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Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis
Journal of Environmental Sciences 2011, 23(11) 1904–1910
Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis Qin Zhang1,2 , Xiaodi Cheng1 , Chen Zheng1 , Xionghan Feng1 , Guohong Qiu1 , Wenfeng Tan1 , Fan Liu1,∗ 1. Key Laboratory of Subtropical Agriculture and Environment, Ministry of Agriculture, College of Resources & Environment, Huazhong Agricultural University, Wuhan 430070, China. E-mail: chincheung [email protected] 2. College of Land Resource and Environment, Jiangxi Agricultural University, Nanchang 330045, China Received 22 December 2010; revised 26 February 2011; accepted 17 March 2011
Abstract Manganese oxides are known as one type of semiconductors, but their photocatalysis characteristics have not been deeply explored. In this study, photocatalytic degradation of phenol using several synthesized manganese oxides, i.e, acidic birnessite (BIR-H), alkaline birnessite (BIR-OH), cryptomelane (CRY) and todorokite (TOD), were comparatively investigated. To elucidate phenol degradation mechanisms, X-ray diffraction (XRD), ICP-AES (inductively coupled plasma-atomic emission spectroscopy), TEM (transmission electronic microscope), N2 physisorption at 77 K and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed to characterize the structural, compositional, morphological, specific surface area and optical absorption properties of the manganese oxides. After 12 hr of UV-Vis irradiation, the total organic carbon (TOC) removal rate reached 62.1%, 43.1%, 25.4%, and 22.5% for cryptomelane, acidic birnessite, todorokite and alkaline birnessite, respectively. Compared to the reactions in the dark condition, UVVis exposure improved the TOC removal rates by 55.8%, 31.9%, 23.4% and 17.9%. This suggests a weak ability of manganese oxides to degrade phenol in the dark condition, while UV-Vis light irradiation could significantly enhance phenol degradation. The manganese minerals exhibited photocatalytic activities in the order of: CRY > BIR-H > TOD > BIR-OH. There may be three possible mechanisms for photochemical degradation: (1) direct photolysis of phenol; (2) direct oxidation of phenol by manganese oxides; (3) photocatalytic oxidation of phenol by manganese oxides. Photocatalytic oxidation of phenol appeared to be the dominant mechanism. Key words: manganese oxides; adsorption; oxidation; photocatalysis; degradation; phenol DOI: 10.1016/S1001-0742(10)60655-9 Citation: Zhang Q, Cheng X D, Zheng C, Feng X H, Qiu G H, Tan W F, Liu F, 2011. Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis. Journal of Environmental Sciences, 23(11): 1904–1910
Introduction Manganese (Mn) oxide is a class of active oxides widespread in the environment (Banerjee and Nesbitt, 1999). Because Mn oxide possessed special physiochemical properties, severe defects and nonstoichiometry in the structure, it has been extensively studied as adsorbent, oxidant, catalysis and rechargeable battery (Post, 1999; Suib, 2008). Recently, the photocatalysis property of Mn oxide has attracted increasing attention due to its semiconducting electronic structure (Iyer et al., 2010; Kwon et al., 2009). Photocatalysis property is determined by the electronic structure of the catalyst (Machida et al., 2001). In Mn oxide, d–d electronic transitions are known to occur under illumination due to partially filled d-orbital, so Mn oxide can be effectively used as photocatalyst without doping of other cations (Iyer et al., 2010). The photochemical reaction occurred due to excitation of the occupied * Corresponding author. E-mail: [email protected]
electrons on O-2p of the Mn oxide to the unoccupied orbits (mainly Mn-3d) when the light energy was greater than the difference of the orbit energy gap (Kwon et al., 2009; Sherman, 2005). It is proved by the fact that the photo-induced electric current of Mn oxide was detected (Sakai et al., 2005). Mn oxide have been used to photocatalytically degrade 2-propanol, halogenated methane and dimethyl methylphosphonate (DMMP) in gas phase under visible illumination (Cao and Suib, 1994; Chen et al., 1997; Segal et al., 1999). Under UV illumination, CRY can more effectively photocatalytically degrade 2,4dichlorophenoxyacetic acid and Methylene Blue than TiO2 (P-25 Degussa), it may be caused by the polarity of Methylene Blue and the hydrophobicity of CRY (Lemus et al., 2008). It indicated that the photocatalysis activity of Mn oxide might be even greater than TiO2 or ZnO because their special photochemical characteristics. Most studies on photocatalysis of Mn oxide have mainly focused on gaseous photocatalytic degradation of organic compound, but there is rare research on photocatalysis of Mn oxide
Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis
in solution. In addition, because Mn oxide has significant capabilities in adsorption, oxidation and photocatalysis, many reactions may be involved in degradation of organic compounds in water, including: (1) surface adsorption; (2) oxidation-reduction; (3) photocatalytic oxidation. These reactions occur rapidly and simultaneously, and often difficult to distinguish the primary from the secondary. All these reactions constitute the foundation of the degradation of organic compounds on Mn oxide, so it is important to evaluate the photocatalysis of Mn oxide for its potential applications in pollution control and treatment. Phenol commonly exists in the environment with highly toxicity, structurally stability, resistance to biodegradation (Kavitha and Palanivelu, 2004; Yang et al., 2008), and phenol is also an intermediate of degradation of aromatic polymer (Gondal et al., 2007). Therefore, phenol has been used as a model compound in simulating organic pollutants in studies (Guo et al., 2006). In this study, the degradation of phenol by several common Mn oxides with or without light irradiation was explored, and the effects of mineral structure and underlying reaction mechanism were also investigated. Results from this study can provide evidence of environmental chemical behavior of Mn minerals in the environment, and provide a new approach to the control and treatment of organic pollutant in wastewater.
