M. Dore

Conditions optimales d'application du système oxydant ozone-peroxyde d'hydrogène

Among the oxidation treatments used in potable water production, none allows significant removal of organic matter of natural and human origin under acceptable economic conditions. Ozone, having the highest normal oxido-reduction potential of all, shows only minor Total Organic Carbon removal. It has no effect on saturated chlorinated solvents found at abnormally high levels in some natural waters before and even after a classic treatment of purification. With an oxidation treatment, the only way to obtain high removal of ozone refractory compounds is to generate very highly reactive but poorly selective radical species like the hydroxyl radical (Hoigne, 1979). On means to produce those radicals in the aqueous phase is to combine two oxidants (Prengle, 1977; Nakayama, 1979; Hango, 1981). This work deals with the ozone-hydrogen peroxide (O3/H2O2) system. Initially, the optimal conditions for applying the O3/H2O2 system were sought using oxalic acid. The effect of the pH, the H2O2 concentration, the concentration and the nature of the organic compound, the radical traps (HCO3) and the mode of injection of the H2O2 on the oxidation velocities and yields are highlighted. In a closed reactor with oxalic acid and 1,1,2 trichloroethane the oxidation velocity is faster with a pH of 7.5 (Figs 4 and 5), an initial H2O2 concentration of 0.6-0.7·10−4 M (Figs 8 and 9) and a consumption of 0.5 mol H2O2 mol−1 of ozone introduced. In an open reactor, optimal conditions with oxalic acid are obtained when the H2O2 is injected by impulse at a rate of 0.6-0.7·10−4M so that 0.5 mol H2O2 are consumed per mol of O3 injected (Figs 11 and 12). The bicarbonates (Fig. 10) cause a sharp drop in the yield and velocity of the oxidation of the oxalic acid and the trichloroethane. Secondly, the O3/H2O2 system is next applied to oxidizing other organochlorinated compounds that are refractory to ozonation, in the same optimal conditions as defined above. It leads to the degradation of the pentachloroethane (Fig. 14) but does not oxidize the carbon tetrachloride (Fig. 15) and hexachloroethane. The type of organic compound to be oxidized, its concentration and the concentration of inhibiting mineral compounds (bicarbonates) do not seem to modify the above defined optimal conditions but do influence the yield and the oxidation velocity of the system. The results are discussed using a reaction scheme (Fig. 16) draw up in accordance with the data gathered from the bibliography. The radical mechanism leading to the degradation of organo-chlorinated compounds cannot be initiated by the cleavage of the CCl bond. It is probable that the CH bond is the first site on which the hydroxyl radicals react and start the radical oxidation mechanism. This latter hypothesis remains to be verified.

Ozone and hydroxyl radicals induced oxidation of glycine

The study of the oxidation of glycine by ozone and hydroxyl radicals has been undertaken. Different experimental procedures involving the H2O2/UV process or ozone in the presence of hydrogen peroxide on one hand, and ozone alone on the other hand allowed to distinguish between the two oxidation pathways. It has been shown that nitrites and nitrates are observed byproducts resulting from molecular ozonation whereas ammonium ions constitute the only inorganic nitrogen species from hydroxyl radicals induced decomposition of glycine. Therefore, the ozonation process promoting radical pathways will favor ammonia production vs. nitrates. Among the organic compounds resulting from oxidation by hydroxyl radicals, oxalic acid was the major byproduct detected in the presence of oxygen. Oxamic acid and formic acid were also observed. In addition, the nitrogen mass balance indicated the formation of unidentified nitrogen byproducts derived from glycine. These nitrogen compounds could result from the interaction of oxygen with nitrogen radical, but, mainly, with α-amino radical of glycine. This reaction is very different from the molecular ozonation which directs the attack on the nitrogen functional group before the C–N bond cleavage of the amino acid molecule.

Mecanisme d'action des radicaux OH sur les acides glycolique, glyoxylique, acetique et oxalique en solution aqueuse: Incidence sur la consammation de peroxyde d'hydrogene dans les systemes H2O2UV et O3H2O2

The aim of this work is to study the mechanism of the reaction of hydroxyl radicals with simple aliphatic acids (acetic, glycolic, glyoxylic and oxalic acids; see Table 1) in aqueous solution. The decomposition of organic acids by hydroxyl radicals has been studied using the photolysis of hydrogen peroxide and the ozone/hydrogen peroxide system.

