Hybrid GasâLiquid Electrical Discharge Reactors ... - ACS Publications

Ind. Eng. Chem. Res. 2004, 43, 1975-1989

1975

Hybrid Gas-Liquid Electrical Discharge Reactors for Organic Compound Degradation David R. Grymonpre´ ,† Wright C. Finney,† Ronald J. Clark,‡ and Bruce R. Locke*,† Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida State University and Florida A & M University, 2525 Pottsdamer Street, Tallahassee, Florida 32310-6046, and Department of Chemistry, Florida State University, Tallahassee, Florida 32306-4390

A gas-liquid hybrid pulsed corona discharge reactor that utilizes high voltage needle-point electrodes submerged in the aqueous phase coupled with planar ground electrode suspended in the gas phase above the water surface has been developed and analyzed for the removal of low concentrations of phenol. Two types of ground electrodes were evaluated. One type consisted of a solid disk made of stainless steel, and the second type consisted of a disk made of high porosity reticulated vitreous carbon (RVC). The liquid-phase discharge leads to the formation of hydrogen peroxide and hydroxyl radicals, and the gas-phase discharge leads to the formation of ozone. The reticulated carbon electrode produced a higher number and more uniform distribution of plasma channels in the gas phase above the liquid surface. This case also led to the largest amount of ozone dissolved in the liquid phase. The combined action of the reactive species formed in the gas and the liquid phases on the degradation of phenol, the formation of primary byproducts, and the removal of total organic carbon was evaluated for a variety of system conditions, including the addition of ferrous sulfate (for Fenton’s reactions), activated carbon (for adsorption and reaction), and various electrode gap spacing. A mathematical model, including sensitivity analysis, has been developed to illustrate the major reaction pathways. Introduction Recent interest in the application of advanced oxidation technologies for the degradation of organic compounds in water treatment has spurred a wide range of studies to develop novel combinations of the various techniques and to investigate the chemical reactor design and analysis of such systems. Advanced oxidation, or more recently advanced oxidation/reduction, technologies include a wide range of processes that can broadly be classified into chemical (e.g., direct ozone, direct hydrogen peroxide, and combinations of ozone and hydrogen peroxide),1-3 photochemical and photocatalytic (e.g., UV/ozone/hydrogen peroxide and TiO2),4,5 mechanical (e.g., ultrasonic),6 and electrical (e.g., electrohydraulic discharge,7 corona discharge,8 and glow discharge electrolysis9,10). A variety of studies have been conducted to investigate novel combinations of such processes (as well as combinations with more conventional physical, chemical, and biological water treatment methods11,12) in order to enhance the formation of the highly reactive radicals (e.g., hydroxyl radical) and molecular species (e.g., ozone and hydrogen peroxide) formed in such systems and to optimize the degradation of the target pollutants. Gas-phase electrical discharge reactors (including dielectric barrier discharge, DC, AC, and pulsed corona * To whom correspondence should be addressed. Tel.: (850) 410-6165. Fax: (850) 410-6150. E-mail: [email protected]. Corresponding author address: Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida State University, 2525 Pottsdamer Street, Tallahassee, FL 32310-6046. † Department of Chemical Engineering, FAMU-FSU College of Engineering, Florida State University and Florida A & M University. ‡ Department of Chemistry, Florida State University.

discharge) have long been used as the most efficient means for the formation of ozone.13-15 Although ozone is a relatively selective oxidant, the combination of ozone and hydrogen peroxide, along with certain catalysts and/ or ultraviolet light, can lead to hydroxyl radical formation. In addition, direct reactions of ozone with byproduct species may enhance hydroxyl radical reactions with the primary species by reducing the competition for radicals. The application of a short high voltage electrical pulse (pulsed corona or corona-like discharge) directly in the aqueous phase has been demonstrated to lead to the production of hydroxyl and other hydrogen and oxygen radicals.16-20 Previous electrical discharge reactors for the treatment of liquid-phase organic compounds have utilized two basic types of electrode configurations. The first type of discharge utilized high voltage needle electrodes and stainless steel planar ground electrodes both fully submerged within the liquid phase.8,16,20-23 In this type of electrode configuration the discharge was formed in only the liquid phase. Another type of corona discharge reactor used point electrodes in the gas phase above the water surface, and the ground electrode was either submerged in the liquid or placed below the liquid phase.24,25 This reactor electrode configuration leads to only gas-phase discharge. Furthermore, several studies have considered bubbling oxygen through the submerged hollow needle electrodes.21 Although bubbling oxygen through the submerged needle electrodes leads to the formation of ozone, the amount of hydrogen peroxide formed in this case was suppressed in comparison to the case without gas flow in the same electrode system.26 Another novel system utilized a high voltage discharge in foam in order to form ozone in the gas and hydrogen peroxide in the liquid.27 The present study seeks to combine the advantages of gas-phase discharge (i.e., for ozone and oxygen radical

10.1021/ie030620j CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004

1976 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004

Figure 1. Schematic of the electrode positions in the pulsed corona reactor #1: (a) ground electrode submerged as used in previous corona discharge research and (b) ground electrode suspended above the liquid surface as used in the present study.

