Photolysis of enrofloxacin and removal of its photodegradation products from water by reverse osmosis and nanofiltration membranes

Photolysis of enrofloxacin and removal of its photodegradation products from water by reverse osmosis and nanofiltration membranes

Separation and Purification Technology 115 (2013) 1–8 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology journa...

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Separation and Purification Technology 115 (2013) 1–8

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Photolysis of enrofloxacin and removal of its photodegradation products from water by reverse osmosis and nanofiltration membranes Davor Dolar a,⇑, Krešimir Košutic´ a, Martina Periša b, Sandra Babic´ b a b

Department of Physical Chemistry, Faculty of Chemical Engineering and Technology, Marulic´ev trg 19, HR-10000 Zagreb, Croatia Department of Analytical Chemistry, Faculty of Chemical Engineering and Technology, Marulic´ev trg 19, HR-10000 Zagreb, Croatia

a r t i c l e

i n f o

Article history: Received 8 February 2013 Received in revised form 21 April 2013 Accepted 22 April 2013 Available online 30 April 2013 Keywords: Enrofloxacin Photolysis Photodegradation products Removal by RO/NF membranes

a b s t r a c t Photolysis in natural aquatic environment might be an important elimination process for the light sensitive antibiotics, such as fluoroquinolones. Photolytic degradation of antibiotics present in wastewaters and natural aquifers can result in generation of new products that may be even more toxic than the parent compound. Therefore, it is important to identify these products and to remove them before entering, for example, drinking water sources, whereby the role of membrane processes is irreplaceable. In this study photodegradation of enrofloxacin (ENRO) in aquatic media was performed at two different pH values, 4 and 8. Four and three photodegradation products (PDPs) with different molecular weight were identified at pH 4 and 8, respectively. All of them, together with the enrofloxacin as the parent compound were removed almost completely (>99%) by reverse osmosis (RO) and tight nanofiltration (NF) membranes, while by loose NF membranes >92%, except for the smallest PDP with HL membrane (37%) at pH 8. The minimum amount of the compounds in the permeate is the proof that most of the selected RO/NF membranes make an effective barrier for them. The size exclusion was the main rejection mechanism, while the physico-chemical interactions and electrostatic repulsion/attraction probably had influence on the overall rejection by loose NF membranes for the smaller photodegradation products of ENRO. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Antibiotics are commonly used in human and animal medicine, and the presence of these compounds in the environment is of concern due to their role in the development of antimicrobial resistance among microorganisms. The fluoroquinolone class of antibiotics is the fourth largest class used in human medicine and is also widely used in veterinary therapy [1]. Their presence in the environment thus may pose serious threats to the ecosystem and human health. Enrofloxacin (ENRO) is a synthetic chemotherapeutic agent belonging to the second generation of the fluoroquinolone carboxylic acid derivatives [2]. It has antibacterial activity against a broad spectrum of Gram-negative and Gram-positive bacteria, and it is effective against Escherichia coli, Enterobacter, Salmonella, Staphylococcus and many others. Therefore, ENRO was frequently detected in various wastewaters (slaughterhouse [3], municipal sewage [4–7], hospital [7], pharmaceutical industry [7] and from livestock [7]) at concentrations in the range of nano- and even micrograms

⇑ Corresponding author. Tel.: +385 1 4597 233; fax: +385 1 4597 250. E-mail address: [email protected] (D. Dolar). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.04.042

per liter (up to 407 ng L 1 in the slaughterhouse wastewater, 1.47 lg L 1 in the wastewater from pharmaceutical industry and 100 ng L 1 in the municipal sewage). Previous studies have shown that some antibiotics, including fluoroquinolones, are poorly eliminated in the sewage treatment plants (SWT) and may reach surface waters [8–11]. Consequently, ENRO was detected in the surface waters around the world [3,12– 15]. Concentrations of ENRO in Nansha River (China) ranged from 3 to 20 ng L 1 [3], while higher concentrations were detected in Italy (the Po river, 37 ± 5 ng L 1 and the river Ticino, 27 ± 4 ng L 1) [13]. The highest concentrations (67.0–102.5 ng L 1) were found in Mondego River near Coimbra (Portugal) by Pena et al. [15]. All steps in SWT (pre-treatment, primary settling, biological reactor and the secondary settling [5,16]) are open and coupled with the incomplete removal of antibiotics. During these steps ENRO can undergo photodegradation, due to the direct exposure to solar irradiation. Exposure to sunlight can result in different degradation products which end up in the surface or ground waters. Photolysis has already been confirmed as one of the most important ways of antibiotics elimination in the natural aquatic environment [1,17], although the structure, residual antibacterial activity and persistence of the photoproducts are still under examination. There is even some evidence that degradation products of

