Competition Kinetics Method Research Articles

OP45 - Sulphur and selenium species as targets and protective species in inflammatory diseases: Why speed matters: understanding key mechanisms of protein modification

Activated leukocytes generate multiple oxidants to kill invading pathogens including O 2 -• , H 2 O 2 , NO • , peroxynitrous acid (ONOOH) and hypohalous acids (HOCl, HOBr, HOSCN) via the activity of NADPH oxidases, nitric oxide synthases (e.g. iNOS) and myeloperoxidase (MPO). Inappropriate generation can however result in tissue damage, with this associated with multiple human inflammatory pathologies (e.g. cardiovascular disease, some cancers, rheumatoid arthritis, asthma, cystic fibrosis, neurodegenerative conditions). Sulphur residues in peptides and proteins are key targets. Competition kinetic methods have yielded rate constants, k, for reaction of HOCl with Cys and GSH of 3.6 x 10 8 and 1.2x10 8 M- 1 s- 1 , respectively. These values are ~10-fold higher than reported previously, and are some of the fastest reactions of non-radical oxidants, emphasizing the key role of thiols in protection. N-Ac-methionine also reacts very rapidly (k 1.7x10 8 M- 1 s- 1 ). ONOOH reacts rapidly with Cys and Met with k ~ 2x10 3 and 2x10 2 M- 1 s- 1 respectively. Selenium compounds should react faster than the sulphur analogues. Seleno-compounds, including novel seleno sugars react with HOCl with k ~10 8 M- 1 s- 1 and ~ 100-fold faster than the sulphur analogues. A similar rate enhancement is seen with ONOOH, with k for ONOOH and selenocysteine ~2x10 5 M- 1 s- 1 . The selenoxides formed on oxidation of these species, can be readily reduced by thiols (e.g. GSH) and some enzyme systems, making these catalytic protective agents. The seleno-sugars decrease oxidant-mediated damage to isolated proteins and human plasma at low concentrations. Examination of these seleno sugars in an animal model of wound healing has shown that topical application markedly enhances wound closure in both normal and diabetic animals. These studies demonstrate that novel selenium-containing molecules may be potent modulators of damage at sites of acute and chronic inflammation, and be beneficial in multiple human pathologies.

Degradation of tyrosol by a novel electro-Fenton process using pyrite as heterogeneous source of iron catalyst

Tyrosol (TY) is one of the most abundant phenolic components of olive oil mill wastewaters. Here, the degradation of synthetic aqueous solutions of 0.30 mM TY was studied by a novel heterogeneous electro-Fenton (EF) process, so-called EF-pyrite, in which pyrite powder was the source of Fe2+ catalyst instead of a soluble iron salt used in classical EF. Experiments were performed with a cell equipped with a boron-doped diamond anode and a carbon-felt cathode, where TY and its products were destroyed by hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between Fe2+ and H2O2 generated at the cathode. Addition of 1.0 g L−1 pyrite provided an easily adjustable pH to 3.0 and an appropriate 0.20 mM Fe2+ to optimize the EF-pyrite treatment. The effect of current on mineralization rate, mineralization current efficiency and specific energy consumption was examined under comparable EF and EF-pyrite conditions. The performance of EF-pyrite was 8.6% superior at 50 mA due to self-regulation of soluble Fe2+ by pyrite. The TY decay in this process followed a pseudo-first-order kinetics. The absolute rate constant for TY hydroxylation was 3.57 × 109 M−1 s−1, as determined by the competition kinetics method. Aromatic products like 3,4-dihydroxyphenylethanol, 4-hydroxyphenylacetic acid, 4-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid and catechol, as well as o-benzoquinone, were identified by GC–MS and reversed-phase HPLC. Short-chain aliphatic carboxylic acids like maleic, glycolic, acetic, oxalic and formic were quantified by ion-exclusion HPLC. Oxalic acid was the major and most persistent product found. Based on detected intermediates, a plausible mineralization pathway for TY by EF-pyrite was proposed.

