Environ. Sci. Technol. 2000, 34, 509-513
Oxidative Processes Occurring When Pulsed High Voltage Discharges Degrade Phenol in Aqueous Solution B I N G S U N , † M A S A Y U K I S A T O , * ,† A N D J. S. CLEMENTS‡ Department of Biological & Chemical Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan, and Department of Physics and Astronomy, Appalachian State University, Boone, North Carolina 28608
In this investigation, results obtained using a pulsed discharge for organic compound removal are presented. The degradation of phenol by a streamer corona discharge and spark discharge, the effects of hydrogen peroxide additive on removal efficiency, and photochemical oxidation by ultraviolet light from the discharge plasma channel were investigated. The intermediate products and final byproducts formed by the spark discharge were also studied. A preliminary study of the degradation mechanism inside and outside the plasma channel was carried out. It was found that the removal efficiency of organic contaminants in the aqueous solution was higher for the spark discharge than for the streamer corona discharge and was greatly influenced by the discharge type and additive. The energy efficiency was the highest (6.91 × 10-3 µmol/J) for the case with hydrogen peroxide injection and spark discharge. The main intermediate products produced by the spark discharge during the treatment process were hydroquinone, pyrocatechol, and p-benzoquinone. These intermediate products disappeared when the treatment time was increased.
1. Introduction In recent years, environmental pollution has become a global problem. Much research has been reported on the removal of trace contaminants in aqueous solutions, for example, oxidation using ozone (1), electron beam irradiation (2, 3), carbon absorption (4, 5), TiO2 photocatalysis (6), sonochemistry (7), and UV photolysis (8, 9). Most of these methods focused on hydroxyl radical production directly in the aqueous solution. This is because the hydroxyl radical is a very powerful, nonselective oxidant that has the potential to kill bacteria and to oxidize organic compounds. Recently, electrical treatment by applying high voltage in the aqueous solution, such as electrochemistry (10, 11), corona streamer discharge (12-16), spark and/or arc discharge (17-19), has been used to degrade trace contaminants. The high voltage discharge method is a plasma process based on pulsed power technology. This method injects energy into an aqueous solution through a plasma channel formed by a high voltage pulsed discharge between two * Corresponding author phone: +81-277-30-1468; fax: +81-27730-1469; e-mail:
[email protected]. † Gunma University. ‡ Appalachian State University. 10.1021/es990024+ CCC: $19.00 Published on Web 12/30/1999
2000 American Chemical Society
submerged electrodes. When a spark discharge occurs, the following five individual effects of the spark process occur simultaneously (19, 20): (1) overpressure shock wave, (2) strong electrical field, (3) production of various free radicals, (4) intense ultraviolet radiation, and (5) ozone production with oxygen bubbling. These effects have various important roles in different application regions in the liquid. The intense shock wave generated by high voltage discharge technology has been used in industry for at least three decades (21, 22). The characteristics of the shock wave generated by the high voltage discharge have been studied by a number of authors (18, 20, 23-26). It has been shown that the intense shock wave generated by an expanding plasma channel has a pressure on the order of several thousands of atmospheres or even tens of thousands of atmospheres. In addition, the plasma channel formed by the pulsed discharge is a high temperature (several tens of thousands of degrees Kelvin) radiation source with a wide emission spectrum (19, 23). This radiation could cause a photolysis effect, leading to dissociation of water molecules. The characteristics of active species (radicals) generated by high voltage discharges have been investigated by a number of researchers (12-16, 19, 27). Water disinfection and the degradation of organic water pollutants using traditional UV-activated hydrogen peroxide and/or ozone have been reported (8, 28-31). However, in