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The effect of pH, nitrate, iron (III) and bicarbonate on photodegradation of oxytetracycline in aqueous solution
Journal of Photochemistry and Photobiology A: Chemistry 356 (2018) 239–247
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
The effect of pH, nitrate, iron (III) and bicarbonate on photodegradation of oxytetracycline in aqueous solution Chanjuan Lia , Dahai Zhanga , Jialin Pengb , Xianguo Lia,* a b
Key Laboratory of Marine Chemistry Theory and Technology (Ocean University of China), Ministry of Education, Qingdao 266100, China Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
A R T I C L E I N F O
Article history: Received 13 October 2017 Received in revised form 2 January 2018 Accepted 3 January 2018 Available online 3 January 2018 Keywords: Oxytetracycline Photodegradation pH Comprehensive effect Simulated sunlight
A B S T R A C T
Photodegradation is a very important elimination pathway of Oxytetracycline (OTC) in aquatic environments. The photochemical behavior of OTC in absence/presence of nitrate, iron (III) and bicarbonate, especially their combinations, was systematically studied in aqueous solution by employing a Suntest-CPS+ sunlight simulator in this work. Meanwhile, the effect of solution pH (4.8–9.1) and irradiation intensity (250–500 W/m2) on the removal efficiency of OTC have also been assessed. The results showed that OTC degradation followed a pseudo-first-order kinetics in most conditions except for irradiation in neutral/alkaline solutions and in presence of NO3, where a two-step pseudo-first-order kinetics was obeyed. OTC degradation was highly pH-dependent and increased significantly with increasing pH. The presence of NO3 and HCO3 promoted the photochemical loss of OTC in aqueous solution by generating hydroxyl radicals and adjusting solution pH, respectively; whereas iron (III) was found to play a negative role under the conditions studied. The influence of NO3 coupled with Fe3+ was not a simple additive effect and the overall performance was to inhibit OTC degradation. In addition, combination of HCO3 and NO3 exhibited an antagonistic effect. Results from irradiation in co-presence of NO3, Fe3+ and HCO3 suggested that direct photolysis of OTC was much more efficient than its indirect photolysis. These results provide a meaningful reference for understanding the fate and transformation of OTC in natural water systems. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Antibiotics, a class of pharmaceutical and personal care products(PPCPs), are produced and consumed in great quantities worldwide, and are becoming the focus of attention because of the issue of antibiotics-resistant bacteria and genes. China, as the largest producer and user of antibiotics all around the world, consumed 92,700 tons of 36 frequently used antibiotics, with about 53,800 tons released into the general environment in 2013 [1,2]. Among those antibiotics, oxytetracycline (OTC) is an important member of tetracyclines (TCs) that is extensively used for treating human and animal diseases and applied as a feed additive for promoting animal growth in a variety of aquacultures [3]. The applied TCs are poorly ingested by organisms, and the majority is excreted in faeces and urine without metabolism. TCs are therefore widely detected in various environmental components, such as surface waters and sewage water [4,5]. Zou et al.
* Corresponding author. E-mail addresses: [email protected] (C. Li), [email protected] (X. Li). https://doi.org/10.1016/j.jphotochem.2018.01.004 1010-6030/© 2018 Elsevier B.V. All rights reserved.
reported that the concentration of OTC in Bohai Bay was up to 0.27 mg/L [6]; while OTC has been detected in very high concentrations ranging from 20 to 800 mg/L in one of the biggest OTC producer’s wastewater treatment plant in China [7]. Conventional water treatment methods, such as filtration, coagulation, flocculation and biodegradation showed very poor efficiencies to remove OTC due to its chemical stability and bioresistance [8]. The extensive usage and frequent detection of antibiotics poses a great threat to the general ecosystem and human health through the development of antibiotics-resistant bacteria and pathogens [9]. For example, OTC has been involved in the aetiology of several diseases, such as hypouricemia and hypokalemia [10]. It can also exhibit significantly toxic effects to algae, which as primary producers play a key role in the whole aquatic ecosystem [11]. Aquatic environment is vital to the survival of humans, animals and plants, and waters are important media for the occurrence of OTC. Therefore, it is meaningful to assess the behavior and fate of OTC in aquatic environments. Photochemical degradation is an important pathway of OTC elimination in surface waters [3,12,13]. In recent years, photodegradation of OTC using advanced oxidation processes (AOPs),
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such as Photo-Fenton oxidation, UV/TiO2, UV/MgAl calcined hydrotalcites, UV/H2O2, UV/Persulfate (PS), has been extensively studied by several groups due to the efficient formation of strongly oxidative radicals to highly effectively remove OTC [9,14–18]; whereas researches related to the environmental fate and transformation of OTC were quite sparse. Psoralen and Riboflavin, used as phototherapy drugs, have been reported that they could act as photosensitizers themselves [19,20]. Upon UV or visible light irradiation, their triplet excited state can react with oxygen to generate reactive oxygen species (ROSs, such as 1O2), which is a central intermediate for cytotoxic action in potential photodynamic therapy. The common pathways involved in the phototransformation of OTC in surface waters were direct photolysis [13,21], indirect photolysis with ROSs (such as HO, 1O2, O2)[10] and even self-sensitized degradation[22]. These photochemical processes can be substantially impacted by complex aquatic constituents, such as humic acids (HA), Fe3+, NO3, HCO3, Cl and SO42 etc., which can act as either sensitizers to enhance the indirect photodegradation or as irradiation filters and/or HO scavengers to inhibit photolysis of organic compounds. Among these common constituents, Fe3+, NO3 and HCO3 are ubiquitous in natural aquatic environments and previous studies have shown that they play an important role in the photodegradation of organic pollutants [23–26]. However, most of these studies were focused on the individual effect of each component on photolysis, and their interaction on photodegradation of OTC is poorly understood. Therefore, the underlying mechanism for the comprehensive effects of nitrate, ferric ion and bicarbonate needs to be further clarified. OTC is an amphoteric molecule which consists of a 4-ring system with multiple function groups such as tertiary amino- and hydroxyl- groups (SI, Fig.S1). Solution pH, as one of the important factors for photodegradation of organic pollutants [27,28], could simultaneously affect the speciation and structure as well as degradation mechanism of OTC. pH may have different effects on OTC photodegradation under different operating conditions, such as light source, temperature, presence/absence of oxygen. For example, Liu et al. suggested that OTC photodegradation followed a pseudo-first-order kinetics at various pH values from 3.0 to 11.0 in UV-C system and UV/H2O2 system [29]; Jin et al. reported that OTC photolysis followed a second-order kinetics in oxygen-free condition and pointed out that thermodynamic collision process occurred in alkaline condition [13]. Recently, diverse researches focused on the photocatalytic degradation of OTC have showed that the degradation rate of OTC in aqueous solution was greatly affected by pH, and OTC degradation was optimal at the zero-point charge (pHzpc) due to its higher adsorption onto the surface of catalyst, leading to increased rate of OTC degradation [30–32]. The effect of pH on degradation of OTC under simulated sunlight was rather scarce. Thus, as one of the key factors of aquatic systems, pH deserves extensive attention for studies on photodegradation of OTC. With this background in our mind, the objective of the present study was to investigate the behavior and kinetics of OTC photodegradation in simulated natural water system. The main scope of this work was to investigate direct and indirect photolysis of OTC in presence/absence of nitrate, iron (III) and bicarbonate in aqueous solution under simulated sunlight irradiation, with particular emphasis on the interaction of these three ions on photochemical degradation of OTC. The influences of pH as well as irradiation intensity were also evaluated. We believe the study is of significance for understanding the environmental fate of OTC in surface waters and helps to better evaluate the environmental fate and persistence of other antibiotics in natural waters.