Ozonation of Aqueous Solutions of Nitrogen Heterocyclic Compounds : Benzotriazoles, Atrazine and Amitrole

Abstract Study of the ozonation of aqueous solutions of four heterocyclic nitrogen compounds, at slightly acidic pH in a heterogeneous gas-liquid system, showed that the reactivities of these compounds are different. Amitrole, a five-membered ring heterocycle, has been found to be a highly reactive compound, while atrazine, a six-membered ring heterocycle, has been found to be a poorly reactive compound. The benzotriazoles, benz-fused five-membered ring heterocycles, present intermediate reactivities. Moreovoer, the chemical natures of the ozonation by-products have been found to be different between amitrole and atrazine. With atrazine, ozone did not open the heterocyclic ring, and led to the formation of a trioxotriazine. With amitrole, ozone broke the heterocyclic ring and formed mainly formamide.

Oxydation des phenols par le peroxyde d'hydrogene en milieu aqueux en presence de fer supporte sur alumine

The aim of this work was to study, in order to specify its reactional mechanism, the oxidation by hydrogen peroxide of phenols in an aqueous medium in the presence of various heterogeneous catalysts and in particular alumina supported iron. In previous works we studied the action of the Fenton reactant (H2O2 + Fe2+) on phenols and on organic acids and compared this system to the catalytic oxidation on a solid catalyst (iron/alumina, iron-copper/alumina) by hydrogen peroxide. The structure of the supported metal was determined by Mossbauer Spectroscopy thanks to which we were able to establish on the one hand the relationships between the structure and the mode of preparation of the catalyst and on the other the modifications of the catalyst surfaces after an oxidation reaction of organic compounds in an aqueous medium. The results of this work showed that the catalytic oxidation of phenol is very weak. It depends on the mode of preparation of the catalyst and the nature of the supported metals, the mode of thermic treatment of the catalyst and the reaction temperature and, above all, on the presence of polyhydroxybenzenes at the beginning of the reaction. Contrarily to phenol, polyhydroxybenzenes are readily degraded in heterogeneous catalysis by hydrogen peroxide. The reaction rate is a function on the one hand of the catalytic properties and on the other of the reaction conditions (pH, temperature, presence of bicarbonates in the solution …). In general the reaction rate in heterogeneous catalysis always seems to be a function of the degree of hydroxylation of the organic compounds in contact with the catalyst in the presence of H2O2. Two types of oxidation mechanisms in heterogeneous catalysis can be envisaged: • -radical mechanism: the radicals formed by the decomposition of H2O2 on the active sites of the catalyst react with the organic compounds; • -non-radical mechanism (Hamiltons reaction). The radical pathway is the result of reaction of hydroxyl radicals with organic compounds. The resulting by-products of this first attack (mainly carbonyl compounds) are subjected to a second radical oxidation or play the part of reductor of oxidized sites. As for the non-radical mechanism, many pathways can occur and among them are the: • -reaction between oxidizing entities and adsorbed organic molecules on the catalyst area; • -oxidation reaction by an oxidative complex of pyrocatecol or hydroquinone (Hamilton reaction); • -formation of inorganic peroxo-compounds adsorbed to the catalyst area. The proposed reactional scheme allows us to explain the whole of the obtained results and particularly the difference in the catalytic activity between phenol and dihydroxybenzene.

Experimental and theoretical studies of the mechanism of the initial attack of ozone on some aromatics in aqueous medium

Abstract An experimental study showing the selectivity of the direct reaction of ozone on some aromatics in aqueous medium is presented. The mechanism of the initial attack of ozone on phenol, benzaldehyde and acetophenone is discussed. Molecular orbital calculations are used to study the possible reaction paths for the ozonation of phenol. The changes in relative reactivity of phenol, benzaldehyde and acetophenone are discussed in terms of charges and frontier orbitals.

Oxidation of Parachloronitrobenzene in Dilute Aqueous Solution by O3 + UV and H2O2 + UV : A Comparative Study

Abstract This laboratory study was designed to investigate the degradation of 4-chloronitrobenzene ([CNB] = 2.4 × 10−6 mol L−1; pH = 7.5) by H2O2/UV and by O3/UV oxidation processes which involve the generation of very reactive and oxidizing hydroxyl free radicals. The effects of the oxidant doses (H2O2 or aqueous O3), liquid flow rate (or the contact time), and bicarbonate ions acting as OH· radical scavengers on the CNB removal rates were studied. For a constant oxidant dose, the results show that the O3/UV system appears to be more efficient than the H2O2/UV system to remove CNB because of the greatest rate of OH· generation by ozone photodecomposition compared to H2O2 photolysis. However, for a given amount of oxidant decomposed, the H2O2/UV oxidant system was found to be more efficient than O3/UV. Moreover, high levels of bicarbonate ions in solution (4 × 10−3 mol L−1) significantly decrease the efficiency of CNB removal by H2O2/UV and by O3/UV oxidation processes.