Nitroxyl (HNO) Reacts with Molecular Oxygen and Forms Peroxynitrite at Physiological pH: BIOLOGICAL IMPLICATIONS

Nitroxyl (HNO), the protonated one-electron reduction product of NO, remains an enigmatic reactive nitrogen species. Its chemical reactivity and biological activity are still not completely understood. HNO donors show biological effects different from NO donors. Although HNO reactivity with molecular oxygen is described in the literature, the product of this reaction has not yet been unambiguously identified. Here we report that the decomposition of HNO donors under aerobic conditions in aqueous solutions at physiological pH leads to the formation of peroxynitrite (ONOO(-)) as a major intermediate. We have specifically detected and quantified ONOO(-) with the aid of boronate probes, e.g. coumarin-7-boronic acid or 4-boronobenzyl derivative of fluorescein methyl ester. In addition to the major phenolic products, peroxynitrite-specific minor products of oxidation of boronate probes were detected under these conditions. Using the competition kinetics method and a set of HNO scavengers, the value of the second order rate constant of the HNO reaction with oxygen (k = 1.8 × 10(4) m(-1) s(-1)) was determined. The rate constant (k = 2 × 10(4) m(-1) s(-1)) was also determined using kinetic simulations. The kinetic parameters of the reactions of HNO with selected thiols, including cysteine, dithiothreitol, N-acetylcysteine, captopril, bovine and human serum albumins, and hydrogen sulfide, are reported. Biological and cardiovascular implications of nitroxyl reactions are discussed.

Electrochemical advanced oxidation for cold incineration of the pharmaceutical ranitidine: Mineralization pathway and toxicity evolution

Ranitidine (RNTD) is a widely prescribed histamine H2-receptor antagonist whose unambiguous presence in water sources appointed it as an emerging pollutant. Here, the degradation of 0.1mM of this drug in aqueous medium was studied by electrochemical advanced oxidation processes (EAOPs) like anodic oxidation with electrogenerated H2O2 and electro-Fenton using Pt/carbon-felt, BDD/carbon-felt and DSA-Ti/RuO2–IrO2/carbon-felt cells. The higher oxidation power of the electro-Fenton process using a BDD anode was demonstrated. The oxidative degradation of RNTD by the electrochemically generated OH radicals obeyed a pseudo-first order kinetics. The absolute rate constant for its hydroxylation reaction was 3.39×109M−1s−1 as determined by the competition kinetics method. Almost complete mineralization of the RNTN solution was reached by using a BDD anode in both anodic oxidation with electrogenerated H2O2 and electro-Fenton processes. Up to 11cyclic intermediates with furan moiety were detected from the degradation of RNTD, which were afterwards oxidized to short-chain carboxylic acids before their mineralization to CO2 and inorganic ions such as NH4+, NO3− and SO42−. Based on identified products, a plausible reaction pathway was proposed for RNTD mineralization. Toxicity assessment by the Microtox® method revealed that some cyclic intermediates are more toxic than the parent molecule. Toxicity was quickly removed following the almost total mineralization of the treated solution. Overall results confirm the effectiveness of EAOPs for the efficient removal of RNTD and its oxidation by-products from water.

Oxidative degradation study on antimicrobial agent ciprofloxacin by electro-fenton process: Kinetics and oxidation products

Oxidative degradation of the antimicrobial agent ciprofloxacin hydrochloride (CIP) has been investigated using electro-Fenton (EF) treatment with a constant current in the range 60–500mA. The process generates highly oxidant species OH in situ via electrochemically monitored Fenton reaction. The EF experiments were performed using cells with a carbon felt cathode and Pt anode. Effect of applied current and catalyst concentration on the kinetics of oxidative degradation and mineralization efficiency have been investigated. Degradation of CIP followed pseudo-first order reaction kinetics. The rate constant of the oxidation of CIP by OH has been determined to be (1.01±0.14) × 1010 M−1 s−1 by using competitive kinetics method. An optimum current of 400mA and a catalyst concentration of Fe2+ at 0.1mM are found to be optimal for an effective degradation of CIP under our operating conditions. A remarkably high degree of mineralization (>94%) was obtained at 6h of treatment under these conditions. A number of stable intermediate products have been identified using HPLC and LC-MS/MS analyses. Based on the identified reaction intermediates, a plausible reaction pathway was proposed for the mineralization process. The high degree of mineralization obtained in this work highlights the potential application of EF process in the efficient removal of fluoroquinolone based drugs in aqueous medium.

