General-acid-catalyzed reactions of hypochlorous acid and acetyl hypochlorite with chlorite ion

Predicting chlorine demand by peracetic acid in drinking water treatment

Peracetic acid (PAA) may be used in drinking water treatment for pre-oxidation and mussel control at the intake. PAA may exert a downstream chlorine demand, but full details of this reaction have not been reported. There are three possible mechanisms of this demand: (1) PAA may react directly with chlorine; (2) PAA exists in equilibrium with hydrogen peroxide, which is known to react with chlorine; and (3) as H2O2 reacts with chlorine, PAA will hydrolyze to form more H2O2 to re-establish PAA/H2O2 equilibrium, thereby serving as an indirect reservoir of chlorine demand. While the H2O2 reaction with chlorine is well known, the other mechanisms of possible PAA-induced chlorine demand have not previously been investigated. The observed molar stoichiometric ratio of PAA to free chlorine (n) for the presumed direct PAA + free chlorine reaction was determined to be approximately 2, and the corresponding observed reaction rate coefficients at pH 6, 7, 8, and 9 were 2.76, 3.14, 1.61, 10.1 M-n·s-1, respectively (at 25 °C). With these estimated values, a kinetic model was built to predict the chlorine demand by PAA. The results suggest that chlorine demand from PAA is likely to be negligible over the course of several days (e.g., < 20% chlorine loss) for most conditions except for high pH (e.g., >8) and high PAA:Cl2 molar ratios (e.g., >2:1).

Kinetic Role of Reactive Intermediates in Controlling the Formation of Chlorine Dioxide in the Hypochlorous Acid-Chlorite Ion Reaction

An advanced experimental protocol is reported for studying the kinetics and mechanism of the complex redox reaction between chlorite ion and hypochlorous acid under acidic condition. The formation of ClO₂ is followed directly by the classical two-component stopped-flow method. In sequential stopped-flow experiments, the target reaction is chemically quenched using NaI solution and the concentration of each reactant and product is monitored as a function of time by utilizing the principles of kinetic discrimination. Thus, in contrast to earlier studies, not only the formation of one of the products but the decay of the reactants was also directly followed. This approach provides a firm basis for postulating a detailed mechanism for the interpretation of the experimental results under a variety of conditions. The intimate details of the reaction are explored by simultaneously fitting 78 kinetic traces, i.e., the concentration vs. time profiles of ClO₂^-, HOCl, and ClO₂, to an 11-step kinetic model. The most important reaction steps were identified, and it was shown that two reactive intermediates have a pivotal role in the mechanism. While chlorate ion predominantly forms via the reaction of Cl₂O, chlorine dioxide is exclusively produced in reaction steps involving Cl₂O₂. This study leads to clear conclusions on how to control the stoichiometry of the reaction and achieve optimum conditions to produce chlorine dioxide and to reduce the formation of the toxic chlorate ion in practical applications.

Kinetic and mechanistic understanding of chlorite oxidation during chlorination: Optimization of ClO2 pre-oxidation for disinfection byproduct control

Chlorine dioxide (ClO₂) applications to drinking water are limited by the formation of chlorite (ClO₂^-) which is regulated in many countries. However, when ClO₂ is used as a pre-oxidant, ClO₂^- can be oxidized by chlorine during subsequent disinfection. In this study, a kinetic model for the reaction of chlorine with ClO₂^- was developed to predict the fate of ClO₂^- during chlorine disinfection. The reaction of ClO₂^- with chlorine was found to be highly pH-dependent with formation of ClO₃^- and ClO₂ in ultrapure water. In presence of dissolved organic matter (DOM), 60-70% of the ClO₂^- was transformed to ClO₃^- during chlorination, while the in situ regenerated ClO₂ was quickly consumed by reaction with DOM. The remaining 30-40% of the ClO₂^- first reacted to ClO₂ which then formed chlorine from the DOM-ClO₂ reaction. Since only part of the ClO₂^- was transformed to ClO₃^-, the sum of the molar concentrations of oxychlorine species (ClO₂^- + ClO₃^-) decreased during chlorination. By kinetic modelling, the ClO₂^- concentration after 24 h of chlorination was accurately predicted in synthetic waters but was largely overestimated in natural waters, possibly due to a ClO₂^- decay enhanced by high concentrations of chloride and in situ formed bromine from bromide. Understanding the chlorine-ClO₂^- reaction mechanism and the corresponding kinetics allows to potentially apply higher ClO₂ doses during the pre-oxidation step, thus improving disinfection byproduct mitigation while keeping ClO₂^-, and if required, ClO₃^- below the regulatory limits. In addition, ClO₂ was demonstrated to efficiently degrade haloacetonitrile precursors, either when used as pre-oxidant or when regenerated in situ during chlorination.

