Free radical destruction of N-nitrosodimethylamine in water

Absolute rate constants for the reactions of the hydroxyl radical, hydrated electron, and hydrogen atom with N-nitrosodimethylamine (NDMA) in water at room temperature have been determined using electron pulse radiolysis and transient absorption spectroscopy (*OH and e- aq) and EPR free induction decay attenuation (*H) measurements. Specific values of (4.30 +/- 0.12) x 10(8), (1.41 +/- 0.02) x 10(10), and (2.01 +/- 0.03) x 10(8) M(-1) s(-1) were measured, respectively. DMPO spin-trapping experiments demonstrated that the hydroxyl radical reaction with NDMA occurs by hydrogen atom abstraction from a methyl group, and the rate constant for the subsequent reaction of this radical transient with dissolved oxygen was measured as (5.3 +/- 0.6) x 10(6) M(-1) s(-1). This relatively slow rate constant implies that regeneration of the parent nitrosoamine from the oxidized transient could occur in natural waters containing dissolved organic compounds. The reaction of the hydrated electron with NDMA was to form a transient adduct anion, which could subsequently transfer this excess electron to regenerate the parent chemical. Such regeneration reactions would significantly reduce the effectiveness of any applied advanced oxidation technology remediation effort on NDMA-contaminated natural waters.     

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.     

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 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.     

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.     

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.     

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.     

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.     

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.     

Relative rate constants of contaminant candidate list pesticides with hydroxyl radicals

The objective of this study was to establish the relative rate constants for the reactions of selected pesticides (linuron, diuron, prometon, terbacil, diazinon, dyfonate, terbufos, and disulfoton) listed on the U.S. EPA Contaminant Candidate Listwith UV and hydroxyl radicals (*OH). Batch experiments were conducted in phosphate buffered solution at pH 7. All pesticides were found to be very reactive toward *OH as indicated by rate constant values above 10(9) M(-1) s(-1). Using molinate as a reference compound, kOH ranged from 2.7 x 10(9) to 12.0 x 10(9) M(-1) s(-1) for the contaminants while slightly higher values from 2.9 x 10(9) to 14.3 x 10(9) M(-1) s(-1) were obtained using nitrobenzene as a reference compound. A method was established that accounts for direct photolysis when calculating kOH using UV/H2O2 process for compounds which degrade significantly by a direct photolysis mechanism.     

Kinetics of contaminant degradation by permanganate

To provide a more complete understanding of the kinetics of in situ chemical oxidation (ISCO) with permanganate (MnO4-), we measured the kinetics of oxidation of 24 contaminants-many for which data were not previously available. The new data reported here were determined using an efficient method based on continuous measurement of the MnO4- concentration by absorbance spectrometry. Under these conditions, the kinetics were found to be first-order with respect to both contaminant and MnO4- concentrations, from which second-order rate constants (k") were readily obtained. Emerging contaminants forwhich k" was determined (at 25 degrees C and pH 7) include 1,4-dioxane (4.2 x 10(-5) M(-1) s(-1)), methyl t-butyl ether (MTBE) (1.0 x 10(-4) M(-1) s(-1)), and methyl ethyl ketone (MEK) (9.1 x 10(-5) M(-1) s(-1)). Contaminants such as 2,4,6-trinitrotoluene (TNT), the pesticides aldicarb and dichlorvos, and many substituted phenols are oxidized with rate constants comparable to tetrachloroethene (PCE) and trichloroethene (TCE) (i.e., 0.03-1 M(-1) s(-1)) and therefore are good candidates for remediation with MnO4- in the field. There are several--sometimes competing--mechanisms by which MnO4- oxidizes contaminants, including addition to double bonds, abstraction of hydrogen or hydride, and electron transfer.     

