Environ. Sci. Technol. 2006, 40, 5977-5983
Photodegradation Processes of the Antiepileptic Drug Carbamazepine, Relevant To Estuarine Waters S E R G E C H I R O N , * ,† CLAUDIO MINERO,‡ AND DAVIDE VIONE‡ Laboratoire Chimie et Environnement, Universite´ de Provence, 3 place Victor Hugo, 13331 Marseille Cedex 3, France, and Dipartimento di Chimica Analitica, Universita` di Torino, Via Pietro Giuria 5, 10125 Torino, Italy
The photodegradation of carbamazepine was studied in artificial estuarine water, under conditions relevant to the Rhoˆ ne delta. Chloride substantially enhances the photodegradation of carbamazepine, most likely because of the interaction between Fe(III) colloids and Cl- ions under irradiation, yielding Cl2•-. For a given compound, prerequisites for the described degradation enhancement by chloride to be significant are faster degradation via reaction with Cl2•- compared to charge-transfer processes on the surface of Fe(III) colloids and an important role of indirect phototransformation compared to direct photolysis. A major photodegradation intermediate of carbamazepine is acridine, formed by direct photolysis, while hydroxylated/ oxidized compounds are formed in the presence of •OH, and chloroderivative formation is observed in the presence of Fe(III) and chloride.
Introduction Photochemistry plays an important role in the transformation of organic compounds, of both natural and anthropogenic origin, in surface waters. In particular, many pesticides and pharmaceuticals undergo photodegradation as their main removal pathway in surface waters (1-10). The photochemical pathways involved are the direct photolysis of sunlightabsorbing molecules, the photodegradation sensitized by dissolved organic matter (DOM), and the reaction with transient species (e.g. •OH, 1O2, CO3•-) generated by irradiation of various photoactive compounds (DOM itself, nitrate, Fe(III) species) (11-15). The chemical composition of natural waters has a substantial impact on photochemical processes due to the varying concentration values of photoactive compounds in different water bodies, and because of the interaction between photogenerated reactive species and natural water components (16). One of the most important changes in the chemical composition of surface waters can be observed in estuarine areas, where enrichment of chloride from saltwater deeply alters the ionic composition of river water and causes additional phenomena such as coagulation and flocculation of colloids. A sharp increase of DOM in estuarine waters is also observed (17). Understandably, the role of chloride ions on photochemical transformation processes deserves much interest due to its potential impact on photochemistry in * Corresponding author phone: +33-4-91-10-85-25; fax: +33-491-10-63-77; e-mail:
[email protected]. † Universite ´ de Provence. ‡ Universita ` di Torino. 10.1021/es060502y CCC: $33.50 Published on Web 08/30/2006
2006 American Chemical Society
estuarine environments. However, the present knowledge on this issue is still limited. Chloride can inhibit the direct photolysis of chrysene on smectite, which probably involves 1O , quenched by halide ions (18). By contrast chloride has 2 no effect on the photodegradation of orbifloxacin, which likely proceeds via bond breaking followed by reaction between the resulting radical and chloride (if available) or oxygen (19). Furthermore, chloride has a very limited effect on the direct photolysis of 2,4-dichlorophenoxyacetic acid (20). Regarding sensitized photolysis, Cl- has been found to favor the photodegradation of fipronil by DOM, although the mechanistic details have not been investigated (21). Moreover, Cl- enhances the photooxidation of elemental mercury, Hg(0), possibly because complex formation between Cl- and Hg(II) decreases the reduction potential of Hg(0) (22, 23). Finally, Cl- inhibits many TiO2-induced photocatalytic degradations, possibly because of competition with the substrates for adsorption onto the semiconductor surface (24). Interestingly, photochemical processes in the presence of Cl- can result in substrate chlorination (19, 25-27), showing that photochlorination is a potential abiotic source of organohalogens to the environment. The chloride effect is likely to vary depending on the substrate under consideration and the adopted irradiation conditions, but it should be extremely interesting to find out indications of general or otherwise wide validity. Contribution to this issue is the purpose of the present work, which studies the photodegradation of a common pollutant (some tens of ng L-1 in Rhoˆne delta water), the antiepileptic drug carbamazepine, under conditions that are relevant to estuarine waters. A mechanistic interpretation of the results is also reported, together with a complete study of the photodegradation intermediates of carbamazepine under different conditions. The latter data can give insight into the compounds that might arise from the photoinduced degradation of carbamazepine in natural waters and support the proposed photodegradation pathways.
