Environ. Sci. Technol. 2009, 43, 176–180
Photodegradation of Triphenylamino Methane (Magenta) by Photosensitizer in Oxygenated Solutions AJITH. C. HERATH, R.M.G RAJAPAKSE, ANURA WICRAMASINGHE, AND VERANJA KARUNARATNE* Department of Chemistry and Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka
Received June 26, 2008. Revised manuscript received August 24, 2008. Accepted October 15, 2008.
Reactive oxygen species (ROS), namely superoxide anion (O2•-, singlet oxygen (1O2), are potentially important substances for the mineralization of toxic organic molecules. The utility of hematoporphyrindihydrochloride (HPDHC) as a photosensitizer to generate ROS and their subsequent role in the destruction of magenta (MaG) in aqueous media is the main concern. The light irradiation of oxygenated aqueous solution of HPDHC and 1.5 × 10-5 mol dm-3 MaG at pH 3 yielded micromolar levels of NO3- ions. A higher rate of photodegradation (1.02 mol dm-3 min-1) at pH 3 was observed compared to that of at pH 6 (0.68 mol dm-3 min-1). Experiments were carried out in the presence of 1, 4-diazabicyclo[2.2.2]octane (DABCO) as singlet oxygen (1O2) quencher and bezoquinone (BQ) as superoxide anion (O2•-) quencher. Only BQ was able to stop photodegradation suggesting that the photooxidation of MaG is mainly caused by O2•-, which is generated by an electron transfer from the excited HPDHC to ground-state oxygen. The presence of iron(II) at pH 3.0, compared to that without iron(II),showed a higher rate of photodegradation due to the formation of extremely reactive hydroxyl radicals (HO · ) upon dismutation of O2•- anion through H2O2 intermediate. The formation of O2•-, H2O2, and HO• is therefore evident, which may act as active sites for subsequent photodegradation of MaG.
Introduction
Cs3PW12O40 to produce O2•- in aqueous medium (13, 14) and the utility for efficient destruction of organic pollutants in the presence of molecular oxygen. However, there is a problem of separating these photocatalysts from the medium which has been circumvented to some extent by immobilizing the catalyst on a solid support such as polypropylene films, zeolite, and silica (15, 16). The utility of porphyrin analogues and phthalocyanines as sensitizers (17) in the photosensitized oxidative degradation of organic pollutants has been an alternative approach to address the problem of pollution abatement. Porphyrins are a ubiquitous class of deeply colored fluorescent pigments, with natural or synthetic origin. They can participate in various photophysico-chemical processes that are promising for photodynamic therapy of cancer (18) and photodynamic action against disease causing microbes and mosquito larvae (19-21). Interestingly, lack of toxicity of porphyrins in the dark and their photodegradation in aqueous media (22, 23) are advantageous in addressing some issues such as photodecontamination of microbially or chemically polluted water. Such processes are characterized by a very low environmental impact. The photodegradation of organic compounds in oxygenated media is accomplished by the participation of 1O2, formed when the energy of the triplet-excited sensitizer is transferred to ground-state triplet oxygen (3O2) (24, 25). However, many experiments support in the formation of O2•– in a competitive reaction via an electron transfer process from sensitizers excited-state to the ground-state oxygen (26-28). Superoxide anion in aquous solution depending on the pH of the medium may be a precursor in producing hydrogen peroxide and subsequently the more potent oxidant OH•, which are capable of attacking almost all-organic molecules (29). Triphenylmethane carbonium dyes such as maG and crystal violet (CrV) are commercially available synthetic dyes frequently used in the textile industry. The highly delocalized carbonium ion of these compounds imparts a high absorption coefficient in the visible region. However, these compounds have been known to be toxic and therefore their destruction has been of recent concern. Miroslav and Jelena reported the photodegradation of MaG in aqueous medium by TiO2, induced by light (30). This paper discusses the photosensitized generation of ROS, their subsequent role, and mode of action in the destruction of MaG and the analysis of photoproducts at different pH media. The effect of Fe(II) and Fe(III) on the rate of degradation of MaG is also of research interest.
Many of the synthetic dyes and pigments used in the textile manufacturing processes are toxic, nonbiodegradable, and resistant to direct degradation mediated by sunlight, and consequently, they appear as a class of persistent pollutants. The discharge and subsequent accumulation of these toxic compounds in aquatic media pose hideous risks to the environment (1). Conventional nondestructive treatment methods, such as adsorption flocculations, have shown to be inefficient (2, 3). In recent years, there has been an increased interest in searching for reliable and efficient techniques for the destruction of pollutants. In this connection, photoexcitable nanocrystaline semiconductors such as TiO2 and ZnO (4-8) and Fenton related oxidation reactions (9-12) are promising treatment strategies for the mineralization of organic materials. Many researchers have encouraged the application of polyoxometalates such as K3PW12O40,
Experimental Details
* Corresponding author phone/fax: +94 81 2389129; e-mail:
[email protected].