formation in the gas phase and at the interface) and liquid-phase discharge (i.e., for hydroxyl radical and hydrogen peroxide formation) through the use of a hybrid gas-liquid corona discharge reactor whereby the high voltage electrode is placed in the liquid phase and the ground electrode is placed in the gas phase (Figure 1). This electrode configuration leads to simultaneous formation of ozone and other gas-phase reactive species as well as hydrogen peroxide and other liquid-phase reactive species. The effects of the addition of iron salts (to enhance Fenton’s chemistry) and activated carbon (adsorption and surface-phase reactions) on liquid-phase phenol oxidation in the gas-liquid hybrid reactor equipped with either stainless steel or reticulated vitreous carbon ground electrodes are reported. Experimental Methods The power supply and all experimental methods and materials used in the present study are identical to those used in previous work.8,16,26,28,29 Two reactors were used in the present study. Reactor #1 was similar to that used in previous work26,28,29 except the electrode configuration utilized a stainless steel ground electrode placed above the surface of the liquid solution as shown in Figure 1b. The experiments reported in previous work26,28,29 with this reactor utilized the electrode configuration shown in Figure 1a where the stainless steel ground was completely submerged within the liquid phase and the liquid volume was 1 L. For the experiments reported in the present study, the high voltage electrode was submerged in the aqueous solution, and the stainless steel ground electrode was placed ∼3-5 mm above the solution in the gas phase. To maintain the 5 cm total electrode gap distance, the solution volume was adjusted to 780 mL. With this air-gap electrode configuration, discharges form in the aqueous phase, along the gas-liquid interface, and in the gas phase above the solution interface. In all cases where oxygen was flowing in the space above the liquid phase, the flow rate of oxygen was 150 SCCM. In all experiments the applied voltage was 45 kV produced from a 2 nF capacitor with 60 Hz frequency. All experiments were conducted three times and gave reproducibility within 5%. All experiments were conducted with the initial solution conductivity adjusted with either KCl or FeSO4 to 150 µS/cm. The initial phenol concentration was 100 ppm, and the initial pH was 5. All results are reported as normalized concentrations to the initial phenol concentration. The changes in pH were similar to those reported in the previous study.26,28,29 Reactor #2, shown in Figure 2 and in Figure 3, had a total capacity of approximately 2.5 L, although the solution volume was typically 0.5 L. The main body of

Figure 2. Detailed schematic of the pulsed corona reactor #2. The ground electrode position can be adjusted as in Figure 1 for reactor #1.

the reactor consisted of a jacketed glass tube with two Teflon end caps threaded into the reactor custom-made by Ace Glass, Inc. (Vineland, NJ). The inner diameter of the glass jacketed reactor was 10 cm, and the height was 37 cm. The Teflon end piece for the bottom of the reactor had machine drilled holes allowing for the placement of up to five point electrodes. Each point electrode comprised of a stainless steel tube that was sealed with epoxy where the electrode was connected at the outside of the reactor. A thin Nichrome (nickel chromium) wire was inserted in the stainless steel tube so that approximately 3-5 mm of wire protruded above the end of the tube. The tip of the Nichrome wire served as the high voltage point electrode. It was possible to operate the reactor with any combination of up to five high voltage electrodes at the bottom of the reactor, and, depending on the number of electrodes used, some of the holes were closed off with Nylon fittings. Five holes were drilled into the Teflon cap on the top of the reactor. A glass stirring shaft fitted with a Teflon impeller was inserted through the center hole. This stirrer provided adequate mixing to the bulk liquid phase. Two of the other holes were used to support the ground electrode as well as to pass the electrical connection to the ground electrode. The ground electrode material used in this reactor was a 10 pores per inch (39 pores per meter) reticulated vitreous carbon (RVC) disk from Electrosynthesis Corporation (Lancaster, NY) with a hole in the center for the stirring rod. The RVC disk was attached to two stainless steel rods that were fixed in place through two holes in the top Teflon cap as shown in the Figure 3. The two remaining holes in the top Teflon cap were used for oxygen input and output and for removing samples from the liquid. When gas was introduced to this reactor, the inlet tube was positioned directly above the RVC ground electrode. The second reactor was used in a manner similar to the first reactor, such that the ground

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 1977

Figure 3. Photograph of hybrid gas-liquid discharge reactor (reactor #2) showing details of point high voltage electrodes, jacketed glass reactor, and close-up and SEM views of ground RVC electrodes.

electrode was either submerged in the solution or suspended above the solution in the gas phase. For a few studies the electrode gap, the total distance between high voltage and ground electrodes, was varied from 5 to 8 cm; however, the distance between the water surface and the ground electrode was maintained at 3-5 mm. RVC is a highly conductive, relatively chemically inert, glassy carbon material that has low microscopic porosity and large macroscopic porosity. The sharp edges are particularly good for electrical discharge applications. The mathematical model and the sensitivity analysis of the reaction model used in the present study are the same as those reported in the previous work.26,28,29 All reaction numbers, rate constants, and the numerical order and labels for the reactions are the same as reported in the previous study.26 The sensitivity coefficients used in the present study are defined by

S(C;ki) )

∂lnC ∂lnki

where C is the concentration of phenol and ki are the various model reaction rate constants as described in the previous paper.26 As in the previous study, the nondimensional sensitivity coefficients for the concentration of phenol with respect to the model parameters are determined.26 The peak values of these sensitivity coefficients are given, and only the reactions with values

within 2-3 orders of magnitude of these peak values are reported here. Results and Discussion Stainless Steel Electrode (Reactor #1). Hydrogen Peroxide and Ozone Formation. Figure 4 shows the concentration of hydrogen peroxide as a function of time within reactor #1 for various electrode configurations. The formation of hydrogen peroxide when both electrodes were submerged26 is shown on this figure as a reference. This figure shows that when oxygen flowed above the liquid solution, similar amounts of hydrogen peroxide were formed in comparison to the condition when both electrodes were submerged with no oxygen flowing through the high voltage electrode. No noticeable changes in the pH or conductivity of the solution were found in these experiments. Other independent measurements of hydrogen peroxide in hybrid gas/liquid corona reactors confirm the major finding here that the amount of hydrogen peroxide formed in the liquid phase is the same in the reactor with a gas gap containing oxygen (or combinations of oxygen and noble gases) as in the case when both electrodes were submerged in the liquid phase.31 Table 1 shows measurements of ozone in the liquid phase with both of the ground electrode configurations. This table shows that when both of the electrodes were submerged in the aqueous solution and oxygen was bubbled through the high voltage electrode, 0.16 mg/L


