Solar transformation and photocatalytic treatment of cocaine in water: Kinetics, characterization of major intermediate products and toxicity evaluation

Applied Catalysis B: Environmental 104 (2011) 37–48

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Solar transformation and photocatalytic treatment of cocaine in water: Kinetics, characterization of major intermediate products and toxicity evaluation Cristina Postigo a,∗ , Carla Sirtori b , Isabel Oller b , Sixto Malato b , Manuel Ignacio Maldonado b , Miren López de Alda a , Damià Barceló a,c,d a

Institute of Environmental Assessment and Water Research (IDAEA, CID-CSIC), Department of Environmental Chemistry, C/Jordi Girona, 18-26, 08034 Barcelona, Spain Plataforma Solar de Almería (PSA-CIEMAT), Carretera Senés, km 4, 04200 Tabernas, Almería, Spain Catalan Institute for Water Research (ICRA), Parc Científic i Tecnològic de la Universitat de Girona, Edifici H2O, 17003 Girona, Spain d King Saud University, Box 2454, Riyadh 11451, Kingdom of Saudi Arabia b c

a r t i c l e

i n f o

Article history: Received 5 January 2011 Received in revised form 17 February 2011 Accepted 22 February 2011 Available online 1 March 2011 Keywords: Cocaine Solar photo-Fenton Solar photocatalysis with TiO2 Solar photolysis Phototransformation products

a b s t r a c t The present manuscript describes for the first time the transformation and mineralization of cocaine (COC) in water (distilled water (DW) and synthetic municipal wastewater effluent (SWeff)) by natural solar irradiation and two solar photocatalytic processes: heterogeneous photocatalysis with titanium dioxide (TiO2 ) and homogeneous photocatalysis by photo-Fenton. The solar photocatalytic processes were run at equivalent pilot-plant scale by means of compound parabolic collectors, which allowed for comparison of solar transformation kinetics and compound mineralization. Direct photolysis resulted in almost complete and partial disappearance of COC in SWeff and DW, respectively, after 33 h of normalized irradiation time, and negligible mineralization. Solar transformation of COC by heterogeneous photocatalysis with TiO2 was completed after 28 and 50 min of illumination in DW and SWeff, respectively, whereas about half of the irradiation time was needed with photo-Fenton, which was also proved to be more effective in compound mineralization. Kinetics parameters were calculated for process comparison. Additionally, the phototransformation intermediates generated during each treatment were investigated and characterized by means of ultra-performance liquid chromatography coupled to quadrupole-time of flight tandem mass spectrometry (UPLC-QqTOF-MS/MS). Identity confirmation was possible for some of them with the analysis of commercially available analytical standards. The main COC phototransformation pathways were observed to be ester bond cleavage, hydroxylation, and demethylation. Finally, the application of an acute toxicity test (Vibrio fischeri) to selected water samples resulted in inhibition percentages of bacterial bioluminescence in most cases below 20% after 30 min of sample contact, which indicates low acute toxicity of the photointermediates generated during the different treatments. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Illicit drugs and their metabolites constitute a group of environmental emerging organic contaminants of concern [1]. In recent years, several works have reported on the occurrence of these substances in environmental waters in various countries all over the world [2]. The main source of illicit drugs and metabolites into the aquatic environment are the municipal wastewater treatment plants effluents, since these compounds are not completely eliminated with conventional treatment processes [3]. Among the investigated illicit drugs, cocaine (COC) is one of the best efficiently removed in conventional activated sludge (CAS) processes, with elimination percentages usually above 90% [3,4]. Despite the high elimination rates observed, COC has been measured at levels above

∗ Corresponding author. Tel.: +34 93 400 61 00; fax: +34 93 204 59 04. E-mail addresses: [email protected], [email protected] (C. Postigo). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.02.030

500 ng/L in wastewater treatment plants effluents [5,6]. Additionally, it is one of the most ubiquitous illicit drugs in natural surface waters, where it reaches levels up to 115 ng/L [4,7]. Despite the fact that environmental concentrations of COC are relatively low, the presence of this compound and/or its natural transformation products, not yet investigated, in the aquatic environment may have negative effects on the aquatic organisms that are continuously exposed to them. Hence, knowledge about its natural transformation is essential for appropriate environmental risk assessment. Moreover, the study of alternative water treatment technologies, such as those based on advanced oxidation processes (AOPs), may contribute to reduce its presence in the environment. AOPs are characterized by the production of hydroxyl radicals (HO• ) by diverse reaction systems that oxidize the organic matter present in water. These processes have been successful in degrading diverse types of organic contaminants, such as pharmaceuticals and pesticides, until their mineralization [8]. Furthermore, the use of sunlight as source of irradiation to run the AOPs reduces the pro-

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C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

cess cost and makes it more affordable for its commercial use as a technology to treat waters [9]. The illumination of a wide band-gap semiconductor with light energy greater than its band gap energy (BGE) produces excited high-energy states of electron and hole pairs (e− /h+ ). Photogenerated holes oxidize the organic molecules directly, or the OH− ions and the H2 O molecules adsorbed on the catalyst’s surface, to HO• . TiO2 is the most used photocatalyst due to its low cost and low toxicity, and its chemical stability and high reactivity. Additionally, its BGE of 3.2 eV makes it capable of getting photoexcited with the solar spectrum [10]. Solar irradiation also enhances the Fenton reaction (H2 O2 + Fe2+ → Fe3+ + HO• + OH− ) and hence the formation of oxidative species, mainly HO• . At acidic pH (optimum pH = 2.8), and in absence of other ligands, the most abundant ferric ion–hydroxy complex present in solution is [Fe(OH)]2+ , which is highly photoactive and hence reacts in the presence of light as follows: [Fe(OH)]2+ + H2 O + hv → Fe2+ + H+ + 2HO• . Thus, the ferrous ion required for the Fenton reaction is produced and also HO• that may further react with the organic matter present in the water. This is the so-called photo-Fenton process. If other ligands different from hydroxyl ligands are present in solution different photoreactive ferric complexes, e.g., Fe3+ -oxalate or Fe3+ -citrate, may be formed and dissociated in the same way producing ferrous ion and an oxidized ligand [11]. Once formed, HO• are extremely unstable and react rather indiscriminately and not selectively with the organic and inorganic species present in the solution, generating simultaneously diverse products through different reaction pathways. Since HO• naturally occurs in environmental waters, together with other reactive oxidative radicals, they are responsible to some extent for the natural abiotic transformation of organic contaminants in the aquatic environment, and in consequence, for their environmental fate. In this context, the present study aimed at investigating for the first time the solar photolytic and photocatalytic transformation of COC in two different aqueous matrices: distilled water (DW) and simulated municipal wastewater treatment plant effluent (SWeff). The photocatalytic treatments investigated were heterogeneous photocatalysis with TiO2 and homogeneous photocatalysis by photo-Fenton, in both cases performed at pilot plant scale by means of compound parabolic collectors (CPC) and assisted by natural illumination. The specific objectives of this study were (i) to evaluate the natural and chemical solar transformation of COC in DW and SWeff, including transformation kinetics and the effect of the matrix on the transformation process, (ii) to identify the phototransformation products originated in each process, since oxidative processes driven by HO• are not compound selective, and (iii) to evaluate the acute toxicity of the photoproducts generated. The study was performed at higher concentration than usually found in natural surface waters or effluents from municipal wastewater treatment plants because working at so low concentration could hide most of the photoproducts. This aspect may slightly affect the photo-transformation kinetics but not the degradation pathway observed or the matrix effect on it.

