Ind. Eng. Chem. Res. 2008, 47, 5761–5781
5761
Chemical Reactivity of Discharges and Temporal Post-Discharges in Plasma Treatment of Aqueous Media: Examples of Gliding Discharge Treated Solutions Jean-Louis Brisset,*,† David Moussa,† Avaly Doubla,‡ Eugen Hnatiuc,§ Bogdan Hnatiuc,§ Georges Kamgang Youbi,†,⊥ Jean-Marie Herry,⊥ Murielle Naı¨tali,⊥ and Marie-Noelle Bellon-Fontaine⊥ Laboratoire d’Electrochimie Interfaciale et de Chimie Analytique (LEICA), UFR Sciences, UniVersite´ de Rouen 76821, Mont Saint-Aignan Cedex, France; Laboratoire de Chimie Mine´rale, UniVersite´ de Yaounde´-I; BP 812, Yaounde´, Cameroon; Technical UniVersity Gh. Asashi, 51-53 Bdu Mangeron, 700 050 Iasi, Romania; and UMR763 BHM INRA-AgroParisTech, 25 AVenue de la Re´publique, 91744 Massy Cedex, France
Environmental applications of electric discharges are being considered increasingly more often: they imply the chemical properties of the activated species generated in and by the discharge. An overview of the resulting chemical effects is presented, based on rationalized classification, i.e., acid-base effects, oxidizing properties, complex forming reactions, and radical reactions. The gliding discharge is considered to be a specifically suitable plasma source for the treatment of liquids for pollutant abatement in the scope of sustainable environment, and this justifies an overview of the chemical properties. Special emphasis is devoted to temporal post-discharge reactions (TPDRs), which occur when the target is no longer exposed to the plasma source, and several typical examples are detailed. These recently evidenced TPDRs seem to present some general character. They are the key parameters to estimating the efficiency of a discharge treatment; they also have major technical and economical importance for the application of the plasma treatment to pollutant and/or micro-organism abatement at atmospheric pressure and quasi-ambient temperature. 1. Introduction Electric discharges occupy a special place among the new technologies developed in the scope of sustaining development, because they allow one to create active species that are able to react with selected target molecules. This feature is specially interesting for the destruction of pollutants or hazardous molecules. Among the new emerging techniques available, the ones operated at atmospheric pressure are largely favored for their ease of use and simplicity. The use of electric discharges is probably one of the most promising techniques available to solve environmental problems. We will focus on discharges in gases at atmospheric pressure used for pollutant abatement in liquid targets, and we will especially address gliding discharges, with an emphasis on the huge range of their possible applications. In addition, some recently evidenced features (i.e., the postdischarge phenomena) that make specific devices particularly attractive will be detailed. Electrical discharge in gases is the source of electrochemical reactions: therefore, they are governed, as any electrochemical process would be, by (i) the nature of the (active) electrode(s) and its (their) interaction with the surrounding medium, (ii) the nature and composition of the medium (i.e., the fed gas in the * To whom correspondence should be addressed. E-mail address:
[email protected]. Present address: LMDF (UPRES 2123), 55 rue St Germain, 27000 Evreux, France. † Laboratoire d’Electrochimie Interfaciale et de Chimie Analytique (LEICA), UFR Science. ‡ Laboratoire de Chimie Mine´rale. § Technical University Gh. Asashi. ⊥ UMR763 BHM INRA-AgroParisTech.
present case; the aqueous and nonaqueous solvent for electrochemistry in liquid media), (iii) the particular properties of the target molecules (i.e., organic compounds dispersed in an aqueous phase for the considered cases), and (iv) the electric energy displayed to the electrochemical reactor. These points will be considered hereafter in the scope of gas discharges. A matching consequence is the occurring continuity between classical electrochemistry1 in the liquid phase and the various types of electrical discharges in gases. This feature is illustrated with the characteristic plots of current i versus applied potential U, which underlines the relevant character of each type of discharge. On applying a weak potential drop (usually
