Degradation kinetics of phenol and its decomposition products in the electrolyte cathode of direct-current atmospheric-pressure discharge

Abstract

The application of high energy chemistry methods to water treatment for the removal of organic com pounds has been intensively studied in recent years. These methods include the treatment of solutions with gas discharge generated ozone (O 3 ), O 3 together with ultraviolet irradiation (O 3 /UV), hydrogen peroxide (H 2 O 2 ) in combination with UV, and H 2 O 2 /UV [1]. Different types of atmospheric pressure gas discharges in oxygen and air and over solutions generate all of the above factors and also cause the formation of other possible active species capable of decomposing any organic matter [2,3]. Laboratory studies of both a wide range of solutions of individual substances [4] and real wastewater [5,6] showed high purification efficiency. Phenol, which is highly toxic, is a typical water pollutant [7], and its decomposition has been intensively studied with the use of a hybrid positive pulse corona in oxygen (simultaneous discharge in the solution and gas) [8], a dielectric barrier discharge in air [9], electrolysis by a contact glow discharge [10], and a gliding arc discharge in oxygen with the injection of a solution [11]. The aim of this work was to study and simulate the degradation kinetics of phenol in the simplest discharge system, a direct current electrolyte cathode discharge, which was not explored as applied to the decomposition of phenol. EXPERIMENTALThe experimental setup was described elsewhere [12]. A direct current atmospheric pressure discharge in air was excited by applying a constant voltage across a metal anode and the surface of a solution. The anode-electrolyte surface distance was 4 mm. The discharge current was 40 mA. A phenol solution in doubly distilled water with a concentration of 0.532 mmol/L was used. The electrolyte volume was 80 mL. After a certain discharge treatment time, the solution was analyzed to determine the concentrations of phenol, carboxylic acids, and aldehydes. A fresh portion of the solution was used for each particular time.The concentrations of phenol and aldehydes were measured based on the intensities of their fluorescence excited at an absorption band maximum with the aid of a Flyuorat 02 fluorimeter (Russia). Phenol was extracted with butyl acetate and back extracted into an aqueous solution with the subsequent acidification of the back extract. In the case of aldehydes, the lumines cent compound was the product of their interaction with 1,3 cyclohexanedione in the presence of ammo nium ions. The concentration of carboxylic acids was determined based on the absorption of a compound formed by the reaction of acids with ammonium metavanadate at a wavelength of 400 nm. A Hitachi U 2001 spectrophotometer (Japan) was used for this pur pose. Random error was checked based on five mea surements at a confidence level of 0.95. Figure 1 shows the kinetics and the degree of decomposition of phenol under the action of the dis charge. We found that the kinetics is adequately described by a first order rate law with respect to the concentration of phenol C = C 0 exp(-K × ...

Phenol decomposition in water cathode of DC atmospheric pressure discharge in air

“…There are some fragmentary data on separate products, but the detailed studies of product kinetics are not available. Recent studies have reported the effective rate constants of phenol decomposition for DC discharge in air and also revealed that the phenol decomposition results in the formation of aldehydes and carboxylic acids [13,14]. Our calculation of product balance, however, expects that other compounds have to be formed as well.…”