Complete mineralization of the antibiotic amoxicillin by electro-Fenton with a BDD anode

On April 30, 2014, the World Health Organization’s first global report on the presence of antibiotics in waters focused on their negative consequences, which may include the growth of microorganisms with antimicrobial resistance. The β-lactam antibiotic amoxicillin (AMX) is widely used in human and veterinary medicine, and it has been recently detected in sewage treatment plants and effluents. In this paper, the degradation of acidic aqueous solutions of AMX by electro-Fenton process has been studied at constant current. Experiments have been performed in an undivided cell equipped with a carbon-felt cathode and a Pt or boron-doped diamond (BDD) anode. In such systems, the organic molecules are mainly oxidized by hydroxyl radical (•OH) simultaneously formed on the anode surface from water oxidation as well as in the bulk from Fenton’s reaction between Fe2+ catalyst and electrogenerated H2O2. The decay and mineralization of AMX was monitored by means of high performance liquid chromatography (HPLC) and TOC measurements. The evolution of the concentration of the final aliphatic carboxylic acids and inorganic ions like ammonium, nitrate and sulfate was assessed by HPLC and ion chromatography, respectively. The effect of the anode material, initial AMX concentration and current density was thoroughly studied. The AMX decay always followed a pseudo-first-order kinetics using either Pt or BDD, and the apparent rate constant increased with applied current. A quicker mineralization was reached with BDD because of the larger production of active •OH. The absolute rate constant between hydroxyl radical and AMX determined by the competition kinetics method using p-hydroxybenzoic acid as the reference compound was found to be (2.02 ± 0.01) × 109 M−1 s−1.

Competitive kinetics study of sulfide oxidation by chlorine using sulfite as reference compound

To design and optimize hydrogen sulfide scrubbers working with chlorine, the knowledge of the kinetics of the hydrogen sulfide oxidation is necessary. In this work, the kinetics of the hydrogen sulfide oxidation by sodium hypochlorite was experimentally investigated using a reactor without headspace (100mL gas-tight syringe) and the competitive kinetics method. The sulfite ion was selected as the reference compound. First, the apparent stoichiometries of sulfite anion and hydrogen sulfide chlorinations were determined performing single-compound experiments. Then, the kinetics of the hydrogen sulfide chlorination was studied in the pH range 6–12 performing simultaneous sulfite and sulfide chlorinations. The results demonstrated that sulfide and sulfite oxidation kinetic rates have the same order of magnitude, which validates the choice of the sulfite anion as the reference compound. Kinetic simulations emphasized that the kinetic rates of the oxidation of both compounds were limited by acid–base reactions. The sulfide oxidation in the pH range 6–12 is mainly due to the hydrosulfide (HS−) oxidation by the hypochlorous acid (ClOH) with an associated kinetic constant of 1.2×109Lmol−1s−1 at 25°C.

Application of the electro-Fenton process to mesotrione aqueous solutions: Kinetics, degradation pathways, mineralization and evolution of the toxicity

<p>The kinetics and the mechanism of degradation of the mesotrione herbicide by three electrochemical advanced oxidation processes (EAOPs) systems, namely electro-Fenton (EF) with Pt anode (EF-Pt), anodic oxidation with BDD anode (AO) and EF-boron doped diamond anode (EF-BDD), were investigated in acidic aqueous solutions. The degradation of mesotrione obeyed apparent first-order reaction kinetics, and its absolute rate constant value with hydroxyl radicals at pH 3.0, determined by the competitive kinetics method, was found to be 8.20 × 10<sup>8</sup> L mol<sup>–1</sup> s<sup>–1</sup>. Fourteen different mesotrione degradation products were separated and/or identified using HPLC / MS<sup>n</sup> analyses. A rationalized scheme was proposed for the reaction pathways of mesotrione degradation in the EF process. The mesotrione mineralization yield values ranged between about 58 and 97%<strong> </strong>for a 6-h electrolysis time, according to the type of EAOP system and the electrolysis current intensity value. The evolution of the toxicity of mesotrione aqueous solutions with electrolysis time (<em>t</em>) was investigated during treatment by the EF-Pt and EF-BDD systems. Toxicity was measured with the Microtox® method, based on the luminescence inhibition of marine bacteria <em>Vibrio fischeri</em>. The curves % inhibition <em>vs.</em> time were found to depend on the toxicity of formed mesotrione degradation products.</p>

Concentration-dependent photodegradation kinetics and hydroxyl-radical oxidation of phenicol antibiotics

