Article pubs.acs.org/est
Arsenite Oxidation-Enhanced Photocatalytic Degradation of Phenolic Pollutants on Platinized TiO2 Jaesung Kim and Jungwon Kim* Department of Environmental Sciences and Biotechnology, Hallym University, Chuncheon, Gangwon-do 200-702, South Korea S Supporting Information *
ABSTRACT: The effect of As(III) on the photocatalytic degradation of phenolic pollutants such as 4-chlorophenol (4-CP) and bisphenol A (BPA) in a suspension of platinized TiO2 (Pt/TiO2) was investigated. In the presence of As(III), the photocatalytic degradation of 4-CP and BPA was significantly enhanced, and the simultaneous oxidation of As(III) to As(V) was also achieved. This positive effect of As(III) on the degradation of phenolic pollutants is attributed to the adsorption of As(V) (generated from As(III) oxidation) on the surface of Pt/TiO2, which facilitates the production of free OH radicals (•OHf) that are more reactive than surface-bound OH radicals (•OHs) toward phenolic pollutants. The generation of •OHf was indirectly verified by using coumarin as an OH radical trapper and comparing the yields of coumarinOH adduct (i.e., 7-hydroxycoumarin) formed in the absence and presence of As(V). In repeated cycles of 4-CP degradation, the degradation efficiency of 4-CP gradually decreased in the absence of As(III), whereas it was mostly maintained in the presence of As(III), which was either initially present or repeatedly injected at the beginning of each cycle. The positive effect of As(III) on 4-CP degradation was observed over a wide range of As(III) concentrations (up to mM levels) with Pt/TiO2. However, a high concentration of As(III) (hundreds of μM) inhibited the degradation of 4-CP with bare TiO2. Therefore, Pt/TiO2 can be proposed as a practical photocatalyst for the simultaneous oxidation of phenolic pollutants and As(III) in industrial wastewaters.
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INTRODUCTION Titanium dioxide (TiO2) photocatalysis has been extensively studied as a viable method for the remediation of contaminated water because TiO2 is highly active, stable, nontoxic, and inexpensive, despite the fact that TiO2 photocatalysis can only be achieved under UV light.1−3 Wastewater treatment using TiO2 photocatalysis can be achieved through both oxidative degradation and reductive transformation of aquatic pollutants. The oxidative degradation process is initiated by oxidizing species such as valence band holes (hVB+), OH radicals (•OH), and superoxide radicals (O2•−),4−9 whereas conduction band electrons (eCB−) are primarily involved in the reductive transformation process.10−12 Despite the formation of various oxidizing species, the oxidative degradation of multicomponent wastewater samples using TiO2 photocatalysis is inefficient because several components (pollutants) compete for the same oxidizing species. However, the economics of overall wastewater treatment will be improved if the oxidation of one component enhances the in situ degradation of other components. Arsenic (As), which mainly exists as arsenite (As(III)) or arsenate (As(V)), depending on the environmental conditions, © 2014 American Chemical Society
is one of the most serious pollutants present both in natural waters and industrial wastewaters.13 The concentration of As varies from micromolar (up to 70 μM in natural waters) to millimolar levels (in industrial wastewaters).14−16 Because As(III) is more toxic, more mobile, and less adsorptive on absorbents than As(V), the oxidation of As(III) to As(V) is highly desirable prior to adsorptive removal. Therefore, a variety of preoxidation methods using photocatalysts,17−19 photochemical systems,20−22 electrochemical systems,23 and chemical reagents24 have been developed. The surface platinization of TiO2 has been frequently attempted to enhance the photocatalytic activity of TiO2 for the degradation of diverse aquatic pollutants such as alkylamines,25,26 resorcinol,27 and pharmaceuticals.28 Pt deposits on TiO2 favor the formation of oxidizing species by accelerating electron transfer from the conduction band to molecular oxygen.29 Recently, it has also been reported that the surface Received: Revised: Accepted: Published: 13384
