Ceramic Honeycomb Catalytic Ozonation of Acetic

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Solid base MgO/ceramic honeycomb catalytic ozonation of acetic acid in water Tongdong Shen, Qiangwei Wang, and Shaoping Tong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02469 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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Industrial & Engineering Chemistry Research

Solid base MgO/ceramic honeycomb catalytic ozonation of acetic acid in water Tongdong Shen, Qiangwei Wang, Shaoping Tong*

College of Chemical Engineering, State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China Corresponding Author E-mail address: [email protected] (Shaoping Tong)

ABSTRACT: MgO/ceramic honeycomb (MgO/CH) composite material was prepared by the excessive impregnation method, and its catalytic activity in ozonation of acetic acid at initial neutral pH value was investigated. It was found that the efficiency of ozonation in the presence of MgO/CH (81.6% acetic acid removal in 30 min) was greatly enhanced compared to that (only 18.7%) in the absence of MgO/CH under the same conditions. Besides the increase in pH value of the solution caused by MgO/CH, the surface active sites of MgO/CH also accelerated ozone decomposition, thus bringing about the further high efficiency of ozonation. Therefore, MgO/ceramic honeycomb is a potential catalyst in ozonation process.

KEYWORDS: MgO/ceramic honeycomb, catalytic ozonation, pH value, ozone

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decomposition

1.

INTRODUCTION

Ozone is widely used as an effective oxidizing agent in drinking water and wastewater treatment

1, 2

. The direct reaction of ozone with organic compounds sometimes generates

some by-products which are inert to ozone, acetic acid as an example, thus leading to a low oxidative efficiency

3, 4

. Hence, the ozone-based advanced oxidation processes

(AOP-O3), such as catalytic ozonation, H2O2/O3, UV/O3, have been developed, in which ozone is effectively transformed into hydroxyl radicals (⋅OH)

5-9

. The reaction of

hydroxyl radicals with most organic compounds is always fast and non-selective, thus improving the degree of mineralization 10, 11. Heterogeneous catalytic ozonation has been considered as a promising process owing to its high efficiency and easy operation. The key factor for this process is to prepare an efficient catalyst. In most of cases, Lewis acid oxides, such as FexOy, MnxOy, are used to enhance the efficiency of ozonation

5, 12, 13

.

However, the research about basic oxide catalytic ozonation is seldom reported and only MgO has been studied in recent years.

Magnesium oxide (MgO), a typical basic oxide catalyst, has been demonstrated to have good catalytic activity for ozonation of organic pollutants 14-17. The presence of magnesia could greatly improve the efficiency of ozonation of phenol in water, and could keep the pH value almost at 10.80

14

. It is well-known that ozone can effectively decompose to

2


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generate hydroxyl radicals in water when pH is higher than 10. However, the possible role of ozone decomposition on the surface of magnesia was not considered. MgO nanocrystal powder is an effective catalyst in ozonation of both dye solution and phenol from saline wastewater

15, 16

. The effects of different initial pH values on degradation

rates of organic pollutants were studied. The enhancement of ozonation efficiency at the optimum pH was mainly attributed to generation of hydroxyl radicals from ozone decomposition by MgO nanocrystals. However, the evolution of pH value with time during MgO catalytic ozonation was not measured. Actually, a decomposition rate of ozone in water highly depends on pH of solution. Furthermore, the decomposition of dissolved ozone on the surface of MgO nanocrystals was also not discussed. Therefore, it is necessary to consider the evolution of both pH value and the concentration of dissolved ozone during the process of catalytic ozonation, which is significant to clarify the mechanism of MgO catalytic ozonation.

The separation of MgO powder from treated wastewater is costly and laborious. The common method is to deposit MgO on a suitable support. MgO/GAC composite was prepared by a sol-gel-thermal deep-coating method and showed the high activity in ozonation of catechol

18

. Ceramic honeycomb is a stable material, and also always be

used as a catalyst support owing to its excellent heat resistance, acid-alkali resistance and wear-resistance, high geometrical surface 19. Ceramic honeycomb has been directly used to improve the ozonation efficiency 5. Ceramic honeycomb could improve the efficiency

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of ozonation of nitrobenzene when the solution pH was close to pHPZC of the catalyst, and the surface hydroxyl groups are responsible for the catalytic activity of ceramic honeycomb

20

. Ceramic honeycomb impregnated with some chemical elements, namely

Mn, Cu, Fe, Ni, Zn, Ag and C compounds.

