Environ. Sci. Technol. 2010, 44, 9075–9079
Photocatalytic Dechlorination of Polychlorinated Biphenyls Using Leuco-methylene Blue Sensitization, Broad Spectrum Visible Lamps, or Light Emitting Diodes MARYAM IZADIFARD, COOPER H. LANGFORD,* AND GOPAL ACHARI Departments of Chemistry and Civil Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4
Received June 12, 2010. Revised manuscript received August 27, 2010. Accepted October 13, 2010.
Photocatalytic routes to dechlorinate polychlorinated biphenyls (PCBs) have considerable potential for development. This paper describes efficient dye-photocatalyzed processes which can be driven by long wavelength light sources including lightemitting diodes (LEDs), fluorescent lamps, and quite probably sunlight. The reduced form of methylene blue (MB), leucomethylene blue (LMB), has previously been found to photoinduce dechlorination of chloroaromatics with an electron transfer from its triplet excited state. Sodium borohydride, used in this case is an efficient sacrificial reductant, which can maintain LMB as the major species in competition with air oxidation of LMB to MB. There is also evidence that it plays a further (chain reaction) role in promoting the LMB photodechlorination process as well. The generality of the photoelectron transfer from reduced members of the phenothiazine dye family is demonstrated with phenothiazine and leuco-methylene green when a wavelength (UV) is chosen to produce the highly reductive triplet. It is likely that dechlorination can be initiated by many triplet excited states with adequate reduction potential.
Introduction Hawari et al. (1) developed a very promising photodechlorination scheme using 254 nm Hg lamps in a basic 2-propanol medium. It has since been implemented on bench pilot scales (2); yet, there has been a continuing effort to photodechlorinate PCBs with wavelength light sources longer than that of deep UV lamps. So far, the strategy has been to use longer wavelength light sources such as “black light” lamps, fluorescent lamps, and xenon arc lamps along with various sensitizers to compensate for lack of absorbance (λ > 300 nm) of PCBs (1, 3-8). This is a good strategy if the final goal is to use sunlight energy to dechlorinate PCBs; otherwise, the reaction at 254 nm is very fast and efficient which compensates for the high power consumption of UV light sources. Other Hg vapor-based or xenon arc lamps offer, at best, small gains in efficiency. If nonsolar longer wavelength sources are to be used, approaches need to utilize new light sources or a much longer wavelength photochemistry with * Corresponding author phone: 403 220 3228; fax: 403 289 9488; e-mail:
[email protected]. 10.1021/es1019993
2010 American Chemical Society
Published on Web 11/03/2010
a comparable rate and efficiency to that developed by Hawari et al. at 254 nm. This report describes systems that show promise of meeting one or both of these criteria. Light-emitting diodes (LEDs), a new generation of light sources, are potential replacements for conventional gas discharge light sources. Efficient electrical to light energy conversion, long lifetime, directed output, availability of specific wavelengths to efficiently match chromophores, less heat production, and DC operation for remote locations are attractive characteristics of LEDs. Although deep UV light sources accomplish direct photodissociation of PCBs, high intensity UV LEDs are not yet commercially available (9). However, high intensity near UV and visible LEDs are commercially available, and production of commercial LEDs with shorter wavelengths and high output is in progress. With currently economical, commercially available LEDs, for which cost per unit power output decreases with increasing wavelength, sensitizers are required to dechlorinate PCBs. Methylene blue (MB) has been reported to be a sensitizer (but see below) for dechlorination of chlorinated compounds under fluorescent lamp irradiation in the presence of a sacrificial electron donor (10-12). In our previous study (13), we reported the mechanism of reaction. Leuco-methylene blue (LMB), the reduced form of MB, was the functional sensitizer for that system. It was shown that the red region of the light source is responsible for reduction of MB to LMB with a mechanism as shown in Scheme 1a (14), and the extreme blue and near UV region is responsible for the reductive dechlorination of PCBs. LMB can also be produced thermally using a reducing agent such as glucose or sodium borohydride (Scheme 1b). We report here a study of performance of PCB sensitization with thermally produced LMB using conventional fluorescent lamps and a blue LED light source. To optimize absorbance for LMB, LED use would exploit a 385 nm LED-based reactor. This is feasible with commercially available high intensity 385 nm LEDs but not available to us at present. The concept is demonstrated here with a less efficient choice of a 436 nm blue LED reactor and model experiments with computations showing the relative performance of 436 and 385 nm irradiation wavelengths. PCB 138 (2,2′,3,4,4′,5′- hexachlorobiphenyl) was the representative PCB of choice for this study, and sodium borohydride (NaBH4), which is a good water-soluble reducing agent, was used to thermally reduce MB to LMB at a sufficient rate to maintain the LMB concentration at >95% in the presence of air. Fluorescent lamps were used in the first instance as an economical light source, paralleling sunlight, to study the efficiency and mechanism of the reaction. This was followed with use of the 436 nm LED reactor. Finally the efficiency of the reaction for illumination in both systems
SCHEME 1. Photoassisted and Thermal Reduction of MB to LMB (modified from the method of Mills et. al. (14))a
a
Glu (glucose) and NaBH4 are alternate reducing agents.
