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APPLIED CHEMISTRY Degradation Performance of 4-Chlorophenol as a Typical Organic Pollutant by a Pulsed High Voltage Discharge System Le C. Lei,* Yi Zhang, Xing W. Zhang, Ying X. Du, Qi Z. Dai, and Song Han Institute of EnVironmental Pollution Control Technologies, Xixi Campus, Zhejiang UniVersity, Hangzhou 310028, P. R. China
Degradation of aqueous 4-chlorophenol (4-CP) with air bubbling was experimentally investigated in a multipoint-plate pulsed high voltage reactor. Several important parameters, including electrode distance, liquid conductivity, gas flow rate, and initial concentration, were investigated for 4-CP removal and energy efficiency. After discharge for 90 min, more than 95% of 4-CP was removed at the electrode distance of 1.5 cm, liquid conductivity of 1.06 µS/cm, gas flow rate of 0.75 m3/h, and initial concentration of 60 ppm with specific energy density of 450 J/mL. The energy efficiency of 4-CP was about 2.15 × 10-9 mol/J, and the mineralization of total organic carbon (TOC) reached to 55.8%. Hydrogen peroxide, ozone, nitrite, and nitrate were observed in the liquid phase. 4-chlorocatechol, hydroquinone, 5-chloro-3-nitropyrocatechol, chloride ion, formic, and acetic and oxalic acids were the main byproducts during 4-CP degradation. The degradation pathways were deduced including hydroxylation, N atom attack, and oxygen/ozone oxidation. A kinetic model was established to predict the degradation pathway, and the experimental data were fitted to the developed mathematical model well. 1. Introduction In recent years, considerable research has been reported on the pulsed high voltage discharge method used in the removal of organic contaminants in aqueous solutions.1,2 The pulsed high voltage discharge method is a plasma process based on pulsed power technology.3,4 When a spark discharge occurs, there are several individual effects produced simultaneously,5-7 such as an overpressure shock wave, strong electrical field, intense ultraviolet radiation, production of various free radicals, and ozone with oxygen bubbling. The pulsed high voltage discharges in water have been widely investigated. Willberg et al. investigated the degradation of 4-chlorophenol (4-CP) in an electrohydraulic discharge (EHD) reactor and discussed the energy efficiency of the EHD process.2 Sun et al. carried out a preliminary study of the degradation mechanism inside and outside the plasma channel with different discharge modes.8 Grymonpre´ et al. developed a mathematical model describing the phenol degradation mechanism in a batch reactor.9 In a pulsed high voltage discharge system, the energy efficiency was an important parameter to describe the relationship between energy and removal efficiency, which related to discharge modes, additive (such as H2O2, O3, and Fe2+), and treated compounds.2,8,10-11 In this system, mechanism and model studies with various pure gases bubbling (such as Ar, O2, O3, and N2) for liquidphase pollutant degradation were studied previously.12-14 However, the practical application of this method for industrial treatment was rarely reported, and several problems such as operation parameters, byproducts of pollutant, and mechanism model prediction needed to be studied. As we known, air is a * To whom correspondence should be addressed. Tel.: +86 571 88273090. Fax: +86 571 88273693. E-mail address:
[email protected].