However,

no

21-25

, were good catalysts for ozonation of some organic

studies

reported

catalytic

activity

of

ceramic

honeycomb-impregnated MgO in catalytic ozonation systems so far.

In this work, MgO/ceramic honeycomb catalyst was prepared by the excessive impregnation method, and its catalytic activity in ozonation of acetic acid was investigated. The evolutions of pH value of solution and the concentration of dissolved ozone with reaction time during ozonation were emphatically analyzed to explore the mechanism of MgO/ceramic honeycomb catalytic ozonation.

2.

EXPERIMENTAL SECTION

2.1. Chemicals and reagents.

Magnesium nitrate, acetic acid (AcOH), sodium hydroxide, sulfuric acid, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium carbonate, dimethyl sulfoxide, methane sulfonic acid, potassium iodide, sodium thiosulfate were of analytical grade. Methanol was chromatographically pure. Indigo carmine was biological stain. All chemicals were used without further purification. All solutions were prepared with double distilled water. Ceramic honeycomb was bought from Hangzhou Kate

4


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Environmental Technology Co., Ltd, and its characteristics were described in Table S1.

2.2. Preparation of catalyst.

Before preparation of the MgO/ceramic honeycomb composite material, the ceramic honeycomb was cleaned with distilled water in an ultrasonic bath for 1 h, and then calcined in air at 600℃ for 2 h. The cleaned ceramic honeycomb was labeled as CH.

The MgO/ceramic honeycomb composite catalyst was prepared by the excessive impregnation method. The CH (80.0 g) was immersed into a beaker with magnesium nitrate solution (0.7 mol⋅L-1 Mg(NO3)2: 280mL) in an ultrasonic bath for 1 h, and then was kept without ultrasound for additional 7 h. The CH impregnated with magnesium nitrate solution was dried in air at 110℃ for 8 h, and then calcined in air at 600℃ for 4 h. The prepared catalyst was labeled as MgO/CH. As a reference in XRD analysis, magnesium oxide powder was also prepared by direct calcination of magnesium nitrate in air at 600 ℃ for 4 h.

MgO content in the MgO/CH composite material was calculated according to the following formula:

ωMgO = ( m1 − m0 ) / m1 ×100%

(1)

Where ω MgO is mass percent of MgO, m1 and m0 are the mass of MgO/CH and CH, respectively.

The crystal phase of the catalyst was analyzed by X-ray diffraction (XRD) 5



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(PNAlyticalX'Pert Pro X-ray diffractometer with Cu Kα radiation, λ=1.5418 Å). The surface morphology was visualized by scanning electron microscopy (SEM) (Hitachi S-4700 operated at 15 kV) and transmission electron microscopy (TEM) (Tecnai G2F30S-Twin operated at 300 kV). The chemical composition was measured using energy dispersive analysis of X-ray (EDAX) integrated with SEM. The specific surface area and pore volume were calculated by Brunauer–Emmett–Teller (BET) model. The surface basicity was measured by the temperature programmed desorption of carbon dioxide (CO2-TPD) on an automated gas sorption analyzer (3020E, FINESORB, China). The pHPZC (point of zero charge) was determined by the pH drift method 26 .

2.3. Catalytic ozonation procedure.

The ozonation procedure was implemented in a glass cylindrical reactor as detailed elsewhere

27

. The prepared block catalyst of 20.0g was located at the bottom of the

reactor. And 500 mL model solution (The initial concentration of AcOH: 100 mg⋅L-1) was added to the reactor. Ozone was produced from pure oxygen by an ozone generator (CFS-1A, Ozonia, Switzerland) and continuously bubbled into the solution through a porous ceramic plate fixed at the bottom of the reactor. The gas flow was controlled at 1.0 L⋅min-1, and the amount of ozone was 45.5 mg⋅min-1. Ozone in off-gas was trapped by KI solution. At suitable intervals, 5 mL sample was taken and subsequently bubbled by nitrogen to remove the dissolved ozone. Then the sample was filtered with a 0.22 µm-pore size syringe filter for analysis. At the same time, the pH value of the solution 6


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was measured by a pH analyzer (pHS-3C, LeiCi Instrument Plant, China). In the experiment of adsorption test, pure oxygen was diffused into the solution under the same experimental conditions. The initial pH value of the solution was adjusted by NaOH or H2SO4 solution. The reaction temperature was controlled at 25±1 ℃.

