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Photoassisted Fenton Degradation of Nonbiodegradable Azo Dye (Orange II) in Fe-Free Solutions Mediated by Cation Transfer Membranes J. Fernandez,† J. Bandara,† A. Lopez,‡ Ph. Buffat,§ and J. Kiwi*,† Institute of Physical Chemistry II, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland, Department of Chemistry and Technology, National Research Council, Water Research Institute, Via de Blasio 5, 70123 Bari, Italy, and Institute of Electron Microscopy, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland Received April 8, 1998. In Final Form: October 14, 1998
Photoassisted degradation of nonbiodegradable Orange II is shown to be catalyzed by Nafion cationtransfer membranes exchanged with Fe ions in the presence of H2O2. The Nafion membranes in the oxidative media used degraded Orange II with similar kinetics as found in the homogeneous Fe3+/H2O2 photoassisted catalysis, avoiding the drawbacks of the homogeneous treatment. The treatment of this model textile dye is shown to proceed via a Fenton-like process without sludge production because of the selective H2O2 decomposition on the Fe ions exchanged on the membrane. The effect of the concentration of H2O2, solution pH, azo dye concentration, and light intensity (visible light) on the degradation of Orange is reported in detail. The activity of the membranes during the Orange II decomposition was tested for 1500 h and was observed to remain fairly stable within this period. The Fe/Nafion membranes consisted mainly of Fe2O3 (78%) before reaction and Fe2O3 (14%) after light irradiation during Orange II oxidation, as found by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The size of the Fe particles in the Nafion was investigated by transmission electron microscopy (TEM) and turned out to be 37 ( 4 Å. These Q-sized Fe particles on the Nafion absorbed directly the light energy, avoiding the losses due to absorption by the contaminants as it is the case in homogeneous photoassisted Fenton processes. A simplified reaction mechanism for Orange II decomposition is suggested that is consistent with the experimental findings for solutions up to pH 4.8. The Fe redox reactions in the membranes under light were studied via XPS and spectrophotometric techniques. The effect of pretreatment of the azo dye making possible subsequent biological degradation was tested by BOD5. A drastic increase of the BOD5 values for the pretreated solutions was found with respect to the zero BOD5 value observed for nonpretreated Orange II.
Introduction (•OH)
chemDuring the past decade, hydroxyl radical istry has attracted wide interest in oxidation technologies in wastewater treatment and the Fenton reagent1 has been commonly used as source of •OH radicals in acidic media
Fe2+ + H2O2 f Fe3+ + OH- + •OH
(1)
Recently,2 it has been shown that UV-vis light accelerates reaction 1, improving the degradation rates of many organic compounds, e.g., the nonbiodegradable azo-dye Orange II taken as a model compound in this study.3,4 Iron salts as shown in eq 1 have been known to be the most common catalyst for H2O2 decomposition in acidic media.1 They have been used under light irradiation for this purpose in concentrations of 20-50 mg Fe ion/L and in dark reactions at much higher concentrations. Only in † Institute of Physical Chemistry II, Swiss Federal Institute of Technology. ‡ Water Research Institute. § Institute of Electron Microscopy, Swiss Federal Institute of Technology.
(1) Walling, Ch. Acc. Chem. Res. 1975, 6, 125. (2) Ruppert, B.; Bauer, R.; Heisler, G. J. Photochem. Photobiol. A. 1993, 73, 75. (3) Bandara, J.; Morrison, C.; Pulgarin, P.; Kiwi, J. J. Photochem. Photobiol. A. 1996, 99, 57. (4) Nadtochenko, V.; Kiwi, J. Faraday Trans. 1997, 93, 2373.