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was tested with a commercial PCB mixture, Aroclor 1254, and with a dioxin-like PCB congener, PCB77 (3,3′,4,4′tetrachlorobiphenyl).
Materials and Methods Materials. Standard PCB congeners used in this study: 2,4′diCB (PCB 8), 4,4′-diCB (PCB 15), 2,4,4′-triCB (PCB 28), 2,4′,5triCB (PCB 31), 3,3′,4-triCB (PCB 35), 3,4,4′-triCB (PCB 37), 2,2′,3,5′-tetraCB (PCB 44), 2,2′,4,5′-tetraCB (PCB 49), 2,3′,4,4′tetraCB (PCB 66), 2,3′,4′,5-tetraCB (PCB 70), 3,3′,4,4′-tetraCB (PCB 77), 2,2′,3,4,5′- pentaCB (PCB 87), 2,2′,3,5′,6-pentaCB (PCB 95), 2,2′,3,4′,5′-pentaCB(PCB 97), 2,2′,4,4′,5-pentaCB (PCB 99), 2,2′,4,5,5′-pentaCB (PCB101), 2,3,3′,4,4′-pentaCB (PCB 105), 2,3′,4,4′,5-PentaCB (PCB 118), 2,2′,3,4,4′,5′-hexaCB (PCB 138), and Aroclor 1254 were obtained from Chromatographic Specialties Inc. Sodium borohydride (Aldrich, 98%), triethylamine (Aldrich), methylene blue (Aldrich), methylene green (EMD chemicals, 98%), phenothiazine (Aldrich, 98+%), acetonitrile (EMD chemicals, HPLC grade), and hexane (EMD chemicals, ACS reagent grade) were used as purchased. Distilled deionized water was used as required in the experiments. Photolysis Procedure, Sample, and Data Analyses. A solution containing the desired concentration of a PCB, methylene blue or methylene green, and sodium borohydride in 10 mL of mixed solvent comprising of 5 mL of acetonitrile and 5 mL of water was prepared in a pyrex tube; the concentrations of the reagents used for each experiment are given in Results and Discussion. Before irradiation, the PCB 138 solution containing MB and NaBH4 was left in a closed cell for complete reduction of MB to LMB; because of the absorption tail of LMB in the visible region, the solution appears yellow. If required, 10 min of nitrogen bubbling was used for deaeration of the samples. The solutions were subjected to irradiation for appropriate times in a Rayonet photoreactor with 14 8-W (S3b8) ordinary fluorescent lamps or in a blue LED reactor (the design of this LED reactor has been described previously by Ghosh et al. (15)) equipped with 81Gilway “super bright” (Peabody, MA) E472 blue LEDs with their output centered at 436 nm. A magnetic bar was used to stir the mixture during irradiation. Light intensities entering the reaction vessel were measured by conventional ferrioxalate actinometry following the procedure from Calvert and Pitts (16). The intensity of light entering the vessel in the Rayonet reactor was measured to be 2.80 × 1016 photons/s over the range of ferrioxalate (UV to ∼520 nm). For the LED reactor, intensity of light was 3.76 × 1015 photons/s at 436 ( 20 nm. Samples taken at different time intervals were extracted with hexane (by shaking in a shaker for 15 min) and analyzed on an Agilent 6890 chromatograph (GC) with an electron capture detector. A DB-608 column (30 m length, 0.25 mm ID, and 0.25 µm film thickness) was used. The GC conditions and the temperature program are reported elsewhere (17). Comparisons were made to reactions in deaerated basic 2-propanol and found to closely approximate literature data (1), (2), (13).
