Accepted Manuscript Title: Photocatalytic degradation of sulfamethoxazole in aqueous solution using a floating TiO2 -expanded perlite photocatalyst ˙ Author: Maciej Długosz Paweł Zmudzki Anna Kwiecie´n Krzysztof Szczubiałka Jan Krzek Maria Nowakowska PII: DOI: Reference:
S0304-3894(15)00412-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.05.016 HAZMAT 16818
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
27-12-2014 4-5-2015 9-5-2015
˙ Please cite this article as: Maciej Dlugosz, Pawel Zmudzki, Anna Kwiecie´n, Krzysztof Szczubialka, Jan Krzek, Maria Nowakowska, Photocatalytic degradation of sulfamethoxazole in aqueous solution using a floating TiO2-expanded perlite photocatalyst, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.05.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photocatalytic degradation of sulfamethoxazole in aqueous solution using a floating TiO2expanded perlite photocatalyst Maciej Długosza, Paweł Żmudzkib, Anna Kwiecieńb, Krzysztof Szczubiałka* a, Jan Krzekb, Maria Nowakowska* a a
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
b
Faculty of Pharmacy, Jagiellonian University Medical College, Medyczna 9, 30-688 Kraków, Poland *Corresponding Authors Krzysztof Szczubiałka, e-mail: [email protected], tel. +48 12 6632062, fax +48 12 6340515 Maria Nowakowska, e-mail: [email protected], , tel. +48 12 6632250, fax +48 Highlights ► 12 6340515 Highlights ► Sulfamethoxazole was degraded using a floating photocatalyst under UV irradiation ► The photocatalyst was obtained by supporting TiO2 onto expanded perlite ► The mechanism of sulfamethoxazole photodegradation in water was proposed ► The photodegradation rate of sulfamethoxazole is greater at higher pH
ABSTRACT Photocatalytic degradation of an antibiotic, sulfamethoxazole (SMX), in aqueous solution using a novel floating TiO2-expanded perlite photocatalyst (EP-TiO2-773) and radiation from the near UV spectral range was studied. The process is important considering that SMX is known to be a widespread and highly persistent pollutant of water resources. SMX degradation was described using a pseudo-first-order kinetic equation according to the Langmuir-Hinshelwood model. The products of the SMX photocatalytic degradation were identified. The effect of pH on the kinetics and mechanism of SMX photocatalytic degradation was explained.
Keywords: sulfamethoxazole, antibiotic, titanium dioxide, expanded perlite, floating photocatalyst, photocatalytic degradation 1. Introduction The occurrence of pharmaceuticals as well as their metabolites and transformation products in the environment is becoming a matter of a grave concern because they are extensively and increasingly used in human and veterinary medicine and are continuously released into the environment. Antibiotics are a group of emerging persistent pollutants that pose a large threat to the environment and society due to their biological activity. One of the most important negative consequences of the antibiotic pollution is an increased microbial resistance [1]. The problem is severe inter alia since microbial populations are capable of horizontal gen transfer of the genes favoring survival [2]. These resistance genes are often persistent and are not eliminated, even in the absence of antibiotic pressure [3]. Sulfonamides are the oldest group of antibiotics used in human and veterinary medicine. They are polar, amphoteric, photo- and thermally stable substances that are readily soluble in water [4]. For this reason, they possess high migration ability in the environment [5]. The most commonly employed of this group of antibiotics is sulfamethoxazole (SMX). It is extensively used in human and veterinary medicine. It is particularly often used in bronchitis and in the treatment of urinary tract infections as well as growth promoter [6]. Advanced oxidation processes (AOPs) have received great interest in recent years as methods complementary to conventional water treatment or as alternative treatment strategies for industrial wastewater prior to discharge into sewage or into aquatic environments. Ozonation, Fenton and photo-Fenton oxidation, photolysis and H2O2-enhanced photolysis, heterogeneous photocatalysis, and electrochemical oxidation were frequently studied AOPs [7,8,9,10,11].
One of the AOP methods that can be useful for the antibiotic oxidation is microheterogeneous semiconductor photocatalysis. It employs photoinduced charge separation of electrons and holes, which can react with water or oxygen to form reactive oxygen species (ROS), predominantly hydroxyl radicals. High oxidizing potential of hydroxyl radicals (2.80 V) allows mineralizing most of the low molecular weight organic molecules. The reports on application of this method for abatement of pharmaceuticals such as amoxicillin, ampicillin, cloxacillin [12], sulfamethoxazole [13], and tetracycline [14] confirm efficacy of this method. For all these experiments a significant reduction in total organic carbon (TOC) was confirmed what is considered to be essential parameter in view of the potential application of a method. Unfortunately even the finest TiO2 powders tend to sediment after some time resulting in a decrease of the amount of absorbed light and in the loss of efficacy and efficiency. To overcome this drawback TiO2 powders can be immobilized on various surfaces such as glass plates [15] or silica gel beads [16] to be further used in tubular photoreactors. They can be also immobilized on floating supports and used in floating bed photoreactors [17]. The floating particles are localized at the air/water interface where the solar light has the highest intensity. Moreover, the floating particles can be easily collected from the surface. For this purpose materials such as cenospheres [18], polystyrene beads [19] or expanded perlite [20,21] were already used. The direct [22,23] and photocatalyzed by TiO2 [13,24] photodegradation of SMX has been already studied and the photoproducts were identified [13,25]. It was found that the TiO2photocatalyzed degradation of SMX results in the formation of products with lower toxicity which are biodegradable, in contrast to parent SMX [24]. This paper presents SMX photocatalytic degradation studies employing a novel floating photocatalyst composed of TiO2 immobilized on expanded perlite (EP). The application of TiO2
supported on the floating material for SMX degradation has not been described yet. Kinetics of the process at different pH values were determined and photodegradation products at pH 5.1 were identified based on UPLC-MS/MS analyses. We have found the formation of different photoproducts for SMX degradation using this photocatalyst than those described previously [13]. The results indicate that the process has the potential to be applied for facile removal of antibiotic pollutants from shallow water reservoirs. Taking into account the ability of the studied photocatalyst to float and consequently the possibility of its multiple use [21], its application is expected to be more economical than that of TiO2 in suspension.
2. Experimental 2.1. Materials Titanium(IV) isopropoxide (TIP, 97%), sulfamethoxazole (SMX, analytical grade), phosphoric acid (≥85%), acetonitrile (HPLC grade) were supplied by Sigma-Aldrich. Isopropanol, HCl, KCl, Na2CO3, NaHCO3, acetic acid (all analytical grade) were supplied by POCh (Gliwice, Poland). The sample of expanded perlite (EP) was provided by Zębiec sp. z o.o. (Zębiec, Poland). Deionized water was used in all experiments.
2.2. Apparatus The UV-Vis absorption spectra of SMX solutions were collected using Varian Cary 50 UV-Vis spectrophotometer (Varian Inc., Palo Alto, CA, USA). The samples were irradiated using the Rayonet RPR-100 photoreactor (Southern New England Ultraviolet Company, Branford, CT, USA) equipped with six (8 W each) lamps emitting radiation in the range of 316-400 nm with maximum intensity at 350 nm. The HPLC-PDA system consisted of a Waters (Waters
Corporation, Milford, MA, USA) 515 HPLC pump and a Waters 2996 Photodiode Array detector (Waters Corporation, Milford, MA, USA). Waters Symmetry C18, 5 µm, 3.9×150 mm column was used which was maintained at room temperature. The eluent was acetonitrile:water (35:65 v/v) for pH = 1 and 5.1 samples and acetonitrile:1% acetic acid (35:65) for pH = 10 samples. The flow rate was set at 0.7 ml/min. The detection wavelength was set at 270 nm. The products of SMX photodegradation were identified using UPLC-MS/MS technique. The UPLCMS/MS system consisted of a Waters ACQUITY UPLC (Waters Corporation, Milford, MA, USA) coupled to a Waters TQD mass spectrometer (electrospray ionization mode ESI-tandem quadrupole). Chromatographic separations were carried out using the Acquity UPLC BEH C18 column; 2.1 × 100 mm, and 1.7 µm particle size, equipped with Acquity UPLC BEH C18 VanGuard pre-column; 2.1 × 5 mm, and 1.7 µm particle size. The column was maintained at 40°C, and eluted under gradient conditions from 95% to 0% of eluent A over 10 min, at a flow rate of 0.3 mL min-1. Eluent A: formic acid/water (0.1%, v/v); eluent B: formic acid/acetonitrile (0.1%, v/v). Chromatograms were made using Waters eλ PDA detector (Waters Corporation, Milford, MA, USA). Spectra were analyzed in 200 – 700 nm range with 1.2 nm resolution and sampling rate 20 points/s. MS detection settings of Waters TQD mass spectrometer were as follows: source temperature 150 °C, desolvation temperature 350°C, desolvation gas flow rate 600 L h-1, cone gas flow 100 L h-1, capillary potential 3.00 kV, cone potential 20 V. Nitrogen was used for both nebulizing and drying gas. The data were obtained in a scan mode ranging from 50 to 1000 m/z in time 0.5 s intervals. Collisionally Activated Dissociations (CAD) analyses were carried out with the energy of 30 eV and all the fragmentations were observed in the source. Consequently, the ion spectra were obtained by scanning from 50 to 1000 m/z range. Data acquisition software was MassLynx V 4.1 (Waters Corporation, Milford, MA, USA).