this way suitable H2O2 decomposition kinetics with meaningful yields of •OH radicals, eq 1, were attained in photoassisted Fenton reactions.5,6 Fe sludge disposal and/ or regeneration after the Fenton reactions is a serious problem during pollutant degradation in homogeneous media. The removal of iron ions is a relatively simple operation. It is commonly carried out by precipitation and redissolution of the Fe ions after the treatment of large volumes, but it implies the use of large amounts of chemicals and manpower. This costly regeneration step therefore has to be avoided. After the Fenton treatment of wastewater the remaining Fe ions are found at concentrations far above the levels allowed in wastewater by the EEC regulations of 2 ppm.7 To overcome such drawbacks, we undertook the present study. In this study we sought to explore the possibility of highly dispersed Fe ions supported onto dissolved Nafion membranes having similar effect as Fe ions on the decomposition of H2O2 in homogeneous solution. The immobilization the Fe ions on a membrane allows the catalysis of the disappearance of Orange II under light in novel as compared to homogeneous solutions, avoiding the drawbacks of the disposal of the iron ions at the end of the treatment. This new (5) Edwards, J.; Curci, R. Catalytic Oxidation with H2O2 as Oxidant.; Strukul, G., Ed.; Kluwer: Dordrecht, 1982. (6) Halmann, M. Photodegradation of Water Pollutants; CRC Press: Boca Raton, FL, 1996. (7) European Economic Community, EEC List of Council Directives 76/4647, Brussels, Belgium, 1982.
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approach will be applied to a nonbiodegradable azo dye (Orange II) found in the effluents of the textile industry8,9 as part of the studies conducted by our group in the emerging field of advanced oxidation technologies (AOT’s). Membrane-related research has attracted much attention in recent years as a structured medium for photochemical reactions. Nafion perfluorinated membranes (Dupont) have been used during the past decade in a variety of catalytical/electrocatalytical integrated chemical reactions.10 Polymer membranes have also been used for charge transfer in inorganic and biological systems.11 Very few studies of chemical transformations on photoactivation loaded membranes containing CdS or TiO212-15 reactions have been reported until now. Reactions on Fe-loaded polymer membrane systems have recently reported.16 So far, photochemically activated degradation of pollutants on Fe-loaded Nafion membranes has not been reported. In the present study, the nonbiodegradable azo dye Orange II has been chosen as a probe and its degradation in aqueous solution has been investigated. Azo dyes account for more than 22% of the world dye production and are commonly found in effluents of the textile industry.5-7 The hydroxylation/oxidation of Orange II3,4 provides a suitable test for the oxidative degradation of this material under relevant conditions and in technical systems. The pretreatment proposed in suspensions18 has been shown to be effective for subsequent biological treatmentsa lowcost processsto complete the abatement of pollutants. This approach will be extended to a catalytic reaction occurring in two phases with no iron ions in the solution. Experimental Section Experiments were conducted with Nafion perfluorinated membrane (Dupont 117, 0.007 in. thick, Aldrich no. 7.467-4) containing hydrophilic sulfonate groups immobilized on the fluorocarbon matrix. This cation transfer membrane was exchanged a few minutes with FeCl3‚6H2O (Fluka) at room temperature. The Nafion membrane was immersed in HCl before the ion exchange, and after the ion exchange, the membrane was washed with water followed by immersion in NaOH (1 M) to convert the exchanged Fe3+ to its hydrated form. The Fe content in the Nafion was determined after digesting the membrane in concentrated HNO3 (Teflon coated autoclave) under pressure and temperature. This solution was subsequently diluted and the Fe content measured by atomic absorption spectroscopy (AAS) in a Philips 20 AS instrument provided with a flame detector. The membranes used throughout this work had a loading of 1.78 wt %. Photolysis experiments were carried out by means of a Hanau Suntest lamp (AM 1, tunable in light intensity) equipped with an IR filter to remove infrared radiation. The radiant flux of the Suntest solar simulator was measured with a power meter of the YSI Corp., Colorado. Spectrophotometric analyses were performed via a HewlettPackard 386/20 N diode array. The detection of Orange II in solution was carried out via an HPLC (Varian 9065 diode array) provided with a Phenomenex C-18 inverse-phase column. GC (8) Morrison, C.; Bandara, J.; Kiwi, J. J. Adv. Oxid. Technol. 1996, 1, 160. (9) Roques, H. Chemical Water Treatment; VCH: New York, 1996. (10) Bard, J. Integrated Chemical Systems; John Wiley & Sons: New York, 1994. (11) Lawson, R.; Kiang, B.; Martin, R. Chem. Mater. 1993, 5, 400. (12) Meissner, G.; Memming, R.; Kastening, B. Chem. Phys. Lett. 1983, 96, 34. (13) Bellobono, I.; Carrara, A.; Barni, B.; Gazzotti, A. J. Photochem. Photobiol. A. 1994, 84, 90. (14) Pozzo, R.; Baltanas, M.; Cassano, A. Catal. Today. 1997, 39, 219. (15) Miyoshi, H.; Nippa, S.; Uchida, H. Bull. Chem. Soc. Jpn. 1994, 63, 3880. (16) Parton, R.; Vankelecom, I.; Casselman, M.; Uytterhoeven, J.; Jacobs, P. Nature. 1994, 394, 541. (17) Waller, F. J. Catal. Rev. Sci. Eng. 1986, 28, 1. (18) Pulgarin, C.; Kiwi, J. Langmuir 1995, 11, 519.
