8 mg/L ozone doses were required for 95% oxidation of MC-LR, CYN, and ANTX, respectively. For the Finnish water (13.1 mg/L DOC), >2 mg/L ozone dose was required for each toxin. The contribution of hydroxyl radicals to toxin oxidation during ozonation of natural water was greatest for ANTX > CYN > MC-LR. Overall, the order of reactivity of cyanotoxins during ozonation of natural waters corresponds to the relative magnitudes of the second-order rate constants for their reaction with ozone and *OH. Ozone primarily attacks the structural moieties responsible for the toxic effects of MC-LR, CYN, and ANTX, suggesting that ozone selectively detoxifies these cyanotoxins.     

Oxidation of antibacterial molecules by aqueous ozone: moiety-specific reaction kinetics and application to ozone-based wastewater treatment

Ozone and hydroxyl radical (*OH) reaction kinetics were measured for 14 antibacterial compounds from nine structural families, to determine whether municipal wastewater ozonation is likely to result in selective oxidation of these compounds' biochemically essential moieties. Each substrate is oxidized by ozone with an apparent second-order rate constant, k'(O3,app) > 1 x 10(3) M(-1) s(-1), at pH 7, with the exception of N(4)-acetylsulfamethoxazole (K'(O3,app) is 2.5 x 102 M(-1) s(-1)). k'(O3,app) values (pH 7) for macrolides, sulfamethoxazole, trimethoprim, tetracycline, vancomycin, and amikacin appear to correspond directly to oxidation of biochemically essential moieties. Initial reactions of ozone with N(4)-acetylsulfamethoxazole, fluoroquinolones, lincomycin, and beta-lactams do not lead to appreciable oxidation of biochemically essential moieties. However, ozone oxidizes these moieties within fluoroquinolones and lincomycin via slower reactions. Measured k'(O3,app) values and second-order *OH rate constants, k'(*OH,app) were utilized to characterize pollutant losses during ozonation of secondary municipal wastewater effluent. These losses were dependent on k'(O3,app), but independent of k'(*OH,app). Ozone doses > or =3 mg/L yielded > or =99% depletion of fast-reacting substrates (K'(O3,app) > 5 x 10(4) M(-1) s(-1)) at pH 7.7. Ten substrates reacted predominantly with ozone; only four were oxidized predominantly by .OH. These results indicate that many antibacterial compounds will be oxidized in wastewater via moiety-specific reactions with ozone.     

Quantitative correlation of absolute hydroxyl radical rate constants with non-isolated effluent organic matter bulk properties in water

Absolute second-order rate constants for the reaction between the hydroxyl radical (*OH) and eight water samples containing non-isolated effluent organic matter (EfOM) collected at different wastewater and reclamation sites were measured by electron pulse radiolysis. The measured rate constants ranged from 0.27 to 1.21 x 10(9) Mc(-1) s(-1), with an average value of 0.86 (+/-0.35) x 10(9) Mc(-1) s(-1). These absolute values were 3-5 times faster than previously reported values using natural organic matter and wastewater isolates. The obtained rate constants were correlated (R2 > 0.99) to bulk EfOM properties through an empirical equation that included terms relating to the polarity, apparent molecular weight, and fluorescence index of the effluent organic matter. The obtained data were used to model steady state *OH concentrations during UV advanced oxidation. The steady-state *OH concentration was lower than that obtained using previously reported values for the reaction with dissolved organic matter, indicating that accurate measurement of reaction rate constants at specific sites would greatly improve the design and prediction of the removal of organic contaminants. These results will improve the ability of researchers to accurately model scavenging capacities during the advanced oxidation processtreatment of wastewaters.     