2.3. Synthesis and characterization of the photocatalyst Expanded perlite (EP) coated with TiO2 was prepared by hydrolysis of titanium isopropoxide (TIP) precursor in the presence of EP as described earlier [21]. Briefly, EP was sieved through a 40 mesh screen to select only large floating grains and to facilitate further photocatalyst handling. To separate the floating grains the material was placed in a 2 l beaker filled with water and left for 12 hours. The floating fraction was collected and dried at 333 K overnight. The photocatalyst was synthesized by the hydrolysis of the TIP precursor in the presence of EP. The support was dried under vacuum at 373 K for 2 h directly prior to the TiO2 deposition. Then, 8 g of EP were immersed in the excess of TIP (75 ml) under nitrogen and left to equilibrate for 15 minutes. Distilled water was added dropwise to the vigorously shaken mixture. The EPsupported titanium(IV) oxide (EP-TiO2) was then sieved and rinsed with water. After the hydrolysis the EP-TiO2 grains showed the tendency to aggregate while retaining the ability to float. The aggregates could be broken apart, however, during the sieving procedure. The 2 g samples of EP-TiO2 were dried in the oven at 353 K for 2 h and then calcinated at 773 K for 2 more hours. The obtained photocatalyst sample (EP-TiO2-773) was washed again to remove the excess of TiO2 which was detached during sintering. The final concentration of TiO2 in the final EP-TiO2 composite was 39.4 wt%, as described in our previous work [21]. Before analyses EPTiO2-773 was milled.
2.4. Sulfamethoxazole photodegradation In a typical experiment 100 mg of EP-TiO2-773 photocatalyst was added to 30 ml of SMX solution (c = 0.1 mg/ml) and placed in a quartz beaker. The samples were bubbled with oxygen for half an hour prior and during irradiation. Mixing was achieved by oxygen bubbling through
the reaction mixture. The samples were irradiated for 2 h. The 1.5 ml aliquots of the suspension were subtracted every 10 minutes for an hour and then after another hour. The samples were centrifuged at 18,000 rpm prior to HPLC measurements to separate the solid particles of TiO2 that could detach from EP. The experiments were carried out at pH 1, 5.1, and 10. The pH were adjusted using KCl-HCl buffer, deionized water and carbonate buffer, respectively. To estimate the role of hydroxyl radicals in the SMX photodegradation, the photoreaction was carried out in the presence of 0.5 M isopropanol, a hydroxyl radical scavenger. SMX concentration was determined using HPLC-PDA technique. For the identification of the products 30 ml of 0.1 mg/ml SMX aqueous solution was irradiated in the presence of 300 mg of EP-TiO2-773 for 2 h using six lamps (8 W each) emitting radiation at 350 nm.
2.5. Studies on the SMX adsorption on EP-TiO2-773 100 mg of EP-TiO2-773 was added to 30 ml of 0.1 mg/ml SMX solution. The suspension was shaken for 24 h in dark. The 1.5 ml aliquots were collected, centrifuged, and the SMX concentration was determined using the HPLC-PDA system.
3. Results and discussion 3.1. Structural and spectral properties of SMX at various pH Depending on the pH range the SMX may exist in a cationic, neutral or anionic form (see Scheme 1)
Scheme 1. The SMX structures at different pH values [26]. Fig. 1 shows the electronic absorption spectra of SMX at various pH, the absorption spectrum of EP-TiO2-773 photocatalyst, and the spectral distribution of the light source used for the irradiation. The pH of each solution was adjusted accordingly to ensure the presence in a solution primarily one form of the drug.
Fig. 1. Normalized SMX absorption spectra (c= 0.1 mg/ml) at pH 1, 5.1, and 10, the normalized photocatalyst diffuse reflectance spectrum, and the normalized spectral distribution of the lamp used for irradiations. The SMX absorption spectrum is characterized by a strong band with maximum intensity around 260 nm which originates from the π → π* electron transition [27]. The spectrum is batochromically shifted with decreasing pH (Fig. 1), however, this effect is quite moderate and does not significantly increase the absorption of the light used for irradiation by the cationic form of SMX compared to its anionic form. From the analysis of the plots presented in Fig. 1 one can
conclude that the light used for the irradiations can be efficiently absorbed by the EP-TiO2-773 photocatalyst while its absorption by SMX can be neglected. As found from the comparison of the SEM images of EP-TiO2-773 photocatalyst and EP before coating it was found that the perlite support was covered with TiO2 which resulted in the roughening of the initially smooth perlite surface [21]. The photocatalyst obtained contained 39.40 wt% TiO2 [21]. It can be expected that the photocatalyst can efficiently absorb solar light in the near-UV spectral range. This absorption is intensified by the calcination process which slightly shifts absorption of the photocatalyst to the longer wavelengths, as found in our previous study [21].
3.2. SMX adsorption on EP-TiO2-773 We found that SMX was adsorbed on the surface of the EP-TiO2-773. Some differences are observed between efficiency of that process for various SMX forms present at different pH values (Table 2). The highest amount; 9.2·10-4 mg/mg EP-TiO2-773 (3.1 wt%) of SMX was adsorbed at pH 5.1, about 7.3·10-4 mg/mg EP-TiO2-773 (2.4 wt%) was adsorbed at pH 1, and about 3.6·10-4 mg/mg EP-TiO2-773 (1.2 wt%) was adsorbed at pH 10. These differences can be explained considering the pH effect on the charge of both the EP-TiO2-773 surface and that of SMX molecule. It is known that TiO2 surface charge changes with changing pH. For anatase, that is the main crystalline phase of our photocatalyst [21], the average point zero charge (PZC), that is the pH value at which the surface bears no charge, equals 5.9 [28]. At higher pH the surface charge of anatase becomes negative while for lower pH it is positively charged. As indicated above, in our experiments the highest adsorption is observed for pH 5.1. Under these conditions the SMX molecule is neutral and the surface charge of TiO2 coating the EP beads is positive so there is no Columbic interaction (i.e. neither attraction nor repulsion) between them.
At pH 1 and 10 both the surface of TiO2 and SMX molecules have positive and negative charge, respectively, and repel each other. This limits their adsorption and makes it lower than that observed at pH 5.1. The addition of isopropanol, increasing the hydrophobicity of the solvent significantly decreased SMX adsorption at pH 5.1 to 2.1·10-4 mg/mg EP-TiO2-773 (0.7 w%) (Table 2). That confirms that nonspecific hydrophobic interactions between surface of EP-TiO2773 and SMX serve as a driving force for adsorption of SMX on EP-TiO2-773 surface.