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Figure 1. Experimental setup used during the Orange II abatement throughout this study. was used to follow the CO2 evolved during dye degradation via a Gow-Mac thermal conductivity detector on a Poropak Q column. The Orange II peak was detected at λ ) 486 nm with a retention time of 11.1 min. The solution gradient was regulated during the analysis with a buffer consisting of ammonium acetate and acetonitrile. Electron microscopy (TEM) was carried out by means of a Philips 20 MS instrument with a resolution limit of 3 Å. The Nafion membranes were cut in water at an angle of 45° by a diamond sharp edge after deposition of the epoxy resin (EM bed 810). The membranes were then set for TEM on a C grid, which in turn was supported on a Cu grid. Photoelectron spectroscopy (XPS) was carried out using a Leybold-Heraeus instrument referenced to the Mg KR1,2 line at 1253.6 eV. The binding energies of the iron oxide surface species were referenced to the Au 4f7/2 level of 83.8 eV. The quantitative evaluation of the experimental data was carried out with a Shirley-type background correction, as will be described below in the text. This correction was necessary because of the electrostatic charging of the particles during the measurements. The short wavelength radiation from the Suntest solar simulator (λ < 310 nm) was removed from reaching the samples by the Pyrex wall of the reaction vessels. The radiant flux reaching the solutions in the photolysis vessels (60 mL Pyrex) was set at 50 and 80 mW/cm2. The Nafion iron-loaded membrane was positioned immediately behind the wall of the reaction vessel to act as the only light absorber, as shown in Figure 1 in this two phase system. BOD5 measurements have been carried out in an Oxytop WTW 1230 Hg-free unit using a 20% inoculum in a solution containing phosphate buffer and the necessary nutrient salts and trace elements. Water sewage was decanted for 24 h, filtered through cotton, and used as inoculum.
Results and Discussion A. Degradation Studies of Orange II Involving the Main Solution Parameters. Figure 2 shows the disappearance of Orange II (pH 2.8) under light irradiation on an Fe-loaded Nafion membrane by a (Suntest lamp) as a function of time for different concentrations of H2O2 in the solution. The detection of Orange II remaining in the solution was carried out via HPLC as mentioned in the Experimental Section. With increasing concentration of H2O2, the degradation of Orange II was accelerated up to H2O2 (2.42 mM) and the Orange II disappearance was observed in ∼30 min. No further acceleration in the
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Figure 2. Disappearance of Orange II under Suntest simulated solar light followed by high-pressure liquid chromatography (HPLC) as a function of the concentration of H2O2 used, up to 100 min irradiation. The Orange II formula is also shown in this figure. Other experimental conditions are indicated in the figure.
degradation kinetics of Orange II was observed at higher H2O2 concentration (4.85 mM) in Figure 2. At even higher oxidant concentration (9.70 mM), a scavenging of the •OH radicals in solution is observed,5,6 which is detrimental to the degradation kinetics of Orange II
H2O2 + •OH f HO2• + H2O
(2)
Figure 3 presents the decoloration of Orange II in solution as a function of irradiation time when the solution pH is systematically varied. The concentration of Orange II (0.05 M) was selected because it corresponds to the level of pollution found in strongly colored polluted waters located close to textile manufacturing sites. The decoloration shown in Figure 3 is probably initiated by the OH-radical attack on the phenolic OH-bond •OH + H-OPh f H2O + •O-Ph, leading to the formation of water as the lowest energy reaction product. The Fe3+ ions interact electrostatically with the sulfonic groups in the Nafion, giving the ensemble a net charge of 2+ for the (Fe3+-SO3-)2+ ion pair. Despite this strong electrostatic interaction, the finely dispersed iron ions on the Nafion loaded membrane become almost inactive at pH > 4.8 because of the Fe-oxyhydroxy precipitation in solution at higher pH values. This shows that diffusion of OH- ions from the solution precipitate the Fe groups exchanged on the Nafion matrix.19 (19) Olah, A.; Prakash, S.; Sommer, J. In Superacids; WileyInterscience: New York, 1985; Chapter 1.