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some compounds can be as active and/or toxic as their parent compound [18]. Ultimately, the contaminated surface waters can enter drinking water treatment plants (DWTPs), which are also not very efficient in removing these compounds, reaching the water distribution systems. Degradation products of ENRO can also be considered as contaminants contributing to these complex mixtures. Although there are papers (most of them by the Sturini group) dealing with investigation of photodegradation pathways and identification of degradation products of ENRO photolysis [17,19–21], according to the authors’ knowledge there is only one paper from our group dealing with the removal of photodegradation products (more precisely the anthelmintic drugs) [22]. Hence, it is important to investigate new techniques for removing antibiotics and their photodegradation products (PDPs) before entering surface waters, which can be implemented in SWT or DWTPs or that can replace existing treatment plants. Since ENRO is zwitterion, kinetics of its photolysis as well as the nature of photodegradation products is greatly affected by the pH value. Due to the previously expressed statements and importance of removal of photodegradation products, the aim and the novelty of this study was to investigate the removal of enrofloxacin’s photodegradation product with reverse osmosis (RO) and nanofiltration (NF) membranes at two different pH values (4 and 8).

2. Methods and materials 2.1. Chemicals Enrofloxacin was supplied by Sigma–Aldrich. Standard ENRO solution (10 mg L 1) was prepared using Milli-Q water at two different pH values, 4 and 8. Although such high concentrations of pharmaceuticals rarely could be found in real samples [23,24], ENRO solution of 10 mg L 1 was used in order to reach faster membrane saturation (small concentration can lead to overestimation of the rejection [25]) and to allow determination of the photodegradation products. All the solvents used were HPLC-grade supplied by Kemika (Zagreb, Croatia). For water pH-value adjustment, hydrochloric acid (0.1 mol L 1) and sodium hydroxide solutions (0.1 mol L 1) were used. The chemical structure and physico-chemical characteristics of ENRO are given in Table 1. The 3D structures, molecular size (length, width and height) of all compounds were determined with software package ‘‘HyperChem 8.0’’. The molecular mechanics was applied to optimize the conformation of each compound. The conformations with minimal energy were found using the Polak–Ribiere algorithm, with a convergence limit of 0.4184 kJ mol 1 or a maximum number of calculation cycles set at 390.

Table 1 Physico-chemical characteristics of enrofloxacin. Chemical structure O F N CH3-CH2

N

CAS number: 93106-60-6

COOH N

Molecular formula Molecular weight (MW) log KO/W pKa [26] Water solubility

C19H22F1N3O3 359.40 g mol 0.70 3.85; 6.19; 7.59; 9.86 3397 mg L

1

1

Data obtained by EPI SUITETM 4.10 (03.01.2012); log KO/W is estimated value for unionized form.

