Photolysis and TiO2 Photocatalytic Treatment of Naproxen: Degradation, Mineralization, Intermediates and Toxicity Fabiola Méndez-Arriaga, Jaime Gimenez, and Santiago Esplugas* Chemical Engineering Departament, University of Barcelona, Marti i Franques 1, 08028, Barcelona, Spain
Abstract: The heterogeneous photocatalytic degradation of Naproxen using TiO2 as photocatalyst was investigated. Effects of TiO2 loading, temperature, volumetric flow and dissolved oxygen concentration, as operational parameters, were studied. The experiments were carried out in a 0.078 L Duran reactor equipped with a 1kW Xe-lamp. After 3 hours of photolysis, a Naproxen aqueous solution (0.8 mmol//L) results in a 90% removal with only a 5% of mineralization whereas the TiO2 photocatalysis leads lower removal (40%) with better mineralization (20%). Identification of byproducts has shown that demethylation and decarboxylation are the principal initial processes in the degradation of NPX. The toxicity of the treated solution was evaluated using the Microtox test based on the bioluminescent bacteria Vibro fisheri, in order to compare the acute toxicity of Naproxen and its photoproducts. Photocatalysis did not show an improvement in the biodegradability under the operational conditions tested. However, mineralization data are promising for future studies.
Introduction Nowadays the excessive use of pharmaceutical compounds had leaved an unexpected consequence on the aquatic environment. Drugs and/or their metabolites go into the sewage treatment plants (STPs) after human or animal consume, from several sources like hospital, industry or municipal wastewaters (1). The incomplete elimination of pharmaceutical compounds in STPs is already reported in several countries (2, 3, 4). These kinds of pollutants are able to across the STP without further degradation and finally are discharged to surface waters as rivers or lakes. Naproxen (NPX) is a Non-Steroidal Anti-Inflammatory (NSAI) drug and it is one of the most frequently found as recalcitrant in surface waters (5, 6). It is widely prescribed for the skeleton-muscle pain or inflammatory rheumatic disorders due to its analgesic and antipyretic effects. NPX is an aromatic compound characterized for its 2-arylpropionic acid group, typical of the NSAI drugs. NPX has been already detected in STP influents into the range of 250 ng/L to 1.5 µg/L, and its removal has been estimated around 71%. Its presence has already shown harmful toxicological consequences (7) and adverse ecological impact on the microbiological aquatic systems (8). Since STPs are not sufficiently effective to eliminate pharmaceutical compounds like NPX, new alternative techniques, as the so-called Advanced Oxidation Processes (AOPs), have been tested to diminish its concentration in water. AOPs make use of different reacting systems mainly characterized for the promoKeywords: NSAID, Naproxen, photocatalysis, photolysis *Corresponding author; E-mail address:
[email protected]
ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
tion of hydroxyl radicals (.OH), and due to its nonselective and powerful oxidative characteristics, several AOPs have reached good removal rates for recalcitrant pharmaceutical pollutants in wastewater. For instance amoxilline, ibuprofen or bisphenol-A using O3, photocatalysis and sonolysis respectively (9, 10, 11) are fine example of AOP environmental aquatic remediation. In the case of NSAID several conventional, advanced or combination of oxidative treatments have been studied. For example NPX has been degraded by chlorination, biofilm (12), photolysis (13, 14, 18) and combined heterogeneous photocatalysis with separation by nanofiltration (15). Some results of the treatments above mentioned have shown higher toxicity of the effluent due to the remained byproducts. NPXbyproducts produced from the chlorination and photolysis treatment have shown low biodegradability and high ecotoxicity on algae, rotifers and microcrustaceans organisms. Although some degradation processes of NPX have been developed, a study aiming the biodegradability of byproducts from photocatalysis irradiated treatment is still lacking. In the last basis, the objective of this investigation is to undertake a study of the heterogeneous photocatalytic degradation of NPX in aqueous suspension with TiO2 in order i) to identify the main byproducts remained after photocatalytic treatment and ii) to evaluate the biodegradability and toxicity of the treated dissolution.
