Fe(II) in Acidic Environment in a Rotating

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Ozonation of Phenol with O3/Fe(II) in Acidic Environment in a Rotating Packed Bed Zequan Zeng,†,‡ Haikui Zou,†,‡ Xin Li,†,‡ Baochang Sun,†,‡ Jianfeng Chen,†,‡ and Lei Shao*,†,‡ †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, P. R. China ‡ Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, P. R. China ABSTRACT: The treatment of acidic phenolic wastewater by ferrous-catalyzed ozonation (O3/Fe(II)) process in a rotating packed bed (RPB) was studied, and the O3/Fe(II) process was compared with the O3 process. It was observed that the phenol degradation rate in the O3/Fe(II) process was roughly 10% higher than that of O3 process in acidic environment in the RPB. It is also found that the degradation efficiency of phenol was significantly affected by the rotation speed and inlet ozone concentration. Phenol degradation efficiency increased with increasing initial pH of the phenolic solution, rotation speed, and concentrations of the inlet ozone gas, as well as a decreasing liquid flow rate and initial concentrations of phenol. Phenol degradation efficiency reached maximum at a temperature of 25 °C and an initial Fe(II) concentration of 0.4 mM. The result of the contrast experiment showed that the biological oxygen demand/chemical oxygen demand (BOD/COD) of the phenol solution increased from 0.2 to 0.59 after the solution was treated by O3/Fe(II) process. The intermediates of the ferrouscatalyzed ozonation process were identified by gas chromatography/mass spectroscopy (GC/MS), and it is deduced that the pathway of phenol degradation in ferrous-catalyzed ozonation is different from that in ozonation. Hydroquinone and 1,4-benzoquinone were the main intermediates, and a small amount of polymeric intermediates was found in the O3/Fe(II) process.

1. INTRODUCTION Phenolic compounds are toxic pollutants commonly present in organic wastewater. Phenol is one of the 129 priority-control pollutants listed by United States Environmental Protection Agency (U.S. EPA). Biological degradation method is one of the most widely used wastewater treatment technologies with low operation cost and high treatment capacity. Ammary et al.1 investigated the treatment of phenolic wastewater by using activated sludge process and obtained a phenol removal of 63%. Hussain et al.2 tested the anaerobic treatment of phenolic wastewater and obtained chemical oxygen demand (COD) removal of 40%. However, the bactericidal properties of phenolic wastewater limited the application of biological degradation method. Ozonation and advanced oxidation processes (e.g., Fenton, O3/H2O2, H2O2/UV) were thus proposed for the pretreatment of phenolic wastewater. Zazo et al.3 examined the oxidation of phenol by using Fenton reagent and obtained a phenol removal and a total organic carbon (TOC) removal of 100% and 39%, respectively. However, the application of the Fenton process is limited by the large consumption of H2O2 and the oxidant residual. Ozone (E0 = 2.08 V) can destroy the structure of many organic matters and increase the amount of the dissolved oxygen, which can improve the efficiency of biological treatment. Ozone therefore has been widely used for treatment of wastewater in recent years.4−7 A disadvantage of ozonation for wastewater treatment is the energy cost for ozone generation, especially in acidic environment, because ozonation efficiency is much higher in alkaline solution than in acidic solution as a result of the improvement of the hydroxyl radicals © 2012 American Chemical Society

(E0 = 2.80 V) generation at a higher pH (eqs 1 and 2, k1, k2: reaction rate constants).8 O3 + OH− → O2 + HO2− ,

k1 = 70 M−1s−1

O3 + HO2− → 2O2 + ·OH,

k 2 = 1.6 × 109 M−1s−1

(1)

(2)

Many kinds of organic wastewaters are acidic, especially in conventional bisphenol A and p-nitrophenol production processes. The ozonation efficiency is limited in acidic wastewater due to the scarcity of hydroxyl radicals. Therefore, metalcatalyzed homogeneous ozonation has been developed to promote the efficiency of ozonation process in acidic wastewaters in recent years.9−11 Transition metals such as Fe(II), Fe(III), Mn(II) are proposed for the process of homogeneous catalytic ozonation. Fe0 and Fe(II) are promising and effective catalysts due to their nontoxicity and benefit for coagulation.12−14 Ferrous ions act as the initiator of radical chain reaction to yield more hydroxyl radicals.15−17 Cortes18 compared the O3/high pH and O3/Fe(II) processes for the oxidation of chlorobenzenes and found that the organic oxidation was more efficient and the removal of TOC and COD was higher in the O3/ Fe(II) process. Because ozone reacts with phenol rapidly, the ozonation process is limited by the low gas−liquid mass transfer efficiency in conventional contactors. Therefore, a Received: Revised: Accepted: Published: 10509

