Journal of Hazardous Materials 296 (2015) 248–255
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Integrating electrochemical oxidation into forward osmosis process for removal of trace antibiotics in wastewater Pengxiao Liu a,c , Hanmin Zhang a,∗ , Yujie Feng b,∗∗ , Chao Shen a , Fenglin Yang a a Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China b State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73, Huanghe Road, Nangang District, Harbin 150090, China c Beijing Aerospace Institute for Metrology and Measurement Technology, China Academy of Launch Vehicle Technology, Beijing 100076, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Forward osmosis with the function of electrochemical oxidation was established. • FOwEO has the capability to thoroughly remove trace antibiotics from wastewater. • Rejection and degradation efficiencies for antibiotics were improved in FOwEO. • A synergetic effect between FO and EO was well achieved in FOwEO.
a r t i c l e
i n f o
Article history: Received 14 January 2015 Received in revised form 26 March 2015 Accepted 16 April 2015 Available online 18 April 2015 Keywords: Wastewater Trace antibiotics Forward osmosis Concentrate disposal
a b s t r a c t During the rejection of trace pharmaceutical contaminants from wastewater by forward osmosis (FO), disposal of the FO concentrate was still an unsolved issue. In this study, by integrating the advantages of forward osmosis and electrochemical oxidation, a forward osmosis process with the function of electrochemical oxidation (FOwEO) was established for the first time to achieve the aim of rejection of trace antibiotics from wastewater and treatment of the concentrate at the same time. Results demonstrated that FOwEO (current density J = 1 mA cm−2 ) exhibited excellent rejections of antibiotics (>98%) regardless of different operation conditions, and above all, antibiotics in the concentrate were well degraded (>99%) at the end of experiment (after 3 h). A synergetic effect between forward osmosis and electrochemical oxidation was observed in FOwEO, which lies in that antibiotic rejections by FO were enhanced due to the degradation of antibiotics in the concentrate, while the electrochemical oxidation capacity was
∗ Corresponding author. Tel.: +86 411 84706173; fax: +86 411 84708083. ∗∗ Corresponding author. Tel.: +86 451 86283068; fax: +86 451 87162150. E-mail addresses: [email protected] (H. Zhang), [email protected] (Y. Feng). http://dx.doi.org/10.1016/j.jhazmat.2015.04.048 0304-3894/© 2015 Elsevier B.V. All rights reserved.
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improved in the FOwEO channel, of which good mass transfer and the assist of indirect oxidation owing to the reverse NaCl from draw solution were supposed to be the mechanism. This study demonstrated that the FOwEO has the capability to thoroughly remove trace antibiotics from wastewater. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials and methods
Occurrence of trace antibiotics in the aquatic environment has received global concerns on account of threats to the environment and human health, such as assist in development of antibiotic-resistant bacteria [1], toxicity to non-target organisms [2], contamination of drinking water resources [3], etc. Considering the pathways by which antibiotics enter into the aquatic environment, effluent from municipal wastewater treatment plants (WWTPs) has been identified as a hotspot source due to the insufficient removal of antibiotics by the traditional treatment processes [4]. Control of the discharge of antibiotics from WWTPs could be an effective approach to limit the occurrence of antibiotics in the environment, which consequently contributes to reducing the environmental risk. Membrane filtration technologies like reverse osmosis (RO) and nanofiltration (NF) have been found to be with a good performance in removal of trace pharmaceuticals, and production of high quality water from WWTP secondary effluent [5,6]. However, both RO and NF were energy intensive processes and suffering from severe membrane fouling [7]. In recent decades, forward osmosis (FO), a new membrane process driven by a difference in osmotic pressure across the membrane with advantages of high quality permeate, no need of hydraulic pressures and low membrane fouling propensity [8], has been developed for water treatment, such as treatment of industry wastewater, desalination of sea water, reclamation of impaired water, etc. [9]. More recently, several investigations have specifically focused on the performance of FO for rejection of trace pharmaceutical contaminants. Hancock et al. [10] investigated rejections of 32 trace organic contaminants by FO membrane in both benchand pilot-scale FO installations. Valladares Linares et al. [11] conducted the study on rejection of micropollutants by clean and fouled FO membrane. Jin et al. [12] found that the rejections of four trace pharmaceuticals by four different kinds of FO were in the range of 64–97%. D’Haese et al. [13] studied the influence of membrane fouling on trace organics transport in closed-loop forward osmosis systems. Xie et al. [14–17] conducted several studies to explore the influence of operation conditions, like membrane orientation, temperature, membrane fouling and different draw solution solute, on the rejections of pharmaceutically active compounds by FO membrane. The effectiveness of rejection of trace pharmaceuticals by FO was validated in these studies. However, one issue that still needs to be solved is the disposal of FO concentrate which contains relatively high levels of pharmaceuticals enriched during the filtration process. As an advanced oxidation process, electrochemical oxidation offers advantages for refractory pollutants treatment including the effective control of reaction conditions, in-situ generation of oxidants without addition of chemicals and operation at ambient temperature and pressure [18,19]. Therefore, in this study, by integrating the advantages of forward osmosis and electrochemical oxidation, a forward osmosis process with the function of electrochemical oxidation (FOwEO) was established for the first time to achieve the aim of rejection of trace antibiotics from wastewater and disposal of the concentrate at the same time.
2.1. Trace antibiotic contaminants Four antibiotics, namely, sulfamethoxazole (SMX), trimethoprim (TMP), norfloxacin (NOR) and roxithromycin (ROX), which belong to four different antibiotic groups with different properties, such as relative molecular weight (253–837), charge and hydrophobicity, and were frequently detected in WWTP effluent, were selected as the target trace contaminants. The physico-chemical properties of the four antibiotics are summarized in Table S1 in Supplementary data. 2.2. Forward osmosis membrane The FO membrane used in this study is a commercially available asymmetric cellulose triacetate membrane provided by Hydration Technology Innovations (HTI, Albany, USA). It has been widely used and investigated in previous studies, and more details of the membrane properties could be obtained there [15,16]. 2.3. FOwEO and other devices The forward osmosis process with the function of electrochemical oxidation (FOwEO) is made up of a FO process and an electrolytic cell which is integrated in the feed channel of the membrane cell. The integrated FOwEO cell is the essential component of the FOwEO. It consists of two channels, namely FOwEO channel and draw solution channel, of which the dimensions are both 200 mm long, 20 mm wide, and 8 mm deep. Forward osmosis membrane is set in the middle to separate the two channels. A stainless steel mesh is used as cathode, and horizontally placed above the FO membrane with a 2 mm gap in the FOwEO channel. Above the cathode with a 5 mm gap is the anode which is a titanium mesh coated by IrO2 –Ta2 O5 –SnO2 with a projected surface area of 40 cm2 (20 × 2 cm). Ti/IrO2 –Ta2 O5 –SnO2 anode is provided by Long Sheng Electrode Co., China. The internal schematic of the FOwEO cell is shown in Fig. 1. The flow diagram of FOwEO and the photograph of FOwEO cell are shown in Fig. 2. Two peristaltic pumps (Longer Pump® BT6002J, Baoding Longer Precision Pump Co., Ltd.) were used to pump the feed and draw solutions in counter-current mode with a cross-flow velocity of 8 cm s−1 (flow rate of 770 mL min−1 ) in both channels, respectively. Reynold number (Re) of the flow was calculated to be 908 in the FOwEO channel. A digital direct-current supply was used to provide constant current for the FOwEO cell. The draw solution reservoir was placed on a digital balance, and weight changes were recorded by a computer to calculate the permeate water flux. The temperature of the feed and draw solutions was maintained around 20 ◦ C using a home-made cooling apparatus which cooled off the solution by circulating water. A traditional FO process was applied as a comparison with the FOwEO. The size of the FO cell and other operation procedures were all the same as those of the FOwEO. To compare the electrochemical oxidation efficiencies of antibiotics in FOwEO with those in a traditional electrolytic cell (EC), an undivided batch electrolytic cell was used to treat the feed solution under the same conditions as those
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P. Liu et al. / Journal of Hazardous Materials 296 (2015) 248–255 Table 1 Characteristics of the WWTP secondary effluent.