2. Materials and methods 2.1. Chemicals Oxytetracycline hydrochloride (purity: 96.5%, CAS: 2058-46-0) was purchased from Dr. Ehrenstorfer-Gmbh (Germany). High performance liquid chromatography (HPLC)-graded solvents (acetonitrile, formic acid) were obtained from J&K Chemical Co. Ltd. (Beijing, China). Other chemicals, such as NaNO3, NaHCO3, FeCl36H2O, NaCl and isopropanol are all of analytical grade and were used directly without further purification. HCl (0.1 M) and NaOH (0.2 M) were used for pH adjustment. All the experiments have been performed at pH 7.0 (0.1) except for those designed for studying the effects of initial pH. Individual stock solution of OTC was prepared by dissolving Oxytetracycline hydrochloride in Millie-Q water from a Millipore Gradient A10 system (USA) to achieve a final concentration of 250 mg/L. This solution was freshly prepared before experiments and stored in dark at 4 C. OTC solution for photolysis was prepared by adding an appropriate volume of stock solution into Millie-Q water to obtain a concentration of 20 mg/L, and filled into quartz cuvettes (w28 mm 60 mm) for irradiation. 2.2. Photo-degradation experiments Irradiation was provided by a Suntest CPS+ sunlight simulator (Atlas, Germany) equipped with a 1.1 kW xenon lamp which simulates natural sunlight. Special UV filters are provided to yield desired irradiation spectra. The wavelength of excitation source for all degradation reactions is in a range of 300–800 nm. Schematic and radiation intensity at different wavelengths of the sunlight simulator are given in Supporting information (Fig.S2). Irradiance was maintained constant at 765 W/m2 during all the experiments except for the study that considers the effects of light intensity. Capped quartz tubes with 25 mL OTC solution were soaked in a water bath and secured in the groove of the device, directly under the irradiance. The temperature of the water bath was kept constant at 20 C, controlled by a cooling water circulator. As dark controls, some of the reaction tubes were wrapped with aluminum foil and set aside separately at the same temperature of 20 C. At each designated time interval, one of the reaction tubes was removed periodically. Then 1.5 mL reaction mixture was taken out from the tube using a syringe and filtered through a 0.22 mm membrane before the OTC concentration was quantified by using a high-performance liquid chromatography (HPLC). 2.3. Analytical method The concentration of OTC was analyzed by using a Hitachi L-2000 series HPLC system, equipped with a L-2130 binary pump, a L-2200 auto sampler, a L-2300 column compartment and a L-2455 diode-array detector. The analytical column was a Venusil MP C18 reverse-phase column (250 mm 4.6 mm, 5 mm, Agela Technologies, China) in a thermostatic oven at 30 C. The mobile phase A was acetonitrile and B was ultrapure water with 0.15% formic acid (20:80, v/v). The injection volume was 20 mL with a constant flow rate of 1.0 mL/min for mobile phase. The analytical wavelength of the diode-array detector was set at 267 nm. pH values were measured with a pH meter (pH-220, Qiwei, China). UV–vis spectra of OTC and ions selected for study (nitrate, iron (III), bicarbonate) were analyzed by an ultraviolet spectrophotometer (UV2550, Shimaduz, Japan)
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3. Results and discussion All the experiments were carried out in triplicate with errors below 5% and average values are reported with error bars. Results from dark control samples revealed that the reduction of OTC concentration caused by other degradation paths (such as hydrolysis) and adsorption on the reaction tube under the same condition were less than 3%, negligible compared with that of photolysis. Thus, the effect of hydrolysis was ignored in the following discussion. 3.1. Direct photodegradation 3.1.1. Photolysis kinetics Previous studies have shown that the photodegradation of organic pollutants, such as diclofenac [33], nonylphenol [34], ibuprofen [35], chloramphenicol [36], in aqueous solution generally followed a pseudo-first order kinetics. For OTC photodegradation in aqueous solution, a plot of ln (Ct/C0) vs reaction time t (Fig. 1) also showed that it follows a pseudo-first-order kinetics. The pseudo-first-order rate constant (k) was calculated from the slope of the regression line, where Ct represents the concentration of OTC at time t (min) and C0 is the initial concentration. The halflife (t1/2) of OTC was also calculated and illustrated in Table S1 (SI), along with apparent rate constants. Compared with the previous study on OTC photolysis kinetics from pH 4 to 9, the k values in the present study were slightly lower than those in aqueous solution from irradiation of mercury lamp (Table S2)[24]. The major absorption peaks of OTC solution are under 400 nm (Fig.S5). The mercury lamp filtrated by the borosilicate glass (300–400 nm) perfectly covers the bathochromic absorption band of OTC (365 nm), while the major part of the irradiation from xenon lamp (Fig.S2) is higher than 400 nm. Therefore, it is not surprising that the k values in the present study (with a xenon lamp as the irradiation source) were slightly lower. We found that there were inflexion points in the kinetic profiles of OTC photodegradation at pH 7.1, 7.9 and 9.1 (Fig. 1). The photolysis obviously slowed down after 40 min and this phenomenon was more distinct with increasing pH. Similar results were also reported by Zhang et al. [12] for UV degradation of ciprofloxacin hydrochloride at different pHs. Jiao et al. [24] and Bi et al. [37] have reported that the photolysis of OTC followed a pseudo-first-order kinetics in the range of pH 4–9, which means that differences in the absorbance at the excitation wavelength could not be a reason for inflexion points in the kinetic profiles of OTC photodegradation in this pH range. A previous study suggested that it is difficult for OTC to completely
mineralize to CO2 and H2O due to its stable structure, and OTC is mainly transformed to other byproducts in photochemical processes [24]. Most of OTC molecules are present in the form of zwitterions or anions in neutral or alkaline conditions (Fig.S3), and the photochemical activity of the deprotonated OTC molecules is much higher than the protonated OTC molecules (in acidic condition). Under the same light conditions, the deprotonated OTC molecules are more likely to absorb photons to generate intermediates, and the rapid accumulation of intermediates will compete with OTC for the limited photons, leading to a slower degradation of OTC. On the other hand, dissolved oxygen (DO) is an important factor in photooxidation. DO could capture the electrons and energy from the excited states of other molecules to generate ROSs such as 1O2, O2 and HO, and thus promote the degradation reaction. Previous studies have also reported that 1O2 and O2were involved in the photodegradation of TCs in the presence of oxygen [26,38–40]. Hence, the reduction of DO (or ROSs) with prolonged reaction time in the system would be another reason. Because alkaline conditions are favorable for the formation of ROSs (see Section 3.1.2 for more details), a faster photodegradation rate than that under acidic conditions at the beginning of reaction is certainly expected. With the consumption of DO in reaction tubes, ROSs, generated by photosensitization, reduced and the degradation rate of OTC was retarded. In summary, OTC structure, combined with limited DO content, is responsible for the two stages in the pseudo-first-order kinetics of OTC degradation under neutral and alkaline conditions. 3.1.2. Effect of initial pH In this paper, we use the apparent rate constants, calculated from plots of ln(Ct/C0) vs reaction time t as described in Section 3.1.1, to compare the degradation rate of OTC; and the removal efficiency of OTC is calculated by Eq. (1). Y ð%Þ ¼
C0 Cf 100 C0
ð1Þ
Where Cf is the final concentration of OTC after a certain time of irradiation and C0 is the initial concentration. As stated above, the degradation rate of OTC significantly increased with raising pH in the studied range. The removal efficiency of OTC in 270 min was 61.1%, 73.3%, 81.3%, 90.5% and 95.6% at pH of 4.8, 6.1, 7.1, 7.9 and 9.1, respectively (Fig. S4). We also found that the degradation rate of OTC increased sharply and reached equilibrium around 60 min of irradiation at pH = 9.1; whereas the decay of OTC was still going on even longer than 270 min at pH of 4.8, 6.1 and 7.1 (Fig. S4). The influence of pH on OTC removal under the experimental conditions was consistent with some of previous reports [28,41]. This is clearly an indication that alkaline media are more favorable for the degradation of OTC than an acidic condition. There are four different forms of OTC molecules (Fig.S3), namely, protonated (H3L+), neutral (H2 L) and deprotonated species (HL, L2) [22,42], under various pH values. H3 Lþ
Fig. 1. Photolysis kinetics of OTC at different pH conditions (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and temperature of 20 C).
241
$
pK a1 ¼3:22
H2 L
$
pK a2 ¼7:46
HL
$
pK a3 ¼8:94
L2
Increased degradation rate and removal efficiency of OTC at a higher pH could be attributed to pH-dependent species and absorption spectrum of OTC. With increasing pH, the absorption of deprotonated OTC exhibits a red shift [13,24] and overlaps more with the spectrum of the sunlight simulator, thus favoring the direct photodegradation of OTC. On the other hand, at a higher pH, OTC molecules mainly exist in negative forms, which increase the electron density of OTC ring system and thus facilitate the formation of ROSs from electrophilic attacking of DO, and thus could promote the indirect photodegradation of OTC. It has been established that 1O2 and O2 can be generated in irradiated OTC
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solutions [43]. However, it should be pointed out that no other photosensitive ions were added under the experimental conditions; we then speculate that ROSs might be generated via the following pathway. Under the irradiation of simulated sunlight, the deprotonated OTC molecules are easier to absorb photons and to be excited to singlet state, which can readily transfer to the triplet state (OTC *) via intersystem crossing. OTC * can then act as a self-photosensitizer to produce ROSs as shown in reactions (2)–(6) [4,26]. Zhao et al. confirmed that only the anionic form of OTC could generate singlet oxygen which was one of the important ROSs to photolysis of OTC under simulated solar light [22]. OTC + hv ! OTC
*
(2)
OTC * + 3O2 ! OTC + 1O2
(3)
OTC * + 3O2 ! OTC+ + O2
(4)
2O2 + 2H+ ! H2O2 + O2
(5)
H2O2 + hv ! 2HO
(6)
In order to verify the presence of HO in neutral condition, 100 mM isopropanol, which is commonly used as a HO scavenger, was added to 20 mg/L OTC solution. The result (Fig.S7) showed that the degradation rate of OTC was significantly decreased after addition of isopropanol but the removal efficiency of OTC was reduced by only 8% in 180 min. Therefore, we speculate that only a small amount of HO existed in neutral solution. Since direct degradation was mainly responsible for the decay of OTC under our experimental conditions, the effect of indirect degradation caused by a small amount of HO on the removal efficiency was limited. Based on the above discussion, we concluded that OTC photolysis occurred via direct photodegradation and an indirect process with the participation of ROSs in neutral and basic media; while in acid media, direct photolysis of OTC was the only process. 3.1.3. Effect of light intensity As shown in Fig. 2, with increasing light intensity from 250 W/m2 to 350 W/m2 and 500 W/m2, the removal efficiency of OTC in 270 min increased from 76.6% to 78.3% and 81.2%. Although the OTC degradation rate under diverse light intensity did not significantly differ, the higher light intensity was more favorable
Fig. 2. The influence of light intensity on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at pH of 7.1 and temperature of 20 C).