recent years, a pulsed high voltage process for the treatment of hazardous chemical wastes in water has been developed (16, 32-36). Due to collisions of high energy electrons with molecules, the intense electrical discharge disassociates water molecules to yield active OH radicals. These radicals combine with almost any organic chemical compound in a very efficient manner. When a spark discharge is produced by pulsed high voltage, UV radiation, active species (radicals), and a shock wave occur simultaneously (20). These processes have a synergistic effect in the degradation of organic compounds and also in sterilization. Therefore, this method is considered to be a promising alternative for the treatment of pollutants. Depending on the degradation region, pulse discharge processes can be conveniently grouped into two categories of effects: localized and extended (32, 33). Localized effects occur in the immediate vicinity of the plasma channel and include thermal oxidation within the plasma channel, vacuum UV photolysis at the surface of the plasma channel, and supercritical oxidation within the subsequent steam bubble. As shown in Figure 1, these processes are limited to the combined volumes of the plasma channel and steam bubbles. In contrast, extended effects primarily result from UV radiation and the intense shock wave, which radiate out into the bulk of the solution. Various chemical reactions occur outside the plasma channel. In the present study, a series of experiments and analyses was performed to determine which factors influence the degradation energy efficiency of the pulsed discharge. The photolysis effect of the ultraviolet light outside the plasma and the effect of additives were also studied. The oxidative process was analyzed to find the ways of increasing the energy efficiency. Expanding of the “localized” plasma channel and improvement of the “extended” efficiency are two ways of increasing the energy efficiency. The main byproducts formed from the treatment of phenol by a spark discharge were also identified. VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Variation of voltage and current for the spark discharge in aqueous solution.
FIGURE 1. Oxidation processes inside and outside the discharge plasma channel formed by a pulsed discharge in an aqueous phenol solution.
FIGURE 2. Schematic diagram of the experimental apparatus for organic compound removal by pulsed discharge.
2. Experimental Apparatus The experimental apparatus consists of a pulsed high voltage power supply and a reactor. The reactor vessel contains a needle-plate geometry electrode system as shown in Figure 2. A platinum (or stainless steel) needle 1.0 mm in diameter was used, and the initial curvature radius of the tip of the needle was about 50 µm. The needle was encased in ceramic pipe which was sealed with a silicone insulator, with the needle tip protruding 0.5 mm from the ceramic pipe. The grounded plate electrode was a stainless steel disk 20 mm in diameter. A positive pulse voltage was applied to the needle electrode. The pulse repetition rate was 30 pps for streamer corona discharge and 2.54 pps for spark discharge, respectively. The inner diameter of the reactor was 46 mm and the length was 32 mm. Two rotating spark-gap switches in the HV pulse supply were employed to generate high voltage pulses. The output pulse voltage and current were measured with a high voltage probe (Iwatsu HV-P30) and a wide band current transformer (Pearson Electronics M411), respectively. The peak power was in the megawatt range (19). During phenol treatment, the liquid was circulated with a peristaltic pump and cooled through a coiled-pipe heat exchanger maintained ambient temperature. The circulation rate of the liquid was adjusted as required from 20 to 200 mL/min. The amount of hydrogen peroxide (H2O2) produced by the discharge was measured using a colorimetric method 510
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developed by Ghormley (37). The concentration of the phenol and byproducts in the solution was measured by using high performance liquid chromatography (LC-9A, UV absorption detector, Shimadzu). An aqueous solution consisting of 70% phosphoric acid (0.01 mol/L H3PO4) and 30% acetonitrile was used as the eluting solvent.