1 Materials and methods 1.1 Synthesis of Mn oxide Acidic birnessite (BIR-H), alkaline birnessite (BIROH), cryptomelane (CRY) and todorokite (TOD) were synthesized as reported in our previous work (Feng et al., 2004a, 2004b, 2007). All synthesized Mn oxide was washed with deionized water until conductivity was measured to less than 20 μS/cm, then freeze-dried, ground into powder, and stored in bottles. 1.2 Characterization of Mn oxide XRD was conducted on D8 Advance X-ray diffractometer (Bruker, Germany) with Cu Kα (λ = 0.15406) radiation at 40 kV tube voltage, 40 mA tube current, 1˚/min scanning speed, and step size of 0.02˚. The micro-morphology was observed using JEM-2010 FEF transmission electron microscope (Jeol, Japan). The specific surface area (SSA) was determined using Autosorb-1 automatic specific surface area analyzer (Quantachrome, USA). The UV-Vis diffuse reflectance spectrum (UV-Vis RDS) was determined by Shimadzu 3600 spectrophotometer (Shimadzu, Japan). The average oxidation state of Mn (AOSMn ) and the determination of elemental composition were measured as reported in our previous work (Feng et al., 2004b). The point of zero charge (PZC) was measured by the rapid potentiometric titration method (Tan et al., 2008). The concentration of Mn2+ was analyzed with an atomic absorption spectrometer (Varian AA 240 FS, USA). 1.3 Evaluation of photocatalysis Five milliliter phenol solution (500 mg/L) (pH 6.0, preadjusted by HCl or NaOH solution) was added into 20
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mL 1.25 g/L Mn mineral suspension (pH 6.0) in a 50-mL quartz tube. A total of 12 quartz tubes were placed on a SGY-1 rotary photochemical reactor (Nanjing Stonetech Electric Equipment Company, China) and ventilated with air at a steady rate of 3.3 L/min. Among the 12 tubes, two tubes were used for kinetics study, and the other 10 tubes were used to measure total organic carbon (TOC) at different time intervals. The samples were centrifuged at 14,000 r/min, and then the supernatant was filtered through a 0.22-μm membrane for analyses of phenol and TOC. The residual mineral was rinsed three times and freeze-dried for FT-IR determination. All these experiments were conducted with or without light radiation. Each treatment had two replicates, and averages were recorded. To measure the volatility and direct photolysis of phenol, the blank control experiment was set up only in the absence of Mn oxide. Analysis of phenol was carried out on a highperformance liquid chromatography (HPLC 1200, Agilent, USA). Aliquots of 10 μL were injected to the HPLC, running with the mobile phase of acetonitrile/water (V/V) at 10/90. The separation was performed using an XDB C18 reversed phase column (Agilent, USA) at the flow rate of 1.0 mL/min and column temperature of 30°C. A fluorescence detector was used with the excitation wavelength and emission wavelength set at 270 and 300 nm, respectively. The TOC was determined using a Multi 3100 total organic carbon/nitrogen analyzer (Jena, Germany). The conversion rate R of phenol and TOC was calculated by Eq. (1): R = (C0 − Ct )/C0 × 100
(1)
where, C 0 (mg/L) and C t (mg/L) are the initial and final phenol or TOC concentrations, respectively. It is used to evaluate the activity of photocatalysis.
2 Results 2.1 Characterization of synthesized Mn oxides The XRD spectra of the synthesized Mn oxides (Fig. 1) showed that the diffraction peaks of CRY were in good agreement with the characteristic peaks (JCPDS 74-1451) of monoclinic cryptomelane. The diffraction peaks of BIR-H matched well with hexagonal birnessite (JCPDS 86-0666). The XRD pattern of TOD was similar to JCPDS
Intensity (CPS)
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BIR-OH TOD CRY BIR-H 10
20
30
40 50 60 70 80 2θ (degree) Fig. 1 Powder X-ray diffraction patterns of the tested manganese oxides.
Journal of Environmental Sciences 2011, 23(11) 1904–1910 / Qin Zhang et al.
87-0389, confirming monoclinic todorokite. All the peaks in the XRD pattern of BIR-OH can be attributed to monoclinic alkaline birnessite (JCPDS 43-1456). TEM images for synthesized Mn oxide in this study are shown in Fig. 2. The morphology of CRY was mainly tiny needles sized from tens nm to hundreds nm. BIR-H was shaped in aggregates of lamellar crystals layered over each other. TOD had morphology of platy trilling patterns, while BIROH was mainly stacked hexahedral sheet crystals. All these results are consistent with previous report (Feng et al., 2007), and TEM images indicated the particle size of Mn oxide is in a sequence of BIR-OH > TOD > BIR-H > CRY. Table 1 shows some other properties of synthesized Mn oxide, such as AOSMn , chemical composition, PZC, and SSA. The AOSMn of CRY, BIR-H and BIR-OH had relatively higher AOSMn values, ranging from 3.82 to 3.87, whereas TOD had a relatively low AOSMn value (3.67). These AOSMn and stoichiometry resembled those of Mn oxide reported in other literature (Feng et al., 2004a, 2004b, 2007). The PZC values of all Mn oxide were low, and in the order of BIR-H (1.78) < CRY (2.15) < BIROH (3.35) < TOD (3.50). These results were very close to our previous report (Tan et al., 2008). For SSA, the order was CRY (175.2 m2 /g) > BIR-H (89.8 m2 /g) > TOD (72.1 Table 1
Basic property of manganese oxides
Sample
AOSMn
PZCa
SSA (m2 /g)
Compositionb
CRY BIR-H TOD BIR-OH
3.82 3.86 3.64 3.87
2.15 1.78 3.50 3.35
175.2 89.8 72.1 19.6
K0.10 MnO1.96 (H2 O)0.41 K0.21 MnO2.03 (H2 O)0.71 Mg0.15 MnO1.97 (H2 O)0.83 Na0.32 MnO2.10 (H2 O)0.87
a
Determined by fast potentiometric titration method. Calculated from elemental composition and AOSMn . AOSMn : average oxidation state of Mn; PZC: point of zero charge; SSA specific surface area. b
Fig. 2
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m2 /g) > BIR-OH (19.6 m2 /g), which is in good agreement with results of the TEM images. 2.2 Direct degradation of phenol in the absence of Mn oxide Figure 3 shows the conversion rate of phenol (RPh-OH ) and TOC (RTOC ) with or without light irradiation in the absence of Mn oxide. Without light irradiation, the RPh-OH was only 0.2% after 12 hr. However, irradiation greatly elevated RPh-OH as well as RTOC . After 12 hr, the RPh-OH and RTOC were 99.4% and 12.3%, respectively. 2.3 Degradation of phenol on Mn oxides The kinetics curves of phenol degradation on synthesized Mn oxide with or without light irradiation are shown in Fig. 4. In the dark condition, the phenol degradation on CRY and BIR-H steadily increased with time, and their RPh-OH was 11.5% and 11.8% after 12 hr, respectively. The corresponding RTOC was 6.3% and 11.5%, respectively. R Ph-OH with light R TOC with light R Ph-OH in dark
100
Conversion rate (%)
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80 60 40 20 0 0
2
4
6 8 10 12 Time (hr) Fig. 3 Kinetics curves of conversion of phenol and TOC in absence of Mn oxide.
a
b
c
d
TEM images of the tested manganese oxides. (a) CRY; (b) BIR-H; (c) TOD; (d) BIR-OH.