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to be about 2.33 × 10−3 einstein min−1 L−1. The reactor was open to ambient air to prevent the depletion of dissolved oxygen, and stirred magnetically during the irradiation. Multiple (two or more) experiments were performed for a given condition. Analytical Methods. Sample aliquots (1 mL) were withdrawn from the reactor intermittently (less than six times) during the irradiation and filtered through a 0.45 μm PTFE syringe filter (Millipore) to remove Pt/TiO2 (or bare TiO2) particles. The concentration of 4-CP and BPA was measured using a high performance liquid chromatography (HPLC, Agilent 1120) equipped with a UV−visible detector and a ZORBAX 300SB C-18 column (4.6 × 150 mm2). The eluent consisted of a binary mixture of 0.1% phosphoric acid solution and acetonitrile (80:20 (v/v) for the analysis of 4-CP and 70:30 (v/v) for the analysis of BPA). The concentration of As(V) generated from the oxidation of As(III) was determined using the colorimetric molybdene blue method.33 A molybdate reagent solution (200 μL) and ascorbic acid (10 g/100 mL, 100 μL) were added to the 8-fold diluted sample (4 mL). The solution was mixed vigorously, and its absorbance was measured at 870 nm using a UV−visible spectrophotometer (Shimadzu UV-2600) after 1 h. The generation of OH radicals in the UV-irradiated photocatalyst suspension was monitored by measuring the fluorescence of the coumarinOH adduct (i.e., 7-hydroxycoumarin) that was formed from the reaction between OH radical and coumarin.34,35 The fluorescence emission intensity of 7-hydroxycoumarin was measured at 460 nm using monochromatic light with a wavelength of 332 nm as the excitation source on a spectrofluorometer (Shimadzu RF-5301). The surface analysis of Pt/TiO2 was performed by X-ray photoelectron spectroscopy (XPS, Kratos XSAM 800pci) using the Mg Kα line (1253.6 eV) as the excitation source and by using a high-resolution transmission electron microscope (HRTEM, JEM-2100F) operated at an accelerating voltage of 200 kV.
platinization of TiO2 significantly accelerates the oxidation of As(III) to As(V) by preventing charge recombination between eCB− and intermediate As(IV) species (null reaction), which dominantly occurs on bare TiO2.17 In this work, we investigated the effect of As(III) on the photocatalyic degradation of phenolic pollutants such as 4chlorophenol (4-CP) and bisphenol A (BPA) in a suspension of platinized TiO2 (Pt/TiO2). The degradation of phenolic pollutants was markedly enhanced in the presence of As(III) with the simultaneous oxidation of As(III) to As(V). The effect of As(III) on the degradation of 4-CP was investigated as a function of various experimental parameters such as initial pH (pHi), As(III) concentration, and the presence of Pt on the TiO2 surface. Furthermore, detailed mechanistic investigations of the positive effect of As(III) on 4-CP degradation were explored.
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EXPERIMENTAL SECTION Materials and Chemicals. Materials and chemicals were used as received without further purification. They include titanium dioxide (TiO2, Degussa P25), 4-chlorophenol (C6H5ClO, 4-CP, Aldrich, ≥99%), bisphenol A (C15H16O2, BPA, Aldrich, ≥99%), sodium (meta) arsenite (NaAsO2, As(III), Sigma, 100%), sodium arsenate dibasic heptahydrate (Na2HAsO4·7H2O, As(V), Sigma-Aldrich, ≥98%), chloroplatinic acid hydrate (H2PtCl6·xH2O, Aldrich, ≥99.8%), methanol (CH4O, Wako, ≥99.8%), coumarin (C9H6O2, Sigma, ≥99%), molybdate reagent solution (containing hexaammonium heptamolybdate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O), sulfuric acid (H2SO4), and potassium antimony(III) oxide tartrate (K(SbO)C4H4O6·0.5H2O), Fluka), ascorbic acid (C6H8O6, Junsei, ≥99.6%), and tert-butyl alcohol (C4H10O, TBA, Junsei, ≥99%). The deionized water used was ultrapure (18.3 MΩ·cm) and prepared by a Human-Power I+ water purification system (Human corporation). Platinized TiO2 (Pt/TiO2) was obtained using a photodeposition method.30,31 An aqueous suspension of TiO2 (0.5 g/ 500 mL) was irradiated with a 300-W Xe arc lamp (λ > 300 nm, Oriel) in the presence of chloroplatinic acid hydrate (24.4 μM) as a Pt precusor and methanol (2 M) as an electron donor. After the suspension was irradiated for 3 h, the Pt/TiO2 powder was filtered through a 0.45 μm PVDF disc filter (Pall), thoroughly washed with distilled water, and dried in an oven at 70 °C. A typical Pt loading was estimated to be approximately 0.5 wt % by measuring the concentration of unused chloroplatinic acid hydrate remaining in the filtrate solution using inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Spectro). Photocatalytic Experiments. Pt/TiO2 (or bare TiO2) (15 mg) was dispersed in deionized water by sonication