2. Experimental 2.1. Chemicals and reagents Cocaine hydrochloride salt (purity, 98.5%) was provided as a concession for research purposes (2009C00124) by the Division of Narcotic Drugs and Psychotropic Substances of the Spanish Agency of Pharmaceuticals and Medical Products. High purity (>97%) standard solutions of benzoylecgonine (BE), ecgonine (ECG), ecgonine

methyl ester (EME), and cocaethylene (CE) were purchased from Cerilliant (Round Rock, TX, USA). DW used in the experiments was obtained from the Plataforma Solar de Almería (PSA) distillation plant (conductivity < 10 ␮S/cm, Cl− = 0.7–0.8 mg/L, NO3 − < 0.2 mg/L, organic carbon < 0.5 mg/L). Distilled water was also used to generate SWeff. The chemical composition of the SWeff is detailed in Table S1 (supplementary material) and was derived from the guidelines established by the U.S. Environmental Protection Agency (U.S. EPA) and the OECD for moderately hard synthetic freshwater and synthetic sewage, respectively [12,13]. Heterogeneous photocatalysis was performed using TiO2 Degussa P-25 (Frankfurt, Germany). Reagents used in the photoFenton experiments were iron sulfate heptahydrate (FeSO4 ·7H2 O) and hydrogen peroxide (H2 O2 ) (30%, w/v), and those used for pH adjustment were sulfuric acid (H2 SO4 ) and sodium hydroxide (NaOH). All of them were purchased from Panreac. High performance liquid chromatography (HPLC)-grade acetonitrile (Merck) and water produced by a Mili-Q ultra-pure water system from Millipore (Milford, MA, USA) were used for HPLC analyses. Ultra-performance liquid chromatography (UPLC)-grade acetonitrile and water (Merck) were used in the UPLC analyses. Formic acid (purity, 98%) added to the chromatographic mobile phase was acquired from Fluka. 2.2. Hydrolysis, photolysis and solar photocatalysis experiments All hydrolysis and phototransformation experiments were carried out during summer 2009 at the PSA (latitude 37◦ N, longitude 2.4◦ W). Individual solutions of COC were prepared by dissolving the compound in DW and SWeff at an initial concentration of 10 mg/L in 5 L Pyrex beakers. This value is much higher than reported environmental concentrations [2,14]; however, it was chosen as COC initial concentration (COC0 ) in all experiments performed for better evaluation of transformation kinetics and photointermediates generated. The beakers containing the COC solutions were kept in the dark at room temperature during hydrolysis experiments and they were exposed to direct sunlight for 6 days during the photolysis experiments. Samples were taken periodically after stirring of the waters solutions. It is important to remark that the beakers were closed and the temperature was not artificially controlled during these experiments. In this respect, the highest temperature reached (about 40 ◦ C) is not considered significant to thermically transform COC. During sample collection, the amount of dissolved oxygen in the water could vary (4–6 mg/L). However, since no reactive oxygen species (ROS) can be formed in DW and SWeff by means of illumination with solar photons, this fact did not affect the experiments. A CPC reactor was used for the photochemical assays. The photoreactor is composed of two modules of eight Pyrex glass tubes mounted on a fixed platform tilted 37◦ (local latitude), providing a total irradiated area of 3 m2 . The total volume in each experiment was 35 L, but only 22 L were irradiated. At the beginning of all photochemical experiments, homogenization of the solution that contained the compound and the reagents added to the process was done with the photoreactor covered to avoid any photoreaction during preparation. In the TiO2 heterogeneous photocatalytic experiments, after addition of the drug to the photoreactor, the system was well homogenized for 15 min. Subsequent addition of the catalyst (TiO2 , 200 mg/L) required also homogenization of the system for 15 more min. A homogenized sample was collected before uncovering the photoreactor and starting the photocatalytic experiment. In the photo-Fenton experiments, after homogenization of the cocaine concentration in the photoreactor, the pH of the water was adjusted with sulfuric acid (H2 SO4 , 2 N) in order to carry out the

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

photo-Fenton reaction at a fixed pH, between 2.6 and 2.8. After 15 min of homogenization a sample was taken to confirm the pH, and afterwards, the iron salt (2 mg/L) was also added and well homogenized for 15 min more. A homogenized sample was collected also before adding the first dose of hydrogen peroxide, which was kept always in excess (10–20 mg/L) during the experiment, and uncovering the photoreactor. All the photochemical experiments were performed at different days between 9 am and 4 pm. Solar ultraviolet (UV) radiation was measured by a global UV radiometer (KIPP & ZONEN, model CUV3) mounted on a platform tilted 37◦ (the same as the CPC). Comparison of the data obtained with diverse photochemical experiments carried out on different days is possible using Eq. (1) as described elsewhere [15]; where tn is the experimental time for each sample, Vi is the illuminated volume, VT is the total volume, UV is the average solar UV radiation measured during tn , and t30 W is the normalized illumination time that refers to a constant solar UV power of incidence 30 W/m2 (the typical solar UV power on a perfectly sunny day around noon). t30W,n = t30 W,n−1 + tn

UV Vi , 30 VT

tn = tn − tn−1

(1)

2.3. Analytical determinations COC levels were monitored by reverse-phase liquid chromatography coupled to UV detection using a HPLC-UV system (Agilent Technologies, series 1100). The mobile phase used consisted of a linear gradient of a mixture of acetonitrile/water with formic acid (25 mM) (10/90, initial conditions) and the stationary phase was a Gemini C18 column (150 × 3 mm, 5 ␮m) from Phenomenex (CA, USA). UV detection of COC was done at  = 235 nm. Mineralization was evaluated by measuring the dissolved organic carbon (DOC) of the water samples. DOC measurements were done by direct injection of filtered samples into a Shimadzu-5050A TOC analyzer provided with a non-dispersive infrared (NDIR) detector and calibrated with standard solutions of potassium hydrogen phthalate. Ammonium concentration was determined with a Dionex DX-120 ion chromatograph (IC) equipped with a Dionex Ionpac CS12A 4 × 250 mm column. Anion concentrations (NO3 − and carboxylates) were measured with a Dionex DX-600 ion chromatograph using a Dionex Ionpac AS11-HC 4 × 250 mm column. Total iron concentration in the samples was monitored by colorimetric determination with 1, 10-phenanthroline, according to ISO 6332, using a Unicam-2 spectrophotometer. The concentration of H2 O2 was analyzed by a fast, simple spectrophotometric method using ammonium metavanadate ( = 450 nm), which allows the H2 O2 concentration to be determined immediately based on a red orange peroxovanadium cation formed during the reaction of H2 O2 with metavanadate [16]. These measurements are necessary to confirm the continuous presence of Fe2+ ion and H2 O2 in the system and hence to ensure that the photo-Fenton process takes place. Samples used to identify phototransformation products were 20-fold concentrated by means of solid phase extraction (SPE) with a Baker vacuum system (J.T. Baker, The Netherlands) onto previously conditioned (5 mL of MeOH and 5 mL of deionized water) Oasis HLB cartridges (6cc/200 mg, 30 ␮m) (Waters, Milford, MA). Analyte elution was performed with 4 mL + 4 mL of MeOH. The eluted volume was dried under N2 and then reconstituted to 1 mL with water/MeOH (90/10, v/v). Identification of phototransformation products generated during the different treatments was performed by means of UPLC coupled to quadrupole-time of flight tandem mass spectrometry (UPLC-QqTOF-MS/MS) using a Waters Acquity UPLCTM system coupled to a Waters/Micromass QqToF-MicroTM (Waters/Micromass, Manchester, UK). Chromatographic separation was performed on a

