Suspended Activated Carbon Particles and Ozone Formation in

Ind. Eng. Chem. Res. 2003, 42, 5117-5134

5117

Suspended Activated Carbon Particles and Ozone Formation in Aqueous-Phase Pulsed Corona Discharge Reactors 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

The effects of suspended activated carbon particles and oxygen flow through the high-voltage electrode in liquid-phase pulsed corona discharge reactors were evaluated for the degradation and removal of phenol. Experimental studies showed that phenol can be effectively degraded with a wide range of reactor conditions; however, the most efficient removal of phenol occurred when activated carbon and ferrous sulfate solutions were utilized in the liquid-phase corona reactor. The most efficient TOC removal occurred in the above conditions with the addition of oxygen flow. The oxygen gas flow leads to ozone formation, and the subsequent reactions of the dissolved ozone enhance reactions with the oxidation byproducts of phenol. The ferrous sulfate leads to Fenton’s reactions from the hydrogen peroxide generated by the discharge. Through the combination of experimental measurements and a mathematical model accounting for adsorption, mass transfer, and surface reaction on the activated carbon, it was found that there is a strong possibility that the activated carbon participates in catalytic reactions with phenol and its primary byproducts. Introduction There is a continuing need for the development of efficient and cost-effective removal technologies for hazardous organic contaminants such as phenol, benzene, and polychlorinated biphenyls from groundwater and wastewater. Conventional methods for water remediation, including carbon adsorption,1,2 air stripping, ozone oxidation,3,4 and chlorine treatment, suffer from various limitations. For example, chlorine oxidation has been shown to produce carcinogenic halogenated hydrocarbons.5 In addition, ozone oxidation is limited by high selectivity and slow kinetics,6 and carbon adsorption requires regeneration of the spent carbon. Due to these, and other, limitations, recent work has focused on the development of advanced oxidation technologies (AOTs) that produce hydroxyl radicals that have favorable reaction rates for the oxidation of many organic contaminants. These advanced oxidation technologies include UV/hydrogen peroxide treatment, sonolysis, electrohydraulic discharge,7,8 electron beam irradiation,9 hydrogen peroxide/ozone or peroxone treatment,10,11 glow discharge electrolysis,12,13 and corona discharges.14-23 Combinations of various processes have also been reported, and one intriguing method reports on the combination of ozone and carbon particles to enhance hydroxyl radical formation.24,25 Previous work has also shown that the combination of an aqueous-phase pulsed streamer-like corona discharge with suspended activated carbon particles leads to enhancement of the * To whom correspondence should be addressed: Dr. Bruce R. Locke, Professor, Department of Chemical Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer Street, Tallahassee, FL 32310-6046. Tel: 850-410-6165. Fax: 850-4106150. E-mail: [email protected]. † Florida State University and Florida A & M University. ‡ Florida State University.

overall removal of phenol compared to the pulsed corona discharge alone.21,22 This enhancement is due to the combination of phenol oxidation in the bulk liquid phase induced by the pulsed corona discharge and physical adsorption of the phenol on the activated carbon, as well as to putative surface-phase reactions on the activated carbon induced by the pulsed corona. These surface reactions would then imply that the combination of pulsed corona and activated carbon would lead to a selfregenerating process for the activated carbon. The combination of the activated carbon and potassium salts has been shown to affect the power waveform of the pulsed corona reactor.26 Since liquid-phase corona discharge reactors can also be configured to produce ozone through injection of oxygen (or air) through a highvoltage hollow electrode immersed in water,14 there may be an advantage of using activated carbon in such a situation. Therefore, the overall objective of the present work is to evaluate the combination of pulsed corona discharge with suspended activated carbon particles and ozone formed by bubbling oxygen through the highvoltage discharge electrode. The specific objectives of the present work are to determine the rates of phenol oxidation, byproduct formation, and TOC removal in the corona discharge reactor with oxygen gas bubbled through the high-voltage electrodes both with and without suspended activated carbon particles in solution. In addition, the effects of added salts, including KCl and FeSO4, are assessed to determine the contributions of Fenton’s reactions to the phenol removal. A mathematical model of the bulk and surface reactions induced by the corona discharge is developed, and a model parameter sensitivity analysis is conducted. Model Development In previous studies of the bulk-phase corona-induced chemical reactions,16,22 it was found that the pulsed

10.1021/ie020330n CCC: $25.00 © 2003 American Chemical Society Published on Web 09/19/2003

5118 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003

corona discharge leads to the formation of hydrogen peroxide, hydroxyl radicals, and aqueous electrons [reactions (1)-(3) in Table 1]. The corresponding rate constants for these three reactions were found to depend on the operating variables of the reactor, including peak voltage, discharge power, electrode geometry, electrode gap spacing, number of high-voltage electrodes, and conductivity.16,22 It was assumed in the previous mathematical models developed for the liquid-phase corona reactors that the other major species formed in the reactor are the same as those formed in radiation processes such as electron beam irradiation and pulsed radiolysis.27 The present study extends the chemical reaction system [reactions (1)-(71) in Table 1] given in previous work22 through the inclusion of additional chemical reactions that account for the production and subsequent reactions of ozone. Reaction (72) in Table 1 represents a global average rate of ozone production in the reactor and the rate for this reaction was obtained by fitting the experimental data for phenol concentration in a KCl solution at 60 min. The other reactions, (73)-(89), describe known ozone reactions. The corresponding kinetic rate constants (Table 2) were obtained from the radiation and oxidation chemistry literature for conditions with similar temperature and pH as in the present study. The general mass balance for a well-mixed, constant volume, and constant temperature batch reactor is given by

dci ) ri dt

(1)

where ci is the average concentration in the bulk solution of species i and ri is the average rate of formation of that species in the bulk fluid in the reactor. Mass balances were written for all species in the bulk liquid accounting for all the reactions given in Table 1. The effects of the suspended activated carbon particles are considered in a reaction/diffusion model. This model includes the reactions induced by the pulsed streamer corona in the bulk phase, with possible surface reactions on the activated carbon particles, masstransfer flux to the particle phase, adsorption associated with the activated carbon particles, and diffusion within the particle phase.21,28 The general model and corresponding assumptions have been reported in previous work.21,28,29 This model is based on the volume averaging method extensively applied to reaction/diffusion problems in porous catalysts30-33 and only a few comments necessary to describe the approach and final results will be given here. The material balance on phenol averaged over the bulk fluid phase is given by

dcPh b ) A′Nb|R+δ + rbVL VL dt

(2)

where A′ is the total surface area at the edge of the boundary at r ) R + δ for all particles, rb is the average rate of reaction in the bulk fluid, R is the particle radius, δ is the boundary layer thickness for an individual typical activated carbon particle, Nb is the flux at the interface between the bulk fluid and the stagnant boundary layer around the particle, and VL is the volume of the liquid phase. Incorporating the bulk-phase direct reactions of phenol gives

VL

dcPh b Ph ) A′Nb|R+δ + VL‚[-k53y2cPh b - k54y2cb + br] dt (3)

where k53 and k54 are the bulk reaction rate constants for phenol oxidation to catechol and for phenol oxidation to hydroquinone, respectively,22 y2 is the concentration of the hydroxyl radical in the bulk phase, and “br” represents the other bulk-phase reactions incorporated in the rate equation for phenol. The species continuity equation for phenol averaged over a representative region in the particles32 is given by

(

)

∂cPh 1 ∂ 2 ∂cPh Ph r - kPh )D 2 p c ∂t ∂r ∂r r

(4)

where kPh p is the effective reaction rate constant for phenol accounting for all internal surfaces of the activated carbon particles, D is the effective diffusion coefficient in the particle, and cPh is the local averaged concentration of phenol in the activated carbon particle. The effect of electrohydrodynamic transport of solute in the particles is neglected since it is unlikely that hydrodynamic flow is induced within the small pores of the particle. This is because the volume of the discharge is small compared to the volume of the reactor and therefore the local electric field inside the particle is small. Furthermore, the time scale for the discharge is such that the field is only on for 1 ms/s. Electrophoretic transport of phenol in the particle is neglected since the electric field in the particle is small and the concentration of phenol is much higher than that of the phenolate ion in the pH range of these studies. The governing equations for catechol and hydroquinone are similar to those for phenol. The boundary and initial conditions for phenol in the particle phase are

AD

|

∂cPh ∂r

R

) -AKmt(cPh|R - KPhcPh b )

(5)

∂cPh )0 ∂r 0

|

(6)

cPh(t ) 0) ) 0

(7)

where A is the total external surface area at r ) R of all the activated carbon particles in suspension, KPh is the adsorption equilibrium constant for phenol, and Kmt is the mass-transfer coefficient. The initial condition for phenol in the bulk liquid is Ph cPh b (t ) 0) ) cb0

(8)

The last condition is

A′Nb|R+δ = ANb|R = AD

|

∂cPh ∂r

R

(9)

which implies that the flux at the edge of the boundary layer is equal to the flux at the particle surface. Equation 9 requires the assumptions that there is no accumulation in the boundary layer (i.e., quasi-steady state34) and there is no reaction in the boundary layer.

Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5119 Table 1. List of Bulk-Phase Reactions k1

1. H2O 98 H• + •OH k3

3. H2O 98 H+ + eaq- + •OH

1 1 HO + H 2 2 2 2 2 k4

4. Fe3+ + H2O2 98 Fe2+ + HO2• + H+

k5

6. •OH + Fe2+ 98 Fe3+ + OH-

k7

8. Fe2+ + HO2• 98 Fe3+ + H2O2

5. Fe2+ + H2O2 98 Fe3+ + OH- + •OH 7. Fe3+ + HO2• 98 Fe2+ + O2 + H+ k9

9. Fe3+ + O2-• 98 Fe2+ + O2 k11

11. eaq- + Fe3+ 98 Fe2+ k13

13. •OH + H2O2 98 HO2• + H2O k15

15. O2-• + H+ 98 HO2• k17

17. HO2• + O2-• 98 H2O2 + O2 k19

19. •OH + O2-• 98 O2 + OHk21

21. 2•OH 98 H2O2

k6

k8

k10

10. Fe2+ + O2-• 98 Fe3+ + H2O2 k12

12. H• + Fe3+ 98 Fe2+ + H+ k14

14. HO2• 98 O2-• + H+ k16

16. HO2• + HO2• 98 H2O2 + O2 k18

18. •OH + H2 98 H• + H2O k20

20. •OH + HO2• 98 H2O + O2 k22

22. •OH + OH- 98 H2O + O-•

k23

24. •OH + O-• 98 HO2-

k25

26. eaq- + H• 98 H2 + OH-

23. •OH + H2O2 98 O2-• + H2O 25. •OH + HO2- 98 HO2• + OHk27

27. 2eaq- 98 2OH- + H2 k29

29. eaq- + O2 98 O2-• k31

31. eaq- + H+ 98 H• k33

33. eaq- + HO2-• 98 2OH- + •OH k35

35. eaq- + O-• 98 2OH-

k24

k26

k28

28. eaq- + H2O2 98 •OH + OHk30

30. eaq- + O2-• 98 O22k32

32. eaq- 98 H• + OHk34

34. eaq- + •OH 98 OHk36

36. H• + O2 98 HO2•

k37

38. 2H• 98 H2

k39

40. H• + HO2• 98 H2O2

37. H• + O2-• 98 HO239. H• + •OH 98 H2O k41

41. H• + H2O2 98 H2O + •OH k43

43. H• + H2O 98 H2 + •OH

k38

k40

k42

42. H• + OH- 98 eaq- + H2O k44

44. O-• + H2O 98 •OH + OH-

k45

46. O-• + H2 98 H• + OH-

k47

48. O-• + O2-• 98 2OH- + O2

45. O-• + HO2- 98 O2-• + OH47. O-• + H2O2 98 O2-• + H2O k50

50. H+ + OH- 98 H2O k52

k2

2. H2O 98

52. H2O2 98 H+ + HO2-

k46

k48

k51

51. H+ + HO2- 98 H2O2

5120 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 Table 1. List of Bulk-Phase Reactions

k73

k72 3 72. O2 98 O3 2

73. O3 + HO2- 98 HO2• + O3-• k74

74. O2-• + O3 98 O2 + O3-• k76

76. •OH + HO2- 98 H2O + O2-• k78

78. O3-• + H+ 98 HO3•

k75

75. •OH + O3 98 O2 + HO2• k77

77. O3 + OH- 98 O2-• + HO2• k79

79. HO3• 98 H+ + O3-•

k80

81. •OH + O3 98 HO4•

k82

83. O3 + OH- 98 HO2- + O2

80. HO3• 98 •OH + O2 82. HO4• 98 HO2• + O2 k84

84. O3-• + H2O 98 •OH + O2 + OH-

k81

k83

k85

85. O3-• + •OH 98 O2-• + HO2•

k86

86. O3-• + •OH 98 O3 + OH-

The boundary equations for the other organic species are similar to those given above. The initial concentrations for all species, except phenol, are zero in both the bulk and particle phases.

With use of the averaging method described in previous work,21,28,29 the material balances in the bulk and particle phases can be transformed to the ordinary differential equations

Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5121 Table 2. Reaction Rate Constants for Bulk-Phase Reactions k1 ) 9.25 × 10-10 M s-1 16 k2 ) 8.0 × 10-7 M s-1 16 k3 ) 2.35 × 10-9 M s-1 16 k4 ) 0.01 M-1 s-1 53 k5 ) 76 M-1 s-1 54 k6 ) 4.3 × 108 M-1 s-1 54 k7 ) 1.0 × 104 M-1 s-1 58 k8 ) 1.2 × 106 M-1 s-1 58 k9 ) 1.5 × 108 M-1 s-1 58 k10 ) 1.0 × 107 M-1 s-1 58 k11 ) 6.0 × 1010 M-1 s-1 64 k12 ) 2.0 × 106 M-1 s-1 67 k13 ) 2.7 × 107 M-1 s-1 54 k14 ) 8.0 × 105 s-1 57 k15 ) 5.0 × 1010 M-1 s-1 57 k16 ) 8.5 × 105 M-1 s-1 57 k17 ) 9.7 × 107 M-1 s-1 57 k18 ) 3.9 × 107 M-1 s-1 73 k19 ) 1 × 1010 M-1 s-1 74 k20 ) 1 × 1010 M-1 s-1 74 k21 ) 5.5 × 109 M-1 s-1 54 k22 ) 1.3 × 1010 M-1 s-1 68 k23 ) 2.7 × 107 M-1 s-1 54 k24 ) 2.0 × 1010 M-1 s-1 76 k25 ) 7.5 × 109 M-1 s-1 70 k26 ) 3.4 × 1010 M-1 s-1 79 k27 ) 5.5 × 109 M-1 s-1 54 k28 ) 1.3 × 1010 M-1 s-1 81 k29 ) 1.9 × 1010 M-1 s-1 82 k30 ) 1.3 × 1010 M-1 s-1 83 k31 ) 2.3 × 1010 M-1 s-1 54 k32 ) 1 × 103 s-1 79

k33 ) 3.5 × 109 M-1 s-1 49 k34 ) 3.0 × 1010 M-1 s-1 51 k35 ) 2.2 × 1010 M-1 s-1 51 k36 ) 1.2 × 1010 M-1 s-1 54 k37 ) 2.0 × 1010 M-1 s-1 55 k38 ) 5.0 × 109 M-1 s-1 54 k39 ) 7.0 × 109 M-1 s-1 59 k40 ) 2.0 × 1010 M-1 s-1 55 k41 ) 5.0 × 107 M-1 s-1 62 k42 ) 2.2 × 107 M-1 s-1 54 k43 ) 1.0 × 1010 M-1 s-1 65 k44 ) 9.4 × 107 s-1 68 k45 ) 4.0 × 108 M-1 s-1 54 k46 ) 8.0 × 107 M-1 s-1 71 k47 ) 4.0 × 108 M-1 s-1 54 k48 ) 6.0 × 108 M-1 s-1 71 k50 ) 1.4 × 1011 M-1 s-1 72 k51 ) 2.6 × 1010 M-1 s-1 72 k52 ) 3.7 × 10-2 M-1 s-1 72 k53 ) 7.0 × 109 M-1 s-1 54 k54 ) 0.3 × 109 M-1 s-1 54 k55 ) 1.0 × 108 M-1 s-1 75 k56 ) 1.0 × 109 M-1 s-1 75 k57 ) 1.5 × 109 M-1 s-1 35 k58 ) 1.0 × 109 M-1 s-1 35 k59 ) 3.7 × 109 M-1 s-1 75 k60 ) 5.0 × 108 M-1 s-1 80 k61 ) 5.0 × 108 M-1 s-1 80 k62 ) 5.0 × 108 M-1 s-1 35 k63 ) 5.0 × 108 M-1 s-1 35 k64 ) 1.0 × 109 M-1 s-1 56 k65 ) 1.0 × 105 M-1 s-1 35

9RKmtγ dcPh b ) 〈cPh〉 dτ RKmt + 9D

(

)

9RKmtKPhγ + φ53y2 + φ54y2 cPh b + AA (10) RKmt + 9D

(

)

9RKmt 9RKmtKPh Ph d〈cPh〉 Ph ) - φPh c p 〈c 〉 + dτ RKmt + 9D RKmt + 9D b (11)