for 1 min in an ultrasonic cleaning bath. An aliquot of pollutant (i.e., 4CP, BPA, and As(III)) stock solution was subsequently added to the suspension to yield the desired initial concentration. The solution (total volume = 30 mL) was unbuffered. The initial pH (pHi) of the suspension was adjusted with a HClO4 or NaOH solution. The suspension was then stirred for 30 min to allow the adsorption equilibrium of pollutants on the surface of Pt/ TiO2 in the dark. A 300-W Xe arc lamp (Oriel) was used as a light source. The light beam was passed through a 5 cm IR water filter and a cutoff filter (λ > 320 nm) and focused onto a cylindrical Pyrex glass reactor (volume = 55.5 mL and diameter = 3.5 cm). The incident light intensity (320 nm < λ < 500 nm) was measured by using ferrioxalate actinometry32 and estimated
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RESULTS AND DISCUSSION Effect of As(III) on 4-CP Degradation. Figure 1 shows the photocatalytic degradation of 4-CP and BPA (as model phenolic pollutants) in a suspension of Pt/TiO2 with or without As(III). The degradation of both 4-CP and BPA (300 μM) was significantly enhanced in the presence of As(III) (250 μM) (i.e., under the ratio of [phenolic pollutants]/[As(III)] = 1.2). This positive effect of As(III) on 4-CP degradation was also observed when the ratio was 0.2 or 4.0 (see Figure S1 and accompanying discussion in the Supporting Information, SI). In accordance with the enhanced degradation of 4-CP with As(III), both the generation of chloride (Cl−) as a product and the removal of total organic carbon (TOC) were enhanced with As(III) (Figure S2 and Table S1 in the SI). The possibility of Cl− production from the degradation of HClO4 used for the pH adjustment can be ruled out because Cl− was not generated in the absence of 4-CP (Figure S2 in the SI). In addition, the degradation of 4-CP was negligible in the absence of oxygen, which is the favorable condition for reductive transformation process (Figure S3 in the SI). These results clearly indicate that the removal of 4-CP in the presence of As(III) proceeded through oxidation, not by reduction or complexation with As(III). Along with the enhanced 4-CP and BPA degradation in the presence of As(III), As(III) was completely oxidized to As(V) (Figure 1), which makes the Pt/TiO2 system a practical 13385
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functional groups of TiO2 from >TiOH2](2/3)+ (or >Ti OH](1/3)−) to >TiF](1/3)− for fluoride and >TiOPO3](7/3)− for phosphate at acidic pH (reactions 1 and 2).31 This transformation favors the formation of free OH radicals (•OHf) that are more reactive than surface-bound OH radicals (•OHs) toward phenolic pollutants because the substitution of (protonated) surface OH groups by these anions inhibits the reaction between hVB+ and (protonated) surface OH groups (i.e., generation of •OHs, reaction 3), but enhances the reaction between hVB+ and water molecules (i.e., generation of •OHf, reaction 4).30,36,37 >TiOH 2](2/3) + (or >TiOH](1/3) − ) + F− → >TiF](1/3) − + H 2O (or OH−)
(1)
>TiOH 2](2/3) + (or >TiOH](1/3) − ) + H 2PO4 − → >TiOPO3](7/3) − + H 2O + 2H+(or H+)
(2)
>TiOH + hVB+ → >Ti•OHs
(3)
>Tianion + hVB+ + H 2O → >Tianion + •OH f + H+
(4)
(The surface Ti having a +4 formal charge is located at the octahedral site surrounded by five lattice oxygen atoms and one surface group; see Figure S6 in the SI for the pH-dependent speciation of TiO2 surface, fluoride, and phosphate.) It should be noted that •OHf is more reactive than •OHs toward phenolic pollutants, which are only slightly adsorbed on the TiO2 surface, because •OHs can react with substrate near the surface whereas •OHf can diffuse out from the surface and react with substrate in the bulk solution.38,39 Therefore, the degradation of phenolic pollutants can be enhanced through the generation of •OHf by the adsorption of anions on the surface of TiO2.30,36,37,40−44 Similarly to fluoride and phosphate, anionic As(V) (H2AsVO4−) generated from the oxidation of neutral As(III) (H3AsIIIO3) can also be adsorbed on the surface of TiO2 through bidentate complexation (see Figure S6d in the SI for the pH-dependent speciation of As(III) and As(V)).45 In such a case, the adsorption of As(V) anions should deplete the
Figure 1. Photocatalytic degradation of (a) 4-CP and (b) BPA in the absence or presence of As(III). Experimental conditions: [Pt/TiO2] = 0.5 g L−1, [4-CP] or [BPA] = 300 μM, [As(III)] = 250 μM, pHi = 3.0, and λ > 320 nm.