39

Waters Acquity BEH C18 column (2.1 × 100 mm, 1.7 ␮m) that was kept in a column oven at 30 ◦ C. The mobile phase consisted of a linear gradient of A: acetonitrile and B: 25 mM aqueous formic acid for analyses performed in positive electrospray ionization (PI) mode. A linear gradient of A: acetonitrile and B: water was applied for analyses carried out in the negative ionization (NI) mode. Sample volume injection was 5 ␮L. Full-scan analyses were carried out on selected samples in PI and NI modes for identification of photointermediates. NI scans did not show significant peaks compared to blank samples, thus further MS and MS2 analyses were performed in the PI mode. Acquisition in full scan mode was performed in the range m/z 50–700 at different cone voltages (15 V, 25 V and 35 V) and with a capillary voltage of 3000 V. Such a wide range of m/z was scanned in order to look for dimers, since dimerization has been previously reported to occur during phototransformation of organic contaminants, e.g., diclofenac [17] and triazines [18]. MS2 analyses were carried out on identified protonated molecules [M+H+ ] in order to get structural information. Collision induced fragmentation (CID) of selected m/z ions was performed at different collision energies that ranged between 10 and 40 eV, using argon as collision gas at a pressure of 22 psi. Data were collected in the centroid mode, with a scan time of 0.3 s and an interscan delay time of 0.1 s, and with a full width at half maximum (FWHM) resolution of 5000. Other MS parameters were set as follows: 600 L/h for the desolvation gas at a temperature of 350 ◦ C, 50 L/h for the cone gas, 120 ◦ C as source temperature. A valine–tyrosine–valine (Val-Tyr-Val) solution (m/z of [M+H]+ = 380.2185) was used to tune the instrument and also as lock mass to achieve mass accuracy. Reference was analyzed by infusion in the MS analyzer by means of an independent reference probe (LockSprayTM ) with a frequency of 4 s. Elemental compositions and accurate masses of the protonated molecules and their fragments were determined by means of MassLynx V4.1 software. 2.4. Acute toxicity evaluation Acute toxicity of COC and their solar phototransformation products was evaluated with Biofix® Lumi-10, a commercial bioassay based on inhibition of the luminescence emitted by the marine bacteria Vibrio fischeri. The inhibition of light emission was measured after sample contact periods of 5, 15 and 30 min, as detailed in ISO 11348-3:2007 [19]. 3. Results and discussion 3.1. Cocaine hydrolysis and photolysis Slight COC transformation (less than 5% of COC0 ) was observed in DW after 22 h of keeping the solution in the dark at room temperature. However, COC levels decreased to almost half the initial concentration in SWeff under the same conditions. This fact could be attributed to natural hydrolysis of COC in water. Samples collected at the end of the hydrolysis experiment presented two new chromatographic peaks, which corresponded to known COC metabolites, namely, ecgonine methyl ester (labeled as P199) and benzoylecgonine (labeled as P289). These metabolites, generated through chemical hydrolysis of cocaine ester bonds, have been previously reported as the main hydrolytic products of COC [20]. The hydrolysis reaction rates were observed to be faster in SWeff compared to DW, since higher levels of the aforementioned hydrolysis products were measured in SWeff (P199/COC0 × 100 = 12%; P289/COC0 × 100 = 37%) compared to DW (P199/COC0 × 100 = 1%; P289/COC0 × 100 = 0.6%). Hydrolysis rates in SWeff may be increased by the slightly higher pH of this matrix

1.0

1.0

0.8

0.8

0.6

0.6

COC/COC0 in DW

0.4

0.4

DOC/DOC0 in DW COC/COC0in SWeff

0.2

DOC/DOC0 in SWeff

0.0

DOC/DOC0

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

COC/COC0

40

0.2

0.0 0

10

20

30

40

50

3.2.2. Homogeneous photocatalysis by photo-Fenton Transformation of COC in the investigated water matrices with photo-Fenton treatment is shown in Fig. 2b. Phototransformation of COC with solar photo-Fenton was in general very fast. Only 5 min and 25 min of photo-Fenton process were required to completely transform COC0 in DW and SWeff, respectively. About 80% of the DOC mineralization was achieved in DW containing COC after 58 min; however, only 70% of mineralization was attained in SWeff even after two additional hours of photo-Fenton treatment (180 min). Less than 3 mM of H2 O2 were required to achieve such mineralization percentages in the worst case scenario, i.e., SWeff. Since H2 O2 consumption is the main operational cost of photo-Fenton treatment [9], decontamination of waters containing COC would be highly cost-effective.

60

t 30w(hours)* Fig. 1. COC transformation (COC/COC0 ) and mineralization (DOC/DOC0 ) during hydrolysis (shadowed part) and photolysis experiments performed in DW and SWeff. (* the normalized illumination time (t30 W ) was calculated with a variation of Eq. (1), where Vi /VT = 1).

(pH ≈ 7) compared to DW (pH ≈ 6), since previous studies have reported a dramatic instability of COC in water solutions as the pH becomes more basic at ambient temperatures [21,22]. Besides pH, water hardness and/or the ionic strength of the solution might also affect COC hydrolysis and photolysis; however, this has not been investigated in depth yet. Results obtained during photolysis experiments of COC are shown in Fig. 1. Note that t30 W was calculated using a variation of Eq. (1), where Vi /VT was equal to 1. Direct sun-light exposure of the SWeff solution containing COC resulted in a decrease of 90% of COC0 after 20 h of sun-light irradiation time, being almost completely transformed after 27 h. In the case of DW, experimental results indicated that solar photolysis only achieved about 22% phototransformation of COC0 after 60 h of sun-light exposure. DOC concentration was stable throughout the entire photolysis experiments, which were run for up to 6 days (≈62 h of normalized irradiation time) in both investigated matrices. Thus DOC mineralization did not take place with direct solar irradiation, as this process is not able to produce any radical able to completely oxidize COC or its photogenerated metabolites. Although COC0 was decreased by means of hydrolysis and photolysis, these processes require much longer times than photochemical treatments (hours vs min) to transform COC, as it is discussed in Section 3.2. Therefore, both processes were not considered to affect dramatically photocatalytic transformation rates and pathways. 3.2. Solar photochemical transformation 3.2.1. Heterogeneous photocatalysis with TiO2 Phototransformation and mineralization of COC in DW and SWeff with TiO2 heterogeneous photocatalysis is shown in Fig. 2a. After system homogenization before starting the TiO2 photocatalytic treatment, a decrease in COC0 of about 1% and 5% was observed in DW and SWeff, respectively, due to adsorption of the compound onto the TiO2 particles. COC was completely transformed in DW after 29 min of solar photocatalytic treatment whereas 22 min more were necessary for its complete transformation in SWeff. Overall, mineralization of the water DOC occurred at a slower rate than compound transformation. In the case of SWeff, only about 50% of the organic content of the water was mineralized after 3 h of treatment, whereas slightly more than 80% of DOC mineralization occurred in DW after the same normalized irradiation time.