Thiamphenicol and florfenicol are two phenicol antibiotics widely used in aquaculture and are ubiquitous as micropollutants in surface waters. The present study investigated their photodegradation kinetics, hydroxyl-radical (OH) oxidation reactivities and products. Firstly, the photolytic kinetics of the phenicols in pure water was studied as a function of initial concentrations (C0) under UV–vis irradiation (λ>200nm). It was found that the kinetics was influenced by C0. A linear plot of the pseudo-first-order rate constant vs C0 was observed with a negative slope. Secondly, the reaction between the phenicol antibiotics and OH was examined with a competition kinetic method under simulated solar irradiation (λ>290nm), which quantified their bimolecular reaction rate constants of (2.13±0.02)×109M−1s−1 and (1.82±0.10)×109M−1s−1 for thiamphenicol and florfenicol, respectively. Then the corresponding OH oxidated half-lives in sunlit surface waters were calculated to be 90.5–106.1h. Some main intermediates were formed from the reaction, which suggested that the two phenicols underwent hydroxylation, oxygenation and dehydrogenation when OH existed. These results are of importance to assess the phenicol persistence in wastewater treatment and sunlit surface waters.

Degradation of anti-inflammatory drug ketoprofen by electro-oxidation: comparison of electro-Fenton and anodic oxidation processes.

The electrochemical degradation of the nonsteroidal anti-inflammatory drug ketoprofen in tap water has been studied using electro-Fenton (EF) and anodic oxidation (AO) processes with platinium (Pt) and boron-doped diamond (BDD) anodes and carbon felt cathode. Fast degradation of the parent drug molecule and its degradation intermediates leading to complete mineralization was achieved by BDD/carbon felt, Pt/carbon felt, and AO with BDD anode. The obtained results showed that oxidative degradation rate of ketoprofen and mineralization of its aqueous solution increased by increasing applied current. Degradation kinetics fitted well to a pseudo-first-order reaction. Absolute rate constant of the oxidation of ketoprofen by electrochemically generated hydroxyl radicals was determined to be (2.8 ± 0.1) × 10(9) M(-1) s(-1) by using competition kinetic method. Several reaction intermediates such as 3-hydroxybenzoic acid, pyrogallol, catechol, benzophenone, benzoic acid, and hydroquinone were identified by high-performance liquid chromatography (HPLC) analyses. The formation, identification, and evolution of short-chain aliphatic carboxylic acids like formic, acetic, oxalic, glycolic, and glyoxylic acids were monitored with ion exclusion chromatography. Based on the identified aromatic/cyclic intermediates and carboxylic acids as end products before mineralization, a plausible mineralization pathway was proposed. The evolution of the toxicity during treatments was also monitored using Microtox method, showing a faster detoxification with higher applied current values.

Electro-oxidation of the dye azure B: kinetics, mechanism, and by-products

In this work, the electrochemical degradation of the dye azure B in aqueous solutions was studied by electrochemical advanced oxidation processes (EAOPs), electro-Fenton, and anodic oxidation processes, using Pt/carbon-felt and boron-doped diamond (BDD)/carbon-felt cells with H₂O₂ electrogeneration. The higher oxidation power of the electro-Fenton (EF) process using BDD anode was demonstrated. The oxidative degradation of azure B by the electrochemically generated hydroxyl radicals ((•)OH) follows a pseudo-first-order kinetics. The apparent rate constants of the oxidation of azure B by (•)OH were measured according to pseudo-first-order kinetic model. The absolute rate constant of azure B hydroxylation reaction was determined by competition kinetics method and found to be 1.19 × 10(9) M(-1) s(-1). It was found that the electrochemical degradation of the dye leads to the formation of aromatic by-products which are then oxidized to aliphatic carboxylic acids before their almost mineralization to CO₂ and inorganic ions (sulfate, nitrate, and ammonium). The evolution of the TOC removal and time course of short-chain carboxylic acids during treatment were also investigated.