Figure 4. Hydrogen peroxide formation in reactor #1 with a stainless steel ground electrode under various conditions with and without gas flow and activated carbon (150 µS/cm KCl, 45 kV, and where appropriate 150 SCCM oxygen flow). The lines are only for connecting points and are not model results. Table 1. Dissolved Ozone Concentrations for Various Conditionsa FeSO4 (150 µS/cm) (mg/L)

KCl (150 µS/cm) (mg/L)

ground position

condition

submerged in solution26 suspended above solution suspended above solution

O2 bubbled through HV electrode stagnant air

0.21

0.16

0.14

0.12

O2 flowing

0.31

0.25

a

Stainless steel ground electrode in reactor #1 at 45 kV.

of ozone was produced at 60 min in a potassium chloride solution. When both electrodes were submerged and oxygen was bubbled through the high voltage electrode with the solution containing ferrous sulfate, the concentration of ozone increased to 0.21 mg/L. The amount of ozone further increased when the ground electrode was placed in the gas phase above the solution surface and pure oxygen flowed in the gas above the liquid solution. The ozone measured in the potassium chloride solution was 0.25 mg/L in 60 min and 0.31 mg/L in the ferrous sulfate solution when the ground electrode was suspended above the liquid solution. When stagnant air was present above the liquid solution, less ozone was dissolved in the liquid phase due probably to lower oxygen concentration in the gas and perhaps due to the effect of gas composition on the properties of the electrical discharge. Additional details of gas-phase ozone formation and liquid-phase hydrogen peroxide formation are reported in other studies.26,32-34 Phenol Oxidation. Figure 5 shows 35% phenol removal in 60 min when the ground electrode was suspended in the gas phase consisting of pure oxygen and with potassium chloride salt in the solution phase, while Figure 6 shows 97.5% phenol removal under the same conditions with the addition of ferrous sulfate. In the latter case the removal of phenol was higher than in the previously reported case26 where both electrodes were submergedsi.e., where 90% of phenol was removed in 60 min. Although there was a slight increase in TOC removal when the ground electrode was suspended above the liquid (21.7% vs 17.8%), the most significant changes were in the concentrations of the byproducts. When both electrodes were submerged, the oxidation of all species was primarily by hydroxyl radicals produced through Fenton’s reaction, and the concentration of

Figure 5. Phenol removal and product formation in reactor # 1 with a stainless steel ground electrode (150 µS/cm KCl, no activated carbon, 45 kV, and 150 SCCM O2). The lines are only for connecting points and are not model results. The phenol and product concentrations are normalized to the 100 ppm initial phenol concentration.

Figure 6. The effect of iron sulfate on phenol removal and byproduct formation in reactor #1 (150 µS/cm FeSO4, no activated carbon, 45 kV, and 150 SCCM O2). The lines are only for connecting points and are not model results. Phenol and all byproducts are normalized to the initial 100 ppm phenol concentration.

catechol increased to 25 ppm at 30 min and decreased to 20 ppm after 60 min. Hydroquinone gave results similar to catechol increasing to 12 ppm at 30 min and


Figure 7. The effects of activated carbon and oxygen flow on phenol removal in reactor #1 (150 µS/cm KCl, 45 kV, 1 g/L activated carbon). The lines are only for connecting points and are not model results. Table 2. Energy Efficiency for Phenol Removal in 150 µS/cm KCl Solutionsa KCl condition

% removal

no C, stagnant air no C, O2 flowing (150 SCCM) C (1 g/L), O2 flowing (150 SCCM)

24.6 35.6 100

a

% on carbon

energy per pulse (J/pulse)

g/kWh

EE/O (kWh/1000 L per order of magnitude removed)

3.5

1.31 1.29 1.31

0.3 0.7 3.7

791 316 30

Stainless steel ground electrode in reactor #1, initial phenol concentration 100 ppm, 45 kV, pH 5.

Table 3. Energy Efficiency for TOC Removal in 150 µS/cm KCl Solutionsa KCl condition

% removal


10.8 8.6 81.0

a

% on carbon


g/kWh


3.5

1.31 1.29 1.31

0.2 0.3 2.9

1285 805 49


decreasing to 8 ppm after 60 min. However, when the ground electrode was suspended above the solution, the catechol increased to 20 ppm at 30 min and decreased to 9 ppm after 60 min, and the hydroquinone peaked at 10 ppm and reach 3 ppm after 60 min of corona treatment. These results imply that the combination of the hydrogen peroxide and ozone (and potentially other oxidants) formed in the hybrid reactor, causes the extent of oxidation of byproducts proceed further than when only hydrogen peroxide was present. Figure 7 summarizes the effects of the different conditions used to evaluate the potassium chloride solution when the ground electrode was suspended above the liquid solution in the gas phase. The case with the stagnant gas showed the lowest phenol removal. This condition had the lowest ozone and hydrogen peroxide levels of the three experimental conditions. When activated carbon was added to the system with oxygen flowing above the liquid solution, phenol was completely removed from the system in 30 min of corona treatment. After analyzing the activated carbon after the 60 min treatment (following the method reported previously26) only 3.5% of the original carbon was estimated to be adsorbed to the activated carbon after corona treatment. In the previously reported case where both electrodes were submerged (shown in Figure 4 of the previous

study26) there was approximately 15% phenol removal in 60 min,26,29 while in the present case 30% phenol removal was achieved. However, in another previously reported case where oxygen was bubbled through the high voltage electrode about 50% phenol removal was observed.26 In contrast, with activated carbon approximately 95% phenol removal was observed in 60 min in the previously reported case with oxygen bubbled in the hollow electrode compared to over 99% removal in 30 min in the present study. It is possible, as suggested in the previous study,26 that the higher ozone and other oxidants, concentrations in the liquid phase of the hybrid reactor may enhance oxidation with the activated carbon. The results of phenol and TOC removal from potassium chloride solutions with the stainless steel ground electrode suspended above the liquid surface are shown in Tables 2 and 3. The experimental condition with the lowest removal efficiency was the case with a stagnant gas phase. The removal efficiencies for this case were 0.3 g/kWh and 791 kW/1000 L/order for phenol removal and 0.2 g/kWh and 1285 kW/1000 L/order for TOC removal. When oxygen was flowing in the gas phase above the liquid, the removal efficiency increased to 0.7 g/kWh and 316 kW/1000 L/order for phenol removal and 0.3 g/kWh and 805 kW/1000 L/order for TOC removal. This is clearly due to a higher concentration of ozone.