Fig. 1. The internal schematic of the FOwEO cell.
in FOwEO, such as same feed water quality and volume, electrodes and gap, current density, treatment time, etc. The solution in EC was circulated using a peristaltic pump at the same rotate speed in FOwEO to blend. 2.4. Experimental protocols Model feed solutions were prepared to contain 200 g L−1 of each antibiotic, 15 mg L−1 alginate, 10 mmol L−1 NaCl, 1 mmol L−1 NaHCO3 , and 0.5 mmol L−1 CaCl2 with pH adjusted to 7.0. NaCl solutions with concentrations of 1 mol L−1 (M), 2 M or 4 M were used as draw solutions. In FOwEO and FO processes, both membrane orientation, namely, active layer of membrane facing feed
Parameters
Value
pH DOC (mg L−1 ) NH4 + –N(mg L−1 ) NO3 - –N (mg L−1 ) UV254 (cm−1 ) SUVA (L mg−1 m−1 )
7.6 6.8 9.5 16.8 0.193 2.84
± ± ± ± ± ±
0.2 0.1 0.4 0.5 0.003 0.02
solutions (AL-FS) mode and that facing draw solutions (AL-DS) mode, were tested. Two different current density (J = 0.5 mA cm−2 and J = 1 mA cm−2 ) were applied to degrade antibiotics in FOwEO. Each experiment was conducted at room temperature around 20 ◦ C, and repeated twice to ensure the precision of the results. A new FO membrane coupon was used for each experiment. The experimental conditions in FOwEO and FO process were illustrated in Table S2 in Supplementary data. The initial volumes of feed and draw solutions were 1 L and 2 L, respectively. In the feed solutions, 1 mL sample was taken at the beginning and end (after 3 h) of each experiment, respectively, for determination of antibiotics concentrations. 500 mL sample was taken in the draw solutions and immediately extracted by solid phase extraction (SPE). Detailed SPE procedures were described in Supplementary data. In one set of experiment, which was conducted in AL-DS mode with draw solution concentration of 1 M, 1 mL sample was taken at certain time intervals (0 min, 30 min, 60 min, 90 min, 120 min, 180 min) in the feed to investigate the change of antibiotics concentrations during FOwEO or FO process. To investigate the adsorption of antibiotics onto membrane, the FO membrane coupons after experiments were extracted and detected. The extraction procedures were described in Supplementary data. The secondary effluent from WWTP was collected and used in both FO and FOwEO. The characteristics of the secondary effluent are shown in Table 1. Prior to experiments, the effluent was filtered by Whatman filter paper No. 1 to remove suspended matter. Then, it was used as the feed solution without any antibiotics spiked, and treated by FOwEO or FO process for 12 h, respectively. AL-FS mode was applied in this set of experiments, and the concentration of draw solutions was 2 M. Current density of 1 mA cm−2 was used in FOwEO. The rejection of antibiotics in FOwEO and FO processes was defined as: R=
1−
cp cf
100%
(1)
R is the rejection of antibiotics, cp (g L−1 ) is the antibiotic concentration in permeate, and cf (g L−1 ) is the average concentration of antibiotics in the feed during the experiment. As cp can not be detected directly due to the volume of the draw solution initially present, it is calculated as follow: cp =
cd × Vd Vp
(2)
cd (g L−1 ) is the concentration of antibiotics in the draw solutions at the end of experiments, Vd (L) is the volume of the draw solutions at the end of experiments, Vp (L) is the volume of the permeate produced during the experiments. 2.5. Analytical methods
Fig. 2. The flow diagram of FOwEO and the photograph of the FOwEO cell.
High performance liquid chromatography coupled with tandem mass spectrometry (HPLC–MS/MS) was applied to detect the antibiotics. The detailed method was described in our previous study [20]. Chloride ion was determined using ion-selective electrode
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Fig. 3. Change of antibiotics concentrations in the concentrate during FO and FOwEO (experiment conditions: AL-DS mode, 1 M NaCl as draw solution; duplicate experiments were conducted).
