Environ. Sci. Technol. 2007, 41, 5433-5438
Ferrioxalate-Polyoxometalate System as a New Chemical Actinometer JAESANG LEE, JUNGWON KIM, AND WONYONG CHOI* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea
A chemical actinometer that employs an aqueous solution of ferrioxalate and polyoxometalate (POM: SiW12O404-) is proposed as a simple and fast method that quantifies the light intensity of UV radiation in the 300-390 nm range. As a modified ferrioxalate actinometer, illuminated ferrioxalate solution in the presence of POM generates CO2 radical anions that lead to the production of blue-colored POM(SiW12O405-) whose concentration can be easily determined spectrophotometrically (by monitoring absorbance at 730 nm). Photoproduction rate of POM- is linearly correlated with the light intensity. The measured quantum yields, Φ(POM-), range 0.11-0.21 in the 300-390 nm region and is fairly constant around 0.18 in the 335-380 nm range, which makes this actinometry ideally suited for the rapid measurement of the light intensity in the UVA region such as in common black light sources. This alternative actinometry eliminates the postirradiation analytical procedure (which needs phenanthroline as a complexation reagent for Fe2+ and the subsequent color development) that is required in the standard ferrioxalate actinometry, and it enables the in-situ quantification of the light intensity in a simpler way by monitoring the formation of POM- as a proxy indicator.
Introduction Studies of environmental photochemistry involve both sunlight-driven reactions in nature and pollution abatement technologies such as photochemical advanced oxidation processes. In any case, the measurements of the light intensity and the quantum yield of photochemical reactions are essential to obtain detailed information about the kinetics and efficiency of the overall photochemical process. Chemical actinometry utilizes photochemical reactions whose quantum yields are well defined to measure light intensities. A large number of chemical actinometers are known (1), which are based on the quantitative photochemical reactions of hydrogen peroxide (2), azobenzene (3), uranyl oxalate (4), and ferrioxalate (5, 6), acetone/2propanol/carbon tetrachloride mixture (7), to list just a few examples. In particular, ferrioxalate is the most popular actinometer, because it has high and constant quantum yields over the UV-vis region (250-500 nm) (5, 6). Ferrioxalate actinometry is based on the photogeneration of Fe2+ ions through the photoinduced LMCT (ligand-to-metal charge transfer) (reaction 1) and the subsequent reductive reaction via CO2 radical anions, CO2‚- (reaction 3) (6). The concentration of Fe2+ ions has been commonly determined after complexation with 1,10-phenanthroline and the subsequent * Corresponding author phone: +82-54-279-2283; fax: +82-54279-8299; e-mail:
[email protected]. 10.1021/es070474z CCC: $37.00 Published on Web 06/29/2007
2007 American Chemical Society
absorbance measurement (λmax ) 510 nm) (8).
[FeIII(C2O4)3]3- + hν f Fe2+ + C2O4‚- + 2(C2O4)2-
(1)
C2O4‚- f CO2 + CO2‚-
(2)
[FeIII(C2O4)3]3- + CO2 ‚-f Fe2+ + 3(C2O4)2- + CO2
(3)
However, the standard method using 1,10-phenanthroline to quantify ferrous ions is somewhat troublesome because it needs postirradiation analytical procedures (e.g., addition of several chemical agents, dark conditions required for analytical works, and long equilibration time (at least half an hour) is required to confirm the complete colorization of Fe2+-phenanthroline complex), which are laborious, timeconsuming, and possible sources of experimental errors (912). In addition, impure metal ions could form complexes with phenanthroline to induce some analytical interference (11). In this work, we propose the use of a ferrioxalatepolyoxometalate system as an alternative actinometer, which is based on the in-situ photogeneration of reduced polyoxometalate (blue color) whose concentration can be readily determined spectrophotometrically without any posttreatment. Polyoxometalate (POM) is a well-organized metal-oxygen cluster anion that can be reduced reversibly and stepwise without decomposition (13, 14), and it can be utilized in various electron-transfer processes. For example, POM promotes the electron transfer from TiO2 conduction band to dioxygen or methyl orange as an efficient electron carrier (15-17), or it initiates a variety of redox reactions under UVilluminated conditions as a homogeneous photocatalyst (1820). POM can also accelerate the dark oxidation reaction occurring on zerovalent iron by serving as an electron shuttle (21). In particular, the accumulation of electrons on POM induces the formation of POM-, which is indicated by gradual blue coloration (13). The blue color of POM- is attributed to intraelectron transfer between adjacent metal ions (M-M charge-transfer band) (13). In the illuminated (ferrioxalate + POM) solution, the reductive colorization of POM can be induced from the reaction with CO2‚- that is generated via reactions 1-2.