3.3. Kinetics of phototransformation reactions The photocatalytic transformation of COC with TiO2 followed apparent first-order kinetics, as is usual in heterogeneous photocatalysis for other pollutants when initial concentration is low enough and no catalyst saturation occurs. It should be emphasized that phototransformation intermediates formed during COC decomposition could also be competitive on the surface of the TiO2 . Their concentration varies throughout the reaction up to their mineralization and thus, Eq. (2) (based on Langmuir-Hinshelwood kinetic model, used commonly for describing photocatalysis by TiO2 kinetics [23]) can describe the kinetics: r=

k KC

1 + KC +

nr

K C (i i=1 i i

= 1, n)

(2)

where kr is the reaction rate constant, K is the reactant (COC) adsorption constant, C is COC concentration at any time, Ki is the intermediates adsorption constant and Ci is intermediates concentration at any time. When C0 (10 mg/L of COC) is low enough, Eq. (2) can be simplified (1 + KC + . . . = 1) to a first order reaction rate equation (see Eq. (3)): r = kap C

(3)

This was confirmed by the linear behavior of ln(C0 /C) as a function of t30 W . Similar behavior was observed with the photo-Fenton treatment. Assuming that the reaction between HO• radicals and COC is the rate-determining step when COC concentration is so low, the rate equation is then written as: 

r = kHO [HO• ]C = kap C

(4)

where C is COC concentration, kHO is the photo-Fenton reaction rate constant and kap  is a pseudo first order constant. Table 1 shows the kinetic parameters (kap ) obtained for each solar photocatalytic treatment in each aqueous matrix. It is important to remark that the concentration of HO• in each catalytic system depends on the catalyst concentration (TiO2 in photocatalytic treatments (200 mg/L), and Fe2+ (2 mg/L) and H2 O2 in photo-Fenton) and photons absorbed by each system. In the light of the results, it can be concluded that transformation of COC is faster with the photo-Fenton treatment than with TiO2 heterogeneous photocatalysis, being about four times and two times faster in DW and in SWeff water, respectively. This assessment is supported by the half-life time of COC observed in TiO2 heterogeneous photocatalysis experiments (6.5 min in DW and 10 min in SWeff) and in photo-Fenton experiments (1.5 min in DW and 5.5 min in SWeff). The observed results may be explained by the higher solar light harvesting that the photo-Fenton process presents as compared to TiO2 photocatalysis, which finally produces larger quantities of HO• in less time. In solar photoFenton treatments, the effective wavelength can reach up to 600 nm

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

41

b

a 1.0

COC/COC in DW DOC/DOC in DW

1.0

1.0

0.8

0.8

COC/COC in DW DOC/DOC in DW COC/COC in SW

DOC/DOC0

0.8 0.6

0.4

0.4

0.2

0.2

0.2

0.0 25 50 75 100 125 150 175 200 225

0.0

0.6

0.4

0.4

0.2

0

DOC/DOC in SW

0.6

0.6

0.0

COC/COC0

COC/COC0

DOC/DOC in SW

DOC/DOC0

COC/COC in SW

0.8

1.0

0

25

50

75

0.0 100 125 150 175 200

t 30w(min)

t 30w(min)

Fig. 2. COC transformation (COC/COC0 ) and mineralization (DOC/DOC0 ) during (a) heterogeneous photocatalysis with TiO2 and (b) homogeneous photocatalysis by photoFenton in DW and SWeff (t30 W : normalized illumination time, see Eq. (1)). Table 1 Kinetic parameters obtained for phototransformation of COC in DW and SWeff with solar photocatalytic treatments (kap : pseudo-first order reaction rate constant and r2 : coefficient of determination, t30 W,75%DOC : normalized irradiation time required to mineralize 75% of the initial DOC). DW

TiO2 photocatalysis Photo-Fenton

SWeff

kap (min−1 )

r2

t30 W,75%DOC (min)

kap (min−1 )

r2

t30 W,75%DOC (min)

0.179 0.847

0.9916 0.8853

85 45

0.098 0.184

0.9846 0.9776

>225 >180

depending on the presence of different iron complexes, whereas in TiO2 photocatalysis it is below 390 nm. Additionally, contrary to photo-Fenton treatments, in TiO2 photocatalysis, reactions do not take place in dark zones [9]. As the mineralization does not follow simple models like first or zero order kinetics, overall rate constants cannot be calculated. Therefore, the normalized irradiation time necessary to mineralize 75% of the initial DOC was provided to compare experiments (see Table 1), and in this case, photo-Fenton was also more effective than TiO2 photocatalysis in compound mineralization. Lower transformation and mineralization rates of the compound in SWeff compared to DW systems indicate the nonselective attack of HO• , which also react with other inorganic and organic species that are present in the SWeff, being the obtained kinetics more realistic than in DW experiments. The formation of nitrogen inorganic species (NH4 + and NO3 − ) and carboxylic acids produced from COC transformation was monitored by IC during the treatments carried out with DW, while in SWeff experiments it was not possible due to the interferences provoked by its chemical composition. The heteroatoms present in the molecule of COC are released mainly in the form of NO3 − and NH4 + as the compound gets mineralized. On the other hand, carboxylic acids are the intermediate products formed prior to COC complete mineralization to CO2 and H2 O. Since solar photolysis treatments of COC did not result in a significant compound mineralization, nitrogen inorganic species and carboxylic acids were only slightly accumulated after almost 4 days of irradiation time, being the concentrations of NO3 − and NH4 + below 0.2 mg/L, and not higher than 0.5 mg/L for the detected carboxylic acids: formic, pyruvic, acetic and oxalic acid, except for maleic acid, which presented levels up to 1.8 mg/L. The levels of carboxylic acids during photo-Fenton and TiO2 heterogeneous photocatalysis were comparatively much higher than in photolysis, as a consequence of a more intense mineralization of DOC during these processes. Among the identified carboxylic

