Mechanism of Reductive Decomposition of Pentachlorophenol by Ti

Jun 28, 2010 - The reductive decomposition of pentachlorophenol (PCP) by photocatalysis with Ti-doped β-Bi2O3 was investigated under visible light (Î...
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Environ. Sci. Technol. 2010, 44, 5581–5586

Mechanism of Reductive Decomposition of Pentachlorophenol by Ti-Doped β-Bi2O3 under Visible Light Irradiation LIFENG YIN, JUNFENG NIU,* ZHENYAO SHEN, AND JING CHEN State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, P. R. China

and positive-hole (hvb+) pair (eq 1). The negative-electron reacts with oxygen molecules to form superoxide anions (eq 2). The positive-hole reacts with hydroxyls to form •OH (eq 3). The hydroxyl radical attacks and oxidizes the organic pollutant molecule to produce reaction intermediates (eq 4) and finally mineralization products (eq 5). + Cat. (photocatalyst) + hυ f ecb + hvb

(1)

ecb + O2 f O 2

(2)

+ hvb + OH- f •OH

(3)

•OH + S (organic pollutant) f Q (intermediates)

(4)

Received March 30, 2010. Revised manuscript received June 17, 2010. Accepted June 17, 2010.

•OH + Q f CO2 + H2O

(5)

The reductive decomposition of pentachlorophenol (PCP) by photocatalysis with Ti-doped β-Bi2O3 was investigated under visible light (λ > 420 nm) irradiation. The results indicated that hydroxyl radical (•OH) and singlet oxygen (1O2) could not be detected with electron spin resonance (ESR) on the photocatalyst under light irradiation. An electron scavenger weakened the photocatalytic activity of the photocatalyst for the decomposition of PCP; however, scavengers of reactive oxygen species (ROS) enhanced the activity. The decomposition intermediates of PCP detected by liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) suggested the existence of phenol, cyclohexanone, cyclohexanol, glycol, and propylene. All the evidence suggested that reductive dechlorination was the major route in the decomposition of PCP, during which the photogenerated electron under visible light irradiation acted as reductant. The reliability of the proposed reductive mechanism was further verified by comparing the reduction potential (Ere) of PCP with the conduction band potential (Ecb) of the photocatalyst. The decomposition pathway of PCP with electron reduction under visible light irradiation was also investigated.

The most powerful oxidative system of photocatalysis is based on the generation of •OH. However, the role of •OH in the photocatalytic decomposition of PCP under visible light irradiation has not been fully identified. Another elimination pathway for PCP in aqueous solution is reductive decomposition. Some researchers have reported the reductive dechlorination of PCP under anaerobic conditions at neutral pH (9, 10). In the presence of reductive reaction as a major step, PCP was sequentially degraded to low chlorinated phenols and sometimes even mineralized to methane and carbon dioxide. Reduction of PCP has also been found in the presence of enhanced zero valence iron (11), acidogenic sludge (12), and polycarboxylic acids (13). The negative-electron from photocatalysis is also a powerful reductant, however, the role of reduction reaction in the photocatalysis process of PCP is still unrevealed. The aim of this study was to investigate the decomposition mechanisms of PCP by Ti-doped β-Bi2O3 under visible light (λ > 420 nm) irradiation. Based on the above considerations, we addressed in the present study the following questions: (i) Whether or not PCP was decomposed by •OH or other redox species? (ii) What is the major photocatalytic decomposition pathway of PCP under visible light irradiation? (iii) What is the driving power for the first step of decomposition? To find out the detailed decomposition pathway for PCP, the identification of the reactive radicals and intermediates, electrochemical detection, and energy band calculation were conducted.