for OTC removal. The photon generation rate changes with different light intensities, and at a higher intensity, higher photon flux was provided [44]; more excited OTC molecules can then be produced and be involved in reactions via direct or indirect photodegradation. The slight difference of removal efficiency also indicated that light intensity was not a key factor to the photolysis of OTC under the experimental conditions. 3.2. Univariate effect of NO3 , Fe3+ and HCO3 3.2.1. The effect of NO3on photodegradation of OTC HO, a non-selective oxidant, is highly reactive to many organic chemicals with a secondary rate constant in the range of 108–1010 M1 s1 [17]. Nitrate is one of the important sources of hydroxyl radicals in natural water[45]; therefore it may play a key role in the degradation of OTC. To observe the influence of NO3 on the degradation of OTC, we conducted experiments in presence of nitrate with four different concentrations from 0 to 1.0 mM, which were similar to the real level typically observed in natural aquatic environment [46]. As shown in Fig. 3, with increasing nitrate concentration from 0.1 mM to 0.5 mM and 1.0 mM, the removal efficiency of OTC after 180 min irradiation enhanced from 70.6% to 80.2% and 82.9%. The overall kinetics of OTC photolysis in presence of nitrate can be well described by a two-step pseudo-first-order model (Fig.S8). Reaction rate constants (k1,a and k1,b for the first and second phases) were 0.01730 min1/0.00386 min1, 0.01751 min1/0.00435 min1, 0.02349 min1/0.00512 min1, 1 1 0.02827 min /0.00555 min at nitrate concentrations of 0 mM, 0.1 mM, 0 0.5 mM and 1.0 mM, respectively. NO3 can absorb light in UV range (shown in Fig.S5) and act as a sensitizer to promote the photoreaction. The light shielding effect of NO3 was negligible due to the very low molar extinction coefficient of NO3 in the UV–visible region [47,48]. The photolysis of NO3 leads to the formation of HO via reaction Eqs. (7)–(9) [44,47] and the produced HO can promote the indirect photodegradation of OTC. Although NO2 can be generated from NO3 (Eq. (8)), it would further polymerize to form N2O4 and finally to NO2 by the process of dismutation [49]. Thus, the effect of the NO2 was ignored in the transformation process of OTC in the present study. NO3 ! NO2 + O
NO3
! NO2+ O
(7)
(8)
Fig. 3. The influence of NO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 6.9).
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O + H2O ! HO+ OH
243
(9) NO3
hardly influenced the Previous studies have shown that destruction of OTC in the UV/H2O2, UV/PS[4] and UV/TiO2/5A or UV/TiO2/13X system [15] due to the high radical quantum yield of H2O2, PS and TiO2 under UV activation. However, NO3 could enhance OTC degradation in UV/NO3 system due to the generation of HO by the excited NO3, as discussed above. According to Eq. (9), HO should be proportional to NO3concentration. If OTC is reacted with equimolar HO, the degradation rate of OTC should also be proportional to NO3 concentration [44,50]. Fig. 4 presents a good linear relationship between rate constants and NO3 concentrations (R1,a2 = 0.97962, R1,b2 = 0.88872), as also represented by Eqs. (10) and (11). k
1,a
= 0.01155c + 0.01701
(10)
k
1,b
= 0.0016c + 0.00406
(11)
Where k1,a and k1,b represent the pseudo-first-order degradation rate constants of OTC in the two stages in min1 and c is the nitrate concentration in mM. Consequently, the greater degradation rate of OTC with increasing concentration of NO3was mainly due to the higher yield of hydroxyl radical within the studied concentration range. 3.2.2. The effect of Fe3+ on photodegradation of OTC Iron is ubiquitous in natural aquatic environment at concentrations ranging from 107 to 104 M [51]. In the present study, we used FeCl3 as the source of Fe3+ to study the effect of iron (III) on OTC degradation. In order to determine the impact of Cl, separate experiments with different amounts of NaCl added to OTC solution were also conducted. Results showed that the removal of OTC caused by Cl was negligible (details shown in Fig. S6), indicating that the degradation of OTC in FeCl3 solution at pH 7.0 (0.1) was solely due to the presence of Fe3+. It is true that chlorine may affect the photochemistry under acidic condition owing to that Cl can react with HO to form ClOH and finally transformed into Cl2 . However, the rapid regeneration of HO through the dissociation of the formed ClOH occurred instead of forming of Cl2 through the oxidation of Cl with HO at pH 7 [17]. Fig. 5 shows that Fe3+ inhibited OTC degradation at concentrations of 2.5 mM, 10 mM and15 mM. No significant difference in pseudo-first-order rate constants was observed with the addition of different concentrations of Fe3+ (Table S1).
Fig. 4. Effect of NO3 concentration on rate constant (k) for photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 6.9).
Fig. 5. The influence of Fe3+on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 6.9).
The existing literature proved that Fe3+ could produce HO via reactions (12)–(14) [34,51], and a positive role of Fe3+ to the photodegradation of OTC is then expected. Fe3+ + H2O ! Fe2+ + HO + H+
(12)
Fe3+ + H2O Ð Fe (OH)
(13)
Fe (OH)
2+
2+
+ H+
! Fe2+ + HO
(14) 3+
However, under our experimental conditions, Fe at different concentrations revealed an inhibition for OTC degradation. We tentatively attribute this result to pH used in this work, since the role of Fe3+ on photodegradation strongly depends on its speciation, which is pH dependent. At a pH of 7.0(0.1), Fe3+ in aqueous solution at our studied concentration range would be mainly in colloidal forms such as Fe(OH)3. The existing of Fe(OH)3 would result in substantially decreased production of HO because of its particulate nature. More importantly, Fe(OH)2+ is the predominant photoreactive species in Fe(III)-aquo complexes (Fe3+, Fe(OH)2+, Fe(OH)2+, dimer Fe(OH)24+) [44], whereas the photoactivity of Fe(OH)2+ in aqueous solution was significantly restrained at pH > 5.0 [51]. Therefore, the promotion effect of Fe3+ was not found under our conditions. In addition, although the light absorption of Fe3+ at our concentration range is less than that of OTC, the absorption spectrum of Fe3+ has a great overlapping with that of OTC (Fig.S5). Slightly light-shielding may slow down the degradation rate of OTC; similar results have also been reported by Peng et al. for photodegradation of nonylphenol [34]. 3.2.3. The effect of HCO3on photodegradation of OTC HCO3 is a rich anion in natural water systems with a concentration range from 0.4 mM to 4.4 mM [52]. Although HCO3 does not absorb light in solar spectrum, it can react with strongly oxidizing radicals (HO, SO4) or excited aromatic ketones to produce selective carbonate radicals (CO3), triggering indirect degradation of electron-rich organic pollutants [4]. Also, the addition of bicarbonate will increase the pH of the solution, and then affect the photochemical activity of organic compounds. The pKa1 and pKa2 of H2CO3 are 6.37 and 10.33, respectively [53]. Under our experimental condition (pH = 7.0 0.1), HCO3 is the major species, accounting for 81%; the rest are available in the form of dissolved carbon dioxide, accounting for about 19%. Dosage of bicarbonate added to the solution has little effect on the concentration of HCO3 in our case. Experiments with two
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concentrations (0 mM, 3.0 mM) of HCO3 were then operated at pH of 7.0. In order to verify whether the effect of HCO3 was due to its role on solution pH, a separate experiment with 3 mM HCO3 without pH adjustment (pH = 8.5) was also conducted. Results (Fig. 6) showed that the degradation of OTC was slightly increased with prolonged irradiation when the concentration of HCO3 was increased from 0 mM to 3.0 mM at both pH 7.0 and 8.5. However, compared with that at pH 7.0, it significantly increased at pH 8.5 when the concentration of HCO3 was 3.0 mM. The degradation rate constants were calculated according to a pseudofirst-order kinetics (Table S1). Fig. 6 reveals that the influence of HCO3 on degradation of OTC at pH 8.5 was mainly attributed to its pH-adjusting role. Similar results have also been obtained by other researchers for the degradation of tetracycline and furfural [26,54]. As stated above, there is a small amount of HO in neutral solution (pH 7.0); the addition of bicarbonate can produce selective CO3 via HO-scavenging according to Eq. (15) [55]. It was reported that the second-order rate constant for the reaction between OTC and CO3 is 2.9 108 M1s1 [4], which is one order of magnitude lower than that of 6.96–7.18 109 M1 s1 [9] for the reaction between OTC and HO. Therefore, a decreased degradation rate at the beginning of the reaction at pH 7.0 is certainly expected. However, with prolonged irradiation time and the elimination of HO (due to the exhaust of DO), the excited aromatic ketones or the excited triplet state of a sensitizer (such as OTC* generated in the solution) could react with bicarbonate to produce CO3 [55], leading to a slight promotion effect due to its considerable secondorder reaction with OTC. HO + HCO3 ! H2O + CO3
Table 1 The concentration of NO3, Fe3+ and HCO3 used in the univariate and multivariate experiments. No. of run
1 2 3 4 5 6 7 8 9 10 11 12
Concentration used OTC(mg/L)
NO3(mM)
Fe3+(mM)
HCO3(mM)
20 20 20 20 20 20 20 20 20 20 20 20
0 0.1 0.5 1.0 0 0 0 0 1.0 1.0 0 1.0
0 0 0 0 2.5 10.0 15.0 0 15.0 0 15.0 15.0
0 0 0 0 0 0 0 3.0 0 3.0 3.0 3.0
(15)
3.3. Multivariate effects of NO3, Fe3+ and HCO3 Based on results in Section 3.2, we selected 1.0 mM NO3, 15 mM Fe3+ and 3.0 mM HCO3 to study their comprehensive effects on OTC degradation. The experimental design was shown in Table 1. A pseudo-first-order kinetics was also adopted to calculate the rate constants of OTC degradation; and the correlation coefficient (R2) for the fitting was more than 0.95 in any condition (Table S1). 3.3.1. The comprehensive effect of NO3 and Fe3+ on OTC photodegradation As shown in Fig. 7, the influence of NO3 coupled with Fe3+ was not a simple additive effect on OTC photodegradation. The
Fig. 6. The influence of HCO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0 and 8.5).