Identification of Ozonation Products of Aromatic Hydrocarbon Micropollutants: Effect on Chlorination and Biological Filtration

A study was made of the identification of substances produced by the ozonation of some aromatic compounds in a dilute aqueous medium: the aim was to determine the influence of ozonation on post–chlorination as well as on the mechanism of elimination of organic matter in secondary filtration on activated carbon. The nature of ozonation products such as aldehyde ketone and acid accounts for the following results –a change in chlorine demand and trihalomethane production –a loss in physical adsorption on activated carbon –the significance of biodegradation in secondary filtration on activated carbon

Chloropicrin formation during oxidative treatments in the preparation of drinking water

Chlorination of water can lead to the formation of chloropicrin. The numerous potential precursors (of various reactivities) observed during this study, confirm this hypothesis. Combination of ozonation and chlorination can also lead to the formation of this compound, dangerous to health; however, the conditions of the formation and particularly the impact of a nitration reaction in the gas phase are still not clearly defined.

Chloration de composés organiques: demande en chlore et réactivite vis-a-vis de la formation des trihalométhanes. Incidence de l'azote ammoniacal

The purpose of this work is to contribute to knowledge of the condition of formation of volatile and non-volatile organochlorinated compounds during surface water chlorination. With this aim in view, the chlorination of a number of organic substances in diluted aqueous solutions (10−6-10−5 moll −1) and in a neutral medium was studied. Special attention was given to their reactivity in relation to the formation of trihalomethanes. The results obtained show great differences in the reactivity of chlorine towards chemical substances liable to be present in the waters. The high chlorine demands, 5–12 mol of chlorine per mol of compound after a 15 h reaction at pH 7, where obtained with those aromatic compounds having activating groups such as — OH and — NR2 (phenol and aniline derivatives). On the other hand, a number of compounds (alipahitcs in general; acids, aldehydes, alcohols…) are relatively inert towards chlorination. As far as chloroform production is concerned, the study shows that many organic compounds are liable to lead to the production of low quantities of chloroform in a neutral medium (molar yields < 5%). However, only a few specific structures such as metapolyhydroxybenzenes and metachlorophenols constitute good precursors of the haloform reaction. The study of the formation kinetics of chloroform, carried out with some precursors of different reactivity: acetone, acetylacetone, resorcinol, phloroglucinol and 3,5-dichlorophenol allowed us to determine the kinetic constants of chloroform formation at pH 7.5 and 20° C: Acetone v=k[Acetone] k=1,7 x 10-7s-1 Acetylacetone k[Trichloro-l,l,l acetone] k=5,6 X 10-6s-1 Resorcinol v=k[Resorcinol][Chlore] k=(1,5±0,5) x 10 3 | mol-1s-1 Phloroglucinol v=k[Phloroglucinol][Chloro] k=1,5±0.5 | mol-1s-1 Dichloro-3,5 phenol v=k[Dichloro-3,5 phenol][Chlore] k=0,5±0,25 | mol-1s-1 The results obtained during this work also show that the chlorine found in the chloroform produced from a precursor, represents only a small proportion of the chlorine demand. Even with a highly reactive precursor such as resorcinol. It was shown that the liberation of chloroform is accompanied by the formation of trichloroacetic acid (molar yield < 10%) and of monochloromaleic acid (molar yield 60%). Moreover, tetrahalogenated and pentahalogenated hydrocarbons (C3Cl4 and C3 HCl5) resulting from 3,5 -dichlorophenol chlorination were made manifest. As for the chlorin- ation of acetylacetone, a mechanism passing through the formation of 1,1-tri- chloroacetone was proposed. Lastly, in the general framework of the interactions between chlorine, ammonia nitrogen and the organic compounds which are frequently found in surface water chlorination, the study allowed us to show that the chlorination of highly reactive precursors (such as metapolyhydroxybenzenes and metachlorophenols) can lead to the production of important amounts of chloroform before reaching breakpoint. These results compared to the values of the velocities of the various reactions between chlorine and the ammoniacal compounds.