Figure 8. The effects of activated carbon and oxygen flow with iron salt on phenol removal in reactor #1 (150 µS/cm FeSO4, 150 SCCM oxygen flow, 1 g/L activated carbon, 45 kV). The lines are only for connecting points and are not model results. Table 4. Energy Efficiency for Phenol Removal in 150 µS/cm FeSO4 Solutionsa FeSO4

a

condition

% removal

no C, stagnant air no C, O2 flowing (150 SCCM) C, O2 flowing (150 SCCM)

89.8 97.5 100

% on carbon


g/kWh


3.5

1.39 1.31 1.31

1.4 2.2 4.0

107 56 22


Table 5. Energy Efficiency for TOC Removal in 150 µS/cm FeSO4 Solutionsa FeSO4

a

condition

% removal


28.9 21.7 75.9

% on carbon


g/kWh


3.5

1.39 1.31 1.31

0.5 0.6 3.2

463 390 41


The highest removal efficiency occurred when activated carbon was suspended in the reactor and oxygen was flowing in the region above the liquid surface. This condition resulted in removal efficiencies of 3.7 g/kWh and 30 kW/1000 L/order for phenol removal and 2.9 g/kWh and 49 kW/1000 L/order for TOC removal. With activated carbon the phenol removal efficiency is higher when the ground electrode was suspended above the liquid surface (3.7 g/kWh) than in the previously reported case where oxygen was bubbled through the high voltage electrode (Table 3 in the previous paper,26 3.1 g/kWh). This latter result can be partially explained by the higher amounts of hydrogen peroxide formed in the present case in comparison to bubbling the gas through the electrode. Figure 8 shows phenol removal for the various experimental conditions with the ground electrode suspended above the surface of the liquid with ferrous sulfate solutions. Similar to the results with potassium chloride (Figure 7), the case with stagnant gas in the region above the liquid solution gave the lowest phenol removal of the three conditions shown. This lower amount of phenol removal compared to the other two ferrous sulfate solutions is again due to the smaller amounts of ozone and hydrogen peroxide formed during

corona treatment. The ferrous sulfate solution gave a much higher phenol removal in comparison to the potassium chloride solution. When activated carbon was added to the ferrous sulfate solutions, 93% of the phenol was removed in the first 15 min, and after 45 min, greater than 99% of the phenol was removed. In this case, 3.5% of the original phenol was found on the surface of the activated carbon after corona treatment. The phenol removal for the cases of oxygen flowing with activated carbon particles with KCl (Figure 7) and with ferrous sulfate (Figure 8) are within experimental error (about 5%). Tables 4 and 5 summarize phenol and TOC removal as well as removal efficiency for the ferrous sulfate solutions when the stainless steel ground electrode was suspended above the liquid surface. For the experimental condition with stagnant air in the gas phase, the removal efficiencies were 1.4 g/kWh and 107 kW/1000 L/order for phenol and 0.5 g/kWh and 463 kW/1000 L/order for TOC removal. The removal efficiency increased to 2.2 g/kWh when oxygen flowed through the region above the aqueous solution. Model-Data Comparison (No Activated Carbon). Figure 9 shows the model-data comparison for the experimental conditions when the stainless steel elec-


Figure 9. Model data comparison for phenol oxidation under 45 kV corona treatment in reactor #1 (150 µS/cm FeSO4 or 150 KCl).

Figure 10. Model predictions of the reaction rates for phenol and catechol with hydroxyl radicals and ozone for cases with and without ozone in reactor #1 (all conditions appropriate for experiments with 45 kV and no activated carbon).

trode was placed in the gas above the solution surface and 150 SCCM O2 was flowing in the gas phase. The experimental results for the phenol removal from the potassium chloride solution were used to determine the rate of ozone formation when the stainless steel ground electrode was suspended above the surface of the liquid solution. The value determined for the production of ozone was 1.03 × 10-7 M s-1. Iron reactions were added to the model, and the model was used to predict the experimental results based on previously developed model,26,29 as shown in Figure 9. Although the model prediction matches the experimental data well at 60 min of corona treatment, the model underpredicts the removal for earlier times. Figures 7 and 8 (as well as comparison with the previous results, Figures 4 and 5 of previous work26) show that the extent of phenol removal for the case with ferrous sulfate was higher when oxygen, leading to ozone, was present than when ozone was not present. The effect of ozone on phenol removal may be due to the competition for ozone and hydroxyl radicals among phenol and byproducts. Figure 10 shows the computed reaction rates of phenol and catechol with hydroxyl radicals and ozone molecules for experimental conditions with and without ozone (including ferrous sulfate). As shown in this figure, the rate of reaction of phenol with the hydroxyl radicals is larger when ozone is present than when ozone is not present, and conversely the rate of reaction between catechol and hydroxyl

radicals is smaller when ozone is present. In addition, when ozone is present, the reaction rate between the phenol and ozone is small, but the reaction rate between the catechol and ozone is higher than the reaction rate between catechol and hydroxyl radicals. These model results indicate that when the ozone is present, ozone preferentially reacts with the primary byproducts, allowing more of the hydroxyl radicals to react directly with the phenol, increasing the overall phenol removal. Sensitivity Analysis (No Activated Carbon). The normalized sensitivity coefficients for the phenol removal from the potassium chloride solution with the stainless steel ground electrode suspended in the gas phase above the liquid solution are given in Figure 11. Since KCl is used as the conductivity salt, only a small amount of hydroxyl radicals is formed, and therefore it is expected that the phenol would be preferentially oxidized by ozone. This expectation is also supported by analysis of the sensitivity coefficients. The normalized sensitivity coefficient with the highest maximum value corresponds to the rate of production of ozone (-0.55) reaction 72, given by k

