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Degradation of benzophenone in aqueous solution by Mn-Fe-K modified ceramic honeycomb-catalyzed ozonation
ISSN 1001-0742
Journal of Environmental Sciences
Vol. 18, No. 6, pp. 1065-1072,2006
Article ID: 1001-0742(2006)06-1065-08
CLC number: X703
C N 11-26291x
Document code: A
Degradation of benzophenone in aqueous solution by Mn-Fe-K modified ceramic honeycomb-catalyzed ozonation HOU Yan-jun, MA Jun', SUN Zhi-zhong, YU Ying-hui, ZHAO Lei (School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China. E-mail: majun- [email protected]; [email protected])
Abstract: Comparative studies of ozonation alone, ceramic honeycomb-catalyzed and Mn-Fe-K modified ceramic honeycomb catalyzed ozonation processes have been undertaken with benzophenone as the model organic pollutant. The experimental results showed that the presence of Mn-Fe-K modified ceramic honeycombs significantly increased the removal rate of benzophenone and TOC compared with that achieved by ozonation alone or ceramic honeycomb-catalyzed ozonation. The electron paramagnetic resonance (EPR) experiments verified that higher benzophenone removal rate was attribute to more hydroxyl radicals generated in the Mn-Fe-K modified ceramic honeycomb-catalyzedozonation. Under the conditions of this experiment, the degradation rate of all the three ozonation processes are increasing with the amount of catalyst, temperature and value of pH increased in the solution. We also investigated the effects of different process of ozone addition, the optimum conditions for preparing catalyst and influence of the Mn-Fe-K modified ceramic honeycomb after multiple-repeated use. Keywords catalytic ozonation; Mn-Fe-K modified ceramic honeycomb; benzophenone
Introduction Ozonation is an attractive and increasingly important method for the degradation of organic pollutants in aqueous solution. Many studies showed that hydroxyl radical produced by the ozone has strong oxidation potential (Eo = 2.80 V) and degrade organic pollutants quickly. However, many refkactory pollutants, for example pesticide, are very difficult to degrade by ozone alone. Thus, more and more attention is being focused on heterogeneous catalytic ozonation processes for its high catalytic efficiency as well as its easy operation (Hoigne, 1998; Leagube et d.,1999; Andreozzi and Marotta, 2000). Paillard et d.(199 1) reported that Ti02-catalyzed ozonation was more effective for degradation of oxalic acid than ozone alone. The experimental results showed that the oxidation was hardly influenced by the hydroxyl radical scavengers such as bicarbonates. Bhat and Gurol (1 995) studied the ozonation of chlorobenzene in the presence of goethite and found that catalyzed ozonation was more effective and led to a higher degree of mineralization of the compound than ozone alone. Mathias et nl. (2004) indicated that A1,0, was an effective heterogeneous catalyst for the ozonation of refractory organic compounds such as oxalic, acetic and salicylic with higher DOC removal, accordingly, a reaction pathway was presented by which ozone reacted with OH- ions on the surface and radical species were formed. Qu et d. (2004) found that the use of Cu/A1203 substantially enhanced the mineralization of alachlor in ozonation and more hydroxyl radicals generated in the catalyzed ozonation could bring on a higher alachlor removal rate. Ma and
Graham (1997, 1999, 2000) showed that MnOz formed in situ exhibited high activity for ozonation of atrazine compared to the case of ozonation alone, it was also indicated that humic substances and radical scavengers influenced the degradation of atrazine. After continuous investigations, the results suggested the generation of hydroxyl radical via ozone reacting with the surface-bound OK ions on the surface of the manganese dioxide and MnOJGAC catalyzed ozonation (Ma et d.,2005a). Ma et d. (2005b) studied the catalytic ozonation of trace nitrobenzene in water over FeOOWGAC catalyst and indicated that the pseudo-first order rate of ozone decomposition increased obviously for FeOOWGAC catalyst and the oxide surface at nearly zero charged point was favorable for the catalytic ozonation of nitrobenzene. In our previous work (Sun et d., 2005), ceramic honeycombs were selected as catalysts in ozonation system. The experimental results showed that that ceramic honeycomb-catalyzed ozonation was more effective for degradation of nitrobenzene than ozone alone. Although the ceramic honeycomb exhibit some catalyzing effectiveness for the degradation of organic pollutants in aqueous solution, the higher consume of ozone and nevertheless lower removal efficiency are the limitations in its practical application. To overcome the shortcomings of ceramic honeycomb, a modified catalyst was prepared by adulterating the Mn-Fe-K on ceramic honeycomb. It is expected that higher catalytic activity can be achieved because it improves the surface structure and composition of ceramic honeycomb.
1 Materials and method
Foundation item: The National Natural Science Foundation of China (No. 50378028); *Corresponding author
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HOU Yan-jun et al.
1.1 Materials Benzophenone was selected as the model organic compound, the model-polluted water was prepared by diluting benzophenone (Shanghai Chemical Reagent Company, pre-distillation treated, with purity of 99.80%) in distilled water until a concentration 10 m a was obtained. Sulfuric acid and sodium hydroxides (analytical grade) were added in the aqueous solution to control the pH. Other chemical reagents used in the experiment, such as sodium thiosulphate, potassium indide, etc., were analytical grade reagents. All glass wares were soaked in chromic acid, then rinsed with tap water and distilled water. Ceramic honeycombs with the constituent of cordierite were applied in this system (Shanghai Pengyi Fire-resistant Material Factory). Monolithic honeycomb was cylindrical with a diameter of 50 mm and a length of 50 mm; the cell densities of the ceramic honeycombs were 400 square cells per square inch with a wall thickness of 0.4 mm. The impregnation method was used to produce the Mn-Fe-K modified ceramic honeycomb catalysts. Treated ceramic honeycombs was immersed in the mixed aqueous solution containing appropriate amounts of manganese nitrate, ferric nitrate and potassium nitrate with constant shaking for 2 h to allow for enough impregnation and adsorption ;then dried for 12 h at 25°C and 12 h at 85°C and calcined for 4 h at 600°C. The above process was repeated three times to produce steady Mn-Fe-K modified ceramic honeycomb catalysts. The main crystalline phase of the catalysts was 2MgO .2A1203-5Sioz. The surface area of the Mn-Fe-K modified cordierite ceramics was 2.77 mz/g (by brunauer emmett and teller (BET)). The absolute content of Mn was 0.71%, the absolute of Fe was1.64 %, the absolute content of K was 0.09 % (by electron spectroscope for chemical analysis (ESCA)). 1.2 Analytical methods The concentration of ozone in the gas was measured by iodometric titration method (Ozone Standards Committee Method). Dissolved ozone in the water was detected by spectrophotometerusing the Indigo method (Bader and Hoigne, 1981). The concentration of benzophenone was analyzed by SPD-1OW high performance liquid chromatography (HPLC) (Shimadzu, Japan). The main carboxylic acids produced during the ozonation period were analyzed using Dix-500 ion chromatograph (Dionex, USA). The pH in aqueous solution was measured by Delta 320 pH acidometer (Shanghai Leici Apparatus Factory, China). The consumed ozone was obtained by the following calculation: consumed ozone = introduced ozone - (ozone in off-gas + ozone residual in water). X-ray power difiaction (XRD) was used to
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analyze crystallinity and crystal phase of ceramic. Brunauer emmett and teller (BET) was used to determine the specific surface area of catalysts through low-temperature nitrogen adsorption. Scanning electronic microscope (SEM) was used to study the microstructure of the catalysts. Electron spectroscope for chemical analysis (ESCA) was used to study the absolute content of the elements of catalysts. EPR (electron paramagnetic resonance) experiment (Bruker Elexsys E5OO ESR spectroscopy, Germany) was applied with spintrapping reagent 5,5-dimethyl- 1-pyrolin-N-oxide (DMPO). EPR conditions: microwave power 20 mW, modulation fiequency 100 kHz, sweep width 100.0 G; central magnetic field 3480 G; scan 40 s each time; scan times 20. 1.3 Ozonation procedure Experiments were carried out in a cylindrical reactor (inside diameter 50 nun and volume 3 L) made of stainless steel (Fig. l), which was shielded to control reaction temperature. Before the experimental operation, the reactor was pre-ozonated for 4 min and washed several times with distilled water to minimize the potential side effects. The Mn-Fe-K modified ordierite honeycombs were placed into the reaction column. The ozone was generated fiom ozone generator (Qinghua Tongli) and was fed into the cylinder via a porous fiit at bottom of column. The oxygen flux, electric current and aerating time were regulated to control the applied ozone dose. In this experiment, the model water (3 L) spiked with the benzophenone concentration of 10 mg/L was pumped into the column by a magnetic pump (Xishan Pump Co. Ltd., Shanghai) and then circulated. The experiment was performed in continuous (catalytic ozonation process) or semicontinuous (ozonation process). Water samples were taken from the contactor column to analyze the concentration of residual benzophenone. The oxidation reaction was stopped by the addition of a small amount of sodium thiosulphate solution.