Reactions of hydroxyl radical with humic substances: bleaching, mineralization, and production of bioavailable carbon substrates

In this study, we examine the role of the hydroxyl (OH*) radical as a mechanism for the photodecomposition of chromophoric dissolved organic matter (CDOM) in sunlit surface waters. Using gamma-radiolysis of water, OH* was generated in solutions of standard humic substances in quantities comparable to those produced on time scales of days in sunlit surface waters. The second-order rate coefficients of OH* reaction with Suwannee River fulvic (SRFA; 2.7 x 10(4) s(-1) (mg of C/L)(-1)) and humic acids (SRHA; 1.9 x 10(4) s(-1) (mg of C/L)(-1)) are comparable to those observed for DOM in natural water samples and DOM isolates from other sources but decrease slightly with increasing OH* doses. OH* reactions with humic substances produced dissolved inorganic carbon (DIC) with a high efficiency of approximately 0.3 mol of CO2/mol of OH*. This efficiency stayed approximately constant from early phases of oxidation until complete mineralization of the DOM. Production rates of low molecular weight (LMW) acids including acetic, formic, malonic, and oxalic acids by reaction of SRFA and SRHA with OH* were measured using HPLC. Ratios of production rates of these acids to rates of DIC production for SRHA and for SRFA were similar to those observed upon photolysis of natural water samples. Bioassays indicated that OH* reactions with humic substances do not result in measurable formation of bioavailable carbon substrates other than the LMW acids. Bleaching of humic chromophores by OH* was relatively slow. Our results indicate that OH* reactions with humic substances are not likely to contribute significantly to observed rates of DOM photomineralization and LMW acid production in sunlit waters. They are also not likely to be a significant mechanism of photobleaching except in waters with very high OH* photoformation rates.     

Reactions of hydroxyl radicals and ozone with acenaphthene and acenaphthylene

Acenaphthene and acenaphthylene are polycyclic aromatic hydrocarbons (PAHs) emitted into the atmosphere from a variety of incomplete combustion sources such as diesel exhaust. Both PAHs are present in the gas phase under typical atmospheric conditions and therefore can undergo atmospheric gas-phase reactions with the hydroxyl (OH) radical and for acenaphthylene with ozone. Using a relative rate method, rate constants have been measured at 296 +/- 2 K for the OH radical reactions with acenaphthene and acenaphthylene of (in units of 10(-11) cm3 molecule(-1) s(-1)) 8.0 +/- 0.4 and 12.4 +/- 0.7, respectively, and for the O3 reaction with acenaphthylene of (1.6 +/- 0.1) x 10(-16) cm3 molecule(-1) s(-1). The products of the gas-phase reactions of acenaphthene and acenaphthylene and their fully deuterated analogues have been investigated using in situ atmospheric pressure ionization tandem mass spectrometry (API-MS) and gas chromatography-mass spectrometry (GC-MS). The major products identified from the OH radical-initiated reaction of acenaphthene and acenaphthylene were a 10 carbon ring-opened product and a dialdehyde, respectively. The major product observed from the API-MS analysis of the O3 reaction with acenaphthylene was a secondary ozonide, which was not observed by GC-MS.     

Temperature and wavelength dependence of nitrite photolysis in frozen and aqueous solutions

While the photolysis of nitrite is an important source of hydroxyl radical (*OH) in some natural waters, its wavelength and temperature dependence have not been fully described in solution. In addition, there are no studies of this reaction on ice, although there is evidence of nitrite production in snow. To address these gaps, we have measured the wavelength and temperature dependence of the quantum yields of *OH from the photolysis of frozen and aqueous NO2-. From our solution and ice results, we derive a master equation that describes the *OH quantum yield from NO2 photolysis as a function of both temperature (240-295 K) and illumination wavelength (302-390 nm): phi(NO1- -->OH*)(T,lamda) = (Y0 + a/(1 + exp((lamda-c)/b)))exp-(((e lamda) + f)/R) x (1/295 - 1/T)) where Y0 = 0.0204 +/- 0.0010, a = 0.0506 +/- 0.0022, b = 11.2 +/- 1.2, c = 332 +/- 1, e = 20.5 +/- 3.2, f = 7553 +/- 1204, uncertainties represent 1 standard error, Tis the temperature (K), Ris the gas constant (8.314 J mol(-1) K(-1)), and lamda is the wavelength (nm). Using these results we predict the pseudo-steady-state concentrations of nitrite on sunlit polar snow grains and compare the relative importance of the photolysis of nitrite, nitrate, and hydrogen peroxide as sources of snow-grain *0H.     