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Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis
100
100 CRY
BIR-H
R (%)
R (%)
RPh-OH with light
80
80
60 RPh-OH with light
40
RTOC with light RPh-OH in dark RTOC in dark
60
40
RTOC with light RPh-OH in dark RTOC in dark
20
20
0
0 0
2
4
8
10
12
6 8 Time (hr) Fig. 4 Kinetics curves of degradation of phenol and TOC on Mn oxide.
10
12
6 Time (hr)
8
10
0
12
100
2
4
6 Time (hr)
100 TOD
BIR-OH RPh-OH with light
80
60
RPh-OH with light
80
RTOC with light RPh-OH in dark RTOC in dark
R (%)
R (%)
1907
RTOC with light RPh-OH in dark RTOC in dark
60
40
40
20
20
0
0 0
2
4
6 Time (hr)
8
10
12
Compared to CRY and BIR-H, BIR-OH and TOD exhibited a weaker ability to degrade phenol, and the RPh-OH was only 4.1% and 2.8% after 12 hr. The corresponding RTOC was only 4.0% and 2.0%. With light irradiation for 12 hr, RPh-OH for these Mn oxide followed the order of CRY (92.1%) > BIR-H (77.3%) > BIR-OH (57.4%) > TOD (45.8%); the RTOC was in the order of: CRY (62.1%) > BIR-H (43.1%) > TOD (25.4%) > BIR-OH (22.5%). These results proved that irradiation could effectively enhance the degradation of phenol on Mn oxide, especially for BIR-H and CRY.
3 Discussion 3.1 Volatility and photo-degradation of phenol Phenol is a kind of volatile organic compound (VOC), and can be directly degraded under illumination (Celin et al., 2003). Thus, it is important to measure the evaporation and photolysis of phenol. In dark experiments, there was no evident increase in RPh-OH corresponding to the time of air ventilation, meaning air ventilation did not enhance the evaporation of phenol and that air almost did not oxidize phenol after 12 hr ventilation. Although RPh-OH was 99.4% after 12 hr of light exposure, only 12.3% of TOC content was removed, implying that most phenol was degraded to an intermediate but not CO2 and H2 O. There
0
2
4
are two possible mechanisms for direct photo-degradation of phenol: (1) direct photolysis of phenol (Zepp and Cline, 1977), or (2) the oxygen in the solution may first be oxidized into ozone or other secondary oxidants, and those oxidants further oxidize phenol (Legrini et al., 1993). 3.2 Adsorption of phenol and intermediates on Mn oxides Mn oxide always exhibits strong adsorption capability (Takahashi et al., 2007), so whether it adsorbs phenol or its intermediates will directly impact the evaluation of photocatalysis. The FT-IR spectra of Mn oxide before and after the reaction are shown in Fig. 5. No obvious changes were observed in the spectra before or after the reaction, which demonstrated that almost no phenol or its intermediates were absorbed by the tested Mn oxide. This may be due to the same charges of Mn oxide, phenol and intermediates. Mn oxide possesses a great number of negative charges at pH 6.00 due to its low PZC value (Table 1). In addition, organic compounds always are electrically neutral or negatively charged in solution due to dissociation of H+ (Cavani et al., 2010; Guo et al., 2006; Yang et al., 2008). Electrostatic repulsion made against adsorption of phenol and intermediates on the surfaces of Mn oxide has very little impact on the degradation of phenol and TOC, whether with or without light irradiation.
Journal of Environmental Sciences 2011, 23(11) 1904–1910 / Qin Zhang et al.
1908
1.6
1.6
Absorbance
528 1.2 467 727
1657 1554
3425 2351
0.8
a
1.4
b
1.2
Absorbance
CRY
1.4
1.0
c
0.6
0.2
0.4 1000
2000 3000 Wavenumber (cm-1)
1120 1024
a
1632
b c
0.2
4000
1.0
1000
2000 3000 Wavenumber (cm-1)
4000
2.0 3402 3244 a
644 764
1634
2351 b c
0.4
BIR-OH
1.8 Absorbance
TOD
517 434555 0.8 Absorbance
512
3427
0.8 0.6
0.0
BIR-H
1.0
0.4
0.6
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a
1.6 1.4 1.2
451496
b
1632 1554 1412
3442
1.0
c
0.8 0.6
0.2
0.4 0.0
0.2 2000 3000 4000 1000 2000 3000 4000 -1 Wavenumber (cm ) Wavenumber (cm-1) Fig. 5 FT-IR spectra of the tested Mn oxides. Line a: before reaction; line b: after reaction without UV; line c: after reaction with UV. 1000
3.3 Oxidation of phenol on Mn oxides In the dark experiment, the degradation of phenol on Mn oxide resulted from the overall effect of adsorption and catalytic oxidation of Mn oxide. As described above, the adsorption of phenol and its intermediates on Mn oxide is ignorable, so the oxidation of Mn oxide was the main mechanism for phenol degradation. This indicates that oxidation of phenol could occur at low temperatures by obtaining electrons by Mn(III, IV), or at a high temperatures (> 140°C) by releasing lattice oxygen (Luo et al., 2008). It is impossible to release lattice oxygen at the temperature (25°C) in this study, so the phenol oxidation in the dark reaction is caused by reduction of Mn(III, IV). Based on the RTOC , the oxidation capability of Mn oxide follows the order of BIR-H > CRY > BIR-OH >TOD, which is consistent with our previous reports about oxidation of Cr(III) and As(III) (Feng et al., 2006, 2007). In addition,
the changes in Mn2+ concentration during reaction are measured (Fig. 6a). All the Mn2+ concentrations during reaction are all very low and below 1.0 mg/L, which is consistent with the results of low RTOC . In general, Mn oxide has a low ability to oxidize phenol. 3.4 Photocatalytic oxidation of Mn oxides The photocatalysis of Mn oxide (RPhoto ) can be evaluated by the difference of RTOC with and without light (Table 2). After a 12 hr reaction, the RPhoto for BIR-OH, TOD, BIR-H and CRY is as high as 17.9%, 23.4%, 31.9% and 55.8%, respectively. However, considering the removal of TOC under direct photolysis, these differences cannot be totally attributed to photocatalysis of Mn oxide. As RTOC under direct photolysis was found to be 12.3% after 12 hr light irradiation (Fig. 4), the net RPhoto for BIR-OH, TOD, BIRH and CRY, which are obtained by subtracting 12.3%, is
2.0
20 CRY BIR-H
1.5
With light Mn2+ concentration (mg/L)
Mn2+ concentration (mg/L)
In dark
TOD BIR-OH
1.0
0.5
0.0
15
10
5
0
CRY BIR-H
0 3 6 9 6 9 12 Time (hr) Time (hr) Fig. 6 Changes in Mn2+ concentration in solution during degradation of phenol by Mn oxides in dark or with light. 0
3
TOD BIR-OH 12
No. 11 Table 2
Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis Effect of irradiation on the degradation of phenol with Mn oxides after 12 hr reaction
2.2
CRY
BIR-H
TOD
BIR-OH
2.0
RPh-OH in dark (A) RPh-OH with light (B) (B)–(A) RTOC in dark (A ) RTOC with light (B ) RPhoto ((B )–(A ))
11.5 92.1 80.6 6.3 62.1 55.8
11.8 77.3 65.5 11.5 43.4 31.9
2.8 45.8 43.0 2.0 25.4 23.4
4.1 57.4 53.3 4.6 22.5 17.9
1.8 Absorbance
Conversion rate (%)