Results and Discussion Dechlorination of PCB 138 with MB and NaBH4 Using Fluorescent Lamps. PCB 138 was dechlorinated (>95%, based on disappearance of PCB 138) under broad-band fluorescent lamp irradiation (14 lamps) in the presence of methylene blue and NaBH4 in about 10 min. Using the highest concentration of MB feasible (see the next section), PCB 138 was 97% dechlorinated after 2 min of irradiation; a lower concentration of MB was used here in order to facilitate analysis of product congener distribution. The decrease in PCB 138 was accompanied by an increase in less chlorinated congeners and biphenyl. Figure 1 shows major products of 9076
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FIGURE 1. Dechlorination of an air saturated solution of PCB 138 (7.92 × 10-5 M) using MB (2.33 × 10-3 M) and NaBH4 (5.90 × 10-1 M) irradiated with 14 ordinary fluorescent lamps. Calibration of concentrations was done with PCB 138 and other congeners estimated based on relative sensitivity. dechlorination of PCB 138 and their related peak areas under visible light irradiation as a function of time. On the basis of the retention time of the available standards, the major peaks were recognized, but there are several unrecognized PCB congeners, which were all undetectable after 30 min irradiation. The final product is biphenyl, reaching an approximate steady state at 30 min and remaining near there under irradiation for more than 90 min (not shown in figure). Performing a similar experiment in a deaerated solution shows that the reaction is faster in a deaerated solution, and PCB 138 is consumed (>95% loss of 138) in less than 1 min. However, in contrast to photoreduction of MB to LMB in the presence of a sacrificial electron donor such as triethylamine (13), when NaBH4 is used, the deaeration of the solution is not essential and the efficiency of the reaction in a closed vial is sufficient from a practical point of view. This implies that reduction of MB to LMB by NaBH4 is faster than reoxidation of LMB by air (an important point for possible applications). Therefore, the remaining experiments were performed without deaeration but in a closed cell. Dechlorination of PCB 138 with MB and NaBH4 Using a Blue LED Reactor. Since LEDs produce nearly monochromatic light, MB has to be thermally reduced to LMB to be able to use a sole blue LED reactor. If the reaction mixture can be irradiated in a reactor equipped with two wavelength LEDs (e.g., red and blue LEDs), then aliphatic amines could be used for the MB photoreduction. Because of the mechanistically central role of blue LEDs (436 nm), thermal reduction was chosen for this study. In a blue LED reactor described in Materials and Methods, PCB 138 was successfully dechlorinated by LMB in about 40 min (Figure 2). The distribution of the congeners is similar to that for dechlorination of PCB 138 under conventional fluorescent lamps, except that the reaction is slower because of the lower intensity due to limiting output to only the tail of the absorption spectrum of LMB (the concentration of LMB is the optimal concentration for this system, vide infra). To the best of our knowledge, this is the first time that an LED light source has been used for dechlorination of PCBs. This experiment was performed to show that LED light sources also can be used for dechlorination of PCBs. It has been shown previously that in order to take full advantage of the LED light sources, a dye or a reagent with absorption spectra optimally matched to the LED output is required (15). For LMB a reactor with shorter wavelength 385 nm LEDs would be more efficient because of the absorption region of LMB compared to the output of blue LEDs. In order to show the relative performance of 436 and 385 nm LEDs, two