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Figure 1. Diagram of the experimental setup.

solutions with different concentrations. The initial pH of the solutions was adjusted to a desired value with H2SO4 (95−97%, Beijing Chemical Works, China). Figure 1 shows a sketch of the experimental setup. The RPB consists of two parts, the stationary casing and the packed rotor. The rotor has an inner diameter of 40 mm, an outer diameter of 120 mm, and an axial length of 15 mm. The diameter of the stationary housing is 170 mm. The volumes of the packed-bed rotator and the housing case are 150 and 340 mL, respectively. Stainless steel wire mesh (Beijing Hongyahong Mesh Sale Center, Beijing, China) was used as the packing material. The voidage and surface area per unit volume of the dry packing in the rotor are 0.97 m3/m3 and 522 m2/m3, respectively. Ozone was generated from pure oxygen by using an ozone generator (Shanmeishuimei Environmental Technologies Co. Ltd., Beijing, China), and the ozone containing gas (O3 and O2) was mixed with N2 to obtain a desired ozone concentration before the mixed gas stream was introduced into the RPB via a gas inlet with a gas flow rate of 150 L/h. The phenol solution (2 L, 60−140 mg/L) was pumped into the RPB via a liquid inlet with the liquid flow rate of 15−30 L/h. When the gas and liquid streams contacted countercurrently in the packed rotor, O3 was absorbed into the phenol solution to degrade phenol. Then, the gas and liquid streams left the RPB via the gas outlet and liquid outlet, respectively. The gas stream was further introduced into a KI (10 g/L, analytical grade, Beijing Chemical Works, China) solution to absorb the effluent O3. The effluent liquid (10 mL) was sampled when ozone concentration (CA) at the gas outlet reached a steady state. To stop the reaction in the samples, 0.1 mL of sodium thiosulfate

gas−liquid contactor that can enhance the mass transfer is desirable. The rotating packed bed (RPB) is a novel gas−liquid contactor that was first proposed by Ramshaw and Mallinson.19 RPBs can significantly intensify mass transfer and mixing processes and were used for absorption,20−23 stripping,24,25 and distillation,26,27 etc. Recently, RPBs have been employed in ozonation processes as gas−liquid contactors due to their excellent gas−liquid mass transfer effect.28−30 Lin et al.31,32 examined the absorption of O3 by H2O2 or Fe2+ solution in RPB and reported that the gas phase mass transfer coefficient kGa and O3 absorption ratio reached 1.1 s−1 and nearly 100%, respectively. However, the study on metal-catalyzed homogeneous ozonation for wastewater treatment in RPBs has not been reported yet. The objective of this work was to investigate the pretreatment of phenolic wastewater in an RPB by using ozonation (O3-RPB system) and ferrous-catalyzed homogeneous ozonation (O3/Fe(II)-RPB system). Phenol as a typical phenolic organic compound was chosen as the model contaminant. The effects of different operation parameters on the degradation efficiency of phenol were examined. Furthermore, the degradation efficiencies in RPB system and in a stirred tank reactor (STR) were compared, and the intermediates of phenol degradation under O3/Fe(II) action were identified to elucidate the process pathway and mechanism.

2. EXPERIMENTAL SECTION 2.1. Experimental procedures. 2.1.1. O3-RPB. Phenol (analytical grade, Sinopharm Chemical Reagent Co. Ltd., China) was dissolved into deionized water to prepare the phenol 10510