as high as 5.6%, 11.1%, 19.6% and 43.5%, respectively. Hence, the capacity of photocatalysis can be ordered as CRY > BIR-H > TOD > BIR-OH. Considering the loss of light intensity caused by the presence of mineral particles, the real contribution of photocatalytic oxidation of Mn oxide may be higher than the net RPhoto . Accordingly, it is evident that all tested Mn oxide possesses photolysis ability, which is greatly affected by the crystal structure. Moreover, Fig. 6 shows the changes in Mn2+ concentration during reaction with light. The Mn2+ concentrations in the reaction with light are much higher than those without light, and all are above 10.2 mg/L after 12 hr reaction. These indicate the different mechanism in the reaction with light. Due to the low oxidation capability of Mn oxides on phenol, a small amount of Mn4+ reduce to Mn2+ , so the Mn2+ concentration is low. However, the photocatalytical reaction occurred due to excitation of the occupied electrons on O-2p of the Mn oxides to the unoccupied orbits (mainly Mn-3d) by the irradiation. Meantime, a positively-charged hole were formed on the Mn atom, which was quickly filled by excited electrons and reduced Mn(IV) to highly soluble Mn(III) or Mn(II) (Kwon et al., 2009). Hence, the Mn2+ concentration in the reaction with light increased with the reaction time, and was much higher than that in the dark reaction because of high photocatalytical capability of Mn oxides. Usually, the photocatalytic activities increased with the SSA because large SSA means more active reaction sites. In our experiments, the photocatalytic activities and SSA of Mn oxide minerals exhibited the same order of CRY > BIR-H > TOD > BIR-OH, indicating the great effect of SSA on photocatalytic activities. However, it is noteworthy that the SSA of TOD (72.1 m2 /g) is much higher than that of BIR-OH (19.6 m2 /g) but the RTOC of TOD (23.4%) is little high than that of BIR-OH (17.9%). Obviously, besides the SSA, the crystal structure of Mn oxide also plays an important role in the photocatalytic activity of Mn oxides. In addition, the optical absorption behaviors of Mn oxides significantly affect the photocatalytic activity of the photocatalyst (Asahi et al., 2001; Li et al., 2008). UVVis RDS spectra of the Mn oxide (Fig. 7) show that the light absorbance of CRY and BIR-H are far greater than those of TOD and BIR-OH. The increase in absorption intensity promotes photocatalytic activity due to the enhanced formation of photoelectrons and photoholes (Asahi et al., 2001), so the higher photocatalytic activity of CRY and BIR-H may be partially caused by their higher light adsorption. Although both CRY and BIR-H have strong absorption between 200 and 500 nm, the absorption of
1909
CRY
1.6 BIR-H TOD
1.4
BIR-OH
1.2 1.0 200
300
400
500 600 700 800 Wavelength (nm) Fig. 7 UV-Vis DRS spectra of the tested manganese oxides.
CRY in the range of 500–800 nm is much greater than that of BIR-H. In addition, the mercury high-pressure lamp used in this study has high irradiation at 366, 436, 546 and 578 nm, and may have resulted in the high photocatalytic oxidation of CRY.
4 Conclusions In a dark condition, the degradation of phenol was low for BIR-OH, TOD, BIR-H and CRY. Light irradiation significantly improved the degradation of phenol and TOC on Mn oxides, and the photocatalysis of Mn oxides were greatly impacted by their crystal structure. After 12 hr of light irradiation, the capacities of photocatalysis were in the order of CRY > BIR-H > TOD > BIR-OH. The strong photocatalysis of CRY might be contributed to its strong absorption in a wide range of wavelengths. In addition, there are three possible mechanisms of photochemical degradation of phenol in the presence of Mn oxide under light irradiation: direct photolysis of phenol, oxidation by Mn oxide, and photocatalytic oxidation assisted by Mn oxide. Among these mechanisms, photocatalytic oxidation was the dominant one. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 40830527, 40771102), and the New Century Excellent Talents in University of China (No. NCET-09-0399).
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Li S J, Ma Z C, Zhang J, Wu Y S, Gong Y M, 2008. A comparative study of photocatalytic degradation of phenol of TiO2 and ZnO in the presence of manganese dioxides. Catalysis Today, 139(1-2): 109–112. Luo J, Zhang Q H, Garcia-Martinez J, Suib S L, 2008. Adsorptive and acidic properties, reversible lattice oxygen evolution, and catalytic mechanism of cryptomelane-type manganese oxides as oxidation catalysts. Journal of the American Chemical Society, 130(10): 3198–3207. Machida M, Murakami S, Kijima T, Matsushima S, Arai M, 2001. Photocatalytic property and electronic structure of lanthanide tantalates, LnTaO4 (Ln = La, Ce, Pr, Nd, and Sm). Journal of Physical Chemistry B, 105(16): 3289– 3294. Post J E, 1999. Manganese oxide minerals: crystal structures and economic and environmental significance. Proceedings of the National Academy of Sciences of the United States of America, 96(7): 3447–3454. Sakai N, Ebina Y, Takada K, Sasaki T, 2005. Photocurrent generation from semiconducting manganese oxide nanosheets in response to visible light. Journal of Physical Chemistry B, 109(19): 9651–9655. Segal S R, Suib S L, Tang X, Satyapal S, 1999. Photoassisted decomposition of dimethyl methylphosphonate over amorphous manganese oxide catalysts. Chemistry of Materials, 11(7): 1687–1695. Sherman D M, 2005. Electronic structures of iron(III) and manganese(IV) (hydr)oxide minerals: thermodynamics of photochemical reductive dissolution in aquatic environments. Geochimica et Cosmochimica Acta, 69(13): 3249–3255. Suib S L, 2008. Porous manganese oxide octahedral molecular sieves and octahedral layered materials. Accounts of Chemical Research, 41(4): 479–487. Takahashi Y, Manceau A, Geoffroy N, Marcus M A, Usui A, 2007. Chemical and structural control of the partitioning of Co, Ce, and Pb in marine ferromanganese oxides. Geochimica et Cosmochimica Acta, 71(4): 984–1008. Tan W F, Lu S J, Liu F, Feng X H, He J Z, Koopall L K, 2008. Determination of the point-of-zero, charge of manganese oxides with different methods including an improved salt titration method. Soil Science, 173(4): 277–286. Yang S X, Zhu W P, Wang J B, Chen Z X, 2008. Catalytic wet air oxidation of phenol over CeO2 -TiO2 catalyst in the batch reactor and the packed-bed reactor. Journal of Hazardous Materials, 153(3): 1248–1253. Zepp R G, Cline D M, 1977. Rates of direct photolysis in aquatic environment. Environmental Science & Technology, 11(4): 359–366.
Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis Qin Zhang1,2 , Xiaodi Cheng1 , Chen Zheng1 , Xionghan Feng1 , Guohong Qiu1 , Wenfeng Tan1 , Fan Liu1,∗ 1. Key Laboratory of Subtropical Agriculture and Environment, Ministry of Agriculture, College of Resources & Environment, Huazhong Agricultural University, Wuhan 430070, China. E-mail: chincheung [email protected] 2. College of Land Resource and Environment, Jiangxi Agricultural University, Nanchang 330045, China Received 22 December 2010; revised 26 February 2011; accepted 17 March 2011
Abstract Manganese oxides are known as one type of semiconductors, but their photocatalysis characteristics have not been deeply explored. In this study, photocatalytic degradation of phenol using several synthesized manganese oxides, i.e, acidic birnessite (BIR-H), alkaline birnessite (BIR-OH), cryptomelane (CRY) and todorokite (TOD), were comparatively investigated. To elucidate phenol degradation mechanisms, X-ray diffraction (XRD), ICP-AES (inductively coupled plasma-atomic emission spectroscopy), TEM (transmission electronic microscope), N2 physisorption at 77 K and UV-visible diffuse reflectance spectroscopy (UV-Vis DRS) were employed to characterize the structural, compositional, morphological, specific surface area and optical absorption properties of the manganese oxides. After 12 hr of UV-Vis irradiation, the total organic carbon (TOC) removal rate reached 62.1%, 43.1%, 25.4%, and 22.5% for cryptomelane, acidic birnessite, todorokite and alkaline birnessite, respectively. Compared to the reactions in the dark condition, UVVis exposure improved the TOC removal rates by 55.8%, 31.9%, 23.4% and 17.9%. This suggests a weak ability of manganese oxides to degrade phenol in the dark condition, while UV-Vis light irradiation could significantly enhance phenol degradation. The manganese minerals exhibited photocatalytic activities in the order of: CRY > BIR-H > TOD > BIR-OH. There may be three possible mechanisms for photochemical degradation: (1) direct photolysis of phenol; (2) direct oxidation of phenol by manganese oxides; (3) photocatalytic oxidation of phenol by manganese oxides. Photocatalytic oxidation of phenol appeared to be the dominant mechanism. Key words: manganese oxides; adsorption; oxidation; photocatalysis; degradation; phenol DOI: 10.1016/S1001-0742(10)60655-9 Citation: Zhang Q, Cheng X D, Zheng C, Feng X H, Qiu G H, Tan W F, Liu F, 2011. Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis. Journal of Environmental Sciences, 23(11): 1904–1910
Introduction Manganese (Mn) oxide is a class of active oxides widespread in the environment (Banerjee and Nesbitt, 1999). Because Mn oxide possessed special physiochemical properties, severe defects and nonstoichiometry in the structure, it has been extensively studied as adsorbent, oxidant, catalysis and rechargeable battery (Post, 1999; Suib, 2008). Recently, the photocatalysis property of Mn oxide has attracted increasing attention due to its semiconducting electronic structure (Iyer et al., 2010; Kwon et al., 2009). Photocatalysis property is determined by the electronic structure of the catalyst (Machida et al., 2001). In Mn oxide, d–d electronic transitions are known to occur under illumination due to partially filled d-orbital, so Mn oxide can be effectively used as photocatalyst without doping of other cations (Iyer et al., 2010). The photochemical reaction occurred due to excitation of the occupied * Corresponding author. E-mail: [email protected]
electrons on O-2p of the Mn oxide to the unoccupied orbits (mainly Mn-3d) when the light energy was greater than the difference of the orbit energy gap (Kwon et al., 2009; Sherman, 2005). It is proved by the fact that the photo-induced electric current of Mn oxide was detected (Sakai et al., 2005). Mn oxide have been used to photocatalytically degrade 2-propanol, halogenated methane and dimethyl methylphosphonate (DMMP) in gas phase under visible illumination (Cao and Suib, 1994; Chen et al., 1997; Segal et al., 1999). Under UV illumination, CRY can more effectively photocatalytically degrade 2,4dichlorophenoxyacetic acid and Methylene Blue than TiO2 (P-25 Degussa), it may be caused by the polarity of Methylene Blue and the hydrophobicity of CRY (Lemus et al., 2008). It indicated that the photocatalysis activity of Mn oxide might be even greater than TiO2 or ZnO because their special photochemical characteristics. Most studies on photocatalysis of Mn oxide have mainly focused on gaseous photocatalytic degradation of organic compound, but there is rare research on photocatalysis of Mn oxide
Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis
in solution. In addition, because Mn oxide has significant capabilities in adsorption, oxidation and photocatalysis, many reactions may be involved in degradation of organic compounds in water, including: (1) surface adsorption; (2) oxidation-reduction; (3) photocatalytic oxidation. These reactions occur rapidly and simultaneously, and often difficult to distinguish the primary from the secondary. All these reactions constitute the foundation of the degradation of organic compounds on Mn oxide, so it is important to evaluate the photocatalysis of Mn oxide for its potential applications in pollution control and treatment. Phenol commonly exists in the environment with highly toxicity, structurally stability, resistance to biodegradation (Kavitha and Palanivelu, 2004; Yang et al., 2008), and phenol is also an intermediate of degradation of aromatic polymer (Gondal et al., 2007). Therefore, phenol has been used as a model compound in simulating organic pollutants in studies (Guo et al., 2006). In this study, the degradation of phenol by several common Mn oxides with or without light irradiation was explored, and the effects of mineral structure and underlying reaction mechanism were also investigated. Results from this study can provide evidence of environmental chemical behavior of Mn minerals in the environment, and provide a new approach to the control and treatment of organic pollutant in wastewater.