Ionizing radiation-induced destruction of benzene and dienes in aqueous media

Pulse radiolysis with spectrophotometric and conductometric detection was utilized to study the formation and reactions of radicals from benzene and dienes in aqueous solutions. The benzene OH adduct, *C6H6OH, reacts with O2 (k = 3 x 10(8) L mol(-1) s(-1)) in a reversible reaction. The peroxyl radical, HOC6H6O2*, undergoes O2*- elimination, bimolecular decay, and reaction with benzene to initiate a chain reaction, depending on the dose rate, benzene concentration, and pH. The occurrence of the chain reaction is demonstrated in low-dose-rate gamma radiolysis experiments where the consumption of O2 was monitored. 1,4-Cyclohexadiene, 1,4-hexadiene, and 1,4-pentadiene form OH-adducts and undergo H-abstraction by O*- radicals. The OH-adducts react with O2 to form peroxyl radicals. These peroxyl radicals, however, do not undergo unimolecular O2*- elimination but rather decay by second-order processes, which lead to subsequent steps of O2*- elimination.     

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.     

Oxidation of pharmaceuticals during ozonation and advanced oxidation processes

This study investigates the oxidation of pharmaceuticals during conventional ozonation and advanced oxidation processes (AOPs) applied in drinking water treatment. In a first step, second-order rate constants for the reactions of selected pharmaceuticals with ozone (k(O3)) and OH radicals (k(OH)) were determined in bench-scale experiments (in brackets apparent k(O3) at pH 7 and T = 20 degrees C): bezafibrate (590 +/- 50 M(-1) s(-1)), carbamazepine (approximately 3 x 10(5) M(-1) s(-1)), diazepam (0.75 +/- 0.15 M(-1) s(-1)), diclofenac (approximately 1 x 10(6) M(-1) s(-1)), 17alpha-ethinylestradiol (approximately 3 x 10(6) M(-1) s(-1)), ibuprofen (9.6 +/- 1.0 M(-1) s(-1)), iopromide (<0.8 M(-1) s(-1)), sulfamethoxazole (approximately 2.5 x 10(6) M(-1) s(-1)), and roxithromycin (approximately 7 x 10(4) M(-1) s(-1)). For five of the pharmaceuticals the apparent k(O3) at pH 7 was >5 x 10(4) M(-1) s(-1), indicating that these compounds are completely transformed during ozonation processes. Values for k(OH) ranged from 3.3 to 9.8 x 10(9) M(-1) s(-1). Compared to other important micropollutants such as MTBE and atrazine, the selected pharmaceuticals reacted about two to three times faster with OH radicals. In the second part of the study, oxidation kinetics of the selected pharmaceuticals were investigated in ozonation experiments performed in different natural waters. It could be shown that the second-order rate constants determined in pure aqueous solution could be applied to predict the behavior of pharmaceuticals dissolved in natural waters. Overall it can be concluded that ozonation and AOPs are promising processes for an efficient removal of pharmaceuticals in drinking waters.     

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.     

Aqueous chlorination kinetics of some endocrine disruptors

The aqueous chlorination kinetics of six endocrine disruptors (EDs: 4-n-nonylphenol, beta-estradiol, estrone, estriol, 17alpha-ethinylestradiol, progesterone) were studied in the 3.50-12.00 pH range, at 20+/-2 degrees C, in the presence of an excess of total chlorine. Under these conditions, all molecules with a phenolic group in their structure were rapidly oxidized by chlorine, whereas progesterone remained unchanged. In the first step, apparent kinetic rate constants were determined at various pH levels. Then each elementary reaction kinetic rate constant, i.e., the reaction of hypochlorous acid (HOCl) with ionized EDs and neutral EDs and an acid-catalyzed reaction of HOCl with neutral EDs, was calculated in the second step. The results showed that chlorination exhibits a second-order reaction rate. The rate constants for the acid-catalyzed reaction ranged from 3.02 x 10(4) M(-1) s(-1) (for 4-n-nonylphenol) to 1.82-2.62 x 10(5) M(-1) s(-1) (for hormones). The rate constants of HOCI reactions with ionized EDs were found to be equal to 7.5 x 10(4) M(-1) s(-1) (for 4-n-nonylphenol) and between 3.52 and 4.15 x 10(5) M(-1) s(-1) (for hormones), while the rate contants of HOCI with neutral EDs were much lower, i.e., between 1.31 M(-1) s(-1) (for 4-n-nonylphenol) and 3.74-4.82 M(-1) s(-1) (for hormones). At pH 7, the apparent-second-order rate constants were calculated to range from 12.6 to 131.1 M(-1) s(-1). For a total chlorine concentration of 1 mg/L, the corresponding half-life times at pH 7 were about 65 min for 4-n-nonylphenol and 6-8 min for hormones.     