Figure 3. Disappearance of Orange II followed by absorption in a 0.1 cm cell as a function of solution pH under light irradiation up to 1600 min. Other experimental conditions are marked in the figure.
Figure 4 presents the decoloration of Orange II mediated by Nafion/Fe membranes under light irradiation for different concentrations of the azo dye. No Fe-Orange II complex formation occurred in solution, since the red color characteristic of the Fe-Orange II complex3,4 was not observed. The negative sulfonic groups in the cation transfer membrane precluded any proximity between the Orange II also negatively charged (due to its own sulfonic group; see Figure 2) and the sulfonic groups of the Nafion membrane. The decoloration of Orange II in Figure 4 sets in favorably after 30 min. The first step during the degradation of aromatic compounds has been traditionally ascribed to an hydroxylation process5,6,9
Orange II + •OH f Orange II-OH f azo bond breaking/ring hydroxylation (3) The different form and slopes for the different dye concentrations indicate inter- and intramolecular processes affecting the scission kinetics of the azo bond. The nature of the intermediates of Orange II degradation under oxidative conditions has been reported from our laboratory20 and will not be addressed in this study. The inset to Figure 4 shows the finer details for the changes in the UV-vis absorption spectrum during the disappearance of Orange II (0.2 mM). Figure 5 shows the effect of light irradiation and the effect of the intensity of the light applied on the Nafion/Fe loaded membranes during Orange II decoloration. It is readily seen that in the dark only a very modest decoloration of Orange II is observed. Orange II abatement under light irradiation in the (20) Bandara, J.; Kiwi, J. J. Chem. Soc., Perkin Trans. 2, submitted.
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Figure 4. First-order plot for the kinetics of Orange II degradation mediated by Nafion/Fe membrane under light for different concentrations of the dye. The inset shows the details of the decrease of the Orange II absorbance in a 0.1 cm cell.
presence of H2O2 has recently been reported.21 The inset to Figure 5 shows that the disappearance of Orange II follows a shape normally presented by the pseudo-firstorder kinetics decay process
rate ) dCdye/dt ) kCdye
(4)
where k is the pseudo-first-order rate constant and Cdye is the volume concentration of Orange II in solution. If during the degradation of Orange II and upon H2O2 addition a steady state •OH radical concentration is attained in solution,
k ) k′(COH)
(5)
rate ) k′(COH)(Cdye)
(6)
then
The inset to Figure 5 shows more clearly the effect of the light intensity on the decoloration kinetics and the absence of any effect in the dark. B. Total Organic Carbon Decrease (TOC) during Light-Induced Orange II Degradation. Figure 6 presents the decrease in TOC with time as a function of the concentration of Orange II used in Nafion/Fe loaded membranes under solar-simulated radiation. A rapid decrease in the initial TOC values is observed within the (21) Bandara, J.; Herrera, F.; Kiwi, J.; Pulgarin, C. J. Chem. Res., Synop. 1998, 234.
Figure 5. Effect of the light intensity on the Orange II degradation mediated by Nafion and Nafion/Fe loaded membranes as a function of time and the intensity of the applied light irradiation (open points). The degradation in the dark (full points) is shown as reference.
time of Orange II decoloration as described in section A above. The subsequent slower decrease observed in the TOC values is due to the long-lived intermediates generated in the solution (up to 1600 min as will be shown in the next paragraph when discussing Figure 6). The generation of CO2 during the degradation taking place along with the formation of long-lived intermediates in solution was detected by the GC technique (see Experimental Section). The partial mineralization results are not shown in this figure, since these experimental data are only of marginal interest. The addition of the TOC and the amount of CO2 at each of the experimental points in Figure 6 indicated mass balance for the C during the degradation process in solution. These experiments were carried out by taking the initial TOC and the TOC/CO2 at a specified time for a single run. Fresh solutions were used for different analyses times. When considering the TOC values reported in Figure 6 and the data for Orange II disappearance (Figures 2-5), it is seen that the latter process occurs without significant decrease in the TOC values in solution. This is an important aspect for any practical application for the results obtained in this study, since decoloration of dyes in solution is shown to proceed without considerable abatement of the C content in solution. This approach used in this study therefore makes less costly the removal of nonbiodegradable azo dyes during wastewater detoxification. C. Optical Changes in Nafion/Fe Loaded Membranes. Spectroscopic and XPS Evidence Observed for the Fe Oxidation States during Orange II
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Figure 6. Decrease in the TOC mg C/L with time for three initial concentrations of Orange II. For other experimental conditions see details in the figure.