Electron pulse radiolysis determination of hydroxyl radical rate constants with Suwannee River fulvic acid and other dissolved organic matter isolates

Pulse radiolysis experiments were conducted on dissolved organic matter (DOM) samples isolated as hydrophobic and hydrophilic acids and neutrals from different sources (i.e., stream, lake, wastewater treatment plant). Absolute bimolecular reaction rate constants for the reaction of hydroxyl radicals (*OH) with DOM (k*(OH), DOM) were determined. k*(OH, DOM) values are expressed as moles of carbon. Based on direct measurement of transient DOM radicals (DOM*) and competition kinetic techniques, both using pulse radiolysis, the k*(OH, DOM) value for a standard fulvic acid from the Suwannee River purchased from the International Humic Substances Society was (1.60 +/- 0.24) x 10(8) M(-1) s(-1). Both pulse radiolysis methods yielded comparable k*(OH, DOM) values. The k*(OH, DOM) values for the seven DOM isolates from different sources ranged from 1.39 x 10(8) M(-1) s(-1) to 4.53 x 10(8) M(-1) s(-1), and averaged 2.23 x 108 M(-1) s(-1) (equivalent to 1.9 x 10(4) (mgC/L)(-1) s(-1)). These values represent the first direct measurements of k*(OH, DOM,) and they compare well with literature values obtained via competition kinetic techniques during ozone or ultraviolet irradiation experiments. More polar, lower-molecular-weight DOM isolates from wastewater have higher k*(OH, DOM) values. In addition, the formation (microsecond time scale) and decay (millisecond time scale) of DOM* transients were observed for the first time. DOM* from hydrophobic acids exhibited broader absorbance spectra than transphilic acids, while wastewater DOM isolates had narrower DOM* spectra more skewed toward shorter wavelengths than did DOM* spectra for hydrophobic acids.     

Atmospheric HFEs degradation in the gas phase: reactions of HFE-7100 and HFE-7200 with Cl atoms at low temperatures

Kinetic rate coefficients for the reactions of HFE-7100 (1) (C4F9OCH3) and HFE-7200 (2) (C4F9OC2H5) with Cl atoms have been measured using a discharge flow mass spectrometric technique (DFMS) at 1 Torr total pressure. The reactions have been studied under pseudo-first-order kinetic conditions in excess of HFEs over Cl atoms and the study has been extended from 333 down to 234 K to approach the tropospheric temperature profile. At room temperature the measured rate constants are k (1) = (1.43 +/- 0.28) x 10(-13) cm3molecule(-1)s(-1) and k (2) = (2.1 +/- 0.1) x 10(-12) cm3molecule(-1)s(-1). The Arrhenius expressions from our results are (units in cm3molecule(-1)s(-1)): k (1) = (2.3 +/- 1.4) x 10(-10) exp - (2254 +/- 177)/T(234-315 K) and k (2) = (3.7 +/- 0.5) x 10(-11) exp - (852 +/- 38)/T(234-333 K) (errors are sigma). The reactions proceed through the abstraction of an H atom to form HCl and the corresponding halo-alkyl radical. At 298 K and 1 Torr, yields on HCl of 0.88 +/- 0.09 and 0.95 +/- 0.10 (errors are 2sigma) were obtained for HFE-7100 and HFE-7200 reactions, respectively.     

Oxidation kinetics of selected taste and odor compounds during ozonation of drinking water