Heat preservation
4
t
r' KI aqueous
Oxygen
Ozonizer
Fig. 1 Schematic diagram of Mn-Fe-K modifiedceramic honeycomb omnation system
Degradation of benzophenone in aqueous solution by Mn-Fe-K
No.6
Heterogeneous catalytic ozonation of benzophenone is a gas-liquid-solid reaction where many chemical and physical steps such as fluid dynamics, catalyst material, catalytic surface area, chemical kinetics, the temperature, pH of the solution, etc., might affect the process rate. With the aim to test the catalytic activity, the effects of various factors on the oxidation were investigated under the same operating conditions such as the identical geometric structure of cordierite honeycomb and Mn-Fe-K modified cordierite honeycomb modules, the constant fluid rate (100 L h ) and the same ozone applied (0.35 mg/(L min)).
2 Results and discussion 2.1 Mn-Fe-K modified ceramic honeycombs catalytic oxidation of benzophenone The efficiency of heterogeneous catalyzed ozonation by Mn-Fe-K modified ceramic honeycombs for benzophenone ozonation in aqueous solution was clearly evidenced by the experimental data shown in Fig.2. The reactivity was strongly enhanced by the addition of catalysts in ozonation system, and the oxidation of benzophenone was about 8 1.4% in the Mn-Fe-K modified ceramic honeycombs catalytic ozonation system compared to 68.8% with ceramic honeycombs catalytic ozonation system and 48.3% with ozone alone in 120 min. The TOC removal in Mn-Fe-K modified ceramic honeycombs catalytic ozonation system was 56.2% compared to 43.1% with ceramic honeycombs catalytic ozonation system and 20.9% with ozone alone in 120 min. Benzophenone decreased mainly in first 10 min and TOC decreased continuously. The results show the high catalytic capacity of Mn-Fe-K modified ceramic honeycombs loo
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on ozonation mineralization of benzophenone. At pH=6.87 and T==21 k l)T, a slight removal of benzophenone was shown in the presence of five Mn-Fe-K modified ceramic honeycombs in series fixed in the cylinder. The reduction probably caused by the adsorption on the catalyst surface appeared negligible compared to ozonation system in 120 min reaction. Therefore, these results suggested that the degradation rate of benzophenone was attributed to the catalytic activity of Mn-Fe-K modified ceramic honeycombs. The adsorption of benzophenone on the Mn-Fe-K modified ceramic honeycombs is small but the adsorption of degradated intermediate of benzophenone ozonation on the Mn-Fe-K modified ceramic honeycombs may be biggish. So we studied the TOC removal at different experiments. Fig.3 shows the TOC removal of ozone alone, ozone + adsorption (that means ozonated alone 120 min, then blowed away the ozone in water by Nzand adsorped 120 min by Mn-Fe-K ceramic honeycombs) and Mn-Fe-K ceramic honeycombs catalyzed ozonation after 120 min. It can be found that the contribution of ozone alone to TOC was 20.9%, the contribution of adsorption of degradated intermediate by Mn-Fe-K ceramic honeycombs to TOC was 26.2% and the contribution of catalysis to TOC was 56.2% . Table 1 shows the mainly three kinds of organic by-products in the decomposition of benzophenone (Ma and Gao, 2003): acetic acid, oxalic acid and propionic acid. The concentrations of those organic acids produced in the ozonation alone, ceramic honeycomb-catalyzed and Mn-Fe-K modified ceramic honeycomb catalyzed ozonation processes were different. After 30 rnin catalyzed ozonation, the Mn-Fe-K modified ceramic honeycombs led to the lowest concentration of acetic acid, oxalic acid and propionic acid in water. The result agrees with the TOC removal efficiency of three ozonation processes, and shows hrther the high catalytic capacity of Mn-Fe-K modified ceramic honeycombs on ozonation
r-----l A
A
60 0
s I
30
u" 40
X
X
t
-5
I
60 t, rnin
90
120
Fig.2 Degradation of benzophenone in the presence of Mn-Fe-K modified ceramic honeycombs Reaction conditions: temp. (21 1 ) T ; pH 6.87; initial benzophenone: 10 mg/L; number of two kinds of ceramic honeycombs in series: 5; background: distilled water; experiments: ( ) ozonation alone; ( ) benzophenone ceramic honeycombs catalyzed; ( A ) benzophenone h4n-Fe-K ceramic honeycombs catalyzed; ( X ) TOC ozonation alone; (0 ) TOC ceramic honeycombs catalyzed; (0)TOC Mn-Fe-K ceramic honeycombs catalyzed
+
+
1
20
i-
0
Ozone alone
Ozone+
Catalytic
adsorption
ozonation
Fig.3 TOC removal at different experiments after 120 min Adsorption conditions: temp. 21 k IT ; pH 6.87; initial benzophenone: 10 mgL; number of two kinds of ceramic honeycombs in series: 5 ; background: distilled water
HOU Yan-jun et al.