Fig. 7. The comprehensive influence of NO3and Fe3+on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
degradation rate of OTC with co-presence of Fe3+ and NO3 was slightly higher than that with Fe3+ but much lower than that with NO3. And, we found no significant difference in the removal efficiency of OTC with co-addition of NO3 and Fe3+. This phenomenon was probably due to the fact that the slightly increased degradation rate caused by HO from NO3 (Eqs. (7)–(9)) could not completely compensate for the inhibition effect of Fe3+ as discussed in Section 3.2, resulting in a slightly negative effect on the degradation of OTC. In addition, both the absorption spectra of NO3 and Fe3+ (Fig.S5) overlap with that of OTC. Simultaneous addition of NO3and Fe3+ in the solution enhanced the light shielding effect, resulting from their competition with OTC molecules for photon absorption. That hinders OTC molecules to reach the excited state, consequently leading to a lower degradation rate than that of the direct photolysis of OTC. 3.3.2. The comprehensive effect of NO3 and HCO3 on OTC photodegradation As discussed in Section 3.2, NO3 and HCO3 promote photodegradation of OTC by generating HO and CO3, respectively. Fig. 8 depicts that combination of HCO3and NO3 exhibit a slightly antagonistic effect. Our results differed from those obtained by Dionysiou et al. [4] who have reported that the generation of CO3 presented a positive role on OTC degradation in UV/NO3/HCO3 system. It could be attributed to higher pH (pH = 9.0) used in their study. As stated above, alkalinity favors electrophilic attacking of HO and CO3, and thus promoted the indirect photodegradation of OTC. Moreover, the difference in
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alone, we found essentially no change in photodegradation rate of OTC with coexistence of Fe3+ and HCO3 (Fig. 9). A reasonable argument is that the concentration of Fe3+ is far less than that of HCO3; the influence of HCO3 surpasses the inhibition effect of Fe3+ and their comprehensive effect is mainly attributed to HCO3. The result also implies that reaction with CO3 is an important pathway for OTC photodegradation.
Fig. 8. The comprehensive influence of NO3 and HCO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
radical quantum yield caused by the wavelength of light used in the work of Dionysiou et al. may be another reason. The quenching effect of HCO3 on HO, produced from NO3 as depicted in Eqs. (7)– (9), leads to a decreasing of the indirect photolysis of OTC. Furthermore, compared to the presence of HCO3 alone, the co-existence of HCO3 and NO3 result in an increasing of the concentration of inorganic components in the solution, which is likely to decrease the amount of photons entered into the solution because of light attenuation [56]. The concentrations of excited state of OTC and CO3 are decreased; and a declined degradation rate in subsequent processes of photodegradation is then expected. However, the degradation rate of OTC with co-presence of HCO3and NO3 is still faster than its direct photolysis, indicating that the influence of CO3 on the degradation of electron-rich organic pollutants is not negligible. 3+
3.3.3. The comprehensive effect of Fe and HCO3 on OTC photodegradation As discussed in Section 3.2.2, Fe3+ inhibits OTC degradation due to its competitive effect on photon absorption and lower transmittance of solution caused by Fe(III)-aquo complexes; while the degradation of OTC in presence of HCO3 occurs mainly via direct photolysis and partly via indirect photolysis with participation of CO3. The comprehensive effect of coexisted HCO3 and Fe3+ is supposed to inhibit the degradation of OTC due to the light attenuation. However, compared with the presence of HCO3
Fig. 9. The comprehensive influence of Fe3+ and HCO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
3.3.4. The comprehensive effect of NO3, Fe3+ and HCO3 on OTC photodegradation Fig. 10 shows that the comprehensive effect of these three ions is not significant, even though we expect an enhanced degradation and removal efficiency of OTC. Combining with Figs. 7–9, we found no significant effect overall on the degradation of OTC for any of these four combinations. Their combined effect on OTC degradation is a suppression at the beginning of irradiation and then a promotion after prolonged irradiation for any combination with presence of HCO3, indicating that HCO3 may play an important role in these complex factors. In summary, results from irradiation in the co-presence of NO3, Fe3+ and HCO3 indicate that the direct photolysis of OTC is much more efficient than its indirect photochemical processes. 4. Conclusions We studied the direct and indirect photodegradation of OTC in aqueous solution by using a sunlight simulator in the present work. The comprehensive effects of typical aquatic constituents (NO3, Fe3+ and HCO3) in realistic levels as well as solution pH and irradiation intensity were systematically assessed. We anticipate providing a meaningful reference for understanding the environmental behavior and fate of OTC in aquatic systems. The conclusions are as follows: (1) OTC degradation rate and removal efficiency are significantly enhanced with increasing solution pH but slightly promoted with increasing irradiation intensity. (2) OTC degradation proceeds via a direct photolysis and HO- or/ and CO3-involved indirect photolysis; while direct photolysis is dominant and the photodegradation follows a two-step pseudo-first-order kinetics in neutral or alkaline aquatic environments. (3) Under our experimental conditions, OTC photodegradation rate increases with increasing NO3 concentration, owing to more HO generated; and the promotion effect of HCO3 is
Fig. 10. The comprehensive influence of NO3, Fe3+ and HCO3on photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
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mainly attributed to its adjusting role on solution pH; while Fe3+ inhibits the degradation of OTC by light shielding. (4) Results from irradiation in co-presence of NO3, Fe3+ and HCO3 indicate that the direct photolysis of OTC is much more efficient than its indirect photolysis in neutral solutions. Given the extensive use and widespread presence of OTC in natural waters, further studies on monitoring of the degradation intermediates and by-products are needed for understanding its transformation mechanism and for better assessing the risks associated with antibiotics released into the aquatic environment. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Grant No. 41276067) and the Fundamental Research Funds for the Central Universities (201762006, 201762030) of Ocean University of China. We also thank Professor Dr. Jiangtao Wang (College of Chemistry and Chemical Engineering, Ocean University of China) for helps in HPLC measurement. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jphotochem.2018. 01.004. References [1] Q.Q. Zhang, G.G. Ying, C.G. Pan, Y.S. Liu, J.L. Zhao, Comprehensive evaluation of antibiotics emission and fate in the river basins of China: source analysis, multimedia modeling, and linkage to bacterial resistance, Environ. Sci. Technol. 49 (2015) 6772–6782. [2] M. Duan, H. Li, J. Gu, X. Tuo, W. Sun, X. Qian, X. Wang, Effects of biochar on reducing the abundance of oxytetracycline antibiotic resistance genes, and human pathogenic bacteria in soil and lettuce, Environ. Pollut. 224 (2017) 787– 795. [3] K.F. Rubert, J.A. Pedersen, Kinetics of oxytetracycline reactionwith a hydrous manganese oxide, Environ. Sci. Technol. 40 (2006) 7216–7221. [4] Y. Liu, X. He, X. Duan, Y. Fu, D. Fatta-Kassinos, D.D. Dionysiou, Significant role of UV and carbonate radical on the degradation of oxytetracycline in UV-AOPs: kinetics and mechanism, Water Res. 95 (2016) 195–204. [5] Y. Li, X. Qiao, Y.N. Zhang, C. Zhou, H. Xie, J. Chen, Effects of halide ions on photodegradation of sulfonamide antibiotics: formation of halogenated intermediates, Water Res. 102 (2016) 405–412. [6] S. Zou, W. Xu, R. Zhang, J. Tang, Y. Chen, G. Zhang, Occurrence and distribution of antibiotics in coastal water of the Bohai Bay, China: impacts of river discharge and aquaculture activities, Environ. Pollut. 159 (2011) 2913–2920. [7] K. Li, A. Yediler, M. Yang, S. Schulte-Hostede, M.H. Wong, Ozonation of oxytetracycline and toxicological assessment of its oxidation by-products, Chemosphere 72 (2008) 473–478. [8] A.J. Watkinson, E.J. Murby, S.D. Costanzo, Removal of antibiotics in conventional and advanced wastewater treatment: implications for environmental discharge and wastewater recycling, Water Res. 41 (2007) 4164–4176. [9] Y. Liu, X. He, Y. Fu, D.D. Dionysiou, Kinetics and mechanism investigation on the destruction of oxytetracycline by UV-254 nm activation of persulfate, J. Hazard. Mater. 305 (2016) 229–239. [10] M.F. Zaranyika, P. Dzomba, J. Kugara, Degradation of oxytetracycline in the aquatic environment: a proposed steady state kinetic model that takes into account hydrolysis, photolysis, microbial degradation and adsorption by colloidal and sediment particles, Environ. Chem. 12 (2015) 174–188. [11] L. Fu, T. Huang, S. Wang, X. Wang, L. Su, C. Li, Y. Zhao, Toxicity of 13 different antibiotics towards freshwater green algae Pseudokirchneriella subcapitata and their modes of action, Chemosphere 168 (2017) 217–222. [12] G. Zhang, B. Wu, S. Zhang, Effects of acetylacetone on the photoconversion of pharmaceuticals in natural and pure waters, Environ. Pollut. 225 (2017) 691– 699. [13] X. Jin, H. Xu, S. Qiu, M. Jia, F. Wang, A. Zhang, X. Jiang, Direct photolysis of oxytetracycline: influence of initial concentration, pH and temperature, J. Photochem. Photobio. A Chem. 332 (2017) 224–231. [14] J.H.O.S. Pereira, D.B. Queirós, A.C. Reis, O.C. Nunes, M.T. Borges, R.A.R. Boaventura, V.J.P. Vilar, Process enhancement at near neutral pH of a homogeneous photo-fenton reaction using ferricarboxylate complexes: application to oxytetracycline degradation, Chem. Eng. J. 253 (2014) 217–228.
