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.     

Kinetics and mechanism of degradation of dichlorvos in aqueous solutions containing reduced sulfur species

Reactions of dichlorvos with five reduced sulfur species (hydrogen sulfide, bisulfide, thiosulfate, thiophenol, and thiophenolate) were examined in well-defined anoxic aqueous solutions to investigate their role in its degradation. Reactions were monitored at varying concentrations of reduced sulfur species over pH range to obtain the second-order reaction rate constants. Experiments at 25 degrees C demonstrated that degradation of dichlorvos promoted by bisufide, thiosulfate, and thiophenolate were of much greater importance than hydrolysis under the experimental conditions in our study. In contrast, hydrogen sulfide and thiophenol were not effective in the degradation of dichlorvos. The activation parameters of the reaction of dichlorvos with bisulfide, thiosulfate, and thiophenolate were also determined from the measured second-order rate constants over a temperature range of 12-50 degrees C. The relative reactivity of the reduced sulfur species decreases in the following order: PhS- > HS- approximately equal to S2O3(2-). When the second-order rate constants at 25 degrees C are multiplied by the environmentally relevant concentration of the reduced sulfur species, predicted half-lives of dichlorvos ranged from hours to days. The results indicated that reduced sulfur species could play a very important role in the chemical fate of dichlorvos in coastal marine environments.     

Nucleophilic substitution of phosphorothionate ester pesticides with bisulfide (HS-) and polysulfides (S(n)2-)

The reactions of five organophosphorus insecticides (OPs) (chlorpyrifos-methyl, parathion-methyl, fenchlorphos, chlorpyrifos, and parathion) with hydrogensulfide/ bisulfide (H2S/HS-) and polysulfides (S(n)2-) were examined in well-defined aqueous solutions over a pH range from 5 to 9. The rates are first-order in the concentration of the different reduced sulfur species. Experiments at 25 degrees C demonstrated that the reaction of the five OPs with the reduced sulfur species follows a SN2 mechanism. The activation parameters of the reaction of OPs with bisulfide were determined from the measured second-order rate constants over a temperature range of 5-60 degrees C. The determined second-order rate constants show that the reaction of an OP with polysulfides is from 15 to 50 times faster than the reaction of the same OP with bisulfide. The dominant transformation products are desalkyl OPs, which indicate that the nucleophilic substitution of reduced sulfur species occurs at the carbon atom of the alkoxy groups. And also the results show that these reduced sulfur species are much better nucleophiles, and thus degrade these pesticides faster than the well-studied base hydrolysis by OH-. When the determined second-order rate constants are multiplied with the concentration of HS- and S(n)2- reported in salt marshes and porewater of sediments, predicted half-lives show that abiotic degradation by sulfide species may be of comparable importance to microbially mediated degradation in anoxic environments.     

Nucleophilic aliphatic substitution reactions of propachlor,alachlor, and metolachlor with bisulfide (HS-) and polysulfides (Sn2-)

Reactions of bisulfide and polysulfides with alachlor, propachlor, and metolachlor were examined in aqueous solution to investigate the role reduced sulfur species could play in effecting abiotic transformations of chloroacetanilide herbicides. Experiments at 25 degrees C demonstrated that reactions were approximately first-order in HS- concentration and revealed that polysulfides are considerably more reactive than HS-. delta H not equal to values for reactions of the three chloroacetanilides with HS- are statistically indistinguishable at the 95% confidence level, as are delta S not equal to values, despite significant differences in second-order rate constants (kHS-). Transformation products were characterized by GC/MS (in some cases following methylation) and were found to be consistent with substitution of chlorine by the sulfur nucleophile. Products containing multiple sulfur atoms were observed for the reactions of chloroacetanilides with polysulfides, while products resulting from reaction with HS- only possessed a single sulfur atom. When second-order rate constants at 25 degrees C are multiplied by HS- and polysulfide concentrations reported in salt marsh porewaters, predicted half-lives range from minutes to hours. HS- and especially polysulfides could thus exert a substantial influence on the fate of chloroacetanilide herbicides in aquatic environments. Oxidation of the resulting sulfur-substituted products could generate ethane sulfonic acid derivatives, previously reported as prevalent chloroacetanilide degradates.     

