Environ. Sci. Technol. 2005, 39, 8512-8517
Phenol Degradation by a Nonpulsed Diaphragm Glow Discharge in an Aqueous Solution YONG JUN LIU AND XUAN ZHEN JIANG* Department of Chemistry, Zhejiang University, Hangzhou 310027, China
In the present study, a nonpulsed direct current diaphragm glow discharge process was developed for the first time for phenol degradation in an aqueous solution. The discharge was generated in a small hole in a dielectric diaphragm interposed between two submersed graphite electrodes. The experimental results revealed that supplied voltage, initial pH, iron salts, and radical scavengers impact the phenol degradation significantly. Enhancing the applied voltage, lowering the solution pH, and adding appropriate amounts of Fe2+ or Fe3+ to the solution were found to be favorable for phenol degradation. Carbonate ions or n-butanol in the solution can decelerate the phenol removal. When the treatment time is increased, the pH value of the solution decreased, leading to an increase in the phenol decomposition. It was revealed by high performance liquid chromatography and ionic chromatography that the main intermediates of phenol decomposition are hydroquinone, pyrocatechol, p-benzoquinone and organic acids. In comparison with the highvoltage corona discharge plasma in distilled water, this process offers simple technology, higher energy efficiency, easier scaleup, and easier applicability to salt-containing wastewater with no electrode erosion and electromagnetic radiation.
1. Introduction Interest in the application of electrical discharges for the degradation of organic pollutants in aqueous solution has grown enormously, mainly because of its high removal efficiency and environmental compatibility (1). Many processes, such as electro-hydraulic discharge (EHD), pulsed corona discharge (PCD), and glow discharge electrolysis (GDE) are being developed (2-13). Among them, PCD has received the most extensive investigations because it can directly inject the electrical energy into an aqueous solution through plasma channels, leading to the formation of various chemically active species with almost no energy wasted in heating the bulk liquid (1). The chemically active species, in turn, rapidly degrade the organic pollutants dissolved in an aqueous solution. Previous studies have demonstrated that various compounds, such as phenol, acetophenone, and organic dyes, etc., could be satisfactorily removed in PCD (5-7, 14-19). However, the PCD process seems to perform best in water with lower conductivity (5, 16), which is not suitable for real wastewater that contains salts. As another well-known plasma generation process in water, GDE seems to have advantage in this respect. In GDE, a rod anode is * Corresponding author phone/fax: 0086-571-87951611; e-mail:
[email protected]. 8512
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 21, 2005
FIGURE 1. Experimental setup of the diaphragm glow discharge reactor: (1 and 2) graphite electrodes; (3) dielectric diaphragm with small hole; (4) reaction vessel; (5) glow discharge; (6) cooling water in; (7) cooling water out; (8) gas outlet; (9) electrolytic solution; (10) thermometer. placed near the surface of an electrolyte, and a sheath of plasma is sustained between the anode tip and the surface of the electrolyte as a high direct current (DC) voltage is applied to the system. In this case, the current flows as a glow discharge; therefore it is called glow discharge plasma or glow discharge electrolysis (20). In GDE, the charged species in the plasma are accelerated due to the cathode drop, entering the solution with an energy distribution high enough to bring about oxidative reactions in the aqueous solution (20-23). Several authors have investigated its chemical effects and applications in water treatment (8-13, 20-23). The most obvious advantage of GDE is that it can be operated under high-salt-containing water in contrast to PCD, because the aqueous solution itself serves as the cathode in GDE. Therefore, higher salt concentration makes it easier for the discharge to take place. But this process uses expensive platinum as the working electrode, which greatly limits its economical feasibility for industrial applications (23). To solve the above problems, we have designed a new diaphragm glow discharge plasma reactor in which the electrodes do not directly contact with the highly reactive plasma. The discharge mode used in this work was termed as diaphragm glow discharge (DGD). Our experimental results showed that the DGD reactor could be operated in an electrolytic solution with no electrode corrosion. To the best of our knowledge, this kind of reactor for the treatment of water-containing salts has not been reported to date (24-28). Phenol is one of the most important indicators of water pollution. Many countries have listed it as a priority pollutant. Therefore, in the present work, we chose phenol as a model pollutant dissolved in a sulfate solution to evaluate the operating parameters and provide some fundamental information on this new process for phenol degradation.
2. Experimental Methods The experimental apparatus consisted of a DC high-voltage power supply and a reactor. The schematic diagram of the reactor is shown in Figure 1. The reaction vessel was a jacketed glass cylinder with an inner diameter of 65 mm. A quartz diaphragm about 1 mm in thickness with a small hole (ca. 2 mm in diameter) in its center separated the reaction vessel 10.1021/es050875j CCC: $30.25
2005 American Chemical Society Published on Web 09/22/2005
FIGURE 2. Characteristic curve of current vs voltage (solution temperature, 298 K; [Na2SO4], 4.0 g/L; volume of the electrolyte, 250 mL). into two compartments. Two planar graphite electrodes were placed in each compartment at a distance of 50 mm between the two electrodes. Each electrode had a submersed area of approximately 2 cm2. The diaphragm was impermeable apart from the hole. The solution for treatment was prepared by dissolving phenol to 300 mg/L in distilled water, its conductivity was raised to about 6500 µS cm-1 by adding Na2SO4, and its pH was adjusted by adding either dilute sodium hydroxide or dilute sulfuric acid. A 250-mL portion of the solution was poured into the reaction vessel for treatment. The solution was treated by applying DC voltage across the electrodes through a DC power supply (variable voltage 0-1000 V and maximum current of 0.3 A). The solution was stirred by two magnetic stirrers, one in the anolyte and one in the catholyte compartment, and was maintained at 298 ( 2 K by running water in the outer jacket. The temperature was monitored by a mercury thermometer. The treated solutions in the two compartments were mixed together before any measurement in this study. The pH was measured by a pH meter (pHS-25 B, Shanghai Dapu Instrumental Factory, China). During the reaction, 20-µL mixtures were periodically sampled and analyzed by high performance liquid chromatography (Angilent 1100 coupled with a UV detector with the wavelength set at 254 nm) to follow the decrease in the phenol concentration and to analyze its degradation products. The separation was performed using an ODS-18 (Elite, Dalian, China) reversed-phase column at a flow rate of 0.8 mL min-1 (mobile phase, aqueous solution containing 40% acetonitrile). Identification and quantitative analysis of the intermediates were based on the peak retention time and calibration with standards. The organic acids resulting from the phenol degradation were identified by ionic chromatography (IC; Dionex Dx-120) with an Ion Pac AS-14 column, and an aqueous solution of NaHCO3 was used as the mobile phase, with a flow rate of 1.0 mL/ min.
