Environ. Sci. Technol. 2000, 34, 2024-2028
Light Induced Elimination of Monoand Polychlorinated Phenols from Aqueous Solutions by PW12O403-. The Case of 2,4,6-Trichlorophenol
monochlorophenols (4), chloracetic acid (5), and organochlorine pesticides (6). In all cases the final photodegradation leads to complete mineralization of substrates, i.e., formation of CO2, H2O, and Cl-.
E V A G E L I A A N D R O U L A K I , †,‡ ANASTASIA HISKIA,† DIMITRA DIMOTIKALI,‡ CLAUDIO MINERO,§ PAOLA CALZA,§ EZIO PELIZZETTI,§ AND E L I A S P A P A C O N S T A N T I N O U * ,† Institute of Physical Chemistry, NCSR Demokritos, 153-10 Athens, Greece, Chemical Engineering Department, NTU, 157-80 Athens, Greece, and Department of Analytical Chemistry, University of Torino, 10125 Torino, Italy
Chlorinated phenols are used in the pesticide field as fungicides and disinfectants and are also important chemicals in a number of industrial processes. Chlorinated phenol residues are also liberated via chemical or biological degradation of several other groups of pesticides (for example, phenoxy, organophosphorus, and carbamate compounds). Moreover it has been shown that chlorinated phenols are chemical precursors of the highly toxic polychlorinated dibenzo-p-dioxins (7-10). Among the chlorinated phenols, 2,4,6-TCP is of special note because of its potential carcinogenicity (7).
Light induced catalytic decomposition of several mono-, di-, and trichlorophenols and phenol in the presence of PW12O403- in aqueous solutions (pH 1) leads to mineralization of substrates. The method is an example of Advanced Oxidation Processes (AOP) that cause mineralization of organic pollutants through the generation of very active, mainly OH, radicals. Generally, chlorination of phenolic ring enhances the decomposition, whereas the effect of chlorine substituents in the ortho position is less pronounced. However, the rates of decomposition of chlorinated phenols are very much the same. Dioxygen’s main function seems to be the regeneration of the catalyst, with limited participation in the initial stages of the photoreactions. A detailed study of 2,4,6-trichlorophenol (2,4,6-TCP) photodecomposition showed that key reactions involved were hydroxylation, substitution of chlorine by OH radicals mainly in the ortho and para positions, and breaking of the aromatic ring. Ring-opened products detected were maleic, oxalic, acetic, and formic acids. Acetic acid has been so far a common intermediate in the photodecomposition of aromatic compounds with this method. The ultimate products were CO2, H2O, and Cl-.
Introduction Polyoxometalates (POM) offer a wide range of characteristics such as molecular composition, size, shape, charge density, redox potentials, acidity, and solubility that render them potentially promising catalysts. In addition, POM can accept and release a certain number of electrons without decomposition (1). It is well-known that illumination of POM at the OfM CT band (i.e. below 400 nm) enhances their oxidizing ability, making them useful in the oxidation of a great variety of organic compounds including various organic pollutants (2). We and others have demonstrated the ability of POM to cause photocatalytic decomposition of phenol, p-cresol (3), * Corresponding author phone: 011-3-01-6503642-3; fax: 0113-01-651176-6; e-mail:
[email protected]. † NCSR Demokritos. ‡ NTU. § University of Torino. 2024
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The method is an example of “Advanced Oxidation Processes” (AOP) that cause mineralization of organic pollutants through the generation of very active, mainly OH, radicals.
The mineralization of monochlorophenols in the presence of PW12O403-, SiW12O404-, and W10O404- has been reported (4a,b). Misono and co-workers have also studied the photodegradation of 4-chlorophenol by PW12O403- (4c,d). It has been shown that PW12O403-, SiW12O404-, and W10O404- have similar performance in the light induced mineralization of several organic pollutants (3, 4a,b, 5). In this study, PW12O403has been used as catalyst in the photodegradation of polychlorinated phenols, namely, 2,4-dichlorophenol (2,4DCP), 2,6-dichlorophenol (2,6-DCP), 3,4-dichlorophenol (3,4DCP), 3,5-dichlorophenol (3,5-DCP), 2,4,6-trichlorophenol (2,4,6-TCP), and included monochlorophenols and phenol for comparison. Limited work was done with SiW12O404-. Specifically this paper reports on the following: (a) the photodecomposition of chlorophenols with cut off filters 320 and 345 nm by direct photolysis and through the catalyst PW12O403-, (b) the effect of the number and position of chlorine substituents on the rates of photodecomposition, (c) the role of dioxygen, and (d) the final degradation products and the intermediates involved in the case of 2,4,6-TCP.