Nucleophilic aromatic substitution reactions of chloroazines with bisulfide (HS-) and polysulfides (Sn2-)

Reactions of bisulfide and polysulfides with chloroazines (important constituents of agrochemicals and textile dyes) were examined in aqueous solution at 25 degrees C. For atrazine, rates are first-order in polysulfide concentration, and polysulfide dianions are the principal reactive nucleophiles; no measurable reaction occurs with HS-. Second-order rate constants for reactions of an array of chloroazines with polysulfides are several orders of magnitude greater than for reactions with HS-. Transformation products indicate the substitution of halogen(s) by sulfur. Ring aza nitrogens substantially enhance reactivity through a combination of inductive and mesomeric effects, and electron-withdrawing or electron-donating substituents markedly enhance or diminish reactivity, respectively. The overall second-order nature of the reaction, the products observed, and reactivity trends are all consistent with a nucleophilic aromatic substitution (S(N)Ar) mechanism. Rate constants for reactions with HS- and Sn2- (n = 2-5) correlate only weakly with lowest unoccupied molecular orbital energies, suggesting that the electrophilicity of a chloroazine is not the sole determinant of its reactivity. When second-order rate constants are extrapolated to HS- and Sn2- concentrations reported in salt marsh pore waters, half-lives of minutes to years are obtained. Polysulfides in particular could play an important role in effecting abiotic transformations of chloroazines in hypoxic marine waters.     

Hydrosulfide oxidation pathways in oxic solutions containing iron(III) chelates

The role of dissolved oxygen (DO2) on the oxidation of hydrosulfide ions (HS-; C(HS-)0 = 50-150 micromol/L) into polysulfides (S(n)2-; n = 2-9), colloidal sulfur, and oxysulfur species with iron(III) trans-1,2-diaminocyclohexanetetraacetate (iron(III)-cdta; C(Fe(III)0 = 50-300 micromol/L) complexes in alkaline solutions (pH 9-10.2) was investigated at 25 +/- 1 degree C. At higher pH, oxygen was seen to slow down the hydrosulfide conversion rate. For instance, the HS- half-life was 24.8 min in a DO2-saturated iron(III)-cdta solution compared to 11.3 min in the corresponding anoxic solution (pH 10.2, C(HS-)0 = 80 micromol/L, C(Fe(III))0 = 200 micromol/L). In anoxia, HS- oligomerizes into chain-like polysulfides which behave as autocatalysts on the HS- conversion rates. The presence of DO2 disrupts the HS- oligomerization process by generating thiosulfate precursors from polysulfides, a pathway that impedes the HS- uptake. At lower alkaline pH where the hydroxide-free Fe(3+)cdta(4-) is the prevailing iron(III)-cdta species, the "iron(II)-cdta + DO2" oxidative reaction becomes crucial. Oxidative regeneration of iron(III) as Fe(3+)cdta(4-) (being more reactive than Fe(3+)OH(-)cdta(4-)) offsets to some extent the restrictive role of oxygen on the accumulation of polysulfides. Thiosulfate and sulfate were the main end-products for the current experimental conditions to the detriment of colloidal sulfur, which did not form in DO2-saturated solutions.     

Pesticide processing potential in prairie pothole porewaters

Prairie pothole lakes (PPLs) are located within the extensively farmed Great Plains region of North America, and many are negatively impacted by nonpoint source pesticide pollution. To date, the environmental fate of pesticides in these lakes remains largely unknown. In this study, two PPLs in the Cottonwood Lake area of North Dakota were sampled, and transformations of four chloroacetanilide pesticides in sediment porewaters were examined. The reduced sulfur species in the porewaters, such as bisulfide (HS(-)) and polysulfides (S(n)(2-)), readily transformed the target pesticides into sulfur-substituted products. Although HS(-) and S(n)(2-) played a dominant role, other reactive constituents in PPL porewaters also contributed to the transformation. Results from this study revealed that abiotic reactions with reduced sulfur species could represent an important removal pathway for pesticides entering PPLs.     