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Table 1 Comparison of concentration of three by-products between three OZOM~~OUproeeapes ~
~
~~~~
By-products
Acetic acid, m a
Oxalic acid, mg/L
Propionic acid, mg/L
Ozone alone
5.41
2.43
0.95
Catalyzed by ceramic honeycombs
1.71
0.64
0.33
Catalyzed by Mn-Fe-K modified ceramic honeycombs
0.86
0.21
0.17
mineralization of benzophenone. It has been reported that the heterogeneous catalytic ozonation with the use of metal oxides as catalysts was based on an ozone decomposition reaction followed by the generation of hydroxyl radicals (Hoigne and Bader, 1997; Ma and Graham, 1999; Fernando et d., 2002). The surface microstructure of the catalysts is shown in Fig.4. Studied the microstructure we can find that two possible factors may influence the degradation efficiency of benzophenone. Firstly, the numerous pores presented on the surface could remarkably increase the geometric area of catalysts, Mu-Fe-K modified cordierite honeycombs (Fig.4) provided larger surface area (2.77 mYg) than cordierite honeycombs (0.35 m*/g),thus the adsorption of ozone molecules on the surface of catalysts took place; secondly, Mn-Fe-K modified cordierite honeycombs with the dominant constituent of Mg2ALSisOle exhibited higher catalytic activity, the solid surface-solution interface can catalyze oxidation reactions. It has been reported that the surface area provided by pores in catalyst structure was a more important factor in the catalytic oxidation process than the external surface area of catalysts because adsorption was often one of the stages of heterogeneous catalytic ozonation, and these processes were possibly essential to overall catalytic reactions (Mirat et al., 1996; Legube et al., 1999). The catalysts with the main crystalline phase of 2MgO 2A1203* 5Si02 provided both Bronsted acid (-Si-) and Lewis acid (-Al-, -Mg-, -Mn- and -Fe-) sites, which were thought to be the catalytic centers on the surface (Barbaba et al., 2003). When the ozone molecules were adsorbed on the catalyst surface, the groups on the catalyst surface may initiate the decomposition of ozone and generated surface bound hydroxyl radicals. Then the adsorbed organic molecules were oxidized by adjacent hydroxyl radicals. Hydroxyl radicals significantly improved ozone oxidation of benzophenone because of its unselective oxidizing reaction with organic molecules. To confirm the effect of hydroxyl radicals on oxidation reaction in the aqueous solution, EPR experiment was carried out. Benzophenone solution, 0, and the catalyst powder were mixed with DMPO dissolved in ultra-pure water. Immediately after the mixing, 25 pl of the sample solution was transferred into a capillary tube, and EPR spectra were recorded in the X-band on a Bruker ESP spectrometer at room
Fig.4 SEM picture of surface structure of Mn-Fe-K modified ceramic honeycomb
temperature. As shown in Fig.5, a much stronger signal of hydroxyl radicals, which had the typical spectrum of 1 :2:2:1, was presented in catalytic ozonation process. This indicated that hydroxyl radicals was surely produced with Mn-Fe-K modified ceramic honeycombs used in ozonation process and played a more important role in catalytic ozonation process compared with ozone alone which gave no signal of hydroxyl radicals at all. 800
I
1
400
tj =
200 0 -200
-400 -600
1
‘
-800 3430
I
I
I
I
I
3450
3470
3490
3510
3530
Gauss
Fig.5 Intensity of DMPO-OH adduct signals with Mn-Fe-K modified ceramic honeycombs catalytic ozonation DMPO and 0, concentration are 80 and 0.125 rnrnol/L, respectively; catalyst dosage is 1 giL
2.2 Effect of catalyst dosage It is shown in Fig.6 that the degradation percentage of benzophenone was 45.1 YO in ozone alone system. Under the same condition, five MnFe-K modified ceramic honeycombs situated in series at the cylinder bottom, the degradation efficiency increased remarkably from 45.1% to 78.1% after 10 min oxidation reaction. Considering the facts that the adsorption of benzophenone on catalysts was negligible (Fig.2) and the benzophenone degradation
Degradation of benzophenone inI aqueous sohtion by Mn-Fe-K
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was improved with the increase of catalysts dosages at room temperature, these results confirmed that Mn-Fe-K modified cordierite honeycombs provided catalytic activity and enhanced benzophenone removal under the same experimental conditions.
r----l o.8
t
0 0
2
1
3
4
5
Catalyst8 dosage, pieccs
Fig.6 Effect of Mn-Fe-K modified ceramic honeycomb dosage on the degradation of benzophenone Reaction condition: temp. (21 f1)T; pH 6.87; initial benzophenone: 10 mg/L; background: distilled water; experiments: (+) catalytic ozonation with ceramic honeycombs; ( A ) catalytic ozonation with Mn-Fe-K ceramic honeycombs
2.3 Temperature Fig.7 shows that the temperature was an important factor for the degradation of benzophenone, both in ozonation alone and in catalytic ozonation system. As far as the present system was concerned, the effect of this variable was positive to oxidation reaction with temperature ranging from 10 to 40°C. The increase of temperature fi-om 10 to 40°C led to a significant increase of benzophenone conversion from 35.0% to 76.1% in ozonation alone, from 33.5% to 84.2% in ceramic honeycombs catalytic ozonation system and 37.8% to 92.2% in ceramic honeycombs
0.8
9 0.6
+
o.8
+
W
0.4
go.6
0
' 10
I
I
20
30
I
40
T,c Fig.7 Effect of temperature of water sample on the degradation of benzophenone Initial benzophenone: 10 mg/L; number of two kinds of ceramic honeycombs in series: 5; pH 6.87; background distilled water; experiments: (+) ozonation alone; (W) catalytic ozonation with ceramic honeycombs; ( A ) catalytic ozonation with Mn-Fe-K ceramic honeycombs
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catalytic ozonation system, respectively. When at lower temperature ranges, the increase of temperature yielded the increase of both the ozone decomposition rate and the chemical reaction rate. But the ozone solubility would decrease with the temperature increasing continuously, which would cause negative effect on the degradation rate of benzophenone. From the results obtained in present ozonation system, the room temperature was optimal considering the operational conditions. 2.4 pH It was reported that solution pH significantly influenced ozone decomposition in water (Elovitz and Kawaser, 2000). In aqueous systems ozone oxidation follows two simultaneous mechanisms: direct reaction and indirect reaction respectively. The rate of decomposition of ozone increased rapidly with the increase of pH, which was due to the initiation of OHthrough the reaction of 0,with OH (Staehelin and Hoigne, 1982; Sehested et d.,1984; Hoigne and Bader, 1997). However, at pH<3.0 hydroxyl radicals influenced the decomposition of ozone to less extent, so catalytic ozonation of benzophenone has been investigated in the pH range of 3.0-1 1.O in this study. As observed in Fig.8, the increase of pH led to a marked increase of benzophenone degradation from 28.2% to 86.8% in the range 3.09.0) the degradation rate was
1
1
..----
OA2
f
tt
4
W
+
A
4
0 1
I
I
I
3
5
I PH
9
I 11
Fig.8 Effect of pH on the degradation of benzophenone Initial benzophenone: 10 mg/L: temp. (21 k l)T; number of two kinds of ceramic honeycombs in series: 5; background: distilled water; reaction time 10 min; experiments: (+) ozonation alone; ( W ) catalytic ozonation with ceramic honeycombs; (A) catalytic ozonation with Mn-Fe-K ceramic honeycombs
HOU Yan-jun et al.