acids, those that were produced in higher proportions during the photocatalytic mineralization processes were the oxalic acid and the maleic acid. 3.4. Major phototransformation intermediates and phototransformation routes Identification of phototransformation intermediates of COC was performed on preconcentrated DW and SWeff samples rather than on non-preconcentrated ones because analysis in full scan of the former evidenced the existence of a greater number of peaks, and hence degradation products, than the latter and also because the lower intensity of some of the photodegradation intermediates observed in non-preconcentrated samples affected negatively to the mass accuracy of the results. Total ion chromatograms (TIC) obtained after full-scan analyses of representative preconcentrated DW samples are shown as supplementary information in Fig. S1. Additionally, the evolution of the most abundant phototransformation intermediates is shown in Fig. S2. Photointermediates are formed in the initial steps of the phototransformation treatments. During solar photolysis, photointermediate levels continually increase as the COC0 decreases. During solar photocatalysis, the highest levels of photointermediates were observed when more than half of the COC0 was transformed. Most of these photointermediates were readily phototransformed afterwards, remaining in solution for just a short period of time after COC was completely eliminated, except for P199 in the photo-Fenton treatment, which may be more recalcitrant, or, what is more likely, may be generated by transformation of other photointermediates. Experimental and theoretical masses (m/z), the error between them in mDa and ppm, the double bond equivalent (DBE), and the proposed elemental composition of the protonated phototransformation intermediates and their main fragment ions formed during phototransformation of COC are shown in Table 2. Accurate masses of parent ions were reported with errors below 5 ppm, which guar-

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C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

Table 2 Accurate mass measurement of protonated molecules and fragment product ions of COC and its phototransformation intermediates obtained with UPLC-ESI-QqTOF-MS/MS analyses. Comp.b

tR (min)

Precursor ion Product ion

Molecular formula

Mass (m/z)

Error

DBE

Experimental

Calculated

(mDa)

(ppm)

COC

4.60

[M+H] [M+H–CH3 OH]+ [M+H–C7 H6 O2 ]+ [M+H–CH3 OH–C7 H6 O2 ]+ [M+H–C10 H17 NO3 ]+ [M+H–C12 H16 O4 ]+

C17 H22 NO4 C16 H18 NO3 C10 H16 NO2 C9 H12 NO C7 H5 O C5 H8 N

304.1546 272.1293 182.1187 150.0928 105.0343 82.0663

304.1549 272.1287 182.1181 150.0919 105.0340 82.0657

−0.3 0.6 0.6 0.9 0.3 0.6

−1.0 2.2 3.3 6.0 2.9 7.3

7.5 8.5 3.5 4.5 2.5 2.5

P181 (AEME)

1.43

[M+H]+ [M+H–CH3 OH]+ [M+H–CO–CH3 OH]+ [M+H–C2 H10 NO• ]• + [M+H–C3 H9 NO2 ]+

C10 H16 NO2 C9 H12 NO C8 H12 N C8 H6 O C7 H7

182.1190 150.0930 122.0966 118.0423 91.0551

182.1181 150.0919 122.0970 118.0419 91.0548

0.9 1.1 −0.4 0.4 0.3

4.9 7.3 −3.3 3.4 3.3

3.5 4.5 3.5 6.0 4.5

P185 (ECG)

0.67

[M+H]+ [M+H–H2 O]+ [M+H–C4 H8 O3 ]+

C9 H16 NO3 C9 H14 NO2 C5 H8 N

186.1138 168.1034 82.0665

186.1130 168.1025 82.0657

0.8 0.9 0.8

4.3 5.4 9.7

2.5 3.5 2.5

P197

1.66

[M+H]+ [2M+H]+ [M+H–OH]+ [M+H–C2 H6 O• ]• + [M+H–C2 H10 NO2 • ]• + [M+H–C3 H9 NO2 ]+

C10 H16 NO3 C20 H31 N2 O6 C10 H15 NO2 C8 H10 NO2 C8 H6 O C7 H7 O

198.1131 395.2186 181.1093 152.0711 118.0418 91.0550

198.1130 395.2182 181.1103 152.0712 118.0419 91.0548

0.5 0.4 −1.0 −0.1 −0.8 0.2

0.1 1.0 −5.5 −0.7 −0.1 2.2

3.5 6.5 4.0 4.5 6.0 4.5

P199 (EME)

0.69

[M+H]+ [M+H–H2 O]+ [M+H–CH4 O–H2 O]+ [M+H–C5 H10 O3 ]+

C10 H18 NO3 C10 H16 NO2 C9 H12 NO C5 H8 N

200.1282 182.1184 150.0925 82.0650

200.1287 182.1181 150.0919 82.0657

−0.5 0.3 0.6 −0.7

−2.5 1.6 4.0 −8.5

2.5 3.5 4.5 2.5

P289a (BE)

3.49

[M+H]+ [M+Na]+ [M+H–H2 O]+ [M+H–C7 H6 O2 ]+ [M+H–C7 H6 O2 –H2 O]+ [M+H–C9 H15 NO3 ]+

C16 H20 NO4 C16 H19 NO4 Na C16 H18 NO3 C9 H14 NO2 C9 H12 NO C7 H5 O

290.1378 312.1198 272.1297 168.1030 150.0908 105.0349

290.1392 312.1212 272.1287 168.1025 150.0919 105.0340

−1.4 −1.4 1.0 0.5 −1.1 0.9

−4.8 −4.5 3.7 3.0 −7.3 8.6

7.5 7.5 8.5 3.5 4.5 5.5

P289b (nor-COC)

4.88

[M+H]+ [M+H–C7 H6 O2 ]+ [M+H–CH3 OH–C7 H6 O2 ]+ [M+H–CO–CH3 OH–C7 H6 O2 ] [M+H–C9 H15 NO3 ]+

C16 H20 NO4 C9 H14 NO2 C8 H10 NO C7 H10 N C7 H5 O

290.1378 168.1031 136.0757 108.0813 105.0333

290.1392 168.1025 136.0762 108.0813 105.0340

−1.0 0.6 −0.5 0.0 −0.7

−3.4 3.6 −3.7 0.0 −6.7

7.5 3.5 4.5 3.5 5.5

P305 (OH-BE)

2.67 2.93 3.67

[M+H]+ [M+Na]+ [M+H–H2 O]+ [M+H–C7 H4 O2 ]+ [M+H–C7 H6 O3 ]+ [M+H–C7 H8 NO4 ]+ [M+H–C9 H15 NO3 ]+

C16 H20 NO5 C16 H19 NO5 Na C16 H18 NO4 C9 H16 NO3 C9 H14 NO2 C9 H12 NO C7 H5 O2

306.1344 328.1171 288.1253 186.1125 168.1035 150.0915 121.0291

306.1341 328.1161 288.1236 186.1130 168.1025 150.0919 121.0290

0.3 3.0 1.7 −0.5 1.0 −0.4 0.1

1 1.0 5.9 −2.7 5.9 −2.7 0.8

7.5 7.5 8.5 2.5 3.5 4.5 5.5

P317 (CE)