Catalysis of elemental sulfur nanoparticles on chromium(VI) reduction by sulfide under anaerobic conditions

Chromate (CrVI) reduction by sulfide was conducted in anaerobic batch experimental systems. The molar ratio of the reduced CrVI to the oxidized S(-II) was 1:1.5 during the reaction, suggesting that the product of sulfide oxidation was elemental sulfur. Under the anaerobic condition, the reaction was pseudo first order initially with respect to CrVI, but the rate was dramatically accelerated at the later stage of the reaction. The rate acceleration was due to catalysis by elemental sulfur nanoparticles; dissolved species such as monomeric elemental sulfur and polysulfides appeared to be ineffective catalysts. Elemental sulfur nanoparticles were capable of adsorbing sulfide and such adsorbed sulfide exhibited much higher reactivity toward CrVI reduction than the aqueous-phase sulfide, resulting in the observed rate acceleration. Kinetic data under various reactant concentrations can be represented by the following empirical kinetic equation: -d[CrVI]/dt = k1 [CrVI][H2S]0.63 + k3[CrVI][triple bond S--SH]0.57. The first term on the right-hand side corresponds to the noncatalytic pathway, with k1 = 1.0 x 10(-3) (microM)(-0.63) min(-1) at pH 7.60 and 8.2 x 10(-5) (microM)-0.63 min(-1) at pH 8.10. The second term, k3[CrVI][triple bond S--SH]b, is the catalytic term with [triple bond S--SH] representing the adsorbed concentration of sulfide on the elemental sulfur nanoparticles (microM). The catalytic term is more important at the later stage of the reaction, as indicated by the observed kinetics and the enhancement of the reaction rate by externally added elemental sulfur nanoparticles. At pH 8.10, k3 = 0.0057 (microM)(-0.57) min(-1).     

Chlorination of phenols: kinetics and formation of chloroform

The kinetics of chlorination of several phenolic compounds and the corresponding formation of chloroform were investigated at room temperature. For the chlorination of phenolic compounds, second-order kinetics was observed, first-order in chlorine, and first-order in the phenolic compound. The rate constants of the reactions of HOCl with phenol and phenolate anion and the rate constant of the acid-catalyzed reaction were determined in the pH range 1-11. The second-order rate constants for the reaction HOCl + phenol varied between 0.02 and 0.52 M(-1) s(-1), for the reaction HOCl and phenolate between 8.46 x 10(1) and 2.71 x 10(4) M(-1) s(-1). The rate constant for the acid-catalyzed reaction varied between 0.37 M(-2) s(-1) to 6.4 x 10(3) M(-2) s(-1). Hammett-type correlations were obtained for the reaction of HOCl with phenolate (log(k) = 4.15-3.00 x sigma sigma) and the acid-catalyzed reaction of HOCl with phenol (log(k) = 2.37-4.26 x sigma sigma). The formation of chloroform could be interpreted with a second-order model, first-order in chlorine, and first-order in chloroform precursors. The corresponding rate constants varied between k > 100 M(-1) s(-1) for resorcinol to 0.026 M(-1) s(-1) for p-nitrophenol at pH 8.0. It was found that the rate-limiting step of chloroform formation is the chlorination of the chlorinated ketones. Yields of chloroform formation depend on the type and position of the substituents and varied between 2 and 95% based on the concentration of the phenol.     

Potential for abiotic reduction of pesticides in Prairie pothole porewaters

Prairie pothole lakes (PPLs) are critical hydrological and ecological components of central North America and represent one of the largest inland wetland systems on Earth. These lakes are located within an agricultural region, and many of them are subject to nonpoint-source pesticide pollution. Limited attention, however, has been paid to understanding the impact of PPL water chemistry on the fate and persistence of pesticides. In this study, the abiotic reductive transformation of seven dinitroaniline pesticides was investigated in PPL sediment porewaters containing naturally abundant levels of reduced sulfur species (i.e., bisulfide (HS(-)) and polysulfides (S(n)(2-))) and dissolved organic matter (DOM). Target dinitroanilines underwent rapid degradation in PPL porewaters and were transformed into corresponding amine products. While the largest fraction of the transformation could be attributed to reduced sulfur species, experimental evidence suggested that other reactive entities in PPL porewaters, such as DOM and mineral phases, might also affect the reaction rates of dinitroanilines. Results from this study highlight the importance of reductive transformation as an abiotic natural attenuation pathway for pesticides entering the PPL sedimentary environment.     

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.     

