Participation des radicaux carbonate à l’oxydation de l’atrazine lors de l’ozonation de solutions aqueuses contenant des ions hydrogénocarbonate Contribution of carbonate radicals to atrazine oxidation during ozonation of aqueous solutions containing bicarbonate ions

Résumé

L’étude porte sur la détermination de la contribution des espèces O₃, OH° et CO₃°- dans la dégradation de l’atrazine lors de l’ozonation de solutions contenant différentes concentrations en ions hydrogénocarbonate et en carbone organique. Le suivi de la concentration en atrazine et en ozone dissous, et les expressions cinétiques ont permis de calculer les concentrations en radicaux hydroxyle et carbonate au cours des réactions. A partir des données expérimentales obtenues sur des eaux pures additionnées de carbone organique et inorganique, les résultats indiquent que l’élimination du micropolluant résulte de l’action de l’ozone (pour une faible part), des radicaux hydroxyle issus de la décomposition de l’ozone, mais aussi pour une part très significative, des radicaux carbonate. La participation des radicaux CO₃°- diminue lorsque la concentration en carbone organique augmente. Les radicaux carbonate peuvent être responsable de plus de 40 % de la dégradation de l’atrazine lors de l’ozonation en présence de 7 mM d’ions hydrogénocarbonate et 129 µM d’ions glycolate utilisés comme molécule modèle pour l’apport de carbone organique. Les résultats obtenus sur des eaux naturelles confirment les conclusions déduites des expériences sur des eaux de composition connue.

Mots-clés:

• Ozonation,
• radicaux hydroxyle,
• radicaux carbonate,
• atrazine

Abstract

The inhibiting effect of bicarbonate ions on the oxidation of organic molecules by the hydroxyl radicals is well known. However, the carbonate radicals resulting from the consumption of the OH° radicals by these ions have only rarely been considered to participate in the reactions of organic pollutant removal. In this study, the contribution of O₃, OH° and CO₃°- radicals in the degradation of atrazine during ozonation of aqueous solutions containing various concentrations of bicarbonate ions and organic carbon, was determined.

Experiments were performed in a bubble column fed continuously by ozone gas and an aqueous solution containing atrazine (0.05 µM) as the model molecule. Three sets of experiments were carried out at pH 8:

1. pure water with different concentrations of bicarbonate ions (0.35-70 mM);

2. pure water with different concentrations of bicarbonate ions (0.35 and 7 mM) and glycolate ions (0-129 µM) selected as the organic carbon source;

3. surface and tap waters.

For different contact times in the ozonation reactor, the concentrations of atrazine and dissolved ozone were determined. Inputting these data into kinetic equations enabled us to calculate the concentrations of hydroxyl and carbonate radicals during ozonation. In the absence of organic carbon, the concentration of hydroxyl radicals was determined by assuming steady state conditions (equation II). The concentration of carbonate radicals was deduced from the slope of the evolution of atrazine concentration ([A]₀ -[A])/[A] versus the contact time, and the values of ozone and OH° radical concentration (equation IV). In the presence of organic matter, the concentration of OH° radicals was calculated from the evolution of ([A]₀ -[A])/[A] versus the contact time, and by replacing the carbonate radical concentration by the expression involving the OH° radicals (equations VI-IX). From the steady state assumption, the concentration of carbonate radicals followed the hydroxyl radical evolution.

In the absence of organic carbon, the results confirmed the global inhibiting effect of bicarbonate ions on the removal of atrazine molecules. However, the concentrations of the carbonate radicals were much higher than the OH° radical concentrations (above 4x10^-10 M compared to 2x10^-12 M, respectively). The concentration of these latter radicals decreased as the concentration of bicarbonate ions increased. Under these conditions, the carbonate radicals were mainly responsible for the removal of atrazine.

From the experiments in pure water with given concentrations of bicarbonate and glycolate ions, the hydroxyl radical concentration increased with the concentration of glycolate ions, thus confirming the promoter property of this organic molecule, which favours the removal of atrazine. However, in most cases, the presence of organic carbon was found to be unfavourable to the concentration of carbonate radicals. Therefore, the participation of the carbonate radicals decreased with increasing organic carbon content. Nevertheless, their contribution to atrazine degradation can reach more than 40% during ozonation in the presence of bicarbonate ions (7 mM) and glycolate ions (129 µM) used as an organic carbon source.

The results obtained from the experiments carried out in natural waters agree with the main conclusions from the above experiments. The removal of atrazine involved ozone, hydroxyl radicals as well as carbonate radicals. The contribution of carbonate radicals decreased when the ratio of inorganic carbon to organic carbon decreased. In the surface and tap waters tested, this contribution was found to be 10 to 40 % and 43 % respectively.

Keywords:

• Ozonation,
• hydroxyl radicals,
• carbonate radicals,
• atrazine