alternative for the simultaneous oxidation of phenolic pollutants and As(III). The observed deficiency of generated [As(V)], calculated as ∼55 μM (initial [As(III)] − steady-state [As(V)]), is due to the adsorption of As(V) on the surface of Pt/TiO2 (see Figure S4 in the SI). The negligible oxidation of both 4-CP and As(III) in the absence of either Pt/TiO2 or UV irradiation establishes that the photocatalytic oxidation of both 4-CP and As(III) proceeds without any photochemical or catalytic pathway (Figure S5 in the SI). Effect of As(V) Generated from As(III) Oxidation on 4CP Degradation. The adsorption of anions such as fluoride and phosphate on the surface of TiO2 changes the surface
Figure 2. (a) As 3d peak in XPS spectra, (b) scanning TEM bright field image, and (c−e) Ti, O, and As elemental mapping of the Pt/TiO2 sample obtained after 15 min of UV irradiation in the presence of 4-CP and As(III). Experimental conditions: [Pt/TiO2] = 0.5 g L−1, [4-CP] = 300 μM, [As(III)] = 250 μM, pHi = 3.0, and λ > 320 nm. 13386
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(protonated) surface OH groups of TiO2 and increase the density of >TiOAsVO3](7/3)− (reaction 5). >TiOH 2](2/3) + (or > TiOH](1/3) − ) + H 2As V O4 − → >TiOAs V O3](7/3) − + H 2O + 2H+(or H+)
(5)
•
Under this condition, the production of OHf becomes dominant by the preferential reaction of hVB+ with water molecules, which enhances the degradation of phenolic pollutants. The adsorption of As(V) on the Pt/TiO2 surface during the course of 4-CP degradation in the presence of As(III) was confirmed by XPS and HRTEM analysis, the results of which are shown in Figure 2. The As(V) 3d binding energy of 45.1 eV corresponds to that of As(V) adsorbed on the surface of TiO2 (Figure 2a).46 Figure 2b shows HRTEM image of Pt/TiO2 sample obtained after 15 min of UV irradiation in the presence of 4-CP and As(III). Figure 2c,d show the elemental distribution of Ti and O, respectively, within the Pt/TiO2 particles corresponding to Figure 2b. The distribution of As atoms (Figure 2e) exactly overlaps with that of Ti and O atoms (Figure 2c,d), which indicates that As species (certainly as As(V) species) are adsorbed over the entire surface of Pt/TiO2 particles. To confirm the effect of As(V) adsorption on • OHf generation and 4-CP degradation, the production of OH radicals and degradation of 4-CP were compared in the absence and presence of As(V) (Figure 3a,b). The generation of OH radicals, which is directly proportional to the fluorescence emission intensity of the coumarinOH adduct (i.e., 7hydroxycoumarin), was enhanced in the presence of As(V) (Figure 3a). It should be noted that the fluorescence method using coumarin as an OH radical trapper primarily detects OH radicals in the bulk solution (i.e., free OH radicals) because the adsorption of coumarin on the TiO2 surface is not favored.47 Furthermore, the adsorption of coumarin was negligible in the presence of As(V) under the experimental conditions identical to those of Figure 3a except for UV irradiation. Under the same incident