Kinetics and mechanistic aspects of As(III) oxidation by aqueous chlorine, chloramines, and ozone: relevance to drinking water treatment

Kinetics and mechanisms of As(III) oxidation by free available chlorine (FAC-the sum of HOCl and OCl-), ozone (O3), and monochloramine (NH2Cl) were investigated in buffered reagent solutions. Each reaction was found to be first order in oxidant and in As(III), with 1:1 stoichiometry. FAC-As(III) and O3-As(III) reactions were extremely fast, with pH-dependent, apparent second-order rate constants, k'app, of 2.6 (+/- 0.1) x 10(5) M(-1) s(-1) and 1.5 (+/- 0.1) x 10(6) M(-1) s(-1) at pH 7, whereas the NH2Cl-As(III) reaction was relatively slow (k'app = 4.3 (+/- 1.7) x 10(-1) M(-1) s(-1) at pH 7). Experiments conducted in real water samples spiked with 50 microg/L As(III) (6.7 x 10(-7) M) showed that a 0.1 mg/L Cl2 (1.4 x 10-6 M) dose as FAC was sufficient to achieve depletion of As(III) to <1 microg/L As(III) within 10 s of oxidant addition to waters containing negligible NH3 concentrations and DOC concentrations <2 mg-C/L. Even in a water containing 1 mg-N/L (7.1 x 10(-5) M) as NH3, >75% As(III) oxidation could be achieved within 10 s of dosing 1-2 mg/L Cl2 (1.4-2.8 x 10(-5) M) as FAC. As(III) residuals remaining in NH3-containing waters 10 s after dosing FAC were slowly oxidized (t1/2 > or = 4 h) in the presence of NH2Cl formed by the FAC-NH3 reaction. Ozonation was sufficient to yield >99% depletion of 50 microg/L As(III) within 10 s of dosing 0.25 mg/L O3 (5.2 x 10(-6) M) to real waters containing <2 mg-C/L of DOC, while 0.8 mg/L O3 (1.7 x 10(-5) M) was sufficientfor a water containing 5.4 mg-C/L of DOC. NH3 had negligible effect on the efficiency of As(III) oxidation by O3, due to the slow kinetics of the O3-NH3 reaction at circumneutral pH. Time-resolved measurements of As(III) loss during chlorination and ozonation of real waters were accurately modeled using the rate constants determined in this investigation.     

Studies on the atmospheric degradation of chlorpyrifos-methyl

The gas-phase atmospheric degradation of chlorpyrifos-methyl (a widely used organophosphate insecticide in Southern European regions) has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. Photolysis under sunlight conditions and reaction with ozone were shown to be unimportant. The rate constant for reaction of chlorpyrifos-methyl with OH radicals was measured using a conventional relative rate method with cyclohexane and n-octane employed as reference compounds with k = (4.1 ± 0.4) × 10(-11) cm(3) molecule(-1) s(-1) at 300 ± 5 K and atmospheric pressure. The available evidence indicates that tropospheric degradation of chlorpyrifos-methyl is mainly controlled by reaction with OH radicals and that the tropospheric lifetime is estimated to be around 3.5 h. Significant aerosol formation was observed following the reaction of chlorpyrifos-methyl with OH radicals, and the main carbon-containing products detected in the gas phase were chlorpyrifos-methyl oxone and 3,5,6-trichloro-2-pyridinol.     

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.     

Kinetic study on thermal denaturation of hen egg-white lysozyme involving precipitation

A kinetic study on the thermal denaturation accompanying precipitation of hen egg-white lysozyme was performed at temperatures between 50 and 90 degrees C. Visible precipitation occurred at lysozyme concentrations higher than 10(-5)M. Even at the concentration of 10(-6)M where no visible precipitation was observed, irreversible and reversible denaturation could be clearly discriminated. The former involves two different reactions with activation energies of approximately 93 and 50 kJ x mol(-1). On the other hand, enthalpy and entropy changes in the latter are 443 kJ x mol(-1) and 1280 J x K(-1) x mol(-1), respectively, indicating a large conformational change. The contradiction that the denaturation or deactivation reaction fitted first-order reaction kinetics while its rate constant depended on the protein concentration, was resolved by newly proposed schemes. The apparent first-order rate constant obtained experimentally depended on the initial protein concentration being on the order of almost unity. Moreover, it was revealed that the apparent first-order reaction involved a second-order reaction that characterized the aggregation of denatured protein molecules. The theory developed here explained reasonably the thermal denaturation accompanying precipitation that occurs at high protein concentration and at high temperature, and was also successfully applied to the lower concentration range with no accompanying precipitation.     