FIGURE 1. Structures of hematoporphyrindihydrochloride (left) and magenta (right).
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Materials. All chemicals and solvents were of reagent grade from Aldrich/Fluka and were used as received. The sensitizer, HPDHC, (Figure 1) was purchased from Aldrich. Double distilled water was used in the preparation of 0.1 mol dm-3
10.1021/es801765t CCC: $40.75
2009 American Chemical Society
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buffer solutions of CH3COOH/CH3COONa, KH2PO4/NaOH, and Na2CO3/NaOH. All experiments were carried out at ambient temperature. Methods. UV-visible spectra were recorded on a Shimadzu 1600 UV-visible spectophotometer. All electrochemical experiments were performed on a BAS-100A electrochemical analyzer with a conventional three-electrode system comprising of an Ag/AgCl reference electrode, a Pt counter electrode and a glassy carbon working electrode. For each experiment, the electrode was polished with an aqueous suspension of Al2O3 (0.05 µm) and the solutions were degassed with N2 prior to analysis making sure that the O2 reduction wave disappeared totally. During recording cyclic voltammograms (CVs), a blanket of N2 was maintained above the solution. CVs were recorded for solutions containing 1.0 × 10-4 mol dm-3 HPDHC at pH values of 3.0, 6.0, and 8.0 within the potential range of -1.0 to 1.2 V. The scan rate chosen was 50 mV s-1. Photoirradiation experiments were carried out using a homemade apparatus consisting of a 20 W fluorescent lamp. The radiation emitted by this source was focused to the wall of a 50 cm3 Pyrex glass cell containing the reaction mixture. The radiation from the lamp was filtered through a circulatory water jacket placed between the source and the reaction vessel, which prevented the warming of the solution. During irradiation, the samples were continuously bubbled with air, oxygen or nitrogen as required. Photodegradation of MaG (1.5 × 10-5 mol dm-3) was monitored by UV-visible spectrometry at regular time intervals over a period of 20 h for solutions at pH 3.0, 6.0, and 8.0. Photosensitized experiments in the presence of 5.0 × 10-6 mol dm-3 HPDHC were continued separately for aerated solutions of MaG at pH 3.0 in the presence of 5.0 × 10-5 mol dm-3 BQ and 1.0 × 10-4 mol dm-3 DABCO. Concentration of BQ was varied from 2.5 × 10-6 mol dm-3 to 10 × 10-6 mol dm-3, and the change in absorbance of each oxygenated solution of MaG was periodically monitored at 544 nm. Stock solutions of 50.0 cm-3 of 0.0076 mol dm-3 Fe2+(aq) and anhydrous Fe3+(aq) were prepared immediately prior to experiments using FeSO4 · 7H2O and anhydrous FeCl3, respectively, and few drops of 5.0 mol dm-3 H2SO4 were added to suppress the hydrolysis of Fe2+ in the solution making sure that the final pH was not less than 3.0. For the analysis of NO3-, a 10.0 cm-3 portion of the MaG solution, at preselected time intervals during the irradiation, was passed through an activated charcoal column to remove any MaG left behind after irradiation. A known amount of standard nitrate solution was passed through the same column and analyzed for nitrate to confirm that no loss of NO3- occurred during the cause of dye adsorption process. The quantification of NO3- was carried out according to an established method (31).
Results and Discussion Visible Light Irradiation of MaG. The Figure 2. shows the absorption spectral changes recorded during the irradiation of an aqueous solution of MaG at pH 3.0 under various conditions. The visual discharge of pink color of the solution and the gradual decrease of the optical absorption at 540 nm in the absorption spectra were observed only in the presence of HPDHC, O2, and light; the color of the solution completely disappeared after a 20 h irradiation period. Control experiments carried out in the absence of HPDHC, light, or O2 (in the latter case the solution was saturated with N2) showed no detectable color bleaching. Photooxidation of MaG in oxygenated solutions in the presence of a photosensitizer is caused by the initial generation of 1O2 or O2•-. To establish the possible involvement of these ROS in the degradation of MaG, irradiation experiments at pH 3.0 were carried out in the presence of
FIGURE 2. Photodegradation of magenta at pH 3.0 as a function of irradiation time under different conditions (inset shows the absorption spectral change under conditions (9) as a function of time).
