Mechanism Involving Hydrogen Sulfite Ions, Chlorite Ions, and Hypochlorous Acid as Key Intermediates of the Autocatalytic Chlorine Dioxide–Thiourea Dioxide Reaction

Molecular mechanism of the metal-independent production of hydroxyl radicals by thiourea dioxide and H2O2

It is well-known that highly reactive hydroxyl radicals (HO•) can be produced by the classic Fenton system and our recently discovered haloquinone/H2O2 system, but rarely from thiol-derivatives. Here, we found, unexpectedly, that HO• can be generated from H2O2 and thiourea dioxide (TUO2), a widely used and environmentally friendly bleaching agent. A carbon-centered radical and sulfite were detected and identified as the transient intermediates, and urea and sulfate as the final products, with the complementary application of electron spin-trapping, oxygen-18 isotope labeling coupled with HPLC/MS analysis. Density functional theory calculations were conducted to further elucidate the detailed pathways for HO• production. Taken together, we proposed that the molecular mechanism for HO• generation by TUO2/H2O2: TUO2 tautomerizes from sulfinic acid into ketone isomer (TUO2-K) through proton transfer, then a nucleophilic addition of H2O2 on the S atom of TUO2-K, forming a S-hydroperoxide intermediate TUO2-OOH, which dissociates homolytically to produce HO•. Our findings represent the first experimental and computational study on an unprecedented new molecular mechanism of HO• production from simple thiol-derived sulfinic acids, which may have broad chemical, environmental, and biomedical significance for future research on the application of the well-known bleaching agent and its analogs.

Chlorate induced false reduction in chemical oxygen demand (COD) based on standard dichromate method: Countermeasure and mechanism

Deliberate addition of mildly oxidative chlorate (ClO₃^-), so-called "chemical oxygen demand (COD) remover", into wastewater in China or electrochemical production of ClO₃^- from Cl^- induces the false COD reduction, which would bring about false appearance of effluents meeting the COD discharge standards. In this study, an easy sulfite-based reduction method was developed for the first time to remove ClO₃^- from the water samples before COD determination to eliminate this interference of ClO₃^-. In this reaction system, keeping the reaction temperature of sulfite reducing ClO₃^- at 60 ^°C was crucial for fast ClO₃^- removal rate, fixed molar [sulfite]_ini/[chlorate]_ini ratio value and the synchronous exhaustion of sulfite and ClO₃^-, which were of great significance for the real application of this improved COD determination method. The ClO₃^- interference on COD determination could be successfully eliminated after 20 min reduction of ClO₃^- by sulfite at pH_ini 4.0∼6.0 with the molar [sulfite]_ini/[chlorate]_ini ratio value in the range of 5∼6 when concentration of ClO₃^- was below 5 mM. Despite of the involvement of SO₄^·- in the sulfite reducing ClO₃^- system, the degradation of organic matters by SO₄^·- could be greatly impeded due to the lessened dissolved oxygen for SO₄^·- production at high reaction temperature and the scavenging of SO₄^·- by sulfite. In this reaction system, ClO₂, ClO₂^- and ClO^- were also generated and could be further reduced by sulfite stoichiometrically via oxygen transfer process with Cl^- as the final product. In general, this study pioneered an effective, fast and convenient method for COD determination of the ClO₃^--laden wastewaters and evaluating the real electrochemical wastewater treatment performance in terms of COD removal.

On the Kinetics and Mechanism of the Thiourea Dioxide-Periodate Autocatalysis-Driven Iodine-Clock Reaction

The thiourea dioxide-periodate reaction has been investigated under acidic conditions using phosphate buffer within the pH range of 1.1-2.0 at 1.0 M ionic strength adjusted by sodium perchlorate. Absorbance-time series are monitored as a function of time at 468 nm, the isosbestic point of the I₂-I₃^- system. The profile of these kinetic runs follows either sigmoidal-shaped or rise-and-fall traces depending on the initial concentration ratio of the reactants. The clock species iodine appears after a well-defined but reproducible time lag even in substrate excess, meaning that the system may be classified as an autocatalysis-driven clock reaction. It is also demonstrated that the age of the thiourea dioxide solution markedly shortens the Landolt time, suggesting that the original form of thiourea dioxide (TDO) rearranges into a more reactive form and reacts faster than the original one. The behavior found is consistent with that recently observed in other oxidation reactions of TDO. To characterize the system quantitatively, a 22-step kinetic model is constructed from adapting the kinetic model of the TDO-iodate reaction published recently by supplementing it with six different reactions of periodate. By the help of seven fitted rate coefficients a sound agreement between the measured and calculated absorbance-time traces is obtained.