Kinetics of hydrolysis and precipitation of ferric iron in seawater

We have investigated the kinetics of iron hydrolysis and precipitation and dissolution of the solid formed via a novel chemical method, namely accessibility of iron to the fungal siderophore desferrioxamine B (DFB), with spectrophotometric detection of the ferrioxamine complex. Our results support a mechanism in which precipitation of dissolved inorganic iron in seawater is first order with respect to total (dissolved and precipitated) iron, with a second-order rate constant of (4.1 +/- 1.1) x 10(7) M(-1) s(-1) at pH 8.1. The rate of dissolution was first order with respect to the total iron concentration and the first-order rate constant decreased from 2.3 x 10(-4) s(-1) after aging for 1 min to 4.8 x 10(-6) s(-1) after aging for 1 week. The proposed reaction mechanism simulated a solubility limit condition in agreement with experimental observations, from which we calculated the solubility of ferric iron to be 1.2 x 10(-13) M when the precipitate had been aged for 1 week. This is approximately 2 orders of magnitude less than reported in previous studies, possibly due to the chemically based method for dissolved iron determination used here compared with traditional physical separation methods. Our results confirm that Fe(III) hydrolysis in seawater is fast and show thatthe precipitated solid is quite labile initially but rapidly becomes much less so, with important implications for sequestration by organic ligands such as siderophores.     

Formation of chloroform and chlorinated organics by free-chlorine-mediated oxidation of triclosan

The widely used antimicrobial agent triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) readily reacts with free chlorine under drinking water treatment conditions. Overall second-order kinetics were observed, first-order in free chlorine and first-order in triclosan. Over the pH range of 4-11.5, the kinetics were pH sensitive as a result of the pH dependent speciation of both triclosan and free chlorine. Using a Marquardt-Levenberg routine, it was determined that this pH effect indicates that the dominant reaction in this system is between the ionized phenolate form of triclosan and hypochlorous acid (HOCl). The overall second-order rate coefficient was determined to be kArO- = 5.40 (+/- 1.82) x 10(3) M(-1) s(-1). Three chlorophenoxyphenols and two chlorophenols were identified by gas chromatographic-mass spectroscopic analysis. The chlorophenoxyphenol compounds include two monochlorinated triclosan derivatives (5,6-dichloro-2-(2,4-dichlorophenoy)phenol and 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol) and one dichlorinated derivative (4,5,6-trichloro-(2,4-dichlorophenoxy)phenol); these species form via bimolecular electrophilic substitution of triclosan. 2,4-Dichlorophenol was detected under all reaction conditions and forms via ether cleavage of triclosan. In experiments with excess free chlorine, 2,4,6-trichlorophenol was formed via electrophilic substitution of 2,4-dichlorophenol. Chloroform formation was observed when an excess of free chlorine was present. A Hammett-type linear free-energy relationship (LFER) using Brown-Okamoto parameters (sigma+) was established to correlate the reactivity of HOCI and the phenolate forms of triclosan and other chlorophenols (log kArO- = -(10.7 +/- 2.2)Sigmasigma(+)o,m,p + 4.43). This LFER was used to obtain estimates of rate coefficients describing the reactivity of the intermediates 5,6-dichloro-2-(2,4-dichlorophenoy)phenol (kArO- approximately equal to 6 x 10(2)), 4,5-dichloro-2-(2,4-dichlorophenoxy)phenol (kArO- approximately equal to 3 x 10(2)), and 4,5,6-trichloro-(2,4-dichlorophenoxy)phenol (kArO- approximately equal to 4 x 10(1)).     