2.4. Analytical procedures.

The concentration of AcOH was analyzed by a high performance liquid chromatography (ThemoFisher Dionex Ultimate 3000, USA). Hypersil Gold C18 column was used with a mixture eluent of 0.0134 M phosphate buffer and methanol (95:5, v/v) at 1.0 mL⋅min-1 flow rate, UV-detection at 210 nm. The concentration of ozone in gas was detected by an online ozone monitor (IDEAL, USA) using ultraviolet absorption method, and the concentration of dissolved ozone was measured by indigo method

28

. The amount of

hydroxyl radicals generation was determined by the concentration of methanesulfonic acid produced from the reaction of dimethylsulfoxide (DMSO) with hydroxyl radicals 29. The concentration of methanesulfonic acid was analyzed by an ion chromatography (Dionex DX1500, USA) (the eluent was 18 mmol⋅L-1 potassium hydroxide, the flow rate was 1.0 mL⋅min-1.). Each experiment was repeated three times, and the results were the average of duplicate experiments of three times. The range of error for each result was always less than 8%.

3.

RESULTS AND DISCUSSIONS

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3.1. Characteristics of MgO/CH catalyst

The XRD patterns of CH and MgO/CH are shown in Figure 1. Pattern a (MgO/CH) exhibited new peaks at 42.8° and 62.1° when being compared to pattern b (CH), which matched the periclase MgO (JCPD45-0946). The sharp and intense peaks shown in pattern b were in good agreement with cordierite (JCPD13-0294). The XRD of MgO powder as a reference indicated pattern (c) was periclase crystalline particle, which was also in accordance with another research 17.

The surface morphologies of CH and MgO/CH are displayed in Figure 2. As observed from Figure 2a1 and 2a2, the surface of CH was dense and agglomerated. Figures 2b1 and 2b2 show that the surface of MgO/CH was composed of irregularly shaped MgO grains with a size of about 100nm. In HRTEM image b3, the lattice fringe spacing of MgO planes on the surface of CH were 0.244 nm and 0.149 nm, corresponding to (111) and (220) lattice spacing of MgO, respectively. A comparison of micrographs of CH and MgO/CH indicated that MgO particles were distributed on the surface of CH.

The elemental analysis of CH and MgO/CH are listed in Table 1. The weight percentage of magnesium (Mg) in MgO/CH (8.83%) was higher than that in CH (7.45%), which also demonstrated that MgO was successfully coated on the CH by the excessive impregnation method.

8


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Table 1. EDAX analysis of CH and MgO/CH.

Element Mg

Al

Si

O

C

CH

7.45

18.72

26.51

41.07

6.25

MgO/CH

8.83

18.38

25.92

40.45

6.42

(Wt %)

The physicochemical properties of CH and MgO/CH are listed in Table 2. The content of MgO (1.1% by weight) in MgO/CH was in accordance with the result of EDAX. The specific surface areas of CH and MgO/CH were 0.1292 and 0.5446 m2⋅g-1, respectively. And the total pore volumes of CH and MgO/CH were 0.0297 and 0.1251 cm3⋅g-1, respectively. The increase in specific surface area and total pore volume of MgO/CH could be attributed to the dispersion of MgO on the surface of CH because MgO had character of high specific surface area and a certain size of pore volume 30. The changes in specific surface area and pore volume are opposite to Gholamreza Moussavi et al.’ work

18

. In their work the specific surface area and total pore volume of GAC both

decreased after immobilization of MgO, which might be due to the bigger pore size of active carbon. The pHpzc values of CH and MgO/CH were determined to be 6.6 and 11.1, respectively. The pHpzc value of MgO was above 12 reported elsewhere

17

. Hence, the

increment of pHpzc value of MgO/CH could be owing to the dispersion of MgO on the surface of CH. Higher pHpzc corresponds to higher basic character

31

, meaning that

MgO/CH had stronger basicity than CH. And it was confirmed by the next CO2-TPD 9



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analysis.