Introduction Pentachlorophenol (PCP) is a manufactured chemical widely used in herbicides, defoliants, pesticides, and wood preservatives. It is a highly toxic contaminant that damages the human immune system and reproductive system. Additionally, PCP exposed in the environment can be transformed to more toxic polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (1). To remove PCP from the environment, many techniques, involving discharge (2), biodegradation (3), direct photolysis (4), and photocatalysis (5-7), have been widely investigated. Photocatalytic degradation is an effective technique to eliminate PCP from aqueous solution. By using metalmodified TiO2 (5), NaBiO3 (6), or Bi2WO6 (7) as the photocatalyst, PCP can be decomposed under visible light or UV-A irradiation. Generally, the photocatalytic decomposition of organic pollutants is ascribed to the oxidation mechanism by hydroxyl radical (•OH) (8). When the photocatalyst absorbs light radiation, it will generate the negative-electron (ecb-) * Corresponding author telephone and fax: +86-10-5880 7612; e-mail: [email protected]. 10.1021/es101006s

 2010 American Chemical Society

Published on Web 06/28/2010

Experimental Section Materials. Nanoporous Ti-doped β-Bi2O3 (NTB) was prepared and used as the visible light responsive photocatalyst. The preparation of NTB was conducted via a solvothermal process with Bi(NO3)3 · 5H2O, Ti(OC4H9)4, and nonionic F108 (see Supporting Information). The band gap absorption edge of NTB was measured to be 90%. The column temperature began at 50 °C for 1 min, ramped to 270 at 20 °C/min, and kept at 270 °C for 2 min. Injection was performed in splitless mode using helium as a carrier gas at a rate of 5.0 mL/min (50 mL/min including makeup gas). According to the NIST mass spectral database, the mass spectra were collected by selective ion monitoring (SIM). Free Radical Measurements. Radicals were detected using a Bruker ESP-300E ESR spectrometer equipped with a quantaRay Nd:YAG laser system as the irradiation light source at λ 532 or 355 nm. ESR measurements were conducted at room temperature under the following conditions: center field of 3480.00 G, microwave power of 10 mW, receiver gain 100 000, modulation frequency 100.0 kHz, modulation amplitude 2.071 G, conversion of 40.96 ms, sweep width 100.0 G, sweep time 41.943 s. The solution of 5,5-dimethyl-1-pyrroline-Noxide (DMPO) was used as the spin trap for superoxide (O2-) and •OH (16, 17). The solution of 2,2,6,6-tetramethyl-4hydroxy-piperidinyloxy (TEMP) was employed to trap singlet oxygen (1O2) (18). The time between the reaction initiation and the onset of ESR scanning was controlled to less than 2 min. To further determine the reactive radicals involved in the photocatalytic system and ensure the presence of reactive oxygen species (ROS), three selective radical scavengers including formic acid, sodium azide, and hydrogen peroxide (H2O2), were used to capture 1O2, •OH, and hvb+, and electron formed in the photoreaction of PCP. Under the same conditions described in the Photocatalytic Decomposition section, certain amounts of formic acid, sodium azide, and H2O2 were added into the reaction solution before irradiation. 5582

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FIGURE 1. ESR signals for detecting •OH and O2- in dark (control), under irradiation at λ 532 nm, and at 355 nm. The determinations of O2- and •OH were conducted by using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin-trapping agent in dimethyl sulfoxide (DMSO) and aqueous solution, respectively. The concentration change of PCP was monitored and analyzed by HPLC. Electrochemical Detection. The redox property of PCP was investigated using a cyclic voltammetry procedure standardized by IUPAC (International Union of Pure and Applied Chemistry). An Ag/AgCl electrode (Fan Chern Co., Ltd., Taiwan) was used as a reference electrode. A platinum plate worked as the counter electrode. Cyclic voltammetry was conducted with a scanning potentiostat, EG&G model CHI660D. The voltage swept from -1.5 to +1.5 V and the scan rate was 5.0 mV/s. The experiments of cyclic voltammetry were carried out in water systems. Current-voltage curves were obtained for 20 mg/L PCP aqueous solution in the range described above.