2.2. Photolysis experiments The photolysis experiments were conducted in a Suntest CPS + simulator (Atlas, Germany) equipped with a xenon lamp and a temperature sensor. The device emitted radiation in the wavelength range of 300–800 nm to simulate the natural sunlight. During the experiments, the radiation intensity was maintained at 500 W m 2 and the reaction temperature was kept at (25 ± 2) °C. Four hundred milliliters of ENRO solution were exposed to simulated sunlight for a continuous duration of 2 h. Water as a solvent can only absorb the light less than 220 nm, while ENRO can absorb light in the wavelength range of 230–350 nm. Thus, the solvent cannot influence the light absorption behavior of ENRO. In order to confirm that the degradation of ENRO was affected only by the light and not by the temperature or hydrolysis, a control sample was also analyzed. It had the same composition as the initial ENRO solution and was exposed under the same conditions but protected from light (a vessel with a control sample was wrapped in the aluminum foil). Irradiated solution containing ENRO photodegradation products was used to investigate their removal by RO/NF membranes. 2.3. Membranes The commercially available membranes with different characteristics examined in this experiment included two RO membranes: the LFC-1 and SWC4 + (Hydranautics, Oceanside, CA, USA), and four NF membranes: NF90 and NF (Dow/FilmTec, Midland, MI, USA), and HL and DK (Desal, Osmonics, GE Infrastructure Water Process Techn., Vista, CA, USA). All membranes were stored in a dark cold place (refrigerator) before use. The main nominal and physico-chemical characteristics of the membranes used are presented in Table 2. Removal of ENRO and its PDPs was tested in a laboratory set-up (described in details in Dolar et al. [36]) at a working pressure of 10 bar and flow rate of 750 mL min 1. The six homemade RO/NF cells of the same type and dimensions were connected in parallel. The RO/NF cell consists of two detachable parts. The upper part is a high pressure chamber provided with inlet and outlet openings for the flow of the feed solution under pressure. The lower part contains an outlet opening for the membrane permeate. The membrane is mounted on a stainless steel porous plate embedded in the lower part of the cell, and the surface layer of the porous membrane faces the feed solution on the high pressure side of the membrane. There is no possibility to control concentration polarization in this apparatus. Therefore, membrane cells were connected in parallel to decrease the concentration polarization. The surface area of the membranes was 10.7 cm2. Virgin preserved membranes were washed with demineralized water without pressure and then pressurized at 15 bar for approximately 3 h in order to stabilize the permeate flux. Samples of enrofloxacin irradiated solution were taken after 4 h of recirculation in order to establish saturation. Dolar et al. [39] showed that the organic saturation on membranes was achieved after 4 h of recirculation in most cases. Recovery is a parameter of economic importance, and represents the fraction of the feed flow through the membrane. According to Mulder [40], a recovery value in the laboratory scale does not necessary have to be measured, since it usually approaches zero. In this study, recovery with tested membranes were 0.070 ± 0.002% (DK), 0.034 ± 0.002% (SWC4), 0.053 ± 0.002% (LFC-1), 0.141 ± 0.012% (NF), 0.184 ± 0.024% (HL) and 0.125 ± 0.019% (NF90). 2.4. Analytical determination To determinate the removal efficiency of both ENRO and its photodegradation products by RO/NF membranes, samples were

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D. Dolar et al. / Separation and Purification Technology 115 (2013) 1–8 Table 2 Membrane nominal and physicochemical characteristics.

a

MWCO RNaCl (%)b RCaCl2 (%)b Jw (L m 2 h 1)c Pore size (nm)d Contact angle (°) [27] RMS roughness (nm)e Zeta potential [28–35] pH 4 (mV) pH 8 (mV) pH 9 (mV) [27] a b c d e

DK

SWC4+

LFC-1

NF

HL

NF90

150–300 61.6 83.3 30.58 ± 0.87 1.06 + 1.36 2.0 40.6 ± 5.2 16.4 ± 3.1

100 98.6 98.3 14.31 ± 0.31 0.32 48.8 ± 7.1 135.6 ± 12.0

100 95.1 97.1 21.17 ± 3.37 0.81 20.1 ± 3.7 135.8 ± 12.8

150–300 64.8 46.1 70.59 ± 2.96 0.76 + 1.48 1.84 – –

150–300 12.1 65.2 87.53 ± 2.58 0.85 + 1.4 2.1 27.5 ± 4.3 7.2 ± 2.6

100–200 84.7 96.9 59.5 ± 1.36 0.81 44.7 ± 1.9 129.5 ± 23.4

0

0; 6 15; -17 20.9

3; 3.2 2; 5 13.2

6

20 18.5

17(pH 7.8) –

5; 4 15; 23 26.0

1; 3; 4 7; 16; 37.0

27

From literature: [36,37]. Determined in this study. Pure water flux (experimental data from this study (N = 5)). Pore size were determined in this study by the modified examination method based on the specific solutes (markers) transport [38]. Root mean square (RMS) [27].