method [21]. Free chlorine was measured using spectrophotometric method with N,N-diethyl-1,4-phenylenediamine (DPD) [22]. UV–vis spectrophotometer, UV-1700 (Shimadzu, Japan), was used to analyze the absorbance at 254 nm (UV254 ). Dissolved organic carbon (DOC) was measured using a total organic carbon analyzer (TOC-VCPH, Shimadzu, Japan). The value of specific UV absorbance (SUVA) was calculated as follow: SUVA = 100
UV254 DOC
(3)
3. Results and discussion 3.1. Antibiotics removal in the concentrate during FOwEO Change of antibiotics concentrations in the concentrate during FO and FOwEO was investigated to explore the antibiotics removal (see Fig. 3). As a comparison, in FO process, antibiotics concentrations presented a generally increasing trend, which could be ascribed to the reduction in volume of the concentrate with water permeation, and the accumulation of antibiotics in concentrate due to the membrane rejection. Besides, adsorption of antibiotics onto membrane during the FO process was verified through membrane extraction experiments. The adsorption amount was in the range of 7–451 ng cm−2 (detailed data are shown in Table S3 in Supplementary data). In FOwEO with J = 0.5 mA cm−2 , a decreasing concentration was observed for all the four antibiotics, where NOR showed the best removal efficiency (99% after 30 min), SMX and TMP were also highly removed (90% and 81% after 3 h), while ROX exhibited a relatively poor removal (46%). The degradation of trace antibiotics by electrochemical oxidation was validated. However, the removal efficiencies varied among them, of which the reason was supposed
to be the diversity of physical–chemical properties and structures of the four antibiotics. For instance, ROX was quite recalcitrant to be damaged because of the steady structure of the 14-membered ring. Evidence by other studies also showed that ROX was hardly degraded through biodegradation or UV photolysis [23,24]. At J = 1 mA cm−2 , the increase in applied charge exerted a notable effect on the removals of all the four antibiotics. Faster removal rates and higher removal efficiencies were achieved compared with those at the lower current density. A removal efficiency of 90% to each target pollutant was attained after 60 min, and 99% was achieved at the end of experiment. ROX, which was hardly degraded at J = 0.5 mA cm−2 , was well degraded at J = 1 mA cm−2 . In summary, it was demonstrated that FOwEO (J = 1 mA cm−2 ) achieved the aim of eliminating the trace antibiotics in the concentrate during the proceeding filtration process. To further evaluate the electrochemical oxidation capacity in FOwEO, other experiments were conducted. 3.2. Electrochemical oxidation efficiencies for antibiotics in FOwEO Electrochemical oxidation efficiencies for antibiotics in FOwEO were investigated and compared with those in an undivided batch electrolytic cell (EC) under the same experimental conditions, and results were shown in Fig. S1 in Supplementary data. It can be observed that the oxidation efficiencies of antibiotics in FOwEO were higher than those in EC. The phenomena could be ascribed to two reasons. First, compared with EC, FOwEO had a better mass transfer. In FOwEO, the feed solution was circulated and flowed over the anode at the rate of 8 cm s−1 with Re of 907, while the circulated flow in EC was comparatively moderate because of the larger volume of the reactor (Re = 174).
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Fig. 4. The reverse NaCl from draw solution and produced free chlorine in the concentrate during FOwEO (experiment conditions: AL-FS mode, 2 M NaCl as draw solution, pure water as initial feed, electricity was supplied after 60 min, current density J = 1 mA cm−2 ).
In addition, during FOwEO, with the water constantly permeating into the draw solution, the feed solution was continuously concentrated, as a result, the conductivity of the feed solution increased with time, which also enhanced the electrochemical oxidation capacity. Normally, the electrochemical oxidation process involves two ways, namely, direct oxidation with electron transferring from pollutant to the anode and indirect oxidation with heterogeneous reactive species formed from water discharge at the anode [25]. For direct oxidation mechanism, current density and mass transfer are two main factors governing the oxidation efficiencies when the electrodes and target pollutants are selected [26]. Therefore, when same current density was applied for EC and FOwEO, the latter that had a better mass transfer was supposed to exhibit a higher electrochemical oxidation efficiencies of antibiotics. Besides, it was demonstrated that the antibiotics decay in EC was in good agreement with pseudo
Fig. 5. The proposed mechanism of antibiotics oxidation in the concentrate during FOwEO.
Fig. 6. Rejections of the four antibiotics in different operation conditions in FO process ((a): in AL-DS mode; (b): in AL-FS mode; duplicate experiments were conducted).
first-order reaction in our study (see Fig. S2 in Supplementary data), which also implied the oxidation reaction was diffusion-controlled [27]. The second reason was considered to be that indirect oxidation in FOwEO was strengthened. For indirect oxidation mechanism, active chlorine species, such as Cl2 (aq), HClO, ClO− , ClO2 − , are very important species that have high standard potentials (E0 (Cl2 ) = 1.36 V, E0 (HClO) = 1.49 V, E0 (ClO− ) = 0.89 V) and can react with pollutants in the wastewater [28]. It was believed that the generation of active chlorine species could be derived from the oxidation of Cl− on the surface of anode [25]. Therefore, addition of chloride ions in the electrolyte could lead to an increase in removal efficiencies of pollutants [29]. For instance, during the anodic degradation of oxalic acid (OA) process, Scialdone et al. [30] found that higher abatement of OA was attained when NaCl was added into the electrolyte in an electrolysis system with DAS (Ti/IrO2 –Ta2 O5 ) as anodes. Bagastyo et al. [19] applied electrochemical oxidation for treatment of reverse osmosis concentrate and found that the high removal of organics, nitrogen and color appeared to be associated with the high chlorine formation, suggesting that indirect oxidation was likely the main removal mechanism. In another aspect, it was well known that reverse draw solute flux occurred inevitably due to the large concentration difference between the draw solution and feed solution during forward osmosis process, especially for the monovalent salt NaCl as draw solution [14,31]. Therefore, in FOwEO, the reverse NaCl from draw solution would be partially transformed into active chlorine species by the anode in the FOwEO channel, which was believed to be able to enhance the indirect oxidation efficiencies to some extent. To verify
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the diffusive driving force of solutes from the membrane into the draw solution; hence, leading to a decline in rejection [14,33]. The results of membrane extraction experiments have confirmed that the accumulation of antibiotics on the supporting layer of FO membrane (in AL-DS mode) was higher than that on the active layer (in AL-FS mode) (data were shown in Table S3 in Supplementary data). From Fig. 6(a), it also can be observed that the rejections of SMX, TMP and NOR decreased with the increase of draw solution concentration. The possible reason was that at AL-DS mode, when higher concentration of draw solution was used, higher reverse salt flux through the membrane was attained, which resulted in a higher ionic strength within the porous support layer of the membrane, thus weakening the electrostatic interaction and leading to a lower rejection of charged species [33]. As for ROX, with high relative molecular weight (837), steric exclusion was bound to be the predominant mechanism in rejection by FO membrane. As shown in Fig. 6(b), an overall rejection of antibiotics was obtained in the range of 89–98% in AL-FS mode, and no obvious change could be observed with the increase of draw solution concentration. It seems that different membrane orientation makes a quite different rejection of trace antibiotics. Fig. 7 compares the rejections during FOwEO and FO in different membrane orientations. Results showed that rejections during FOwEO (>98%) were higher than those during FO process (54–98%) in spite of membrane orientations. The enhancement was in the range of 1–45%. Table 2 compares the antibiotic concentrations in the effluents from FOwEO and FO processes. The antibiotic levels in the effluent from FOwEO were at least one order of magnitudes lower than those from FO process. The remarkable rejection efficiencies of antibiotics in FOwEO were attributed to the simultaneous degradation of antibiotics in the concentrate through the electrochemical oxidation during the filtration. In a word, the rejection of antibiotics in FOwEO was enhanced due to the degradation of antibiotics in the concentrate through electrochemical oxidation. And as discussed previously, the electrochemical oxidation efficiencies of antibiotics in FOwEO were also improved by the assist of forward osmosis process. Therefore, a synergetic effect between forward osmosis and electrochemical oxidation was well achieved in FOwEO. In addition, it was notable that as indirect oxidants like active chlorine species were produced during FOwEO process, the active layer of FO membrane may be damaged in long-term running. This could be a potential drawback of FOwEO, which needs further investigation in future study.