Results and Discussion ESR Analysis. ROS have frequently been proposed as the intermediates in photocatalytic reactions. In our case, the formation of general intermediate species in the PCP decomposition process including O2-, 1O2, and •OH, was investigated. The ESR spin-trap technique was employed to detect the reactive species generated in NTB suspension under light irradiation at λ 532 and 355 nm. As shown in Figure 1, •OH could not be detected in the solution under the irradiation of UV-light (λ 355 nm) and visible light (λ 532 nm), which was different from the well-known photocatalytic oxidation mechanism for UV-light responsive photocatalysts.1O2 was not detected as well. However, in the experiments, O2- was detected with a strong signal, which might be from reduction of O2 (see eq 2). Actually, some studies have also reported that no •OH was detected as the reactive radical in their cases of photocatalysis. The redox species in the photocatalytic surrounding might also involve superoxide anion (19), singlet oxygen (20), and electron (21). Although O2- was detected in our experiments, it has not been made clear whether superoxide anion is strong enough to decompose certain chlorinated organic compounds (19, 20). As a matter of fact, it has been indicated that 2,4,6-dichlorophenol and PCP are too stable to be oxidized by O2- or its contributor H2O2 in aqueous solution (22). Thus, the obtained results from this study suggested that electron might be one of possible activated species in this photocatalytic system. Although •OH and 1O2 were not detected in the photocatalysis system via ESR, the role of •OH and 1O2 was further verified with a radical scavenger. If •OH and 1O2 acted as the key intermediate species to attack PCP, the use of radical

FIGURE 2. Effects of radical scavengers (a) and pH value (b) on the decomposition efficiency of PCP by Ti-doped β-Bi2O3 in aqueous solution. scavengers for ROS should weaken the photocatalytic efficiency. The effects of radical scavengers on PCP photodecomposition are shown in Figure 2a. Here formic acid, sodium azide, and hydrogen peroxide were employed as the scavengers for •OH,1O2, and electron (23-25), respectively. It was observed that the addition of 26.2 and 54.8 mM formic acid enhanced the photodecomposition efficiency of NTB and the addition of 2.0 and 4.0 mM sodium azide improved the photodecomposition efficiency slightly, which indicated that 1 O2, •OH, and the corresponding hvb+ might not be the main active species involved in this process or were not responsible for the decomposition of PCP. At least, the concentration of hvb+, 1O2, or •OH was too low to oxidize PCP in our experiments. The result was in agreement with the ESR analysis. Besides, since formic acid and sodium azide were used as hvb+, •OH, and 1O2 scavengers to reduce the recombination between ROS and electron/hydrated electron, the use of formic acid and sodium azide might lead to more electrons/hydrated electron survival from the recombination with ROS, which would be beneficial to the reduction of PCP by increasing electron/hydrated electron yield. By contrast, 4.7 and 9.5 mM H2O2, which acted as an electron scavenger, caused a significant decrease of PCP removal from 72.0 to 30.1% after 30 min, indicating that reductive reaction might play a predominant role in PCP photocatalytic decomposition under visible light irradiation. Photocatalytic oxidation and photocatalytic reduction are two sides of a coin. Potentially, photocatalysis can provide the negative electron for decomposing PCP. In some previous studies, photocatalytic reductions of carbon dioxide (26), Hg (II) (27), and Cr (VI) (28) have been reported. Because chlorophenols are electronegative, the greater the number of the chlorine substituents on the aromatic ring, the more prone the chlorophenols are to reduction than to oxidation (12). Effect of pH. As shown in Figure 2b, the photocatalytic activity of NTB was enhanced with the increase of pH value in aqueous solution. In acid solution with pH 2.3, the decomposition rate of PCP was about half of that in basic solution with pH 11.2. However, the decomposition rate did not change any further when the pH value was above 11.5. Previous studies have reported that higher pH value is of benefit to the reductive dechlorination of PCP to 2,4,6-TCP due to the higher Gibbs free energy yield (29). Besides, based on the energy band theory, basic surrounding is beneficial to the photocatalytic reduction whereas acid surrounding is good for photocatalytic oxidation (30). Obviously, in our case, the photodegradation of PCP favored the basic surrounding, which might be due to the reductive decomposition routes of PCP instead of the photocatalytic oxidation routes. Reliability of PCP Reduction. The comparison between the potential of NTB conduction band (Ecb) and the reductive potential of PCP (Ere) was conducted to further verify the reliability of the proposed reductive mechanism. In a