(

)

dcHq 9RKmtγ b 〈cHq〉 ) dτ RKmt + 9D

(

)

9RKHqKmtγ + φ66fpy6 + φ70y2 cHq b + BB (12) RKmt + 9D

(

)

9KHqKmtR Hq -9RKmt d〈cHq〉 Hq ) - φHq c 〈c 〉 + p dτ RKmt + 9D KmtR + 9D b (13)

(

)

dcCc 9RKmtγ b ) 〈cCc〉 dτ RKmt + 9D

(

)

9RKCcKmtγ + φ66foy6 + φ69y2 cCc b + CC (14) RKmt + 9D

(

)

9KCcKmtR Cc -9RKmt d〈cCc〉 Cc Cc ) - φp 〈c 〉 + c dτ RKmt + 9D KmtR + 9D b (15) The brackets 〈 〉 define the integral average over the volume of a typical spherical particle. The superscripts “Hq” and “Cc” represent hydroquinone and catechol, respectively, y6 is the concentration of ferric ions, and

k66f ) 4.4 × 102 M-1 s-1 50 k66r ) 1.1 × 103 M-1 s-1 50 k67f ) 4.4 × 104 M-1 s-1 52 k67r ) 1.2 × 10-3 M-1 s-1 35 k68 ) 1.0 × 109 M-1 s-1 56 k69 ) 1.1 × 1010 M-1 s-1 57 k70 ) 5.0 × 109 M-1 s-1 60 k71 ) 1.2 × 109 M-1 s-1 61 k72 ) 1.69 × 10-7 M s-1 (this work) k73 ) 5.5 × 106 M-1 s-1 63 k74 ) 1.6 × 109 M-1 s-1 66 k75 ) 1.1 × 108 M-1 s-1 69 k76 ) 7.5 × 109 M-1 s-1 70 k77 ) 7.0 × 101 M-1 s-1 63 k78 ) 5.2 × 1010 M-1 s-1 66 k79 ) 2.3 × 102 s-1 63 k80 ) 1.1 × 105 s-1 69 k81 ) 2.0 × 109 M-1 s-1 63 k82 ) 2.8 × 104 s-1 63 k83 ) 4.0 × 101 M-1 s-1 63 k84 ) 2.5 × 101 M-1 s-1 63 k85 ) 6.0 × 1010 M-1 s-1 69 k86 ) 2.5 × 109 M-1 s-1 69 k87 ) 1.3 × 103 M-1 s-1 77 k88 ) 3.1 × 105 M-1 s-1 78 k89 ) 1.5 × 106 M-1 s-1 78

AA, BB, and CC are the additional bulk-phase reactions for phenol, hydroquinone, and catechol, respectively. These six equations (eqs 10-15) coupled with the basic set of equations for the other species in the bulk (eqs 1) are solved using a Gear routine with Mathematica Version 3.0 (from Wolfram Research, Champaign, IL). Depending upon the specific experimental conditions under consideration, between 28 and 35 simultaneous ordinary differential equations were solved. The initial conditions for the model were as follows: iron concentration was 485 µM, phenol concentration in the bulk phase was 100 ppm, pH ) 5, dissolved oxygen concentration was 250 µM,35 and the concentrations of all the other species were zero. The adsorption constants for hydroquinone and catechol were determined experimentally to be equal at approximately 1000 (ppm surface)/(ppm bulk). The adsorption constant for phenol was 424 (ppm surface)/(ppm bulk).28 The other constants used in the model were average particle diameter 0.0002 m, total outer particle surface area 0.024 m2, volume of the liquid in the reactor 0.001 m3, effective diffusivity36 3.3 × 10-8 m2 s-1, and mass-transfer resistance coefficient37 4 × 10-6 m/s. A parametric sensitivity analysis was conducted to determine the sensitivity of the phenol concentration to the reaction rate coefficients.22,38 The peak values of the normalized sensitivity coefficients are reported here, and for most cases these peak values occur at 60 min, where phenol is at the lowest concentration. In general, the relative values of the sensitivity coefficients do not change over the course of time as long as the concentration of phenol does not reach zero. If the concentration of phenol reaches zero, then the relative values of the sensitivity coefficients do change, and in these cases the sensitivity coefficients are plotted versus time. Only the reactions with sensitivity coefficients within 2-3 orders of magnitude of the most sensitive reaction are reported here.


Figure 1. Phenol, TOC, and primary byproducts for the case with 150 µS/cm KCl, 1 g/L activated carbon, and no O2.

Experimental Methods The power supply and reactor used in the present study are identical to those used in previous work.15,16,21 The only modification made to the reactor was the introduction of oxygen flow through a hollow tube highvoltage electrode (made from a stainless steel hypodermic needle) in a manner similar to that used by Clements et al.14 Furthermore, the applied voltage and current waveforms were measured using the same methods as in previous work15,16,21 and the solution conditions used in the present study were found not to affect the current or voltage waveforms. All experiments were conducted with fixed applied voltages of 45 kV and with pulsed discharges of 60 Hz frequency and a storage capacitor of 2 nF. All experiments were conducted with the initial solution conductivity adjusted with either KCl or FeSO4 to 150 µS/cm. The primary chemicals used included KCl, FeSO4, phenol, resorcinol, hydroquinone, catechol, acetic acid, acetonitrile, and activated carbon (all were obtained from Fisher Scientific). All chemicals except activated carbon were used as received from the manufacturer. The activated carbon was prepared in the same manner as described in previous work21 and the particle size range was 75-300 µm. Deionized water with a conductivity of less than 1 µS/ cm was used to prepare all solutions and to wash the activated carbon. Three or more trials were performed for most sets of experimental conditions, and averages of these data are reported. All data reported had errors of less than 5% at the 95% confidence level. Initial and final measurements were made of conductivity (ColeParmer Model 1484-10 conductivity meter), pH (Fisher Accumet 950 pH meter), and temperature (standard mercury thermometer). The concentrations of the organic species were measured with a Perkin-Elmer HPLC equipped with a Supelco Supercosil C18 column (25.0 cm × 4.6 mm) and with a mobile phase containing 0.5% acetic acid, 5.0% acetonitrile, and 94.5% deionized water. The flow rate of the mobile phase was 1.0 mL/ min. A Perkin-Elmer Spectrophotometer LC80 was coupled to the HPLC and all measurements were made with a fixed wavelength of 280 nm. The recorded peaks were identified and quantified from a set of calibration standards of phenol and the primary oxidation byprod-

ucts including catechol and hydroquinone. A Shimadzu TOC-5050 total organic carbon (TOC) analyzer was used to measure TOC with 24-µL sample injections. Changes in pH with corona-induced Fenton’s reactions were described in previous work Grymonpre´ et al.,22 and in the present study activated carbon was not found to influence pH changes in the corona reactor. Hydrogen peroxide was measured by a modification of a method developed by Ghormoley and described by Hochanadel.39 It can be noted that measurements of hydrogen peroxide by this method give similar results19 to the method which utilizes the reaction by titanyl ions.40 Ozone was measured in the liquid phase using method 4500-O3 B, Indigo Colorimetric Method.41 The adsorption equilibrium isotherm for phenol on the washed activated carbon was measured using phenol solutions of 25, 50, and 100 ppm with 1 g/L washed activated carbon. Phenol concentration was measured by HPLC before adding the activated carbon. After activated carbon was added, the particles were mixed and suspended in solution for 24 h (at which time equilibrium was reached). The solution was maintained at 20.0 °C using a constant temperature water bath and water jacket. The difference between the initial and final amounts of phenol in solution was used to determine the amount of phenol adsorbed on to the surface of the activated carbon. In some experiments measurements were made to assess the residue remaining on the surface of the activated carbon after corona treatment. In such cases, after 60 min of corona treatment, the activated carbon was recovered, dried, and analyzed for chemical species on the surface. The dried activated carbon was suspended in deionized water and the equilibrated solution was analyzed for organic species using HPLC. For some selected cases, after the above analysis, the activated carbon particles were washed with deionized water, and the adsorption equilibrium coefficient was re-evaluated to assess possible irreversible effects of the corona discharge on the activated carbon. Results and Discussion Potassium Chloride Solutions with and without Activated Carbon. Figure 1 shows the concentrations of phenol, TOC, and primary byproducts for the experi-

Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 5123

Figure 2. Phenol equilibrium isotherm.