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not much obviously enhanced compared to the case of ozonation alone, this phenomenon might be attributed to the scavenging potential of carbonate or the interactions between hydroxyl radicals and H 0 2 radicals (Gottschak et al., 2000). Because the solution was prepared with distilled water, the concentration of inorganic carbon was low and the inhibiting impact of carbonate on the oxidation could be ignored. 2.5 Effect of different processes of ozone addition The different feeding modes affect the degradation of benzophenone obviously when they were fed in the same ozone dosage. Fig.9 shows that to the ozone alone, ozone/ceramic honeycombs and ozone/Mn-Fe-K modified ceramic honeycombs systems, when the gross of ozone was the same, feed in the ozone several times led to a marked increase of benzophenone degradation compared with feed in the ozone one time. The removal efficiencies of benzophenone by ozone alone, ozone/ceramic honeycombs and ozone/Mn-Fe-K modified ceramic honeycombs with feed in the ozone several times within 10 min were 66.39%, 89.71%, 93.52% , respectively, while that feed in the ozone one time were 45.13%, 64.44% and 77.16%. That is, the removal efficiency of benzo- p henone of feeding in the ozone several times would be 1.4times that of feeding in the ozone one time. These results also suggested that the removal efficiencies of benzophenone by ozone alone, ozone/ceramic honeycombs and ozone&-Fe-K modified ceramic honeycombs with feed in the ozone one time within initial 2 min were 24.38%, 37.88% and 40.68%, respectively, while that feed in the ozone several times were 18.09% , 25.23% , 27.14% . That is, the removal efficiency of benzophenone of feeding in the ozone one time would be almost 1.4 times that of feeding in the ozone several times, and the removal of benzophenone of feeding in the ozone one time had appeared obviously within initial 2 min in the three systems. This may because while the ozone was fed in one time, there were hydroxyl radicals with higher concentration produced quickly within initial 2 min. But only part of the hydroxyl radicals reacted with benzophenone, another part of the hydroxyl radicals reacted each other so that the concentration of hydroxyl radicals descended sharply and could not remove the benzophenone effectively any more. While the ozone was fed in several times, the hydroxyl
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X
0”
4
0.4
0.2
0
! -
0
I
I
I
I
2
4
6
8
I
10
t, min
Fig9 Effect of different processes of ozone addition on the degradation of benzophenone Initial benzophenone: 10 m a ; temp. (21 zk 1)F;number of two kinds of ceramic honeycombs in series: 5; pH 6.87; background distilled water; reaction time 10 min; experiments: (*)ozonation alone feed one time; ( ) catalytic ozonation with ceramic honeycombs feed one time; ( A ) catalytic omnation with Mn-Fe-K ceramic honeycombs feed one time; ( X ) ozonation alone feed several time; (0) catalytic ozonation with ceramic honeycombs feed several time; ( 0 ) catalytic ozonation with Mn-Fe-K ceramic honeycombs feed several time
radicals consumed one time could be supplied quickly by the addition of ozone next time and the hydroxyl radicals can be kept on a relative steady concentration and in favor of the ozonation of benzophenone continuously. 2.6 Effect of different factors on preparations of CatdYStS
Table 2 illustrates the effect of different calcining temperatures on the catalyzing effectiveness (dipping solution proportion, Mn:Fe=l:4). 600T was more favorable toward increasing the catalyzing activity than the lower or higher temperature. It is possible that lower temperature calcined results in unstable surface structure and chemical constituents that may changes during the ozonation. High temperature may result in the partly melt of the catalyst surface and reduces the specific surface area of catalysts which affect the catalyzing activity obviously. The components proportion of mixed aqueous solutions of nitrate is another factor affecting the catalyzing activity. As shown in Table 3, under the same calcining temperature 600°C , the catalyzing ability of the catalyst changed significantly with
Table 2 Activity of catalysts with dinerent calcining temperature Mn-Fe-K modified ceramic honeycombs Calciningtemperature 3009:
4509:
6009:
7509:
Metal content of Mn/Fe/K
1.52/1.04/0.53
1.10/1.32/0.44
1.37/1.16/0.52
1.29/1.14/0.43
Specific surface area, m2/g
3.46
4.19
2.14
2.02
Removal efficiencyof benzophenone, %
57.1
74.1
77.6
56.4
Degradation of benzophenone in aqueous solution by Mn-Fe-K ...---
NO.~
different component proportion of mixed aqueous solutions of nitrate. Meanwhile, it is shown that the oxidation rate by Mn-Fe-K modified ceramic honeycombs dipped in (Mn:Fe=l:4) nitrate aqueous solution was about 19.9% higher than that dipped in (Mn :Fe=4 :1) nitrate aqueous solution. It is possible
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that the structure and chemical constituent of catalyst surface which changed with the different component proportion of mixed aqueous solutions of nitrate plays an important role in the oxidation process. The high activity catalysts were in favor of the decomposition of ozone and increased the use ratio of ozone.
Table 3 Activity of Mn-Fe-K ceramic honeycomb catalysts with different component proportion Mn-Fe-K modified ceramic honeycombs Dipping solution proportion (Mn:Fe)
4:1
2: 1
1:1
1:2
1:4
Metal content of Mn/Fe/K
1.62/0.71/0.78
0.82/0.90/0.23
1.45/1.17/0.82
1.47/1.77/0.42
0.71/1.64/0.09
Specifiic surface area, mYg
3.11
2.96
3.44
3.65
2.77
Removal efficiency of benzophenone, %
58.4
62.0
65.4
73.9
78.3
3.7 Catalytical activities after multiple-repeated use Many catalysts exhibited good catalytic activity but poor mechanical strength and could not be used in large-scale water treatment. So the lifetime of Mn-Fe-K modified ceramic honeycombs was investigated under the same operation conditions in this experiment. As shown in Fig.10, there was scarce reduction in catalytic activity after the catalyst was repeated used (about 30 times). Therefore, it was believed that the characteristics of catalysts were not changed after the catalyzed ozonation process. Compared with the catalysts (Ti02, Mn02, FeOOH) used in ozonation reactions, Mn-Fe-K modified ceramic honeycombs, which were commercially available, exhibited more chemical stability, mechanical strength as well as much easier manipulation and may be used in large-scale water treatment in
90
su^
80
n
8
1
P P
70
fkture after farther industrialized researches.
3 Conclusions The presence of Mn-Fe-K modified ceramic honeycombs significantly increased the removal rate of benzophenone and TOC compared with that achieved by ozonation alone or ceramic honeycombcatalyzed ozonation. The EPR experiments verified that higher benzophenone removal rate was attribute to more hydroxyl radicals generated in the catalyzed ozonation. The benzophenone removal was remarkably enhanced with the amount of catalyst, temperature and pH value increasing. When the gross of ozone was same, feed in the ozone several times led to a marked increase of benzophenone degradation compared with feed in the ozone one time in Mn-Fe-K modified ceramic honeycomb-catalyzed ozonation. The optimum condition for preparing catalyst was that the dipping solution proportion is Mn:Fe=l:4 and the sinter temperature is 600°C . The characteristics of catalysts were not changed obviously after the catalyzed ozonation process. The results indicated that the Mn-Fe-K modified ceramic honeycombs were of higher catalytic activity, chemical stability, mechanical strength and may be used in large-scale water treatment in hture after farther industrialized researches.
References:
rr
6
!.
60
%c
8
50
d
1
L
L
L
40
5
10
IS
20
25
30
Use times
Fig. 10 Activity of catalysts on the degradation of benzophenone after repeated uses Initial benzophenone: 10 m e ; temp.: (21 f1)T; number of Mn-Fe-K modified ceramic honeycombs used: 5; pH 6.87; background: distilled water; reaction time 10 min
Andreovi R, Marotta R, 2000. Manganese-catalyzed ozonation of gly -oxalic acid in aqueous solutions[J]. Journal of Chemical Technology and Biotechnology, 75: 59-65. Bader H, Hoigne J, 1981. Determination of ozone in water by the indigo method [J]. Wat Res, 15: 449-456. Barbaba K H, Maria Zhlek, Jacek Nawrocki, 2003. Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment[J]. Applied Catalysis B: Environmental, 46: 639-669. Bhat N, Gurol M D, 1995. Oxidation of chlorobenzene by ozone and heterogeneous catalytic ozonation[C]. 27th Industrial Waste MidAtlantic Conference, Bethlehem, PA, USA, July. 371. Elovitz M S, Kawaser H P, 2000. Hydroxyl radicaVozone ratios during ozonation processes. 11. The effect of temperature, pH, alkalinity, and DOM properties[J]. Ozone Sci Eng, 22: 123-150. Fernando J, Francisco J, Ramon Montero-de-Espinosa, 2002. Catalytic ozonation of oxalic acid in an aqueous TiO, slurry reactor[Jl.