Experimental Section Reagents and Materials. Carbamazepine (purity grade >99%), acridine (>97%), and humic acid sodium salt were purchased from Sigma-Aldrich. Methanol and 2-propanol were LiChrosolv gradient grade from Merck. NaCl (>99.5%), Fe2(SO4)3 (puriss. p.a.), NaNO3 (>99%), and formic acid (>98%) were from Fluka. All reagents were used as received without further purification. Solutions and HPLC eluents were prepared with Milli-Q water. Hematite (R-Fe2O3) was synthesized following the procedure of Leland and Bard (28). Irradiation Experiments. The experiments were carried out with a 0.5 L cylindrical immersion-type photoreactor (Heraeus TQ 150 Model), equipped with a water-cooled, medium-pressure mercury lamp with maximum emission wavelengths at 313, 366, 406, 436, 546, and 578 nm. The radiation path length inside the reactor is 2 cm. The spectrum of the lamp and the UV-vis spectrum of carbamazepine are provided as the Supporting Information. The reactor is made of Pyrex glass in order to cut off the wavelengths shorter than 290 nm. The whole assembly has been mounted on a magnetic stirrer and wrapped with aluminum foil. Initial pH was adjusted by dropwise addition of either 0.1 M NaOH or 0.1 M H2SO4. pH was monitored and remained stable during irradiation. Aliquots of 1 mL were analyzed at selected intervals after a filtration step through 0.45 µm filter membranes (cellulose acetate, Millipore). Analytical Determinations. HPLC-UV and HPLC-MS were adopted. See the Supporting Information for further details. VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Photodegradation rate of carbamazepine (2.1 × 10-4 mol L-1) under different conditions. The composition of the different solutions is reported on the X-axis. Carbamazepine was dissolved in Milli-Q water (MQ, direct photolysis) or in Milli-Q water spiked with Fe(III) (2 mg L-1), NO3- (10 mg L-1), or DOM (25 mg L-1 humic acids). When indicated, 15 g L-1 Cl- was also added. Variability of the results of repeated runs (shown) is around 1-3%.
FIGURE 2. Photodegradation rate of carbamazepine (2.1 × 10-4 mol L-1) as a function of pH (adjusted by addition of H2SO4 or NaOH), in the presence of 2 mg L-1 Fe(III) and of 2 mg L-1 Fe(III) + 15 g L-1 Cl-. The ratio of the degradation rates in the presence and in the absence of chloride, as a function of pH, is also reported.
Results Carbamazepine Direct Photolysis and Photodegradation in Artificial River and Estuarine Water. To begin with, we studied the direct photolysis of carbamazepine (irradiation of the organic substrate alone in ultrapuresMilli-Qswater) and the effect of chloride on its degradation under conditions that are most representative of surface water in the Rhoˆne delta (Southern France). An artificial river water was produced by addition to tap water of 25 mg L-1 humic acids, 2 mg L-1 Fe(III) (as Fe2(SO4)3), and 10 mg L-1 NO3- (as NaNO3). All the added compounds are potentially able to induce photodegradation (11). The runs in the presence of chloride were carried out upon addition of 15 g L-1 Cl- as NaCl, as can be found in the Rhoˆne delta and other estuarine areas (17). The photodegradation rate of 2.1 × 10-4 mol L-1 carbamazepine under the adopted irradiation conditions was 4.7 × 10-9 mol L-1 s-1 in Milli-Q water (direct photolysis), 3.7 × 10-9 mol L-1 s-1 in artificial river water without added Cl-, and as high as 1.0 × 10-8 mol L-1 s-1 in artificial river water spiked with 15 g L-1 Cl- to reproduce estuarine