Reduction of organically complexed ferric iron by superoxide in a simulated natural water

Superoxide (and potentially its conjugate acid hydroperoxyl) is unique among the reactive oxygen species in that its standard redox potential in circumneutral natural waters potentially allows it to reduce ferric iron to the more soluble ferrous state. Here we have observed the superoxide/ hydroperoxyl-mediated reduction of ferric complexes with a variety of synthetic organic ligands and several complexes with natural organic matter (NOM), as well as freshly precipitated amorphous ferric oxyhydroxide, in bicarbonate buffered solutions at pH 8.1. From measurements of superoxide decay in the presence of the complexes, we calculated second-order rate constants for superoxide/ hydroperoxyl-mediated reduction that vary from (9.3+/-0.2) x 10(3) M(-1) s(-1) for the complex between Fe(III) and desferrioxamine B up to (1.9+/-0.2) x 10(5) M(-1) s(-1) for Fe(III)-salicylate and (2.3+/-0.1) x 10(5) M(-1) s(-1) for one of the Fe(III)-NOM complexes. We also verified that ferrous iron was produced from superoxide/hydroperoxyl-mediated Fe(III) reduction using ferrozine to trap free Fe(II). Low yields of the ferrozine complex when compared to the measured rates of superoxide decay suggest that ferric complexes are reduced directlyto corresponding ferrous complexes, with much of the ferrous complex reoxidizing before it is able to release free ferrous iron. This is an important consideration for microorganisms, as the kinetics of trace metal uptake is typically governed by free ion activity.     

Degradation of naled and dichlorvos promoted by reduced sulfur species in well-defined anoxic aqueous solutions

This work examines the reaction of reduced sulfur species (e.g., bisulfide, thiosulfate, thiophenolate) with naled, a registered insecticide, in well-defined anoxic aqueous solutions at 5 degrees C. High concentrations of reduced sulfur species can occur in the porewater of sediments and in anoxic subregions of estuaries. The dominanttransformation product from the reaction of naled with reduced sulfur species is dichlorvos, which indicates that debromination is the major reaction pathway. Dichlorvos is also a registered insecticide which is more toxic than naled. The second-order rate constants for reaction of naled with bisulfide and thiophenolate at 5 degrees C are 10.2 +/- 0.4 M(-1) s(-1) and 27.3 +/- 0.9 M(-1) s(-1), respectively, while the second-order rate constant for the reaction of naled with hydrogen sulfide and thiophenol are not significantly different from zero. The second-order rate constant of the reaction of naled with thiosulfate at 5 degrees C is 5.0 +/- 0.3 M(-1) s(-1). In contrast, the second-order rate constant of the reaction of dichlorvos with bisulfide at 25 degrees C is (3.3 +/- 0.1) x 10(-3) M(-1) s(-1). The activation parameters of the reaction of naled with bisulfide were also determined from the measured second-order rate constants over a temperature range. The results indicate that reduced sulfur species can play a very important role in the transformation of naled and dichlorvos in the coastal marine environment. It can be expected that in the presence of reduced sulfur species, naled is almost immediately transformed into the more toxic dichlorvos, which has an expected half-life of 4 days to weeks.     

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.