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Applied Catalysis B: Environmental, 39: 221-231. Gottschalk C, Libra J A, Saupe A, 2000. Ozoantion of water and wastewater [C]. Wiley-VCH Verlag GmbH, D-69469 Weinheim. 13-14. Hoigne J, Bader H, 1997. The role of hydroxyl radical reactions in ozonation process in aqueous solutions[J]. Water Res, 1 0 377386. Hoigne J, 1998. Chemistry of aqueous ozone and transformation of pollutants by ozonation and advanced oxidation processs [MI. The h a n d h k of environmental chemistry, 5 (c) Quality and treatment of drinking water (Hrubec J. ed.). SpringerverlagBerlin Heidelberg. 84-141. Leagube B, Karpel N, Leitner V, 1999. Catalytic ozonation: a promising advanced oxidation technology for water treatment[Jl. Catalysis Today, 53: 61-72. Ma J, Gao J S, 2003. Efficiency and mechanism of degradation of trace benzophenone in water by O&Oz system[Jl. Journal of Heilongjiang University: Science, 20(1): 86-91. Ma J, Graham N J D, 1997. Preliminary investigation of manganese-catalyzed for the destructionof atrazine[J]. Ozone Sci Eng, 19: 227-233. Ma J, Graham N J D, 1999. Degradation of atrazine by manganese-catalyzed ozonation: influence of humic substances [Jl. Water Res, 33(3): 785-793. Ma J, Graham N J D, 2000. Degradation of atrazine by manganese-catalysed ozonation: influence of radical scavengers [J]. Water Res, 34(15): 3822-3828. Ma J, Sui M H, Zhang T et al., 2005. Effect of pH on MnWGAC
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catalyzed ozonation for degradation of nitrobenzene[J]. Water Res, 39: 779-786, Ma J, Bang T, Chen Z L, 2005. Pathway of aqueous femc hydroxid catalyzed ozone decomposition and ozonation of trace nitrObenzene[J]. Environmental Science, 26(2): 78-82. Mathias E, Franck L, Schrotter J C, 2004. Catalytic ozonation of refractory organic model compounds in aqueous solution by aluminum oxide[Jl. Appl Catal B: Environmental,47: 15-25. Mirat D, Gurol P P, Lin S S et d.,1996. Continuous catalytic oxidation process[P]. Patent number IJS5755977. Paillard H, Dore M, Bourbigot M M, 1991. Prospects concerning applications of catalytic ozonation in drinking water treatment [C]. Proc. loth Ozone World Congress, Monaco, March, 1. 313-329. QuJ H, Li H Y, Liu H Jet al., 2004. Ozonation of alachlor catalyzed by Cu/Ala, in water[Jl. Catalysis Today, 9 0 291-296. Sehested K, Holcman J, Bierbackke E, 1984. Formation of ozone in the reaction of 0;and the decay of the ozonide ion radical at pH 10--13[Jl. J Phys Chm, 88: 269-273. Staehelin J, Hoigne J, 1982. Decomposition of ozone in water: rate of initiation by hydroxide ions and hydrogen peroxide[JJ Environ Sci Technol, 16(10): 676-681. Sun Z Z, Ma J, Wang L B et al., 2005. Degradation of nitrobenzene in aqueous solution by ozone-ceramic honeycomb[Jl. Journal of Environmental Sciences, 17(5): 7 16-72 1. (Received for review December 5,2005. Accepted March 29,2006)
Journal of Environmental Sciences
Vol. 18, No. 6, pp. 1065-1072,2006
Article ID: 1001-0742(2006)06-1065-08
CLC number: X703
C N 11-26291x
Document code: A
Degradation of benzophenone in aqueous solution by Mn-Fe-K modified ceramic honeycomb-catalyzed ozonation HOU Yan-jun, MA Jun', SUN Zhi-zhong, YU Ying-hui, ZHAO Lei (School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China. E-mail: majun- [email protected]; [email protected])
Abstract: Comparative studies of ozonation alone, ceramic honeycomb-catalyzed and Mn-Fe-K modified ceramic honeycomb catalyzed ozonation processes have been undertaken with benzophenone as the model organic pollutant. The experimental results showed that the presence of Mn-Fe-K modified ceramic honeycombs significantly increased the removal rate of benzophenone and TOC compared with that achieved by ozonation alone or ceramic honeycomb-catalyzed ozonation. The electron paramagnetic resonance (EPR) experiments verified that higher benzophenone removal rate was attribute to more hydroxyl radicals generated in the Mn-Fe-K modified ceramic honeycomb-catalyzedozonation. Under the conditions of this experiment, the degradation rate of all the three ozonation processes are increasing with the amount of catalyst, temperature and value of pH increased in the solution. We also investigated the effects of different process of ozone addition, the optimum conditions for preparing catalyst and influence of the Mn-Fe-K modified ceramic honeycomb after multiple-repeated use. Keywords catalytic ozonation; Mn-Fe-K modified ceramic honeycomb; benzophenone
Introduction Ozonation is an attractive and increasingly important method for the degradation of organic pollutants in aqueous solution. Many studies showed that hydroxyl radical produced by the ozone has strong oxidation potential (Eo = 2.80 V) and degrade organic pollutants quickly. However, many refkactory pollutants, for example pesticide, are very difficult to degrade by ozone alone. Thus, more and more attention is being focused on heterogeneous catalytic ozonation processes for its high catalytic efficiency as well as its easy operation (Hoigne, 1998; Leagube et d.,1999; Andreozzi and Marotta, 2000). Paillard et d.(199 1) reported that Ti02-catalyzed ozonation was more effective for degradation of oxalic acid than ozone alone. The experimental results showed that the oxidation was hardly influenced by the hydroxyl radical scavengers such as bicarbonates. Bhat and Gurol (1 995) studied the ozonation of chlorobenzene in the presence of goethite and found that catalyzed ozonation was more effective and led to a higher degree of mineralization of the compound than ozone alone. Mathias et nl. (2004) indicated that A1,0, was an effective heterogeneous catalyst for the ozonation of refractory organic compounds such as oxalic, acetic and salicylic with higher DOC removal, accordingly, a reaction pathway was presented by which ozone reacted with OH- ions on the surface and radical species were formed. Qu et d. (2004) found that the use of Cu/A1203 substantially enhanced the mineralization of alachlor in ozonation and more hydroxyl radicals generated in the catalyzed ozonation could bring on a higher alachlor removal rate. Ma and
Graham (1997, 1999, 2000) showed that MnOz formed in situ exhibited high activity for ozonation of atrazine compared to the case of ozonation alone, it was also indicated that humic substances and radical scavengers influenced the degradation of atrazine. After continuous investigations, the results suggested the generation of hydroxyl radical via ozone reacting with the surface-bound OK ions on the surface of the manganese dioxide and MnOJGAC catalyzed ozonation (Ma et d.,2005a). Ma et d. (2005b) studied the catalytic ozonation of trace nitrobenzene in water over FeOOWGAC catalyst and indicated that the pseudo-first order rate of ozone decomposition increased obviously for FeOOWGAC catalyst and the oxide surface at nearly zero charged point was favorable for the catalytic ozonation of nitrobenzene. In our previous work (Sun et d., 2005), ceramic honeycombs were selected as catalysts in ozonation system. The experimental results showed that that ceramic honeycomb-catalyzed ozonation was more effective for degradation of nitrobenzene than ozone alone. Although the ceramic honeycomb exhibit some catalyzing effectiveness for the degradation of organic pollutants in aqueous solution, the higher consume of ozone and nevertheless lower removal efficiency are the limitations in its practical application. To overcome the shortcomings of ceramic honeycomb, a modified catalyst was prepared by adulterating the Mn-Fe-K on ceramic honeycomb. It is expected that higher catalytic activity can be achieved because it improves the surface structure and composition of ceramic honeycomb.
1 Materials and method
Foundation item: The National Natural Science Foundation of China (No. 50378028); *Corresponding author
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HOU Yan-jun et al.