A novel photochemical system of ferrous sulfite complex: Kinetics and mechanisms of rapid decolorization of Acid Orange 7 in aqueous solutions

We previously reported the decolorization of the azo dye Acid Orange 7 (AO7) by sulfate radical (SO4−) in the presence of iron(II) sulfite complex and oxygen under UV–vis irradiation (photo-iron(II) sulfite system). This system, however, achieves very limited mineralization of AO7 (in terms of total organic carbon (TOC) removal), which is not in accordance with literature reports on the oxidation of organic contaminants by SO4−. In the present work, kinetics and products under irradiation of xenon lamp (350 W) were analyzed to reveal the reaction pathway of decolorization of AO7. Steady-state approximation (SSA) of SO4− radicals and apparent kinetics of AO7 were examined. The reaction between AO7 and SO4− was found to proceed in two steps, namely, electron transfer and SO4− addition. The second-order rate constant for the reaction between AO7 and SO4− was found to be 8.07 ± 1.07 × 109 M−1 s−1 by SSA and 6.80 ± 0.68 × 109 M−1 s−1 by competition kinetics method. The apparent kinetics of the decolorization of AO7 under irradiation closely fits the mechanism of radical chain reactions of various reactive sulfur species. By liquid chromatography coupled with mass spectrometry, we identified the sulfate adduct AO7–SO4 and confirmed the two-step reaction between AO7 and SO4−. This stable sulfate adduct provides a good explanation of the poor TOC removal during decolorization of AO7 by the photo-iron(II) sulfite system.

Hydroxyl radical ([rad]OH) scavenging in young and mature landfill leachates

The final discharge point for collected landfill leachates is frequently the local municipal wastewater treatment facility. The salinity, color, nutrient, and anthropogenic organics contamination of leachates often necessitate some form of pre-treatment. When advanced oxidation processes (AOPs) are considered for pre-treatment, the unique composition of dissolved organic matter (DOM) and the relatively high concentrations of some inorganic solutes in leachate will inhibit treatment efficiency. The most important benchmark for design of AOPs is the expected steady-state production of free radical (OH). Without a quantitative assessment of total OH consumption in high-strength waste water, like a landfill leachate, efficient AOP treatment is uncertain. For this reason, two landfill leachates, distinct in color, DOM, and age of landfill, were characterized for OH-scavenging using an established competition kinetics method. After stripping the samples of inorganic carbon, the DOM in leachate from mature (stabilized) landfill was found to react with OH at a rate of 9.76 × 108 M−1s−1. However, DOM in leachate from newer landfill was observed to scavenge available OH at a faster rate (8.28 × 109 M−1s−1). The combination of fast rate of reaction with OH and abundance of DOM in the sampled leachate severely limited the contribution of OH to degradation of an O3- and OH-labile organic probe compound (bisphenol-a) in oxidized mature leachate (fOH = 0.03). Substantial dosing of both O3 and H2O2 (>70 mg/L and >24 mg/L, respectively) may be required to see at least 1-log-removal (>90%) of an OH-selective leachate contaminant (i.e., parachlorobenzoic acid) in a mature landfill leachate.

Determination of rapid chlorination rate constants by a stopped-flow spectrophotometric competition kinetics method

Free chlorine is extensively used for water and wastewater disinfection nowadays. However, it still remains a big challenge to determine the rate constants of rapid chlorination reactions although competition kinetics and stopped-flow spectrophotometric (SFS) methods have been employed individually to investigate fast reaction kinetics. In this work, we proposed an SFS competition kinetics method to determine the rapid chlorination rate constants by using a common colorimetric reagent, N,N-diethyl-p-phenylenediamine (DPD), as a reference probe. A kinetic equation was first derived to estimate the reaction rate constant of DPD towards chlorine under a given pH and temperature condition. Then, on that basis, an SFS competition kinetics method was proposed to determine directly the chlorination rate constants of several representative compounds including tetracycline, ammonia, and four α-amino acids. Although Cl2O is more reactive than HOCl, its contribution to the overall chlorination kinetics of the test compounds could be neglected in this study. Finally, the developed method was validated through comparing the experimentally measured chlorination rate constants of the selected compounds with those obtained or calculated from literature and analyzing with Taft's correlation as well. This study demonstrates that the SFS competition kinetics method can measure the chlorination rate constants of a test compound rapidly and accurately.

Electrochemical advanced oxidation of 2-chlorobenzoic acid using BDD or Pt anode and carbon felt cathode