Table 2. Physicochemical properties of CH and MgO/CH. Sample

CH

MgO/CH

WtMgO (%)

0

1.1

SBET (m2⋅g-1)

0.1292

0.5446

total pore volume (cm3⋅g-1)

0.0297

0.1251

pHPZC

6.6

11.1

The surface basicity of CH and MgO/CH are shown in Figure 3. The CO2-TPD profiles of CH and MgO/CH showed several CO2 desorption peaks, indicating the different basic sites on the surfaces of the CH and MgO/CH. The strength of basic site is proportionate to the temperature of the peak. The peaks between 20 and 200℃ can be due to the interaction of CO2 with the weak basic sites which correspond to hydroxyl groups on the surface. The peaks between 200 and 400℃ can be attributed to the presence of medium basic sites which most likely correspond to oxygen in the Mg2+ and O2

－

pairs. The peaks

higher than 400℃ can be ascribed to the effect of strong basic sites that probably －

correspond to isolated O2 . Both profile a and profile b showed a peak between 20 and 200℃, but profile b (MgO/CH) also displayed another peak between 200 and 400℃, which demonstrated that the basicity of MgO/CH was stronger than that of CH owing to the dispersion of MgO on the surface of CH. The larger peak area indicates more basic 10


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sites, showing that MgO/CH had more basic sites than CH.

3.2. Ozonation of acetic acid.

The results of AcOH removal by different processes at initial pH 7.5 are illustrated in Figure 4a. It was found that AcOH removal rates after 30min in ozonation, CH catalytic ozonation, MgO/CH catalytic ozonation and adsorption of MgO/CH were 20.7%, 18.7%, 81.6% and 0.5%, respectively, showing that MgO/CH catalytic ozonation had the highest efficiency. It could be found that the adsorption of AcOH on MgO/CH was negligible and CH also had no activity in ozonation of AcOH. Considering AcOH is a hydroxyl radical probe compound in ozonation, one could infer that the enhancement of AcOH removal by MgO/CH catalytic ozonation might be attributed to the generation of hydroxyl radicals.

The pH value of solution is an important factor in ozonation process. It affects the rate constant of ozone decomposition and the dissociation rate of target compound. Magnesium oxide (MgO), a typical solid base catalyst, can effectively change the pH value of solution

33

. The evolutions of pH value of AcOH solution treated by different

processes were shown in Figure 4b. It could be seen that pH values of the solution in processes of ozonation and CH catalytic ozonation maintained nearly 7.5. And the pH values of solution in processes of MgO/CH adsorption and MgO/CH catalytic ozonation increased from 7.5 to 10.6 and from 7.5 to 9.7, respectively, indicating that the addition of MgO/CH could greatly increase the pH of solution. It is well-known that higher pH value

11



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can improve the efficiency of ozonation

34, 35

. Therefore, the increase of pH caused by

adding MgO/CH should be main reason for the enhancement of ozonation efficiency.

3.3. Effect of pH.

It is not clear so far whether ozone also can decompose on the surface of MgO/CH catalyst, thus contributing to the enhancement of AcOH removal. Experiments of AcOH removal by ozonation and MgO/CH catalytic ozonation at different initial pH values of 5.3, 7.5, 9.5 and 11.2 were conducted, and AcOH removal rate, pH value and concentration of dissolved ozone with reaction time were measured and compared. The results are displayed in Figures S1a-c.

In order to verify whether the prepared catalyst could accelerate the decomposition rate of dissolved ozone or not, we took comparisons in pH, removal rate of acetic acid, concentration of dissolved ozone between ozonation at initial pH 9.5 and MgO/CH catalytic ozonation at initial pH 5.3. As shown in Figure 5a, the pH values of the solution within 15 min in the process of MgO/CH catalytic ozonation were all lower than those in ozonation, meaning that less dissolved ozone in the process of MgO/CH catalytic ozonation was decomposed due to hydroxyl anion (OH-). However, as observed in Figure 5b, the concentration of dissolved ozone in the process of MgO/CH catalytic ozonation, on the contrary, was lower than that in ozonation except at 5 min. And the AcOH removal rate at 15 min in MgO/CH catalytic ozonation also surpassed that in ozonation. These

12


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results indirectly demonstrated that dissolved ozone could decompose on the surface of MgO/CH to produce hydroxyl radicals, thus resulting in the further enhancement of ozonation efficiency.

3.4. Catalytic mechanism Acetic acid is considered as the ⋅OH probe compound in ozonation, thus implying the generation of hydroxyl radicals in this process. Therefore, carbonate anion (CO32-), a typical hydroxyl radical scavenger was used to indirectly verify the reaction of hydroxyl radicals. It was found a sharp decline in acetic acid removal rate in MgO/CH catalytic ozonation at initial pH 7.5 (from 80 % to 25 %) after addition of Na2CO3 (0.5g), verifying the existence of reaction of hydroxyl radicals in the process of MgO/CH catalytic ozonation at initial pH 7.5.