Material and Methods
NPX-Na ((S)-6 methoxy α-methyl 2-naphthaleneacetic sodium salt) was purchased from SigmaAldrich and it was used without pre-treatment. Table 1 shows some chemical and physical properties of
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Figure 1. Experimental laboratory equipment. Table 1. Physical and chemical properties of NPX.
Structure
(C14H14O3)
Aqueous solubility
15.9 (mg/L) (25 ºC)
Melting Point
152-154 ºC
Log KOW
3.18
pKa
4.15
NPX [SRC Phys. Prop. Database, 2007]. Titanium dioxide (TiO2) Degussa P-25 (commercial catalyst ~70% anatase, ~30% rutile, surface area 55 m2/g (19)) was used as received. Fresh daily solutions of NPX were prepared in milliQ water. Irradiation experiments were carried out with a solar simulator (Solarbox, 1500 Co.fo.me.gra, 220V, 50Hz) equipped with a 1 kWXenon lamp (Phillips) and a tubular-horizontal photoreactor (0.078 L illuminated volume) concentrically located between the reflected wallmirrors inside of the box. The photon flux inside of the photoreactor was evaluated by actinometrical uraniloxalic procedure and it was calculated of 6.3 µEinstein/s (290 nm < λ< 400 nm range). One and a half L of NPX-TiO2 aqueous mixture were fed in an external vessel and pumped to the Solarbox. Pure oxygen (Air Liquide ®) purge was employed and O2 concentration was measured on line by Crison Oxi 330i WTW Oxi Cal-SL sensor. Temperature of the stirred vessel was kept constant through an ultra-thermostat bath (Selecta; Frigiterm –10). Figure 1 depicts a scheme of the experimental equipment employed. Samples were withdrawn at several intervals of time and immediately filtered with a Durapore PVDF Millipore filter (0.22 µm). NPX concentration was followed by a HPLC Waters using a C18-RP Trace ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
Extrasil OD52 5 Micromet 25 x 0.46 Teknockroma column. The mobile phase was an acetonitrile (Panreac 99.8%) and 0.05 M phosphate dehydrogen ammonium (Aldrich 98% ACS) solution (0.8/0.2) employed in isocratic mode (1 mL/min). Detection of NPX was carried out at 254 nm. Dissolved organic carbon (DOC) and absorption spectrum were obtained by a Shimadzu TOC-V CNS and a Perkin Elmer UV/Vis Lambda 20 spectrophotometer respectively. Biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) were carried out according to Standard Methods (5120) by respirometric single measuring system OxiTop and Norm. France NFT 90-101 respectively. Toxicity measurements were carried out using the Microtox test measuring the inhibition of bioluminescence of Vibro fisheri at 15 min of exposure time. Identification of intermediate products was carried out by electro spray ionization/ mass spectrometry (ESI-MS) using a Jasco AS-2050 plus IS mass spectrometer.
Results and Discussion Preliminary Experiments: Thermo-Degradation and Adsorption Experiments As preliminary control experiments and in order to evaluate possible thermo-degradation or losses by volatility, 1 L of NPX solution (0.8 mmol/L) without catalyst and under no pH or ionic-strength control, was placed in the stirred tank and heated at 20, 40, 60 and 80 ºC. Once reached the step-temperature, samples were withdrawn and NPX concentration and DOC were measured. pH and temperature were online measured. Figure 2 depicts the NPX, DOC and pH evolution for each 20 ºC-step. It can be noticed that NPX concentration and DOC remained unchanged during every step-change of temperature. Thus, NPX do not suffer any thermo-decomposition or loss by
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Figure 2. Evolution of NPX, DOC and pH for each increase of increase.