3. Results and Analysis 3.1. Spark Discharge Characteristics. Figure 3 presents the time profile of the self-extinguishing pulse voltage and current for the needle-plate electrodes in an aqueous solution. A streamer corona discharge current the order of several amperes was produced after a short time delay when the fast rising voltage was applied to the electrode gap (the corona waveform is too small to observe in Figure 3 because of the large scale of the ordinate). The peak power for the streamer corona discharge was about 40 kW. The current increased as the discharge channels formed and developed. Then, the discharge current increased very rapidly and reached a maximum of 280 amperes when the channel approached the opposite plate electrode and a spark discharge occurred. The peak power reached about 2 MW for the spark discharge. Since the voltage source was a storage capacitor, the current decreased after the maximum was reached. Oscillations occurred in both the voltage and current profiles because of the inductance in the discharge circuit. The value of the voltage decay time constant was dependent on the capacitance of the storage capacitor and on the conductivity of the discharge channel. After a period, tTE, the streamer channel reaches the plate electrode, forming a plasma channel in the gap between the two electrodes. Therefore, the resistance of the electrode gap decreases rapidly, which leads to a rapid increase of the current (∼109-1010 amp/s) and decrease of the voltage (see point E in Figure 3). The spark discharge occurs after the streamer discharge period, which depends on the applied voltage. The peak current for the streamer corona discharge is small, in general, below 100 amperes with nanosecond voltage rise (20), and the lengths of plasma channels are less than the electrode distance. The peak current for spark discharge is several hundred amperes. Therefore, the spark plasma channel temperature is higher and the radiation intensity is stronger than for the streamer discharge. 3.2. Removal of Organic Compounds with the Pulsed Discharge. Both the streamer corona discharge and the spark discharge processes are new oxidation technologies by which energy is injected into an aqueous solution through a plasma channel. Figure 4 shows the effect of input energy (from capacitor) on the removal of phenol. The diamond-shaped points are for the streamer corona discharge, and the circular
streamer corona discharge and spark discharge, respectively. That is, the plasma channel volume is larger for the spark discharge than for the streamer corona discharge. In addition, it is found from Figure 4 that the experimental results and the curves obtained from eqs 1 and 2 are not completely consistent. This is because of additional phenol degradation, by UV and other effects, outside the plasma channel. In fact, the plasma radiation generated by the discharge is considered to be an intense UV source. When a spark discharge occurs, strong ultraviolet radiation with λ > 185 nm is emitted from the discharge plasma channel and escapes into the bulk of the solution (18), photolyzing any contaminants. Therefore, the photolysis effect must be considered. Oxidation Effect of UV Produced by Pulsed Discharge. The general equation for direct photolysis reactions is
FIGURE 4. Effect of input energy on removal efficiency for streamer corona and spark discharges. Where curves A and B show calculated values using eq 2 for streamer corona and spark discharges, and also curves C and D show calculated values by eq 6 for streamer corona and spark discharges, respectively. points represent the spark discharge results. The total treatment time is about 75 min (135 000 discharges; with 30 pps) to achieve 100% removal for the streamer corona discharge, and about 100 min (15 240 discharges; with 2.54 pps) to achieve 100% removal for the spark discharge. As mentioned previously, since a higher temperature plasma channel is created by the spark discharge, the ultraviolet radiation (blackbody radiation source) is stronger. Most part of the vacuum ultraviolet (VUV) (part of less than 185 nm) is absorbed by the water layer immediately surrounding the plasma channel, which leads to expansion of the plasma channel (18). Furthermore, in the spark discharge case, strong ultraviolet light radiates from the discharge channel, leading to the formation of hydroxyl radicals through the decomposition of water molecules. These radicals in the plasma channel can oxidize phenol in the aqueous solution. Case 1: Only Localized Oxidation in Plasma Channel. Assume an initial phenol concentration C0, total treatment solution volume Vt, and plasma channel volume Vch. Also, assume that the phenol within the channel is degraded completely by one pulsed discharge, and the solution in the reactor chamber is well mixed after one discharge (according to the treatment method developed by Willberg et al. (32, 33)). Amounts of the various radicals inside the channel are formed by pulsed discharge, and it is well-known that the temperature inside the channel reaches several thousand degrees. In addition, intense UV radiation also occurs. The radical oxidation, UV photolysis, and thermal destruction occur simultaneously inside the channel. Therefore, it is considered that these degradation reactions are sufficient to degrade phenol completely inside the channel for one pulse. After N discharges, the measured concentration CN will be (32, 33, 35)