Figure 7. Optical absorption of Nafion cation transfer membranes (a) alone, (b) loaded with 1.78% Fe before use (zero time), and (c) with the same loading after use.
Degradation. Figure 7 shows the change in absorption of the Nafion/Fe(III) membrane in the presence of Orange II (0.2 mM) at pH 2.8 between time zero and after 500 h irradiation. A decrease in the absorption of the Nafion/ Fe(III) during the photolysis is observed with the concomitant formation of the Nafion/Fe(II) absorbing at much lower λs. Since the iron has been exchanged at room temperature in aqueous solution, the exchanged-hydrated Nafion/Fe(III) would involve the species Fe(OH)2+, 366nm ) 275 M-1 cm-1, and Fe2(OH)24+, 366nm ) 1000 M-1 cm-1.22 During Orange II decoloration, the orange-brown color of the initial Nafion/Fe(III) gradually disappears because the formation of the Fe(H2O)62+ charge-transfer band with 254nm ) 20 M-1 cm-1.23 This band has no absorption in the visible region. The color of the initial Nafion membrane was regenerated by immersion in a solution of 1 M NaOH after about 25 cycles of 2 h each. During the first 15-20 2 h cycles, the decoloration kinetics was close to the one observed during the first cycle. Then, gradually the Nafion membrane becomes gradually colorless. This indicated the need to regenerate the original color and to reestablish the initial time of the membrane to decelerate Orange II in order to work under reproducible conditions. The timing of the membrane regeneration to its original color and kinetics was seen to be conditioned by two factors. (a) The initially orange-brown coloration of the membrane gradually changed to yellow/colorless with the absorption shown in Figure 7. This indicated that during the photolysis the Fe(III) content in the Nafion membrane decreased, leading to the formation of the colorless Fe(II) ion, since the total iron content in the membrane was seen to remain constant at 1.78%, and (b) surprisingly during the repetitive
degradation cycles, the membrane became kinetically faster with time. A faster Orange II degradation kinetics of ∼40% was observed compared to that of the initial run. Sulfo-Fe(II) complexes build up in the Nafion/Fe(II) species in the Nafion and are probably more stable than the sulfo(III)/Nafion/Fe(III) membranes.19 The photooxidation of Orange II would then be accompanied by the formation of highly stable Nafion/Fe(II) on the polymer surface. The oxidation state determination of the exchanged iron during the reaction was carried out by photoelectron spectroscopy (XPS) in a Leybold-Heraeus instrument (Experimental Section). Figure 8 shows the peak for the Nafion/Fe sample before light irradiation, and the inset in Figure 2 shows the peak of the same sample after 2 h of light irradiation. The inset shows clearly how the peaks in the used sample move to higher binding energies of ∼720 eV because of electrostatic charging during measurements. Corrections for electrostatic charging of the particles during the measurements were carried out by internal referencing to the aliphatic surface carbon at 284.6 eV and by cross checking the peak-to-peak distances between C 1s, O 1s, and Fe 2p3/2. Gaussian-Lorentzian fitting of the XPS peaks were carried out by the Shirleytype background correction, subtracting the X-ray satellite peaks.24 Polynomial second-order fits of the experimental curves were carried out to match the asymmetric shape of the corrected XPS signals. They were referenced to the (22) Faust, C.; Hoigne´, J, J. Atmos. Environ. 1990, 24, 79. (23) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic Press: London, 1982; Chapter 10. (24) Shirley, A. Phys. Rev. 1979, B5, 4709.
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Figure 8. XPS spectra of the Fe 2p3/2 region for Nafion/Fe loaded membranes. The Gaussian-Lorentzian fitting of the experimental curves according to refs 24 and 25 is also shown in this figure.