72 3O 9 2 2 8 O3

(R.72)

The other reactions of importance, although with at least 1 order of magnitude lower sensitivity, are

k3

H2O98H+ + eaq- + •OH k1

H2O98H• + •OH

(R.3) (R.1)

It is interesting to note that the order of importance of the first nine reactions are the same for this case as


Figure 11. Sensitivity coefficients of the reaction rate constants on the phenol concentration in reactor #1 for the case of 45 kV, 150 µS/cm KCl, and no activated carbon or oxygen flow.

Figure 12. Sensitivity coefficients of the reaction rate constants in the presence of iron salt on the phenol concentration in reactor #1 for the case of 45 kV, 150 µS/cm FeSO4, and no activated carbon or oxygen flow.

for the case reported in previous work with the oxygen bubbled through the high voltage electrode (Figure 9).26 Figure 12 shows the normalized sensitivity coefficients for the phenol removal from ferrous sulfate solutions with the stainless steel ground electrode suspended in the gas phase above the solution. The four sensitivity coefficients with the highest absolute values were the production of hydrogen peroxide (-58) reaction 2, k2 1 1H H2O2 + 2 H2O982 2

(R2)

the production of ozone (-12.5) reaction 72, Fenton’s reaction (-5.2) reaction 5, k5

Fe2+ + H2O298Fe3+ + OH- + •OH

Two other reactions of importance in this case are reaction 58 and

(R5)

and the reaction between phenol and the hydroxyl radical (-4.75) reaction 53.

In previous work (Figure 10)35 where the ground electrode was submerged in solution and oxygen was bubbled through the high voltage electrode, the formation of ozone, reaction 72, was the most important reaction followed by the formation of hydrogen peroxide, reaction 2. In that case, smaller amounts of hydrogen peroxide were formed, and the Fenton’s reaction and


Figure 13. Model data comparison for phenol oxidation in reactor #1 for the case of activated carbon (1 g/L), 45 kV, 150 µS/cm KCl or FeSO4, and no oxygen flow.

Figure 14. Sensitivity coefficients of the reaction rate constants on the phenol concentration in reactor #1 for the case of activated carbon (1 g/L), 150 µS/cm KCl, and no oxygen flow.

the hydroxyl reaction with phenol were not in the 10 highest values of the sensitivity coefficients. Model-Data Comparison (with Activated Carbon). Figure 13 shows the model data comparison for the experimental conditions for the stainless steel ground electrode placed in the gas phase above the aqueous solution with 1 g/L activated carbon present suspended in the solution. Both of the model simulations shown in this figure utilized rate constants determined from independent experiments. The model data comparison for the ferrous sulfate solutions matched well using the previously determined ozone formation rate (Figure 9), hydrogen peroxide rate,29 and phenol reaction rate on the surface of the activated carbon.26 The model data comparison for the potassium chloride solutions did not match as well. Although the match at 60 min was good, the experimental data showed a phenol concentration of 0 at 30 min, where the model predicted a value of 16 ppm after 30 min. The model predicts that the percentages of original phenol remaining on the activated carbon after corona treatment are 3.6% and 0.2% for potassium chloride and ferrous sulfate solutions, respectively. These results closely match the values obtained experimentally (3.5% of the original phenol was present on the activated

carbon after the experiments for both iron and potassium salt solutions). Further work is necessary to fully characterize the concentrations of oxygen and ozone in the liquid phase, and it is perhaps possible that the case with KCl shown in Figure 13 deviates from the model due to higher sensitivity of phenol concentration with the rates of formation of these species. It is also possible that other oxidants are formed in the hybrid reactor, and further investigation is necessary to determine the identity and existence of such species. Sensitivity Analysis (with Activated Carbon). The results for the normalized sensitivity coefficients for a potassium chloride solution with suspended activated carbon particles and the stainless steel ground electrode suspended in the gas phase above the surface of the solution are shown in Figure 14. As shown in this figure, the reaction that is the most important to the amount of phenol in solution is the ozone formation (reaction 72), and this reaction has a normalized sensitivity coefficient of -6.8. The next four most important processes are the reaction of phenol on the surface of the activated carbon (-4) (rate constant kpph), the adsorption of phenol to the surface of the activated carbon (-2.4) (equilibrium constant KPH), the reaction between phenol and the


Figure 15. Sensitivity coefficients of the reaction rate constants on the phenol concentration in reactor #1 for the case of iron salt and activated carbon (1 g/L), 150 µS/cm FeSO4, and no oxygen flow.