(a) volatility in the temperature range tested. On the other hand, since in heterogeneous systems the adsorption plays an important role in the evolution of the photodegradation, adsorption experiments at constant temperature were carried out. Several ratios of NPX/TiO2 were mixed in 30 mL vials at free pH conditions (pH 6.15±0.15). In the first experimental series, 1 g/L of TiO2 and several initial concentration of NPX (ranging from 2 to 1000 mg/L) were assorted. In the second series, 500 mg/L of NPX were mixed with TiO2 ranging from 0.03 to 1 g/L. Constant temperature (30 ºC), stirring and close-dark surroundings were always controlled during 24 h. Samples were carefully withdrawn from the supernatant and then filtered with a PVDF 0.22µm filter. The equilibrium amount of NPX was determined by difference between the initial concentration and the measured supernatant concentration for each NPX/TiO2 ratio. Figure 3a depicts the amount of NPX in equilibrium and the amount adsorbed on 1 g/L of TiO2 varying the NPX concentration and Figure 3b shows the adsorbed amount of NPX and the NPX equilibrium concentration for several quantities of catalyst for 500 mg/L of NPX. As it is possible to observe from both figures, the amount remained in the solution is between 92 and 95% independently of the initial concentration of NPX and TiO2. The adsorbed amount is ca. 5 and 8% in the wide range of TiO2 tested. Under free pH conditions, close to the point zero charge of TiO2 (6.5), NPX is fully soluble in water. A low adsorption was observed due to no electrostatic attraction between the surface charge of TiO2 and NPX in anion form is accomplished. A highest chemisorption between NPX and TiO2 would be observed of pH adjusted between 4.1 and 5.3, which corresponds to the pKa of NPX and TiO2 (20) respectively. In this range the positive charges of the surface of the catalyst attracts the anionic form of the compound and the photocatalytic degradation would be expected on the surface of the catalyst. However, due to the typical wastewater streams have pH values ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
(b) Figure 3. (a) Adsorption of NPX on 1g/L TiO2 varying the initial concentration of NPX, without adjust of pH (≅6) and 30 ºC. (b) Adsorption of 500 mg/L of NPX varying the concentration of TiO2, 30 ºC and without adjust of pH (≅6).
between 6 and 9 (21) and in order to avoid extra pH adjustment before biodegradability test, no control of pH was selected for the photocatalytic degradation assuming the low adsorption percentage between NXP and TiO2 and that the most possible way of degradation could be reached by migration of ·OH radicals to the bulk of the suspension.
Degradation of NPX by Photolysis Versus Photocatalytic Process To evaluate the degradation of NPX by photolysis, a 1.5 L NPX solution (0.8 mmol/L) was exposed to the solar irradiation with the Xe-lamp simulator source. On the other hand, the degradation of NPX by photocatalytic means was carried out using 0.1 g/L of TiO2 under the same irradiation conditions before described. In Figure 4 it is depicted the degradation of NPX in both cases. As it is possible to observe, NPX concentrations decrease until 75% when it is continually exposed at Xe lamp during 2 hours in absence of catalyst with an initial rate of degradation of 6 µmol/min. On the contrary, an important difference is observed in the J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
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0.012
7
0.01
6 0.008
5 4
0.006
3
♦ kr (min-1 )
Initial degradation rate (µ µ mol/min)
8
0.004
2 0.002
1 0
0 Photolysis
0.1
0.5
1
TiO2 (g/L)
Figure 4. Degradation of NPX by photolysis and photocatalysis using 0.1 g/L of TiO2.
case of degradation of NPX when catalyst is present leading almost 40% removal in the same interval of time. The chemical transformation of organic compounds by photolysis is carried out through the fundamental deactivation of the excited states undergoing in a certain number of primary photochemical processes: rearrangement, formation of radicals, isomerization, ionization etc. (22). Thus the degradation of organic compounds by photolysis starts when the target compound absorbs specific energy and reaches an activation process promoting the electronically excited molecular states. NPX degradation by photolysis is reached due to its absorption spectrum overlap with the Xe-lamp emission spectrum in the range of 290 and 350 nm. Thus several pathways are followed until almost full degradation of the compound in 3 h is observed. Our experiments have concordance with previous investigations founded in references (13, 14, 17, 18). On the other hand, the degradation of NPX by means of photocatalytic process follows another well differentiated and recognized process. In suspension, the photocatalytic mechanism is based on the energy available to be absorbed for the catalyst (TiO2) normally accepted between 300 and 400 nm. Under exited condition, the valence band-electrons are transferred to the conduction band forming the hole+ − electron pair (hvb − ecb ) (Eq. 1). The superoxide radical anions (Eq. 2) and hydroxyl radicals together with H+ (Eq. 3) are formed by the reduction of the oxygen and by cleavage of adsorbed molecules of water. TiO2 + hv → TiO2* (h+vb /e-cb)
Eq. 1
e- + O2→ ·O2-
Eq. 2
+
h + H2O →·OH + H
+
Eq. 3
When the electron migrates to the surface of catalyst, most of the above reactions are reached on ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
Figure 5. Influence of TiO2 loading on the initial degradation rate and reaction constant.