FIGURE 3. Cyclic voltammogram for reduction of 10 mg/L PCP in aqueous solution at different pH values on an Ag/AgCl electrode at a voltage scan rate of 5 mV/s. The two peaks are irreversible reductions at -0.54 and -0.8 V for pH 10, -0.73, and -0.92 V for pH 8.5, and -1.05 and -1.2 V for pH 7.2 (vs. Ag/ AgCl). photocatalysis process, reduction will occur when the potential of Ecb is lower than the reductive potential of organic pollutants (31). Thus, the measurement of the reductive potential of PCP could provide direct information on the first step driving power. Cyclic voltammetry (CV) was utilized to examine the halfwave reduction and oxidation potential of PCP (Figure 3). Two reductive waves corresponding to the sequential reduction of the two meta-chlorine groups were clearly visible. The reduction of the meta-chlorine groups was attributed to the more negative reduction potential in comparison to the orth- or para- chlorine group on the same ring (32, 33). PCP

FIGURE 4. Energy band structure and orbital contribution of Bi 6s, Bi 6p, O 2p, and Ti 3d of Ti-doped β-Bi2O3 stimulant calculated by plane wave theory. VOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Recombination mechanism of ecb-/hvb+ and Ti3+Bi [Bi (III) replaced by Ti(III)] hvb+ trapper in the bulk of Ti-doped β-Bi2O3. may undergo direct reduction to tetrachlorophenol in neutral conditions. In basic solution, the tri- and tetra-chlorinated phenols were further reduced to di- and trichlorophenols at more negative potentials in a double-electron reduction step. The band structure of NTB was calculated by the DFT method. The results are shown in Figure 4. The partial density of states indicated that the O 2p orbitals made a significant contribution to the valence band top of NTB, and that the highest occupied molecular orbital (HOMO) levels were composed mainly of the hybridized O 2p and Bi 6s orbitals. The bottom of conduction band was located at -0.5 eV and the top of valence band was located at -1.45 eV. Upon photoexcitation, electrons would transfer from the valence band to the conduction band, leaving photogenerated holes in the valence band (8). As determined by the elemental composition and the crystal structure, the band gap of NTB was significantly narrowed down to 1.05 eV as compared with 2.8 eV for that of β-Bi2O3 (34). According to the results in Figure 4, the conduction band energy of NTB in aqueous solution can be expressed as follows (26): Ecb(V vs. NHE) ) -0.5 - 0.059 pH

(6)

Equation 6 and results of Figure 4 suggest that at least one (Ecb is lower than the reductive potential of single position dechlorination at pH 7.2) or two (diposition dechlorination

at pH > 8.5) chlorine groups on PCP could be eliminated by NTB photocatalysis. The Ecb decreases with increasing pH. However, the Ere becomes more positive with increasing pH values due to the uptake of hvb+ in the reduction reaction (see Figure 3). It appears, therefore, Ecb (-0.92 eV) is lower than Ere (-0.83 eV) at pH 7.2 (note that the Ag/AgCl electrode is at +0.22 V vs the NHE), as well as at higher pH values. Thus, the thermodynamic favorability of PCP reduction by NTB is dependent on the pH values used in this study. Ti-Doped Structure Analysis. Semiconductors are usually selected as photocatalysts because semiconductors have a narrow gap between the valence and conduction bands. For photocatalysis to proceed, the semiconductor needs to absorb energy equal to or higher than its energy gap. The movement of electrons forms e-/hvb+ or negatively charged electron/ positively charged hole pairs. Due to the fast recombination of the pair in the bulk of the semiconductor, few charge carriers can escape to the surface of the semiconductor and degrade the organic compounds. If a charge trapper, for hole or electron, can capture one of the two charge carriers, the other carrier will take more chance to move to the surface of the semiconductor (8). For NTB (Figure 5), Ti(IV) was partially introduced to the framework and took the place of Bi(III). Under irradiation excitation, Ti(IV) would change into Ti(III) (35). Three of the four outer electrons bonded with oxygen while another electron would be free and became a hvb+ trapper. After capturing the hvb+, the Ti(III) was oxidized into Ti(IV), in rotation. The free electrons would have more chances to move to the surface of NTB. Part of the free electrons reduced the oxygen to O2- in the aqueous solution (eq 2) and the others were incorporated with electron acceptors. As described in previous studies, O2- and its major attributor H2O2 are not powerful enough to dechlorinate chlorophenols (21). Thus, PCP should act as the electron acceptor rather than the electron donor in our experiments. Electrons nucleophilically attacked the C-Cl bond on the aromatic ring of chlorophenols, which resulted in the dechlorinated products (eq 7) (8). The results of product identification (as shown in Figure 6) also confirmed this hypothesis.