ments with no iron salt. As seen in the figure, 82% of the phenol was removed in 60 min of corona treatment, and TOC followed a similar trend. This curve shows slightly more phenol removal than can be accounted for by physical adsorption alone (no corona reference case), although the difference between the two curves is not large. At first consideration, this result would imply that the removal of phenol was due primarily to physical adsorption; however, only small amounts of phenol and byproducts were found to desorb from samples of the activated carbon taken after 60 min of corona treatment. Approximately 20% of the original phenol was found on the surface of the activated carbon after corona treatment (as shown later in Table 3) along with trace amounts of the primary byproducts. A balance on the total carbon accounting for all chemical species measured (liquid-phase phenol, primary byproducts, and TOC, and particle-phase phenol and byproducts) accounts for only 60% of the original carbon from phenol. This imbalance implies that (1) there is a loss of organic species in the handling of the samples, (2) there is irreversible adsorption onto the activated carbon, or (3) catalytic reactions occur on the surface of the carbon. To assess the first alternative, activated carbon and phenol were combined in solution and allowed to reach equilibrium in the absence of the pulsed streamer corona discharge. The activated carbon particles were thereafter recovered and suspended in fresh deionized water until equilibrium was again reached. Measurements of the phenol mass indicated that all of the phenol originally adsorbed onto the activated carbon could be accounted for, thus confirming that without corona no phenol was lost through sample handling. To consider the effects of corona treatment on the adsorption properties of the activated carbon, equilibrium analysis was conducted before and after corona treatment. Figure 2 shows the data for the equilibrium isotherm for phenol on the activated carbon particles before corona treatment, after 1 h of corona treatment at 45 kV, and after 1 h of corona treatment at 45 kV with oxygen bubbled through the high-voltage point electrode. All the data fall on one curve, indicating little difference between the adsorption characteristics of the

activated carbon before and after corona treatment. This result indicates that any phenol remaining on the surface of the activated carbon after corona treatment should be recoverable, and thus an accurate measurement of the surface concentration can be determined. Furthermore, there should be little phenol irreversibly adsorbed to the surface of the activated carbon. Ferrous Sulfate Solutions with and without Activated Carbon. The role of Fenton’s reactions is shown in Figure 3 where activated carbon particles are suspended in solutions containing ferrous sulfate. Complete phenol removal and 72% TOC removal occurred in 60 min. The activated carbon was analyzed following treatment and only 1.3% of the original phenol was found on the surface of the carbon. Oxygen Bubbling with and without Activated Carbon in Potassium Chloride Solutions. Figure 4 shows the effect of combining suspended activated carbon particles and 150 SCCM oxygen bubbled through the high-voltage electrode on phenol removal without Fenton’s reactions. It can be noted that very little removal of phenol was observed when oxygen was bubbled without the corona discharge.15,42 As shown in this figure, the addition of oxygen bubbled through the high-voltage electrode without activated carbon gave a larger amount of phenol removal (50%) than the case of potassium chloride solution alone (15%). This increased phenol removal is likely due to the production of ozone within the system. The contributions of peroxone reactions10 are very small at this pH compared to those of the direct oxidation reactions. Without gas bubbling and hence no ozone formation, 82% of the phenol was removed in 60 min with the activated carbon, but when oxygen was bubbled through the high-voltage electrode, leading to ozone formation, phenol removal increased to 95% during 60 min of treatment. In the latter case, when the ozone was present with the activated carbon, approximately 5% of the original phenol was recovered from the activated carbon after corona treatment. Table 3 summarizes the results of the experiments with potassium chloride solutions, showing the amount of phenol removed in 60 min of corona treatment, as


Figure 3. Phenol, TOC, and primary byproducts with 150 µS/cm FeSO4, 1 g/L activated carbon, and no O2.

Figure 4. Effects of activated carbon and oxygen flow through the high-voltage electrode on phenol removal with KCl (150 µS/cm). Table 3. Energy Efficiency for Phenol Removal in KCl Solutions KCl condition no C, no O2 C, no O2 no C, O2 C, O2

% % on removal carbon 16.2 82.2 52.5 94.6

20 10

energy per pulse 1.57 J/pulse 1.51 J/pulse 1.37 J/pulse 1.31 J/pulse

EE/O g/kW-h (kW/1000 L) 0.3 2.8 1.2 3.1

815 51 254 48

well as the amount of phenol remaining on the surface of the activated carbon (shown only for the experimental conditions with activated carbon particles), energy per pulse, and energy efficiencies. The energy efficiencies are given in both grams of phenol removed per kilowatthour and electrical energy necessary to reduce the phenol concentration 1 order of magnitude43 per 1000 L, given in units of kW/1000 L. Both of these performance measures are based on the total amount removed from the solution.

As shown in Table 3, the energy efficiencies of phenol removal for the case without carbon and without bubbled oxygen were 0.3 g/kW-h and 815 kW/1000 L/order. When activated carbon was suspended in the reactor, the energy efficiencies increased to 2.8 g/kW-h and 51 kW/1000 L/order. This is due mainly to the adsorption of phenol to the activated carbon, and to the possible surface reactions induced by the corona. Compared to the case without carbon and without bubbled oxygen, adding the oxygen through the high-voltage electrode increased the energy efficiencies to 1.2 g//kW-h and 254 kW/1000 L/order. Although not as large as the increase seen with the activated carbon, the increase seen in the energy efficiency for bubbling oxygen into the reactor can be accounted for by the additional reactions associated with the formation of the ozone. As expected, the addition of both bubbled oxygen and suspended activated carbon gave the best energy efficiencies for the potassium chloride solutions at 3.1


Figure 5. Effects of activated carbon and oxygen flow rate on phenol removal with iron salt (150 µS/cm, FeSO4). Table 4. Energy Efficiency for TOC Removal in KCl Solutions KCl condition no C, no O2 C, no O2 no C, O2 C, O2


20 10


EE/O g/kW-h (kW/1000L) 0.1 2.8 0.2 3.1

1891 62 1143 49

g/kW-h and 48 kW/1000 L/order. These results quantify the benefits of ozone formation and the adsorption and reaction effects on the activated carbon. There was only a small amount of TOC removal, shown in Table 4, for the two experimental conditions without suspended activated carbon. Without bubbling oxygen into the reactor, 2.9% of the TOC was removed. However, when oxygen was added, the TOC removal increased to 12.6%. As stated before, phenol oxidizes to primary byproducts, which then oxidize to organic acids, and there are several steps of oxidation that phenol must go through before complete mineralization is accomplished. Although quantification of the organic acids was not conducted, several peaks on the chromatogram were found to be associated with the major organic acids including oxalic and formic acid. When oxygen was introduced to the reactor, the combination of hydroxyl radicals and ozone lead to more efficient degradation of phenol and byproducts. When activated carbon was added to the reactor, the removal of TOC significantly increased. Since 82% of the TOC was removed from solution for the case without bubbled oxygen, the first possibility is that the TOC adsorbed to the activated carbon. However, only 20% of the original phenol and trace amounts of the other organic species were recovered from the activated carbon after corona treatment. Therefore, approximately 60% of the TOC was completely removed from the system, again strongly supporting the postulated surface reactions on the activated carbon. A similar trend was seen for the experiments with both activated carbon and bubbled oxygen, where approximately 70% of all TOC was removed from the system. Energy efficiencies for TOC removal in potassium chloride solutions are also given in Table 4. These results follow the phenol removal results shown in Table

3. For TOC removal, the most efficient case was the combination of suspended activated carbon and bubbling oxygen through the high-voltage electrode. This condition had energy efficiencies of 3.1 g/kW-h and 49 kW/ 1000 L/order. Oxygen Bubbling with and without Activated Carbon in Ferrous Sulfate Solutions. Figure 5 shows the removal of phenol from ferrous sulfate solutions for experimental conditions with and without bubbled oxygen, and with and without suspended activated carbon. The results for experimental conditions with no activated carbon and no bubbled oxygen, shown here for reference, gave 90% phenol removal in 60 min of corona treatment. When oxygen was bubbled through the high-voltage electrode, the amount of phenol removed decreased to 63%. The concentration of hydrogen peroxide in the pulsed corona reactor when oxygen was not bubbled through the high-voltage electrode increased to 0.003 M with or without suspended carbon as shown in Figure 6. When oxygen was introduced into the reactor by bubbling through the high-voltage electrode, approximately half as much hydrogen peroxide was formed in comparison to the case when oxygen was not bubbled into the reactor. There are two possible reasons for this reduction. One possibility is that the ozone reacts with the hydrogen peroxide and the other possibility is that the discharge is affected by the bubbling gases. To investigate the first option, the model was run under conditions without additives (i.e., iron salts) with various rates of production of ozone and hydrogen peroxide. When the rate of hydrogen peroxide was assumed to be the same as that in the absence of the bubbling gases, the phenol removal predicted by the model for a wide range of ozone generation rates was much larger than experimentally observed. This indicates that the effect of ozone on the hydrogen peroxide is not sufficient to lead to less phenol removal. It is possible that upon bubbling of the gas through the high-voltage electrode some of the energy from the discharge goes to produce ozone. In addition, the nature of the discharge in the gas/liquid environment near the electrode tip may be locally altered, leading to changes in the rate of formation of hydrogen peroxide. The lower hydrogen peroxide formation rate leads to less hydroxyl radicals, via the Fenton’s reac-