5.49

[M+H]+ [M+H–C7 H6 O2 ]+ [M+H–C7 H6 O2 –C2 H6 O]+

C18 H24 NO4 C11 H18 NO2 C9 H12 NO

318.1694 196.1347 150.0919

318.1705 196.1338 150.0919

−1.1 0.9 0.0

−3.5 4.6 0.0

7.5 3.5 4.5

P319a (OH-COC)

3.18 3.48 4.78

[M+H]+ [M+H–CH3 OH]+ [M+H–C7 H4 O2 ]+ [M+H–H2 O–C7 H4 O2 ]+ [M+H–C8 H10 O4 ]+ [M+H–C10 H17 NO3 ]+ [M+H–C12 H14 O5 ]+

C17 H22 NO5 C16 H18 NO4 C10 H18 NO3 C10 H16 NO2 C9 H12 NO C7 H5 O2 C5 H8 N

320.1484 288.1228 200.1296 182.1186 150.0930 121.0289 82.0663

320.1498 288.1236 200.1287 182.1181 150.0919 121.0290 82.0657

−1.4 −0.8 0.9 0.5 1.1 −0.1 0.6

−4.4 −2.8 4.5 2.7 7.3 −0.8 7.3

7.5 8.5 2.5 3.5 4.5 5.5 2.5

4.20 4.51

[M+H]+ [M+H–C7 H6 O2 ]+ [M+H–H2 O–C7 H6 O2 ]+ [M+H–C8 H12 O4 ]+ [M+H–C9 H12 O5 ]+ [M+H–C10 H17 NO4 ]+ [M+H–C12 H14 NO4 ]+

C17 H22 NO5 C10 H16 NO3 C10 H14 NO2 C9 H10 NO C8 H10 N C7 H5 O C5 H8 NO

320.1500 198.1135 180.1028 148.0756 120.0810 105.0337 98.0610

320.1498 198.1130 180.1025 148.0762 120.0813 105.0340 98.0606

0.2 0.5 0.3 −0.6 −0.3 −0.3 0.4

0.6 2.5 1.7 −4.1 −2.5 −2.9 4.1

7.5 3.5 4.5 5.5 4.5 5.5 2.5

3.91

[M+H]+

C17 H22 NO5

320.1489

320.1498

−0.9

−2.8

7.5

P319b a

a

P319c

+

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

43

Table 2 (Continued) Comp.b

tR (min)

Precursor ion Product ion

Molecular formula

Mass (m/z)

Error

DBE

Experimental

Calculated

(mDa)

(ppm)

5.05

[M+H–C7 H6 O2 ]+ [M+H–O–C7 H6 O2 ]+ [M+H–C10 H17 NO4 ]+ [M+H–C12 H14 O5 ]+

C10 H16 NO3 C10 H16 NO2 C7 H5 O C5 H8 N

198.1141 182.1170 105.0337 82.0650

198.1130 182.1181 105.0340 82.0657

1.1 −1.1 −0.3 −0.7

5.6 −6.0 −2.9 −8.5

3.5 3.5 5.5 2.5

a

P335a

2.56 3.05 3.59

[M+H]+ [M+H–C7 H4 O3 ]+ [M+H–C7 H6 O4 ]+ [M+H–C8 H10 O5 ]+ [M+H–C10 H17 NO3 ]+ [M+H–C12 H14 O6 ]+

C17 H22 NO6 C10 H18 NO3 C10 H16 NO2 C9 H12 NO C7 H5 O3 C5 H8 N

336.1444 200.1283 182.1191 150.0924 137.0235 82.0659

336.1447 200.1287 182.1181 150.0919 137.0239 82.0657

−0.3 −0.4 1.0 0.5 −0.4 0.2

−0.9 −2.0 5.5 3.3 −2.9 2.4

7.5 2.5 3.5 4.5 5.5 2.5

a

P335b

2.71

[M+H]+ [M+H–C7 H4 O2 ]+ [M+H–C7 H6 O3 ]+ [M+H–H2 O–C7 H6 O3 ]+ [M+H–C10 H17 NO4 ]+

C17 H22 NO6 C10 H18 NO4 C10 H16 NO3 C10 H14 NO2 C7 H5 O2

336.1454 216.1237 198.1129 180.1027 121.0290

336.1447 216.1236 198.1130 180.1025 121.0290

0.7 0.1 −0.1 0.2 0.0

2.1 0.5 −0.5 1.1 0.0

7.5 2.5 3.5 4.5 5.5

a

P335cc

4.30 5.04

[M+H]+ [M+H–C7 H4 O2 ]+ [M+H–C7 H6 O3 ]+ [M+H–C9 H12 O5 ]+ [M+H–C8 H8 O3 ]+

C17 H22 NO6 C10 H18 NO4 C10 H16 NO3 C8 H10 NO C9 H14 NO3

336.1454 216.1235 198.1132 136.0760 184.0982

336.1447 216.1236 198.1130 136.0762 184.0974

0.7 −0.1 0.2 −0.2 0.8

2.1 −0.5 1.0 −1.5 4.3

7.5 2.5 3.5 4.5 3.5

a

Compound generated only during solar photocatalytic treatments (photo-Fenton and heterogeneous photocatalysis with TiO2 ). COC: cocaine, AEME: anhydroecgonine methyl ester, ECG: ecgonine, EME: ecgonine methyl ester, BE: benzoylecgoine, nor-COC: nor-cocaine, OH-BE: hydroxybenzoylecgonine, CE: cocaethylene, OH-COC: hydroxycocaine. c In this case, peaks at tR = 4.30 and 5.04 presented the same major fragment ions, but m/z 198, which was specific of tR = 4.30 and m/z 184, which was exclusive of the peak at tR = 5.04. b

antee the correct assignment of their molecular formula in all cases. The identity of few of them is also confirmed by the presence of sodium adducts. Slightly higher errors (always below 10 ppm) were occasionally reported for fragment ions; however, they were accepted since they corresponded to characteristic fragments of COC or its major phototransformation intermediates, confirmed with commercially available analytical standards. Main CID fragments of COC according to the fragmentation pathway proposed by Wang et al. [24] are shown as supplementary material in Fig. S3. Overall, all identified phototransformation intermediates of COC were present in both investigated aqueous matrices. However, some of them were exclusively generated by one of the investigated treatments, as it has been indicated in Table 2. Benzoylecgonine (P289a, m/z 290.1392 – C16 H20 NO4 ) and ecgonine methyl ester (P199, m/z 200.1287 – C10 H18 NO3 ), which are known COC metabolites, were generated during all investigated treatments. They were identified also as the main hydrolytic products of COC (see Section 3.1). Their identity was confirmed with the analysis of commercially available standards, by comparing the analyte chromatographic retention time (tR ) and its fragmentation pattern at a given collision energy in the phototransformation samples and in the individual commercial solutions. P199, produced via cleavage of the benzoyl ester bond of COC, was the major phototransformation product of COC in all treatments. As expected, due to the lower photolytic transformation rate of COC, maximum levels of P199 (normalized to COC0 ) were lower in solar photolysis (12%) than in solar photocatalysis (25% and 28% in photo-Fenton and heterogeneous photocatalysis with TiO2 , respectively) (see Fig. S2). P289, generated via cleavage of the methyl ester bond of COC, is the second main COC phototransformation product generated during solar photolysis of aqueous solutions containing COC. However, photogeneration of P289a was less dominant in photocatalytic treatments. P199 and P289a have been also reported as major reaction products of COC in the presence of hydrogen peroxide [25], which is also a strong oxidative agent.