FIGURE 3. Spectral profiles, recorded after 12 h of irradiation, showing the scavenging effect of 5.0 × 0-5 mol dm-3 BQ and 1.0 × 10-4 mol dm-3 DABCO on phtobleaching of MaG at pH 3.0. BQ, a superoxide scavenger or DABCO, a singlet oxygen quencher. The presence of DABCO in the irradiated solution resulted in photodegradation, nearly to 0.1 absorbance, a result comparable to that observed in the presence of HPDHC (Figure 3). These results indicate that, since DABCO did not have any effect on the photodegradation of MaG, the participation of 1O2 can be ruled out in the oxidation process. In the presence of BQ the photodegradation of MaG observed by decreasing of the Soret band was low. The decrease of the rate of photobleaching (Figure 4), with the increase in concentration of BQ in the reaction system can be attributed to the quenching of O2•- formed in the photosensitization process through rapid electron transfer to BQ producing ground-state O2 and BQ radical anion as given below (reaction 1).
These observations suggest that the superoxide may lead to the oxidative degradation of MaG. The BQ radical formation however would inhibit the participation of O2•- in the dye destruction in oxygenated media. VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Effect of different concentrations of BQ on color bleaching of MaG in the presence of HPDHC at pH 3.0.
FIGURE 6. Change in concentration of MaG upon visible light irradiation in the presence of Fe2+ at pH 3.0. favored (pka ) 4.9) and, therefore, HO2• and H2O2 are not available for the reaction and only O2•- would be involved in the dye destruction process. The presence of Fe2+/Fe3+ in H2O2 medium (Fenton’s reagent) acts as an efficient oxidant in the oxidative photodegradation of a wide range of organic compounds (11, 28, 32). Although H2O2 is an oxidizing agent, the H2O2 dependent oxidizing activity is mediated by secondary radicals such as hydroxyl radicals (HO•) generated from superoxide-driven Fenton’s reaction. Haber-Weiss has suggested (33) that extremely reactive HO · results in the decomposition of H2O2 as follows (reaction 4). Fe2+ + H2O2 + H+ f Fe3+ + H2O + HO•
(4)
Other possible reactions that may take place in concordance with the mentioned above are reactions 5 and 6. FIGURE 5. Photodegradation kinetics of MaG at pH 3.0 in the presence of HPDHC and O2. Kinetics of Photodegradation of Magenta. The photodegradation of MaG as a function of time observed in Figure 2 (inset) was used to describe the kinetics of the degradation process. Photodegradation studies of MaG, carried out at two different oxygen concentrations, air saturated ([O2] ) 2.0 × 10-4 mol dm-3) and O2 saturated ([O2] ) 2.0 × 10-3 mol dm-3) at 25 °C revealed that photooxidation rates for both systems were identical with a pseudo first-order rate constant of 1.02 mol dm-3 min-1 (Figure 5), suggesting that degradation of MaG can be achieved at a maximum rate by merely aerating the solution at 1 atm. Photooxidation studies of MaG at pH 3.0 and 6.0 indicated a higher bleaching rate at pH 3.0 in comparison to pH 6.0. The same investigation at pH 8.0 was found to be difficult, since MaG undergoes decoloration, even in the dark, due to the gradual formation of the carbinol form in the alkaline medium. At low pH, O2•- undergoes rapid protonation to produce H2O2 through the intermediate hydroperoxyl radical (HO2•) as indicated bellow (reactions 2 and 3). The color bleaching of MaG, is therefore a result of the interaction with these reactive oxygen species. + • O•2 + H T HO2 H+
HO•2 + O•2 98 H2O2 + O2
(2) (3)
The low rate of photo-oxidation at pH 6.0 can be attributed to the fact that at high pH the protonation of O2•- is not 178
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HO• + H2O2 f HO•2 + H2O Fe
3+
2+ + O•+ O2 2 f Fe
(5) (6)
The irradiation experiments with MaG were extended in order to test indirectly the formation of H2O2 according to reactions 2 and 3. At pH 3.0 it was found that high photodegradation rate of MaG occurred in the presence of O2 and HPDHC with added Fe2+ in comparison to the photodegradation rate in the absence of Fe2+ (Figure 6). Control experiments, (i) in the absence of the sensitizer (ii) in the presence of Fe2+ without sensitizer and (iii) under nitrogen atmosphere upon irradiation, showed low photodegradation. On the basis of these results it is suggested that, in the system HPDHC/Fe2+/O2 at pH 3.0, the formation of H2O2, according to the reaction 3, is followed by the generation of hydroxyl radicals causing much damage to dye molecules. In contrast, Fe2+ alone, in the absence of the sensitizer, does not show any effect on photodegradation, probably due to its optical inactivity in the region of irradiation. The increase of photodegradation rate of MaG (in HPDHC/Fe2+/O2 at pH 3.0) as a function of high concentrations of Fe2+ can be described by high rate of HO• formation. (Figure 7). The effect of sunlight on photo-oxidation of MaG at pH 3.0 in the presence of Fe3+ is quite interesting (Supporting Information Figure S1). It was found that, with the absorption of energy in the UV region, the reaction proceeds in the formation of HO• radicals (34, 35) (reactions S1 and S2 in the Supporting Information). In the absence of oxygen and HPDHC (where the solution was saturated with nitrogen), the rate of photodegradation seems to be compatible with that of HPDHC-Fe2+ system in the presence of oxygen (Figure 8). The irradiation with the visible region of sunlight showed
FIGURE 9. Yield of nitrate upon photooxidation of MaG at pH 3.0 (b) and at pH 8.0 (2). FIGURE 7. The effect of concentration of Fe2+ ions on the photobleaching rate of MaG at pH 3.0 in the presence of HPDHC. b in the absence of the sensitizer.