Experimental Section Materials. All chemicals were reagent grade or pure and used as received. 2,6-Dichlorohydroquinone (2,6-DCHQ) was prepared by the reduction of 2,6-dichlorobenzoquinone (2,6DCQ) with sodium dithionate and was characterized by 1H NMR (11). PW12O403- and SiW12O404- were prepared according to methods reported in the literature (12). Instrumentation. Photolysis experiments were performed with an Oriel 1000 W Xe arc lamp, equipped with a cool water circulating filter to absorb the near-IR radiation. This lamp gives a flat responce from ca. 320 to 750 nm corresponding to irradiance at 0.5 m, ca. 200 mW‚m-2 nm-1 according to the supplier. The incident radiation was reduced to about 40% with a slit diaphragm in order to obtain reasonable photolysis times. Control experiments have shown comparable photodegradation rates of organic pollutants by TiO2 and PW12O403-. HPLC analysis was carried out with a Waters apparatus equipped with a UV detector. GC analysis for the determination of CO2 was carried out using a Varian Model 3300 gas chromatograph equipped with TCD and a 2 m Porapack Q column. Identification of intermediates was performed using a Micromass Platform II quadrupole mass spectrometer equipped with a DB-5 fused silica capillary column. The 10.1021/es990802y CCC: $19.00
2000 American Chemical Society Published on Web 04/08/2000
FIGURE 1. A comparative diagram of the photodegradation of oxygenated solutions of 2,4,6-TCP, in the presence and absence of catalyst, with cut off filters 320 and 345 nm: 2,4,6-TCP 10-3 M, PW12O403- 7 × 10-4 M, pH 1 (HClO4), T ∼ 20 °C.
FIGURE 3. First-order plots for the photodecomposition of phenol and various chlorophenols, in the presence of catalyst: Substrates 10-3 M, PW12O403- 7 × 10-4 M, oxygenated solutions, pH 1 (HClO4), λ > 320 nm, T ∼ 20 °C.
TABLE 1. Initial Rate of Photodegradation of Phenol and Various Chlorophenols in Oxygenated Solutions (First Column), Calculated from the First-Order Plots (Figure 3), and the Initial Rate of Reduction of Catalyst in Deaerated Solutions (Second Column), Calculated from the “Zero-Order” Plots of the Initial Development of the Blue Color with Photolysis Timea initial rate of
FIGURE 2. The gradual formation of the reduced form of catalyst, PW12O404-, upon photolysis of deaerated solutions of 2,4,6-TCP. Substrate 10-3 M, PW12O403- 7 × 10-4 M, pH 1 (HClO4), λ > 320 nm, T ∼ 20 °C. Photolysis time is indicated on spectra. maleic, oxalic, acetic, and formic acids were analyzed by suppressed ion chromatography, performed with a Dionex apparatus. Photolysis Experiment. Aqueous solutions of chlorophenols, 10-3 M, unless it is stated otherwise, in the presence of the catalyst (PW12O403-, 7 × 10-4 M), were prepared by dissolving certain quantities of substrate in HClO4 (0.1 M). Of the above solutions, 4.0 mL was added to an 8 mL spectrophotometer cell (1 cm path). After 20 min of deaeration or oxygenation, the cell was covered with an airtight serum cap. Photolysis was performed at 20 °C with stirring. The concentration of dioxygen saturated solution is ca. 1.2 mM at room temperature (13). Thus, a typical oxygenated photolysis experiment contained 4 mL of dioxygen saturated aqueous solution with 4 mL of dioxygen gas over it. No specific measurements of dioxygen concentration were performed, since we did not involve dioxygen in any specific kinetic studies. To have adequate quantities for the identification of intermediates by GC-MS, samples of 30 mL of the 2,4,6-TCP (10-3 M) were photolyzed under the same conditions. Analysis of the Photolyzed Solutions. Photolyzed solutions (4.0 mL) of chlorinated phenols were analyzed by HPLC with the UV detector wavelength at 280 nm. When GC-MS was used for analysis, the photolyzed solution (30 mL) was extracted with dichloromethane (3 × 20 mL), and the organic layers were combined, dried with sodium sulfate, and evaporated to 100 µL. Chloride ions were analyzed spectrophotometrically (14).