the irradiated surface of TiO2. As well recognized the ·OH radical is one of the most powerful oxidants with a redox potential of 2.80 E0V (20). If organic compounds are adsorbed on the surface of the catalyst, the ·OH non-selective attack promotes the cleavage of compound bounds. If not important adsorption is observed, diffusion of ·OH and/or ·O2- radicals to the bulk of the dissolution will be the dominant process in the degradation. In our case it was possible to observe that 40% of the initial NPX was degraded under photocatalytic process. The low adsorption of NPX on the catalytic surface and the short time-live of the ·OH are some of the reasons to observe the strong difference between the degradation by photolysis and photocatalysis -2 folds higher-. Part of the ·OH generated are recombined, loosed or deactivated before they react with NPX present in its full soluble state in the bulk of the dissolution.
Photocatalytic Mineralization In order to evaluate the effect of the TiO2 loading on the DOC removal, concentrations of catalyst from 0.1 to 1 g/L were tested for 0.8 mmol/L NPX solution. In Figure 5 it is depicted the degradation rate of NPX and the pseudo-first order constant for the initial time of reaction including the observed by photolysis. Results showed that the optimal amount of TiO2 to reach the maximum initial degradation of NPX was the highest tested. An important remark is that the initial degradation for photolysis and photocatalytic experiments is quite similar (between 3 and 7 µmol/ L·min). However, after 180 min, DOC removal and NPX degradation are maxim for a TiO2 concentration of 0.1 g/L and decrease when TiO2 concentration increases to 0.5 g/L. From this value, an increase in the TiO2 concentration does not seem to influence in a high manner, on the DOC removal and NPX degradation (see Table 2). The last can be explained due to at the beginning of the reaction, the most J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
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Table 2. Effect of TiO2 load on NPX degradation and DOC removal at 180 min.
TiO2 g/L Photolysis 0.1 0.5 1
NPX degradation mmol/min 0.0400 0.0177 0.0088 0.0084
DOC removal % 5 20 11 9
important influence in the degradation is the ·OH attack and it is governed for the catalyst loading. However throughout the process, slower degradation rate of NPX can be attributed to the apparition of probably hydrophobic byproducts which present more affinity to the surface of TiO2. Preferential degradation of these hydrophobic compounds reduces the probability to follow the degradation of NPX. On the other hand operating with the minimum amount of TiO2, there was a slow degradation of NPX at the initial time of reaction. This fact may be explained by the scarcely diffusion and contact of the ·OH able to reach the compound. However, after 180 min, the global NPX degradation and DOC removal is the highest observed due to two important degradation pathways could be occurring at the same time: photolysis and ·OH attack of NPX and byproducts. Photolysis showed a DOC removal of 5% and almost double mineralization was observed for photocatalysis using 1 g/L of catalyst -the maximum employed-. However the higher DOC removal is observed using the lowest amount of catalyst (0.1 g/L). Only under low amount of TiO2 both process (photolysis and photocatalysis) contribute to improve the DOC removal. Another experimental series varying the volumetric rate (0.1 to 0.4 L/min) and temperature (20 to 40 oC), with 0.1 g/L of TiO2, were carried out following the evolution of NPX and DOC concentration. In Fig 6a it is depicted the DOC change for several conditions including the photolysis case. It is noticed that DOC removal is not dependent of the volumetric flow or of temperature into 20 to 30 ºC. However if photocatalytic degradation is maintained at 40 ºC, an evident increase of the DOC removal is observed (26%) (see Figure 6b). It is generally accepted that the photocatalytic reaction rate is not considerable modified with the variation of temperature (20). Nevertheless, if volatile compounds are generated, slight increase in the DOC removal could be attributed to the formation of a quiet higher thermolabile dissolution, as shown in the photocatalysis degradation behavior of NPX. In order to evaluate the effect of the presence of oxygen during the photocatalytic degradation, O2 was ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
(a)
(b) Figure 6. (a) Temperature and recirculation rate effect on DOC removal with 0.1 g/L TiO2. (º) photolysis; (♦) 0.1 L/min; (■) 0.2 L/min; (×)20 ºC; (+) 30ºC (ж)40 ºC. (b) NPX conversion (continuous bar line) and DOC removal (doted bar line) at 180 and 240 min of treatment for 20, 30 and 40 ºC using 0.1 g/L of TiO2.