photon flux (i.e., under the condition in which the number of photogenerated hVB+ as an OH radical initiator remained constant in both cases), the higher production of OH radicals in the presence of As(V) indicates that more reactive • OHf toward phenolic pollutants are primarily generated by the adsorption of As(V). In addition, the degradation of 4-CP was also enhanced in the presence of As(V) (Figure 3b), and the extent of the positive effect on 4-CP degradation induced by As(V) was similar to that induced by As(III) (k with As(III) = 0.144 ± 0.009 min−1 vs k with As(V) = 0.145 ± 0.014 min−1, where k is the pseudo-first-order rate constant for the degradation of 4-CP). This result implies that 4-CP in the presence of As(III) was little oxidized by intermediate As(IV) species generated from As(III) oxidation or O2•− (or •OH) generated from the reduction of oxygen by intermediate As(IV) species. The addition of excess tert-butyl alcohol (TBA) as an OH radical scavenger greatly reduced the degradation of 4-CP in the presence of As(III) (Figure 3c). This result indicates that the degradation of 4-CP in the presence of As(III) is primarily initiated by •OH, whereas degradation by hVB+, O2•−, and surface-complex-mediated process (i.e., electron transfer from surface-complexed 4-CP to the conduction band of TiO248) is only a minor pathway. The higher generation of hydroxylated intermediates (i.e., 4-chlorocatechol, 4-chlororesorcinol, hydroquinone, and hydroxyhydroquinone) in the presence of As(III)
Figure 3. (a) Photocatalytic production of coumarinOH adduct (i.e., 7-hydroxycoumarin), (b) degradation of 4-CP in the absence or presence of As(V), and (c) degradation of 4-CP with As(III) in the absence or presence of TBA. Experimental conditions: [Pt/TiO2] = 0.5 g L−1, [coumarin] = 1 mM for part a, [4-CP] = 300 μM for parts b and c, [As(V)] = 250 μM for parts a and b, [As(III)] = 250 μM for part c, [TBA] = 0.1 M for part c, pHi = 3.0, and λ > 320 nm.
implies that the •OH mediated pathways in the degradation of 4-CP are enhanced with As(III) (see Figure S7 in the SI). Overall, it is clear that the enhanced degradation of 4-CP in the presence of As(III) is ascribed to the higher production of • OHf by the adsorption of As(V) generated from the oxidation of As(III). Effect of As(III) on 4-CP Degradation under Various Conditions. Table 1 compares the effects of As(III) on 4-CP degradation kinetics as a function of As(III) concentration and pHi. The results are expressed in terms of the relative pseudofirst-order rate constant (krel) for the degradation of 4-CP, which is defined as the ratio of (k with As(III))/(k without As(III)). Although the effect of As(III) on 4-CP degradation varied depending on [As(III)] and pHi, As(III) showed a positive effect on 4-CP degradation in all cases (i.e., krel >1). The krel value increased with increasing As(III) concentration 13387
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oxidation of As(III) (see Figure S4 in the SI), the production of •OHf, and eventually the degradation of 4-CP.