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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
The effect of pH, nitrate, iron (III) and bicarbonate on photodegradation of oxytetracycline in aqueous solution Chanjuan Lia , Dahai Zhanga , Jialin Pengb , Xianguo Lia,* a b
Key Laboratory of Marine Chemistry Theory and Technology (Ocean University of China), Ministry of Education, Qingdao 266100, China Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
A R T I C L E I N F O
Article history: Received 13 October 2017 Received in revised form 2 January 2018 Accepted 3 January 2018 Available online 3 January 2018 Keywords: Oxytetracycline Photodegradation pH Comprehensive effect Simulated sunlight
A B S T R A C T
Photodegradation is a very important elimination pathway of Oxytetracycline (OTC) in aquatic environments. The photochemical behavior of OTC in absence/presence of nitrate, iron (III) and bicarbonate, especially their combinations, was systematically studied in aqueous solution by employing a Suntest-CPS+ sunlight simulator in this work. Meanwhile, the effect of solution pH (4.8–9.1) and irradiation intensity (250–500 W/m2) on the removal efficiency of OTC have also been assessed. The results showed that OTC degradation followed a pseudo-first-order kinetics in most conditions except for irradiation in neutral/alkaline solutions and in presence of NO3, where a two-step pseudo-first-order kinetics was obeyed. OTC degradation was highly pH-dependent and increased significantly with increasing pH. The presence of NO3 and HCO3 promoted the photochemical loss of OTC in aqueous solution by generating hydroxyl radicals and adjusting solution pH, respectively; whereas iron (III) was found to play a negative role under the conditions studied. The influence of NO3 coupled with Fe3+ was not a simple additive effect and the overall performance was to inhibit OTC degradation. In addition, combination of HCO3 and NO3 exhibited an antagonistic effect. Results from irradiation in co-presence of NO3, Fe3+ and HCO3 suggested that direct photolysis of OTC was much more efficient than its indirect photolysis. These results provide a meaningful reference for understanding the fate and transformation of OTC in natural water systems. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Antibiotics, a class of pharmaceutical and personal care products(PPCPs), are produced and consumed in great quantities worldwide, and are becoming the focus of attention because of the issue of antibiotics-resistant bacteria and genes. China, as the largest producer and user of antibiotics all around the world, consumed 92,700 tons of 36 frequently used antibiotics, with about 53,800 tons released into the general environment in 2013 [1,2]. Among those antibiotics, oxytetracycline (OTC) is an important member of tetracyclines (TCs) that is extensively used for treating human and animal diseases and applied as a feed additive for promoting animal growth in a variety of aquacultures [3]. The applied TCs are poorly ingested by organisms, and the majority is excreted in faeces and urine without metabolism. TCs are therefore widely detected in various environmental components, such as surface waters and sewage water [4,5]. Zou et al.
* Corresponding author. E-mail addresses: [email protected] (C. Li), [email protected] (X. Li). https://doi.org/10.1016/j.jphotochem.2018.01.004 1010-6030/© 2018 Elsevier B.V. All rights reserved.
reported that the concentration of OTC in Bohai Bay was up to 0.27 mg/L [6]; while OTC has been detected in very high concentrations ranging from 20 to 800 mg/L in one of the biggest OTC producer’s wastewater treatment plant in China [7]. Conventional water treatment methods, such as filtration, coagulation, flocculation and biodegradation showed very poor efficiencies to remove OTC due to its chemical stability and bioresistance [8]. The extensive usage and frequent detection of antibiotics poses a great threat to the general ecosystem and human health through the development of antibiotics-resistant bacteria and pathogens [9]. For example, OTC has been involved in the aetiology of several diseases, such as hypouricemia and hypokalemia [10]. It can also exhibit significantly toxic effects to algae, which as primary producers play a key role in the whole aquatic ecosystem [11]. Aquatic environment is vital to the survival of humans, animals and plants, and waters are important media for the occurrence of OTC. Therefore, it is meaningful to assess the behavior and fate of OTC in aquatic environments. Photochemical degradation is an important pathway of OTC elimination in surface waters [3,12,13]. In recent years, photodegradation of OTC using advanced oxidation processes (AOPs),
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such as Photo-Fenton oxidation, UV/TiO2, UV/MgAl calcined hydrotalcites, UV/H2O2, UV/Persulfate (PS), has been extensively studied by several groups due to the efficient formation of strongly oxidative radicals to highly effectively remove OTC [9,14–18]; whereas researches related to the environmental fate and transformation of OTC were quite sparse. Psoralen and Riboflavin, used as phototherapy drugs, have been reported that they could act as photosensitizers themselves [19,20]. Upon UV or visible light irradiation, their triplet excited state can react with oxygen to generate reactive oxygen species (ROSs, such as 1O2), which is a central intermediate for cytotoxic action in potential photodynamic therapy. The common pathways involved in the phototransformation of OTC in surface waters were direct photolysis [13,21], indirect photolysis with ROSs (such as HO, 1O2, O2)[10] and even self-sensitized degradation[22]. These photochemical processes can be substantially impacted by complex aquatic constituents, such as humic acids (HA), Fe3+, NO3, HCO3, Cl and SO42 etc., which can act as either sensitizers to enhance the indirect photodegradation or as irradiation filters and/or HO scavengers to inhibit photolysis of organic compounds. Among these common constituents, Fe3+, NO3 and HCO3 are ubiquitous in natural aquatic environments and previous studies have shown that they play an important role in the photodegradation of organic pollutants [23–26]. However, most of these studies were focused on the individual effect of each component on photolysis, and their interaction on photodegradation of OTC is poorly understood. Therefore, the underlying mechanism for the comprehensive effects of nitrate, ferric ion and bicarbonate needs to be further clarified. OTC is an amphoteric molecule which consists of a 4-ring system with multiple function groups such as tertiary amino- and hydroxyl- groups (SI, Fig.S1). Solution pH, as one of the important factors for photodegradation of organic pollutants [27,28], could simultaneously affect the speciation and structure as well as degradation mechanism of OTC. pH may have different effects on OTC photodegradation under different operating conditions, such as light source, temperature, presence/absence of oxygen. For example, Liu et al. suggested that OTC photodegradation followed a pseudo-first-order kinetics at various pH values from 3.0 to 11.0 in UV-C system and UV/H2O2 system [29]; Jin et al. reported that OTC photolysis followed a second-order kinetics in oxygen-free condition and pointed out that thermodynamic collision process occurred in alkaline condition [13]. Recently, diverse researches focused on the photocatalytic degradation of OTC have showed that the degradation rate of OTC in aqueous solution was greatly affected by pH, and OTC degradation was optimal at the zero-point charge (pHzpc) due to its higher adsorption onto the surface of catalyst, leading to increased rate of OTC degradation [30–32]. The effect of pH on degradation of OTC under simulated sunlight was rather scarce. Thus, as one of the key factors of aquatic systems, pH deserves extensive attention for studies on photodegradation of OTC. With this background in our mind, the objective of the present study was to investigate the behavior and kinetics of OTC photodegradation in simulated natural water system. The main scope of this work was to investigate direct and indirect photolysis of OTC in presence/absence of nitrate, iron (III) and bicarbonate in aqueous solution under simulated sunlight irradiation, with particular emphasis on the interaction of these three ions on photochemical degradation of OTC. The influences of pH as well as irradiation intensity were also evaluated. We believe the study is of significance for understanding the environmental fate of OTC in surface waters and helps to better evaluate the environmental fate and persistence of other antibiotics in natural waters.