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solution (0.1 mol/L, Na2S2O3, analytical grade, Beijing Chemical Works, China) was added into the samples to consume the remnant ozone. 2.1.2. O3/Fe(II)-RPB. The O3/Fe(II)-RPB experiments were performed the same way as the O3-RPB experiments with the addition of Fe(II) catalyst (0−1 mM, FeSO4·7H2O, analytical grade, Beijing Chemical Works, China) prior to being pumped into the RPB. 2.1.3. O3/Fe(II)-STR. In the O3/Fe(II)-STR experiments, a flask with 1 L of the phenol solution (100 mg/L) under magnetic stirring of 1800 rpm was used to simulate a STR. The O3 containing gas (O3, O2, and N2) was introduced into the STR through an aeration stone (diameter 25 mm, length 30 mm) for 2 min to compare the degradation efficiencies of phenol in the STR and the RPB. 2.2. Analyses. The ozone concentration was adjusted by varying the flow rate of O2 and N2. An ozone monitor (Limicen Ozone R&D Center, Guangzhou, China) was used to measure the ozone concentrations in the inlet and outlet air streams. The initial pH of the solution was measured by a pH meter (Model PHS-25, Shanghai Precision & Scientific Instrument Co. LTD, Shanghai, China). The phenol concentration in the solution (CB) was analyzed by a high-performance liquid chromatography (HPLC, Waters 2695, U.S.A.) with 150 × 3.9 mm C18 (4 μm) column (Sun Fire, U.S.A.) and UV/visible detector (Model 2996, Waters, U.S.A.) at 270 nm. A mixture of methyl alcohol and water (65:35, v/v) was used as the mobile phase at a flow rate of 0.6 mL/min. The injection volume was 10 μL. The phenol degradation efficiency (ηB) is defined by eq 3. ηB =

(C B0 − C B) × 100 C B0

Figure 2. Effect of initial pH on ηB in the O3 and O3/Fe(II) processes with or without t-BuOH (T = 25 °C, CFe(II) = 0.2 mM, CA0 = 75 mg/L, CB0 = 100 mg/L, liquid flow rate = 20 L/h, gas flow rate = 150 L/h, rotation speed = 1000 rpm).

and catalytic ozonation of phenol in the RPB with different initial pH. It is observed that a higher ηB was obtained in the O3/Fe(II) process than in the O3 process at initial pH values between 1 and 4. The ηB value increased remarkably with the increase of the initial pH of the solution in both processes as a result of the enhanced catalytic decomposition of ozone by hydroxyl ion and the formation of hydroxyl radicals. Because the hydroxyl radicals are more active than ozone, the efficiency of ozonation is thus higher at a higher pH value. As Figure 2 shows, the ηB in the O3 and O3/Fe(II) processes both declined greatly (decreased by about 20%) with the addition of t-BuOH and an initial pH above 4. It can be concluded that radical reaction plays an important role with the initial pH above 4. The addition of t-BuOH only caused the decrease of the ηB in the O3/Fe(II) process at pH below 4, which means that the addition of Fe(II) improved the ozonation efficiency mainly by increasing the amount of hydroxyl radicals. Pera-Titus33 reported that direct ozonation (oxidation by ozone molecule) is the dominating pathway at pH below 4, and both direct and indirect ozonation (oxidation mainly by hydroxyl radicals) are important at the pH range of 4−9. Ferrous ion can promote the decomposition of ozone and formation of hydroxyl radicals (eqs 4 to 11) and enhance the efficiency of ozonation significantly at pH below 4.12 However, ferrous ions play a less important role due to the presence of a large amount of hydroxyl radicals at pH higher than 4, resulting in a similar efficiency of ozonation between ozonation and catalytic ozonation processes at a higher pH value. The mechanism of catalytic ozonation in the O3/Fe(II) process can be expressed as follows.

(3)

where CB0 is the initial phenol concentration (mg/L) and CB is the outlet phenol concentration. Chemical oxygen demand (COD) was determined spectrophotometrically by a COD analyzer (5B-3A, Lian-hua Tech. Co., Ltd., Lanzhou, China) according to a Chinese standard method (HJ/T 399-2007). The 5-day biochemical oxygen demand (BOD5) test was conducted to measure the biodegradability according to a Chinese standard method: dilution and inoculation method (HJ 505-2009). A gas chromatography instrument coupled with a mass spectrometry detector (GC/MS, QP2010, Shimadzu, Japan) was employed to identify the intermediates. The column was a 30.0 m × 0.25 mm × 0.25 μm capillary column (GsBP-XLB, General Separation Technologies, U.S.A.). The temperature program started at 70 °C (hold 2 min) and increased to 200 °C (hold 5 min) at a rate of 15 °C/min, with a holding time of 2 min for each increment. Helium was used as the carrier gas at a constant flow rate of 1.0 mL/min. The samples (300 mL) were extracted by dichloromethane (100 mL, chromatographic grade, Fisher, U.S.A.) twice, and then the extracted liquid was concentrated by a nitrogen flow gas to 1 mL for GC/MS analysis.