Besides 289a, another isobaric compound at m/z 290.1378 (C16 H20 NO4 ) was produced in all phototransformation treatments. Its late chromatographic elution time (tR = 4.8 min) and its fragmentation pattern allowed the identification of this isobaric compound as nor-cocaine, labeled as P289b. P289b is produced via N-demethylation of COC and has been also reported as a COC metabolization product. Its fragmentation produced a fragment ion at m/z 168.1025 (C9 H14 NO2 ) common to 289a that corresponds to the loss of benzoic acid (C7 H6 O2 ), and fragment ions at m/z 136.0762 and 108.0813, which are characteristic from nor-cocaine [24], which result from the subsequent loss of methanol (CH3 OH) and CO, respectively. Two other COC metabolites, ecgonine (P185, m/z 186.1130 – C9 H16 NO3 ) and cocaethylene (P317, m/z 318.1705 – C18 H24 NO4 ) were also identified and confirmed with the analysis of analytical standard solutions as phototransformation products in all investigated treatments. P185 may be generated by further phototransformation of P289a and/or P199, via benzoyl ester cleavage and demethylation, respectively. On the other hand, P317, minor phototransformation product, was produced by COC methylation. Its fragmentation pattern showed main fragment ions at m/z 196 and 150, which indicates subsequent losses of benzoic acid and the ethyl-ester group. Compound P181, appearing at tR = 1.43, yielded a m/z ratio of 182.1181 with an error smaller than 5 ppm. This accurate mass corresponded to the formula C10 H16 NO2 , which is consistent with the further transformation of P199 by the loss of one water molecule. The molecular formula and its fragmentation pattern under electrospray ionization, which shows major fragment ions at m/z 122 and 118, suggest that P181 is anhydroecgonine methyl ester, which is a pyrolytic product of COC [24,26]. The applied solar photolytic and photocatalytic transformation treatments also generated various monohydroxylated (P319) derivatives of COC. Up to seven different peaks were produced in photocatalytic treatments (m/z 320.1498 – C17 H22 NO5 , protonated molecule), as a consequence of the non-specific attack of

44

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

Fig. 3. Product ion MS/MS chromatogram and spectra obtained for isobaric compounds P319 in a DW sample treated with solar heterogeneous photocatalysis (cone = 25 V; collision energy = 25 eV).

HO• radicals. The structural information obtained by means of MS2 analyses and shown in Fig. 3 was not sufficient to predict the exact position of the HO• attack. However, it allowed dividing these compounds into three different groups (P319a, P319b, and P319c in Table 2) according to the part of the molecule in which the HO• attack was produced. P319a comprises those COC monohydroxylated photointermediates found at tR = 3.18, 3.48 and 4.78 min and whose fragmentation patterns indicate that the hydroxylation took place at the phenyl ring of COC. Their major fragment ions at m/z 121.0290 (C7 H5 O2 ) and 182.1181 (C10 H16 NO2 ) (see Table 2), pro-

duced via ester bond cleavage, correspond to the hydroxy benzoyl cation (COC fragment ion at m/z 105 + 16) and to a fragment ion generated by the loss of hydroxy benzoic acid, respectively (see Fig. S3 and Fig. 3). Thus, the peaks labeled as P319a are presumably ortho, meta- and para- hydroxycocaine, which have been also reported as reaction products of COC and hydrogen peroxide [25] and as urine metabolites of COC [27]. The monohydroxylated derivatives labeled as P319b and P319c are those peaks that appear at tR = 4.20 and 4.51, and 3.91 and 5.05 min, respectively. In these peaks, the HO• attack may occur at the tropane moiety of COC or at its methyl-

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

45

Fig. 4. Product ion MS/MS chromatogram and spectra obtained for isobaric compounds P335 in a DW sample treated with solar photo-Fenton (cone = 25 V; collision energy = 25 eV).

ester moiety, since all they showed as relevant fragments the ions at m/z 198.1130, and m/z 105.0340, both generated via ester bond cleavage. The ion at m/z 198 corresponds to the hydroxylated form of the COC fragment ion m/z 182, produced by the loss of benzoic acid, and the fragment ion at m/z 105 corresponds to the benzoyl cation (see Fig. S3 and Fig. 3). These peaks were labeled as 319b and 319c according to the presence/absence of the fragment ions at m/z 82 (common to COC, see Fig. S3) and at m/z 98 (=m/z 82 + 16). According to the peak signals, the reaction mechanisms that take place during COC phototransformation favor the generation of compounds P319a at times 3.18, 3.48 and 4.78 and P319b at time 4.20. The formation of the latter is also more favored in heterogeneous TiO2 photocatalysis treatments compared to direct solar photolysis and photo-Fenton (see Fig. S2). Up to six COC dihydroxylated derivatives (P335) were generated only during solar photocatalytic treatments (m/z 336.1447 – C17 H22 NO6 , protonated molecule). Fig. 4 shows the chromatographic signals and the fragmentation pattern of each peak,

together with the characterization of the main fragment ions in each case. Compounds found at tR = 2.56, 3.05 and 3.59, labeled as P335a, were the most abundant of the dihydroxylated derivatives generated. Their fragmentation pattern showed an ion at m/z 137.0239 (C7 H5 O3 ), which corresponds to the dihydroxy benzoyl cation, and typical fragment ions of COC (304 → 182, 150, 82). This suggests that dihydroxylation of these isobaric compounds took place at the COC phenyl ring. However, the specific places for HO• radical addition in the ring could not be elucidated with the available fragmentation information. Peaks found at tR = 2.71, 4.30 and 5.04 min presented a fragmentation pattern different from that aforementioned (see Table 2) which is characterized by the formation of an ion at m/z 216.1236 (C10 H18 NO4 ), which would fit with the loss of a hydroxy benzoyl radical or benzoic acid. The loss of a hydroxy benzoyl radical would indicate that one of the HO• radical attacks took place at the COC phenyl ring, whereas the loss of benzoic acid would suggest that the two HO• radical attacks occurred at the tropane 2-carboxilic acid methyl ester moiety. The first reaction

46

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

Fig. 5. Proposed phototransformation pathway of COC in aqueous solution during solar photocatalytic treatments (COC: cocaine, AEME: anhydroecgonine methyl ester, ECG: ecgonine, EME: ecgonine methyl ester, BE: benzoylecgonine, nor-COC: nor-cocaine, OH-BE: hydroxybenzoylecgonine, CE: cocaethylene, OH-COC: hydroxycocaine).

a

50

b 50

Inhibition of bioluminescence (%)