1.1 Materials Benzophenone was selected as the model organic compound, the model-polluted water was prepared by diluting benzophenone (Shanghai Chemical Reagent Company, pre-distillation treated, with purity of 99.80%) in distilled water until a concentration 10 m a was obtained. Sulfuric acid and sodium hydroxides (analytical grade) were added in the aqueous solution to control the pH. Other chemical reagents used in the experiment, such as sodium thiosulphate, potassium indide, etc., were analytical grade reagents. All glass wares were soaked in chromic acid, then rinsed with tap water and distilled water. Ceramic honeycombs with the constituent of cordierite were applied in this system (Shanghai Pengyi Fire-resistant Material Factory). Monolithic honeycomb was cylindrical with a diameter of 50 mm and a length of 50 mm; the cell densities of the ceramic honeycombs were 400 square cells per square inch with a wall thickness of 0.4 mm. The impregnation method was used to produce the Mn-Fe-K modified ceramic honeycomb catalysts. Treated ceramic honeycombs was immersed in the mixed aqueous solution containing appropriate amounts of manganese nitrate, ferric nitrate and potassium nitrate with constant shaking for 2 h to allow for enough impregnation and adsorption ;then dried for 12 h at 25°C and 12 h at 85°C and calcined for 4 h at 600°C. The above process was repeated three times to produce steady Mn-Fe-K modified ceramic honeycomb catalysts. The main crystalline phase of the catalysts was 2MgO .2A1203-5Sioz. The surface area of the Mn-Fe-K modified cordierite ceramics was 2.77 mz/g (by brunauer emmett and teller (BET)). The absolute content of Mn was 0.71%, the absolute of Fe was1.64 %, the absolute content of K was 0.09 % (by electron spectroscope for chemical analysis (ESCA)). 1.2 Analytical methods The concentration of ozone in the gas was measured by iodometric titration method (Ozone Standards Committee Method). Dissolved ozone in the water was detected by spectrophotometerusing the Indigo method (Bader and Hoigne, 1981). The concentration of benzophenone was analyzed by SPD-1OW high performance liquid chromatography (HPLC) (Shimadzu, Japan). The main carboxylic acids produced during the ozonation period were analyzed using Dix-500 ion chromatograph (Dionex, USA). The pH in aqueous solution was measured by Delta 320 pH acidometer (Shanghai Leici Apparatus Factory, China). The consumed ozone was obtained by the following calculation: consumed ozone = introduced ozone - (ozone in off-gas + ozone residual in water). X-ray power difiaction (XRD) was used to
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analyze crystallinity and crystal phase of ceramic. Brunauer emmett and teller (BET) was used to determine the specific surface area of catalysts through low-temperature nitrogen adsorption. Scanning electronic microscope (SEM) was used to study the microstructure of the catalysts. Electron spectroscope for chemical analysis (ESCA) was used to study the absolute content of the elements of catalysts. EPR (electron paramagnetic resonance) experiment (Bruker Elexsys E5OO ESR spectroscopy, Germany) was applied with spintrapping reagent 5,5-dimethyl- 1-pyrolin-N-oxide (DMPO). EPR conditions: microwave power 20 mW, modulation fiequency 100 kHz, sweep width 100.0 G; central magnetic field 3480 G; scan 40 s each time; scan times 20. 1.3 Ozonation procedure Experiments were carried out in a cylindrical reactor (inside diameter 50 nun and volume 3 L) made of stainless steel (Fig. l), which was shielded to control reaction temperature. Before the experimental operation, the reactor was pre-ozonated for 4 min and washed several times with distilled water to minimize the potential side effects. The Mn-Fe-K modified ordierite honeycombs were placed into the reaction column. The ozone was generated fiom ozone generator (Qinghua Tongli) and was fed into the cylinder via a porous fiit at bottom of column. The oxygen flux, electric current and aerating time were regulated to control the applied ozone dose. In this experiment, the model water (3 L) spiked with the benzophenone concentration of 10 mg/L was pumped into the column by a magnetic pump (Xishan Pump Co. Ltd., Shanghai) and then circulated. The experiment was performed in continuous (catalytic ozonation process) or semicontinuous (ozonation process). Water samples were taken from the contactor column to analyze the concentration of residual benzophenone. The oxidation reaction was stopped by the addition of a small amount of sodium thiosulphate solution.
Heat preservation
4
t
r' KI aqueous
Oxygen
Ozonizer
Fig. 1 Schematic diagram of Mn-Fe-K modifiedceramic honeycomb omnation system
Degradation of benzophenone in aqueous solution by Mn-Fe-K
No.6
Heterogeneous catalytic ozonation of benzophenone is a gas-liquid-solid reaction where many chemical and physical steps such as fluid dynamics, catalyst material, catalytic surface area, chemical kinetics, the temperature, pH of the solution, etc., might affect the process rate. With the aim to test the catalytic activity, the effects of various factors on the oxidation were investigated under the same operating conditions such as the identical geometric structure of cordierite honeycomb and Mn-Fe-K modified cordierite honeycomb modules, the constant fluid rate (100 L h ) and the same ozone applied (0.35 mg/(L min)).
2 Results and discussion 2.1 Mn-Fe-K modified ceramic honeycombs catalytic oxidation of benzophenone The efficiency of heterogeneous catalyzed ozonation by Mn-Fe-K modified ceramic honeycombs for benzophenone ozonation in aqueous solution was clearly evidenced by the experimental data shown in Fig.2. The reactivity was strongly enhanced by the addition of catalysts in ozonation system, and the oxidation of benzophenone was about 8 1.4% in the Mn-Fe-K modified ceramic honeycombs catalytic ozonation system compared to 68.8% with ceramic honeycombs catalytic ozonation system and 48.3% with ozone alone in 120 min. The TOC removal in Mn-Fe-K modified ceramic honeycombs catalytic ozonation system was 56.2% compared to 43.1% with ceramic honeycombs catalytic ozonation system and 20.9% with ozone alone in 120 min. Benzophenone decreased mainly in first 10 min and TOC decreased continuously. The results show the high catalytic capacity of Mn-Fe-K modified ceramic honeycombs loo
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on ozonation mineralization of benzophenone. At pH=6.87 and T==21 k l)T, a slight removal of benzophenone was shown in the presence of five Mn-Fe-K modified ceramic honeycombs in series fixed in the cylinder. The reduction probably caused by the adsorption on the catalyst surface appeared negligible compared to ozonation system in 120 min reaction. Therefore, these results suggested that the degradation rate of benzophenone was attributed to the catalytic activity of Mn-Fe-K modified ceramic honeycombs. The adsorption of benzophenone on the Mn-Fe-K modified ceramic honeycombs is small but the adsorption of degradated intermediate of benzophenone ozonation on the Mn-Fe-K modified ceramic honeycombs may be biggish. So we studied the TOC removal at different experiments. Fig.3 shows the TOC removal of ozone alone, ozone + adsorption (that means ozonated alone 120 min, then blowed away the ozone in water by Nzand adsorped 120 min by Mn-Fe-K ceramic honeycombs) and Mn-Fe-K ceramic honeycombs catalyzed ozonation after 120 min. It can be found that the contribution of ozone alone to TOC was 20.9%, the contribution of adsorption of degradated intermediate by Mn-Fe-K ceramic honeycombs to TOC was 26.2% and the contribution of catalysis to TOC was 56.2% . Table 1 shows the mainly three kinds of organic by-products in the decomposition of benzophenone (Ma and Gao, 2003): acetic acid, oxalic acid and propionic acid. The concentrations of those organic acids produced in the ozonation alone, ceramic honeycomb-catalyzed and Mn-Fe-K modified ceramic honeycomb catalyzed ozonation processes were different. After 30 rnin catalyzed ozonation, the Mn-Fe-K modified ceramic honeycombs led to the lowest concentration of acetic acid, oxalic acid and propionic acid in water. The result agrees with the TOC removal efficiency of three ozonation processes, and shows hrther the high catalytic capacity of Mn-Fe-K modified ceramic honeycombs on ozonation
r-----l A
A
60 0
s I
30
u" 40
X
X
t
-5
I
60 t, rnin
90
120
Fig.2 Degradation of benzophenone in the presence of Mn-Fe-K modified ceramic honeycombs Reaction conditions: temp. (21 1 ) T ; pH 6.87; initial benzophenone: 10 mg/L; number of two kinds of ceramic honeycombs in series: 5; background: distilled water; experiments: ( ) ozonation alone; ( ) benzophenone ceramic honeycombs catalyzed; ( A ) benzophenone h4n-Fe-K ceramic honeycombs catalyzed; ( X ) TOC ozonation alone; (0 ) TOC ceramic honeycombs catalyzed; (0)TOC Mn-Fe-K ceramic honeycombs catalyzed
+
+
1
20
i-
0
Ozone alone
Ozone+
Catalytic
adsorption
ozonation
Fig.3 TOC removal at different experiments after 120 min Adsorption conditions: temp. 21 k IT ; pH 6.87; initial benzophenone: 10 mgL; number of two kinds of ceramic honeycombs in series: 5 ; background: distilled water
HOU Yan-jun et al.