analyzed by HPLC-MS/MS. The analysis was performed using an Agilent 6410 triple-quadrupole mass spectrometer equipped with an ESI interface coupled with an Agilent (Santa Clara, CA, USA) Series 1200 HPLC. The column used for separation was Synergy Fusion C18 embedded column (150 mm  2.0 mm, particle size 4 lm) supplied by Phenomenex. The mobile phase consisted of 0.1% formic acid in Milli-Q water as eluent A and 0.1% formic acid in acetonitrile as eluent B was used in the gradient elution mode at a flow rate of 0.2 mL min 1. The elution gradient started with 100% of eluent A and linearly decreased to 92% A over the first 2.30 min. The percentage of eluent A was 90% in 6 min, while for another 5 min the elution gradient decreased to 70% A, continuing to decrease to 40% A within following 4 min. The percentage of eluent A decreased to 5% in 18 min and these conditions were held for 10 min before the initial mobile phase composition was restored at 28.10 min. After the gradient elution, the column was equilibrated for 12 min before the next injection. An injection volume of 5 lL was used in all analyses. The analyses were done in positive-ion mode. The conditions for the analyses were: drying gas temperature of 350 °C; capillary voltage of 4.0 kV; drying gas flow of 11 L min 1 and nebulizer pressure of 2.41 bar. The limit of detection (LOD) was 20 ng L 1 for ENRO, while limit of quantification (LOQ) was 50 ng L 1. LOD and LOQ were estimated on the basis of a signal-to-noise ratio of 3 and 10, respectively. Instrument control, data acquisition and evaluation were done with Agilent MassHunter 2003–2007 Data Acquisition for Triple Quad B.01.04 (B84) software.

3. Results and discussion 3.1. Identification of ENRO photodegradation products Fluoroquinolones are zwitterions and the pH values greatly influences their photolysis, with respect to both the kinetics and the nature of the photodegradation products [17,41,42]. This is relevant because of the large pH range among environmental waters, especially for wastewaters which may have different pH values depending on their origin (e.g. wastewater from the pharmaceutical industry, hospital wastewater or municipal wastewater). Therefore, photolysis of ENRO was performed at two different pH values, acidic (pH 4) and mildly alkaline (pH 8). Irradiation of 400 mL ENRO solution (10 mg L 1) for 2 h under artificial light was not long enough for complete ENRO degradation at both investigated pH values, since 4% and 50% of the initial ENRO concentration remained after irradiation at pH 8 and pH 4,

respectively. The zwitterion existing at pH 8 reacts faster than the cationic form at pH 4. Despite incomplete ENRO degradation, four photodegradation products with different molecular weight (MW), and ciprofloxacin (CIPRO) [1,17,31] as one of them were detected. CIPRO was detected only in acidic conditions due to the slower degradation. Among these photodegradation products, PDP4-1/PDP8-1 (m/z 115) and PDP4-3/PDP8-3 (m/z 374) were detected at both investigated pH values. Another two photodegradation products, PDP4-4 (m/z 344) and PDP8-2 (m/z 390) were detected only at pH 4 and 8, respectively. Table 3 presents the molecular structures calculated with HyperChem software, retention times (tR), precursor ions (m/z) and MW of ENRO and its PDPs at both investigated pHs. The structures of PDPs and photodegradation pathways were suggested by Babic´ et al. [43]. Two main processes resulted primarily from ENRO depending on the pH values: cyclopropane ring cleavage at pH 4 and oxidative photodegradation at pH 8.

3.2. Removal of ENRO and its photodegradation products by RO/NF membranes As summarized by Bellona et al. [44], the solute and membrane parameters are important in rejection of the solute. The composition of the feed water, like ionic strength, hardness, and the presence of organic matter, were not considered as important factors in our study, because experiments were done only with Milli-Q water. Molecular weights (MW), molecular sizes (length and width) and structures of degradation products were taken into account for understanding the rejection mechanism [45]. Generally, the solute parameters for degradation products are hard to define because standards for most of them are not available. But, in our case, parameters for ENRO (Table 1) and CIPRO were both obtainable. The experimental log KO/W for the latter was 0.28 [45]. CIPRO is consisted of one carboxylic group and three amine groups. Therefore, it has four pKa values in the aqueous solution (i.e. 3.01, 6.14, 8.70 and 10.58) [26]. The molecule is positively charged below pH 8.70, and neutrally charged within the range of 8.70 < pH < 10.58. According to this range, CIPRO was positively charged at pH 4. Degradation products can be presented by their shape and structure. The various ways of presenting shape of organic compounds are summarized by Bellona et al. [44], but most of them are not applicable due to the missing parameters. For example, Braeken et al. [46] showed correlation between the molecular size and Stokes diameter depending on diffusivity, which is not available for all organic compounds, especially for degradation

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Table 3 Molecular structures, retention time (tR), precursor ion (m/z) and molecular weight (MW) of ENRO and its PDPs at pH 4 and 8. Name