Figure 16. The time dependence of the sensitivity coefficients for phenol concentration in reactor #1 for the case of iron salt and activated carbon (1 g/L) and 150 µS/cm FeSO4.

hydroxyl radical (-0.95) (reaction 53), and the production of hydrogen peroxide (-0.95) (reaction 2). Although the first three processes were expected to be the most important, the reaction of the phenol and hydroxyl

radical and the production of hydrogen peroxide are not similar to the results seen for similar experimental conditions without carbon (Figure 11). This indicates that when the activated carbon and higher levels of ozone are present, the formation of hydrogen peroxide and subsequent formation of the hydroxyl radicals become important. In previous work where oxygen was bubbled through the high voltage electrode (Figure 17)26 the surface reaction rate was more important than the ozone formation rate (due to the lower rate of ozone formation), while the direct reaction with ozone was slightly higher than that with hydroxyl radicals. Figure 15 shows the normalized sensitivity coefficients for the ferrous sulfate solutions with suspended activated carbon and the stainless steel ground electrode suspended in the gas phase above the surface of the solution. Similar to previous results for the ferrous sulfate solutions (Figure 12), the reaction that is most important to the concentration of phenol in the bulk phase is the production of hydrogen peroxide directly from the corona discharge (reaction 2), and the normalized sensitivity coefficient for this reaction is -5.6. The

Figure 17. Phenol removal and primary byproduct formation in reactor #2 (RVC ground electrode) for the case of 150 µS/cm FeSO4, no activated carbon, 150 SCCM O2, with an electrode gap spacing of 8 cm. Phenol and all byproducts are normalized to the initial 100 ppm phenol concentration.


Figure 18. Phenol removal and primary byproduct formation in reactor #2 (RVC ground electrode) for the case of 150 µS/cm FeSO4, no activated carbon, 150 SCCM O2, with an electrode gap spacing of 5 cm. Phenol and all byproducts are normalized to the initial 100 ppm phenol concentration.

following steps are also important: (1) phenol reaction on the surface of the activated carbon (-3.35), (2) the mass transfer of phenol from the bulk solution to the surface of the activated carbon (-1.8) (Kmt mass transfer coefficient), (3) the production of ozone (-1.8) (reaction 72), (4) the reaction of phenol with a hydroxyl radical (-1.25) (reaction 53), and (5) the reaction between phenol and ozone (-1.1) (reaction 87). The other processes among the 10 with the highest values of the normalized sensitive coefficient are (1) the regeneration of the ferrous ion from dihydroxybenzene (hydroquinone or catechol) (-1.05) (reaction k66fo), (2) the adsorption of phenol to the surface of the activated carbon (-1) (adsorption constant KPH), (3) Fenton’s reaction (-0.95) (reaction 5), and (4) the consumption of hydroxyl radicals via the ferrous ion (+0.78) (reaction 6). Important in these processes is the regeneration of the ferrous ions through reaction between ferric ions and catechol and hydroquinone. These are the main steps for the regeneration of ferrous ions, and when these reactions are not present, the ferrous ions in the model go to zero. Also note that the reaction between ferrous ions and hydroxyl radicals is shown to decrease the removal of the phenol through the sensitivity analysis (as indicated by the “plus” sign). Although this result is intuitive, it is important that this is reflected in the parametric sensitivity analysis. Comparison of Figure 15 with Figure 18 of the previous study26 shows many common reactions in the top 10 most sensitive; however, the order of importance of these reactions is different between these cases. In the previous study where oxygen flowed through the high voltage electrode the resulting lower hydrogen peroxide concentration lead to the lesser importance of the hydroxyl radical reaction with phenol. In the present study, the sensitivities of the hydroxyl radical reaction with phenol as well as the Fenton’s reaction are large. Figure 16 shows the time course of the normalized sensitivity coefficient for the ferrous sulfate solutions with activated carbon in suspension with the ground electrode located in the gas phase above the surface of the liquid solution. Initially, the processes that have the highest values are the mass transfer of the phenol from the solution to the surface of the activated carbon and

the adsorption of the phenol to the surface of the carbon. This shows that at early times (the first 15 min), the main pathway of the phenol removal is the adsorption of the phenol to the activated carbon. After the first 15-20 min, the sensitivity coefficient for the production of hydrogen peroxide and the reaction of phenol on the surface of the activated carbon become the dominant processes reducing the amount of phenol in the bulk solution. The production of hydrogen peroxide creates a pathway for hydroxyl radicals to form in the bulk phase and remove phenol by reaction with the hydroxyl radicals. The reaction of phenol on the activated carbon surface frees up a surface site for another phenol molecule to adsorb onto the carbon and subsequently react on the surface of the carbon. At 40 min, most of the processes reach their peak values, and this is when the phenol concentration in the solution approaches zero. At these low values of phenol concentration the values of the normalized sensitivity coefficients are not meaningful. It is also interesting to note that after the amount of phenol in the aqueous phase approaches zero, the value of the sensitivity coefficient for the adsorption coefficient turns positive, indicating that some phenol desorption occurs. Reticulated Vitreous Carbon Electrodes (Reactor #2). Physical and Electrical Characteristics. Reactor #2 has several advantages over reactor #1. Application of multiple HV electrodes gives the flexibility of expanding the corona contact area. The RVC ground electrode is a macroscopically porous (but microscopically nonporous) electrode (as shown in Figure 3) that allows flow of the solution and solids through the ground electrode increasing the degree of mixing in the reactor. In addition, when the RVC ground electrode was placed in the gas above the liquid, the gases evolved in the discharge can easily pass through the electrode thus eliminating any gas buildup on the underside of the ground electrode. Further, a uniform discharge can be obtained between the surface of the aqueous solution and the small sharp bends that occur at the surfaces of the RVC ground electrode. It was noted that when the stainless steel electrode was used as the ground above the water surface, the discharge channels between the water surface and the electrode moved from place to


Figure 19. Photograph of hybrid gas-liquid discharge reactor (reactor #2) with RVC electrode ground in the gas phase. The photo on the left, with a slightly angled view of the same reactor as on the right, shows the discharges propagating along the surface of the water.