3.3. SMX photocatalytic degradation It was observed that irradiation of SMX + EP-TiO2-773 systems at various pH with light of maximum intensity at λmax = 350 nm, absorbed only by EP-TiO2-773, results in SMX degradation. HPLC analyses of these mixtures carried out after various irradiation times have shown the decrease in the intensity of the peak assigned to SMX and appearance of new peaks, which were ascribed to the products of the photodegradation (see Fig. S1 in Supporting Information). The fastest decrease in the concentration of SMX was observed for the system irradiated at pH 10. For pH 1 and 10 peaks with longer retention times than that of SMX have additionally appeared. This might suggest the formation of products more hydrophobic and/or with higher molecular weight than SMX. The electronic absorption spectra for SMX irradiated in the presence of EP-TiO2-773 for 120 min were recorded (see Fig. S2 in Supporting Information). It was observed that there was no change in SMX absorption when the system was irradiated at pH 1, while irradiation carried out at pH 5.1 resulted in a drop in the intensity of the π-π* band. However, the greatest changes in the spectrum were observed in the system irradiated at pH 10, in which a new absorption band at longer wavelengths appeared with a shoulder at the longer wavelength region, ranging up to 500
nm. The zero-order rate constants and quantum yields of SMX photodegradation in the absence and in the presence of the photocatalyst at pH values of 1, 5.1, and 10 are given in Table 1 [29,30]. They clearly show that the rate constants of SMX photodegradation and respective quantum yields are much higher in the presence of the photocatalyst and grow with increasing pH for the catalyzed systems.
Scheme 2. Proposed pathways of EP-TiO2-773-photocatalyzed degradation of SMX at pH 5.1. The products marked with an asterisk (*) were detected exclusively in the presence of EP-TiO2773, while other products were also detected in direct SMX photolysis.
3.4. SMX photodegradation mechanism There are two possible modes of TiO2 photocatalytic abatement of pollutants in aqueous solutions. The first one involves photogenerated hydroxyl radicals which are the products of the interactions of semiconductor holes with water and allows for a complete mineralization of organic compounds [31]. The second is based on an electron transfer from the electronically excited organic molecule to the conduction band of TiO2. The visible-light-induced photobleaching of dyes in the presence of the semiconductor occurs according to that mechanism [32]. To determine whether that way of action is possible in a studied system the excited state oxidation potential (Eox*) of SMX molecules was calculated according to the equation:
Eox* [V] = Eox - E0-0/e (1)
where Eox is the measured oxidation potential of SMX vs Ag/AgCl electrode (the values for SMX oxidation at various pH were acquired from Arvand et al. [33]) and ∆E0-0 is the zerothzeroth energy of SMX and e is the electron charge. The TiO2 conductance band potential at various pH was calculated according to the equation [34]:
ECB [V] = 0.05 – 0.059 · pH (2)
The obtained results are presented in Table 1. The calculated excited state oxidation potentials of SMX for all pH values are higher than the energy of the conduction band of TiO2 so the electron transfer is thermodynamically possible. Despite meeting these conditions by the system this process is unlikely to occur since it requires efficient adsorption of pollutants on the surface of TiO2, which is not the case for SMX as evidenced by the adsorption experiments (Table 2). Furthermore electron transfer from excited SMX molecule to the TiO2 conduction band is unlikely due to the negligible absorption by SMX of light used for the irradiation of the system (Fig. 1). The above mentioned findings suggest that the hydroxyl radical formation is essential for the degradation of SMX in the system studied. That suggestion was supported by the observation that the addition of a hydroxyl radical quencher such as isopropanol to the SMX + EP-TiO2-773 system inhibited the photocatalyst activity (see Fig. S3 in Supporting Information). Further results allow concluding that the main primary steps in the SMX photodegradation catalyzed by EP-TiO2-773 at pH 5.1 involve isomerization of the oxazole ring and the hydroxylation of the oxazole substituent followed by breaking of the N-O bond which leads to the formation of compounds SP-2 and SP-3 and the hydroxylation of the phenyl ring resulting in formation of products SP-6 and SP-5. Additionally, the products of the hydrolysis of SMX, marked as SP-1, and its dimerization, marked as SP-4, were also observed. Scheme 2 presents the mechanism of the photocatalytic degradation of SMX. The presence of four hydroxylation products: SP-2, SP-3, SP-5, SP-6, and one product requiring prior hydroxylation (SP-4), further supports the mechanism in which hydroxyl radicals are the active species.
Interestingly, that mechanism is similar to the standard metabolism of toxins by living organisms which often oxidize them by means of hydroxylation in Phase I reactions to make them more susceptible to modification in Phase II reactions [35]. Another metabolic pathway is hydrolysis of the compound which occurs also in our system as indicated by formation of SP-1. The previous reports on aerobic biodegradation of SMX by activated sludge also indicate the presence of SP-1 among the metabolites [36]. SMX monohydroxylated (SP-4 and SP-6) and dihydroxylated (SP-2) derivatives, as well as the products of SMX hydrolysis (SP-1) were previously identified by Hu et al. [37] for SMX degradation using TiO2. Their results showed that these derivatives underwent further degradation until complete mineralization which was confirmed by the evolution of sulfate and nitrate anions. Thus, it is reasonable to assume that complete mineralization can be achieved by floating EP-TiO2-773 photocatalyst. The direct photolysis of SMX in the absence of EP-TiO2-773 occurs with the formation of similar products to these formed on irradiation of SMX + EP-TiO2-773. That can be explained considering that SMX in electronically excited states can generate free radicals and singlet oxygen which are electrophiles and might react with SMX with the formation of products similar to these formed in photocatalyzed process, like: SP-1, SP-2, SP-3, SP-5, and SP-7 [38]. Since the photocatalyst generates strongly oxidizing hydroxyl radicals there is a probability that the electrophilic substitution occurs even at these SMX positions which are less prone to this reaction. This results in the formation of two additional oxidation products (SP-4 and SP-6) exclusively in the photocatalytic degradation process. 3.5. Kinetics of reaction of SMX degradation photocatalyzed by EP-TiO2-773
As EP-TiO2-773 is a microheterogeneous photocatalyst the proper description of the kinetics for reaction occurring in such a system is provided by the Langmuir-Hinshelwood model: kC − dC = r dt 1 + KC
(3)
where C is the concentration of substrate, kr is the reaction rate constant and K is the adsorption constant. At the low substrate concentration the term KC can be neglected and the equation can be transformed into pseudo-first order kinetic equation: − ln
C = kt C0
(4)
As this is the case for our experiments therefore pseudo-first order kinetics was used for the determination of all reaction rate constants (see Fig. S4 and S5 in Supporting Information). It was found that EP-TiO2-773 photocatalyst enhanced the photodegradation rate of SMX for all systems, in comparison to the reference system (i.e., without EP-TiO2-773). The rate of reaction is increasing with pH of the medium and it is 6 times higher in alkaline (pH 10) than in acidic medium (pH 1) (Table 2). SMX can also undergo direct photolysis although the rate of that reaction is considerably lower than these determined in the presence of the photocatalyst. Interestingly, the highest rate was observed at pH 5.1 which is important in view of possible practical applications of the EP-TiO2-773 photocatalyst.
One can observe that addition of isopropanol functioning as the hydroxyl radical scavenger at the concentration of 0.5 M resulted in significant decrease in the photodegradation rate of SMX by
EP-TiO2-773. For the system irradiated at pH 5.1 the rate constant dropped from 6.93·10-3 min-1 to 2.89·10-3 min-1 (Fig. S3 in Supporting Information).
