Destruction plasmachimique d’urée et de thiourée par décharge électrique à pression atmosphérique Plasmachemical degradation of urea and thiourea solutes exposed to an electric discharge at atmospheric pressure

Résumé

Le traitement à la pression atmosphérique de solutions aqueuses d’urée et de thiourée par plasma d’arc rampant en atmosphère d’air humide conduit à la dégradation totale de ces composés. Les cinétiques globales d’ordre nul ont des constantes voisines proches de 3•10^‑6 s^‑1. Le procédé met en jeu les espèces actives créées dans la décharge, les radicaux OH et NO, responsables des caractères chimiques principaux du plasma : oxydation du fait de la présence de OH (E°_OH H2O =  2,85 V/ENH) et acidification, provenant de la formation d’acides nitreux et nitrique en solution. Ces résultats sont relatifs à des molécules modèles, mais suggèrent l’extension du procédé à la dégradation de molécules toxiques et d’effluents industriels soufrés.

Mots clés:

• Dépollution,
• thiourée,
• urée,
• solutions aqueuses,
• dégradation oxydante,
• décharge électrique,
• plasma nonthermique,
• pression atmosphérique

Abstract

Electric discharges in humid air (i.e., a gliding arc discharge at atmospheric pressure and quasi-ambient temperature) are considered in the context of evaluating new techniques for pollution abatement. An electric discharge in a gas under specific conditions gives rise to a plasma, which involves activated gas species with enhanced reactivity. The main chemical properties of a discharge in humid air are attributed to NO and OH radicals formed in the discharge, which are able to react with solutes at the plasma/liquid interface. These activated species are formed in advanced oxidation processes and are respectively responsible for acid and oxidizing effects in the target solution: NO gives rise to nitrous and nitric acids, and OH is strongly oxidising [E°(OH/H₂O) =  2.85 V/NHE].

To examine the degradation power of the plasma treatment on molecules of the same family and to evaluate the ability of the gliding arc system to oxidize sulphur-containing solutes (2 x 10^‑3 M) in batch conditions, aqueous urea and thiourea were selected as suitable target solutions. The solutes were completely degraded within 180‑200 minutes of treatment and the concentrations decreased linearly with increasing exposure times in the discharge. This trend accounts for overall zero-order kinetic schemes with the relevant rate constants of k_urea = 5.28 x 10^‑6 s^‑1 and k_thiourea = 2.03 x 10^‑6 s^‑1.

The evolution of solutes with time was followed by total organic carbon (TOC) measurements for urea, and by the conductometric titration of the sulphate ions formed in the case of thiourea. Spectrophotometric measurements of the treated solutions at the solute absorption peaks were found to be unsuitable for analysis purposes due to the formation of nitrite/nitrate ions which absorb in the same wavelength range.

The extension of a gliding arc system from the laboratory level to an industrial scale for pollution abatement of industrial effluents is considered.

Keywords:

• Non thermal plasma,
• oxidation,
• gliding arc discharge,
• humid air plasma,
• thiourea,
• urea,
• pollution abatement