Fig. 7. Comparison of rejections of the four antibiotics in FOwEO and FO (experiment conditions: 4 M NaCl as draw solution in AL-DS mode; 2 M NaCl as draw solution in AL-FS mode; J = 1 mA cm−2 in FOwEO; duplicate experiments were conducted).
this speculation, detection of reverse NaCl and free chlorine in feed was tested during FOwEO with pure water as initial feed. Results demonstrated that both reverse NaCl and free chlorine were found in the feed solution (see Fig. 4). The average reverse NaCl flux was calculated to be 27 g m−2 h−1 , and the concentration of free chlorine was found in the range of 0.05–0.1 mg L−1 after electricity was supplied. Therefore, the antibiotics oxidation mechanism in FOwEO was proposed as shown in Fig. 5. To sum up, the electrochemical oxidation efficiencies of antibiotics in FOwEO were remarkable, and the promotion of electrochemical oxidation capacity due to forward osmosis was also observed, which confirmed the success of the combination of the two processes. However, due to the limitation of current conditions, identification of degradation products of antibiotics produced in FOwEO needs to be further studied in future research. Besides, the influence of electrochemical oxidation on forward osmosis was studied by investigation of the rejections of antibiotics and water flux in FOwEO. 3.3. Rejections of antibiotics during FO and FOwEO Rejections of antibiotics during FO processes were investigated under different membrane orientations and with different concentrations of draw solutions. In AL-DS mode, an overall rejection was obtained in the range of 54–89% (see Fig. 6(a)). The reason for the relatively low antibiotic rejection was that the porous and rough structure of the support layer of the FO membrane [32] could make the antibiotics easily accumulate on the supporting layer and cause severe internal concentration polarization (ICP), which increased
3.4. Water flux during FO and FOwEO Water flux was investigated in all sets of experiments. The enhancement of water flux in FOwEO was expected due to the stainless steel mesh cathode over the membrane, which was believed to be able to conduce to a turbulence flow and alleviate the
Table 2 The antibiotic concentrations (unit: ng L−1 ) in effluents from FO and FOwEO (J = 1 mA cm−2 ). Draw solution
NOR
SMX
TMP
ROX
FO
FOwEO
FO
FOwEO
FO
FOwEO
FO
FOwEO
AL-DS 1M 2M 4M
24.6 ± 3.2 36.5 ± 4.6 66.8 ± 10
n.a. n.a. n.a.
44 ± 3.2 51.2 ± 6.2 92.6 ± 14.1
n.a. n.a. n.a.
34.3 ± 4.5 42.3 ± 4.2 67.6 ± 10.1
n.a. n.a. n.a.
45.6 ± 5.2 42.3 ± 6.7 28.2 ± 3.1
3.2 ± 0.8 4.8 ± 0.6 3.1 ± 0.3
AL-FS 2M 4M
12.3 ± 1.1 8.6 ± 1.3
n.a. n.a.
19.4 ± 1.3 14.2 ± 2.4
n.a. n.a.
20.6 ± 1.3 26.3 ± 2.1
n.a. n.a.
12.3 ± 1.5 2.7 ± 0.2
n.a. n.a.
Note: “n.a.” represents not available, analytes were detected below LOQs, duplicate experiments were conducted.
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Table 3 Antibiotic concentrations in secondary effluent, concentrates and effluents from FO and FOwEO (J = 1 mA cm−2 ). Concentration in different samples (ng L−1 ) Antibiotic
Secondary effluent
SMX TMP NOR ROX
1311 159 134 753
± ± ± ±
65 13 29 35
FO concentrate 2356 291 210 1426
± ± ± ±
91 25 18 72
FO effluent 168 36 13 42
± ± ± ±
19 6 1 3
FOwEO concentrate
FOwEO effluent
n.a. n.a. n.a. n.a.
n.a. n.a. n.a. n.a.
Note: “n.a.” represents not available, analytes were detected below LOQs.
concentration polarization; thus, improving the flux according to our previous study [34]. However, results showed that no significant difference of water flux was observed in FO and FOwEO under the same operation conditions (shown in Fig. 8). Perhaps, the installation of electrodes also resulted in the difficulty in maintaining a high shear flow speed in the FOwEO channel, which neutralized the positive effect of the produced turbulence on the flux. In addition, compared with flux using pure water as feed, the flux decline with model solution feed was supposed to be due to membrane fouling caused by the background organics (alginate) during the filtration.
3.5. Treatment of WWTP secondary effluent in FO and FOwEO To further explore the capability of FOwEO to treating antibiotics in real wastewater, experiments using WWTP secondary effluent as feed were conducted. The antibiotic concentrations in the secondary effluent were in the range of134–1311 ng L−1 (see Table 3). After treated by the FO process, a rejection range of 84–96% was obtained for the four antibiotics, whereas in FOwEO, the target antibiotics were all detected below the limit of quantification (LOQs) in both the permeate and concentrate, which implied the good efficiencies to rejecting antibiotics from the wastewater and eliminating them from the concentrate at the same time. In addition, other characteristics of the concentrates from FO and FOwEO were investigated as well (shown in Table S4 in Supplementary data). Dissolved organic carbon (DOC), NH4 + -N and UV254 in the concentrate from FOwEO were 24%, 47% and 41% lower than those from FO process respectively, indicating the effectiveness of further treatment of the concentrate in FOwEO. 4. Conclusions Results of this study demonstrated that the forward osmosis process with function of electrochemical oxidation (FOwEO) has the capability to thoroughly remove trace antibiotics from wastewater: • In FOwEO (current density J = 1 mA cm−2 ), rejecting trace antibiotics from the wastewater (>98%) and eliminating them from the concentrate (99%) were achieved at the same time in the end of experiment (after 3 h). • Compared with a traditional electrolytic cell (EC), electrochemical oxidation capacity was improved in FOwEO, of which good mass transfer and the assist of indirect oxidation owing to the reverse NaCl from draw solution were proposed to be the mechanism. • Rejections of antibiotics (>98%) during FOwEO were enhanced due to the degradation of antibiotics in the concentrate compared with those (54–98%) during a traditional forward osmosis process (FO). • A synergetic effect between forward osmosis and electrochemical oxidation was well attained in FOwEO. In summary, the combination of forward osmosis with electrochemical oxidation in FOwEO achieved a desirable result in treatment of trace antibiotics from wastewater. Acknowledgments
Fig. 8. The water flux of FO and FOwEO (J = 1 mA cm−2 ) with pure water and model solution as feed. (experimental conditions: (a) AL-DS mode, 1 M NaCl as draw solution; (b) AL-FS mode, 2 M NaCl as draw solution.)
The research is financially supported by the Fundamental Research Funds for the Central Universities and the National Natural Science Foundation of China (NSFC 51278079). And we also gratefully acknowledge the support from the Open Project of State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology, NO. ESK201301).
