Étude comparative de la vitesse de décomposition de H2O2 et de l'atrazine par les systèmes Fe(III)/H2O2, Cu(II)/H2O2 et Fe(III)/Cu(II)/H2O2

Cette étude a eu pour objectif de comparer les vitesses de décomposition du peroxyde d'hydrogène et d'oxydation de l'atrazine par les systèmes catalytiques Fe(III)/H2O2, Cu(II)/H2O2, et Fe(III)/Cu(II)/H2O2. Les expériences ont été réalisées à pH 3,0, à une température de 25,0 (± 0,2) °C, en milieu perchlorate, en présence et en absence d'oxygène dissous. L'étude comparative a confirmé que les vitesses de décomposition de H2O2 et d'oxydation de l'atrazine sont beaucoup plus lentes en présence de Cu(II) qu'en présence de Fe(III) et l'addition de Cu(II) augmente l'efficacité du système Fe(III)/H2O2. Pour nos conditions expérimentales ([composé organique]o < 1 µM) les expériences de cinétique compétitive, réalisées avec des solutions aqueuses contenant trois composés organiques (atrazine, 1,2,4-trichlorobenzène, 2,5-dichloronitrobenzène), ont montré que le radical hydroxyle représente la principale espèce responsable de l'oxydation des composés organiques. Les résultats ont également mis en évidence la formation très rapide d'un composé entre Cu(II) et H2O2 (étude spectrophotométrique) et ont montré l'importance de la concentration en oxygène dissous sur les vitesses globales de décomposition de H2O2 et de l'atrazine par les systèmes Cu(II)/H2O2 et Fe(III)/Cu(II)/H2O2.Toxic and refractory organic pollutants in industrial wastewater can be degraded by advanced oxidation processes (AOPs) alone, or in combination with physico-chemical and biological processes. Of these oxidation methods, Fenton's reagent (Fe(II)/H2O2) and Fenton-like reagents (Fe(III)/H2O2, Mn+ or Mn+1 /H2O2) are effective oxidants of large variety of organic pollutants.The mechanism of decomposition of H2O2 and of oxidation of organic solutes by Fenton's and Fenton-like reactions has been the subject of numerous studies. However, there are still many uncertainties as to the nature of the oxidant species formed and the rate constants of elementary reactions (Table 1).Our recent studies carried out in HClO4 /NaClO4 solutions and in the presence of very low concentrations of organic solutes (atrazine, 1,2,4-trichlorobenzene; concentration < 3 µM) have shown that the reaction of Fe(II) with H2O2 leads to the formation of two intermediates and that the overall initiation step (reaction 1, Table 1) at pH < 3.5 leads to the formation of OH radical (Gallard et al., 1998a). Other work with different organic compounds and higher concentrations of organic solutes indicates that the intermediates (Fe(II)-hydroperoxy complexes, ferrous ion) might also oxidize organic compounds. Ferric ion can also catalyze the decomposition of H2O2. The mechanism is initiated by the formation of two Fe(III)-peroxy complexes at pH < 3.5 (reaction 2a, Table 1) followed by their slow decomposition into Fe(II) and HO2·/O2·- (reaction 2b, Table 1) (Gallard et al., 1999; De Laat and Gallard, 1999; Gallard and De Laat, 1999).The formation of intermediates (complexes, cupryl ion) has also been postulated for the catalytic decomposition of H2O2 by Cu(II). Depending on the experimental conditions (nature and concentrations of organic solutes, pH,…), the degradation of organic compounds might be attributed to the hydroxyl radical (reaction 1, Table 1) or to other species like the cupryl ion (Cu(III)). Production of Cu(III) by reaction of OH· with Cu(II) has also been demonstrated by pulse radiolysis experiments. Kinetic data indicate that the rate of decomposition of H2O2 and the rate of oxidation of organic compounds are faster with Fe(III)/H2O2 than with Cu(II)/H2O2 and that Cu(II) can improve the efficiency of the Fe(III)/H2O2 process.The present study has been undertaken in order to compare the rates of decomposition of H2O2 and the rates of oxidation of atrazine by Fe(III)/H2O2, Cu(II)/H2O2 and Fe(III)/Cu(II)/H2O2 under identical conditions. These conditions (pH 3.0, I=0.1 M, [Atrazine]o < 1 µM) were the same as those used in previous studies of the Fe(II)/H2O2 