common gas present in the environment, and it is convenient and cheap for industrial application. The effect of operation parameters on pollutant removal with air bubbling could provide performance for large-scale applications. And, identifying intermediates was beneficial to investigating the biological toxicity of the treated solution. In addition, the establishment of a kinetic model could be utilized to predict the degradation pathway. In this paper, a pulsed high voltage discharge system with air bubbling was used to degrade 4-CP (a typical organic pollutant for low biodegradability and ubiquitous presence in the natural environment). Special attention was given to some important operation parameters affecting 4-CP removal and energy efficiency, including electrode distance, liquid conductivity, air flow rate, and initial concentration. The active species (hydrogen peroxide, ozone, nitric acid, and nitrous acid) in distilled water and the intermediates of 4-CP (4-chlorocatechol, hydroquinone, 5-chloro-3-nitropyrocatechol, chloride ion, formic, and acetic and oxalic acids) during degradation were determined. Finally, the degradation mechanism of 4-CP with air bubbling was deduced and a kinetic model was established to predict the degradation pathway. 2. Experimental Methods 2.1 Setup. The experimental apparatus consists of a pulsed high voltage power supply and a reactor.12-14 Electrical energy was formed from a large pulse-forming capacitance (4000 pF) pulsed-power circuit. This energy was released as a pulsed electrical discharge using a rotating spark gap. In this study, the input voltage was 16 kV and the pulsed repetition rate was 100 pps for pulsed high voltage discharge. The reactor vessel (Ø 90 mm × 120 mm), which was made of Plexiglas, contained a needles-plate geometry electrode
10.1021/ie070186b CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007
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Figure 1. Schematic diagram of the experimental apparatus.
system, as seen in Figure 1. The high voltage electrodes contained 10 stainless steel acupuncture needles (Ø 0.25 mm) through 10 holes (Ø 1.0 mm) dispersed on the bottom of the reactor uniformly. When air bubbled through the holes, the gas enclosed the needle tip to mix the solution, resulting in the hybrid gas-liquid discharge. The grounded plate electrode was a stainless steel disk (Ø 85 mm). During degradation of 4-chlorophenol (4-CP), the solution (500 mL) was circulated with a peristaltic pump at 400 mL/min and the system was maintained at an ambient temperature of 25 °C by cooling water. The intervals for taking the sample were 0, 5, 10, 15, 20, 25, 30, 40, 50, 60, 75, and 90 min, respectively. In order to investigate the effect of operation parameters, the electrode distance (d) between high voltage electrodes and the grounded plate electrode was changed to 1.5, 2.5, and 5.0 cm; the liquid conductivity (σ) was adjusted by sodium chloride and changed to 1.06, 198, 959, and 1753 µS/cm; the air flow rate (Qair) was changed to 0.25, 0.5, 0.75, and 1.0 m3/h; and the initial concentration (C0) was changed to 30, 60, 90, and 120 ppm, respectively. The parameter was optimized when a condition merely changed one parameter and fixed others. Furthermore, the initial pH was about 5.5, and without adjustment, if any chemical was added into the solution, the liquid conductivity was changed, which may influence the discharge mode. 2.2. Analytical Procedures. The sample was conserved in an icebox to restrain its indirect reactions and detect them within 4 h. 4-CP and its intermediates were analyzed by HPLC (Knauer K-2005), equipped with a MS-2 C18 column (Ø 4.6 × 250 mm) and a UV detector. Nitrate and nitrite ions were detected by ion chromatography (IC; 792 Basic IC, Metrohm, Switzerland) with DS-plusTM auto-suppressor and a Metrosep A Supp 4 column (250 mm × 4.0 mm). Other intermediates were analyzed using a HP6890/5973 GC/MS with an HP/5MS capillary column (30 m × 0.25 mm i.d., film thickness 0.25 µm). Identification of the intermediates was established using an NIST98 MS Data Library. The detailed operation conditions of HPLC, IC, and GC/MS analyses were similar with that used in previous work.12 The concentration of dissolved ozone in distilled water was determined by the indigo method.15 As the nitrites existing in the water interfered with the absorbance of the indigo solution, the real absorbance of the ozone-indigo complex was obtained by eliminating the absorbance of the nitrites-indigo complex. The concentration of hydrogen peroxide was determined colorimetrically using the reaction of H2O2 with titanyl ions by the analysis of the maximum absorbance of the yellow peroxotitanium (IV) complex at a wavelength λ ) 410 nm.16 The carbon dioxide was determined by collecting the gas formed by the pulsed high voltage discharge and analyzing it by absorption in barium hydroxide and by back-titration with standardized oxalic acids.17 The liquid conductivity was measured by DDS11 A conductivity meter. The pH of the solution was measured by a pHS-25 meter.