3.6. The pH influence on photocatalyzed by EP-TiO2-773 degradation of SMX Since changes in pH effect the ionization potential of SMX, as well as the TiO2 surface properties, TiO2 conduction band potential and the formation of hydroxyl radicals, it is difficult to evaluate the exact contribution of each of these processes. Previous reports on pH-dependent TiO2 photodegradation link the degradation rate with the efficiency of adsorption of different pollutant forms on TiO2, as observed for para-hydroxybenzoic acid [39]. That is not the case for SMX since the pH effect on the adsorption efficiency is rather low in all the experiments described in the current paper (see Table 2). Also, the changes in the reaction rates do not follow the trend observed in adsorption of SMX on EP-TiO2-773. On the contrary, the fastest photodegradation occurs for the least adsorbed anionic form (Table 2). That suggests that the pH-induced changes in the reactivity might be related to the changes in the properties of a SMX molecule rather than to the changes of TiO2 surface. Considering the proposed above hydroxyl radical photodegradation mechanism and taking into account the changes in the SMX structure induced by pH change, the pH effect on SMX photocatalyzed degradation rate can be explained in the terms of electrophilic substitution. At the lowest pH the protonated amino and neutral sulfonamide moieties deactivate the phenyl ring thus inhibiting the reaction. There are also more hydrophobic products along with the hydrophilic ones what indicates that processes other than hydroxylation occur (Fig. S1A in Supporting Information). There are also no changes in the UV-Vis spectrum upon irradiation (Fig. S2A in
Supporting Information) what indicates that no structural change occurs that might disrupt the ππ* conjugated structure. Along with the pH increase up to 5.1, the amino moiety is deprotonated and acts as ortho- and para- activating substituent which increases the rate of the ring hydroxylation. Here the products are more hydrophilic due to the extensive hydroxylation and during HPLC analysis they leave the column earlier (Fig. S1B). The drop in the SMX absorption band at 261 nm assigned to π → π* transition (Fig. S2B) indicates disruption of the conjugated structure what might suggest occurrence of cleavage of phenyl ring, as previously observed for phenol or oxazole substituent (see Scheme 2). Further pH increase up to pH 10 results in the formation of the amide anion and increasing electron density in the proximity of the sulfonamide moiety. That change limits the electron withdrawing effect of sulfonamide moiety which makes the phenyl ring more prone to the hydroxylation. Another factor which should be considered might be the generation of the carbonate radicals from the carbonate anions. As noted by Hu et al. [37] these radicals are weaker oxidizing agents than the hydroxyl radicals and if created at the cost of hydroxyl radicals they should inhibit the reaction. But large concentration of the carbonate buffer allows for a large number of carbonate ions to be present at the surface thus generating large number of carbonate radicals. We suspect that the activated phenyl ring is more prone to substitution so that even weaker oxidizing species such as carbonate radicals might suffice for the reaction to occur. This effect do not exclude the simultaneous participation of photogenerated hydroxyl radicals. The HPLC chromatograms show that, along with hydrophilic products of SMX hydroxylation, there are chromatographic peaks for less hydrophilic products. These peaks are observed at longer retention times which might be related to larger molecules caused by SMX oxidative coupling
(Fig. S1C in Supporting Information). Further evidence of this phenomenon is provided by a significant bathochromic shift in electronic absorption spectrum, most probably due to the extension of the π conjugated system (Fig. S2C in Supporting Information). Similar reactions mediated by hydroxyl radicals were already investigated for aniline and phenol derivatives [40,41] and were shown to be pH-dependent. The reaction rates for SMX photolysis increase with changing pH in the following order: k pH 5.1>k pH 1>k pH 10 (see Table 2). The highest reaction rate for SMX observed for pH 5.1 might be explained considering its ability to directly absorb some fraction of light used for irradiation and by involvement in the reaction triplet states of SMX. The SMX anion present at pH 10 medium has lower than neutral molecule populated triplet states as described by Zhou et al. [38] and its absorbance is blue shifted limiting its absorption of the incident irradiation. These, simultaneously occurring two effects make the SMX anion the least reactive species under the experimental conditions. The reaction rate for pH 1 is also inhibited relatively to that occurring at pH 5.1 owing to protonation of the amino group and even bathochromic shift enhancing the absorption of slightly more incident radiation does not compensate that effect.
4. Conclusions SMX is an antibiotic which is widely spread and persistent in the environment, frequently detected in water resources, and very difficult to eliminate. In the current paper we present the EP-TiO2-773 photocatalyst which can be potentially used for SMX abatement as it markedly enhances SMX photodegradation in the aqueous medium in the wide range of pH values on irradiation with light of low energy (3.2 eV) from near UV spectral region. The mechanism of the photocatalyzed degradation indicates that the hydroxyl radicals are the main players in that
process. The products of SMX hydroxylation and hydrolysis are the primary compounds formed during that process. This is similar to the standard metabolism of living organisms which oxidize toxins to their more hydrophilic derivatives in order to increase their reactivity and to decrease their retention in the body. The spectral/photophysical properties of EP-TiO2-773 combined with its floating ability make it facile for utilization in solar light induced remediation of shallow water reservoirs polluted with SMX.
Supporting Information. Fig. S1. HPLC chromatograms measured before and after the 120 min irradiation for EP-TiO2-773 + SMX at pH (A) 1, (B) 5.1, and (C) 10. Initial concentration of SMX was set at c = 0.1 mg/ml and the concentration of EP-TiO2-773 was 3.33 mg/ml. Fig. S2. UV-Vis spectra of the EP-TiO2-773 + SMX recorded before and after 120 min irradiation at pH (A) 1), (B) 5.1, and (C) 10. Initial concentration of SMX was set at c = 0.1 mg/ml and the concentration of EP-TiO2-773 was 3.33 mg/ml. Fig. S3. First order fit of EP-TiO2-773 photocatalyzed SMX photodegradation in water (pH 5.1) in the absence and in the presence of 0.5 M isopropanol as a hydroxyl radical scavenger. Fig. S4. Pseudo-first order kinetics for EP-TiO2-773 photocatalyzed SMX photodegradation under UV irradiation. Fig. S5. Pseudo-first order kinetics of SMX direct photolysis under UV irradiation Table S1. Products of photodegradation of SMX. Scheme S1. Fragmentation pattern of SMX.
Scheme S2. Fragmentation pattern of SP-2. Scheme S3. Fragmentation pattern of SP-3. Scheme S4. Fragmentation pattern of SP-4. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) Acknowledgement The authors gratefully acknowledge the financial support of the Polish Ministry of Science and Higher Education in the form of the grant No. N N209 144436 and the support within the Fokus Action, Faculty of Chemistry Jagiellonian University, KNOW Action. MD gratefully acknowledges the grant of Ministry of Science and Higher Education, K/DSC/002462.
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Table 1. Zero-order rate constants and quantum yields of SMX photodegradation at different pH values in the absence and in the presence of EP-TiO2-773 pH
kSMX
kEP-TiO2-773+SMX
φSMX
φEP-TiO2-773+SMX
φEP-TiO2-773+SMX - φSMX
1
3.06·10-7
1.36·10-6
0.010
0.044
0.034
5.1
9.92·10-7
2.23·10-6
0.032
0.071
0.039
10
8.53·10-9
4.57·10-6
0.000
0.146
0.146
The identification of the photodegradation products of SMX formed after 2 h of irradiation of the SMX + EP-TiO2-773 system in the aqueous medium (at pH 5.1) was performed based on the results of UPLC/MS analysis, supported with fragmentation patterns obtained from MS/MS experiments. The structures of the stable photodegradation products, confirmed by CAD experiments, are shown in Scheme 2 along with the proposed general mechanism. It generally involved loss of 5-methyloxazole moiety and subsequent degradation of sulfonamide moiety, or loss of 4-aminophenyl moiety and subsequent degradation of the product of this transition. All the products, their retention times, fragmentation ions, and fragmentation patterns are given in Supplementary Information (Table S1 and Schemes S1-S4). Table 2. The reaction rate constants for catalyzed reactions and respective blank runs in different media.