Exploring the mechanism of hypochlorous acid decomposition in aqueous solutions

Hypochlorous acid is an intermediate in important industrial processes such as the production of chlorate but is also used for water treatment and disinfection. In aqueous solutions hypochlorous acid may decompose into oxygen or chlorate. Using density functional theory (DFT) modelling we have for the first time established detailed mechanisms for the respective decomposition pathways. Our calculations indicate, that both oxygen and chlorate formation proceed through an identical set of intermediates. At neutral pH the reaction is initiated by a fast equilibrium between HOCl, OCl^-, Cl₂O and Cl₃O₂^-. The subsequent abstraction of Cl^- to form Cl₂O₂ is rate determining for chlorate formation while it is the decomposition of Cl₂O₂ in the case of oxygen formation. Under alkaline conditions, OCl^- decomposition to chlorate proceeds through chlorite. This reaction path is significantly less active. The highest rate for chlorate or oxygen formation is found at pH 7.1. These results highlight the need to consider a complex mixture of different Cl species when addressing the chemistry of hypochlorous acid containing solutions.

Degradation of sulfamethoxazole using ozone and chlorine dioxide - Compound-specific stable isotope analysis, transformation product analysis and mechanistic aspects

The sulfonamide antibiotic sulfamethoxazole (SMX) is a widely detected micropollutant in surface and groundwaters. Oxidative treatment with e.g. ozone or chlorine dioxide is regularly applied for disinfection purposes at the same time exhibiting a high potential for removal of micropollutants. Especially for nitrogen containing compounds such as SMX, the related reaction mechanisms are largely unknown. In this study, we systematically investigated reaction stoichiometry, product formation and reaction mechanisms in reactions of SMX with ozone and chlorine dioxide. To this end, the neutral and anionic SMX species, which may occur at typical pH-values of water treatment were studied. Two moles of chlorine dioxide and approximately three moles of ozone were consumed per mole SMX degraded. Oxidation of SMX with ozone and chlorine dioxide leads in both cases to six major transformation products (TPs) as revealed by high-resolution mass spectrometry (HRMS). Tentatively formulated TP structures from other studies could partly be confirmed by compound-specific stable isotope analysis (CSIA). However, for one TP, a hydroxylated SMX, it was not possible by HRMS alone to identify whether hydroxylation occurred at the aromatic ring, as suggested in literature before, or at the anilinic nitrogen. By means of CSIA and an analytical standard it was possible to identify sulfamethoxazole hydroxylamine unequivocally as one of the TPs of the reaction of SMX with ozone as well as with chlorine dioxide. H-abstraction and electron transfer at the anilinic nitrogen are suggested as likely initial reactions of ozone and chlorine dioxide, respectively, leading to its formation. Oxidation of anionic SMX with ozone did not show any significant isotopic fractionation whereas the other reactions studied resulted in a significant carbon isotope fractionation.

Oxidations at Sulfur Centers by Aqueous Hypochlorous Acid and Hypochlorite: Cl+ Versus O Atom Transfer

Sulfur-containing compounds are known to be susceptible to oxidation by aqueous HOCl, but the factors affecting the rates of these reactions are not well-established. Here we report on the kinetics of oxidation of thiosulfate, thiourea, thioglycolate, (methylthio)acetate, tetrathionate, dithiodiglycolate, and dithiodipropionate at 25 °C and 0.4 M ionic strength. These reactions obey the general rate law -d[OCl^-]/dt = (k_OCl^-[OCl^-] + k_HOCl[HOCl])[substrate] with some exceptions: tetrathionate and the two disulfides undergo rate-limiting hydrolysis at high pH, and dithiodiglycolate has an additional term in the rate law that is second order in [substrate]. The reactions of HOCl are believed to have a Cl⁺ transfer mechanism, and in the case of thiosulfate the rate of hydrolysis of the ClS₂O₃^- intermediate was determined. In the case of thiourea evidence was obtained for thiourea monoxide as a long-lived product. It is shown that sulfite and species with terminal sulfur atoms have k_HOCl values in the vicinity of 1 × 10⁹ M^-1 s^-1, while SCN^- and thioethers react somewhat more slowly; tetrathionate, trithionate, and disulfides react much more slowly. Comparison of the rate constants with those for oxidation of these sulfur substrates by H₂O₂ and [Pt(CN)₄Cl₂]^2- shows that HOCl reacts a few orders of magnitude more rapidly than [Pt(CN)₄Cl₂]^2- and ∼9 orders of magnitude more rapidly than H₂O₂. Many of the k_HOCl values are leveled by the high electrophilicity of HOCl. It is proposed that the k_OCl^- values correspond to oxygen-atom transfer mechanisms, as supported by LFERS (linear free energy relationships) relating these rate constants to those for reactions of H₂O₂ and [Pt(CN)₄Cl₂]^2-.