FIGURE 2. Dechlorination of an air saturated solution of PCB 138 (7.00 × 10-5 M) using MB (1.32 × 10-2 M) and NaBH4 (5.40 × 10-1 M) in the LED reactor. Concentration calibration as in Figure 1. experiments were performed using a spectrofluorometer with a xenon lamp and 10 nm slit width at 436 and 385 nm to provide approximately monochromatic light to simulate LED light sources. The results presented in Figure S1 in Supporting Information show that the reaction at 385 nm is at least 7 times faster than the reaction at 436 nm using the light intensities provided by a spectrofluorometer’s xenon lamp. If corrections for the light intensities are applied, the difference in rate is even more than 7 times. Therefore, using 385 nm LED light sources seems promising. With blue LEDs though, increase in efficiency of the reaction might be achieved by increasing the absorbance of LMB by increasing its concentration. To find the optimum concentration of LMB, a series of 1-h (minimum) experiments were conducted using different concentrations of MB. The results (Figure S4 in Supporting Information) show that the optimal reaction rate occurred at 13 mM of MB for our LED reactor. Further increases in MB concentration decreased the reaction rate, suggesting self-quenching of the excited states. Since the intensity is greater and the spectral range covered is better under 14 fluorescent lamps irradiation in Rayonet, it was used for the following experiments to elucidate the mechanism. Mechanism of the Reaction. The mechanism of dechlorination of PCBs using MB and aliphatic amines under fluorescent light irradiation has recently been proposed (13). The involvement of LMB was thoroughly documented, where aliphatic amines were used as sacrificial electron donors, reducing MB to LMB under red light (14). In the current system, NaBH4 is used as a thermal reducing agent. As soon as NaBH4 is added to the MB solution, the reaction starts and MB is quantitatively reduced to its leuco form in less than 3 min. The produced LMB becomes excited by the blue to near UV region of the light source, promoting electron transfer from triplet excited state of LMB to the PCB 138 molecules (photoinduced electron transfer) as discussed previously (13). After electron transfer from LMB to the PCBs, the resulting oxidized state MB is reduced to LMB again by excess NaBH4, which is able to reduce MB to LMB at a sufficient rate by hydride transfer to maintain the LMB concentration in the presence of air (NaBH4 is consumed in the cycle). Our results show that dechlorination of PCB 138 in the presence of NaBH4 is much faster than that in the presence of TEA as reductant. Performing the experiments using these two reducing agents (the experimental details on MB-TEA and the fluorescent lamps are given in ref 13) under the same conditions, i.e., deaerated solutions, 14 fluorescent
FIGURE 3. (A) Dechlorination of PCB 138 (6.31 × 10-5 M) in the presence of MB (1.98 × 10-3 M) and TEA (0.711 M) with 2 and 14 fluorescent lamps (deaerated). (B) Dechlorination of PCB 138 (7.92 × 10-5 M) in the presence of MB (2.32 × 10-3 M) and NaBH4 (5.89 × 10-1 M) with 2 lamps (deaearated).