k1×103 pH
-1 a
(min )
k1/k1
pH 1
k2×103 m×104 λmax SMX structure (min-1) b (nm) c (mg/mg) d
+
1
5.1
10
3.83±0.06
6.93±0.15
1
1.8
24.44±2.15 6.4
0.86±0.0 265 0
2.17±0.2 261 2
0.14±0.0 256 6
7.3
9.2
3.6
H3N
H2N
H2N
Eox* [V]
O S NH O N O
-2.97 -0.109
O S NH O N O
-3.10 -0.350
O S N O N
-3.22 -0.640 O
0.5 M isoprop anol 2.89±2.15 pH = 5.1
-
-
-
2.1
a
rate constant for EP-TiO2-773-catalyzed photodegradation
b
rate constant for photoreaction in the absence of the photocatalyst
c
wavelength of the maximum absorption of SMX
d
mass of SMX adsorbed on 1 mg of EP-TiO2-773
ECB [V]
S0304-3894(15)00412-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2015.05.016 HAZMAT 16818
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
27-12-2014 4-5-2015 9-5-2015
˙ Please cite this article as: Maciej Dlugosz, Pawel Zmudzki, Anna Kwiecie´n, Krzysztof Szczubialka, Jan Krzek, Maria Nowakowska, Photocatalytic degradation of sulfamethoxazole in aqueous solution using a floating TiO2-expanded perlite photocatalyst, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2015.05.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photocatalytic degradation of sulfamethoxazole in aqueous solution using a floating TiO2expanded perlite photocatalyst Maciej Długosza, Paweł Żmudzkib, Anna Kwiecieńb, Krzysztof Szczubiałka* a, Jan Krzekb, Maria Nowakowska* a a
Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
b
Faculty of Pharmacy, Jagiellonian University Medical College, Medyczna 9, 30-688 Kraków, Poland *Corresponding Authors Krzysztof Szczubiałka, e-mail: [email protected], tel. +48 12 6632062, fax +48 12 6340515 Maria Nowakowska, e-mail: [email protected], , tel. +48 12 6632250, fax +48 Highlights ► 12 6340515 Highlights ► Sulfamethoxazole was degraded using a floating photocatalyst under UV irradiation ► The photocatalyst was obtained by supporting TiO2 onto expanded perlite ► The mechanism of sulfamethoxazole photodegradation in water was proposed ► The photodegradation rate of sulfamethoxazole is greater at higher pH
ABSTRACT Photocatalytic degradation of an antibiotic, sulfamethoxazole (SMX), in aqueous solution using a novel floating TiO2-expanded perlite photocatalyst (EP-TiO2-773) and radiation from the near UV spectral range was studied. The process is important considering that SMX is known to be a widespread and highly persistent pollutant of water resources. SMX degradation was described using a pseudo-first-order kinetic equation according to the Langmuir-Hinshelwood model. The products of the SMX photocatalytic degradation were identified. The effect of pH on the kinetics and mechanism of SMX photocatalytic degradation was explained.
Keywords: sulfamethoxazole, antibiotic, titanium dioxide, expanded perlite, floating photocatalyst, photocatalytic degradation 1. Introduction The occurrence of pharmaceuticals as well as their metabolites and transformation products in the environment is becoming a matter of a grave concern because they are extensively and increasingly used in human and veterinary medicine and are continuously released into the environment. Antibiotics are a group of emerging persistent pollutants that pose a large threat to the environment and society due to their biological activity. One of the most important negative consequences of the antibiotic pollution is an increased microbial resistance [1]. The problem is severe inter alia since microbial populations are capable of horizontal gen transfer of the genes favoring survival [2]. These resistance genes are often persistent and are not eliminated, even in the absence of antibiotic pressure [3]. Sulfonamides are the oldest group of antibiotics used in human and veterinary medicine. They are polar, amphoteric, photo- and thermally stable substances that are readily soluble in water [4]. For this reason, they possess high migration ability in the environment [5]. The most commonly employed of this group of antibiotics is sulfamethoxazole (SMX). It is extensively used in human and veterinary medicine. It is particularly often used in bronchitis and in the treatment of urinary tract infections as well as growth promoter [6]. Advanced oxidation processes (AOPs) have received great interest in recent years as methods complementary to conventional water treatment or as alternative treatment strategies for industrial wastewater prior to discharge into sewage or into aquatic environments. Ozonation, Fenton and photo-Fenton oxidation, photolysis and H2O2-enhanced photolysis, heterogeneous photocatalysis, and electrochemical oxidation were frequently studied AOPs [7,8,9,10,11].
One of the AOP methods that can be useful for the antibiotic oxidation is microheterogeneous semiconductor photocatalysis. It employs photoinduced charge separation of electrons and holes, which can react with water or oxygen to form reactive oxygen species (ROS), predominantly hydroxyl radicals. High oxidizing potential of hydroxyl radicals (2.80 V) allows mineralizing most of the low molecular weight organic molecules. The reports on application of this method for abatement of pharmaceuticals such as amoxicillin, ampicillin, cloxacillin [12], sulfamethoxazole [13], and tetracycline [14] confirm efficacy of this method. For all these experiments a significant reduction in total organic carbon (TOC) was confirmed what is considered to be essential parameter in view of the potential application of a method. Unfortunately even the finest TiO2 powders tend to sediment after some time resulting in a decrease of the amount of absorbed light and in the loss of efficacy and efficiency. To overcome this drawback TiO2 powders can be immobilized on various surfaces such as glass plates [15] or silica gel beads [16] to be further used in tubular photoreactors. They can be also immobilized on floating supports and used in floating bed photoreactors [17]. The floating particles are localized at the air/water interface where the solar light has the highest intensity. Moreover, the floating particles can be easily collected from the surface. For this purpose materials such as cenospheres [18], polystyrene beads [19] or expanded perlite [20,21] were already used. The direct [22,23] and photocatalyzed by TiO2 [13,24] photodegradation of SMX has been already studied and the photoproducts were identified [13,25]. It was found that the TiO2photocatalyzed degradation of SMX results in the formation of products with lower toxicity which are biodegradable, in contrast to parent SMX [24]. This paper presents SMX photocatalytic degradation studies employing a novel floating photocatalyst composed of TiO2 immobilized on expanded perlite (EP). The application of TiO2
supported on the floating material for SMX degradation has not been described yet. Kinetics of the process at different pH values were determined and photodegradation products at pH 5.1 were identified based on UPLC-MS/MS analyses. We have found the formation of different photoproducts for SMX degradation using this photocatalyst than those described previously [13]. The results indicate that the process has the potential to be applied for facile removal of antibiotic pollutants from shallow water reservoirs. Taking into account the ability of the studied photocatalyst to float and consequently the possibility of its multiple use [21], its application is expected to be more economical than that of TiO2 in suspension.
2. Experimental 2.1. Materials Titanium(IV) isopropoxide (TIP, 97%), sulfamethoxazole (SMX, analytical grade), phosphoric acid (≥85%), acetonitrile (HPLC grade) were supplied by Sigma-Aldrich. Isopropanol, HCl, KCl, Na2CO3, NaHCO3, acetic acid (all analytical grade) were supplied by POCh (Gliwice, Poland). The sample of expanded perlite (EP) was provided by Zębiec sp. z o.o. (Zębiec, Poland). Deionized water was used in all experiments.
2.2. Apparatus The UV-Vis absorption spectra of SMX solutions were collected using Varian Cary 50 UV-Vis spectrophotometer (Varian Inc., Palo Alto, CA, USA). The samples were irradiated using the Rayonet RPR-100 photoreactor (Southern New England Ultraviolet Company, Branford, CT, USA) equipped with six (8 W each) lamps emitting radiation in the range of 316-400 nm with maximum intensity at 350 nm. The HPLC-PDA system consisted of a Waters (Waters
Corporation, Milford, MA, USA) 515 HPLC pump and a Waters 2996 Photodiode Array detector (Waters Corporation, Milford, MA, USA). Waters Symmetry C18, 5 µm, 3.9×150 mm column was used which was maintained at room temperature. The eluent was acetonitrile:water (35:65 v/v) for pH = 1 and 5.1 samples and acetonitrile:1% acetic acid (35:65) for pH = 10 samples. The flow rate was set at 0.7 ml/min. The detection wavelength was set at 270 nm. The products of SMX photodegradation were identified using UPLC-MS/MS technique. The UPLCMS/MS system consisted of a Waters ACQUITY UPLC (Waters Corporation, Milford, MA, USA) coupled to a Waters TQD mass spectrometer (electrospray ionization mode ESI-tandem quadrupole). Chromatographic separations were carried out using the Acquity UPLC BEH C18 column; 2.1 × 100 mm, and 1.7 µm particle size, equipped with Acquity UPLC BEH C18 VanGuard pre-column; 2.1 × 5 mm, and 1.7 µm particle size. The column was maintained at 40°C, and eluted under gradient conditions from 95% to 0% of eluent A over 10 min, at a flow rate of 0.3 mL min-1. Eluent A: formic acid/water (0.1%, v/v); eluent B: formic acid/acetonitrile (0.1%, v/v). Chromatograms were made using Waters eλ PDA detector (Waters Corporation, Milford, MA, USA). Spectra were analyzed in 200 – 700 nm range with 1.2 nm resolution and sampling rate 20 points/s. MS detection settings of Waters TQD mass spectrometer were as follows: source temperature 150 °C, desolvation temperature 350°C, desolvation gas flow rate 600 L h-1, cone gas flow 100 L h-1, capillary potential 3.00 kV, cone potential 20 V. Nitrogen was used for both nebulizing and drying gas. The data were obtained in a scan mode ranging from 50 to 1000 m/z in time 0.5 s intervals. Collisionally Activated Dissociations (CAD) analyses were carried out with the energy of 30 eV and all the fragmentations were observed in the source. Consequently, the ion spectra were obtained by scanning from 50 to 1000 m/z range. Data acquisition software was MassLynx V 4.1 (Waters Corporation, Milford, MA, USA).