Molecular structure

tR (min)

m/z

MW (g mol

pH 4 PDP4-1

1.966

115

114

CIPRO

16.266

332

331

PDP4-3

16.567

374

373

PDP4-4

19.968

344

343

1

)

5

D. Dolar et al. / Separation and Purification Technology 115 (2013) 1–8 Table 3 (continued) Name

Molecular structure

ENRO

tR (min)

m/z

MW (g mol

16.895

360

359

pH 8 PDP8-1 PDP8-2

Same as PDP4-1

1.972 8.819

115 390

114 389

PDP8-3 ENRO

Same as PDP4-3 Same as at pH 4

16.231 16.570

374 360

373 359

Carbon

; Nitrogen

; Oxygen

; Fluorine

1

)

.

products. Consequently, photodegradation products are in this paper presented with an effective diameter of an organic component in water (dc) and molecular size (length, width and height), as presented in Table 4. The relationship between the molecular weight of an organic component and its effective diameter in water is: dc = 0.065 (MW)0.438 [47]. Although molecular weight is not a direct measure of a molecule dimension, it still reflects the molecular size in some way, and it represents a readily accessible parameter. This correlation was used because it is valid for the molecular weight range where nanofiltration typically operates (up to ±600 Da) [48]. As can be seen from data presented in the Table 4, all photodegradation products are longer than 1 nm and wider than 0.58 nm, except PDP4-1 and PDP8-1. This is important for their efficient removal via size exclusion, and for determining rejection mechanisms. Table 4 presents three sizes for PDP4-4, since there are three possible structures with the same MW, explained by Babic´ et al. [43]. As can be seen from the Table 4, all three structures are very similar in size. The rejection factors of all photodegradation products with investigated RO/NF membranes at pH 4 and 8 are presented in Table 5. As can be seen from these tables, rejection of all compounds

Table 4 Molecular weight, effective diameter in water (dc) and molecular size (length, width and height) of the PDPs at pH 4 and 8. MW (g mol

a

1

) dc (nm) Lengtha (nm) Widtha (nm) Heighta (nm)

pH 4 PDP4-1 CIPRO PDP4-3 PDP4-4

114 331 373 343

0.517 0.825 0.870 0.838

ENRO

359

pH 8 PDP8-1 PDP8-2 PDP8-3 ENRO

114 389 373 359

0.855

0.530 1.146 1.035 1.333 1.358 1.313 1.330

0.266 0.673 0.643 0.698 0.585 0.640 0.664

0.091 0.255 0.649 0.252 0.252 0.218 0.286

0.517 0.886 0.870 0.855

0.530 1.213 1.035 1.330

0.266 0.737 0.643 0.664

0.091 0.512 0.649 0.286

Determined with Hyperchem software.

was higher than 92%, except for PDP8-1 with HL membrane (36.9%). The size of the molecules is assumed to be one of the main factors that determine retention. Therefore, retention will be

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Table 5 Rejection factors of PDPs with investigated RO/NF membranes at pH 4 and 8. R (%) MW (g mol pH 4 PDP4-1 CIPRO PDP4-3 PDP4-4 ENRO pH 8 PDP8-1 PDP8-2 PDP8-3 ENRO

1

)