and Fe(III)/H2O2 systems.Experiments were carried out in MilliQ water, in the dark, at 25.0 (± 0.2) °C, pH 3.0, ionic strength (I) of 0.1 M, in the presence and in the absence of dissolved oxygen. pH and I were adjusted with perchloric acid and sodium perchlorate. The concentrations of hydrogen peroxide ([H2O2]o ≤ 10 mM) and of atrazine ([atrazine]o ≤ 1 µM) were determined iodometrically and by HPLC, respectively.In the absence of organic solutes, experimental results have shown that the rate of decomposition of H2O2 is faster with Fe(III) than with Cu(II) (Figure 2). In agreement with previous data (De Laat and Gallard, 1999), the initial rate of decomposition of H2O2 by Fe(III) can be described by a pseudo first-order kinetic law with respect to H2O2, and dissolved oxygen (0-1 mM) has no effect on the rate of decomposition. For the Cu(II)/H2O2 system, our spectrophotometric data (Figure 1) gave evidence that the decomposition of H2O2 by Cu(II) goes through the formation of an intermediate which might be a Cu(II)-hydroperoxy complex and which absorbs in the region 350-600 nm. Furthermore, the rate of decomposition of H2O2 by Cu(II) does not follow a first-order kinetic law and is affected by the concentration of dissolved oxygen (Figures 2 et 3).As far as the oxidation of atrazine is concerned, a preliminary study of the oxidation of solutions containing atrazine, 1,2,4 trichlorobenzene and 2,5 dichloronitrobenzene in very dilute aqueous solutions ([organic solutes]o < 3 µM) has been conducted at pH 3.0. Experimental results showed that the relative rates of decomposition of organic solutes by Fe(III)/H2O2, Fe(II)/H2O2 and Cu(II)/H2O2 were identical and could be described by the competitive kinetic expression (Figure 4). These data suggest that the oxidation of the organic solutes by the three systems of oxidation tested can be attributed to a unique oxidant species, the hydroxyl radical, under our experimental conditions.The rate of oxidation of atrazine by Cu(II)/H2O2 was found to be much slower than by Fe(III)/H2O2 (Figure 5), to be dependent on the concentrations of reactants ([Cu(II)]o, [H2O2]o Figure 6) and to decrease in the presence of dissolved oxygen (Figure 7). These data confirm that the rate of decomposition of H2O2 by Cu(II), and as a consequence, the rate of production of OH radicals by Cu(II)/H2O2, are much slower than by Fe(III)/H2O2. In addition, a fraction of Cu(I) may be oxidized by dissolved oxygen and this reaction, which competes with the reaction of Cu(I) with H2O2, may also decrease the rate of formation of OH radical.For the Fe(III)/Cu(II)/H2O2 system, experimental data have demonstrated that the addition of Cu(II) increases the rate of decomposition of H2O2 (Figure 8a) and atrazine (Figure 8b) by Fe(III)/H2O2 and that these increases in reaction rates depend on the concentration of dissolved oxygen. This catalytic effect of Cu(II) has been attributed to a fast regeneration of Fe(II) (which is the major source of OH radical) by the reaction of Cu(I) with Fe(III). Since this reaction competes with oxidation of Cu(I) by O2 and H2O2, the catalytic properties of Fe(III) and Cu(II) mixtures will depend on the experimental conditions, such as the relative concentrations of reactants. In conclusion, this comparative study has confirmed that the rates of decomposition of H2O2 and atrazine, in dilute aqueous solution, by Fe(III)/Cu(II)/H2O2 are faster than by Fe(III)/H2O2 and Cu(II)/H2O2. This study has also demonstrated that dissolved oxygen has a significant effect on the reaction rates in the Cu(II)/H2O2 and Fe(III)/Cu(II)/H2O2.oxidation systems. The effects of dissolved oxygen and of the addition of Cu(II) on the efficiency of the Fe(III)/H2O2 system could be explained by assuming that the OH radical is the major oxidant species under our experimental conditions. However, additional research is needed in order to better understand the mechanism of decomposition of H2O2 by Cu(II) and Cu(I) and to determine the rate constants of individual reactions involved in the Cu(II)/H2O2 and Cu(I)/H2O2 systems