from the singlet excited-state porphyrin molecule to groundstate O2 to form O2•- anion. E[D·+⁄D∗] ) E[D·+⁄D] - ∆H(D∗, D) + T∆S(D * , D)
FIGURE 8. Photodegradation of MaG upon visible light irradiation in the presence of HPDHC, O2 and in the absence of HPDHC and O2 at pH 3.0. The solution contains an identical concentration of 1.4 × 10-4 mol dm-3 Fe2+ and Fe 3+. no detectable photodegradation of MaG (Supporting Information Figure S2). This suggests that, in the former case photodegradation follows a different mechanism in which oxygen is not involved. Cyclic Voltammetric Studies on HPDHC. Cyclic voltammetric experiments were performed on HPDHC in order to investigate the various redox potentials and thereby the energetics of ground and excited states of HPDHC as a way to explaining the different reaction pathways leading to the dye degradation. The cyclic voltammogram of HPDHC at a glassy carbon electrode surface in deareated solution at pH 3.0 consists of an irreversible anodic peak at Epa ) 0.85 V corresponding to porphyrin ring centered oxidation. Based on the voltammetric measurements, anodic peak potentials of HPDHC were found to be pH dependent and are 0.85, 0.86, and 0.92 V vs Ag/AgCl reference at pH 3.0, 6.0, and 8.0, respectively. The results show that the relative ease of oxidation of HPDHC at pH 3.0. Estimation of Excited-State Potential for HPDHC and O2•- Anion Formation. From a thermodynamic point of view, electron transfer from ground-state porphyrin to groundstate oxygen is not energetically favorable due to high oxidation potential of the sensitizer ∼1.0V vs NHE) compared to redox potential of O2/O2•- (-0.16V vs NHE). The excitedstate potential estimated, according to eq 9 (36) for HPDHC (-1.0 V vs NHE), indicates the feasibility of electron transfer
(9)
E is the excited-state potential of the electron donor (D) and ∆H and ∆S are the enthalpy and entropy changes between the excited-state and the ground-state of the molecule, respectively. It is reported that in condensed phase the enthalpy difference between excited and ground-state of the same molecule is practically equal to the zero-zero spectroscopic energy (E0-0) of the excited-state and at pH 3.0, its calculated value is around 2.1 eV for HPDHC. The entropy difference, which arises due to changes in dipolemoment, internal degrees of freedom, and orbital and spin degeneracy, is negligible for most practical systems. Analysis of Photoproducts in the Presence of NO3-. Photodegradation studies carried out by monitoring the loss of optical absorbance of the respective compounds in the visible region were limited to pH values below 7.0, because in basic media MaG is converted to the colorless form due to loss of conjugation. However, NO3- ions, produced in the oxidation of -NH2 groups in the dye in the presence of HPDHC at pH 8.0 revealed the feasibility of photodegradation in basic media. Figure 9 shows the micromolar range of NO3ions formation as a result of degradation of MaG. Control experiments carried out for MaG under nitrogen saturation and in the absence of HPDHC gave no detectable amounts of NO3- at both pH values. In conclusion, the experimental results indicate that the Porphyrin-sensitized oxygenated solutions generate predominantly O2•- anions. The highest photodegradation rate yielding NO3- ions as photoproducts of MaG was achieved at pH 3 and followed pseudo first-order kinetics. The presence of Fe(II) with oxygen and Fe(III) without oxygen showed enhanced rate of photodegradation with the participation of HO• radicals in the presence of visible light and UV light, respectively. Photoproducts of MaG and HPDHC are relatively nontoxic and unlikely to cause any impact to the aquatic environment hence this approach in pollution abatement seems to be more advantageous.
Acknowledgments The generous financial support by NSF of Sri-Lanka is gratefully acknowledged.
Supporting Information Available Detailed analysis and discussion of the involvement of Fe(III) in the photodegration of MaG; figures showing the effect of Fe(III) on the photodegration under various experimental conditions (S1, S2), A reaction describing the generation of VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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hydroxyl radicals (S1), quenching of hydroxyl radicals by 2-butanol (reaction S2). This material is available free of charge via the Internet at http://pubs.acs.org.
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ES801765T