voltage (23), and it is assumed that normal electrolysis of water takes place at the electrodes. When the supplying voltage was increased from 450 to 750 V, the current decreased with the increment in the voltage, and some flashes of light appeared. Occasional electrical breakdown in the gas bubbles formed in the small hole were responsible for the flashes of light. Low electrical conductance of the gas bubbles was responsible for the decrease in current. When the voltage exceeded 750 V, the current increased abruptly with rising voltage, and an intense glow appeared in and around the small hole in the diaphragm, indicating a switchover from normal electrolysis to the glow discharge process. The current was increased due to formation of highly conductive plasma in the hole. The current-voltage characteristics of DGD shown in Figure 2 were more similar to that of glow discharge process, indicating that the mechanisms in the two processes were similar (20, 23). In the present experimental conditions, the current-voltage relationship shown in Figure 2 remained the same when phenol was added into the solution. Therefore, the energy efficiency values given in section 3.7 were calculated from the current and voltage values taken from Figure 2. The following electrochemical reactions take place in different regions during the DGD (20, 22). On the surface of the graphite electrodes, normal electrolysis takes place, where the yield is stipulated by Faradaic law
H2O f H2 (cathode) + O2 (anode)
In the small hole, ionization and thermal decomposition of water vapor occur, resulting in an excess Faradaic yield of H2 and O2 (22)
H2Oliquid + Joule heat f H2Ogas +
(2)
-
H2Ogas f H2O + e (ionization)
(3)
H2Ogas f H2 + O2 (thermal decomposition)
(4)
At the plasma-catholyte and plasma-anolyte interfaces, a charge-transfer Faradaic process takes place where hydroxyl radicals and hydrogen radicals are produced, respectively (22)
H2O+ + H2O f •OH + H3O+ (plasma-catholyte interface) (5) H2O + e- f H• + OH- (plasma-anolyte interface) (6) Like the case of radiation chemistry in aqueous solutions (22, 23), several water molecules at the plasma-liquid interfaces are activated by the energetic electrons, and positive ions result in non-Faradaic yields of hydroxyl radicals and hydrogen and their combination products by the following reactions
H2O+* + nH2O f nH2O* + H2O+ •
H2O* f OH + H
3. Results and Discussion 3.1. Current-Voltage Characteristics. The current versus voltage curve in the case of a 4-g/L sodium sulfate solution (without phenol) is shown in Figure 2. The curve can be divided into several sections. In the range of 0-250 V, the current was roughly proportional to the applied voltage, where the normal electrolysis of water on the electrode surfaces was observed. In the voltage range of 250-450 V some vapor bubbles were produced in the small hole due to Joule heating (20). Because of the dielectric nature of the vapor bubbles, the current no longer increased with rising
(1)
-
(7)
•
•
(8) •
e * + nH2O f m OH + mH + e
-
(9)
OH• + •OH f H2O2 •
(10)
•
H + H f H2
(11)
2•OH + H2O2 f O2 + 2H2O
(12)
Here, the asterisks and n and m represent an energetic state and non-Faradaic constants for H2O+* and e-*, respectively. VOL. 39, NO. 21, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Phenol removal under different supplying voltages (solution temperature, 298 K; [Na2SO4], 4.0 g/L; volume of the electrolyte, 250 mL; initial pH ) 6.5; initial phenol concentration, 300 mg/L). Many of the hydroxyl radicals produced at the plasmaelectrolyte interface diffuse into the solution and oxidize the dissolved pollutants. It should be mentioned here that the hydroxyl radicals are the strongest oxidizing agents, second only to fluorine in oxidation potential. 3.2. Effect of Voltage on Phenol Decomposition. In GDE, 500 or 600 V was often employed for pollutant degradation because the platinum anode would melt at a higher voltage. In DGD, the electrodes are not in contact with the plasma. Therefore, the electrode erosion is not an issue, and consequently, higher voltage could be applied. Figure 3 shows the effect of applied voltage on phenol decomposition when a 250-mL solution containing 300-mg/L phenol was subjected to the discharge treatment. It can be observed from Figure 3 that the rate of phenol decomposition increases with an increase in applied voltage. For example, at 300 V, only 20% of phenol was removed within 30 min of discharge treatment, while approximately 70% of phenol could be degraded in the case of 900 V within the same time. The applied voltage could not be increased beyond 900 V in this study because of the limitation of the power supply. Therefore, we have selected 900 V for our next experiments. It is generally believed that the most responsible oxidant in glow discharge electrolysis is the hydroxyl radical (8-13). Increasing the applied voltage could increase the number of H2O+ ions in the glow discharge (Faradaic charge transfer, cf. current-voltage relationship) and its kinetic energy (increase of the cathode fall (20)). These two factors contribute to the increase in radicals in the bulk solution, which in turn enhances the degradation rate. In the lower-voltage region (
