Élimination du phénol par deux plantes aquatiques : Juncus fontanesii (Gay) et Lemna minor L. Elimination of phenol by two aquatic plants : Juncus fontanesii (Gay) and Lemna minor L.

Résumé

L'élimination du phénol et de ses dérivés, substances organiques toxiques, fait appel à différents processus physico-chimiques ou biologiques. Certaines plantes aquatiques ont la capacité de déplacer des produits chimiques en les métabolisant, en les évaporant ou en les dégradant. Il faut, toutefois, rester à des concentrations inférieures aux seuils de toxicité des espèces employées.

Dans le présent travail, deux plantes aquatiques: le Jonc de Desfontaines (Juncus fontanesii) de la famille des Joncacées et la Lentille d'eau (Lemna minor) de la famille des Lemnacées, ont été testées pour éliminer le phénol. Le travail a été effectué sans addition d'éléments nutritifs ni acclimatation prélable, pour des concentrations variant de 8 à 48 mg/l et pour deux densités surfaciques de la biomasse végétale fraîche : 2,8 et 5,6 kg/m².

Les deux espèces se sont révélées aptes à éliminer totalement le phénol avec des cinétiques différentes. Un phénomène de relargage, important dans le cas de l'emploi de J. fontanesii, a pu être observé. Une comparaison de ce type d'élimination à celui dû aux micro-organismes nous a permis, par utilisation des boues activées, d'aboutir à l'ordre de performance suivant : J. fontanesii > L. minor (faibles densités) > micro-organismes avec barbottage d'air > micro-organismes sous des conditions atmosphériques > témoins (sans plantes) > L. minor (fortes densités) > micro-organismes sous des conditions anaérobies.

Mots-clés:

• Phénol,
• biodégradation,
• plante aquatiques,
• Juncus fontanesii,
• Lemna minor

Abstract

Phenols are considered as toxic organic compounds. They can be treated by different physico-chemical or biological processes. These products can be oxidized by chemicals such as H₂O₂, TiO₂, O₃, etc. The performance of the process depends on pH, temperature and phenol/oxidant ratio. Otherwise, they can be transformed biologically by enzymes, fungi, yeast or plants. Considerable work has already been done with regard to uptake of phenol by aquatic plants.

In our study two aquatic plants: Juncus fontanesii, a rooted species from Joncaceae family and Lemna minor, floating species from Lemnaceae family, have been selected to study their ability to remove phenol from static phenolic solutions. The initial concentration of phenol varied from 8 to 48 mg/l. The density of biomass (wet weight) ranged from 2.8 to 5.6 kg/m². Experiences were carried out without acclimation and without addition of nutritive elements. Controls (without plants) were prepared with the same concentrations. Under these conditions, the results of quantitative analyses show that J. fontanesii is able to remove phenol more rapidly than L. minor and can release a fraction of it to the medium particularly in the first ten hours of contact.

It has been observed that phenol uptake is sensitive to the density of biomass and the initial concentration. In order to examine more closely the effect of these variables, we have carried out experiments where the initial concentration was kept constant (8 mg/l) and biomass density varied. When the density of biomass increases, the kinetic uptake of phenol by J. fontanesii increases too; however, it decreases in the presence of L. minor. In fact, at high densities, L. minor covers fully the surface of the water and causes a screen effect, such that diffusion of atmospheric oxygen into the medium is limited. In addition, L. minor has a short root system, so the amount of oxygen that enters the solution is negligible. Elimination of phenol by L. minor is rapid when the density of biomass ranges from 0.7 to 1.4 kg/m². For both plants, we have noticed the existence of a maximum time limit of degradation and an optimal density beyond which there is no improvement in elimination.

Phenol can be degraded by micro-organisms. In order to elucidate this pathway, an investigation was undertaken using activated sludges in the following situations: under atmospheric conditions, under anaerobic conditions and with bubbling air intermittently.

The comparison of obtained results shows that the rate and kinetics of the elimination decrease in the following order: J. fontanesii > L. minor (low densities) > micro-organisms with air bubbling > micro-organisms under atmospheric conditions > controls (without plants) > L. minor (high densities) > micro-organisms under anaerobic conditions.

Keywords:

• Phenol,
• biodegradation,
• aquatic plants,
• Juncus fontanesii,
• Lemna minor.