Kinetics of the Benzaldehyde-Inhibited Oxidation of Sulfite by Chlorine Dioxide

There has been steady interest in the aqueous reaction of ClO2• with sulfur(IV) since the 1950s, and a wide variety of rate laws and mechanisms have been proposed. In neutral-to-alkaline media, the reaction is challenging to study because of its great rate. Here it is shown that benzaldehyde can be used as an additive to slow the reaction and make its rates more amenable to study. The rates can be quantitatively modeled by a mechanism that includes reversible binding of sulfur(IV) by benzaldehyde and a rate-limiting mixed second-order reaction of ClO2• with SO3(2-). The latter reaction occurs through parallel electron transfer from SO3(2-) to ClO2• and oxygen-atom transfer from ClO2• to SO3(2-).

Photochemical oxidation of chloride ion by ozone in acid aqueous solution

The experimental investigation of chloride ion oxidation under the action of ozone and ultraviolet radiation with wavelength 254 nm in the bulk of acid aqueous solution at pH 0-2 has been performed. Processes of chloride oxidation in these conditions are the same as the chemical reactions in the system O3 - OH - Cl(-)(aq). Despite its importance in the environment and for ozone-based water treatment, this reaction system has not been previously investigated in the bulk solution. The end products are chlorate ion ClO3(-) and molecular chlorine Cl2. The ions of trivalent iron have been shown to be catalysts of Cl(-) oxidation. The dependencies of the products formation rates on the concentrations of O3 and H(+) have been studied. The chemical mechanism of Cl(-) oxidation and Cl2 emission and ClO3(-) formation has been proposed. According to the mechanism, the dominant primary process of chloride oxidation represents the complex interaction with hydroxyl radical OH with the formation of Cl2(-) anion-radical intermediate. OH radical is generated on ozone photolysis in aqueous solution. The key subsequent processes are the reactions Cl2(-) + O3 → ClO + O2 + Cl(-) and ClO + H2O2 → HOCl + HO2. Until the present time, they have not been taken into consideration on mechanistic description and modelling of Cl(-) oxidation. The final products are formed via the reactions 2ClO → Cl2O2, Cl2O2 + H2O → 2H(+) + Cl(-) + ClO3(-) and HOCl + H(+) + Cl(-) ⇄ H2O + Cl2. Some portion of chloride is oxidized directly by O3 molecule with the formation of molecular chlorine in the end.

Kinetics and mechanism of the oxidation of bromide by periodate in aqueous acidic solution

The periodate–bromide reaction has been studied spectrophotometrically mainly in excess of bromide ion, monitoring the formation of the total amount of bromine at 450 nm at acidic buffered conditions and at a constant ionic strength in the presence of a phosphoric acid/dihydrogen phosphate buffer. The stoichiometry of the reaction was established to be strictly IO4(–) + 2Br(–) + 2H(+) → Br2 + IO3(–) + H2O. The formal kinetic order of the reactants was found to be perfectly one and two in the cases of periodate and bromide, respectively, but that of the hydrogen ion lies between one and two. We have also provided experimental evidence that dihydrogen phosphate accelerates the formation of bromine, suggesting the appearance of strong buffer assistance. On the basis of the experiments, a simple two-step kinetic model is proposed involving BrIO3 as a key intermediate that perfectly explains all of the experimental findings. Furthermore, we have also shown that in huge excess of bromide, the apparent rate coefficient obtained from the individual curve fitting method of the absorbance–time series is necessarily independent of the initial periodate concentration that may falsely be interpreted as the rate of bromine formation is also independent of the concentration of periodate.