substrate
degradation of substrates, M‚min-1 × 105 (values within 20%)
reduction of catalyst, M‚min-1 × 105 (values within 15%)
phenol 2-CP 3-CP 4-CP 2,4-DCP 2,6-DCP 3,4-DCP 3,5-DCP 2,4,6-TCP
2.4 8.6 8.2 9.5 8.6 6.6 12.6 12.8 7.5
2.4 4.9 8.3 6.8 4.4 3.7 8.3 13.6 1.8
a Substrates: 10-3 M, PW O 3- 7 × 10-4 M, pH 1 (HClO ), λ > 320 12 40 4 nm, T ∼ 20 °C.
Gases from the headspace were analyzed for CO2 via GCTCD. Carbon dioxide content was calculated using a calibration curve, made of known quantities of CO2, and processed under the same experimental conditions. The degree of reduction of PW12O403- in photolyzed deaerated solutions was calculated from the known extinction coefficients of the blue products (751 nm ) 2 × 103 M-1 cm-1 and 653 nm ) 4.4 × 103 M-1 cm-1 for the one and two-electron reduction products, respectively) (15).
Results and Discussion Photodecomposition of Chlorophenols with Cut Off Filters 320 and 345 nm by Direct Photolysis and through the Catalyst PW12O403-. Chlorinated phenols absorb strongly from about 270 to 290 nm. Photolysis in that region, results in photodecomposition of chlorophenols (16). Despite the lack of absorption of 2,4,6-TCP beyond 320 nm, 25% of the substrate was decomposed after 1 h of irradiation with a 320 nm cut off filter, whereas the decomposition with a 345 nm cut off filter was negligible (Figure 1). Addition of PW12O403- to oxygenated solutions accelerates the photodecomposition significantly (Figure 1). Effective photodecomposition also takes place in the absence of VOL. 34, NO. 10, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. The percent reduction of catalyst and the concomitant photodecomposition of 2,4,6-TCP in deaerated solutions: 2,4,6-TCP 10-3 M, PW12O403- 7 × 10-4 M, pH 1 (HClO4), λ > 320 nm, T ∼ 20 °C (100% reduction indicates complete one-electron reduction of catalyst).
FIGURE 6. Variation of maximum percent reduction of catalyst upon photolysis of deaerated solutions of various chlorophenols with increasing number of chlorine substituents, i.e., phenol, 4-CP, 2,4DCP, 2,4,6-TCP. Substrates 2 × 10-3 M, PW12O403- 7 × 10-4 M, pH 1 (HClO4), λ > 320 nm, T ∼ 20 °C (100% reduction indicates complete one-electron reduction of catalyst). of the one-electron reduced catalyst, PW12O404-, in deoxygenated solutions (except for 2,4,6-TCP which is four times faster). The last item suggests that dioxygen’s main function is the regeneration of the catalyst with limited effect in the initial decomposition of the substrates. The basic photocatalytic reactions involving POM have been reported elsewhere (4b). It is sufficient here to mention that excited POM has been shown to react with substrate S via two modes, direct reaction and indirect reaction through OH radicals (17).