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Integrating electrochemical oxidation into forward osmosis process for removal of trace antibiotics in wastewater Pengxiao Liu a,c , Hanmin Zhang a,∗ , Yujie Feng b,∗∗ , Chao Shen a , Fenglin Yang a a Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China b State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73, Huanghe Road, Nangang District, Harbin 150090, China c Beijing Aerospace Institute for Metrology and Measurement Technology, China Academy of Launch Vehicle Technology, Beijing 100076, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Forward osmosis with the function of electrochemical oxidation was established. • FOwEO has the capability to thoroughly remove trace antibiotics from wastewater. • Rejection and degradation efficiencies for antibiotics were improved in FOwEO. • A synergetic effect between FO and EO was well achieved in FOwEO.
a r t i c l e
i n f o
Article history: Received 14 January 2015 Received in revised form 26 March 2015 Accepted 16 April 2015 Available online 18 April 2015 Keywords: Wastewater Trace antibiotics Forward osmosis Concentrate disposal
a b s t r a c t During the rejection of trace pharmaceutical contaminants from wastewater by forward osmosis (FO), disposal of the FO concentrate was still an unsolved issue. In this study, by integrating the advantages of forward osmosis and electrochemical oxidation, a forward osmosis process with the function of electrochemical oxidation (FOwEO) was established for the first time to achieve the aim of rejection of trace antibiotics from wastewater and treatment of the concentrate at the same time. Results demonstrated that FOwEO (current density J = 1 mA cm−2 ) exhibited excellent rejections of antibiotics (>98%) regardless of different operation conditions, and above all, antibiotics in the concentrate were well degraded (>99%) at the end of experiment (after 3 h). A synergetic effect between forward osmosis and electrochemical oxidation was observed in FOwEO, which lies in that antibiotic rejections by FO were enhanced due to the degradation of antibiotics in the concentrate, while the electrochemical oxidation capacity was
∗ Corresponding author. Tel.: +86 411 84706173; fax: +86 411 84708083. ∗∗ Corresponding author. Tel.: +86 451 86283068; fax: +86 451 87162150. E-mail addresses: [email protected] (H. Zhang), [email protected] (Y. Feng). http://dx.doi.org/10.1016/j.jhazmat.2015.04.048 0304-3894/© 2015 Elsevier B.V. All rights reserved.
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improved in the FOwEO channel, of which good mass transfer and the assist of indirect oxidation owing to the reverse NaCl from draw solution were supposed to be the mechanism. This study demonstrated that the FOwEO has the capability to thoroughly remove trace antibiotics from wastewater. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Materials and methods
Occurrence of trace antibiotics in the aquatic environment has received global concerns on account of threats to the environment and human health, such as assist in development of antibiotic-resistant bacteria [1], toxicity to non-target organisms [2], contamination of drinking water resources [3], etc. Considering the pathways by which antibiotics enter into the aquatic environment, effluent from municipal wastewater treatment plants (WWTPs) has been identified as a hotspot source due to the insufficient removal of antibiotics by the traditional treatment processes [4]. Control of the discharge of antibiotics from WWTPs could be an effective approach to limit the occurrence of antibiotics in the environment, which consequently contributes to reducing the environmental risk. Membrane filtration technologies like reverse osmosis (RO) and nanofiltration (NF) have been found to be with a good performance in removal of trace pharmaceuticals, and production of high quality water from WWTP secondary effluent [5,6]. However, both RO and NF were energy intensive processes and suffering from severe membrane fouling [7]. In recent decades, forward osmosis (FO), a new membrane process driven by a difference in osmotic pressure across the membrane with advantages of high quality permeate, no need of hydraulic pressures and low membrane fouling propensity [8], has been developed for water treatment, such as treatment of industry wastewater, desalination of sea water, reclamation of impaired water, etc. [9]. More recently, several investigations have specifically focused on the performance of FO for rejection of trace pharmaceutical contaminants. Hancock et al. [10] investigated rejections of 32 trace organic contaminants by FO membrane in both benchand pilot-scale FO installations. Valladares Linares et al. [11] conducted the study on rejection of micropollutants by clean and fouled FO membrane. Jin et al. [12] found that the rejections of four trace pharmaceuticals by four different kinds of FO were in the range of 64–97%. D’Haese et al. [13] studied the influence of membrane fouling on trace organics transport in closed-loop forward osmosis systems. Xie et al. [14–17] conducted several studies to explore the influence of operation conditions, like membrane orientation, temperature, membrane fouling and different draw solution solute, on the rejections of pharmaceutically active compounds by FO membrane. The effectiveness of rejection of trace pharmaceuticals by FO was validated in these studies. However, one issue that still needs to be solved is the disposal of FO concentrate which contains relatively high levels of pharmaceuticals enriched during the filtration process. As an advanced oxidation process, electrochemical oxidation offers advantages for refractory pollutants treatment including the effective control of reaction conditions, in-situ generation of oxidants without addition of chemicals and operation at ambient temperature and pressure [18,19]. Therefore, in this study, by integrating the advantages of forward osmosis and electrochemical oxidation, a forward osmosis process with the function of electrochemical oxidation (FOwEO) was established for the first time to achieve the aim of rejection of trace antibiotics from wastewater and disposal of the concentrate at the same time.
2.1. Trace antibiotic contaminants Four antibiotics, namely, sulfamethoxazole (SMX), trimethoprim (TMP), norfloxacin (NOR) and roxithromycin (ROX), which belong to four different antibiotic groups with different properties, such as relative molecular weight (253–837), charge and hydrophobicity, and were frequently detected in WWTP effluent, were selected as the target trace contaminants. The physico-chemical properties of the four antibiotics are summarized in Table S1 in Supplementary data. 2.2. Forward osmosis membrane The FO membrane used in this study is a commercially available asymmetric cellulose triacetate membrane provided by Hydration Technology Innovations (HTI, Albany, USA). It has been widely used and investigated in previous studies, and more details of the membrane properties could be obtained there [15,16]. 2.3. FOwEO and other devices The forward osmosis process with the function of electrochemical oxidation (FOwEO) is made up of a FO process and an electrolytic cell which is integrated in the feed channel of the membrane cell. The integrated FOwEO cell is the essential component of the FOwEO. It consists of two channels, namely FOwEO channel and draw solution channel, of which the dimensions are both 200 mm long, 20 mm wide, and 8 mm deep. Forward osmosis membrane is set in the middle to separate the two channels. A stainless steel mesh is used as cathode, and horizontally placed above the FO membrane with a 2 mm gap in the FOwEO channel. Above the cathode with a 5 mm gap is the anode which is a titanium mesh coated by IrO2 –Ta2 O5 –SnO2 with a projected surface area of 40 cm2 (20 × 2 cm). Ti/IrO2 –Ta2 O5 –SnO2 anode is provided by Long Sheng Electrode Co., China. The internal schematic of the FOwEO cell is shown in Fig. 1. The flow diagram of FOwEO and the photograph of FOwEO cell are shown in Fig. 2. Two peristaltic pumps (Longer Pump® BT6002J, Baoding Longer Precision Pump Co., Ltd.) were used to pump the feed and draw solutions in counter-current mode with a cross-flow velocity of 8 cm s−1 (flow rate of 770 mL min−1 ) in both channels, respectively. Reynold number (Re) of the flow was calculated to be 908 in the FOwEO channel. A digital direct-current supply was used to provide constant current for the FOwEO cell. The draw solution reservoir was placed on a digital balance, and weight changes were recorded by a computer to calculate the permeate water flux. The temperature of the feed and draw solutions was maintained around 20 ◦ C using a home-made cooling apparatus which cooled off the solution by circulating water. A traditional FO process was applied as a comparison with the FOwEO. The size of the FO cell and other operation procedures were all the same as those of the FOwEO. To compare the electrochemical oxidation efficiencies of antibiotics in FOwEO with those in a traditional electrolytic cell (EC), an undivided batch electrolytic cell was used to treat the feed solution under the same conditions as those
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P. Liu et al. / Journal of Hazardous Materials 296 (2015) 248–255 Table 1 Characteristics of the WWTP secondary effluent.