purged into the stirred-tank and maintained its concentration around 40±2 mg/L. Figure 7 depicts the DOC removal and NPX degradation under oxygen saturated conditions. In presence of oxygen the complete degradation of NPX was observed in 120 min with an initial degradation rate of 4.4 µmol/min see Figure 5-. In photocatalysis it is well known that the oxygen enhances the photocatalytic activity (22, 23, 24, 25). Due to NPX was complete removed it is possible to assure that the presence of oxygen promotes less recombination and more efficiency on the generation and reactivity of ·OH and O2.-. Several authors have reported the important influence of the oxygenated surroundings in the photolysis of NPX (18). However oxygen in saturating conditions did not any enhance in the DOC removal (see Figure 7). Photocatalytic reaction under O2 saturated conditions could promotes parallel reactions involving the organic radicals generated among the ·OH, H+, ·O2·-, etc with NPX, however no higher mineralization is achieved due to possible reactions also with the byproducts produced. J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
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Table 3. Intermediate compounds identified by LC/ESI-TOF-MS: NPXo 0.8 mmol/L; pH free; 40 mg/L O2; 40 ºC; 0.1 g/L TiO2, solar simulated irradiation during 3 hours.
ID
Compound
Ret. Time min
Calculate mass 230
Exp. mass m/z 229.0813
1
Naproxen
12
3
2-(6-Hydroxy naphtalen-2-yl) propanoic acid
9
Dimmer C26H26O4
229[M]-,185[M-COO]-,170[M-C2H3O2]-
21
216
215.0713
215[M]-; 171[M-COO]-;157[171-CH3]-
25
402
401.8813
401[M]-;357[M-C2H4O]-;313[357-C2H4O]-; 245[M-C11H9O]-;187[313-C10H7]-
Figure 7. Photocatalytic degradation of NPX (fill dots) and DOC conversion (empty dots) under saturated conditions of dissolved O2 (40 mg/L) using 1 g/L TiO2 at 40 ºC.
Byproducts Generated by Photocatalysis Intermediates generated after 180 min of treatment (samples for aerated and oxygenated conditions with 0.1 g/L TiO2) were identified with a Liquid Chromatography-Electrospray Ionization-Time of Flight mass (LC/ESI-TOF-MS), using a Jasco AS2050 plus IS mass spectrometer into the m/z range of 65 to 1000 in negative ionization mode. Table 3 lists the intermediates detected and Figure 8 depicts the possible reaction mechanism with the corresponding intermediates. The spectrum showed the presence of two principal molecular peaks at m/z 215 and 401 at retention times of 21 and 25 min after the molecular mass of NPX (m/z= 229) observed at 12 min. The remained DOC after treatment of NPX 1 consisted mostly of 2-(6-Hydroxynaphtalen-2-yl) propanoic acid 3 and a dimmer 9 formed from consecutives transformations of the 1-(6-Methoxynaphtalen-2-yl) oxyradical 7. Under O2 saturated conditions it is possible to assure that the photoreactivity of NPX was much higher, in agreement with previous results. However for sample under aerated conditions, the same byproducts were observed only varying the signal intensity. The evolution of the degradation of NPX follows several multi-step and interconnected routes. In general, two well differentiated and simultaneous ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
Main fragments m/z