SWC4+

LFC-1

NF90

NF

HL

DK

114 331 373 343 359

>99.9 >99.9 >99.9 >99.9 >99.9

>99.9 >99.9 >99.9 >99.9 >99.9

>99.9 >99.9 >99.9 >99.9 >99.9

>99.9 >99.9 >99.9 >99.9 >99.9

>99.9 >99.9 >99.9 >99.9 98.5

93.5 >99.9 >99.9 >99.9 99.4

114 389 373 359

>99.9 >99.9 >99.9 >99.9

>99.9 >99.9 >99.9 >99.9

>99.9 >99.9 >99.9 >99.9

94.6 >99.9 >99.9 >99.9

36.9 >99.9 94.9 92.1

95.3 >99.9 >99.9 >99.9

correlated firstly with the molecular weight cut-off (MWCO), since Yang et al. [49] showed that the nominal MWCO is only valid for predicting the retention in the aqueous solutions. Both RO (LFC-1 and SWC4+) and tight nanofiltration (NF90) membranes almost completely (>99.9% of concentrations were below the limit of detection) removed all photodegradation products at pH 4 and 8. MWCOs of the used RO and tight nanofiltration membranes were 100 and 100–200 Da [36,37], respectively. Also, pore sizes of LFC-1 and NF90 membranes were 0.78 nm and 0.79 nm, respectively [36]. It is evident from Table 4 that all PDPs present in the solution after irradiation (except PDP4-1 and PDP81) were bigger (taking into account dc and molecular length) than MWCO and pore sizes of the membranes. Hence, one of the most important rejection mechanisms for these membranes was the size exclusion effect. It was expected that PDP4-1 and PDP8-1 would diffuse through the membrane pores due to the small MW and molecular size, but two additional rejection mechanisms probably had the significant effect on the complete removal. The first mechanism were physico-chemical interactions between compounds present in the solutions confirmed by Dolar et al. [36], while the second one was electrostatic repulsion/attraction between compounds and membranes. As stated by some authors [30,31,34,50], zeta potential of membranes decreases with the increase of pH. Also, most of the membranes used in this study were positively charged at pH 4, while compounds acted as zwitterions approaching to positive charge at neutral conditions due to protonated amino groups in the structure. Degradation product PDP4-1 was positively charged at pH 4 due to protonated amino groups, thus electrostatic repulsions acted as an additional rejection mechanism. At pH 8, membranes were charged more negatively, so the electrostatic attraction, together with the size exclusion, was probably most pronounced. In neutral conditions, ENRO is present as zwitter ion [17,51], while CIPRO is positively charged due to three amino groups. In spite of CIPROs big size, the probable electrostatic repulsion at pH 4 contributed to the rejection. The rejection of CIPRO with loose NF membranes was little higher (compared to Dolar et al. [36,52]), which could be due to more pronounced physico-chemical interactions between molecules and the electrostatic repulsion/attraction between CIPRO and membranes, since concentration of CIPRO in present study was much lower than in previous works. Compared to RO and tight nanofiltration membranes, loose nanofiltration (NF, HL and DK) membranes had MWCO between 150 and 300 Da, and bimodal pore size distributions (PSDs) [36]. This means that most pores are located at 0.7 nm, followed by pores at 1.56 nm. The important fact is that bigger pores are located even up to 2 nm, which makes it significant for the removal of organic compounds with the size exclusion mechanism. Therefore, all compounds found in the solution were close to upper

range of MWCO, while the dc and molecule size (length and width) revealed that they can enter the bigger pores in the membrane skin. Even though, decrease in rejection of the degradation products was not observed. Most of the compounds were almost completely removed at pH 4, probably due to the physico-chemical interactions between compounds and electrostatic repulsion. Rejection of CIPRO with HL and NF membranes was little higher than observed in our previous paper [36]. The possible explanation could lie in the physico-chemical interactions (presence of five compounds) and electrostatic repulsions/attractions. The electrostatic interactions are mentioned because compounds are mostly protonated at pH 4, while zeta potential of HL membrane has a little negative charge (Table 2). Rejection of PDP4-1 with DK membrane, in the amount of 93.5%, was lower than removal with the other loose nanofiltration membranes, because zeta potential of DK membrane at pH 4 is 0 mV [30]. Therefore, the lack of electrostatic repulsion/attraction probably resulted in a lower removal. A very high rejection of PDP4-1 can also be the result of amide group forming, because carbonyl group of imide ring in the polymer chains are able to react with amines of the compound [45]. At pH 8, after 2 h of irradiation, three photodegradation products were present in the solution. Two of them (PDP8-1 and PDP8-3) were equal to those at pH 4 (PDP4-1 and PDP4-3), while PDP8-2 was the new one. With RO and tight NF90 membranes, all photoproducts were almost completely (>99.9%) removed via the size exclusion, probably coupled with the electrostatic attraction (which was more pronounced than at pH 4), since membranes are more negatively charged at pH 8 (data shown in Table 2). Photodegradation products with MW higher than 359 g mol 1 were completely removed with all membranes, except in the case of HL membrane, confirming the same removal mechanism as discussed before, probably coupled with the physico-chemical interactions. Rejection factors for PDP8-1 with loose nanofiltration membranes were smaller than factors for PDP4-1. It was the smallest photodegradation product. A possible explanation could be that it is neutral or has very small positive charge at pH 8. Hence, it could pass through bigger pores in the membrane selective layer (skin). Our observations regarding MW, the molecular size (primarily length and width) and MWCO have confirmed the importance of the dimensional parameters for the removal of molecules with different molecular structures with RO and tight NF membranes, and can be used particularly for the degradation products. Hence, in line with the findings of the other authors, it can be concluded that the retention of pharmaceuticals by these membranes is principally governed by the steric interaction [36,53,54]. As mentioned before, according to the molecule length, width and dc, compounds can enter to bigger pores in the membrane polymer skin, especially in loose nanofiltration membranes. Nevertheless, rejections were very high (>92%). Due to the lack of parameters for PDPs, preferential orientation can be used for analyzing their removal and could influence it. In some situations, molecules can rotate freely, which depends on the preferential orientation. For non-spherical, especially elongated molecules, preferential orientation strongly influences the solute rejection [55]. The location of the hydroxyl groups in the structure of photodegradation products, except PDP4-1 and CIPRO, is in the center of the molecule. Therefore, the preferential orientation of these molecules would be such that the electrical dipole associated with the hydroxyl group will stay as far as possible from the negatively charged membrane [55]. Hence, a more horizontal orientation in relation to the membrane or longest axis parallel to the membrane could be assumed for these compounds. As a consequence, a high rejection was obtained.