Table 1. Relative Pseudo-First-Order Rate Constants (krel) for the Degradation of 4-CP as a Function of Initial pH (pHi) and As(III) Concentration pHi 3.0 3.0 3.0 3.0 3.0 3.0 4.5 6.0 9.0 10.5
[As(III)] (μM) 31.25 62.5 125 250 500 1000 250 250 250 250
k without As(III)a (min−1) 0.051(±0.004) 0.051(±0.004) 0.051(±0.004) 0.051(±0.004) 0.051(±0.004) 0.051(±0.004) 0.035(±0.003) 0.035(±0.002) 0.040(±0.006) 0.055(±0.002)
k with As(III)a (min−1) 0.093(±0.001) 0.106(±0.007) 0.120(±0.013) 0.144(±0.009) 0.118(±0.008) 0.086(±0.003) 0.127(±0.009) 0.107(±0.003) 0.117(±0.004) 0.074(±0.002)
> TiOH 2](2/3) + → > TiOH](1/3) − + H+(pK a1 = 3.936)
(6)
krelb
> TiOH]
(1/3) −
1.82 2.08 2.35 2.82 2.31 1.69 3.63 3.06 2.93 1.35
→ > TiO]
(4/3) −
+
36
+ H (pK a2 = 8.7 )
(7)
In contrast to the results obtained in the pHi range 4.5−10.5 (i.e., krel value decreased as the pHi increased), the krel value at pHi = 3.0 was lower than that at pHi = 4.5 (see Table 1), although the lower pH (i.e., more positively charged surface of Pt/TiO2) can provide a better condition for the adsorption of anionic As(V) species. This finding can be explained in terms of As(V) speciation as a function of the pH. As(V) species exist in anionic forms such as H2AsVO4−, HAsVO42−, and AsVO43− above pH = 4.0. However, the fraction of H3AsVO4, the neutral form of As(V) which is only slightly adsorbed on a positively charged surface, increases with decreasing pH below pH = 4.0 (Figure S6d in the SI). A higher fraction of H3AsVO4 at pHi = 3.0 than that at pHi = 4.5 can help explain why the krel at pHi = 3.0 was lower than that at pHi = 4.5, although the surface charge was more positive at pHi = 3.0 (see Discussion on pH change in the SI for details). We also investigated the effect of As(III) on 4-CP degradation using bare TiO2 as a function of [As(III)] (Figure 4a). The positive effect of As(III) on 4-CP degradation was also observed with bare TiO2 but was limited to low concentrations of As(III) ([As(III)] ≤ 250 μM). The degradation of 4-CP was significantly inhibited at [As(III)] ≥ 500 μM. The observed
a
All R2 values are higher than 0.98. bkrel = (k with As(III))/(k without As(III)). Experimental conditions: [Pt/TiO2] = 0.5 g L−1, [4-CP] = 300 μM, and λ > 320 nm.
up to [As(III)] = 250 μM, but decreased with increasing As(III) concentration from [As(III)] = 250 μM at the same pHi. As(III) can have both negative (as a competitive scavenger of oxidizing species) and positive effect (as a source of As(V) which facilitates the production of •OHf) on 4-CP degradation. The degradation rate of 4-CP should be reduced as the concentration of As(III) increases, if As(III) acts only as a competitive scavenger of oxidizing species. However, the adsorption of As(V) can be more significant when more As(III) is added and oxidized to As(V) (i.e., at a higher concentration of As(V)). This effect in turn would accelerate the degradation of 4-CP by enhancing the production of •OHf. The adsorption of As(V) generated from As(III) oxidation increased with the concentration of As(III) until it was saturated at [As(III)] = 250 μM (Figure S8 in the SI). Below [As(III)] = 250 μM, the krel value increased with increasing As(III) concentration, because the adsorption of As(V), which facilitates the production of •OHf, increased with increasing As(III) concentration. However, above [As(III)] = 250 μM, the extent of positive effect of As(III) on 4-CP degradation should remain constant independent of As(III) concentration, because the adsorption of As(V) was almost the same. However, the extent of negative effect of As(III) on 4-CP degradation (i.e., As(III) acting as a competitive scavenger of oxidizing species) should increase with increasing As(III) concentration. Therefore, the overall positive effect of As(III) on 4-CP degradation should be reduced with increasing As(III) concentration above [As(III)] = 250 μM. In accordance with the As(V) adsorption kinetics on the Pt/TiO2 surface (Figure S8 in the SI), the optimal concentration of As(III) (i.e., the highest krel value) was observed at [As(III)] = 250 μM. The solution pH can also affect krel value at the same concentration of As(III) because the adsorption kinetics of As(V) generated from As(III) oxidation is greatly dependent on the pH-dependent surface charge of Pt/TiO2 and speciation of As(V). In the pHi range 4.5−10.5, the positive effect of As(III) on 4-CP degradation (i.e., krel value) gradually decreased as the pHi increased (see Table 1). As the pH increased, the surface charge of TiO2 became less positive or more negative (reactions 6 and 7, Figure S6a in the SI), which reduced the adsorption of anionic As(V) species (i.e., H2AsVO4−, HAsVO42−, and AsVO43−) generated from the