The amount of hydroxyl radicals could be measured by the reaction of hydroxyl radical (⋅OH) with dimethyl sulfoxide (DMSO) (Equation. 2), in which the amount of hydroxyl radical is the concentration of the produced methanesulfinic acid 36-38. As shown in Figure 6a, the amount of hydroxyl radicals (4.36 mg⋅L-1) generated in MgO/CH catalytic ozonation of DMSO at initial pH 7.5 was 6.5 times larger than that (0.67 mg⋅L-1) in ozonation after reaction of 30 min. Figure 6b shows that the pH value in MgO/CH catalytic ozonation of DMSO maintained nearly 7.5 owing to the accumulation of methanesulfinic acid, and the concentration of dissolved ozone in MgO/CH catalytic

13



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ozonation was always lower than that in ozonation during the reaction process. Therefore, the increase of the amount of hydroxyl radicals in MgO/CH catalytic ozonation of DMSO was mostly due to ozone decomposition on the surface of MgO/CH, which indirectly proved that ozone could decompose on the surface of MgO/CH in neutral solution without the effect of pH increase.

⋅OH + (CH3)2SO → CH3S(O)OH + ⋅CH3

(2)

It was interesting to track and compare the evolution of ⋅OH in the processes of ozonation and MgO/CH catalytic ozonation of AcOH at initial pH 7.5. Elovitz and von Gunten

39

proposed a relative approach to measure the transient concentration of hydroxyl radicals. The ratio of the ⋅OH-exposure and O3-exposure (Rct= ∫ [⋅OH ]dt /

∫ [O ]dt ) is measured to 3

determine the role of hydroxyl radical in ozonation.

 [ AcOH ]t In   [ AcOH ] 0 

  = − kO3 ∫ [ O3 ]dt − k⋅OH ∫ [⋅OH ]dt 

(

= − kO3 + k⋅OH Rct

(3)

) ∫ [O ]dt

(4)

3

The O3-exposure in this work was estimated by subtracting the amount of O3 off-gas from the amount of O3 input gas versus time. Once Rct is calculated and ozone concentration in solution is measured, the ⋅OH concentration can be calculated at any time. This way is quite useful in evaluating the role of ⋅OH in an ozonation process. The rate constants for the reaction of AcOH with hydroxyl radical ( k⋅OH )

10

and ozone ( kO3 )

40

are 1.6×107

L⋅mol-1⋅s-1 and 3×10-5 L⋅mol-1⋅s-1, respectively. The good liner dependence between

14


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(

In [ AcOH ]t / [ AcOH ]0

)

and

∫ [O ]dt 3

is shown in Figure 7. Rct values of ozonation

and MgO/CH catalytic ozonation of acetic acid at initial pH 7.5 were estimated to be 8.10×10-8 and 2.63×10-7, respectively.

Concentrations of ozone in off-gas in the processes of ozonation and MgO/CH catalytic ozonation of AcOH at initial pH value of 7.5 were measured to take a comparison. As shown in Figure 8, the ozone concentration of off-gas in MgO/CH catalytic ozonation was always lower than that in ozonation under the same conditions. At the same time, the concentration of dissolved ozone in MgO/CH catalytic ozonation was also lower than that in ozonation (Figure S1c). From these results, one could conclude that more ozone transferred from gas into water, and that also more dissolved ozone decomposed to produce hydroxyl radicals in the process of MgO/CH catalytic ozonation, thus resulting in a higher efficiency of ozonation for acetic acid degradation.

On the basis of the above experimental results, the possible mechanism for acetic acid removal by MgO/CH catalytic ozonation at initial pH 7.5 was proposed as follows. On one hand, the basic sites on the surface of MgO/CH bonded with hydrogen ion, therefore leading to the increase of the pH of solution. Hydroxyl ion accelerated the decomposition rate of dissolved ozone to produce ⋅OH 34, 35. On the other hand, some part of dissolved ozone absorbed on the surface of MgO/CH, the basic sites of MgO/CH also accelerated the decomposition rate of dissolved ozone, thus further improving the efficiency of ozonation 17, 41, 42. In this process, the proportions of AcOH removal by the two effects 15



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were hard to determine because of their similarity.

3.5. Stability of MgO/CH.

The catalytic activity of MgO/CH was mainly due to MgO coated on the surface of CH. Thus the stability of MgO/CH depended on the stability of MgO and the adhesive force between MgO and CH. Considering its application, MgO/CH was recycled consecutively for 5 times to test its stability under the same conditions. The results are listed in Table 3. It could be seen that the removal rate of acetic acid maintained nearly 80% in five repeated experiments, indicating the prepared MgO/CH was stable. The stability of MgO in ozonation process reported elsewhere

17

was satisfactory. Therefore, MgO/CH is

relatively stable in ozonation process and has a good prospect of application.