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In the case of ENRO and CIPRO, it can be assumed that adsorption did not influence the rejection, because log KO/W was 0.70 and 0.28, respectively. It is known that hydrophilic organic compounds do not adsorb on polymeric membrane matrix [56], and the most of the membranes tested here had relatively hydrophobic character (Table 2). The membrane values for hydrophobicity of other photodegradation products that were almost completely removed with RO and tight NF are not available, and therefore it is hard to discuss the adsorption on membrane polymeric matrix according to hydrophobicity. It is known that the molecular size and hydrophobicity influence the retention of an organic compound (PDPs in our case), but Braeken et al. [46] found that the larger the molecule, the less the effect hydrophobicity has on its retention. Therefore, at this stage of the research, the molecular size of PDPs found after photolysis, especially for larger molecules, is an important parameter for determining the rejection mechanism. Also, structure of the photodegradation products can help us in understanding interactions between a membrane and a compound. The size exclusion effect, the physico-chemical interactions and the electrostatic attractions/repulsions are very well known mechanisms for the rejection of the organic compounds. From the structures presented in Table 3, it is evident that all compounds, except PDP4-1 and PDP8-1, have oxygen and fluorine. They are strongly electronegative atoms important in the creation of the hydrogen bonds, the strongest intermolecular forces after the charge interactions [57]. Therefore, if compounds with strongly electronegative atoms can reach pores present in the skin, especially in case of the loose NF membranes, the hydrogen bonds probably have influence on the rejection. 4. Conclusions ENRO is frequently detected in various wastewaters at concentrations in the range of nano- to even micrograms per liter, and as an emerging contaminant should be eliminated as far as possible. In the natural aquatic environment, photolysis (such as ENRO photolysis), is an important elimination process for light sensitive antibiotics, carried out under simulated solar irradiation for 2 h. The photolytic degradation of ENRO resulted in four and three photodegradation products (PDPs) with a different molecular weight at pH 4 and 8, respectively. The dense RO and tight NF membranes removed ENRO and its all photodegradation products almost completely (>99.9%). It was confirmed that the size exclusion acted as the main rejection mechanism. Finally, the physico-chemical interactions and the electrostatic repulsion/attraction probably influenced the overall rejection by loose NF membranes for the smaller photodegradation products. Acknowledgements This work has been supported by the Croatian Ministry of Science, Education and Sports Projects: 125-1253008-3009 Membrane and adsorption processes for removal of organic compounds in water treatment and 125-1253008-1350 Advanced analytical methods for pharmaceuticals determination in the environment. References [1] S.K. Khetan, T.J. Collins, Human pharmaceuticals in the aquatic environment: a challenge to green chemisty, Chem. Rev. 107 (2007) 2319–2364. [2] Y. Xiao, H. Chang, A. Jia, J. Hu, Trace analysis of quinolone and fluoroquinolone antibiotics from wastewaters by liquid chromatography–electrospray tandem mass spectrometry, J. Chromatogr. A 1214 (2008) 100–108. [3] B. Shao, D. Chen, J. Zhang, Y. Wu, C. Sun, Determination of 76 pharmaceutical drugs by liquid chromatography–tandem mass spectrometry in slaughterhouse wastewater, J. Chromatogr. A 1216 (2009) 8312–8318.

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