2. Materials and methods 2.1. Chemicals Oxytetracycline hydrochloride (purity: 96.5%, CAS: 2058-46-0) was purchased from Dr. Ehrenstorfer-Gmbh (Germany). High performance liquid chromatography (HPLC)-graded solvents (acetonitrile, formic acid) were obtained from J&K Chemical Co. Ltd. (Beijing, China). Other chemicals, such as NaNO3, NaHCO3, FeCl36H2O, NaCl and isopropanol are all of analytical grade and were used directly without further purification. HCl (0.1 M) and NaOH (0.2 M) were used for pH adjustment. All the experiments have been performed at pH 7.0 (0.1) except for those designed for studying the effects of initial pH. Individual stock solution of OTC was prepared by dissolving Oxytetracycline hydrochloride in Millie-Q water from a Millipore Gradient A10 system (USA) to achieve a final concentration of 250 mg/L. This solution was freshly prepared before experiments and stored in dark at 4 C. OTC solution for photolysis was prepared by adding an appropriate volume of stock solution into Millie-Q water to obtain a concentration of 20 mg/L, and filled into quartz cuvettes (w28 mm 60 mm) for irradiation. 2.2. Photo-degradation experiments Irradiation was provided by a Suntest CPS+ sunlight simulator (Atlas, Germany) equipped with a 1.1 kW xenon lamp which simulates natural sunlight. Special UV filters are provided to yield desired irradiation spectra. The wavelength of excitation source for all degradation reactions is in a range of 300–800 nm. Schematic and radiation intensity at different wavelengths of the sunlight simulator are given in Supporting information (Fig.S2). Irradiance was maintained constant at 765 W/m2 during all the experiments except for the study that considers the effects of light intensity. Capped quartz tubes with 25 mL OTC solution were soaked in a water bath and secured in the groove of the device, directly under the irradiance. The temperature of the water bath was kept constant at 20 C, controlled by a cooling water circulator. As dark controls, some of the reaction tubes were wrapped with aluminum foil and set aside separately at the same temperature of 20 C. At each designated time interval, one of the reaction tubes was removed periodically. Then 1.5 mL reaction mixture was taken out from the tube using a syringe and filtered through a 0.22 mm membrane before the OTC concentration was quantified by using a high-performance liquid chromatography (HPLC). 2.3. Analytical method The concentration of OTC was analyzed by using a Hitachi L-2000 series HPLC system, equipped with a L-2130 binary pump, a L-2200 auto sampler, a L-2300 column compartment and a L-2455 diode-array detector. The analytical column was a Venusil MP C18 reverse-phase column (250 mm 4.6 mm, 5 mm, Agela Technologies, China) in a thermostatic oven at 30 C. The mobile phase A was acetonitrile and B was ultrapure water with 0.15% formic acid (20:80, v/v). The injection volume was 20 mL with a constant flow rate of 1.0 mL/min for mobile phase. The analytical wavelength of the diode-array detector was set at 267 nm. pH values were measured with a pH meter (pH-220, Qiwei, China). UV–vis spectra of OTC and ions selected for study (nitrate, iron (III), bicarbonate) were analyzed by an ultraviolet spectrophotometer (UV2550, Shimaduz, Japan)
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3. Results and discussion All the experiments were carried out in triplicate with errors below 5% and average values are reported with error bars. Results from dark control samples revealed that the reduction of OTC concentration caused by other degradation paths (such as hydrolysis) and adsorption on the reaction tube under the same condition were less than 3%, negligible compared with that of photolysis. Thus, the effect of hydrolysis was ignored in the following discussion. 3.1. Direct photodegradation 3.1.1. Photolysis kinetics Previous studies have shown that the photodegradation of organic pollutants, such as diclofenac [33], nonylphenol [34], ibuprofen [35], chloramphenicol [36], in aqueous solution generally followed a pseudo-first order kinetics. For OTC photodegradation in aqueous solution, a plot of ln (Ct/C0) vs reaction time t (Fig. 1) also showed that it follows a pseudo-first-order kinetics. The pseudo-first-order rate constant (k) was calculated from the slope of the regression line, where Ct represents the concentration of OTC at time t (min) and C0 is the initial concentration. The halflife (t1/2) of OTC was also calculated and illustrated in Table S1 (SI), along with apparent rate constants. Compared with the previous study on OTC photolysis kinetics from pH 4 to 9, the k values in the present study were slightly lower than those in aqueous solution from irradiation of mercury lamp (Table S2)[24]. The major absorption peaks of OTC solution are under 400 nm (Fig.S5). The mercury lamp filtrated by the borosilicate glass (300–400 nm) perfectly covers the bathochromic absorption band of OTC (365 nm), while the major part of the irradiation from xenon lamp (Fig.S2) is higher than 400 nm. Therefore, it is not surprising that the k values in the present study (with a xenon lamp as the irradiation source) were slightly lower. We found that there were inflexion points in the kinetic profiles of OTC photodegradation at pH 7.1, 7.9 and 9.1 (Fig. 1). The photolysis obviously slowed down after 40 min and this phenomenon was more distinct with increasing pH. Similar results were also reported by Zhang et al. [12] for UV degradation of ciprofloxacin hydrochloride at different pHs. Jiao et al. [24] and Bi et al. [37] have reported that the photolysis of OTC followed a pseudo-first-order kinetics in the range of pH 4–9, which means that differences in the absorbance at the excitation wavelength could not be a reason for inflexion points in the kinetic profiles of OTC photodegradation in this pH range. A previous study suggested that it is difficult for OTC to completely
mineralize to CO2 and H2O due to its stable structure, and OTC is mainly transformed to other byproducts in photochemical processes [24]. Most of OTC molecules are present in the form of zwitterions or anions in neutral or alkaline conditions (Fig.S3), and the photochemical activity of the deprotonated OTC molecules is much higher than the protonated OTC molecules (in acidic condition). Under the same light conditions, the deprotonated OTC molecules are more likely to absorb photons to generate intermediates, and the rapid accumulation of intermediates will compete with OTC for the limited photons, leading to a slower degradation of OTC. On the other hand, dissolved oxygen (DO) is an important factor in photooxidation. DO could capture the electrons and energy from the excited states of other molecules to generate ROSs such as 1O2, O2 and HO, and thus promote the degradation reaction. Previous studies have also reported that 1O2 and O2were involved in the photodegradation of TCs in the presence of oxygen [26,38–40]. Hence, the reduction of DO (or ROSs) with prolonged reaction time in the system would be another reason. Because alkaline conditions are favorable for the formation of ROSs (see Section 3.1.2 for more details), a faster photodegradation rate than that under acidic conditions at the beginning of reaction is certainly expected. With the consumption of DO in reaction tubes, ROSs, generated by photosensitization, reduced and the degradation rate of OTC was retarded. In summary, OTC structure, combined with limited DO content, is responsible for the two stages in the pseudo-first-order kinetics of OTC degradation under neutral and alkaline conditions. 3.1.2. Effect of initial pH In this paper, we use the apparent rate constants, calculated from plots of ln(Ct/C0) vs reaction time t as described in Section 3.1.1, to compare the degradation rate of OTC; and the removal efficiency of OTC is calculated by Eq. (1). Y ð%Þ ¼
C0 Cf 100 C0
ð1Þ
Where Cf is the final concentration of OTC after a certain time of irradiation and C0 is the initial concentration. As stated above, the degradation rate of OTC significantly increased with raising pH in the studied range. The removal efficiency of OTC in 270 min was 61.1%, 73.3%, 81.3%, 90.5% and 95.6% at pH of 4.8, 6.1, 7.1, 7.9 and 9.1, respectively (Fig. S4). We also found that the degradation rate of OTC increased sharply and reached equilibrium around 60 min of irradiation at pH = 9.1; whereas the decay of OTC was still going on even longer than 270 min at pH of 4.8, 6.1 and 7.1 (Fig. S4). The influence of pH on OTC removal under the experimental conditions was consistent with some of previous reports [28,41]. This is clearly an indication that alkaline media are more favorable for the degradation of OTC than an acidic condition. There are four different forms of OTC molecules (Fig.S3), namely, protonated (H3L+), neutral (H2 L) and deprotonated species (HL, L2) [22,42], under various pH values. H3 Lþ
Fig. 1. Photolysis kinetics of OTC at different pH conditions (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and temperature of 20 C).
241
$
pK a1 ¼3:22
H2 L
$
pK a2 ¼7:46
HL
$
pK a3 ¼8:94
L2
Increased degradation rate and removal efficiency of OTC at a higher pH could be attributed to pH-dependent species and absorption spectrum of OTC. With increasing pH, the absorption of deprotonated OTC exhibits a red shift [13,24] and overlaps more with the spectrum of the sunlight simulator, thus favoring the direct photodegradation of OTC. On the other hand, at a higher pH, OTC molecules mainly exist in negative forms, which increase the electron density of OTC ring system and thus facilitate the formation of ROSs from electrophilic attacking of DO, and thus could promote the indirect photodegradation of OTC. It has been established that 1O2 and O2 can be generated in irradiated OTC
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solutions [43]. However, it should be pointed out that no other photosensitive ions were added under the experimental conditions; we then speculate that ROSs might be generated via the following pathway. Under the irradiation of simulated sunlight, the deprotonated OTC molecules are easier to absorb photons and to be excited to singlet state, which can readily transfer to the triplet state (OTC *) via intersystem crossing. OTC * can then act as a self-photosensitizer to produce ROSs as shown in reactions (2)–(6) [4,26]. Zhao et al. confirmed that only the anionic form of OTC could generate singlet oxygen which was one of the important ROSs to photolysis of OTC under simulated solar light [22]. OTC + hv ! OTC
*
(2)
OTC * + 3O2 ! OTC + 1O2
(3)
OTC * + 3O2 ! OTC+ + O2
(4)
2O2 + 2H+ ! H2O2 + O2
(5)
H2O2 + hv ! 2HO
(6)
In order to verify the presence of HO in neutral condition, 100 mM isopropanol, which is commonly used as a HO scavenger, was added to 20 mg/L OTC solution. The result (Fig.S7) showed that the degradation rate of OTC was significantly decreased after addition of isopropanol but the removal efficiency of OTC was reduced by only 8% in 180 min. Therefore, we speculate that only a small amount of HO existed in neutral solution. Since direct degradation was mainly responsible for the decay of OTC under our experimental conditions, the effect of indirect degradation caused by a small amount of HO on the removal efficiency was limited. Based on the above discussion, we concluded that OTC photolysis occurred via direct photodegradation and an indirect process with the participation of ROSs in neutral and basic media; while in acid media, direct photolysis of OTC was the only process. 3.1.3. Effect of light intensity As shown in Fig. 2, with increasing light intensity from 250 W/m2 to 350 W/m2 and 500 W/m2, the removal efficiency of OTC in 270 min increased from 76.6% to 78.3% and 81.2%. Although the OTC degradation rate under diverse light intensity did not significantly differ, the higher light intensity was more favorable
Fig. 2. The influence of light intensity on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at pH of 7.1 and temperature of 20 C).