Non-heme manganese catalysts for on-demand production of chlorine dioxide in water and under mild conditions

Two non-heme manganese complexes are used in the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00. The catalysts afford up to 1000 turnovers per hour and remain highly active in subsequent additions of chlorite. Kinetic and spectroscopic studies revealed a Mn(III)(OH) species as the dominant form under catalytic conditions. A Mn(III)(μ-O)Mn(IV) dinuclear species was observed by EPR spectroscopy, supporting the involvement of a putative Mn(IV)(O) species. First-order kinetic dependence on the manganese catalyst precludes the dinuclear species as the active form of the catalyst. Quantitative kinetic modeling enabled the deduction of a mechanism that accounts for all experimental observations. The chlorine dioxide producing cycle involves formation of a putative Mn(IV)(O), which undergoes PCET (proton coupled electron-transfer) reaction with chlorite to afford chlorine dioxide. The ClO2 product can be efficiently removed from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of pure chlorine dioxide for on-site use and further production of chlorine dioxide.

Protein sulfenic acid formation: from cellular damage to redox regulation

Protein sulfenic acid formation has long been regarded as unwanted damage caused by reactive oxygen species (ROS). However, over the past 10 years, accumulating evidence has shown that the reversible oxidation of cysteine thiol groups to sulfenic acid functions as a redox-based signal transduction mechanism. Here, we review the mechanisms of sulfenic acid formation by ROS. We present some of the most important roles played by sulfenic acids in living cells as well as the pathways that regulate sulfenic acid formation. We highlight the experimental tools that have been developed to study the cellular sulfenome and show how computational approaches might help to better understand the mechanisms of sulfenic acid formation.

Chlorine dioxide oxidation of dihydronicotinamide adenine dinucleotide (NADH)

The oxidation of dihydronicotinamide adenine dinucleotide (NADH) by chlorine dioxide in phosphate buffered solutions (pH 6-8) is very rapid with a second-order rate constant of 3.9 x 10(6) M(-1) s(-1) at 24.6 degrees C. The overall reaction stoichiometry is 2ClO2(*) per NADH. In contrast to many oxidants where NADH reacts by hydride transfer, the proposed mechanism is a rate-limiting transfer of an electron from NADH to ClO2(*). Subsequent sequential fast reactions with H(+) transfer to H2O and transfer of an electron to a second ClO2(*) give 2ClO2(-), H3O(+), and NAD(+) as products. The electrode potential of 0.936 V for the ClO2(*)/ClO2(-) couple is so large that even 0.1 M of added ClO2(-) (a 10(3) excess over the initial ClO2(*) concentration) fails to suppress the reaction rate.

Pitfall of an Initial Rate Study: On the Kinetics and Mechanism of the Reaction of Periodate with Iodide Ions in a Slightly Acidic Medium

The kinetics of the periodate-iodide reaction has a contradictory history dating back to almost a century. This reaction has been reinvestigated spectrophotometrically in the pH range 3.13-5.55 in both buffered (acetic acid/acetate) and unbuffered solution at T=25.0+/-0.1 degrees C with an I=0.5 M ionic strength. The spectra between 290 and 500 nm were recorded and the reaction was followed until at least 95% of one of the reactants was consumed. The stoichiometry has been found to be strongly dependent on pH, but the rate of the initial step is independent of pH within the pH range studied. An eight-step kinetic model is proposed with four fitted kinetic parameters to take all the important characteristics of the experimental curves into account. On the basis of the model, a perfect reconciliation of the previous contradictory results is presented. It is shown that the kinetic parameters obtained from the initial rate of formation of a product unavoidably leads to misinterpretation of the results in the case of a branching mechanism (and stoichiometry).

Kinetics and mechanism of catalytic decomposition and oxidation of chlorine dioxide by the hypochlorite ion

The oxidation of ClO(2) by OCl(-)is first order with respect to both reactants in the neutral to alkaline pH range: -d[ClO(2)]/dt = 2k(OCl)[ClO(2)][OCl(-)]. The rate constant (T = 298 K, mu = 1.0 M NaClO(4)) and activation parameters are k(OCl) = 0.91 +/- 0.02 M(-1) s(-1), DeltaH = 66.5 +/- 0.9 kJ/mol, and DeltaS(++) = -22.3 +/- 2.9 J/(mol K). In alkaline solution, pH > 9, the primary products of the reaction are the chlorite and chlorate ions and consumption of the hypochlorite ion is not observed. The hypochlorite ion is consumed in increasing amounts, and the production of the chlorite ion ceases when the pH is decreased. The stoichiometry is kinetically controlled, and the reactants/products ratios are determined by the relative rates of the production and consumption of the chlorite ion in the ClO(2)/OCl(-) and HOCl/ClO(2)(-) reactions, respectively.