2.3. Synthesis and characterization of the photocatalyst Expanded perlite (EP) coated with TiO2 was prepared by hydrolysis of titanium isopropoxide (TIP) precursor in the presence of EP as described earlier [21]. Briefly, EP was sieved through a 40 mesh screen to select only large floating grains and to facilitate further photocatalyst handling. To separate the floating grains the material was placed in a 2 l beaker filled with water and left for 12 hours. The floating fraction was collected and dried at 333 K overnight. The photocatalyst was synthesized by the hydrolysis of the TIP precursor in the presence of EP. The support was dried under vacuum at 373 K for 2 h directly prior to the TiO2 deposition. Then, 8 g of EP were immersed in the excess of TIP (75 ml) under nitrogen and left to equilibrate for 15 minutes. Distilled water was added dropwise to the vigorously shaken mixture. The EPsupported titanium(IV) oxide (EP-TiO2) was then sieved and rinsed with water. After the hydrolysis the EP-TiO2 grains showed the tendency to aggregate while retaining the ability to float. The aggregates could be broken apart, however, during the sieving procedure. The 2 g samples of EP-TiO2 were dried in the oven at 353 K for 2 h and then calcinated at 773 K for 2 more hours. The obtained photocatalyst sample (EP-TiO2-773) was washed again to remove the excess of TiO2 which was detached during sintering. The final concentration of TiO2 in the final EP-TiO2 composite was 39.4 wt%, as described in our previous work [21]. Before analyses EPTiO2-773 was milled.
2.4. Sulfamethoxazole photodegradation In a typical experiment 100 mg of EP-TiO2-773 photocatalyst was added to 30 ml of SMX solution (c = 0.1 mg/ml) and placed in a quartz beaker. The samples were bubbled with oxygen for half an hour prior and during irradiation. Mixing was achieved by oxygen bubbling through
the reaction mixture. The samples were irradiated for 2 h. The 1.5 ml aliquots of the suspension were subtracted every 10 minutes for an hour and then after another hour. The samples were centrifuged at 18,000 rpm prior to HPLC measurements to separate the solid particles of TiO2 that could detach from EP. The experiments were carried out at pH 1, 5.1, and 10. The pH were adjusted using KCl-HCl buffer, deionized water and carbonate buffer, respectively. To estimate the role of hydroxyl radicals in the SMX photodegradation, the photoreaction was carried out in the presence of 0.5 M isopropanol, a hydroxyl radical scavenger. SMX concentration was determined using HPLC-PDA technique. For the identification of the products 30 ml of 0.1 mg/ml SMX aqueous solution was irradiated in the presence of 300 mg of EP-TiO2-773 for 2 h using six lamps (8 W each) emitting radiation at 350 nm.
2.5. Studies on the SMX adsorption on EP-TiO2-773 100 mg of EP-TiO2-773 was added to 30 ml of 0.1 mg/ml SMX solution. The suspension was shaken for 24 h in dark. The 1.5 ml aliquots were collected, centrifuged, and the SMX concentration was determined using the HPLC-PDA system.
3. Results and discussion 3.1. Structural and spectral properties of SMX at various pH Depending on the pH range the SMX may exist in a cationic, neutral or anionic form (see Scheme 1)
Scheme 1. The SMX structures at different pH values [26]. Fig. 1 shows the electronic absorption spectra of SMX at various pH, the absorption spectrum of EP-TiO2-773 photocatalyst, and the spectral distribution of the light source used for the irradiation. The pH of each solution was adjusted accordingly to ensure the presence in a solution primarily one form of the drug.
Fig. 1. Normalized SMX absorption spectra (c= 0.1 mg/ml) at pH 1, 5.1, and 10, the normalized photocatalyst diffuse reflectance spectrum, and the normalized spectral distribution of the lamp used for irradiations. The SMX absorption spectrum is characterized by a strong band with maximum intensity around 260 nm which originates from the π → π* electron transition [27]. The spectrum is batochromically shifted with decreasing pH (Fig. 1), however, this effect is quite moderate and does not significantly increase the absorption of the light used for irradiation by the cationic form of SMX compared to its anionic form. From the analysis of the plots presented in Fig. 1 one can
conclude that the light used for the irradiations can be efficiently absorbed by the EP-TiO2-773 photocatalyst while its absorption by SMX can be neglected. As found from the comparison of the SEM images of EP-TiO2-773 photocatalyst and EP before coating it was found that the perlite support was covered with TiO2 which resulted in the roughening of the initially smooth perlite surface [21]. The photocatalyst obtained contained 39.40 wt% TiO2 [21]. It can be expected that the photocatalyst can efficiently absorb solar light in the near-UV spectral range. This absorption is intensified by the calcination process which slightly shifts absorption of the photocatalyst to the longer wavelengths, as found in our previous study [21].
3.2. SMX adsorption on EP-TiO2-773 We found that SMX was adsorbed on the surface of the EP-TiO2-773. Some differences are observed between efficiency of that process for various SMX forms present at different pH values (Table 2). The highest amount; 9.2·10-4 mg/mg EP-TiO2-773 (3.1 wt%) of SMX was adsorbed at pH 5.1, about 7.3·10-4 mg/mg EP-TiO2-773 (2.4 wt%) was adsorbed at pH 1, and about 3.6·10-4 mg/mg EP-TiO2-773 (1.2 wt%) was adsorbed at pH 10. These differences can be explained considering the pH effect on the charge of both the EP-TiO2-773 surface and that of SMX molecule. It is known that TiO2 surface charge changes with changing pH. For anatase, that is the main crystalline phase of our photocatalyst [21], the average point zero charge (PZC), that is the pH value at which the surface bears no charge, equals 5.9 [28]. At higher pH the surface charge of anatase becomes negative while for lower pH it is positively charged. As indicated above, in our experiments the highest adsorption is observed for pH 5.1. Under these conditions the SMX molecule is neutral and the surface charge of TiO2 coating the EP beads is positive so there is no Columbic interaction (i.e. neither attraction nor repulsion) between them.
At pH 1 and 10 both the surface of TiO2 and SMX molecules have positive and negative charge, respectively, and repel each other. This limits their adsorption and makes it lower than that observed at pH 5.1. The addition of isopropanol, increasing the hydrophobicity of the solvent significantly decreased SMX adsorption at pH 5.1 to 2.1·10-4 mg/mg EP-TiO2-773 (0.7 w%) (Table 2). That confirms that nonspecific hydrophobic interactions between surface of EP-TiO2773 and SMX serve as a driving force for adsorption of SMX on EP-TiO2-773 surface.