for OTC removal. The photon generation rate changes with different light intensities, and at a higher intensity, higher photon flux was provided [44]; more excited OTC molecules can then be produced and be involved in reactions via direct or indirect photodegradation. The slight difference of removal efficiency also indicated that light intensity was not a key factor to the photolysis of OTC under the experimental conditions. 3.2. Univariate effect of NO3 , Fe3+ and HCO3 3.2.1. The effect of NO3on photodegradation of OTC HO, a non-selective oxidant, is highly reactive to many organic chemicals with a secondary rate constant in the range of 108–1010 M1 s1 [17]. Nitrate is one of the important sources of hydroxyl radicals in natural water[45]; therefore it may play a key role in the degradation of OTC. To observe the influence of NO3 on the degradation of OTC, we conducted experiments in presence of nitrate with four different concentrations from 0 to 1.0 mM, which were similar to the real level typically observed in natural aquatic environment [46]. As shown in Fig. 3, with increasing nitrate concentration from 0.1 mM to 0.5 mM and 1.0 mM, the removal efficiency of OTC after 180 min irradiation enhanced from 70.6% to 80.2% and 82.9%. The overall kinetics of OTC photolysis in presence of nitrate can be well described by a two-step pseudo-first-order model (Fig.S8). Reaction rate constants (k1,a and k1,b for the first and second phases) were 0.01730 min1/0.00386 min1, 0.01751 min1/0.00435 min1, 0.02349 min1/0.00512 min1, 1 1 0.02827 min /0.00555 min at nitrate concentrations of 0 mM, 0.1 mM, 0 0.5 mM and 1.0 mM, respectively. NO3 can absorb light in UV range (shown in Fig.S5) and act as a sensitizer to promote the photoreaction. The light shielding effect of NO3 was negligible due to the very low molar extinction coefficient of NO3 in the UV–visible region [47,48]. The photolysis of NO3 leads to the formation of HO via reaction Eqs. (7)–(9) [44,47] and the produced HO can promote the indirect photodegradation of OTC. Although NO2 can be generated from NO3 (Eq. (8)), it would further polymerize to form N2O4 and finally to NO2 by the process of dismutation [49]. Thus, the effect of the NO2 was ignored in the transformation process of OTC in the present study. NO3 ! NO2 + O
NO3
! NO2+ O
(7)
(8)
Fig. 3. The influence of NO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 6.9).
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O + H2O ! HO+ OH
243
(9) NO3
hardly influenced the Previous studies have shown that destruction of OTC in the UV/H2O2, UV/PS[4] and UV/TiO2/5A or UV/TiO2/13X system [15] due to the high radical quantum yield of H2O2, PS and TiO2 under UV activation. However, NO3 could enhance OTC degradation in UV/NO3 system due to the generation of HO by the excited NO3, as discussed above. According to Eq. (9), HO should be proportional to NO3concentration. If OTC is reacted with equimolar HO, the degradation rate of OTC should also be proportional to NO3 concentration [44,50]. Fig. 4 presents a good linear relationship between rate constants and NO3 concentrations (R1,a2 = 0.97962, R1,b2 = 0.88872), as also represented by Eqs. (10) and (11). k
1,a
= 0.01155c + 0.01701
(10)
k
1,b
= 0.0016c + 0.00406
(11)
Where k1,a and k1,b represent the pseudo-first-order degradation rate constants of OTC in the two stages in min1 and c is the nitrate concentration in mM. Consequently, the greater degradation rate of OTC with increasing concentration of NO3was mainly due to the higher yield of hydroxyl radical within the studied concentration range. 3.2.2. The effect of Fe3+ on photodegradation of OTC Iron is ubiquitous in natural aquatic environment at concentrations ranging from 107 to 104 M [51]. In the present study, we used FeCl3 as the source of Fe3+ to study the effect of iron (III) on OTC degradation. In order to determine the impact of Cl, separate experiments with different amounts of NaCl added to OTC solution were also conducted. Results showed that the removal of OTC caused by Cl was negligible (details shown in Fig. S6), indicating that the degradation of OTC in FeCl3 solution at pH 7.0 (0.1) was solely due to the presence of Fe3+. It is true that chlorine may affect the photochemistry under acidic condition owing to that Cl can react with HO to form ClOH and finally transformed into Cl2 . However, the rapid regeneration of HO through the dissociation of the formed ClOH occurred instead of forming of Cl2 through the oxidation of Cl with HO at pH 7 [17]. Fig. 5 shows that Fe3+ inhibited OTC degradation at concentrations of 2.5 mM, 10 mM and15 mM. No significant difference in pseudo-first-order rate constants was observed with the addition of different concentrations of Fe3+ (Table S1).
Fig. 4. Effect of NO3 concentration on rate constant (k) for photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 6.9).
Fig. 5. The influence of Fe3+on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 6.9).
The existing literature proved that Fe3+ could produce HO via reactions (12)–(14) [34,51], and a positive role of Fe3+ to the photodegradation of OTC is then expected. Fe3+ + H2O ! Fe2+ + HO + H+
(12)
Fe3+ + H2O Ð Fe (OH)
(13)
Fe (OH)
2+
2+
+ H+
! Fe2+ + HO
(14) 3+
However, under our experimental conditions, Fe at different concentrations revealed an inhibition for OTC degradation. We tentatively attribute this result to pH used in this work, since the role of Fe3+ on photodegradation strongly depends on its speciation, which is pH dependent. At a pH of 7.0(0.1), Fe3+ in aqueous solution at our studied concentration range would be mainly in colloidal forms such as Fe(OH)3. The existing of Fe(OH)3 would result in substantially decreased production of HO because of its particulate nature. More importantly, Fe(OH)2+ is the predominant photoreactive species in Fe(III)-aquo complexes (Fe3+, Fe(OH)2+, Fe(OH)2+, dimer Fe(OH)24+) [44], whereas the photoactivity of Fe(OH)2+ in aqueous solution was significantly restrained at pH > 5.0 [51]. Therefore, the promotion effect of Fe3+ was not found under our conditions. In addition, although the light absorption of Fe3+ at our concentration range is less than that of OTC, the absorption spectrum of Fe3+ has a great overlapping with that of OTC (Fig.S5). Slightly light-shielding may slow down the degradation rate of OTC; similar results have also been reported by Peng et al. for photodegradation of nonylphenol [34]. 3.2.3. The effect of HCO3on photodegradation of OTC HCO3 is a rich anion in natural water systems with a concentration range from 0.4 mM to 4.4 mM [52]. Although HCO3 does not absorb light in solar spectrum, it can react with strongly oxidizing radicals (HO, SO4) or excited aromatic ketones to produce selective carbonate radicals (CO3), triggering indirect degradation of electron-rich organic pollutants [4]. Also, the addition of bicarbonate will increase the pH of the solution, and then affect the photochemical activity of organic compounds. The pKa1 and pKa2 of H2CO3 are 6.37 and 10.33, respectively [53]. Under our experimental condition (pH = 7.0 0.1), HCO3 is the major species, accounting for 81%; the rest are available in the form of dissolved carbon dioxide, accounting for about 19%. Dosage of bicarbonate added to the solution has little effect on the concentration of HCO3 in our case. Experiments with two
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concentrations (0 mM, 3.0 mM) of HCO3 were then operated at pH of 7.0. In order to verify whether the effect of HCO3 was due to its role on solution pH, a separate experiment with 3 mM HCO3 without pH adjustment (pH = 8.5) was also conducted. Results (Fig. 6) showed that the degradation of OTC was slightly increased with prolonged irradiation when the concentration of HCO3 was increased from 0 mM to 3.0 mM at both pH 7.0 and 8.5. However, compared with that at pH 7.0, it significantly increased at pH 8.5 when the concentration of HCO3 was 3.0 mM. The degradation rate constants were calculated according to a pseudofirst-order kinetics (Table S1). Fig. 6 reveals that the influence of HCO3 on degradation of OTC at pH 8.5 was mainly attributed to its pH-adjusting role. Similar results have also been obtained by other researchers for the degradation of tetracycline and furfural [26,54]. As stated above, there is a small amount of HO in neutral solution (pH 7.0); the addition of bicarbonate can produce selective CO3 via HO-scavenging according to Eq. (15) [55]. It was reported that the second-order rate constant for the reaction between OTC and CO3 is 2.9 108 M1s1 [4], which is one order of magnitude lower than that of 6.96–7.18 109 M1 s1 [9] for the reaction between OTC and HO. Therefore, a decreased degradation rate at the beginning of the reaction at pH 7.0 is certainly expected. However, with prolonged irradiation time and the elimination of HO (due to the exhaust of DO), the excited aromatic ketones or the excited triplet state of a sensitizer (such as OTC* generated in the solution) could react with bicarbonate to produce CO3 [55], leading to a slight promotion effect due to its considerable secondorder reaction with OTC. HO + HCO3 ! H2O + CO3
Table 1 The concentration of NO3, Fe3+ and HCO3 used in the univariate and multivariate experiments. No. of run
1 2 3 4 5 6 7 8 9 10 11 12
Concentration used OTC(mg/L)
NO3(mM)
Fe3+(mM)
HCO3(mM)
20 20 20 20 20 20 20 20 20 20 20 20
0 0.1 0.5 1.0 0 0 0 0 1.0 1.0 0 1.0
0 0 0 0 2.5 10.0 15.0 0 15.0 0 15.0 15.0
0 0 0 0 0 0 0 3.0 0 3.0 3.0 3.0
(15)
3.3. Multivariate effects of NO3, Fe3+ and HCO3 Based on results in Section 3.2, we selected 1.0 mM NO3, 15 mM Fe3+ and 3.0 mM HCO3 to study their comprehensive effects on OTC degradation. The experimental design was shown in Table 1. A pseudo-first-order kinetics was also adopted to calculate the rate constants of OTC degradation; and the correlation coefficient (R2) for the fitting was more than 0.95 in any condition (Table S1). 3.3.1. The comprehensive effect of NO3 and Fe3+ on OTC photodegradation As shown in Fig. 7, the influence of NO3 coupled with Fe3+ was not a simple additive effect on OTC photodegradation. The
Fig. 6. The influence of HCO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0 and 8.5).