3. RESULTS AND DISCUSSION 3.1. Effect of the Initial pH. To investigate the effect of initial pH on ηB, several ozonation and catalytic ozonation (O3/Fe(II)) experiments with or without radical scavenger (tertiary butanol, t-BuOH) were carried out with the initial pH of 1−7. t-BuOH reacts with hydroxyl radicals very quickly (k = 5 × 108 M−1 s−1) and is insensitive to O3 (k = 0.03 M−1 s−1). Figure 2 shows the effect of initial pH on ηB for the ozonation

Fe 2 + + O3 → Fe3 + + ·O3−

(4)

·O3− + H+ → O2 + ·OH

(5)

Fe2 + + O3 → FeO2 + + O2 2+

FeO

+ H 2O → Fe

3+

+ ·OH + OH

(6) −

Fe3 + + O3 + H 2O → FeO2 + + H+ + ·OH + O2 10511

(7) (8)

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(9)

Fe 2 + + H 2O2 → Fe3 + + ·OH + OH−

(10)

Fe3 + + H 2O2 → Fe 2 + + H+ + ·HO2

(11)

3.2. Effect of the Initial Fe(II) Concentration. The influence of initial Fe(II) concentration (CFe(II) = 0−1.0 mM/L) on ηB for the catalytic ozonation of phenol in the RPB is illustrated in Figure 3. It is observed that the optimum Fe(II)

Figure 4. Variations of ηB and kGa with rotation speed in the O3 and O3/Fe(II) processes (T = 25 °C, pH = 2, CFe(II) = 0.4 mM, CA0 = 85 mg/L, CB0 = 100 mg/L, liquid flow rate = 20 L/h, gas flow rate = 150 L/h).

speed can increase the ozone mass transfer rate by increasing the renewal rate of gas−liquid interface. It can be seen from Figure 4 that kGa increased from 0.049 s−1 to 0.071 s−1 in the O3 process and from 0.054 s−1 to 0.111 s−1 in the O3/Fe(II) process with the increase of the rotation speed from 150 to 1400 rpm. According to the study of Guo et al.,34 the liquid residence time in an RPB decreased with an increasing rotation speed. Though an increase in the rotation speed would decrease the residence time of phenol solution in the RPB and thus reduce the contact time between ozone and phenol solution, Figure 4 reveals that the enhancement of gas−liquid interfacial area and interfacial renewal rate has a greater effect on the mass transfer between ozone and phenol solution than the decrease of the phenol solution residence time in the RPB. It is noted that only a slight increase in ηB and kGa were attained when the rotation speed increased from 1000 to 1400 rpm. This phenomenon indicates that the contact time factor played a more important role under high rotation speed. 3.4. Effects of the Inlet Ozone Concentration. Figure 5 shows the effect of the inlet ozone concentration (CA0 = 38−67 mg/L) on ηB (Figure 5A), kGa, and the absorbed O3 dosage per volume of liquid (mO3) (Figure 5B). The mO3 was defined as follows:

Figure 3. Variations of ηB with initial Fe(II) concentration (CFe(II)) in the O3/Fe(II) process under different rotation speed (T = 25 °C, pH = 2, CA0 = 60 mg/L, CB0 = 100 mg/L, liquid flow rate = 20 L/h, gas flow rate = 150 L/h).

doses were 0.4 mM with the degradation efficiency of phenol reaching peaks. It is also found that phenol degradation efficiency in the O3/Fe(II) process was higher than that in the O3 process (CFe(II) = 0). The degradation efficiency of phenol increased from 22.7% (CFe(II) = 0) to 29.9% (CFe(II) = 0.4 mM) and from 30.7% (CFe(II) = 0) to 45.2% (CFe(II) = 0.4 mM) at a rotation speed of 500 and 1000 rpm, respectively. These results clearly reveal that Fe(II) can improve the generation of radicals and therefore enhance the degradation efficiency of phenol. However, when the Fe(II) dose exceeded the optimum amount (0.4 mM), radicals were consumed by excess Fe(II) as expressed in eqs 12 and 13, leading to the decrease of the degradation efficiency of phenol.15 Fe 2 + + ·OH → Fe3 + + OH−