3.3. SMX photocatalytic degradation It was observed that irradiation of SMX + EP-TiO2-773 systems at various pH with light of maximum intensity at λmax = 350 nm, absorbed only by EP-TiO2-773, results in SMX degradation. HPLC analyses of these mixtures carried out after various irradiation times have shown the decrease in the intensity of the peak assigned to SMX and appearance of new peaks, which were ascribed to the products of the photodegradation (see Fig. S1 in Supporting Information). The fastest decrease in the concentration of SMX was observed for the system irradiated at pH 10. For pH 1 and 10 peaks with longer retention times than that of SMX have additionally appeared. This might suggest the formation of products more hydrophobic and/or with higher molecular weight than SMX. The electronic absorption spectra for SMX irradiated in the presence of EP-TiO2-773 for 120 min were recorded (see Fig. S2 in Supporting Information). It was observed that there was no change in SMX absorption when the system was irradiated at pH 1, while irradiation carried out at pH 5.1 resulted in a drop in the intensity of the π-π* band. However, the greatest changes in the spectrum were observed in the system irradiated at pH 10, in which a new absorption band at longer wavelengths appeared with a shoulder at the longer wavelength region, ranging up to 500
nm. The zero-order rate constants and quantum yields of SMX photodegradation in the absence and in the presence of the photocatalyst at pH values of 1, 5.1, and 10 are given in Table 1 [29,30]. They clearly show that the rate constants of SMX photodegradation and respective quantum yields are much higher in the presence of the photocatalyst and grow with increasing pH for the catalyzed systems.
Scheme 2. Proposed pathways of EP-TiO2-773-photocatalyzed degradation of SMX at pH 5.1. The products marked with an asterisk (*) were detected exclusively in the presence of EP-TiO2773, while other products were also detected in direct SMX photolysis.
3.4. SMX photodegradation mechanism There are two possible modes of TiO2 photocatalytic abatement of pollutants in aqueous solutions. The first one involves photogenerated hydroxyl radicals which are the products of the interactions of semiconductor holes with water and allows for a complete mineralization of organic compounds [31]. The second is based on an electron transfer from the electronically excited organic molecule to the conduction band of TiO2. The visible-light-induced photobleaching of dyes in the presence of the semiconductor occurs according to that mechanism [32]. To determine whether that way of action is possible in a studied system the excited state oxidation potential (Eox*) of SMX molecules was calculated according to the equation:
Eox* [V] = Eox - E0-0/e (1)
where Eox is the measured oxidation potential of SMX vs Ag/AgCl electrode (the values for SMX oxidation at various pH were acquired from Arvand et al. [33]) and ∆E0-0 is the zerothzeroth energy of SMX and e is the electron charge. The TiO2 conductance band potential at various pH was calculated according to the equation [34]:
ECB [V] = 0.05 – 0.059 · pH (2)
The obtained results are presented in Table 1. The calculated excited state oxidation potentials of SMX for all pH values are higher than the energy of the conduction band of TiO2 so the electron transfer is thermodynamically possible. Despite meeting these conditions by the system this process is unlikely to occur since it requires efficient adsorption of pollutants on the surface of TiO2, which is not the case for SMX as evidenced by the adsorption experiments (Table 2). Furthermore electron transfer from excited SMX molecule to the TiO2 conduction band is unlikely due to the negligible absorption by SMX of light used for the irradiation of the system (Fig. 1). The above mentioned findings suggest that the hydroxyl radical formation is essential for the degradation of SMX in the system studied. That suggestion was supported by the observation that the addition of a hydroxyl radical quencher such as isopropanol to the SMX + EP-TiO2-773 system inhibited the photocatalyst activity (see Fig. S3 in Supporting Information). Further results allow concluding that the main primary steps in the SMX photodegradation catalyzed by EP-TiO2-773 at pH 5.1 involve isomerization of the oxazole ring and the hydroxylation of the oxazole substituent followed by breaking of the N-O bond which leads to the formation of compounds SP-2 and SP-3 and the hydroxylation of the phenyl ring resulting in formation of products SP-6 and SP-5. Additionally, the products of the hydrolysis of SMX, marked as SP-1, and its dimerization, marked as SP-4, were also observed. Scheme 2 presents the mechanism of the photocatalytic degradation of SMX. The presence of four hydroxylation products: SP-2, SP-3, SP-5, SP-6, and one product requiring prior hydroxylation (SP-4), further supports the mechanism in which hydroxyl radicals are the active species.
Interestingly, that mechanism is similar to the standard metabolism of toxins by living organisms which often oxidize them by means of hydroxylation in Phase I reactions to make them more susceptible to modification in Phase II reactions [35]. Another metabolic pathway is hydrolysis of the compound which occurs also in our system as indicated by formation of SP-1. The previous reports on aerobic biodegradation of SMX by activated sludge also indicate the presence of SP-1 among the metabolites [36]. SMX monohydroxylated (SP-4 and SP-6) and dihydroxylated (SP-2) derivatives, as well as the products of SMX hydrolysis (SP-1) were previously identified by Hu et al. [37] for SMX degradation using TiO2. Their results showed that these derivatives underwent further degradation until complete mineralization which was confirmed by the evolution of sulfate and nitrate anions. Thus, it is reasonable to assume that complete mineralization can be achieved by floating EP-TiO2-773 photocatalyst. The direct photolysis of SMX in the absence of EP-TiO2-773 occurs with the formation of similar products to these formed on irradiation of SMX + EP-TiO2-773. That can be explained considering that SMX in electronically excited states can generate free radicals and singlet oxygen which are electrophiles and might react with SMX with the formation of products similar to these formed in photocatalyzed process, like: SP-1, SP-2, SP-3, SP-5, and SP-7 [38]. Since the photocatalyst generates strongly oxidizing hydroxyl radicals there is a probability that the electrophilic substitution occurs even at these SMX positions which are less prone to this reaction. This results in the formation of two additional oxidation products (SP-4 and SP-6) exclusively in the photocatalytic degradation process. 3.5. Kinetics of reaction of SMX degradation photocatalyzed by EP-TiO2-773
As EP-TiO2-773 is a microheterogeneous photocatalyst the proper description of the kinetics for reaction occurring in such a system is provided by the Langmuir-Hinshelwood model: kC − dC = r dt 1 + KC
(3)
where C is the concentration of substrate, kr is the reaction rate constant and K is the adsorption constant. At the low substrate concentration the term KC can be neglected and the equation can be transformed into pseudo-first order kinetic equation: − ln
C = kt C0
(4)
As this is the case for our experiments therefore pseudo-first order kinetics was used for the determination of all reaction rate constants (see Fig. S4 and S5 in Supporting Information). It was found that EP-TiO2-773 photocatalyst enhanced the photodegradation rate of SMX for all systems, in comparison to the reference system (i.e., without EP-TiO2-773). The rate of reaction is increasing with pH of the medium and it is 6 times higher in alkaline (pH 10) than in acidic medium (pH 1) (Table 2). SMX can also undergo direct photolysis although the rate of that reaction is considerably lower than these determined in the presence of the photocatalyst. Interestingly, the highest rate was observed at pH 5.1 which is important in view of possible practical applications of the EP-TiO2-773 photocatalyst.
One can observe that addition of isopropanol functioning as the hydroxyl radical scavenger at the concentration of 0.5 M resulted in significant decrease in the photodegradation rate of SMX by
EP-TiO2-773. For the system irradiated at pH 5.1 the rate constant dropped from 6.93·10-3 min-1 to 2.89·10-3 min-1 (Fig. S3 in Supporting Information).