POM + CO2‚- f POM- + CO2
(4)
We selected SiW12O404- (abbreviated as SiW124- throughout the text) as a model POM that is stable up to pH 5.5 (20). The reducing power of CO2‚- is strong enough to drive reaction 4; compare E0(CO2/CO2‚-) ) -1.9 VNHE (22) versus E0(SiW124-/ SiW125-) ) +0.054 VNHE (14). The kinetics of the reaction between SiW124- and CO2‚- is also very fast.
SiW124- (colorless) + CO2‚- f
SiW125- (colored) + CO2 [k ) 8.4 × 108 M-1 s-1 (14)] (5)
The reoxidation of POM- by dissolved oxygen occurs at a much slower rate.
SiW125- + O2 f
SiW124- + O2- [k ) 125 M-1 s-1 at pH 1 (14)] (6)
The concentrations of reduced POM can be easily determined by monitoring the absorbance at 730 nm. Since the photoinduced production of Fe2+ is always accompanied by VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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CO2‚- that subsequently reacts with POM to produce POM(reaction 4), the spectrophotometric quantification of POMcan serve as an in-situ proxy indicator of [Fe2+]. This work demonstrates the successful performance of the ferrioxalatePOM system as a new chemical actinometer.
Experimental Section Chemicals that were used as received and include iron(III) perchlorate hydrate (Fe(ClO4)3‚xH2O, Aldrich), potassium oxalate monohydrate (K2C2O4‚H2O, Aldrich), and tungstosilicic acid hydrate (SiW12O40H4, Fluka). Water was ultrapure (18 MΩ‚cm) and prepared by a Barnstead purification system. Other chemicals used were of the highest purity available. The actinometer solution was prepared as follows: 2 mL of Fe(ClO4)3 (50 mM) and 1.2 mL of K2C2O4 (1 M) stock solutions were added to a 50 mL glass vial, and then diluted to 18 mL. Before the addition of SiW124-, the pH of the solution was checked and adjusted around 4.8 with 1 M perchloric acid to avoid self-decomposition of SiW124- that may occur above 5.5 (20). 2 mL of SiW124- stock solution (10 mM) was subsequently added to the above ferrioxalate solution to make 20 mL actinometer solution ([Fe3+]0 ) 5 mM, [C2O42-]0 ) 60 mM, [SiW124-]0 ) 1 mM), and then the pH of the mixture was adjusted again to 4.5 with 1 M perchloric acid and 1 M sodium hydroxide. No precipitate formation was observed during the preparation of the actinometer solution. An aliquot of the actinometer solution was transferred to a 3 mL quartz cuvette for irradiation. The actinometer solution was freshly prepared with every experiment. A 300 W xenon-arc lamp (Oriel) coupled with a grating monochromator (Oriel, model 77250) was employed as a monochromatic light source and the actinometer solution in a quartz cuvette (1 cm path length and 3 mL volume) was irradiated. The entrance and exit slit widths were set to 1.5 mm to obtain the 10 nm bandpass (fwhm) at the selected wavelength. The cross-section of the incident light beam onto the reactor was 0.5 × 1.5 cm. The incident light intensities were varied using a set of neutral density filters that were placed between the monochromator exit and the reactor. The monochromatic UV light intensity onto the reactor cell was measured using the ferrioxalate actinometry (5). The quantum yields for POM- production, Φ(POM-), were determined spectrophotometrically by monitoring the absorbance at 730 nm ( ) 2100 M-1 cm-1 (20)). The slope obtained from the linear correlation between the formation rate of POM- and the light intensity yields Φ(POM-) of the ferrioxalate-POM actinometry because the incident photons with λ < 400 nm are almost completely absorbed by the present actinometer solution (see Figure 1a). The quantum yields were determined with varying the monochromatic irradiation wavelength in 10-20 nm intervals from 300 through 400 nm. The UV/visible absorption spectra for the in-situ generated POM- in the above reactor cell were recorded immediately after the irradiation using a UV-visible spectrophotometer (UV-2401PC, Shimadzu). The absorbance measurements were usually done within a minute following the irradiation. Although the reoxidation (discoloration) of POM- by ambient oxygen slowly occurred, it was insignificant as long as the measurement can be done rapidly enough: the typical absorbance change was less than 0.01 in a minute. Triplicate experiments were performed for the determination of Φ(POM-) value at each wavelength and confirmed the reproducibility. As a polychromatic (white) light source, when needed, another 300 W xenon-arc lamp (Oriel) without the grating monochromator was used. In this case, the white light was filtered through a 10 cm IR water filter and a cutoff filter (λ > 300 nm). The filtered light was focused onto the 3 mL quartz cuvette. The incident light intensity of the white light was estimated to be about 1.1 × 10-3 einstein/min‚L in the 5434