Figure 6. Hydrogen peroxide formation with 150 µS/cm KCl with and without activated carbon and oxygen flow. Table 5. Energy Efficiency for Phenol Removal in FeSO4 Solutions

Table 6. Energy Efficiency for TOC Removal in FeSO4 Solutions

FeSO4 condition no C, no O2 C, no O2 no C, O2 C, O2

% % on removal carbon 89.9 100 63.1 98.6

1.3 5


FeSO4 EE/O g/kW-h (kW/1000L) 1.8 3.7 1.0 3.1

95 23 194 44

tions, and lower phenol removed when oxygen was bubbled into the reactor. It can be noted that the concentrations of ozone in the reactor after 60 min of exposure to corona discharge with FeSO4 and KCl were found to be within experimental error at 0.21 )/- 0.04 mg/L and 0.16 ( 0.04 mg/L, respectively. Therefore, direct reactions of ozone with the iron salt are negligible. The experimental condition with bubbled oxygen and no suspended activated carbon particles in ferrous sulfate solutions showed higher phenol removal rates than the same conditions in potassium chloride solutions. This is due to an increased amount of hydroxyl radicals formed by Fenton’s reaction. Figure 5 also shows the negative effect of bubbling oxygen through the high-voltage electrode for ferrous sulfate solutions when activated carbon was suspended in the solution. Although both experimental conditions with activated carbon had close to 100% removal after 60 min, the condition without bubbled oxygen showed almost complete removal after 30 min, while the condition with bubbled oxygen required the entire 60 min. This effect is again due to the detrimental effect that bubbling oxygen has on hydrogen peroxide production. This reduction in phenol removal is shown in Table 5, where the amount of phenol removal and the energy efficiencies are summarized for various experimental conditions. This table shows that, in a ferrous sulfate solution with suspended activated carbon and no bubbled oxygen, the phenol removal efficiency was 3.7 g/kW-h and 23 kW/1000 L/order, which was the highest energy efficiency seen in this reactor. When oxygen was bubbled into this reactor, the energy efficiency decreased to 3.1 g/kW-h and 44 kW/1000 L/order. Without the activated

condition no C, no O2 C, no O2 no C, O2 C, O2


1.3 5


EE/O g/kW-h (kW/1000L) 0.3 2.3 0.2 2.5

701 69 915 66

carbon, the energy efficiencies were lower than those with the activated carbon. Table 6 shows the amount of TOC removed from the ferrous sulfate solutions as well as the energy efficiencies for the TOC removal. It is interesting to note that even though the experimental conditions with bubbled oxygen gave less phenol removal than without bubbled oxygen, the TOC removal was similar with or without bubbled oxygen (with and without activated carbon). This can be explained by the fact that TOC measures all organic molecules that are in solution, and even though the phenol removal is different, indicating that the progression through the various oxidation steps is different, very little of the original organic carbon has been totally mineralized. This would result in different solution chemistry (different ratios of phenol, primary byproducts, organic acids, etc.), but similar amounts of total organic carbon. It should also be noted that the highest energy efficiency for TOC removal occurred when activated carbon was suspended in the reactor, oxygen was bubbled through the high-voltage electrode, and the salt solution was potassium chloride. This again shows the importance of the induced surface reactions on the activated carbon and the overall influence of the ozone reactions. The induced surface reaction is important due to the fact that upon desorption of the postcorona carbon sample, little phenol was present and no byproducts were present. The ozone reactions enhance the removal of the phenol oxidation products, thus adding to the total TOC removed. Model-Data Comparison and Sensitivity Analysis. Figure 7 shows the model-data comparison for the experimental conditions when oxygen was bubbled into the solution through the high-voltage point electrode.


Figure 7. Model-data comparison for phenol removal with 45-kV corona treatment and 150 SCCM O2 flow and no activated carbon.

Figure 8. Sensitivity analysis for phenol concentrationspeak value with 150 SCCM O2 flow and KCl.

The curve representing the model calculation for the phenol removal in potassium chloride solution was fit to the experimental data point at 60 min. In fitting this curve, the reaction rate for reaction 72 (in Table 1) was determined to be 1.69 × 10-7 M s-1 when oxygen was injected into the solution through the high-voltage point electrode. The model curve for the ferrous sulfate solution (in Figure 7) is the model prediction for the phenol removal using the reaction rate constants for ozone and hydrogen peroxide formation as used in the case with potassium chloride. At early times, the model prediction gives concentrations close to the experimental data, but as time progresses, the model predicts a higher rate of phenol removal than the experimental data shows. This is due to the limited oxidation pathways used in the model chemical reactions. The model only includes the oxidation reactions for phenol and the primary products (catechol and hydroquinone). There are many other possible oxidation products from phenol.44 If these other species were included as part of the oxidation pathway, they would compete with the organic compounds already present in the model to be oxidized

with ozone and hydroxyl radicals. The competition for oxidizing species will reduce the rate of phenol removal given by the model. Figure 8 shows the results for the parametric sensitivity analysis for the potassium chloride solution when oxygen was bubbled through the high-voltage electrode. The reaction that had the highest sensitivity coefficient was the production of ozone within the solution. The peak-normalized sensitivity coefficient for this reaction was -1.3. All other sensitivity coefficients are much smaller, indicating the central importance of the direct ozone reactions on phenol oxidation. It should be noted that the reaction including the production of hydrogen peroxide is tenth on the list with a value of -0.002. The 10 highest normalized sensitivity coefficients for ferrous sulfate solutions with oxygen bubbled through the high-voltage electrode are shown in Figure 9. Consistent with the results shown in Figure 8 for the case with the potassium chloride solution, the normalized sensitivity coefficient with the highest peak value for the case with ferrous sulfate was the production of ozone. Experimental results show that ozone was pro-


Figure 9. Sensitivity analysis for phenol concentrationspeak values (150 SCCM O2 flow, FeSO4, ground submerged).

Figure 10. Model-data comparison for phenol removal with 45-kV corona treatment and 1 g/L activated carbon.

duced under these conditions and that lower quantities of hydrogen peroxide were formed when oxygen was bubbled through the high-voltage point electrode than when no oxygen was bubbled in to the system through the high-voltage point electrode. These results are consistent with the sensitivity analysis. The peak sensitivity coefficient for the production of ozone in the ferrous sulfate solution was -2.2, which is higher than the sensitivity coefficient when potassium chloride was in solution. The sensitivity coefficient with the second highest peak value was the reaction rate constant for the production of hydrogen peroxide. Although the sensitivity of phenol concentration to this rate constant is 3 times smaller than that to the rate constant for ozone formation, the reactions of hydroxyl radicals are very important in governing the phenol removal. Figure 10 shows the model-data comparison for the experiments with activated carbon in suspension without oxygen bubbled through the electrode. The model results include the effects of the bulk chemical reactions and the reaction-diffusion model in the particle phase.

The only unknown parameter in the model was the surface reaction rate, which was considered to be an irreversible, first-order decay of the adsorbed organic molecules (phenol, hydroquinone, and catechol). Fitting the model to the experimental data for the potassium chloride solution gave a value of 5 × 10-4 s-1 for all surface reactions. When the iron species was added, and no other parameters were adjusted, the model prediction was nearly identical to the actual results from the experimental data. This result lends support to the validity of the bulk reaction model and the hypothesis that reactions occur on the surface of the activated carbon. The model results indicate that the percentage of the original phenol present on the surface of the activated carbon after corona treatment is 21% for the potassium chloride solution and 0.5% for the ferrous sulfate solution. These results are within 1% of the experimental values (21% vs 20% and 0.5% vs 1.3%). These results also imply that no other parameters are affected when the carbon particles were added. This result is contrary


Figure 11. Effects of surface reactions on phenol concentration for cases with iron or potassium salts.