(12)

Fe2 + + FeO2 + + 2H+ → 2Fe3 + + H 2O

(13)

mO3 = (CA0 − CA )(G /L)

(14)

where CA0 is the inlet ozone concentration (mg/L), CA is the outlet ozone concentration, G is the gas flow rate (L/h), and L is the liquid flow rate (L/h). It can be seen from Figure 5A that with an increasing inlet ozone concentration from 38 mg/L to 67 mg/L phenol degradation efficiency increased from 23.3% to 38.8% in the O3 process and from 28.8% to 51.3% in the O3/Fe(II) process. Although kGa decreased with an increasing inlet ozone concentration, mO3 increased from 36 mg/L to 56 mg/L and from 44 mg/L to 70 mg/L in the O3 and O3/Fe(II) processes, respectively (Figure 5B). The reason for this is that a high concentration of inlet ozone can offer a high driving force for the mass transfer of ozone into the phenol solution. As mass transfer is the limiting step of the ozonization of phenol, the increase of the driving force can enhance the absorption of ozone and the reaction between ozone and the phenol solution, resulting in the increase of ηB.

3.3. Effects of the Rotation Speed. Figure 4 shows the gas phase overall volumetric mass transfer coefficient (kGa, s−1) and phenol degradation efficiency (ηB, %) under the rotation speed of 150−1400 rpm. kGa in the RPB was calculated with the formula proposed by Lin et al.31 It is observed that increasing the rotation speed enhanced the degradation of phenol. With the increase of the rotation speed from 150 to 1400 rpm, the phenol degradation efficiency increased from 32.2% to 61.7% and from 39.3% to 69.8% in the O3 and O3/Fe(II) processes, respectively. The increase of rotation speed enhances the mass transfer between ozone and phenol solution due to the decrease of the size of liquid droplets and the thickness of liquid films. In addition, the increase of rotation 10512

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able for phenol degradation, leading to the decrease of the phenol degradation efficiency. 3.6. Effects of Initial Phenol Concentration. Figure 7 presents the effect of the initial phenol concentration

Figure 7. Variations of ηB with initial phenol concentration (CB0) in the O3 and O3/Fe(II) processes (T = 25 °C, pH = 2, CFe(II) = 0.4 mM, CA0 = 75 mg/L, liquid flow rate = 20 L/h, gas flow rate = 150 L/h, rotation speed = 1000 rpm).

(CB0 = 60−140 mg/L) on the ηB. It is found that the ηB values decreased from 64.7% to 41.1% and from 76.6% to 46.8% with an increasing initial phenol concentration from 60 mg/L to 140 mg/L in both the O3 and the O3/Fe(II) processes respectively. It is also observed that phenol degradation efficiency in the O3/Fe(II) process was 5−12% higher than that in the O3 process at a certain initial phenol concentration, which reveals that the ferrous-catalyzed ozonization process has a better degradation efficiency than the ozonization process for phenol. 3.7. Effects of the Reaction Temperature. Figure 8 shows the effect of the reaction temperature (T = 15 °C − 25 °C)

Figure 5. Variations of ηB (A), kGa, and mO3 (B) with inlet ozone concentration (CA0) in the O3 and O3/Fe(II) processes (T = 25 °C, pH = 2, CFe(II) = 0.4 mM, CB0 = 100 mg/L, liquid flow rate = 20 L/h, gas flow rate = 150 L/h, rotation speed = 1000 rpm).

Figure 6. Variations of ηB with liquid flow rate (L) in the O3 and O3/Fe(II) processes (T = 25 °C, pH = 2, CFe(II) = 0.4 mM, CA0 = 60 mg/L, CB0 = 100 mg/L, rotation speed = 1000 rpm, gas flow rate = 150 L/h).

Figure 8. Variations of η B with temperature (T) in the O3 and O3/Fe(II) processes (pH = 2, CFe(II) = 0.4 mM, CA0 = 75 mg/L, CB0 = 100 mg/L, liquid flow rate = 20 L/h, gas flow rate = 150 L/h, rotation speed = 1000 rpm).