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FIGURE 1. (a) Evolution of the UV-visible absorption spectra of ferrioxalate/SiW124- solution during the UV illumination (solid lines) with λ > 300 nm and in the following dark period (dashed lines) ([oxalate]0 ) 60 mM, [Fe3+]0 ) 5 mM, [SiW124-]0 ) 1 mM, pHi ) 4.5, I ) 1.1 × 10-3 einstein/min‚L). The inset compares the absorption spectra of 1 mM SiW124- and 5 mM Fe(III)-oxalate solution. (b) Time profile of SiW125- generation in the illuminated ferrioxalate-SiW124solution and its decay in the following dark period in the presence (B) or absence (n) of dissolved oxygen. active wavelength region (300-460 nm) using the ferrioxalate actinometry. Since the present quantum yields were determined using low-intensity monochromatic light in a small cuvette (3 mL), the practical applicability of the ferrioxalate-POM actinometry was tested with a larger reactor (60 mL) irradiated by a polychromatic light source (4 W black light bulbs (BLB), Philips TL4W) at ambient temperature. The BLB emission wavelengths ranged in 350-400 nm and the relative intensity profile of the emission was measured by a Spectropro-500 spectrometer. By varying the number of BLBs, the light intensity onto the reactor was controlled. With this reactor system, the light intensities were determined by both the ferrioxalate actinometry and the proposed ferrioxalate-POM actinometry and compared each other to assess the reliability of the ferrioxalate-POM system as a practical actinometry.
Results and Discussion Figure 1 shows the time-dependent generation of SiW125(POM-) (via reaction 5) in the ferrioxalate-POM solution (SiW124-/Fe3+/oxalate) under white light irradiation and its
decay (via reaction 6) in the following dark period, which was monitored by measuring the absorbance at 730 nm. The actinometer solution consisted of 5 mM ferrioxalate and 1 mM POM absorbs up to 450 nm and exhibits an almost complete absorption in the wavelength below 400 nm. The POM alone solution does absorb UV light below 360 nm but its absorbance is far smaller than that of ferrioxalate (Figure 1a inset). UV irradiation onto SiW124- alone, SiW124-/ Fe3+ or SiW124-/oxalate solution did not induce any color appearance, which indicates that the presence of Fe(III)-oxalate complexes is essential for the photoinduced reduction of SiW124-. The ferrioxalate-POM actinometer solution develops negligible coloration under room light condition (∆A730 less than 0.003 in 2 h) and the strict dark condition is not required for this actinometry. As another POM candidate in the ferrioxalate-POM actinometry, we also tested PW12O403- and PMo12O403- that can be similarly reduced by CO2 anion radicals [e.g., k(PW123- + CO2‚-) ) 2.96 × 109 M-1 s-1 (14); k(PW124- + O2) ) 2.9 M-1 s-1 (14)]. However, UV irradiation for 30 min led to an appearance of precipitates in these ferrioxalate-POM solutions. Therefore, SiW124- was selected for this ferrioxalate-POM actinometry. CO2 anion radicals that are generated in-situ from the UV-irradiated ferrioxalate solution have a reducing power strong enough to reduce O2 (23), methyl viologen (24), metal ions (Ni2+ (25), Cd2+, Zn2+ (26)), and NDMA (27). As shown in Figure 1b, the reduction of SiW124- by CO2‚- proceeds rapidly and reaches the saturation within a few minutes of white light irradiation whereas the reoxidation of SiW125(i.e., discoloration) in the dark is far slower and takes more than 3 h. The absence of dissolved O2 (under continuous N2 sparging condition) decelerates the discoloration process in the dark but does not stop it. It is known that good electron acceptors such as H2O2, Fe3+, and Ag+ are able to reoxidize POM- (19). The fact that blue-colored SiW125- could be bleached even under the anoxic condition implies that SiW125can be oxidized by Fe(III)-oxalate.
