Etude de la dégradation de quelques composés organochlorés volatils par photolyse du peroxyde d'hydrogène en milieux aqueux hotooxidation of volatile chlorinated hydrocarbons in aqueous solution in the presence of hydrogen peroxide

Résumé

Le travail a eu pour but d'étudier l'efficacité de la photolyse du peroxyde d'hydrogène sur la dégradation de quelques composés organochlorés aliphatiques saturés (chiorométhanes et chloroéthanes) en milieu aqueux (pH 7,5). Les expériences ont été réalisées en réacteur statique, avec une Lampe basse pression à vapeur de mercure et avec des concentrations initiales en produit chloré de l'ordre de 10^-6 mol l^-1 et en H₂0₂ comprises entre 10^-5 et 10^-3 mol L^-.

Les résultats montrent que le système H₂0₂/UV peut oxyder les composés organochlorés étudiés à l'exception des composés ne possédant pas d'atome d'hydrogène (CCL₄ et C₂ CL₆). Les rendements d'oxydation obtenus avec Le réacteur utilisé dépendent du temps de réaction, de la concentration initiale en H₂0₂, du flux photonique et peuvent être nettement diminués par la présence de pièges à radicaux (ions bicarbonates) dans le milieu réactionnel.

Par ailleurs, une étude cinétique de la photolyse du peroxyde d'hydrogène en absence de matière organique est également présentée.

Mots-clés:

• Composés chlorés volatils,
• peroxyde d'hydrogène,
• irradiation UV,
• cinétique

Abstract

The aim of this work was to study oxidation of certain volatile polychlorinated hydrocarbons, using hydrogen peroxide photoactivated by UV. This research was carried out with different mixtures of diluted aqueous solutions of chloromethanes (CHCl₃, CCl₄) and chloroethanes (C₂H₃Cl₃, C₂H₂Cl₄, C₂HCl₅, CCl₆), which are typical halogenated compounds most frequently found in contaminated groundwater. The effect of the hydrogen peroxide concentration, the light intensity and the bicarbonate concentration on the rate of 1,1,2-trichloroethane (TCE) oxidation was determined. A kinetic study on hydrogen peroxide photolysis in a solution free of organic compounds was also carried out.

EXPERIMENTATION

Experiments were conducted in a batch reactor (V = 4 l), equipped with an immersed mercury low-pressure lamp. The intensity emitted at 253.7 nm was roughly 2 10¹⁹ photons s^-1. The temperature of the reaction mixture was maintained with a regulation system at 16 ± 0.5 °C (figure 1).

The solutions were prepared in a phosphate buffer µ = 2 10^-2 M, pH = 7.5). The outer surface of the lamp was masked with strips of aluminium, so as to obtain various percentages of initial energy (20 to 100 %).

The concentration of the hydrogen peroxide of the samples was determined by spectrophotometry and the chlorinated compounds were analysed by electron capture gas chromatography.

RESULT

Kinetics of hydrogen peroxide photolysis : H₂0₂ was decomposed by UV tb produce two hydroxyl radicals. In diluted solutions ([H₂0₂] < 10^-3 M), the concentration decreases in accordante with a first order law. The rate constant depends on the initial light intensity (I_o), on the characteristics of the reactor (volume and distance between the lamp and the watt. of the reactor) and on the motar extinction coefficient of the irradiated solution (equation C). The decomposition rate appears to be dependent on pH, the rate of constant rire has been found to be proportional to the dissociation of hydrogen peroxide into its basic form (E_H2O2 = 20 mol^-1 cm^-1,E_H2O2_ = 240 mol^-1 cm^-1) (figure 2 and 3).

Oxidation of the chlorinated compounds : H₂0₂/UV is very efficient for the removal of organic compounds. Preliminary experiments showed that both UV and H₂O₂ treatments do not decompose halogenated compounds. Hydroxyl radicals are extremly reactive and attack organic compounds preferentialty by abs-tracting a hydrogen atom from an organic molecule. This is confirmed by the results which show that chloromethanes and chloroethanes with an H atom are eliminated, but net compounds such as tetrachloride and hexachloroethane (figure 4 to 7).

The effectiveness of an H₂0₂/UV system depends on various parameters. Studies on the TCE elimination show that the oxidation yields an increase when the reaction time, the UV irradiation dose (figure 9b) and the hydrogen peroxide concentration (figure 8) increase. However, the efficiency decreases in the presence of radical traps such as bicarbonate and carbonate ions (figure 10).

Keywords:

• Volatile chlorinated compound,
• hydrogen peroxide,
• UV-irradiation,
• kinetic