place. When RVC was used as the ground, the discharge channels formed between the water surface and the electrode were more numerous and tended to stay in place. Figure 19 shows the intense plasma channels formed between the liquid surface and the ground electrode. It is interesting to note that the liquid-phase discharge is not substantially changed by the addition of the air gap. The plasma channels formed in the gas phase are uniformly distributed in the cross section when using the RVC electrodes. The sharp edges of the RVC are ideal for the formation of gas-phase plasma.36-37 However, in this electrode configuration the intensity of the plasma formed between the water surface and the RVC is much higher, as observed visually, than when entire electrode system is placed in the gas phase. Recent work has shown that the rate of formation of hydrogen peroxide in the hybrid reactor is the same as that in the case when both electrodes are submerged.31,38 Phenol Removal. When both electrodes were submerged in the aqueous solutions, there was little difference between the stainless steel ground electrode and the RVC ground electrode, although the RVC electrode does prevent the buildup of gas under the ground electrode. The main advantage of the RVC ground electrode was seen when the ground electrode was placed in the gas phase above the surface of the aqueous solution. Figure 17 shows the results for phenol removal when the ground electrode was placed in the gas phase with an electrode spacing of 8 cm. This figure shows that approximately 90% of the phenol was removed after 60 min of corona treatment, and the peak levels of catechol and resorcinol were 13 and 4 ppm, respectively. For this experimental condition, it should be noted that 780 mL of solution was used to adjust the top of the aqueous solution to ∼3-5 mm below the RVC ground electrode. The energy per pulse for this experiment was

1.17 J/pulse, corresponding to a phenol removal efficiency of 1.7 g/kWh. The removal of phenol shown in Figure 17 for the case with ferrous sulfate is almost identical to the case when both electrodes were submerged.35 When both electrodes were submerged, 90% phenol removal was achieved after 60 min of corona treatment, and 24 ppm catechol and 11 ppm hydroquinone (peak values) were formed during this period. Although the phenol removal and removal efficiency were similar for both cases of submerged and nonsubmerged electrodes (1.7 g/kWh for both), the reaction pathways were very different. When both electrodes were submerged, hydrogen peroxide was the main species formed, the average electric field was higher, and the energy per pulse was higher at 1.57 J/pulse. When the RVC ground electrode reactor was used and the ground electrode was located in the gas phase above the liquid solution, hydrogen peroxide was formed in the liquid phase, and ozone was simultaneously formed in the gas phase. The main advantage of the air gap is due to these ozone reactions, and this is shown not only by the phenol removal but also in the levels of the primary oxidation products. Catechol and hydroquinone levels were 50% lower when ozone was involved in the reacting chemical species. It is important to note that ozone is not the only possible reactant that can be formed or whose rate of formation may be enhanced in the air gap. It is possible that higher levels of hydroxyl radicals or other radicals can be formed in this intense plasma region; however, future work will be necessary to fully assess these possibilities. Figure 18, in comparison to Figure 17, shows the importance of the gap spacing on the reaction dynamics. The 5 cm gap case gave a much more rapid phenol removal than the 8 cm gap case. Furthermore, the primary byproductsshydroquinone and catecholsare


Figure 20. Phenol removal and primary byproduct formation in reactor #2 (RVC ground electrode) for the case of 150 µS/cm FeSO4, 1 g/L activated carbon, 150 SCCM O2, with an electrode gap spacing of 5 cm. Phenol and all byproducts are normalized to the initial 100 ppm phenol concentration.

completely oxidized to further products in the 5 cm electrode spacing case. The results shown in Figure 18 also indicate the advantage of using the RVC ground electrode reactor instead of the stainless steel ground reactor. Figure 6 shows the results for similar experimental conditions (compared to those in Figure 18) with the use of the stainless steel ground electrode. The phenol removal using the stainless steel ground electrode was 97% after 60 min, the concentration of catechol after 60 min was 10 ppm, and the TOC removal was only 20% after 60 min of corona treatment. For the RVC case, Figure 20 shows that 97% of the phenol was removed in the first 30 min of corona treatment and that all of the phenol and primary oxidation products are completely removed from the reactor after 45 min. The total organic carbon (TOC) level for this condition is 53% lower than the initial concentration. For this experiment, 454 mL of total solution was used to maintain a 3-5 mm distance between the surface of the solution and the RVC ground electrode. The removal efficiency for this experimental condition was 1.60 g/kWh and 27 kWh/1000 L/order of magnitude (EE/O). The main difference between the two reactors was the amount of ozone that was produced, and thus the amount of ozone that was dissolved in the aqueous solution. Using the stainless steel ground electrode, the ozone measured in the aqueous phase was 0.31 mg O3/L after 30 min of corona treatment. This number increased to 3.0 ( 0.3 mg O3/L when using the RVC electrode. Qualitative observations during experiments of the discharge in the gas phase showed that very few streamers occurred above the water surface when using the stainless steel ground electrode. When the discharges occurred, they were concentrated at two or three specific points on the edge of the electrode. When the RVC ground electrode was used, a more uniform and continuous discharge occurred in the gas phase between the liquid surface and the ground electrode. The discharge also occurred at very many points along the bottom surface of the ground electrode. This increased number of discharge channels when using the RVC electrode likely caused a larger amount of ozone to be produced in the gas phase. Also, when the numerous discharges with the RVC ground electrode hit the water