pathways could be assured: demethylation by ·OH attack and decarboxylation by photolysis. As explained above the degradation of NPX was directly influenced for the TiO2 loading, decreasing as higher TiO2 used. At high TiO2 concentration the low decrease of NPX can be due to scattering effect and degradation by photoinitiation was not forceful enough. The initial degradation of Naproxen 1 leads demethylated 2 and decarboxilated 5 radical derivatives. The possible hydrogen addition, from the cleavage of water through the h+ in the surface of catalyst (Eq 3), to 2 radical generates 2-(6 hydroxynaphtalen-2-yl) propionic acid 3. Although the initial ·OH attack could be possible in ο or α−methyl position, only evidence of demethylation in ο−methyl position is observed in the molecular ion at m/z 215 in the electronic impact mass spectrum with principal fragments of 171 and 143. By elemental analysis and in agreement with the molecular formula of fragments, (6-methoxynaphthalen-2-yl)-propionic acid or α-demethylnaproxen was not noticed to be present. Isidori et al. (2005) have reported that NSAID as NPX are rapidly photodemethylated in the environment. However no-proof of 2-(6 hydroxynaphtalen-2-yl) propionic acid (ο-demethylnaproxen) as intermediate of photolysis has been reported. We could assure that the ·OH radical attack can be the promoting step for its formation in photocatalytic experiences. The above is also related to the highest degradation rate of NPX at low TiO2 concentrations due to the photolytic via showed a major contribution. The methyl radical generated follows several reaction pathways –recombination, hydrogen subtraction, ·OH attack- (see Figure 8, left) driving to the formation of low-weight soluble and/or volatile compounds like methane, ethane, its concomitant alcohols, aliphatic acids, or ketone. Compounds like ethanol, highly soluble in water, remains in dissolution. However it is possible that some other of these compounds leave the dissolution by volatility or evaporation. J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
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CH3
1 Demethilation
O
Decarboxylation
-
H3C
O Na+
O
CH
O
+
CH3
O
H3C
5*
O
+ O2
CH3 CH3
O
CH3
H2C
H3C
6
-
O Na+
HO
+ ·OH
O
4
O
3
-
C
O
HO
+ h+
-
O
O H3C
O
+ 2
-
O
+ CH3
CO2
CH3
CH3
O CH3 O
CH3
H3C + ·OH + h+
7*
O OH
HO CH2
CH2 H3C
H3C
+ ·CH3 + h+
OH
C
-C 2H3OH
O
O
CH3
+ h+
Dimmerization
HO CH2
+8
H3C
H3C
8
O
O
OH
O
CH2
CH2
+h+ OH
9
H3C
O
O
CH3
Figure 8. Intermediates observed at 180 min of photocatalytic treatment and propose mechanism of reaction.
This is in agreement with the slight increase in the DOC removal observed in experiments reached at 40 ºC, for instance, for ketone which presents a boiling point of 56 ºC. It was also observed –as latter described- that these low-weigh compounds, through its reactive radicals transformations, are involucrate in the post-sequential formation of more different byproducts. The other main degradation via was the decarboxylation of NPX. In agreement with other authors its generation way is through photoinitiation absorption of light (14, 17, 18). Isidori et al. (17) and Jimenez et al. (18) have reported the formation of 2ethyl-6-methoxynaphtalene (ethylnerolin) by photolysis, however this byproduct could drive to produce CO2 or bicarbonate 4 by ·OH attack in the central C atom in presence of TiO2. Interesting, the last can be corroborated for the evolution of the pH during the reaction for all condition tested (see Figure 9). pH increases in an unusual behavior of oxidation photoISSN 1203-8407 © 2008 Science & Technology Network, Inc.