3.5. Effect of the Liquid Flow Rate. Figure 6 presents the effect of the liquid flow rate (L = 15−30 L/h) on ηB. Phenol degradation efficiency decreased from 38.3% to 28.0% and from 42.6% to 33.3% with an increasing liquid flow rate from 15 L/h to 30 L/h in the O3 and O3/Fe(II) processes, respectively. It can be seen that the phenol degradation efficiency decreased with the increase of the liquid flow rate. An increasing liquid flow rate results in a shorter residence time, which is unfavor-

on the ozonization processes. It is interesting that the maximum ηB was achieved at a reaction temperature of 25 °C in both the O3 and O3/Fe(II) processes. This may be attributed to the low pH value in this study. A rise of the reaction temperature would reduce the solubility of ozone and increase the phenol reaction 10513

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Table 1. Experimental Results for Contrast Experimenta O3/Fe(II)-RPB O3/Fe(II)-STR O3-RPB a

phenol degradation (%)

error (%)

COD (mg/L)

error (%)

BOD (mg/L)

error (%)

BOD/COD value

57 36 44

2.1 3.8 2.8

174 192 187

4.6 3.8 3.3

102.7 92.1 95.4

4.1 8.2 6.1

0.59 0.48 0.51

The COD and the BOD/COD value of 100 mg/L phenol solution are 236 mg/L and 0.2, respectively.

Figure 9. GC/MS total ion chromatograms for phenol degradation in the O3-RPB (a) and O3/Fe(II)-RPB (b) systems.

Table 2. Indentified Intermediates in the O3-RPB and O3/ Fe(II)-RPB Systems no.

retention time (min)

compd identified by MS

D1 D2 D3 D4 D5 D6

4.022 7.064 9.429 11.282 12.694 7.752

p-benzoquinone hydroquinone dimethyl phthalate [1,1′-biphenyl]-2−2′-diol [1,1′-biphenyl]-4,4′-diol catechol

O3 alone

O3/ Fe(II)

exist exist

exist exist exist exist exist

exist

Figure 10. Proposed reaction pathways for phenol degradation in the O3/Fe(II)-RPB process.

rate and ozone decomposition rate. The former is unfavorable for the degradation of phenol, while the latter would enhance the degradation of phenol. The ozone decomposition (ozone reacts with hydroxyl ions to generate hydroxyl radicals) could be neglected and the solubility of ozone played a more important role in the degradation of phenol at low pH. Therefore, the ηB decreased with the increasing temperature when the reaction temperature was above 25 °C. 3.8. Contrast Experiment. A contrast experiment was carried out with the initial pH, temperature, CFe(II), CA0, gas flow rate, liquid flow rate (for the RPB), rotation speed (for the RPB), and magnetic stirring speed (for the STR) at 2, 25 °C, 0.4 mM, 60 mg/L, 150 L/h, 30 L/h, 1500 rpm, and 1800 rpm, respectively.

The ozonation time of STR system was set to 2 min, so the applied ozone amount per unit volume phenol solution in the STR system was equal to that in the RPB system. As Table 1 shows, the highest phenol degradation rate, COD removal rate, and BOD/COD value were attained in the O3/Fe(II)-RPB process. The BOD/COD value of phenol solution increased from 0.2 to 0.59 after the solution was treated by the O3/Fe(II)-RPB process. The great increase in the BOD/ COD value suggests that the O3/Fe(II)-RPB process can significantly improve the biodegradability of wastewater. 10514

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process and O3-RPB process. The main reaction pathway of phenol degradation in the O3/Fe(II)-RPB process is shown in Figure 10. In the O3/Fe(II)-RPB process, a portion of 1,4-benzoquinone was deoxidated to hydroquinone. In addition, phenoxyl radicals, 1-hydroxy-4-phenoxyl radicals, and 2-hydroxy-4-phenoxyl radicals were generated due to the large amount of hydroxyl radicals in the O3/Fe(II)-RPB process. The proposed mechanism of polymeric intermediates formation is presented in Figure 11.