Fig. 1. The internal schematic of the FOwEO cell.
in FOwEO, such as same feed water quality and volume, electrodes and gap, current density, treatment time, etc. The solution in EC was circulated using a peristaltic pump at the same rotate speed in FOwEO to blend. 2.4. Experimental protocols Model feed solutions were prepared to contain 200 g L−1 of each antibiotic, 15 mg L−1 alginate, 10 mmol L−1 NaCl, 1 mmol L−1 NaHCO3 , and 0.5 mmol L−1 CaCl2 with pH adjusted to 7.0. NaCl solutions with concentrations of 1 mol L−1 (M), 2 M or 4 M were used as draw solutions. In FOwEO and FO processes, both membrane orientation, namely, active layer of membrane facing feed
Parameters
Value
pH DOC (mg L−1 ) NH4 + –N(mg L−1 ) NO3 - –N (mg L−1 ) UV254 (cm−1 ) SUVA (L mg−1 m−1 )
7.6 6.8 9.5 16.8 0.193 2.84
± ± ± ± ± ±
0.2 0.1 0.4 0.5 0.003 0.02
solutions (AL-FS) mode and that facing draw solutions (AL-DS) mode, were tested. Two different current density (J = 0.5 mA cm−2 and J = 1 mA cm−2 ) were applied to degrade antibiotics in FOwEO. Each experiment was conducted at room temperature around 20 ◦ C, and repeated twice to ensure the precision of the results. A new FO membrane coupon was used for each experiment. The experimental conditions in FOwEO and FO process were illustrated in Table S2 in Supplementary data. The initial volumes of feed and draw solutions were 1 L and 2 L, respectively. In the feed solutions, 1 mL sample was taken at the beginning and end (after 3 h) of each experiment, respectively, for determination of antibiotics concentrations. 500 mL sample was taken in the draw solutions and immediately extracted by solid phase extraction (SPE). Detailed SPE procedures were described in Supplementary data. In one set of experiment, which was conducted in AL-DS mode with draw solution concentration of 1 M, 1 mL sample was taken at certain time intervals (0 min, 30 min, 60 min, 90 min, 120 min, 180 min) in the feed to investigate the change of antibiotics concentrations during FOwEO or FO process. To investigate the adsorption of antibiotics onto membrane, the FO membrane coupons after experiments were extracted and detected. The extraction procedures were described in Supplementary data. The secondary effluent from WWTP was collected and used in both FO and FOwEO. The characteristics of the secondary effluent are shown in Table 1. Prior to experiments, the effluent was filtered by Whatman filter paper No. 1 to remove suspended matter. Then, it was used as the feed solution without any antibiotics spiked, and treated by FOwEO or FO process for 12 h, respectively. AL-FS mode was applied in this set of experiments, and the concentration of draw solutions was 2 M. Current density of 1 mA cm−2 was used in FOwEO. The rejection of antibiotics in FOwEO and FO processes was defined as: R=
1−
cp cf
100%
(1)
R is the rejection of antibiotics, cp (g L−1 ) is the antibiotic concentration in permeate, and cf (g L−1 ) is the average concentration of antibiotics in the feed during the experiment. As cp can not be detected directly due to the volume of the draw solution initially present, it is calculated as follow: cp =
cd × Vd Vp
(2)
cd (g L−1 ) is the concentration of antibiotics in the draw solutions at the end of experiments, Vd (L) is the volume of the draw solutions at the end of experiments, Vp (L) is the volume of the permeate produced during the experiments. 2.5. Analytical methods
Fig. 2. The flow diagram of FOwEO and the photograph of the FOwEO cell.
High performance liquid chromatography coupled with tandem mass spectrometry (HPLC–MS/MS) was applied to detect the antibiotics. The detailed method was described in our previous study [20]. Chloride ion was determined using ion-selective electrode
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Fig. 3. Change of antibiotics concentrations in the concentrate during FO and FOwEO (experiment conditions: AL-DS mode, 1 M NaCl as draw solution; duplicate experiments were conducted).