catalytic processes. Thus bicarbonate remains in solution increasing the pH. This parallel process competes on the CO2 formation and mineralization shows an inefficient increase. In presence of O2, the photodecarboxylated radical 5 is the precursor of two of the most important and higher yield-amount photolytic byproducts: 1-(6methoxynaphtalen-2-yl)ethanone (acetylnerolin) and 1-(6-methoxynaphtalen-2-yl)ethanol with 34% and 7% respectively (18). Under O2 conditions the alcohol and ketone intermediates are formed by oxygen trapping of the benzilic radical following the break-down of the unstable hydroperoxide. By consecutive steps, closely related to peroxide chemistry and constant hydrogen source, 1-(6-methoxynaphtalen-2-yl)oxyethyl was also formed and reported as photolytic byproduct. Jimenez et al. (18) assure that the source of hydrogen is essential to obtain the oxyethyl moiety. In photocatalytic-enhance conditions this requirement is enough reached due to the cleavage of O-H bond of water. J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
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pH
7.50
6.50
5.50 0
30
60
90
120
150
180
Time (min)
Figure 9. pH evolution for several operational conditions. (♦) Photolysis, 2 L/min, 30 ºC; (■) 1g/L TiO2, 2 L/min; (▲) 0.5 g/L TiO2, 2 L/min, 30 ºC; (◊) 0.1 g/L TiO2, 2 L/min, 30 ºC; (○) 0.1 g/L TiO2, 1 L/min, 20 ºC; (●) 0.1 g/L TiO2, 4 L/min; (□) 0.1 g/L TiO2, 4 L/min, 20 ºC; (∆) 0.1 g/L TiO2, 4 L/min, 40 ºC.
On the other hand DellaGreca et al. (14) assured that oxyethyl compound and other dimmers reported are only generated under presence of inorganic salts in dissolution in coincidence with our conditions. In our molecular ion detection chromatogram, any of three above described byphotoproducts ((1-(6methoxynaphtalen-2-yl)ethanone, 1-(6-methoxynaphtalen-2-yl)ethanol or 1-(6-methoxynaphtalen-2yl)oxyethyl)) were observed. However due to the constant O-H bond is cleaved through the photocatalytic action, the hydroxyolefin radical is formed leading the benzyl cation with an unpaired electron. In our experience certain sticky and greasy (oily) appearance was observed in the dissolution, due to probably parent olefin radical compound formed. Actually DellaGreca et al. (14) report the olefin side chain also as byproduct of photolysis. Although the olefin-byproduct was not a photocatalytic intermediate, its short-life time could cleavage the C-C bond of the naphtalen ring followed by the coupling with the analogue non-dehydroxyolefined to afford the dimmer 8. Dimmer 9 could be formed from hydrogen subtraction. This step, which introduces the methyl function on the oxymethyl moiety, could be followed by a second step consisting in the reaction of the above radical intermediate with the CH3OHCH2· radical, formed from the initial demethylation photocatalyzed process and its post-radical activity. Thus under photocatalytic O2-enhanced process, the continuous source of ·H and ·OH, makes possible the formation of 9 by union of CH3OHCH2· radical. Compound 9 showed the molecular ion at m/z 401 in the electronic impact mass spectrum with the follow m/z fragments: 313, 357, 245 and 187. Dimmer 9 was the other byproduct remained in the dissolution; however DellaGreca et al. (14) reported the formation of ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
two dimmers from the photolysis via no equivalent to here founded. Summarizing, byproduct 9 is strong dependent on the ethoxy-group due to the O2 saturated conditions and the action of inorganic salts present is water. Although we did not observe any photolysisbyproduct, it is possible to assure that the transformation of NPX is strong associated with the involvement of different short-lived intermediates, as above described. It is possible to observe that the presence of the TiO2 promotes different and pseudo-complementary degradation pathways of NPX with additional important increase of DOC removal. The important contribution of the photocatalytic degradation is the further reacting of the methyl radical. The difference also can be strongly attributed to the presence of O2 in dissolution which leads the formation of byproducts and the reactivity continues with the ·OH and with the promotion of the h+ from the catalyst. This can indicate that by photocatalysis the pathway goes further in the oxidative reaction.