conditions. The average quantum yield for carbamazepine direct photolysis in Milli-Q water under the adopted lamp is φav ) 1.5 × 10-4 (see the Supporting Information for further details). Additionally, the photodegradation of carbamazepine is a bit faster in Milli-Q water than in artificial river water without added Cl-. The most likely explanation is that, while Fe(III), humic acids and nitrate are able to induce photodegradation processes on their own, they also absorb radiation that is therefore no more available for direct photolysis, which can produce an overall inhibitory effect. Last and most important, a considerable enhancement of carbamazepine photodegradation is observed upon addition of chloride levels (15 g L-1) that are commonly found in estuarine waters. Further experiments were carried out to explain the effect of chloride on carbamazepine phototransformation. In this case the different photoactive compounds (Fe(III), humic acids, and nitrate) were added separately to Milli-Q water, and the effect of chloride addition was assessed case by case (Figure 1). Fe(III) is the only compound for which Cl- addition strongly enhances photodegradation, while there is no Cl- effect on direct photolysis. Furthermore, the photodegradation rate of carbamazepine in the presence of Fe(III) + Cl- (8.9 × 10-9 mol L-1 s-1) is quite similar to the rate observed in the presence of artificial river water + Cl5978
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FIGURE 3. Photodegradation rate of carbamazepine (1.9 × 10-4 mol L-1) as a function of [Fe(III)], in the presence of 15 g L-1 Cl-. (1.0 × 10-8 mol L-1 s-1). It can therefore be hypothesized that the enhancement of carbamazepine photodegradation by chloride in artificial river water is due to an interaction between Fe(III) and Cl-. Figure 1 also shows that carbamazepine photodegradation by nitrate is very fast (1.1 × 10-8 mol L-1 s-1 with no added chloride), while the degradation rate in artificial river water containing the same amount of nitrate is much slower (3.7 × 10-9 mol L-1 s-1 without added chloride). The most likely reason is that nitrate induces photodegradation through the generation of •OH, which is efficiently scavenged by DOM in artificial as well as in real river water (also see the Supporting Information) (14). The Chloride Effect: Roles of pH and [Fe(III)]. It is interesting to study the pH dependence of carbamazepine photodegradation in the presence of Fe(III) and of Fe(III) + Cl-. Figure 2 reports, as a function of pH, the photodegradation rate of 2.1 × 10-4 mol L-1 carbamazepine in the presence of 2 mg L-1 Fe(III), with and without 15 g L-1 Cl-. The ratio of the photodegradation rates in the presence and in the absence of chloride is also reported for the figure. The carbamazepine photodegradation rate is higher under acidic conditions, both in the absence and in the presence of chloride. However it is most interesting to observe that, while chloride inhibits photodegradation under acidic conditions, it has an enhancing effect at pH > 5. Figure 3 reports the photodegradation rate of 45 mg L-1 (1.9 × 10-4 mol L-1) carbamazepine, in the presence of 15
SCHEME 1. Intermediates of Carbamazepine Photodegradation as Detected by HPLC-MS under the Different Adopted Conditions (Both Direct and Indirect Photolysis)a
a The roman number is the way each intermediate is referred to in the text. The scheme also shows the proposed interconversion pathways of the detected compounds.