POM* + S f POM(e-) + S(+)
FIGURE 5. The gradual reduction of PW12O403- and Cl- production, upon photolysis of deaerated solution of 2,4-DCP: Catalyst 7 × 10-4 M, 2,4-DCP 4 × 10-4 M, pH 1 (HClO4), λ > 320 nm, T ∼ 20 °C (100% reduction indicates complete one-electron reduction of catalyst). dioxygen. A deep blue color of the one-electron reduced tungstate, PW12O404-, develops upon photolysis of a deaerated aqueous solution of 2,4,6-TCP (Figure 2). Under the experimental conditions used, the decomposition of chlorophenols followed first-order kinetics (Figure 3). The Effect of Chlorination on the Rates of Photodecomposition of the Title Substrates. The Role of Dioxygen. Table 1 compares (a) the rates of decomposition of oxygenated aqueous solutions of chlorinated phenols and phenol as they were calculated from the first-order plots of Figure 3 and (b) the initial rates of reduction of catalyst from the corresponding deoxygenated solutions, calculated from the zero-order plots of the initial development of the blue color with photolysis time. From Figure 3 and Table 1, one can observe the following general trends for the photodecomposition of aqueous solutions (pH 1) of phenol and chlorinated phenols in the presence of PW12O403-: (a) the rates of photodecomposition of chlorinated phenols are within the same order of magnitude. In that respect, as a referee has kindly pointed out, the lack of selectivity is an advantage for waste management. (b) The photodecomposition of substrates follows first-order kinetics as stated earlier, (c) chlorination of the phenolic ring enhances slightly the rate of decomposition, whereas the effect of chlorine substituents in the ortho position is less pronounced, and (d) the rates of decomposition of chlorophenols in oxygenated solutions are not dramatically different from the rates of production 2026
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redox reaction (1)
POM* + H2O h POM(e-) + OH + H+ formation of OH (2) OH + S f hydroxylation, H abstraction, Cl substitution, oxidation products (3) POM(e-) + oxidant f POM + oxidant products regeneration of catalyst (4) Several attempts have been made to correlate the rates of photodecomposition of chlorinated phenols with the position and the number of chlorine atoms in the ring. If we assume, as is generally true, that the main oxidant in Fentons reagent, UV-H2O2, UV/near visible-TiO2, and UV/ near visible-POM are the OH radicals, then we could expect similar behavior in the photodecomposition process reported here. As is known for electrophilic reagents, the OH group in the phenolic ring is o, p directing with activation, whereas the Cl substituent is o, p directing with deactivation. OH radicals are electrophilic, and so one might assume that phenol would degrade quicker than chlorophenols. However, the opposite was observed. There are, of course, several other factors affecting the initial rates of photodecomposition, the most important being: (a) the nature of the associated complex or associated equilibrium involved between the catalyst and substrate for the systems using TiO2 or POM and (b) the pH of the solution. Our experimental results for deoxygenated samples were based on the rates of reduction of catalyst in aqueous solutions, monitored by the initial development of the blue color (751 nm) (Figure 2). These experiments were repeated three times, and the average rates of reactions were recorded (Table 1). These results are easily obtained and have low
SCHEME 1
experimental error. The order of decreasing initial rates of photodecomposition in deaerated solutions is as follows: 3,5-DCP > 3-CP > 3,4-DCP > 4-CP > 2-CP > 2,4-DCP > 2,6-DCP > phenol > 2,4,6-TCP. Because of the low solubilities of trichlorophenols we used an order of magnitude lower concentrations, i.e., 7 × 10-5 M, for both 2,4,5-TCP and 2,4,6-TCP to compare their photodegradation rates, keeping all other parameters the same. The rates were 5.19 × 10-5 M min-1 and 1.25 × 10-5 M min-1 for 2,4,5-TCP and 2,4,6-TCP, respectively. Thus 2,4,6TCP reacts four times slower than 2,4,5-TCP, indicating that ortho chlorosubstituents retard the initial rates of photodecomposition through stereochemical inhibition (18) and/ or intramolecular hydrogen bonding (20). Similar results have been reported with TiO2 (19). Misono and co-workers, on the other hand, have indicated that the rate of decomposition of several chlorophenols by PW12O403- increases with the extent of chlorine substitution (4d). Matthews observed that the rate of production of CO2 for several chlorophenols and chlorobenzenes increases when the number of chlorine substituents in the aromatic ring increases from one to two, whereas addition of a third chlorine group results in slight decrease (21). Thus, it appears from this work and the work of others that chlorination of phenol generally enhances the rates of photodecomposition, whereas the effect of chlorine substituents in the ortho position is less pronounced (18, 19, 21). It should be mentioned that our experiments were made at pH 1 where all chlorophenols and phenol are in un-ionized form, according to their pKa range (6.1 < pKa < 9.9) (22). Details of the Photocatalytic Decomposition of 2,4,6TCP by PW12O403-. The Role of Dioxygen. Effective decomposition of the target compound is accomplished by PW12O403- in the presence and absence of dioxygen. Dioxygen accelerates the decomposition of 2,4,6-TCP by about four times. Figure 4 plots the percent reduction of the catalyst and the percent decomposition of deoxygenated aqueous solutions of 2,4,6-TCP vs the photolysis time, whereas in Figure 5 the percent reduction of catalyst and the Cl- production are monitored for 2,4-DCP. Figures 4 and 5 show that within ca. 30 min of irradiation, the percent reduction of the catalyst PW12O404- reaches a plateau, whereas the initial decomposition of 2,4,6-TCP (Figure 4) and the formation of Cl- (Figure 5) continued. This suggests, as has been observed in other cases (4), that a dynamic equilibrium is established in which the rate of the primary photoreactions that reduce the catalyst and oxidize the substrate (eqs 1-3) are matched by a thermal reoxidation of the catalyst by some intermediate produced during photolysis (eq 4). The case of reoxidation of catalyst by H+ is ruled out, since H+ oxidizes only the two-electron reduced tungstate, PW12O405-, as per the redox potentials of the species (23). Further, Figure 6 shows the extent of reduction of catalyst for several chlorophenols. No correlation is found with the number of chlorine substituents, suggesting that liberated
chlorine is not the oxidant of the reduced catalyst. The low percent reduction of catalyst with 2,4,6-TCP (Figure 6) could be attributed to the formation of the highly oxidizing 2,6DCQ. The acceleration of the decomposition of 2,4,6-TCP by dioxygen suggests its participation not only in the reoxidation of the catalyst (eq 4) but also in the initial stages of the reaction (Table 1). Reductive activation of dioxygen proceeds in this manner (3-5).