Figure 4. Time profiles of (a) 4-CP degradation and (b) As(III) oxidation in a suspension of bare TiO2 as a function of As(III) concentration. Experimental conditions: [bare TiO2] = 0.5 g L−1, [4CP] = 300 μM, pHi = 3.0, and λ > 320 nm. 13388
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Figure 5. Repeated cycles of (a) 4-CP degradation and (b−d) simultaneous oxidation of 4-CP and As(III). For part a, only an aliquot of 4-CP was added at the beginning of each cycle. For part b, an aliquot of 4-CP was added at the beginning of each cycle, but As(III) was added only at the beginning of first cycle. For parts c and d, aliquots of both 4-CP and As(III) were added at the beginning of each cycle. Experimental conditions: [Pt/ TiO2] = 0.5 g L−1, [4-CP]initial or [4-CP]added = 300 μM, [As(III)]initial = 250 μM for parts b and c, [As(III)]added = 250 μM for part c, [As(III)]initial or [As(III)]added = 30 μM for part d, pHi = 3.0, and λ > 320 nm.
inhibitory effect of As(III) on 4-CP degradation at [As(III)] ≥ 500 μM can be attributed to the slow oxidation of As(III) on bare TiO2. As shown in Figure 4b, the complete oxidation of As(III) was not achieved after 15 min, and a significant amount of As(III) still remained in the solution at [As(III)] ≥ 500 μM. Under this condition, the residual As(III) would continuously compete with 4-CP for oxidizing species, inhibiting the degradation of 4-CP. However, the oxidation of As(III) on Pt/TiO2 was very fast and was completed within 15 min even at [As(III)] = 1 mM, considering the adsorption of As(V) on the Pt/TiO2 surface (Figure S9 in the SI). Pt deposits on TiO2 can accelerate electron transfer from the conduction band to molecular oxygen by retarding charge recombination between eCB− and hVB+.29 With Pt/TiO2 used in this study, the enhanced electron transfer by Pt deposits was confirmed by comparing the photocurrents (Iph) generated on Pt/TiO2 and bare TiO2 (see Figure S10 and accompanying discussion in the SI). As a result, more oxidizing species can be generated on Pt/TiO2 than on bare TiO2. In addition, Pt deposits can also prevent the charge recombination reaction between intermediate As(IV) species and eCB−, which regenerates As(III) and hinders the oxidation of As(IV) to As(V).17 The rapid oxidation of As(III) to As(V) on Pt/TiO2 not only diminishes the negative effect of As(III) as a competitive scavenger of oxidizing species, but also leads to the rapid adsorption of As(V) on the surface of Pt/ TiO2 (positive effect). Therefore, the positive effect of As(III) on 4-CP degradation with Pt/TiO2 was observed over a wider range of As(III) concentrations (up to mM levels) compared to that with bare TiO2 (only up to μM levels).
Repeated Cycles of Simultaneous Oxidation of 4-CP and As(III). To verify the practical viability of the process for the simultaneous oxidation of 4-CP and As(III), the photocatalytic reaction was repeated up to five cycles in a single batch reactor by injecting 4-CP alone or both 4-CP and As(III) every 30 min (Figure 5). The degradation efficiency of 4-CP in the absence of As(III) gradually decreased with an increasing number of cycles (from 72% to 18% after five cycles, Figure 5a). This behavior appears to be due to the surface-mediated charge recombination between oxidizing species and eCB− through intermediates (generated from 4-CP degradation) adsorbed on Pt/TiO2, which creates a null reaction.31,40 However, the degradation of 4-CP was nearly complete in all cycles in the initial presence of 250 μM As(III) with the complete oxidation of As(III) in the first cycle (Figure 5b). In addition, when both 4-CP and As(III) were repeatedly injected at the beginning of each cycle, the simultaneous oxidation of 4CP and As(III) was also nearly complete in all cycles, not only at a high concentration of As(III) (250 μM, As(III) concentration level in industrial wastewaters, Figure 5c), but also at a low concentration of As(III) (30 μM, As(III) concentration level in natural waters, Figure 5d). A stable degradation efficiency throughout the repeated cycles in the presence of As(III) should be because As(V) adsorbed on Pt/ TiO2 prevents the surface-mediated charge recombination process by inhibiting the adsorption of intermediates. Environmental Implications. If the oxidation of only As(III) (but not both As(III) and phenolic pollutants) is considered, then the Pt/TiO2/As(III)/phenolic pollutant 13389
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catalytic degradation of 4-CP and BPA with 4-CP (or BPA) alone or both 4-CP and BPA. This material is available free of charge via the Internet at http://pubs.acs.org/.