g L-1 Cl-, as a function of [Fe(III)]. The rate increases, although not dramatically, with increasing [Fe(III)]. This finding is important because, in the context of the increasing salinity that is found in estuarine environments when approaching the sea, the coagulation/flocculation of colloids decreases the total Fe loading of river water. In the case of the Rhoˆne delta, total Fe decreases from around 1.5 mg L-1 at the delta inlet to 0.1 mg L-1 at the outlet to the Mediterranean Sea, while [Cl-] shows a corresponding increase from a few mg L-1 to 35 g L-1. This scenario is very common in estuarine areas (29). From a photochemical point of view, the decrease in Fe through estuarine waters can be compensated by the increasing [Cl-] that enhances some photodegradation processes induced by Fe(III) colloids. Interestingly, the degradation rate of 1.9 × 10-4 mol L-1 carbamazepine increased from 4.1 × 10-9 mol L-1 s-1 in the presence of 2 mg L-1 Fe(III) and no chloride to 6.0 × 10-9 mol L-1 s-1 in the presence of 0.2 mg L-1 Fe(III) and 15 g L-1 Cl-. Intermediates of Carbamazepine Photodegradation. Various intermediates of carbamazepine photodegradation under the different adopted conditions have been detected
by HPLC-MSn and identified on the basis of their mass fragmentation spectra. Byproduct molecular weight was assigned on the basis of the pseudomolecular ions [M + H]+ and [M + Na]+. The byproduct structures were tentatively elucidated according to their MS2/MS3 mass fragmentation pattern using the pseudomolecular ion as precursor ion. A comprehensive view of carbamazepine photodegradation pathways under different conditions is provided in Scheme 1. Additional data concerning mass spectra, ions used for byproduct identification, and total ion current chromatograms under different conditions are reported as Supporting Information. Under conditions of carbamazepine direct photolysis the detected intermediates are I-VI. As far as I (MW 254) is concerned, structural MS2 analyses were consistent with previously published data (30), and I was identified as 10hydroxycarbamazepine. The MS2 spectrum of II (MW 223) showed losses of 18 and 28 mass units due to losses of H2O and carboxyaldehyde moieties, respectively. The ion at m/z 180 accounted for the unmodified acridine structure. II was assigned the structure of hydroxyacridine-9-carboxaldehyde. VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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III (MW 195) was tentatively identified as acridone since its MS2 spectrum was characterized by a base peak at m/z 167 related to the CO group removal. IV (MW 179) was identified as acridine by comparing its chromatographic and mass spectrometric behavior with that of an authentic standard. The concentration of acridine, a stable azaarene compound with known mutagenic and carcinogenic activity (31), steadily increased against the photolysis time and accounted for 10% of the initial carbamazepine concentration after 8 h irradiation. The structure of V (MW 450) was assumed according to its MS spectrum that was characterized by a base peak at m/z 248. This ion was the sodium adduct of the ion at m/z 225 resulting from the C9-C9 bond cleavage. In addition, the MS2 spectrum revealed losses of 58 and 116 mass units related to one and two NCHOHNH2 losses, respectively. V would in part decompose to yield VI (MW 360). In the presence of carbamazepine and Fe(III) at pH 2.0, without chloride, three major additional byproducts were detected: VII (MW 252), VIII (MW 266), and IX (MW 250). VII has a MS2 spectrum depicting fragment ions at m/z 210 (CONH2 loss) and m/z 236 (OH loss). VIII and IX correspond to byproducts with a high degree of transformation, and their assignment is more speculative. However, VIII and IX were the only compounds detected in (+) APCI apart from the parent compound. VIII and IX should be quinonid derivatives since quinones are readily amenable by APCI. In addition, losses of 28 mass units (one in case of VIII and two in case of IX) corresponding to CO losses are characteristic of quinone derivatives. At pH 2.0 with Fe(III) + Cl-, two chlorinated compounds were also detected: X (MW 288) and XI (MW 304). The chlorination pathway was clearly evidenced by the occurrence of the chlorine isotopic pattern in the X and XI mass spectra. The MS2 spectrum of X (MW 288) showed a chlorine loss with an ion at m/z 254, and MS3 structural analyses of X obtained by using the m/z 254 ion as the precursor ion were consistent with the fragmentation pattern observed for I. X could be tentatively identified as chloro-10-hydroxycarbamazepine. At pH 7.5 in the presence of Fe(III) and no chloride, three additional isomers (XII) appeared with a molecular weight of 270. The MS2 spectrum of XII was featured by a base peak at m/z 253 related to a first OH loss. The MS3 fragmentation pattern using the m/z 253 precursor ion was consistent with that observed in the MS2 mass spectrum of VII. XII was tentatively identified as dihydroxycarbamazepine isomers since the oxidation of the aromatic ring of hydroxycarbamazepine is expected to yield up to 4 isomers with the OH function in different positions. At pH 7.5 in the presence of Fe(III) + Cl-, in addition to I, II, IV, V, VII, VIII, and XII, the chlorinated compounds X and XI were also detected.