Figure 2. Typical pulsed voltage and current waveforms of spark discharge (a) and corona discharge (b).
The output waveforms of pulsed voltage and current from the HV pulsed supply to the reactor were measured with a digital oscilloscope produced by Lecroy Co., which had a high-voltage probe (P 150-GL/5k, 1/5000) produced by the Japanese Hydrazin Company. Figure 2 presents the typical pulsed voltage and current waveforms of spark discharge (a) and corona discharge (b) for the needles-plate electrode system in an aqueous solution with the same input energy. The voltage for spark discharge retained at high value constantly within 1500 ns, which was longer than that for the corona discharge (250 ns). The peak voltage and current for spark discharge were both higher than that for corona discharge. The duration of one pulse spark discharge was nearly 5000 ns and the peak power for the spark discharge was about 2.68 × 106 W, while the duration of one pulse corona discharge was nearly 2000 ns and the peak power for the corona discharge was about 1.73 × 106 W. 2.3. Definition of the Specific Energy Density (SED) and Energy Efficiency (G50). The input energy per discharge can be obtained by the following equation,18-19
Ein )
∫0ΓUI dt
(1)
where U and I are the voltage (V) and current (A) at time t (ns), respectively. Ein is the input energy per discharge (J/ discharge). Γ is the total time (ns) of one discharge. The SED can be obtained by the following equation
SED )
EinN Vt
(2)
where, Vt is the total treatment solution volume (mL) and N is
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Figure 3. Effect of electrode distance on 4-CP removal against the discharge time. The lines are only for curve fitting and are not model results (operating conditions: σ ) 1.06 µS/cm; Qair ) 0.75 m3/h; C0 ) 60 ppm).
Figure 4. Effect of liquid conductivity on 4-CP removal against the discharge time. The lines are only for curve fitting and are not model results (operating conditions: d ) 1.5 cm; Qair ) 0.75 m3/h; C0 ) 60 ppm).
the discharge number
N ) 60‚f‚t
(3)
where f is the pulsed repetition rate (Hz) and t is the time for 4-CP degradation (min). The energy efficiency G50 (10-9 mol/J) is defined as the molecules number of 4-CP converted per input energy. Here, the yield value was calculated when 50% 4-CP was degraded:10
1 C 2 0 G50 ) SED50
(4)
where C0 is the initial 4-CP concentration (M). 3. Results and Discussion 3.1. Effect of Operation Parameters on 4-CP Removal. Figures 3-6 show the effect of operation parameters on 4-CP removal with elapsed discharge time. Table 1 depicts the Ein, SED50, G50, initial and final pH, and liquid conductivity on different operation conditions. Figure 3 shows the effect of electrode distance on removal of 4-CP at 1.06 µS/cm, 0.75 m3/ h, and 60 ppm. The removal efficiency increased with increasing residence time under each electrode distance condition and the
Figure 5. Effect of air flow rate on 4-CP removal against the discharge time. The lines are only for curve fitting and are not model results (operation conditions: d ) 1.5 cm; σ ) 1.06 µS/cm; C0 ) 60 ppm).