[FeIII(C2O4)3]3- + SiW125- f Fe2+ + 3(C2O4)2- + SiW124- (7) The slow reoxidation of SiW125- allows the static spectrophotometric method to be applied for the quantification of POM-. It should also be noted that the cycles of photoreduction and reoxidation (POM f POM- f POM) could be repeated several times without a significant loss of the activity (Figure 1b inset). Quantum yields (Φ) for the production of SiW125- in the ferrioxalate-SiW124- solution could be determined at selected wavelengths in 300-400 nm range by dividing the production rate of SiW125- (mol/L‚min) by the light intensity (einstein/ L‚min). Figure 2 shows that the production rate of SiW125in the ferrioxalate-SiW124- solution increases linearly with the incident light intensity at selected wavelengths. The quantum yields determined from the line slope in Figure 2 are listed in Table 1. The regression lines were forced to pass the origin. The linearity was reasonably good in 300-390 nm range but not satisfactory at 400 nm. The value of Φ(SiW125-) ranges 0.11-0.21 in the 300-390 nm region with its maximum at 350 nm, and it is fairly constant around 0.18 in the 335380 nm range for which the proposed ferrioxalate-SiW124actinometry can most properly be used. Figure 3 shows that both the production of SiW125- in the ferrioxalate-SiW124- solution and the production of Fe2+ in the ferrioxalate solution (determined by the standard method using 1,10-phenanthroline) linearly increase with the illumination time under white light (Figure 3a) or monochromatic light (at 350 nm) (Figure 3b). The conventional
FIGURE 2. The production rate of SiW125- as a function of the light intensity at selected wavelengths, (a) 300-350 nm and (b) 365-400 nm ([oxalate]0 ) 60 mM, [Fe3+]0 ) 5 mM, [SiW124-]0 ) 1 mM, pHi 4.5).
TABLE 1. Quantum Yields for the Production of SiW125- in the UV-Irradiated Ferrioxalate/SiW124- Solutiona wavelength (nm)b
(SiW125-)
300 320 335 350 365 380 390 400
0.11 ( 0.001 0.13 ( 0.005 0.18 ( 0.009 0.21 ( 0.009 0.18 ( 0.001 0.16 ( 0.004 0.14 ( 0.003 0.09 ( 0.002
a [oxalate] ) 60 mM, [Fe3+] ) 5 mM, [SiW 4-] ) 1 mM, pH 4.5, 0 0 12 0 i air-equilibrated b This is the center wavelength of the irradiation. Each wavelength window is 10 nm wide (fwhm). The light intensity at each wavelength was varied in the range of (0.6-8.0) × 10-4 einstein/min‚L to determine the quantum yield.