system is inefficient because the presence of 4-CP inhibits the oxidation of As(III) (see Figure S11 and accompanying discussion in the SI). The external addition of chemical reagents containing As species (produced for laboratory experiments, agricultural, and/or medical use) to wastewaters containing phenolic pollutants may also be impractical because As(V) remains in the treated water and post-treatment for As(V) removal is required. However, the practical viability of the simultaneous oxidation of phenolic pollutants and As(III) using Pt/TiO2 can be established by mixing As(III)- and phenolic pollutant-contaminated industrial wastewaters (or natural waters). In such a case, the degradation efficiency and stability of phenolic pollutants can be improved without the cost of As(III) oxidation, which eventually reduces the overall cost of wastewater treatment (i.e., the cost of the simultaneous oxidation of As(III) and phenolic pollutants < the cost of the oxidation of As(III) + the cost of the degradation of phenolic pollutants). However, this method is economically feasible only if two wastewaters (or natural waters) containing As(III) and phenolic pollutants are close together (i.e., the cost of wastewater (or natural water) transport is low). Above all, this Pt/TiO2 photocatalytic system should be the most efficient for the treatment of wastewater originally containing high concentrations of both As(III) and phenolic pollutants. We investigated the effect of As(III) on the photocatalytic degradation of phenolic pollutants such as 4-CP and BPA in a suspension of Pt/TiO2. Most often, the oxidation rate of one pollutant was reduced in the presence of the other pollutant because the pollutants competed for oxidizing species (see Figure S12 in the SI for an example). However, the degradation of phenolic pollutants was markedly enhanced in the presence of As(III) with the simultaneous oxidation of As(III) to As(V). This enhancement is ascribed to the adsorption of As(V) generated from As(III) oxidation on the surface of Pt/TiO2, which facilitates the formation of free OH radicals that are more reactive than surface-bound OH radicals toward phenolic pollutants. This study provides an example demonstrating that the simultaneous oxidation of phenolic pollutants and As(III) can be more efficient than the oxidation of phenolic pollutants alone.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program (NRF-2013R1A1A1007312) and Space Core Technology Development Program (NRF2014M1A3A3A02034875) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.
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ASSOCIATED CONTENT
* Supporting Information S
Photocatalytic degradation of 4-CP in the absence or presence of As(III) depending on [4-CP]; discussion on figure S1; generation of Cl− during 4-CP degradation in the absence or presence of As(III); removal efficiency of 4-CP and TOC in the absence or presence of As(III); photocatalytic degradation of 4CP in the absence of oxygen; adsorption of As(V) on the Pt/ TiO2 surface as a function of pH; degradation of 4-CP and oxidation of As(III) in the absence of either Pt/TiO2 or UV irradiation; pH-dependent speciation of TiO2 surface, fluoride, phosphate, arsenite, and arsenate; generation of intermediates from the degradation of 4-CP in the absence or presence of As(III); adsorption of As(V) generated from As(III) oxidation on the surface of Pt/TiO2; discussion on pH change; photocatalytic oxidation of As(III) in the presence of 4-CP depending on [As(III)] and adsorption of As(V) on the Pt/ TiO2 surface as a function of [As(V)]; photocurrent (Iph) collected on a Pt electrode through Fe3+/Fe2+ electron shuttle in the suspensions of bare TiO2 and Pt/TiO2; discussion on figure S10; photocatalytic oxidation of As(III) in the absence or presence of pollutant; discussion on figure S11; and photo13390
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Environmental Science & Technology
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