1 Materials and methods 1.1 Synthesis of Mn oxide Acidic birnessite (BIR-H), alkaline birnessite (BIROH), cryptomelane (CRY) and todorokite (TOD) were synthesized as reported in our previous work (Feng et al., 2004a, 2004b, 2007). All synthesized Mn oxide was washed with deionized water until conductivity was measured to less than 20 μS/cm, then freeze-dried, ground into powder, and stored in bottles. 1.2 Characterization of Mn oxide XRD was conducted on D8 Advance X-ray diffractometer (Bruker, Germany) with Cu Kα (λ = 0.15406) radiation at 40 kV tube voltage, 40 mA tube current, 1˚/min scanning speed, and step size of 0.02˚. The micro-morphology was observed using JEM-2010 FEF transmission electron microscope (Jeol, Japan). The specific surface area (SSA) was determined using Autosorb-1 automatic specific surface area analyzer (Quantachrome, USA). The UV-Vis diffuse reflectance spectrum (UV-Vis RDS) was determined by Shimadzu 3600 spectrophotometer (Shimadzu, Japan). The average oxidation state of Mn (AOSMn ) and the determination of elemental composition were measured as reported in our previous work (Feng et al., 2004b). The point of zero charge (PZC) was measured by the rapid potentiometric titration method (Tan et al., 2008). The concentration of Mn2+ was analyzed with an atomic absorption spectrometer (Varian AA 240 FS, USA). 1.3 Evaluation of photocatalysis Five milliliter phenol solution (500 mg/L) (pH 6.0, preadjusted by HCl or NaOH solution) was added into 20
1905
mL 1.25 g/L Mn mineral suspension (pH 6.0) in a 50-mL quartz tube. A total of 12 quartz tubes were placed on a SGY-1 rotary photochemical reactor (Nanjing Stonetech Electric Equipment Company, China) and ventilated with air at a steady rate of 3.3 L/min. Among the 12 tubes, two tubes were used for kinetics study, and the other 10 tubes were used to measure total organic carbon (TOC) at different time intervals. The samples were centrifuged at 14,000 r/min, and then the supernatant was filtered through a 0.22-μm membrane for analyses of phenol and TOC. The residual mineral was rinsed three times and freeze-dried for FT-IR determination. All these experiments were conducted with or without light radiation. Each treatment had two replicates, and averages were recorded. To measure the volatility and direct photolysis of phenol, the blank control experiment was set up only in the absence of Mn oxide. Analysis of phenol was carried out on a highperformance liquid chromatography (HPLC 1200, Agilent, USA). Aliquots of 10 μL were injected to the HPLC, running with the mobile phase of acetonitrile/water (V/V) at 10/90. The separation was performed using an XDB C18 reversed phase column (Agilent, USA) at the flow rate of 1.0 mL/min and column temperature of 30°C. A fluorescence detector was used with the excitation wavelength and emission wavelength set at 270 and 300 nm, respectively. The TOC was determined using a Multi 3100 total organic carbon/nitrogen analyzer (Jena, Germany). The conversion rate R of phenol and TOC was calculated by Eq. (1): R = (C0 − Ct )/C0 × 100
(1)
where, C 0 (mg/L) and C t (mg/L) are the initial and final phenol or TOC concentrations, respectively. It is used to evaluate the activity of photocatalysis.
2 Results 2.1 Characterization of synthesized Mn oxides The XRD spectra of the synthesized Mn oxides (Fig. 1) showed that the diffraction peaks of CRY were in good agreement with the characteristic peaks (JCPDS 74-1451) of monoclinic cryptomelane. The diffraction peaks of BIR-H matched well with hexagonal birnessite (JCPDS 86-0666). The XRD pattern of TOD was similar to JCPDS
Intensity (CPS)
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BIR-OH TOD CRY BIR-H 10
20
30
40 50 60 70 80 2θ (degree) Fig. 1 Powder X-ray diffraction patterns of the tested manganese oxides.
Journal of Environmental Sciences 2011, 23(11) 1904–1910 / Qin Zhang et al.
87-0389, confirming monoclinic todorokite. All the peaks in the XRD pattern of BIR-OH can be attributed to monoclinic alkaline birnessite (JCPDS 43-1456). TEM images for synthesized Mn oxide in this study are shown in Fig. 2. The morphology of CRY was mainly tiny needles sized from tens nm to hundreds nm. BIR-H was shaped in aggregates of lamellar crystals layered over each other. TOD had morphology of platy trilling patterns, while BIROH was mainly stacked hexahedral sheet crystals. All these results are consistent with previous report (Feng et al., 2007), and TEM images indicated the particle size of Mn oxide is in a sequence of BIR-OH > TOD > BIR-H > CRY. Table 1 shows some other properties of synthesized Mn oxide, such as AOSMn , chemical composition, PZC, and SSA. The AOSMn of CRY, BIR-H and BIR-OH had relatively higher AOSMn values, ranging from 3.82 to 3.87, whereas TOD had a relatively low AOSMn value (3.67). These AOSMn and stoichiometry resembled those of Mn oxide reported in other literature (Feng et al., 2004a, 2004b, 2007). The PZC values of all Mn oxide were low, and in the order of BIR-H (1.78) < CRY (2.15) < BIROH (3.35) < TOD (3.50). These results were very close to our previous report (Tan et al., 2008). For SSA, the order was CRY (175.2 m2 /g) > BIR-H (89.8 m2 /g) > TOD (72.1 Table 1
Basic property of manganese oxides
Sample
AOSMn
PZCa
SSA (m2 /g)
Compositionb
CRY BIR-H TOD BIR-OH
3.82 3.86 3.64 3.87
2.15 1.78 3.50 3.35
175.2 89.8 72.1 19.6
K0.10 MnO1.96 (H2 O)0.41 K0.21 MnO2.03 (H2 O)0.71 Mg0.15 MnO1.97 (H2 O)0.83 Na0.32 MnO2.10 (H2 O)0.87
a
Determined by fast potentiometric titration method. Calculated from elemental composition and AOSMn . AOSMn : average oxidation state of Mn; PZC: point of zero charge; SSA specific surface area. b
Fig. 2
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m2 /g) > BIR-OH (19.6 m2 /g), which is in good agreement with results of the TEM images. 2.2 Direct degradation of phenol in the absence of Mn oxide Figure 3 shows the conversion rate of phenol (RPh-OH ) and TOC (RTOC ) with or without light irradiation in the absence of Mn oxide. Without light irradiation, the RPh-OH was only 0.2% after 12 hr. However, irradiation greatly elevated RPh-OH as well as RTOC . After 12 hr, the RPh-OH and RTOC were 99.4% and 12.3%, respectively. 2.3 Degradation of phenol on Mn oxides The kinetics curves of phenol degradation on synthesized Mn oxide with or without light irradiation are shown in Fig. 4. In the dark condition, the phenol degradation on CRY and BIR-H steadily increased with time, and their RPh-OH was 11.5% and 11.8% after 12 hr, respectively. The corresponding RTOC was 6.3% and 11.5%, respectively. R Ph-OH with light R TOC with light R Ph-OH in dark
100
Conversion rate (%)
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80 60 40 20 0 0
2
4
6 8 10 12 Time (hr) Fig. 3 Kinetics curves of conversion of phenol and TOC in absence of Mn oxide.
