Non-thermal Plasma Technology for Degradation of Organic Compounds in Wastewater Control: a Critical Review

Non-thermal plasma is an emerging technique in environmental pollution control technology, produced by the high-voltage discharge processes and therefore a large amount of high energy electrons and active species are generated. The degradation of difficult-degraded organic pollutions will be greatly enhanced by the active species generated from non-thermal plasma process. However, research on non-thermal plasma technology on organic wastewater cleaning remains scarce. In this work, recent developments of plasma on wastewater control technology are critically reviewed, and the operational principles and parameters are outlined with special focus on the degradation of organic compounds in the wastewater treatment processes. *Corresponding author Email: [email protected] INTRODUCTION The degradation of hazardous organic pollutants in water is one of the critical and urgent topics in environmental research. However, these hazardous can not be effectively removed or recovered by conventional biological or chemical method [1,2]. Therefore, considerable attention has been focused on the so-called advanced oxidation processes (AOPs) based on the generation of highly reactive species, especially hydroxyl radicals [2,3]. As a very powerful and non-selective oxidant with sufficient potential to completely oxidize organic compounds into carbon dioxide and water [3-5], the hydroxyl radical plays an important role in pollution control system for organic degradation [6]. The oxidation potentials of major chemical reactants, including hydroxyl radical, are shown in Table 1 [7,8]. There are a number of AOPs to generate hydroxyl radicals. Ultraviolet photolysis, wet oxidation, and TiO2 photocatalysis [9], for example, have all reached the stage of practical applications. However, the hydroxyl radical reactions activated by these methods are known to be slow [10]. Ozone oxidation, another AOP technique, takes a time-consuming series of chain reactions to produce hydroxyl radicals in neutral and basic solutions, but has the unstable nature of ozone to worry about [10]. Compared to its fellow AOP approaches, non-thermal plasma can achieve greater output and better efficiency. Non-thermal plasma produced in water solutions forms the basis of an innovative AOPs of water treatment [3,4]. The non-thermal plasma pollution control process can remove chemical and biological wastes in all three – gas, liquid, and solid – states. The method can decompose both highand low-concentration organic wastewater even in a large flux [2]. Non-thermal plasma is produced during the discharge processes where a large amount of high energy electrons and active species (such as active radicals and molecules) are generated [1-4,11] for direct, and effective oxidation of dissolved organic molecules. The paper reviews the current researches on chemical reactions in the aqueous phase under highvoltage electrical discharges with special focus directing onto the degradation of organic compounds in water by non-thermal plasma technology. PRINCIPLE OF NON-THERMAL PLASMA TECHNOLOGY IN WATER Consisting of both positive and negative ions and 428 J. Environ. Eng. Manage., 17(6), 427-433 (2007) Table 1. Oxidation potential of major chemical reactants [7,8] Oxidant Oxidation potential (V) hydroxyl radical 2.80