Formation of methyl iodide on a natural manganese oxide

This paper demonstrates that manganese oxides can initiate the formation of methyl iodide, a volatile compound that participates to the input of iodine into the atmosphere. The formation of methyl iodide was investigated using a natural manganese oxide in batch experiments for different conditions and concentrations of iodide, natural organic matter(NOM) and manganese oxide. Methyl iodide was formed at concentrations ≤1 μg L-1 for initial iodide concentrations ranging from 0.8 to 38.0 mg L-1. The production of methyl iodide increased with increasing initial concentrations of iodide ion and Mn sand and when pH decreased from 7 to 5. The hydrophilic NOM isolate exhibited the lowest yield of methyl iodide whereas hydrophobic NOM isolates such as Suwannee River HPOA fraction produced the highest concentration of methyl iodide. The formation of methyl iodide could take place through the oxidation of NOM on manganese dioxide in the presence of iodide. However, the implication of elemental iodine cannot be excluded at acidic pH. Manganese oxides can then participate with ferric oxides to the formation of methyl iodide in soils and sediments. The formation of methyl iodide is unlikely in technical systems such as drinking water treatment i.e. for ppt levels of iodide and low contact times with manganese oxides

Oxidation of iodide and iodine on birnessite (δ-MnO2) in the pH range 4-8

The oxidation of iodide by synthetic birnessite (δ-MnO2) was studied in perchlorate mediain the pH range 4-8. Iodine (I2) was detected as an oxidation product that was subsequently further oxidized to iodate (IO3). The third order rate constants, second order on iodide and first order on manganese oxide, determined by extraction of iodine in benzene decreased with increasing pH (6.3-7.5) from 1790 to 3.1 M2 s1. Both iodine and iodate were found to adsorb significantly on birnessite with an adsorption capacity of 12.7 mM/g for iodate at pH5.7. The rate of iodine oxidation by birnessite decreased with increasing ionic strength, which resulted in a lower rate of iodate formation. The production of iodine in iodide-containing waters in contact with manganese oxides may result in the formation of undesired iodinated organic compounds (taste and odor, toxicity) in natural and technical systems. The probability of the formation of such compounds is highest in the pH range 5-7.5. For pH 7.5, iodide is not oxidized to a significant extent

Mind the Gap: Persistent and Mobile Organic Compounds—Water Contaminants That Slip Through

The discharge of persistent and mobile organic chemicals (PMOCs) into the aquatic environment is a threat to the quality of our water resources. PMOCs are highly polar (mobile in water) and can pass through wastewater treatment plants, subsurface environments and potentially also drinking water treatment processes. While a few such compounds are known, we infer that their number is actually much larger. This Feature highlights the issue of PMOCs from an environmental perspective and assesses the gaps that appear to exist in terms of analysis, monitoring, water treatment and regulation. On this basis we elaborate strategies on how to narrow these gaps with the intention to better protect our water resources

Global patient outcomes after elective surgery: prospective cohort study in 27 low-, middle- and high-income countries.

BACKGROUND: As global initiatives increase patient access to surgical treatments, there remains a need to understand the adverse effects of surgery and define appropriate levels of perioperative care. METHODS: We designed a prospective international 7-day cohort study of outcomes following elective adult inpatient surgery in 27 countries. The primary outcome was in-hospital complications. Secondary outcomes were death following a complication (failure to rescue) and death in hospital. Process measures were admission to critical care immediately after surgery or to treat a complication and duration of hospital stay. A single definition of critical care was used for all countries. RESULTS: A total of 474 hospitals in 19 high-, 7 middle- and 1 low-income country were included in the primary analysis. Data included 44 814 patients with a median hospital stay of 4 (range 2-7) days. A total of 7508 patients (16.8%) developed one or more postoperative complication and 207 died (0.5%). The overall mortality among patients who developed complications was 2.8%. Mortality following complications ranged from 2.4% for pulmonary embolism to 43.9% for cardiac arrest. A total of 4360 (9.7%) patients were admitted to a critical care unit as routine immediately after surgery, of whom 2198 (50.4%) developed a complication, with 105 (2.4%) deaths. A total of 1233 patients (16.4%) were admitted to a critical care unit to treat complications, with 119 (9.7%) deaths. Despite lower baseline risk, outcomes were similar in low- and middle-income compared with high-income countries. CONCLUSIONS: Poor patient outcomes are common after inpatient surgery. Global initiatives to increase access to surgical treatments should also address the need for safe perioperative care. STUDY REGISTRATION: ISRCTN5181700

Women from the Middle East and North Africa in Europe: Understanding their marriage and family dynamics

The aim of this article is to assist the understanding of social workers in Europe of marriage and family dynamics among women from Middle East and North African countries who have moved to Europe. The focus of this article is on husband selection processes and family dynamics after marriage in Egypt, which is used as a case study reflecting culture and norms surrounding marriage in this region. This article reports on the findings of doctoral studies which examined marriage patterns and family dynamics in North Africa and in particular in Egypt where more in-depth data were available. The authors reflect issues surrounding values and process of marriage not only in terms of the implications for practice with social work clients or service users, but also in relation to the potential of women from this region who may join the social care workforce

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 Sigmasigma) and the acid-catalyzed reaction of HOCl with phenol (log(k) = 2.37-4.26 x Sigmasigma). 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

Chlorination of natural organic matter: kinetics of chlorination and of THM formation

The kinetics of the formation of trihalomethanes (THMs) and of chlorine consumption for the chlorination of natural organic matter with an excess of chlorine (50 muM > [Cl-2](o) > 210 muM) was investigated. THM precursors could be divided into a fast and a slowly reacting fraction. Long term chlorine demand and the formation of THM could be described by second order kinetics. Rate constants were between 0.01 and 0.03 M-1 s(-1) in the pH range 7-9 for surface waters and humic materials extracted from surface waters. A groundwater gave a higher rate constant of 0.124 M-1 s(-1). Resorcinol-type structures were tested with respect to kinetics and yield of THM formation. They could possibly be responsible for the fast reacting THM precursors, which represent 15-30% of the THM precursors of natural waters. Additional classes of compounds that might contribute to the initial THM formation include readily enolizable compounds such as beta -diketones and beta -ketoacids. Experiments with phenol showed that slowly reacting THM precursors may consist of phenolic compounds. The influence of pretreatments (UV/visible irradiation, ozone and chlorine dioxide) on chlorine demand and THM formation from NOM was also studied: UV/visible irradiation does not alter THM formation but leads to a higher chlorine demand. Preoxidation with ozone leads to a lower THM formation with an unaltered chlorine demand and preoxidation with chlorine dioxide reduces THM formation and the chlorine demand. (C) 2001 Elsevier Science Ltd. All rights reserved