Discussion Chloride-Enhanced Photodegradation of Carbamazepine. When considering the chloride-enhanced carbamazepine photodegradation by Fe(III), it should be observed that the relevant experiments have been carried out at pH 7.5. Under such conditions Cl- is not able to scavenge •OH (32) because the corresponding reaction 1 is reversible and the adduct species ClOH•- is poorly reactive toward organic compounds (33). The scavenging of •OH by chloride is operational in acidic solution only, under which conditions ClOH•- evolves into Cl• (reaction 2 (32-34))
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Cl- + •OH a ClOH•-
(1)
ClOH•- + H+ a Cl• + H2O
(2)
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The lack of •OH scavenging by chloride at neutral to basic pH excludes a potentially negative effect of the ion on photodegradation. It should also be considered that, at pH 7.5, most Fe(III) would be in the colloidal form as Fe(III) (hydr)oxides (35), and the presence of chloride would probably enhance photodegradation by these species. This hypothesis can be verified by testing the effect of chloride on carbamazepine photodegradation in the presence of hematite (R-Fe2O3), adopted as a model colloidal form of Fe(III) oxide. Addition of 15 g L-1 chloride enhances the photodegradation rate of 50 mg L-1 carbamazepine (2.1 × 10-4 mol L-1), in the presence of 100 mg L-1 hematite, from 9.0 × 10-9 to 1.9 × 10-8 mol L-1 s-1. The chloride-enhanced carbamazepine photodegradation in the presence of hematite bears similarity with the effect of nitrite on the photodegradation of phenol by both R-Fe2O3 and β-FeOOH (36). The degradation of phenol upon irradiation of the cited Fe(III) (hydr)oxides with visible light would mainly take place upon charge-transfer processes (37, 38), which are however quite slow because the oxidation of phenol requires H+ transfer together with electron abstraction (39). In contrast, nitrite is readily photooxidized by R-Fe2O3 and β-FeOOH to •NO2 (36, 40), which is then able to transform phenol faster than the Fe(III) oxides, mainly by nitration: phenol photodegradation by R-Fe2O3 and β-FeOOH in the presence of nitrite is an order of magnitude faster than in its absence and leads to an almost quantitative yield of nitrophenols (36). The Fe(III) (hydr)oxides are also able to oxidize Cl- to Cl• (26), which then yields Cl2•- (34).
)FeIII-OH + Cl- + hν f Fe2+ + OH- + Cl•
(3)
Cl- + Cl• a Cl2•-
(4)
The radicals Cl2•- are oxidizing and chlorinating agents (25, 26, 41) and are likely responsible for the observed chloride effect on carbamazepine photodegradation in the presence of hematite and of Fe(III) colloids. In the presence of Fe(III) without Cl-, photodegradation of carbamazepine is likely to take place by charge-transfer processes (very probably slow) and by reaction with •OH, produced in a limited amount upon UV photolysis of the Fe3+-OH- surface groups of the (hydr)oxides (37).
Fe3+-OH- + hν(UV) f Fe2+ + •OH
(5)
In the presence of chloride, the charge-transfer reactions induced by Fe(III) colloids would be able to efficiently oxidize Cl- to Cl•/Cl2•- (reactions 3 and 4 (26)). The radical Cl2•would then degrade carbamazepine faster than the Fe(III) colloids in the absence of Cl-. Note that the rate constant for the reaction between carbamazepine and Cl2•- is unfortunately not available, but tryptophan has a somewhat similar structure (benzene ring fused with a N-containing heteroaromatic ring and a lateral chain with an amino group) and undergoes fast reaction with Cl2•- (2.6 × 109 M-1 s-1 (42)), although slower than the reaction with •OH (1.3 × 1010 M-1 s-1 (43)). Coherently with this hypothesis, carbamazepine chloroderivatives were detected in the presence of Fe(III) + Cl- (see later), which is consistent with the generation of Cl2•- (25, 26). Moreover, the carbamazepine photodegradation process accounted for by •OH, photoformed in reaction 5, is likely to proceed at about the same rate in the presence and in the absence of chloride, which is not able to scavenge •OH in neutral to basic solution (32, 33). The pH Effect. Figure 2 shows that carbamazepine photodegradation is faster in acidic conditions, in the presence of both Fe(III) and Fe(III) + Cl-, and that chloride inhibits photodegradation by Fe(III) at pH < 5 and enhances it at higher pH. Focusing first on the trend observed in the