Figure 12. Sensitivity analysis for phenol concentrationspeak values (1 g/L carbon and KCl).

to previous modeling work,28 which suggested that the electric field might alter the adsorption properties of the activated carbon. The current model includes a more rigorous set of reactions than those used in the previous work and incorporates a more detailed kinetic model with improved Fenton’s chemistry.22 Figure 11 shows the model simulations with and without surface reactions when 1 g/L activated carbon is suspended in the solution. The curves with the surface reactions are identical to the model results given in Figure 10. The model was also calculated for the two cases, with potassium chloride and with ferrous sulfate, with the surface reaction rate set to zero. In the potassium chloride solution when no surface reactions are present, the model indicates that the phenol removal quickly reaches a constant value of 60% after 15 min of corona treatment. This curve is significantly different than the results seen in experiments, where the amount

of phenol removal is continuously increasing with time. Similar to the case with the potassium chloride solution, the model simulation without the surface reactions in the ferrous sulfate case is very different from the model results including the surface reactions, and thus also very different from the experimental results. In general, the model without the surface reactions cannot adequately predict the phenol removal, giving further support to the hypothesis that surface reactions occur on the activated carbon. Figure 12 shows the normalized sensitivity coefficients for the phenol concentration with respect to the bulk and particle reaction rate constants, the interphase mass-transfer coefficient, and the adsorption constant. The results shown here are for the case where phenol is removed from a potassium chloride solution with suspended activated carbon particles and no oxygen input into the system. The only three parameters that


Figure 13. Sensitivity analysis of phenol concentrationspeak values (1 g/L carbon, FeSO4).

Figure 14. Sensitivity analysis for phenol concentration (1 g/L activated carbon and FeSO4).

have a significant effect on phenol are the fluid-particle mass-transfer coefficient, the phenol adsorption constant, and the reaction rate constant for phenol on the surface of the activated carbon. All other parameters give sensitivity values at least 1 order of magnitude smaller, indicating that, in the potassium chloride solution, the adsorption and particle surface reaction were responsible for nearly all the phenol removal from the system. Figure 13 shows the peak sensitivity coefficients for phenol concentration in the ferrous sulfate solution containing suspended activated carbon particles and without oxygen flowing through the high-voltage electrode. This figure shows that when ferrous sulfate is present in solution, both bulk and surface oxidation reactions occur. The key bulk-phase reactions in this case are the production of hydrogen peroxide from the corona discharge (-7), the reaction between phenol and the hydroxyl radical (-1.25), Fenton’s reaction (-1.25), and the regeneration of the ferrous ions from catechol (-1.25). The main parameters associated with the

activated carbon in this case are the phenol reaction on the carbon surface (-3.5), the mass-transfer coefficient (-1.1), and the phenol adsorption constant (-0.95). Although it appears that the oxidation reactions in the bulk phase were as, or more, important than the adsorption and reaction rates on the activated carbon, the results of the normalized sensitivity coefficients versus time (Figure 14) further show the importance of the processes associated with the suspended activated carbon. Initially, the sensitivity of most of the parameters, except for the mass-transfer coefficient and the phenol adsorption constant, are very small. The sensitivity coefficient for these two parameters quickly increases, indicating that when the phenol concentration is at its highest, the most important processes are mass transfer and adsorption. At later times, the sensitivity coefficient for the production of the hydrogen peroxide increases, and the senstivity for the reaction rate constant on the surface of the carbon also increases. After approximately 40 min, the experimental and theoretical values of phenol concentration approach


Figure 15. Model-data comparison for phenol removal with 45-kV corona treatment and 150 SCCM O2 and 1 g/L activated carbon.

Figure 16. Sensitivity analysis of phenol concentrationspeak values (1 g/L carbon, 150 SCCM O2, KCl).

zero; thus, the values of the sensitivity coefficients after this time will have little effect on the phenol concentration. Therefore, although the bulk-phase reactions are significant, the main pathway for the removal of phenol under these conditions is through the adsorption of phenol to the surface of the activated carbon and subsequent surface reactions. Figure 15 shows the model-data comparison when activated carbon was suspended in the solution and oxygen was bubbled through the high-voltage point electrode. The model results shown here are predictions based on rate constants determined from independent experiments. It can be recalled that the hydrogen peroxide rate of formation was determined by direct measurement without phenol or iron present. The rate of ozone formation was determined without the activated carbon and surface reaction rate constants were determined without oxygen bubbling through the highvoltage electrodes. At 60 min, both the experimental data and the theoretical predictions match well. For earlier times, there are small differences between the experimental data and the theoretical predictions. Note

that when the rate constant for the production of ozone was determined for the case without activated carbon, the theoretical curve was fit to the experimental data at 60 min. This may partially explain why the experimental data and theoretical predictions do not match identically at other times. The model predicts that the percentages of the original phenol remaining on the surface of the carbon after corona treatment are 9.8% and 6.4% for the potassium chloride and ferrous sulfate solutions, respectively. These values are very close to those measured experimentally (10% for KCl solutions and 5% for FeSO4 solutions). Sensitivity analysis for the case of potassium chloride solutions combined with suspended activated carbon and oxygen bubbled through the high-voltage electrode is shown in Figure 16. The most sensitive processes are the reaction of phenol on the particle surface (-1.85), the production of ozone (-1.75), and the adsorption of phenol to the particle (-1.6). The normalized sensitivity coefficients for the case when suspended activated carbon is combined with oxygen bubbled through the high-voltage electrode than for the two conditions


Figure 17. Sensitivity analysis of phenol concentrationspeak values (1 g/L carbon, 150 SCCM O2, FeSO4).

separately. Also, note that the production of hydrogen peroxide and Fenton’s reaction are not in the 10 largest normalized sensitivity coefficients. This again shows that when iron is not present in the system, the ozone reactions dominate the bulk-phase reactions. Figure 17 shows the top 10 normalized sensitivity coefficients for the experimental condition when activated carbon was suspended in the ferrous sulfate solution and oxygen was bubbled through the highvoltage electrode. The three processes with the highest normalized sensitivity coefficients are the reaction of phenol on the surface of the activated carbon (-2.9), the production of ozone (-2.9), and the adsorption of the phenol to the activated carbon (-2.1). Similar to the case with the potassium chloride solution, the values of the sensitivity coefficients in this case are higher than the sensitivity coefficients for the case of suspended carbon alone or the bubbled oxygen alone. The experimental results for the combined case gave a relatively low amount of hydrogen peroxide formed in the solution, and this is reflected in the normalized sensitivity coefficient. The value of the sensitivity coefficient for the production of hydrogen peroxide is -0.65. Although this is still one of the more important reactions for this experimental condition, the final solution is much more sensitive to the three other above-mentioned processes. It is important to note that other studies have also reported the possibility of activated carbon serving as a catalyst for the oxidation of organic compounds in aqueous solutions.45-48 Oxidative coupling of phenols on the surface of activated carbon was found to occur in water solutions with high oxygen content.47 In addition, chlorophenol was oxidized over activated carbon in packed columns.48 Wet oxidation of phenol with activated carbon in trickle-bed reactors at 140 °C and 1-9 bar was found to follow first-order kinetics.45,46 Although the rates of these reactions are relatively slow compared to those observed in the pulsed corona experiments reported in the present study, this previous work supports the general concept of activated carbon functioning not only as a passive adsorbent but also as a catalyst. The mechanism(s) for organic compound oxidation on activated carbon is not fully established; how-

ever, recent studies have demonstrated that activated carbon can serve as a catalyst to convert ozone into hydroxyl radicals.24 Thus, there is certainly the possibility that activated carbon can readily participate in radical-based reactions under highly oxidative conditions as occur in liquid-phase pulsed corona reactors. Conclusions The addition of suspended activated carbon particles to the liquid-phase pulsed corona discharge reactor is shown to enhance the removal of phenol from the aqueous solution. This occurs by adsorption of the organic molecules and possible reactions of the phenol on the carbon surface. After the carbon particles have been suspended in the solution treated by the pulsed corona, the physical properties of the carbon remain unchanged. An analysis of the carbon showed that little of the phenol initially adsorbed on the activated carbon was still present after treatment was completed. A mathematical model that included the bulk-phase reactions and the reaction-diffusion processes in the activated carbon was fit to the experimental data at one set of operating conditions to determine the surface reaction rate. Once this rate was determined, the experimental conditions were changed and the model was found to closely predict the phenol concentration that was experimentally measured. Sensitivity analysis also demonstrates the importance of the surface reaction to the removal of phenol from the pulsed corona reactor. Oxygen bubbling through the high-voltage needle electrodes leads to the formation of ozone. Subsequent reactions of dissolved ozone in the liquid phase enhance the degradation of phenol through competitive reactions with phenol and the primary reaction products of phenol oxidation. Acknowledgment We gratefully acknowledge partial support from the U.S. Air Force, Tyndall Air Force Base, and the Department of Chemical Engineering, FAMU-FSU College of Engineering.