Fig. 7. The comprehensive influence of NO3and Fe3+on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
degradation rate of OTC with co-presence of Fe3+ and NO3 was slightly higher than that with Fe3+ but much lower than that with NO3. And, we found no significant difference in the removal efficiency of OTC with co-addition of NO3 and Fe3+. This phenomenon was probably due to the fact that the slightly increased degradation rate caused by HO from NO3 (Eqs. (7)–(9)) could not completely compensate for the inhibition effect of Fe3+ as discussed in Section 3.2, resulting in a slightly negative effect on the degradation of OTC. In addition, both the absorption spectra of NO3 and Fe3+ (Fig.S5) overlap with that of OTC. Simultaneous addition of NO3and Fe3+ in the solution enhanced the light shielding effect, resulting from their competition with OTC molecules for photon absorption. That hinders OTC molecules to reach the excited state, consequently leading to a lower degradation rate than that of the direct photolysis of OTC. 3.3.2. The comprehensive effect of NO3 and HCO3 on OTC photodegradation As discussed in Section 3.2, NO3 and HCO3 promote photodegradation of OTC by generating HO and CO3, respectively. Fig. 8 depicts that combination of HCO3and NO3 exhibit a slightly antagonistic effect. Our results differed from those obtained by Dionysiou et al. [4] who have reported that the generation of CO3 presented a positive role on OTC degradation in UV/NO3/HCO3 system. It could be attributed to higher pH (pH = 9.0) used in their study. As stated above, alkalinity favors electrophilic attacking of HO and CO3, and thus promoted the indirect photodegradation of OTC. Moreover, the difference in
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alone, we found essentially no change in photodegradation rate of OTC with coexistence of Fe3+ and HCO3 (Fig. 9). A reasonable argument is that the concentration of Fe3+ is far less than that of HCO3; the influence of HCO3 surpasses the inhibition effect of Fe3+ and their comprehensive effect is mainly attributed to HCO3. The result also implies that reaction with CO3 is an important pathway for OTC photodegradation.
Fig. 8. The comprehensive influence of NO3 and HCO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
radical quantum yield caused by the wavelength of light used in the work of Dionysiou et al. may be another reason. The quenching effect of HCO3 on HO, produced from NO3 as depicted in Eqs. (7)– (9), leads to a decreasing of the indirect photolysis of OTC. Furthermore, compared to the presence of HCO3 alone, the co-existence of HCO3 and NO3 result in an increasing of the concentration of inorganic components in the solution, which is likely to decrease the amount of photons entered into the solution because of light attenuation [56]. The concentrations of excited state of OTC and CO3 are decreased; and a declined degradation rate in subsequent processes of photodegradation is then expected. However, the degradation rate of OTC with co-presence of HCO3and NO3 is still faster than its direct photolysis, indicating that the influence of CO3 on the degradation of electron-rich organic pollutants is not negligible. 3+
3.3.3. The comprehensive effect of Fe and HCO3 on OTC photodegradation As discussed in Section 3.2.2, Fe3+ inhibits OTC degradation due to its competitive effect on photon absorption and lower transmittance of solution caused by Fe(III)-aquo complexes; while the degradation of OTC in presence of HCO3 occurs mainly via direct photolysis and partly via indirect photolysis with participation of CO3. The comprehensive effect of coexisted HCO3 and Fe3+ is supposed to inhibit the degradation of OTC due to the light attenuation. However, compared with the presence of HCO3
Fig. 9. The comprehensive influence of Fe3+ and HCO3on the photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
3.3.4. The comprehensive effect of NO3, Fe3+ and HCO3 on OTC photodegradation Fig. 10 shows that the comprehensive effect of these three ions is not significant, even though we expect an enhanced degradation and removal efficiency of OTC. Combining with Figs. 7–9, we found no significant effect overall on the degradation of OTC for any of these four combinations. Their combined effect on OTC degradation is a suppression at the beginning of irradiation and then a promotion after prolonged irradiation for any combination with presence of HCO3, indicating that HCO3 may play an important role in these complex factors. In summary, results from irradiation in the co-presence of NO3, Fe3+ and HCO3 indicate that the direct photolysis of OTC is much more efficient than its indirect photochemical processes. 4. Conclusions We studied the direct and indirect photodegradation of OTC in aqueous solution by using a sunlight simulator in the present work. The comprehensive effects of typical aquatic constituents (NO3, Fe3+ and HCO3) in realistic levels as well as solution pH and irradiation intensity were systematically assessed. We anticipate providing a meaningful reference for understanding the environmental behavior and fate of OTC in aquatic systems. The conclusions are as follows: (1) OTC degradation rate and removal efficiency are significantly enhanced with increasing solution pH but slightly promoted with increasing irradiation intensity. (2) OTC degradation proceeds via a direct photolysis and HO- or/ and CO3-involved indirect photolysis; while direct photolysis is dominant and the photodegradation follows a two-step pseudo-first-order kinetics in neutral or alkaline aquatic environments. (3) Under our experimental conditions, OTC photodegradation rate increases with increasing NO3 concentration, owing to more HO generated; and the promotion effect of HCO3 is
Fig. 10. The comprehensive influence of NO3, Fe3+ and HCO3on photodegradation of OTC (C0 (OTC) = 20 mg/L, at irradiation intensity of 765 W/m2 and pH of 7.0).
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mainly attributed to its adjusting role on solution pH; while Fe3+ inhibits the degradation of OTC by light shielding. (4) Results from irradiation in co-presence of NO3, Fe3+ and HCO3 indicate that the direct photolysis of OTC is much more efficient than its indirect photolysis in neutral solutions. Given the extensive use and widespread presence of OTC in natural waters, further studies on monitoring of the degradation intermediates and by-products are needed for understanding its transformation mechanism and for better assessing the risks associated with antibiotics released into the aquatic environment. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (Grant No. 41276067) and the Fundamental Research Funds for the Central Universities (201762006, 201762030) of Ocean University of China. We also thank Professor Dr. Jiangtao Wang (College of Chemistry and Chemical Engineering, Ocean University of China) for helps in HPLC measurement. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jphotochem.2018. 01.004. References [1] Q.Q. Zhang, G.G. Ying, C.G. Pan, Y.S. Liu, J.L. Zhao, Comprehensive evaluation of antibiotics emission and fate in the river basins of China: source analysis, multimedia modeling, and linkage to bacterial resistance, Environ. Sci. Technol. 49 (2015) 6772–6782. [2] M. Duan, H. Li, J. Gu, X. Tuo, W. Sun, X. Qian, X. Wang, Effects of biochar on reducing the abundance of oxytetracycline antibiotic resistance genes, and human pathogenic bacteria in soil and lettuce, Environ. Pollut. 224 (2017) 787– 795. [3] K.F. Rubert, J.A. Pedersen, Kinetics of oxytetracycline reactionwith a hydrous manganese oxide, Environ. Sci. Technol. 40 (2006) 7216–7221. [4] Y. Liu, X. He, X. Duan, Y. Fu, D. Fatta-Kassinos, D.D. Dionysiou, Significant role of UV and carbonate radical on the degradation of oxytetracycline in UV-AOPs: kinetics and mechanism, Water Res. 95 (2016) 195–204. [5] Y. Li, X. Qiao, Y.N. Zhang, C. Zhou, H. Xie, J. Chen, Effects of halide ions on photodegradation of sulfonamide antibiotics: formation of halogenated intermediates, Water Res. 102 (2016) 405–412. [6] S. Zou, W. Xu, R. Zhang, J. Tang, Y. Chen, G. Zhang, Occurrence and distribution of antibiotics in coastal water of the Bohai Bay, China: impacts of river discharge and aquaculture activities, Environ. Pollut. 159 (2011) 2913–2920. [7] K. Li, A. Yediler, M. Yang, S. Schulte-Hostede, M.H. Wong, Ozonation of oxytetracycline and toxicological assessment of its oxidation by-products, Chemosphere 72 (2008) 473–478. [8] A.J. Watkinson, E.J. Murby, S.D. Costanzo, Removal of antibiotics in conventional and advanced wastewater treatment: implications for environmental discharge and wastewater recycling, Water Res. 41 (2007) 4164–4176. [9] Y. Liu, X. He, Y. Fu, D.D. Dionysiou, Kinetics and mechanism investigation on the destruction of oxytetracycline by UV-254 nm activation of persulfate, J. Hazard. Mater. 305 (2016) 229–239. [10] M.F. Zaranyika, P. Dzomba, J. Kugara, Degradation of oxytetracycline in the aquatic environment: a proposed steady state kinetic model that takes into account hydrolysis, photolysis, microbial degradation and adsorption by colloidal and sediment particles, Environ. Chem. 12 (2015) 174–188. [11] L. Fu, T. Huang, S. Wang, X. Wang, L. Su, C. Li, Y. Zhao, Toxicity of 13 different antibiotics towards freshwater green algae Pseudokirchneriella subcapitata and their modes of action, Chemosphere 168 (2017) 217–222. [12] G. Zhang, B. Wu, S. Zhang, Effects of acetylacetone on the photoconversion of pharmaceuticals in natural and pure waters, Environ. Pollut. 225 (2017) 691– 699. [13] X. Jin, H. Xu, S. Qiu, M. Jia, F. Wang, A. Zhang, X. Jiang, Direct photolysis of oxytetracycline: influence of initial concentration, pH and temperature, J. Photochem. Photobio. A Chem. 332 (2017) 224–231. [14] J.H.O.S. Pereira, D.B. Queirós, A.C. Reis, O.C. Nunes, M.T. Borges, R.A.R. Boaventura, V.J.P. Vilar, Process enhancement at near neutral pH of a homogeneous photo-fenton reaction using ferricarboxylate complexes: application to oxytetracycline degradation, Chem. Eng. J. 253 (2014) 217–228.
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