absence of Cl-, carbamazepine photodegradation rate decreases with increasing pH. At the most acidic pH values Fe(III) is mainly in the dissolved form (Fe3+, FeOH2+, and FeSO4+ because pH was adjusted with H2SO4). FeOH2+ is the most photoactive Fe(III) species (35). Under such conditions the photodegradation of carbamazepine by Fe(III) would mainly be accounted for by photoformed •OH, consistently with the detection of various hydroxylated intermediates. Upon pH increase, dimeric and oligomeric Fe(III) compounds up to Fe(III) colloids tend to prevail over mononuclear species (35). The main difference between monomeric and dimeric/ oligomeric species or colloids is that the irradiation of the former yields •OH with reasonable to elevated quantum yield, while •OH production by dimers/oligomers/colloids is quite low. In contrast, the latter are able to induce charge-transfer reactions (35). The decrease of carbamazepine degradation rate with increasing pH confirms that photoinduced chargetransfer processes as directly involving the substrate are relatively slow, at least when compared with •OH-induced degradation. The addition of chloride to Fe(III) in acidic solution would result in •OH scavenging via reactions 1, 2, and 4, which replace •OH with the less reactive species Cl2•-, causing slower photodegradation of the substrate. Similar results have also been obtained in the presence of phenol upon irradiation of acidic Fe(III) + Cl- (25). Under neutral to basic conditions Cl- is no longer able to scavenge •OH, and, in addition, •OH photoproduction by Fe(III) decreases. Accordingly, the inhibition by Cl- of •OH-initiated carbamazepine photodegradation is no longer operational around and above neutrality. Under such circumstances chloride has a positive effect on carbamazepine photodegradation. As already discussed, this is most likely due to the intermediate role played by Cl2•-, formed upon charge-transfer processes (reactions 3 and 4 (26)). Carbamazepine transformation by Cl2•-, albeit slower than degradation by •OH, would be faster than charge-transfer processes. The enhancement of the degradation of organic substrates by radicals formed upon oxidation of small inorganic anions (Cl-, NO2-) has already been described in the literature (36, 44). These processes are environmentally significant because they affect the photodegradation rate of the substrates (and hence their lifetimes in surface waters) and can yield chloroand nitroderivatives of high environmental concern (25, 26, 36, 40). Formation Pathways of Carbamazepine Intermediates. Irradiation of Carbamazepine Alone. Direct photolysis of carbamazepine proceeds through two main routes. Hydration of the C10-C11 double bond is a first minor pathway leading to I. This reaction also occurs in the dark at pH 7. The other compounds (II-VI) are formed via a main photodegradation pathway that involves a ring contraction process as already suggested (45). Ring contraction would possibly involve the formation of carbamazepine-9-carboxaldehyde resulting from a hydroxylation step at the 10 position. This intermediate might evolve in three different ways. (i) The first possibility would be a further hydroxylation step with loss of the CONH2 lateral chain leading to II and III. III was sporadically detected in our experiments and might also arise upon oxidation of IV. (ii) The second possibility is the simultaneous loss of the carboxyaldehyde group and the CONH2 lateral chain yielding IV. (iii) The third possibility is dimerization. This pathway includes a simultaneous reduction of the CONH2 lateral chain, leading to V and VI. Further details of the direct photolysis process are discussed in the Supporting Information. Fe(III), pH 2, No Chloride. In addition to I, IV, and V, likely formed upon direct photolysis, VII-IX were detected as additional intermediates. VII would probably derive from a •OH attack on a C aromatic ring and was clearly assigned 6 the structure of hydroxycarbamazepine. The described
hydroxylation pathway is reasonable because of the sustained photoproduction of •OH upon photolysis of FeOH2+ (and of Fe3+ at a lesser extent) (35). The quinonid compound VIII might form upon oxidation of an undetected hydroquinone structure, arising on further hydroxylation of VII. An intramolecular reaction with H2O loss might then be responsible for the transformation of VIII into IX. Hydroquinone oxidation to VIII might be carried out by •OH, but many hydroquinones are known to undergo oxidation to pbenzoquinones in the presence of Fe(III) under acidic conditions (46). Actually, the two compounds VIII and IX were not detected upon carbamazepine photodegradation in the presence of nitrate, which is able to photochemically yield •OH, suggesting that VIII would possibly be formed by Fe(III). The process would most likely imply a transfer of electrons, which might also take place in natural waters in the presence of other oxidizing species (e.g. derived from the Mn cycle (47)). Fe(III)+Cl-, pH 2. The formation of the chlorinated intermediates X and XI is consistent with the production