The Hatta number (Ha) of a certain reaction is generally used for reactor evaluation. When Ha < 0.3, the reaction takes place entirely in the bulk liquid, and the reactor with large liquid volume (such as STR) would benefit the reaction. When Ha > 3, the reaction takes place entirely in liquid film, and the reactor with large interfacial area (such as RPB) would benefit the reaction. As the reaction of ozone with phenol is a second order reaction, the Hatta number is given by35 Ha =

k[phenol]DO3 kL

4. CONCLUSIONS An O3/Fe(II) process was employed to study the degradation of phenol in acidic environment in RPB, and a comparison between the O3/Fe(II) process and the O3 process was conducted for the degradation effect of phenol under different conditions. It was found that the O3/Fe(II) process was more effective than the O3 process in acidic environment due to the promotion of the generation of hydroxyl radicals by Fe(II), and the O3/ Fe(II) (0.4 mM) process had a roughly 10% higher phenol degradation rate than did the O3 process. It was determined that the optimum conditions for the phenol degradation in acidic environment were the temperature of 25 °C, the Fe(II) initial concentration of 0.4 mM, low phenol concentration and liquid flow rate, and high rotation speed and inlet ozone concentration. A contrast experiment was carried out to compare the catalytic ozonation efficiencies in the RPB and in the STR, and it was found that phenol degradation rate and COD removal rate in the RPB were 19% and 8% higher than that in the STR. The BOD/COD value of phenol solution increased from 0.2 to 0.59 after the solution was treated by the O3/Fe(II)-RPB process. The GC/MS analysis showed that the mechanism of phenol oxidation in the O3/Fe(II)-RPB process was different from that in the O3-RPB system. More hydroquinone and a small amount of polymeric intermediates were found in the O3/Fe(II)-RPB process. This work demonstrates that it is feasible to combine the high gravity technology with catalytic ozonation to treat refractory pollutants.

(15)

where k is the rate constant of the reaction between ozone and phenol (L/mol s), [phenol] is the concentration of phenol in the bulk liquid (mol/L), DO3 is the diffusivity coefficient of ozone into water (m2/s), and kL is the mass transfer coefficient (m/s). Hatta number was estimated at 7.35 for the reaction of ozone with phenol by the above equation with k = 5 × 104 L/mol·s, kL = 4 × 10−5 m/s, and DO3 = 1.73 × 10−9 m2/s.36 Therefore, RPBs can significantly intensify mass transfer and reaction in phenol ozonation due to the existence of a huge interfacial area. 3.9. Phenol Degradation Mechanism in the O3/Fe(II) Process. The degradation products of phenol in the O3-RPB process and the O3/Fe(II)-RPB process were identified by GC/MS, and the GC/MS total ion chromatograms and the indentified intermediates are shown in Figure 9 and Table 2, respectively. As Figure 9 shows, the main intermediate was p-benzoquinone in the O3-RPB process, while two main intermediates p-benzoquinone and hydroquinone were identified in the O3/Fe(II)-RPB process. In addition to p-benzoquinone and hydroquinone, small quantities of dimethyl phthalate and polymerization were found as the intermediates in the O3/Fe(II)-RPB process. As is well-known, phenol is first oxidated to 1,4-benzoquinone, hydroquinone, and catechol, and then to organic acids in the ozonation process.37 However, there are some differences between the phenol degradation pathways of O3/Fe(II)-RPB

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 64421706. Fax: +86 10 64434784. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 20676006, 20990221), National High-Tech Research Program of China (863 Program; No. 2009AA033301), and Program for New Century Excellent Talents in University of China (NCET-07-0053).

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REFERENCES

(1) Ammary, B. Y. Treatment of olive mill wastewater using an anaerobic sequencing batch reactor. Desalination 2005, 177 (1−3), 157. (2) Hussain, A.; Kumar, P.; Mehrotra, I. Nitrogen biotransformation in anaerobic treatment of phenolic wastewater. Desalination 2010, 250 (1), 35. (3) Zazo, J. A.; Casas, J. A.; Mohedano, A. F.; Rodriguez, J. J. Semicontinuous Fenton oxidation of phenol in aqueous solution: A kinetic study. Water Res. 2009, 43 (16), 4063.

Figure 11. Proposed secondary reaction pathways for phenol degradation in the O3/Fe(II)-RPB process. 10515

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dx.doi.org/10.1021/ie300476d | Ind. Eng. Chem. Res. 2012, 51, 10509−10516