method [21]. Free chlorine was measured using spectrophotometric method with N,N-diethyl-1,4-phenylenediamine (DPD) [22]. UV–vis spectrophotometer, UV-1700 (Shimadzu, Japan), was used to analyze the absorbance at 254 nm (UV254 ). Dissolved organic carbon (DOC) was measured using a total organic carbon analyzer (TOC-VCPH, Shimadzu, Japan). The value of specific UV absorbance (SUVA) was calculated as follow: SUVA = 100
UV254 DOC
(3)
3. Results and discussion 3.1. Antibiotics removal in the concentrate during FOwEO Change of antibiotics concentrations in the concentrate during FO and FOwEO was investigated to explore the antibiotics removal (see Fig. 3). As a comparison, in FO process, antibiotics concentrations presented a generally increasing trend, which could be ascribed to the reduction in volume of the concentrate with water permeation, and the accumulation of antibiotics in concentrate due to the membrane rejection. Besides, adsorption of antibiotics onto membrane during the FO process was verified through membrane extraction experiments. The adsorption amount was in the range of 7–451 ng cm−2 (detailed data are shown in Table S3 in Supplementary data). In FOwEO with J = 0.5 mA cm−2 , a decreasing concentration was observed for all the four antibiotics, where NOR showed the best removal efficiency (99% after 30 min), SMX and TMP were also highly removed (90% and 81% after 3 h), while ROX exhibited a relatively poor removal (46%). The degradation of trace antibiotics by electrochemical oxidation was validated. However, the removal efficiencies varied among them, of which the reason was supposed
to be the diversity of physical–chemical properties and structures of the four antibiotics. For instance, ROX was quite recalcitrant to be damaged because of the steady structure of the 14-membered ring. Evidence by other studies also showed that ROX was hardly degraded through biodegradation or UV photolysis [23,24]. At J = 1 mA cm−2 , the increase in applied charge exerted a notable effect on the removals of all the four antibiotics. Faster removal rates and higher removal efficiencies were achieved compared with those at the lower current density. A removal efficiency of 90% to each target pollutant was attained after 60 min, and 99% was achieved at the end of experiment. ROX, which was hardly degraded at J = 0.5 mA cm−2 , was well degraded at J = 1 mA cm−2 . In summary, it was demonstrated that FOwEO (J = 1 mA cm−2 ) achieved the aim of eliminating the trace antibiotics in the concentrate during the proceeding filtration process. To further evaluate the electrochemical oxidation capacity in FOwEO, other experiments were conducted. 3.2. Electrochemical oxidation efficiencies for antibiotics in FOwEO Electrochemical oxidation efficiencies for antibiotics in FOwEO were investigated and compared with those in an undivided batch electrolytic cell (EC) under the same experimental conditions, and results were shown in Fig. S1 in Supplementary data. It can be observed that the oxidation efficiencies of antibiotics in FOwEO were higher than those in EC. The phenomena could be ascribed to two reasons. First, compared with EC, FOwEO had a better mass transfer. In FOwEO, the feed solution was circulated and flowed over the anode at the rate of 8 cm s−1 with Re of 907, while the circulated flow in EC was comparatively moderate because of the larger volume of the reactor (Re = 174).
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Fig. 4. The reverse NaCl from draw solution and produced free chlorine in the concentrate during FOwEO (experiment conditions: AL-FS mode, 2 M NaCl as draw solution, pure water as initial feed, electricity was supplied after 60 min, current density J = 1 mA cm−2 ).
In addition, during FOwEO, with the water constantly permeating into the draw solution, the feed solution was continuously concentrated, as a result, the conductivity of the feed solution increased with time, which also enhanced the electrochemical oxidation capacity. Normally, the electrochemical oxidation process involves two ways, namely, direct oxidation with electron transferring from pollutant to the anode and indirect oxidation with heterogeneous reactive species formed from water discharge at the anode [25]. For direct oxidation mechanism, current density and mass transfer are two main factors governing the oxidation efficiencies when the electrodes and target pollutants are selected [26]. Therefore, when same current density was applied for EC and FOwEO, the latter that had a better mass transfer was supposed to exhibit a higher electrochemical oxidation efficiencies of antibiotics. Besides, it was demonstrated that the antibiotics decay in EC was in good agreement with pseudo
Fig. 5. The proposed mechanism of antibiotics oxidation in the concentrate during FOwEO.
Fig. 6. Rejections of the four antibiotics in different operation conditions in FO process ((a): in AL-DS mode; (b): in AL-FS mode; duplicate experiments were conducted).
first-order reaction in our study (see Fig. S2 in Supplementary data), which also implied the oxidation reaction was diffusion-controlled [27]. The second reason was considered to be that indirect oxidation in FOwEO was strengthened. For indirect oxidation mechanism, active chlorine species, such as Cl2 (aq), HClO, ClO− , ClO2 − , are very important species that have high standard potentials (E0 (Cl2 ) = 1.36 V, E0 (HClO) = 1.49 V, E0 (ClO− ) = 0.89 V) and can react with pollutants in the wastewater [28]. It was believed that the generation of active chlorine species could be derived from the oxidation of Cl− on the surface of anode [25]. Therefore, addition of chloride ions in the electrolyte could lead to an increase in removal efficiencies of pollutants [29]. For instance, during the anodic degradation of oxalic acid (OA) process, Scialdone et al. [30] found that higher abatement of OA was attained when NaCl was added into the electrolyte in an electrolysis system with DAS (Ti/IrO2 –Ta2 O5 ) as anodes. Bagastyo et al. [19] applied electrochemical oxidation for treatment of reverse osmosis concentrate and found that the high removal of organics, nitrogen and color appeared to be associated with the high chlorine formation, suggesting that indirect oxidation was likely the main removal mechanism. In another aspect, it was well known that reverse draw solute flux occurred inevitably due to the large concentration difference between the draw solution and feed solution during forward osmosis process, especially for the monovalent salt NaCl as draw solution [14,31]. Therefore, in FOwEO, the reverse NaCl from draw solution would be partially transformed into active chlorine species by the anode in the FOwEO channel, which was believed to be able to enhance the indirect oxidation efficiencies to some extent. To verify
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the diffusive driving force of solutes from the membrane into the draw solution; hence, leading to a decline in rejection [14,33]. The results of membrane extraction experiments have confirmed that the accumulation of antibiotics on the supporting layer of FO membrane (in AL-DS mode) was higher than that on the active layer (in AL-FS mode) (data were shown in Table S3 in Supplementary data). From Fig. 6(a), it also can be observed that the rejections of SMX, TMP and NOR decreased with the increase of draw solution concentration. The possible reason was that at AL-DS mode, when higher concentration of draw solution was used, higher reverse salt flux through the membrane was attained, which resulted in a higher ionic strength within the porous support layer of the membrane, thus weakening the electrostatic interaction and leading to a lower rejection of charged species [33]. As for ROX, with high relative molecular weight (837), steric exclusion was bound to be the predominant mechanism in rejection by FO membrane. As shown in Fig. 6(b), an overall rejection of antibiotics was obtained in the range of 89–98% in AL-FS mode, and no obvious change could be observed with the increase of draw solution concentration. It seems that different membrane orientation makes a quite different rejection of trace antibiotics. Fig. 7 compares the rejections during FOwEO and FO in different membrane orientations. Results showed that rejections during FOwEO (>98%) were higher than those during FO process (54–98%) in spite of membrane orientations. The enhancement was in the range of 1–45%. Table 2 compares the antibiotic concentrations in the effluents from FOwEO and FO processes. The antibiotic levels in the effluent from FOwEO were at least one order of magnitudes lower than those from FO process. The remarkable rejection efficiencies of antibiotics in FOwEO were attributed to the simultaneous degradation of antibiotics in the concentrate through the electrochemical oxidation during the filtration. In a word, the rejection of antibiotics in FOwEO was enhanced due to the degradation of antibiotics in the concentrate through electrochemical oxidation. And as discussed previously, the electrochemical oxidation efficiencies of antibiotics in FOwEO were also improved by the assist of forward osmosis process. Therefore, a synergetic effect between forward osmosis and electrochemical oxidation was well achieved in FOwEO. In addition, it was notable that as indirect oxidants like active chlorine species were produced during FOwEO process, the active layer of FO membrane may be damaged in long-term running. This could be a potential drawback of FOwEO, which needs further investigation in future study.