Exact Concentration Dependence of the Landolt Time in the Thiourea Dioxide-Bromate Substrate-Depletive Clock Reaction

The thiourea dioxide (TDO)-bromate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen-phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The title system shows a remarkable resemblance to the classical Landolt reaction, namely, the clock species (bromine) may only appear after the substrate TDO is completely consumed. Thus, the title system can be classified as substrate-depletive clock reaction. Despite the well-known slow rearrangement characteristic of TDO in acidic solution, it is surprisingly found that the Landolt time of the title reaction does not depend at all on the age of TDO solution applied. It is, however, shown experimentally that the inverse of Landolt time linearly depends on the initial bromate concentration as well as on the square of the hydrogen ion concentration. In addition to this, it is also noticed that dihydrogen phosphate markedly affects the Landolt time as well, and this feature may easily be taken into consideration by the H₂PO₄^- dependence of the rate of bromate-bromide reaction quantitatively. Based on the experiments, a simple three-step kinetic model is proposed from which a complex formula is derived to indicate the exact concentration dependence of the Landolt time.

Autocatalysis-Driven Clock Reaction III: Clarifying the Kinetics and Mechanism of the Thiourea Dioxide-Iodate Reaction in an Acidic Medium

The thiourea dioxide-iodate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid-dihydrogen phosphate buffer within the pH range of 1.1-1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The system was found to exhibit clock behavior, having a well-defined and reproducible time lag called Landolt time, though elementary iodine may even be detected in substrate excess; hence, under these conditions, the reaction can be classified as an autocatalysis-driven clock reaction. It is clearly demonstrated that the previously proposed kinetic model suffers from serious drawbacks from both theoretical and experimental points of view. The reaction may be characterized by either sigmoidal-shaped or rise-and-fall kinetic traces, depending on the initial concentration ratio of the reactants. Iodide significantly accelerates the appearance of the clock species iodine acting therefore as an autocatalyst. The age of stock TDO solution also has a great, so far completely overlooked impact on the Landolt time. On the basis of evaluating simultaneously the kinetic curves, a 16 step kinetic model including 5 well-known rapidly established equilibria is proposed with 7 fitted rate coefficients in which the rate coefficients of both forms of TDO were determined.

Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by Iodine in a Slightly Acidic Medium

The thiourea dioxide (TDO)-iodine reaction was investigated spectrophotometrically monitoring the consumption of total amount of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered slightly acidic medium. The nitrogen- and carbon-containing products were found to be ammonium ion and dissolved carbon dioxide, respectively, while from sulfur part sulfate ion was exclusively detected, when fresh TDO solution was used. The stoichiometry of the reaction was established as 2I₂ + TDO + 4H₂O → SO₄^2- + 2NH₄⁺ + 4I^- + CO₂ + 4H⁺ indicating a strict 2:1 stoichiometric ratio. However, using aged TDO solution this stoichiometric ratio is shifted to lower values suggesting the formation of elementary sulfur augmented by the 2TDO + I₂ + 4H₂O → S + SO₄^2- + 4NH₄⁺ + 2I^- + 2CO₂ hypothetical limiting stoichiometry. We also confirmed experimentally that in aqueous solution TDO slowly rearranges into an unindentified species. This species then produces elementary sulfur at a later stage of the aging process via subsequent reactions accounting for a loss of reducing power. The direct reaction between TDO and iodine was found to be relatively rapid and completed within seconds in absence of initially added iodide ion. Formation of the latter ion, however, strongly inhibits the oxidation process; hence, the system is autoinhibitory with respect to iodide ion. Furthermore, increase of pH markedly accelerates the reaction as well. These observations suggest that a short-lived steady-state intermediate (iodinated TDO) is produced in a rapid pre-equilibrium, where iodide and hydrogen ions are also involved. A nine-step kinetic model, to be able to describe the most important characteristics of the experimental curves with four fitted parameters, is proposed and discussed.

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-.