PW12O404- + O2 f PW12O403- + O2-
(5)
O2- + S f oxidation products
(6)
Intermediates in the Photodecomposition of 2,4,6-TCP. A detailed study of the photodegradation of 2,4,6-TCP reveals the formation of several intermediates shown in Scheme 1. 2,6-Dichlorohydroquinone (2,6-DCHQ), 2,6-dichlorobenzoquinone (2,6-DCQ), 3,5-dichlorocatechol (3,5-DCC), a dihydroxytrichlorobenzene, a trihydroxydichlorobenzene, maleic, oxalic, acetic, and formic acids were identified by GCMS and ion exchange chromatography. No chlorocyclopentadiene carboxylic acids, which are metabolites of direct photolysis of 2,4,6-TCP (16), were detected among the intermediates in these studies. The incident photons were overwhelmingly absorbed by the catalyst, so that direct photolysis was negligible. The formation and decay of two key intermediates involved in the photodisintegration process, 2,6-DCQ and 2,6-DCHQ, are shown in Figure 7. Measured amounts were very low due to their instability under photocatalytic conditions, as has been observed by others (19). The presence of 2,6-DCQ at zero photolysis time indicates contamination of 2,4,6-TCP (purity 98%) in the preparation process (7).
FIGURE 7. Formation and decay of 2,6-DCQ and 2,6-DCHQ upon photolysis of aqueous oxygenated solution 2,4,6-TCP 10-3 M, in the presence of PW12O403- 7 × 10-4 M, pH 1 (HClO4), λ > 320 nm, T ∼ 20 °C. VOL. 34, NO. 10, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(2)
(3) (4)
(5)
FIGURE 8. Formation of CO2, Cl- and decay of 2,4,6-TCP upon photolysis of aqueous oxygenated solution of substrate, in the presence of catalyst: 2,4,6-TCP 4 × 10-4 M, PW12O403- 7 × 10-4 M, pH 1 (HClO4), λ > 320 nm, T ∼ 20 °C.
(6)
Key reactions involved are as follows: (a) hydroxylation of the aromatic ring, (b) substitution of chlorine by OH, (c) oxidation of chlorinated HQ to the corresponding quinone, and (d) breaking of the aromatic ring, producing carboxylic acids. Notice that acetic acid has been the common chemical identified in the photodecomposition of aromatic compounds with this method, implying that some reduction step must be involved prior to their mineralization (3, 4a,b). Figure 8 shows the photodegradation of 2,4,6-TCP and the concominant formation of Cl- and CO2. The formation of CO2 is detected after an induction period, indicating that the process of mineralization goes through several intermediates, as shown above. For 2,4,6-TCP disappearance occurred in almost 30 min of illumination in oxygenated solution, whereas mineralization required a much longer time (Figure 8).
(9)
Acknowledgments We thank Ministry of Development, General Secretariat of Research and Technology of Greece, and the Italian-Greek exchange program for supporting part of this work. We thank S. Boyatzis for helping out with the GC-MS.
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Received for review July 16, 1999. Revised manuscript received December 7, 1999. Accepted January 14, 2000. ES990802Y