Table 3. Stability of MgO/CH in catalytic ozonation of AcOH for 5 consecutive times. Reaction conditions: initial pH, 7.5; [AcOH]0, 100 mg⋅L-1; catalyst dosage, 20.0 g; gas flow rate, 1.0 L⋅min-1; amount of ozone, 45.5 mg⋅min-1.

recycle number

AcOH removal (%)

pH（t=30min）

1

81.6

9.70

2

81.4

9.55

3

82.7

9.49

4

74.7

9.07

5

79.1

9.27

16


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4.

CONCLUSIONS

Ceramic honeycomb-supported MgO was prepared, and further used as a catalyst in ozonation of AcOH at initial neutral pH. It was found that MgO/CH could greatly improve the efficiency of ozonation of AcOH. The experimental results indicated two factors might be attributed to the enhancement of ozonation: (1) the increase of pH after addition of MgO/CH; (2) the decomposition of dissolved ozone on the surface of MgO/CH catalyst. Both two factors resulted in efficient generation of hydroxyl radicals, thus improving the oxidative efficiency of AcOH. The recycling test showed that MgO/CH was relatively stable. In conclusion, MgO/CH is a potential catalyst for ozonation of water or wastewater containing ozone-inert contaminants.

AUTHOR INFORMATION

Corresponding Author *Tel/Fax: 86-571-88320960. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

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This work was supported by the National Natural Science Foundation of China (Grant numbers 21176225).

ASSOCIATED CONTENT

Supporting information. Catalysis of MgO/CH at different pH, discussion the results at pH 11.2, catalytic activities of CH at different pH values, apparent reaction rate constants in section 3.3, catalytic activity and pH variation in tap water-prepared solution, discussion about the repeated test, characteristics of the ceramic honeycomb, AcOH degradation and MgO catalytic ozonation in other works. These materials are available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. XRD patterns of CH and MgO/CH.

Figure 2. SEM, TEM, HRTEM images of CH (a1-a3) and MgO/CH (b1-b3).


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Figure 3. CO2-TPD profiles of CH (a) and MgO/CH (b).



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Figure 4. (a) AcOH removal rates by ozonation, catalytic ozonation, and adsorption of MgO/CH. (b) Evolution of pH value of the solution for different processes. Reaction conditions: initial pH, 7.5; [AcOH]0, 100 mgL-1; catalyst dosage, 20.0 g; gas flow rate, 1.0 Lmin-1; amount of ozone, 45.5 mgmin-1.


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Figure 5. (a) Variations of pH value within 15 min in MgO/CH catalytic ozonation at initial pH 5.3 and ozonation at initial pH 9.5. (b) Concentrations of dissolved ozone and AcOH removal rates during 15 min in MgO/CH catalytic ozonation at initial pH 5.3 and ozonation at initial pH 9.5. Reaction conditions: [AcOH]0, 100 mgL-1; catalyst dosage, 20.0 g; gas flow rate, 1.0 Lmin-1; amount of ozone, 45.5 mgmin-1



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Figure 6. (a) Generation amounts of OH in ozonation and MgO/CH catalytic ozonation of DMSO. (b) Evolutions of pH value and dissolved ozone concentration in ozonation and MgO/CH catalytic ozonation of DMSO. Reaction conditions: initial pH, 7.5; [DMSO]0, 200 mgL-1; catalyst dosage, 20.0 g; gas flow rate, 1.0 Lmin-1; amount of ozone, 45.5 mgmin-1.


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Figure 7. Effect of MgO/CH catalyst on Rct. Reaction conditions: initial pH, 7.5; [AcOH]0, 100 mg L-1; catalyst dosage, 20.0 g; gas flow rate, 1.0 L min-1; ozone input rate, 45.5 mg min-1.

Figure 8. Ozone concentration of off-gas in the processes of ozonation and MgO/CH catalytic ozonation of acetic acid. Reaction conditions: initial pH, 7.5; [AcOH]0, 100 mgL-1; catalyst dosage, 20.0 g; gas flow rate, 1.0 Lmin-1; amount of ozone, 45.5



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mgmin-1.

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Ceramic Honeycomb Catalytic Ozonation of Acetic

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