3.6. The pH influence on photocatalyzed by EP-TiO2-773 degradation of SMX Since changes in pH effect the ionization potential of SMX, as well as the TiO2 surface properties, TiO2 conduction band potential and the formation of hydroxyl radicals, it is difficult to evaluate the exact contribution of each of these processes. Previous reports on pH-dependent TiO2 photodegradation link the degradation rate with the efficiency of adsorption of different pollutant forms on TiO2, as observed for para-hydroxybenzoic acid [39]. That is not the case for SMX since the pH effect on the adsorption efficiency is rather low in all the experiments described in the current paper (see Table 2). Also, the changes in the reaction rates do not follow the trend observed in adsorption of SMX on EP-TiO2-773. On the contrary, the fastest photodegradation occurs for the least adsorbed anionic form (Table 2). That suggests that the pH-induced changes in the reactivity might be related to the changes in the properties of a SMX molecule rather than to the changes of TiO2 surface. Considering the proposed above hydroxyl radical photodegradation mechanism and taking into account the changes in the SMX structure induced by pH change, the pH effect on SMX photocatalyzed degradation rate can be explained in the terms of electrophilic substitution. At the lowest pH the protonated amino and neutral sulfonamide moieties deactivate the phenyl ring thus inhibiting the reaction. There are also more hydrophobic products along with the hydrophilic ones what indicates that processes other than hydroxylation occur (Fig. S1A in Supporting Information). There are also no changes in the UV-Vis spectrum upon irradiation (Fig. S2A in
Supporting Information) what indicates that no structural change occurs that might disrupt the ππ* conjugated structure. Along with the pH increase up to 5.1, the amino moiety is deprotonated and acts as ortho- and para- activating substituent which increases the rate of the ring hydroxylation. Here the products are more hydrophilic due to the extensive hydroxylation and during HPLC analysis they leave the column earlier (Fig. S1B). The drop in the SMX absorption band at 261 nm assigned to π → π* transition (Fig. S2B) indicates disruption of the conjugated structure what might suggest occurrence of cleavage of phenyl ring, as previously observed for phenol or oxazole substituent (see Scheme 2). Further pH increase up to pH 10 results in the formation of the amide anion and increasing electron density in the proximity of the sulfonamide moiety. That change limits the electron withdrawing effect of sulfonamide moiety which makes the phenyl ring more prone to the hydroxylation. Another factor which should be considered might be the generation of the carbonate radicals from the carbonate anions. As noted by Hu et al. [37] these radicals are weaker oxidizing agents than the hydroxyl radicals and if created at the cost of hydroxyl radicals they should inhibit the reaction. But large concentration of the carbonate buffer allows for a large number of carbonate ions to be present at the surface thus generating large number of carbonate radicals. We suspect that the activated phenyl ring is more prone to substitution so that even weaker oxidizing species such as carbonate radicals might suffice for the reaction to occur. This effect do not exclude the simultaneous participation of photogenerated hydroxyl radicals. The HPLC chromatograms show that, along with hydrophilic products of SMX hydroxylation, there are chromatographic peaks for less hydrophilic products. These peaks are observed at longer retention times which might be related to larger molecules caused by SMX oxidative coupling
(Fig. S1C in Supporting Information). Further evidence of this phenomenon is provided by a significant bathochromic shift in electronic absorption spectrum, most probably due to the extension of the π conjugated system (Fig. S2C in Supporting Information). Similar reactions mediated by hydroxyl radicals were already investigated for aniline and phenol derivatives [40,41] and were shown to be pH-dependent. The reaction rates for SMX photolysis increase with changing pH in the following order: k pH 5.1>k pH 1>k pH 10 (see Table 2). The highest reaction rate for SMX observed for pH 5.1 might be explained considering its ability to directly absorb some fraction of light used for irradiation and by involvement in the reaction triplet states of SMX. The SMX anion present at pH 10 medium has lower than neutral molecule populated triplet states as described by Zhou et al. [38] and its absorbance is blue shifted limiting its absorption of the incident irradiation. These, simultaneously occurring two effects make the SMX anion the least reactive species under the experimental conditions. The reaction rate for pH 1 is also inhibited relatively to that occurring at pH 5.1 owing to protonation of the amino group and even bathochromic shift enhancing the absorption of slightly more incident radiation does not compensate that effect.
4. Conclusions SMX is an antibiotic which is widely spread and persistent in the environment, frequently detected in water resources, and very difficult to eliminate. In the current paper we present the EP-TiO2-773 photocatalyst which can be potentially used for SMX abatement as it markedly enhances SMX photodegradation in the aqueous medium in the wide range of pH values on irradiation with light of low energy (3.2 eV) from near UV spectral region. The mechanism of the photocatalyzed degradation indicates that the hydroxyl radicals are the main players in that
process. The products of SMX hydroxylation and hydrolysis are the primary compounds formed during that process. This is similar to the standard metabolism of living organisms which oxidize toxins to their more hydrophilic derivatives in order to increase their reactivity and to decrease their retention in the body. The spectral/photophysical properties of EP-TiO2-773 combined with its floating ability make it facile for utilization in solar light induced remediation of shallow water reservoirs polluted with SMX.
Supporting Information. Fig. S1. HPLC chromatograms measured before and after the 120 min irradiation for EP-TiO2-773 + SMX at pH (A) 1, (B) 5.1, and (C) 10. Initial concentration of SMX was set at c = 0.1 mg/ml and the concentration of EP-TiO2-773 was 3.33 mg/ml. Fig. S2. UV-Vis spectra of the EP-TiO2-773 + SMX recorded before and after 120 min irradiation at pH (A) 1), (B) 5.1, and (C) 10. Initial concentration of SMX was set at c = 0.1 mg/ml and the concentration of EP-TiO2-773 was 3.33 mg/ml. Fig. S3. First order fit of EP-TiO2-773 photocatalyzed SMX photodegradation in water (pH 5.1) in the absence and in the presence of 0.5 M isopropanol as a hydroxyl radical scavenger. Fig. S4. Pseudo-first order kinetics for EP-TiO2-773 photocatalyzed SMX photodegradation under UV irradiation. Fig. S5. Pseudo-first order kinetics of SMX direct photolysis under UV irradiation Table S1. Products of photodegradation of SMX. Scheme S1. Fragmentation pattern of SMX.
Scheme S2. Fragmentation pattern of SP-2. Scheme S3. Fragmentation pattern of SP-3. Scheme S4. Fragmentation pattern of SP-4. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) Acknowledgement The authors gratefully acknowledge the financial support of the Polish Ministry of Science and Higher Education in the form of the grant No. N N209 144436 and the support within the Fokus Action, Faculty of Chemistry Jagiellonian University, KNOW Action. MD gratefully acknowledges the grant of Ministry of Science and Higher Education, K/DSC/002462.
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Table 1. Zero-order rate constants and quantum yields of SMX photodegradation at different pH values in the absence and in the presence of EP-TiO2-773 pH
kSMX
kEP-TiO2-773+SMX
φSMX
φEP-TiO2-773+SMX
φEP-TiO2-773+SMX - φSMX
1
3.06·10-7
1.36·10-6
0.010
0.044
0.034
5.1
9.92·10-7
2.23·10-6
0.032
0.071
0.039
10
8.53·10-9
4.57·10-6
0.000
0.146
0.146
The identification of the photodegradation products of SMX formed after 2 h of irradiation of the SMX + EP-TiO2-773 system in the aqueous medium (at pH 5.1) was performed based on the results of UPLC/MS analysis, supported with fragmentation patterns obtained from MS/MS experiments. The structures of the stable photodegradation products, confirmed by CAD experiments, are shown in Scheme 2 along with the proposed general mechanism. It generally involved loss of 5-methyloxazole moiety and subsequent degradation of sulfonamide moiety, or loss of 4-aminophenyl moiety and subsequent degradation of the product of this transition. All the products, their retention times, fragmentation ions, and fragmentation patterns are given in Supplementary Information (Table S1 and Schemes S1-S4). Table 2. The reaction rate constants for catalyzed reactions and respective blank runs in different media.
k1×103 pH
-1 a
(min )
k1/k1
pH 1
k2×103 m×104 λmax SMX structure (min-1) b (nm) c (mg/mg) d
+
1
5.1
10
3.83±0.06
6.93±0.15
1
1.8
24.44±2.15 6.4
0.86±0.0 265 0
2.17±0.2 261 2
0.14±0.0 256 6
7.3
9.2
3.6
H3N
H2N
H2N
Eox* [V]
O S NH O N O
-2.97 -0.109
O S NH O N O
-3.10 -0.350
O S N O N
-3.22 -0.640 O
0.5 M isoprop anol 2.89±2.15 pH = 5.1
-
-
-
2.1
a
rate constant for EP-TiO2-773-catalyzed photodegradation
b
rate constant for photoreaction in the absence of the photocatalyst
c
wavelength of the maximum absorption of SMX
d
mass of SMX adsorbed on 1 mg of EP-TiO2-773
ECB [V]