FIGURE 4. Dechlorination of PCB 138 (5.54 × 10-5 M) using (a) PT (4.52 × 10-3 M) and NaBH4 (5.20 × 10-1 M), (b) PT (4.62 × 10-3 M), (c) MB (4.28 × 10-3 M) and NaBH4 (5.39 × 10-1 M), and (d) NaBH4 (5.40 × 10-1 M) under visible light (14 lamps) in nondeaerated solutions. lamps, and the same concentration of MB and PCB 138, shows that with NaBH4, PCB 138 is >95% dechlorinated in less than 1 min (because the reaction was so fast, the kinetics using only two lamps are shown in Figure 3), while with TEA, >95% dechlorination requires at least 15 min after an induction period which is required for LMB formation. The rate of the reaction in the presence of NaBH4 is quite high, more so than seems assignable to the rates of rereduction to replenish LMB. It is possible that NaBH4 has a role beyond that of a reducing agent for MB. This hypothesis received confirmation from an experiment with phenothiazine (PT) as a sensitizer. Phenothiazine Study. Phenothiazine (PT) has a similar structure to LMB after MB reduction and an air-stable reduced oxidation state with no absorbance at wavelengths longer than 400 nm. PT has been used as a sensitizer in alkaline 2-propanol by Hawari et al. (1) for photodechlorination of Aroclor 1254 using both the UV tail in sunlight and 350 nm lamps as light sources. Because PT is stable in the reduced form, there is no requirement for a reducing agent, and it is expected to give results similar to those with LMB after MB reduction. Reaction in the presence of PT without NaBH4 is not found to be efficient, and it is not comparable to the reaction in the presence of LMB produced by reaction of MB and NaBH4 (Figure 4). Once NaBH4 is added to the PT system, PCB 138 is >95% dechlorinated in less than 10 min, which seems similar to that of the PT in the alkaline VOL. 44, NO. 23, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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SCHEME 2. NaBH4 Mechanistic Possibilities: Intersystem Crossing (ISC) between Singlet and Triplet Excited States (18-23)
2-propanol system (Hawari (1)). After 20 min of irradiation, there are still residual lesser-chlorinated biphenyls while for a similar irradiation time, in the case of LMB, there is no chlorinated compound remaining. The additional role of BH4- may be donation of an H atom to the aryl radical to produce a reactive BH3 · - radical; see Scheme 2. NaBH4 Mechanistic Possibilities. NaBH4 has been used by several researchers to enhance dechlorination of excited polychlorinated biphenyls. On the basis of proposals in the literature, the photoreduction of PCBs by NaBH4 may occur by three different competing mechanisms: (1) photoinduced hydride attack on a chlorobiphenyl excited state (18), (2) photoinduced electron transfer from BH4- to the chlorobiphenyl excited state (19, 20), and (3) homolysis of C-Cl bond and then reaction of PCBs with BH4-. All require excitation of the PCB by UV (21-23). A summary of possible mechanisms is shown in Scheme 2. The last two processes which involve electron transfer from BH4- to PCBs might be initiation steps for a chain mechanism proposed by Barltrop et al. (21): + •Cl• + BH4 f Cl + BH3 + H •Ar• + BH4 f ArH + BH3
BH•3
•
-
ArCl + f Ar + BH3 + Cl 2BH3 f B2H6 in H2O f borate + H2 The proposal of a chain reaction can be supported by reaction quantum yields of higher than 1.0 and by inhibitory effects of free radical scavengers (e.g., acrylonitrile) on the efficiency of the reaction. Epling et al. (24) reported a chain mechanism for photodechlorination of chlorobenzene (Φr ) 2.14) and for debromination of bromobiphenyls (Φr ) 4.5, 5.6, 9.8, and 12.7 for different brominated compounds) under 254 nm irradiation. Their results also revealed inhibitory effects of acrylonitrile on the reaction. A chain reaction, as proposed by Barltrop et al. (21) for halogenated aromatic hydrocarbons, was proposed for both cases (Scheme 2). Epling et al. also studied 254 nm photodechlorination of PCBs in the presence of NaBH4 (18) and reported that the dechlorination of PCBs does not proceed via a free radical chain reaction because the measured quantum yields were lower than that observed for the chloro and bromo aromatics and there was no decrease in the efficiency of dehalogenation with scavengers present. However, in their report on dechlorination of PCBs in a micellar system (24), they proposed a mixed mechanism or a predominately electron transfer pathway for dechlorination of PCBs but did not make any final conclusions. Note that all these experiments were performed under 254 nm irradiation using low pressure mercury lamps, and the role of NaBH4 was to enhance the known photodehalogenations of haloaromatic compounds. All three mechanisms mentioned previously require initial PCB excitation by light absorption. Under visible light irradiation, PCBs fail to absorb. With the conventional fluorescent light sources, UV wavelengths are limited to the wavelengths in the near UV region with low intensities; probability of excitation of PCBs with these light sources is 9078
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SCHEME 3. A Plausible Mechanism for Reductive Dechlorination of PCBs with LMBa
a NaBH was used to reduce MB. The dotted lines show how a chain 4 could arise.
