КИНЕТИКА И КАТАЛИЗ, 2015, том 56, № 5, с. 577-581
УДК 541.128
THE PHOTOCATALYTIC INTERACTION OF Cr(VI) IONS AND PHENOL ON POLYMER-MODIFIED TiO2 UNDER VISIBLE LIGHT IRRADIATION © 2015 D. Zhang1, *, A. Wei1, J. Zhang1, R. Qiu2, 3
1Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, 650500, P.R. China 2School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou, 510275, P.R. China 3 Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology (Sun Yat-sen University), Guangzhou, 510275, P.R. China *E-mail: zddkmust@163.com Received 28.10.2014
To investigate the interaction of Cr(VI) and phenol in the photocatalytic process, the photocatalytic reaction of the mixed solution of these two pollutants was carried out using poly(fluorene-co-thiophene) modified TiO2 photocatalyst and visible light. In this mixed system, the removal efficiency in the presence of Cr(VI) and phenol were higher than that produced by individual systems, indicating a synergetic effect between the reduction of Cr(VI) and degradation of phenol. In the mixed system, Cr(VI) ions were reduced by the photogenerated electrons, and phenol was oxidized by photo-generated holes. In the presence of Cr(VI) ions, both the photocatalytic degradation and mineralization efficiency of phenol were promoted, but the promoting role was not the same. There was an optimum value of phenol/Cr(VI) molar ratio (1 : 1) in the degradation process. If the ratio was less than 1 : 1, the promoting effect decreased slowly but was still fairly distinct. The mineralization efficiency of phenol increased gradually with the increasing concentration of Cr(VI) ions, but when phenol/Cr(VI) was less than 1 : 1, the rate of increase became slower.
Keywords: photocatalysis, polymer-modified TiO2, phenol, Cr(VI), interaction, visible light irradiation. DOI: 10.7868/S0453881115050196
So far, the removal of toxic contaminants from water remains a huge challenge to treatment of industrial and municipal wastewaters. In most cases, inorganic and organic substances pollute wastewaters. The major inorganic pollutants are toxic heavy metal ions such as Cr(VI), Pb(II), Hg(II), and Cd(II), which, due to their toxicity, bioaccumulation and persistence, pose a serious problem to the aquatic environment [1]. Likewise, serious environmental hazards can be expected from the presence of organic pollutants like phenol, poly-chlorinated biphenyls in water [2, 3]. The situation where organic substances and metal ions coexist in wastewater is, however, quite frequent. Accordingly, simultaneous removal of organic matter and metal ions from wastewater is of special importance for pollution control and remediation.
To address this problem, many scientists and their co-workers have made considerable efforts in wastewater treatment. Prairie et al. [4, 5] observed that the concentration of metal ions such as Cr(VI), Au(III), Hg(II), Pd(II), Pt(IV), and Ag(I) can be significantly reduced in the presence of salicylic acid after a brief period of UV illumination. They were the first to describe a synergetic relationship between reduction of metal ions and oxidation of organic matter in the pho-tocatalytic process. To explain the synergetic effects
suggestion was made that both members of a photogenerated electron-hole pair are consumed in separate redox half-reactions preventing recombination of the electron-hole pair [6]. The use of photocatalysis, induced by visible light to remove simultaneously an organic substance and metal ions from polluted water, attracted the attention of investigators [7—12]. In our earlier work [13—17], we found that by using poly(fuorene-co-thiophene) modified TiO2 (PFT/TiO2 hereafter) with visible light it is possible to prepare a good photocatalyst capable to remove inorganic and organic pollutants from wastewater. It is, therefore, highly desirable to investigate the interaction between inorganic and organic compounds in the photocatalytic process.
The objective of the present work was to investigate the photocatalytic interaction of Cr(VI) ion and phenol in the PFT/TiO2 system under visible light. Since Cr(VI) cation is one of the most toxic ions present in the environment it was chosen as a model heavy metal ion. At concentrations higher than 0.05 mg/L this cation exerts toxic effects on most organisms. Phenol is an aromatic compound and when present in wastewa-ters generated by various production complexes it causes a number of adverse health and environmental effects [18].
Fig. 1. Schematic drawing of the experimental setup. 1 — Column glass reactor, 2 — reaction box with tinfoil pasted on the inside wall, 3 — LED light device, 4 — magnetic rotor, 5 — magnetic stirrer.
EXPERIMENTAL
Materials and methods
Catalyst preparation and characterization. Nanosized titania was purchased from "Degussa Co." (P-25, 80% anatase, 20% rutile; ¿*BET ~ 50 m2/g, mean particle size ~30 nm) and dried at 120°C for 12 h before use. The sensitization of TiO2 by PFT with thiophene content of30% was described in our earlier study [13]. The photocata-lysts thus prepared were characterized by scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy, photoluminescent spectroscopy (PLS), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) techniques [13— 15]. All the results showed that TiO2 sensitized by PFT was an excellent visible-light-responded photocatalyst.