Fe 2p3/2 binding energies:25 Fe metal, 706.7 eV; FeO, 709.6 eV; Fe2O3, 710.9 eV; Fe3O4, 711.4 eV. No evidence for the zero oxidation state of Fe was found. The quantitative evaluation of the Fe(III) and Fe(II) species revealed at time zero (Figure 2) the following: 78% Fe(III) with binding energies (BE) of 710.0 eV and a minor component consisting of Fe(III)/Fe(II) of 22% with binding energies of 713.8 eV. The former species appeared as Fe2O3 (Fe(IV) signals) while the latter species consisted mainly of the Fe3O4 composite oxide found in the corrected XPS signals. After 2 h of degradation (Figure 2), the XPS peaks consisted of species corresponding to Fe(III)/Fe(II) of ∼86% and Fe(III) species comprising only ∼14%. This is indicative of Fe(II) species growth during the photocatalysis in the Nafion membrane confirming the spectroscopic observations reported in Figure 7. Quantitative surface concentration of the elements on the Nafion/Fe membrane was carried out before and after reaction (Figure 2) and evaluated according to the literature.26 The results are reported in Table 1. The percentage of Mo and N practically does not change during the degradation. Since the media of the reaction was acidic it is not surprising that the N species found appeared as ammonium N in the XPS spectrogram. No buildup of N residues on the surface catalyst was observed during the degradation process, indicating the efficiency of the catalyst used during Orange II decomposition. The surface atomic concentration of S was detected at the sulfite peak position. It was seen to increase slightly after the reaction because of the S content of the Orange II. The surface C content reported in Table 1 seems to be derived from absorption from atmospheric CO2 decreasing during the (25) Proctor, A.; Sherwood, P. Anal. Chem. 1980, 52, 2315. (26) Stanbury, M. Advances in Inorganic Chemistry; Skies, E., Ed.; Academic Press: London, 1989; Series 33, pp 69-137.
Table 1. XPS Results Showing Surface Concentrations of Various Elements (in atom %) Present in Nafion/Fe Membranes Fe (2p) F (1s) O (1s) N (1s) S (2p) C (1s) Mo (1s)
before reaction
after 2 h reaction
0.59 7.18 12.0 2.5 0.80 74.0 0.22
1.3 26.9 8.9 2.4 1.54 58.8 0.17
process. The lack of surface enrichment in C after the reaction on the Nafion explains the long-term stability of the catalytic membrane. This will be shown by long-term experiments below in section E. The modest increase in the F surface after the reaction was a side effect of the decrease observed for the C content on the Nafion. This leads to the exposure of a larger fraction of F surface atoms instead of the initial C surface species. D. Electron Microscopy(TEM) of the Highly Dispersed Q-Sized Iron Clusters in the Nafion Membranes. The left-hand side of Figure 9 shows the pores of the C grid and a total lack of Fe clusters. The righthand side of Figure 9 shows the Fe clusters on the Nafion with a mean particle size of 37 ( 4 Å. A few pockets of agglomerated Fe clusters were seen in the samples scanned. One of them is seen in the center of Figure 9. But these agglomerates have no quantitative importance in comparison with the smooth uniform dispersion of most of the Fe clusters on the Nafion seen in the right-hand side of Figure 9. A darker band for the Nafion/resin interface is in the middle of the electron micrograph. This darker transition area is due to a thin film formed during the reaction of the epoxy resin with the Nafion during the preparation of the samples. The Fe clusters were seen to
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Figure 9. Transmission electron micrograph of the Fe clusters on the Nafion as seen in the right-hand side of this figure. Magnification is 368 000 times. For other details, see text.