very low. Our results show that NaBH4 alone produces no dechlorination of PCB under visible light (Figure 4). For a system with visible light sources, BH4- could only be involved if aryl radicals are produced indirectly. Aryl radicals presumably react with BH4- to produce BH3 · - for which there is precedent to suggest electron transfer to PCB and the chain reaction proposed by Barltrop et al. (22). The quantum yield of disappearance of PCB 138 under fluorescent lamp irradiation was measured as a test for chain reaction. The quantum yield with the highest practical concentration of LMB was measured to be approximately 1.6 with insufficient confidence to give strong support to a chain mechanism. On the other hand, oxygen retarded the reaction by a factor of ∼7. Dechlorination of PCB 138 at 350 nm. Since the blue to near UV region of the fluorescent lamps is responsible for the reaction of LMB and PCB 138 and the reaction is more efficient at shorter wavelength (see spectrum in Supporting Information), an experiment at 350 nm was performed to compare the efficiency of the BH4- reaction with that of direct dechlorination of PCB 138 in alkaline 2-propanol at 254 nm. For the reactions performed in closed cells irradiated with eight lamps (254 nm in alkaline 2-propanol or 350 nm with MB and BH4-) and without deaeration, PCB 138 is >95% consumed in less than 2 min. The light intensities were measured to be 1.18 × 1016 and 1.08 × 1016 photons/s for 350 and 254 nm lamps, respectively, similar light intensities. Therefore, the efficiency of the borohydride reaction is comparable to a system with a known chain reaction. Plausible Mechanism of the Reaction. The results provide evidence for the involvement of borohydride beyond reduction of MB in the mechanism of dechlorination. Scheme 3 offers a plausible mechanism for the reaction, but at this point, more definitive evidence for a chain reaction mechanism is desired. Processes shown with solid lines basically present the photocatalytic dechlorination of PCBs with LMB,
while oxidized LMB is efficiently reduced back to LMB by excess borohydride. Dotted lines show how a chain reaction could be added to the previous processes. Using Methylene Green as an Alternative Dye to MB. Methylene green (MG) with a similar structure to MB was also evaluated for dechlorination of PCB 138 under fluorescent lamps and in the LED reactor. As suggested by MG absorptivity at a shorter wavelength than for MB, LMB itself has a much higher molar absorptivity for the visible and near UV region than LMG (see Figure S5 in Supporting Information). Although MG is easily reduced to its leuco form with NaBH4, there is neither reaction under fluorescent nor under LED light; no dechlorination was observed, presumably because LMG has no absorption in the region of interest. The generality of the basic mechanism was confirmed by dechlorination of PCB 138 under 350 nm irradiation. PCB 138 was completely dechlorinated in the presence of LMG in 10 min, with 14 “black light” lamps with emission centered at 350 nm. Dechlorination of a Nonortho PCB, PCB 77, and Aroclor 1254. LMB produced by the borohydride reduction was successfully used for dechlorination of PCB 77, which is one of the most toxic PCB congeners, and for Aroclor 1254, a commercial congener mixture with high toxicity. The results showed that PCB 77 was dechlorinated with an efficiency similar to PCB 138 in both the Rayonet and LED reactors, which was expected based on the reduction potentials of these congeners. Aroclor 1254 was completely dechlorinated under irradiation with 14 ordinary fluorescent lamps in about 30 min (Figure S2 in Supporting Information). It was also possible to dechlorinate Aroclor 1254 in the blue LED reactor (Figure S3) but it takes a longer time. Using 385 nm LEDs is recommended to achieve a faster reaction. Because using sunlight energy for dechlorination of PCBs was one of the objectives, one test was performed with a Newport 92215A solar simulator under conditions as described in the caption of Figure 1. Dechlorination was >95% complete in less than 10 min. All the results underscore the application of this system for LED or sunlight treatment of PCBs. For example, replacing Hg lamps with 385 nm LEDs in a pilot scale system for soil PCB cleanup as described earlier (2) may be feasible for the practical dechlorination of PCBs.
Acknowledgments Support in the form of grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.
Supporting Information Available Additional information as noted in the text. This information is available free of charge via the Internet at http:// pubs.acs.org/.
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