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Table 1 Comparison of concentration of three by-products between three OZOM~~OUproeeapes ~
~
~~~~
By-products
Acetic acid, m a
Oxalic acid, mg/L
Propionic acid, mg/L
Ozone alone
5.41
2.43
0.95
Catalyzed by ceramic honeycombs
1.71
0.64
0.33
Catalyzed by Mn-Fe-K modified ceramic honeycombs
0.86
0.21
0.17
mineralization of benzophenone. It has been reported that the heterogeneous catalytic ozonation with the use of metal oxides as catalysts was based on an ozone decomposition reaction followed by the generation of hydroxyl radicals (Hoigne and Bader, 1997; Ma and Graham, 1999; Fernando et d., 2002). The surface microstructure of the catalysts is shown in Fig.4. Studied the microstructure we can find that two possible factors may influence the degradation efficiency of benzophenone. Firstly, the numerous pores presented on the surface could remarkably increase the geometric area of catalysts, Mu-Fe-K modified cordierite honeycombs (Fig.4) provided larger surface area (2.77 mYg) than cordierite honeycombs (0.35 m*/g),thus the adsorption of ozone molecules on the surface of catalysts took place; secondly, Mn-Fe-K modified cordierite honeycombs with the dominant constituent of Mg2ALSisOle exhibited higher catalytic activity, the solid surface-solution interface can catalyze oxidation reactions. It has been reported that the surface area provided by pores in catalyst structure was a more important factor in the catalytic oxidation process than the external surface area of catalysts because adsorption was often one of the stages of heterogeneous catalytic ozonation, and these processes were possibly essential to overall catalytic reactions (Mirat et al., 1996; Legube et al., 1999). The catalysts with the main crystalline phase of 2MgO 2A1203* 5Si02 provided both Bronsted acid (-Si-) and Lewis acid (-Al-, -Mg-, -Mn- and -Fe-) sites, which were thought to be the catalytic centers on the surface (Barbaba et al., 2003). When the ozone molecules were adsorbed on the catalyst surface, the groups on the catalyst surface may initiate the decomposition of ozone and generated surface bound hydroxyl radicals. Then the adsorbed organic molecules were oxidized by adjacent hydroxyl radicals. Hydroxyl radicals significantly improved ozone oxidation of benzophenone because of its unselective oxidizing reaction with organic molecules. To confirm the effect of hydroxyl radicals on oxidation reaction in the aqueous solution, EPR experiment was carried out. Benzophenone solution, 0, and the catalyst powder were mixed with DMPO dissolved in ultra-pure water. Immediately after the mixing, 25 pl of the sample solution was transferred into a capillary tube, and EPR spectra were recorded in the X-band on a Bruker ESP spectrometer at room
Fig.4 SEM picture of surface structure of Mn-Fe-K modified ceramic honeycomb
temperature. As shown in Fig.5, a much stronger signal of hydroxyl radicals, which had the typical spectrum of 1 :2:2:1, was presented in catalytic ozonation process. This indicated that hydroxyl radicals was surely produced with Mn-Fe-K modified ceramic honeycombs used in ozonation process and played a more important role in catalytic ozonation process compared with ozone alone which gave no signal of hydroxyl radicals at all. 800
I
1
400
tj =
200 0 -200
-400 -600
1
‘
-800 3430
I
I
I
I
I
3450
3470
3490
3510
3530
Gauss
Fig.5 Intensity of DMPO-OH adduct signals with Mn-Fe-K modified ceramic honeycombs catalytic ozonation DMPO and 0, concentration are 80 and 0.125 rnrnol/L, respectively; catalyst dosage is 1 giL
2.2 Effect of catalyst dosage It is shown in Fig.6 that the degradation percentage of benzophenone was 45.1 YO in ozone alone system. Under the same condition, five MnFe-K modified ceramic honeycombs situated in series at the cylinder bottom, the degradation efficiency increased remarkably from 45.1% to 78.1% after 10 min oxidation reaction. Considering the facts that the adsorption of benzophenone on catalysts was negligible (Fig.2) and the benzophenone degradation
Degradation of benzophenone inI aqueous sohtion by Mn-Fe-K
No.6
was improved with the increase of catalysts dosages at room temperature, these results confirmed that Mn-Fe-K modified cordierite honeycombs provided catalytic activity and enhanced benzophenone removal under the same experimental conditions.
r----l o.8
t
0 0
2
1
3
4
5
Catalyst8 dosage, pieccs
Fig.6 Effect of Mn-Fe-K modified ceramic honeycomb dosage on the degradation of benzophenone Reaction condition: temp. (21 f1)T; pH 6.87; initial benzophenone: 10 mg/L; background: distilled water; experiments: (+) catalytic ozonation with ceramic honeycombs; ( A ) catalytic ozonation with Mn-Fe-K ceramic honeycombs
2.3 Temperature Fig.7 shows that the temperature was an important factor for the degradation of benzophenone, both in ozonation alone and in catalytic ozonation system. As far as the present system was concerned, the effect of this variable was positive to oxidation reaction with temperature ranging from 10 to 40°C. The increase of temperature fi-om 10 to 40°C led to a significant increase of benzophenone conversion from 35.0% to 76.1% in ozonation alone, from 33.5% to 84.2% in ceramic honeycombs catalytic ozonation system and 37.8% to 92.2% in ceramic honeycombs
0.8
9 0.6
+
o.8
+
W
0.4
go.6
0
' 10
I
I
20
30
I
40
T,c Fig.7 Effect of temperature of water sample on the degradation of benzophenone Initial benzophenone: 10 mg/L; number of two kinds of ceramic honeycombs in series: 5; pH 6.87; background distilled water; experiments: (+) ozonation alone; (W) catalytic ozonation with ceramic honeycombs; ( A ) catalytic ozonation with Mn-Fe-K ceramic honeycombs
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catalytic ozonation system, respectively. When at lower temperature ranges, the increase of temperature yielded the increase of both the ozone decomposition rate and the chemical reaction rate. But the ozone solubility would decrease with the temperature increasing continuously, which would cause negative effect on the degradation rate of benzophenone. From the results obtained in present ozonation system, the room temperature was optimal considering the operational conditions. 2.4 pH It was reported that solution pH significantly influenced ozone decomposition in water (Elovitz and Kawaser, 2000). In aqueous systems ozone oxidation follows two simultaneous mechanisms: direct reaction and indirect reaction respectively. The rate of decomposition of ozone increased rapidly with the increase of pH, which was due to the initiation of OHthrough the reaction of 0,with OH (Staehelin and Hoigne, 1982; Sehested et d.,1984; Hoigne and Bader, 1997). However, at pH<3.0 hydroxyl radicals influenced the decomposition of ozone to less extent, so catalytic ozonation of benzophenone has been investigated in the pH range of 3.0-1 1.O in this study. As observed in Fig.8, the increase of pH led to a marked increase of benzophenone degradation from 28.2% to 86.8% in the range 3.0
1
1
..----
OA2
f
tt
4
W
+
A
4
0 1
I
I
I
3
5
I PH
9
I 11
Fig.8 Effect of pH on the degradation of benzophenone Initial benzophenone: 10 mg/L: temp. (21 k l)T; number of two kinds of ceramic honeycombs in series: 5; background: distilled water; reaction time 10 min; experiments: (+) ozonation alone; ( W ) catalytic ozonation with ceramic honeycombs; (A) catalytic ozonation with Mn-Fe-K ceramic honeycombs
HOU Yan-jun et al.