Electrochemical advanced oxidation processes, anodic oxidation and electro-Fenton, have been applied to oxidative degradation of 2-chlorobenzoic acid (2-CBA) in acidic aqueous medium by using Pt or boron-doped-diamond (BDD) anode. Electrolyses were conducted in an open and cylindrical cell with a carbon-felt cathode for H2O2 generation. The main oxidizing species is hydroxyl radical (OH) formed at the BDD surface in both processes and in the bulk from the Fenton’s reaction between initially added and then regenerated Fe2+ (catalyst) and electrogenerated H2O2. Decay of 2-CBA concentration followed a pseudo-first order reaction kinetics. The absolute rate constant of the hydroxylation of 2-CBA was determined by using the competition kinetics method and found to be 2.04×109M−1s−1. The comparative study of TOC removal measurements during electro-Fenton treatment showed a higher mineralization rate with BDD anode than Pt anode due to the higher oxidizing power of the former anode. The chlorine present in 2-CBA is completely released in the form of chloride ion at Pt anode, which is slowly oxidized to Cl2 at the BDD anode. Aromatic oxidation intermediates such as 2-chlorophenol, catechol and benzoquinone are identified by reversed-phase liquid chromatography. Oxalic acid is detected by ion-exclusion-chromatography as the main carboxylic acid. This ultimate end product is finally oxidized into CO2 by hydroxyl radicals. Based on the identified intermediates and inorganic end products, and TOC removal results, a plausible mineralization pathway of 2-CBA by electrogenerated hydroxyl radicals was proposed.

Levofloxacin oxidation by ozone and hydroxyl radicals: Kinetic study, transformation products and toxicity

This work was carried out to investigate the fate of the antibiotic levofloxacin upon oxidation with ozone and hydroxyl radicals. A kinetic study was conducted at 20°C for each oxidant. Ozonation experiments were performed using a competitive kinetic method with carbamazepin as competitor. Significant levofloxacin removal was observed during ozonation and a rate constant value of 6.0×104M−1s−1 was obtained at pH 7.2. An H2O2/UV system was used for the formation of hydroxyl radicals HO. The rate constant of HO was determined in the presence of a high H2O2 concentration. The kinetic expressions yielded a kHO/LVF value of 4.5×109M−1s−1 at pH 6.0 and 5.2×109M−1s−1 at pH 7.2. These results were used to develop a model to predict the efficacy of the ozonation process and pharmaceutical removal was estimated under different ozonation conditions (i.e. oxidant concentrations and contact times). The results showed that levofloxacin was completely degraded by molecular ozone during ozonation of water and that hydroxyl radicals had no effect in real waters conditions.Moreover, LC/MS/MS and toxicity assays using Lumistox test were performed to identify ozonation transformation products. Under these conditions, four transformation products were observed and their chemical structures were proposed. The results showed an increase in toxicity during ozonation, even after degradation of all of the observed transformation products. The formation of other transformation products not identified under our experimental conditions could be responsible for the observed toxicity. These products might be ozone-resistant and more toxic to Vibrio fisheri than levofloxacin.

Dimer radical cation of 4‐thiouracil: a pulse radiolysis and theoretical study

Pulse radiolysis with optical absorption detection has been used to study the reactions of hydroxyl radical (OH•) with 4‐thiouracil (4TU) in aqueous medium. The transient absorption spectrum for the reaction of OH• with 4TU is characterized by λmax 460 nm at pH 7. A second‐order rate constant k(4TU+OH) of 1.7 × 1010 M−1 s−1 is determined via competition kinetics method. The transient is envisaged as a dimer radical cation [4TU]2•+, formed via the reaction of an initially formed radical cation [4TU]•+ with another 4TU. The formation constant of [4TU]2•+ is 1.8 × 104 M−1. The reactions of dibromine radical ion (Br2•−) at pH 7, dichlorine radical ion (Cl2•−) at pH 1, and azide radical (N3•) at pH 7 with 4TU have also produced transient with λmax 460 nm. Density functional theory (DFT) studies at BHandHLYP/6–311 + G(d,p) level in aqueous phase showed that [4TU]2•+ is characterized by a two‐centerthree electron (2c‐3e) [−S∴S−] bond. The interaction energy of [−S∴S−] bond in [4TU]2•+ is −13.01 kcal mol−1. The predicted λmax 457 nm by using the time‐dependent DFT method for [4TU]2•+ is in agreement with experimental λmax. Theoretical calculations also predicted that compared with [4TU]2•+, 4‐thiouridine dimer is more stable, whereas 4‐thiothymine dimer is less stable. Copyright © 2013 John Wiley & Sons, Ltd.

Degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid by TiO2 photocatalysis: Kinetics, photoproducts and comparison to structurally related compounds

The wide occurrence of sunscreen agent micropollutants in natural environment received extensive attention in recent years due to their potential endocrine disrupting effect. The present study focuses on the kinetics and mechanism of photocatalytic degradation of sunscreen agent 2-phenylbenzimidazole-5-sulfonic acid (PBSA) in illuminated TiO2 suspensions. Photocatalysis of PBSA was systematically investigated under different process conditions and water matrices. Experimental results demonstrated that PBSA photocatalytic reactions followed pseudo-first-order kinetics. Radical scavenging experiments indicated that hydroxyl radical (HO) is the predominant reactive species responsible for an appreciable degradation of PBSA. Second-order rate constant of PBSA-HO reaction was determined to be 5.8x109M−1s−1 by competition kinetics method. Major intermediates included hydroxylated products, benzamide, hydroxylated benzamidine, hydroxylated 2-pheny-1H-benzimidazole as well as phenylimidazolecarboxylic derivatives which were elucidated by means of high performance liquid chromatograph–mass spectrometry (HPLC–MS) technique. Four carboxylic acids, oxalic, malonic, acetic and maleic acids, were detected during PBSA photocatalysis by HPLC–UV analysis. Ion chromatography (IC) results revealed that the sulfonic group of PBSA was primarily converted to sulfate ion while nitrogen atoms were released predominantly as ammonium and to a lesser extent as nitrate. The reduction of TOC processed much more slowly compared to PBSA degradation, however, approximately 80% TOC was removed after 720min irradiation. A comparison of photocatalytic degradation of PBSA and structurally related compounds revealed that the 5-sulfonic moiety in PBSA had negligible effect on the photocatalysis of 2-pheny-1H-benzimidazole while 2-phenyl substituent stabilized the benzimidazole ring system to photocatalytic degradation.

Electrochemical degradation of the antibiotic sulfachloropyridazine by hydroxyl radicals generated at a BDD anode

The treatment of aqueous solutions of the antibiotic sulfachloropyridazine (SCP) was carried out at the natural pH of the solution (pH 4.5) with hydroxyl radicals (OH) generated at a BDD anode surface by electro-oxidation using an undivided electrochemical cell equipped with a three-dimensional carbon-felt cathode. Hydroxyl radicals are powerful oxidants and react with the antibiotic leading to its overall mineralization. The kinetic study showed that oxidative degradation of SCP follows pseudo first-order reaction kinetics, with a relatively short degradation time. The degree of mineralization of SCP solutions increased with the applied current, being higher than 95% after 8h of electrolysis at 350mA or higher current. To determine the degradation pathway upon the action of hydroxyl radicals, the cyclic and aliphatic by-products, as well as the released inorganic ions, were identified and quantified over electrolysis time. The values of the rate constants of reactions between OH and the SCP and its intermediates were determined by the competition kinetics method using p-hydroxybenzoic acid. The absolute rate constant for the OH-mediated degradation of SCP was found to be 1.92×109M−1s−1. Toxicity assessment by the Microtox® method during the electro-oxidation of SCP solutions revealed the formation of compounds that can be more toxic than the parent molecule, but the overall results confirm the effectiveness of this electrochemical process for the removal of the antibiotic SCP and its by-products from aqueous media.

Complete removal of the insecticide azinphos-methyl from water by the electro-Fenton method – A kinetic and mechanistic study

The removal of organophosphorous insecticide azinphos-methyl (AZPM) from water has been investigated by the electro-Fenton method which produces hydroxyl radicals electro-catalytically. The reaction between these radicals and AZPM has led to the oxidation of AZPM up to carbon dioxide and inorganic ions indicating its complete removal from water. The oxidation kinetics was fitted to pseudo-first order reaction and absolute rate constant of the second order reaction kinetic was determined as (6.82 ± 0.18) × 109 M−1 s−1 by using competitive kinetics method. The oxidation of AZPM by hydroxyl radicals leads to the formation of different intermediates species; such as aromatic derivatives, short-chain carboxylic acids and inorganic ions as end products. The identification and quantification of these intermediates were deeply investigated by HPLC, GC–MS and ion chromatography analyses. Based on the identified intermediates, a general oxidation mechanism was proposed. Mineralization ability of the process was also tested by using aqueous AZPM solutions and its commercial formulation, Gusathion M WP 25 (GMWP25). Mineralization efficiency was followed by the chemical oxygen demand (COD) analysis of treated solutions. The overall mineralization of synthetic AZPM solution and that of commercial formulation at the end of treatment highlights the outstanding mineralization power of the electro-Fenton process.