surface, small ripples were seen that did not seem to occur with the stainless steel electrode. These ripples could have increased the mass transfer of the ozone from the gas phase to the aqueous phase, increasing the amount of ozone in the aqueous phase. It is also possible that local electrohydrodynamic flow may enhance the transport at the gas-liquid interface.39 The G50 value for this condition was 6.55 × 10-9 mol/ J, which is 1 order of magnitude lower than the results reported by Hoeben et al.40 It should be noted that the current experimental condition was conducted with a phenol solution that was 10 times higher in concentration, 11/2 times larger in treated volume, 1/2 of the pulse frequency, and 3/4 of the treatment time compared to the gas-phase pulsed corona. Also, the numbers reported by Hoeben et al.40 were adjusted to reflect only the energy used directly in the reactor by subtracting the energy lost in the power supply. The results reported here include both the power delivered to the reactor as well as the power lost in the pulse forming circuit. The effects of the addition of activated carbon to the reactor are shown in Figure 20. Under these conditions, 87% of the phenol was removed after 15 min of corona treatment, and nearly all of the phenol was removed after 30 min of corona treatment. After 60 min of corona treatment, less than 1% of the original phenol was present on the surface of the activated carbon. When compared to Figure 18 (similar conditions but without activated carbon), the phenol was removed at a slightly higher rate when the activated carbon was present. The phenol removal efficiency for the present experimental condition with activated carbon is 3.2 g/kWh and 24 kW/ 1000 L/order of magnitude. Both these numbers represent higher phenol removal efficiency with activated carbon compared to those without. The primary oxidation products were quickly produced and are almost completely removed after 45 min. Also, 85% of the TOC was removed after 60 min, again showing the advantage of the RVC ground electrode. Under similar operating conditions except with the use of the stainless steel ground electrode, the TOC removal after 60 min was 76%. This increase in TOC removal with the RVC ground electrode can be directly attributed to the higher production of ozone with the RVC electrode.


Figure 21. Model data comparison for phenol oxidation in reactor #2 (RVC ground electrode) for the case of 45 kV corona treatment, 150 µS/cm KCl, and 150 SCCM O2. Phenol and all byproducts are normalized to the initial 100 ppm phenol concentration.

Model-Data Comparison. Figure 21 shows the model simulation for the experimental condition where ferrous sulfate was present in the aqueous solution with no carbon present, the RVC electrode was suspended in the gas above the solution, and O2 was flowing in the gas phase. The model result was obtained by adjusting the rate constant for ozone formation. The rate constant used for the formation of ozone was 6.0 × 10-7 M s-1. The byproduct concentrations predicted by the model are much smaller than those found experimentally. One of the main limitations of the model is the extent of the oxidation of phenol. Currently, the model includes the oxidation of phenol to primary byproducts and then the oxidation of the byproducts to other products. The model does not include any reactions for these “other” products. Although this model was valid before when phenol byproducts were not completely oxidized, the current model does not include enough of the oxidation reactions of byproducts to be valid when all of the byproducts are completely oxidized during the 60 min corona treatment. It is also possible that the reaction rate constants for ozone and hydroxyl radicals need further refinement for this case. Conclusions The present study demonstrates the development of a new reactor electrode configuration that can be used to efficiently remove phenol from aqueous solutions. This is the first demonstration of the combination of a gas-phase nonthermal plasma discharge with a liquidphase pulsed streamer corona discharge to remove contaminants from the aqueous phase. A high voltage point electrode was submerged in the liquid phase, and the planar ground electrode was placed in the gas above the liquid surface. This configuration resulted in an electrical discharge in the water that produced hydrogen peroxide and an electrical discharge in the gas phase that produced ozone. The ozone then dissolved in the liquid phase, leading to much higher concentrations of reactive species in the liquid phase than were seen previously when both electrodes were submerged in the aqueous solution. This new electrode configuration led to high phenol removal efficiencies under several different operating

conditions. Computer modeling and sensitivity analysis described the major reactions involved when both hydrogen peroxide and ozone were produced as well as detailing the effects of Fenton’s chemistry when iron salt was added and the effects of adsorption and reaction with activated carbon. The greatest amount of dissolved ozone was found when reticulated vitreous carbon was used as the ground electrode material. This created a uniform gas-phase discharge between the liquid surface and the ground electrode and also created small ripples on the surface of the water that helped increase the mass transfer of the ozone to the liquid phase. It is also important to note that the hybrid gas-liquid discharge reactor may also lead to the formation of oxidants other than ozone in the gas, at the gas-liquid interface, and in the liquid. Further work is necessary to fully address the complete mechanisms in such reactors. Acknowledgment We gratefully acknowledge partial support from the U.S. Air ForcesTyndall Air Force Base and the Department of Chemical Engineering, FAMU-FSU College of Engineering. Literature Cited (1) Langlais, B.; Reckhow, D. A.; Brink, D. R. Ozone in Water Treatment, Application and Engineering; Lewis Publishers: Chelsea, 1991. (2) Glaze, W. H.; Kang, J. W. Advanced Oxidation Processes. Description of a Kinetic Model for the Oxidation of Hazardous Materials in Aqueous Media with Ozone and Hydrogen Peroxide in a Semibatch Reactor. Ind. Eng. Chem. Res. 1989, 28, 1573. (3) Kwon, B. G.; Lee, D. S.; Kang, N.; Yoon, J. Characteristics of P-Chlorophenol Oxidation by Fenton’s Reagent. Water Res. 1999, 33, 2110. (4) Peyton, G. R.; Glaze, W. H. Destruction of Pollutants in Water with Ozone in Combination with Ultraviolet Radiation, 3. Photolysis of Aqueous Ozone. Environ. Sci. Technol. 1988, 22, 761. (5) Ollis, D. F.; Al-Ekabi, H. A. Photocatalytic Treatment and Purification of Air and Water; Elsevier Science: Amsterdam, 1993. (6) Petrier, C.; Micolle, M.; Merlin, G.; Luche, J. L.; Reverdy, G. Characteristics of Pentachlorophenate Degradation in Aqueous Solution by Means of Ultrasound. Environ. Sci. Technol. 1992, 26, 1639. (7) Hoffmann, M. R.; Hua, I.; Hochemer, R.; Willberg, D.; Lang, P.; Kratel, A. Chemistry Under Extreme Conditions in Water

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Received for review July 28, 2003 Revised manuscript received January 2, 2004 Accepted January 21, 2004 IE030620J

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