a
b
c
d
TEM images of the tested manganese oxides. (a) CRY; (b) BIR-H; (c) TOD; (d) BIR-OH.
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Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis
100
100 CRY
BIR-H
R (%)
R (%)
RPh-OH with light
80
80
60 RPh-OH with light
40
RTOC with light RPh-OH in dark RTOC in dark
60
40
RTOC with light RPh-OH in dark RTOC in dark
20
20
0
0 0
2
4
8
10
12
6 8 Time (hr) Fig. 4 Kinetics curves of degradation of phenol and TOC on Mn oxide.
10
12
6 Time (hr)
8
10
0
12
100
2
4
6 Time (hr)
100 TOD
BIR-OH RPh-OH with light
80
60
RPh-OH with light
80
RTOC with light RPh-OH in dark RTOC in dark
R (%)
R (%)
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RTOC with light RPh-OH in dark RTOC in dark
60
40
40
20
20
0
0 0
2
4
6 Time (hr)
8
10
12
Compared to CRY and BIR-H, BIR-OH and TOD exhibited a weaker ability to degrade phenol, and the RPh-OH was only 4.1% and 2.8% after 12 hr. The corresponding RTOC was only 4.0% and 2.0%. With light irradiation for 12 hr, RPh-OH for these Mn oxide followed the order of CRY (92.1%) > BIR-H (77.3%) > BIR-OH (57.4%) > TOD (45.8%); the RTOC was in the order of: CRY (62.1%) > BIR-H (43.1%) > TOD (25.4%) > BIR-OH (22.5%). These results proved that irradiation could effectively enhance the degradation of phenol on Mn oxide, especially for BIR-H and CRY.
3 Discussion 3.1 Volatility and photo-degradation of phenol Phenol is a kind of volatile organic compound (VOC), and can be directly degraded under illumination (Celin et al., 2003). Thus, it is important to measure the evaporation and photolysis of phenol. In dark experiments, there was no evident increase in RPh-OH corresponding to the time of air ventilation, meaning air ventilation did not enhance the evaporation of phenol and that air almost did not oxidize phenol after 12 hr ventilation. Although RPh-OH was 99.4% after 12 hr of light exposure, only 12.3% of TOC content was removed, implying that most phenol was degraded to an intermediate but not CO2 and H2 O. There
0
2
4
are two possible mechanisms for direct photo-degradation of phenol: (1) direct photolysis of phenol (Zepp and Cline, 1977), or (2) the oxygen in the solution may first be oxidized into ozone or other secondary oxidants, and those oxidants further oxidize phenol (Legrini et al., 1993). 3.2 Adsorption of phenol and intermediates on Mn oxides Mn oxide always exhibits strong adsorption capability (Takahashi et al., 2007), so whether it adsorbs phenol or its intermediates will directly impact the evaluation of photocatalysis. The FT-IR spectra of Mn oxide before and after the reaction are shown in Fig. 5. No obvious changes were observed in the spectra before or after the reaction, which demonstrated that almost no phenol or its intermediates were absorbed by the tested Mn oxide. This may be due to the same charges of Mn oxide, phenol and intermediates. Mn oxide possesses a great number of negative charges at pH 6.00 due to its low PZC value (Table 1). In addition, organic compounds always are electrically neutral or negatively charged in solution due to dissociation of H+ (Cavani et al., 2010; Guo et al., 2006; Yang et al., 2008). Electrostatic repulsion made against adsorption of phenol and intermediates on the surfaces of Mn oxide has very little impact on the degradation of phenol and TOC, whether with or without light irradiation.
Journal of Environmental Sciences 2011, 23(11) 1904–1910 / Qin Zhang et al.