Fig. 7. Comparison of rejections of the four antibiotics in FOwEO and FO (experiment conditions: 4 M NaCl as draw solution in AL-DS mode; 2 M NaCl as draw solution in AL-FS mode; J = 1 mA cm−2 in FOwEO; duplicate experiments were conducted).
this speculation, detection of reverse NaCl and free chlorine in feed was tested during FOwEO with pure water as initial feed. Results demonstrated that both reverse NaCl and free chlorine were found in the feed solution (see Fig. 4). The average reverse NaCl flux was calculated to be 27 g m−2 h−1 , and the concentration of free chlorine was found in the range of 0.05–0.1 mg L−1 after electricity was supplied. Therefore, the antibiotics oxidation mechanism in FOwEO was proposed as shown in Fig. 5. To sum up, the electrochemical oxidation efficiencies of antibiotics in FOwEO were remarkable, and the promotion of electrochemical oxidation capacity due to forward osmosis was also observed, which confirmed the success of the combination of the two processes. However, due to the limitation of current conditions, identification of degradation products of antibiotics produced in FOwEO needs to be further studied in future research. Besides, the influence of electrochemical oxidation on forward osmosis was studied by investigation of the rejections of antibiotics and water flux in FOwEO. 3.3. Rejections of antibiotics during FO and FOwEO Rejections of antibiotics during FO processes were investigated under different membrane orientations and with different concentrations of draw solutions. In AL-DS mode, an overall rejection was obtained in the range of 54–89% (see Fig. 6(a)). The reason for the relatively low antibiotic rejection was that the porous and rough structure of the support layer of the FO membrane [32] could make the antibiotics easily accumulate on the supporting layer and cause severe internal concentration polarization (ICP), which increased
3.4. Water flux during FO and FOwEO Water flux was investigated in all sets of experiments. The enhancement of water flux in FOwEO was expected due to the stainless steel mesh cathode over the membrane, which was believed to be able to conduce to a turbulence flow and alleviate the
Table 2 The antibiotic concentrations (unit: ng L−1 ) in effluents from FO and FOwEO (J = 1 mA cm−2 ). Draw solution
NOR
SMX
TMP
ROX
FO
FOwEO
FO
FOwEO
FO
FOwEO
FO
FOwEO
AL-DS 1M 2M 4M
24.6 ± 3.2 36.5 ± 4.6 66.8 ± 10
n.a. n.a. n.a.
44 ± 3.2 51.2 ± 6.2 92.6 ± 14.1
n.a. n.a. n.a.
34.3 ± 4.5 42.3 ± 4.2 67.6 ± 10.1
n.a. n.a. n.a.
45.6 ± 5.2 42.3 ± 6.7 28.2 ± 3.1
3.2 ± 0.8 4.8 ± 0.6 3.1 ± 0.3
AL-FS 2M 4M
12.3 ± 1.1 8.6 ± 1.3
n.a. n.a.
19.4 ± 1.3 14.2 ± 2.4
n.a. n.a.
20.6 ± 1.3 26.3 ± 2.1
n.a. n.a.
12.3 ± 1.5 2.7 ± 0.2
n.a. n.a.
Note: “n.a.” represents not available, analytes were detected below LOQs, duplicate experiments were conducted.
254
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Table 3 Antibiotic concentrations in secondary effluent, concentrates and effluents from FO and FOwEO (J = 1 mA cm−2 ). Concentration in different samples (ng L−1 ) Antibiotic
Secondary effluent
SMX TMP NOR ROX
1311 159 134 753
± ± ± ±
65 13 29 35
FO concentrate 2356 291 210 1426
± ± ± ±
91 25 18 72
FO effluent 168 36 13 42
± ± ± ±
19 6 1 3
FOwEO concentrate
FOwEO effluent
n.a. n.a. n.a. n.a.
n.a. n.a. n.a. n.a.
Note: “n.a.” represents not available, analytes were detected below LOQs.
concentration polarization; thus, improving the flux according to our previous study [34]. However, results showed that no significant difference of water flux was observed in FO and FOwEO under the same operation conditions (shown in Fig. 8). Perhaps, the installation of electrodes also resulted in the difficulty in maintaining a high shear flow speed in the FOwEO channel, which neutralized the positive effect of the produced turbulence on the flux. In addition, compared with flux using pure water as feed, the flux decline with model solution feed was supposed to be due to membrane fouling caused by the background organics (alginate) during the filtration.
3.5. Treatment of WWTP secondary effluent in FO and FOwEO To further explore the capability of FOwEO to treating antibiotics in real wastewater, experiments using WWTP secondary effluent as feed were conducted. The antibiotic concentrations in the secondary effluent were in the range of134–1311 ng L−1 (see Table 3). After treated by the FO process, a rejection range of 84–96% was obtained for the four antibiotics, whereas in FOwEO, the target antibiotics were all detected below the limit of quantification (LOQs) in both the permeate and concentrate, which implied the good efficiencies to rejecting antibiotics from the wastewater and eliminating them from the concentrate at the same time. In addition, other characteristics of the concentrates from FO and FOwEO were investigated as well (shown in Table S4 in Supplementary data). Dissolved organic carbon (DOC), NH4 + -N and UV254 in the concentrate from FOwEO were 24%, 47% and 41% lower than those from FO process respectively, indicating the effectiveness of further treatment of the concentrate in FOwEO. 4. Conclusions Results of this study demonstrated that the forward osmosis process with function of electrochemical oxidation (FOwEO) has the capability to thoroughly remove trace antibiotics from wastewater: • In FOwEO (current density J = 1 mA cm−2 ), rejecting trace antibiotics from the wastewater (>98%) and eliminating them from the concentrate (99%) were achieved at the same time in the end of experiment (after 3 h). • Compared with a traditional electrolytic cell (EC), electrochemical oxidation capacity was improved in FOwEO, of which good mass transfer and the assist of indirect oxidation owing to the reverse NaCl from draw solution were proposed to be the mechanism. • Rejections of antibiotics (>98%) during FOwEO were enhanced due to the degradation of antibiotics in the concentrate compared with those (54–98%) during a traditional forward osmosis process (FO). • A synergetic effect between forward osmosis and electrochemical oxidation was well attained in FOwEO. In summary, the combination of forward osmosis with electrochemical oxidation in FOwEO achieved a desirable result in treatment of trace antibiotics from wastewater. Acknowledgments
Fig. 8. The water flux of FO and FOwEO (J = 1 mA cm−2 ) with pure water and model solution as feed. (experimental conditions: (a) AL-DS mode, 1 M NaCl as draw solution; (b) AL-FS mode, 2 M NaCl as draw solution.)
The research is financially supported by the Fundamental Research Funds for the Central Universities and the National Natural Science Foundation of China (NSFC 51278079). And we also gratefully acknowledge the support from the Open Project of State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology, NO. ESK201301).
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