Biodegradability and Toxicity Biodegradability and toxicity assays were carried out at different time intervals of the photocatalytic treatment and also for pure NPX solution without previous treatment. BOD5/COD for the initial NPX solution was 0.02 and for 30, 60, 120 and 240 min of photocatalytic treatment, the BOD5/COD values never were higher than 0.05. This can suggest that no-biodegradable photoproducts were formed during the photocatalytic treatment. In Figure 10, changes in toxicity during the photocatalytic treatment are presented. As it is possible to observe, in presence of TiO2 an important increase in the toxicity is observed after 4 h irradiation. Under saturated dissolved oxygen it is able to achieve complete degradation of NPX, however the increase of the toxicity is observed due to the partial decomposition of NPX that can lead to the formation of final products more toxic than the original NPX compound. In our experience, also the by-products identified did not show as biodegradables and no improvement was reached in the EC50 values, always lower than 8%. Our results are in agreement with the reported by several authors regarding the toxicity of byproducts from photolysis degradation (14, 17). Photoproducts by solar simulator irradiation of NPX were significantly more toxic than the parent compounds for several organisms tested (B. calyciflorus, T. platyurus and C. Dubia, Daphnia magna, Vibrio fisheri) for toxicity test, acute bioassays, chronic bioassays and genotoxicity/mutagenesis testing respectively. The photo J. Adv. Oxid. Technol. Vol. 11, No. 3, 2008
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Figure 10. Changes of the EC50 (bars) and % inhibition (points and line) of bioluminescence of Vibro fisheri as function of irradiation time in the presence of 0.1 g/L TiO2 and 40 mg/L O2.
decarboxylation products with ethyl, 1-hydroxyethyl and acetyl side chains have shown a high cytotoxicity. Moreover, complex ciclodextrin-NPX was also proved to improve drug phototoxicity without successful (18). The diminished phototoxicity of the complexed NPX is associated with the involvement of different shortlived intermediates, probably similar as intermediates founded by photocatalysis. Similar quiet results are reported for chlorinated process or by biofilm (12). Due these compounds exhibit a low final BOD5/ COD, a photocatalytic treatment for degradation of NPX can be suggested for a post-biological processes, only if the oxidation state is more advanced. It is possible that the byproducts of NPX are quite difficult to improve its biodegradability, however due to by photocatalysis the mineralization increase, it is possible to propose it as good DOC removal treatment. With photocatalytic process not only the complete NPX concentration is able to disappear but also mineralization is achieved. The focus could be the optimization and solar alternative process to improve the efficiency of the photocatalytic treatment for NPX degradation.
shown a nature of photoinitiation production. Photolysis plays an important role on the degradation of NPX affecting the reaction rate and the main subproducts formed. The byproducts of degradation of NPX can be summarized as product of several influences: by photoinitiation, ·OH attack and high h+ reactivity under O2 saturated conditions, inorganic ions. As possible to conclude, the main subproducts of photocatalytic treatment involve also the photolysis byproducts in a further oxidation. In oxidant medium (O2 saturated), an improvement in the transformation of NPX is reached, due to lesser hole-electron recombination, but no enhancement in mineralization is observed. Intermediate products can represent worst biodegradability conditions for the treated effluent. The toxicity of the treated solution is not reduced during photocatalytic process and no coupling for detoxification with a biological system can be assured before optimization.
Acknowledgements Generous support by the Spanish Ministry of Education and Science (CICYT projects CTQ200500446/PPQ and CTQ2004-02311/PPQ) and University of Barcelona (predoctoral fellowship) is gratefully acknowledged.
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Conclusions Photolysis of NPX causes a reduction of 80% of its initial concentration after 3 hours of treatment. Photocatalysis using TiO2 proved more degradation efficiency only in the DOC removal. However, 50% of NPX can be degraded (26% mineralization) by photocatalytic means due the absorption competition among the NPX and the TiO2 into the UV-Vis range. Demethylation and decarboxylation are the initial process in the degradation of NPX. The radical formed from the decarboxilation has already reported as a product of photolysis. The production of demethylated radical derivative is strong dependent on the ·OH radical attack while decarboxilated radical derivative ISSN 1203-8407 © 2008 Science & Technology Network, Inc.
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Received for review March 26, 2008. Revised manuscript received June 16, 2008. Accepted June 18, 2008.
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