The applicability of ozonation to mitigate taste and odor problems in drinking water was investigated. Second-order rate constants of eleven taste and odor compounds with ozone and hydroxyl radicals were determined under laboratory conditions. Measured rate constants for the reaction with hydroxyl radicals are between 3 x 10(9) and 10(10) M(-1)s(-1) and for ozone: kbeta-cyclocitral = 3890 +/- 140 M(-1)s(-1); kgeosmin = 0.10 +/- 0.03 M(-1)s(-1); k3-hexen-1-ol = 5.4 +/- 0.5 x 10(5) M(-1)s(-1); kbeta-ionone = 1.6 +/- 0.13 x 10(5) M(-1)s(-1); k2-isopropyl-3-methoxypyrazine = 50 +/- 3 M(-1)s(-1); k2-methylisoborneol = 0.35 +/- 0.06 M(-1)s(-1); k2,6-nonadienal = 8.7 +/- 0.4 x 10(5) M(-1)s(-1); k1-penten-3-one = 5.9 +/- 0.1 x 10(4) M(-1)s(-1); k2,6-di-tert-butyl-4-methylphenol (BHT) = 7.4 +/- 0.2 x 10(4) M(-1)s(-1); k2,4,6-tribromoanisole = 0.02 +/- 0.01 M(-1)s(-1); k2,4,6-trichloroanisole = 0.06 +/- 0.01 M(-1)s(-1). Experiments conducted in natural waters showed that the removal efficiency during ozonation can be reliably predicted with the determined second-order rate constants. Ozonation is a powerful tool capable of oxidizing most of the taste and odor compounds to more than 50% under typical drinking water treatment conditions. For ozone-resistant taste and odor compounds, the application of advanced oxidation processes may be appropriate.     

Aqueous photochemical reaction kinetics and transformations of fluoxetine

Fluoxetine (FLX) was shown to be photoreactive in sunlit surface waters. FLX degraded in deionized water when exposed to simulated sunlight with a half-life of 55.2+/-3.6 h(-1). Photodegradation products were identified using high performance liquid chromatography-UV (HPLC-UV) and liquid chromatography tandem mass spectrometry (LC-MS-MS) using electrospray (ES) ionization. Defluorination of the trifluoromethyl group in FLX and in fluometuron and flutalanil,two other compounds containing this functional group, is suggested to be a common direct photolysis pathway for trifluoromethylated compounds. Products resulting from O-dealkylation of FLX were also observed. The rate of degradation was faster in synthetic field water where .OH was the likely dominant oxidant in the system. The bimolecular rate constant for the reaction between FLX and .OH was measured as (8.4+/-0.5) x 10(9) and (9.6 +/-0.8) x 10(9) M(-1) s(-1) using two different methods of competition kinetics. Indirect photodegradation reactions could lead to the production of hydroxylated and O-dealkylated compounds. Although direct photolysis could potentially limitthe persistence of FLX in surface waters, its degradation by indirect photolysis would proceed faster. Thus, this latter process could be important in the elimination of FLX in surface waters.     

Photosensitizer method to determine rate constants for the reaction of carbonate radical with organic compounds

Carbonate radical (CO3*-) is a powerful oxidant that is present in sunlit surface waters and in waters treated by advanced oxidation processes. The production of CO3*- in aqueous solution through oxidation of carbonate anion by excited triplet states of aromatic ketones was investigated in this study to provide new methods for the determination of rate constants and to explore a possible photoinduced pathway of CO3*- formation in the aquatic environment. Rate constants for triplet quenching by carbonate anion of up to 3.0 x 10(7) M(-1) s(-1) and CO3*- yields approaching unity, determined using laser flash photolysis, allowed us to conclude that such a formation mechanism might be significant in sulit natural waters. Kinetic methods based on either flash photolysis or steady-state irradiation and on the use of aromatic ketones as photosensitizers gave bimolecular rate constants in the range of 4 x 10(6) to 1 x 10(8) M(-1) s(-1) for the reaction of CO3*- with several s-triazine and phenylurea herbicides. For various anilines and phenoxide anions, rate constants determined by these methods agreed well with published values. Moreover, it could be shown for the first time by a direct method that dissolved natural organic matter (DOM) reduces the lifetime of CO3*- and a second-order rate constant of (280 +/- 90) (mg of C/L)(-1) s(-1) was obtained for Suwannee River fulvic acid.     