Photocatalytic reaction. The photocatalytic experiment was performed in a column glass reactor. Three light emitting devices 1 W LED (light-emitting diode, "Shenzhen Lanbaoli Photo-electricity Technology Co. Ltd.", China) with a radiant wavelength of450—475 nm were used as the visible light source, and the photon flux determined by a UV-A Ultraviolet Meter ("Photoelectric Instrument Factory" of Beijing Normal University, China) was about 2 mW/cm2 at 420 nm and 0 mW/cm2 at 365 nm. Adjacent light emitting devices were placed tightly against the reactor, and were spaced 120 degrees apart (Fig. 1). The column glass container (2.5 cm i.d. and 10 cm in height) was placed concentrically inside the box, and filled with 20 mL of the solution.
The photocatalytic experiment was carried out by adding 20 mg of photocatalyst to 20 mL of mixed solution containing Cr(VI) and phenol. There were 3 replicates for each set. Prior to the photocatalytic experiment, the solution containing photocatalyst and pollutants was magnetically stirred in darkness for 30 min to establish adsorption-desorption equilibrium. In all batch experiments suspension containing 1 g of catalyst per liter of suspension was used and the Cr(VI) concentration in suspension was 15 mg/L. After a specified interval of irradiation time, a 2-ml aliquot of the reaction mixture was withdrawn for analysis.
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Reduction efficiency of Cr(VI), %
»— Cr alone *- Cr/phenol = 3:1
Cr/phenol = r- Cr/phenol =1:1
Cr/phenol = ►— Cr/phenol =
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Irradiation time, min
Fig. 2. Effect of different concentration of phenol on the reduction of Cr(VI). Catalyst dosage — 1 g/L, pH 3.
Analysis. Prior to analysis, the samples withdrawn were centrifuged at 6000 r/min for 10 min to remove any nanosized catalyst particles and then filtered through a 0.22 ^m Millipore membrane filter. The Cr(VI) content was determined with the UV-2501 spectrophotometer ("Shimadzu", Japan) measuring the intensity of absorption at 540 nm due to the complex formed with 1,5-diphenylcarbohydrazide. The concentration of phenol was also determined spectro-photometrically using the band at 510 nm as suggested by the 4-aminoantipryrine method [19]. A calibration based on Beer—Lambert law was used to quantify the results of measurements. The initial concentration of Cr(VI) (C0) was measured after the adsorption-des -orption equilibrium was attained in darkness. At a specified interval of irradiation time, the concentration of Cr(VI) (Ct) was also measured. Then, the reduction efficiency (n) of Cr(VI) was calculated: n = (1 - Ct/C0) x 100%.
RESULTS AND DISCUSSION
Effect of phenol on photocatalytic reduction of Cr(VI). The concentration of Cr(VI) in the initial solution was 15 mg/L. Phenol in different amounts was added into this solution to obtain a series of mixtures, in which the molar ratios Cr(VI)/phenol were 2 : 1, 3 : 1, 1 : 2, and 1 : 3. The pH was adjusted to 3, which is the best value for photocatalytic reduction of Cr(VI). To investigate the effect of phenol on photocatalytic reduction of Cr(VI), the catalyst dosage of 1 g/L and the illumination time of 120 min were chosen.
The results are shown in Fig. 2. It can be observed that the photocatalytic reduction efficiency of Cr(VI) gradually increased with increasing phenol concentration, i.e. with reducing Cr(VI)/phenol molar ratio. Moreover, the photocatalytic reduction efficiency of
Reaction rate constant, min-1 Degradation efficiency of phenol, %
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Cr alone 3 : 1 2 : 1 1 : 1 1 : 2 1 : 3 Molar ratio of Cr(VI)/phenol
Fig. 3. Relationship between the Cr(VI)/phenol molar ratio and the reaction rate constant. Catalyst dosage — 1 g/L, pH 3, irradiation time — 120 min.
Phenol 3 : 1 alone
2 : 1 1 : 1 1 : 2 1 : 3 Molar ratio of phenol/Cr(VI)
Fig. 4. Degradation efficiency of phenol observed with different molar ratios of phenol/Cr(VI). Catalyst dosage — 1 g/L, pH 3, irradiation time — 120 min.
Cr(VI) was higher in the mixed system than in the system with Cr(VI) alone. When Cr(VI)/phenol ratio was 1 : 3, the reduction efficiency of Cr(VI) was 2.5 times that of the individual Cr(VI) system and reached 100% after 100 min irradiation.
It can be concluded that the addition of phenol plays a significant role in promoting the reduction of Cr(VI). Vinu and Madras obtained similar results [8]. They show
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