Modeling the kinetics of ferrous iron oxidation by monochloramine

The maintenance of disinfectants in distribution systems is necessary to ensure drinking water safety. Reactions with oxidizable species can however lead to undesirable disinfectant losses. Previous work has shown that the presence of Fe(II) can cause monochloramine loss in distribution system waters. This paper further examines these reactions and presents a reaction mechanism and kinetic model. The mechanism includes both aqueous-phase reactions and surface-catalyzed reactions involving the iron oxide product. In addition, it considers competitive reactions involving the amidogen radical that lead to a nonelementary stoichiometry. Using the method of initial rates, the aqueous-phase reactions were found to have first-order dependencies on Fe(II), NH2Cl, and OH- and a rate coefficient (kNH2Cl,soln) of 3.10 (+/-0.560) x 10(9) M(-2) min(-1). The surface-mediated reactions were modeled by assuming the formation of two surface species: >FeOFe+ and >FeOFeOH. Using numerical techniques, combined rate coefficients for the surface-mediated processes were determined to be 0.56 M(-3) min(-1) and 3.5 x 10(-18) M(-4) min(-1), respectively. The model was then used to examine monochloramine and Fe(II) stability under conditions similar to those observed in distribution systems. Our findings suggest the potential utility of monochloramine as an oxidant for Fe(III) removal in drinking water treatment.     

Triclosan reactivity in chloraminated waters

Triclosan, widely employed as an antimicrobial additive in many household personal care products, has recently been detected in wastewater treatment plant effluents and in source waters used for drinking water supplies. Chloramines used either as alternative disinfectants in drinking water treatment or formed during chlorination of nonnitrified wastewater effluents have the potential to react with triclosan. This study examined triclosan reactivity in chloraminated waters over the pH range of 6.5-10.5. Experimental and modeling results show that monochloramine directly reacts with the phenolate form of triclosan; however, the reaction is relatively slow as evinced by the second-order rate constant k(ArO)-NH2Cl = 0.025 M(-1) s(-1). Kinetic modeling indicates that for pH values less than 9.5, reactions between triclosan and two monochloramine autodecomposition intermediates, hypochlorous acid (k(ArO)-HOC = 5.4 x 10(3) M(-1) s(-1)) and dichloramine (k(ArO)-NHCl2 = 60 M(-1) s(-1)), are responsible for a significant percentage of the observed triclosan decay. The products of these reactions include three chlorinated triclosan byproducts as well as 2,4-dichlorophenol and 2,4,6-trichlorophenol. Low levels of chloroform were detected after 1 week at pH values of 6.5 and 7.5. The slow reactivity of triclosan in the presence of chloramines explains the recalcitrance of this species in nonnitrified wastewater effluents.     

Equilibrium and kinetics of bromine chloride hydrolysis

Aqueous-phase halogen reactions play an important role in tropospheric ozone depletion that is observed during Arctic sunrise where bromine chloride is a key intermediate. The temperature dependencies of BrCl(aq) equilibration with BrCl2-, HOBr(aq), Br2(aq), Cl2(aq), HOCl(aq), Br-, and other species (Br3-, Br2Cl-, Cl3-, OBr-, and OCI-) are determined as a function of Cl- concentration from pH 0 to pH 7. Values for K1 (=[BrCl2-]/([BrCl(aq)][Cl-])) at mu = 1.0 M are 3.8 M(-1) at 25.0 degrees C, 4.7 M(-1) at 10.0 degrees C, and 5.5 M(-1) at 0.0 degrees C, with deltaH1 degrees = -9.9 kJ mol(-1) and deltaS1 degrees = -22 J K(-1) mol(-1). BrCl(aq) hydrolysis equilibria have little or no temperature dependence with Kh1 (=[HOBr(aq)][Cl-][H+]/[BrCl(aq)]) = 1.3 x 10(-4) M2 from 25.0 to 5.0 degrees C, mu = 1.0 M. When conditions are adjusted to give a rapid partial hydrolysis of BrCl in equilibrium with HOBr and Cl- at p[H+] 4.31, a relatively slow reaction (kobsd = 2.4 s(-1)) to form HOCl and Br- is observed. This takes place via BrCl reaction with Cl- to form Cl2, which hydrolyzes in the rate-determining step to give HOCl. On the other hand, the rate of complete BrCl hydrolysis to form HOBr and Cl- at p[H+] 6.4 is extremely rapid with a first-order rate constant of 3.0 x 10(6) s(-1) at 25.0 degrees C. The reverse reaction between HOBr, Cl-, and H+ has a rate constant of 2.3 x 10(10) M(-2) s(-1), so that in seawater, where [Cl-]/[Br-] = 700, the formation of BrCl is much faster than the formation of Br2 from HOBr, Br-, and H+. Rapid formation of BrCl(aq) and its subsequent reaction with Br- is a viable pathway to give Br2(aq). Photolysis of Br2(g) is believed to initiate the reactions associated with ozone depletion.