of the radical Cl2•- upon chloride oxidation by photoformed •OH. XI possibly derives from an additional oxidation step by •OH on the aromatic ring of X. Fe(III), pH 7.5, No Chloride. The formation of the three XII isomers under the reported conditions might reflect the reactivity of •OH, generated upon photolysis of the surface Fe3+-OH- groups of the Fe(III) colloids (21), or the chargetransfer oxidation of carbamazepine. Fe(III)+Cl-, pH 7.5. In addition to VII, VIII, and XII, the chlorinated compounds X and XI were also detected. The formation of the chloroderivatives X and XI in the presence of Fe(III) + Cl- at both pH 2.0 and 7.5 is consistent with the formation of Cl2•- in both systems. A comparison can now be made with previous works that studied the degradation of carbamazepine under different conditions. H2O2/UV treatment of carbamazepine (45) has yielded acridine (IV), hydroxylated IV, and an unhydroxylated analogue of II as well as ring-opening intermediates such as catechol, not detected in the present work. Ozonation mainly resulted in an attack on the double bond and in the subsequent formation of benzaldehyde derivatives, formed upon breaking of the 7-atom ring (9). These intermediates are very different from those found in the present work, indicating that the direct UV photolysis, unlike O3, leads to a rearrangement rather than a breaking of the seven-member ring, while reaction with •OH results in the hydroxylation of both/either the double bond and the aromatic rings. Quite interestingly compound I and some analogues of VII and XII, all hydroxylated derivatives, have also been found as carbamazepine metabolites in a wastewater treatment plant (8), along with epoxy derivatives that were not detected in the present work. It is noteworthy that 10,11-epoxycarbamazepine has been detected upon carbamazepine photodegradation (2), suggesting that the nature of the main detected intermediates (accumulating because of fast formation or slow degradation) strongly depends on the adopted experimental conditions. Environmental Implications. This paper shows that Clcan enhance the photoinduced degradation of the model pollutant carbamazepine by Fe(III) colloids and oxides, most likely via the intermediate production of the oxidizing and chlorinating agent Cl2•-. It can be assumed that chloride ions are able to enhance photodegradation by Fe(III) of those substrates for which the reaction with photogenerated Cl2•is faster than the charge-transfer processes occurring on the surface of Fe(III) colloids. This way, the role of Cl2•- as photogenerated co-oxidant would allow the overcoming of a kinetic limitation to the photodegradation of organic substrates by Fe(III) colloids. The enhancement effect that Cl- may have on the photodegradation by Fe(III) would VOL. 40, NO. 19, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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therefore be important for those substrates for which Fe(III)-induced photoreactions are significant compared to direct photolysis. Chloride-enhanced photodegradation, also leading to the production of chloroderivatives, can become operational as a consequence of the increase in chloride concentration found in estuarine systems. The reported experimental results, together with the data concerning water composition in the Rhoˆne delta, allow the hypothesis that [Cl-] could in some cases more than compensate the decrease of total dissolved Fe, due to coagulation, which is observed while the Rhoˆne river approaches the sea. The same would probably happen in other estuarine areas. Additional work on further substrates and different conditions will be required to assess the breadth of the described effect of chloride and its impact in field conditions. Most notably a major intermediate of carbamazepine photodegradation, arising upon direct photolysis, is the toxic, mutagenic, and carcinogenic acridine. Accordingly, the direct photodegradation of carbamazepine yields a compound, the health and environmental impact of which is much higher than that of the parent substrate. The other detected intermediates might also have a significant environmental impact, which is however not known at the moment.
Acknowledgments The work carried out in Torino was financially supported by Universita` di Torino-Ricerca locale, the INCA Consortium, and PNRA-Progetto Antartide.
Supporting Information Available Lamp and carbamazepine spectra, further discussion including carbamazepine direct photolysis, chromatograms, and mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review March 3, 2006. Revised manuscript received June 20, 2006. Accepted July 7, 2006. ES060502Y
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