be uniformly deposited on the Nafion. The particle size distribution was observed to be narrowly centered around the median value reported above. The extreme sizes for the Fe clusters on the Nafion were found to be between 10 and 40 Å. The high dispersion of the Fe particles on the Nafion accounted for the shift in the BE seen above in section c with respect to the values of the BE reported for compact metal Fe oxides. The XPS signals, according to the known literature values,24,25 correspond to the high electron density of larger size aggregates. In the present case, the low-density small Fe clusters in the exchanged Fe samples made the determination of the BE more difficult than for samples with a higher degree of metallic character. E. Long-Range Stability of the Degradation Mediated by Nafion/Fe Membranes. Mechanistic and Biological Implications of the Biphasic Catalysis. Figure 10 presents the repetitive Orange II degradation in 2 h cycles. After 500 h of reaction (and just before the last trace shown in the right-hand side in Figure 10), the Nafion/membranes were regenerated in solution by immersing in 1 M NaOH. The timing for the regeneration procedure has been elaborated in section C above. During each degradation run in Figure 10, the solution pH was seen to increase from 2.8 to ∼3.2. This corresponds to an increase by a factor of ∼4 of the OH- concentration in the solution during Orange II degradation. This experimental observation leads us to consider reaction 7,
Nafion/Fe(II) + H2O2 f Nafion/Fe(III) + •OH + OH- (7) and not reaction 8,
Nafion/Fe(III) + H2O2 f Nafion/Fe(II) + HO2• + H+ (8) as the main pathway for Orange II degradation. The hydroxylation of the ring on Orange II degradation has been previously reported from our laboratory.3,8,21 Three experimental observations further substantiate that Orange II degradation occurs mainly through the step shown by eq 7 because (a) methanol (0.26 M) precluded the abatement of the azo dye observed in Figure 1 owing to its •OH radical scavenging properties, (b) there is a shift in absorption in the Nafion/Fe membrane during the dye abatement process, as described previously in section C above, and (c) if the superoxyde radical HO2• (pKa ) 4.8) is photoproduced in eq 8, this radical has a considerably lower one-electron standard reduction potential (HO2•/O2•-) of E° ) 0.75 V vs NHE than the •OH radical with (•OH/OH-) E° ) 1.90 V vs NHE.26 The fast oxidation of Orange II3,4,20,21 cannot possibly be accounted for in terms of this kinetically slower and less energetically HO2• radical. On the basis of the experimental results in sections A-E, a simplified reaction mechanism is suggested below in Scheme 1 for the Nafion/Fe mediated Orange II degradation. The Fe(II) species induced by light in the Nafion (Scheme 1, left-hand side) is suggested to generate the radicals •OH from H O decomposition. The degradation of the dye 2 2 is due to these •OH radicals (eq 7). Scheme 1 is a simplified model, since it is difficult to develop a more quantitative model of what is really happening in this complex system. By way of the experimental setup shown in Figure 1, a 10 min pretreatment of Orange II was applied with H2O2
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Orange via a low-cost process. Short-time use of the relatively expensive photons may still be of practical use. Solar energy photons can also be used directly in the pretreatment suggested in this work. Natural sunlight is equal to or higher than the simulated sunlight intensity one used throughout this work. Conclusions
Figure 10. Nafion/Fe mediated Orange II abatement under light as a function of time at the Orange II peak of λ ) 486 nm up to the fifth cycle.
(2.42 M). Biological oxygen demand (BOD5) was carried out for this partially degraded Orange II as described in the Experimental Section. A BOD5 value of ∼100 mg O2/L for Orange II was observed after 10 min of pretreatment compared to a value of zero observed for nonpretreated Orange II solutions. This is indicative of the presence of highly biodegradable intermediates in solution after the applied pretreatment. Azo bonds are not susceptible to common biological/ bacterial attack.27 Therefore, the pretreatment found in this study makes possible the degradation of otherwise recalcitrant azo compounds in a very short time period. After pretreatment, common wastewater biological treatment would fully complete the degradation process of (27) Pitter, P.; Chudoba, P. Biodegradability of Organic Substances in the Aquatic Environment; CRC Press: Boca Raton, FL, 1994.
This study shows for the first time the use the use immobilized Fe clusters on membrane supports that are catalytically active in the decomposition of H2O2 during photoassisted degradation of water contaminants in a biphasic system. The Fe(III) in the Nafion membrane absorbed the light irradiation energy and was seen to be the precursor of reactions leading to effective H2O2 decomposition. The main parameters affecting the degradation of the model azo dye have been reported in detail. A model for the coupling of reactions in the membrane and in solution is suggested based on the experimental findings. This approach could be used as a base of a costeffective technique to carry out Fenton-type degradation of pollutants without the end-of-pipe discharge of the iron sludge into the environment.7 The chosen azo dye provided a suitable probe for oxidative degradation under conditions that could be of importance in natural and technical systems. Acknowledgment. This work was supported by the European Community Environmental Program ENVCT95-0064 and the INTAS Cooperation Program with Russia and Eastern Europe 94-0642. The help of D. Laub with the TEM and of P. Albers with the XPS measurements is appreciated. LA980382A