Literature Cited (1) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (2) Snoeyink, V. L. Adsorption of Organic Compounds. In Water Quality and Treatment: A Handbook of Community Water Supplies; Pontius, F. W., Ed.; McGraw-Hill: New York, 1990. (3) Hoigne, J. Chemistry Of Aqueous Ozone And Transformation Of Pollutants By Ozonation And Advanced Oxidation. In The Handbook of Environmental Chemistry Vol. 5 Part C Quality and Treatment of Drinking Water II; Hrubec, J., Ed.; SpringerVerlag: Berlin, 1998. (4) Langlais, B.; Reckhow, D. A.; Brink, D. R. Ozone in Water Treatment, Application and Engineering; Lewis Publishers: Chelsea, 1991. (5) Singer, P. C. Control of Disinfection By-Products in Drinking Water. J. Environ. Eng. 1994, 120, 727. (6) Glaze, W. H. Drinking-water Treatment with Ozone. Environ. Sci. Technol. 1987, 21, 224. (7) Willberg, D. M.; Lang, P. S.; Hochemer, R. H.; Kratel, A.; Hoffmann, M. R. Electrohydraulic Destruction Of Hazardous Wastes. CHEMTECH 1996, 26, 52. (8) Hoffmann, M. R.; Hua, I.; Hochemer, R.; Willberg, D.; Lang, P.; Kratel, A. Chemistry Under Extreme Conditions in Water Induced Electrohydraulic Cavitation and Pulsed-Plasma Discharges. In Chemistry Under Extreme or Non-Classical Conditions; van Eldik, R., Hubbard, C. D., Eds.; John Wiley and Sons: New York, 1997. (9) Cooper, W. J.; Curry, R. D.; O’Shea, K. E. Environmental Applications of Ionizing Radiation; John Wiley & Sons: New York, 1998. (10) 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. (11) Glaze, W. H.; Kang, J. W. Advanced Oxidation Processes. Test of a Kinetic Model for the Oxidation of Organic Compounds with Ozone and Hydrogen Peroxide in a Semibatch Reactor. Ind. Eng. Chem. Res. 1989, 28, 1580. (12) Hickling, A. Electrochemical Processes in Glow Discharge at the Gas-Solution Interface. In Modern Aspects of Electrochemistry; Bockris, J. O. M., Conway, B. E., Eds.; Plenum Press: New York, 1971. (13) Zhong-ai, H.; Xiao-yan, W.; Jin-zhang, G.; Hua-ling, D.; Jing-guo, H.; Xiao-quan, L.; Jing-wan, K. A study on water treatment induced by plasma with contact glow discharge electrolysis. Plasma Sci. Technol. 2001, 3, 927. (14) Clements, J. S.; Sato, M.; Davis, R. H. Preliminary Investigation of Prebreakdown Phenomena and Chemical Reactions Using a Pulsed High-Voltage Discharge in Water. IEEE Trans. Ind. Appl. 1987, IA-23, 224. (15) Sharma, A. K.; Locke, B. R.; Arce, P.; Finney, W. C. A Preliminary Study of Pulsed Streamer Corona Discharge for the Degradation of Phenol in Aqueous Solutions. Hazard. Waste Hazard. Mater. 1993, 10, 209. (16) Joshi, A. A.; Locke, B. R.; Arce, P.; Finney, W. C. Formation of Hydroxyl Radicals, Hydrogen Peroxide and Aqueous Electrons by Pulsed Streamer Corona Discharge in Aqueous Solution. J. Hazard. Mater. 1995, 41, 3. (17) Sun, B.; Sato, M.; Clements, J. S. Oxidative Processes Occurring When Pulsed High Voltage Discharges Degrade Phenol in Aqueous Solution. Environ. Sci. Technol. 2000, 34, 509. (18) Sunka, P.; Babicky, V.; Clupek, M.; Lukes, P.; Simek, M.; Schmidt, J.; Cernak, M. Generation of Chemically Active Species by Electrical Discharges in Water. Plasma Sources Sci. Technol. 1999, 8, 258. (19) Lukes, P. Water Treatment by Pulsed Streamer Corona Discharge. Ph.D. Dissertation, Institute of Chemical Technology, Prague, Czech Republic, 2001. (20) Sano, N.; Kawashima, T.; Fujikawa, J.; Fugimoto, T.; Kitai, T.; Kanki, T. Decomposition of Organic Compounds in Water by Direct Contact of Gas Corona Discharge: Influence of Discharge Conditions. Ind. Eng. Chem. Res. 2002, 41, 5906. (21) Grymonpre´, D.; Finney, W. C.; Locke, B. R. Aqueous-Phase Pulsed Streamer Corona Reactor using Suspended Activated Carbon Particles for Phenol Oxidation: Model-Data Comparison. Chem. Eng. Sci. 1999, 54, 3095.

(22) Grymonpre´, D. R.; Sharma, A. K.; Finney, W. C.; Locke, B. R. The Role of Fenton’s Reaction in Liquid-Phase Pulsed Corona Reactors. Chem. Eng. J. 2001, 82, 189. (23) Hoeben, W. F. L. M.; van Veldhuizen, E. M.; Rutgers, W. R.; Kroesen, G. M. W. Gas-Phase Corona Discharges for Oxidation of Phenol in an Aqueous Solution. J. Phys. D: Appl. Phys. 1999, 32, L133. (24) Jans, U.; Hoigne, J. Activated Carbon and Carbon Black Catalyzed Transformation of Aqueous Ozone into OH-Radicals. Ozone Sci. Eng. 1998, 20, 67. (25) Jans, U.; Hoigne, J. Atmospheric Water: Transformation Of Ozone Into OH-Radicals By Sensitized Photoreactions Or Black Carbon. Atmos. Environ. 2000, 34, 1069. (26) Grymonpre´, D. R.; Finney, W. C.; Locke, B. R. Activated Carbon Particles in Aqueous Phase Pulsed Streamer Corona Discharge. J. Adv. Oxid. Technol. 1999, 4, 408. (27) Farhataziz; Rodgers, M. A. J. Radiation Chemistry; VCH Publishers: New York, 1987. (28) Grymonpre´, D. R. The Effects of Activated Carbon on Aqueous Phase Pulsed Streamer Corona. M.S. Thesis, Florida State University, Tallahassee, FL, 1998. (29) Grymonpre´, D. R. An Experimental and Theoretical Analysis of Phenol Degradation by Pulsed Corona Discharge. Ph.D. Dissertation, Florida State University, Tallahassee, FL, 2001. (30) Whitaker, S. The Transport Equations for Multi-phase Systems. Chem. Eng. Sci. 1973, 28, 139. (31) Ochoa, J. A.; Stroeve, P.; Whitaker, S. Diffusion and Reaction in Cellular Media. Chem. Eng. Sci. 1986, 41, 2999. (32) Whitaker, S. Mass Transport and Reaction in Catalyst Pellets. Trans. Porous Media 1987, 2, 269. (33) Whitaker, S. The Method of Volume Averaging: An Application to Diffusion and Reaction in Porous Catalysts. In Proceedings of the National Science Council, Part A: Physical Science and Engineering; National Science Council: Taipei, Taiwan, Republic of China, 1991. (34) Whitaker, S. The Species Mass Jump Condition at a Singular Surface. Chem. Eng. Sci. 1992, 47, 1677. (35) Chen, R.; Pignatello, J. J. Role of Quinone Intermediates as Electron Shuttles in Fenton and Photoassisted Fenton Oxidations of Aromatic Compounds. Environ. Sci. Technol. 1997, 31, 2399. (36) Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; Robert E. Krieger Publishing Co: Malabar, 1970. (37) Sitaraman, R.; Ibrahim, S. H.; Kuloor, N. R. A Generalized Equation for Diffusion in Liquids J. Chem. Eng. Data 1963, 8, 198. (38) Varma, A.; Morbidelli, M.; Wu, H. Parametric Sensitivity in Chemical Systems; Cambridge University Press: Cambridge, 1999. (39) Hochanadel, C. J. Effects of Cobalt Gamma Radiation on Water and Aqueous Solutions. J. Phys. Chem. 1952, 56, 587. (40) Eisenberg, G. M. Colorimetric Determination of Hydrogen Peroxide. Ind. Eng. Chem. 1941, 15, 327. (41) Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the Examination of Water and Wastewater; American Public Health Association, American Water Works Association, Water Environment Federation: Washington, DC, 1992. (42) Sharma, A. K. High Voltage Pulsed Streamer Corona Discharges for the Removal of Organic Contaminants from Aqueous Solutions. M.S. Thesis, FAMU-FSU College of Engineering, Florida Agricultural and Mechanical University, Tallahassee, FL, 1993. (43) Bolton, J. R.; Valladares, J. E.; Zanin, J. P.; Cooper, W. J.; Nickelsen, M. G.; Kajdi, D. C.; Waite, T. D.; Kurucz, C. N. Figures-of-Merit for Advanced Oxidation Technologies: A Comparison of Homogeneous UV/H2O2, Heterogeneous UV/TiO2, and Electron Beam Processes. J. Adv. Oxid. Technol. 1998, 3, 174. (44) Hoeben, W. F. L. M. Pulsed Corona-Induced Degradation of Organic Materials in Water. Ph.D. Dissertation, Technische Universiteit Eindhoven, Eindhoven, The Netherlands, 2000. (45) Fortuny, A.; Font, J.; Fabregat, A. Wet air oxidation of phenol using active carbon as catalyst. Appl. Catal., B 1998, 19, 165. (46) Fortuny, A.; Miro, C.; Font, J.; Fabregat, A. Three-Phase Reactors For Environmental Remediation: Catalytic Wet Oxidation of Phenol Using Activated Carbon. Catal. Today 1999, 48, 323.