1908
1.6
1.6
Absorbance
528 1.2 467 727
1657 1554
3425 2351
0.8
a
1.4
b
1.2
Absorbance
CRY
1.4
1.0
c
0.6
0.2
0.4 1000
2000 3000 Wavenumber (cm-1)
1120 1024
a
1632
b c
0.2
4000
1.0
1000
2000 3000 Wavenumber (cm-1)
4000
2.0 3402 3244 a
644 764
1634
2351 b c
0.4
BIR-OH
1.8 Absorbance
TOD
517 434555 0.8 Absorbance
512
3427
0.8 0.6
0.0
BIR-H
1.0
0.4
0.6
Vol. 23
a
1.6 1.4 1.2
451496
b
1632 1554 1412
3442
1.0
c
0.8 0.6
0.2
0.4 0.0
0.2 2000 3000 4000 1000 2000 3000 4000 -1 Wavenumber (cm ) Wavenumber (cm-1) Fig. 5 FT-IR spectra of the tested Mn oxides. Line a: before reaction; line b: after reaction without UV; line c: after reaction with UV. 1000
3.3 Oxidation of phenol on Mn oxides In the dark experiment, the degradation of phenol on Mn oxide resulted from the overall effect of adsorption and catalytic oxidation of Mn oxide. As described above, the adsorption of phenol and its intermediates on Mn oxide is ignorable, so the oxidation of Mn oxide was the main mechanism for phenol degradation. This indicates that oxidation of phenol could occur at low temperatures by obtaining electrons by Mn(III, IV), or at a high temperatures (> 140°C) by releasing lattice oxygen (Luo et al., 2008). It is impossible to release lattice oxygen at the temperature (25°C) in this study, so the phenol oxidation in the dark reaction is caused by reduction of Mn(III, IV). Based on the RTOC , the oxidation capability of Mn oxide follows the order of BIR-H > CRY > BIR-OH >TOD, which is consistent with our previous reports about oxidation of Cr(III) and As(III) (Feng et al., 2006, 2007). In addition,
the changes in Mn2+ concentration during reaction are measured (Fig. 6a). All the Mn2+ concentrations during reaction are all very low and below 1.0 mg/L, which is consistent with the results of low RTOC . In general, Mn oxide has a low ability to oxidize phenol. 3.4 Photocatalytic oxidation of Mn oxides The photocatalysis of Mn oxide (RPhoto ) can be evaluated by the difference of RTOC with and without light (Table 2). After a 12 hr reaction, the RPhoto for BIR-OH, TOD, BIR-H and CRY is as high as 17.9%, 23.4%, 31.9% and 55.8%, respectively. However, considering the removal of TOC under direct photolysis, these differences cannot be totally attributed to photocatalysis of Mn oxide. As RTOC under direct photolysis was found to be 12.3% after 12 hr light irradiation (Fig. 4), the net RPhoto for BIR-OH, TOD, BIRH and CRY, which are obtained by subtracting 12.3%, is
2.0
20 CRY BIR-H
1.5
With light Mn2+ concentration (mg/L)
Mn2+ concentration (mg/L)
In dark
TOD BIR-OH
1.0
0.5
0.0
15
10
5
0
CRY BIR-H
0 3 6 9 6 9 12 Time (hr) Time (hr) Fig. 6 Changes in Mn2+ concentration in solution during degradation of phenol by Mn oxides in dark or with light. 0
3
TOD BIR-OH 12
No. 11 Table 2
Roles of manganese oxides in degradation of phenol under UV-Vis irradiation: Adsorption, oxidation, and photocatalysis Effect of irradiation on the degradation of phenol with Mn oxides after 12 hr reaction
2.2
CRY
BIR-H
TOD
BIR-OH
2.0
RPh-OH in dark (A) RPh-OH with light (B) (B)–(A) RTOC in dark (A ) RTOC with light (B ) RPhoto ((B )–(A ))
11.5 92.1 80.6 6.3 62.1 55.8
11.8 77.3 65.5 11.5 43.4 31.9
2.8 45.8 43.0 2.0 25.4 23.4
4.1 57.4 53.3 4.6 22.5 17.9
1.8 Absorbance
Conversion rate (%)
as high as 5.6%, 11.1%, 19.6% and 43.5%, respectively. Hence, the capacity of photocatalysis can be ordered as CRY > BIR-H > TOD > BIR-OH. Considering the loss of light intensity caused by the presence of mineral particles, the real contribution of photocatalytic oxidation of Mn oxide may be higher than the net RPhoto . Accordingly, it is evident that all tested Mn oxide possesses photolysis ability, which is greatly affected by the crystal structure. Moreover, Fig. 6 shows the changes in Mn2+ concentration during reaction with light. The Mn2+ concentrations in the reaction with light are much higher than those without light, and all are above 10.2 mg/L after 12 hr reaction. These indicate the different mechanism in the reaction with light. Due to the low oxidation capability of Mn oxides on phenol, a small amount of Mn4+ reduce to Mn2+ , so the Mn2+ concentration is low. However, the photocatalytical reaction occurred due to excitation of the occupied electrons on O-2p of the Mn oxides to the unoccupied orbits (mainly Mn-3d) by the irradiation. Meantime, a positively-charged hole were formed on the Mn atom, which was quickly filled by excited electrons and reduced Mn(IV) to highly soluble Mn(III) or Mn(II) (Kwon et al., 2009). Hence, the Mn2+ concentration in the reaction with light increased with the reaction time, and was much higher than that in the dark reaction because of high photocatalytical capability of Mn oxides. Usually, the photocatalytic activities increased with the SSA because large SSA means more active reaction sites. In our experiments, the photocatalytic activities and SSA of Mn oxide minerals exhibited the same order of CRY > BIR-H > TOD > BIR-OH, indicating the great effect of SSA on photocatalytic activities. However, it is noteworthy that the SSA of TOD (72.1 m2 /g) is much higher than that of BIR-OH (19.6 m2 /g) but the RTOC of TOD (23.4%) is little high than that of BIR-OH (17.9%). Obviously, besides the SSA, the crystal structure of Mn oxide also plays an important role in the photocatalytic activity of Mn oxides. In addition, the optical absorption behaviors of Mn oxides significantly affect the photocatalytic activity of the photocatalyst (Asahi et al., 2001; Li et al., 2008). UVVis RDS spectra of the Mn oxide (Fig. 7) show that the light absorbance of CRY and BIR-H are far greater than those of TOD and BIR-OH. The increase in absorption intensity promotes photocatalytic activity due to the enhanced formation of photoelectrons and photoholes (Asahi et al., 2001), so the higher photocatalytic activity of CRY and BIR-H may be partially caused by their higher light adsorption. Although both CRY and BIR-H have strong absorption between 200 and 500 nm, the absorption of
1909
CRY
1.6 BIR-H TOD
1.4
BIR-OH
1.2 1.0 200
300
400
500 600 700 800 Wavelength (nm) Fig. 7 UV-Vis DRS spectra of the tested manganese oxides.
CRY in the range of 500–800 nm is much greater than that of BIR-H. In addition, the mercury high-pressure lamp used in this study has high irradiation at 366, 436, 546 and 578 nm, and may have resulted in the high photocatalytic oxidation of CRY.
4 Conclusions In a dark condition, the degradation of phenol was low for BIR-OH, TOD, BIR-H and CRY. Light irradiation significantly improved the degradation of phenol and TOC on Mn oxides, and the photocatalysis of Mn oxides were greatly impacted by their crystal structure. After 12 hr of light irradiation, the capacities of photocatalysis were in the order of CRY > BIR-H > TOD > BIR-OH. The strong photocatalysis of CRY might be contributed to its strong absorption in a wide range of wavelengths. In addition, there are three possible mechanisms of photochemical degradation of phenol in the presence of Mn oxide under light irradiation: direct photolysis of phenol, oxidation by Mn oxide, and photocatalytic oxidation assisted by Mn oxide. Among these mechanisms, photocatalytic oxidation was the dominant one. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 40830527, 40771102), and the New Century Excellent Talents in University of China (No. NCET-09-0399).
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