FIGURE 6. Identified products and reaction pathway for the decomposition of PCP by Ti-doped β-Bi2O3 photocatalysis. LC-MS and GC-MS were used to determine the various intermediate products. 5584

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2ecb + R-Cl + H+ ) R + Cl-

(7) (2)

Photocatalytic Decomposition Pathway of PCP. The intermediates during PCP decomposition by NTB are shown in Figure 6. The identification of primary decomposition products allowed us to propose interlinking pathways for the decomposition of PCP. The first step in the pathway was the dechlorination of PCP to lower chlorinated phenols. In our experiments, 2,4,6-trichlorophenol, 2,4-dichlorophenol, 2-chlorophenol, and 4-chlorophenol were identified as the major dechlorinated products obtained in the reaction. Based on the ESR analysis, the dechlorination of PCP was attributed to electron reduction. Electron, as a strong nucleophile, directly attacked the C-Cl bond on the aromatic ring. In contrast to the photocatalytic oxidation routes of PCP (5, 36), oxidation products, including tetrachlorobenquinone (TCQ) and tetrachlorohydroquinone, were not detected in our case, suggesting that little oxidation occurred in the first step of decomposition. Sequentially, phenol and cyclohexanone were detected as the reaction proceeded. Phenol came from the dechlorination of monochlorophenol, and cyclohexanone was the reduction product from phenol. Similarly, if •OH catalyzed by the photocatalyst existed in the aqueous solution, the decomposition product of pentachlorophenol including benzoquinone or hydroquinol, could be found (37). The absence of oxidates suggested that the whole reaction followed a reductive decomposition pathway. The formation of cyclohexanone and cyclohexanol also indicated that reductive dechlorination indeed occurred in our experiments. An analogous reduction process of PCP was observed previously during the reductive dechlorination of PCP with Fe and Mg (38). Our results suggested that the oxidative dechlorination of PCP was probably, at least, not a significant reaction in the predictable decomposition pathway. Interestingly, we found that the high decomposition efficiency of PCP was not in virtue of the photocatalytic oxidation as for the decomposition of other organic pollutants, but rather of photocatalytic reduction. The mechanism was proved by the identification of intermediate products and free radicals. It is proposed that the special framework of Ti-doped β-Bi2O3 and visible light irradiation lead to the presence of photocatalytic reduction mechanism. The photocatalytic reduction pathway provides us with a new perspective to analyze the decomposition of PCP and other chlorinated phenols under sunlight irradiation in nature.

(3)

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(5)

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(7)

(8)

(9)

(10)

(11)

(12) (13)

(14)

(15)

Acknowledgments This work was supported by the National High Technology Research and Development Program (863 Program, 2006AA06Z323), National Key Technology R&D Program (2008BAC32B06-3) of China and special fund of State Key Laboratory of Water Environment Simulation (08ESPCTZ). The band structure results were obtained through the use of the ABINIT code, a common project of the Universite´ Catholique de Louvain, Corning Incorporated, the Universite´ de Lie`ge, the Commissariat a` l’Energie Atomique, Mitsubishi Chemical Corp., the Ecole Polytechnique Palaiseau (URL http://www.abinit.org).

Supporting Information Available Reagent lists, synthesis procedures, analytical methods, data modeling, and additional experimental results. This information is available free of charge via the Internet at http:// pubs.acs.org/.

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