5134 Ind. Eng. Chem. Res., Vol. 42, No. 21, 2003 (47) Vidic, R. D.; Suidan, M. T.; Brenner, R. C. Oxidative Coupling of Phenols on Activated Carbon: Impact on Adsorption Equilibrium. Environ. Sci. Technol. 1993, 27, 2079. (48) Lucking, F.; Koser, H.; Jank, M.; Ritter, A. Iron Powder, Graphite and Acitvated Carbon as Catalysts for the Oxidation of 4-Chlorophenol with Hydrogen Peroxide in Aqueous Solutions. Water Res. 1998, 32, 2607. (49) Felix, W. D.; Gall, B.; Dorfman, L. M. Pulse Radiolysis Studies. IX. Reactions of the Ozonide Ion in Aqueous Solution. J. Phys. Chem. 1967, 71, 384. (50) Baxendale, J. H.; Hardy, H. H.; Sutcliffe, L. H. Trans. Faraday Soc. 1951, 47, 963. (51) Matheson, M. S.; Rabani, J. Pulse Radiolysis of Aqueous Hydrogen Solutions. I. Rate Constants for Reaction of eaq- with Itself and other Transients II. The Interconvertiblility of eaq- and H. J. Phys. Chem. 1965, 69, 1324. (52) Mentasti, E.; Pelizzetti, E.; Saini, G. J. J. Chem. Soc., Dalton Trans. 1973, 19, 2609. (53) Walling, C.; Goosen, A. J. Mechanism Of Ferric Ion Catalyzed Decomposition Of Hydrogen-Peroxide-Effect Of Organic Substrates J. Am. Chem. Soc. 1973, 95, 2987. (54) Walling, C. Fenton’s Reagent Revisited. Acc. Chem. Res. 1975, 8, 125. (55) Feng, P. Y.; Brynjolfsson, A.; Halliday, J. W.; Jarrett, R. D. High-Intensity Radiolysis of Aqueous Ferrous Sulfate-Cupric Sulfate-Sulfuric Acid Solutions. J. Phys. Chem. 1970, 74, 1221. (56) Patel, K. B.; Willson, R. L. Semiquinone Free Radicals and Oxygen. Pulse Radiolysis Study of One Electron-Transfer Equilibria. J. Chem. Soc., Faraday Trans. 1973, 69, 814. (57) Bielskie, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B. Reactivity of HO2/O2- Radicals in Aqueous Solution. J. Phys. Chem. Ref. Data 1985, 14, 1041. (58) Rush, J. D.; Bielski, B. H. J. Pulse Radiolytic Studies of The Reactions of HO2/O2- With Fe(II)/Fe(III) Ions - The Reactivity of HO2/O2- With Ferric Ions And Its Implication on the Occurrence of the Haber-Weiss Reaction. J. Phys. Chem. 1985, 89, 5062. (59) Thomas, J. K. Rates of Reaction of the Hydroxyl Radical. Trans. Faraday Soc. 1965, 61, 702. (60) Al-Suhybani, A. A.; Hughes, G. Pulse Radiolysis of Deaerated Hydroquinone Solutions. J. Chem. Soc. Pak. 1968, 8, 107. (61) Gehringer, P.; Eschweiler, H. The Use Of RadiationInduced Advanced Oxidation For Water Reclamation. Water Sci. Technol. 1996, 34, 343. (62) Elliot, A. J. A Pulse Radiolysis Study of the Temperature Dependence of Reactions Involving H, OH, and eqa- in Aqueous Solutions. Rad. Phys. Chem. 1989, 34, 753. (63) Staehelin, J.; Hoigne, J. Decomposition of Ozone in Water: Rate of Initiation by Hydroxide Ions and Hydrogen Peroxide. Environ. Sci. Technol. 1982, 16, 676. (64) Jonah, C. D.; Miller, J. R.; Matheson, M. S. The Reaction of the Precursor of the Hydrated Electron with Electron Scavengers. J. Phys. Chem. 1977, 81, 1618. (65) Hartig, K. G.; Getoff, N. Reactivity of Hydrogen Atoms with Liquid Water. J. Photochem. 1982, 18, 29. (66) Buehler, R. E.; Staehelin, J.; Hoigne, J. Ozone Decomposition in Water Studied by Pulse Radiolysis 1.HO2/O2- and HO3/ O3- as Intermediates. J. Phys. Chem. 1984, 88, 2560. (67) Baxendale, J. H.; Dixon, R. S.; Stott, D. A. Reactivity of Hydrogen Atoms with Fe3+, FeOH2+ and Cu2+ in Aqueous Solutions. Trans. Faraday Soc. 1968, 64, 2398.

(68) Zehavi, D.; Rabani, J. Pulse Radiolytic Investigation of OaqRadical Ions. J. Phys. Chem. 1971, 75, 1738. (69) Sehested, K.; Holcman, J.; Bjergbakke, E.; Hart, E. J. A Pulse Radiolytic Study of the Reaction OH + O3 in Aqueous Medium. J. Phys. Chem. 1984, 88, 4144. (70) Christensen, H.; Sehested, K.; Corfitzen, H. Reactions of Hydroxyl Radicals with Hydrogen Peroxide at Ambient and Elevated Temperatures. J. Phys. Chem. 1982, 86, 1588. (71) Hickel, B.; Sehested, K. Activation Energies for the Reactions O- + H2 and O- + D2 in Aqueous Solution. J. Phys. Chem. 1991, 95, 744. (72) 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. (73) Christensen, H.; Sehested, K. Reaction of Hydroxyl Radicals with Hydrogen at Elevated Temperatures, Determination of the Activation Energy. J. Phys. Chem. 1983, 87, 118. (74) Elliot, A. J.; Buxton, G. V. Temperature Dependence of the Reactions of OH+O2 and OH + HO2 in Water up to 200 °C. J. Chem. Soc., Faraday Trans. 1992, 88, 2465. (75) Raghavan, N. V.; Steenken, S. Electrophilic Reaction of the OH Radical With Phenol - Determination Of The Distribution Of Isomeric Dihydroxycyclohexadienyl Radicals J. Am. Chem. Soc. 1980, 102, 3495. (76) Rabani, J.; Matheson, M. S. The Pulse Radiolysis of Aqueous Solutions of Potassium Ferrocyanide. J. Phys. Chem. 1966, 70, 761. (77) Hoigne, J.; Bader, H. Rate Constants of Reactions of Ozone with Organic and Inorganic Compounds in Water - II Dissociating Organic Compounds. Water Res. 1983, 17, 185. (78) Gurol, M. D.; Nekouinaini, S. Kinetic Behavior of Ozone in Aqueous Solutions of Substituted Phenols. Ind. Eng. Chem. 1984, 23, 54. (79) Schwarz, H. A. Reaction of the Hydrated Electron with Water. J. Phys. Chem. 1992, 96, 8937. (80) O’Neill, P.; Steenken, S.; Schulte-Frohlinde, D. Formation of Radical Cations of Methoxylated Benzenes by Reaction with OH Radicals, Tl2+, Ag2+, and SO4*- in Aqueous Solution. An Optical and ConductometricPulse Radiolysis and in situ Radiolysis Electron Spin Resonance Study. J. Phys. Chem. 1975, 79, 2773. (81) Elliot, A. J.; McCracken, D. R.; Buxton, G. V.; Wood, N. D. Estimation of Rate Constants for Near-Diffusion-Controlled Reactions in Water. J. Chem. Soc., Faraday Trans. 1990, 86, 1539. (82) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals in Aqueous Solutions. J. Phys. Chem. Ref. Data 1988, 17, 513. (83) Gruenbein, W.; Henglein, A.; Stevens, G.; Beck, G. Vielfachpuls-Radiolyse: Sukzessive Anlagerung zweier hydratisieter, Elektronen an Sauerstoff, Nitrobenzol und Nitromethane. Ber. Bunsen-Ges. Phys. Chem. 1971, 75, 126.

Received for review May 2, 2002 Revised manuscript received July 21, 2003 Accepted July 23, 2003 IE020330N

Suspended Activated Carbon Particles and Ozone Formation in

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