ferrioxalate actinometry can be replaced by the ferrioxalateSiW124- actinometry as long as the production rate of Fe2+ in the former is linearly correlated with the production rate of SiW125- in the latter as demonstrated here. Note that the VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Effects of the Concentrations of SiW124- and Fe3+ on Φ(SiW125-) in Ferrioxalate/SiW124- Actinometrya wavelength (nm)
Φ(SiW125-)
[SiW124-]0 (mM)
[Fe3+]0 (mM)
320
0.13 ( 0.005 0.18 ( 0.003 0.18 ( 0.004 0.18 ( 0.009 0.21 ( 0.001 0.22 ( 0.002 0.20 ( 0.003 0.12 ( 0.002 0.21 ( 0.009 0.24 ( 0.003 0.13 ( 0.001 0.16 ( 0.004 0.16 ( 0.001 0.11 ( 0.001
1 2 3 1 2 3 1 1 1 1 1 1 1 1
5 5 5 5 5 5 2.5 10 5 2.5 10 5 2.5 10
335
350 380
a
FIGURE 3. The photoinduced production of SiW125- and Fe2+ in the ferrioxalate-POM and ferrioxalate solution, respectively, under (a) white light (I ) 1.1 × 10-3 einstein/min‚L) and (b) monochromatic light (at 350 nm) irradiation (I ) 3.9 × 10-4 einstein/min‚L) ([oxalate]0 ) 60 mM, [Fe3+]0 ) 5 mM, [SiW124-]0 ) 1 mM, pHi 4.5). linearity between [SiW125-] and the irradiation time is maintained, even up to 90% photoconversion of SiW124-. Incidentally, it should be mentioned that the ferrioxalatePOM actinometry data presented in Figures 2 and 3 showed more variation from the linearity trend compared with those of the ferrioxalate actinometry. This seems to be a tradeoff for the rapid and simple measurement. The slow reoxidation of POM- in the ambient condition, interfering light absorption by POM and POM-, and the complexity of radical chemistry involving CO2‚- might be responsible for this variation. Some aspects are discussed below. The ferrioxalate-POM actinometry is as simple as measuring absorbance with a UV-visible spectrophotometer but the underlying chemistry is rather complex. In the ferrioxalate actinometry, incident photons are absorbed by the ferrioxalate complex alone. On the other hand, in ferrioxalatePOM actinometry, photons are absorbed by not only the ferrioxalate complex but also by POM whose absorption band tails up to near 360 nm (see the inset of Figure 1a) although the latter fraction should be much smaller than the former. Therefore, the overall performance of the ferrioxalate-POM actinometry should depend on the POM concentration. Table 2 shows the effect of POM concentration on Φ(SiW125-) at 320 and 335 nm where the light absorption by POM could be non-negligible. Although a higher POM concentration should shield more UV light, Φ(SiW125-) slightly increased with increasing [POM] in the range of 1-3 mM, contrary to the expectation. This implies that the increased UV shielding 5436
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[oxalate]0 ) 60 mM, pHi 4.5, air-equilibrated.
at higher POM concentration is counterbalanced by other effects. The reaction of photogenerated CO2‚- can be branched into three paths: (i) CO2‚- + O2 [k ) 2.5 × 109 M-1 s-1 (28)]; (ii) CO2‚- + FeIII(C2O4)33- [k . 8 × 109 M-1 s-1 (29)]; (iii) CO2‚- + SiW124- [k ) 8.4 × 108 M-1 s-1 (14)]. Upon increasing the concentration of POM, the generation of CO2‚from ferrioxalate might be somewhat reduced because of the light shielding by POM, but this effect seems to be offset by the much enhanced rate of the third path (or reaction 5). As a result, the concentration of SiW124- seems to have a minor effect on the efficiency of the ferrioxalate-POM actinometry in the 1-3 mM range, and the concentration of SiW124- was typically set to 1 mM in this study. The application of this ferrioxalate-POM actinometry is limited in the 300390 nm range because POM strongly absorbs UV light below 300 nm and the photogenerated POM- interferes by absorbing visible light above 390 nm (see Figure 1a). The concentration of ferric ions is also an important parameter in determining the overall performance of the ferrioxalate-POM actinometry. With the concentration of SiW124- fixed at 1 mM, Table 2 also shows the effect of ferric ion concentration (2.5, 5, 10 mM). Increasing [Fe3+] from 2.5 to 5 mM little changed Φ(SiW125-) but the further increase up to 10 mM markedly reduced the quantum yield. The linear correlation between the production rate of POMand the light intensity was fully demonstrated in all cases regardless of the wavelength and [Fe3+]. More CO2‚- radicals should be generated with higher concentration of ferrioxalate since more photons are absorbed. In turn, more CO2‚radicals should induce more POM-. However, the fact that the generation rate of POM- was reduced when increasing [Fe3+] from 5 to 10 mM indicates that excess ferrioxalate hinders the