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

40

40

30

30

20

20

10

10

0

47

Dissapearance of COC

0 S0 S1 S2 S3 S4 S5 S6 S7 S8 S9

S0 S1 S2 S3 S4 S5 S6 S7

-

-

t30w

+

t30w

+

Fig. 6. Inhibition of Vibrio fischeri bioluminescence (%) after 30 min of contact with selected samples collected during (a) solar photolysis and (b) solar heterogeneous photocatalysis.

scenario should be responsible for the peak at tR = 2.71 min, labeled as P335b, since fragmentation of this compound showed also major fragment ions at m/z 198.1130 (C10 H16 NO3 ) and 121.0290 (C7 H5 O2 ), which correspond to the hydroxylated form of tropane 2-carboxilic acid methyl ester and to the hydroxy benzoyl radical, respectively. The second reaction scenario would fit with the fragmentation pattern observed for the peaks appearing at tR = 4.30 and 5.04 min, labeled as P335c. Subsequent loss of water and methanol from the ion at m/z 216 will cause the fragment ions at m/z 198.1130 and 184.0974 (C9 H14 NO3 ), respectively, which are the main fragments ions to see in the fragmentation spectra of peaks 335c (see Fig. 4). The formation of these photointermediates was slightly more abundant during photo-Fenton than with TiO2 photocatalysis treatments. A calculated mass m/z 306.1341 was obtained for the protonated molecule of the photointermediates P305 (tR = 2.67, 2.93 and 3.67 min), which gave the best-fit formula C16 H20 NO5. This formula contains one more oxygen atom than the photointermediates P289. Since no increase in the molecule DBE was observed, oxidation of the molecule should have been produced via hydroxylation of P289. MS2 analyses allowed characterizing these compounds as monohydroxylated derivatives of P289a, since same fragment ions or fragmentation pathways were observed (see Table 2). Additionally, the presence of the fragment ion at m/z 121.0290 in all MS2 spectra indicated that the hydroxylation took place at the phenyl ring of P289a. The photointermediate P197 (m/z 198) was also generated during all investigated treatments in both water matrices. An additional confirmation for P197 was the formation of its dimer (m/z 395.2182) at the ionization source, since it was found at the same tR . This photointermediate could be originated from different sources, such as hydroxylation of P181 and/or via ester bond cleavage of one of the monohydroxylated forms of COC generated during the transformation treatments. Along with the photoproducts already mentioned, the presence of other compounds was also detected; however, their molecular structures could not be assigned. Their molecular formula, DBE, experimental and calculated masses, and mass errors are shown as supplementary information (Table S2).

Fig. 5 shows the main proposed phototransformation pathways of COC during solar photocatalytic treatments, including only those photointermediates whose molecular structure could be assigned. Considering the identified photointermediates and their abundance, the primary phototransformation route would lead to the formation of P185, P199 and P289a via COC ester bonds cleavage. Further transformation would proceed through N-demethylation of COC (P289b), multistep hydroxylation reactions through attack of HO• producing monohydroxylated derivatives of COC and P289 (P305 and P319) and dihydroxylated derivatives of COC (P335), and further oxidation steps (P181 and P197). Methylation of COC also took place during the phototransformation treatments (P317). Phototransformation of COC followed same transformation routes in DW than in SWeff. Further transformation of most of these compounds into aliphatic products, e.g., carboxylic acids before complete mineralization would imply cleavage of their benzene ring. 3.5. Toxicity evaluation The acute toxicity of COC and its phototransformation intermediates was monitored with the V. fischeri toxicity test. Results showed that the bioluminescence emitted by the bacteria was never inhibited above 44% after 30 min of sample contact. Bioluminescence inhibition values were usually below 20%, indicating that COC and its intermediates have low acute toxicity effects on the tested bacteria. Overall, higher inhibition percentages were obtained in DW samples compared to SWeff samples, where stimulation percentages were mainly observed. This fact may be attributed to the higher content of organic matter in SWeff than in DW, which may be available for the cells as a food resource. Bioluminescence inhibition percentages obtained in selected DW samples during the photolysis and heterogeneous photocatalysis with TiO2 is shown in Fig. 6. In the light of the results, bioluminescence emitted by the bacteria seemed to be more inhibited during heterogeneous photocatalysis with TiO2 than in solar photolysis. As it is shown in Fig. 6, inhibition percentage values observed in the samples treated with TiO2 photocatalysis were not affected by the complete disappearance of COC after sample 4, which points

48

C. Postigo et al. / Applied Catalysis B: Environmental 104 (2011) 37–48

out the generated photointermediates as the main responsible for bioluminescence inhibition. This is also confirmed by the initial samples (S0), which only contained COC, since they did not inhibit the bacterial bioluminescence. Toxicity results obtained for the photo-Fenton treatment were not depicted, since inhibition of the bioluminescence was not observed in any of the analyzed samples, thus it seems to have less acute toxic effects. However, since no big differences were observed in photointermediates production among the different investigated treatments, this fact may be attributed to the rapid transformation of the most harmful chemical species during photo-Fenton treatments. 4. Conclusions COC dissolved in water both in absence and presence of solar irradiation suffered transformation, but not mineralization. However, complete elimination of COC in solar photocatalytic treatments was achieved in DW and SWeff with short illumination times. Among the tested solar photocatalytic treatments, photoFenton was observed to be more efficient than TiO2 photocatalysis. In both cases, transformation followed pseudo-first order reaction kinetics, with lower reaction rates in SWeff than in DW. Kinetics observed in SWeff may be comparable to those expected in real wastewater effluents with around 30 mg/L of TOC, which was the TOC level achieved in the simulated matrix. In those wastewater effluents with higher TOC levels or salts concentrations, transformation kinetics are expected to be lower. Same phototransformation products were identified in both investigated matrices, however some of them were exclusively formed during the solar photocatalytic processes. The main phototransformation routes identified, i.e., ester bond cleavage, hydroxylation and N-demethylation, are believed also to take place at environmental COC levels due to photolysis and in real wastewater effluent samples treated with the investigated photocatalytical processes. The removal efficiency, together with the low toxicity of the treated samples shown by the V. fischeri bioassay applied, points the investigated solar photocatalytic treatments as good alternative ways to eliminate cocaine from aqueous solutions. Additionally, since the COC0 used in the phototransformation experiments was much higher than reported environmental cocaine levels, the environmental risk that poses this compound and its phototransformation products to aquatic organisms is expected to be even lower than that observed in this study. One of the main COC photointermediates, benzoylecgonine (P289a), has been proposed as consumption indicator of COC, in order to estimate cocaine consumption at the community level, by means of a sewage epidemiology approach [28]. Besides COC phototransformation, results of the hydrolysis study confirm that COC is highly hydrolyzed in benzoylecgonine and ecgonine methyl ester in aqueous solutions with high organic content, and thus high pH. Therefore, further studies are required in this line in order to account for the conversion of cocaine to benzoylecgonine and for the potential hydrolysis of benzoylecgonine, to improve accuracy of drug use estimations performed with the sewage epidemiology. Acknowledgments This study has been financially supported by the MICINN projects CEMAGUA (CGL2007-64551/HID) and SCARCE (Consolider-Ingenio 2010 CSD2009-00065) and the “Programa de

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