removal efficiency of 4-CP at 1.5 cm was the highest, reaching 95.7% after about 90 min. In addition, the discharge mode was changed from spark discharge to corona discharge when the electrode distance increased from 1.5 to 5.0 cm and the input energy per pulse decreased from 0.415 to 0.325 J/pulse. The values of G50 as seen in Table 1 were about 2.15 × 10-9, 1.25 × 10-9, and 0.53 × 10-9 mol/J at 1.5, 2.5, and 5.0 cm, respectively, which indicated that higher energy efficiency was obtained at spark discharge mode. After discharge, the pH decreased and liquid conductivity increased after discharge. Lower final pH and higher final liquid conductivity was obtained with shorter electrode distance, which indicated that higher removal efficiency resulted in lower final pH and higher final liquid conductivity. Liquid conductivity is one of the important parameters to influence the discharge mode and radical emission intensity.6 At an electrode distance of 1.5 cm, the effect of liquid conductivity on the removal of 4-CP is depicted in Figure 4. With the increase of liquid conductivity, the discharge mode was spark discharge at 1.06 µS/cm, then changed to sparkstreamer discharge at 198 µS/cm, and finally to corona discharge at the conductivity higher than 959 µS/cm. When the discharge time was about 90 min, the removal efficiency of 4-CP with spark discharge (95.7%) was 7.7% higher than that with sparkstreamer discharge (88.0%) and 22.1% higher than that with corona discharge (73.6%). During discharge at an initial liquid conductivity of 1.06 µS/cm and electrode distance of 1.5 cm, the liquid conductivity increased to 683 µS/cm, and the discharge mode changed from spark discharge to spark-streamer discharge. The increase of liquid conductivity and change of discharge mode resulted in the slow 4-CP removal trend obtained after 60 min. In addition, when the corona discharge occurred, the removal efficiency in the case of 1753 µS/cm was similar with that of 959 µS/cm. The energy efficiency with corona discharge (0.55 × 10-9 mol/J) was also much lower than that with spark discharge. The lower removal efficiency and energy efficiency with the corona discharge mode may derive from the inhibition of streamer formation at high liquid conductivity. As shown in Figure 2, in the case of spark discharge, the high current was utilized for streamer propagation, but in the case of corona discharge, the current only played a role in transferring the electron between two electrodes. Furthermore, the intensity of ultraviolet and shockwave energy, generated by electrohydraulic effects, was much higher with spark discharge than that with
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Figure 6. Effect of initial concentration on 4-CP removal against the discharge time. The lines represent the model results (operating conditions: d ) 1.5 cm; σ ) 1.06 µS/cm; Qair ) 0.75 m3/h).
corona discharge, and ultraviolet photolysis and oxidation by an active radical such as •OH were enhanced.13 Figure 5 shows the removal efficiency of 4-CP at the air flow rates of 0.25, 0.50, 0.75, and 1.00 m3/h, electrode distance of 1.5 cm, and liquid conductivity of 1.06 µS/cm. The removal efficiency of 4-CP was enhanced with the air flow rate as it increased from 0.25 to 0.75 m3/h, while the value of SED50 for 50% 4-CP removal in Table 1 decreased from 340 to 109 J/mL. But when the flow rate was about 1.00 m3/h, the value of SED50 only decreased a little (100 J/mL). The energy efficiency of 4-CP in the case of 0.75 m3/h was 3.1 and 1.6 times that in the case of 0.25 (0.69 × 10-9 mol/J) and 0.50 m3/h (1.34 × 10-9 mol/J), respectively, but only 0.20 × 10-9 mol/J less than that in the case of 1.00 m3/h (2.35 × 10-9 mol/J). Therefore, it could imply that the optimum flow rate of air was 0.75 m3/h and the enhanced effect was unapparent when the flow rate was too large. The effect of the initial concentration of 4-CP at 1.5 cm, 1.06 µS/cm, and 0.75 m3/h is presented in Figure 6. Under the same discharge time, the removal efficiency of 4-CP decreased with an increase in the initial concentration, while the absolute removal amount increased. As the number of 4-CP molecules increased along with increasing initial concentration, the discharge power (Ein) in the reactor did not vary accordingly as shown in Table 1, which led to the drop in the removal efficiency. With the initial concentration of 4-CP increasing from 30 to 120 ppm, the value of G50 increased from 1.76 × 10-9 to 2.66 × 10-9 mol/J, which was why the absolute removal amount increased. The result that a higher final liquid conductivity was obtained in Table 1 at a higher initial concentration also validated the finding that a greater absolute amount was degraded to form plenty of substances and ions. Compared to oxygen bubbling, for corona discharge, the G50 with air bubbling (