Atmospheric fate of dichlorvos: photolysis and OH-initiated oxidation studies

The OH-initiated oxidation of dichlorvos (a widely used insecticide) has been investigated under atmospheric conditions at the large outdoor European photoreactor (EUPHORE) in Valencia, Spain. The rate constant of OH reaction with dichlorvos, k, was measured by using a conventional relative rate technique where 1,3,5-trimethylbenzene (TMB) and cyclohexane were taken as references. With the use of the rate constants of 5.67 x 10(-11) and of 6.97 x 10(-12) cm3 molecule(-1) s(-1) for the reactions OH + TMB and OH + cyclohexane, respectively, the resulting value of the OH reaction rate constant with dichlorvos was derived to be k = (2.6 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1). The tropospheric lifetime of dichlorvos with respect to reaction with OH radical has been estimated to be around 11 h. The major carbon-containing products observed for the OH reaction with dichlorvos in air under sunlight condition were phosgene and carbon monoxide. The formation of a very stable toxic primary product such as phosgene associated with the relatively short lifetime of dichlorvos may make the use of this pesticide even more toxic for humans when released into the atmosphere.     

Night-time atmospheric fate of acrolein and crotonaldehyde

The absolute rate coefficients for the gas-phase reaction of the NO3 radical with acrolein and crotonaldehyde have been measured overthe temperature range 249-330 K, using a discharge flow system and monitoring the NO3 radical by laser induced fluorescence (LIF). The obtained rate coefficients at 298 K for NO3 reactions with acrolein and crotonaldehyde were (3.30 +/- 0.39) x 10(-15) cm3 molecule(-1) s(-1) for acrolein and (1.35 +/- 0.04) x 10(-14) cm3 molecule(-1) s(-1) for crotonaldehyde, and the proposed Arrhenius expressions are k(T) = (1.72 +/- 0.5) x 10(-13) exp[(-1190 +/- 43)/T] and k(T) = (5.02 +/- 0.7) x 10(-13) exp[(-1076 +/- 47)/T], respectively, in units of cm3 molecule(-1) s(-1). In addition, the products and mechanisms were investigated using an environmental chamber/FTIR absorption system. Formaldehyde, CO, and acryloylperoxy nitrate were identified as the main products for the acrolein reaction with molar yields of 31.6 +/- 2.0, 20.9 +/- 1.9, and 47 +/- 3, respectively. In the crotonaldehyde reaction the main products detected were crotonylperoxy nitrate and CO with yields of 93.6 +/- 4.3 and 8.3 +/- 1.1, respectively. On the basis of the rate constant measured, the activation energy calculated, and the identified products, abstraction of the aldehydic H seems to be the main degradation pathway at room temperature for the reaction of acrolein with NO3. For crotonaldehyde, the mechanism is unclear on the basis of the experimental results. The atmospheric implications of the reactions in question are also discussed.     

Gas-phase chemistry of (alpha-terpineol with ozone and OH radical: rate constants and products

A bimolecular rate constant, kOH+alpha-terpineol, of (1.9 +/- 0.5) x 10(-10) cm3 molecule(-1) s(-1) was measured using gas chromatography/mass spectrometry and the relative rate technique for the reaction of the hydroxyl radical (OH) with alpha-terpineol (1-methyl-4-isopropyl-1-cyclohexen-8-ol) at (297 +/- 3) K and 1 atm total pressure. Additionally, a bimolecular rate constant, kO3+alpha-terpineol, of (3.0 +/- 0.2) x 10(-16) cm3 molecule(-1) s(-1) was measured by monitoring the first order decrease in ozone concentration as a function of excess alpha-terpineol. To better understand alpha-terpineol's gas-phase transformation in the indoor environment, the products of the alpha-terpineol + OH and alpha-terpineol + 03 reactions were also investigated. The positively identified alpha-terpineol/OH reaction products were acetone, ethanedial (glyoxal, HC(=O)C(=O)H), and 2-oxopropanal (methyl glyoxal, CH3C(=O)C(=O)H). The positively identified alpha-terpineol/O3 reaction product was 2-oxopropanal (methyl glyoxal, CH3C(=O)C(=O)H). The use of derivatizing agents O-(2,3,4,5,6-pentalfluorobenzyl)hydroxylamine (PFBHA) and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) clearly indicated that several other reaction products were formed. The elucidation of these other reaction products was facilitated by mass spectrometry of the derivatized reaction products coupled with plausible alpha-terpineol/OH and alpha-terpineol/O3 reaction mechanisms based on previously published volatile organic compound/ OH and volatile organic compound/O3 gas-phase reaction mechanisms.