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not much obviously enhanced compared to the case of ozonation alone, this phenomenon might be attributed to the scavenging potential of carbonate or the interactions between hydroxyl radicals and H 0 2 radicals (Gottschak et al., 2000). Because the solution was prepared with distilled water, the concentration of inorganic carbon was low and the inhibiting impact of carbonate on the oxidation could be ignored. 2.5 Effect of different processes of ozone addition The different feeding modes affect the degradation of benzophenone obviously when they were fed in the same ozone dosage. Fig.9 shows that to the ozone alone, ozone/ceramic honeycombs and ozone/Mn-Fe-K modified ceramic honeycombs systems, when the gross of ozone was the same, feed in the ozone several times led to a marked increase of benzophenone degradation compared with feed in the ozone one time. The removal efficiencies of benzophenone by ozone alone, ozone/ceramic honeycombs and ozone/Mn-Fe-K modified ceramic honeycombs with feed in the ozone several times within 10 min were 66.39%, 89.71%, 93.52% , respectively, while that feed in the ozone one time were 45.13%, 64.44% and 77.16%. That is, the removal efficiency of benzo- p henone of feeding in the ozone several times would be 1.4times that of feeding in the ozone one time. These results also suggested that the removal efficiencies of benzophenone by ozone alone, ozone/ceramic honeycombs and ozone&-Fe-K modified ceramic honeycombs with feed in the ozone one time within initial 2 min were 24.38%, 37.88% and 40.68%, respectively, while that feed in the ozone several times were 18.09% , 25.23% , 27.14% . That is, the removal efficiency of benzophenone of feeding in the ozone one time would be almost 1.4 times that of feeding in the ozone several times, and the removal of benzophenone of feeding in the ozone one time had appeared obviously within initial 2 min in the three systems. This may because while the ozone was fed in one time, there were hydroxyl radicals with higher concentration produced quickly within initial 2 min. But only part of the hydroxyl radicals reacted with benzophenone, another part of the hydroxyl radicals reacted each other so that the concentration of hydroxyl radicals descended sharply and could not remove the benzophenone effectively any more. While the ozone was fed in several times, the hydroxyl
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X
0”
4
0.4
0.2
0
! -
0
I
I
I
I
2
4
6
8
I
10
t, min
Fig9 Effect of different processes of ozone addition on the degradation of benzophenone Initial benzophenone: 10 m a ; temp. (21 zk 1)F;number of two kinds of ceramic honeycombs in series: 5; pH 6.87; background distilled water; reaction time 10 min; experiments: (*)ozonation alone feed one time; ( ) catalytic ozonation with ceramic honeycombs feed one time; ( A ) catalytic omnation with Mn-Fe-K ceramic honeycombs feed one time; ( X ) ozonation alone feed several time; (0) catalytic ozonation with ceramic honeycombs feed several time; ( 0 ) catalytic ozonation with Mn-Fe-K ceramic honeycombs feed several time
radicals consumed one time could be supplied quickly by the addition of ozone next time and the hydroxyl radicals can be kept on a relative steady concentration and in favor of the ozonation of benzophenone continuously. 2.6 Effect of different factors on preparations of CatdYStS
Table 2 illustrates the effect of different calcining temperatures on the catalyzing effectiveness (dipping solution proportion, Mn:Fe=l:4). 600T was more favorable toward increasing the catalyzing activity than the lower or higher temperature. It is possible that lower temperature calcined results in unstable surface structure and chemical constituents that may changes during the ozonation. High temperature may result in the partly melt of the catalyst surface and reduces the specific surface area of catalysts which affect the catalyzing activity obviously. The components proportion of mixed aqueous solutions of nitrate is another factor affecting the catalyzing activity. As shown in Table 3, under the same calcining temperature 600°C , the catalyzing ability of the catalyst changed significantly with
Table 2 Activity of catalysts with dinerent calcining temperature Mn-Fe-K modified ceramic honeycombs Calciningtemperature 3009:
4509:
6009:
7509:
Metal content of Mn/Fe/K
1.52/1.04/0.53
1.10/1.32/0.44
1.37/1.16/0.52
1.29/1.14/0.43
Specific surface area, m2/g
3.46
4.19
2.14
2.02
Removal efficiencyof benzophenone, %
57.1
74.1
77.6
56.4
Degradation of benzophenone in aqueous solution by Mn-Fe-K ...---
NO.~
different component proportion of mixed aqueous solutions of nitrate. Meanwhile, it is shown that the oxidation rate by Mn-Fe-K modified ceramic honeycombs dipped in (Mn:Fe=l:4) nitrate aqueous solution was about 19.9% higher than that dipped in (Mn :Fe=4 :1) nitrate aqueous solution. It is possible
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that the structure and chemical constituent of catalyst surface which changed with the different component proportion of mixed aqueous solutions of nitrate plays an important role in the oxidation process. The high activity catalysts were in favor of the decomposition of ozone and increased the use ratio of ozone.
Table 3 Activity of Mn-Fe-K ceramic honeycomb catalysts with different component proportion Mn-Fe-K modified ceramic honeycombs Dipping solution proportion (Mn:Fe)
4:1
2: 1
1:1
1:2
1:4
Metal content of Mn/Fe/K
1.62/0.71/0.78
0.82/0.90/0.23
1.45/1.17/0.82
1.47/1.77/0.42
0.71/1.64/0.09
Specifiic surface area, mYg
3.11
2.96
3.44
3.65
2.77
Removal efficiency of benzophenone, %
58.4
62.0
65.4
73.9
78.3
3.7 Catalytical activities after multiple-repeated use Many catalysts exhibited good catalytic activity but poor mechanical strength and could not be used in large-scale water treatment. So the lifetime of Mn-Fe-K modified ceramic honeycombs was investigated under the same operation conditions in this experiment. As shown in Fig.10, there was scarce reduction in catalytic activity after the catalyst was repeated used (about 30 times). Therefore, it was believed that the characteristics of catalysts were not changed after the catalyzed ozonation process. Compared with the catalysts (Ti02, Mn02, FeOOH) used in ozonation reactions, Mn-Fe-K modified ceramic honeycombs, which were commercially available, exhibited more chemical stability, mechanical strength as well as much easier manipulation and may be used in large-scale water treatment in
90
su^
80
n
8
1
P P
70
fkture after farther industrialized researches.
3 Conclusions The presence of Mn-Fe-K modified ceramic honeycombs significantly increased the removal rate of benzophenone and TOC compared with that achieved by ozonation alone or ceramic honeycombcatalyzed ozonation. The EPR experiments verified that higher benzophenone removal rate was attribute to more hydroxyl radicals generated in the catalyzed ozonation. The benzophenone removal was remarkably enhanced with the amount of catalyst, temperature and pH value increasing. When the gross of ozone was same, feed in the ozone several times led to a marked increase of benzophenone degradation compared with feed in the ozone one time in Mn-Fe-K modified ceramic honeycomb-catalyzed ozonation. The optimum condition for preparing catalyst was that the dipping solution proportion is Mn:Fe=l:4 and the sinter temperature is 600°C . The characteristics of catalysts were not changed obviously after the catalyzed ozonation process. The results indicated that the Mn-Fe-K modified ceramic honeycombs were of higher catalytic activity, chemical stability, mechanical strength and may be used in large-scale water treatment in hture after farther industrialized researches.
References:
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6
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60
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8
50
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1
L
L
L
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30
Use times
Fig. 10 Activity of catalysts on the degradation of benzophenone after repeated uses Initial benzophenone: 10 m e ; temp.: (21 f1)T; number of Mn-Fe-K modified ceramic honeycombs used: 5; pH 6.87; background: distilled water; reaction time 10 min
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