E)
Eone 1 CN ln Vt (-b) C0
(1)
where b ) -ln(1 - Vch/Vt), E is the total energy per volume (J/mL), and Eone is the energy of one pule discharge. Clearly, E ) Eone‚N/Vt. The removal efficiency R is
R)1-
CN ) 1 - e-bN C0
(2)
Curves A (streamer discharge) and B (spark discharge) in Figure 4 are the results obtained from eqs 1 and 2, and b is 1.21 × 10-5 discharge-1 and 8.93 × 10-5 discharge-1 for the
dCN ) φ(λ)I0(λ,t){1 - exp(-2.303lCN)} dt
(3)
where CN is the phenol concentration, φ is the photolysis quantum yield, I0 is the intensity of the UV source, is the substrate extincion coefficient ((mol/L)-1 cm-1), and l is the radiation path length. Since I0 varies over the lifetime of the pulse, it is difficult to solve this equation. Instead, according to the treatment method developed by Willberg et al. (32, 33), we use the total energy per pulse, a well-defined quantity, and replace the time t with the number of discharges N, which yields the following equation (33)
-
dCN ) φ(λ)Q(λ){1 - exp(-2.303plCN)} dN
(4)
where Q(λ) is the radiant energy per discharge, and p is the molar absorption of phenol. Case 2: Both Localized Oxidation in the Plasma Channel and UV Photolysis Reaction. The general equation now becomes
CN C0
) e-Nb -
φQ 1 - e-Nb
+
φQN-1
C0 1 - e-b
∑ (e
C0 n)0
-Nb -2.303plCN-1-n
e
) (5)
Obviously, eq 5 is so complex that it cannot be solved simply. However, for large values of p, l, and CN, the last term in eq 5 can be neglected. Therefore the removal efficiency R′ becomes
R′ ) 1 -
(
)
CN k0 1 - e-Nb ) 1 - e-Nb C0 C0 1 - e-b
(6)
where k0 ) φQ (zero-order rate constant of the direct photolysis). Next we use experimental data to calculate values for k0, yielding k0 ) 1.28 × 10-3 µ(mol/L)/discharge for streamer corona discharge, k0 ) 1.52 × 10-2 µ(mol/L)/ discharge for the spark discharge. Substituting these values for k0 into eq 6 yields the removal efficiency as a function of input energy, see curves C (streamer discharge) and D (spark discharge) in Figure 4. The UV radiation play a more important role in the spark discharge (24%) than in the streamer discharge (15%). In addition, the shock wave produced by the spark discharge could cause cavitational effects similar to those observed in sonochemical experiments (7, 38). The sonolytic oxidation of organic compounds proceeds primarily via reactions with hydroxyl radicals. According to results by Chang et al. (20), an arc discharge generates a strong shock wave with cavitation zone, and the bubbles formed by discharge are plasma bubbles as well as to form transient supercritical water conditions. In the cavitation zone, the VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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with degradation products, this photolysis process may be expressed mathematically as
dCN ) -fhν,PHQφ{1 - e-2.303‚l{p[CN]+H2O2[H2O2]}} dN - kOH,PH[OH][CN], (7) where
fhν,PH )
FIGURE 5. Effect of input energy on removal efficiency with and without H2O2 additive for the spark discharge. strong ultraviolet light emission and high radical density are observed. The combined action of these factors leads to a higher energy efficiency for the spark discharge, i.e., with the streamer corona discharge, the ultraviolet radiation is weaker, and almost no shock wave is created. Therefore, more radicals are produced with the spark discharge and the spatial distribution of radicals is wider, i.e., they are formed outside the channel in addition to inside of it (19). This favors the destruction of organic compounds. Therefore, as shown in Figure 4, the removal efficiency is higher with the spark discharge than with the streamer discharge. 3.3. Effect of Chemical Additive on Removal Efficiency. One expects to find some compounds which can promote chemical reactions and increase energy efficiency. In this investigation, the effect of a chemical additive on the pulsed spark discharge removal efficiency was also studied. The combination of spark discharge with additive (hydrogen peroxide) was able to completely degrade a 50 ppm phenol solution with