production of POM-. In fact, the UV photon absorption by ferrioxalate complexes is almost completely saturated in the presence of [Fe3+] ) 5 mM (see Figure 1a inset). Therefore, further increasing [Fe3+] beyond this level does not increase the generation rate of CO2‚- but induces negative effects. With excess ferric ions present, their inhibition effects can be twofold: (1) excess ferrioxalate scavenges POM- through reaction 7 (ferrioxalate + POM-); (2) excess ferrioxalate scavenges CO2‚- through reaction 3 (ferrioxalate + CO2‚-) and consequently inhibits the production of POM-. Since Φ(SiW125-) of the ferrioxalatePOM actinometry depends on the concentrations of ferrioxalate and POM, the optimal concentrations of Fe(III)oxalate and SiW124- are proposed to be 5 and 1 mM, respectively. The validity of the ferrioxalate-POM system as a practical actinometry was confirmed by the comparison with the ferrioxalate actinometry. The two actinometries determined
the light intensities (from 4 W BLB lamps) incident onto a reactor (60 mL) on the basis of Φ(SiW125-) determined in this work (Table 1) and the literature quantum yields of the ferrioxalate actinometry. The BLB lamp has emission spectra ranging from 350 to 400 nm. The wavelength-dependent Φλ(SiW125-) values were roughly averaged to be 0.17 through Σ(Iλ‚Φλ) with taking the relative emission intensity at selected wavelengths (Iλ: 0.02, 0.57, 0.31, 0.09, and 0.02 at 350, 365, 380, 390, and 400 nm, respectively) into account. The light intensities (I) determined at two different irradiation conditions showed a reasonable agreement between the proposed ferrioxalate-POM and the ferrioxalate actinometry (in this order) as follows.
I ) (1.56 ( 0.04) versus (1.41 ( 0.04) × 10-4 einstein/L‚min (with 4 BLBs) I ) (2.79 ( 0.06) versus (2.67 ( 0.03) × 10-4 einstein/L‚min (with 6 BLBs) It surely demonstrates that the ferrioxalate-POM actinometry can be employed as a simple and reliable actinometer. However, the ferrioxalate-POM actinometry significantly underestimated the light intensity when it is lower than 0.7 × 10-4 einstein/L‚min, which seems to be the lower intensity limit of the ferrioxalate-POM actinometry. We have demonstrated that the ferrioxalate-POM system can be used as a simple and rapid chemical actinometry that can quantify the UV light intensity in the 300-390 nm region. Each chemical actinometer has its own working wavelength region (WR) such as potassium iodide in N2O-saturated aqueous solution (Φ ) 0.235, WR:254 nm), glucose (Φ ) 0.33, WR:200-300 nm), p-benzoquinone (Φ ) 0.47, WR:260380 nm), and potassium ferrioxalate (Φ ) 1.25-0.9, WR: 250-500 nm) (1). Although the ferrioxalate-POM actinometry works for the narrower wavelength region (300-390 nm) compared with that of the ferrioxalate actinometry (250500 nm), it has strong merits of simplicity and rapidity in the analytical procedure. This alternative actinometry eliminates the postirradiation analytical procedure that is required in the standard ferrioxalate actinometry and enables the in-situ quantification of the light intensity by a simple absorbance measurement. This new actinometry can be conveniently carried out with a standard spectrophotometric quartz cuvette in which both the irradiation and the subsequent in-situ absorbance measurement are done. No rigorous dark condition is required for this actinometry because the photosensitive phenanthroline complex is not used. In addition, it does not suffer from the slow color development of the Fe(II)-phenanthroline complex, the competitive complexation of phenanthroline by Fe(III), and the interference from impurity metal ions that may be present in the fresh ferrioxalate solution (9-11). This new actinometry is ideally suited for the rapid measurement of the light intensity in the UVA region. Measuring the light intensity from BLBs whose irradiation is usually centered around 360-370 nm is a good example. BLBs are widely used as a primary UV light source in a variety of photochemical processes. This new actinometry can be immediately applied to many important environmental photochemical processes such as semiconductor photocatalysis and photo-Fenton reaction that effectively utilize UVA photons.
Acknowledgments This work was supported by the Basic Research program of KOSEF (grant no. R01-2006-000-10019-0), the SRC/ERC program of MOST/KOSEF (grant no. R11-2000-070-080010), and the Brain Korea 21 program.
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Received for review February 24, 2007. Revised manuscript received May 17, 2007. Accepted May 25, 2007. ES070474Z
